Effects of growth factors on insulin-like growth factor binding protein (IGFBP) secretion by primary porcine satellite cell cultures

Insulin-like growth factor-binding proteins (IGFBP) regulate the biological functions of insulin-like growth factors (IGF) and may affect cell growth through IGF-independent actions. Growth factors and hormones have been shown to alter IGFBP production by target cells suggesting that the effects of these factors may be partially mediated by the local production of IGFBP. Growth factors, including IGF-I, transforming growth factor-β₁ (TGF-β₁), and basic fibroblast growth factor (bFGF) have potent effects on satellite cell proliferation and differentiation, and some of these factors have been shown to alter IGFBP production in various cell types. Consequently, some of their actions on muscle satellite cells may be mediated by the local production of IGFBP. In this study, we measured the effects of IGF-I, bFGF, and TGF-β₁ on IGFBP production by primary porcine satellite cell (PSC) cultures after first determining physiologically active concentrations of these growth factors to use according to [³H]thymidine incorporation dose responses. There is little information on the effects of these growth factors on IGFBP production in primary porcine myogenic cells due to the confounding affects of contaminating nonmuscle fibroblasts. Comparative studies show that primary porcine satellite cells produce IGFBP-3 and -5 whereas porcine muscle-derived nonfusing cells (FIB) produce IGFBP-2 and -4 but not IGFBP-3 or -5. Because of this, our investigations have focused on growth factor-induced production of IGFBP-3 and -5 in primary porcine satellite cells cultures. Both IGF-I and bFGF exhibited dose-dependent increases in [³H]thymidine incorporation with increasing concentration from 1 to 50 ng/mL (P < 0.05), whereas TGF-β₁ caused a dose-dependent decrease from 0.01 to 0.5 ng/mL (P < 0.05). When 20 ng/ mL of IGF-I was added to the media, IGFBP-3 was increased approximately 65% (P < 0.05) and IGFBP-5 was increased approximately twofold (P < 0.05). The addition of 0.5 ng/mL TGF-β₁ caused more than a two-fold increase in IGFBP-3 (P < 0.05) and approximately an 80% increase in IGFBP-5 (P < 0.05), whereas 50 ng/ mL of bFGF caused approximately 40% (P < 0.05) and 70% (P < 0.05) increases in IGFBP-3 and -5, respectively. Neither IGFBP-3 nor -5 was detectable in the conditioned media from fibroblasts whether or not IGF-I, TGF- β₁ or bFGF were present. These data suggest that the effects of IGF-I, TGF- β₁ and bFGF on porcine satellite cells may in part be through the autocrine/ paracrine production of IGFBP-3 and -5 by porcine satellite cells.