Inducible overexpression of human protein kinase C alpha in NIH 3T3 fibroblasts results in growth abnormalities

ELAC (3,12-di-O-acetyl-8-O-tigloilingol), a plant-derived lathyrane diterpene, induces subventricular zone neural progenitor cell proliferation through PKCβ activation

BACKGROUND AND PURPOSE

Pharmacological strategies aimed to facilitate neuronal renewal in the adult brain, by promoting endogenous neurogenesis, constitute promising therapeutic options for pathological or traumatic brain lesions. We have previously shown that non-tumour-promoting PKC-activating compounds (12-deoxyphorbols) promote adult neural progenitor cell (NPC) proliferation in vitro and in vivo, enhancing the endogenous neurogenic response of the brain to a traumatic injury. Here, we show for the first time that a diterpene with a lathyrane skeleton can also activate PKC and promote NPC proliferation.

EXPERIMENTAL APPROACH

We isolated four lathyranes from the latex of Euphorbia plants and tested their effect on postnatal NPC proliferation, using neurosphere cultures. The bioactive lathyrane ELAC (3,12-di-O-acetyl-8-O-tigloilingol) was also injected into the ventricles of adult mice to analyse its effect on adult NPC proliferation in vivo.

KEY RESULTS

The lathyrane ELAC activated PKC and significantly increased postnatal NPC proliferation in vitro, particularly in synergy with FGF2. In addition ELAC stimulated proliferation of NPC, specifically affecting undifferentiated transit amplifying cells. The proliferative effect of ELAC was reversed by either the classical/novel PKC inhibitor Gö6850 or the classical PKC inhibitor Gö6976, suggesting that NPC proliferation is promoted in response to activation of classical PKCs, particularly PKCß. ELAC slightly increased the proportion of NPC expressing Sox2. The effects of ELAC disappeared upon acetylation of its C7-hydroxyl group.

CONCLUSIONS AND IMPLICATIONS

We propose lathyranes like ELAC as new drug candidates to modulate adult neurogenesis through PKC activation. Functional and structural comparisons between ELAC and phorboids are included.

Characterization of patterns of expression of protein kinase C-alpha, -delta, -eta, -gamma and -zeta and their correlations to p53, galectin-3, the retinoic acid receptor-beta and the macrophage migration inhibitory factor (MIF) in human cholesteatomas

Cholesteatoma is a benign disease characterized by the presence of an unrestrained growth and the accumulation of keratin in the middle ear cavity. Due to roles in cell proliferation, apoptosis and differentiation members of the protein kinase C (PKC) family could be involved in disease progression. This study focuses on the expression of protein kinase C-alpha, -delta, -eta, -gamma and -zeta in the epithelial tissues of 56 human cholesteatomas and their correlations with those of previously characterized distributions of p53, galectin-3, retinoic acid receptor-beta (RARbeta) and macrophage migration inhibitory factor (MIF). We have previously reported this marker set to be correlated with keratinocyte differentiation in human cholesteatomas. Our present data clearly show that the percentage of PKC-alpha (but not PKC-delta, -gamma, -eta and -zeta)-immunopositive cells in epithelial tissue fro recurrent cholesteatomas was significantly higher than in non-recurrent cases. Correlations between the PKC isoenzymes and the biological markers were non-uniform. PKC-alpha (but not PKC-delta, -gamma, -eta and -zeta) expression in epithelial cholesteatoma cells correlated significantly and positively with the percentages of p53-immunopositive cells. The patterns of PKC-alpha and -delta expression, but not of PKC-gamma, -eta and -zeta, correlated significantly and positively with galectin-3 expression. In addition, the correlation levels between the expression of PKC-alpha and -delta and that of galectin-3 varied depending on the infection and recurrence status. Presence of RARbeta correlated significantly (and positively) with the expression of PKC-gamma and -zeta and also in relation to the infection and recurrence status. MIF correlated presence significantly (and positively) with that of the five PKCs under study, depending on whether the cholesteatomas were non-infected or infected as well as non-recurrent or recurrent. In conclusion, the present study suggests that modifications occurring at the level of keratinocyte differentiation in human cholesteatomas involve distinct effectors, to which the activation of PKC-alpha, -delta, -eta, -gamma and -zeta can be added.

Diacylglycerol (DAG)-lactones, a new class of protein kinase C (PKC) agonists, induce apoptosis in LNCaP prostate cancer cells by selective activation of PKCalpha

Phorbol esters, the archetypical (PKC) activators, induce apoptosis in androgen-sensitive LNCaP prostate cancer cells. In this study we evaluate the effect of a novel class of PKC ligands, the diacylglycerol (DAG)-lactones, as inducers of apoptosis in LNCaP cells. These unique ligands were designed using novel pharmacophore- and receptor-guided approaches to achieve highly potent DAG surrogates. Two of these compounds, HK434 and HK654, induced apoptosis in LNCaP cells with much higher potency than oleoyl-acetyl-glycerol or phorbol 12,13-dibutyrate. Moreover, different PKC isozymes were found to mediate the apoptotic effect of phorbol 12-myristate 13-acetate (PMA) and HK654 in LNCaP cells. Using PKC inhibitors and dominant negative PKC isoforms, we found that both PKCalpha and PKCdelta mediated the apoptotic effect of PMA, whereas only PKCalpha was involved in the effect of the DAG-lactone. The PKCalpha selectivity of HK654 in LNCaP cells contrasts with similar potencies in vitro for binding and activation of PKCalpha and PKCdelta. Consistent with the differences in isoform dependence in intact cells, PMA and HK654 show marked differences in their abilities to translocate PKC isozymes. Both PMA and HK654 induce a marked redistribution of PKCalpha to the plasma membrane. On the other hand, unlike PMA, HK654 translocates PKCdelta predominantly to the nuclear membrane. Thus, DAG-lactones have a unique profile of activation of PKC isozymes for inducing apoptosis in LNCaP cells and represent the first example of a selective activator of a classical PKC in cellular models. An attractive hypothesis is that selective activation of PKC isozymes by pharmacological agents in cells can be achieved by differential intracellular targeting of each PKC.

Pharmacological evidence for system-dependent involvement of protein kinase C isoenzymes in phorbol ester-suppressed gap junctional communication

Several phorbol esters are potent activators of protein kinase C. They down-regulate gap junctional intercellular communication and induce phosphorylation of connexin43, but the sensitivity and extent of responses vary much between systems. We asked whether the total protein kinase C enzyme activity or the protein kinase C isoenzyme constitution was of importance for such variations. Some fibroblastic culture systems were compared. It was concluded that the total protein kinase C enzyme activity did not determine the sensitivity to phorbol esters. Furthermore, the use of isotype-specific inhibitors of protein kinase C indicated that protein kinase C alpha, delta, and epsilon may be involved to different extents in different fibroblastic systems in the response to phorbol esters.

Docosahexaenoic acid modulates phorbol ester-induced activation of extracellular signal-regulated kinases 1 and 2 in NIH/3T3 cells

Phosphorylation of extracellular signal-regulated kinases (ERK1/ERK2) has been implicated in cell proliferation of mammalian cells. In the present study, we investigated the role of docosahexaenoic acid (DHA) in the modulation of ERK1/ERK2 phosphorylation, stimulated either with phorbol 12-myristate 13-acetate (PMA) or transforming growth factor-alpha (TGFalpha) in NIH/3T3 cells. We observed that both PMA and TGFalpha induced ERK1/ERK2 phosphorylation within 5 min of stimulation. PMA acts upstream of MEK and via activation of protein kinase C (PKC), as GF109203X, a potent PKC inhibitor, and U0126, a MEK inhibitor, abolished its actions on ERK1/ERK2 phosphorylation. TGFalpha did not act via PKC because GF109203X failed to curtail the degree of ERK1/ERK2 phosphorylation in these cells. DHA alone failed to induce the phosphorylation of these mitogen-activated protein (MAP) kinases; however, this fatty acid significantly curtailed the PMA- but not TGFalpha-induced MAP kinase enzyme activity and phosphorylation in NIH/3T3 cells. Furthermore, we observed that DHA significantly inhibited PMA-induced translocation of two PKC isoforms, PKC alpha and PKC epsilon, from cytosol to plasma membrane. Interestingly, DHA failed to inhibit the PMA-induced translocation PKC delta isoform in these cells. Furthermore, DHA decreased PMA-induced proliferation of NIH/3T3 cells. In this study, we show for the first time that DHA inhibits MAP kinase ERK1/ERK2) activation and proliferation of NIH/3T3 cells via its inhibitory action on PKC alpha and epsilon isoforms.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.

Heparin down-regulates the phorbol ester-induced protein kinase C gene expression in human endothelial cells: enzyme-mediated autoregulation of protein kinase C-alpha and -delta genes

Overexpression of protein kinase C-alpha and protein kinase C-delta has been shown to modulate a number of biological effects, including the cell growth and differentiation. We hypothesized that heparin, a potent antimitogenic drug, could affect the cell proliferation by inhibiting the expression of specific protein kinase C genes. Heparin, markedly but not completely, inhibited the serum-stimulated protein kinase C-alpha and -delta mRNA expression. Protein kinase C inhibition or down-regulation significantly decreased the serum-induced protein kinase C isoenzyme gene expression. Heparin failed to inhibit the residual effect of serum that was resistant to the above-mentioned treatments. Phorbol 12-myristate 13-acetate elicited an increase of protein kinase C isoenzyme gene expression that was completely prevented by protein kinase C inhibition or down-regulation. Heparin dose-dependently counteracted and ultimately abolished the increase in the protein kinase C isoenzyme gene expression elicited by phorbol 12-myristate 13-acetate. These results suggest that the inhibition of an autoregulatory role wielded by protein kinase C on the protein kinase C-alpha and -delta gene expression might represent a possible mechanism by which glycosaminoglycans modulate the cell growth.

Role of protein kinase C isoforms in locomotion of Walker 256 carcinosarcoma cells

Treatment with low (nanomolar) concentrations of phorbol-12-myristate-13-acetate (PMA) for 5 to 30 min suppresses locomotion of Walker 256 carcinosarcoma cells, suggesting that activation of protein kinase C (PKC) is a stop signal for tumor cell locomotion. We have compared the effects of PMA on cell shape and motility with down-regulation of specific PKC isoforms. Using specific antibodies, we show that Walker carcinosarcoma cells express PKC isoforms alpha, betaI, betaII, gamma, lambda, mu, eta and zeta. Short-term incubation with PMA induced a marked shift of isoforms alpha, betaI, betaII, gamma and eta to the particulate fraction. Long-term incubation with PMA (0.1 microM, 6 hr) resulted in significant reduction of expression of conventional PKCs alpha, betaI, betaII and gamma and of the novel PKC eta to 10% to 26% of controls. Down-regulation of PKC alpha, betaI and betaII by long-term incubation with PMA was reversible after removal of PMA, whereas that of isoforms gamma and eta was not. The motile properties of cells after down-regulation of PKC isoforms were investigated. Concomitant with down-regulation of PKC isoforms, long-term incubation of cells with PMA resulted in recovery of the polar shape and the ability to migrate. Motility and polarized shape of the down-regulated cells were no longer susceptible to short-term treatment with PMA, showing that active PKC is indeed responsible for the inhibitory effects of PMA. Effects of long-term incubation with PMA on cell shape and motility were reversible. Our findings strongly suggest that PKCs alpha, betaI and betaII activated by PMA are involved in stopping Walker carcinosarcoma cell locomotion.

Downregulation of the colon tumour-suppressor homeobox gene Cdx-2 by oncogenic ras

Downregulation of the colon tumour-suppressor homeobox gene Cdx-2 by oncogenic ras Constitutive activation of the ras proto-oncogene is a frequent and early event in colon cancers, but the downstream nuclear targets are not fully understood. The Cdx-1 and Cdx-2 homeobox genes play crucial roles in intestinal cell proliferation and differentiation. In addition, Cdx-2 is a colonic tumour-suppressor gene, whereas Cdx-1 has oncogenic potential. Here, we show that constitutive activation of ras alters Cdx-1 and Cdx-2 expression in human colonic Caco-2 and HT-29 cells that harbour a normal ras proto-oncogene. Oncogenic ras downregulates Cdx-2 through activation of the PKC pathway and a decline in activity of the Cdx-2 promoter AP-1 site. This decline results from a PKC-dependent decrease in the relative expression of c-Jun, an activator of Cdx-2 transcription, compared to c-Fos, an inhibitor of Cdx-2. Unlike Cdx-2, Cdx-1 is upregulated by oncogenic ras and this effect is mediated by activation of the MEK1 pathway. These results indicate that oncogenic ras activation has opposite effects on Cdx-1 and Cdx-2 expression through distinct signalling pathways and they provide the first evidence for a functional link between ras activation and the downregulation of the Cdx-2 tumour-suppressor gene in colon cancer cells.

Selective loss of substrate recognition induced by the tumour-associated D294G point mutation in protein kinase Calpha

The tumour-associated D294G mutant of protein kinase Calpha (PKCalpha) was recently shown not to be translocated to the plasma membrane on stimulation with PMA, in contrast with the wild-type enzyme. Using recombinant wild-type and mutant PKCalpha, we establish here that, although the PKCalpha intrinsic lipid-dependent catalytic activity remains unaltered by the D294G mutation, the mutant enzyme exhibits a selective loss of substrate recognition. Indeed, whereas the mutant enzyme is still able to phosphorylate histone IIIS with comparable efficiency to that of the wild-type enzyme, it exhibits a lack of kinase activity towards the previously cloned 35F and 35H substrates for PKC. Overlay experiments demonstrate that this selective loss of kinase activity is correlated with a decrease in binding of D294G PKCalpha to the 35F and 35H proteins compared with that of the wild-type enzyme. Because the 35H and 35F proteins are predicted to be PKCalpha-anchoring proteins, these findings suggest a selective loss of PKCalpha-protein interactions that might fail to stabilize the location of the PKCalpha mutant at the plasma membrane.

Protein kinase C alpha modulates growth and differentiation in Caco-2 cells

BACKGROUND & AIMS

Caco-2 cells have been used extensively to elucidate events involved in intestinal cell proliferation and differentiation. Because individual isoforms of protein kinase C (PKC) and p21waf1, a cyclin-dependent kinase inhibitor, may regulate these processes, their role(s) on the growth and differentiation of Caco-2 cells were assessed.

METHODS

Protein abundance and subcellular distribution of several PKC isoforms, as well as the expression of p21waf1, were examined in preconfluent and postconfluent cells.

RESULTS

In cells at confluence (approximately 7 days postplating) and during their postconfluent phase (up to 20 days postplating), both total protein expression of PKC-alpha and its particulate distribution increased compared with their 3-day postplated counterparts. These findings were in agreement with those obtained by immunocytochemistry of PKC-alpha. In contrast, neither the total expression nor the subcellular distribution of PKC-betaI, -betaII, -delta, or -zeta changed significantly during these time periods. In addition, the expression of p21waf1, which can be induced by PKC-alpha, increased in postconfluent cells.

CONCLUSIONS

PKC-alpha, but not other isoforms of PKC, may modulate the proliferation and differentiation of Caco-2 cells. This regulation appears to be mediated, at least in part, via a mechanism involving p21waf1.

Activation of the mitogen-activated protein kinase/extracellular signal-regulated kinase pathway by conventional, novel, and atypical protein kinase C isotypes

Phorbol ester treatment of quiescent Swiss 3T3 cells leads to cell proliferation, a response thought to be mediated by protein kinase C (PKC), the major cellular receptor for this class of agents. We demonstrate here that this proliferation is dependent on the activation of the extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK/MAPK) cascade. It is shown that dominant-negative PKC-alpha inhibits stimulation of the ERK/MAPK pathway by phorbol esters in Cos-7 cells, demonstrating a role for PKC in this activation. To assess the potential specificity of PKC isotypes mediating this process, constitutively active mutants of six PKC isotypes (alpha, beta, delta, epsilon, eta, and zeta) were employed. Transient transfection of these PKC mutants into Cos-7 cells showed that members of all three groups of PKC (conventional, novel, and atypical) are able to activate p42 MAPK as well as its immediate upstream activator, the MAPK/ERK kinase MEK-1. At the level of Raf, the kinase that phosphorylates MEK-1, the activation cascade diverges; while conventional and novel PKCs (isotypes alpha and eta) are potent activators of c-Raf1, atypical PKC-zeta cannot increase c-Raf1 activity, stimulating MEK by an independent mechanism. Stimulation of c-Raf1 by PKC-alpha and PKC-eta was abrogated for RafCAAX, which is a membrane-localized, partially active form of c-Raf1. We further established that activation of Raf is independent of phosphorylation at serine residues 259 and 499. In addition to activation, we describe a novel Raf desensitization induced by PKC-alpha, which acts to prevent further Raf stimulation by growth factors. The results thus demonstrate a necessary role for PKC and p42 MAPK activation in 12-O-tetradecanoylphorbol-13-acetate induced mitogenesis and provide evidence for multiple PKC controls acting on this MAPK cascade.

PKC isoenzyme expression and cellular responses to phorbol ester in JEG-3 choriocarcinoma cells

Protein kinase C (PKc) is a key regulatory enzyme involved in the transduction of extracellular growth signals to the cell nucleus. It occurs in several isoforms, the exact functional roles of which have not been established as yet. The tumor-promoting agent 12-O-tetradecanoyl-phorbol acetate (TPA) is the classic activator of PKC and modulates the activity of the activating protein-1 (AP-1) transcription factor complex via this pathway. AP-1, in turn, induces cell proliferation in many tissues. In the present study, the PKC isoenzyme expression pattern in JEG-3 choriocarcinoma cells was analyzed. The results were compared with those obtained in HEC-1B endometrium adenocarcinoma cells, which had previously been characterized in this respect. To gain insight into the possible functional consequences of different PKC expression patterns, cell proliferation rates and AP-1 activity in response to TPA in both cell lines was studied. Western blot analysis of the PKC isoenzyme expression pattern revealed that JEG-3 cells are deficient in the PKC alpha, delta, and epsilon isoforms. These isoenzymes are strongly expressed in HEC-1B cells, with the alpha and delta being constitutively active. As opposed to HEC-1B cells, JEG-3 cells did not show an enhanced proliferation rate in response to TPA. Furthermore, TPA-treated JEG-3 cells did not exhibit any change in cell shape and refractility as observed in HEC-1B cells. AP-1 activity, as determined by a transfected AP-1-luciferase reporter plasmid, was induced 10-fold by TPA in JEG-3 cells, yet only threefold in HEC-1B cells. It is concluded from these data that differential expression of a subset of PKCs, e.g., the alpha, delta, and epsilon isoforms, may serve as an indicator of the proliferative potential in response to growth factors and mitogens. Furthermore, our data indicate that the inducibility of AP-1 activity does not necessarily reflect the proliferative capacity of a given cell type in response to classical tumor promoters such as phorbol ester.

Enhancement of migration by protein kinase Calpha and inhibition of proliferation and cell cycle progression by protein kinase Cdelta in capillary endothelial cells

Activation of protein kinase C (PKC) induces angiogenesis, migration, and proliferation of endothelial cells (EC), but can also prevent growth factor-induced EC proliferation. To determine whether these disparate effects are mediated by substrates of individual PKC isoenzymes, PKCalpha and PKCdelta were overexpressed in rat microvascular EC. Basal and stimulated migration were enhanced in PKCalpha EC compared with either PKCdelta or control EC. Serum-induced growth of PKCdelta EC was decreased, while that of PKCalpha cells was similar to control EC. Phorbol ester markedly inhibited PKCdelta EC growth but enhanced growth of PKCalpha and control EC. To determine possible causes for this altered proliferation, the effect of PKCdelta on adhesion, mitogen-activated protein kinase activity, and cell cycle progression was measured. Adherence of PKCdelta EC to vitronectin was significantly enhanced. Serum-induced extracellular signal-regulated kinase-2 activity was increased equally in both PKCalpha and PKCdelta EC above that of control, while extracellular signal-regulated kinase-1 activity was similar in all EC. Cell cycle analysis suggested that PKCdelta EC entered S phase inappropriately and were delayed in passage through S phase. Thus, PKCalpha may mediate some proangiogenic effects of PKC activation; conversely, PKCdelta may direct antiangiogenic aspects of overall PKC activation, including slowing of the cell cycle progression.

The catalytic domain of protein kinase C-delta in reciprocal delta and epsilon chimeras mediates phorbol ester-induced macrophage differentiation of mouse promyelocytes

The overexpression of protein kinase C-delta (PKC-delta), but not PKC-epsilon, enables the mouse myeloid cell line 32D to differentiate into macrophages when treated with phorbol esters such as 12-O-tetradecanoylphorbol-13-acetate (TPA). To determine the domain of PKC-delta that is responsible for this isotype-specific function, cDNAs that encode reciprocal chimeras of PKC-delta and -epsilon (PKC-delta epsilon and PKC-epsilon delta) were constructed by exchanging regulatory and kinase domains using polymerase chain reaction technology. Both chimeras were stably expressed in 32D cells using the pLTR expression vector and displayed protein kinase activity upon TPA treatment. TPA treatment of L epsilon delta, cells that overexpressed the PKC-epsilon delta chimera, induced a dramatically increased cell volume, surface adherence, surface expression of Mac-1 and Mac-3, lysozyme production, and phagocytosis. These are the characteristics of the macrophage phenotype found in TPA-treated 32D cells that overexpressed PKC-delta. In contrast, little effect was seen in L delta epsilon, 32D cells that overexpressed PKC-delta epsilon, with or without TPA treatment. A PKC inhibitor directed toward the catalytic domain of PKC, GF109203X, and a selective inhibitor of PKC-delta, Rottlerin, blocked the TPA-induced differentiation of PKC-epsilon delta-overexpressing 32D cells. These results demonstrate that the catalytic domain of PKC-delta contains the primary determinants for its activity in phorbol ester-induced macrophage differentiation.

Protein kinase C (PKC) isoenzyme expression pattern as an indicator of proliferative activity in uterine tumor cells

The protein kinase C (PKC) signal transduction pathway is the prototype of a growth factor-responsive intracellular signaling system, which is activated by various cytokines, growth factors and tumor promoters, such as the phorbol ester 12-O-tetradecanoyl-phorbol acetate (TPA). To date, a large number of different PKC isoforms has been identified, the physiological relevance of which is unknown. Moreover, the expression pattern of PKC isoforms in uterine cells has not been studied as yet. To study the functional role of differential PKC isoform expression in uterine tumor progression, we have compared the proliferative response to TPA, changes in cell morphology induced by TPA, and the PKC isoform expression pattern in two uterine tumor cell lines of different origin. The moderately differentiated endometrial HEC-1-B adenocarcinoma cell line showed a marked increase in proliferative activity and a profound morphological change in response to TPA. In contrast, TPA did not induce cell proliferation and/or morphological changes in the well-differentiated SKUT-1-B mixed mesodermal cell line. Analysis of the PKC isoform expression profile by Western blot revealed that PKC alpha, betaI, delta, epsilon, and zeta were expressed at a much higher level in HEC-1-B as compared to SKUT-1-B cells. PKC beta11 was the only isoenzyme to exhibit a higher expression level in SKUT-1-B cells. This is the first study analyzing the PKC isoform expression profile in uterine tumor cells. Our data demonstrate that the proliferative response to TPA correlates with the expression levels of the majority of PKC isoforms in these cells. Overexpression of PKC isoforms indicates a higher proliferative capacity, and may, thus, represent an important step in the pathogenesis of certain uterine malignancies.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

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.

Activation of c-fos expression by transforming Ha-ras in HC11 mouse mammary epithelial cells is PKC-dependent and mediated by the serum response element

The mechanism by which transforming Ha-ras induces c-fos expression in HC11 mouse mammary epithelial cells was investigated with regard to controversial data concerning the role of protein kinase C (PKC) and the required promoter elements of the fos gene. HC11 cells carrying a glucocorticoid-inducible Ha-ras (val12) construct were transfected with a chloramphenicol acetyltransferase (CAT) reporter gene under the control of a human fos promoter which includes the serum response element (SRE), the adjacent c-fos AP-1 site (FAP) and the cAMP response element (CRE). Induction of the Ha-ras gene by dexamethasone lead to a transactivation of expression of the transfected fos promoter construct which was inhibited by the PKC inhibitor BM41440 and abrogated in PKC-'depleted' cells. A similar transactivation was observed when the fos promoter construct was co-transfected with a constitutively active ras expression vector. Again, this effect was depressed by the PKC inhibitor and abolished in PKC-'depleted' cells. 'PKC-depletion' was achieved by long-term exposure to 12-O-tetradecanoylphorbol-13-acetate. This procedure was shown to deplete cells of PKC alpha and to reduce significantly PKC epsilon. Long-term exposure to bryostatin 1 selectively depletes PKC alpha. Depletion of PKC alpha by bryostatin 1 does not reduce the transcriptional activation of the SRE-FAP-TK-CAT (TK: thymidine kinase) construct by Ha-ras. In order to delineate the promoter elements mediating the transcriptional activation, constructs which lack the FAP and the CRE sites but contain an intact SRE were co-transfected with the ras construct. Elimination of the FAP and CRE sequences did not affect the transcriptional activation by Ha-ras (val12). It is concluded that in HC11 cells, transforming Ha-ras activates c-fos expression in a PKC-dependent manner, presumably implying PKC epsilon, and that the SRE is sufficient to mediate transcriptional activation.

Protein kinase C: is its pivotal role in cellular activation over-stated?

Evidence has emerged over the past decade to suggest that protein kinase C (PKC) is a widespread family of kinases responsible for many diverse and critical cellular functions. With the development of selective agents to activate or inhibit the individual PKC isoenzymes, it is now apparent that much of the literature that implicated PKC in many cellular functions needs to be appraised. In this article, Sandra Wilkinson and Trevor Hallam discuss the problems of the existing methods and the recent evidence that suggests that PKC isotypes are necessary for some, but not all, of those cellular responses where PKC had been thought to play an important role. Selective inhibitors of PKC isoenzymes may have potential for therapeutic use in auto-immune diseases, transplant rejection and oncology.

Activation and substrate specificity of the human protein kinase C alpha and zeta isoenzymes

Protein kinase C (PKC), a class of serine/threonine kinases activated by Ca2+ and/or phospholipids, is involved in a variety of cellular processes such as proliferation, differentiation and secretion. Nine members of the PKC gene family are known; these are differentially expressed in eukaryotic cells and can be divided into two sub-groups: the Ca(2+)-dependent (classical) PKC isoenzymes alpha, beta I, beta II and gamma, and the Ca(2+)-independent neoPKC isoenzymes delta, epsilon, zeta, eta and theta. A detailed biochemical characterisation of these PKC isoenzymes is one prerequisite for the elucidation of their distinct roles within cellular signal transduction. In this study, we report the cloning of a human PKC-zeta cDNA, its expression in recombinant baculovirus-infected insect cells and the partial purification of the PKC-zeta isoenzyme. In comparison to highly purified human PKC alpha, a representative of the classical PKC subgroup, purified PKC zeta was characterised with respect to activator requirement, substrate specificity, proteolytic activation and sensitivity towards PKC inhibitors. In contrast to PKC alpha, PKC zeta exhibits a constitutive kinase activity which is independent of Ca2+, phosphatidylserine and diacylglycerol. Arachidonic acid alone or a combination of gamma-linolenic acid and phosphatidylserine slightly enhance PKC zeta activity. In the presence of the classical PKC activators phosphatidylserine/diacylglycerol, PKC alpha phosphorylates a PKC-alpha pseudosubstrate-derived peptide, an epidermal-growth-factor-receptor-derived peptide, histone III-S and myelin basic protein to an equal extent, whilst PKC zeta phosphorylates only the PKC-alpha-derived peptide. However, arachidonic acid greatly diminishes PKC-alpha activity towards the epidermal-growth-factor-receptor-derived peptide, histone III-S and myelin basic protein, but enhances PKC-zeta activity towards the PKC-alpha-derived peptide. These results indicate a possible modulation of substrate specificity of these two PKC isoenzymes by (the binding of) different activators (to their regulatory domains). In the case of PKC zeta, this finding is strengthened by the fact that the epidermal growth factor receptor-derived peptide, which is not a substrate for the holoenzyme, is significantly phosphorylated by a protein fragment generated by limited proteolysis and comprising only the kinase domain. Furthermore, PKC zeta, in contrast to PKC alpha, is insensitive to PKC inhibitors known to interfere either with the regulatory or the catalytic domain and cannot be activated by phorbol ester treatment of NIH 3T3 cells or insect cells, overexpressing the respective PKC isoenzyme. The potential implications of these findings on the mechanism(s) of activation and the substrate specificity of PKC zeta are discussed.

Over-expression of protein kinase C-alpha enhances platelet-derived growth factor- and phorbol ester- but not calcium ionophore-induced formation of prostaglandins in NIH 3T3 fibroblasts

Over-expression of human protein kinase C-alpha in murine NIH 3T3 fibroblasts is associated with an increased platelet-derived growth factor- and phorbol ester-mediated formation of prostaglandins, whereas the calcium ionophore-induced release of arachidonic acid metabolites is unaffected; however, the differences of arachidonic acid and prostaglandin formation are much more pronounced with platelet-derived growth factor than with phorbol ester. Platelet-derived growth factor induces an identical elevation of intracellular free calcium in control and protein kinase C-alpha over-expressing cells: the phorbol ester has no effect on intracellular free calcium in both cell lines. These results demonstrate that protein kinase C-alpha may couple to arachidonic acid cascade in NIH 3T3 fibroblasts.

Activation of purified human protein kinase C alpha and beta I isoenzymes in vitro by Ca2+, phosphatidylinositol and phosphatidylinositol 4,5-bisphosphate

The increasing number of eukaryotic protein kinase C (PKC) isoenzymes which have been described has raised great interest in potential differences in the cellular expression, the mode of activation and the substrate specificity of these isoenzymes. The last two aspects have mostly been studied with isoenzymes purified from rat or bovine brain or from recombinant-baculovirus-infected insect cells. In this study, we have expressed the human PKC isoenzymes alpha and beta I in recombinant-baculovirus-infected insect cells. The isoenzymes were purified to homogeneity by a four-step procedure which included a reversible Ca(2+)-dependent association/dissociation to and from the endogenous membranes of the lysed insect cells. Characterization of the purified enzymes with respect to ATP requirement and substrate specificity, using the epidermal-growth-factor receptor peptide and histone III-S respectively, revealed no isoenzyme-specific differences. Activation by trypsin or Ca2+ and a variety of different phospholipids and phosphoinositides (in a mixed-micellar assay) gave the following results. Proteolytic cleavage of the PKC isoenzymes by trypsin generated fully activated phospholipid-independent PKC beta I, whereas PKC alpha reached only 50% of the activity obtained in the presence of phospholipids. PKC alpha and beta I showed no difference in their dependence on Ca2+, diacylglycerol (DAG) and phosphatidylserine (PS). Replacement of either DAG or PS by phosphatidylglycerol, cardiolipin, phosphatidylcholine and several phosphoinositides revealed that PtdIns(4,5)P2 can act as a PKC activator similar to DAG, whereas PtdIns can substitute for PS as a cofactor of activation. Thus, at least for the PKC isoenzymes alpha and beta I, a combination of PtdIns and PtdIns(4,5)P2 can fully replace PS and DAG in vitro as the classical activators of PKC.

Protein kinase C isoenzymes: divergence in signal transduction?

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