Pituitary Insulin: Insulin-Like Growth Factors

Insulin-like growth factor (IGF)-l and IGF-ll peptides as well as their mRNAs are produced in many organs, including the pituitary. Although IGFl and IGFII peptides are localized in endocrine cells of the anterior pituitary, IGF-l mRNA can be detected throughout the adenohypophysis, and IGF-ll mRNA is abundant in intermediate and neural lobes. It is well-established that both circulating and intrinsic IGF-l are negative regulators of pituitary GH production. Other functions of intrinsic IGFs in normal and tumorous pituitary are just emerging. IGF-l may play a role in the stimulation of PRL synthesis and mediation of proliferative effects of estrogen on lactotroph. Compared with IGF-l, the function of IGE-Il has not been clarified so far. The growth-promoting actions of IGFs are mediated by IGF-l receptor. The role of local and circulating IGFBPs in pituitary are not yet documented. IGFBPs in other tissues have inhibitory and stimulatory effects on IGFs, and can act independently from the IGFs as well. IGFs have been reported to promote cell proliferation in many tumors. However, the extent to which IGFs contribute to pituitary tumor development and growth remains obscure.