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We have previously determined the crystal structure of a non-structural 5B (NS5B) protein, an RNA-dependent RNA polymerase (RdRp) of hepatitis C virus (HCV). NS5B protein with the hydrophobic C-terminal 21 amino acid residues truncated, designated NS5B 570 , shows a typical nucleotide polymerase structure resembling a right-hand shape. In the crystal structure, a C-terminal region between Leu545 and His562 occupies a putative RNA-binding cleft of this polymerase and seems to inhibit the polymerase activity. Varieties of recombinant NS5B proteins (NS5B 552 , NS5B 544 , NS5B 536 or NS5B 531 , with C-terminal 39, 47, 55 or 60 amino acid residues truncated, respectively) were systematically constructed to elucidate effects of the region on the polymerase activity. NS5B 544 , NS5B 536 and NS5B 531 showed markedly higher RdRp activities compared to the activities of NS5B 570 or NS5B 552 . Furthermore, when the hydrophobic amino acid residues Leu547, Trp550 and Phe551 (LWF) in NS5B 570 and NS5B 552 were changed to alanine, their activities were higher than that of the original NS5B 570 . The crystal structures of the various recombinant NS5B proteins were also determined. Structural comparison of the NS5B proteins indicates that the activation was caused by elimination of a unique hydrophobic interaction between the three C-terminal residues and a shallowly concave pocket consisting of thumb and palm domains.

Cellular interactions between hematopoietic cells and stromal cells play important roles in the proliferation and differentiation of hematopoietic cells. The proliferation of a human erythroleukemia cell line, HEL cells, which can differentiate into macrophage- and megakaryocyte-like cells, and erythroid precursors was dramatically induced on coculture with a hematopoietic-supportive stromal cell line, HESS-5 cells, which can support long-term hematopoiesis in vitro without fetal bovine serum. HEL cells proliferated when they were cocultured with but not without direct cell contact. Because the coculture supernatants with direct cell contact and cytokines such as interleukins and growth factors did not exhibit growth-stimulating activity toward HEL cells, it was suggested that some molecule that has growth-stimulating activity exists on the surface of the cells. Extracellular matrix components such as fibronectin, laminin, vitronectin, and collagen did not affect the proliferation of HEL cells. An anti-CD18 monoclonal antibody, which recognizes the common β chain of the β2 integrin subfamily, induced dramatic proliferation of HEL cells. Moreover, the proliferation of HEL cells was inhibited by an antisense oligonucleotide of CD18 mRNA. As judged from these observations, the proliferation of HEL cells was mediated by CD18 molecules expressed on HEL cells. On the contrary, the common counter-receptor of the β2 integrin subfamily, intercellular adhesion molecule-1, which is expressed on CHO-K1 cells, did not stimulate the growth of HEL cells. It is known that other counter molecules of the β2 integrin subfamily, such as complement C3bi and fibrinogen, are not produced by stromal cells. These findings suggest that the proliferation of HEL cells may be induced through an interaction between a novel molecule of the β2 integrin subfamily on HEL cells and the counter-receptor on HESS-5 cells. The β2 integrin subfamily may regulate the growth of hematopoietic cells in hematopoiesis in vivo and/or cause the abnormal growth of leukemia cells.

Two species of L-histidine decarboxylase (HDC) mRNA were found in the KU-812-F basophilic cell line, but only the 2.4-kilobase (kb) one encodes the functional HDC (Mamune-Sato, R., Yamauchi, K., Tanno, Y., Ohkawara, Y., Ohtsu, H., Katayose, D., Maeyama, K., Watanabe, T., Shibahara, S., and Takishima, T. (1992) Eur. J. Biochem. 209, 533-539). The 3.4-kb one encodes a truncated HDC protein and is also found in human leukemia-derived cell lines HEL and KCL-22. To clarify the mechanisms that regulate transcription of the HDC gene and generate the two species of mRNA, we have isolated genomic DNA clones coding for the HDC from human genomic libraries. Structural analysis of the isolated clones revealed that the human HDC gene is composed of 12 exons spanning approximately 24 kb. Genomic DNA blot analysis suggested that HDC is encoded by a single copy gene. The structural analysis also demonstrated that the heterogeneity of the HDC mRNA is caused by an insertion of the seventh intron sequence and alternative use of the splicing acceptor site at the 12th exon. The transcription start site of the HDC gene and the nucleotide sequences of the promoter and first exon regions were determined. We found a TATA-like sequence, a GC box, four CACC boxes, four GATA consensus sequences, and six leader-binding protein-1 binding motifs in the promoter region of the HDC gene.

The primary structure of nucleoside diphosphate (NDP) kinase from spinach leaves has been deduced from its cDNA sequence. A λgt11 cDNA library derived from spinach leaves was screened using an antibody against NDP kinase I, which we previously purified to electrophoretic homogeneity ( T. Nomura, T. Fukui, and A. Ichikawa, 1991 , Biochim. Biophys. Acta 1077, 47–55). The cDNA sequences of positive clones contained the amino acid coding region (444 base pairs) for NDP kinase I as well as 5′ and 3′ noncoding regions of 33 and 361 base pairs, respectively. The cDNAs hybridized to a 1.1-kb mRNA. NDP kinase I contains 148 amino acid residues with a molecular mass of 16,305, which is in excellent agreement with that of the purified enzyme (16 kDa). Homology was found between the sequence of spinach NDP kinase I and those of the rat, Myxococcus xanthus , and Dictyostelium discoideum NDP kinases, as well as the human Nm 23-gene product and the awd protein of Drosophila melanogaster .

In the preceding paper (Kawai, H. et al. (1992) Biochim. Biophys. Acta 1133, 172-178), we reported that in mastocytoma P-815 cells dexamethasone and 12-O-tetradecanoylphorbol-13-acetate (TPA) synergistically enhanced the de novo synthesis of L-histidine decarboxylase (HDC). Here we found that Ca2+ acted synergistically with cAMP in the induction of HDC mRNA and HDC activity in mastocytoma P-815 cells, and that the mechanism underlying the enzyme induction by Ca2+ plus cAMP was distinguishable from that by dexamethasone plus TPA. Ca2+ ionophore A23187, itself having no significant activity, markedly enhanced the induction of HDC activity by N6,O2'-dibutyryl cAMP (db cAMP) or cAMP-inducible prostaglandins such as PGE1, PGE2 and PGI2 in the presence of the phosphodiesterase inhibitor, Ro201724. However, A23187 had little effect on increases in HDC activity induced by other known stimulants, such as TPA, dexamethasone and sodium butyrate. These results suggest that A23187 has a specific effect on the induction of HDC activity due to an increased level of cAMP. The finding that both A23187 and cAMP enhanced HDC activity suggests that both Ca2+/calmodulin and cyclic nucleotide dependent protein kinase play essential roles in the process of enhancement of HDC activity. To examine this possibility, we studied the effects of W-7, an inhibitor of calmodulin, removal of extracellular Ca2+, and H-8, an inhibitor of cAMP-dependent protein kinase, on the enhancing activity of A23187 plus db cAMP. The enhancement of HDC activity by A23187 plus db cAMP was inhibited by W-7, removal of extracellular Ca2+, and H-8. The increase in HDC activity was due to the de novo synthesis of the enzyme, since it was suppressed by the addition of cycloheximide or actinomycin D, and was well correlated with the marked accumulation of a 2.7 kilobase HDC mRNA. Furthermore, the mechanism underlying the induction of HDC by db cAMP plus A23187 is distinguishable from that in the case of dexamethasone plus TPA, since preexposure to dexamethasone plus TPA for 12 h, for a plateau level to be reached, did not affect the subsequent increase in HDC activity due to db cAMP plus A23187.

12-O-Tetradecanylphorbol-13-acetate (TPA) markedly enhanced the increase in l -histidine decar☐ylase (HDC) activity induced by dexamethasone in mouse mastocytoma P-815 cells, even with a concentration of the latter that had the maximal effect, whereas it induced a rapid and transient increase in HDC activity, which peaked after 3 h in the absence of dexamethasone. The synergistic effect of TPA on HDC activity induced by dexamethasone was detected after 4 h, a plateau level being reached by 6 h, which was similar to the time course with dexamethasone alone. TPA enhanced the induction of HDC activity by various glucocorticoids, but had no effect on the induction by dibutyryl cAMP, prostaglandin E2 or sodium butyrate. Both 1-oleoyl-2-acetylglycerol, a protein kinase C activator, and okadaic acid, a protein phosphatase inhibitor, enhanced the increase in HDC activity induced by dexamethasone, but 4α-phorbol-12,13-didecanoate, an inactive derivative of TPA, did not. Protein kinase C inhibitors, such as staurosporin, H-7 and K252a, suppressed the increase in HDC activity induced by TPA with or without dexamethasone. The enhancement of HDC activity by dexamethasone was completely suppressed by cycloheximide or actinomycin D. Furthermore, TPA markedly enhanced the accumulation of HDC mRNA due to dexamethasone (5 to 10-fold, from 6 to 12 h after). TPA did not cause a significant increase in the level of either [3H]dexamethasone binding capacity or preformed HDC activity in cells. These results taken together suggest that dexamethasone-induced de novo synthesis of HDC in mastocytoma P-815 cells is up-regulated by TPA-activated protein kinase C through the mechanism involving an increased rate of transcription.

We have characterized a soluble pertussis toxin (PT)-sensitive GTP-binding protein (G-protein) present in mouse mastocytoma P-815 cells. 65% of total ADP-ribosylation of PT substrate having a molecular mass of 40 kDa on SDS-polyacrylamide gel electrophoresis in cell homogenate was detected in the supernatant after centrifugation at 100,000 x g for 90 min. [32P]ADP-ribosylation of cytosolic PT substrate was significantly enhanced on the addition of exogenous beta gamma complex. The molecular mass of the cytosolic PT substrate was estimated to be about 80 kDa on an Ultrogel AcA 44 column, but the beta gamma complex was not detected in the cytosol by using the anti-beta gamma complex antibody. Furthermore, the cytosolic PT substrate was found to have some unique properties: [35S]GTP gamma S binding was not inhibited by GDP and [32P]ADP-ribosylation was not affected by GTP gamma S treatment. Only after the cytosolic PT substrate had been mixed with exogenous beta gamma complex, did it copurify with exogenous beta gamma complex by several column chromatographies including an Octyl-Sepharose CL-4B column. The PT substrate was identified as Gi2 alpha by Western blot analysis and peptide mapping with S. aureus V8 protease. These results suggest that Gi2 alpha without beta gamma complex exists with an apparent molecular mass of about 80 kDa in the cytosolic fraction of P-815 cells.

Prostaglandin E1 (PGE1) specifically bound to the membrane prepared from murine renal outer medulla. The extent of binding of [3H]PGE1 to the membrane was increased about 4-fold by guanosine triphosphate (GTP) and its analogs, but the dissociation of bound [3H]PGE1 from the membrane was in turn enhanced by GTP gamma S. Scatchard plot analyses revealed that GTP gamma S increased the binding affinity more than 2-fold without a major change in the number of binding sites. When [3H]PGE1-bound proteins were cross-linked in the membrane by dithiobis (succinimidyl propionate), bound [3H]PGE1 was no longer dissociated by GTP gamma S treatment, suggesting that cross-linking produced a stable complex of PGE receptor with a GTP-binding protein. The cross-linked [3H]PGE1-specifically bound proteins solubilized from the membranes labeled with [3H] PGE1 in the presence or absence of GTP gamma S were eluted as an apparently single radioactive peak at the same position of Mr = 15000 by gel filtration, indicating that the PGE receptor forms a complex with a GTP-binding protein regardless of the treatment with GTP gamma S by which [3H]PGE1 binding is promoted. The ability of GTP gamma S to stimulate [3H]PGE1 binding was eliminated by pretreatment of the membrane with pertussis toxin, but not cholera toxin, indicating that the PGE receptor is coupled to a pertussis toxin-sensitive GTP-binding protein.

Histidine decarboxylase was purified from mouse mastocytoma P-815 cells to electrophoretic homogeneity by ammonium sulfate fractionation, dialyses at pH 7.5 and 6.0, chromatographies on DEAE-Sepharose CL-6B, Phenyl-Sepharose CL-4B and Hydroxylapatite, Phenyl-Superose HPLC, Mono Q HPLC, and Diol-200 gel filtration HPLC. Under the assay conditions used, the pure enzyme exhibited a specific activity of 800 nmol/min/mg, which constituted 12,500-fold purification compared to the crude extract, with a 7% yield. The two-step dialysis turned out to be essential for removing the factor(s) which interfered with the enzyme purification. The optimum pH for the enzyme reaction was 6.6 and the isoelectric point of the enzyme was pH 5.4. The molecular mass of the enzyme was found to be approximately 53 kDa on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, 110 kDa on gel filtration, and 115 kDa on polyacrylamide gradient gel electrophoresis in the absence of sodium dodecyl sulfate. The Km value for histidine was estimated to be 0.26 mM at pH 6.8.

The in vitro expansion of CD34+ cells is important for clinical applications such as transplantation and gene therapy with CD34+ cells isolated from human umbilical cord blood. In the present study, we developed a xenogenic coculture system involving HUCB-CD34+ cells and a murine stromal cell line, HESS-5 cells, in the presence of recombinant human (rh) cytokines. We examined the effects of combinations of cytokines, such as rh-IL-3, rh-SCF, rh-granulocyte colony-stimulating factor (G-CSF), rh-granulocyte-macrophage-CSF and h-erythropoietin (EPO), on the expansion of CD34high+ cells and colony-forming progenitor cells (CFCs). The proliferation of CD34high+ cells and CFCs was dramatically promoted on coculture with HESS-5 cells, and the expansion ratio of the CD34high+ cells showed good correlation with that of high-proliferative potential colony-forming cells (HPP-CFCs). The most potent combination of cytokines in this xenogenic coculture system for the expansion of CD34high+ cells and HPP-CFCs was rh-IL-3 and rh-SCF. The proliferation of CD34high+ cells was supported in the presence of HESS-5 cells with direct cell contact, but not observed in the indirect coculture involving a microporous membrane. Furthermore, we developed a unique coculture method, designated as the bilayer coculture method, involving CD34+ cells and HESS-5 cells using a microporous membrane. This expansion system will be applicable to the expansion of the primitive progenitor cells of HUCB-CD34+ cells and is worthy of consideration for the clinical application of HUCB-CD34+ cells.

To isolate the interferon-inducible protein 10 (IP-10) receptor gene, we searched for cells that respond to IP-10. Among several human and murine T cell lines, only CTLL2 cells ( a murine cytotoxic T cell line) responded to IP-10 with transient elevation of intracellular Ca2+. The murine IP-10 receptor gene has been cloned from cDNA derived from CTLL2 cells using the reverse transcriptase-polymerase chain reaction protocol with two degenerate primers corresponding to conserved regions of chemokine receptors. The cDNA encoding the murine IP-10 receptor has an open reading frame of 1101 bp corresponding to a protein of 367 amino acids that exhibits 86 % identity with the human IP-10 receptor. It mediates Ca2+mobilization in response to IP-10, but does not recognize other rodent chemokines, including GRO, RANTES, monocyte chemoattractant protein-1 (MCP-1), and macrophage inflammatory protein-1α (MIP-1α). Northern blot analysis revealed that murine IP-10 and its receptor mRNA were constitutively expressed in the spleen and thymus from normal mouse, while IP-10 and its receptor mRNA were derived from stromal cells and lymphocytes in both tissues, respectively.In vivotreatment with concanavalin A (Con A) for 12 hrs revealed that splenocytes significantly induce IP-10 receptor mRNA expression and show a good chemotactic response to IP-10. Therefore, it is supposed that IP-10 and its receptor are important for lymphocyte trafficking to lymphoid organs and that the IP-10 receptor on lymphocytes is rapidly inducible on inflammation or in immunological events.

We have expressed and characterized human recombinant 74-kDa (rHDC74) and 54-kDa (rHDC54) L-histidine decarboxylases (HDCs) in Sf9 cells. By immunoblot analysis, rHDC74 and rHDC54 were shown to be localized predominantly in the particulate and soluble fractions, respectively. rHDC74 exhibited histamine-synthesizing activity equivalent to that of rHDC54. The existence of 74- and 54-kDa HDCs was also confirmed in the particulate and supernatant fractions of the cell lysate, respectively, from the human basophilic leukemia cell line KU-812-F. The ratio of HDC activity to immunoreactivity was similar for the two forms of the enzyme. The specific activity of purified rHDC54 (1.12 μmol/mg/min) was comparable to those of HDCs from other mammalian tissues or cells. The purified rHDC54 was eluted as a monomer form from a Superdex-200 column; the molecular mass of the enzyme was approximately 54 kDa on SDS-polyacrylamide gel electrophoresis without 2-mercaptoethanol. The HDC activity of rHDC54 significantly decreased on dialysis against buffer without pyridoxal 5′-phosphate; addition of pyridoxal 5′-phosphate to the dialysate readily increased in the enzyme activity to the original activity. Taken together, these results suggest that human HDC functions as both 74- and 54-kDa forms having equivalent HDC activity, which are localized in the particulate and soluble fractions, respectively, and that the latter form exhibits its activity as a monomer form.

L-Histidine decarboxylase (HDC) catalyzes the formation of histamine from L-histidine. This biogenic amine is known to exert various effects in physiological and pathological reactions. In contrast to the well-known mechanism of histamine action through its interaction with specific receptors, the mechanisms regulating HDC gene expression are not elucidated. We have purified HDC from mouse mastocytoma cells, and isolated mouse HDC cDNA, and found that the primary translated product is posttranslationally processed to yield a mature active enzyme. In mastocytoma cells, we demonstrated that the induction of HDC activity and HDC mRNA synergistically occurred on treatment with dexamethasone+TPA, and also cAMP+Ca2+. To clarify the mechanism of up-regulation by these stimuli of the transcription of the HDC gene, we have isolated a genomic DNA clone encoding 5'-flanking region sequence and the first two exons. The transcription start site and the nucleotide sequences of the promoter regions including TATA- and GC-boxes were determined. With mastocytoma cells transiently transfected with 5' deletion constructs of HDC-CAT fusion gene, it was found that the sequences from -132 to -53 and -267 to -53 are essential for the regulatory elements involved in the increased transcription of the HDC gene with dexamethasone+TPA and cAMP+Ca2+, respectively. Furthermore, we have isolated a genomic DNA from human basophilic cells, and analysed its structure to elucidate the mechanisms regulating the tissue specificity of HDC gene expression.

The possibility of post-translational processing of mouse mastocytoma histidine decarboxylase (HDC; EC 4.1.1.22) was investigated. The molecular mass of the recombinant HDC expressed in Sf9 cells using HDC cDNA from mouse mastocytoma cells was determined to be 74 kDa by SDS-PAGE. In contrast to the native HDC from mastocytoma cells, the recombinant 74 kDa HDC was essentially inactive and precipitable in Sf9 cells. On the other hand, deletion mutants of the recombinant HDC lacking a C-terminal region equivalent to 10 (64 kDa) or 20 kDa (54 kDa) in size were present as active forms in the soluble fraction of Sf9 cells. To examine the C-terminal deletion of the 74 kDa species yielding the 53 kDa species by means of the immunoblotting analysis, two peptides (corresponding to residues 323–337 and 572–586 of the recombinant 74 kDa HDC peptide) were synthesized, and rabbit antiserum specific for each peptide was prepared. On immunoblotting analysis, anti-peptide 323–337 antiserum recognized both the recombinant 74 kDa and native enzyme subunit peptides, but anti-peptide 572–586 antiserum recognized only the recombinant 74 kDa peptide, i.e., not the native enzyme subunit peptide. Furthermore, HDC activity in the crude extract from Sf9 cells was not precipitable with antipeptide 572–585 antiserum. These results strongly suggest that the 53 kDa subunit peptide of native mastocytoma HDC is derived from the unidentified inactive 74 kDa HDC peptide, probably by post-translational processing of HDC in its C-terminal region.

cDNA clones encoding a glutamic acid-rich protein were isolated from a bovine retina cDNA expression library. The cDNA sequence contained an open reading frame of 1770 base pairs encoding a protein of 590 amino acids (64,509 Da) and untranslated regions of 60 and 490 base pairs at the 5' and 3' ends, respectively. The cDNA hybridized to a 2.4-kilobase retinal mRNA. The amino acid sequence derived from the cDNA sequence contains a glutamic acid-rich domain in which 68 of 109 amino acids are glutamic acid. In addition, this domain contains four repeats of a peptide of 11 amino acids and two repeats of a peptide of 26 amino acids. A polyclonal antibody raised against a decapeptide corresponding to the undecapeptide repeat sequence reacted with a protein in an extract of bovine rod outer segments, whose molecular mass, 65 kDa, corresponded to that of the above glutamic acid-rich protein. The retinal glutamic acid-rich protein showed homology with glutamic acid-rich proteins from bovine brain and the C-terminal region of mammalian neurofilaments.

The primary structure of L-histidine decarboxylase (HDC: L-histidine carboxy-lyase, EC 4.1.1.22) from mouse mastocytoma P-815 cells has been determined by parallel analysis of the amino acid sequence of the protein and the nucleotide sequence of the corresponding cDNA. HDC contains 662 amino acid residues with a molecular mass of 74017, which is larger by about 21 000 Da than that of the previously purified HDC subunit (53 kDa), suggesting that HDC might be posttranslationally processed. The HDC cDNA hybridized to a 2.7 kilobase mRNA of mastocytoma cells. Homology was found between the sequences of mouse mastocytoma HDC and fetal rat liver HDC.

Addition of prostaglandin E 1 (PGE 1 ) to intact mouse mastocytoma P-815 cells stimulated adenosine 3', 5'-monophosphate (cAMP) accumulation and retarded cellular growth. To study the effects of prostaglandin (PG) binding on cAMP accumulation, specific [ 3 H]PG binding to intact mastocytoma cells was examined. Intact mastocytoma cells have two types of binding sites for PGE 1 with high ( K d = 2.14 × 10 −9 M) and low ( k d = 1.05 × 10 −8 M) affinities and one type of binding site for PGF 2α . Mastocytoma cells, however, did not have a binding site for PGA 1 or PGD 2 . The order of potencies of nonradioactive prostaglandins in competing with [ 3 H]PGE 1 for binding sites was as follows: PGE 1 ⩾ PGE 2 ⪢ PGF 2α ⩾ PGA 1 ⩾ PGD 2 ⩾ PGI 2 . In contrast, the relative potencies of the prostaglandins in enhancing cAMP accumulation were PGI 2 ⩾ PGE 1 ⩾ PGE 2 ⪢ PGA 1 ⩾ PGF 2 α = PGD 2 , indicating that the receptors for E type and I type of PGs were different. Refractoriness of mastocytoma cells to PGE 1 stimulation of cAMP accumulation developed within 1 min of incubation of the cells at 37°, but disappeared immediately after decreasing the temperature to below 23°. A change in the number of PGE 1 receptors was not observed. cAMP concentrations were quickly restored by increasing temperatures from below 23° to 37°. This refractoriness did not develop in the presence of phosphodiesterase inhibitors. Furthermore, the activity of phosphodiesterase in mastocytoma cells was enhanced within 2 min by PGE 1 treatment.

The 3 H-labeled prostaglandin D 2 ([ 3 H]PGD 2 ) binding protein in the membrane fraction of mastocytoma P-815 cells was characterized. The specific binding of [ 3 H]PGD 2 to the cells or the membranes reached a maximum at pH 5.6, and was saturable, displaceable and of high affinity when incubated at 0 or 37°C. The B max values for [ 3 H]PGD 2 binding in the two preparations at pH 5.6 were much higher at 0°C than at 37°C, whereas the K d values were almost equal (85.3 nM for the cells and 80.5 nM for the membranes, respectively). High specific [ 3 H]PGD 2 binding activity in the mildly acid-treated cells was still observed when the external pH was raised from 5.6 to 7.2. Furthermore, specific [ 3 H]PGD 2 binding to the membranes (at 0°C, pH 5.6) increased on addition of phosphatase inhibitors (NaF and molybdate) in the presence of 10 μM ATP, but practically disappeared on pretreatment of the membranes with phosphatase. On incubation of the membrane with [γ- 32 P]ATP and molybdate, the stimulated incorporation of the [ 32 P]phosphate into several peptides, including ones having an M r of around 100 000–120 000, was observed. These results suggest that [ 3 H]PGD 2 binding in the mastocytoma P-815 cell membrane is controlled through phosphorylation-dephosphorylation of the receptor itself.

The growth of mouse mastocytoma P-815 cells in culture (37 degrees, 42 hr) was inhibited by exogenous adenosine (0.2 to 1.0 mM) and more effectively by AMP (0.01 to 0.1 mM), but not by adenine. The inhibited growth (a 25% inhibition by 0.5 mM adenosine and a 80% inhibition by 0.25 mM AMP) was restored to a near control level by the addition of uridine (0.5 mM) to the medium. The pretreatment (37 degrees, 3 hr) of the cells with adenosine or AMP caused a 60% inhibition of incorporation (37 degrees, 2 hr) of [U-14C]aspartate into uracil nucleotides, accumulating 14C-orotate and orotidine. Both dipyridamole, an inhibitor of adenosine uptake, and exogenous adenosine deaminase suppressed the growth inhibition induced by not only adenosine but also AMP. 2-Chloroadenosine, which is resistant to the action of adenosine deaminase, was a more potent growth inhibitor, while 3'AMP and 2'-AMP, which are not hydrolyzed to adenosine by membrane 5'-nucleotidase, were ineffective. Adenosine 5'-sulfate and other 5'-substituted adenosines were also ineffective. These observations indicate that AMP inhibits the growth of mastocytoma P-815 cells as a result of its continuous conversion to adenosine and a constant exposure of the cells to a low concentration of adenosine which readily permeates the cell membrane. In addition, adenosine, AMP and their agarose-linked forms rapidly (37 degrees, 20 min) elevated cellular levels of cAMP. This effect was not suppressed by dipyridamole. Apparently adenosine and AMP also act extracellularly for growth inhibition by regulating cAMP levels.

Hydrocortisone-stimulated synthesis of histidine decarboxylase (EC 4.1.1.22) was studied in rat stomach. Single or daily administration of hydrocortisone increased the histidine decarboxylase activity but not the histamine content in the fundic glands of stomachs from fed and fasted rats with or without adrenalectomy, and also in those from adrenalectomized fasted rats with concomitant treatment with pirenzepine, an inhibitor of gastrin secretion. The increase in histidine decarboxylase activity was maximal at 2 h after the administration of hydrocortisone, and the activity had returned almost to the pretreatment level by 4h after the administration. Furthermore, the increase was not augmented by daily administration of hydrocortisone for 3 d. Cycloheximide prevented the increase in gastric histidine decarboxylase due to hydrocortisone. Rat fundic gland tissue was found to possess binding sites for [3H] glucocorticoids in the cytosol. These results suggest that hydrocortisone stimulates the de novo synthesis of histidine decarboxylase in the fundic gland of rat stomach, though the response is not mediated by changes in the gastrin level.

Background: Hepatitis C virus (HCV) is the major etiological agent of hepatocellular carcinoma, and HCV RNA-dependent RNA polymerase (RdRp) is one of the main potential targets for anti-HCV agents. HCV RdRp performs run-off copying replication in an RNA-selective manner for the template–primer duplex and the substrate, but the structural basis of this reaction mechanism has still to be elucidated. Results: The three-dimensional structure of HCV RdRp was determined by X-ray crystallography at 2.5 A resolution. The compact HCV RdRp structure resembles a right hand, but has more complicated fingers and thumb domains than those of the other known polymerases, with a novel α -helix-rich subdomain ( α fingers) as an addition to the fingers domain. The other fingers subdomain ( β fingers) is folded in the same manner as the fingers domain of human immunodeficiency virus (HIV) reverse transcriptase (RT), another RNA-dependent polymerase. The ribose-recognition site of HCV RdRp is constructed of hydrophilic residues, unlike those of DNA polymerases. The C-terminal region of HCV RdRp occupies the putative RNA-duplex-binding cleft. Conclusions: The structural basis of the RNA selectivity of HCV RdRp was elucidated from its crystal structure. The putative substrate-binding site with a shallow hydrophilic cavity should have ribonucleoside triphosphate (rNTP) as the preferred substrate. We propose that the unique α fingers might represent a common structural discriminator of the template–primer duplex that distinguishes between RNA and DNA during the replication of positive single-stranded RNA by viral RdRps. The C-terminal region might exert a regulatory function on the initiation and activity of HCV RdRp.

Growth-inhibited mouse mastocytoma P-815 cells at stationary phase contained more histamine, serotonin and adenosine 3',5'-monophosphate (cAMP), and higher activities of histidine decarboxylase and adenylate cyclase than the cells during exponential growth. The elevation of endogenous cAMP levels induced by several growth-inhibiting agents such as N6, O2'-dibutyryl cAMP (Bt2cAMP), prostaglandin E1, AMP and 2-chloroadenosine stimulated several functions characteristic of mastocytoma P-815 cells in culture, elevating the synthesis of histamine and serotonin, the activity of chymotrypsin-like protease, and the incorporation of [35S]sulfate into acidic glycosaminoglycans. 1-Methyl-3-isobutyl-xanthine (MIX), a potent inhibitor of cAMP phosphodiesterase, potentiated stimulatory effect of these agents. The results indicate that cAMP regulates the growth and functions of mastocytoma P-815 cells. [35S]-Sulfated acidic glycosaminoglycans synthesized in cells at stationary phase or in cells treated with Bt2cAMP plus MIX mainly localized in the 3000-10000 x g sedimentable fraction of cell homogenates, and had a molecular weight of 200000 to 400000 based on gel filtration. This acidic glycosaminoglycan was resistant to chondroitinase ABC and the heparin-degrading enzyme present in the 20000 x g sedimentable fraction of the cells, and was identified as a highly sulfated macromolecular heparin based on behaviors on DEAE-cellulose column and on acidic electrophoresis. Cycloheximide suppressed the stimulatory effect of Bt2cAMP on the synthesis of histamine and [35S]-sulfated acidic glycosaminoglycan.

Dexamethasone at a concentration as low as 10 nM significantly increased both the histamine content and histidine decarboxylase activity of cultured mastocytoma P-815 cells. Both effects were clearly seen using several glucocorticoids, which were as effective as dexamethasone. In contrast to that of histamine, the serotonin level of mastocytoma P-815 cells was decreased by treatment with dexamethasone. The dexamethasone-induced increases in histamine content and histidine decarboxylase activity were completely suppressed by the addition of cycloheximide and actinomycin D. Mastocytoma P-815 cells were found to possess binding sites for [3H]dexamethasone in the cytosol (Kd = 15.7 nM) and the nuclei (Kd = 1.26 nM). These results show that glucocorticoids significantly stimulate de novo synthesis of histidine decarboxylase.

Prostaglandin E1 (PGE1) receptors from mouse mastocytoma P-815 cells were found to bind to a wheat germ agglutinin (WGA)-Agarose column, suggesting that the receptors are glycoproteins. To further elucidate the role of carbohydrate moieties in the PGE1 receptors for their binding activity to ligand, the P-815 cells were treated with tunicamycin, swainsonine or monensin. Tunicamycin, an inhibitor of N-glycosylation, dose- and time-dependently inhibited the binding of PGE1 to mastocytoma P-815 cells. Neither swainsonine, an inhibitor of Golgi mannosidase II, nor monensin, an inhibitor of processing beyond the high mannose stage, altered PGE1 binding properties of the cells. The inhibition of PGE1 binding by tunicamycin was observed when incorporation of [3H]glucosamine into macromolecules was inhibited. The inhibitory effect was not on their affinity but on their number of binding sites. Subcellular distributions of [3H]PGE1-binding activity showed that decreases in the binding activity by tunicamycin were highest in plasma membrane fractions. Treatment of membranes with various endo- and exoglycosidases did not affect PGE1 binding. PGE1-stimulated cyclic AMP accumulation in the cells was also inhibited by tunicamycin. These results suggest that PGE1 receptors of mastocytoma P-815 cells are glycoproteins and that inhibition of N-glycosylation of PGE1 receptors by tunicamycin results in the arrest of the translocation of newly synthesized receptors to the surface of mastocytoma P-815 cells.