Phorbol esters, but not insulin, promote depletion of cytosolic protein kinase C in rat adipocytes

The protein kinase C pseudosubstrate peptide (PKC19-36) inhibits insulin-stimulated protein kinase activity and insulin-mediated translocation of the glucose transporter glut 4 in streptolysin-O permeabilized adipocytes

The effect of insulin on protein kinase activity and plasma membrane translocation of the glucose transporter GLUT 4 has been studied in adipocytes permeabilized by Streptolysin-O. Insulin increased protein kinase activity, and this was completely inhibited by the PKC pseudosubstrate inhibitor peptide (PKC19-36). Insulin-mediated translocation of GLUT 4 was also inhibited by the PKC inhibitor peptide. Both these insulin effects were blocked by a PKCbeta neutralizing antibody. Our results are consistent with the hypothesis that insulin activates PKCbeta activity in adipocytes in situ, and that this PKC activation is a component of the system whereby insulin regulates translocation of GLUT 4 to the plasma membrane.

Protein kinase C: mediator or inhibitor of insulin action?

The role of protein kinase C in insulin signal transduction is controversial. It has been postulated that protein kinase C is activated by insulin and that the kinase is directly involved in insulin-mediated metabolic processes. In opposition to this view is the hypothesis that protein kinase C is not activated by insulin and, more importantly, may be responsible for attenuation of the insulin signal. The evidence for and against protein kinase C as a mediator of the insulin signal will be put in perspective followed by discussion of the possible role of the kinase in the pathogenesis of insulin resistance in type II diabetes.

Inhibitors such as staurosporine, H-7 or polymyxin B cannot be used in skeletal muscle to prove the role of protein kinase C on insulin action

The precise role of protein kinase C in insulin action in skeletal muscle is not well defined. Based on the fact that inhibitors of protein kinase C block some insulin effects, it has been concluded that some of the biological actions of insulin are mediated via protein kinase C. In this study, we present evidence that inhibitors of protein kinase C such as staurosporine, H-7 or polymyxin B cannot be used to ascertain the role of protein kinase C in skeletal muscle. This is based on the following experimental evidences: a) staurosporine, H-7 and polymyxin B markedly block in muscle the effect of insulin on System A transport activity; however, this effect of insulin is not mimicked in muscle by TPA-induced stimulation of protein kinase C, b) H-7 and polymyxin B block insulin action on System A transport activity in an additive manner to the inhibitory effect of phorbol esters, c) staurosporine, H-7 and polymyxin B block the effect of insulin on lactate production, a process that is activated by insulin and TPA in an additive fashion, and d) staurosporine completely blocks the tyrosine kinase activity of insulin receptors partially purified from rat skeletal muscle.

Stimulation of proliferation of HL60 cells by low concentrations of 12-O-tetradecanoylphorbol-13-acetate and its relationship to the mitogenic effects of insulin

The effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) on the growth and differentiation of cultured human acute promyelocytic leukemia (HL60) cells have been studied using cells growing in a fully defined medium consisting of RPMI 1640 supplemented with selenium dioxide, insulin, and either transferrin or ferric citrate. High concentrations of TPA (greater than 1 nM) cause the expected inhibition of proliferation and induction of macrophage-like differentiation. In contrast, in cells deprived of insulin, which continue to grow at a slow rate, lower concentrations of TPA stimulate proliferation without inducing differentiation. A TPA concentration between 0.03 and 0.3 nM will approximately double the long-term rate of thymidine incorporation into DNA and the rate of increase in cell density. Low-TPA becomes progressively less able to stimulate further proliferation as the insulin concentration is increased and is virtually without effect on cells stimulated by an optimal insulin concentration (5 micrograms ml-1). Insulin itself stimulates proliferation to a greater extent than low-TPA, increasing the long-term rate of thymidine incorporation and the rate of increase in cell density by three- to fourfold. The ability of higher concentrations of TPA to induce differentiation is independent of the presence of insulin. Low-TPA also stimulates the short-term incorporation of thymidine (during a 1-h pulse after 1 or 2 days incubation) by three- to fourfold, as compared to a sevenfold stimulation by insulin. The proliferation response to low TPA concentrations provides a useful model for dissecting the signalling pathways that control cell proliferation following stimulation by insulin and activators of protein kinase C.

Phorbol esters do not influence the maturation and the beta-adrenergic responsiveness of differentiated adipose precursor cells, cultured in a charcoal-treated serum medium

Phorbol esters are a well defined group of tumor-promoting substances, which influence the proliferation, the differentiation and the hormonal responsiveness of a wide variety of cell types through the activation of protein-kinase C. Opposite effects have been described on the beta-adrenoceptors and the adenylate cyclase activity, depending on the cell type. Since there is no information about the influence of these substances on lipolysis in adipose precursor cells, the aim of the present study was to examine the effect of 7 days exposure to phorbol esters on the differentiation and the lipolytic responsiveness of rat preadipocytes, cultured in 10% charcoal-treated fetal calf serum. Our results demonstrate that prolonged exposure to these tumor promoters does not influence the differentiation, the beta-adrenoceptors number and the lipolytic response to adenylate cyclase or beta-adrenergic agonists in differentiated cells.

Insulin and phorbol ester stimulate phosphorylation of a 15,000 dalton membrane protein in rat diaphragm in a similar manner

The effects of insulin on the phosphorylation of a 15 kilodalton (kDa) membrane protein in rat diaphragm in situ have been investigated. Incubation of the diaphragm with insulin or tumor-promoting phorbol ester increased the 32P-labelling of the 15 kDa protein at serine residues by 50 +/- 8% and 64 +/- 11%, (mean +/- S.E.), respectively. Thermolytic peptide mapping of the 15 kDa protein after insulin treatment of the diaphragm yielded two major phosphopeptides, one of which was absent from digests from control diaphragms. The same two phosphopeptides were identified after incubation of the diaphragm with phorbol ester and after phosphorylation of sarcolemma in vitro with [gamma-32P]ATP and protein kinase C. Additional experiments indicated that pretreatment of diaphragms with insulin or phorbol ester both increased the state of phosphorylation of the 15 kDa sarcolemma protein on phosphorylation sites regulated by protein kinase C. The stimulatory effect of insulin was decreased by staurosporine or by preincubation of the diaphragms with phorbol esters. These results indicate that the insulin-induced increases in protein kinase C activity previously found in rat diaphragm (Walaas et al. (1987) FEBS Lett. 220, 311-318) may be involved in insulin-mediated regulation of phosphorylation of the 15 kDa protein in situ.

Impaired translocation of protein kinase C activity in human non-insulin-dependent diabetes mellitus

Calcium- and phospholipid-dependent protein kinase (protein kinase C; PKC) may be an important mediator in transduction of some of the cellular actions of insulin. We studied PKC activity in freshly isolated circulating mononuclear cells obtained from healthy subjects and patients with non-insulin-dependent (type II) diabetes mellitus (NIDDM). The kinase activity was measured using a specific nonapeptide substrate, Ala-Ala-Ala-Ser-Phe-Lys-Ala-Lys-Lys-amide. There was negligible calcium- and phospholipid-independent kinase activity in cytosolic and particulate fractions of cells from both control and diabetic subjects. Total (cytosolic and particulate) PKC activity of mononuclear cells from poorly controlled diabetic patients was significantly reduced compared with controls; this reduction was mainly due to a decrease in the cytosolic kinase activity. Tumor-promoting phorbol ester (TPA, 0.1 mumol/L) induced translocation of PKC activity in control cells; in contrast, this subcellular redistribution was not observed in cells from a majority of poorly controlled diabetic subjects. Increased calcium influx into the cells caused by the calcium ionophore A23187-triggered translocation of PKC activity in control cells, while it was ineffective in cells from poorly controlled diabetic patients. Cells from well-controlled diabetic patients demonstrated TPA-induced translocation of the PKC activity approaching that of control cells. The total PKC activity in cells from patients with good glycemic control was normal. Impaired activation of PKC is thus associated with the insulin resistance found in patients with poorly controlled NIDDM.(ABSTRACT TRUNCATED AT 250 WORDS)

Refractoriness to growth hormone is associated with increased intracellular calcium in rat adipocytes

In adipocytes that have been deprived of growth hormone (GH) for at least 3 hr, GH elicits a transient insulin-like response that is followed by a period of refractoriness to further insulin-like stimulation. Exposure of adipocytes to GH in the first hour of a 3-hr incubation prevents the appearance of insulin-like sensitivity. Intracellular Ca2+ concentration [( Ca2+]i) was measured in individual adipocytes that were loaded with fura-2 hexakis(acetoxymethyl) ester after preincubation in the presence (refractory) or absence (sensitive) of recombinant human GH at 100 ng/ml. Using a dual nitrogen laser imaging microscope with computer-assisted image processing to measure fluorescence changes, we observed that resting [Ca2+]i was 220 +/- 10 nM in refractory adipocytes and 110 +/- 6 nM in sensitive adipocytes (P less than 0.001). GH had no acute effect on [Ca2+]i in sensitive adipocytes but caused a sustained 3-fold increase in [Ca2+]i in refractory cells within 3 min (P less than 0.001). Insulin did not change [Ca2+]i in either sensitive or refractory adipocytes. In refractory cells treated with insulin and GH simultaneously, insulin completely blocked the rise in [Ca2+]i due to GH. Oxytocin elicited a prompt increase in [Ca2+]i followed by a quick return to resting levels in both sensitive and refractory cells. These findings indicate that basal [Ca2+]i is increased in refractory cells and that GH produces a sustained rise in [Ca2+]i only in refractory adipocytes. We suggest that the sustained increase in [Ca2+]i produced by GH in refractory cells prevents the expression of the insulin-like response.

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.

Sulfonylureas activate glucose transport and protein kinase C in rat adipocytes

Glyburide and tolbutamide, at concentrations of 20 to 40 mumol/L and 1 to 2 mmol/L, respectively, stimulated glucose transport in rat adipocytes. Concomitantly, protein kinase C was activated, as evidenced by translocation of immunoreactive enzyme from cytosol to membranes. Glucose transport effects of the sulfonylureas were blocked by three inhibitors of protein kinase C (H-7, staurosporine, and sangivamycin), and by phorbol ester-induced down-regulation of protein kinase C. These findings suggest that sulfonylureas may stimulate glucose transport in rat adipocytes through activation of protein kinase C.

Protein kinase C inhibitors block insulin and PMA-stimulated hexose transport in isolated rat adipocytes and BC3H-1 myocytes

Effects of protein kinase C (PKC) inhibitors and "down-regulation" on insulin and PMA-stimulated 2-deoxyglucose transport were determined in isolated rat adipocytes or BC3H-1 myocytes. In both model systems, H-7, sangivamycin, and staurosporine, inhibitors of the catalytic domain of PKC, each effectively blocked insulin and PMA-stimulated hexose uptake at similar concentrations. In the myocytes, staurosporine completely blocked the insulin effect retained post-chronic phorbol myristate acetate (PMA)-induced "down-regulation." These findings indicate (1) that chronic pretreatment with PMA may not lead to a complete loss of PKC activity in the myocyte, and (2) that PKC is involved in insulin-stimulated hexose transport in both isolated rat adipocytes and BC3H-1 myocytes.

Protein kinase C activation patterns are determined by methodological variations. Studies of insulin action in BC3H-1 myocytes and rat adipose tissue

In BC3H-1 myocytes, insulin has been reported to (a) increase diacyglycerol (DAG) production and provoke increases in protein kinase C enzyme activity of crude or DEAE-Sephacel-purified cytosol and membrane fractions in BC3H-1 myocytes (Cooper et al. (1987) J. Biol. Chem. 262, 3633-3739), but (b) decrease cytosolic, and transiently increase membrane, immunoreactive protein kinase C (Acevedo-Duncan et al. (1989) FEBS Lett. 244, 174-176). Presently, we used a Mono-Q column to purify protein kinase C and found that, similar to immunoblot findings, enzyme activity decreased in the cytosol, and increased in the membrane during insulin treatment. Similar differences in protein kinase C activation patterns were observed in rat adipose tissue: insulin stimulated cytosolic protein kinase C enzyme activity as measured after DEAE-Sephacel chromatography, but decreased cytosolic enzyme activity when measured after Mono-Q chromatography or by immunoblotting. We presently evaluated the possibility that insulin-induced increases in endogenous DAG may influence protein kinase C during assay in vitro. Crude cytosol from BC3H-1 myocytes contained 25-35% of total and [3H]glycerol-labelled DAG and insulin increased this DAG. Considerable amounts of [3H]glycerol-labelled DAG were present in insulin-stimulated protein kinase C-containing column fractions following DEAE-Sephacel chromatography of cytosol fractions, whereas lesser amounts were recovered after Mono-Q column chromatography. This difference in recovery of DAG and activation of the enzyme by this endogenous DAG may explain why we were able to discern insulin-induced (presumably translocation 'provoked') decreases in cytosolic protein kinase C in the present Mono-Q column preparations of both BC3H-1 myocytes and rat adipose tissue.

Protein kinase C activators selectively inhibit insulin-stimulated system A transport activity in skeletal muscle at a post-receptor level

We have investigated the role of phorbol esters on different biological effects induced by insulin in muscle, such as activation of system A transport activity, glucose utilization and insulin receptor function. System A transport activity was measured by monitoring the uptake of the system A-specific analogue alpha-(methyl)aminoisobutyric acid (MeAIB), by intact rat extensor digitorum longus muscle. The addition of 12-O-tetradecanoylphorbol 13-acetate (TPA, 0.5 microM) for 60 or 180 min did not modify basal MeAIB uptake by muscle, suggesting that insulin signalling required to stimulate MeAIB transport does not involve protein kinase C activation. However, TPA added 30 min before insulin (100 nM) markedly inhibited insulin-stimulated MeAIB uptake. The addition of polymyxin B (0.1 mM) or H-7 (1 mM), protein kinase C inhibitors, alone or in combination with TPA leads to impairment of insulin-stimulated MeAIB uptake. This paradoxical pattern is incompatible with a unique action of Polymyxin B or H-7 on protein kinase C activity. Therefore these agents are not suitable tools with which to investigate whether a certain insulin effect is mediated by protein kinase C. TPA did not cause a generalized inhibition of insulin action. Thus both TPA and insulin increased 3-O-methylglucose uptake by muscle, and their effects were not additive. Furthermore, TPA did not modify insulin-stimulated lactate production by muscle. In keeping with this selective modification of insulin action, treatment of muscles with TPA did not modify insulin receptor binding or kinase activities. In conclusion, phorbol esters do not mimic insulin action on system A transport activity; however, they markedly inhibit insulin-stimulated amino acid transport, with no modification of insulin receptor function in rat skeletal muscle. It is suggested that protein kinase C activation causes a selective post-receptor modification on the biochemical pathway by which insulin activates system A amino acid transport in muscle.

Insulin, oxytocin, and vasopressin stimulate protein kinase C activity in adipocyte plasma membranes

Incubation of isolated rat adipocytes with insulin, vasopressin, or oxytocin increased plasma membrane-bound protein kinase C (PKC) activity by 100-400%. PKC activity was assayed by a procedure that is virtually background-free, thus permitting assay of protein kinase activity in highly diluted samples of solubilized membranes. Hormone-dependent increases in PKC activity were limited to plasma membranes. Stimulation of the kinase was half-maximal with 70 pM insulin, and the hormone effect was rapid. Oxytocin and vasopressin produced effects on PKC similar to insulin, but the magnitude of the vasopressin stimulation exhibited seasonal variations. Treatment of cells with phorbol 12-myristate 13-acetate (PMA) resulted in a loss of PKC activity from the cytosol and a gain in plasma membrane activity, indicative of translocation of the enzyme. With activity measurements it was not possible to determine if insulin stimulated a translocation of the kinase. However, Western blot analysis of plasma membranes with polyclonal antibodies directed against PKC suggest that at least some of the insulin-stimulated PKC activity resulted from enzyme translocation.

Insulin stimulates the translocation of protein kinase C in rat adipocytes

Insulin-induced changes in protein kinase C were examined in cytosol and membrane fractions of rat adipocytes enzymatically after Mono Q column chromatography and by immunoblotting. During a 5-20 min period of insulin treatment, cytosolic protein kinase C decrease by approximately 50%, whereas membrane protein kinase C increased nearly 2-fold. These findings suggest that insulin stimulates the translocation of protein kinase C in rat adipocytes.

A further comparison of insulin- and phorbol ester-stimulated glucose transport in adipocytes

Insulin and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) stimulatory effects on adipocyte glucose transport were compared for their sensitivity to: (1) sphingosine and staurosporine, two potent protein kinase C (PKC) inhibitors; and (2) phenylarsine oxide (PhAsO), a dithiol reagent blocking insulin-stimulated glucose transport. None affected basal 2-deoxyglucose transport, cell viability, cellular ATP content, or insulin binding. Insulin- and PMA-stimulated 2-deoxyglucose transport were both markedly inhibited by sphingosine (5-50 microM) and staurosporine (0.1-2 microM), although with differences in the extents of maximal inhibitions (65 and 48% vs. 88 and 98%) and the concentrations of the drugs causing the half-maximal inhibitions observed in the experiments (2- to 3-fold higher for insulin). Insulin and PMA both altered PKC along with glucose transport, either by increasing its activity in the cytosol or by promoting its translocation to membrane. Insulin- and PMA-stimulated 2-deoxyglucose transport were both inhibited selectively by PhAsO (0.1-1 microM), at almost identical maximal inhibitions (84 and 90%) and IC50 values (0.18 and 0.16 microM). Furthermore, insulin- and PMA-induced increases in transport Vmax (6.5- and 3.4-fold) were both reduced by 89% by PhAsO, which, however, failed to affect the decrease in transport Km (1.7-fold) exclusively induced by insulin. Likewise, PhAsO did not affect insulin or PMA activation of PKC. The results suggest that insulin activates adipocyte glucose transport through: (1) a PKC-dependent mechanism requiring cellular dithiols, responsible for a part of the hormone-induced increase in transport Vmax; and (2) a PKC-independent mechanism responsible for both a further increase in transport Vmax and a decrease in transport Km.

Sphingosine, an inhibitor of protein kinase C, suppresses the insulin-like effects of growth hormone in rat adipocytes

Insulin, human growth hormone (hGH), and phorbol 12-myristate 13-acetate all stimulate lipogenesis in rat adipocytes preincubated without hGH for 4 hr. As previous data suggested that protein kinase C plays an important role in the action of insulin and in the insulin-like effects of hGH in rat adipocytes, we tested the effects of sphingosine, a potent inhibitor of protein kinase C, on the lipogenic activity of both hormones. At 50 microM, sphingosine had no effect on basal lipogenesis but completely abolished the action of phorbol 12-myristate 13-acetate and decreased by 65% and 89%, respectively, the effects of hGH and insulin. At higher concentrations (100 microM), sphingosine abolished both basal and hormone-stimulated lipogenesis; this effect was partially reversible after washing the cells. Similar effects of sphingosine on basal and stimulated glucose uptake were seen in parallel, suggesting that sphingosine inhibits lipogenesis at the glucose-uptake step in rat adipocytes. N-Acetylsphingosine and sphingomyelin, two analogs of sphingosine that are inactive on protein kinase C, did not inhibit lipogenesis induced by hGH, insulin, or phorbol 12-myristate 13-acetate. Sphingosine did not inhibit insulin binding to rat adipocytes at concentrations up to 200 microM but decreased hGH binding to its receptors by 44% at 50 microM. These data suggest a direct link between the inhibition of protein kinase C and that of lipogenesis and provide new evidence for the involvement of protein kinase C in the mechanism of action of growth hormone and insulin in rat adipocytes.

Acridine orange, an inhibitor of protein kinase C, abolishes insulin and growth hormone stimulation of lipogenesis in rat adipocytes

To determine whether protein kinase C plays a role in the actions of insulin and growth hormone in rat adipocytes, we tested the effect of acridine orange, a potent inhibitor of kinase C, on the lipogenic activity of both hormones. This compound completely inhibited the effects of insulin, growth hormone and phorbol ester 12-myristate 13-acetate, whereas 9-acridine carboxylic acid, an analog of acridine orange which does not inhibit kinase C, had no effect. Acridine orange did not act through inhibition of hormone binding. These data are consistent with the involvement of kinase C in the action of insulin and growth hormone on lipogenesis in rat fat cells.

Immunological evidence that insulin activates protein kinase C in BC3H-1 myocytes

Effects of insulin on immunoreactive protein kinase C were examined in BC3H-1 myocytes. Insulin provoked rapid dose-dependent decreases in cytosolic enzyme, and transient increases and subsequent decreases in membrane-associated enzyme. Phorbol esters provoked similar changes. Our findings suggest that insulin provokes the translocative activation of protein kinase C.

Effect of phorbol esters on the distribution and total activity of protein kinase C in the perfused rat heart

1. The perfused rat heart was treated with the tumour-promoter and protein kinase C activator, phorbol 12-myristate 13-acetate and the distribution of protein kinase C activity between cytosolic and particulate fractions determined. 2. Phorbol ester treatment led to a rapid loss of protein kinase C activity from the cytosol (t0.5 = 2 min) with a corresponding translocation into the particulate fraction. Translocated protein kinase C activity was tightly bound to the particulate fraction, could only be extracted with buffers containing 2% Triton X-100 and could therefore be misinterpreted as being down-regulated. 3. Claims of rapid down-regulation of protein kinase C activity by phorbol esters need to be supported by rigorous procedures for extraction of the particulate material.

Insulin and glucose modulate protein kinase C activity in rat adipocytes

In the presence of 1 mM glucose, insulin (10 ng/ml) increases both catalytic and receptor-binding properties of adipocyte cytosolic protein kinase C (PKC). Preincubation of adipocytes with 10 mM glucose raises basal PKC catalytic activity and prevents further stimulation of this enzyme by insulin. The effect of hyperglycemia is likely to be mediated by direct conversion of glucose into diacylglycerol. Thus, an incorporation of 14C-glucose into diacylglycerol is enhanced 10-fold in the presence of 10 mM glucose. These observations indicate that, in normal adipocytes, both insulin and glucose activate PKC; hyperglycemia eliminates the ability of insulin to stimulate this enzyme, thereby interfering with insulin action.

Evidence that phorbol ester-activated pathways are not directly involved in the action of insulin in rat adipocytes

The effects of 12-phorbol 13-myristate acetate (PMA) on glucose transport, glucose metabolism and lipolysis in rat adipocytes were examined. Alone, PMA (100 ng/ml) stimulated 2-deoxyglucose transport, glucose oxidation and lipogenesis by an amount corresponding to about 30-50% of the maximal insulin effect. The effect of PMA on the insulin-stimulated processes was additive at all insulin concentrations. Adenosine deaminase-stimulated lipolysis was enhanced by about 50% by PMA (100 ng/ml). Thus, in contrast to insulin, PMA had a lipolytic effect. The antilipolytic effect of insulin was unaffected by the presence of PMA, both ED50 and maximal inhibition of insulin were unchanged. In conclusion, it is suggested that phorbol esters may activate pathways other than those activated by insulin in rat adipocytes.

The role of protein kinase C activation in signal transmission by interleukin 2

Role of protein kinase C (PKC) in interleukin (IL) 2-induced proliferation was investigated by utilizing two murine IL 2-dependent cell lines, CT6 and CTLL-2 cell lines. CT6 cells showed a marked proliferative response to phorbol 12-myristate 13-acetate (PMA), while CTLL-2 did not. PMA induced PKC translocation from cytosol to membrane only in a PMA-responsive cell line. IL 2 failed to stimulate PKC translocation in both cell lines. H-7, a potent and specific PKC inhibitor, however, inhibited the proliferation of both cell lines induced by IL 2. Taken collectively, IL 2 may induce PKC activation without its translocation.

Phorbol esters, protein kinase C, and thyroxine 5'-deiodinase in brown adipocytes

Protein kinase C activity has been identified in the rat brown adipocyte. About 60% of this activity is found in the cytosolic fraction under basal conditions, and 12-O-tetradecanoylphorbol 13-acetate (TPA) causes a rapid shift from the cytosol to the particulate fraction. Norepinephrine and phenylephrine cause a similar redistribution that can be blocked by prazosin but not by alprenolol. alpha 1-Adrenergic agonists cause three- to fivefold stimulation of type 2 iodothyronine 5'-deiodinase activity in brown adipocytes. TPA has no effect on basal deiodinase activity and reduces the response of the enzyme to alpha 1-adrenergic agonists. These results suggest that the translocation of protein kinase C from cytosol to particulate fraction is not sufficient to increase deiodinase activity but can modulate the alpha 1-adrenergic agonist-mediated responses in these cells.

Effect of phorbol esters on the hormonal responsiveness of isolated white fat cells

Phorbol 12-myristate 13-acetate (PMA) did not alter the basal cyclic AMP accumulation or lipolysis in isolated rat fat cells. Similarly, PMA was unable to modify the cyclic AMP accumulation induced by isoprenaline and forskolin or the inhibition of cyclic AMP accumulation and lipolysis induced by prostaglandin E2 and phenylisopropiladenosine. PMA inhibited the alpha 1-adrenergic increase in the labeling of phosphatidylinositol in a dose-dependent fashion. By itself, PMA was without significant effect on the labeling of phosphatidylinositol and phosphatidylethanolamine but increased the labeling of phosphatidylcholine and its precursor phosphatidic acid.

Evidence for the presence of cytosolic inhibitors for phospholipid/Ca2+-dependent protein kinase in mastocytoma cell

1. Two kinds of C-kinase inhibitors were indicated to be present in mastocytoma cells. 2. One is a protein (mol. wt 64,000) which is weak but specific for C-kinase. 3. The other component (mol. wt 23,000) does not seem to be a protein and inhibits A-kinase as well as C-kinase.

Diacylglycerols stimulate short-circuit current across frog skin by increasing apical Na+ permeability

The phorbol ester TPA (12-O-tetradecanoylphorbol-13-acetate) stimulates baseline Na+ transport across frog skin epithelium and partially inhibits the natriferic response to vasopressin. The effects are produced largely or solely when TPA is added to the mucosal surface of the tissue. Although TPA activates protein kinase C, it has other effects, as well. Thus, the biochemical basis for the effects and the ionic events involved have been unclear. Furthermore, the physiologic implications have been obscure because of the sidedness of TPA's actions. We now report that two synthetic diacylglycerols (DAG) replicate the stimulatory and inhibitory effects of TPA on frog skin. DAG is the physiologic activator of PKC. In this tissue, it produces half-maximal stimulation at a concentration of less than or equal to 19 microM. In contrast to TPA, DAG is about equally effective from either tissue surface. In a series of eight experiments, DAG was found to depolarize the apical membrane. Diacylglycerol also increases the paracellular conductance of frog skins bathed with mucosal Cl- Ringer's solution. The latter effect can be minimized by replacing NO3- for Cl- in the mucosal solution. Under these conditions, combined intracellular and transepithelial measurements indicated that DAG increased both the apical Na+ permeability and intracellular Na+ concentration. These results are qualitatively similar to the effects of cyclic 3',5'-AMP on this tissue, suggesting that activation of PKC by DAG causes phosphorylation of the same or nearby gating sites phosphorylated by cAMP. We propose that apical Na+ entry is regulated in part by activation of PKC, and that insulin may be a physiologic trigger of this activation.

Role of kinase C in the insulin-like effects of human growth hormone in rat adipocytes

Insulin and to a smaller extent, human growth hormone (hGH), both stimulate lipogenesis in isolated rat adipocytes preincubated 4 hours in the absence of hormone. The non-additivity of maximal doses suggested that hGH may share a subset of the metabolic pathways stimulated by insulin. We explored whether kinase C may be involved in the common lipogenic effect of both hormones. The stimulation of lipogenesis by phorbol ester 12-myristate 13-acetate (PMA) (an activator of kinase C) was not additive to the stimulation by either insulin or hGH. Downregulation of kinase C resulted in a marked decrease of the maximal insulin effect (44 +/- 9%) and even more of the hGH effect (64 +/- 14%). These data suggest that kinase C either mediates part of, or modulates, the effect of insulin and hGH on lipogenesis.

Insulin increases membrane protein kinase C activity in rat diaphragm

Calcium/phospholipid-dependent protein kinase activity (protein kinase C) was identified in rat diaphragm membrane and cytosol fractions by means of in vitro phosphorylation either of histones or of a specific 87 kDa protein substrate, combined with phosphopeptide-mapping techniques. Both insulin and tumor-promoting phorbol ester treatment of the diaphragm preparations led to increased protein kinase C activity in the membrane fractions. In contrast to the phorbol ester, however, insulin did not induce a concomitant decrease in cytosolic activity, indicating that translocation of the enzyme had not taken place. Thus, insulin appears to increase specifically membrane protein kinase C activity in rat skeletal muscle, possibly through a mechanism not identical to that induced by phorbol esters.

Insulin-dependent alterations of phorbol ester binding to adipocyte subcellular constituents. Evidence for the involvement of protein kinase C in insulin action

The binding of tritiated phorbol-12,13-dibutyrate (3H-PBu2) was employed to estimate the mass of protein kinase C associated with plasma membranes and cytosol isolated from untreated and insulin-treated adipocytes. Binding of 3H-PBu2 to both plasma membranes and cytosol was rapid, achieving a steady state within minutes. Treatment of cells with physiological concentration of insulin (0.67 nM) caused a 42% increase (from 0.92 +/- 0.08 to 1.30 +/- 0.12 pmol 3H-PBu2/mg protein, p less than 0.0001) and a 27% decrease (from 0.41 +/- 0.07 to 0.30 +/- 0.05 pmol 3H-PBu2/mg protein, p less than 0.020) in phorbol ester bound to cytosol and plasma membranes, respectively. The half-maximal concentrations of unlabelled PBu2 needed to displace 3H-PBu2 bound to cytosol from control and insulin-treated cells were 54 and 13 pM, respectively. These data indicate that insulin modifies protein kinase C in adipocytes.

Insulin stimulation of iodothyronine 5'-deiodinase in rat brown adipocytes

Insulin (100-3333 microU/ml) stimulates iodothyronine 5'-deiodinase 3 to 4 fold in dispersed rat brown adipocytes. Deiodinase activity increased steadily from 1 to 4 hours. Insulin increased enzyme activity via an increase in the Vmax while the Km remained unchanged. Omission of glucose from the medium did not affect the insulin response. Studies with alpha-amanitin suggested that the increase in deiodinase activity was not due to an increase in the rate of transcription. The insulin effect was not additive to that of alpha 1-catecholamines, suggesting the two stimulators might have one or more common elements.

Protein kinase C is not required for insulin stimulation of hexose uptake in muscle cells in culture

The L6 skeletal muscle cell line has been identified as a suitable model to study the action of insulin on glucose uptake in muscle [Klip, Li & Logan (1984) Am. J. Physiol. 247, E291-E296]. The signals that transfer information from occupied insulin receptors to glucose transporters remain unknown. Here we report that activation of protein kinase C by exogenous phorbol esters results in stimulation of glucose uptake. Protein C kinase activity was induced to migrate from the cytosolic fraction to the microsomal fraction after 40 min of exposure of intact cells to 4 beta-phorbol 12,13-dibutyrate. In contrast, incubation with insulin did not alter the subcellular distribution of the kinase. Prolonged preincubation of L6 cells with phorbol esters resulted in depletion of kinase C activity, whereas neither the basal rate of glucose uptake nor its stimulation by insulin were affected. This suggests that protein kinase C is expressed in L6 cells, and that insulin stimulation of hexose transport does not involve protein kinase C.

Evidence that protein kinase C is involved in regulating glucose transport in the adipocyte

The role of protein kinase C in the mechanism of stimulation of glucose transport in rat adipocytes was investigated. Glucose transport was stimulated by dioleoylglycerol (DOG), tetradecanoyl phorbol acetate (TPA) and phospholipase C (PLC). Agents that inhibit protein kinase C (polymyxin B, gossypol and quercitin) also inhibited glucose transport that had been stimulated by DOG, TPA, PLC and insulin.