Resensitization of lutropin-desensitized tumour Leydig-cell adenylate cyclase with human erythrocyte membranes

The inhibitory GTP-binding protein (Gi) occurs in rat Leydig cells and is differentially modified by lutropin and 12-O-tetradecanoylphorbol 13-acetate

Luteinizing-hormone (LH)-stimulated cyclic AMP production in rat testis Leydig cells was desensitized by both LH and 12-O-tetradecanoylphorbol 13-acetate (TPA). However, TPA, but not LH, enhanced the subsequent response to cholera toxin. Treatment of the cells with pertussis toxin potentiated cyclic AMP production in both control and LH-desensitized cells, but did not potentiate further the responses obtained by TPA pretreatment. The results implicate the presence of an inhibitory GTP-binding protein (Gi), which may be inhibited by TPA. The presence of a Gi-like protein within the plasma membrane of Leydig cells was demonstrated by pertussis-toxin-catalysed [32P]ADP-ribosylation of a Mr-40000-41000 protein.

Adrenal adenylate cyclase and steroidogenic activities of 63 day old ovine fetuses: in vitro effects of ACTH1-24 and forskolin

This study examines the activity of the adenylate cyclase system and that of some enzymes of the steroidogenic pathway of adrenal cells from 62-63 day old ovine fetuses. Synthetic corticotropin (ACTH1-24), cholera toxin and forskolin stimulated both cAMP and corticoid productions by freshly isolated adrenal cells. The cAMP response to ACTH1-24 was lower than that to forskolin. However, forskolin-induced steroidogenesis was significantly lower than the ACTH1-24-induced steroid output. Freshly isolated cells metabolized quickly [14C]-labeled pregnenolone mainly through the 17-deoxy pathway. The amounts of cortisol and of corticosterone formed, in the presence of exogenous pregnenolone, were roughly 15-fold higher than under maximal stimulation by ACTH1-24. When the cells were cultured for 6 days in the absence or presence of ACTH1-24 (10(-8) M) or forskolin (10(-5) M), a small development of the cAMP response to these factors was observed in the course of the experiment. However, the mechanism of this development appeared different, according to the conditions of culture. The amounts of corticosterone secreted on day 6 by ACTH1-24- or forskolin-treated cells were 2- to 4-fold higher than on day 1, whereas cortisol outputs were much lower on day 6 than on day 1. The response to ACTH1-24 of cells maintained in ACTH-free media decreased dramatically during the culture in terms of both cortisol and of corticosterone. On day 6 of the experiment, the metabolism of [14C]pregnenolone was lower than on day 1 under all 3 conditions of culture. Only the 3 beta-hydroxysteroid dehydrogenase/isomerase activity could be maintained by continuous treatment with forskolin. However, both ACTH1-24 and forskolin enhanced the production of pregnenolone from an endogenous substrate. In conclusion, these results present evidence that: 1) the adenylate cyclase system is not a bottleneck in the steroidogenic response to ACTH1-24 of freshly isolated adrenal cells from 62-63 day old ovine fetuses; 2) the main rate-limiting step for steroidogenesis by these cells is the availability of pregnenolone; 3) neither ACTH1-24 nor forskolin is able to maintain the activity of most enzymes involved in the metabolization of pregnenolone by cultured cells while increasing pregnenolone availability; 4) some inhibiting factors are involved in the loss of adrenal cells responsiveness to ACTH between days 50 and 100 of gestation, and they probably act mainly on the adenylate cyclase system.

Similarities and differences in phorbol ester- and luteinizing-hormone-induced desensitization of rat tumour Leydig-cell adenylate cyclase

The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) was shown to mimic luteinizing hormone (LH; lutropin) in causing desensitization of LH-mediated cyclic AMP production in tumour Leydig cells. However, there were differences between LH- and TPA-induced desensitization: (1) TPA induced a more rapid effect than LH; (2) adenosine did not inhibit TPA-induced desensitization, whereas it completely inhibited the LH-induced desensitization; (3) adenylate cyclase activity in plasma membranes from TPA-desensitized cells was not decreased, whereas similar preparations from LH-desensitized cells lost their response to LH and to LH plus guanosine 5'-[beta gamma-imido]triphosphate; TPA-, but not LH-, treated cells had a decreased capacity to respond to cholera toxin and forskolin. These results indicate that LH and phorbol esters induce desensitization of adenylate cyclase in rat tumour Leydig cells by different mechanisms.

Adenosine potentiates lutropin-stimulated cyclic AMP production and inhibits lutropin-induced desensitization of adenylate cyclase in rat Leydig tumour cells

The action of adenosine on lutropin (LH)-stimulated cyclic AMP production and LH-induced desensitization of adenylate cyclase in rat Leydig tumour cells was investigated. Adenosine and N6-(phenylisopropyl)adenosine caused a dose-dependent potentiation of LH-stimulated cyclic AMP production at concentrations (0.01-10 microM) which alone did not produce an increase in cyclic AMP production. However, 2-deoxyadenosine had no effect either alone or in combination with LH on cyclic AMP production. The potentiation produced by adenosine was unaffected by concentrations of the specific nucleoside-transport inhibitor dipyridamole, which inhibited [3H]adenosine uptake by up to 90%. The phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine, but not RO-10-1724, inhibited the adenosine-induced potentiation. In the presence of adenosine, the kinetics of LH-stimulated cyclic AMP production were linear with time up to 2h, compared with those with LH alone, which showed a characteristic decrease in rate of cyclic AMP production after the first 15-20 min. Consistent with the altered kinetics, adenosine also inhibited the LH-induced desensitization of adenylate cyclase. These results suggest that adenosine has effects on rat tumour Leydig cells through receptors on the external surface of the plasma membrane. This receptor has characteristics similar to those of the R-type receptors, which have been shown either to stimulate or to inhibit adenylate cyclase. However, the effects of adenosine in the present studies does not involve a direct inhibition or activation of adenylate cyclase, but may involve an as yet undefined receptor-mediated modulation of adenylate cyclase.

Involvement of the regulatory protein (Ns) in the maturation of ACTH-sensitive adenylate cyclase of ovine fetal adrenal during late gestation

The responsiveness to several stimuli of the adenylate cyclase of crude adrenal membranes from fetal and neonate lambs has been measured under different conditions. The response to Gpp(NH)p, ACTH1-24 + Gpp(NH)p and NaF was significantly lower in membranes from fetuses than from neonate lambs, whereas the response to forskolin was similar. Addition of human erythrocyte ghosts to fetal adrenal membranes enhanced the stimulation of cyclase by NaF and Gpp(NH)p, which became similar to that observed in membranes from neonates. However, such a "complementation' did not enhance the response of the enzyme to ACTH1-24. The adenylate cyclase activity displayed a biphasic response to GTP under basal conditions or when stimulated by forskolin. The magnitude of the inhibition achieved with 10(-3) M GTP was similar in fetal and neonatal adrenal membranes. These data, together with previous results (Endocrinology, 1981, 108, 2114-2119), show that the maturation of the ACTH-sensitive adenylate cyclase system of the ovine fetal adrenal gland involves an enhancement of the number of ACTH receptors and a development of the Ns subunit, whereas the catalytic site is hardly (if at all) involved in this process.

A transplantable rat Leydig cell tumor—2. Adenylyl cyclase activation by prostaglandin E1, isoproterenol and glucagon

The responsiveness of the membrane bound adenylyl cyclase (AC) system to various hormones and hormone analogues in a transplantable rat Leydig cell tumor (H-540) has been investigated and compared with that of interstitial tissue from normal adult rats. The results can be summarized as follows: The AC of the Leydig cell tumors was stimulated by isoproterenol, epinephrine, norepinephrine, dopamine, glucagon, PGE1, PGF2 alpha and LH/hCG. The AC of the Leydig cell tumors exhibited a much greater response to PGE1 than the AC in normal interstitial tissue (5-fold and 2-fold stimulation, respectively). The AC of the Leydig cell tumours also revealed a somewhat higher response to isoproterenol than the AC of interstitial tissue (2.5-fold and 2-fold, respectively). Adenylyl cyclase in both tissues were equally stimulated by glucagon (2-fold). The LH response in the Leydig cell tumors was only half of that found in normal Leydig cells. This study indicates, that in spite of the fact that tumor Leydig cells respond to LH and produce testosterone, the response pattern of the AC is different from that of normal Leydig cells.

A transplantable rat Leydig cell tumor--1. LH and prolactin receptors and effects of the endocrine status of the host animal

We have examined the binding capacity and properties (affinity, specificity) of LH and prolactin (Prl) receptors in a transplantable rat Leydig cell tumor (H-540) grown in intact, castrated and hypophysectomized rats. LH receptors in adult rat testis and Prl receptors in the rat ventral prostate were examined simultaneously for comparison. The results can be summarized as follows: The qualitative properties (affinity, specificity) of LH and Prl receptors in tumor Leydig cells appear to be identical to those of corresponding receptors in non-tumor tissues. The levels of LH receptors in tumor Leydig cells are only some 1% of that present in normal Leydig cells from adult rats. Tumor Leydig cells grown in hypophysectomized rats had even lower levels of LH receptors; ca. 1/3 of that found in tumors from intact rats. The levels of Prl receptors in the tumor Leydig cells are almost as high as in normal Leydig cells from adult rats. In tumors grown in hypophysectomized rats, the levels of Prl receptors were much lower (ca. 20%) than in tumors from intact or castrated rats. There were great variations in the number of LH and Prl receptors in individual tumors, and there was a positive correlation (r = 0.88; P less than 0.01) between LH and Prl receptors in individual tumors. No differentiation toward a "LH receptor tumor" or "Prl receptor tumor" was observed. Thus, receptors for LH and Prl in tumor cells are qualitatively normal, but the number is greatly (LH) or moderately (Prl) reduced. These receptors in the tumor Leydig cells are stimulated by pituitary hormones.

Characterization of the homologous and heterologous desensitization of rat Leydig-tumour-cell adenylate cyclase

The homologous and heterologous desensitization of rat Leydig-tumour-cell adenylate cyclase induced by lutropin (LH) was characterized with the aid of forskolin and cholera toxin. Forskolin stimulated cyclic AMP production in a dose-dependent manner, with linear kinetics up to 2h. Forskolin also potentiated the action of LH on cyclic AMP production, but was only additive with cholera toxin. Preincubation of rat Leydig tumour cells with LH (1.0 micrograms/ml) for 1 h produced a desensitization of the subsequent LH (1.0 micrograms/ml)-stimulated cyclic AMP production, whereas the responses to cholera toxin (5.0 micrograms/ml), forskolin (100 microM), LH plus forskolin or cholera toxin plus forskolin were unaltered. In contrast, preincubation with LH for 20h produced a desensitization to all the stimuli tested. When rat Leydig tumour cells were preincubated for 1h with forskolin or dibutyryl cyclic AMP, the only subsequent response that was significantly altered was that to LH plus forskolin after preincubation with forskolin. However, preincubation for 20h with forskolin or dibutyryl cyclic AMP induced a desensitization to all stimuli subsequently tested. LH produced a rapid (0-1h) homologous desensitization, which was followed by a slower (2-8h)-onset heterologous desensitization. Forskolin and dibutyryl cyclic AMP were only able to induce heterologous desensitization. The rate of desensitization induced by either forskolin or dibutyryl cyclic AMP was similar to the rate of heterologous desensitization induced by LH. These results demonstrate that in purified rat Leydig tumour cells LH produces an initial homologous desensitization of adenylate cyclase that involves a cyclic AMP-independent lesion at or proximal to the guanine nucleotide regulatory protein (G-protein). This is followed by heterologous desensitization, which can also be induced by forskolin or dibutyryl cyclic AMP, thus indicating that LH-induced heterologous desensitization of rat Leydig-tumour-cell adenylate cyclase involves a cyclic AMP-dependent lesion that is after the G-protein.

Desensitization of the cAMP system in mouse Leydig cells by hCG, cholera toxin, dibutyryl cAMP and cAMP: localization of the 'lesion' to the guanine nucleotide regulatory protein-adenylate cyclase complex

The mechanism of hCG-induced desensitization of the cAMP system was studied in Percoll-purified mouse Leydig cells. Pretreatment of Leydig cells with hCG resulted in a time- and dose-dependent decrease in the capacity of hCG-induced cAMP formation. Maximal desensitization (approximately 90%) was induced by only partial prior stimulation. Desensitization, however, was not observed without a prior increase in cAMP or testosterone production. Pretreatment of the cells with N6,O2'-dibutyryl cAMP (DBcAMP) also induced a dose- and time-dependent densensitization. cAMP was only effective in the presence of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (MIX). Cholera toxin desensitized the hormone-induced cAMP response as drastically as hCG. Cholera toxin was unable to reverse the refractory state induced by one of the agonists. hCG-induced desensitization was not associated with a loss in [125I]hCG binding or an increase in maximal phosphodiesterase activity, and appeared not to be dependent on protein synthesis. Membranes from hCG, cholera toxin of DBcAMP-desensitized cells showed an impaired adenylate cyclase activity in response to hCG, hCG plus beta-gamma-imidoguanosine 5'-triphosphate (GPPNP) and NaF. In conclusion, hCG-induced desensitization of the adenylate cyclase system in mouse Leydig cells can be mimicked by cholera toxin, DBcAMP and cAMP, indicating a cAMP-mediated process. The site of the 'lesion' has to be localized to the guanine nucleotide regulatory protein-adenylate cyclase complex rather than to its uncoupling from the hormone receptor.

Challenge of hepatocytes by glucagon triggers a rapid modulation of adenylate cyclase activity in isolated membranes

Membrane fractions obtained from hepatocytes treated with glucagon exhibited a decreased glucagon (with or without GTP)-stimulated adenylate cyclase activity. A maximum effect was seen in around 5 min. No change in the rate of cyclic AMP production was observed for the basal, NaF-, p[NH]ppG (guanosine 5'-[beta, gamma-imido]-triphosphate)- and GTP-stimulated states of the enzyme. The lag observed in the p[NH]ppG-stimulated adenylate cyclase activity of native membranes was abolished when membranes from glucagon-pretreated cells were used. When Mn2+ replaced Mg2+ in the assays, the magnitude of the apparent desensitization was decreased. Mn2+ abolished the lag of onset of p[NH]ppG-stimulated activity in native membranes. The desensitization process was dose-dependent on glucagon, which exhibited a Ka of 4 X 10(-10) M. Depletion of intracellular ATP did not affect this process. It is suggested that this desensitization occurs at the level of the guanine nucleotide-regulatory protein.