Hepatic protein kinase C: translocation stimulated by prolactin and partial hepatectomy

Subcellular redistribution of protein kinase C isozymes is associated with rat liver cirrhotic changes induced by carbon tetrachloride or thioacetamide

BACKGROUND AND AIMS

Protein kinase C (PKC) plays a key role in the alteration of signal transduction in the liver, which may contribute to the development of liver cirrhosis. The aim of the present study was to examine the subcellular redistribution of PKC isozymes in rat liver cirrhosis, which is induced by two different cirrhotic chemical agents, carbon tetrachloride (CCl4) and thioacetamide (TAA).

METHODS AND RESULTS

Thioacetamide and CCl4 were administered to rats for 8 and 30 weeks, respectively before rats were killed and autopsies performed at 9, 20 and 30 weeks later. The TAA induced a fibrotic pattern in the liver that differed from that produced by CCl4, notably in the formation of fibrous connective tissue and the proliferation of bile ductule cells. Cholangiofibrosis and clear-cell foci were also observed in TAA-treated rats at 30 weeks. Histological examination revealed that severe cirrhotic changes were present 9 weeks after the commencement of CCl4 treatment and 30 weeks after TAA treatment.

DISCUSSION

When the subcellular redistribution of PKC isozymes (PKCalpha, -beta1, -delta, and -epsilon) was examined, all the PKC isozymes in CCl4-treated rats were found to be translocated to the membrane fraction, which may mean PKC activation, and then downregulated by proteolytic degradation after 9 weeks of treatment, which coincided with peak cirrhotic changes. All rats treated with CCl4 recovered to the control level after 20 weeks of treatment. In the case of TAA-treated rats, PKC isozymes were translocated to the particulate fraction of the liver after 9 weeks of treatment and this persisted in most of the rats for the duration of the experiment.

CONCLUSIONS

From these results, it would appear that PKC translocation preceded morphologic changes, and that an altered subcellular distribution of the PKC isozyme may be associated with the response to liver damage and carcinogenesis.

Role of diacylglycerol (DAG) in hormonal induction of S phase in hepatocytes: the DAG-dependent protein kinase C pathway is not activated by epidermal growth factor (EGF), but is involved in mediating the enhancement of responsiveness to EGF by vasopressin, angiotensin II, and norepinephrine

The role of diacylglycerol (DAG) in hormonal induction of S phase was investigated in primary cultures of rat hepatocytes. In this model, several agonists that bind to G protein-coupled receptors act as comitogens when added to the cells soon after plating (i.e., in Go/early Gl phase), while the cells are most responsive to the mitogenic effect of epidermal growth factor (EGF) at 24-48 h of culturing (i.e., mid/late Gl). It was found that the cellular concentration of DAG rose markedly and progressively during the first 24 h of culturing. Exposure of the hepatocytes at 3 h to alpha1-adrenergic stimulation (norepinephrine with timolol), vasopressin, or angiotensin II further increased this rise, producing a sustained increase in the DAG level. Norepinephrine, which was the most efficient comitogen, produced the most prolonged DAG elevation. In contrast, no significant increase of DAG was found in response to EGF, neither at 3 nor at 24 h, using concentrations that markedly stimulated the ERK subgroup of the mitogen-activated protein kinases (MAPK) and DNA synthesis. Addition of Bacillus cereus phosphatidylcholine-specific phospholipase C (PC-PLC) strongly elevated DAG, while Streptomyces phospholipase D (PLD) increased phosphatidic acid (PA) but not DAG. B. cereus PC-PLC and the protein kinase C (PKC) activator tetradecanoyl phorbol-acetate (TPA), like norepinephrine, vasopressin, and angiotensin II, stimulated MAPK and enhanced the stimulatory effect of EGF on DNA synthesis. The PKC inhibitor GF109203X did not diminish the effect of EGF on MAPK or DNA synthesis, but strongly inhibited the effects of norepinephrine, vasopressin, angiotensin II, TPA and B. cereus PC-PLC on MAPK and almost abolished the enhancement by these agents of EGF-stimulated DNA synthesis. These results suggest that although generation of DAG is not a direct downstream response mediating the effects of the EGF receptor in hepatocytes, a sustained elevation of DAG with activation of PKC markedly increases the responsiveness to EGF. Mechanisms involving DAG and PKC seem to play a role in the comitogenic effects of various agents that bind to G protein-coupled receptors and activate the cells early in Gl, such as norepinephrine, angiotensin II, and vasopressin.

Differential effects of phorbol ester on growth and protein kinase C isoenzyme regulation in human hepatoma Hep3B cells

PMA has both mitogenic and antiproliferative effects on human hepatoma Hep3B cells. In response to low PMA concentration (10 nM), Hep3B cells displayed an increasing proliferation potentiation. At high PMA concentration (1 microM) Hep3B cells exhibited modest cytostatic effects. Determinations of protein kinase C (PKC) activity in PMA-treated cells revealed that alterations in PKC activity are associated with proliferative capacity. The decrease in PKC activity mediated by a high dose of PMA was accompanied by cell growth inhibition. Increases in PKC activity mediated by a low dose of PMA were consistent with proliferation stimulation. Immunoblot analysis showed that there are at least six PKC isoenzymes: alpha, delta, epsilon, mu, zeta and iota/lambda, constitutively expressed in Hep3B cells. Cellular fractionation and immunocytochemical staining results demonstrated that both 10 nM and 1 microM PMA treatments induced a marked translocation of PKC-alpha from cytosol to membrane or nuclear fraction within 5-30 min. At the same time PKC-delta and epsilon were translocated from the membrane to nuclear fraction. In addition, prolonged treatment with 1 microM PMA, but not with 10 nM PMA, selectively mediated the down-regulation of these three PKC isoenzymes. The distinct effects of different concentrations of PMA on cell proliferation and PKC-alpha, delta and epsilon isoenzyme modulation support the involvement of these three PKC isotypes in the mechanism of action of Hep3B cells in cell growth events.

Differential expression of protein kinase C isoforms in diethylnitrosamine-initiated rat liver

Although protein kinase C (PKC), a family of 12 related isoforms, plays an important role in carcinogenesis, little is known about the specific role of each isoform in the initiation stage of hepatocarcinogenesis. The subcellular distribution of PKC isoforms in the early stages of diethylnitrosamine (DEN)-initiated hepatocarcinogenesis was therefore examined. Three-week-old female Sprague-Dawley rats were intraperitoneally injected twice in 1 week with DEN; all animals were sacrificed at 1, 2 and 24 h and 3 and 7 days after the second injection. PKCalpha and -beta expression in both cytosolic and particulate fractions decreased as a result of 1 h of DEN treatment and this effect lasted for 7 days. In both fractions, PKCepsilon expression showed a marked increase by DEN treatment, while the expression of PKCdelta and -zeta was almost unchanged. These results suggest that differential expression of PKC isoforms may play an important role in the early stage of DEN-initiated hepatocarcinogenesis in rats.

Prolactin inhibits EGF-induced DNA synthesis in mammary epithelium via early signaling mechanisms: possible involvement of protein kinase C

Prolactin and prolactin agonists inhibited EGF-induced DNA synthesis in mammary epithelium, whereas other pituitary hormones had no effect on EGF-induced DNA synthesis. The inhibitory effect of prolactin was seen for EGF and TGF-alpha, but not for IGF-I or cholera toxin. Autoradiography indicated that prolactin decreased the ability of EGF to induce cells to progress to S phase of the cell cycle, and time course studies indicated that the effects of prolactin were not due to an altered timing of DNA synthesis induction. Prolactin addition within 30 min of adding EGF was necessary to inhibit EGF-induced DNA synthesis. Conditioned media from prolactin-treated cells from which prolactin had been neutralized with the extracellular domain of the prolactin receptor had no effect on EGF-induced DNA synthesis, suggesting that the effect was due to prolactin, not an autocrine factor induced by prolactin. Prolactin induced a rapid association of protein kinase C with the membrane fraction of NMuMG cells, as well as increased threonine phosphorylation of the EGF receptor. Protein kinase C inhibitors eliminated most of the inhibitory effect of prolactin on EGF-induced DNA synthesis. The protein kinase C inhibitor Calphostin C restored high-affinity EGF binding in prolactin-treated cells and reversed the inhibitory effect of prolactin on EGF-induced EGF receptor tyrosine phosphorylation.

Activation of a novel form of phospholipase A2 during liver regeneration

Activation of phospholipase A2 (PLA2) occurs following mitogenic stimulation of cells. This study examined PLA2 activation during liver regeneration. Increased activity was detected within 1 h after partial hepatectomy, was maximal by 6 h, and returned to control levels by 24 h. Fractionation of cell-free extracts revealed multiple peaks of PLA2 activity. One peak appeared identical to the previously described cPLA2, and was modestly stimulated during regeneration. A higher molecular weight form (hPLA2) was stimulated approximately 5-fold during regeneration. This enzyme was Ca(2+)-dependent and selective for arachidonoylphosphatidylethanolamine. The activation of this novel form of PLA2 represents an early event in liver regeneration, and is likely to contribute to the proliferative response.

Regulation of heat-shock protein (hsp70) gene expression by hGH and IL2 in rat Nb2 lymphoma cells

A comparative study of hGH and IL2 post-signaling effects, as examined by RNA expression (Nb29) and protein levels of the heat-shock protein hsp70, was performed in a hormone-dependent rat lymphoma cell line, Nb2-11C. Optimal doses of hGH or IL2 increased Nb29 expression in a dose-dependent manner. Addition of both mitogens to cell cultures affected Nb29 expression and mitogenesis synergystically, indicating a possible interaction between the post-receptoral mechanisms of these mitogens. Pretreatment of the cells with cholera toxin (CT) inhibited Nb29 expression, protein levels and mitogenesis of hGH- or IL2-induced cells up to 50%, indicating the involvement of Gs-proteins in the post-signaling processes of both hGH and IL2. Incubation of cell cultures with low concentrations of pertussis toxin (IAP) (0.01 ng/ml) markedly increased Nb29 expression in hGH but not in IL2-induced cells, suggesting specific involvement of the Gi-protein in post-signaling processes of hGH-induced cells. Addition of the PKC activator 12-O-tetra-decanoyl phorbol ester (TPA) to control cell cultures markedly increased the expression of Nb29 RNA levels but not mitogenesis, indicating that induction of these proteins in the cells is not sufficient for cell proliferation. Furthermore, incubation of hGH- or IL2-induced cells with the potent PKC inhibitor staurosporin (ST) decreased the levels of Nb29 in both hGH- and IL2-induced cells, although the effect of the mitogens differed significantly in their inhibition slopes. These results indicate that activation of PKC is one of the signaling pathways differentially involved in hGH and IL2 stimulation of cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid stimulation of mitogen-activated protein kinase of rat liver by prolactin

Intraperitoneal prolactin administration to female rats caused a rapid and transient stimulation of hepatic mitogen-activated kinase (MAP kinase) activity measured in vitro as cytosolic phosphotransferase capacity towards two specific substrates. Myelin basic protein kinase activity of MAP kinase immunoprecipitates confirmed the specificity and magnified the prolactin effect. Immunoblot experiments with anti-(MAP kinase) and anti-phosphotyrosine antibodies showed changes in both electrophoretic mobility and phosphotyrosine content of 40 and 44 kDa isoenzymes suggesting that prolactin affects these isoforms. Concomitant with the increase in MAP kinase activity, prolactin induced tyrosine phosphorylation in a number of liver proteins, suggesting a rapid involvement of tyrosine kinases which might be correlated in some way with MAP kinase activation. Protein kinase C activity, which has been implicated in the regulation of MAP kinase and in mediating the prolactin effect, does not seem to participate in MAP kinase activation.

Does oligosaccharide-phosphatidylinositol (glycosyl-phosphatidylinositol) hydrolysis mediate prolactin signal transduction in granulosa cells?

Initial biosynthetic radiolabelling experiments with cultured granulosa cells revealed the presence of an oligosaccharide-phosphatidylinositol (glycosyl-phosphatidylinositol; (Ose)nPtdIns) structurally related to (Ose)nPtdIns-lipids isolated from other cell types. Prolactin (PRL) stimulated [3H]glucosamine-(Ose)nPtdIns turnover and the rapid generation of [3H]myristoyl-diacylglycerol in cultured follicle-stimulating hormone-(FSH)-primed granulosa cells endowed with PRL receptors. In parallel experiments performed with [3H]myo-inositol-labelled granulosa cells, treatment with PRL stimulated (Ose)nPtdIns hydrolysis in a similar manner, whereas no effect on phosphoinositide (PtdIns, PtdInsP and PtdInsP2) turnover could be observed. These results strongly suggest that the cleavage of (Ose)nPtdIns by phosphodiesterase followed by the subsequent generation of diacylglycerol and a soluble phosphoinositol-oligosaccharide (inositol-phosphoglycan; (Ose)nInsP) moiety could be part of the signal-transduction mechanism linking PRL receptors to their biological effects in granulosa cells. To test this hypothesis, we examined the effect of PRL and purified (Ose)nInsP moiety (from rat liver membranes) on granulosa cell 3 beta-hydroxysteroid dehydrogenase/delta 5-4 isomerase (3 beta-HSD) enzyme activity. Results presented show that, in FSH-primed granulosa cells, PRL (40 nM) and (Ose)nInsP (5 microM) prevented gonadotropin-stimulated 3 beta-HSD activity. Furthermore, in undifferentiated granulosa cells where PRL receptors are absent, no effect of the hormone on 3 beta-HSD activity could be observed, whereas (Ose)nInsP (1-10 microM) inhibited enzyme activity in a dose-dependent manner.

Possible role of Na+ influx in phorbol ester-induced down-regulation of protein kinase C in HL60 cells

Amiloride, an inhibitor of Na+/H+ exchange, inhibited down-regulation of protein kinase C in HL60 cells induced by tumor-promoting phorbol ester in dose-dependent manner judging from immunoblot analysis. This inhibition was observed with regard to type I (gamma), type II (beta), and type III (alpha) isozymes of protein kinase C. On the other hand, monensin, a Na+ ionophore, accelerated the down-regulation of protein kinase C induced by phorbol ester. When we examined 22Na+ uptake by HL60 cells, the higher uptake was observed after stimulation with phorbol ester compared to the control cells and this 22Na+ uptake was strongly inhibited by the addition of amiloride. However, monensin further stimulated the 22Na+ uptake observed in phorbol ester-treated cells. These data suggest that the increase in intracellular Na+ concentration may be one of the triggers for the induction of down regulation of protein kinase C.

Prolactin-stimulated mitogenesis of cultured astrocytes is mediated by a protein kinase C-dependent mechanism

Prolactin (PRL) has been reported to activate cellular proliferation in nonreproductive tissue, such as liver, spleen, and thymus. Recently, we have extended the possible role of PRL as a mammalian mitogen by demonstrating a mitogenic effect of PRL in cultured astrocytes. Although the cellular mechanisms by which PRL regulates cell growth are not fully understood, protein kinase C (PKC) has been implicated as one of the transmembrane signaling systems involved in the regulation of PRL-induced cell proliferation in Nb2 lymphoma cells and liver. In the present studies, we examined the possible role of PKC in PRL-induced proliferation of cultured astrocytes. Incubation of cultured astrocytes with 1 nM PRL resulted in a rapid translocation of PKC from the cytosol to the membrane, with maximal PKC activity in the membrane occurring 30 min after exposure to PRL. Translocation of PKC activity occurred over a physiological range of PRL, with maximal PKC activation occurring at 1 nM. At concentrations greater than 10 nM PRL, there was a decrease in the amount of PKC activity associated with the membrane fraction compared with that of cells stimulated with 1 nM PRL. Incubation of astrocytes with PRL in the presence of the PKC inhibitors staurosporine, 1-(-5-isoquinolinesulfonyl)-2-methylpiperazine, or polymyxin B blocked the PRL-induced increase in cell number with IC50 values of approximately 2 nM, 10 microM, and 6 microM, respectively. PKC is the only known cellular receptor for 12-O-tetradecanoylphorbol 13-acetate (TPA), which stimulates the translocation of PKC from the cytosol to the membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of prolactin receptors in hepatic nuclei

Prolactin is a trophic hormone which may act directly at the hepatocyte nucleus. In this study, specific prolactin binding sites were sought in purified rat liver nuclei. Saturable and specific, high affinity 125I-prolactin binding sites were demonstrated to be on or within the nucleus. Prolactin binding was competitively inhibited by rat and ovine prolactins but not by rat growth hormone. Using immunogold electron microscopy, we detected prolactin receptors throughout the nucleus, in association with heterochromatin. Furthermore, endogenous immunoreactive prolactin was demonstrated to be within hepatic nuclei. We conclude that rat liver nuclei possess prolactin binding sites which likely participate in hormone-directed growth processes.

Prolactin as an immunoregulatory hormone in mammals and birds

The immunoregulatory function of prolactin (PRL) and the mechanism of its action in mammals seem to be well documented. Reciprocal interdependence between PRL secretion and immune system function is essential for normal ontogeny, development and aging. PRL receptors in lymphocytes participate in the transduction of its regulatory signal into the intracellular enzymatic machinery including that of the nucleus, leading to the expression of some genes and to the synthesis of new proteins. Activation of phosphoinositide turnover and subsequent increase in protein kinase-C activity seems to be a possible mechanism acting in the regulatory influence of PRL on mammalian immune cells. These cells in turn, under mitogen or antigen stimulation, secrete a substance with PRL-like activity. The regulatory function of PRL within the avian immune system is less well known, but it seems to have some features in common with those in mammals. Direct mitogenic action on thymocytes and splenocytes in the chicken might indicate the existence of PRL receptors in these cells and could explain the immunostimulatory effect of PRL observed in vivo, which is dependent on the time of hormone administration. As the avian PRL stimulates mitogenesis of rat Nb2 lymphoma cells, the mechanism of direct PRL action on immune cells in mammals and birds seems to be similar. PRL in chickens also modifies the level and the diurnal rhythm of corticosterone which, in turn, influences the immunoregulatory effect exerted by PRL. Thus, PRL seems to be an important factor, influencing directly or indirectly the avian immune system.

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

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

Changes in phosphatidylethanolamine metabolism in regenerating rat liver as measured by 31P-NMR

31P-NMR spectra of regenerating rat liver in vivo show increases in resonance intensities in the phosphomonoester (PME) region and decreases in the phosphodiester (PDE) region as early as 12 h post partial hepatectomy, which return to normal by 8 days. The compounds primarily responsible for these changes have been identified in perchloric acid extracts as the phosphomonoester phosphoethanolamine and the phosphodiester glycerophosphoethanolamine (GPE), indicating altered phosphatidylethanolamine metabolism. A corresponding increase in diacylglycerol (DAG) levels during regeneration indicates a possible role for a phosphatidylethanolamine-specific phospholipase C in cellular proliferation. These results suggest that changes in phospholipid metabolites previously associated with neoplastic tissue can also be induced by normal tissue undergoing rapid cellular proliferation. The spectral changes observed in the regenerating rat liver are similar to changes seen in spectra from the livers of human patients in several disease states, indicating that 31P-NMR may allow non-invasive study of cell turnover in liver disease.

A specific binding site in Nb2 node lymphoma cells mediates the effects of didemnin B, an immunosuppressive cyclic peptide

Didemnin B (DB) is a cyclic depsipeptide with a variety of biologic effects, including potent antiviral, antitumor, and immunosuppressive activities. Although its mechanism of action has been attributed to inhibition of DNA and protein synthesis, the exact cellular site of interaction has not been previously defined. Since DB is strongly antiproliferative in Nb2 node lymphoma cells, we investigated potential DB binding sites in these cells, using [3H]-DB (2.7 mCi/mg) as the radiolabeled ligand. Time course studies with Nb2 cells showed that steady state [3H]-DB binding was attained after 4 h. Scatchard analysis with resting cells yielded a Kd of 180 nM (200 ng/ml), and 7 x 10(6) binding sites/cell. The IC50 of DB inhibition of ongoing protein and DNA synthesis in Nb2 cells, measured 24 h after prolactin (PRL) stimulation, was also in the range of 100 ng/ml. Didemnin analogs, with alterations at critical amino acid residues, inhibited the synthesis of DNA and protein and competed with [3H]-DB binding with the same rank order of potency. This implies that this binding site may mediate the inhibition of macromolecule synthesis. Subcellular fractionation of [3H]-DB labeled Nb2 cells revealed that specific binding occurred predominantly in the 100,000 g cytosolic fraction. Comparison with cyclophilin and the FK506 binding protein, both cytosolic receptors, suggests that the DB binding site may also belong to the family of immunophilins.

Reduced calcium and inhibition of protein kinase C mimic the enhancement of ornithine decarboxylase activity of prolactin in Ambystoma tigrinum tissues

We previously reported that prolactin (PRL) could increase the activity of ornithine decarboxylase (ODC) in liver slices taken from larval tiger salamanders (Ambystoma tigrinum). This action of the hormone was inhibited by oxytocin (OT), the calcium ionophore A23187, and diacyglycerol (DG) and was duplicated by 10 microM verapamil (VML), a calcium channel blocker. Here, we expand these results to show that 1) a higher dose of VML (50 microM) produces an additive effect with PRL; 2) addition of small amounts of calcium (0.1 mM) to the liver culture medium blocks PRL action; 3) neither nifedipine (NIF), a different type of calcium channel blocker, nor EDTA alter PRL action; and 4) gossypol, a reported inhibitor of protein kinase C, mimics PRL action. Additionally, we show that PRL increases ODC activity in tiger salamander tail skin in vitro, a tissue previously demonstrated to be a PRL target tissue in this species. The same set of treatments which we have shown to modify PRL effects on ODC in liver slices affects PRL action in the tail skin in a parallel manner. Thus, the mechanism whereby PRL enhances ODC activity appears to be the same in both these tissues. These results are discussed in conjunction with the findings from similar studies using mammalian tissues in an attempt to assess the current picture of the mechanism of PRL action and the possible role of inositol phospholipid turnover, calcium, and protein kinase C in the action of this hormone.

Prolactin-provoked alterations of cytosolic, membrane, and nuclear protein kinase C following partial hepatectomy

The adenohypophyseal polypeptide hormone prolactin is a potent liver mitogen, stimulating cell cycle progression, an effect that appears coupled to increasing protein kinase C activity in membrane and nuclear fractions. Here, we examine whether hepatocyte proliferation, stimulated by partial hepatectomy, is associated with altered serum prolactin or protein kinase C activation. Within 5-15 min of liver resection, serum prolactin concentrations elevate significantly. Protein kinase C activity in hepatic cytosol decreases significantly, and membrane and nuclear PKC activity increase by 30 min. Hypophysectomy prior to partial hepatectomy abrogates any effect of liver resection on protein kinase C activation in the hepatic remnant. Based upon these data, it is suggested that the rapid increase in serum prolactin seen after partial hepatectomy may be linked to protein kinase C activation, which in turn stimulates the hepatic proliferative response that is essential for hepatic regeneration.

Prolactin activates protein kinase C and stimulates growth-related gene expression in rat liver

We have examined the effect of prolactin (PRL) on growth-related gene expression, protein kinase C (PKC) activity and diacylglycerol (DAG) mass in rat liver. Hepatic levels of messenger (m)RNA for c-myc, ornithine decarboxylase (ODC) and beta-actin increased in a dose-dependent manner within 1 h after PRL administration. Prolactin also caused a transient elevation of liver DAG levels and particulate-associated PKC activity. The PRL-provoked increases in DAG mass and particulate PKC activity were coincident and maximal at 20 min and began declining toward control levels by 30 min. These results suggest a temporal relationship between PRL-stimulated DAG accumulation and PKC activation. Furthermore, the subsequent rapid induction of growth-related gene expression provides new information on the role of PRL as a hepatic mitogen.

Prolactin induces proliferation of vascular smooth muscle cells through a protein kinase C-dependent mechanism

The effects of prolactin (PRL) on A10 (aortic smooth muscle) cell proliferation were examined by measuring both [3H]thymidine incorporation and increases in cell number. PRL induced a significant proliferative response from 10(-11) to 10(-7) M, with optimal activity at 10(-10) M. PRL also enhanced platelet-derived growth factor (PDGF)-induced proliferation. The possibility that PRL induces proliferation through a protein kinase C (PKC)-mediated mechanism was also examined. PRL caused activation of PKC from 10(-12) to 10(-8) M. Antiserum to PRL, a monoclonal antibody directed against the PRL receptor and the immunosuppressive agent cyclosporine A, were able to inhibit PRL-induced proliferation and activation of PKC. The PKC inhibitors, staurosporine, sphingosine, and 1-(-5-iso-quinoline-sulfonyl)-2-methylpiperazine (H-7) also antagonized both proliferation and PKC activation. These data strongly suggest that PRL-induced A10 cell proliferation is mediated through the PKC pathway and that this may play a role in vascular smooth muscle cell hyperplasia, characteristic of the pathogenesis of cardiovascular diseases such as hypertension and atherosclerosis.

Prolactin stimulation of protein kinase C activity in the rat hypothalamus

Stimulation of cultured hypothalamic slices with PRL causes a rapid translocation of a Ca2+/phospholipid dependent protein kinase from the cytosol to the membrane fraction. The translocation of PKC from the cytosol to the membrane occurred at physiological concentrations of PRL with a maximal response occurring at 10(-10) M. At concentrations above this, there was less PKC activity translocated from the cytosol to the membrane. When injected into the medial preoptic area of the hypothalamus, PRL resulted in a similar translocation of PKC activity. These data clearly indicate that PRL can activate PKC in the rat hypothalamus, and suggest that PKC may be one of the transmembrane signaling mechanisms involved in the regulation of brain function by prolactin.

Studies on protein kinase C tightly-bound to rat liver plasma membrane and its protease-activated form

1. Rat liver plasma membrane contained two types of protein kinase C which could be extracted by Ca2(+)-chelator and detergent, respectively. The activities of these two enzymes were nearly equivalent. 2. The detergent-extracted protein kinase C, tightly-bound to membrane, was separated into two subtypes by hydroxyapatite column chromatography. Based on the elution profile and the Ca2+/phospholipid requirement, the major and the minor components were identified as type III and type II protein kinase C, respectively. 3. The detergent-extracted protein kinase C was converted to an active fragment with Mr 45,000 by limited proteolysis with trypsin. Incubation under physiological level of ionic strength increased the stability of this active enzyme and protected it from further inactivation by trypsin. 4. Phosphorylation of H1 histone by the protease-activated kinase was stimulated 1.5-2-fold by phosphatidylserine. However, this enzyme phosphorylated multiple proteins in rat liver subcellular fractions in Ca2(+)- and phospholipid-independent manner. 5. These results suggest that the protein kinase C (mainly type III enzyme) tightly-bound to rat liver plasma membrane may have important role through protein phosphorylation by the native or the protease-activated kinase.

Prolactin-stimulated ornithine decarboxylase induction in rat hepatocytes: coupling to diacylglycerol generation and protein kinase C

The trophic effects of prolactin (PRL) in rat liver have been linked to activation of protein kinase C (PKC). Since alterations in PKC activity imply its activation by 1,2-diacylglycerol (DAG), we tested whether PRL treatment stimulated DAG generation coupled to induction of a growth response in primary hepatocytes. Addition of PRL to hepatocyte cultures significantly increased [3H]-glycerol incorporation into DAG within 5 minutes which was followed by a loss of cytosolic PKC activity by 10 minutes. Prolactin also significantly enhanced radiolabel incorporation into triacylglycerol and phospholipids within 10 minutes and induced ODC activity at 6 hours. Therefore, prolactin-stimulated alterations in PKC activity are preceded by enhanced DAG generation. Moreover, these events appear to be coupled to PRL-stimulated entry of hepatocytes into cell cycle.

Prolactin and known modulators of rat splenocytes activate nuclear protein kinase C

Prolactin (PRL) and other trophic factors rapidly activate a nuclear pool(s) of protein kinase C (nPKC) in purified splenocyte nuclei. The PRL also enhanced [2-3H]glycerol incorporation into nuclear mono- and triacylglycerol. An assay was devised which not only probed the ability of the hormone to activate protein kinase C (PKC) but also demonstrated the presence of nuclear substrates. Using this methodology, a biphasic concentration-response curve to PRL was observed. Heterologous species of PRL and various growth factors also activated nPKC. The PRL-induced nPKC stimulation was antagonized by various immunomodulators, G protein-coupling inhibitors, PKC inhibitors, a calmodulin inhibitor, and a peripheral benzodiazepine agonist and antagonist. A monoclonal antibody to PKC, anti-rat PRL antiserum and a monoclonal anti-rat PRL receptor antibody antagonized PRL-induced PKC-dependent nuclear phosphorylation, further implicating nPKC and a PRL receptor-mediated activation process. Nuclear PKC may be a major target for trophic regulation in response to both positive and negative growth signals.

Comparison of substrate recognition by protein kinase C (type III) between rat liver cytosolic and particulate fractions

1. Phosphorylation of rat liver endogenous substrates by protein kinase C (type III) was compared between cytosolic and particulate (mitochondria, microsomes and plasma membrane) fractions. 2. The rate and the maximum level of protein phosphorylation were several-fold higher in particulate fractions than in cytosolic fraction. 3. Protein phosphorylation in cytosolic fraction was dependent on both Ca2+ and phospholipid, but only Ca2+ was necessary in phosphorylation of particulate fractions. 4. These results suggest that protein kinase C (type III) has much more target proteins in particulate fractions rather than in cytosolic fraction and Ca2+ was important regulator in particulate protein phosphorylation.

An early elevation of diacylglycerol and phosphatidate in regenerating liver

Liver diacylglycerol and phosphatidate are elevated following partial hepatectomy. These increases precede those in DNA synthesis and triacylglycerol accumulation. Possible factors involved in the increase in the lipids and the possible role of the lipids in liver regeneration are discussed.

1,2-sn-diacylglycerol accumulates in choline-deficient liver. A possible mechanism of hepatic carcinogenesis via alteration in protein kinase C activity?

Choline deficiency is associated with triacylglycerol accumulation in the liver, and is the only nutritional state known to trigger hepatic cancer spontaneously. In two different experiments, rats were pair-fed for 6 weeks with control (0.2% choline), or choline-deficient (CD) (0.002% choline) diets. Hepatic choline and phosphocholine declined in CD animals to 54% and 16% of control levels, respectively. In control livers, 1,2-sn-diacylglycerol (1,2-sn-DAG) concentration was (in nmol/g wet wt) 144 (+/- 25; mean +/- SE); while in CD livers it was 792 (+/- 140) in the first experiment. In the second experiment the values were 375 (+/- 26) and 1147 (+/- 74), respectively. 1,2-sn-DAG, a precursor of triacylglycerol, is an endogenous activator of protein kinase C (PKC). PKC is the presumed site of action of the tumor-promoting phorbol esters. We suggest that the 1,2-sn-DAG accumulating in CD liver could bind PKC, altering its activity, and thus contribute to the carcinogenic effect of CD diets.

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.

New aspects of prolactin and immunity: a lymphocyte-derived prolactin-like product and nuclear protein kinase C activation

In addition to its growth regulating properties, new evidence reviewed here by Diane Haddock Russell demonstrates that prolactin has important immunoregulatory properties. In immune-compromised dwarf mice, prolactin restores immunocompetence. Human lymphocytes have prolactin receptors and mitogen-stimulated lymphocytes make and secrete a prolactin-like activity. Prolactin can stimulate the activation of nuclear protein kinase C in spleen and liver isolated nuclear preparations. This activation is blocked by prolactin receptor monoclonal antibody, suggesting that there is a receptor-mediated activation process in the nucleus. The discovery of the ability of prolactin and growth factors to activate nuclear protein kinase C may constitute a breakthrough in our understanding of how these hormones regulate trophic responses.

Rapid activation of protein kinase C in isolated rat liver nuclei by prolactin, a known hepatic mitogen

Rat liver nuclei pure by enzymatic and electron microscope criteria contain protein kinase C (PKC) that can be activated several hundredfold within 3 min of addition of prolactin or phorbol 12-tetradecanoate 13-acetate. Rat prolactin stimulated PKC maximally at 10(-12) M, whereas ovine prolactin was maximally stimulatory at 10(-10) M. Activation was time and dose dependent, exhibited a biphasic pattern, and was blocked by anti-prolactin antiserum, by PKC inhibitors such as 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) and sphingosine, and by cyclosporine. Moreover, the ability of prolactin to activate nuclear PKC was inhibited totally by a monoclonal antibody to the rat liver prolactin receptor, implicating a prolactin receptor-mediated activation process. Epidermal growth factor (EGF), a liver mitogen, caused a lesser but significant activation of nuclear PKC. However, EGF and suboptimal prolactin were synergistic. Human growth hormone, which has lactogenic properties, stimulated PKC activity, whereas nonlactogenic substances such as ovine growth hormone, insulin, dexamethasone, and 8-bromo-cAMP were inactive. That this may be a general mechanism for prolactin is suggested by the ability of prolactin to stimulate PKC 140-fold in rat splenocyte nuclei. Prolactin has comitogenic properties in lymphocytes.