Effects of IGF-I and -II, IGF binding protein-3 (IGFBP-3), and transforming growth factor-β (TGF-β) on growth and apoptosis of human osteosarcoma Saos-2/B-10 cells: Lack of IGF-independent IGFBP-3 effects

Abstract

Objective: Insulin-like growth factor binding protein-3 (IGFBP-3) inhibits cell growth. Previous reports have suggested the existence of plasma membrane IGFBP-3 receptors that could mediate direct, IGF-independent effects. Thus far, however, the only well-defined putative IGFBP-3 receptor is the type V transforming growth factor-β (TGF-β) receptor, a membrane glycoprotein that mediates TGF-β-induced growth inhibition in selected cells. The aim of the study was to test whether IGFBP-3 and TGF-β exert short-term effects in an osteosarcoma cell line that produces no IGF but contains type 1 IGF receptors. Design: DNA synthesis and apoptosis in Saos-2/B-10 cells were measured in response to IGF-I, IGF-II, IGFBP-3 and TGF-β2, and to type 1 IGF receptor ligands with poor affinity for IGFBP-3 ([QAYL]-IGF-I and insulin). Results: IGF-I and IGF-II stimulated thymidine incorporation into DNA and suppressed apoptosis in a dose-dependent manner with maximal effects at 1 and 3 nM respectively. TGF-β2 slightly increased thymidine incorporation into DNA but had no effect on apoptosis. IGFBP-3 had no effect by itself. Whereas it blocked the above effects of 1 nmol/l IGF-I, it did not block those of 1 nmol/l [QAYL]-IGF-I or 100 nmol/I insulin. Conclusions: IGFBP-3 does not affect DNA synthesis or apoptosis in an IGF-independent manner in IGF-responsive osteosarcoma cells. It therefore appears to act essentially by sequestration of IGF.