| IRS-1 is a major early signal mediator of IGF-I and is engaged in various IGF-I stimulated cellular events, including cell proliferation, differentiation and transformation. Considerable evidence suggests that IGF-I plays an important role in tumor cell growth and IRS-1 has been shown to be involved in this process.; In our studies, we found that IGF-I stimulation promoted IRS-1 down regulation in human prostate primary cancer epithelial cells. This down regulation occurs primarily through increasing the degradation of IRS-1 proteins, not repressing IRS-1 protein synthesis. We examined potential mechanisms and identified two independent pathways that contributed to this degradation. Using proteasome specific inhibitors—lactocystin and MG132, we found that IRS-1 could be degraded through the 26S proteasome pathway. This result was further confirmed by detectable ubiquitination of IRS-1 proteins in the presence of IGF-I. Furthermore, our studies demonstrated a calcium-dependent calpain digestion of IRS-1 that also occurred during thapsigargin mediated cell apoptosis. Co-precipitation of IRS-1 and calpain proteins was found in the presence or absence of IGF-I. In summary, we identified two important mechanisms for the regulation of IRS-1: targeting to the 26 S proteasome and calpain mediated digestion. |
