LH/hCG-receptor is coupled to both adenylate cyclase and protein kinase C signaling pathways in isolated mouse Leydig cells

C-Fos regulation by the MAPK and PKC pathways in intervertebral disc cells

BACKGROUND

The gene encoding c-fos is an important factor in the pathogenesis of joint disease in patients with osteoarthritis. However, it is unknown whether the signal mechanism of c-fos acts in intervertebral disc (IVD) cells. We investigated whether c-fos is activated in relation to mitogen-activated protein kinases (MAPKs) and the protein kinase C (PKC) pathway in nucleus pulposus (NP) cells.

METHODOLOGY/RESULTS

Reverse transcription-polymerase chain reaction and western blotting analyses were used to measure the expression of c-fos in rat IVD cells. Transfections were performed to determine the effects of c-fos on target gene activity. The effect of c-fos protein expression was examined in transfection experiments and in a 3- (4,5-dimethylthiazol-2-yl) -2,5-diphenyltetrazolium bromide cell viability assay. Phorbol 12-myristate 13-acetate (PMA), the most commonly used phorbol ester, binds to and activates protein kinase C (PKC), causing a wide range of effects in cells and tissues. PMA induced the expression of c-fos gene transcription and protein expression, and led to activation of the MAPK pathways in NP cells. The c-fos promoter was suppressed completely in the presence of the MAPK inhibitor PD98059, an inhibitor of the MEK/ERK kinase cascade, but not in the presence of SKF86002, SB202190, or SP600125. The effects of the PKC pathway on the transcriptional activity of the c-fos were evaluated. PKCγ and PKCδ suppressed the promoter activity of c-fos. Treatment with c-fos inhibited aggrecan and Col2 promoter activities and the expression of these genes in NP cells.

CONCLUSIONS

This study demonstrated, for the first time, that the MAPK and PKC pathways had opposing effects on the regulation of c-fos in NP cells. Thus, the expression of c-fos can be suppressed in the extracellular matrix of NP cells.

Inhibitory actions of mibefradil on steroidogenesis in mouse Leydig cells: involvement of Ca(2+) entry via the T-type Ca(2+) channel

Intracellular cAMP and Ca(2+) are involved in the regulation of steroidogenic activity in Leydig cells, which coordinate responses to luteinizing hormone (LH) and human chorionic gonadotropin (hCG). However, the identification of Ca(2+) entry implicated in Leydig cell steroidogenesis is not well defined. The objective of this study was to identify the type of Ca(2+) channel that affects Leydig cell steroidogenesis. In vitro steroidogenesis in the freshly dissociated Leydig cells of mice was induced by hCG incubation. The effects of mibefradil (a putative T-type Ca(2+) channel blocker) on steroidogenesis were assessed using reverse transcription (RT)-polymerase chain reaction analysis for the steroidogenic acute regulatory protein (StAR) mRNA expression and testosterone production using radioimmunoassay. In the presence of 1.0 mmol L(-1) extracellular Ca(2+), hCG at 1 to 100 IU noticeably elevated both StAR mRNA level and testosterone secretion (P < 0.05), and the stimulatory effects of hCG were markedly diminished by mibefradil in a dose-dependent manner (P < 0.05). Moreover, the hCG-induced increase in testosterone production was completely removed when external Ca(2+) was omitted, implying that Ca(2+) entry is needed for hCG-induced steroidogenesis. Furthermore, a patch-clamp study revealed the presence of mibefradil-sensitive Ca(2+) currents seen at a concentration range that nearly paralleled those inhibiting steroidogenesis. Collectively, our data provide evidence that hCG-stimulated steroidogenesis is mediated at least in part by Ca(2+) entry carried out by the T-type Ca(2+) channel in the Leydig cells of mice.

Protein kinase C increases 11beta-hydroxysteroid dehydrogenase oxidation and inhibits reduction in rat Leydig cells

Glucocorticoid hormone controls Leydig cell steroidogenic function through a receptor-mediated mechanism. The enzyme 11beta-hydroxysteroid dehydrogenase (11betaHSD) plays an important role in Leydig cells by metabolizing glucocorticoids, and catalyzing the interconversion of corticosterone (the active form in rodents) and 11-dehydrocorticosterone (the biologically inert form). The net direction of this interconversion determines the amount of biologically active ligand, corticosterone, available for glucocorticoid receptor binding. We hypothesize that 11betaHSD oxidative and reductive activities are controlled separately in Leydig cells, and that shifts in the favored direction of 11betaHSD catalysis provide a mechanism for the control of intracellular corticosterone levels. Therefore, in the present study, we tested the dependency of 11betaHSD oxidative and reductive activities on protein kinase C (PKC) and calcium-dependent signaling pathways. 11betaHSD oxidative and reductive activities were measured in freshly isolated intact rat Leydig cells using 25 nM radiolabeled substrates after treatment with protein kinase modulators. We found that PKC and calcium-dependent signaling had opposing effects on 11betaHSD oxidative and reductive activities. Stimulation of PKC using the PKC activator, 6-[N-decylamino]-4-hydroxymethylinole (DHI), increased 11betaHSD oxidative activity from a conversion rate of 5.08% to 48.23% with an EC50 of 1.70 +/- 0.44 microM (mean +/- SEM), and inhibited reductive activity from 26.90% to 3.66% conversion with an IC50 of 0.22 +/- 0.05 microM. This indicated that PKC activation in Leydig cells favors 11betaHSD oxidation and lower levels of corticosterone. The action of DHI was abolished by the PKC inhibitor bisindolylmaleimide I. In contrast, addition of calcium to Leydig cells increased 11betaHSD reductive activity while decreasing oxidative activity, thereby favoring reduction and conversion of inert 11-dehydrocorticosterone into active corticosterone. The opposite effect was seen after elimination of calcium-dependent signaling, including removal of calcium by EGTA or addition of the calmodulin (calcium binding protein) inhibitor SKF7171A, or the calcium/calmodulin-dependent protein kinase I (CaMK II) inhibitor, KN62. We conclude that 11betaHSD oxidative and reductive activities are separately regulated and that, in contrast to calcium-dependent signaling, PKC stimulates 11betaHSD oxidation while inhibiting 11betaHSD reduction. Maintenance of a predominantly oxidative 11betaHSD could serve to eliminate adverse glucocorticoid-induced action in Leydig cells.

A cellular automaton model of cellular signal transduction

On the basis of cellular automata models, a software specifically tailored to model biochemical reactions involved in cellular signal transduction was implemented on a personal computer. Recent data regarding desensitization processes in mouse Leydig cells are used to simulate the underlying reactions of signal transduction. Pretreatment of real Leydig cells with different molecules results in a modification of the signal transduction cascade leading to a diminished response of the cells during subsequent stimulations. The model is capable of simulating the complex behavior of this intracellular second messenger production in a qualitative and semi-quantitative way. The results indicate that quantitative simulations on a molecular level will be possible once appropriate computer hardware is available. The simulations and results of the cellular automaton presented are easily described and comprehended, which make it a useful tool that will facilitate research in cellular signal transduction and other fields covering complex reaction networks.

Role of proteoglycans on testosterone synthesis by purified Leydig cells from immature and mature rats

In order to characterize an involvement of proteoglycans (PG) in the regulation of Leydig cell function, we have examined the effects of para-nitrophenyl-beta-D-xyloside (PNPX), a specific inhibitor of PG synthesis and para-nitrophenyl-beta-D-galactoside (PNPG), an inefficient structural analogue, on testosterone production by purified Leydig cells from immature and mature rats, in the presence or not of various concentrations of hCG during 24 h. Whatever the age, the addition of PNPX induces a decrease of [35S] and [3H] incorporations into cell layer associated-PG; these latter being less numerous (-50 and -25%, respectively in immature and mature rat), and less sulfated (-40%) when compared to control Leydig cells. In immature Leydig cells, the inhibition of PG synthesis decreases both the basal and weakly stimulable-hCG or -(Bu)2cAMP or -LH testosterone synthesis. In mature Leydig cells, the PG inhibition has no effect on testosterone production both in the absence of hCG and in the presence of weak amounts of hCG but increases it in the presence of subsaturating hCG concentrations. Whatever the age, the inhibition of PG synthesis is ineffective in the presence of saturating amounts of either hCG or (Bu)2cAMP. These effects are maintained in the presence of MIX, PMA, but are not observed in the presence of 22R-hydroxycholesterol. Therefore, our results suggest that in rat Leydig cells, the inhibition of PG synthesis affects the signal transduction at a step distal to cyclic AMP and more precisely, the cholesterol supply to the mitochondria by acting on its cellular distribution (free and esterified cholesterol).

Mechanisms of desensitization of follicle-stimulating hormone (FSH) action in a murine granulosa cell line stably transfected with the human FSH receptor complementary deoxyribonucleic acid

The desensitization of follicle-stimulating hormone (FSH)-evoked cAMP synthesis occurs upon continuous or repeated hormonal stimulation, and it involves the hormone-receptor interaction and post-receptor events. These mechanisms were studied in a murine granulosa cell line (KK-1) stably transfected with the human FSH receptor (hFSHR) complementary deoxyribonucleic acid (cDNA) under a powerful viral promoter. Hence, the FSHR transcriptional regulation was eliminated from the experimental model. Stimulation of the cells with recombinant human FSH (rhFSH) or a phorbol ester, 12-O-tetradecanoylphorbol-13 acetate (TPA), resulted in clear desensitization, i.e. subsequent rhFSH-stimulated cAMP formation was 73.4 +/-2.2%, (P < 0.001) and 66.3 +/-3.4%, (P < 0.0001), respectively, of that of cells preincubated in medium. TPA prestimulation evoked also clear inhibition (65-74% of control) of rhFSH or forskolin (a non-specific activator of adenylate cyclase) induced progesterone production. The suppression by TPA preincubation of the rhFSH-induced cAMP synthesis was completely abolished by the protein kinase C (PKC) inhibitor staurosporine (STR). Preincubation with STR exhibited a significant (P < 0.0001) increasing effect on the rhFSH-stimulated cAMP accumulation. The specific involvement of PKC was further evidenced by other inhibitors, all of them exerted significant elevation of cAMP synthesis following rhFSH restimulation. Furthermore, only the PKC beta isoform appeared to be constitutively expressed in these cells during desensitization. Prestimulation of the G-protein activity by sodium fluoride (NaF) or cholera toxin (CT), followed by rhFSH challenge, accounted for a decrease in the cAMP-mediated responsiveness, down to 69.4 +/- 2.8 or 74.2 +/- 1.9%, of control (P < 0.001), respectively, indicating that the post-receptor events are critical for desensitization. [125I]iodo-rhFSH binding to the cells did not change significantly during desensitization and the different stimulations. In contrast, approximately 50% increase (P < 0.001) occurred in the steady-state levels of FSHR mRNA in the cells stimulated with FSH. This was apparently due to prolonged half-time of mRNA, and not to altered transcription, since the FSHR cDNA was driven by a powerful viral promoter. In accordance, the cells transfected with Simian Virus (SV40) promoter-driven luciferase gene did not display alterations in luciferase activity following stimulatory treatments. The effects of the post-receptor stimulations (NaF or CT) on [125I]iodo-rhFSH binding were minor (8-12% reduction). Taken together, these data provide evidence that the agonist-responsive hFSHR desensitization appears through a PKC-beta isoform-mediated modulation of cAMP production. The desensitization of FSH action involves modifications of functional properties of the existing components of the FSH signal transduction complex, and does not require concomitant suppression of transcription or translation of the FSHR gene.

PKC isoenzyme expression and cellular responses to phorbol ester in JEG-3 choriocarcinoma cells

Protein kinase C (PKc) is a key regulatory enzyme involved in the transduction of extracellular growth signals to the cell nucleus. It occurs in several isoforms, the exact functional roles of which have not been established as yet. The tumor-promoting agent 12-O-tetradecanoyl-phorbol acetate (TPA) is the classic activator of PKC and modulates the activity of the activating protein-1 (AP-1) transcription factor complex via this pathway. AP-1, in turn, induces cell proliferation in many tissues. In the present study, the PKC isoenzyme expression pattern in JEG-3 choriocarcinoma cells was analyzed. The results were compared with those obtained in HEC-1B endometrium adenocarcinoma cells, which had previously been characterized in this respect. To gain insight into the possible functional consequences of different PKC expression patterns, cell proliferation rates and AP-1 activity in response to TPA in both cell lines was studied. Western blot analysis of the PKC isoenzyme expression pattern revealed that JEG-3 cells are deficient in the PKC alpha, delta, and epsilon isoforms. These isoenzymes are strongly expressed in HEC-1B cells, with the alpha and delta being constitutively active. As opposed to HEC-1B cells, JEG-3 cells did not show an enhanced proliferation rate in response to TPA. Furthermore, TPA-treated JEG-3 cells did not exhibit any change in cell shape and refractility as observed in HEC-1B cells. AP-1 activity, as determined by a transfected AP-1-luciferase reporter plasmid, was induced 10-fold by TPA in JEG-3 cells, yet only threefold in HEC-1B cells. It is concluded from these data that differential expression of a subset of PKCs, e.g., the alpha, delta, and epsilon isoforms, may serve as an indicator of the proliferative potential in response to growth factors and mitogens. Furthermore, our data indicate that the inducibility of AP-1 activity does not necessarily reflect the proliferative capacity of a given cell type in response to classical tumor promoters such as phorbol ester.

Protein kinase C (PKC) isoenzyme expression pattern as an indicator of proliferative activity in uterine tumor cells

The protein kinase C (PKC) signal transduction pathway is the prototype of a growth factor-responsive intracellular signaling system, which is activated by various cytokines, growth factors and tumor promoters, such as the phorbol ester 12-O-tetradecanoyl-phorbol acetate (TPA). To date, a large number of different PKC isoforms has been identified, the physiological relevance of which is unknown. Moreover, the expression pattern of PKC isoforms in uterine cells has not been studied as yet. To study the functional role of differential PKC isoform expression in uterine tumor progression, we have compared the proliferative response to TPA, changes in cell morphology induced by TPA, and the PKC isoform expression pattern in two uterine tumor cell lines of different origin. The moderately differentiated endometrial HEC-1-B adenocarcinoma cell line showed a marked increase in proliferative activity and a profound morphological change in response to TPA. In contrast, TPA did not induce cell proliferation and/or morphological changes in the well-differentiated SKUT-1-B mixed mesodermal cell line. Analysis of the PKC isoform expression profile by Western blot revealed that PKC alpha, betaI, delta, epsilon, and zeta were expressed at a much higher level in HEC-1-B as compared to SKUT-1-B cells. PKC beta11 was the only isoenzyme to exhibit a higher expression level in SKUT-1-B cells. This is the first study analyzing the PKC isoform expression profile in uterine tumor cells. Our data demonstrate that the proliferative response to TPA correlates with the expression levels of the majority of PKC isoforms in these cells. Overexpression of PKC isoforms indicates a higher proliferative capacity, and may, thus, represent an important step in the pathogenesis of certain uterine malignancies.

Mechanisms of hormone and growth factor action in the bovine corpus luteum

The binding of hormones and growth factors to their cell surface receptors leads to an orderly cascade of events leading to activation of cytoplasmic effector molecules. The mechanism of action of luteinizing hormone involves the stimulation of multiple signal transduction effector systems including adenylyl cyclase and inositol phospholipid-specific phospholipase C (PLC). This results in the formation of second messengers that activate cAMP-dependent, Ca(2+)-dependent and lipid-dependent protein kinases. Prostaglandin F(2alpha) activates PLC which increases intracellular calcium and activates protein kinase C. This results in the activation of a series of protein kinases in the mitogen-activated protein (MAP) kinase cascade, leading to the activation of nuclear transcription factors c-fos and c-jun. Hormone responsive effector systems, therefore, operate by activating families of protein kinases which regulate cell metabolism, secretion, and gene transcription. Growth factors activate specific receptor protein tyrosine kinases which recruit additional signaling molecules (phospholipase Cgamma, phosphatidylinositol 3-kinase, Shc, Grb2, etc.) initiating a cascade of events mediated via MAP kinases. The signaling pathways activated by hormones interact or cross talk with the signaling pathways activated by growth factors. The diversity of cellular signaling mechanisms elicited by hormones and the potential for interactions with signals generated by growth factor receptor tyrosine kinases, may allow fine tuning of cellular responses during the life span of the corpus luteum.

Selective translocation of non-conventional protein kinase C isoenzymes by gonadotropin-releasing hormone (GnRH) in the gonadotrope-derived alpha T3-1 cell line

Gonadotropin-releasing hormone acts via G-protein coupled receptors to stimulate polyphosphoinositide-specific phospholipase C (PIC) with consequent elevation of cytosolic Ca2+ and activation of protein kinase C (PKC). Whereas Ca2+ is known to mediate stimulation of exocytotic gonadotropin release by GnRH, the identity of the PKC isoenzymes activated by GnRH and their physiological role in gonadotropes are poorly understood. In many systems translocation of PKC (from cytosolic to particulate fractions of cellular homogenates) has been taken as evidence of hormonal activation of PKC and down regulation of PKC (by prolonged treatment with PKC-activating phorbol esters) has been used extensively to investigate the role of PKC in hormone action. Here we have assessed the influence of GnRH and phorbol esters on translocation and down regulation of PKC isoenzymes identified by Western blotting with isoenzyme-specific antibodies in alpha T3-1 cells (a gonadotrope-derived cell line). These cells were found to posses PKCs alpha, epsilon and zeta but not beta, delta (present in rat pituitaries) or gamma (present in rat brains). In short-term stimulations (10 min), the PKC-activating phorbol esters, PMA and PDBu, caused concentration-dependent increases in the proportion of PKC alpha and PKC epsilon recovered from the particulate fraction of alpha T3-1 cells, but did not induce measurable translocation of PKC zeta. The inactive phorbol ester 4 alpha PDBu did not cause translocation of any of these isoenzymes. GnRH treatment induced a concentration-dependent increase in the proportion of particulate PKC epsilon and PKC zeta but had no measurable effect on PKC alpha translocation. In longer incubations (6-48 h) GnRH failed to cause measurable down-regulation of these isoenzymes whereas PMA treatment led to a clear down regulation of PKCs alpha and epsilon (albeit with different kinetics). The data demonstrate the differential activation and down regulation of PKC isoenzymes by GnRH versus PMA, which are clearly pertinent to the design of experiments intended to address the role of such isoenzymes in GnRH action. Moreover, they provide the first demonstration of hormonal regulation of an atypical PKC isoenzyme (PKC zeta) in pituitary cells.

Lys583 in the third extracellular loop of the lutropin/choriogonadotropin receptor is critical for signaling

The lutropin/choriogonadotropin receptor (LH/CG-R) contains a relatively large extracellular domain, in addition to the seven transmembrane helices (TMH), three extracellular loops (ECL), and three intracellular loops typical of G protein-coupled receptors. While high affinity ligand binding has been attributed to the N-terminal extracellular domain, there is evidence that portions of the three ECLs may function in ligand binding and transmembrane signaling. We have investigated the role of several ionizable amino acid residues of rat LH/CG-R in human choriogonadotropin (hCG) binding and hCG-mediated cAMP production. COS-7 cells were transfected with the pSVL expression vector containing cDNAs of either wild-type or mutant rat LH/CG-R. Several point mutants of Lys583, located at the interface of ECL III and TMH VII, bound hCG like wild-type receptor but exhibited greatly diminished ligand-mediated signaling. Neither the point mutant, Lys401-->Asp (ECL I), nor the double mutant, Asp397-->Lys/Lys583-->Asp (ECLs I and III, respectively, showed significant hCG binding to intact cells; in detergent-solubilized cells, only the double mutant bound hCG. The mutants Arg341-->Glu (interface of the extracellular domain and TMH I) and Lys488-->Glu (ECL II) proved to be similar to wild-type receptor in binding and signaling. Our results establish that Lys583 is important in signaling but not ligand binding. Its location on the opposite side of the membrane from GS precludes a direct interaction, thus emphasizing the importance of a conformational change in the receptor and suggesting that ligand binding to receptor and ligand-mediated receptor activation are dissociable phenomena.