The orphan nuclear receptor BmHR3A of Bombyx mori: hormonal control, ovarian expression and functional properties

Ovarian development in the domesticated silkmoth, Bombyx mori, is induced b y the molting hormone 20-hydroxy-ecdysone (20E) shortly after larval-pupal ecdysis. Studies of the ecdysone response in Drosophila and other insects h ave shown that 20E exerts its effects initially by the induction of a small number of early genes, including the orphan nuclear receptors HR3, that tr ansduce and amplify the hormone signal. Here we show that the silkmoth orph an receptor BmHR3A acts in the 20E-induced regulatory cascade in the ovary during pupal and pharate adult development in a manner different than that observed in the classical ecdysone regulatory hierarchy in Drosophila saliv ary glands at the end of the third instar. While other isoforms of BmHR3 ar e induced as early gene products in the ecdysone response, BmHR3A is induce d 2 days after 20E administration in the silkmoth ovary and, thus. behaves as late product. The period of accumulation of BmHR3A in ovarian follicular cells occurs during vitellogenesis and coincides with the period of transc riptional expression of the ESP (egg-specific protein) gene, whose product constitutes a major component of the egg yolk, while it is reciprocal to th e period of expression of BmGATA beta, a gene encoding a regulator of late chorion gene expression. Bandshift experiments demonstrate that BmHR3A bind s specifically to RORE (Retinoic acid-related Orphan receptor Response Elem ent)-like sequences in the promoters of both genes, thus suggesting a direc t role for BmHR3A in regulating the expression of BmGATA beta and ESP genes during vitellogenesis. Finally, we show that BmHR3A functions as a constit utive transcriptional activator in a B. mori derived cell line. We propose that BmHR3A may function as a regulator of vitellogenesis in the silkmoth o vary. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.