ROLES OF INSULIN-LIKE GROWTH-FACTOR (IGF) RECEPTORS AND IGF-BINDING PROTEINS IN IGF-II-INDUCED PROLIFERATION AND DIFFERENTIATION OF L6A1 RAT MYOBLASTS

Insulin-like growth factor II (IGF-II) stimulates the proliferation an d differentiation of rat myoblasts. Previous studies suggest that thes e responses are mediated by the IGF-I receptor, but the IGF-II/mannose 6-phosphate receptor was recently implicated in differentiation of mo use myoblasts. L6A1 myoblasts synthesize IGF-binding protein-4 (IGFBP- 4), IGFBP-5, and IGFBP-6, which modulate IGF action. We studied the ro les of IGF receptors and IGFBPs in L6A1 myoblast proliferation and dif ferentiation by comparing the effects of IGF-II and a number of IGF-II mutants with decreased affinities for IGF receptors and/or IGFBPs. IG F-II induced concentration-dependent proliferation with a maximum incr ease of 47%; half-maximal proliferation was seen with approximately 50 ng/ml. [Arg(54),Arg(55)]IGF-II bound to the IGF-I receptor with sligh tly lower affinity than IGF-II, did not bind to the IGF-II/mannose 6-p hosphate receptor, and bound to IGFBPs secreted by myoblasts with appr oximately 16-fold decreased affinity. It induced proliferation with eq ual potency to IGF-II. [Leu(27)]IGF-II, which did not bind to the IGF- I receptor but bound to the IGF-II/mannose 6-phosphate receptor and IG FBPs with slightly lower affinity than IGF-II, had a markedly impaired proliferative effect, inducing proliferation only at high concentrati ons. [Thr(48),Ser(49),Ile(50)]IGF-II, which bound to the IGF-I recepto r with slightly lower affinity than IGF-II but did not substantially b ind to the IGF-II/mannose 6-phosphate receptor or IGFBPs, induced prol iferation with approximately 5-fold greater potency than IGF-II. The o rder of potency in inducing myoblast differentiation was the same, alt hough there was less difference in the relative potencies of IGF-II an d mutants. Coincubation of recombinant human (rh) IGFBP-6 in molar exc ess with IGF-II inhibited myoblast proliferation and differentiation. rhIGFBP-6 was slightly less potent in inhibiting proliferation induced by [Arg(54)Arg(55)]IGF-II. rhTGFBP-6 did not inhibit proliferation or differentiation induced by [Thr(48),Ser(49),Ile(50)]IGF-II. These res ults suggest that 1) IGF-II-induced proliferation and differentiation of L6A1 myoblasts are predominantly mediated by the IGF-I receptor; 2) the IGF-II/mannose B-phosphate receptor is not required for these act ions of IGF-II; 3) nevertheless, the IGF-II/mannose 6-phosphate recept or may be capable of mediating these actions; and 4) IGFBPs secreted b y myoblasts inhibit IGF actions.