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Specifications are the regulatory and legal standards that a product must meet to be suitable for use in humans. Specifications evolve in parallel with drug development and are refined prior to marketing authorization and, in some cases, after marketing. Recent changes in regulatory procedures for biotechnology-derived protein products have placed much emphasis on the use of characterization and final product specifications to provide assurance of overall quality of these products. In addition, harmonized guidelines for the testing and specifications for biotechnology products have been developed through the International Conference on Harmonization process. The availability of sensitive, quantitative, and specific analytical methods for characterization has made this possible, thus providing regulatory flexibility in the development of biotechnology-derived protein products. Further refinement of these analytical tools will undoubtedly enhance this regulatory flexibility.

Forskolin (Fsk) has been demonstrated to interact directly with the enzyme adenylyl cyclase (EC 4.6.1.1) in diverse tissues. However, the ability of Fsk to bind to and activate adenylyl cyclase varies depending on the tissue being studied. Different adenylyl cyclase subtypes have been cloned and expressed in a recombinant Sf9 expression system. This provides an opportunity to study the effects of chemically reactive derivatives of Fsk on individual adenylyl cyclase subtypes in the absence of Gs alpha. Reaction of type I adenylyl cyclase with an isothiocyanate derivative of Fsk (6-[[N-(2-isothiocyanatoethyl)amino]carbonyl]forskolin) causes irreversible inhibition of Fsk binding with an IC50 of 300 nM and irreversible inhibition of Fsk activation with an IC50 of 10 microM, suggesting that there are two sites of 6-[[N-(2-isothiocyanatoethyl)amino]carbonyl]forskolin interaction. These studies establish the usefulness of the isothiocyanate derivative of Fsk in localizing the site(s) of Fsk interaction with type I adenylyl cyclase.

Three series of analogs were regioselectively prepared from a protected forskolin precursor to afford 7-carbamoyl-7-desacetylforskolins (series 1), 6-carbamoyl-7-desacetylforskolins (series 2), and 6-carbamoylforskolins (series 3). The analogs were pharmacologically evaluated for binding (IC50) to and activation (EC50) of type I adenylyl cyclase in membranes from stably transfected Sf9 cell lines expressing a single adenylate cyclase subtype. The following ranges were determined for the IC50's and EC50's of each individual series: series 1, IC50 = 43-1600 nM, EC50 = 0.5-9.6 microM; series 2, IC50 = 65-680 nM, EC50 = 0.63-6.5 microM; series 3, IC50 = 21-271 nM, EC50 = 0.5-8.1 microM (forskolin IC50 = 41 nM and EC50 = 0.5 microM). Activation paralleled binding; however, some analogs exhibited poor binding and good activation whereas others demonstrated good binding but poor activation. Steric bulk tended to diminish binding and activation when at the 6- or 7-position, although bulk was accommodated at the 6-position if the 7-site was reacetylated. Acylation of the 7-position by the carbamoyl linker or acetyl was important for obtaining good binding and activation; however, the effect was more pronounced with binding. For both binding and activation, small, linear, lipophilic substituents (propyl, allyl, isopropyl) are well tolerated at the 7-position but less so in the 6-position, even when the 7-site is reacetylated. Planar aromatic moieties (phenyl and 2-pyridinyl) demonstrated moderate to good potency for binding and activation when located at either the 6- or 7-positions. There is an overall trend toward increasing potency for both binding and activation with polar substituents.

An improved affinity support for the purification of adenylyl cyclase was prepared from 7-desacetyl-7-aminoethylaminocarbonyl forskolin. This analog allows convenient synthesis of an affinity matrix that is chemically stable, with-standing repeated use for up to two years, and efficient, yielding purifications of adenylyl cyclase from solubilized bovine brain membranes of 2,000-6,000 fold in a single step. Immunoblotting data suggest that the majority of the enzyme purified in this fashion differs from forms described previously. Since the specific activity of this preparation is substantially higher than that described in previous reports, it is possible that the purification described here selects, presumably on the basis of affinity for forskolin, for a form of adenylyl cyclase with higher specific activity than any described previously.

To identify specific drug targets of the antimitotic drug estramustine, a photoaffinity analogue, 17-O-[[2-[3-(4-azido-3-[125I] iodophenyl)propionamido]ethyl]carbamyl]estradiol-3-N-bis(2- chloroethyl)carbamate, was synthesized and reacted in competition assays with cytoskeletal protein preparations. By attaching the photoaffinity ligand to the 17 beta-position of the steroid D-ring, the cytotoxic properties of the drug were maintained. In cytoskeletal protein preparations from human prostate carcinoma cells (DU 145) or a clonally selected, estramustine-resistant cell line (E4), the major microtubule-associated protein (MAP) present was MAP4. In both cytoskeletal fractions and reconstituted microtubules, 17-O-[[2-[3-(4-azido-3-[125I]iodophenyl)propionamido] ethyl]carbamyl]estradiol-3-N-bis(2-chloroethyl)carbamate bound to both MAP4 and tubulin. From competition assays, the apparent binding constant for MAP4 from DU 145 cells was 15 microM. Similar calculations for tubulin gave values of 13 microM (bovine brain), 19 microM (DU 145 wild-type cells), and 25 microM (E4 cells). The identification of these cytoskeletal proteins as specific drug targets provides a direct explanation for the antimicrotubule and antimitotic effects of estramustine.

Several forms of adenylyl cyclase (types I, II, V, and VI) have been expressed using the recombinant baculovirus expression system in Sf9 cells. The activation of type I adenylyl cyclase by forskolin and Gs alpha was not greater than additive. In contrast, there was synergistic activation of type II, V, and VI adenylyl cyclases by Gs alpha and forskolin. Gs alpha potentiated the effect of forskolin on type II adenylyl cyclase to the greatest extent. Type I and II adenylyl cyclases were photolabeled specifically by an iodinated photoaffinity derivative of forskolin ([125I]-6-AIPP-Fsk). Type I adenylyl cyclase was photolabeled efficiently in the absence of Gs alpha, and the addition of Gs alpha only slightly increased the labeling efficiency. In contrast, type II adenylyl cyclase was not photolabeled efficiently in the absence of Gs alpha, and the addition of Gs alpha greatly enhanced the labeling efficiency. Photolabeling of type V and VI adenylyl cyclases was detected only in the presence of Gs alpha. Neither calcium/calmodulin nor G protein beta gamma subunits modulated the photolabeling of type I or II adenylyl cyclases. Another iodinated derivative of forskolin, [125I]-6-IHPP-fsk, bound to Sf9 cell membranes expressing type I adenylyl cyclase with high affinity in a filtration binding assay, and the specific binding was not enhanced by the addition of Gs alpha. In contrast, specific binding of [125I]-6-IHPP-Fsk to membranes expressing type II adenylyl cyclase was detected only in the presence of Gs alpha.(ABSTRACT TRUNCATED AT 250 WORDS)

Forskolin is a potent inhibitor of mammalian passive glucose transporters. Here we show that forskolin is a remarkably specific inhibitor of energized D-galactose transport by the GalP sugar-H+ symport protein of Escherichia coli. Surprisingly, it does not inhibit transport of L-arabinose or D-xylose by the related E. coli AraE and XylE transporters, even though the amino acid sequences of their proteins are 30-64% identical to GalP and to the mammalian GLUT family. However, unlike GLUT1, photoactivation of the [3H]forskolin-GalP complex fails to incorporate radioactivity covalently into the protein, in contrast to the effective incorporation of radioactivity from [3H]cytochalasin B into both proteins. However, 3-[125I]iodo-4-azidophenethylamido-7-O-succinyldesacetylforskol in ([125I]APS-forskolin), which labels GLUT1, is a potent labeling reagent for GalP and, to a lesser extent, for AraE. The appropriate sugar substrates of each transporter protect it against the [125I]APS-forskolin. Equilibrium binding studies using membranes from an E. coli strain that overexpresses GalP reveal a single set of high affinity binding sites for [3H]forskolin with a Kd of 1.3-1.4 microM, probably forming a 1:1 complex, compared with a value of 7.5 microM for GLUT1. Sugar substrates of GalP and cytochalasin B displace forskolin from the protein. The nonhomologous sugar-H+ symporters for L-rhamnose (RhaT), L-fucose (FucP) and lactose (LacY) in E. coli are insensitive to forskolin. Forskolin and [125I]APS-forskolin, therefore, constitute novel probes for exploring the structure-activity relationship of the bacterial GalP protein. GalP will provide an excellent model for the human glucose transporters and for elucidating the molecular basis of subtle differences in substrate and inhibitor recognition by individual members of this widespread family of transport proteins.

An iodinated derivative of forskolin, 6-O-[[2-[3-(4-azido-3-[125I] iodophenyl)propionamido]ethyl]carbamyl]forskolin ([125I]6-AIPP-Fsk), photolabels the multidrug efflux pump P-glycoprotein in membranes prepared from the multidrug-resistant cell lines KB-V1 and KB-C1. The labeling site for [125I]6-AIPP-Fsk was localized by immunoprecipitation of tryptic fragments of P-glycoprotein labeled in KB-C1 membranes. A 6-kDa, photolabeled, tryptic fragment was immunoprecipitated by antiserum raised against residues 348-419 of P-glycoprotein, PEPG9, but not by antisera raised against flanking regions PEPG7 and PEPG11. A peptide that corresponds to residues 343-359 of P-glycoprotein inhibited immunoprecipitation of the 6-kDa fragment by antiserum against PEPG9 but had no effect on the immunoprecipitation of photolabeled fragments by antiserum against PEPG7. A second peptide, corresponding to residues 360-376, had no effect on the immunoprecipitation by antiserum against PEPG9. [125I]6-AIPP-Fsk labels the carboxyl-terminal half of P-glycoprotein, because low molecular mass tryptic fragments were immunoprecipitated by three carboxyl-terminal antisera. Therefore, [125I]6-AIPP-Fsk labels both halves of P-glycoprotein, and labeling in the amino-terminal half can be localized to residues 291-359, which span proposed transmembrane regions 5 and 6. KB-V1 membranes photolabeled with [125I]6-AIPP-Fsk and [125I]iodoarylazidoprazosin were digested with either Staphylococcus aureus V8 protease or chymotrypsin and had similar digestion patterns, suggesting that the two drugs label the same sites on P-glycoprotein.

7-Bromoacetyl-7-desacetylforskolin (BrAcFsk), an alkylating derivative of forskolin, activated adenylyl cyclase and irreversibly blocked high affinity forskolin binding sites in human platelet membranes and rat brain membranes (Laurenza et al., 1990). Photoincorporation of an iodinated arylazido derivative of forskolin, 125I-6-AIPP-Fsk, into adenylyl cyclase in bovine brain membranes was irreversibly inhibited by BrAcFsk but not by 1,9-dideoxy-BrAcFsk, suggesting that BrAcFsk was reacting specifically with a nucleophilic group(s) at the forskolin binding site of adenylyl cyclase. Immunoblotting with antiforskolin antiserum demonstrated that partially purified bovine brain adenylyl cyclase had incorporated BrAcFsk. The interaction of BrAcFsk with the glucose transporter in human erythrocyte membranes was examined in a similar manner. Photoincorporation of 125I-7-AIPP-Fsk, an iodinated arylazido derivative of forskolin which is specific for the glucose transporter, into the glucose transporter was not irreversibly inhibited by BrAcFsk, suggesting that, in contrast to adenylyl cyclase, there is no reactive nucleophilic group at the forskolin binding site on the human erythrocyte glucose transporter. The immunoblotting procedure with antiforskolin antiserum confirmed that BrAcFsk was not covalently attached to human erythrocyte glucose transporter.

It has been inferred that residue changes that affect T cell recognition of synthetic peptides will have a similar effect in the intact protein. However, since small peptides do not require antigen processing it is possible that residue changes in synthetic peptides will not have an equivalent effect in the intact protein. Mutant proteins of staphylococcal nuclease (Nase) and 15mer synthetic peptides with corresponding substitutions were compared to determine if residue changes within an immunodominant epitope have an effect on the generation of naturally processed peptides. Five different substitutions in the synthetic peptide resulted in loss of reactivity of individual Nase-specific clones. When the same single amino acid changes were made in the intact protein, the naturally-processed peptides were also unable to stimulate the Nase-specific clones. However, two other substitutions in the synthetic peptide were stimulatory for a T cell clone even though the same changes in the intact protein were non-stimulatory. These results suggest that certain residue changes affect recognition of the naturally processed peptide but not the synthetic peptide with the same amino acid change. In addition, these results demonstrate that the effects of amino acid substitutions in synthetic peptides on T cell recognition may not always reflect the effects of these substitutions in the intact protein. Substitutions located outside Nase-specific T cell epitopes were also examined. Thirty different mutant proteins were all stimulatory. Moreover, a number of these mutants proteins were 50- to 100-fold more efficient in their stimulatory capacity than the native Nase protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminoalkylcarbamate derivatives of forskolin have been synthesized at the 6- and 7-hydroxyl positions which have different selectivity for adenylyl cyclase and a glucose transporter, respectively. They were radioiodinated using the Bolton-Hunter reagent to yield [125I]-2-[3-(4-hydroxy-3-iodophenyl)propanamido]-N-ethyl-6- (aminocarbonyl)forskolin ([125I]6-IHPP-Fsk) and [125I]-2-[3-(4-hydroxy-3-iodophenyl)(propanamidol]-N-ethyl-7- (aminocarbonyl)-7-desacetylforskolin ([125I]7-IHPP-Fsk) and tested as autoradiographic probes for adenylyl cyclase and a glucose transporter. In slide-mounted rat brain sections [125I]6-IHPP-Fsk binding was potently inhibited by 1 microM 6-HPP-Fsk (95%) but unaffected by 500 mM D-glucose. In contrast, [125I]7-IHPP-Fsk was only partially inhibited by 1 microM 6-HPP-Fsk (37%), but residual [125I]7-IHPP-Fsk binding was further inhibited 56% by 500 mM D-glucose. These data suggest that while [125I]6-IHPP-Fsk binds exclusively to adenylyl cyclase, a significant fraction of [125I]7-IHPP-Fsk is binding to a glucose transporter in brain. Autoradiographic patterns of [125I]6-IHPP-Fsk and glucose-sensitive [125I]7-IHPP-Fsk binding were different. [125I]6-IHPP-Fsk binding was heterogeneously distributed and resembled [3H] forskolin binding. Highest densities of binding sites were noted in olfactory tubercle, caudate putamen, nucleus accumbens, pyramidal and granule cell layers of hippocampus, molecular layer of cerebellum and substantia nigra. In contrast, of glucose-sensitive [125I]7-IHPP-Fsk, binding appeared more homogeneous and similar to [3H]cytochalasin B, a compound which inhibits glucose transport. Highest densities of binding were noted in caudate putamen, nucleus accumbens, cerebral cortex and molecular layer of cerebellum.(ABSTRACT TRUNCATED AT 250 WORDS)

A frog used for "hunting magic" by several groups of Panoan-speaking Indians in the borderline between Brazil and Peru is identified as Phyllomedusa bicolor. This frog's skin secretion, which the Indians introduce into the body through fresh burns, is rich in peptides. These include vasoactive peptides, opioid peptides, and a peptide that we have named adenoregulin, with the sequence GLWSKIKEVGKEAAKAAAKAAGKAALGAVSEAV as determined from mass spectrometry and Edman degradation. The natural peptide may contain a D amino acid residue, since it is not identical in chromatographic properties to the synthetic peptide. Adenoregulin enhances binding of agonists to A1 adenosine receptors; it is accompanied in the skin secretion by peptides that inhibit binding. The vasoactive peptide sauvagine, the opioid peptides, and adenoregulin and related peptides affect behavior in mice and presumably contribute to the behavioral sequelae observed in humans.

Two radioiodinated derivatives of forskolin, [125I]6-IHPP-Fsk and [125I]7-IHPP-Fsk, were synthesized as specific ligands for adenylyl cyclase and glucose transporter, respectively. [125I]6-IHPP-Fsk bound to bovine brain homogenates with a Kd of 9 nM and binding was inhibited by forskolin but not 1,9-dideoxyforskolin, cytochalasin B, or D-glucose. [125I]7-IHPP-Fsk bound to bovine brain homogenates at two classes of binding sites with Kd's of 56 nM and 4.7 microM; cytochalasin B and D-glucose inhibited 75% of the high affinity binding while having no effect on the low affinity binding. [125I]6-IHPP-Fsk and [125I]7-IHPP-Fsk were used to localize adenylyl cyclase and glucose transporter in rat brain by receptor autoradiography. The pattern of binding obtained with [125I]6-IHPP-Fsk was similar to that observed using [3H]forskolin to detect adenylyl cyclase. In contrast, the pattern of binding obtained with [125I]7-IHPP-Fsk was similar to that observed by others using [3H]cytochalasin B to detect glucose transporter. These iodinated ligands are selective for adenylyl cyclase and glucose transporter and require significantly shorter exposure times to yield autoradiographs than tritiated ligands.

7-(2-Aminoethyl)aminocarbonyl-7-desacetylforskolin (7-AEC-Fsk) and 6-(2-aminoethyl)aminocarbonylforskolin (6-AEC-Fsk) were synthesized and tested for their ability to activate adenylyl cyclase and inhibit the high affinity binding of [3H]forskolin to bovine brain membranes. Forskolin and 7-AEC-Fsk were equipotent in activating adenylyl cyclase, with EC50 values of about 4 microM, whereas 6-AEC-Fsk had an EC50 of about 2 microM. 6-AEC-Fsk and 7-AEC-Fsk stimulated adenylyl cyclase about 7-fold over basal levels at 100 microM, whereas forskolin produced a 5-fold stimulation. Forskolin and 6-AEC-Fsk inhibited the binding of [3H]forskolin to bovine brain membranes with Kd values of 41 nM and 28 nM, respectively, whereas 7-AEC-Fsk had a Kd of 83 nM. The 3-(3-iodo-4-hydroxyphenyl)propionamide derivative of 6-AEC-Fsk (6-I-HPP-Fsk) was more potent than forskolin in inhibiting [3H]forskolin binding to bovine brain membranes, with a Kd of 14 nM. 6-AEC-Fsk was reacted with 125I-labeled Bolton-Hunter reagent to produce 6-125I-HPP-Fsk with a specific activity of 2175 Ci/mmol. 6-125I-HPP-Fsk bound to bovine brain membranes with a Kd of 13 nM and a Bmax of 3.8 pmol/mg of protein. Forskolin inhibited the binding of 6-125I-HPP-Fsk to bovine brain membranes with a Kd of 31 nM, whereas 1,9-dideoxyforskolin only slightly inhibited the binding at 10 microM. The binding of 6-125I-HPP-Fsk was not inhibited by agents that inhibit forskolin binding to the glucose transporter, such as D-glucose or cytochalasin B. There was no displaceable binding of 6-125I-HPP-Fsk to red blood cell membranes, which contain a large concentration of the glucose transporter. Pretreatment of bovine brain membranes with an alkylating derivative of forskolin, 7-bromoacetyl-7-desacetylforskolin (BrAcFsk), led to an irreversible decrease in the binding of [3H]forskolin and 6-125I-HPP-Fsk. The time dependence and concentration dependence for the BrAcFsk-induced decrease in [3H]forskolin binding sites were identical to those observed for the decrease in 6-125I-HPP-Fsk binding sites. 6-125I-HPP-Fsk binding was determined in human platelet membranes in the presence of Mg2+ alone and in combination with guanosine 5'-O-(3-thio)triphosphate (GTP gamma S) or AIF4-. The presence of GTP gamma S or AIF4- increased the binding of 6-125I-HPP-Fsk by 4.5-fold and 4-fold, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

(Aminoalkyl)carbamates of forskolin were synthesized at the 6- and 7-hydroxyl positions of forskolin with the length of the alkyl chain varying from ethyl to heptyl. Two of these derivatives, 7-[[(2-aminoethyl)amino]carbonyl]-7-desacetylforskolin (2) and 6-[[(2-aminoethyl)amino]carbonyl]forskolin (3), were used to synthesize iodinated derivatives of forskolin that bind with high affinity to adenylyl cyclase in bovine brain membranes and the glucose transporter in human erythrocyte membranes, respectively. Hydroxyphenyl derivatives of forskolin were prepared from the (aminoalkyl)carbamates and tested for their ability to bind to adenylyl cyclase in bovine brain membranes and the glucose transporter in human erythrocyte membranes. The 6-derivative (18) of forskolin had a Kd of 9 nM at adenylyl cyclase and was more potent than either the 7-derivatives or the 6-derivatives of 7-desacetylforskolin. The 7-derivatives were more potent at binding to the glucose transporter than forskolin. In contrast, the 6-derivatives had Kd's greater than 100 microM at the glucose transporter. Isothiocyanates and N-bromoacetyl derivatives were synthesized from 2 and 3 as potential alkylating agents for forskolin binding sites. The alkylating agents produced an irreversible loss of forskolin binding to adenylyl cyclase. In contrast, the alkylating agents bound reversibly to the glucose transporter.

Neutralizing antibodies (NA) against HIV-1MN and HIV-1IIIB, and antibodies binding to synthetic peptides (BA) derived from the gp120 envelope V3 region principal neutralizing determinants (PND) of the HIV-1MN, HIV-1IIIB, and HIV-1Z3 virus strains were assayed in HIV-1 antibody-positive sera from the United States, Haiti, Brazil, Zaire, and Zimbabwe. The ability of soluble PND peptide to block neutralization of the corresponding virus by representative sera was also tested. In each country, NA and BA titers were highest against the HIV-1MN strain, and compared with other countries, NA and BA titers against HIV-1MN were higher in sera from the United States and Haiti. When NA titers were compared with BA titers against either HIV-1MN or HIV-1IIIB, no correlation was found for the HIV-1IIIB strain, but there was a significant correlation for HIV-1MN. Addition of the HIV-1MN strain peptide to a neutralization assay for HIV-1MN resulted in a four- to tenfold reduction in NA titers in sera from the United States, Zaire, and Brazil. The results suggest that HIV-1MN and closely related variants are prevalent in many parts of the world, and that antibodies directed against the PND account for most of the neutralizing activity in sera of infected individuals.

Forskolin and 1,9-dideoxyforskolin, an analogue that does not activate adenylyl cyclase, were tested for their ability to enhance the cytotoxic effects of adriamycin in human ovarian carcinoma cells, SKOV3, which are sensitive to adriamycin and express low levels of P-glycoprotein, and a variant cell line, SKVLB, which overexpresses the P-glycoprotein and has the multidrug resistance (MDR) phenotype. Forskolin and 1,9-dideoxyforskolin both increased the cytotoxic effects of adriamycin in SKVLB cells, yet had no effect on SKOV3 cells. Two photoactive derivatives of forskolin have been synthesized, 7-O-[[2-[3-(4-azido-3- [125I]iodophenyl)propionamido]ethyl] carbamyl]-7-deacetylforskolin, 125I-7-AIPP-Fsk, and 6-O-[[2-[3-(4-azido-3- [125I]iodophenyl)propionamido]ethyl]carbamyl]forskolin, 125I-6-AIPP-Fsk, which exhibit specificity for labeling the glucose transporter and adenylyl cyclase, respectively (Morris et al., 1991). Both photolabels identified a 140-kDa protein in membranes from SKVLB cells whose labeling was inhibited by forskolin and 1,9-dideoxyforskolin. There was no specific labeling of proteins in membranes from the SKOV3 cells. The overexpressed 140-kDa protein in SKVLB membranes was identified as the P-glycoprotein by immunoblot analysis and immunoprecipitation using anti-P-glycoprotein antiserum. Total inhibition of photolabeling of the P-glycoprotein was observed with verapamil, nifedipine, diltiazem, and vinbalastine, and partial inhibition was observed with colchicine and cytochalasin B. Forskolin was less effective at inhibiting the photolabeling of the P-glycoprotein than 1,9-dideoxyforskolin or a lipophilic derivative of forskolin. The data are consistent with forskolin binding to the P-glycoprotein analogous to that of other chemosensitizing drugs that have been shown to partially reverse MDR.(ABSTRACT TRUNCATED AT 250 WORDS)

Two photolabels, N-(3-(4-azido-3-125I-phenyl)-propionamide)-6- aminoethylcarbamylforskolin(125I-6-AIPP-Fsk) and N-(3-(4-azido-3-125I-phenyl)propionamide)-7-aminoethylcarbamyl-7- desacetylforskolin (125I-7-AIPP-Fsk) were synthesized with specific activities of 2200 Ci/mmol and used to label adenylyl cyclase and the glucose transporter. The affinities of the photolabels for adenylyl cyclase were determined by their inhibition of [3H]forskolin binding to bovine brain membranes. 6-AIPP-Fsk and 7-AIPP-Fsk inhibited [3H]forskolin binding with IC50 values of 15 nM and 200 nM, respectively. 125I-6-AIPP-Fsk labeled a 115-kDa protein in control and GTP gamma S-preactivated bovine brain membranes. This labeling was inhibited by forskolin but not by 1,9-dideoxyforskolin or cytochalasin B. 125I-6-AIPP-Fsk labeling of partially purified adenylyl cyclase was inhibited by forskolin but not by 1,9-dideoxyforskolin. 125I-7-AIPP-Fsk specifically labeled a 45-kDa protein and not a 115-kDa protein in control and GTP gamma S-preactivated brain membranes. This labeling was inhibited by forskolin, 1,9-dideoxyforskolin, cytochalasin B, and D-glucose but not cytochalasin E or L-glucose. Human erythrocyte membranes were photolyzed with 125I-6-AIPP-Fsk and 125I-7-AIPP-Fsk. 125I-7-AIPP-Fsk, but not 125I-6-AIPP-Fsk, strongly labeled a broad 45-70-kDa band. Forskolin, 7-bromoacetyl-7-desacetylforskolin, 1,9-dideoxyforskolin, cytochalasin B, and D-glucose, but not cytochalasin E or L-glucose, inhibited 125I-7-AIPP-Fsk labeling of the 45-70-kDa band. 125I-6-AIPP-Fsk and 125I-7-AIPP-Fsk are high affinity photolabels with specificity for adenylyl cyclase and the glucose transporter, respectively.

Brefeldin A (BFA) causes rapid redistribution of Golgi proteins into the ER, leaving no definable Golgi apparatus, and blocks transport of proteins into post-Golgi compartments in the cell. In this study we follow the disassembly of the Golgi apparatus in BFA-treated, living cells labeled with NBD-ceramide and demonstrate that forskolin can both inhibit and reverse this process. Long, tubular processes labeled with NBD-ceramide were observed emerging from Golgi elements and extending out to the cell periphery in cells treated with BFA for 5 min. With longer incubations in BFA, the NBD label was dispersed in a fine reticular pattern characteristic of the ER. Treatment with forskolin inhibited these effects of BFA as well as BFA's earliest morphologic effect on the Golgi apparatus: the redistribution to the cytosol of a 110-kD Golgi peripheral membrane protein. In addition, forskolin could reverse BFA's block in protein secretion. Forskolin inhibition of BFA's effects was dose dependent and reversible. High concentrations of BFA could overcome forskolin's inhibitory effect, suggesting forskolin and BFA interact in a competitive fashion. Remarkably, in cells already exposed to BFA, forskolin could reverse BFA's effects causing the 110-kD Golgi peripheral membrane protein to reassociate with Golgi membrane and juxtanuclear Golgi complexes to reassemble. Neither membrane permeant cAMP analogues nor cAMP phosphodiesterase inhibitors could replicate or enhance forskolin's inhibition of BFA. 1,9-Dideoxyforskolin, which does not activate adenylyl cyclase, was equally as effective as forskolin in antagonizing BFA. A derivative of forskolin, 7-HPP-forskolin, that is less potent than forskolin at binding to adenylyl cyclase, was also equally effective as forskolin in antagonizing BFA. In contrast a similar derivative, 6-HPP-forskolin, that is equipotent with forskolin at binding to adenylyl cyclase, did not inhibit BFA's effects. These results suggest that forskolin acts as a competitive antagonist to BFA, using a cAMP-independent mechanism to prevent and reverse the morphologic effects induced by BFA.

Neutrophils (PMN) treated with cAMP elevating agents were evaluated for their chemotactic responsiveness to FMLP and leukotriene B4 (LTB4). PGE1 and isoproterenol, increased PMN cyclic AMP production and inhibited chemotaxis to both FMLP and LTB4. In contrast, forskolin, which activates adenylate cyclase directly, inhibited chemotaxis to FMLP but not to LTB4. The phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX), was required for inhibition of PMN chemotaxis to FMLP by forskolin, PGE1, and isoproterenol. Isoproterenol and PGE1 inhibited PMN chemotaxis to LTB4 in the absence of IBMX and chemotaxis was further inhibited in the presence of IBMX. PMN cAMP levels were stimulated 2- to 3-fold with isoproterenol, 6- to 10-fold with PGE1, and 5- to 7-fold with forskolin over basal levels in the presence of IBMX. These observations demonstrate that total cellular cAMP concentration is not correlated with inhibition of PMN chemotaxis to all stimuli; forskolin, which increased cyclic AMP 5- to 7-fold over basal levels, did not inhibit chemotaxis to LTB4, whereas isoproterenol, which increased cyclic AMP only 2- to 3-fold over basal levels, inhibited chemotaxis to LTB4. PMN cAMP extrusion was determined under basal conditions and in the presence of PGE1, isoproterenol, or forskolin. PMN extruded cAMP under all conditions examined.

Chemical and proteolytic digestion of intact erythrocyte glucose transporter as well as purified transporter protein has been used to localize the derivatization site for the photoaffinity agent 3-[125I]iodo-4-azido-phenethylamino-7-O-succinyldeacetylforskol in [( 125I]IAPS-forskolin). Comparison of the partial amino acid sequence of the labelled 18 kDa tryptic fragment with the known amino acid sequence for the HepG2 glucose transporter confirmed that the binding site for IAPS-forskolin is between the amino acid residues Glu254 and Tyr456. Digestion of intact glucose transporter with Pronase suggests that this site is within the membrane bilayer. Digestion of labelled transporter with CNBr generated a major radiolabelled fragment of Mr approximately 5800 putatively identified as residues 365-420. Isoelectric focusing of Staphylococcus aureus V8 proteinase-treated purified labelled tryptic fragment identified two peptides which likely correspond to amino acid residues 360-380 and 381-393. The common region for these radiolabelled peptides is the tenth putative transmembrane helix of the erythrocyte glucose transporter, comprising amino acid residues 369-389. Additional support for this conclusion comes from studies in which [125I]APS-forskolin was photoincorporated into the L-arabinose/H(+)-transport protein of Escherichia coli. Labelling of this transport protein was protected by both cytochalasin B and D-glucose. The region of the erythrocyte glucose transporter thought to be derivatized with IAPS-forskolin contains a tryptophan residue (Trp388) that is conserved in the sequence of the E. coli arabinose-transport protein.

An antibody (RM) raised against the carboxyl-terminal decapeptide of the alpha subunit of the stimulatory guanine nucleotide regulatory protein (Gs alpha) has been used to study the interaction of Gs alpha with bovine brain adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1]. RM antibody immunoprecipitated about 60% of the solubilized adenylate cyclase preactivated with either GTP-gamma-S or AlF4-. In contrast, RM antibody immunoprecipitated about 5% of the adenylate cyclase not preactivated (control) and 15% of the adenylate cyclase pretreated with forskolin. Adenylate cyclase solubilized from control membranes or GTP-gamma-S preactivated membranes was partially purified by using forskolin-agarose affinity chromatography. The amount of Gs alpha protein in the partially purified preparations was determined by immunoblotting with RM antibody. There was 3-fold more Gs alpha detected in partially purified adenylate cyclase from preactivated membranes than in the partially purified adenylate cyclase from control membranes. Partially purified adenylate cyclase from preactivated membranes was immunoprecipitated with RM antibody and the amount of adenylate cyclase activity immunoprecipitated (65% of total) corresponded to the amount of Gs alpha protein immunoprecipitated. Only 15% of the partially purified adenylate cyclase from control membranes was immunoprecipitated. The presence of other G proteins in the partially purified preparations of adenylate cyclase was investigated by using specific antisera that detect Go alpha, Gi alpha, and G beta. The G beta protein was the only subunit detected in the partially purified preparations of adenylate cyclase and the amount of G beta was about the same in adenylate cyclase from preactivated membranes and from control membranes.(ABSTRACT TRUNCATED AT 250 WORDS)

1. Forskolin, a naturally occurring diterpene that activates adenylate cyclase, HL706, a water-soluble derivative of forskolin (6 beta-[(piperidino)acetoxy]-7-desacetylforskolin) that is less potent than forskolin in activating adenylate cyclase, and 1,9-dideoxyforskolin, an analogue that does not activate adenylate cyclase, were examined for effects on the nicotinic receptor-mediated 22Na+ flux, a high potassium-induced 45Ca2+ flux through L-type calcium channels, and a high potassium-induced 86Rb+ efflux through a calcium-dependent potassium channels in PC12 cells. 2. Forskolin and analogues at 30 microM completely blocked carbamylcholine-elicited flux of 22Na+ through the nicotinic receptor-gated channel. 1,9-Dideoxyforskolin had an IC50 value of 1.6 microM with forskolin and HL706 being two- to three fold less potent. 3. Forskolin and its analogues appear to be noncompetitive blockers of the neuronal nicotinic receptor-channel complex in PC12 cells, but unlike many noncompetitive blockers, did not markedly enhance desensitization. Instead, forskolin, but not HL706 or 1,9-dideoxyforskolin, slightly antagonized the desensitization evoked by high concentrations of carbamylcholine. N-Ethylcarboxamidoadenosine, an adenosine analogue that elevates cyclic AMP and 8-bromo-cyclic AMP had no effect on desensitization. 4. Forskolin, HL706, and 1,9-dideoxyforskolin in the presence of carbamylcholine inhibited the binding of a noncompetitive blocker, [3H]perhydrohistrionicotoxin, to the muscle-type nicotinic receptor-channel complex in Torpedo electroplax membranes with IC50 values of 20 microM. Forskolin had no effect on [3H]perhydrohistrionicotoxin binding in the absence of carbamylcholine, while HL706 and 1,9-dideoxyforskolin still inhibited binding in the absence of carbamylcholine. 5. Forskolin, but not HL706 or 1,9-dideoxyforskolin had a slight inhibitory effect on the binding of [125I]alpha-bungarotoxin to acetylcholine recognition sites in Torpedo membranes. 1,9-Dideoxyforskolin at 30 microM, but not forskolin or HL706, markedly inhibited depolarization-evoked 45Ca+ flux and 86Rb+ efflux in PC12 cells, suggesting that 1,9-dideoxyforskolin has nonspecific inhibitory effects on a variety of ion channels.

We have identified cell surface receptors for Act-2, a secreted protein expressed upon activation of T cells, B cells, and monocytes. Although 125I-Act-2 showed little, if any, specific binding to resting peripheral blood lymphocytes (PBL) receptors were readily detected on PHA/PMA-activated PBL and a variety of cell lines including MT-2, HL60, DMSO differentiated HL60, HeLa, and K562 cells. The equilibrium dissociation constant (Kd) is 3-12 nM for MT-2, K562, and PBL activated with PHA/PMA for 40-80 h. We have also identified a rabbit polyclonal antiserum that can block Act-2 binding to its receptors. The ability to detect specific Act-2 receptors and the development of a blocking antiserum should prove valuable in efforts to molecularly clone the Act-2 receptor and to dissect the biological actions of Act-2.

The 7-bromoacetyl-7-desacetyl (BrAcFsk) and 7-chloroacetyl-7-desacetyl (CIAcFsk) analogs of forskolin were synthesized as alkylating agents to study the high affinity binding sites for forskolin. BrAcFsk and CIAcFsk activated adenylate cyclase in human platelet membranes with EC50 values of about 20 and 12 microM, respectively. Both analogs increased cyclic AMP in human platelets; however, they were less potent that forskolin. Forskolin inhibited [3H]forskolin binding to human platelet membranes with an IC50 of 20 nM, whereas BrAcFsk and CIAcFsk inhibited [3H] forskolin binding with IC50 values of 0.1 microM. Pretreatment of intact platelets with 10 microM BrAcFsk caused a 90% irreversible loss in [3H]forskolin binding sites, whereas pretreatment with 10 microM CIAcFsk led to a loss of 55% of the binding sites. The loss of binding sites occurred within 5 min for BrAcFsk and within 30 min for CIAcFsk. The time required for the loss of binding sites produced by either alkylating agent was increased by the inclusion of 200 microM forskolin in the pretreatment buffer. The inactive bromoacetyl analog of forskolin 7-bromoacetyl-7-desacetyl-1.9-dideoxyforskolin (1,9-dideoxy-BrAcFsk) did not activate adenylate cyclase, inhibit [3H]forskolin binding, or cause an irreversible loss of [3H]forskolin binding sites. Adenylate cyclase was assayed in membranes from platelets treated with either 10 microM BrAcFsk or 10 microM 1,9-dideoxy-BrAcFsk. The stimulation of adenylate cyclase by prostaglandin E1, guanosine-5'-O-(3-thio)triphosphate, and AIF4 was inhibited by about 50% in membranes from platelets treated with BrAcFsk. However, the stimulation of adenylate cyclase by forskolin was unaffected by preincubation with BrAcFsk. Pretreatment of human platelets with 1,9-dideoxy-BrAcFsk had no effect on the stimulation of adenylate cyclase by prostaglandin E1, AIF4, or forskolin.

The diterpene forskolin markedly activates adenylate cyclase in membranes from various rat brain regions and elicits marked accumulations of radioactive cyclic AMP in adenine-labeled slices from cerebral cortex, cerebellum, hippocampus, striatum, superior colliculi, hypothalamus, thalamus, and medulla-pons. In cerebral cortical slices, forskolin has half-maximal effects at 20-30 microM on cyclic AMP levels, both alone and in the presence of the phosphodiesterase inhibitor ZK 62771. The presence of a very low dose of forskolin (1 microM) can augment the response of brain cyclic AMP-generating systems to norepinephrine, isoproterenol, histamine, serotonin, dopamine, adenosine, prostaglandin E2, and vasoactive intestinal peptide. Forskolin does not augment responses to combinations of histamine-norepinephrine adenosine-norepinephrine, or histamine-adenosine. For norepinephrine and isoproterenol in rat cerebral cortical slices and for histamine in guinea pig cerebral cortical slices, the presence of 1 microM-forskolin augments the apparent efficacy of the amine, whereas for adenosine, prostaglandin E2, and vasoactive intestinal peptide, the major effect of 1 microM-forskolin is to increase the apparent potency of the stimulatory agent. In rat striatal slices, forskolin reveals a significant response of cyclic AMP systems to dopamine and augments the dopamine-elicited activation of adenylate cyclase in rat striatal membranes. The activation of cyclic AMP systems by forskolin is rapid and reversible, and appears to involve both direct activation of adenylate cyclase and facilitation and/or enhancement of receptor-mediated activation of the enzyme.

High affinity binding sites for [3H]forskolin have been identified in rat brain membranes. These sites have a Kd of 15 nM and a Bmax of about 200 fmol/mg protein. The binding of [3H]forskolin to those high affinity sites in rat brain membranes is increased about two-fold by addition of MgCl2 or MnCl2. Smaller increases are observed in the presence of calcium, sodium, or potassium. The binding of [3H]forskolin is also increased in the presence of NaF or GppNHp, agents that are known to activate adenylate cyclase through the stimulatory guanine nucleotide regulatory protein (Ns). The increase in [3H]forskolin binding in the presence of NaF or GppNHp is due to an increase in the number of binding sites with no change in the apparent Kd for the binding sites. The NaF- and GppNHp-stimulated binding requires the presence of magnesium or manganese. The binding of [3H]forskolin to rat brain membranes is reduced in membranes that are heated or pretreated with chymotrypsin, trypsin, or N-ethylmaleimide. NaF stabilizes the binding sites to thermal denaturation. The data demonstrate that the number of high affinity forskolin binding sites are increased under conditions that promote the activation of the catalytic protein of adenylate cyclase by the Ns protein. It is suggested that the high affinity forskolin binding sites are associated with a complex of the catalytic protein and the activated Ns protein.

Forskolin, a diterpene of the labdane family, activates adenylate cyclase in broken cell preparations as well as in intact tissues. This activation does not require the guanine nucleotide regulatory subunit of the enzyme and probably occurs via an interaction with the catalytic subunit of adenylate cyclase. Activation of adenylate cyclase by forskolin results in marked increases in levels of intracellular cyclic AMP in a variety of eukaryotic cells. Low concentrations of forskolin which alone elicit small increases in intracellular cyclic AMP greatly potentiate hormonal activation of adenylate cyclase in a number of intact cells. Forskolin elicits cellular responses which have been proposed to be dependent o cyclic AMP as a second messenger. Forskolin, thus provides an invaluable tool for the investigation of the role of cyclic AMP in physiological responses to hormones, both through it direct activation of adenylate cyclase and through its ability to potentiate hormonal activation of adenylate cyclase.

Calcium stimulates adenylate cyclase activity in rat cerebral cortical membranes with either ATP or AppNHp as substrate. In contrast, isoproterenol stimulates the cerebral cortical enzyme with ATP as substrate but not with AppNHp as substrate unless exogenous GTP is added. In rat striatal membranes, calcium or dopamine stimulate adenylate cyclase activity with ATP as substrate, but not with AppNHp as substrate. GTP restores the dopamine but not the calcium response. The inhibitory guanine nucleotide GDP-beta S antagonizes dopamine and GppNHp stimulation of the brain adenylate cyclases, but not stimulation by calcium of either rat cerebral cortical or striatal enzymes. Results indicate that GTP is not requisite to calcium-calmodulin activation of adenylate cyclases in brain membranes. In addition, calcium-calmodulin cannot activate striatal adenylate cyclases with a nonphosphorylating nucleotide, AppNHp, as substrate.

A protein previously isolated from octopus optic lobe is shown to have the biochemical characteristics of a calmodulin-like protein. The amino acid composition of the octopus calmodulin is similar to that of another sea invertebrate calmodulin, from Renilla reniformis, in that both contain a single residue of tyrosine which distinguishes them from the vertebrate calmodulins which contain two tyrosines. The 1H NMR spectra of the octopus calmodulin and bovine brain calmodulin are compared in their apo- and calcium-saturated conformations. A comparison of these spectra indicates that the single tyrosine of the octopus calmodulin is in a structurally homologous position to tyrosine-138 of bovine brain calmodulin. 1H NMR and UV difference spectroscopy also demonstrate that the solution conformations of the apo- and calcium-saturated forms of octopus calmodulin are very similar to those of bovine brain calmodulin. It is concluded that both proteins undergo similar calcium-induced changes in tertiary structure, which result in near identical solution conformations.

Forskolin (7 beta-acetoxy-8,13-epoxy-1 alpha, 6 beta, 9 alpha-trihydroxylabd-14-en-11-one), a diterpene from the Indian plant Coleus forskohlii, activates cyclic AMP generating systems in a number of mammalian tissues in a rapid and reversible fashion. Derivatives of forskolin have been tested for their ability to stimulate membrane adenylate cyclase from rat brain and rabbit heart, as well as cyclic AMP generation in guinea pig brain vesicular preparations, a model system for intact cells. Derivatives at the 6 beta- and 7 beta-hydroxy functions retain activity, but none have greater activity than that of forskolin. Reduction of the 11-keto function affords an active 11 beta-hydroxy derivative. Reduction of the 14,15-vinyl (alpha) substituent reduces activity, while epoxidation abolishes activity. Derivatization or lack of the 1 alpha- and 9 alpha-hydroxy functions results in a marked reduction in activity, emphasizing the importance of the alpha aspect of the molecule. However, the 1 alpha, 6 beta-di-O-acetyl derivative does retain activity. None of the inactive derivatives, which include the 14,15-epoxy, the 1,9-dideoxy, and the 1,6-diketo derivatives, antagonize the stimulatory effects of forskolin.

The diterpene, forskolin [half-maximal effective concentration (EC50), 5-10 microM] activates adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1] in rat cerebral cortical membranes in a rapid and reversible manner. Activation is not dependent on exogenous guanyl nucleotides and is not inhibited by guanosine 5'-O-(2-thiodiphosphate) when assayed with adenosine 5'-[beta, gamma-imido]triphosphate as substrate. GTP and GDP potentiate responses to forskolin. The activations of adenylate cyclase by forskolin and guanosine 5'-[beta, gamma-imido]triphosphate p[NH]ppG are not additive, whereas activations by forskolin and fluoride are additive or partially additive. The responses of adenylate cyclase to forskolin or fluoride are not inhibited by manganese ions, whereas the response to p[NH]ppG is completely blocked. Activation of adenylate cyclase by forskolin is considerably greater than the activation by fluoride in membranes from rat cerebellum, striatum, heart, and liver, while being about equal or less than the activation by fluoride in other tissues. Forskolin (EC50, 25 microM) causes a rapid and readily reversible 35-fold elevation of cyclic AMP in rat cerebral cortical slices that is not blocked by a variety of neurotransmitter antagonists. Low concentrations of forskolin (1 microM) augment the response of cyclic AMP-generating systems in brain slices to norepinephrine, isoproterenol, histamine, adenosine, prostaglandin E2, and vasoactive intestinal peptide. Forskolin would appear to activate adenylate cyclase through a unique mechanism involving both direct activation of the enzyme and facilitation or potentiation of the modulation of enzyme activity by receptors or the guanyl nucleotide-binding subunit, or both.

Forskolin clearly has effects on all three known components of adenylate cyclase, Ni, Ns, and the catalytic subunit (C). Forskolin can activate the catalytic activity of adenylate cyclase directly in the absence of the Ni or Ns subunit, and, therefore, forskolin is acting at a site that is on the catalytic subunit or a closely associated protein. A lack of forskolin stimulation of cyclic AMP in intact cells does not necessarily imply that it requires or acts via the Ns protein, since, as shown for the cyc- S49 cells, the enzyme may be in an inhibited state because of the presence of Ni. The presence of a site on adenylate cyclase that can regulate not only the absolute activity of the enzyme but also its sensitivity to hormones raises the possibility that there may be substances endogenous to the cell that can functionally interact at this site. It is too early to speculate as to the nature of these substances. However, they could be extracellular, originating from other cells, or they could be intracellular. The final determination will rely on the identification and physical disposition of the forskolin-binding site and also the identification of endogenous compounds with forskolin-like activities.

[12-3H]Forskolin (27 Ci/mmol) has been used to study binding sites in rat brain tissue by using both centrifugation and filtration assays. The binding isotherm measured in the presence of 5 mM MgCl2 by using the centrifugation assay is described best by a two-site model: Kd1 = 15 nM, Bmax1 (maximal binding) = 270 fmol/mg of protein; Kd2 = 1.1 microM; Bmax2 = 4.2 pmol/mg of protein. Only the high-affinity binding sites are detected when the binding is determined by using a filtration assay; Kd = 26 nM, Bmax = 400 fmol/mg of protein. Analogs of forskolin that do not activate adenylate cyclase (EC 4.6.1.1) do not compete effectively for [3H]forskolin binding sites. Analogs of forskolin that are less potent than forskolin in activating adenylate cyclase are also less potent in competing for forskolin binding sites. The presence of 5 mM MgCl2 or MnCl2 was found to enhance binding. In the presence of 1 mM EDTA the amount of high-affinity binding is reduced to 110 fmol/mg of protein with no change in Kd. There is no effect of CaCl2 (20 mM) or NaCl (100 mM) on the binding. No high-affinity binding can be detected in membranes from ram sperm, which contains an adenylate cyclase that is not activated by forskolin. It is proposed that the high-affinity binding sites for forskolin are associated with the activated complex of catalytic subunit and stimulatory guanine nucleotide binding protein.

Analogues of forskolin that are more soluble in water than forskolin have been synthesized and tested for their ability to interact with adenylate cyclase. These analogues are esterified with various heterocyclic amino acids at the 6 beta-hydroxyl position of forskolin or at the 6 beta-hydroxyl or 7 beta-hydroxyl position of 7-desacetyl forskolin. Analogues were tested for their ability to activate rat brain adenylate cyclase, activate detergent-solubilized rat brain adenylate cyclase, increase cyclic AMP in intact S49 wild-type cells, and inhibit the binding of 3H-forskolin to rat brain membranes. Forskolin activated rat brain adenylate cyclase with an EC50 of 4 microM and increased cyclic AMP in intact S49 cells with an EC50 of 5 microM. Analogues esterified at the 7 beta-hydroxyl position had EC50 values that ranged from 4 microM to 15 microM for activating adenylate cyclase in membranes and solubilized preparations, and for increasing cyclic AMP in S49 cells. Analogues esterified at the 6 beta-hydroxyl position with no acyl group at the 7 beta-hydroxyl position were generally less potent than the corresponding 7-acyl analogues with EC50 values that ranged from 30 microM to 100 microM. Interestingly, the diacyl analogues of forskolin containing an acetate group at the 7 beta-hydroxyl position and esterified with heterocyclic amino acids at the 6 beta-hydroxyl position were very potent at stimulating adenylate cyclase, with EC50 values that ranged from 1 microM to 25 microM. The 7-acyl analogues and the 6,7-diacyl analogues inhibited the binding of 3H-forskolin to rat brain membranes with IC50 values that ranged from 20 microM to 70 microM, while the 6-acyl analogues had much higher IC50 values that ranged from 100 nM to 375 nM. Aqueous solutions of forskolin were also produced by dissolving forskolin in solutions of hydroxypropyl-gamma-cyclodextrin. These aqueous solutions of forskolin were equipotent with alcoholic solutions of forskolin in stimulating adenylate cyclase. In conclusion, water-soluble derivatives of forskolin may be useful for increasing cyclic AMP in broken cell preparations or in intact cell preparations where the presence of organic solvents, which are necessary to solubilize forskolin, are detrimental. Alternatively, aqueous solutions of forskolin can be produced by dissolving forskolin in solutions of hydroxypropyl-gamma-cyclodextrin.

The binding of [3H]forskolin to proteins solubilized from bovine brain membranes was studied by precipitating proteins with polyethylene glycol and separating [3H]forskolin bound to protein from free [3H]forskolin by rapid filtration. The Kd for [3H]forskolin binding to solubilized proteins was 14 nM which was similar to that for [3H]forskolin binding sites in membranes from rat brain and human platelets. Forskolin analogs competed for [3H]forskolin binding sites with the same rank potency in both brain membranes and in proteins solubilized from brain membranes. [3H]forskolin bound to proteins solubilized from membranes with a Bmax of 38 fmol/mg protein which increased to 94 fmol/mg protein when GppNHp was included in the binding assay. In contrast, GppNHp had no effect on [3H]forskolin binding to proteins solubilized from membranes preactivated with GppNHp. Solubilized adenylate cyclase from non-preactivated membranes had a basal activity of 130 pmol/mg/min which was increased 7-fold by GppNHp. In contrast, adenylate cyclase from preactivated membranes had a basal activity of 850 pmol/mg/min which was not stimulated by GppNHp or forskolin. Thus, the number of high affinity binding sites for [3H]forskolin in solubilized preparations correlated with the activation of adenylate cyclase by GppNHp via the guanine nucleotide binding protein (GS).

Forskolin and four analogues of forskolin, 7-beta-[gamma-(N'-methylpiperazino)-butyryloxy]-7-desacet ylforskolin, 7-desacetylforskolin, 7-tosyl-7-desacetylforskolin, and 1,9-dideoxyforskolin, were tested for their ability to activate adenylate cyclase, inhibit glucose transport, and inhibit cytochalasin B binding in rat adipocyte membranes. Forskolin was the most potent analogue in activating adenylate cyclase with an EC50 of 2 microM, whereas 7-beta-[gamma-(N'-methylpiperazino)butyryloxy]-7-desacety lforskolin and 7-desacetylforskolin were less potent, with EC50 values of 3 microM and 20 microM, respectively. The 7-tosyl-7-desacetylforskolin and 1,9-dideoxyforskolin did not stimulate adenylate cyclase even at the highest concentrations tested (100 microM). In contrast, forskolin and all of the analogues were able to fully inhibit glucose transport in adipocyte plasma membranes. The order of potency for the inhibition was forskolin greater than 7-beta-[gamma-(N'-methylpiperazino)butyryloxy]-7-desacety lforskolin greater than 7-desacetylforskolin greater than 7-tosyl-7-desacetylforskolin greater than 1,9-dideoxyforskolin, and the EC50 values were 0.24 microM, 1.8 microM, 7.1 microM, 8.8 microM, and 12.8 microM, respectively. Cytochalasin B binding to rat adipocyte membranes was inhibited by forskolin and the four analogues with the same order of potency as observed for the inhibition of glucose transport. Thus, the site of action of forskolin which is responsible for the inhibition of glucose transport and cytochasin B binding exhibits structural requirements for forskolin and its analogues that are different from those of the site responsible for the activation of adenylate cyclase.

Displaceable binding of [3H]forskolin to human platelet membranes can be detected in the presence of magnesium. There is an increase in the number of [3H]forskolin binding sites when membranes are incubated with GppNHp or NaF in the presence of magnesium. Prostaglandin E1, which stimulates human platelet adenylate cyclase, does not affect the binding of [3H]forskolin in the absence of GppNHp. However, the dose-response curve for the GppNHp-dependent increase in [3H]forskolin binding sites is shifted to lower concentrations in the presence of prostaglandin E1. Prostaglandin E1 potentiates the effect of GppNHp on [3H]forskolin binding most likely by facilitating the binding of the guanine nucleotide at the stimulatory quanine nucleotide regulatory protein of adenylate cyclase.

Forskolin, a naturally occurring diterpene, directly stimulates adenylyl cyclase and has been used extensively to increase cAMP and to elicit cAMP-dependent physiological responses. More recently, forskolin has been shown to inhibit a number of membrane transport proteins and channel proteins through a mechanism that does not involve the production of cAMP. Many of these channel proteins are predicted to have similar topographies in the membrane bilayer and it is tempting to speculate that forskolin may be binding at structurally homologous sites. Kenneth Seamon and colleagues discuss the cAMP-independent effects of forskolin and the structural similarity between forskolin and other physiologically important substances such as hexoses and steroids with respect to potential forskolin binding sites.