Differential regulation of protein kinase C isozymes by thyrotropin-releasing hormone in GH4C1 cells

Cellular distribution of isoforms of protein kinase C (PKC) in pancreatic acini

As in a previous study (Biochim, Biophys. Acta 1224 (1994) 127-138), we used quantitative immunoblot analysis and found that rat pancreatic acini possess four different isoforms of PKC-alpha, delta, epsilon and zeta. The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) caused translocation of each isoform from the cytosol to the membrane fraction. CCK-8 increased diacylglycerol (DAG) and caused translocation of PKC-sigma and PKC-epsilon but not that of PKC-alpha or PKC-zeta. L-364,718, a CCK receptor antagonist, prevented as well as reversed the effects of CCK-8 on DAG and on translocation of PKC-sigma and PKC-epsilon. To explore the possibility that different isoforms of PKC might have different distributions in rat pancreas, we used immunocytochemistry to determine the cellular distribution of different isoforms of PKC in intact pancreas as well as pancreatic acini. In intact pancreas, PKC-alpha and PKC-sigma were detected in islet cells but not in duct or acinar cells. PKC-epsilon was detected in the apical region of acinar cells and PKC-zeta was detected over the luminal surfaces of acinar cells and the ductules that extend from the acinus. Neither PKC-epsilon nor PKC-zeta was detected in islets. In pancreatic acini PKC-alpha and PKC-sigma were detected in islets or fragments of islets that contaminated the preparation but were not detected in acinar cells. PKC-epsilon was detected in the apical region of acinar cells and adding 1 microM TPA or 1 microM CCK-8 accentuated the immunostaining but did not alter its cellular distribution. L-364,718 reversed the changes in immunostaining caused by CCK-8. PKC-zeta was detected over the luminal surface of the acinar cells. TPA, but not CCK-8 or CCK-8 followed by L-364,718, increased the number of acini that showed staining of the luminal surfaces of acinar cells. Thus, the present results demonstrate that different isoforms of PKC are distributed differently in rat pancreas and that the different patterns of distribution can explain, at least in part, the different responses to CCK-8.

A composite Ets/Pit-1 binding site in the prolactin gene can mediate transcriptional responses to multiple signal transduction pathways

Binding sites for the tissue-specific transcription factor, Pit-1, are required for basal and hormonally induced prolactin gene transcription. Although Pit-1 is phosphorylated in response to several signaling pathways, the mechanism by which Pit-1 contributes to hormonal induction of gene transcription has not been defined. Recent reports suggest that phosphorylation of Pit-1 may not be required for hormonal regulation of the prolactin promoter. Analysis of the contribution of individual Pit-1 binding sites has been complicated due to the fact that some of the elements appear to be redundant. To better understand the role of Pit-1 sites in mediating hormonal regulation of the prolactin gene, we have performed enhancer tests using the three most proximal Pit-1 binding sites of the rat prolactin gene which are designated the 1P, 2P, and 3P sites. The results demonstrate that multimers of the 3P Pit-1 binding site are much more responsive to several hormonal and intracellular signaling pathways than multimers of the 1P or 2P sites. The 3P DNA element was found to contain a consensus binding site for the Ets family of proteins. Mutation of the Ets binding site greatly decreased the ability of epidermal growth factor, phorbol esters, Ras, or the Raf kinase to induce reporter gene activity. Mutation of the Ets site had little effect on basal enhancer activity. In contrast, mutation of the consensus Pit-1 binding site in the 3P element essentially abolished all basal enhancer activity. Overexpression of Ets-1 in GH3 pituitary cells enhanced both basal and Ras induced activity from the 3P enhancer. These data describe a composite element in the prolactin gene containing binding sites for two different factors and the studies suggest a mechanism by which Ets proteins and Pit-1 functionally cooperate to permit transcriptional regulation by different signaling pathways.

Carbachol-induced desensitization of rat basophilic leukemia (RBL-2H3) cells transfected with human m3 muscarinic acetylcholine receptors

1. Carbachol-induced homologous desensitization of the secretory response was investigated by transfecting RBL-2H3 cells with cDNA encoding the human m3 muscarinic acetylcholine receptor (RBL-m3). 2. Exposure of RBL-m3 cells to 100 microM carbachol for 30 min in Ca2+-free medium inhibited the secretion induced by the subsequent addition of 10 microM carbachol plus Ca2+. 3. Desensitized cells bound [3H]quinuclidinyl benzilate with a similar Bmax and Kd to those of control cells. 4. The carbachol-induced transient increase in levels of inositol 1,4,5-trisphosphate was not changed by desensitization. 5. Homologous desensitization persisted when desensitized cells were permeabilized with Staphylococcal alpha-toxin.

Protein kinase C (PKC)-induced PKC down-regulation. Association with up-regulation of vesicle traffic

Phorbol esters cause long term activation of protein kinase C (PKC) and frequently the down-regulation of PKC protein levels in mammalian cells. Mammalian PKC-gamma, -delta, and -eta down-regulated in response to phorbol esters when expressed in Schizosaccharomyces pombe. However, PKC-epsilon does not down-regulate in S. pombe, in contrast to the behavior of this isotype in mammalian cells. Co-expression of PKC-gamma or -delta with PKC-epsilon in S. pombe renders PKC-epsilon susceptible to down-regulation. A protein kinase defective form of PKC-delta does not down-regulate efficiently in S. pombe but, like PKC-epsilon, is susceptible when co-expressed with PKC-gamma or full-length PKC-delta. Thus, down-regulation is a consequence of the catalytic function of certain PKC isotypes with other isotypes being affected in trans. PKC down-regulation parallels a striking accumulation of vesicles in S. pombe, suggesting a direct relationship between these events.

Regulation of non-classical protein kinase C isoenzymes in a human T cell line

We have examined the expression and responses to activation, of novel/atypical protein kinase C (PKC) isoforms epsilon, zeta, and delta in the T cell lymphoma cell line K-4. The effects of 1-h phorbol 12-myristate 13-acetate (PMA) and OKT3 activation of K-4 cells on PKC isoform distribution were examined. In addition, the effects of PMA-mediated down-regulation on the expression of PKC epsilon and zeta were determined using high concentrations of PMA over 24- and 48-h time periods in these cells. PKC zeta expression was not altered by incubation of K-4 cells with up to 200 ng/ml PMA over a 24- or 48-h period. PKC epsilon was down-regulated in a concentration-dependent manner by PMA after both 24- and 48-h of activation. Expression of PKC epsilon was not completely depressed, however, even at the highest concentration of the phorbol ester after 48-h incubation with PMA. The presence of PKC epsilon, zeta, and delta was confirmed by immunohistochemistry with distinct patterns of expression observed. PMA-induced PKC activation for a 1-h period resulted in a translocation of PKC delta from resting cytoplasmic/nuclear staining to a cytoplasmic aggregate. Following 1-h activation through the T cell receptor-associated complex CD3, PKC delta translocated from a peri-nuclear/cytoplasmic compartment to a putative cytoskeletal location in K-4 cells. This translocation was time dependent and redistributed to a cytoplasmic aggregate prior to the cytoskeleton. Similarly, following 1-h activation through the T cell receptor, PKC zeta redistributed directly to what is possibly a cytoskeletal cell compartment. The cytoplasmic distribution of PKC zeta was unaltered following activation with PMA over a 1-h time period. There was no apparent redistribution of PKC epsilon cytoplasmic staining pattern following a 1-h direct or indirect activation. These results underline the differences in individual PKC isoform distribution, and responses to different stimuli, thereby providing additional evidence for the use of discrete PKC isoform signaling pathways in T cells. Furthermore, this data underlines the differences in PMA-mediated PKC activation and activation through the T cell receptor.

Activation of the transcription factor NF-KB in GH3 pituitary cells

Since several genes expressed in the pituitary can bind the transcription factor NF-KB, its presence and regulation was examined in the GH3 pituitary cell line. An electrophoretic mobility shift assay using nuclear extracts and an oligonucleotide probe corresponding to the Ig KB binding site was employed to identify activated NF-KB. One complex possessed properties characteristic of NF-KB: co-migration with an NF-KB complex and binding specificity restricted to NF-KB binding DNA sequences. Antibodies to the NF-KB subunits NFKB1p50 (p50) and RelA (p65) interacted with the extract-DNA complex. Activation of NF-KB in GH3 cells was increased by PMA or the cytokine tumor necrosis factor alpha. A synergy between PMA and TNF or a calcium mobilizing agent was seen in NF-KB activation. Further TNF activation was enhanced by TRH. These observations indicate the presence of NF-KB in GH3 cells and demonstrate its activation by hormones/second messengers that act on pituitary cells.

Effects of cholecystokinin (CCK) and other secretagogues on isoforms of protein kinase C (PKC) in pancreatic acini

We used rat pancreatic acini and measured the effects of various agents on digestive enzyme secretion, diacylglycerol (DAG) and the cellular distribution of protein kinase C (PKC) enzyme activity as well as isoforms of PKC determined by quantitative immunoblot analysis. TPA, but not CCK-8, caused translocation of PKC enzyme activity from the cytosol fraction to the membrane fraction. Immunoblot analysis detected PKC-alpha, PKC-delta, PKC-epsilon and PKC-zeta. PKC-beta, PKC-gamma and PKC-eta were not detected. TPA caused translocation of all isoforms from cytosol to membrane, whereas CCK-8 caused translocation of PKC-delta and PKC-epsilon, carbachol caused translocation of PKC-epsilon, and bombesin and secretin caused no detectable translocation of any isoform. Specific receptor antagonists could prevent, as well as reverse completely, the translocation of PKC isoforms caused by CCK-8 or carbachol. Agonists added in sequence with an interposed addition of a specific receptor antagonist caused cycling of PKC-epsilon between cytosol and membrane fractions. Each receptor-mediated agonist that caused translocation of PKC also increased DAG, and with CCK-8 and carbachol cycling of PKC-epsilon between cytosol and membrane was accompanied by corresponding cyclic changes in cellular DAG. CCK-JMV-180, bombesin and secretin increased DAG but did not cause translocation of any PKC isoform. Translocation of a PKC isoform could be accounted for by whether the increased DAG originated from PIP2 (accompanied by translocation) or from phosphatidylcholine (no accompanying translocation). Thus it appeared that DAG, in pancreatic acini, is functionally compartmentalized depending on the source of the lipid. Studies using CCK-8 and CCK-JMV-180 indicated that occupation of the low affinity state of the CCK receptor by either peptide increased DAG from phosphatidylcholine, whereas occupation of the very low affinity state by CCK-8 increased DAG from PIP2 and caused translocation of PKC-delta and PKC-epsilon. TPA stimulated amylase secretion, indicating that activation of PKC can stimulate enzyme secretion; however, with the various receptor-mediated secretagogues there was no consistent, unequivocal correlation between translocation of an isoform of PKC and accompanying changes in enzyme secretion.

Developmental expression of protein kinase C isozymes in oligodendrocytes and their differential modulation by 4 beta-phorbol-12,13-dibutyrate

Myelin gene expression in normal oligodendrocytes (OLG) depends on developmentally regulated protein kinase C (PKC) enzyme activity (Asotra and Macklin: J Neurosci Res 34:571-588, 1993). We studied the developmental expression of the Ca(++)-dependent PKC-alpha, -beta 1, -beta II and -gamma isozymes, and the Ca(++)-independent PKC-delta, -epsilon, -zeta and -eta isozymes in enriched rat brain OLG cultures. In A2B5+ O-2A progenitors, only PKC-delta, PKC-epsilon and PKC-zeta were detected immunocytochemically. In 04+ proligondendrocytes, PKC-beta I, -delta and -zeta were expressed moderately and low levels of PKC-alpha and -epsilon were detected. GD3+ OLG, GC+ OLG and MBP+ OLG showed increased levels of PKC-alpha, -beta I, -delta and -zeta isozymes. PKC-beta II, -gamma and -eta were poorly expressed in OLG. On immunoblots, PKC-alpha was present early and increased continually up to 18 days but PKC-beta I increased until 12 days in cultured OLG. High levels of PKC-delta, PKC-epsilon and PKC-zeta, the most abundant PKC isozymes in OLG, were maintained up to 12 days and were then slightly reduced. Interestingly, relatively high levels of PKC-alpha, PKC-beta I, PKC-beta II, PKC-gamma and PKC-epsilon isozymes were detected in purified myelin membrane although greater levels of PKC-delta were found in OLG than in purified myelin. Thus, most of the PKC isozymes found in cultured OLG were also present in myelin, although at different levels. Treatment with 50 nM 4 beta-phorbol-12,13-dibutyrate (PDB) caused a delayed downregulation of PKC-delta levels after 8 hr without modulating the expression of other PKC isozymes in 1-day OLG; in the 3-day-old and 6-day-old OLG, PDB downmodulated PKC-beta I, -delta and epsilon isozymes with only a minor effect on PKC-alpha and no reduction in PKC-zeta. Induction or downmodulation of individual PKC isozymes by phorbol esters appears to depend on the differentiation state of OLG. These data suggest that PKC-beta I, -delta and -epsilon isozymes have an important function in different cellular events of OLG differentiation. We conclude that the PKC-dependent modulation of myelin gene expression in OLG results predominantly from the Ca(++)-dependent PKC-beta I isozyme activity and the CA(++)-independent PKC-delta and PKC-epsilon activitives in a cell differentiation state-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Translocation and downregulation of protein kinase C isoenzymes-alpha and -epsilon by phorbol ester and bryostatin-1 in human keratinocytes and fibroblasts

Protein kinase C isoenzymes can be subdivided into two classes, based on their requirement for calcium. Protein kinase C-alpha, beta I, -beta II, and -gamma are calcium dependent, whereas protein kinase C-gamma, -epsilon, -zeta, -eta, and -theta are calcium independent. We have examined the expression, translocation, downregulation, and activation of calcium-dependent and -independent protein kinase C isoenzymes in human skin keratinocytes and fibroblasts. Human keratinocytes and fibroblasts expressed protein kinase C-alpha, -delta, -epsilon, and -zeta mRNA and protein, whereas protein kinase C-eta (L) was detected only in keratinocytes. Protein kinase C-beta I, -beta II, -gamma, and -theta were not detected in either cell type. The protein kinase C activators 12-0-tetradecanoylphorbol 13-acetate and bryostatin-1 (50 nM, for 5 min) induced translocation of protein kinase C-alpha and -epsilon cytosol to membrane in both keratinocytes and fibroblasts. 12-0-tetradecanoylphorbol 13-acetate and bryostatin-1, for 18 h, induced complete downregulation (i.e., loss) of protein kinase C-alpha and -epsilon in keratinocytes, but only partial downregulation was observed in fibroblasts. The subcellular distribution of protein kinase C-delta, -zeta or protein kinase C-eta, in keratinocytes or fibroblasts, did not change in response to 12-0-tetradecanoylphorbol 13-acetate or bryostatin-1. These data indicate differential expression, subcellular distribution, and regulation of protein kinase C isoenzymes in human skin cells.

Ca(2+)-dependent protein kinase C isoforms in rat pituitary hyperplasia: effect of in vivo treatment with quinagolide

Ca(2+)-dependent protein kinase C (PKC) activity, diacylglycerol levels and PKC alpha, beta I, beta II and gamma expression were analyzed in the pituitary of female rats treated with estradiol alone (2 months) or in combination with quinagolide in the second month. Polymerase chain reaction (PCR) and Western blot analysis revealed the presence of PKC alpha, beta I and beta II isoenzymes in the rat pituitary gland but not of PKC gamma isoenzymes. Increases in pituitary weight and plasma prolactin levels induced by estradiol were associated with an increase in diacylglycerol pituitary content (1.55 +/- 0.06 versus 1.12 +/- 0.17 nmol diacylglycerol/mg protein in controls, P < 0.01). Cotreatment with quinagolide reversed these effects. Changes in PKC activity were accompanied by parallel changes in PKC alpha and beta I expressions. Estradiol treatment increased the expression of these isoforms whereas cotreatment with quinagolide antagonized these effects. PKC beta II expression was not affected. In conclusion, Ca(2+)-dependent PKC activity and protein expression are increased in hyperplastic pituitary cells, suggesting the involvement of this class of PKCs in the rat pituitary cell proliferation and/or hormonal secretion. This is further assessed by the fact that the dopamine receptor agonist treatment decreases activity and expression of these PKCs in parallel with the decrease in hormonal secretion and cell proliferation.

Effects of Ca2+ on the activation of conventional and new PKC isozymes and on TPA and endothelin-1 induced translocations of these isozymes in intact cells

The effects of Ca2+ on the translocation of conventional and new protein kinase C isozymes in intact cells were studied by using C6 glioma cells as a model system. Two conditions which monitor intracellular Ca2+ were performed: one is extracellular Ca(2+)-depletion by treating the cells with physiological saline solution (PSS) without Ca2+ but containing 0.5 mM EGTA, the other is treating the cells with 1 microM ionomycin to induce Ca(2+)-influx. In addition, the TPA and endothelin-1 induced translocations of conventional and new PKC isozymes under these two conditions were also comparatively studied. When the intact cells were treated with Ca(2+)-free, EGTA containing PSS, the membrane-bound conventional PKC alpha (cPKC alpha) was greatly reduced and cytosolic cPKC alpha was slightly increased. However, neither membrane bound nor cytosolic new PKC delta (nPKC delta) was affected by extracellular Ca(2+)-depletion. On the other hand, when the cells were treated with 1 microM ionomycin, the translocation of cPKC alpha itself was observed while nPKC delta was not affected. In extracellular Ca(2+)-depletion, the translocation of cPKC alpha induced by 100 nM TPA still occurred although the extent of translocation was smaller than that induced by TPA under normal Ca2+ conditions; however, that induced by 30 nM ET-1 was blocked. After the cells were treated with 1 microM ionomycin, the translocation of cPKC alpha induced by 30 nM TPA was further increased compared to 1 microM ionomycin or 30 nM TPA alone, while that induced by ET-1 was only slightly further increased. All these results suggested that in intact cells, the activation of cPKC alpha was operated by both the intracellular Ca2+ level and diacylglycerol and that of nPKC delta was operated by diacylglycerol alone as predicted by their properties from purified enzyme or cDNA. In addition, the translocation of cPKC alpha induced by the natural activator ET-1 seemed to be more dependent on Ca2+ than TPA in intact cells.

Staurosporine up-regulates the expression of phorbol dibutyrate binding sites in human platelets

Tumor-promoting phorbol esters bind to and activate protein kinase C (PKC). Staurosporine, a potent PKC inhibitor, interferes with PKC catalytic activity without altering phorbol ester binding sites in cell-free systems. We found that, unlike cell-free systems, treatment of intact platelets with staurosporine enhances the expression of phorbol 12, 13-dibutyrate (PDBu) binding sites. Incubation of platelets at 37 degrees with staurosporine (25 nM to 1 microM and 2 nM tritiated PDBu ([3H]PDBu) increased the amount of [3H]PDBu specifically bound to intact platelets by approximately 10 to 200% of control values. This effect was rapid and plateaued after 10 min of cell treatment. Scatchard analysis of the data showed that staurosporine (500 nM) significantly increased the total binding capacity Bmax from 42.9 +/- 15.4 x 10(3) to 78 +/- 7.3 x 10(3) sites per platelet and reduced the apparent dissociation constant value Kd from 30.8 +/- 8.6 nM to 9.4 +/- 3.4 nM. Enhanced PDBu binding capacity and affinity were also observed with human mononuclear and polymorphonuclear leukocytes. Fractionation of staurosporine-treated platelets showed an increased binding capacity of the particulate fraction (102%) and decreased binding capacity of the soluble fraction (60%) compared to controls, with no change in the affinity of PDBu binding to these fractions. Chelation of internal calcium with BAPTA did not significantly attenuate the staurosporine-mediated rise in PBDu binding but prevented the platelet-activating factor-induced response, indicating that cytosolic calcium does not play an important role in these staurosporine effects. These results show that, in addition to interfering with PKC protein-phosphorylating activity, staurosporine enhances PDBu binding affinity and capacity in intact platelets. This latter effect appears to be due to translocation of soluble PDBu binding sites, presumably PKC units.

Phorbol esters showing selective activation of PKC isozymes in vitro regulate thyroid function and insulin-like growth factor binding protein secretion

We have examined the effects of phorbol derivatives which show selective activation of protein kinase C (PKC) isozymes in vitro, on several parameters of thyroid function. Functions examined were iodide uptake and organification, iodocompound secretion and insulin-like growth factor binding protein (IGFBP) secretion, all of which have been shown previously to be modulated by 12-O-tetradecanoylphorbol 13-acetate (TPA), a pan activator of PKC isozymes. All of the agents examined, including DOPPA (12-deoxyphorbol-13-O-phenylacetate-20 acetate), which is specific for the beta 1 isozyme in vitro, were able to mimic the effects of TPA. These effects were evident by 2 h in the iodide uptake and organification assays, by 4 h in the secretion assays and by 8 h in the IGFBP secretion assays. The phorbol derivatives differed from TPA in their ability to down-regulate total PKC activity, DOPPA being weakly effective at 8 h (14.7% inhibition) when TPA had effected > 70% down-regulation of PKC. As the effects of DOPPA were detected by 8 h at the latest, these data indicate that the effects observed were due to PKC activation rather than down-regulation. Furthermore, the differences in down-regulation profiles between DOPPA and TPA suggest that in vivo, DOPPA may maintain its in vitro specificity. We conclude that inhibition of thyroid iodide uptake and its organification, stimulation of iodocompound secretion and stimulation of IGFBP-2 and IGFBP-3 secretion may be effected through the modulation of a limited number of PKC isozymes and possibly initially, only through PKC beta 1.

Bradykinin and angiotensin II: activation of protein kinase C in arterial smooth muscle

The effects of bradykinin (BK) and angiotensin II (ANG II) were compared in cultured rat mesenteric arterial smooth muscle cells. BK and ANG II activated a phosphoinositide-specific phospholipase C, leading to the rapid release of [3H]inositol phosphates, an increase in intracellular calcium, and formation of sn-1,2-diacylglycerol (DAG). DAG formation was biphasic with a transient peak at 5 s followed by a sustained increase from 60 to 600 s. The BK-mediated increases in inositol triphosphate and DAG were dose dependent with half-maximal increases at concentrations of 5 and 2 nM, respectively. Both hormones were found to activate protein kinase C (PKC) as assessed by phosphorylation of the 68- to 72-kDa intracellular PKC substrate myristoylated alanine-rich C kinase substrate. However, despite similar phosphorylation of this substrate, only ANG II produced a significant increase in membrane-bound PKC activity. The mechanism accounting for the inability of BK to increase membrane-bound PKC activity is unclear. Our studies excluded differential translocation of PKC to the nuclear membrane, production of an inhibitor of membrane-bound PKC activity, and expression of BK and ANG II receptors on different cells as the mechanism. Vascular smooth muscle cells were found to express at least four different PKC isozymes: alpha, delta, zeta, and a faint band for epsilon. All of the isozymes except zeta-PKC were translocated by treatment with the phorbol ester 4 beta-phorbol 12-myristate 13-acetate. However, neither ANG II nor BK produced significant translocation of any measured isozyme; therefore, we could not exclude the possibility that ANG II and BK activate different isozymes of PKC. Both hormones were found to have a similar small and inconsistent effect in stimulating [3H]thymidine incorporation. These observations demonstrate that BK and ANG II have similar biochemical effects on vascular smooth muscle cells and imply that, in selected vessels, the vasodilatory effects of BK mediated by the endothelium may be partially counterbalanced by a vasoconstrictor effect on the underlying vascular smooth muscle cells.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein kinase C--a question of specificity

Following the initial identification of protein kinase C (PKC) by Nishizuka and co-workers in the late seventies, a wealth of information on this protein kinase has accumulated. Perhaps most striking was the realization that PKC is not just a single polypeptide but in fact consists of a large family of related proteins. These PKC isotypes are unique, not only with respect to primary structure, but also on the basis of expression patterns, subcellular localization, activation in vitro and responsiveness to extra-cellular signals. This review focuses on the heterogeneity within the PKC family and highlights some of the recent evidence that the isotypes might have separate and unique functions in the cell.

Reconstitution of protein kinase C alpha function by the protein kinase C beta-I carboxy terminus

The Ca(2+)- and phospholipid-dependent Ser/Thr kinase protein kinase C (PKC) plays important roles in the transduction of cellular signals. Various PKC isoforms exist in mammalian cells which share conserved and variable regions as defined by cDNA sequence comparisons. To test whether carboxyl (C) terminal sequences of distinct isoforms can complement each other to yield functional chimeric molecules, we have constructed a PKC chimera in which amino acids 595-672 at the C-terminus of bovine PKC alpha (a) were replaced with the corresponding C-terminal amino acids (598-671) of rat PKC beta-I (b) to yield the chimera alpha/beta-I (ab). The chimera was then characterized biochemically and functionally, and compared with the parental isoforms. Since structure/function analysis of PKC in mammalian experimental systems is complicated by multiple PKC isoforms and by cellular complexity, we stably introduced the PKC constructs into the yeast Saccharomyces cerevisiae, a simple, lower eukaryote with a short doubling time and well established molecular genetics. In yeast, the faithfully expressed PKCab chimera and normal PKC isoforms bound radiolabelled phorbol ester and were recognized on immunoblots by PKC-specific antibodies. The chimera phosphorylated substrate peptides in a PMA- and Ca(2+)-dependent manner, and, upon activation, increased the cell doubling time and the rate of Ca2+ uptake into cells. In addition, PKCab displayed characteristics distinct from normal PKCb, but virtually indistinguishable from normal PKCa. Our findings indicate the reconstitution of PKCa function by the PKCb carboxyl terminus.

Desensitization of cell signalling mediated by phosphoinositidase C

The waning of responses to cell-surface receptor activation during persistent stimulation with agonists (desensitization) is a feature common to many forms of transmembrane signalling. However, information is scarce regarding the regulatory processes that modulate the extensive group of receptors linked via phosphoinositidase C to the production of inositol 1,4,5-trisphosphate and 1,2-diacylglycerol. This situation is now beginning to change. Recent data indicate (1) that very rapid desensitization, possibly associated with receptor phosphorylation, regulates receptors linked to phosphoinositidase C, (2) that different receptors are desensitized to varying extents, (3) that receptor internalization can mediate desensitization at later times and (4) that signalling can be regulated at additional sites downstream of phosphoinositidase C. As Richard Wojcikiewicz and colleagues discuss here, these diverse regulatory events provide the means by which the breakdown of phosphoinositides and cellular responsiveness to their products are controlled during cell stimulation.

Role of protein kinase C during interferon-gamma- and phorbol ester-stimulated immunocytochemical expression of ICAM-1 in human renal carcinoma cells

Incubation of the human renal carcinoma cell line CaKi-1 with interferon-gamma (IFN-gamma) or the phorbol ester, phorbol-12-myristate 13-acetate (PMA) strongly stimulated the immunocytochemical expression of the intercellular adhesion molecule-1 (ICAM-1) in a dose-dependent manner. Since PMA is capable of activating the Ca2+/phospholipid-dependent protein kinase C (PKC), we investigated the role of this kinase during IFN-gamma signal transduction. Calcium ionophore A23187 significantly enhanced IFN-gamma- and PMA-induced ICAM-1 staining. While staurosporine, H7 and sphingosine, three known PKC inhibitors, blocked the PMA effect, only staurosporine abrogated the action of IFN-gamma. Finally, 24 h of PMA pretreatment with subsequent IFN-gamma stimulation enhanced ICAM-1 staining above values from cultures where IFN-gamma was omitted. This occurred despite the fact that 24 h of PMA pretreatment abolished the effect of IFN-gamma on PKC activation, as determined by acetylated myelin basic protein 4-14 phosphorylation. In conclusion, these results suggest that additional events other than PKC activation are required for complete regulation of ICAM-1 antigen by IFN-gamma in the whole cell population. Hence, other Ca(2+)-dependent signalling pathway(s) mediated by IFN-gamma receptors must act. Further studies are needed to elucidate these specific pathway(s) activated during IFN-gamma stimulation in our model.

The protein kinase C family

Protein kinase C represents a structurally homologous group of proteins similar in size, structure and mechanism of activation. They can modulate the biological function of proteins in a rapid and reversible manner. Protein kinase C participates in one of the major signal transduction systems triggered by the external stimulation of cells by various ligands including hormones, neurotransmitters and growth factors. Hydrolysis of membrane inositol phospholipids by phospholipase C or of phosphatidylcholine, generates sn-1,2-diacylglycerol, considered the physiological activator of this kinase. Other agents, such as arachidonic acid, participate in the activation of some of these proteins. Activation of protein kinase C by phorbol esters and related compounds is not physiological and may be responsible, at least in part, for their tumor-promoting activity. The cellular localization of the different calcium-activated protein kinases, their substrate and activator specificity are dissimilar and thus their role in signal transduction is unlike. A better understanding of the exact cellular function of the different protein kinase C isoenzymes requires the identification and characterization of their physiological substrates.

Expression and partial characterization of rat protein kinase C-delta and protein kinase C-zeta in insect cells using recombinant baculovirus

Expression of rat protein kinase C-delta (PKC-delta) and PKC-zeta in insect cells using recombinant baculovirus resulted in the production of proteins with a molecular size of approximately 76 kD and 78 kD, respectively, as determined by immunoblotting with subtype-specific antisera. Although the PKC-zeta cDNA encoded for 592 amino acids, a 76 kD protein was also generated by in vitro transcription/translation. Extracts of cells expressing PKC-delta were able to bind phorbol ester to levels comparable to extracts of cells expressing PKC-alpha. No phorbol ester binding was, however, detected in insect cell extracts expressing PKC-zeta. However, similar levels of protein kinase activity were detected in lysates of cells expressing PKC-delta or PKC-zeta when protamine sulfate was used as exogenous substrate. Compared to protamine sulfate, both, myelin basic protein (MBP) or histone, were poor substrates for PKC-delta and PKC-zeta. In contrast to PKC-zeta, the PKC-delta enzyme activity phosphorylated MBP or histone in a phosphatidylserine-(PS)/diacylglycerol(DG)-dependent manner, albeit not to the same extent as PKC-alpha. Lack of stimulation of the enzyme activity of PKC-zeta by PS/DG, was confirmed by endogenous phosphorylation of insect cell proteins by PKC-zeta, whereas several insect cell proteins were phosphorylated by PKC-delta in a PS/DG-dependent manner, including a protein of 78 kD. Our data demonstrate that the 76 kD PKC-zeta, in contrast to PKC-delta, is unable to bind phorbol esters and displays a protein kinase activity that is independent of PS or PS/DG. In addition, staurosporine was about 2-4 order of magnitudes less effective in inhibiting the protein kinase activities of PKC-delta and PKC-zeta when compared to PKC-alpha.

Serine/threonine protein kinases

Signal transduction in the nervous system is heavily dependent on the three multifunctional serine/threonine protein kinases, PKA, PKC, and CaM-KII. Recent studies have furthered our understanding of how the multiple isoforms of these kinases and their subcellular localizations, regulatory properties, and substrate determinants are important for the specificity of kinase functions.

Protein kinase C mRNA levels and activity in reconstituted normal human epidermis: relationships to cell differentiation

Although keratinocytes are a major target of phorbol ester actions, the activity and the expression of the eight cloned protein kinase C (PKC) isoenzymes have not been studied in detail in human epidermis. Starting from normal human keratinocytes, we reconstituted in culture a multilayered epithelial tissue which presents many hystological, biochemical, and molecular features of the authentic epidermis and we used it as a model to investigate the PKC activity and mRNA levels. We found that i) PKC activity is higher in differentiated than in non-differentiated cells; ii) the mRNA levels of PKC delta and -eta/L, while are differently affected by spontaneous keratinocyte differentiation, are down-regulated during phorbol esters-induced cell differentiation. Our findings could represent a basis to investigate the involvement of PKC isoforms in the keratinocyte differentiation process.