Effects of aging on cholesterol content and cholesterol-metabolizing enzymes in the rat adrenal gland.

Previous studies from this laboratory have documented a progressive decline in basal and ACTH-stimulated corticosterone production in isolated adrenocortical cells as rats age. In the current study we examined the possibility that the aging process exerts this effect by interfering with the mechanism(s) by which cholesterol is processed and/or synthesized by the adrenal gland. Freshly excised adrenals from 2-, 5-, 12-, and 18-month-old rats were used for the measurement of cholesteryl ester, free cholesterol, cholesteryl esterases, and acyl co-enzyme A (CoA)-cholesterol acyltransferase activities as well as key enzymes involved in cholesterol biosynthesis. The results showed that cholesteryl ester content increased in a linear manner with advancing age, while neutral cholesteryl esterase activity decreased steadily until at 18 months of age it reached 40% that of 2-month-old control rats. In contrast, lysosomal acid cholesteryl esterase did not change with age, and acyl CoA: Cholesterol acyltransferase showed only a 33% decrease at 12 months of age. The activity of 3-hydroxy-3-methylglutaryl CoA reductase, the rate-limiting enzyme in cholesterol biosynthesis, decreased steadily with advancing age, and at 18 months of age, activity was only half of that in 2-month-old control rats. In contrast, the activities of other enzymes involved in the de novo synthesis of cholesterol, namely acetoacetyl CoA thiolase and HMG CoA synthase, were similar in 2- and 12-month-old rats, while mevalonate kinase activity was significantly lower in the 12-month-old rats. After depletion of plasma lipoprotein cholesterol by 4-aminopyrazolo-[3,4-d]pyrimidine, the intraadrenal cholesteryl ester content in young and aged animals fell significantly. Furthermore, such treatment enhanced the activities of all of the cholesterol de novo synthetic enzymes examined. In addition, HMG CoA synthase and HMG CoA reductase activities rose to much greater levels in both young and old rats compared to acetoacetyl CoA thiolase and mevalonate kinase. Finally, markedly higher activities of HMG CoA reductase were observed in 12- and 18-month-old rats after 4-aminopyrazolo-[3,4-d]pyrimidine treatment. Similar results were seen using 17 alpha-ethinyl estradiol to deplete cholesterol and adrenal sterol ester stores. The metabolism of endogenous cholesterol and exogenous hydroxysterols (which bypass the cAMP-dependent transport of endogenous cholesterol to mitochondrial side-chain cleavage enzyme complex) to corticosterone by collagenase-dispersed adrenocortical cells isolated from rats of various ages were also studied.(ABSTRACT TRUNCATED AT 400 WORDS)