MOLECULAR ANALYSIS OF THE MODE OF ACTION OF RH-5992, A LEPIDOPTERAN-SPECIFIC, NONSTEROIDAL ECDYSTEROID AGONIST

The dibenzoyl hydrazine, RH-5992, induces precocious molting in all le pidopteran larvae tested including the tobacco hornworm, Manduca sexta and the spruce budworm, Choristoneura fumiferana. Synthesis of a new cuticle including formation of a new head capsule that is incompletely sclerotized and lack of ecdysis are some of the major phenotypic effe cts. Similar to 20E, RH-5992 induces Manduca hormone receptor 3 (MHR3) mRNA and suppresses 14 kDa larval cuticular protein (LCP-14) transcri pt levels in 4th instar epidermis of Manduca, cultured in vitro. The E D(50) for induction of MHR3 was 1.4 x 10(-7) M, 10 times less than tha t of 20E, When the epidermis was exposed to 20E for 17 h and then cult ured in hormone-free medium for a further 48 h, the LCP-14 mRNA level went up to nearly 60% of the maximal level reached in the untreated co ntrol, However, when RH-5992 was used, the level remained low even aft er 48 h, Dopa decarboxylase (DDC) transcription normally occurs at the end of the molt in preparation for sclerotization and in the epidermi s cultured in vitro requires initial exposure to 20E for 17 h followed by its removal. Both in vivo and transient ill vitro treatment with R H-5992 prevented DDC expression for up to 48 h after the removal of th e compound. Thus, RH-5992 mimics the action of 20E on the expression o f these 3 genes, but its effects persist in the tissue much longer tha n 20E.