Identification of structural characteristics that contribute to a difference in antiapoptotic function between human insulin and insulin-like growth factor 1 receptors.

To determine whether potentiation of cell survival is an intrinsic function among the insulin receptor tyrosine kinase (RTK) family, we compared the ability of insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF-IR) cytoplasmic tails to attenuate apoptosis. Expression and activation of IGF-IR, in interleukin-3 (IL-3)-dependent 32Dcl.3 cells, prevents death under conditions of IL-3 withdrawal. In contrast, a chimeric receptor comprising the extracellular portion of IGF-IR fused to the cytoplasmic tail of IR (chIR) fails to promote cell survival when activated with ligand. Both chIR and IGF-IR exhibit comparable levels of enzymatic activity as evidenced by their ability to autophosphorylate and transphosphorylate the shc protein in vivo. Both chIR and IGF-IR can activate the MAPK signal transduction pathway; however, neither RTK is capable of promoting mitogenesis in the absence of IRS proteins. Structure function analysis of the IR cytoplasmic tail reveals that replacing the COOH-terminal 94 amino acids of the IR cytoplasmic tail with the comparable sequence from IGF-IR confers full antiapoptotic function. Furthermore, mutation of only two amino acids within IR, Phe-1264 and His-1265 to tyrosine (chIR/YY) is sufficient to impart a cell survival activity comparable to wild-type IGF-IR. Amino acid residues Phe-1264 and His-1265 of IR are in a region comparable to Tyr-1250 and Tyr-1251 within human IGF-IR. The amino acid sequence of IR from other species contains at least one tyrosine residue in this region, suggesting that differences in antiapoptotic function we observed may represent a characteristic unique to human members of this RTK family. The ability of IGF-IR or chIR/YY to prevent apoptosis is not blocked by addition of the PI3K inhibitor wortmannin. These studies define a critical region responsible for mediating cell survival through a novel interaction that is independent of mitogenesis.