Modulation of cultured mouse Leydig cells adenylate cyclase by forskolin and hCG

Comparative effects of sub-stimulating concentrations of non-human versus human Luteinizing Hormones (LH) or chorionic gonadotropins (CG) on adenylate cyclase activation by forskolin in MLTC cells

We have compared various Luteinizing Hormone (LH) and Chorionic Gonadotropin (CG) preparations from non-human and human species in their ability to synergize with 10 µM forskolin (FSK) for cyclic AMP intracellular accumulation, in MLTC cells. LH from rat pituitary as well as various isoforms of pituitary ovine, bovine, porcine, equine and human LHs and equine and human CG were studied. In addition, recombinant human LH and CG were also compared with the natural human and non-human hormones. Sub-stimulating concentrations of all LHs and CGs (2-100 pM) were found to stimulate cyclic AMP accumulation in MLTC cells in the presence of an also non-stimulating FSK concentration (10 µM). Like rat LH, the most homologous available hormone for mouse MLTC cells, all non-human LHs and CG exhibit a strong potentiating effect on FSK response. The human, natural and recombinant hLH and hCG also do so but in addition, they were found to elicit a permissive effect on FSK stimulation. Indeed, when incubated alone with MLTC cells at non-stimulating concentrations (2-70 pM) hLH and hCG permit, after being removed, a dose-dependent cyclic AMP accumulation with 10 µM FSK. Our data show a clearcut difference between human LH and CG compared to their non-human counterparts on MLTC cells adenylate cyclase activity control. This points out the risk of using hCG as a reference ligand for LHR in studies using non-human cells.

Endocrine cell lines from the placenta

Cell-lines derived from human placenta and chorion have been used extensively to model the endocrine functions of human trophoblast. In general terms, the endocrine functions of the primary cells and tissues are at least partially replicated within the cell-lines, suggesting that they may be used as appropriate models. There are, however, two major provisos that compromise this generalisation. Firstly, the endocrine function of placenta represents a complex interaction between cytotrophoblast, syncytiotrophoblast and multiple regulators, so a single cell population digested from the normal environment is unlikely to represent this. Secondly, the characterisation of primary trophoblast populations and of cell-lines is incomplete, complicating the assignment of functions to trophoblast populations. Despite these difficulties, useful information has been obtained from the available cell-lines, regardless of whether they have arisen spontaneously, been transformed in vitro, or derived from cancers in vivo.

Differences in LH receptor down-regulation between rat and mouse Leydig cells: effects of 3',5'-cyclic AMP and phorbol esters

The role of cyclic AMP and phorbol esters in luteinizing hormone (LH) receptor down-regulation in Leydig cells has been studied. Dibutyryl cyclic AMP (db-cAMP) (0.01, 0.1 and 1 mM), forskolin (80 microM) and cholera toxin (1.19 nM) caused a 30-50% loss of [125I]hCG binding sites and an inhibition of receptor-[125I]hCG complex internalization in mouse tumour Leydig (MA10, MLTC-1) cells during 2 h. In contrast, db-cAMP had no effect on the level of binding sites or internalization of the hormone receptor complex in rat testis Leydig cells or a rat tumour (R2C) Leydig cell. Phorbol 12-myristate 13-acetate (PMA) at concentrations from 10(-9) to 10(-5) M had no effect on hormone binding or hormone-receptor complex internalization in any of the Leydig cells. In contrast a 2 h preincubation of MLTC-1 cells with 10(-7) M PMA caused a loss of subsequent LH-stimulated cyclic AMP and pregnenolone production. These results indicate that LH receptor down-regulation is mediated by cyclic AMP dependent kinase, but not protein kinase C, in mouse Leydig cells. No down-regulation of rat Leydig cell LH receptor occurs with either kinase.

Inhibition of luteinizing hormone-human chorionic gonadotropin binding by retinoids in a Leydig cell line

Treatment of K9 mouse Leydig cells with 3 x 10(-6) M retinol (R) and retinoic acid (RA) resulted in 75% and 65% reduction of 125I-labeled hCG binding respectively, when assayed at 35 degrees C. This effect was dose-dependent and was first detected 12 h after initiation of treatment: it was maximal at 48 h for RA. R and RA had no significant effect on the rate of internalization and degradation of 125I-hCG as measured by disappearance of acid-releasable (i.e. surface-bound) radioactivity from the cells and by the appearance of trichloracetic acid-soluble label in the medium. When exposed to increasing concentrations of hCG for 24 h, both retinoid-treated and control cells 'down-regulated' their gonadotropin receptors with the same dose-dependent pattern. The kinetics of reappearance of the receptors was similar for retinoid-treated and control cells, but for treated cells the maximal number of receptors reinitiated at 24 h never exceeded 40% of the values observed with control cells. Scatchard plot analysis confirmed a decrease in hCG receptor number from approximately 26,000 to approximately 6400 and approximately 3500 sites per cell after R and RA treatment. Kd values for 125I-hCG binding were 2 x 10(-10) M, 7.3 x 10(-11) M and 6.9 x 10(-11) M for control, R- and RA-treated cells respectively. On the basis of our data it is likely that retinoid-induced reduction in 125I-hCG binding to K9 Leydig cells is due to decreased receptor synthesis.

A cAMP independent inhibitory action of high doses of forskolin in rat Leydig cells

In addition to well known direct stimulatory and potentiatory actions of forskolin, we have previously reported that low doses of this diterpene (10(-9), 10(-12) M) markedly inhibit the production of cAMP and testosterone in rat Leydig cells through a pertussis toxin sensitive G-protein (A. Khanum and M. L. Dufau, J. Biol. Chem. 261, 1986). A different type of inhibitory effect of forskolin is described in this study. Forskolin (10(-5) M) markedly stimulates basal adenylate cyclase activity (about 200%) in rat Leydig cell membranes and potentiates the stimulatory effect of gonadotropin (10(-9), 10(-7) M) on adenylate cyclase in presence or in absence of GTP (10(-5) M). Similarly a time-dependent stimulation of forskolin (10(-5) M) alone is noted on all cAMP pools and testosterone production. Using a supramaximal steroidogenic dose of hCG (0.26 nM) or choleragen (0.1 microM), forskolin potentiates the gonadotrophin and toxin-induced responses of all cAMP pools significantly while inhibiting testosterone production. Moreover, forskolin also inhibits 8-Bromo-cAMP stimulated steroidogenesis. In contrast, pregnenolone synthesis was not altered by the diterpene. We have demonstrated in this study that the inhibitory effect of high doses of forskolin on steroidogenesis is distal to cAMP generation, and resulted from a steroidogenic block residing beyond pregnenolone synthesis.

Forskolin inhibition of cyclic AMP generation in J774 macrophages

Forskolin inhibited cyclic AMP generation in J774 macrophage cells in response to isoproterenol. Forskolin, 10 nM-0.1 mM, also inhibited the adenylate cyclase activity of membrane preparations. The basal activity and the isoproterenol-, cholera toxin-, fluoride- or GppNHp-stimulated activities were maximally depressed by 10 microM forskolin (30-70% inhibition, EC50 = 0.3-0.5 microM). This effect was achieved similarly in membranes from pertussis toxin-treated cells. Forskolin required guanine nucleotides for inhibition. In the absence of GTP the decrease in basal activity was reversed into stimulation (EC50 = 10 microM forskolin). Reversal of inhibition into activation also followed the addition of 1 mM MnCl2 (EC50 = 10 microM forskolin). 1,9-Dideoxyforskolin was ineffective to alter adenylate cyclase activity. In contrast, a water-soluble derivative of forskolin was as active as forskolin to regulate activity. The results suggest that forskolin may interact with adenylate cyclase to cause either activation or inhibition depending on the degree of activation of Ns and on its interaction with the catalyst.

Steroidogenesis expression depends on negative control(s): analysis in Leydig X adrenal intraspecific cell hybrids

Hybrids constructed by fusing mouse Leydig cells with mouse adrenal Y1 cells were able to randomly express all the parental specific traits but for the response to gonadotropin (hCG) and corticotropin (ACTH): three of them, YDYL 14, 17 and 19, metabolized both progesterone and dehydroepiandrosterone into testosterone accounting for 17 alpha-hydroxylase, 17-20-lyase, 17-ketoreductase and 3 beta-hydroxysteroid dehydrogenase activities. Under basal conditions, 17 alpha-hydroxylase and 17-20-lyase activities were high in the three clones as compared to parental Leydig cells, and were no longer stimulated by cAMP in YDYL 17 and 19. The hybrids responded to various hormones such as prostaglandin E2 (PGE2), vasoactive intestinal peptide (VIP) and prolactin (PRL) which are not directly implicated in the expression of steroidogenesis; they generally retained the Y1 morphological response to 8-bromo cAMP. On extended culture, reexpression of ACTH sensitivity occurred in one clone, YDYL 9. This reexpression was correlated with a Robertsonian translocation between mouse chromosomes 2 and 11, while extinction required the presence of an intact mouse chromosome 11.

Concurrent LH and forskolin action on adenylate cyclase activation and progesterone synthesis in corpora lutea from pregnant ewes

The present communication documents LH- and forskolin-induced activation of adenylate cyclase (AC) system and progesterone synthesis in corpora lutea from pregnant ewes. The activation of AC in plasma membranes by LH or forskolin was amplified by Gpp(NH)p. These results suggest that regulatory nucleotide component (Ns) of the AC complex is required for forskolin. Simultaneous addition of maximal concentrations of forskolin (10(-4) M), Gpp(NH)p (10(-4) M) and LH (10(-7) M) led to greater than additive (i.e. synergistic) responses: the experimental value was 4.71 +/- 0.19 nmoles cAMP/mg of membrane protein, whereas the theoretical additive effect was 3.17 +/- 0.10 nmoles/mg of membrane protein (p less than 0.001). These data reveal that more Ns or C component is being activated in these cells when combined treatments with these agents are applied. In intact cells maximum stimulatory concentrations of forskolin or LH caused similar increase in progesterone production with similar time courses. In striking contrast, the exposure of the luteal cells to LH and forskolin simultaneously led to a decrease in progesterone synthesis as early as 1h30 (40%, p less than 0.001). Thus, the synergism observed between LH and forskolin on the stimulation of plasma membranes AC activity did not occur in steroidogenesis. The AC responses in crude plasma membranes form these cells to different stimulants were enhanced (i.e. 15%, p less than 0.2 for Gpp(NH)p, 33%, p less than 0.01 for LH plus Gpp(NH)p and 52%, p less than 0.01 for forskolin). These findings suggest that an early desensitization of the AC system cannot explain the impaired steroidogenic response observed.

Construction of a Leydig cell line synthesizing testosterone under gonadotropin stimulation: a complex endocrine function immortalized by cell hybridization

Hybridization between a mouse Leydig tumor cell line, MA-10, which produces cyclic AMP and progesterone under human chorionic gonadotropin (hCG) stimulation, and freshly isolated mouse Leydig cells gave rise to 54 hybrid clones, one of which, LK17, was capable of hCG-stimulated testosterone production. Subcloning of this hybrid resulted in the emergence of a subclone, K9, whose testosterone production is more than 10 times that of parent clone LK17, after hCG stimulation, with an ED50 of 37 pM. Testosterone synthesis by K9 cells was multiplied by 25 after gonadotropin stimulation, and binding of hCG declined after prolonged exposure to the hormone. These similarities with murine Leydig cells in primary culture make the K9 clone an attractive alternative for physiological studies.

Systematic shut-off of the hormone receptors in intraspecific adrenal x Leydig cell hybrids

The mouse Y1 adrenal cell line was fused with mouse Leydig cells in primary culture. The selected hybrids were examined for their response to gonadotropin (hCG) and ACTH. None of them bound specifically [125I]hCG, nor did they augment their cAMP production in response to gonadotropin or ACTH stimulation, whereas their adenylate cyclase remained responsive to forskolin and cholera toxin, thus indicating a repression of hCG receptor synthesis and probably a loss of ACTH receptors, rather than a lesion of the coupling between the hormone receptor complex and the adenylate cyclase. Basal pregnenolone production in 17 hybrids was close to that of Leydig and Y1 cells and was enhanced after 8-bromo adenosine 3',5'-monophosphate (8-Br-cAMP) stimulation in 11 of them. Therefore, the negative control leading to the extinction of both parental functions acts preferentially at the first step of steroidogenesis, i.e., the gene(s) coding for the hormone receptors.