Tumor-promoting phorbol esters stimulate the phosphorylation of ribosomal protein S6 in quiescent Reuber H35 hepatoma cells

C57BL/6 congenic mouse NRASQ61K melanoma cell lines are highly sensitive to the combination of Mek and Akt inhibitors in vitro and in vivo

RAS is frequently mutated in various tumors and known to be difficult to target. NRAS^Q61K/R are the second most frequent mutations found in human skin melanoma after BRAF^V600E . Aside from surgery, various approaches, including targeted therapies, immunotherapies, and combination therapies, are used to treat patients carrying NRAS mutations, but they are inefficient. Here, we established mouse NRAS^Q61K melanoma cell lines and genetically derived isografts (GDIs) from Tyr::NRAS^Q61K mouse melanoma that can be used in vitro and in vivo in an immune-competent environment (C57BL/6) to test and discover novel therapies. We characterized these cell lines at the cellular, molecular, and oncogenic levels and show that NRAS^Q61K melanoma is highly sensitive to the combination of Mek and Akt inhibitors. This preclinical model shows much potential for the screening of novel therapeutic strategies for patients harboring NRAS mutations that have limited therapeutic options and resulted in poor prognoses.

Cell signaling in protein synthesis ribosome biogenesis and translation initiation and elongation

Protein synthesis is a highly energy-consuming process that must be tightly regulated. Signal transduction cascades respond to extracellular and intracellular cues to phosphorylate proteins involved in ribosomal biogenesis and translation initiation and elongation. These phosphorylation events regulate the timing and rate of translation of both specific and total mRNAs. Alterations in this regulation can result in dysfunction and disease. While many signaling pathways intersect to control protein synthesis, the mTOR and MAPK pathways appear to be key players. This chapter briefly reviews the mTOR and MAPK pathways and then focuses on individual phosphorylation events that directly control ribosome biogenesis and translation.

Physiological roles of ribosomal protein S6: one of its kind

The phosphorylation of ribosomal protein S6 (rpS6), which occurs in response to a wide variety of stimuli on five evolutionarily conserved serine residues, has attracted much attention since its discovery more than three decades ago. However, despite a large body of information on the respective kinases and the signal transduction pathways, the role of this phosphorylation remained obscure. It is only recent that targeting the genes encoding rpS6, the phosphorylatable serine residues or the respective kinases that the unique role of rpS6 and its posttranslational modification have started to be elucidated. This review focuses primarily on the critical role of rpS6 for mouse development, the pathways that transduce various signals into rpS6 phosphorylation, and the physiological functions of this modification. The mechanism(s) underlying the diverse effects of rpS6 phosphorylation on cellular and organismal physiology has yet to be determined. However, a model emerging from the currently available data suggests that rpS6 phosphorylation operates, at least partly, by counteracting positive signals simultaneously induced by rpS6 kinase, and thus might be involved in fine-tuning of the cellular response to these signals.

Association of the p56lck protein tyrosine kinase with the Fc gamma RIIIA/CD16 complex in human natural killer cells

The multimeric Fc gamma RIIIA (CD16) complex is expressed on the surface of natural killer (NK) cells and is composed of a 50-70-kDa transmembrane glycoprotein Fc gamma receptor (CD16), the T cell receptor (TCR)-zeta chain, and the Fc epsilon RI gamma chain. Cross-linking Fc gamma RIIIA initiates the rapid tyrosine phosphorylation of multiple substrates including the zeta subunit and causes subsequent cell activation and antibody-dependent cellular cytotoxicity (ADCC). The subunits of the Fc gamma RIIIA complex lack intrinsic protein tyrosine kinase (PTK) activity, suggesting that receptor-induced tyrosine phosphorylation events are mediated by a nonreceptor PTK. We report here that the human Fc gamma RIIIA is complexed with p56lck, a src-family PTK previously found associated with the CD4 and CD8 receptors on T cells. Upon engagement of the CD16 receptor, p56lck is rapidly (within 30 s) and transiently phosphorylated on tyrosine residues. Several Fc gamma RIIIA-associated proteins are identified in immune complex kinase assays including the TCR-zeta subunit, a p70-90 zeta-associated protein (ZAP), p50a (acidic) and p50b (basic), and p56lck. We demonstrate that the src-family protein tyrosine kinase inhibitor, herbimycin A, blocks increased intracellular calcium levels and ADCC caused by CD16 cross-linking on NK3.3 cells. Likewise cross-linking CD16 with the protein tyrosine phosphatase CD45, abrogates CD16-induced calcium mobilization. These data suggest that p56lck is physically associated with Fc gamma RIIIA (CD16) and functions to mediate signaling events related to the control of NK cellular cytotoxicity.

Altered protein kinase C activity and its endogenous protein phosphorylation in rat liver after administration of ethionine

Ethionine, an ethyl analogue of methionine, induces fatty liver in rats. The effects of ethionine administration on protein kinase C (PKC) in rat liver was examined. By a single administration at a dose of 0.5 mg/g body wt., liver PKC activity was increased in both cytosolic and total particulate fractions. The increase in cytosol was significant, even at 4 h after administration, when compared with control rat liver cytosol. On the other hand, a 4-day consecutive administration (0.5 mg/g per day) resulted in decreased PKC activity, particularly in cytosol, when compared with the control. Protein phosphorylation in liver catalyzed by PKC was found to be enhanced by ethionine, irrespective of the mode of administration. The enhanced phosphorylation was observed in both cytosolic and total particulate fractions. The change of PKC activity, and the phosphorylation of its endogenous substrates, are postulated to be involved in the pathogenesis of ethionine-induced fatty liver of rats.

Involvement of protein kinase C in the regulation of ornithine decarboxylase mRNA by phorbol esters in rat hepatoma cells

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) stimulates a rapid increase in ornithine decarboxylase (EC 4.1.1.17; ODC) activity in target cells. Here we demonstrate that this process involves a rapid accumulation of ODC mRNA, which is maximal 3 h after treatment (three- to eightfold greater than control cells) and decays to control levels within 18 h. Stimulation of ODC mRNA by TPA is blocked by phorbol dibutyrate down-regulation of protein kinase C (PKC). ODC mRNA was also induced by the PKC activators, phospholipase C and 1-oleoyl-2-acetyl-rac-glycerol, and blocked by kinase inhibitors (trifluoroperazine, H7, and palmitoyl-L-carnitine), consistent with a requirement for PKC activation in the induction mechanism. However, the non-PKC-specific protein kinase inhibitor HA1004 also suppressed expression of ODC mRNA in response to TPA, under conditions where it did not inhibit PKC, suggesting that additional kinases may be involved in the intracellular signalling process. The stability of the ODC mRNA (control value = 6.2 +/- 1.6 h) is not significantly changed by either TPA (5.7 +/- 0.8 h) or by cycloheximide (6.0 h). These results are inconsistent with any contribution from altered mRNA half-life towards the accumulation of ODC mRNA following treatment with phorbol ester tumor promoters.

Regulation of ornithine decarboxylase mRNA by phorbol esters and insulin in normal and C-kinase-deficient rat hepatoma cells

Tumor-promoting phorbol esters and insulin produce similar effects in Reuber H35 rat hepatoma cell proliferation, including increased ornithine decarboxylase (ODC) enzyme activity, DNA synthesis, and mitogenesis. We investigated ODC mRNA accumulation in cells treated with either insulin or 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Both agents caused rapid accumulation of ODC mRNA: for TPA, it was maximal 3 hr after treatment (4-6-fold greater than control cells) and returned quickly to control levels; for insulin, it was significantly longer, continuing to increase for at least 6 hr. Simultaneous treatment with TPA and insulin led to additive effects on ODC mRNA. Induction of ODC by TPA was blocked by down-regulation or inhibition of protein kinase C (PKC), consistent with a PKC-mediated mechanism. In contrast, PKC down-regulation had little effect on ODC induction by insulin. Furthermore, although both agents stimulated ribosomal S6 protein phosphorylation in cells containing normal amounts of PKC, the response to TPA was abolished in PKC-depleted cells; the effect of insulin was only slightly inhibited. TPA caused a rapid redistribution of essentially all of the PKC activity from the cytosolic to the membrane fraction of the cells, whereas insulin had no effect on PKC distribution. These results suggest that although insulin and TPA share some common cytoplasmic signalling pathways, their effects on phosphorylation of nuclear proteins and transcription of ODC may be mediated by distinct factors.

Purification and identification of creatine phosphokinase B as a substrate of protein kinase C in mouse skin in vivo

We previously described epidermal proteins with molecular weights of 40,000 (p40) and 34,000 (p34) as target proteins of protein kinase C in mouse skin carcinogenesis in vivo. In the present work, p40 was purified from mouse brain by the use of 32P-labeled p40 of BALB/MK-2 cells as a tracer. Following four lines of evidence indicate that p40 is creatine phosphokinase B. 1) The amino acid sequences of all peptide fragments of p40 from mouse brain were located in the primary structure of creatine phosphokinase B. 2) p40 of BALB/MK-2 cells was immunoprecipitated with goat antibody against human creatine phosphokinase B. 3) p40 of BALB/MK-2 cells was absorbed to and eluted from a creatine affinity column. 4) Purified creatine phosphokinase B was phosphorylated in vitro by purified protein kinase C, but not by cAMP-dependent kinase or casein kinase II.

Role of Ca2+/phospholipid-dependent protein kinase in catecholamine secretion from bovine adrenal medullary chromaffin cells

The role of Ca2+/phospholipid-dependent protein kinase (protein kinase C) in catecholamine secretion from bovine adrenal medullary chromaffin cells was examined using four protein kinase C inhibitors: polymyxin B, sphingosine, staurosporine, and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7). For this purpose, digitonin-permeabilized chromaffin cells were used. Secretion of catecholamines from these cells was stimulated by the addition of micromolar amounts of exogenous free Ca2+. 12-O-Tetradecanoylphorbol-13-acetate (TPA) and arachidonic acid, activators of protein kinase C, enhanced the catecholamine secretion evoked by Ca2+. But phorbol-12, 13-diacetate, a phorbol ester analog that does not activate protein kinase C, had no effect on Ca2(+)-evoked secretion. Polymyxin B at a low concentration (1 microM) abolished the enhancement of secretion by TPA or arachidonic acid without affecting the secretion evoked by Ca2+. However, polymyxin B at higher concentrations (10-100 microM) greatly reduced Ca2+-evoked catecholamine secretion. Sphingosine 10 microM-1 mM), Staurosporine (100 nM-1 microM, and H-7 (100-500 microM) inhibited TPA- or arachidonic acid-enhanced secretion but not Ca2(+)-evoked secretion. In cells in which protein kinase C was down-regulated by TPA, specific binding of [3H]phorbol-12,13-dibutyrate to the cells almost disappeared and the enhancement of secretion by TPA was no longer observed, whereas Ca2(+)-evoked secretion was maintained. These results strongly suggest that protein kinase C is not essential for the Ca2(+)-dependent catecholamine secretion from bovine adrenal chromaffin cells, but acts instead as a modulator.

Mitogen-responsive S6 kinase

Many cell lines respond to mitogenic stimuli (serum, growth factors) with rapid phosphorylation of the ribosomal protein S6 at several serine sites. We have tried to identify the protein kinase(s) mediating this effect of growth stimuli. Examining post-DEAE chromatography fractions of S49 kin- cell extracts, we could detect a highly active effector-independent S6 kinase with specificity for serine residues. The study was extended to the presumably homologous human enzyme, using HeLa S3 cells as model system. Activity yields increased up to sevenfold when exhausted HeLa cells were supplied with fresh medium plus serum. The enzyme uses ATP, not GTP, as cosubstrate, 40-S or 80-S (reassociated from subunits) ribosomal particles being substrate. The optimal K+ concentration, measured at 3 mM Mg2+, is 35 mM. Under optimized assay conditions S6 phosphorylation proceeded faster in vitro than it appeared to do in vivo. The apparent Mr of the enzyme, as estimated by gel filtration on Sephadex G-100, is 56,000 (determination in the presence of 200 mM KCl in 25 mM phosphate buffer). Tighter binding to DEAE-Sephacel and higher specificity for S6 distinguishes this enzyme from the following S6-phosphorylating protein kinases: protein kinase C, protease-activated kinase II, histone-4 phosphotransferase and an enzyme with the properties of casein kinase I. In published summaries of observations shown here and in a follow-up study with chick embryo fibroblasts, the enzyme(s) has been referred to as mitogen-responsive S6 kinase(s) [Martini, O. H. W. and Lawen, A. (1985) in Hormones and cell regulation (Dumont, J. E., Hamprecht, B. and Nunez, J., eds) vol. 9, pp. 411-412, Elsevier Company, North-Holland, Amsterdam; Lawen, A. and Martini, O. H. W. (1985) FEBS Lett. 185, 272-276].

A role for protein kinase C-mediated phosphorylation in the mobilization of arachidonic acid in mouse macrophages

Mouse peritoneal macrophages respond to activators of protein kinase C and to zymosan particles and calcium ionophore by rapid enhancement of a phospholipase A pathway and mobilization of arachidonic acid. The pattern of protein phosphorylation induced in these cells by 4 beta-phorbol 12-myristate 13-acetate (PMA), 1,2-dioctanoyl-sn-glycerol, exogenous phospholipase C and by zymosan and ionophore A23187 was found to be virtually identical. The time course of phosphorylation differed among the phosphoprotein bands and in only some of those identified (i.e., those of 45 and 65 kDa) was the phosphorylation sufficiently rapid to be involved in the activation of the phospholipase A pathway. Phosphorylation of lipocortin I or II could not be detected. Down-regulation of kinase C by a 24-h pretreatment with PMA resulted in extensive inhibition of both protein phosphorylation and the mobilization of arachidonic acid in response to PMA or dioctanoylglycerol. The phosphorylation of the 45 kDa protein in response to zymosan and A23187 was also inhibited by pretreatment with PMA, while only arachidonic acid release induced by zymosan was inhibited by this pretreatment. Depletion of intracellular calcium had little effect on kinase C-dependent phosphorylation, although arachidonic acid mobilization is severely inhibited under these conditions. Bacterial lipopolysaccharide and lipid A induced a phosphorylation pattern different from that induced by PMA, and down-regulation of protein kinase C did not affect lipopolysaccharide-induced protein phosphorylation. The results indicate (i) that protein kinase C plays a critical role also in zymosan-induced activation of the phospholipase A pathway mobilizing arachidonic acid; (ii) that such activation requires calcium at some step distal to kinase C-mediated phosphorylation and (iii) that phosphorylation of lipocortins does not explain the kinase C-dependent activation.

Properties of the 12-O-tetradecanoylphorbol-13-acetate-stimulated S6 kinase from rat astroglial cells

The S6 kinase activity of astroglial cells in primary culture stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA) has been studied. This activity was eluted as a single peak at 0.15 M NaCl from a DEAE-Sephacel column. The chromatography of this peak on phosphocellulose revealed an activity eluted at 0.15 M NaCl. This partially purified enzyme had a sedimentation coefficient of 3.7S; Km values were 2 X 10(-5) M for ATP and 10(-6) M for 40S ribosomal subunits. The optimal Mg2+ concentration requirement was 2-3 mM. Mn2+ and Co2+ could substitute for Mg2+ (optimum concentrations 1.5 and 0.8 mM, respectively), but these cations were strong inhibitors in the presence of Mg2+. The enzyme was inhibited by N-ethylmaleimide, indicating that it contained thiol groups. This S6 kinase used ATP, but not GTP, as a phosphate donor, and exhibited great specificity for S6 as phosphate acceptor. Whole histones and protamine were slightly phosphorylated whereas phosvitin, histone H1, and surprisingly the peptide Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala were not phosphorylated. The TPA-stimulated S6 kinase resembles the insulin-, fibroblast growth factor- and cyclic AMP-stimulated enzymes, suggesting that several pathways might activate the same entity.

Insulin and phorbol ester stimulate phosphorylation of acetyl-CoA carboxylase at similar sites in isolated adipocytes. Lack of correspondence with sites phosphorylated on the purified enzyme by protein kinase C

1. The phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA) stimulates fatty acid synthesis from glucose in isolated adipocytes with a half-maximal effect at 0.72 microM. In seven batches of cells, the maximal effects of TPA and insulin were 8.5 +/- 1.1-fold and 27.1 +/- 2.1-fold respectively. Insulin also stimulated fatty acid synthesis from acetate 8.9 +/- 0.5-fold (three experiments), but TPA did not significantly increase fatty acid synthesis from this precursor. 2. In contrast to insulin, TPA treatment of isolated adipocytes did not produce an activation of acetyl-CoA carboxylase which was detectable in crude cell extracts. 3. The total phosphate content of acetyl-CoA carboxylase, isolated from adipocytes in the presence of protein phosphatase inhibitors, was estimated by 32P-labelling experiments to be 2.6 +/- 0.1 (5), 3.4 +/- 0.2 (5), and 3.8 +/- 0.2 (3) mol/mol subunit for enzyme from control, insulin- and TPA-treated cells respectively. Insulin and TPA stimulated phosphorylation within the same two tryptic peptides. 4. Purified acetyl-CoA carboxylase is phosphorylated in vitro by protein kinase C at serine residues which are recovered in three tryptic peptides, i.e. peptide T1, which appears to be identical with the peptide Ser-Ser(P)-Met-Ser-Gly-Leu-His-Leu-Val-Lys phosphorylated by cyclic-AMP-dependent protein kinase, and peptides Ta and Tb, which have the sequences Ile-Asp-Ser(P)-Gln-Arg and Lys-Ile-Asp-Ser(P)-Gln-Arg respectively, and which appear to be derived from a single site by alternative cleavages. None of these correspond to the peptides whose 32P-labelling increase in response to insulin or TPA. Peptides Ta/Tb are not significantly phosphorylated in isolated adipocytes, even after insulin or TPA treatment. Peptide T1 is phosphorylated in isolated adipocytes, but this phosphorylation is not altered by insulin or TPA. 5. These results show that TPA mimics the effect of insulin on phosphorylation, but not activation, of acetyl-CoA carboxylase, i.e. that these two events can be dissociated. In addition, phorbol ester stimulates phosphorylation of acetyl-CoA carboxylase in isolated adipocytes, but this is not catalyzed directly by protein kinase C, and acetyl-CoA carboxylase does not appear to be a physiological substrate for this kinase.

Comparative studies on phosphorylation of synthetic peptide analogue of ribosomal protein S6 and 40-S ribosomal subunits between Ca2+/phospholipid-dependent protein kinase and its protease-activated form

Ca2+/phospholipid-dependent protein kinase (protein kinase C) and trypsin-activated protein kinase C (protein kinase M) phosphorylated the synthetic peptide R1-A13 (Arg-Arg-Leu-Ser-Ser-Leu-Arg-Ala-Ser-Thr-Ser-Lys-Ala) which contains both cAMP- and insulin-regulated phosphorylation sites in rat liver ribosomal protein S6 [Wettenhall, R. E. H. & Morgan, F. J. (1984) J. Biol. Chem. 259, 2084-2091]. Both enzymes showed essentially the same kinetic properties; V and apparent Km were determined to be 0.16 mumol min-1 mg-1 and 30 microM, respectively. At first, tryptic phosphopeptides were prepared at the early stage of phosphorylation and purified by high-performance liquid chromatography (HPLC). Through these analyses, four radioactive peptides were isolated. When protein kinase C was employed, phosphorylation was observed on all four peptides in a Ca2+/phospholipid-dependent manner. Irrespective of the protein kinase employed, phosphate incorporation into these peptides increased linearly with time; the peptide concentration did not affect the ratio of phosphate distribution into these four peptides. Analysis of amino acid composition and phosphoamino acid of radioactive peptides obtained after extensive phosphorylation showed that phosphates were incorporated into Ser-4, Ser-5, Ser-9 and Ser-11. The latter three serine residues were major phosphorylated sites. When rat liver 40-S ribosomal subunits were employed as substrate for protein kinases C and M, a radioactive protein with Mr,app = 31,000, which corresponded to S6 protein, was detected on an autoradiogram of a sodium dodecyl sulfate/polyacrylamide slab gel. The rate of phosphorylation with protein kinase M was twice as fast as that with protein kinase C. The elution profile of radioactive tryptic peptides in HPLC suggest that phosphorylation occurred on the sites in S6 protein corresponding to Ser-5, Ser-9 and Ser-11 as major sites and Ser-4 as the minor one. These results indicate that protein kinase C has an ability to recognize at least four sites derived from hormone-dependent phosphorylation sites in ribosomal protein S6 irrespective of the mode of activation of this enzyme.

Transient induction of ornithine decarboxylase mRNA in rat hepatoma cells treated with 12-O-tetradecanoylphorbol-13-acetate

The contribution of changes in mRNA levels to the induction of ornithine decarboxylase (ODC) by 12-O-tetradecanoylphorbol-13-acetate (TPA) in rat H35 hepatoma cells was analyzed by Northern blot and quantitative dot blot hybridization. ODC mRNA accumulated rapidly in TPA-treated cultures. The increase in message was transient, reaching a peak within about 3 h, then declining to control levels after 18 h. Maximal accumulation of ODC-specific mRNA varied from 3- to 8-fold above control. The TPA dose-response for ODC message accumulation was half-maximal at approximately 0.18 microM TPA. The increase was completely blocked by actinomycin D, suggesting that TPA stimulates the transcription of ODC genes. Inhibition of protein synthesis by cycloheximide (10 micrograms/ml) led to a superinduction of ODC mRNA in the presence of TPA, which suggested that a short-lived protein may be responsible for negative control of ODC expression.

Induction and suppression of type I collagenase in cultured human cells

A number of peptide growth factors have been shown to induce the secretion of type I collagenase into the medium of human fibroblast cultures (Chua et al., 1985). In this study the ability of eye-derived growth factor, lectin and tumor-promoting agents on collagenase induction in human fibroblast cells were examined. These agents were found to be able to induce collagenase production to a similar extent as epidermal growth factor. Dexamethasone at 10-100 nM was found to suppress collagenase induction in human fibroblast cells. The cell-type specificity of this enzyme induction by growth factors was studied by using a human epidermoid carcinoma cell line, A-431. An Mr 55,000 band appeared in the medium of A-431 cells upon 22 h exposure to EGF. Two-dimensional peptide pattern of the Mr 55,000 band in A-431 cells was identical to the counterpart in HF cells. Our results indicated that the induction of collagenase was not unique to human fibroblast cultures.

Characteristics of thyroid protein kinase C. Different Ca2 requirement for the phosphorylation of endogenous proteins and of H1 histone

Thyroid protein kinase C (PKc) from cytosols of porcine and rat thyroid glands has been characterized using histone H1 or endogenous proteins as substrates. As in many other tissues histone H1 is by far the preferred exogenous substrate of thyroid PKc. Kinetic studies with H1 showed that, compared to rat thyroids, porcine glands are particularly rich in PKc, the predominant kinase activity in this tissue. The cAMP-dependent protein kinase (PKa) level, on the contrary, is very similar in both rat and porcine thyroids. Consequently, for the same type of tissue, there may be great species differences in the PKc level and the ratios between PKc and PKa kinase activities. Chromatographic properties of thyroid PKc are similar to those described in other tissues (one major peak followed by a small shoulder) except that elution of the main peak can vary depending on the nature of the salt gradient (approximately 55 mM for NaCl and 15 mM for sodium phosphate). In the first case PKc is completely separated from the PKa activity, in the second it is coeluted with the peak of PKa type I. The one-dimensional PAGE pattern of proteins phosphorylated by porcine PKc is very similar to the pattern obtained by rat enzyme. Protein bands of 18 kDa, 22-25 kDa and 32-36 kDa are specific substrates of the thyroid PKc, after in vitro phosphorylation of cytosol proteins. A great difference in Ca2+ requirement for PKc activation was noted, depending whether histone H1 or endogenous proteins were substrates. As in other tissues, calcium was absolutely necessary for phosphorylation of histone H1 by PKc. The addition of calcium was not absolutely necessary when endogenous proteins were the substrates, either for the activation of the enzyme or for phosphorylation of the PKc-specific substrates. Almost the same rate of phosphorylation was obtained with or without calcium in the incubation medium. However the one-dimensional PAGE pattern of phosphorylated proteins was different in the presence or absence of calcium. While addition of calcium was not absolutely necessary for the phosphorylation of a great number of proteins by the PKc, its presence was indispensable for the phosphorylation of certain endogenous substrates. However, calcium alone, in the absence of phospholipids had no effect on the phosphorylation of these proteins. Endogenous proteins, phosphorylated by the PKc only when calcium was present, were resolved by the two-dimensional PAGE into several distinct spots with molecular masses of 32-35 kDa and pI range of 5-7.5.(ABSTRACT TRUNCATED AT 400 WORDS)

Demonstration of protein kinase C activity in crustacean Y-organs, and partial definition of its role in regulation of ecdysteroidogenesis

Ecdysteroid-producing Y-organs from the crab Cancer antennarius were shown to possess enzyme activity that was stimulated in vitro by addition of Ca2+, phosphatidylserine, or the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA; ED50, 4 nM). In the presence of calcium and phosphatidylserine, PMA increased protein kinase C activity dose-dependently to a maximum 4-fold increase at 100 nM PMA. Stimulated protein kinase C activity was unaffected by calmodulin (100 nM) but was inhibited by 100 nM trifluoperazine. Pretreatment of cultured Y-organ segments with PMA elevated basal protein kinase C activity, whereas molt-inhibiting hormone (MIH) and calcium ionophore A23187 did not affect activity. PMA (1-100 nM) increased Y-organ steroidogenesis dose-dependently and alleviated suppression due to MIH or lysine vasopressin; PMA effects on steroidogenesis became evident after 2 h of incubation. Another phorbol activator of protein kinase C (phorbol 12, 13-dibutyrate) and a permeable synthetic diacylglycerol (1-oleoyl-2-acetyl-glycerol) stimulated ecdysteroidogenesis while an inactive phorbol (4 alpha-phorbol 12,13-didecanoate) and diolein were ineffective. The inhibitory effects on steroidogenesis of cholera toxin, forskolin, dibutyryl cAMP, and 3-isobutyl-1-methylxanthine were countered by PMA, but PMA did not alter basal or peptide hormone-stimulated Y-organ cAMP levels. Stimulatory effects on steroidogenesis of PMA and of A23187 were not additive, and PMA did not alter inhibition caused by lanthanum (calcium channel blocker) or trifluoperazine (calmodulin inhibitor). PMA increased the incorporation of [3H]leucine into Y-organ protein by 112%, and countered the suppressive effect of MIH on protein synthesis; PMA did not affect RNA synthesis. When Y-organs were suppressed with cycloheximide, PMA was unable to stimulate steroidogenesis. Actinomycin D alone had no effect on steroidogenesis but prevented stimulation by PMA. The results indicate that Y-organs contain protein kinase C activity which stimulates ecdysteroid production and protein synthesis by a mechanism not directly interactive with the cAMP or Ca2+-calmodulin systems.

Calcium, cyclic AMP and protein kinase C--partners in mitogenesis

Evidence is steadily mounting that the proto-oncogenes, whose products organize and start the programs that drive normal eukaryotic cells through their chromosome replication/mitosis cycles, are transiently stimulated by sequential signals from a multi-purpose, receptor-operated mechanism (consisting of internal surges of Ca2+ and bursts of protein kinase C activity resulting from phosphatidylinositol 4,5-bisphosphate breakdown and the opening of membrane Ca2+ channels induced by receptor-associated tyrosine-protein kinase activity) and bursts of cyclic AMP-dependent kinase activity. The bypassing or subversion of the receptor-operated Ca2+/phospholipid breakdown/protein kinase C signalling mechanism is probably the basis of the freeing of cell proliferation from external controls that characterizes all neoplastic transformations.

Prolactin stimulation of ornithine decarboxylase and mitogenesis in Nb2 node lymphoma cells: the role of protein kinase C and calcium mobilization

The tumor promotor 12-O-tetradecanoylphorbol-13-acetate (TPA) in combination with calcium ionophores has been shown to bypass the requisite antigen- or lectin-induced signal for lymphocyte mitogenesis. This suggests that protein kinase C activation and calcium mobilization may be early events required for lymphocyte proliferation. Therefore, the relationship(s) of protein kinase C activation and calcium mobilization to ornithine decarboxylase induction and cellular proliferation were examined in a rat node lymphoma cell line (Nb2) which is dependent upon prolactin (PRL) for mitogenesis. TPA enhanced PRL-stimulated Nb2 node lymphoma cell ornithine decarboxylase induction and [3H]thymidine incorporation. Addition of a calcium ionophore (A23187) to cultures containing TPA plus PRL increased ornithine decarboxylase above PRL alone or PRL plus TPA but inhibited proliferation compared to the PRL plus TPA regimen. Exposure of cells to TPA or TPA plus A23187 increased [3H]thymidine incorporation in a similar manner to that demonstrated for low-dose PRL. However, optimal concentrations were only 20-25% as effective as mitogens as was optimal PRL. Protein kinase C and calmodulin antagonists inhibited PRL-stimulated ornithine decarboxylase induction and proliferation. Ca2+ chelation or cation channel antagonism inhibited both PRL-stimulated responses. The cyclic AMP analogue, 8Br-cAMP, inhibited PRL-stimulated ornithine decarboxylase activity as well as cellular proliferation processes assessed by [3H]thymidine incorporation. Finally, tumor-promoting phorbol esters inhibited 125I-rPRL binding. These data strongly suggest that protein kinase C activation and calcium mobilization are requisite events for PRL-stimulated ornithine decarboxylase induction and cellular proliferation in Nb2 node lymphoma cells. An additional component that is linked to alterations in K+ channeling is also implicated. These data support a role for protein kinase C in PRL-coupled mitogenesis. However, other critical Ca2+ and/or ion-induced events are also required.

Effect of polymyxin-B on T-lymphocyte protein synthesis

The role of protein kinase-C (PK-C) protein phosphorylation on the mitogen triggered responses of T-lymphocytes was examined by observing the effect of polymyxin-B (an inhibitor of PK-C) on mitogen induced protein and DNA synthesis. Polymyxin-B inhibited 3H-thymidine incorporation by PHA activated T-lymphocytes over a range of PHA concentrations. 3H-leucine incorporation by PHA activated T-lymphocytes was inhibited by polymyxin-B in a dose dependent manner. A partially purified PK-C fraction from polymyxin-B treated PHA activated T-lymphocytes demonstrated less than 25% of the phosphorylating activity of untreated lymphocytes. These results suggest that protein synthesis during the T-lymphocyte activation process is dependent on PK-C activity.

Redistribution of protein kinase C during mitogenesis of human B lymphocytes

G0 human tonsillar B-lymphocytes were stimulated to divide by the polyclonal mitogen Staphylococcus Aureus Cowan strain 1 (SAC) and by the combined use of 12-O-tetradecanoyl phorbol-13-acetate (TPA) and the calcium ionophore ionomycin. The activities of protein kinase C, which requires Ca++ and phospholipid as co-factors, and a proteolytically cleaved form of this enzyme (protein kinase M), which is independent of calcium and phospholipid control, were determined in soluble and particulate fractions obtained from activated B cells. Treatment of G0 B cells with SAC or TPA together with ionomycin caused redistribution of protein kinase C from the soluble to the particulate fraction where the 80,000-Dalton protein kinase C was cleaved to give rise to a 50,000-Dalton form of the kinase which was also found in the cytoplasm. These data suggest that redistribution and proteolytic cleavage of protein kinase C are key signal transduction events in B cell mitogenesis.

A model for intracellular translocation of protein kinase C involving synergism between Ca2+ and phorbol esters

The activation of protein kinase C by diacylglycerol and by tumour promoters has implicated this enzyme in transmembrane signalling and in the regulation of the cell cycle. In vitro studies revealed that catalytic activity requires the presence of calcium and phospholipids with a preference for phosphatidylserine. Diacylglycerol and tumour promoters such as phorbol esters bind to the enzyme, leading to its activation while sharply increasing its affinity for Ca2+ and phospholipid. Addition of diacylglycerol analogues or phorbol esters to intact cells results in the phosphorylation of specific polypeptides. Several cellular processes, including hormone and neurotransmitter release and receptor down-regulation, are modulated by the activation of protein kinase C, while phorbol ester-induced stimulation of the enzyme in whole cells has been associated with its translocation from the cytoplasm to the plasma membrane. Moreover, the use of Ca2+ ionophores has revealed an apparent synergism between Ca2+ mobilization and protein kinase C activation. This synergism has recently also been found to apply to receptor down-regulation (ref. 23 and accompanying paper). Here we describe a reconstitution system in which intracellular translocation of protein kinase C and the synergism between Ca2+ and enzyme activators can be studied. The results suggest a rationale for concomitant Ca2+ mobilization and diacylglycerol formation in response to some hormones, neurotransmitters and growth factors.

Protein kinase C: properties and possible role in cellular division and differentiation

Protein kinase C was first described some eight years ago. Recent results indicate that this kinase may have a crucial role in signal transduction for substances involved in cellular differentiation and division. Protein kinase C is activated by attachment to plasma membranes, in the presence of calcium and diacylglycerol. The activator is produced in the membrane following the signal-induced breakdown of phosphoinositides. Tumor promoters, such as phorbol ester, can substitute for diacylglycerol. The recent findings that: tyrosine kinases might be involved in the phosphoinositide turnover and, phosphorylation of growth factor receptors by protein kinase C regulates some of their functions, indicate more and more clearly that this kinase is involved in the control of cell growth division and differentiation. Purification procedures, properties and mechanisms of regulation will be summarized and discussed.

Phosphorylation of the ribosomal protein S6 during agonist-induced exocytosis in exocrine glands is catalyzed by calcium-phospholipid-dependent protein kinase (protein kinase C). Experiments with guinea pig parotid glands

The ribosomal protein S6 in exocrine cells is phosphorylated during stimulation of exocytosis by cAMP-dependent or calcium-dependent agonists. Under both conditions the same tryptic S6 phosphopeptides (termed A, B, and C) were found [Padel, Kruppa, Jahn & Söling (1983) FEBS Lett. 159, 112-118]. Studies have now been made of the phosphorylation pattern of protein S6 from purified guinea pig parotid ribosomes following in vitro phosphorylation with calmodulin-dependent, phospholipid-dependent, and cAMP-dependent protein kinases. Only the phospholipid-dependent enzyme led to the phosphorylation of peptides A, B, and C, while the cAMP-dependent enzyme phosphorylated only peptides A and C, and the calmodulin-dependent enzyme did not phosphorylate any of the phosphopeptides found in S6 from unstimulated or stimulated intact cells. Guinea pig parotid microsomes contain substantial phospholipid-dependent protein kinase activity. Stimulation of intact parotid glands with tetradecanoylphorbol acetate led to a significant phosphorylation of S6 and a similar tryptic S6 phosphopeptide pattern as seen with carbamoylcholine. It is concluded that activation of phospholipid-dependent protein kinase is responsible for the phosphorylation of protein S6 during stimulation with calcium-dependent and cAMP-dependent secretagogues.

The phosphorylation of eukaryotic ribosomal protein S6 by protein kinase C

Purified Ca2+-dependent and phospholipid-dependent protein kinase (protein kinase C) from bovine brain catalysed the phosphorylation of ribosomal protein S6 when incubated with 40S ribosomal subunits from rat liver or from hamster fibroblasts. The phosphorylation was dependent on Ca2+ and phospholipid, and occurred under ionic conditions similar to those which support protein biosynthesis in vitro. Protein kinase C phosphorylated at least three sites on ribosomal protein S6 when incubated with unphosphorylated ribosomes, and increased the extent of phosphorylation of ribosomes previously phosphorylated predominantly on two sites by cyclic-AMP-dependent protein kinase, converting some molecules to the tetraphosphorylated or pentaphosphorylated form. This indicates that protein kinase C can phosphorylate sites on ribosomal protein S6 other than those phosphorylated by the cyclic-AMP-dependent protein kinase, and this conclusion was confirmed by analysis of tryptic phosphopeptides. These results strengthen the possibility that protein kinase C might be involved in catalysing the multisite phosphorylation of ribosomal protein S6 in certain circumstances in vivo.

Inositol trisphosphate and diacylglycerol as intracellular second messengers in liver

Receptor occupation by a variety of Ca2+-mobilizing hormones, such as alpha 1-adrenergic agents, vasopressin and angiotensin II, causes a rapid phosphodiesterase-mediated hydrolysis of phosphatidylinositol-4,5-bisphosphate in the plasma membrane with the production of the water soluble compound myo-inositol-1,4,5-trisphosphate (IP3) and the lipophilic molecule 1,2-diacylglycerol (DG). This review summarizes the recent evidence obtained in the liver that defines the roles of these products as intracellular messengers of hormone action. Intracellular Ca2+ mobilization is mediated by IP3, which releases Ca2+ from a subpopulation of the endoplasmic reticulum, resulting in a rapid increase of the cytosolic free Ca2+ concentration ( [Ca2+]i). Further effects of receptor occupancy are inhibition of the plasma membrane Ca2+-ATPase, despite net Ca2+ efflux, and an increased permeability of the plasma membrane to extracellular Ca2+. The activation of the phospholipid-dependent protein kinase C by DG does not alter Ca2+ fluxes across the plasma membrane. In contrast to some secretory cells, a synergism between protein kinase C activation and increased [Ca2+]i is not observed in liver. Activation of protein kinase C profoundly inhibits the response to alpha 1-adrenergic agonists, with only minimal effects on the vasopressin response. It is concluded that in liver the two inositol-lipid messenger systems, IP3 and DG, exert their effects by essentially separate pathways.

Protein kinase C: rapid enzyme purification and substrate-dependence of the diacylglycerol effect

Protein kinase C has been purified by a rapid method resulting in a high-yield, stable enzyme preparation. The catalytic and regulatory properties of this enzyme preparation were characterized employing histone H1 and HMG8, a proteolytic fragment of H1. The enzyme had a lower Km for HMG8, and was stimulated more effectively by diacylglycerol and phorbol esters in the presence of this substrate. Furthermore, these activators markedly increased the Km for HMG8 but not for H1. Protein kinase C and cyclic AMP-dependent protein kinase phosphorylate serine residues which are located in different, single tryptic peptides from HMG8.