Hormone- and tumor promoter-induced activation or membrane association of protein kinase C in intact cells

Effect of phorbol ester and platelet-derived growth factor on protein kinase C in rat hepatic stellate cells

BACKGROUND/AIMS

Hepatic stellate cells (HSC) play a key role in hepatic fibrogenesis and thus, it is important to understand the intracellular signalling pathways that influence their behaviour. This study investigated the expression and regulation of protein kinase C (PKC) in HSC.

RESULTS

Western blot analysis indicates that rat HSC express at least four PKC isoforms, PKC-alpha, PKC-delta, PKC-epsilon and PKC-zeta. PKC-alpha and PKC-zeta were located predominantly in the cytosol and were redistributed to the membrane by the PKC agonist, phorbol 12-myristate 13-acetate (PMA), while PKC-delta and PKC-epsilon were highly membrane-bound and did not undergo translocation by PMA. PKC-alpha, PKC-delta and PKC-zeta were rapidly downregulated by PMA. However, PKC-epsilon was resistant to downregulation. We also examined phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS), a specific substrate of PKC, as another approach to assess activation of PKC. Platelet-derived growth factor (PDGF) and PMA increased the phosphorylation of MARCKS, suggesting that PDGF can induce PKC activation. PDGF-induced stimulation of extracellular signal-regulated kinase, phosphatidylinositol 3-kinase and p70-S6 kinase was not abrogated by downregulation of PKC-alpha, PKC-delta and PKC-zeta. Prolonged PKC inhibition did not inhibit the fibrogenic phenotype.

CONCLUSION

Multiple PKC isoforms are expressed in rat HSC and are differentially regulated by PMA. PDGF activates certain mitogenic signalling pathways independent of PKC-alpha, PKC-delta and PKC-zeta. Specific PKC isoforms may modulate different cell functions in HSC.

Impaired alpha1-adrenergic responses in aged rat hearts

To determine age-related changes in the cardiac effect of alpha1-adrenergic stimulation, both cardiomyocyte Ca2+-transient and cardiac protein kinase C (PKC) activity were measured in 3-month- (3MO) and 24-month- (24MO) old Wistar rats. Ca2+ transients obtained under 1 Hz pacing by microfluorimetry of cardiomyocyte loaded with indo-1 (405/480 nm fluorescence ratio) were compared in control conditions (Kreb's solution alone) and after alpha1-adrenergic stimulation (phenylephrine or cirazoline, an alpha1-specific agonist). PKC activity and PKC translocation index (particulate/total activity) were also assayed before and after alpha1-adrenergic stimulation. In 3MO, cirazoline induced a significant increase in Ca2+ transient for a 10(-9) M concentration which returned to control values for larger concentrations. In contrast, in 24MO, we observed a constant negative effect of cirazoline on the Ca2+ transient with a significant decrease at 10(-6) M compared with both baseline and Kreb's solution. Preliminary experiments showed that, in a dose-response curve to phenylephrine, the response of Ca2+ transient was maximal at 10(-7) M. This concentration induced a significant increase in Ca2+ transient in 3MO and a significant decrease in 24MO. The same concentration was chosen to perform PKC activity measurements under alpha1-adrenergic stimulation. In the basal state, PKC particulate activity was higher in 24MO than that in 3MO but was not different in cytosolic fractions; so that the translocation index was higher in 24MO (P < 0.01). After phenylephrine, a translocation of PKC toward the particulate fraction was observed in 3MO but not in 24MO. In conclusion, cardiac alpha1-adrenoceptor response was found to be impaired in aged hearts. The negative effect of alpha1-adrenergic stimulation on Ca2+ transient in cardiomyocytes obtained from old rats can be related to an absence of alpha1-adrenergic-induced PKC translocation.

Aging-related alterations in the distribution of Ca(2+)-dependent PKC isoforms in rabbit hippocampus

The immunocytochemical and subcellular localization of the Ca(2+)-dependent protein kinase C (cPKC) isoforms (PKCalpha, beta1, beta2, and gamma) was examined in rabbit hippocampus of young (3 months of age; n = 11) and aging (36 months of age; n = 14) subjects. Detailed immunocytochemical analyses revealed a significant increase in PKCbeta1, beta2, and gamma immunoreactivity in principal cell bodies and associated dendrites, and interneurons of the hilar region in the aging rabbits. The number of PKCalpha- and gamma-positive interneurons in the aging stratum oriens declined significantly. PKCalpha was least affected in principal cells, showing an increase in immunostaining in granule cells only. Weakly PKC-positive principal cells intermingled between densely stained ones were seen in parts of the hippocampus in most of the aging rabbits, showing that the degree of aging-related alterations in PKC-immunoreactivity varies between neurons. Changes in PKC expression in the molecular and subgranular layer of the aging dentate gyrus suggested a reorganization of PKC-positive afferents to this region. Western blot analysis revealed a significant loss of PKC in the pellet fraction for all isoforms, and a tendency for increased levels of cytosolic PKC. However, no significant changes were found in total PKC content for any PKC isoform. A concurrent dramatic loss of the PKC anchoring protein receptor for activated C kinase (RACK1) in the pellet fraction was shown by Western blotting. These findings suggest that the loss of RACK1 contributes to the dysregulation of the PKC system in the aging rabbit hippocampus. The enhanced PKC-immunoreactivity might relate to reduced protein-protein interactions of PKC with the anchoring protein RACK1 leading to increased access of the antibodies to the antigenic site. In conclusion, the results suggest that memory deficits in aging rabbits are (in part) caused by dysregulation of subcellular PKC localization in hippocampal neurons.

Role of protein kinase C in the signal pathways that link Na+/K+-ATPase to ERK1/2

We have shown before that Na(+)/K(+)-ATPase acts as a signal transducer, through protein-protein interactions, in addition to being an ion pump. Interaction of ouabain with the enzyme of the intact cells causes activation of Src, transactivation of EGFR, and activation of the Ras/ERK1/2 cascade. To determine the role of protein kinase C (PKC) in this pathway, neonatal rat cardiac myocytes were exposed to ouabain and assayed for translocation/activation of PKC from cytosolic to particulate fractions. Ouabain caused rapid and sustained stimulation of this translocation, evidenced by the assay of Ca(2+)-dependent and Ca(2+)-independent PKC activities and by the immunoblot analysis of the alpha, delta, and epsilon isoforms of PKC. Dose-dependent stimulation of PKC translocation by ouabain (1-100 microm) was accompanied by no more than 50% inhibition of Na(+)/K(+)-ATPase and doubling of [Ca(2+)](i), changes that do not affect myocyte viability and are known to be associated with positive inotropic, but not toxic, effects of ouabain in rat cardiac ventricles. Ouabain-induced activation of ERK1/2 was blocked by PKC inhibitors calphostin C and chelerythrine. An inhibitor of phosphoinositide turnover in myocytes also antagonized ouabain-induced PKC translocation and ERK1/2 activation. These and previous findings indicate that ouabain-induced activation of PKC and Ras, each linked to Na(+)/K(+)-ATPase through Src/EGFR, are both required for the activation of ERK1/2. Ouabain-induced PKC translocation and ERK1/2 activation were dependent on the presence of Ca(2+) in the medium, suggesting that the signal-transducing and ion-pumping functions of Na(+)/K(+)-ATPase cooperate in activation of these protein kinases and the resulting regulation of contractility and growth of the cardiac myocyte.

Depletion of taurine in experimental diabetic neuropathy: implications for nerve metabolic, vascular, and functional deficits

In diabetes, increased oxidative stress, disruption of signal transduction pathways, and endothelial dysfunction have been critically implicated in the pathogenesis of experimental diabetic neuropathy (EDN). The development of nerve conduction slowing in diabetes is accompanied by depletion of the beta-amino acid taurine. Since taurine functions as an antioxidant, calcium modulator, and vasodilator, taurine depletion may provide a pathogenetic link between nerve metabolic, vascular, and functional deficits complicating diabetes. The mechanism(s) of nerve taurine depletion, the localization of critical taurine deficits, and its pathophysiological significance in EDN are however unknown. This study explored the pathophysiological effects of selective nerve taurine replacement in streptozotocin-diabetic (STZ-D) rats. A polyclonal human taurine transporter (TT) antibody was also generated in order to determine potential loci of critical taurine depletion. Two weeks of STZ-D reduced sciatic motor nerve conduction velocity (NCV) by 23% (P < 0.01), decreased composite nerve blood flow by 38% (P < 0.01), and reduced nerve taurine content by 29% (P < 0.05). In STZ-D rats, a 1% taurine diet corrected nerve taurine depletion, prevented motor NCV slowing, and partially attenuated composite nerve blood flow deficits. After 6 weeks of STZ-D, a 1% taurine diet ameliorated motor NCV slowing and endoneurial nutritive blood flow deficits, prevented digital sensory NCV slowing, and reduced ouabain-sensitive nerve (Na,K)-ATPase activity. Immunohistochemical studies localized taurine and the TT to the vascular endothelium and Schwann cells of the sciatic nerve. In conclusion, taurine depletion in the vascular endothelium and Schwann cells of the sciatic nerve may contribute to the neurovascular and metabolic deficits in EDN.

Dopamine-induced translocation of protein kinase C isoforms visualized in renal epithelial cells

Short-term regulation of sodium metabolism is dependent on the modulation of the activity of sodium transporters by first and second messengers. In understanding diseases associated with sodium retention, it is necessary to identify the coupling between these messengers. We have examined whether dopamine, an important first messenger in tubular cells, activates and translocates various protein kinase C (PKC) isoforms. We used a proximal tubular-like cell line, LLCPK-1 cells, in which dopamine was found to inhibit Na(+)-K(+)-ATPase in a PKC-dependent manner. Translocation of PKC isoforms was studied with both subcellular fractionation and confocal microscopy. Both techniques revealed a dopamine-induced translocation from cytosol to plasma membrane of PKC-alpha and -epsilon, but not of PKC-delta, -gamma, and -zeta. The process of subcellular fractionation resulted in partial translocation of PKC-epsilon. This artifact was eliminated in confocal studies. Confocal imaging permitted detection of translocation within 20 s. Translocation was abolished by a phospholipase C inhibitor and by an antagonist against the dopamine 1 subtype (D(1)) but not the 2 subtype of receptor (D(2)). In conclusion, this study visualizes in renal epithelial cells a very rapid activation of the PKC-alpha and -epsilon isoforms by the D(1) receptor subtype.

1,25(OH)(2)-vitamin D(3) affects the subcellular distribution of protein kinase C isoenzymes in rat duodenum: influence of aging

We have previously shown that the steroid hormone 1, 25-dihydroxy-vitamin D(3) [1,25(OH)(2)D(3)] stimulates total cell protein kinase C (PKC) activity in rat duodenum, an effect that is severely impaired in old animals. We further examined the role of 1, 25(OH)(2)D(3) on PKC as it relates to aging by measuring hormone-induced changes in subcellular localization of PKC activity and isoenzymes in duodenal mucosae from young (three-month-old) and aged (24-month-old) rats. Short treatment of duodenum with 1, 25(OH)(2)D(3) (0.1 nM, 1 min) increased membrane-associated PKC activity, whereas it decreased the activity in the cytosol of young rats but was without significant effect in aged animals. Furthermore, the ability to translocate was present in young animals after a short treatment with the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA; 100 nM) or dioctanoyl-glycerol (50 microM), whereas the ability was absent in aged rats, suggesting that PKC function was impaired with aging independent of agonist stimulation. The expression of specific PKC isoenzymes and changes in their subcellular distribution after short exposure of the duodenum to the hormone were determined. Western blot analysis of total homogenates using antibodies to various PKC isoforms allowed detection of PKC alpha, beta, and delta. The expression of the straight theta and the zeta isoforms was in addition demonstrated by reverse transcription-polymerase chain reaction. The pattern of isoenzymes present in the duodenum was unaffected by aging. In young rats, 1, 25(OH)(2)D(3) translocates PKC alpha, beta, and delta to the membrane and nucleus; however, no translocation of PKC isoforms was observed in 24-month-old animals in response to the hormone. In summary, in rat duodenum, 1,25(OH)(2)D(3) modulation of PKC activity and isoenzyme subcellular distribution are impaired with aging and may explain age-induced alterations in the intestinal processes under the control of the hormone.

The effect of immunization with porins on gut pathophysiological response in rats infected with Salmonella typhimurium

Attachment of Salmonella typhimurium to epithelial surfaces elicit significant alterations in different cell signalling events which lead to the development of disease. The present investigation was conducted to evaluate the effect of immunization of rats with porins, on gut physiologic markers following challenge with S. typhimurium. Male albino Wistar rats were immunized with purified porins and challenged by intragastric infection with S. typhimurium. Electrolyte transport, levels of different second messengers and inflammatory mediators were studied. A net absorption of transepithelial fluxes of Na+ and Cl- in immunized-challenged group and secretion in infected group was found. Ca2+ and 3-O-methyl-D-glucose fluxes did not show any change. Significant increase in the levels of [Ca2+]i, cAMP, membrane form of protein kinase C, prostaglandins, NADPH oxidase, Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, total oxygen free radicals, reactive nitrogen intermediates, citrulline and lipid peroxidation was found in the infected group. However, in the immunized-challenged group, the values of all the parameters were found to be almost the same as that of control as well as immunized groups. Na+, K+-ATPase and calmodulin levels were unaltered in all the groups of animals. The results of this study thus suggest that immunization of rats with purified Salmonella porins followed by subsequent challenge with the organism might be helpful for the prevention of multiple physiologic derangements in isolated ileal cells.

Induction of cardiac nitric oxide synthase 2 in rats exposed to chronic hypoxia

Induction of nitric oxide synthase (NOS2, also designated as iNOS) in the heart is known to occur in response to various stimuli. It is not known, however, whether in vivo hypoxia leads to cardiac NOS2 induction. We thus investigated the effects of normobaric hypoxia (10% O(2)for 8, 15 and 21 days) on NOS2 protein expression and enzyme activity in rat right ventricle (RV) and left ventricle (LV). Chronic hypoxia induced RV hypertrophy: the RV weight to body weight ratio was increased by 45% upon 15 days of exposure, with no change thereafter and no change in left ventricular (LV) weight. Treatment of hypoxic rats with l -NAME for 1 month decreased pulmonary artery pressure and RV hypertrophy compared to hypoxic non-treated rats. NOS2 activity detected by [(3)H]l -arginine to [(3)H]l -citrulline conversion increased in RV during hypoxia, with a maximum at 15 days (+161% of control rats P<0.05), whereas it increased less (by 60%) in LV. In parallel, after 15 days of hypoxia there was a three-fold increase in NOS2 protein abundance detected by Western blotting using an isoform-specific antibody in the RVs (two-fold increase in the LV). Immunochemistry with the specific antibody demonstrated the expression in cardiomyocytes isolated from both ventricles of normoxic and hypoxic rats. Protein kinase C (PKC) content and activity was unchanged in LV of hypoxic rats, but increased in RV as compared with normoxic rats. These results clearly show that, in the heart, NOS2 is upregulated by hypoxia with an expression in cardiomyocytes of both ventricles. In addition, NOS2 is more inducible in the right hypertrophied ventricle than in the left non-hypertrophied hypoxic ventricle.

Glycated albumin stimulation of PKC-beta activity is linked to increased collagen IV in mesangial cells

Albumin modified by Amadori-glucose adducts induces coordinate increases in the expression of extracellular matrix proteins, transforming growth factor (TGF)-beta1, and the TGF-beta type II receptor in glomerular mesangial cells. Because activation of protein kinase C (PKC) accompanies the increased mesangial cell expression of matrix proteins and TGF-beta1 induced by high ambient glucose, we postulated that glycated albumin (GA) modulates PKC activity and that PKC participates in mediating the GA-induced stimulation of matrix production. To test this hypothesis, we examined the effects of PKC inhibitors on collagen type IV production by mouse or rat mesangial cells incubated with GA, and the influence of GA on PKC activity in these cells. Increased collagen type IV production evoked by GA in 5.5 and 25 mM glucose in mouse mesangial cells was prevented by both general (GF-109203X) and beta-specific (LY-379196) PKC inhibitors. Total PKC activity, measured by phosphorylation of a PKC-specific substrate, increased with time after exposure of rat mesangial cells to GA compared with the nonglycated, glucose-free counterpart. GA caused an increase in PKC-beta1 membrane-bound fraction and in total PKC activity in media containing physiological (5.5 mM) glucose concentrations in rat mesangial cells, confirming that the glucose-modified protein, and not a "hyperglycemic" milieu, was responsible. The findings indicate that Amadori-modified albumin stimulates mesangial cell PKC activity, and that activation of the PKC-beta isoform is linked to the stimulation of collagen type IV production.

Effect of ageing on the expression of protein kinase C and its activation by 1,25(OH)2-vitamin D3 in rat skeletal muscle

To characterize age-induced effects on muscle protein kinase C (PKC) and its regulation by the steroid hormone 1,25(OH)2-vitamin D3 [1,25(OH)2D3], changes in PKC activity and the expression and translocation of the specific PKC conventional isoforms alpha and beta, novel isoforms delta, epsilon, and theta and atypical isoform zeta were studied in homogenates and subcellular fractions from skeletal muscle of young (3 months) and aged (24 months) rats treated in vitro with 1,25(OH)2D3. The hormone (10(-9) M) increased total and membrane PKC activity, within 1 min, and these effects were completely blunted in muscle from aged rats. The presence of PKC isoenzymes was shown by Western blot analysis with the use of specific antibodies. The expression of PKC alpha, beta and delta was greatly diminished in old rats, whereas age-related changes were less pronounced in the isoforms epsilon, theta and zeta. After a short exposure (1 min) of muscle to 1,25(OH)2D3, increased amounts of PKC alpha and beta in muscle membranes and reverse translocation (from membrane to cytosol) of PKC epsilon were observed only in young animals. The data indicate that, in rat muscle, ageing impairs calcium-dependent PKC (alpha and beta) and calcium-independent PKC (delta, epsilon, theta and zeta) signal transduction pathways under selective regulation by 1,25(OH)2D3.

Characterization of the functional properties of smooth muscle caldesmon domain 4a: evidence for an independent inhibitory actin-tropomyosin binding domain

Recent analysis has shown the presence of three sequences in the C-terminal 170 amino acids of human caldesmon (domain 4) which are involved in actin binding and tropomyosin-dependent inhibition of actomyosin ATPase. Two are in domain 4b (amino acids 715-793) and one is in domain 4a (amino acids 636-714). In the present work we have compared recombinant peptides containing either domain 4a or domain 4b to address the question as to whether domain 4a alone has any inhibitory activity. We have produced three new recombinant fragments containing domain 4a: H10 [622-708], H12 [506-708] and H13 [622-726] and we have characterized their functional properties. All three fragments bound to actin and tropomyosin. Caldesmon, but not domain 4b, was able to displace the fragments H10, H12 and H13 from actin. Thus the isolated caldesmon domain 4a peptides bind to the same region on actin as in the whole molecule while domains 4a and 4b occupy different sites on the actin molecule. Unlike domain 4b, none of the domain 4a fragments inhibited the actomyosin ATPase in the absence of tropomyosin. However both domain 4a and 4b fragments displayed an inhibitory activity in the presence of tropomyosin. H13 and H12 were more potent inhibitors than H10. Ca2+-calmodulin bound to H13 and reversed the inhibitory activity of this fragment but did not bind to H10 and H12. We conclude that domain 4a can act as an independent inhibitory actin-tropomyosin binding domain, but its properties are very different from the extreme C-terminal domain 4b.

Protein kinase C modulation of ventilatory response to hypoxia in nucleus tractus solitarii of conscious rats

This study aimed to determine the role of protein kinase C (PKC) in signal transduction mechanisms underlying ventilatory regulation in the nucleus tractus solitarii (NTS). Microinjection of phorbol 12-myristate 13-acetate into the commissural NTS of nine chronically instrumented, unrestrained rats elicited significant cardiorespiratory enhancements that lasted for at least 4 h, whereas administration of vehicle (n = 15) or the inactive phorbol ester 4alpha-phorbol 12,13-didecanoate (n = 7) did not elicit minute ventilation (VE) changes. Peak hypoxic VE responses (10% O2-balance N2) were measured in 19 additional animals after NTS microinjection of bisindolylmaleimide (BIM) I, a selective PKC inhibitor (n = 12), BIM V (inactive analog; n = 7), or vehicle (Con; n = 19). In Con, VE increased from 139 +/- 9 to 285 +/- 26 ml/min in room air and hypoxia, respectively, and similar responses occurred after BIM V. BIM I did not affect room air VE but markedly attenuated hypoxia-induced VE increases (128 +/- 12 to 167 +/- 18 ml/min; P < 0. 02 vs. Con and BIM V). When BIM I was microinjected into the cerebellum (n = 4), cortex (n = 4), or spinal cord (n = 4), VE responses were similar to Con. Western blots of subcellular fractions of dorsocaudal brain stem lysates revealed translocation of PKCalpha, beta, gamma, delta, epsilon, and iota isoenzymes during acute hypoxia, and enhanced overall PKC activity was confirmed in the particulate fraction of dorsocaudal brain stem lysates harvested after acute hypoxia. These studies suggest that, in the adult rat, PKC activation in the NTS mediates essential components of the acute hypoxic ventilatory response.

1,25(OH)2-vitamin D3 affects the subcellular distribution of protein kinase C isoenzymes in muscle cells

Previous studies have shown the involvement of protein kinase C (PKC) in 1,25-dihydroxy-vitamin D3 [1,25(OH)2D3] regulation of DNA synthesis (long-term effect) and Ca2+ channel activity (short-term effect) in cultured myoblasts. Both events mediate stimulation of myoblast cell proliferation and growth by 1,25(OH)2D3. To characterise further the role of PKC in the hormone mode of action in muscle cells, the presence of PKC isoenzymes in chicken embryo myoblasts and changes in their total cell and subcellular levels after treatment (72 h and 5 min) with 1,25(OH)2D3 (1 nM), 12-O-tetradecanoyl phorbol 13-acetate (TPA; 100 nM) and 1,2-dioctanoyl-rac-glycerol (DOG; 50 microM) were investigated. Western blot analysis provided evidence on the expression of PKC alpha, beta and delta isoforms in avian myoblasts. Two immunoreactive bands of 80 kDa (intact molecule) and 50 kDa (catalytic fragment) were detected for each isoenzyme. 1,25(OH)2D3 and DOG, which increased myoblast PKC activity parallel with the stimulation of DNA synthesis and culture growth and the phorbol ester TPA which induced the opposite changes, exerted differential effects on PKC isoenzymes. Long-term (72 h) treatment with 1,25(OH)2D3 and DOG did not change total PKC isoform levels but decreased the 80 kDa species and increased the release of the catalytic fragment of PKC delta and beta, whereas TPA augmented the total amounts of the three PKC isoforms, increasing the band of 80 kDa of PKC beta and delta and the 50 kDa species for PKC alpha. Subcellular distribution studies showed that the 80 kDa molecule is only present in the cytosolic fraction whereas in the particulate fractions the 50 kDa fragments are detected. Increased amounts of the catalytic fragments of PKC beta and delta both in the nucleus and membranes were observed after 72 h treatment with DOG while 1,25(OH)2D3 increases PKC beta in the nucleus and PKC delta in membranes. TPA induced the appearance of the 50 kDa species of PKC alpha in the nuclear and membrane fractions. The phorbol ester also decreased the catalytic fragments of PKC beta and delta in membranes. Increased levels of PKC beta, and to a lesser extent of PKC delta, in membranes and cytosol could be detected after short exposure (5 min) of myoblasts to 1,25(OH)2D3, DOG and TPA. In conclusion, the data indicate the operation in myoblasts of PKC signal transduction pathways mediated by the Ca(2+)-dependent PKCs alpha and beta and the Ca(2+)-independent PKC delta. Moreover, the results suggest that the beta and delta isoforms of PKC could play a role in the regulation of muscle cell metabolism by 1,25(OH)2D3.

Modulatory effects of PKC activity on increased 92-kDa gelatinase secretion by neonatal alveolar macrophages

We previously demonstrated that alveolar macrophages (AMs) from neonatal rats can secrete more 92-kDa gelatinase than AMs from adult rats. In this study, we investigated the role of the protein kinase C (PKC) pathway in the transductional regulation of 92-kDa gelatinase secretion by rat AMs, and we also evaluated maturational changes in this role with increasing postnatal age. After AM stimulation by phorbol 12-myristate 13-acetate (PMA), we observed a dose-dependent increase in gelatinase secretion that was significantly more marked in AMs from 6-day-old rats than in AMs from adult rats and that was inhibited by the PKC inhibitor calphostin C. Adenosine 3',5'-cyclic monophosphate mimetics or concanavalin A failed to induce an increase in gelatinase secretion by AMs. Time-dependent variations in PKC activity after PMA stimulation differed significantly between 6-day-old rats and adult rats; PKC activity decreased in adult AMs (50%) but remained stable in 6-day-old AMs. We therefore investigated age-related differences in the intracellular proteolytic degradation of PKC, which is thought to be mediated by calpains. Leupeptin, used as a calpain inhibitor, inhibited the decrease in PKC activity after exposure of adult AMs to PMA and induced a greater than threefold increase in PMA-induced gelatinase secretion. Calpain activity was significantly lower in AM extracts from 6-day-old than from adult rats. The physiological implication of these developmental changes in 92-kDa gelatinase regulation was demonstrated by investigation of AMs from 1-day-old rats that showed a high level of spontaneous PKC-dependent gelatinase secretion coexisting with very low calpain activity. We conclude that sustained PKC activity is a key factor in the increased gelatinase secretion by AMs seen during the postnatal period and is due, at least in part, to reduced PKC degradation.

Effects of interferon and PKC modulators on human glioma protein kinase C, cell proliferation, and cell cycle

The in-vitro effects of human interferon alpha-2b (HuIFN alpha-2b), protein kinase C (PKC) agonist [TPA (12-0-tetra-decanoyl-phorbol-13-acetate)] and PKC inhibitor (calphostin C) on human glioma (U-373 MG) PKC activity, cell proliferation and cell cycle were compared. HuIFN alpha-2b and TPA increased PKC activity, elevated the number of cells in DNA synthesis (S) phase and decreased cell proliferation by similar magnitudes. Calphostin C inhibited PKC activity, increased the number of cells in S phase and produced strong cytotoxic effects (IC50 150 nM). Higher concentrations of calphostin C with or without serum induced an additional block in gap2 and mitosis. We conclude that HuIFN alpha-2b's mode of action may be directly or indirectly affecting PKC. The response produced by HuIFN alpha-2b is similar to TPA (potent PKC activation and S phase arrest).

Increased flux through the hexosamine biosynthesis pathway inhibits glucose transport acutely by activation of protein kinase C

The hexosamine biosynthesis pathway and protein kinase C (PKC) activation mediate hyperglycaemia-induced impaired glucose transport, but the relative role of each pathway is unknown. Following a 2 h preincubation of rat adipocytes in the presence of either high glucose (30 mM) plus insulin (0.7 nM) or glucosamine (3 mM), both high glucose and glucosamine inhibited subsequent basal and insulin-stimulated glucose transport, measured at 5.0 mM glucose. Azaserine, an inhibitor of the enzyme glutamine:fructose-6-phosphate aminotransferase, abolished the effect of high glucose, but not that of glucosamine. Ro-31-8220, an inhibitor of PKC, reversed the effects of both high glucose and glucosamine, suggesting that flux through the hexosamine biosynthesis pathway impaired glucose transport acutely by activating PKC. Both high glucose and glucosamine caused a 3-fold increase in PKC activity; this effect of high glucose, but not that of glucosamine, was partially decreased by azaserine. Neither high glucose nor glucosamine altered basal or insulin-stimulated plasma membrane GLUT1 levels, whereas both treatments decreased basal, but not insulin-stimulated, GLUT4 levels. Azaserine abolished the effect of high glucose, but not that of glucosamine, on basal plasma membrane GLUT4 levels. Ro-31-8220, which returned glucose transport to control values, caused a further decrease in plasma membrane GLUT4 levels. It is concluded that, in rat adipocytes, an acute increase in flux through the hexosamine biosynthesis pathway inhibits glucose transport by activation of PKC.

rho, a small GTP-binding protein, is essential for Shigella invasion of epithelial cells

Shigella, the causative agents of bacillary dysentery, are capable of invading mammalian cells that are not normally phagocytic. Uptake of bacteria by the mammalian cells is directed by bacterial factors named IpaB, IpaC, and IpaD invasins, in which Ipa invasins secreted into the bacterial environment can interact with alpha5beta1 integrin. We report here that Shigella invasion of epithelial cells requires rho activity, a ras-related GTP-binding protein. The invasive capacity of Shigella flexneri for Chinese hamister ovary (CHO) cells and other epithelial cells were greatly reduced when treated with Clostridium botulinum exoenzyme C3 transferase. Conversely, uptake of bacteria by CHO cells was promoted upon microinjection of an activated rho variant, Val14RhoA. Attachment of S. flexneri to CHO cells can elicit tyrosine phosphorylation of pp125FAK and paxillin, localized accumulation of F-actin, vinculin, and talin, and activation of protein kinase C, which were all blocked by the treatment with C3 transferase. Our results indicate that cellular signal transduction regulated by rho is essential for Shigella invasion of epithelial cells.

gamma Isoform-selective changes in PKC immunoreactivity after trace eyeblink conditioning in the rabbit hippocampus

An immunocytochemical examination of the rabbit hippocampus was done to determine which of the Ca(2+)-dependent protein kinase C (PKC) isoforms (PKC alpha, -beta I, -beta II, or -gamma) are involved in associative learning. The hippocampally dependent trace eyeblink conditioning task was used for behavioral training, and pseudoconditioned and naive animals served as controls. Significant increases (P < 0.05) in staining intensity were found with antibodies reactive with the catalytic or the regulatory domain of PKC gamma in conditioned animals compared with naive and pseudoconditioned subjects at a 24-h post-conditioning time point. The increase was found in CA1 and CA3 pyramidal cell bodies, in apical dendrites and the proximal part of the basilar dendrites, and in cell bodies of dentate granule cells. In contrast, no conditioning-specific changes were found for PKC alpha, -beta I, or -beta II in in hippocampal neurons. The increase in PKC gamma immunoreactivity (ir) was significantly less (P < 0.05) in poor learners than in good learners. The correlation between the degree of PKC gamma-ir and the total number of conditioned responses across training sessions was both positive and significant. These results suggest that PKC gamma is the major Ca(2+)-dependent PKC isoform involved in hippocampal neurons during acquisition of associative memories. Immunoblots revealed no conditioning-induced increase in the total amount or translocation of PKC gamma at the 24-h time point, and no proteolytic PKC fragments were observed. In agreement with the Western blot data, PKC activity did not differ among naive, pseudoconditioned, and trace conditioned animals. The conditioning-induced increase in antibody binding to the gamma-isoform must therefore be due to an increased access to the antigenic site(s) as a result of alteration in the tertiary structure of PKC gamma or in quaternary interactions of PKC gamma in situ.

Characterization of calcium-dependent forms of protein kinase C in adult rat ventricular myocytes

The presence and subcellular localization of the Ca2+-dependent protein kinase C (PKC) isoforms alpha and beta were investigated in freshly isolated adult rat cardiac ventricular myocytes. PKC activity was measured in cytosolic and particulate fractions prepared from control myocytes and those treated with either phorbol ester (phorbol 12-myristate 13-acetate, PMA) or a permeant synthetic diacylglycerol analog (1-oleoyl-2-acetylglycerol, OAG) in the absence or presence of an inhibitor of diacylglycerol kinase activity, compound R59022. Preliminary studies detected no Ca2+-/phospholipid-dependent histone kinase activity in either subcellular fraction. To reproducibly observe Ca2+-/phospholipid-dependent protein kinase activity, partial purification using a MonoQ HR 5/5 column and the presence of the peptide inhibitor of the cAMP-dependent protein kinase were essential. MonoQ chromatography of cytosolic and particulate fractions resulted in three peaks of Ca2+/phospholipid-dependent protein kinase activity. In the cytosolic fraction a large peak of activity eluted at 230-300 mM NaCl. Isoform-specific antisera indicated both PKC alpha and PKC beta were present. In the particulate fraction two peaks of Ca2+-/phospholipid-dependent protein kinase activity, both containing PKCa immunoreactivity, were observed. The larger peak eluted at 230-300 mM NaCl. In addition, a peak eluting at lower salt concentrations contained a Ca2+-/phospholipid-independent histone kinase activity. This peak of kinase activity contained PKC alpha immunoreactive bands of 80- and 50-kDa. The 80-kDa band was the holoenzyme of PKC alpha whereas the band of lower molecular mass was likely a proteolytic fragment. In both cytosolic and particulate fractions, the peak of kinase activity eluting at 230-300 mM NaCl contained PKC alpha in the form of an 80-kDa doublet; this suggested the presence of autophosphorylated PKC. Incubation of the myocytes with PMA, but not OAG, resulted in translocation of PKC from the cytosolic to the particulate fraction. Curiously, a transient decrease in PKC activity was observed in both subcellular fractions following treatment with either OAG or ethanol (1%). Results from this study show that freshly isolated adult rat cardiac ventricular myocytes contain both PKC alpha and PKC beta, and that these isoforms translocate to the particulate fraction in response to treatment with PMA, but not OAG.

Inhibition of protein kinase C and proto-oncogene expression by crocetin in NIH/3T3 cells

Crocetin, a carotenoid isolated from the seeds of Gardenia jasminoides, was found to be a potent inhibitor of tumor promotion induced by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse skin. When mouse fibroblast NIH/3T3 cells were treated with TPA alone, protein kinase C (PKC) translocated from the cytosolic fraction to the particulate fraction. Pretreatment with 60 and 120 microM crocetin for 15 min inhibited the TPA-induced PKC activity in the particulate fraction by 50% and 66%, respectively, but did not affect the level of PKC protein. Crocetin also reduced the level of TPA-stimulated phosphorylation of cellular proteins. Cells pretreated with crocetin (120 microM) had 55% less PKC [3H]phorbol dibutyrate-binding capacity. Suppression of TPA (100 ng/mL)-induced c-jun and c-fos gene expression was also observed in the mouse fibroblast cells pretreated with crocetin (30, 60, and 120 microM). Our results provided a basis for understanding the inhibitory effect of crocetin on TPA-mediated tumor promotion.

Protein kinase C isoform expression in normal and failing rabbit hearts

Protein kinase C (PKC) is activated by alpha-adrenergic stimulation. Molecular analysis showed that PKC consists of a family of at least 12 isozymes. Studies of their distribution in the heart showed conflicting results. The first goal of our study was thus to characterize cardiac PKC in normal rabbits. PKC plays an important role in gene expression, cell growth, and differentiation and is involved in the hypertrophy phase of cardiac overload, but since its expression has never been evaluated in heart failure, the second goal of our study was to evaluate PKC activity and isoform expression in rabbits with heart failure induced by a double hemodynamic overload (aortic insufficiency followed by an aortic stenosis). In the first part of the study, PKC isoform expression analyzed in normal rabbits by immunoblotting showed that isoforms alpha, beta, epsilon, and zeta were expressed along with PKC gamma, which had never been detected in the heart. PKC gamma expression was also identified by polymerase chain reaction, and immunofluorescence techniques showed a localization on intercalated disks associated with the membrane localization observed with the other isoforms. In the second part of the study, PKC activity, content, and isoform expression showed a decrease of 37% in the failing group. PKC immunodetection with a monoclonal antibody (Mab 1.9) recognizing the catalytic domain of all PKC isoforms revealed a 20% decrease in the failing ventricles compared with normal left ventricles. Expressed PKC isoforms quantified by Western blot showed, in the failing heart group compared with the control group, a decrease of 27%, 32%, 16%, and 9% of PKC alpha, PKC beta 1, PKC gamma, and PKC epsilon, respectively, whereas PKC zeta was not significantly modified. These results show that, in heart failure, PKC activity and expression of Ca(2+)-dependent PKC isoforms are decreased. This may lead to alterations of PKC-induced phosphorylations.

Regulatory domain of human protein kinase C alpha dominantly inhibits protein kinase C beta-I-regulated growth and morphology in Saccharomyces cerevisiae

This study demonstrates that the isolated regulatory (R) domain (amino acids 1-270) of human protein kinase C alpha (PKC alpha) is a potent inhibitor of PKC beta-I activity in a yeast expression system. The PKC alpha R domain fused to glutathione-S-transferase competitively inhibited the activity of yeast-expressed rat PKC beta-I in vitro (Ki = 0.2 microns) and was 400-fold more potent than a synthetic pseudosubstrate peptide corresponding to amino acids 19-36 from PKC alpha. In contrast, the fusion protein did not affect the activity of the purified catalytic subunit of cAMP-dependent protein kinase. The PKC alpha R domain (without glutathione-S-transferase [GST]) also was tested for its ability to inhibit PKC beta-I activity in vivo, in a yeast strain expressing rat PKC beta-I. Upon treatment with a PKC-activating phorbol ester, yeast cells expressing rat PKC beta-I were growth-inhibited and a fraction of the cells appeared as long chains. Coexpression of the R domain with rat PKC beta-I blocked the phorbol ester-induced inhibition of yeast cell growth and the phorbol ester-dependent alterations in yeast cell morphology. These results indicate that the R domain of PKC alpha acts as a dominant inhibitor of PKC activity in vivo and thus provides a useful genetic tool to assess the roles of PKC in various signal transduction processes.

Stimulation of protein kinase C redistribution and inhibition of leukotriene B4-induced inositol 1,4,5-trisphosphate generation in human neutrophils by lipoxin A4

1. To test the hypothesis that protein kinase C (PKC) is involved in the inhibitory actions of lipoxin A4 (LXA4) on second messenger generation, we studied the effects of LXA4 on PKC in human neutrophils and on leukotriene B4 (LTB4)-stimulated inositol-1,4,5-trisphosphate (Ins(1,4,5)P3) generation. 2. LXA4, 1 microM, caused a fall in cytosolic PKC-dependent histone phosphorylating activity to 23.5% of basal levels. 3. LXA4, caused an increase in particulate PKC-dependent histone phosphorylating activity with a bell-shaped dose-response fashion; maximal stimulation was observed at 10 nM LXA4. 4. Western blot analysis with affinity-purified antibodies to alpha- and beta-PKC showed that only the beta-PKC isotype was translocated by LXA4. 5. LXA4 inhibited LTB4-stimulated Ins(1,4,5)P3 generation in a bell-shaped fashion with maximal inhibition at 1 nM LXA4. The observed inhibition was dose-dependently removed by pre-incubation with a PKC inhibitor (Ro-31-8220). 6. These results show that LXA4 activates PKC in whole cells and supports a role for PKC activation in the inhibitory action of LXA4 on LTB4-induced Ins(1,4,5)P3 generation. 7. LXA4 (1-1000 nM) pre-incubation did not affect specific binding of [3H]-LTB4 to neutrophils. Thus, the inhibitory effect of LXA4 on LTB4-stimulated Ins(1,4,5)P3 generation could not be attributed to an effect on LTB4 receptors.

Modulation of basal nitric oxide-dependent cyclic-GMP production by ambient glucose, myo-inositol, and protein kinase C in SH-SY5Y human neuroblastoma cells

Defective tissue perfusion and nitric oxide production and altered myo-inositol metabolism and protein kinase C activation have been invoked in the pathogenesis of diabetic complications including neuropathy. The precise cellular compartmentalization and mechanistic interrelationships of these abnormalities remain obscure, and nitric oxide possesses both neurotransmitter and vasodilator activity. Therefore the effects of ambient glucose and myo-inositol on nitric oxide-dependent cGMP production and protein kinase C activity were studied in SH-SY5Y human neuroblastoma cells, a cell culture model for peripheral cholinergic neurons. D-Glucose lowered cellular myo-inositol content, phosphatidylinositol synthesis, and phosphorylation of an endogenous protein kinase C substrate, and specifically reduced nitric oxide-dependent cGMP production a time- and dose-dependent manner with an apparent IC50 of approximately 30 mM. The near maximal decrease in cGMP induced by 50 mM D-glucose was corrected by the addition of protein kinase C agonists or 500 microM myo-inositol to the culture medium, and was reproduced by protein kinase C inhibition or downregulation, or by myo-inositol deficient medium. Sodium nitroprusside increased cGMP in a dose-dependent fashion, with low concentrations (1 microM) counteracting the effects of 50 mM D-glucose or protein kinase C inhibition. The demonstration that elevated D-glucose diminishes basal nitric oxide-dependent cGMP production by myo-inositol depletion and protein kinase C inhibition in peripheral cholinergic neurons provides a potential metabolic basis for impaired nitric oxide production, nerve blood flow, and nerve impulse conduction in diabetes.

Dysregulation of signal transduction pathways as a potential mechanism of nervous system alterations in HIV-1 gp120 transgenic mice and humans with HIV-1 encephalitis

HIV-1 associated central nervous system (CNS) disease involves neuronal damage and prominent reactive astrocytosis, the latter characterized by strong upregulation of the glial fibrillary acidic protein (GFAP) in astrocytes. Similar alterations are found in transgenic mice expressing the HIV-1 envelope protein gp120 in the CNS. Because alterations of astrocyte functions could contribute to neuronal impairment, we compared brains of gp120 transgenic mice and gp120-transfected C6 astrocytoma cells with controls and found that gp120 induced a prominent elevation of steady state GFAP mRNA levels, primarily due to transcript stabilization. Increased levels of GFAP mRNA were also found in nontransfected C6 cells exposed to recombinant gp120. Exposure of C6 cells or primary mouse astrocytes to soluble gp120 led to activation of PKC as indicated by redistribution and increase in PKC immunoreactivity at the single cell level. gp120 effects were diminished by inhibitors of protein kinase C (PKC) but not inhibitors of protein kinase A. PKC activity was upmodulated in gp120-transfected C6 cells and in the CNS of gp120 transgenic mice. Further, brain tissue from patients with HIV-1 encephalitis and from gp120 transgenic mice showed increased PKC immunoreactivity. Taken together, these results indicate that gp120-induced increases in PKC activity may contribute to the gliosis seen in gp120 transgenic mice as well as in HIV-1-infected humans and raise the question of whether dysregulation of signal transduction pathways represents a general mechanism of HIV-associated pathogenesis.

Inhibition of mitogenesis and c-fos induction in mesangial cells by heparin and heparan sulfates

When rat renal mesangial cells (RMC) or vascular smooth muscle cells are released from quiescence by serum stimulation they express c-fos mRNA transiently at 30 to 60 minutes and progress in synchrony to S phase. Heparin causes significant suppression of [3H]-thymidine incorporation into DNA in S phase and a decrease and delay of entry of cells into S/G2. Added at the time of serum stimulation, heparin (1 microgram/ml or less) causes a decrease in the subsequent expression of c-fos mRNA in RMC, and a similar effect is observed with heparan sulfate chains isolated from RMC-cultures themselves. Although these cells internalize and degrade heparin, the timing of the maximal effect indicates an extracellular action of heparin. In keeping with this idea, 125I-heparin binds specifically to a single class of high affinity sites on the cell surface. The effect of heparin on c-fos induction may be independent of interaction with cytokines or cytokine receptors; its magnitude is not diminished when heparin-binding substances are removed from serum by heparin-Sepharose. Furthermore, direct activation of protein kinase C (PKC) with a phorbol ester in the absence of serum likewise induces c-fos and 1 microgram/ml heparin inhibits this response by 65%. Phorbol ester caused an increase in the proportion of histone H1-active PKC associated with the cell membrane fraction, from approximately 25% to 70% of total activity. Heparin affected neither the total activity of the kinase nor the proportion associated with the membrane. When PKC was inhibited with staurosporine, only very low levels of c-fos were induced by serum. We conclude that low concentrations of heparin and heparan sulfate suppress the mitogenic response of mesangial cells to serum and inhibit c-fos mRNA induction through an effect of cell surface-bound glycosaminoglycan on a signalling pathway downstream of PKC.

Pronounced activation of protein kinase C, ornithine decarboxylase and c-jun proto-oncogene by paraquat-generated active oxygen species in WI-38 human lung cells

Paraquat (methyl viologen, PQ) is a widely used herbicide that produces oxygen-derived free radicals and severely injures human lungs. In this study we examined the effects of PQ on the protein kinase C (PKC), ornithine decarboxylase (ODC) and c-jun oncogene expression in WI-38 human lung cells. Exposure of cells to 25-200 microM PQ resulted in an increase of [3H]phorbol dibutyrate (PDBu) binding and PKC redistribution in a dose-dependent manner. Interestingly, a superoxide dismutase mimic, 4-hydroxyl-2,2,6,6-tetramethyl-piperidine-1-oxyl (Tempol, 2.5 mM) and catalase (400 micrograms/ml) could significantly reduce the PQ-stimulated increase of phorbol ester binding and particular PKC phosphorylating activity, but dimethylsulfoxide (DMSO, 1.5%), an effective .OH trapping agent, failed to prevent this stimulation. In addition, an endogenous substrate of PKC, 80 kDa protein, was found to be highly phosphorylated in intact WI-38 cells treated with 50 microM PQ. The increase of phosphorylated proteins could be completely or partly abolished by Tempol or catalase, but only the phosphorylation of 80 kDa protein was diminished by protein kinase C inhibitor, 1-(5-isoquinolinyl-sulfonyl)-2-methylpiperazine (H-7). A maximal peak of ODC activity was observed at 6 h of treatment with 50 microM PQ. PQ induced activity was reduced at the following rates, Tempol 85%, DMSO 80% and catalase 45%, but H-7 failed to do so. Furthermore, we found that the level of c-jun mRNA was transiently increased by PQ and the peak appeared at 1 h of treatment. When correlated with the PKC result, Tempol, catalase and H-7 all effectively blocked PQ-elicited c-jun transcript expression, but DMSO only exhibited a weakly inhibitory effect. We therefore propose that superoxide anion (O2- and H2O2 generated by PQ could activate PKC and lead to induction of c-jun gene expression; on the other hand, O2- and .OH might trigger other kinase pathways to elevate ODC activity. Finally, the sequential expression of c-jun oncogene and ODC may cooperate to relieve the oxidative damages elicited by PQ.

Sphinganine potentiation of cellular differentiation induced by various anti-leukemia drugs in human leukemia cell line HL-60

A slight induction of cellular differentiation (myelocytes and granulocytes) of HL-60 cells occurred after treatment with anti-tumor agents etoposide (VP-16), mitoxantrone (MXT), mitomycin C (MMC), actinomycin D (Act-D) or novobiocin (NOVO). Addition of sphinganine (SP), an inhibitor of protein kinase C (PKC) enhanced (2-3 fold) the VP-16, MXT, MMC or Act-D-induced differentiation but not the NOVO-induced differentiation. No induction of differentiation was observed with 5-fluorouracil (5-FU) in the absence or presence of SP. The addition of SP in the fresh medium after the removal of VP-16, MXT, or MMC (0.5 h treatment) enhanced the induction of differentiation. In contrast, SP post-treatment did not have any effect on enhancing the differentiation which was induced by Act-D short exposure (0.5 h). In an attempt to characterize the biochemical requirements for potentiation of VP-16-induced differentiation, we examined the effects of calcium depletion using calcium chelator ethylene glycol-bis(beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) or calcium channel blocker verapamil. Potentiation of VP-16-induced differentiation by SP was not observed in EGTA- or verapamil-treated cells. Calcium supplementation to the cells during the treatment with EGTA restored the SP-potentiation of VP-16-induced differentiation. Our results also showed that the induction of differentiation was accompanied by a decrease in PKC activity (70% of the control). PKC activity decreased to a greater extent (50% of control) in SP potentiation of differentiation induction. Our results suggested that calcium-dependent biological action of antitumor agents and the inhibition of PKC activity are required for SP-potentiation of differentiation induction.

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

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

PGF2 alpha activation of Na/H antiporter and ammoniagenesis in parent/variant LLC-PK1 cells

A novel variant of the LLC-PK1 cell line was used to examine directly the mechanism whereby PGF2 alpha and TPA inhibit renal ammoniagenesis. The variant cells, which exhibit a growth pattern and morphology similar to the parent cell line, were isolated by a self selection process utilizing long-term cultures of parent cells maintained under conditions of continuous gentle rocking of the media fluid. Incubation of both parent and variant LLC-PK1 cells for one hour in a glutamine supplemented Krebs-Hensleit media of low pH (pH 6.8) increased ammonia and alanine production in comparison to the basal rates at pH 7.4. The phorbol ester TPA and also PGF2 alpha inhibited the low pH-induced increases in ammonia and alanine formation in parent cells; however, neither TPA nor PGF2 alpha inhibited ammonia or alanine metabolism in variant cells. TPA and PGF2 alpha activated PKC similarly in the parent and variant cells as demonstrated by a significant increase in membrane bound enzyme activity. BCECF labeling of cells indicated that the parent and variant cells possess an amiloride sensitive Na+/H+ antiporter of comparable activity. Exposure of parent cells to PGF2 alpha or TPA resulted in the activation of Na+/H+ antiporter activity. By contrast, neither compound stimulated antiporter activity in variant cells. These studies strongly suggest that PKC mediated activation of the Na+/H+ antiporter accounts for the inhibition of ammonia production produced by both PGF2 alpha and TPA. In addition, this novel variant of LLC-PK1 cells should provide a valuable tool to investigate various normal and pathophysiological functions involving mediation by PKC and/or Na+/H+ antiporter activity.

Regulation of melanogenesis induced by 5-methoxypsoralen without ultraviolet light in murine melanoma cells

Melanogenesis in melanoma cells can be enhanced by psoralens in the absence of UV light. Melanin biosynthesis is regulated by a number of melanocyte-specific proteins, including tyrosinase, DOPAchrome tautomerase (DCT), and tyrosinase-related protein-1 (TRP-1, gp75). To get more insight on the molecular mechanisms involved in psoralens-induced melanogenesis, we determined tyrosinase and DCT activities as well as mRNA and protein levels of tyrosinase, DCT, and TRP-1 in S91 mouse melanoma cells treated by 5-MOP. High concentration of 5-MOP (5 x 10(-5) M) induced a time-dependent increase of tyrosinase activity and melanin content, which was correlated to an increase of both mRNA and protein levels of tyrosinase. These results demonstrate that the 5-MOP stimulation of melanogenesis is related to increased tyrosinase synthesis. In addition, 5-MOP stimulated TRP-1 synthesis and induced a dose-dependent decrease of DCT activity without any modification in the expression of the protein. We explored then the signalling pathways involved in 5-MOP-induced melanogenesis and, particularly, the role of cyclic AMP and protein kinase C (PKC). A small stimulation of cyclic AMP production was observed in presence of 5-MOP. Furthermore, 1-oleoyl-2-acetylglycerol (OAG), a PKC activator, potentiated the 5-MOP stimulation of tyrosinase activity, while calphostin, a specific PKC inhibitor, inhibited the 5-MOP induction of tyrosinase activity. Phorbol-myristate acetate (PMA), described as a strong activator of PKC, inhibited also the effect of 5-MOP when used at long term. Taken together, these results demonstrate that in murine melanoma cells 5-MOP stimulates melanogenesis by increasing activity and synthesis of tyrosinase. Tyrosinase and TRP-1 expression are coordinately regulated by 5-MOP. Furthermore, a negative correlation between melanogenesis and DCT activity was observed under 5-MOP stimulation. At least, PKA and PKC systems appear to play an important role in the melanogenic effect of 5-MOP.

Translocation of protein kinase Cs in Aplysia neurons: evidence for complex regulation

We examined the activation of protein kinase C (PKC) produced by phorbol esters in Aplysia nervous tissue. Translocation of PKC in intact ganglia requires higher concentrations of phorbol esters than would be expected from: (1) their affinity for Aplysia PKCs measured in vitro; (2) their physiological effects on cultured Aplysia neurons; and (3) their actions on PKC in synaptosomes. Although phorbol esters enter intact ganglia slowly, delayed access to neurons is insufficient to account for the high concentrations needed for translocation. Increasing accessibility to the neural components of ganglia increases the rate at which translocation occurs, but does not affect the concentration of phorbol ester required. We suggest that this might best be explained by the presence of a competitive inhibitor at the binding site for phorbol esters in PKC. An indication for an inhibitor is that the concentration of phorbol esters needed for translocation in homogenates of nervous tissue is markedly decreased by diluting the extract. Preliminary characterization shows that the inhibitory activity is unusual: in addition to being competitive with lipid activators, it is soluble and tissue-specific. This type of inhibitor may be an important regulator of protein phosphorylation by PKC in neurons.

Regulation of endothelin-induced Ca2+ mobilization in smooth muscle cells by protein kinase C

We have investigated the role of protein kinase C (PKC) in regulating vascular smooth muscle cell responses to endothelin (ET). During the initial phase of the response, ET stimulated rapid formation of diacylglycerol due to rapid and transient activation of phosphatidyl inositol-specific phospholipase C and to rapid and prolonged activation of phospholipase D. Concurrently, ET stimulated translocation of PKC activity that reached a peak at 1 min and remained elevated for at least 20 min. Activation of PKC produced early inhibitory effects. Treatment of cells with phorbol 12-myristate 13-acetate (PMA) 5 min before stimulation with ET inhibited total inositol phosphate formation by > 50%. Because each inositol phosphate isomer was equally affected, the target appears to be either phospholipase C or some upstream component of the receptor coupling mechanism. Activation of PKC was important for sustained response to ET. Treatment of cells with staurosporine significantly reduced sustained elevation of cytosolic free Ca2+ concentration ([Ca2+]i) normally seen with ET. We had previously shown that sustained elevation of [Ca2+]i initiated by ET was due to continued activity of L-type Ca2+ channels. Our current data suggest that PKC is important in this response. For example, staurosporine inhibited both ET-induced 45Ca2+ and Mn2+ entry occurring 10 min after stimulation of influx mechanisms by the agonist. Similarly, pretreatment of cells for 18 h with phorbol dibutyrate depleted the cells of PKC and blocked the sustained activity of Ca2+ entry mechanisms stimulated by ET. Finally, PMA initiated a slowly developing, sustained elevation of [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Bidirectional modulation of parathyroid hormone-responsive adenylyl cyclase by protein kinase C

The temporal pattern with which phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), modulates parathyroid hormone (PTH)-responsive adenylyl cyclase (AC) was evaluated in a clonal osteoblast-like cell line (UMR-106). Brief (< or = 1 h) exposure of UMR-106 cells to PMA enhanced PTH stimulation of AC, whereas more prolonged PMA treatment decreased the PTH response, with maximum inhibition occurring at < or = 6 h. PMA treatment also resulted in initial activation followed by downregulation of PKC. Exposure of cells to 1,2-dioctanoyl-sn-glycerol, which activated but did not downregulate PKC, resulted in bidirectional modulation of PTH-responsive AC identical to that produced by PMA. Prolonged PMA exposure decreased PTH receptor number, as determined by radioligand binding studies, and reduced PTH receptor mRNA levels, assessed by Northern blot analysis. Forskolin activation of the catalytic subunit of AC was also decreased after prolonged PMA treatment. The results suggest that activation of PKC sequentially stimulates and then inhibits PTH responsiveness. Inhibition of the PTH response occurs by PKC actions exerted on the PTH receptor and the AC catalytic subunit.

Polyunsaturated fatty acids do not activate protein kinase C in the testis of the goldfish (Carassius auratus)

Earlier studies have established that polyunsaturated fatty acids (PUFA) such as eicosapentaenoic acid and docosahexaenoic acid inhibit steroid production in the goldfish testis. As PUFA inhibit testicular steroidogenesis in the rat through activation of protein kinase C (PKC), the present studies were undertaken to characterize the properties of PKC in the goldfish testis and to test the effects of selected PUFA on PKC activity. PKC activity was quantified in goldfish testis homogenate following partial purification by DEAE-cellulose chromatography by determining the transfer of radiolabelled phosphate from [γ - (32)P]ATP to histone III-S. Testicular PKC activity was defined by the amount of protein phosphorylation in the presence of phosphatidylserine, phasphatidylcholine, Ca(2+) ions and diolein (a 1,2-diacylglycerol analog) above that obtained in response to Ca(2+) ions alone. Western blot analysis of a crude testis homogenate using an antibody specific to the α and β isoforms of mammalian PKC led to the identification a single band of protein (80 kD) that co-migrated with PKC from rabbit brain cytosol. Addition of arachidonic, eicosapentaenoic or docosahexaenoic acids failed to activate PKC. However, PKC activity stimulated by phospholipid, Ca(2+) ions and diolein was inhibited in a dose related fashion by all of these fatty acids. These studies suggest that the inhibitory effects of EPA and DHA on testicular steroidogenesis are not mediated by activation of PKC. The lack of effect of PUFA on PKC activity in the goldfish testis suggests that either the distribution of PKC isoforms differs between the testis of mammals and fish or that PKC is not activated by PUFA in the goldfish.

Muscarinic receptor-mediated increase in zeta-PKC expression in SK-N-SH human neuroblastoma cells

Anti-peptide antibodies specific for each protein kinase C (PKC) isozyme were used to screen SK-N-SH human neuroblastoma cells. These cells were found to express only alpha- and zeta-PKC. Stimulation of these cells with phorbol esters caused alpha- but not zeta-PKC to translocate from cytosolic to membrane fractions. Stimulation of these cells with carbachol, which releases inositol trisphosphate and diacylglycerol, caused a transient translocation of alpha-PKC but not of zeta-PKC. Carbachol did, however, cause a gradual increase in immunoreactive zeta-PKC which reached maximal values 10-20 min after stimulation. These results implicate zeta-PKC in a receptor-mediated signalling event.

Persistent activation of protein kinase C during the development of long-term facilitation in Aplysia

We investigated activation of the two major neuronal protein kinase C (PKC) isoforms in Aplysia, Ca(2+)-activated Apl I and Ca(2+)-independent Apl II, during the induction and maintenance of behavioral sensitization of Aplysia defensive reflexes. Activation of PKC occurred during the training stimulus and persisted for at least 2 hr thereafter but was not maintained for 24 hr. The persistent activation required protein synthesis and was blocked by cyproheptidine, an agent that also blocked the initial activation of PKC. Persistent activation involved both an increase in membrane-associated Apl I and an increase in an autonomous kinase activity that may be related to a post-translational modification of Apl II. These results are consistent with the hypothesis that in addition to its role in producing the presynaptic facilitation of mechanosensory-motor neuron synapses that underlie short-term facilitation, PKC is needed for maintaining synaptic changes in an intermediate period that precedes the modifications accompanying consolidation of memory.

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.

Effect of modulation of protein kinase C activity on cisplatin cytotoxicity in cisplatin-resistant and cisplatin-sensitive human osteosarcoma cells

The effect of modulation of protein kinase C (PKC) activity by 12-O-tetradecanoylphorbol-13-acetate (TPA) on cisplatin cytotoxicity was examined in a human osteosarcoma U2-OS cell line and in a U2-OS variant (U2-OS/Pt) selected after continuous exposure to increasing concentrations of cisplatin. U2-OS/Pt cells showed a 7.5-fold resistance to the drug. A 24 h exposure of cells to TPA caused a potentiation of cisplatin cytotoxicity in sensitive and in resistant cells; under these conditions, PKC activity was shown to be down-regulated. In contrast, a short-term exposure of cells to TPA did not affect cisplatin cytotoxicity in U2-OS or in U2-OS/Pt cells. These results support the involvement of PKC in cellular response to cisplatin. However, this enzyme is probably not directly implicated in the mechanisms of acquired resistance in this cell system.

Alkylcatechols regulate NGF gene expression in astroglial cells via both protein kinase C- and cAMP-independent mechanisms

It has been previously demonstrated that, in mouse astroglial cells and fibroblast cells in culture, alkylcatechols cause a rapid increase in the nerve growth factor (NGF) mRNA level followed by a marked increase in the amount of NGF released into the medium. To understand the mechanism of this alkylcatechol effect on NGF gene expression in astroglial cells, we examined the effects of protein kinases that influence intracellular signal transduction and of their inhibitors. The reagents to increase the intracellular content of cyclic AMP (cAMP) such as dibutyryl cyclic AMP (Bt2cAMP), forskolin, or cholera toxin did not mimic alkylcatechol induction of NGF gene expression. Phorbol ester, a direct activator of protein kinase C (PKC), caused an increase in the NGF synthesis/secretion. The stimulatory effect of homocatechol (4-methylcatechol) on NGF synthesis was not completely inhibited by staurosporine, an inhibitor of PKC. The treatment of cells with homocatechol resulted in the translocation of PKC from cytosol to membrane-associated fractions, although the levels of the subcellular location of PKC were not correlated with the level of the induction of NGF gene expression. The concomitant administration of homocatechol (10(-4) M) and PMA (10(-8) - 10(-6) M) evoked a drastic and prolonged increase in the NGF mRNA level, and also markedly increased the amounts of NGF secreted by the cells (approximately 150-fold). This synergism was inhibited in part by staurosporine, but the level of increase in NGF mRNA and in NGF protein was rather greater than that of activation of PKC.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytosolic calcium homeostasis in bovine parathyroid cells and its modulation by protein kinase C

1. The effects of protein kinase C (PKC) activators and inhibitors on the mechanisms regulating cytosolic Ca2+ homeostasis in dissociated bovine parathyroid cells loaded with fura-2 were examined. 2. Stepwise increases in the concentration of extracellular Ca2+ (from 0.5 to 2 or 3 mM) elicited transient followed by sustained increases in the concentration of intracellular free Ca2+ ([Ca2+]i). Cytosolic Ca2+ transients reflected the mobilization of intracellular Ca2+ and influx of extracellular Ca2+ whereas sustained increases in [Ca2+]i resulted from the influx of extracellular Ca2+. Brief (1-2 min) pretreatment with phorbol myristate acetate (PMA) shifted the concentration-response curve for extracellular Ca(2+)-induced cytosolic Ca2+ transients to the right without affecting the maximal response. Cytosolic Ca2+ transients elicited by extracellular Mg2+ were similarly affected by PMA. 3. These effects of PMA were mimicked by various other activators of PKC with the rank order of potency PMA > phorbol dibutyrate > bryostatin , > (-)indolactam V > mezerein. Isomers or analogues of these compounds that do not alter PKC activity (4 alpha-phorbols and (+)indolactam V) did not alter [Ca2+]i. 4. PKC activators depressed evoked increases in [Ca2+]i when influx of extracellular Ca2+ was blocked with Gd3+. Cytosolic Ca2+ transients elicited by extracellular Mg2+ in the absence of extracellular Ca2+ were similarly inhibited by PKC activators. Activation of PKC thus inhibits the mobilization of intracellular Ca2+ elicited by extracellular divalent cations. 5. Increases in the concentration of extracellular Ca2+ caused corresponding increases in the formation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3). Pretreatment with PMA shifted the concentration-response curve for extracellular Ca(2+)-induced [3H]InsP3 formation to the right without affecting the maximal response. 6. PKC activators also caused some depression of steady-state increases in [Ca2+]i elicited by extracellular Ca2+. In contrast, PMA did not affect increases in [Ca2+]i elicited by ionomycin or thapsigargin. 7. Ba2+ was used to monitor divalent cation influx. PMA decreased the rate of rise of the fluorescent signal elicited by extracellular Ba2+. 8. All these effects of PKC activators on [Ca2+]i were blocked or reversed by staurosporine at concentrations (30-100 nM) that inhibited PKC activity in parathyroid cells. Staurosporine alone potentiated cytosolic Ca2+ responses evoked by submaximal concentrations of extracellular divalent cations. 9. PKC thus depresses both the mobilization of intracellular Ca2+ and the influx of extracellular Ca2+ in parathyroid cells. The effects on [Ca2+]i provide evidence for a Ca2+ receptor on the surface of parathyroid cells that uses transmembrane signalling mechanisms common to some other Ca(2+)-mobilizing receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of endothelin 1 gene by fluid shear stress is transcriptionally mediated and independent of protein kinase C and cAMP

Fluid shear stress induces a number of morphological and functional changes in vascular endothelium, including a rapid and significant down-regulation of endothelin 1 (ET-1) mRNA and peptide release in bovine aortic endothelial cells. We show here that both the cell alignment and ET-1 down-regulation depend on on-going protein synthesis, and that the latter is the result of a decrease in transcription, as shown by nuclear run-off assay, and not the result of changes in ET-1 mRNA half-life. The treatment of endothelial cells with either phorbol 12-myristate 13-acetate (100 nM) to activate protein kinase C (PKC) or forskolin (10 microM) to stimulate adenylate cyclase sharply decreased ET-1 mRNA. However, the phorbol-induced ET-1 decrease was, unlike the shear-induced down-regulation, independent of active protein synthesis. Physiological shear stress (20 dynes/cm2) did not significantly activate PKC, as assessed by PKC translocation and enzymatic activity assay and failed to increase intracellular cAMP content. Furthermore treatment with calphostin C (1 microM) did not prevent the shear-induced down-regulation of ET-1. DNA transfection experiments suggest that the shear stress-responsive element of the ET-1 gene is contained in the sequence between -2.5 kb and -2.9 kb of the 5'-upstream region. Neither the transcription factor AP-1 binding site nor the GATA-2-factor binding site, necessary for the basal level of transcription of ET-1 gene, is sufficient to confer shear-responsiveness to the reporter gene. These results suggest that shear stress regulates the transcription of the ET-1 gene via an upstream cis element by a distinct mechanism not dependent on the PKC or cAMP pathways.

A study of cross-resistance pattern and expression of molecular markers of multidrug resistance in a human small-cell lung-cancer cell line selected with doxorubicin

A doxorubicin-resistant variant of the human small-cell lung-cancer cell line N592 was selected by in vitro continuous exposure to increasing drug concentrations. The aim of this study was to examine the cross-resistance pattern, cellular pharmacokinetics of doxorubicin and expression of molecular factors of resistance. The sub-line N592/DX exhibited a multidrug-resistance phenotype, which was somewhat atypical, since it included cisplatin. Development of doxorubicin resistance could not be attributed to differential doxorubicin uptake or retention. Verapamil partially reverted doxorubicin resistance without affecting cellular pharmacokinetics. These findings are consistent with undetectable levels of mdr-1-gene expression in these cells. A molecular analysis of other putative mechanisms of multidrug resistance indicated no alterations in GSH levels or GSH-related enzymes, but a marginal reduction of topoisomerase II alpha expression in the resistant sub-line. This reduction, which was associated with an increase in topoisomerase I, does not explain the high degree of resistance. This study supports the view that alternative, unidentified mechanisms, which may be of clinical relevance, must be involved in the development of multidrug resistance of small-cell lung cancer.