Inhibition of p70S6K1 Activation by Pdcd4 Overcomes the Resistance to an IGF-1R/IR Inhibitor in Colon Carcinoma Cells

Agents targeting insulin-like growth factor 1 receptor (IGF-1R) are being actively examined in clinical trials. Although there has been some initial success of single-agent targeting IGF-1R, attempts in later studies failed because of resistance. This study aimed to understand the effects of programmed cell death 4 (Pdcd4) on the chemosensitivity of the IGF-1R Inhibitor OSI-906 in colorectal Cancer cells and the mechanism underlying this impact. Using OSI-906-resistant and -sensitive colorectal Cancer cells, we found that the Pdcd4 level directly correlates with cell chemosensitivity to OSI-906. In addition, tumors derived from Pdcd4 knockdown cells resist the growth inhibitory effect of OSI-906 in a colorectal Cancer xenograft mouse model. Moreover, Pdcd4 enhances the antiproliferative effect of OSI-906 in resistant cells through suppression of p70S6K1 activation. Knockdown of p70S6K1, but not p70S6K2, significantly increases the chemosensitivity of OSI-906 in cultured colorectal Cancer cells. Furthermore, the combination of OSI-906 and PF-4708671, a p70S6K1 inhibitor, efficiently suppresses the growth of OSI-906-resistant colon tumor cells in vitro and in vivo. Taken together, activation of p70S6K1 that is inhibited by Pdcd4 is essential for resistance to the IGF-1R Inhibitor in colon tumor cells, and the combinational treatment of OSI-906 and PF-4708671 results in enhanced antiproliferation effects in colorectal Cancer cells in vitro and in vivo, providing a novel venue to overcome the resistance to the IGF-1R Inhibitor in treating colorectal Cancer.