The stimulatory effect of corticotropin on cortisol biosynthetic pathway in guinea-pig adrenocortical cells

Gap junction-mediated cell-to-cell communication in bovine and human adrenal cells. A process whereby cells increase their responsiveness to physiological corticotropin concentrations

We have studied the role of gap junction-mediated intercellular communication on the steroidogenic response of bovine (BAC) and human (HAC) adrenal fasciculo-reticularis cells in culture to corticotropin (ACTH). Indirect immunofluorescence analyses showed that intact human and bovine adreno-cortical tissue as well as HAC and BAC in culture expressed the gap junction protein connexin43 (also termed alpha 1 connexin). Both HAC and BAC were functionally coupled through gap junctions as demonstrated by microinjection of a low molecular mass fluorescent probe, Lucifer yellow. The cell-to-cell transfer of the probe was blocked by 18 alpha-glycyrrhetinic acid (GA), an inhibitor of gap junction-mediated intercellular communication. GA markedly decreased the steroidogenic response (cortisol production) of both HAC and BAC to low (10 pM) but not to high (5 nM) concentrations of ACTH. GA had no inhibitory effect on the steroidogenic response to 8 Br-cAMP (at either low or high concentrations) and did neither modify the binding of 125I-ACTH to its receptor nor the ACTH-induced cAMP production. BAC cultured at high or low cell densities (2.4 x 10(5) vs. 0.24 x 10(5) cells/cm2) exhibited distinct levels of intercellular communication and were differently responsive to sub-maximal ACTH concentrations. The ACTH ED50 values for cortisol production were 8.5 +/- 1.3 and 45 +/- 14 pM (P < 0.02) for BAC cultured at high and low density, respectively. In the presence of GA, there was a shift of the ACTH concentration-response curves in the two culture conditions. The ACTH ED50 of high density and low density cultured BAC increased 25- and 5-fold, respectively, and became similar (220 +/- 90 and 250 +/- 120 pM). These results demonstrate that gap junction-mediated communication between hormone-responsive and nonresponsive cells is one mechanism by which adrenal cells increase their responsiveness to low ACTH concentrations.

Effect of chronic ACTH treatment on guinea-pig adrenal steroidogenesis: steroid plasma levels, steroid adrenal levels, activity of steroidogenic enzymes and their steady-state mRNA levels

We report here the effects of a 7-day treatment of guinea-pigs with ACTH on adrenal mRNA levels for steroid-transforming enzymes. Adrenal 3 beta-hydroxysteroid dehydrogenase 4-ene-5-ene-isomerase (3 beta-HSD), 17-hydroxylase, 17,20-lyase, 21-hydroxylase and 11-hydroxylase activities were also examined as well as plasma and adrenal steroid levels. Our data reveal that chronic ACTH-treatment stimulated all post-pregnenolone enzyme activities in glomerulosa-fasciculata cells. Plasma steroid levels increased 8 h after the last injection of ACTH and returned to the control levels 24 h later whereas, in the adrenal, the content in steroids in the group sacrificed 8 h after the last injection of ACTH were similar to the values of the control group and decreased markedly 24 h later. It is suggested that the steroid turn-over in the adrenal may be affected by the chronic ACTH-treatment. On the other hand, despite the significant stimulation in steroid-transforming enzyme activities, our data reveal that chronic ACTH administration caused a decrease in mRNA levels for P450c21 and P450c17 while P450scc, 3 beta-HSD and P450c11 remained unchanged. Taken together, these results suggest that in vivo chronic ACTH-treatment of guinea-pigs increases adrenal steroidogenic capacity by increasing steroid secretion and steroid enzyme activity. Moreover, the chronic treatment with ACTH may have a post-transcriptional effect on steroidogenic enzymes gene expression by affecting the half-life of their mRNAs.

Enhanced 17 alpha-hydroxylation of pregnenolone and increased androgen production by rabbit adrenocortical cells stimulated chronically with corticotropin

The postulated chronic stimulatory effect of corticotropin (ACTH) on pregnenolone production and on 17 alpha-hydroxylase activity was evaluated on adrenocortical cells obtained from control and chronically ACTH-treated rabbits. The cells were incubated with various concentrations of ACTH added alone or together with trilostane, so as to inhibit further conversion of pregnenolone and dehydroepiandrosterone. The maximal steroidogenic effect of ACTH (determined in the absence of trilostane) was increased 2-fold in adrenocortical cells from ACTH-treated animals; furthermore, cortisol production was increased whereas that of corticosterone decreased. While the generation of pregnenolone was of comparable magnitude for cells from both experimental groups, chronic in vivo treatment with ACTH was followed by a 40-fold enhancement in 17-hydroxypregnenolone production. Concomitantly, maximal DHEA production documented in the presence of ACTH and trilostane was enhanced more than 200-fold, from 0.45 +/- 0.20 pmol in control rabbits to 147 +/- 67 pmol in cells from ACTH-treated animals. The corresponding values of DHEA-sulphate production were 0.86 +/- 0.12 and 432 +/- 334 pmol, respectively. Thus, a prolonged stimulatory effect of ACTH on rabbit adrenocortical cells consists in an enhancement of the capacity to generate pregnenolone, and to convert this compound into 17-hydroxylated steroids.

Enhanced androgen production by rabbit adrenocortical cells stimulated chronically with corticotropin: evidence for increased 17 alpha-hydroxylase activity

The effects of prolonged treatment with corticotropin (ACTH1-24, 200 micrograms s.c. daily during 12 days) on the production of androgens and glucocorticoids were studied on rabbit dispersed adrenocortical cells. The steroidogenic capacity of adrenocortical cells, expressed in terms of the maximal response to ACTH of glucocorticoid (i.e. corticosterone and cortisol) production, was significantly increased after treatment with ACTH. This was associated with a loss of sensitivity to this peptide: indeed, the concentration of ACTH required to induce a half maximal secretory response was one order of magnitude higher with cells from ACTH-treated animals. Among the C21 steroids measured the changes observed involved the 17 alpha-hydroxylated compounds (cortisol, cortisone, 11-deoxycortisol) while corticosterone production was significantly depressed. This effect of prolonged ACTH treatment on steroidogenic pathways involving 17 alpha-hydroxylation, was further evidenced by a clear-cut enhancement in androgen secretion (dehydroepiandrosterone, androstenedione and testosterone) by adrenocortical cells from ACTH-treated animals. The changes observed after treatment of the animal with ACTH were equally obvious, whether the adrenocortical cells were incubated with ACTH or with dibutyryl-c-AMP.

The enhancement of pregnenolone production as the main mechanism of the prolonged stimulatory effect of ACTH on cortisol production by guinea-pig adrenocortical cells

The prolonged stimulatory influence of corticotropin (ACTH) on the adrenocortical steroidogenic response to ACTH was studied in guinea-pig adrenocortical cells harvested from control and ACTH-treated animals (ACTH1-24, 50 micrograms s.c. twice daily on the day preceding the in vitro experiment). The maximal capacity to produce cortisol in response to ACTH (by 10(5) cells and 2 h incubation) was increased from 341.8 +/- 36.3 ng (control group) to 663.3 +/- 37.6 ng for cells obtained from guinea-pigs treated in vivo with ACTH. In the presence of trilostane, added to the cells in order to block the conversion of pregnenolone to cortisol, the net maximal output of pregnenolone and 17-hydroxypregnenolone in response to ACTH was significantly increased in adrenocortical cells from ACTH-treated animals (449.5 +/- 35.8 ng pregnenolone and 85.7 +/- 10.5 ng 17-hydroxypregnenolone vs 269.1 +/- 36.3 ng pregnenolone and 43.7 +/- 8.51 ng 17-hydroxypregnenolone for cells from control guinea-pigs). It appeared therefore that the total production of pregnenolone (as estimated by the sum of pregnenolone and 17-hydroxypregnenolone produced by the cells incubated with trilostane) nearly reached the level of the maximal production of cortisol in response to ACTH and was also significantly enhanced for cells from ACTH-treated animals (532.2 +/- 38.4 ng vs 312.8 +/- 40.0 ng for cells from control group). By contrast, no effect was documented on 17 alpha-hydroxylase activity since 17 alpha-hydroxylation index was similar for both types of adrenocortical cells (16.3 +/- 2.05% for ACTH-treated animals and 14.2 +/- 2.83% for control group). It was concluded therefore that the prolonged stimulatory influence of ACTH on pregnenolone production is the main mechanism of the enhancement of cortisol synthesis by guinea-pig adrenocortical cells previously stimulated by ACTH.

On the specificity of the inhibitory effect of trilostane and aminoglutethimide on adrenocortical steroidogenesis in guinea pig

The effect of trilostane and aminoglutethimide on steroidogenesis was studied on isolated guinea pig adrenocortical cells in order to verify whether, in addition to the inhibitory influence of trilostane on 3beta-hydroxysteroid dehydrogenase and isomerase and the inhibition by aminoglutethimide of pregnenolone formation, these inhibitors may also affect other enzymatic steps of cortisol synthesis. While trilostane completely abolished cortisol production in response to ACTH with concomitant enhancement in pregnenolone and 17-hydroxypregnenolone formation, the conversion of progesterone, 17-hydroxyprogesterone and 11-deoxycortisol into cortisol was not affected by the presence of the inhibitor. Contrasting with this specific inhibitory effect of trilostane, aminoglutethimide inhibited not only pregnenolone formation but also several other enzymatic steps involved in the conversion of this precursor of steroidogenesis into cortisol. Among these additional effects of aminoglutethimide on steroidogenesis, the inhibition of 11 beta-hydroxylation was clearly demonstrated.

The prolonged stimulatory effect of ACTH on 11 beta-hydroxylation, and its contribution to the steroidogenic potency of adrenocortical cells

The mechanism of the prolonged stimulatory influence of corticotropin (ACTH) on the capacity of adrenocortical cells to produce cortisol in response to ACTH and more specifically the role of 11 beta-hydroxylation, was studied on guinea-pig adrenocortical cells dispersed from control and ACTH-treated animals. As a result of the previous in vivo exposure to ACTH, the net maximal production of glucocorticoids in response to ACTH (by 10(5) cells and 2 h incubation) increased from 660 +/- 33.9 ng (control group) to 1105 +/- 117.9 ng for cells from ACTH-treated animals (P less than 0.001), whereas the apparent affinity of the steroidogenic response remained unchanged. In addition there occurred an increased conversion of exogenous pregnenolone into cortisol by cells from ACTH-treated animals, indicating a prolonged stimulatory influence of ACTH on the post-pregnenolone pathway of cortisol biosynthesis. The activity of 11 beta-hydroxylation step was therefore examined by incubating the adrenocortical cells from control and ACTH-treated animals in the presence of increasing amounts of 11-deoxycortisol. The maximal capacity of 11-deoxycortisol conversion into cortisol was increased as a result of the in vivo exposure to ACTH, averaging 3423 +/- 211 ng cortisol formed from 5 micrograms 11-deoxycortisol by 10(5) cells from ACTH-treated animals vs 2074 +/- 185 ng for cells from control guinea-pigs (P less than 0.001). However, the conversion of lower amounts of 11-deoxycortisol into cortisol, reproducing quantitatively the maximal effect of ACTH on cortisol biosynthesis, was only barely increased in cells from ACTH-treated animals (P greater than 0.05). Therefore it was concluded that ACTH increases in a lasting way not only the overall steroidogenic capacity of adrenocortical cells but also the maximal efficiency of 11 beta-hydroxylation. Since the latter effect cannot account quantitatively for the magnitude of the lasting effect of ACTH on the maximal capacity of adrenocortical cells to produce cortisol in response to ACTH, it appears that the prolonged influence of ACTH on cortisol biosynthesis should also involve a stimulatory influence of the peptide on earlier step(s) of steroidogenesis.