Effects of phorbol esters on steroidogenesis in small bovine luteal cells

Mono-(2-ethylhexyl) phthalate stimulates basal steroidogenesis by a cAMP-independent mechanism in mouse gonadal cells of both sexes

Phthalates are widely used as plasticizers in a number of daily-life products. In this study, we investigated the influence of mono-(2-ethylhexyl) phthalate (MEHP), the active metabolite of the frequently used plasticizer di-(2-ethylhexyl) phthalate (DEHP), on gonadal steroidogenesisin vitro. MEHP (25–100 μM) stimulated basal steroid synthesis in a concentration-dependent manner in immortalized mouse Leydig tumor cells (MLTC-1). The stimulatory effect was also detected in KK-1 granulosa tumor cells. MEHP exposure did not influence cAMP or StAR protein levels and induced a gene expression profile of key steroidogenic proteins different from the one induced by human chorionic gonadotropin (hCG). Simultaneous treatment with MEHP and a p450scc inhibitor (aminoglutethimide) indicated that MEHP exerts its main stimulatory effect prior to pregnenolone formation. MEHP (10–100 μM) up-regulated hormone-sensitive lipase and 3-hydroxy-3-methylglutaryl coenzyme A reductase, suggesting that MEHP increases the amount of cholesterol available for steroidogenesis. Our data suggest that MEHP, besides its known inhibitory effect on hCG action, can directly stimulate gonadal steroidogenesis in both sexes through a cAMP- and StAR-independent mechanism. The anti-steroidogenic effect of DEHP has been proposed to cause developmental disorders such as hypospadias and cryptorchidism, whereas a stimulation of steroid synthesis may prematurely initiate the onset of puberty and theoretically affect the hypothalamic–pituitary–gonadal axis.

Paradoxical effect of 3-isobutyl-1-methylxanthine on cytochrome P450 cholesterol side-chain cleavage mRNA accumulation in porcine granulosa cells

Earlier studies in immature porcine granulosa cells cultured in serum-free medium showed dual actions of the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA). In cells incubated for 24 h, TPA inhibited follicle-stimulating hormone (FSH)-stimulated cytochrome P450 cholesterol side-chain cleavage (P450scc) mRNA accumulation. In contrast, at 4 h, TPA increased P450scc mRNA concentration in the absence and presence of FSH or 8-bromo-cAMP; in addition, TPA augmented FSH-stimulated cAMP accumulation. The actions of TPA were then examined in the presence of the phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX). With IBMX present, TPA caused a smaller relative augmentation of cAMP accumulation during a 4-h incubation period, suggesting that TPA may both increase cAMP synthesis and inhibit its degradation. The stimulatory effect of FSH or 8-bromo-cAMP on P450scc mRNA concentration was not modified by IBMX. However, TPA no longer augmented the FSH- or 8-bromo-cAMP-stimulated P450scc mRNA accumulation when IBMX was present. In cells treated with FSH for 24 h, IBMX augmented progesterone production, but paradoxically accentuated the inhibitory effect of TPA on steroidogenesis. These results indicate that IBMX converts TPA from a stimulatory into an inhibitory agent by an action unrelated to cAMP, and points to the need for caution in interpreting experiments with this drug.

Steroidogenic enzyme activity after acute activation of protein kinase (PK) A and PKC in ovine small and large luteal cells

Intracellular effector systems which utilize PKA and PKC can be pharmacologically activated by forskolin and phorbol 12-myristate 13-acetate (PMA) and appear to be important for regulation of steroidogenesis by cells of the corpus luteum. In this study the effect of pharmacologic activation of PKA (forskolin) or PKC (PMA) on the activity of adenylate cyclase, cholesterol esterase, P450 cholesterol side chain cleavage (P450scc) and 3 beta-hydroxysteroid dehydrogenase/delta 5, delta 4 isomerase (3 beta HSD) was determined. Basal adenylate cyclase activity (as measured by intracellular and secreted cAMP) was extremely low in both large and small luteal cells. Forskolin stimulated adenylate cyclase activity in both large and small luteal cells but progesterone production was increased only in small cells. PMA inhibited progesterone production by large and forskolin-stimulated small cells without altering adenylate cyclase activity. Basal cholesterol esterase activity was greater in small than in large cells and was stimulated by forskolin only in small cells. PMA did not significantly alter cholesterol esterase activity in either cell type. Activity of P450scc or 3 beta HSD was measured by conversion of hydroxylated cholesterol derivatives (P450scc) or pregnenolone (3 beta HSD) to progesterone. Although basal progesterone production was 47 times greater in large than small cells, there was only 5.1 (P450scc) and 6.4 (3 beta HSD) times greater enzyme activity in large than in small luteal cells. Activation of PKA and/or PKC did not alter the activity of P450scc or 3 beta HSD in either cell type.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of CYP11A gene expression in bovine ovarian granulosa cells in primary culture by cAMP and phorbol esters is conferred by a common cis-acting element

Production and secretion of steroid hormones throughout the ovarian cycle occurs in a highly episodic and coordinated fashion that requires precise and finely tuned regulatory mechanisms. The regulation of ovarian steroidogenesis by the gonadotropin follicle stimulating hormone (FSH) and luteinizing hormone (LH) as well as by other factors occurs, at least in part, at the level of expression of the genes encoding steroidogenic enzymes. The present study is aimed at the elucidation of regulatory mechanisms by which cyclic adenosine monophosphate (cAMP) and protein kinase C regulate cytochrome P450scc (CYP11A) gene expression in bovine granulosa cells in primary culture. As a first step we characterized the bovine granulosa cell cultures with regard to regulation of P450scc activity and mRNA levels upon treatment with forskolin and/or the phorbol ester TPA. Forskolin, a potent stimulator of cAMP generation, increased both progesterone secretion and P450scc mRNA levels. In contrast, treatment with TPA alone decreased both basal progesterone production and P450scc mRNA accumulation. Co-treatment with forskolin and TPA decreased progesterone and P450scc mRNA levels as compared to forskolin treatment alone. The possibility that TPA interfered with the forskolin-stimulated cAMP production could be excluded because simultaneous treatment of granulosa cells with TPA and forskolin potentiated the formation of cAMP. In order to identify regulatory sequences within the 5' flanking region of the bovine CYP11A gene, chimeric DNA constructs comprizing regions of the CYP11A gene fused to a beta-globin-derived reporter gene were transfected into granulosa cells in primary culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of the phospholipase C second messenger system in the regulation of steroidogenesis in small bovine luteal cells

We have previously suggested that the interaction between luteinizing hormone (LH) and its receptor, in addition to stimulating adenylate cyclase, is able to trigger a negative regulatory signal at a step beyond cAMP synthesis (Benhaim et al. (1987) FEBS Lett. 223, 321-326). The present study was conducted to determine whether the phospholipase C system is involved in this phenomenon. Small bovine luteal cells from pregnant cows were incubated with phospholipase C, A23187, an ionophore of calcium and/or phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), in the presence or absence of bovine luteinizing hormone or dibutyryl cyclic AMP (dbcAMP). A23187 associated with PMA was able to mimic the stimulatory effect of phospholipase C on basal progesterone production, whereas neither A23187 nor PMA alone had any effect. In the presence of high doses of LH, phospholipase C inhibited progesterone and cAMP production in a dose-dependent manner. A23187 and PMA were able to mimic the inhibition of progesterone synthesis but stimulated LH-induced cAMP accumulation. When cells were stimulated by high doses of dbcAMP, phospholipase C and A23187 but not PMA inhibited progesterone synthesis. These observations suggest that (1) phospholipase C can mimic the post-cAMP negative regulatory signal induced in vitro by high doses of LH, in the presence of an activation of PKC; (2) phospholipase C is also able to mimic in vitro the luteolytic properties of prostaglandin F2 alpha that we previously described (Benhaim et al. (1987) Prostaglandins 33, 227-239); and (3) under basal conditions or in the presence of low doses of LH, the phospholipase C system slightly stimulates steroidogenesis.