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

In the present study, we test the hypothesis that the stimulation of osteoprogenitor differentiation by the glucocorticoid dexamethasone (Dex) is mediated, at least in part, through components of the insulin-like growth factor (IGF) system. Because it has been suggested that osteoprogenitors and adipocyte progenitors originate from the same precursor cells, and that their differentiation in many systems is reciprocally regulated, the effects of Dex 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 also evaluated to what degree the effects of IGF-1 and IGF-2 on differentiation of osteoblasts and adipocytes were affected by the serum concentration of the culture media. Bone cell populations were isolated from vertebrae of 3-month-old female Wistar rats using an explant culture technique. Osteoprogenitor differentiation was evaluated by a colony assay: Bone-forming osteoblastic colonies (bone nodules) derived from single osteoprogenitors were identified by alkaline phosphatase (ALP) staining and/or staining for mineralized matrix according to the von Kossa technique. Unmineralized nodules and osteoblastic colonies were subsequently identified by their distinctive color and morphology after methylene blue counterstaining. Differentiated adipocytes were identified by Sudan IV staining. IGF-1 and IGF-2 stimulated both osteoprogenitor and adipocyte progenitor differentiation in a dose-dependent pattern. The stimulation of osteoprogenitor differentiation by IGF was not dependent on Dex, but differentiation of adipocytes was. The stimulatory effects of IGF-1 and IGF-2 on osteoprogenitor differentiation were greater in media containing 2.5% fetal bovine serum (FBS) than in media containing 5% or 10% FBS, whereas stimulation of adipocyte formation was greater in media containing 10% FBS. Neutralizing antibody against the type 1 IGF receptor (IGF-1R) partially blocked IGF- and Dex-induced osteoprogenitor differentiation, but did not affect IGF-induced adipocyte formation. This suggests that IGF-stimulated osteoprogenitor differentiation is mediated through IGF-1R, that the stimulation of adipocyte formation by IGF is not, that the stimulatory effects of Dex on osteoprogenitor differentiation are partially mediated through IGF-1R, and that the effects on adipocyte differentiation appear to be mediated through signaling pathways other than the IGF-1R.