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International audience; Human HspB1 (also denoted Hsp27) is an oligomeric anti-apoptotic protein that has tumorigenic and metastatic roles. To approach the structural organizations of HspB1 that are active in response to apoptosis inducers acting through different pathways, we have analyzed the relative protective efficiency induced by this protein as well its localization, oligomerization and phosphorylation. HeLa cells, that constitutively express high levels of HspB1 were treated with either etoposide, Fas agonist antibody, staurosporine or cytochalasin D. Variability in HspB1 efficiency to interfere with the different apoptotic transduction pathways induced by these agents were detected. Moreover, inducer-specific dynamic changes in HspB1 localization, native size and phosphorylation were observed, that differed from those observed after heat shock. Etoposide and Fas treatments gradually shifted HspB1 towards large but differently phosphorylated oligomeric structures. In contrast, staurosporine and cytochalasin D induced the rapid but transient formation of small oligomers before large structures were formed. These events correlated with inducer-specific phosphorylations of HspB1. Of interest, the formation of small oligomers in response to staurosporine and cytochalasin D was time correlated with the rapid disruption of F-actin. The subsequent, or gradual in the case of etoposide and Fas, formation of large oligomeric structures was a later event concomitant with the early phase of caspase activation. These observations support the hypothesis that HspB1 has the ability, through specific changes in its structural organization, to adapt and interfere at several levels with challenges triggered by different signal transduction pathways upstream of the execution phase of apoptosis.

International audience; It is well established that cells exposed to the limiting oxygen microenvironment (hypoxia) of tumors acquire resistance to chemotherapy, through mechanisms not fully understood. We noted that a large number of cell lines showed protection from apoptotic stimuli, staurosporine, or etoposide, when exposed to long-term hypoxia (72 h). In addition, these cells had unusual enlarged mitochondria that were induced in a HIF-1-dependent manner. Enlarged mitochondria were functional as they conserved their transmembrane potential and ATP production. Here we reveal that mitochondria of hypoxia-induced chemotherapy-resistant cells undergo a HIF-1-dependent and mitofusin-1-mediated change in morphology from a tubular network to an enlarged phenotype. An imbalance in mitochondrial fusion/fission occurs since silencing of not only the mitochondrial fusion protein mitofusin 1 but also BNIP3 and BNIP3L, two mitochondrial HIF-targeted genes, reestablished a tubular morphology. Hypoxic cells were insensitive to staurosporine- and etoposide-induced cell death, but the silencing of mitofusin, BNIP3, and BNIP3L restored sensitivity. Our results demonstrate that some cancer cells have developed yet another way to evade apoptosis in hypoxia, by inducing mitochondrial fusion and targeting BNIP3 and BNIP3L to mitochondrial membranes, thereby giving these cells a selective growth advantage.

International audience; Cellular prion protein (PrP(c)) undergoes a disintegrin-mediated physiological cleavage, generating a soluble amino-terminal fragment (N1), the function of which remained unknown. Recombinant N1 inhibits staurosporine-induced caspase-3 activation by modulating p53 transcription and activity, whereas the PrP(c)-derived pathological fragment (N2) remains biologically inert. Furthermore, N1 protects retinal ganglion cells from hypoxia-induced apoptosis, reduces the number of terminal deoxynucleotidyltransferase-mediated biotinylated UTP nick end labeling-positive and p53-immunoreactive neurons in a pressure-induced ischemia model of the rat retina and triggers a partial recovery of b-waves but not a-waves of rat electroretinograms. Our work is the first demonstration that the alpha-secretase-derived PrP(c) fragment N1, but not N2, displays in vivo and in vitro neuroprotective function by modulating p53 pathway. It further demonstrates that distinct N-terminal cleavage products of PrP(c) harbor different biological activities underlying the various phenotypes linking PrP(c) to cell survival.

International audience; The neuropeptide neurotensin (NT) has been recently shown to protect pancreatic beta cells from toxic agents-induced apoptosis through interaction with the NT receptor-2 (NTSR2) and activation of the phosphatidylinositol-3 kinase pathway. However, expression of the NT receptor-3/sortilin (NTSR3) in the mouse pancreatic beta cell line -TC3 led us to investigate its possible functional role in these cells. By using siRNA, immunoprecipitation, co-localization and caspase-3 assays,we provide evidence for a functional endogenous interaction between NTSR2 and NTSR3. Expression of both receptors is necessary for the protective action of NT on staurosporine-induced caspase-3 activity in -TC3 cells. Moreover, NTSR2 and NTSR3 co-immunoprecipitate and are co-localized at the plasma membrane. Thus, the NT response in beta cells is controlled by the formation of a functional complex between NTSR2 and NTSR3.

International audience; BACKGROUND INFORMATION: Caspase-dependent and -independent death mechanisms are involved in apoptosis in a variety of human carcinoma cells treated with antineoplastic compounds. Our laboratory has reported that p53 is a key contributor of mitochondrial apoptosis in cervical carcinoma cells after staurosporine exposure. However, higher mitochondrial membrane potential dissipation and greater DNA fragmentation were observed in p53wt (wild-type p53) HeLa cells compared with p53mt (mutated p53) C-33A cells. Here, we have studied events linked to the mitochondrial apoptotic pathway. RESULTS: Staurosporine can induce death of HeLa cells via a cytochrome c/caspase-9/caspase-3 mitochondrial-dependent apoptotic pathway and via a delayed caspase-independent pathway. In contrast with p53wt cells, p53mt C-33A cells exhibit firstly caspase-8 activation leading to caspase-3 activation and Bid cleavage followed by cytochrome c release. Attenuation of PARP-1 [poly(ADP-ribose) polymerase-1] cleavage as well as oligonucleosomal DNA fragmentation in the presence of z-VAD-fmk points toward a major involvement of a caspase-dependent pathway in staurosporine-induced apoptosis in p53wt HeLa cells, which is not the case in p53mt C-33A cells. Meanwhile, the use of 3-aminobenzamide, a PARP-1 inhibitor known to prevent AIF (apoptosis-inducing factor) release, significantly decreases staurosporine-induced death in these p53mt carcinoma cells, suggesting a preferential implication of caspase-independent apoptosis. On the other hand, we show that p53, whose activity is modulated by pifithrin-alpha, isolated as a suppressor of p53-mediated transactivation, or by PRIMA-1 (p53 reactivation and induction of massive apoptosis), that reactivates mutant p53, causes cytochrome c release as well as mitochondrio-nuclear AIF translocation in staurosporine-induced apoptosis of cervical carcinoma cells. CONCLUSIONS: The present paper highlights that staurosporine engages the intrinsic mitochondrial apoptotic pathway via caspase-8 or caspase-9 signalling cascades and via caspase-independent cell death, as well as through p53 activity.

Apoptotic volume decrease (AVD) is prerequisite to apoptotic events that lead to cell death. In a previous study, we demonstrated in kidney proximal cells that the TASK2 channel was involved in the K+ efflux that occurred during regulatory volume decrease. The aim of the present study was to determine the role of the TASK2 channel in the regulation of AVD and apoptosis phenomenon. For this purpose renal cells were immortalized from primary cultures of proximal convoluted tubules (PCT) from wild type and TASK2 knock-out mice (task2-/-). Apoptosis was induced by staurosporine, cyclosporin A, or tumor necrosis factor alpha. Cell volume, K+ conductance, caspase-3, and intracellular reactive oxygen species (ROS) levels were monitored during AVD. In wild type PCT cells the K+ conductance activated during AVD exhibited characteristics of TASK2 currents. In task2-/- PCT cells, AVD and caspase activation were reduced by 59%. Whole cell recordings indicated that large conductance calcium-activated K+ currents inhibited by iberiotoxin (BK channels) partially compensated for the deletion of TASK2 K+ currents in the task2-/- PCT cells. This result explained the residual AVD measured in these cells. In both cell lines, apoptosis was mediated via intracellular ROS increase. Moreover AVD, K+ conductances, and caspase-3 were strongly impaired by ROS scavenger N-acetylcysteine. In conclusion, the main K+ channels involved in staurosporine, cyclosporin A, and tumor necrosis factor-alpha-induced AVD are TASK2 K+ channels in proximal wild type cells and iberiotoxin-sensitive BK channels in proximal task2-/- cells. Both K+ channels could be activated by ROS production.

To characterize the molecular links between type-1 autosomal dominant optic atrophy (ADOA) and OPA1 dysfunctions, the effects of pathogenic alleles of this dynamin on mitochondrial morphology and apoptosis were analyzed, either in fibroblasts from affected individuals, or in HeLa cells transfected with similar mutants. The alleles were missense substitutions in the GTPase domain (OPA1(G300E) and OPA1(R290Q)) or deletion of the GTPase effector domain (OPA1(Delta58)). Fragmentation of mitochondria and apoptosis increased in OPA1(R290Q) fibroblasts and in OPA1(G300E) transfected HeLa cells. OPA1(Delta58) did not influence mitochondrial morphology, but increased the sensitivity to staurosporine of fibroblasts. In these cells, the amount of OPA1 protein was half of that in control fibroblasts. We conclude that GTPase mutants exert a dominant negative effect by competing with wild-type alleles to integrate into fusion-competent complexes, whereas C-terminal truncated alleles act by haplo-insufficiency. We present a model where antagonistic fusion and fission forces maintain the mitochondrial network, within morphological limits that are compatible with cellular functions. In the retinal ganglion cells (RGCs) of patients suffering from type-1 ADOA, OPA1-driven fusion cannot adequately oppose fission, thereby rendering them more sensitive to apoptotic stimuli and eventually leading to optic nerve degeneration.

To characterize the molecular links between type-1 autosomal dominant optic atrophy (ADOA) and OPA1 dysfunctions, the effects of pathogenic alleles of this dynamin on mitochondrial morphology and apoptosis were analyzed, either in fibroblasts from affected individuals, or in HeLa cells transfected with similar mutants. The alleles were missense substitutions in the GTPase domain (OPA1(G300E) and OPA1(R290Q)) or deletion of the GTPase effector domain (OPA1(Delta58)). Fragmentation of mitochondria and apoptosis increased in OPA1(R290Q) fibroblasts and in OPA1(G300E) transfected HeLa cells. OPA1(Delta58) did not influence mitochondrial morphology, but increased the sensitivity to staurosporine of fibroblasts. In these cells, the amount of OPA1 protein was half of that in control fibroblasts. We conclude that GTPase mutants exert a dominant negative effect by competing with wild-type alleles to integrate into fusion-competent complexes, whereas C-terminal truncated alleles act by haplo-insufficiency. We present a model where antagonistic fusion and fission forces maintain the mitochondrial network, within morphological limits that are compatible with cellular functions. In the retinal ganglion cells (RGCs) of patients suffering from type-1 ADOA, OPA1-driven fusion cannot adequately oppose fission, thereby rendering them more sensitive to apoptotic stimuli and eventually leading to optic nerve degeneration.

The cellular prion protein (PrP(c)) undergoes various endopro-teolytic attacks within its N-terminal domain, leading to the production of C-terminal fragments (C) tethered to the plasma membrane and soluble N-terminal peptides (N). One of these cleavages occurs at position 110/111, thereby generating C1 and N1 products. We have reported that disintegrins ADAM-10, -9, and -17 participate either directly or indirectly to this proteolytic event. An alternative proteolytic event taking place around residue 90 yields C2 and N2 fragments. The putative function of these proteolytic fragments remained to be established. We have set up two novel human embryonic kidney 293 cell lines stably overexpressing either C1 or C2. We show that C1 potentiates staurosporine-induced caspase-3 activation through a p53-dependent mechanism. Thus, C1 positively controls p53 transcription and mRNA levels and increases p53-like immunoreactivity and activity. C1-induced caspase-3 activation remained unaffected by the blockade of endocytosis in HEK 293 cells and was abolished in p53-deficient fibroblasts. Conversely, overexpression of the C2 fragment did not significantly sensitize HEK 293 cells to apoptotic stimuli and did not modify p53 mRNA levels or activity. Therefore, the nature of the proteolytic cleavage taking place on PrP(c) yielded C-terminal catabolites with distinct function and could be seen as a switch mechanism controlling the function of the PrP(c) in cell survival.

International audience; We recently argued for a major role of p53 in staurosporine(ST)-induced apoptosis of immortalized epithelial cells, depending on their p53 status. Here, we studied the effects of PRIMA-1 (p53 reactivation and induction of massive apoptosis) and Pifithrin-alpha (p fifty-three inhibitor) in combination with ST to reinforce our previous results by respectively restoring or inhibiting the p53 transcriptional activity in different cell lines.PRIMA-1 does modify neither expression of apoptosis-related proteins nor the percentage of wild-type p53 HeLa and CaSki cells with [symbol: see text]delta psi m and DNA cleavage, whilst it increases by 45% Bax expression and apoptosis of mutated p53 C33A cells. Pifithrin-alpha, does modify neither Bax expression nor apoptosis level of C33A cells, but readily inhibits both [symbol: see text]delta psi m and DNA fragmentation of p53wt cells with decreasing Bax expression. These data support the evidence that PRIMA-1 could be a good candidate, as an anti-cancer drug targeting mutant p53, in order to increase ST efficiency. Moreover, Pifithrin-alpha could be used in combination with ST and PRIMA-1 to prevent side effects of anti-tumor therapies in cells expressing mutant P53.

Tightly controlled proteolysis is a defining feature of apoptosis and caspases are critical in this regard. Significant roles for non-caspase proteases in cell death have been highlighted. Staurosporine causes a rapid induction of apoptosis in virtually all mammalian cell types. Numerous studies demonstrate that staurosporine can activate cell death under caspase-inhibiting circumstances. The aim of this study was to investigate the proteolytic mechanisms responsible for cell death under these conditions. To that end, we show that inhibitors of serine proteases can delay cell death in one such system. Furthermore, through profiling of proteolytic activation, we demonstrate, for the first time, that staurosporine activates a chymotrypsin-like serine protease-dependent cell death in HL-60 cells independently, but in parallel with the caspase controlled systems. Features of the serine protease-mediated system include cell shrinkage and apoptotic morphology, regulation of caspase-3, altered nuclear morphology, generation of an endonuclease and DNA degradation. We also demonstrate a staurosporine-induced activation of a putative 16 kDa chymotrypsin-like protein during apoptosis.

We have assessed the contribution of apoptosis-inducing factor (AIF) and inhibitor of caspase-activated DNase (ICAD) to the nuclear morphology and DNA degradation pattern in staurosporine-induced apoptosis. Expression of D117E ICAD, a mutant that is resistant to caspase cleavage at residue 117, prevented low molecular weight (LMW) DNA fragmentation, stage II nuclear morphology, and detection of terminal deoxynucleotidyl transferase staining. However, high molecular weight (HMW) DNA fragmentation and stage I nuclear morphology remained unaffected. On the other hand, expression of either D224E or wild type ICAD had no effect on DNA fragmentation or nuclear morphology. In addition, both HMW and LMW DNA degradation required functional executor caspases. Interestingly, silencing of endogenous AIF abolished type I nuclear morphology without any effect on HMW or LMW DNA fragmentation. Together, these results demonstrate that AIF is responsible for stage I nuclear morphology and suggest that HMW DNA degradation is a caspase-activated DNase and AIF-independent process.

International audience; Amphiregulin (AR) and insulin-like growth factor-1 (IGF1) are growth factors known to promote non-small cell lung cancer (NSCLC) survival. We have previously published that 1) AR and IGF1, secreted by H358 NSCLC cells, cooperate to protect those cells and H322 NSCLC cells from serum-starved apoptosis; 2) H358 cells resist Bax-induced apoptosis through an inhibition of Bax conformational change. We show here that the antiapoptotic activity of the AR/IGF1 combination is specifically abolished by the PKC inhibitors calphostin C and staurosporine, but not by the MAPK and phosphatidylinositol 3-kinase inhibitors PD98059 and wortmannin, suggesting the involvement of a PKC-dependent and MAPK- and phosphatidylinositol 3-kinase-independent survival pathway. The PKCdelta inhibitor rottlerin restores apoptosis induced by serum deprivation. In addition, phosphorylation of PKCdelta and PKCzeta/lambda, but not of PKCalpha/beta(II), increases in serum-starved H358 cells and in H322 cells treated with an AR/IGF1 combination and is blocked by calphostin C. The combination of AR and IGF1 increases p90(rsk) and Bad phosphorylation as well as inhibiting the conformational change of Bax by a PKC-dependent mechanism. Finally, PKCdelta, PKCzeta, or p90(rsk) small interfering RNAs block the antiapoptotic activity of AR/IGF1 combination but have no effect on partial apoptosis inhibition observed with each factor used alone. Constitutively active PKC expression inhibits serum deprivation-induced apoptosis, whereas a catalytically inactive form of p90(rsk) restores it. Thus, AR and IGF1 cooperate to prevent apoptosis by activating a specific PKC-p90(rsk)-dependent pathway, which leads to Bad and Bax inactivation. This signaling pathway is different to that used by single factor.

International audience; Amphiregulin (AR) and insulin-like growth factor-1 (IGF1) are growth factors known to promote non-small cell lung cancer (NSCLC) survival. We have previously published that 1) AR and IGF1, secreted by H358 NSCLC cells, cooperate to protect those cells and H322 NSCLC cells from serum-starved apoptosis; 2) H358 cells resist Bax-induced apoptosis through an inhibition of Bax conformational change. We show here that the antiapoptotic activity of the AR/IGF1 combination is specifically abolished by the PKC inhibitors calphostin C and staurosporine, but not by the MAPK and phosphatidylinositol 3-kinase inhibitors PD98059 and wortmannin, suggesting the involvement of a PKC-dependent and MAPK- and phosphatidylinositol 3-kinase-independent survival pathway. The PKCdelta inhibitor rottlerin restores apoptosis induced by serum deprivation. In addition, phosphorylation of PKCdelta and PKCzeta/lambda, but not of PKCalpha/beta(II), increases in serum-starved H358 cells and in H322 cells treated with an AR/IGF1 combination and is blocked by calphostin C. The combination of AR and IGF1 increases p90(rsk) and Bad phosphorylation as well as inhibiting the conformational change of Bax by a PKC-dependent mechanism. Finally, PKCdelta, PKCzeta, or p90(rsk) small interfering RNAs block the antiapoptotic activity of AR/IGF1 combination but have no effect on partial apoptosis inhibition observed with each factor used alone. Constitutively active PKC expression inhibits serum deprivation-induced apoptosis, whereas a catalytically inactive form of p90(rsk) restores it. Thus, AR and IGF1 cooperate to prevent apoptosis by activating a specific PKC-p90(rsk)-dependent pathway, which leads to Bad and Bax inactivation. This signaling pathway is different to that used by single factor.

International audience; The mitochondrial localization of p53 is an important event in p53-dependent apoptosis. Some p53 mutants defective for transcription also facilitate apoptosis through changes of the mitochondria. Here, apoptosis of HeLa and CaSki cells (p53(wt)), C33A and HaCat cells (p53(mt)) and SaOs-2 cells (p53 deficient) was induced by 300 nM staurosporine. We showed that wild-type p53, as well as p53 mutants, were transiently located to the mitochondria with changes in the mitochondrial membrane potential (Delta Psi m). However, in C33A cells harboring a p53 mutated on its DNA binding domain, Delta Psi m collapse and Sub-G1 DNA content were reduced compared to p53(wt) cells, whereas no significant difference was observed in HaCat cells with a p53 mutated on UV hot spots. In addition, inhibition of the mitochondrial permeability transition pores by cyclosporine A significantly reduced the Delta Psi m loss and the sub-G1 DNA content in p53 positive cells. These results indicate that Delta Psi m collapse is an early and necessary event, which plays an important role in apoptosis of immortal mammalian cells.

International audience; Apoptotic cell death often requires caspases. Caspases are part of a family of related molecules including also paracaspases and metacaspases. Are molecules of this family generally involved in cell death? More specifically, do non-apoptotic caspase-independent types of cell death require paracaspases or metacaspases? Dictyostelium discoideum lends itself well to answering these questions because 1) it undergoes non-apoptotic developmental cell death of a vacuolar autophagic type and 2) it bears neither caspase nor metacaspase genes and apparently only one paracaspase gene. This only paracaspase gene can be inactivated by homologous recombination. Paracaspase-null clones were thus obtained in each of four distinct Dictyostelium strains. These clones were tested in two systems, developmental stalk cell death in vivo and vacuolar autophagic cell death in a monolayer system mimicking developmental cell death. Compared with parent cells, all of the paracaspase-null cells showed unaltered cell death in both test systems. In addition, paracaspase inactivation led to no alteration in development or interaction with a range of bacteria. Thus, in Dictyostelium, vacuolar programmed cell death in development and in a monolayer model in vitro would seem not to require paracaspase. To our knowledge, this is the first instance of developmental programmed cell death shown to be independent of any caspase, paracaspase or metacaspase. These results have implications as to the relationship in evolution between cell death and the caspase family.

We have previously demonstrated that GH is subject to rapid receptor-dependent nuclear translocation. Here, we examine the importance of ligand activation of the GH-receptor (GHR)-associated Janus kinase (JAK) 2 and receptor dimerization for hormone internalization and nuclear translocation by use of cells stably transfected with cDNA for the GHR. Staurosporine and herbimycin A treatment of cells did not affect the ability of GH to internalize but resulted in increased nuclear accumulation of hormone. Similarly, receptor mutations, which prevent the association and activation of JAK2, did not affect the ability of the hormone to internalize or translocate to the nucleus but resulted in increased nuclear accumulation of GH. These results were observed both by nuclear isolation and confocal laser scanning microscopy. Staurosporine treatment of cells in which human GH (hGH) was targeted to the cytoplasm (removal of secretion sequence) or to the nucleus (addition of the nuclear localization sequence of SV40 large T antigen) resulted in preferential accumulation of hGH in the nucleus. We further investigated the requirement of receptor dimerization for GH nuclear translocation using the non-receptor-dimerizing hGH antagonist, hGH-G120R, conjugated to fluorescein isothiocyanate. Confocal laser scanning microscopy demonstrated efficient internalization of both hGH and hGH-G120R but lack of nuclear translocation of hGH-G120R. Thus, we conclude that activation of JAK2 kinase and the subsequent tyrosine phosphorylation is not required for nuclear translocation of GH but is pivotal for the removal of the hormone from the nucleus, and that GH translocates into the nucleus in a GHR dimerized-dependent fashion.

We previously showed that Hsp27 protects against apoptosis through its interaction with cytosolic cytochrome c. We have revisited this protective activity in murine cell lines expressing different levels of Hsp27. We report that Hsp27 also interferes, in a manner dependent on level of expression, with the release of cytochrome c from mitochondria. Moreover, a decreased level of endogenous Hsp27, which sensitized HeLa cells to apoptosis, reduced the delay required for cytochrome c release and procaspase 3 activation. The molecular mechanism regulating this function of Hsp27 is unknown. In our cell systems, Hsp27 is mainly cytosolic and only a small fraction of this protein colocalized with mitochondria. Moreover, we show that only a very small fraction of cytochrome c interacts with Hsp27, hence excluding a role of this interaction in the retention of cytochrome c in mitochondria. We also report that Bid intracellular relocalization was altered by changes in Hsp27 level of expression, suggesting that Hsp27 interferes with apoptotic signals upstream of mitochondria. We therefore investigated if the ability of Hsp27 to act as an expression-dependent modulator of F-actin microfilaments integrity was linked to the retention of cytochrome c in mitochondria. We show here that the F-actin depolymerizing agent cytochalasin D rapidly induced the release of cytochrome c from mitochondria and caspase activation. This phenomenon was delayed in cells pretreated with the F-actin stabilizer phalloidin and in cells expressing a high level of Hsp27. This suggests the existence of an apoptotic signaling pathway linking cytoskeleton damages to mitochondria. This pathway, which induces Bid intracellular redistribution, is negatively regulated by the ability of Hsp27 to protect F-actin network integrity. However, this upstream pathway is probably not the only one to be regulated by Hsp27 since, in staurosporine-treated cells, phalloidin only partially inhibited cytochrome c release and caspase activation. Moreover, in etoposide-treated cells, Hsp27 still delayed the release of cytochrome c from mitochondria and Bid intracellular redistribution in conditions where F-actin was not altered.

Cell activation during fertilization of the egg of Xenopus laevis is accompanied by various metabolic changes, including a permanent increase in intracellular pH (pHi) and a transient increase in intracellular free calcium activity ([Ca2+]1,). Recently, it has been proposed that protein kinase C (PKC) is an integral component of the Xenopus fertilization pathway (Bement and Capeo, J. Cell Biol. 108, 885-892, 1989). Indeed, activators of PKC trigger cortical granule exocytosis and cortical contraction, two events of egg activation, without, however, releasing the cell cycle arrest (blocked in second metaphase of meiosis). In the egg of Xenopus, exocytosis as well as cell cycle reinitiation are supposed to be triggered by the intracellular Ca2+ transient. We report here that PKC activators do not induce the intracellular Ca2+ transient, or the activation-associated increase in pHi. These results suggest that the ionic responses to egg activation in Xenopus do not appear to depend on the activation of PKC. In addition, in eggs already pretreated with phorbol esters, those artificial activators that act by releasing Ca2+ intracellularly, triggered a diminished increase in pHi. Finally, sphingosine and staurosporine, two potent inhibitors of PKC, were found to trigger egg activation, suggesting that a decrease in PKC activity might be an essential event in the release of the metaphase block, in agreement with recent findings on the release of the prophase block in Xenopus oocytes (Varnold and Smith, Development 109, 597–604, 1990).