TGF-ß-induced expression of IGFBP-3 regulates IGF1R signaling in human osteosarcoma cells.

ABSTRACT

Signaling pathways initiated by transforming growth factor-ß (TGF-ß) and insulin-like growth factors (IGFs) are important in osteosarcoma cell growth. We have investigated a role for endogenous IGF binding protein-3 (IGFBP-3) in mediating cross-talk between TGF-ß receptor and type I IGF receptor (IGF1R) signaling pathways in MG-63 osteosarcoma cells. TGF-ß1 indirectly activated the Ras/Raf/MAPK pathway and induced the expression of IGFBP-3, an important regulator of IGF1R activity. IGFBP-3 attenuated TGF-ß1 activation of ERK1/2 and Akt in MG-63 cells, and inhibited TGF-ß1-induced cell cycle progression and proliferation. This effect of IGFBP-3 was blocked by inhibiting IGF1R signaling. TGF-ß1 phosphorylated Smad2 on the non-receptor substrate sites (Ser245/250/255). Blocking the TGF-ß1-induced expression of IGFBP-3 enhanced pSmad2(Ser245/250/255) and increased its nuclear accumulation. These results suggest an important role for TGF-ß1 in osteosarcoma cell growth, with the induction of IGFBP-3 by TGF-ß1 serving in a negative-feedback loop to control cell growth by preventing activation of the IGF1R.