Insulin and insulin-like growth factor I (IGF-I) participate in energy metabolism, regulate cellular growth and differentiation, and are thought to act locally in a paracrine manner through specific receptors. Systemic levels of these peptides in humans and primates are directly associated with levels of activity of the reproductive axis. To date, it is unclear whether these peptides participate in reproductive function by acting at the level of the GnRH neuron. In this study we examined the effects of IGF-I, IGF-II and insulin on immortalized GnRH-secreting neurons, the GTI-7 cell line. The GTI-7 cells expressed all three members of the insulin receptor family as determined by analysis of 125I-IGF-I, 125I-IGF-II and 125I-insulin binding sites. Insulin receptors bound insulin, IGF-II and IGF-I with a ratio of potency of 1:5:20. IGF-I and IGF-II receptors bound both IGF-I and IGF-II. The ratio of potency of IGF-I/IGF-II was 1:5 for the IGF-I receptor and 100:1 for the IGF-II receptor. The binding characteristics of the growth factors at 22°C suggested the possibility that these cells may secrete IGF binding proteins. To ensure that changes in GnRH levels in the media were due to secretion and not to changes in cell number, the mitogenic effect of these peptides on GTI cells was evaluated. Both insulin and IGF-I were strong mitogens (48-hour incubation), restoring cell number to that of serum-replete cultures at a dose of 0.1 ng/ml. A 100-fold higher dose of IGF-II was required to produce a similar level of mitogenicity, implicating an action through the IGF-I and/or insulin receptor. Due to these mitogenic effects, the effect of insulin, IGF-I and IGF-II on GnRH secretion was studied after short-term exposure. Insulin and IGF-I did not affect GnRH secretion, but IGF-II had a biphasic effect on GnRH release after 2 h of incubation (a maximum stimulatory effect occurred with a 0.1 ng/ml dose). In order to examine the signal transduction mechanism, the role of cytoplasmic calcium mobilization in IGF-II-induced GnRH secretion was examined in single cells using calcium imaging. The effect of IGF-II on GnRH secretion appeared to operate via a calcium independent mechanism. The studies document an insulin/IGF system in the GTI-7 neuronal cell line and show that insulin and IGFs can exert direct effects on the immortalized GnRH neurons. The ability of these agents to stimulate mitogenesis and/or secretion suggests that these peptides may play some role in the early development or maturation of GnRH neurons in vivo.
- Gonadotropic-hormone-releasing hormone
- GTI cells
- Insulin-like growth factors
ASJC Scopus subject areas