IGFBP-3 and TGF-β1 regulate collagen IαI production via a TGF-βRI-dependent and Smad 3-depedent mechanism. (A) IGFBP-3 (50 nM) stimulated collagen IαI production is inhibited by the TGF-βRI kinase inhibitor, LY-364947, or the Smad 3 inhibitor, SIS3. Inset: representative immunoblot of collagen IαI and β-actin. (B) TGF-β1 (1 nM) stimulated collagen IαI production is inhibited by the TGF-βRI kinase inhibitor, LY-364947, or the Smad 3 inhibitor, SIS3. Inset: representative immunoblot of collagen IαI and β-actin. Quiescent normal muscle cells were stimulated for 24 hours with 50 nM IGFBP-3 or 1 nM TGF-β1 in the presence and absence of test agents. Collagen IαI protein levels were measured in conditioned media by immunoblot analysis. Results are expressed as percent of untreated normal margin muscle cells. Values represent the mean ± SEM of 4–5 separate experiments. *P < 0.05 versus untreated cells; **P < 0.05 versus IGFBP-3 or TGF-β1 stimulated cells.