Regulation of Y-organ ecdysteroidogenesis by molt-inhibiting hormone in crabs: Involvement of cyclic AMP-mediated protein synthesis

Abstract

The crustacean neuropeptide, molt-inhibiting hormone (MIH), directly inhibits Y-organ ecdysteroidogenesis, an effect mediated by cyclic AMP (cAMP) and antagonized by calcium-calmodulin. We investigated regulation of Y-organ protein, RNA, and DNA syntheses by MIH, cAMP, and calcium in relation to steroidogenesis in vitro. Ecdysteroid production and [ 3H]leucine incorporation into protein were inhibited 50–60 and 80–90%, respectively, by MIH activity in eyestalk extracts (4 eyestalk equivalents), 10 −6 M forskolin, or a combination of 10 −2 M dibutyryl cAMP and 10 −4 M 3-isobutyl-1-methylxanthine (dbcAMP-IBMX). Calcium ionophore A23187 (10 −4 M) stimulated ecdysteroidogenesis two-fold, did not affect the relatively high basal (control) rate of protein synthesis, and reduced the inhibitory effects of forskolin on steroidogenesis and protein synthesis. Incorporation of [ 3H]uridine into RNA was unaffected by MIH, forskolin, or A23187 but was reduced 50% by dbcAMP-IBMX. Basal rates of [ 3H]thymidine incorporation into DNA were low and were not affected by treatments. The effects of MIH were specific; extracts of brain or muscle did not alter Y-organ steroidogenesis or protein synthesis, while muscle extract increased precursor incorporation into RNA. Eyestalk extract did not affect [ 3H]leucine incorporation into protein of brain, muscle, or gill. Cycloheximide (5 μg/ml) depressed protein synthesis 90% and steroidogenesis 60%, enhanced the inhibition induced by MIH, and blocked the stimulation of steroidogenesis induced by A23187; effects on basal steroidogenesis were evident after 1 hr. Actinomycin D (1 μg/ml) depressed RNA synthesis 86% but did not alter basal, MIH-inhibited, or A23187-stimulated ecdysteroidogenesis during incubations. These results indicate that MIH suppresses Y-organ steroidogenesis in part by inhibiting protein synthesis at the translational level; the effect is mediated by cAMP. The stimulation of steroidogenesis by calcium, mediated by lowering cAMP, also appears to depend in part upon protein synthesis.