Signaling Pathways for Ecdysteroid Hormone Synthesis in Crustacean Y-organs

Abstract

The Y-organs of crustaceans secrete steroid hormones (ecdysteroids) which are responsible for molting and regeneration. The Y-organs in turn are controlled (negatively) by the eyestalk peptide, molt-inhibiting hormone (MIH). We are exploring the signaling paths in Y-organ cells that lead to ecdysteroid generation when activated by the absence of MIH. The objective is to understand the connections between MIH-receptor occupancy and the depression of genes that express ecdysteroidogenic enzymes. MIH action is mediated by a rise in cyclic 5′ adenosine monophosphate (cAMP); cGMP also is involved in some species. That a cyclic nucleotide is a central regulatory component is indicated by the following selection of results: dibutyryl cAMP, activators of adenylyl cyclase or inhibitors of cyclic nucleotide phosphodiesterase each mimic the inhibitory action of MIH. Cyclic AMP inhibits the receptor-mediated uptake of cholesterol (the obligate ecdysteroid precursor), by decreasing the number of receptor sites for the lipoprotein carrier of cholesterol. MIH via cAMP also depresses de novo protein synthesis upon which ecdysteroidogenesis depends in part. A role for cellular free calcium (Ca++) is indicated by the ability of Ca++ (or a Ca++ionophore) to stimulate ecdysteroidproduction, thereby antagonizing MIH action. The mechanism involveslowering cAMP levels by enhancing phosphodiesterase activity via calmodulin, not by affecting adenylate cyclase activity. Ca++ counters the suppressive action of MIH or cAMP on protein synthesis. Consistent with the MIH-Ca++ mutual antagonism, MIH increases Ca++ efflux from 45Ca-preloaded cells. Y-Organ cells contain protein kinase C (PKC), the activation of which increases ecdysteroid production. PKC activity is not affected by MIH, but is stimulated by Ca++. These and related experiments indicate that the PKC-activated increase in ecdysteroidogenesis involves events downstream from the production of cAMP and the degradation of cAMP by Ca++. In relation to the latter, specific and non-specific inhibitors of protein tyrosine kinases (PTK) inhibit ecdysteroid synthesis dose-dependently. The relationship of PTK with MIH-cAMP and Ca++-PKC systems is under study.