Inhibition of MEK sensitizes paclitaxel-induced apoptosis of human colorectal cancer cells by downregulation of GRP78.

Here we report that paclitaxel induces variable degrees of apoptosis in human colorectal cancer cells. Paclitaxel induces multiple arms of the endoplasmic reticulum stress response, including upregulation of the 78-kDa glucose-regulatory protein (GRP78) and eukaryotic initiation factor alpha phosphorylation. Inhibition of the MEK/ERK pathway sensitized colorectal cancer cells to paclitaxel-induced apoptosis. A similar result was obtained by the inhibition of GRP78 using small interfering RNA molecules. Knockdown of MEK resulted in a significant downregulation of paclitaxel-induced upregulation of GRP78 indicating that activation of GRP78 is a downstream event of MEK/ERK pathway activation. These results indicate that GRP78 might be a novel mechanism underlying the resistance of colorectal cancer cells to microtubule-targeting drugs. A combination of compounds capable of suppressing GRP78 might be a golden approach for improving the effectiveness of taxanes.