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

Abstract

Signaling pathways initiated by transforming growth factor-β (TGF-β) 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-β receptor and type I IGF receptor (IGF1R) signaling pathways in MG-63 osteosarcoma cells. TGF-β1 indirectly activated the Ras/Raf/MAPK pathway and induced the expression of IGFBP-3, an important regulator of IGF1R activity. IGFBP-3 attenuated TGF-β1 activation of ERK1/2 and Akt in MG-63 cells, and inhibited TGF-β1-induced cell cycle progression and proliferation. This effect of IGFBP-3 was blocked by inhibiting IGF1R signaling. TGF-β1 phosphorylated Smad2 on the non-receptor substrate sites (Ser245/250/255). Blocking the TGF-β1-induced expression of IGFBP-3 enhanced pSmad2(Ser245/250/255) and increased its nuclear accumulation. These results suggest an important role for TGF-β1 in osteosarcoma cell growth, with the induction of IGFBP-3 by TGF-β1 serving in a negative-feedback loop to control cell growth by preventing activation of the IGF1R.

Keywords: EGF; HGF; IGF; IGF binding protein-3; IGF1R; IGF1R KI; IGF1R kinase inhibitor; IGFBP-3; Insulin-like growth factor; Insulin-like growth factor binding protein-3; MAPK; Osteosarcoma; PI3-K; R-Smads; RNA interference; RNAi; SAPK/JNK; Signaling; TGF-β; TGF-β receptor; TGF-β type I receptor; TGF-β type II receptor; Transforming growth factor-β; TβR; TβRI; TβRII; c-Jun N-terminal kinase; epidermal growth factor; hepatocyte growth factor; insulin-like growth factor; mitogen activated protein kinase; phosphoinositide 3-kinase; receptor-activated Smads; transforming growth factor-β; type I IGF receptor.