Influence of insulin, insulin-like growth factor 1, and sex steroids on the regulation of the vitellogenin receptor in developing oocytes of largemouth bass (Micropterus salmoides).

摘要

Fish vitellogenin synthesized and released from the liver of oviparous animals is taken up into oocytes by the vitellogenin receptor (vtgr ). This is an essential process in providing nutrient yolk to developing embryos to ensure successful reproduction. Here we disclose the LMB vtgr cDNA sequence that shares high homology with other fish species and conserved domains that classify it as a member of the low density lipoprotein receptor (LDLR) superfamily. The overarching goals of this research were to begin to characterize the expression of LMB vtgr in ovarian tissues during normal reproduction, determine the role of hormones in regulating this gene at the transcriptional level and discern the ability of select environmental chemicals to interfere with pathways that control vtgrexpression.;During the normal reproductive cycle we discovered that vtgr was most highly expressed in ovarian tissues containing oocytes in primary stages of growth which correlated with patterns observed for estrogen receptor (esr) subtypes, androgen receptor (ar) and insulin-like growth factor-1 ( igf1). Using ex vivo experiments it was determined that both Insulin and IGF1 induce the expression of vtgr in ovarian follicles in early growth stages. Preliminary studies suggest involvement of MAPK signaling in IGF1 driven vtgr expression.;Interestingly, while both E2 and 11-KT singly had no effect on vtgr mRNA levels, they were able to suppress INS-induced vtgr mRNA levels. Identification of the first vertebrate vtgr promoter revealed a number of DNA motifs that are potential targets of hormone pathways including GATA-1 and ERE, among others. Functional promoter analysis studies revealed a primary role for esr1 and esr2a in repressing vtgr transcription. The involvement of E2 in downregulation of vtgr mRNA was confirmed in ovarian tissues of LMB exposed in vivo. Finally, the xenoestrogen 17α-ethinyl estradiol (EE2), but not bisphenol-A (BPA), was able to modulate promoter activity via Esrs and repress vtgr expression in vivo in a manner similar to E2.;These results provide for the first time evidence for the ordered stage-specific expression of LMB vtgr during the normal reproductive process and the complex interactions between Ins, Igf and E2 signaling pathways. Furthermore, we identify the vtgr as a potential target of xenoestrogens which brings to light the involvement of this gene in effects imparted by these chemicals.