The Mechanism of Action of Adrenocorticotropic Hormone

Abstract

Adrenocorticotropic hormone (ACTH) and its mediator cyclic adenosine 3′ : 5′-monophosphate (cyclic AMP) are known to stimulate steroidogenesis at a site prior to the formation of 3β-hydroxy-pregnen-5-en-20-one (Δ5-pregnenol-one). In a study of the mechanisms of this effect, it was found that cyclic AMP added directly to isolated adrenal mitochondria failed to enhance transformation of endogenous mitochondrial cholesterol to Δ5-pregnenolone. On the other hand, when adrenal mitochondria were isolated from hypophysectomized rats treated with either ACTH or N6, O2′-dibutyryl cyclic adenosine 3′ : 5′-monophosphate (dibutyryl cyclic AMP) in vivo, they produced significantly more Δ5-pregnenolone in vitro than mitochondria from untreated rats. It was found that the adrenal mitochondria from the treated animals contained significantly more cholesterol than those from controls. When hypophysectomized rats were treated with α-ethyl-α-(p-aminophenyl)glutarimide (aminoglutethimide), which blocks mitochondrial conversion of cholesterol to Δ5-pregnenolone, and given ACTH or dibutyryl cyclic AMP as well, the effect of the latter substances on mitochondrial cholesterol accumulation was markedly accentuated. The cholesterol accumulated in mitochondria was predominantly in the free form. Mitochondria from rats treated with aminoglutethimide and either ACTH or dibutyryl cyclic AMP were isolated and washed to free them of the adrenal inhibitor. Upon in vitro incubation, these mitochondria formed markedly increased quantities of Δ5-pregnenolone, which reflected the increased cholesterol precursor pool that was accumulated during prior in vivo treatment. The results suggest that ACTH and its mediator cyclic AMP stimulate steroidogenesis by regulating the mitochondrial precursor pool of cholesterol, rather than through a direct effect on the mitochondrial enzyme system that transforms cholesterol to Δ5-pregnenolone.