The type I insulin-like growth factor receptor (IGF-IR) plays a critical role in signaling survival and proliferation in many cell types. Activation of IGF-IR by its ligands promotes cell proliferation via mitogen-activated protein kinase (MAPK) cascade and cell survival via phosphoinositide 3-kinase (PI3K) cascade. The IGF-IR emerges as a powerful growth factor for many tumor cells. A truncated IGF-IR 486/STOP, described as a dominant negative IGF-IR mutant, was shown to induce apoptosis and inhibit tumor growth in vivo while endogenous IGF-IR was activated. To investigate the mechanism (s) of the action of 486/STOP, we have introduced 486/STOP into the prostate tumor model cell line M12 and its derivative M12lisn that expresses high levels of wild type IGF-IR. We have found that 486/STOP induces apoptosis in M12 and M12lisn cells in culture and that 486/STOP acts through activation of the pro-apoptotic p38-MAPK without interfering with wild type IGF-IR activation. In addition, our results have indicated that 486/STOP induced activation of p38-MAPK increases through activation of endogenous IGF-IR. These data suggest that activation of the IGF-IR by 486/STOP can selectively enhance the previously reported IGF-IR pro-apoptotic signaling pathways.