Characterization of insulin-like growth factor I (IGF-I) receptor mutants for their effects on IGF-I- and interleukin 4-mediated DNA synthesis of 32Dcells

Recently we demonstrated that overexpression of the wild type insulin-like growth factor I receptor (IGF-IRWT) in 32D myeloid progenitor cells led to cell proliferation in response to interleukin 4 (IL-4) as well as insulin-l ike growth factor I (IGF-I) in the absence of insulin receptor substrate ex pression (Soon, L,, Flechner, L., Gutkind, J, S., Wang, L, H,, Baserga, R,, Pierce, J, H,, and Li, W. (1999)Mol. Cell. Biol. 19, 3816-3828), To unders tand the structural importance of insulin-like growth factor I receptor (IG F-IR) in mediating IL-4- and IGF-I-induced DNA synthesis, we transfected va rious mutants of IGF-IR to 32D cells. Our results show that most mutants, i ncluding Y1250F, Y1251F, Y1250F/Y1251F, S1280A/S1281A/S1282A/S1283A, Y1316F , and 1245d, still retained mitogenic response toward IGF-I or IL-4. Howeve r, the Y950F, Y1131F, and Y1135F mutants were not able to respond to either ligand, The H1293F/K1294R and 1293d mutants reduced response toward IGF-I but not to IL-4, Phosphorylation of She was greatly reduced in those three mutants that lost mitogenic response. The MAPK activity was much lower in Y 1131F and Y1135F mutants, indicating the importance of the Shc/MAPK pathway in IGF-I-induced mitogenesis. Importantly, the synergistic effect of these two factors on DNA synthesis was not affected in cells expressing most of the mutants, even in those three that had lower mitogenic response toward a single ligand, These results suggest that an unidentified pathway(s) may b e induced upon co-addition of IGF-I and IL-4 that sustains the intact mitog enesis.