Differences in phorbol ester-induced decrease of the activity of protein kinase C isozymes in rat hepatocytes

Leishmania mexicana lipophosphoglycan differentially regulates PKCalpha-induced oxidative burst in macrophages of BALB/c and C57BL/6 mice

Leishmania are protozoan parasites that infect macrophages and their survival is partially achieved through inhibition of the cellular oxidative burst by parasite lipophosphoglycan (LPG). PKCalpha is the predominant PKC isoenzyme required for macrophage oxidative burst, yet it is not known if different susceptibility of BALB/c and C57BL/6 mice to Leishmania mexicana could be related to PKCalpha. We analysed the effect of L. mexicana promastigotes and parasite LPG on expression of PKCalpha and on its activity in macrophages of both mouse strains. Our data show that expression of the isoenzyme was not altered either by LPG or by L. mexicana promastigotes. Yet LPG exerted opposing effects on PKCalpha activity of macrophages between both strains: in susceptible BALB/c cells, it inhibited PKCalpha activity, whereas in the more resistant strain it augmented enzymatic activity 2.8 times. In addition, LPG inhibited oxidative burst only in susceptible BALB/c macrophages and the degree of inhibition correlated with parasite survival. Promastigotes also inhibited PKCalpha activity and oxidative burst in macrophages of BALB/c mice, whereas in C57BL/6, they enhanced PKCalpha activity and oxidative burst inhibition was less severe. Our data indicate that control of PKCalpha-induced oxidative burst by L. mexicana LPG relates with its success to infect murine macrophages.

Posttranslational modifications on protein kinase c isozymes. Effects of epinephrine and phorbol esters

The posttranslational modifications induced on PKC isozymes as result of their activation were investigated. Reciprocal immunoprecipitations followed by Western blot analysis demonstrated that all PKC isozymes expressed in rat hepatocytes are modified by tyrosine nitration and tyrosine phosphorylation in different ways upon exposure of cells to a direct PKC activator (TPA), or to an extracellular ligand known to activate PKC-dependent pathways (epinephrine). Our data demonstrate for the first time that all PKC isozymes are also dynamically modified by O-linked beta-N-acetylglucosamine (O-GlcNAc); the presence of this modification was confirmed in part by FT-ICR mass spectrometry analysis. Interestingly, the O-GlcNAc modified Ser or Thr were mapped at similar positions in several PKC isozymes. The biochemical meaning of these posttranslational modifications was investigated for PKC alpha and delta. It was found that the PKC phosphorylation status of both isozymes in tyrosine and serine residues seems to regulate directly the enzyme activity since catalytic inactivation correlate with dephosphorylation of Ser at the C-terminus autophosphorylation sites of each PKC isozyme, and with an increase in the level of tyrosine phosphorylation. Whereas none of the other posttranslational modifications showed per se a direct effect in PKC delta activity, increased tyrosine nitration and O-GlcNAc modifications correlate negatively with PKCalpha activity.

Expression and purification of receptor for activated C-kinase 1 (RACK1)

Receptor for activated C-kinase (RACK1) binds to protein kinase C and functions as an anchor for several other cellular components. Most in vitro studies of RACK1 have been carried out with RACK1 fused to a soluble fusion protein partner, such as GST or MBP. Here, we show that fusion complexes may exist as large soluble aggregates and thereby lead to false conclusions about the biological activity of RACK1. We developed a purification procedure that gave soluble monodisperse molecules of the protein. The RACK1 gene was cloned and expressed in a pMAL vector. After purification of the resulting MBP-RACK1 fusion protein, RACK1 was excised from MBP by thrombin, rendering RACK1 in a soluble monodisperse form as monitored by fluorimetric static light scattering, gel filtration, and ultracentrifugation. Circular dichroism analysis revealed that RACK1 was properly folded with a T(m) of approximately 62 degrees C and contained the predicted portions of secondary structures. The biological activity of the purified protein was verified by binding to activated protein kinase C. The production of soluble, high-purity RACK1 will allow structural studies and functional in vitro studies to identify interacting partners to this important scaffold protein.

p32 (gC1qBP) is a general protein kinase C (PKC)-binding protein; interaction and cellular localization of P32-PKC complexes in ray hepatocytes

The aim of this study was to identify cellular proteins that bind protein kinase C (PKC) and may influence its activity and its localization. A 32-kDa PKC-binding protein was purified to homogeneity from the Triton X-100-insoluble fraction obtained from hepatocytes homogenates. The protein was identified by NH(2)-terminal amino acid sequencing as the previously described mature form of p32 (gC1qR). Recombinant p32 was expressed as a glutathione S-transferase fusion protein, affinity-purified, and tested for an in vitro interaction with PKC using an overlay assay approach. All PKC isoforms expressed in rat hepatocytes interacted in vitro with p32, but the binding dependence on PKC activators was different for each one. Whereas PKCdelta only binds to p32 in the presence of PKC activators, PKCzeta and PKCalpha increase their binding when they are in the activated form. Other PKC isoforms such as beta, epsilon, and theta bind equally well to p32 regardless of the presence of PKC activators, and PKCmu binds even better in their absence. It was also found that p32 is not a substrate for any of the PKC isoforms tested, but interestingly, its presence had a stimulatory effect (2-fold for PKCdelta) on PKC activity. We also observed in vivo interaction between PKC and p32 by immunofluorescence and confocal microscopy. A time course of phorbol ester treatment of cultured rat hepatocytes (C9 cells) showed that PKCtheta and p32 are constitutively associated in vivo, whereas PKCdelta activation is required for its association with p32. Our data also showed that phorbol ester treatment induces a transient translocation of p32 from the cytoplasm to the cell nucleus. Together, these findings suggest that p32 may be a regulator of PKC location and function.

Tight-junction protein zonula occludens 2 is a target of phosphorylation by protein kinase C

Zonula occludens 2 (ZO-2) protein is a tight-junction phos phorylated protein that belongs to the membrane-associated guanylate kinase ('MAGUK') family. Here we study the interaction between ZO-2 and protein kinase C (PKC). We have constructed two ZO-2 fusion proteins of the middle (3PSG) and C-terminal (AP) regions of the molecule and demonstrate that they are phosphorylated by PKC isoenzymes beta, epsilon, lambda and zeta. To understand the physiological significance of the interaction between ZO-2 and PKC, we analysed the phosphorylation state of ZO-2 immunoprecipitated from monolayers with mature tight junctions or from cells that either lack them or have them disassembled through Ca(2+) chelation. We found that in the latter condition the phosphorylation level of ZO-2 is significantly higher and is due to the action of both PKC and cAMP-dependent protein kinase. These results therefore suggest that the phosphorylated state of ZO-2 restrains its capacity to operate at the junctional complex.

Protein kinase C activity modulates dendritic differentiation of rat Purkinje cells in cerebellar slice cultures

The molecular mechanisms underlying dendritic differentiation in neurons are currently poorly understood. We used slice cultures from rat cerebellum of postnatal day 8 to investigate the effect of protein kinase C (PKC) activity on dendritic development of Purkinje cells. After 12 days in culture under control conditions, Purkinje cells had developed a typical dendritic tree consisting of a few long primary dendrites with shorter side branches. Following treatment with the PKC agonist, phorbol-12-myristate-13-acetate (PMA), the dendritic tree area was strongly reduced to 32% of control and primary dendrites were short with only a few side branches. Delayed addition of PMA after 6 days resulted in a retraction of existing dendrites, whereas discontinuation of PMA treatment after 6 days resulted in a recovery of the dendritic tree to almost control values. In the presence of the PKC inhibitor, 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide (GF109203X), the dendritic tree area was increased to 158% of control with much more ramified branches after 12 days. The overall morphology of the cultures and the survival of Purkinje cells were unaffected by PKC modulators. Our data show that increased activity of PKC inhibits, and reduced activity of PKC promotes dendritic growth. This suggests that PKC activity is a critical regulator of dendritic growth and differentiation in cerebellar Purkinje cells.

Characterization of calreticulin as a protein interacting with protein kinase C

A protein kinase C (PKC)-binding protein was purified to homogeneity from the Triton-insoluble fraction from rat hepatocytes homogenates. The protein was identified as the mature calreticulin chain by N-terminal amino acid sequencing and by its immunoreactivity with anti-calreticulin antibody raised against the C-terminal KDEL (single-letter code) sequence. The calculated molecular mass was 46. 6 kDa but the protein migrates in SDS/PAGE as a doublet with apparent molecular masses of 60 and 55 kDa. Studies in vitro with purified calreticulin with the use of an overlay assay approach demonstrated that it binds to activated PKC isoenzymes expressed in rat hepatocytes. Phosphorylation of purified calreticulin with a PKC isoenzyme-specific immune complex kinase assay showed that it is also a very good substrate for all PKC isoforms in vitro. The treatment of intact cells with phorbol ester or with adrenaline (epinephrine) plus propranolol increased calreticulin phosphorylation, which was blocked by the pretreatment of cells with the PKC-specific inhibitor Ro 31-8220. The analysis of calreticulin immunoprecipitates from control or treated cells indicated that PKCalpha, PKCbeta, PKCtheta;, PKCzeta and PKCmu, but not PKCdelta or PKCepsilon, co-immunoprecipitated with calreticulin. Taken together, our results indicate that PKC interacts in vivo with calreticulin and suggest that they can operate in common signalling pathways.

Activation of a cGMP-stimulated cAMP phosphodiesterase by protein kinase C in a liver Golgi-endosomal fraction

The ability of Ca2+/phospholipid-dependent protein kinase (protein kinase C, PKC) to stimulate cAMP phosphodiesterase (PDE) activity in a liver Golgi-endosomal (GE) fraction was examined in vivo and in a cell-free system. Injection into rats of 4 beta-phorbol 12-myristate 13-acetate, a known activator of PKC, caused a rapid and marked increase in PKC activity (+325% at 10 min) in the GE fraction, along with an increase in the abundance of the PKC alpha-isoform as seen on Western immunoblots. Concurrently, 4 beta-phorbol 12-myristate 13-acetate treatment caused a time-dependent increase in cAMP PDE activity in the GE fraction (96% at 30 min). Addition of the catalytic subunit of protein kinase A (PKA) to GE fractions from control and 4 beta-phorbol 12-myristate 13-acetate-treated rats led to a comparable increase (130-150%) in PDE activity, suggesting that PKA is probably not involved in the in-vivo effect of 4 beta-phorbol 12-myristate 13-acetate. In contrast, addition of purified PKC increased (twofold) PDE activity in GE fractions from control rats but affected only slightly the activity in GE fractions from 4 beta-phorbol 12-myristate 13-acetate-treated rats. About 50% of the Triton-X-100-solubilized cAMP PDE activity in the GE fraction was immunoprecipitated with an anti-PDE3 antibody. On DEAE-Sephacel chromatography, three peaks of PDE were sequentially eluted: one early peak, which was stimulated by cGMP and inhibited by erythro-9 (2-hydroxy-3-nonyl) adenine (EHNA); a selective inhibitor of type 2 PDEs; and two retarded peaks of activity, which were potently inhibited by cGMP and cilostamide, an inhibitor of type 3 PDEs. Further characterization of peak I by HPLC resolved a major peak which was activated (threefold) by 5 microM cGMP and inhibited (87%) by 25 microM EHNA, and a minor peak which was insensitive to EHNA and cilostamide. 4 beta-Phorbol 12-myristate 13-acetate treatment caused a selective increase (2.5-fold) in the activity associated with DEAE-Sephacel peak I, without changing the K(m) value. These results suggest that PKC selectively activates a PDE2, cGMP-stimulated isoform in the GE fraction.

Increased expression of liver PKC alpha in hypoinsulinemic diabetic rats: a post-translational effect

Ca2+-dependent protein kinase C (cPKC) activity and expression have been studied in livers from hypoinsulinemic streptozotocin (STZ)-induced diabetic and untreated control rats. In diabetic rats, cPKC activity was slightly decreased in liver total particulate and nuclear fractions but was unchanged in mitochondrial-lysosomal, microsomal and cytosolic fractions. On Western immunoblot analysis, PKC alpha was identified as two distinct proteins of 90 and 81 kDa. In diabetic rats, the abundance of the 90 kDa protein was increased in most subcellular fractions with a maximum in the cytosolic and microsomal fractions (180%) but that of the 81 kDa protein was unchanged. PKC beta2 was detected as a single 81 kDa protein in cytosolic and microsomal fractions with unchanged levels in diabetic rats. Liver PKC alpha mRNA levels as measured by reverse transcription and competitive PCR amplification were similar in diabetic and control rats. The increased expression of PKC alpha protein in diabetic rats was reversed by insulin but not by phlorizin, suggesting that it did not result from hyperglycemia. We conclude that STZ-induced diabetes induces the expression of a biologically inactive form of PKC alpha which differs from active PKC alpha by an undefined post-translational modification, possibly an increase in phosphorylation state.

Alpha1-adrenoceptor subtype activation increases proto-oncogene mRNA levels. Role of protein kinase C

Noradrenaline increased the mRNA levels of c-fos and c-jun in rat-1 fibroblast lines stably expressing the cloned alpha1-adrenoceptor subtypes. The efficacy to induce the expression of c-fos mRNA was similar for the three cell lines (alpha1d = alpha1b = alpha1a) but different for c-jun (alpha1a > or = alpha1b > alpha1d). The EC50 values were also different: approximately 5 nM (c-fos) and approximately 300 nM (c-jun) for cells transfected with the alpha1a subtype, approximately 30 nM (c-fos) and approximately 300 nM (c-jun) for cells transfected with the alpha1b subtype and approximately 300 nM (c-fos and c-jun) for those transfected with the alpha1d subtype. Staurosporine and protein kinase C down-regulation blocked such effects, indicating a role of this protein kinase. Endothelin-1 (10 nM) also increased the levels of c-fos and c-jun mRNAs. These actions of endothelin-1 were unaffected by staurosporine and protein kinase C down-regulation. It is concluded that activation of any of the three cloned subtypes can increase the levels of c-fos and c-jun mRNAs and that protein kinase C plays a major role in mediating such effects.

Purification and characterization of receptors for activated protein kinase C from rat hepatocytes

It has been proposed that protein kinase C may be targeted to specific locations via interactions with anchoring proteins located at various subcellular sites. A group of proteins collectively termed RACKs (Receptors for Activated C-Kinase) have been identified. Here, we made use of a rapid and simple method to purify several RACKs from rat hepatocytes, taking advantage of the ability of these proteins to be precipitated with Triton X-100. The method can be used for the isolation of other proteins that share these properties. Four proteins were purified to apparent homogeneity with M(r) values of 14, 15, 16, and 34 kDa. Amino acid composition and biochemical characteristics of these proteins are presented.

Evidence for a role of protein kinase C in the activation of the pyruvate dehydrogenase complex by insulin in Zajdela hepatoma cells

The signal transduction pathway involved in the activation of pyruvate dehydrogenase (PDH) by insulin is still unknown. In this study, we have examined the possible involvement of protein kinase C (PKC) in the process. In addressing this question, we examined (1) the insulin-like effects of the PKC activator 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) on the PDH complex, (2) the effects of various PKC inhibitors on the PDH activation by insulin, and (3) the response of PKC-depleted cells to insulin. We used as an experimental model Zajdela hepatoma cultured (ZHC) cells, which have been demonstrated to be responsive to physiological doses of insulin. Half-maximal and maximal stimulations of the PDH complex by insulin were observed at 0.05 and 5 nmol/L, respectively. Stimulation of PDH activity by insulin (5 nmol/L) occurred within 5 minutes of incubation and was maximal (+70%) at 7.5 minutes. In the presence of PMA (162 nmol/L), enzyme activity increased within 30 seconds, was maximal (+90%) at 5 minutes, and was no longer detectable after 10 minutes. Total PDH activity was unchanged by insulin or PMA treatment. The effects of PMA and insulin on basal PDH activity were not additive. Moreover, various inhibitors of PKC--staurosporine, sphingosine, acridine orange--completely blocked the stimulation of PDH activity induced by insulin or PMA. A 17-hour treatment of ZHC cells with 500 nmol/L PMA efficiently downregulated PKC, as attested by the marked decrease in the enzyme activity and the loss of phorbol 12,13-dibutyrate binding to intact cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the mechanism of desensitization of the parathyroid hormone-stimulated calcium signal in UMR-106 cells: reversal of desensitization by alkaline phosphatase but not by protein kinase C downregulation

The involvement of protein kinase C (PKC), cAMP-dependent protein kinase (PKA), and other phosphorylation mechanisms in the rapid desensitization of the [Ca2+]i response to parathyroid hormone (PTH) stimulation was investigated in osteoblast-like UMR-106 cells. A 5 minute preincubation of the cell suspension with phorbol 12,13-dibutyrate (PDB) decreased the response to PTH in a concentration-dependent manner. 1-Oleoyl-2-acetyl-r-glycerol (OAG) pretreatment likewise decreased the PTH response. Staurosporine, a potent protein kinase inhibitor, completely prevented the desensitization caused by PDB. These PDB and staurosporine effects were also observed in 3 mM EGTA-containing medium ([Ca2+]free < 10(-8) M). A 5 minute pretreatment of cells with 1 microM forskolin had no effect on the calcium response to PTH. Homologous and PDB-induced desensitizations differed in several respects. Staurosporine pretreatment resulted in only a slight restoration of the PTH response under conditions of homologous desensitization. Chronic treatment with phorbol ester prevented the desensitization of the PTH response by acute phorbol treatment but not the homologous desensitization. Both homologous and PDB-induced desensitization were relieved by alkaline phosphatase treatment, consistent with the involvement of phosphorylation in the desensitization. This alkaline phosphatase effect on desensitization was inhibited by L-phenylalanine. These results suggest that PTH receptor homologous desensitization involves phosphorylation process(es) other than or in addition to those of PKC.

Hypoxia-induced accumulation of erythropoietin mRNA in isolated hepatocytes is inhibited by protein kinase C

To define the role of protein kinase C (PKC) in oxygen-dependent production of erythropoietin (EPO) in the liver, we have determined EPO messenger ribonucleic acid (mRNA) expression in primary cultures of juvenile rat hepatocytes incubated at different oxygen tensions in the absence and presence of phorbol esters, vasopressin, and structurally different kinase inhibitors. Upon reduction of oxygen concentrations from 40% to 3% EPO mRNA in cultured hepatocytes increased markedly within 1.25 h, reached maximal values after 2.5 h and remained elevated for up to 72 h. Treatment of hepatocytes during 1.25-5 h of hypoxic exposure with phorbol 12-myristate-13 acetate (PMA) attenuated hypoxia-induced EPO mRNA levels dose-dependently by a maximum of approximately 50%. This inhibitory effect of PMA disappeared upon treatment for more than 5 h and was completely lost after incubation for 9 and 18 h in the presence of 10(-6) M and 10(-7) M PMA, respectively. Phorbol 12,13-dibutyrate and vasopressin also inhibited EPO mRNA accumulation, whereas 4 alpha-phorbol 12,13-didecanoate was ineffective. Western blot analysis of PKC isozymes revealed the presence of PKC alpha, beta II, delta, epsilon and zeta and provided no evidence that the PMA-induced inhibition of EPO expression was associated with depletion of any of these isozymes. Conversely, PMA-induced inhibition of EPO mRNA accumulation was paralleled by translocation of PKC alpha from cytosol to membranes and the time- and dose-dependent attenuation of the inhibitory effect of PMA on EPO mRNA levels was paralleled by down-regulation of PKC alpha. A dose-dependent inhibition of EPO mRNA formation, independent of effects on total RNA synthesis, as determined by [3H]uridine incorporation, was also found in the presence of the kinase inhibitor staurosporine (ED50 approximately 2 x 10(-8) M) and three structurally related derivatives with increased selectivity for PKC (RO 317549, ED50 approximately 1 x 10(-6) M; RO 318220, ED50 approximately 1 x 10(-6) M and CGP 41251, ED50 approximately 4 x 10(-6) M). The markedly lower potency of the latter three compounds as compared to staurosporine suggests that this suppression of EPO gene induction was not mediated by inhibition of PKC. In summary the data indicate that PKC alpha is a negative modulator of EPO gene expression in hepatocytes. A kinase other than PKC, however, appears to be an essential element of hypoxic signalling.

Activated protein kinase C binds to intracellular receptors in rat hepatocytes

The aim of this study was to identify in rat hepatocytes cellular polypeptides that bind protein kinase C (PKC) and may influence its activity and its compartmentation. At least seven proteins, with apparent M(r) values between 12,000 and 36,000, that behave like Receptors for Activated C-Kinase (RACKs) were found in the Triton-X-100-insoluble fraction of these cells; i.e. PKC bound to these polypeptides when it was in its active form. RACKS seem to be PKC substrates. Studies using isotype-specific PKC antibodies suggested some selectivity of RACKs, i.e. RACKs in the M(r) approximately 28,000-36,000 region bound PKC-alpha and PKC-beta in the presence of phosphatidylserine, diolein and Ca2+, whereas those of M(r) approximately 12,000-14,000 bound all isoforms studied, and, in contrast with the other RACKs, they did this even in the absence of Ca2+. Peptide I (KGDYEKILVALCGGN), which has a sequence suggested to be involved in the PKC-RACKs interaction [Mochly-Rosen, Khaner, Lopez and Smith (1991) J. Biol. Chem. 266, 14866-14868], inhibited PKC activity. Preincubation of RACKs with antisera directed against peptide I prevented PKC binding to them. The data suggest that peptide I blocks PKC binding to RACKs by two mechanisms: inhibition of PKC activity and competition with a putative binding site.

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.

Evidence that growth hormone stimulates protein kinase C activity in isolated rat hepatocytes

The mechanism of action of growth hormone (GH) is not known, although indirect evidence suggests that protein kinase C (PKC) might play an important role in the insulin-like actions of GH. In this investigation, we directly examined the effects of GH relative to those of insulin on PKC activity in isolated rat hepatocytes. Human GH (10(-7) mol/L) significantly increased the activity of PKC in both cytosolic and particulate fractions. The effect was maximal at 1 minute, disappeared at 5 minutes, and then increased again at 30 minutes in both fractions. At 1 minute, maximal and half-maximal stimulation of PKC activity occurred at hGH concentrations of 10(-7) and 5 x 10(-9) mol/L, respectively. Insulin (10(-7) mol/L) also induced a significant and transient increase in enzyme activity at 2 minutes in cytosolic and particulate fractions; at 30 minutes, PKC activity was decreased in the soluble fraction (-17%) and increased in the particulate fraction (+65%). Measurement of specific [3H]-phorbol dibutyrate (PDBu) binding suggested translocation of PKC from the cytosol to the membrane fraction after 30 minutes of incubation, only after insulin treatment. The early effects of GH and insulin on PKC activity were additive in both the particulate and cytosolic fractions. Although the later effects of GH and insulin on PKC were quite different, both hormones rapidly activated PKC in isolated hepatocytes, suggesting that PKC might be involved in triggering the insulin-like actions of GH.

Angiotensin II and active phorbol esters induce proto-oncogene expression in isolated rat hepatocytes

In isolated rat hepatocytes PMA, angiotensin II and to a lesser extent other hormones induce an early genetic response (increased expression of c-fos, c-mos, c-myc and beta-actin) without altering the expression of the glyceraldehyde 3-phosphate dehydrogenase gene. PMA, PDB and O-met-PMA, but not alpha-phorbol, stimulated c-fos expression. The effect of angiotensin II was inhibited by the AT1 antagonist, Losartan (DuP 753) (Ki approx. 25 nM), but not by the AT2 antagonist PD123177. Angiotensin II was much more effective than vasopressin or epinephrine in inducing proto-oncogene expression which suggests that angiotensin II receptors may exert actions in addition to those shared with the receptors for the other calcium-mobilizing hormones. The effect of PMA and angiotensin II on c-fos expression took place rapidly, with half times of 7 and 12 min, respectively. Actinomycin D markedly diminished basal c-fos expression whereas cycloheximide had the opposite effect. Actinomycin D diminished the effect of PMA and angiotensin II but it did not block them. PMA and the calcium-mobilizing hormones increased c-fos expression above the level observed with cycloheximide alone. These data suggest that PMA and the calcium-mobilizing hormones increased both transcription of the c-fos gene and stabilization of the proto-oncogene mRNA.

Modulation by protein kinase C of the hormonal responsiveness of hepatocytes from lean (Fa/fa?) and obese (fa/fa) Zucker rats

The effect of phorbol myristate acetate (PMA) on the hormonal responsiveness of hepatocytes from lean and obese Zucker rats was studied. Phenylephrine-stimulated phosphatydylinositol labeling and phosphorylase activation were antagonized by PMA in cells from obese and lean animals; bigger residual effects were observed in cells from obese animals even at high PMA concentrations. Cyclic AMP accumulation induced by isoproterenol, glucagon, forskolin and cholera toxin was higher in cells from lean animals than in those from obese rats. PMA diminished glucagon- and cholera toxin-induced cyclic AMP accumulation; cells from lean animals were more sensitive to PMA. Two groups of isoforms of protein kinase C (PKC) were observed in hepatocytes from Zucker rats using DEAE-cellulose column chromatography: PKC 1 and PKC 2. The PKC 1 isozymes were separated into four peaks using hydroxylapatite: aa, 1a (PKC-beta), 1b (PKC-alpha) and 1c. Short treatment with PMA decreased the activity of PKC 1 (peaks 1b (PKC-alpha) and 1c) and to a lesser extent of PKC 2; cells from lean animals were more sensitive to PMA than those obtained from obese rats. Our results indicate that cells from genetically obese Zucker rats are in general less sensitive to this activator of protein kinase C than those from their lean littermates. The possibility that alterations in the phosphorylation/dephosphorylation cycles, that control metabolism and hormonal responsiveness, may contribute to this obese state is suggested.