Insulin-like growth factor-1 and-2 stimulate osteoprogenitor proliferationand differentiation and adipocyte formation in cell populations derived from adult rat bone

In the present study, we test the hypothesis that the stimulation of osteop rogenitor differentiation by the glucocorticoid dexamethasone (Dex) is medi ated, at least in part, through components of the insulin-like growth facto r (IGF) system. Because it has been suggested that osteoprogenitors and adi pocyte progenitors originate from the same precursor cells, and that their differentiation in many systems is reciprocally regulated, the effects of D ex and IGF on adipocyte formation were also evaluated in the same cultures. In view of the presence of IGFs and their binding proteins in serum, we al so evaluated to what degree the effects of IGF-1 and IGF-2 on differentiati on of osteoblasts and adipocytes were affected by the serum concentration o f the culture media. Bone cell populations were isolated from vertebrae of 3-month-old female Wistar rats using an explant culture technique. Osteopro genitor differentiation was evaluated by a colony assay: Bone-forming osteo blastic colonies (bone nodules) derived from single osteoprogenitors were i dentified by alkaline phosphatase (ALP) staining and/or staining for minera lized matrix according to the von Kossa technique. Unmineralized nodules an d osteoblastic colonies were subsequently identified by their distinctive c olor and morphology after methylene blue counterstaining, Differentiated ad ipocytes were identified by Sudan IV staining. IGF-1 and IGF-2 stimulated b oth osteoprogenitor and adipocyte progenitor differentiation in a dose-depe ndent pattern. The stimulation of osteoprogenitor differentiation by IGF wa s not dependent on Dex, but differentiation of adipocytes was. The stimulat ory effects of IGF-1 and IGF-2 on osteoprogenitor differentiation were grea ter in media containing 2.5% fetal bovine serum (FBS) than in media contain ing 5% or 10% FBS, whereas stimulation of adipocyte formation was greater i n media containing 10% FBS, Neutralizing antibody against the type 1 IGF re ceptor (IGF-1R) partially blocked IGF- and Dex-induced osteoprogenitor diff erentiation, but did not affect IGF-induced adipocyte formation. This sugge sts that IGF-stimulated osteoprogenitor differentiation is mediated through IGF-1R, that the stimulation of adipocyte formation by IGF is not, that th e stimulatory effects of Dex on osteo-progenitor differentiation are partia lly mediated through IGF-1R, and that the effects on adipocyte differentiat ion appear to be mediated through signaling pathways other than the IGF-1R. (C) 2000 by Elsevier Science Inc. All rights reserved.