Regulation of gonadotropin subunit genes in tilapia

A steroidogenic tilapia gonadotropin (taGtH = LH) was purified from pituita ries of hybrid tilapia (Oreochromis niloticus X O. aureus) and a homologous RTA was established, This RIA enabled the study of the endocrine regulatio n of GtH release, the transduction pathways involved in its secretion and i ts profile during the spawning cycle. Discrepancies between steroid and taG tH peaks during the cycle led to the conclusion that an additional gonadotr opin similar to salmonid FSH operates early in the cycle. In order to ident ify this hormone and to study the endocrine control of synthesis of ah gona dotropin (GtH) subunits, a molecular approach was taken. The cDNA sequences and the entire gene sequences encoding the FSH beta and LH beta subunits, as well as an incomplete sequence of the glycoprotein hormone alpha subunit (GP alpha), were cloned. Salmon gonadotropin-releasing hormone (sGnRH) ele vated mRNA steady-state levels of all three GtH subunits in cultured pituit ary cells. Pituitary adenylate cyclase-activating polypeptide (PACAP) and n europeptide Y (NPY) also stimulated the expression of these subunits and po tentiated the effect of GnRH, except that NPY did not affect FSH beta. The GnRH and NPY effects were found to be mediated mainly through protein kinas e C (PKC), while protein kinase A (PKA) cascade was involved to a lesser ex tent. Mitogen-activated protein kinase (MAPK) cascade takes part in mediati ng GnRH effects, possibly via PKC. Testosterone (T) and estradiol (E2), but not 11-ketotestosterone (KT), are able to elevate GP alpha and LH beta mRN As in pituitary cells of early maturing or regressing males. Low levels of T exposure are associated with elevated FSH beta mRNA in cells of mature fi sh, while higher levels suppress it, but elevate LH beta mRNA. In vivo obse rvations also showed the association of low T levels with increased FSH bet a mRNA and high T levels with elevated LH beta mRNA. In accordance with the se findings, analysis of LH beta and FSH beta 5 ' gene-flanking regions rev ealed on both gene promoters a GtH-specific element (GSE), half site estrog en response elements (ERE), cAMP response element (CRE) and AP1. In vitro e xperiments showed that recombinant human activin-A leads to higher levels o f GP alpha, FSH beta and LH beta mRNAs in pituitary cell culture. Porcine i nhibin marginally decreased the mRNA levels of GP alpha and FSH beta, but a t a low level (1 ng/ml) it stimulated that of LH beta. These results shed s ome light on certain hypothalamic and gonadal hormones regulating the expre ssion of GtH subunit genes in tilapia. In addition, they provide evidence f or their differential regulation, and insight into their mode of action. (C ) 2001 Elsevier Science Inc. All rights reserved.