The effect of ACTH on HMG-CoA reductase activity in hamster adrenals

Effect of ACTH suppression on adrenal 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in 4-aminopyrazolopyrimidine-treated rats

4-Aminopyrazolopyrimidine (4-APP) treatments to rats for 3 days induced 2-fold increase of circulating ACTH and 11-fold increase of adrenal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA compared to NaCl-treated controls. This in vivo model was used to study the effect of the suppression of ACTH secretion on the adrenal HMG-CoA reductase mRNA level. Dexamethasone (Dex) administration to 4-APP-treated rats caused a rapid and parallel decline of the levels of plasma ACTH and adrenal HMG-CoA reductase mRNA to 50% within 2.5 h, whereas the free and esterified cholesterol content was increased 5 and 9.4 times respectively. These changes could be counteracted by the co-administration of ACTH with Dex. Aminoglutethimide (AG) administration to 4-APP-treated rats, which increased the adrenal esterified cholesterol content (7.5 times), decreased the HMG-CoA reductase mRNA level (44%), despite plasma ACTH level remaining elevated. Moreover, the participation of newly synthesized protein(s) in the lowering of adrenal HMG-CoA reductase mRNA level induced by ACTH suppression is suggested by the fact that cycloheximide (Cyclo), when co-administered with AG, completely blocked the decrease of HMG-CoA reductase mRNA level, despite the plasma ACTH level decreasing by 68% and the free and esterified cholesterol content increasing 3.9 and 12.3 times, compared to 4-APP-treated rats. Furthermore, the specificity of these effects was established by the fact that the beta-actin mRNA level was not affected by the administration of either Dex, AG, Cyclo, or AG + Cyclo to 4-APP-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in the rat adrenal gland

We studied the effect of ACTH on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase enzyme. Reductase activity and reductase mass were enhanced by 22- and 6.2-fold respectively in one series of experiments, whereas in another the levels of reductase activity, reductase mass, and reductase mRNA were increased 6.6-, 3.6- and 2.2-fold respectively, following daily administration of exogenous ACTH for 3 days. Daily injection of 4-aminopyrazolopyrimidine (4-APP) to rats for 3 days increased circulating ACTH level 5.4-fold, whereas adrenal HMG-CoA reductase activity, reductase mass and reductase mRNA levels were greatly increased 36-, 10- and 16-fold, respectively. To counteract the effect of elevated plasma ACTH, dexamethasone acetate (Dex) was administered to 4-APP treated rats. At 3 h post Dex administration, plasma ACTH and corticosteroids levels were effectively decreased by 58 and 59%, respectively. The levels of adrenal HMG-CoA reductase mRNA, reductase activity and reductase mass were also diminished by 38, 31 and 40%, respectively. Our results show that rat adrenal HMG-CoA reductase can respond rapidly to hormonal changes, presumably through variations in circulating ACTH levels.

Effect of ACTH on cholesterol and steroid synthesis in adrenocortical tissues

Previous studies have established that under normal conditions, adrenal HMG-CoA reductase activity is higher in hamsters than in rats and humans. The hamster reductase activity follows a diurnal rhythm corresponding to that of plasma ACTH and glucocorticoids [Endocrinology 107 (1980) 215] but not to that of aldosterone. ACTH treatments to hamsters increased reductase activity after a latency of 60 min; this enhancement was prevented by cycloheximide [J. steroid Biochem. 24 (1986) 325]. Immunotitration and immunoblotting studies confirmed that ACTH caused an increase in reductase protein synthesis. In rats, long-term (1-9 days) and short-term (3 h) treatments with ACTH also induced increase in adrenal HMG-CoA reductase activity and reductase protein. In the presence of iodoacetamide and inhibitors of proteolytic enzyme, a main specific band of enzyme was evinced in the area of 102 +/- 6 kDaMr, by Western blotting, for both hamster homogenate and microsomal preparations (Endocrinology, 120 (1987]. Similarly Mr values were found with rat adrenal preparations. The concentration of mRNA, analyzed using the c-DNA pRed-10 coding for the Chinese hamster ovary reductase, was increased in adrenals of hamsters treated with ACTH. The reductase mRNA levels also fluctuated during the day in parallel with those of reductase activity and reductase protein. In conclusion, these results indicate that ACTH and other conditions inducing a change in hamster adrenal HMG-CoA reductase activity provoke parallel changes in reductase mRNA and reductase protein content. ACTH acts on the adrenal reductase of species synthesizing large as well as small quantities of cholesterol, thus indicating the general importance of this hormonal control.

Investigations on the effects of long-term administration of a methionine-enkephalin analogue on the adrenal zona fasciculata of rats treated with dexamethasone or dexamethasone and ACTH

Prolonged (7 days) methionine-enkephalin (DALA) treatment provoked a dose-dependent increase in the volume of zona fasciculata cells of dexamethasone-administered rats, along with a notable rise in the plasma concentration of corticosterone and the activity of 11 beta-hydroxylase. Comparable dose-dependent effects were observed after chronic administration of ACTH to dexamethasone-suppressed rats. The chronic administration of the maximum dose of DALA (500 micrograms/kg/day) was found to significantly further the trophic action of ACTH on the zona fasciculata of dexamethasone-treated animals. It is suggested that enkephalins act independently of and synergistically with ACTH in stimulating the growth and steroidogenic capacity of the rat adrenal zona fasciculata.

Further studies on the inhibitory effects of somatostatin on the growth and steroidogenic capacity of rat adrenal zona glomerulosa

Chronic somatostatin administration was found to partially reverse the ACTH-enhanced growth of the rat zona glomerulosa. The effect of somatostatin was completely superposable to that of captopril, a specific inhibitor of the angiotensin-converting enzyme, and the inhibitory actions of these two agents did not cumulate. Neither somatostatin nor captopril counteracted the ACTH-induced stimulation of the growth and steroidogenic capacity of the rat zona fasciculata. These findings support the view that somatostatin acts as an exclusive and specific modulator of the adrenoglomerulotrophic action of the renin-angiotensin system.

On the control of HMG-CoA reductase, a key regulatory enzyme of adrenal cholesterol synthesis

In this study we have determined the effect of ACTH on the activity of HMG-CoA reductase in microsomes of hamster adrenals. Cycloheximide was used to study the dependence of the increased enzyme activity by ACTH on de novo protein synthesis. Microsomes were prepared and preincubated with and without NaF and in the presence or absence of phosphorylase phosphatase in order to differentiate between expressed (McNaF) and total (McPP) activity. ACTH induced (after 120 and 180 min) significant increases in HMG-CoA reductase activity with a latent period of 60 min for both McNaF and McPP preparations. Cycloheximide alone decreased the activity of the reductase and the coadministration of cycloheximide + ACTH caused a greater loss of activity. Also, both treatments produced an accumulation of free cholesterol in adrenals suggesting an increased turnover of the reductase by these substances. Preincubation of microsomes at 37 degrees C enhanced per se HMG-CoA reductase activity, but the relative increase produced by ACTH treatments or endogenous ACTH remained essentially the same. In conclusion, under experimental conditions used, the enhancement of HMG-CoA reductase activity produced by ACTH seem to be due to increased enzyme synthesis.

Increased 3-hydroxy-3-methyl-glutaryl coenzyme A reductase activity in a virilizing adrenal carcinoma

HMG-CoA reductase activity was determined on microsomal preparations of an adrenal carcinoma and on a control adrenal obtained from palliative surgery for breast carcinoma. In both tissues we also measured [14C]pyruvate incorporation to study the formation of sterols. The endogenous adrenal content of cholesterol and its esters was quantitated. The content of various steroids was also determined in tissues and media before and after incubations in Krebs-Ringer. The carcinoma had a HMG-CoA reductase activity of 972.0 pmol/mg protein/min vs 13.8 for the control adrenal. The tumor incorporated 4.6 pmol of [14C]pyruvate per mg protein per 90 min into digitonin precipitable sterols compared to 0.5 pmol found for the control gland. Free cholesterol and cholesterol esters in tumoral tissue were 0.09/100 mg and 0.02/100 mg tissue respectively, compared to 0.18 and 2.56 in control tissue. The output of corticosteroids and androgens was very high when calculated for the whole tumor. These results suggest that the carcinoma had acquired a high capacity for de novo synthesis of cholesterol which could have served as substrate for the observed high plasma androgen level.