Signaling Pathways for Ecdysteroid Hormone Synthesis in Crustacean Y-organs

The Y-organs of crustaceans secrete steroid hormones (ecdysteroids)which are responsible for molting and regeneration. The Y-organsin turn are controlled (negatively) by the eyestalk peptide,molt-inhibiting hormone (MIH). We are exploring the signalingpaths in Y-organ cells that lead to ecdysteroid generation whenactivated by the absence of MIH. The objective is to understandthe connections between MIH-receptor occupancy and the depressionof genes that express ecdysteroidogenic enzymes. MIH actionis mediated by a rise in cyclic 5' adenosine monophosphate (cAMP);cGMP also is involved in some species. That a cyclic nucleotideis a central regulatory component is indicated by the followingselection of results: dibutyryl cAMP, activators of adenylylcyclase or inhibitors of cyclic nucleotide phosphodiesteraseeach mimic the inhibitory action of MIH. Cyclic AMP inhibitsthe receptor-mediated uptake of cholesterol (the obligate ecdysteroidprecursor), by decreasing the number of receptor sites for thelipoprotein carrier of cholesterol. MIH via cAMP also depressesde novo protein synthesis upon which ecdysteroidogenesis dependsin part. A role for cellular free calcium (Ca⁺⁺) is indicatedby the ability of Ca⁺⁺ (or a Ca⁺⁺ionophore) to stimulate ecdysteroidproduction,thereby antagonizing MIH action. The mechanism involvesloweringcAMP levels by enhancing phosphodiesterase activity via calmodulin,not by affecting adenylate cyclase activity. Ca⁺⁺ counters thesuppressive action of MIH or cAMP on protein synthesis. Consistentwith the MIH-Ca⁺⁺ mutual antagonism, MIH increases Ca⁺⁺ effluxfrom ⁴⁵Ca-preloaded cells. Y-Organ cells contain protein kinaseC (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-activatedincrease in ecdysteroidogenesis involves events downstream fromthe production of cAMP and the degradation of cAMP by Ca⁺⁺.In relation to the latter, specific and non-specific inhibitorsof protein tyrosine kinases (PTK) inhibit ecdysteroid synthesisdose-dependently. The relationship of PTK with MIH-cAMP andCa⁺⁺-PKC systems is under study.