NEUROPEPTIDE-INDUCED INHIBITION OF STEROIDOGENESIS IN CRAB MOLTING GLANDS - INVOLVEMENT OF CGMP-DEPENDENT PROTEIN-KINASE

In crustaceans, ecdysteroid production by the molting glands (Y-organs ) is negatively regulated by a neuropeptide, molt-inhibiting hormone ( MIH). The involvement of cyclic nucleotide-dependent kinases in the me chanism of action of this neuropeptide was investigated with regard to the steroidogenic activity of Carcinus maenas Y-organs. Regardless of the activity level, the major phosphotransferase activity measured in cytosolic fraction was cGMP-dependent, indicating a relatively high c ytosolic concentration of cGMP-kinase in these cells. Phosphotransfera se activity was nearly twofold higher in the intermolt (low steroidoge nic activity) than in premolt (high steroidogenic activity) animals. I n vitro incubation of premolt Y-organs with MIH for 1 hr increased by 3.7-fold the cGMP-kinase activity ratio (-cGMP/+cGMP). Numerous endoge nous protein substrates were predominantly phosphorylated in a cGMP-de pendent manner in cytosolic, particulate, and membrane fractions. Simi lar phosphoprotein patterns were observed in both molting stages. By c ontrast, cAMP-kinase activity, which was low in intermolt Y-organs, in creased significantly in the active steroidogenic premolt Y-organs. Th e increase in cAMP-kinase activity was accompanied by a cAMP-dependent phosphorylation of several specific endogenous proteins. Taken togeth er these results strongly suggest that activation of cGMP-kinase and s ubsequent phosphorylation of an endogenous protein(s) may be responsib le, at least in part, for the MIH-induced inhibition of steroidogenesi s. By contrast, it is most unlikely that cAMP-kinase is involved in th ese processes. (C) 1996 Academic Press, Inc.