Production and hormonal regulation of insulin-like growth factor binding proteins in bovine chondrocytes

Linear growth results from proliferation and differentiation of chondrocytes within the growth plates and is regulated, in part, by the insulin-like growth factors (IGFs). IGF binding proteins (IGFBPs) also appear to play a significant, but yet unclear, role. To examine IGFBP production by chondrocytes, we isolated bovine chondrocytes from adult articular, fetal articular, and fetal growth plate cartilage, and maintained them in primary culture as high-density monolayers or encapsulated in alginate beads. Cells were cultured in serum-free conditions with human GH (hGH), insulin, hIGF-I, or hIGF-II. Human IGF-I resulted in higher DNA content in all three of the chondrocyte types. Conditioned medium samples were analyzed for IGFBPs by Western ligand blotting. Chondrocytes released IGFBPs of 24, 29, 33, 39, and 43 kilodaltons (kDa). Deglycosylation and immunoblotting identified the 39/43-kDa doublet as IGFBP-3 and the 33-kDa band as IGFBP-2. All chondrocyte types released 29- and 24-kDa IGFBP bands constitutively. Adult articular chondrocytes increased production all IGFBPs in response to IGF-I, but particularly the 29-kDa band (17-fold increase). Fetal articular chondrocytes showed a similar pattern, but with less of an increase when treated with IGF-I. Fetal growth plate chondrocytes primarily showed increases in IGFBP-3 and the 24-kDa form (4.7- and 2.7-fold, respectively) in response to IGF-I. Although the role of IGFBPs in IGF mediation of articular and growth plate chondrocyte metabolism requires further research, we show here that bovine chondrocytes produce IGFBPs, and the IGFs regulate this production.