Effects of age and estrogen status on the skeletal IGF regulatory system

Abstract

Human marrow was obtained as material discarded during total hip replacement and was established in culture with phenol red-free α-MEM with 10% fetal bovine serum (FBS) and antibiotics. Insulin-like growth factor I (IGF-I) and its binding proteins were secreted by human marrow cells, in amounts that increased with time in culture. Western ligand blotting showed that insulin-like growth factor binding protein-3 (IGFBP-3) accounted for the majority (∼75%) of the secreted binding proteins. Evidence for marrow secretion of BP-3 protease was found by electrophoretic analysis of mixtures of radiolabeled IGFBP-3 and marrow-conditioned media. The amount of constitutive secretion of IGFBP-3 increased with age of the subject (r=0.97,p=0.0058). A notable exception was marrow from a postmenopausal women on estrogen replacement therapy (ERT) at the time of surgery; her marrow secreted 89.3 ng/mL after 14 d in vitro, only 38% of the IGFBP-3 that was secreted by cultures from two age-matched peers (208.8 and 285.2 ng/mL). This in vivo effect of estrogen was matched by an in vitro experiment in which 10⁻⁸ M 17-β estradiol suppressed IGFBP-3 to 60% of the constitutive level. In all cultures of marrow from postmenopausal women, IL-1β suppressed IGFBP-3 secretion to either undetectable levels or levels between 11% and 35% of control. Thus, human bone marrow cultures demonstrate components of the skeletal IGF regulatory system: IGF-I, IGF-binding proteins, and evidence of IGFBP-3 proteolysis. These results provide evidence of regulation by both systemic (age, estrogen status) and cytokine (IL-1β) factors.