Steroidogenic refractoriness of bovine adrenocortical cells to dibutyryl cyclic AMP

Adrenocorticotropin/3',5'-cyclic AMP-mediated transcription of the scavenger akr1-b7 gene in adrenocortical cells is dependent on three functionally distinct steroidogenic factor-1-responsive elements

The akr1-b7 gene encodes a scavenger enzyme expressed in steroidogenic glands under pituitary control. In the zona fasciculata of the adrenal cortex where its expression is controlled by ACTH, AKR1-B7 detoxifies isocaproaldehyde produced during the first step of steroidogenesis. Three steroidogenic factor-1 (SF-1)-responsive elements (SFREs) are contained within the -510/+41 promoter region, which was previously demonstrated to drive gene expression in transgenic mice adrenal cortex. All these sequences bind at least SF-1 in Y1 adrenocortical cell nuclear extracts and can be activated by overexpression of this factor in HeLa cells. However, the three SFREs show distinct properties regarding akr1-b7 promoter activity in Y1 cells. Whereas the proximal -102 SFRE supports basal promoter activity, the -458 bona fide SFRE is essential for both basal promoter activity and cAMP responsiveness, although it is unresponsive to cAMP when isolated from its promoter context. This suggests that SF-1 is not a cAMP-responsive factor per se. The neighboring SFRE at -503 is a palindromic sequence that binds monomeric and heteromeric SF-1 as well as an adrenal-specific complex. Using MA-10 Leydig cells and Y1-10r9 mutant cells, we provide evidence that its activity in adrenocortical cells depends on the binding of the adrenal-specific factor, which is required for basal and cAMP-induced promoter activity. Furthermore, the -503 site has intrinsic cAMP-sensing ability in Y1 cells, which is correlated with increased adrenal-specific complex binding. Collectively, our results suggest that cAMP responsiveness of the akr1-b7 promoter is achieved through cooperation between the adrenal-specific factor bound to the -503 site and SF-1 bound to the -458 site.

Salt-inducible kinase represses cAMP-dependent protein kinase-mediated activation of human cholesterol side chain cleavage cytochrome P450 promoter through the CREB basic leucine zipper domain

Salt-inducible kinase (SIK), one of the serine/threonine protein kinases, was transiently expressed in Y1 cells during the early phase of the ACTH/cAMP-dependent protein kinase (PKA)-mediated signal transduction. The overexpression of SIK(N), the SIK's N-terminal kinase domain, repressed the expression of the side chain cleavage cytochrome P450 (CYP11A) gene. To elucidate the mechanism of the repression by SIK, several CYP11A promoter constructs were tested for the promoter activities in the presence of PKA and/or SIK(N). A cAMP-response element (CRE)-like sequence present in the promoter was shown to be responsible not only for the PKA-mediated promoter activation but also for the SIK(N)-mediated repression. When the Gal4 DNA binding domain-linked full-length CRE-binding protein (CREB) construct was cotransfected with Gal4 reporter gene, SIK(N) repressed the PKA-induced reporter gene expression. However, SIK(N) could not repress the PKA-induced reporter activity conferred by Gal4 DNA binding domain-linked basic leucine zipper (bZIP)-less CREB or bZIP-disrupted CREB. On the other hand, SIK(N) could repress the kinase-inducible domain-disrupted CREB-dependent reporter gene expression in the presence of PKA. The in vitro kinase reaction studies showed that SIK(N) could not phosphorylate CREB, and PKA failed to phosphorylate SIK(N). Taken together, these results suggest that SIK(N), cooperating with PKA, may act on the CREB's bZIP domain and repress the CREB-mediated transcriptional activation of the CYP11A gene.

Ontogenesis of cholesterol side-chain cleavage activity in the ovine adrenal during late gestation

The present study examined the activity of the cholesterol side-chain cleavage system, and the amount of cytochrome P450scc in adrenal glands of sheep fetuses and newborn lambs as well as the in vitro regulation of these parameters. Freshly isolated fetal adrenal cells incubated in the presence of 1 mM 8Br-cAMP or 25 microM 22R-OH cholesterol, produced 4- to 5-fold less pregnenolone than neonatal cells under similar conditions. Likewise, pregnenolone production by isolated fetal adrenal mitochondria was lower than that of neonatal mitochondria when endogenous cholesterol was used as a substrate or when 22R-OH cholesterol was added to the incubation medium. Also, the amount of P450scc, determined by immunoblot, was lower in fetal mitochondria than in neonatal mitochondria. In culture, ACTH, despite enhancing both the production of pregnenolone and the incorporation of [14C]acetate in cholesterol and its end-products by fetal adrenal cells, neither increased the amount of pregnenolone formed from 22R-OH cholesterol nor the amount of immunoreactive P450scc. By contrast, during the first 48 h of culture under standard conditions, there was a "spontaneous" increase in the activity of P450scc which reached values observed in neonatal adrenal cells. Such a development was inhibited when 5% ovine fetal serum was added to the culture medium. These results reinforce the view that in the ovine fetal adrenal gland, the development of P450scc is not ACTH-dependent but involves most probably a decrease in inhibitory factors present in fetal blood.

Impaired aldosterone secretion from dispersed adrenal capsular cells of chronically alpha-MSH-treated rats

To investigate the chronic effects of alpha-melanocyte-stimulating hormone (alpha-MSH) on aldosterone secretion, synthetic alpha-MSH (8 micrograms/day) was infused in Sprague-Dawley rats by miniosmotic pumps for 5 days. Saline was infused in equivalent volume for 5 days using the same type of pumps in the control group of rats. Aldosterone secretion from the capsular cells of the two groups was examined in the basal state and in response to various stimuli of aldosterone secretion. Aldosterone secretion in vitro from the alpha-MSH-treated rats was significantly impaired in response to all stimuli tested including cyclic AMP, suggesting an intracellular defect in aldosterone synthesis in that group. These results are similar to those observed after chronic adrenocorticotropin administration.

Regulation of the biosynthesis of cytochromes P-450 involved in steroid hormone synthesis

The actions of ACTH to regulate the synthesis of the various enzymes involved in steroid hormone biosynthesis have been studied using bovine adrenocortical cells in monolayer culture. ACTH causes an increase in the synthesis of both the mitochondrial and the microsomal forms of cytochrome P-450 involved in steroid hormone biosynthesis, as well as of the iron-sulfur protein involved in transferring electrons to the mitochondrial forms of cytochrome P-450, namely, adrenodoxin. This increased synthesis is reflective of an increase in translatability of mRNA species specific for these various proteins, and appears in each case to be mediated by cyclic AMP. Whereas the mitochondrial proteins are synthesized as precursors of higher molecular weight which are processed upon insertion into the mitochondria, the microsomal proteins are synthesized as species identical in molecular weight to the mature forms. In order to determine whether the action of ACTH to increase the rate of synthesis of these proteins is the result of an increase in the levels of specific mRNA species, cDNA clones complementary to these mRNA species are being isolated. These probes will also make it possible to characterize the genes encoding the steroidogenic enzymes, as well as to identify regulatory elements which control their transcription.