Regulation of gonadotropin subunit genes in tilapia

A steroidogenic tilapia gonadotropin (taGtH = LH) was purified from pituitaries of hybrid tilapia (Oreochromis niloticus × O. aureus) and a homologous RIA was established. This RIA enabled the study of the endocrine regulation of GtH release, the transduction pathways involved in its secretion and its profile during the spawning cycle. Discrepancies between steroid and taGtH peaks during the cycle led to the conclusion that an additional gonadotropin similar to salmonid FSH operates early in the cycle. In order to identify this hormone and to study the endocrine control of synthesis of all gonadotropin (GtH) subunits, a molecular approach was taken. The cDNA sequences and the entire gene sequences encoding the FSHβ and LHβ subunits, as well as an incomplete sequence of the glycoprotein hormone α subunit (GPα), were cloned. Salmon gonadotropin-releasing hormone (sGnRH) elevated mRNA steady-state levels of all three GtH subunits in cultured pituitary cells. Pituitary adenylate cyclase-activating polypeptide (PACAP) and neuropeptide Y (NPY) also stimulated the expression of these subunits and potentiated the effect of GnRH, except that NPY did not affect FSHβ. The GnRH and NPY effects were found to be mediated mainly through protein kinase C (PKC), while protein kinase A (PKA) cascade was involved to a lesser extent. Mitogen-activated protein kinase (MAPK) cascade takes part in mediating GnRH effects, possibly via PKC. Testosterone (T) and estradiol (E2), but not 11-ketotestosterone (KT), are able to elevate GPα and LHβ mRNAs in pituitary cells of early maturing or regressing males. Low levels of T exposure are associated with elevated FSHβ mRNA in cells of mature fish, while higher levels suppress it, but elevate LHβ mRNA. In vivo observations also showed the association of low T levels with increased FSHβ mRNA and high T levels with elevated LHβ mRNA. In accordance with these findings, analysis of LHβ and FSHβ 5′ gene-flanking regions revealed on both gene promoters a GtH-specific element (GSE), half site estrogen response elements (ERE), cAMP response element (CRE) and AP1. In vitro experiments showed that recombinant human activin-A leads to higher levels of GPα, FSHβ and LHβ mRNAs in pituitary cell culture. Porcine inhibin marginally decreased the mRNA levels of GPα and FSHβ, but at a low level (1 ng/ml) it stimulated that of LHβ. These results shed some light on certain hypothalamic and gonadal hormones regulating the expression of GtH subunit genes in tilapia. In addition, they provide evidence for their differential regulation, and insight into their mode of action.