Regulation of M-phase progression in Chaetopterus oocytes by protein kinase C

Translocation of classical PKC and cortical granule exocytosis of human oocyte in germinal vesicle and metaphase II stage

AIM

Protein kinase C (PKC) is as a family of serine/threonine kinases that can be activated by Ca2+, phospholipid and diacylglycerol. PKC plays an important role in oocyte maturation and activation. This study was undertaken to investigate classical PKC (cPKC) in human oocyte maturation and activation.

METHODS

Germinal vesicle (GV) and metaphase II (MII) stage oocytes were collected from healthy women. The expression and distribution of cPKC were investigated by immunoflourescence. MII oocytes were treated with PKC activator or inhibitor and imaged using a laser confocal scanning microscope (LCSM).

RESULTS

In GV oocytes, PKCalpha, beta1 and gamma were localized to the germinal vesicles, with a weak expression in ooplasm. In MII oocytes, PKCalpha, beta1 and gamma were distributed evenly in ooplasm. After treatment with PKC activator, phorbol 12-myristate 13-acetate (PMA), cPKC translocated to the periphery of oocyte, and cortical granules (CG) exocytosis was found. When the oocytes were treated with PKC inhibitor, staurosporine, no translocation of cPKC and CG exocytosis were found.

CONCLUSION

PKCalpha, beta1 and gamma exist in human oocytes and activation of these subunits could induce CG exocytosis in MII stage.

2-Hydroxyestradiol-17β-induced oocyte maturation in catfish (Heteropneustes fossilis) involves protein kinase C and its interaction with protein phosphatases

In vitro effects of phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, calphostin C (PKC inhibitor) and okadaic acid [OA, a protein phosphatase (PP; PP1 and PP2A) inhibitor] on 2-hydroxyestradiol-17beta (2-OHE(2))-induced oocyte maturation were investigated in the catfish Heteropneustes fossilis. Incubations of postvitellogenic follicles with PMA or OA alone did not induce oocyte maturation. However, co-incubations with 2-OHE(2) and PMA (0.05, 0.5 and 5 microM) or 2-OHE(2) and OA (0.5, 1.0 or 2.0 microM) increased germinal vesicle breakdown (GVBD) significantly over that of 2-OHE(2). Incubation of follicles with calphostin C elicited varied effects on GVBD, low (0.005 and 0.01 microM) and high (5.0 and 10.0 microM) concentrations did not affect GVBD, but medium concentrations (0.05, 0.1, 0.5, 1.0 and 2.5 microM) stimulated it. The medium concentrations elicited a biphasic stimulatory response with peak GVBD at 0.1 microM (54%). Calphostin C (>or=2.5 microM) inhibited the 2-OHE(2)-induced GVBD in a concentration-dependent manner during the 24 h incubation. Pre- or post-treatment with calphostin C inhibited the steroid-induced GVBD only at 6 h. In co-incubation studies, both PMA and OA reversed the inhibitory effect of calphostin C: the former partially and the latter fully. The results of the present study show that PKC appears to modulate the 2-OHE(2)-induced oocyte maturation. The OA-sensitive PP may be involved in the PKC modulation of steroid-induced oocyte maturation.

Effects of MAP kinase pathway and other factors on meiosis ofUrechis unicinctus eggs

The eggs of Urechis unicinctus Von Drasche, an echiuroid, are arrested at P-I stage in meiosis. The meiosis is reinitiated by fertilization. Immunoblotting analysis using anti-ERK2 and anti-phospho-MAPK antibodies revealed a 44 kDa MAP kinase species that was constantly expressed in U. unicinctus eggs, quickly phosphorylated after fertilization, and dephosphorylated slowly before the completion of meiosis I. Phosphorylation of the protein was not depressed by protein synthesis inhibitor Cycloheximide (CHX), but was depressed by the MEK1 inhibitor PD98059. Under PD98059 treatment, polar body extrusion was suppressed and the function of centrosome and spindle was abnormal though GVBD was not affected, indicating that MAP kinase cascade was important for meiotic division of U. unicinctus eggs. Other discovery includes: A23187 and OA could parthenogenetically activate U. unicinctus eggs and phosphorylated 44 kDa MAP kinase species, indicating that the effect of fertilization on reinitiating meiosis and phosphorylation of 44 kDa MAP kinase specie is mediated by raising intracellular free calcium and by phosphorylation of some proteins, and that phosphotase(s) sensitive to OA is responsible for arresting U. unicinctus eggs in prophase I. diC8, an activator of PKC, accelerated the process of U. unicinctus egg meiotic division after fertilization and accelerated the dephosphorylation of 44 kDa MAP kinase specie, which implied that the acceleration effect of PKC on meiotic division was mediated by inactivation of MAP kinase cascade. Elevating cAMP/PKA level in U. unicinctus eggs had no effect on meiotic division of the eggs.

Regulation of cleavage by protein kinase C inChaetopterus

We report that protein kinase C (PKC) plays a regulatory role in early cleavage in Chaetopterus eggs. Using Western blotting, we assayed the expression patterns of conventional PKCs (cPKC), novel PKCs (nPKC), and atypical PKCs (aPKC). During early development after fertilization, PKC protein levels varied independently by isoform. PKC protein expression during differentiation, without cleavage and after parthenogenetic activation, was very similar to that during normal development indicating that PKC gene expression does not require cellularization. Since PKC has been shown to regulate meiosis in this organism, we also assayed the membrane association of these isoforms as an indicator of their activation during meiosis and early cleavage. PKC-gamma transiently associated with membranes and therefore became activated before meiotic division and cleavage, whereas PKC-alpha and -beta transiently dissociated from membranes and therefore became inactivated at these times. Inhibition of these PKC isoforms by bisindolylmaleimide I had no effect on cleavage or early development to the trochophore larva, indicating that PKC-gamma activation is not essential for cleavage or early development. However, their persistent activation by thymeleatoxin blocked cleavage. The results indicate that the dissociation of PKC-alpha and/or -beta from the membrane fraction, and therefore their inactivation, is essential for normal cleavage. Elevated PKC activity is essential for nuclear envelope breakdown and spindle formation at meiosis I. By contrast, down-regulation of this activity is essential for cleavage after fertilization.

Selective enhanced phosphorylation of shrimp beta-tubulin by PKC-delta with PEP(taxol), a synthetic peptide encoding the taxol binding region

Beta-tubulin cDNA from the shrimp Penaeus japonicus was isolated by homology cloning. Expression of cDNA in Escherichia coli yielded a 55 kDa polypeptide, positive for monoclonal antibodies against mammalian beta-tubulin. Autoradiography demonstrated the bacterially expressed hepatopancreas beta-tubulin of P. japonicus is specifically phosphorylated by the delta isoenzyme of protein kinase C (PKC-delta) purified from the plasma membrane of the shrimp heart, in the presence of the receptor for activated PKC (RACK), but not in its absence. Purified shrimp heart PKC-delta is able to phosphorylate bacterially expressed shrimp beta-tubulin without the presence of Ca(++), but requires Mg(++). The kinase activity of purified PKC-delta on bacterially expressed beta-tubulin was enhanced by incubation with PEP(taxol), a synthetic peptide encoding the taxol-binding region of beta-tubulin. In other words, PEP(taxol) modulates the kinase activity of PKC-delta through RACK.

Protein kinase C and meiotic regulation in isolated mouse oocytes

In this study, the possible role of protein kinase C (PKC) in mediating both positive and negative actions on meiotic maturation in isolated mouse oocytes has been examined. When cumulus cell-enclosed oocytes (CEO) were cultured for 17-18 hr in a medium containing 4 mM hypoxanthine (HX) to maintain meiotic arrest, each of the five different activators and five different antagonists of PKC stimulated germinal vesicle breakdown (GVB) in a dose-dependent fashion. One of the activators, phorbol-12-myristate 13-acetate (PMA), also triggered GVB in CEO arrested with isobutylmethylxanthine or guanosine, but not in those arrested with dibutyryl cyclic AMP. When denuded oocytes (DO) were cultured for 3hr in inhibitor-free medium, all PKC activators suppressed maturation (<10% GVB compared to 94% in controls), while the effect of PKC antagonists was negligible. Four of the five antagonists reversed the meiosis-arresting action of HX in DO. PMA transiently arrested the spontaneous maturation of both CEO and DO, with greater potency in DO. The stimulatory action of PMA in HX-arrested oocytes was dependent on cumulus cells, because meiotic induction occurred in CEO but not DO. PKC activators also preferentially stimulated cumulus expansion when compared to antagonists. A cell-cell coupling assay determined that the action of PMA on oocyte maturation was not due to a loss of metabolic coupling between the oocyte and cumulus oophorus. Finally, Western analysis demonstrated the presence of PKCs alpha, beta1, delta, and eta in both cumulus cells and oocytes, but only PKC epsilon was detected in the cumulus cells. It is concluded that direct activation of PKC in the oocyte suppresses maturation, while stimulation within cumulus cells generates a positive trigger that leads to meiotic resumption.

Sequential PKC- and Cdc2-mediated phosphorylation events elicit zebrafish nuclear envelope disassembly

Molecular markers of the zebrafish inner nuclear membrane (NEP55) and nuclear lamina (L68) were identified, partially characterized and used to demonstrate that disassembly of the zebrafish nuclear envelope requires sequential phosphorylation events by first PKC, then Cdc2 kinase. NEP55 and L68 are immunologically and functionally related to human LAP2beta and lamin B, respectively. Exposure of zebrafish nuclei to meiotic cytosol elicits rapid phosphorylation of NEP55 and L68, and disassembly of both proteins. L68 phosphorylation is completely inhibited by simultaneous inhibition of Cdc2 and PKC and only partially blocked by inhibition of either kinase. NEP55 phosphorylation is completely prevented by inhibition or immunodepletion of cytosolic Cdc2. Inhibition of cAMP-dependent kinase, MEK or CaM kinase II does not affect NEP55 or L68 phosphorylation. In vitro, nuclear envelope disassembly requires phosphorylation of NEP55 and L68 by both mammalian PKC and Cdc2. Inhibition of either kinase is sufficient to abolish NE disassembly. Furthermore, novel two-step phosphorylation assays in cytosol and in vitro indicate that PKC-mediated phosphorylation of L68 prior to Cdc2-mediated phosphorylation of L68 and NEP55 is essential to elicit nuclear envelope breakdown. Phosphorylation elicited by Cdc2 prior to PKC prevents nuclear envelope disassembly even though NEP55 is phosphorylated. The results indicate that sequential phosphorylation events elicited by PKC, followed by Cdc2, are required for zebrafish nuclear disassembly. They also argue that phosphorylation of inner nuclear membrane integral proteins is not sufficient to promote nuclear envelope breakdown, and suggest a multiple-level regulation of disassembly of nuclear envelope components during meiosis and at mitosis.

MAP and cdc2 kinase activities at germinal vesicle breakdown in Chaetopterus

In frog oocytes, activation of mitogen-activated protein kinase (MAPK, ERK) leads to activation of cdc2 and germinal vesicle breakdown (GVBD). By contrast, in starfish, MAPK is activated after GVBD. Here we have examined the relative involvements of MAPK and cdc2 in GVBD of Chaetopterus oocytes. MAPK was rapidly tyrosine-phosphorylated and activated (within 1-2 min) in response to exposure of the oocytes either to natural seawater (the normal trigger of GVBD in this organism) or to the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), which can also elicit GVBD. This response preceded the tyrosine dephosphorylation and activation of cdc2 by several minutes. MAPK phosphorylation and activation were transient, lasting only until GVBD occurred and the spindle migrated to the cortex. The enzyme was not phosphorylated again as a result of egg activation. These results are consistent with the hypothesis that the activation of MAPK has a role in GVBD. However, PD 98059, a potent and selective inhibitor of MEK, the protein kinase that phosphorylates and activates MAPK, blocked the phosphorylation of MAPK but did not block GVBD, the dephosphorylation and activation of cdc2, or spindle formation and migration. Oocytes that underwent GVBD in PD 98059 could be fertilized and cleaved normally. Ionophore A23187, although it caused germinal vesicles to disappear and caused transient phosphorylation of MAPK, did not cause dephosphorylation of cdc2, and therefore this disappearance is artifactual. These results suggest that MAPK activation is neither obligatory nor sufficient for either GVBD or meiotic metaphase arrest in Chaetopterus and that activation of MAPK and cdc2 occur on independent, parallel pathways.

PKC--a pivotal regulator of early development

Oocytes, eggs and blastomeres of the embryo are special cells that undergo rapid changes in structure and function at developmental transitions. These changes are frequently regulated by cytoplasmic signaling events, particularly at the developmental transition of fertilization, because the genome is largely inactivated at this time. Protein kinase C (PKC) is a signaling agent that acts after the sperm-induced rise in calcium and has a central role in the remodeling of the structure of the egg into the zygote in many species. PKC also acts during other developmental transitions. This kinase serves as a chronometer, which can choreograph the cell's remodeling events in both space and time. Several technical advancements discussed in this review have permitted a better understanding of the actions of PKC.

Dexniguldipine hydrochloride, a protein-kinase-C-specific inhibitor, affects the cell cycle, differentiation, P-glycoprotein levels, and nuclear protein phosphorylation in Friend erythroleukemia cells

Dexniguldipine hydrochloride (DNIG) is a potent antineoplastic agent with well-documented anti-(protein kinase C) activity and an ability to reverse multidrug resistance. Given the importance of protein kinase C (PKC) activity in proliferation and differentiation, we examined the effect of DNIG on several parameters of Friend erythroleukemia cell (FELC) activity. Particular attention was paid to proliferation, hexamethylene-bisacetamide-(HMBA)-induced differentiation, nuclear localization of protein kinase C, and nuclear protein phosphorylation. P-glycoprotein expression was also followed as an indicator of changes in multidrug resistance. At 2.5 microM, DNIG caused a significant decrease in the rate of FELC proliferation, while maintaining a cellular viability of greater than 80%, whether exposure to the drug was continuous over 96 h or took the form of a 6-h pulse/chase. DNA synthesis was decreased in cells exposed to DNIG for 20 h. Flow cytometry showed a marked increase in the percentage of cells in S phase of the cell cycle. Phosphorylation studies revealed decreased phosphorylation of two nuclear proteins (80 kDa and 47 kDa) following a 4-h exposure to the drug. HMBA-induced differentiation was significantly inhibited with continuous exposure to DNIG, and this effect appears to be a pre-commitment one, as 6-h pulse/chase exposures also resulted in inhibition of differentiation. Cells induced to differentiate with HMBA also demonstrated a decrease in the quantity of the 80-kDa phosphoprotein. Western blotting revealed that, even in the face of decreased phosphorylation, exposure to this PKC inhibitor resulted in an increase in the amount of nuclear PKC alpha. Finally, levels of P-glycoprotein were decreased in the presence of this drug. Our work identifies several effects of the PKC inhibitor DNIG on FELC and suggests several roles for PKC in regulating FELC proliferation and differentiation. Additionally, these results suggest that this PKC inhibitor may increase the effect of other chemotherapeutic drugs, particularly S-phase-specific ones, by increasing the length of S phase and decreasing multidrug resistance. The possibility of combination therapy with DNIG and other antineoplastic agents should be investigated further in light of these findings.

The zeta protein kinase C isoform from the testis of the grey mullet Mugil cephalus with a specific reaction protein of M(r) 48,000 on oolemma

The zeta protein kinase C isoform (PKC-zeta) was purified from the testis of the grey mullet Mugil cephalus and has relative masses (M(r)) of 65,000 and 63,000. The subunits of PKC-zeta from spermatozoa degenerated to M(r) 58,000 and 53,000 after continuous freezing and thawing. Proteins of M(r) 48,000 on the oolemma of the grey mullet Mugil cephalus were found to be the reaction proteins of the PKC-zeta from spermatozoa.

Purification of the delta isoenzyme of protein kinase C from the hepatopancreas of the shrimp Penaeus monodon with phosphorylation on tyrosine residues

The delta isoenzyme of protein kinase C (PKC-delta), purified from the plasma membrane of the hepatopancreas of the shrimp Penaeus monodon is specifically phosphorylated at tyrosine residues, as demonstrated by specific dephosphorylation by phosphotyrosyl protein phosphatase from the hepatopancreas of the shrimp Penaeus monodon. The specific activity of purified PKC-delta was 200 units/mg of protein. The subunits of M(r) 66,000, 62,000, and 58,000 of PKC-delta were not autophosphorylated after the addition of phosphatidylserine and diolein. However, the purified PKC-delta was active and catalyzed the phosphorylation of myelin basic protein. The kinase activity of the purified PKC-delta could be decreased after treatment with phosphotyrosyl protein phosphatase.

Absence of transitory [Ca2+]i flux during early in vitro metacyclogenesis of Trypanosoma cruzi

The phorbol ester TPA (phorbol 12-myristate 13-acetate) substitutes for CO2 as an agonist for transforming Trypanosoma cruzi epimastigotes to the metacyclic trypomastigote stage in a starvation medium consisting of phosphate buffered saline + 10 mM proline, 10 mM sodium acetate and 0.035% NaHCO3. Since TPA is thought to stimulate protein kinase C by mimicking the activity of the secondary messenger diacylglycerol, the above result suggested that T. cruzi metacyclogenesis could be activated by a Ca(2+)-dependent protein kinase C signal induction pathway. Accordingly, cytosolic calcium flux ([Ca2+]i) in epimastigotes, activated with 5% CO2 or TPA (10(-7) M), was measured with the Ca2+ molecular probe, fluo-3AM. In addition, [Ca2+]i was measured in cells incubated with putative metacyclogenic agonists (e.g. proline, glutamate, bioamines, ionophores and catecholamines). None of the compounds studies, except for EGTA, affected cytosolic Ca2+ levels. Control assays with 11 microM thapsigargin, which mobilizes noncytoplasmic Ca2+ stores by inhibiting endoplasmic reticulum Ca(2+)-ATPase, validated our fluorometric assay procedure. Although thapsigargin significantly increases cytoplasmic Ca2+ fluorescence, it has no effect on transformation. The protein kinase C inhibitors staurosporine, H-7 and HA 1004 were tested for their effect on T. cruzi metacyclogenesis. Low concentrations of staurosporine and HA 1004 significantly elevated Peru strain transformation while H-7 had no effect on Peru strain metacyclogenesis. Inhibitor H-7 did significantly depress CL transformation. The results indicate that induction of T. cruzi metacyclic trypomastigote formation by CO2 and TPA is not accompanied by changes in cytosolic Ca2+ and do not provide supporting evidence for participation of a protein kinase C-mediated phosphoinositide cascade in metacyclogenesis.