Insulin-like growth factor-I stimulates gonadotrophin production from eel pituitary cells: a possible metabolic signal for induction of puberty

Abstract

Insulin-like growth factor (IGF)-I has been suggested as a potential signal linking growth and puberty in mammals. Using the juvenile European eel as a model, we employed a long-term, serum-free primary culture of pituitary cells to study the direct effect of IGF-I on gonadotrophin (GtH-II=LH) production. IGF-I increased both cell content and release of GtH-II in a time-and dose-dependent manner. IGF-I and IGF-II had similar potencies but insulin was 100-fold less effective, suggesting the implication of an IGF type 1 receptor. Other growth and metabolic factors, such as basic fibroblast growth factor and thyroid hormones, had no effect on GtH-II production. IGF-I did not significantly increase the number of GtH-II immunoreactive cells, indicating that its stimulatory effect on GtH-II production does not result from gonadotroph proliferation. Comparison of IGF-I and somatostatin (SRIH-14) effects showed that both factors inhibited growth hormone (GH) release but only IGF-I stimulated GtH-II production by eel pituitary cells. This indicates that the effect of IGF-I on gonadotrophs is not mediated by the reduction of GH released by somatotrophs into the culture medium. This study demonstrates a specific stimulatory effect of IGF-I on eel GtH-II production, played out directly at the pituitary level. These data obtained in a primitive teleost suggest that the role of IGF-I as a link between body growth and puberty may have been established early in the evolution of vertebrates.

Insulin-like growth factor-I feedback regulation of growth hormone and luteinizing hormone secretion in the pig: evidence for a pituitary site of action

“…This was dose dependent and was similar to that reported in male castrate sheep (Adam et al, 2000), eel (Huang et al, 1998) and mature gilt (Whitley et al, 1995). The LH response to GnRH was unaffected by IGF-I, suggesting that the action of IGF-I on LH release from anterior pituitary cells in vitro was by direct action, independent of GnRH, rather than by enhanced GnRH-induced LH secretion.…”

Molecular Regulation of Gonadotropin Secretion by Gonadotropin Releasing Hormone in Salmonid Fishes

“…In the pituitary of females, longer incubation might be required for the stimulation of GTH subunit gene expression. In long-term pituitary cell cultures (incubation for 10 days), IGF-I markedly increased release and the cell content of LH in European eels (Huang et al, 1998(Huang et al, , 1999, and the cell content of FSH and LH in coho salmon (Baker et al 2000). Although further study of long-term cell cultures is needed to clarify the effects of IGF-I on GTH genes in females, the present results suggest that IGF-I most probably stimulates Fig.…”

“…In the pituitary cells of European eels (Anguilla anguilla), IGF-I increased the release and cell content of LH in a time-and dose-dependent manner (Huang et al, 1998(Huang et al, , 1999. In salmonids, co-administration of IGF-I and salmon GnRH (sGnRH) to the pituitary cells of rainbow trout elevated sensitivity to sGnRH, as measured by release of FSH and LH at both immature and mature stages (Weil et al 1999).…”

Multiple molecular effect pathways of an environmental oestrogen in fish

“…Since reproductive roles for GH (reviewed in LeGac et al 1993), IGFs (Huang et al 1998, Weber & Sullivan 2005, thyroid hormones (Soyano et al 1993, Tambets et al 1997 and F (reviewed in Mommsen et al 1999) have all been demonstrated in fish, gonadal changes in their expression further indicate multiple pathways of oestrogen effect on sexual development. Given the lack of knowledge on the function of locally produced GH and IGF-I, and of the roles of THRs and GRs in the gonad, however, it is difficult to interpret these effects.…”