On the mechanism of action of adrenocorticotropic hormone. The role of ACTH-stimulated phosphorylation and dephosphorylation of adrenal proteins

The CYP11B subfamily

The biosynthesis of steroid hormones is dependent on P450-catalyzed reactions. In mammals, cholesterol is the common precursor of all steroid hormones, and its conversion to pregnenolone is the initial and rate-limiting step in hormone biosynthesis in steroidogenic tissues such as gonads and adrenal glands. The production of glucocorticoids and mineralocorticoids takes place in the adrenal gland and the final steps are catalyzed by 2 mitochondrial cytochromes P450, CYP11B1 (11β-hydroxylase or P45011β) and CYP11B2 (aldosterone synthase or P450aldo). The occurrence and development of these 2 enzymes in different species, their contribution to the biosynthesis of steroid hormones as well as their regulation at different levels (gene expression, cellular regulation, regulation on the level of proteins) is the topic of this chapter.

Effects of estradiol on corticosterone secretion in ovariectomized rats

The effects of estradiol benzoate (EB) on steroidogenesis in rat zona fasciculata-reticularis (ZFR) cells were studied. Female rats were ovariectomized (Ovx) for 2 weeks and then injected subcutaneously with oil or EB for 3 days before decapitation. ZFR cells were isolated and incubated with adrenocorticotropin (ACTH) or prolactin (PRL) for 1 h. Corticosterone concentrations in plasma and cell media, and adenosine 3',5'-cyclic monophosphate (cAMP) production in ZFR cells were determined by radioimmunoassay. The effects of EB replacement in vivo on the activities of steroidogenic enzymes in ZFR cells were measured by the amounts of intermediate steroidal products separated by thin-layer chromatography. Replacement of EB in vivo resulted in a dose-dependent increase of plasma PRL and corticosterone in Ovx rats. The basal, ACTH-, and PRL-stimulated release of corticosterone by ZFR cells was greater in EB- than in oil-treated animals. Forskolin-induced production of cAMP was greater in the EB-replaced rats than in oil-treated animals, which correlated with the increase of corticosterone production. The 3-isobutyl-l-methylxanthine (IBMX) plus ACTH-, IBMX plus PRL-, and forskolin plus PRL-stimulated productions of cAMP were higher in EB- than in oil-treated rats. The enzyme activities of postpregnenolone were not affected by EB replacement in Ovx rats. These results suggest that the EB-related increase of corticosterone production in Ovx rats is associated with an increase of cAMP generation and the stimulatory effect of PRL on ZFR cells.

Effect of protein kinase inhibitors on ACTH-stimulated aldosterone production in rat zona glomerulosa cells

In order to obtain further evidence for the involvement of protein kinases in the short-term ACTH-stimulated aldosterone synthesis in rat zona glomerulosa cells, the effects of three different compounds with protein kinase inhibitory properties were investigated. Staurosporine, H-7 and trifluoperazine inhibited ACTH-stimulated aldosterone release in a dose-dependent manner. While the inhibitory effect of H-7 was reversible upon washing of the cells with inhibitor-free medium, the inhibition was maintained in cells treated with staurosporine or trifluoperazine. In contrast to the stimulated production, basal release of aldosterone even at the highest drug concentrations tested was not completely inhibited. We thus conclude that protein kinases may play a crucial role in short-term ACTH-stimulated aldosterone production in rat glomerulosa cells.

Isolation and characterisation of calcineurin from adrenal cell cytoskeleton: identification of substrates for Ca2+-calmodulin-dependent phosphatase activity

Ca2+-calmodulin-dependent protein phosphatase activity is found in cytoskeletons of Y-1 mouse adrenal and bovine fasciculata cells. The activity is inhibited by three inhibitors of calmodulin (trifluoperazine, W-7 and pimozide) with EC50 in the low micromolar range. Protein phosphatase activity is inhibited by vanadate, fluoride, Zn2+ and pyrophosphate, stimulated by Mn2+ and found to be tightly bound to the cytoskeleton. Substrates for endogenous phosphatase activity were defined by one- and two-dimensional polyacrylamide gels. Phosphatase activity was seen with proteins that are substrates for both cyclic AMP-dependent and cyclic AMP-independent kinase enzymes. One specific Ca2+-calmodulin-dependent phosphatase, namely calcineurin, was purified to near homogeneity from cytoskeletons of Y-1 cells. The enzyme was found to be a heterodimer (MW 61,000 and 16,000) and the smaller subunit was shown to cross-react with antibodies raised against calcineurin from bovine brain. The purified enzyme catalyzes dephosphorylation of proteins (phosphorylase kinase and casein), phosphoamino acids (tyr greater than thre greater than ser) and a synthetic substrate (p-nitrophenyl phosphate). In addition, a new application of membrane transfer was devised by which the purified enzyme was incubated with a Western blot of cytoskeleton following incubation with [32P]ATP. This method defined four specific substrates of the enzyme (MW 150,000, 55,000, 35,000 and 30,000). Anti-calcineurin revealed that only a single Ca2+-calmodulin-dependent phosphatase is found in adrenal cell cytoskeleton.(ABSTRACT TRUNCATED AT 250 WORDS)

The stimulation by adrenocorticotropin of the phosphorylation of adrenal inhibitor-1: a possible role in steroidogenesis

Adrenocorticotropin (ACTH) acts via protein kinase A and the putative phosphorylation of a regulatory protein(s). We have examined a role in this process for inhibitor-1 which, following phosphorylation by protein kinase A, inhibits a phosphoprotein phosphatase activity. In the tissues we have examined inhibitor-1 was found primarily in the cytosol (90%) with the rest in the mitochondrial pellet. The highest concentration was in the adrenal cortex. Using adrenal cortex slices, the stimulation of steroidogenesis by ACTH and dibutyryl cAMP is paralleled by a corresponding increase in the phosphorylation of inhibitor-1 and this is not affected by inhibitors of protein synthesis which inhibit the steroidogenic response. The increase in the phosphorylation of inhibitor-1 occurs in the cytosol, while that in the mitochondrial pellet is not affected. Exogenous phosphorylated inhibitor-1, however, was found to inhibit phosphoprotein phosphatase activity in the mitochondrial pellet. The results suggest that the ACTH-induced increase in phosphorylated inhibitor-1 in the cytosol can affect susceptible phosphoprotein phosphatase activity both in the cytosol and the mitochondrial pellet and, hence, the level of phosphorylation of regulatory protein(s) involved in steroidogenesis.

cAMP-dependent protein phosphorylation of membrane proteins in the parotid gland, platelets and liver. Comparison of a 22-kDa phosphoprotein from rat parotid microsomes (protein III) with phosphoproteins of similar molecular size from platelet and liver membranes

Stimulation of secretion in exocrine secretory glands leads to the phosphorylation of a 22-kDa membrane protein (protein III) whose function is still unknown [Jahn et al. (1980) Eur. J. Biochem. 112, 345-352; Jahn & Söling (1980) Proc. Natl Acad. Sci. USA 78, 6903-6906]. This report describes the comparison of this protein with phosphorylated membrane proteins of similar molecular mass in platelets and liver. Incubation of platelets with agents which raise the intracellular cAMP concentration results in the phosphorylation of a 22-kDa protein which is also phosphorylated in membrane preparations by endogenous kinases or by exogenous cAMP-dependent protein kinase. It is shown that this protein is distinct from protein III although both proteins have the same molecular mass and are substrates of cAMP-dependent protein kinase. In contrast to platelets, protein III could be demonstrated in liver microsomes. This indicates that the function of protein III is not exclusively linked to the stimulus-secretion coupling in exocrine cells.

Phosphorylation of bovine adrenodoxin. Structural study and enzymatic activity

Adrenodoxin is an iron-sulfur protein which functions as a carrier of reducing equivalents in steroid hydroxylation reactions catalyzed by specific cytochromes P-450 in steroidogenic tissues such as adrenal cortex. Purified bovine adrenocortical adrenodoxin was shown to be selectively phosphorylated upon incubation with purified cAMP-dependent protein kinase, whereas other protein kinases were ineffective. The phosphorylation reaction was completed within 45 min at 30 degrees C and resulted in the optimal incorporation of 1 mol phosphate/mol adrenodoxin. Apoadrenodoxin, lacking the iron-sulfur cluster, was also phosphorylated under similar conditions. An apparent Km of 55 microM with a Vmax of 0.3 pmol 32P incorporated min-1 mg adrenodoxin-1 was calculated. Phosphorylation resulted in a striking change in several molecular properties of adrenodoxin, such as electrophoretic behavior and hydroxyapatite affinity, thus providing the possibility of clearly separating phosphorylated from unphosphorylated adrenodoxin. In addition, phosphoadrenodoxin became refractory to mild trypsin degradation, whereas this was not the case with apoadrenodoxin. The phosphorylated site of adrenodoxin was identified as a serine residue; study of peptide products resulting from CNBr and proteolytic cleavages of phosphoadrenodoxin suggested that Ser-88 was the target of the phosphorylation reaction. The influence of phosphorylation upon adrenodoxin activity was examined using cholesterol side-chain cleavage and 11 beta-hydroxylase (11 beta) systems, reconstituted from purified components. Phosphorylation of adrenodoxin resulted in an average twofold decrease in its Km values for the two specific cytochromes P-450 involved. This effect was paralleled by a positive relationship between the degree of adrenodoxin phosphorylation and its ability to support the overall activity of reconstituted side-chain cleavage and 11 beta-hydroxylase systems. Although it remains to be examined whether adrenodoxin is phosphorylated in the intact cell, the present observations suggest that it represents a potential target in the hormonal regulation of the adrenocortical differentiated functions, especially by stimulatory agents acting through a cyclic-AMP-dependent mechanism, such as adrenocorticotropin.

Steroidogenesis-activator polypeptide isolated from a rat Leydig cell tumor

A cycloheximide-sensitive protein responsive to adenosine 3',5'-monophosphate has been postulated to participate in the regulation of cholesterol side-chain cleavage activity in steroidogenic tissues. Such a steroidogenesis activator polypeptide (SAP) had been isolated from rat adrenocortical tissue and partially characterized. Now a polypeptide with comparable chromatographic behavior and biological activity has been purified from the rat H-540 Leydig cell tumor in quantities sufficient for amino acid sequencing. The activator contains 30 amino acid residues and has a molecular weight of 3215. The synthetic construct based on this sequence is virtually equipotent with native H-540 tumor SAP in an adrenal mitochondrial cholesterol side-chain cleavage assay. Hormonal regulation of the intracellular concentration of this activator may control the rate of cholesterol metabolism in steroidogenic organs.

Corticotropin and angiotensin induce dephosphorylation of a Mr-20,000 protein in bovine adrenal cells

ACTH (1-24) and Angiotensin II, both able to activate steroidogenesis in bovine fasciculata-reticularis cells, each reduced the [32P] incorporation in a cytosolic Mr-20,000 pI 6.8 protein in this cell. Cells preincubated with Sar1-Angiotensin prevented the effect of Angiotensin. Angiotensin 10(-8)M and ACTH 10(-10)M led to an almost complete disappearance of the corresponding radioactive spot on the autoradiograph. The effect was observed as soon as 2 minutes after addition of hormones to the cells. Other activators of steroidogenesis such as 8-bromocyclicAMP (8-BrcAMP), 4 beta-Phorbol-12 beta-Myristate-13 alpha-acetate (PMA) and [9-tryptophan (o-nitrophenylsulfenyl)] substituted ACTH (NPS-ACTH), also reduced the labeling of the Mr-20,000 polypeptide. On the other hand, this effect was not reproduced by insulin or human growth hormone (hGH). On 2-D gels from control, the coincidence of this polypeptide with phosphorylated myosin light chain was not observed. We suggest that the apparent dephosphorylation of this polypeptide may represent a common effect of all steroidogenic agents regardless of their seemingly distinct early actions.

Atrial natriuretic peptide inhibits protein phosphorylation stimulated by angiotensin II in bovine adrenal glomerulosa cells

Bovine adrenal glomerulosa cells were incubated with 32PO4 and either angiotensin II, atrial natriuretic peptide, or both. Solubilized cells were subjected to one-dimensional gel electrophoresis. Autoradiography and scintillation counting of gels showed that angiotensin increased labeling of one band, with an apparent molecular weight of 17,600. Atrial natriuretic peptide inhibited the angiotensin effect. Together with earlier results, this observation suggests that atrial natriuretic peptide affects aldosteronogenesis at the level of protein phosphorylation, but not by altering angiotensin receptors, calcium fluxes or phosphoinositide metabolism.

Transduction of ACTH signal from plasma membrane to mitochondria in adrenocortical steroidogenesis. Effects of peptide, phospholipid, and calcium

The conversion of cholesterol to pregnenolone by adrenocortical mitochondria is the rate-limiting step in steroidogenesis. This process is stimulated dramatically by the action of ACTH through the sequential reactions, in which adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase and ribosomal protein synthesis are all involved. The de novo synthesized protein, the so-called labile protein with a half-life of approx 10 min, is believed to stimulate the cholesterol side chain cleavage reaction by an unknown mechanism. Available evidence indicates that the electron on transfer reaction from NADPH to P-450scc is mediated rapidly by adrenodoxin reductase and p-450 scc. In addition, these redox components are inactivated slowly with a half-life of 3.5 days after hypophysectomy. It is known that the corticoid output from adrenocortical cells starts within 5 min and reaches the maximum after 10-15 min of ACTH administration to animals. One can assume that under normal physiological conditions, both O2 and NADPH are not limiting. Additionally, mitochondrial inner membranes are poor in cholesterol. In this context, the availability of substrate cholesterol to P450scc is the most likely candidate for the regulatory mechanism.

Effect of ACTH on rabbit adrenal microsomal 17 alpha-hydroxylase activity, cytochrome P-450 and protein electrophoretic patterns

To determine whether a change in microsomal proteins can be correlated with adrenocorticotropic hormone (ACTH)-stimulation of rabbit adrenal 17 alpha-hydroxylase activity, rabbit adrenal microsomes were subjected to electrophoresis on polyacrylamide gels in the presence of sodium dodecylsulfate. Microsomes were obtained from rabbits stimulated with ACTH for 0, 2, 4, and 6 days. A protein band with a molecular weight of 53,000 was found to increase 31.1, 27.2 and 61.0 percent in 2-, 4-, and 6-day ACTH-stimulated microsomes as compared to controls; but 17 alpha-hydroxylase activity showed no apparent correlation, increasing 5-6 fold in all experiments. No new protein bands were found after ACTH stimulation, and no other changes in microsomal protein electrophoretic patterns after ACTH stimulation were found to correlate with the increases in 17 alpha-hydroxylase activity. The specific activity (nmol/mg protein) of cytochrome P-450 remained nearly the same throughout the stimulation periods. Tetramethylbenzidine staining for heme prosthetic groups on the electrophoretic gels displayed bands with molecular weights of 61,000, 58,000 and 53,000.

Protein synthesis requirement for acute ACTH stimulation of adrenal corticosteroidogenesis

The rapid, cAMP mediated increase produced by ACTH in adrenal corticosteroidogenesis depends also on the rapid synthesis of protein. This obligatory involvement of protein synthesis has been established in several laboratories mainly on the basis of studies demonstrating a parallelism, under a variety of experimental conditions, of the capacity of adrenal cells to synthesize protein with their ability to increase steroid production in response to ACTH or cAMP. More recent correlative studies have disclosed the existence of two proteins, denoted as p and i. Protein i appears only in ACTH or cAMP stimulated cells and with the same time course and ACTH dose response as the increase in corticosteroid synthesis. Protein p, which closely resembles i in molecular weight and primary structure but differs in pI, is synthesized only in unstimulated cells. Neither the function of these two proteins nor the exact nature of the interplay of their synthesis on regulation has been established, although it seems reasonably certain that i is stimulatory rather than p inhibitory. The protein glycosylation inhibitor tunicamycin was found to inhibit the ACTH produced increase in steroidogenesis and also to inhibit the synthesis of protein i under conditions which did not inhibit overall protein synthesis. Therefore it seems probable that i is an N-glycosylated form of protein p and that adrenal cells are unresponsive to acute ACTH action if they are incapable of synthesizing glycosylated proteins specifically.

Polypeptide activators of cholesterol side-chain cleavage

A rate-determining step in the cAMP-dependent action of ACTH on adrenal steroid biosynthesis is the interaction of cholesterol substrate with the cholesterol side-chain cleavage cytochrome P-450 in the mitochondrion. This interaction is rapidly and reversibly sensitive to inhibitors of protein synthesis. For this reason a hormone-dependent, labile protein activator of cholesterol side-chain cleavage has long been postulated as an obligatory intermediate in the tropic regulation of this reaction. Applying recent advances in liquid chromatography, two-dimensional gel electrophoresis, and enzyme reconstitution into liposomes, several laboratories have now reported the isolation and partial characterization of polypeptide candidates for the status of "labile protein."

LH-dependent steroid production and protein phosphorylation in culture of rat tumour Leydig cells

The possible cause of the decreased responsiveness of cultured Leydig cells to stimulation with LH was investigated with rat tumour Leydig cells cultured at 32 degrees C for 2 days. Pregnenolone production and phosphorylation of proteins were studied in combination with the activity of cyclic AMP-dependent protein kinase. Pregnenolone production in cultured tumour Leydig cells decreased from 45 +/- 16 on day 0 to 14 +/- 7 on day 2 (ng/60 min/mg protein) under basal conditions and from 226 +/- 108 on day 0 to 30 +/- 24 on day 2 (ng/60 min/ng protein) under LH-stimulated conditions (means +/- SD, n = 4). This decrease may be accounted for by decreased capacity of cholesterol side-chain cleavage which decreased (in the presence of 25-hydroxycholesterol) from 1.68 +/- 0.26 on day 0 to 0.74 +/- 0.48 on day 2 (microgram/60 min/mg protein; mean +/- SD, n = 4) and decreased activity of cyclic AMP-dependent protein kinase (as apparent from a direct assay of protein kinase activity and the extent of phosphorylation of LH-dependent phosphoproteins).

Steroidogenic refractoriness of bovine adrenocortical cells to dibutyryl cyclic AMP

The long-term effects of dibutyryl cyclic AMP [(Bu)2-cAMP] on steroidogenesis in bovine adrenocortical cells maintained in primary culture were investigated. During the first 36 h, total steroid production by cells incubated with (Bu)2-cAMP increased progressively. Thereafter, however, there was a marked fall in steroid output. During the first 36 h adrenocortical cells incubated in the presence of (Bu)2-cAMP produced substantially more C19-steroids and 17 alpha-hydroxylated C21-steroids than did cells incubated in the absence of (Bu)2-cAMP. By 48 h, however, such steroid secretion by cells incubated in the continued presence of (Bu)2-cAMP declined toward control levels. By contrast, the secretion of corticosterone and 11-deoxycorticosterone was consistently less by cells maintained in the presence of (Bu)2-cAMP than by cells maintained in its absence. These results suggest that refractoriness results, at least in part, from events which occur distal to the formation of cAMP. The action of ACTH and (Bu)2cAMP to promote the secretion of 17 alpha-hydroxylated C21-steroids and C19-steroids, on the other hand, appears to reflect an increase in the rate of cholesterol side-chain cleavage, as well as an increase in 17 alpha-hydroxylase and possibly also 17, 20-lyase activities.

Cyclic nucleotide-dependent phosphorylation of endogenous proteins in bovine adrenocortical cell membranes

Activation of one or more cyclic AMP-dependent protein kinases has been suggested as an intermediate step in ACTH-stimulated adrenal cell steroidogenesis. Phosphorylation of a number of proteins from different subcellular fractions has been reported but those phosphorylation events which are relevant to the steroidogenic process have not yet been identified. In this paper we report that plasma membrane enriched fractions from bovine adrenal cortex retain the ability to phosphorylate endogenous membrane proteins and that phosphorylation of these acceptors is markedly enhanced by cyclic AMP or, to a lesser extent, by cyclic GMP. Cyclic nucleotide-dependent phosphorylation was most marked in protein acceptors of 191 000, 148 000, 138 000, 107 000, 65 000, 60 000 and 27 000 daltons. Cyclic nucleotide stimulation of phosphorylation was rapid (within 10 s), and is consistent with the rapid onset of ACTH-stimulated steroidogenesis.

ACTH stimulation on cholesterol side chain cleavage activity of adrenocortical mitochondria. Transfer of the stimulus from plasma membrane to mitochondria

Adrenocortical mitochondrial cholesterol side chain cleavage reactions are regulated by the influence of pituitary ACTH. The mechanism of the stimulation involves adenyl cyclase, cAMP-dependent protein kinase, cholesterol esterase, and ribosomal labile protein synthesis. Through these reactions the stimulus reaches the mitochondrial side chain cleavage enzyme system. In this review article, the current implications on the stimulus transfer from the plasma membrane to the mitochondrial inner membrane are summarized. In particular the availability of cholesterol to P-450scc was discussed in terms of the distribution of cholesterol molecules in the membranes.