Inactivation of Akt by the epidermal growth factor receptor inhibitor erlotinib is mediated by HER-3 in pancreatic and colorectal tumor cell lines and contributes to erlotinib sensitivity

Signaling through the receptor for epidermal growth factor receptor (EGFR) is frequently deregulated in solid tumors. Erlotinib (Tarceva, OSI-774, OSI Pharmaceuticals, Inc., Melville, NY) is a low molecular weight, orally bioavailable inhibitor of the EGFR that has been approved for both non small cell lung cancer and pancreatic cancers. Previous studies have indicated that sensitivity to EGFR antagonists correlated with HER-3 signaling for non-small cell lung cancer. Herein, we have sought to understand the signaling pathways that mediate erlotinib sensitivity for pancreatic and colorectal cancers. In a panel of 12 pancreatic tumor cell lines, we find that EGFR is coexpressed with HER-3 in all cell lines sensitive to erlotinib but not in insensitive cell lines. Erlotinib can block HER-3 phosphorylation in these sensitive cell lines, suggesting that HER-3 is transactivated by EGFR. Knockdown of HER-3 in BxPC3, an erlotinib-sensitive pancreatic tumor cell line, results in inhibition of the phosphorylation for both Akt and S6 and is associated with a decrease in cell proliferation and reduced sensitivity to erlotinib. Therefore, EGFR transactivation of HER-3 mediates Akt signaling and can contribute to erlotinib sensitivity for pancreatic tumors. We extended our analysis to a panel of 13 colorectal tumor cell lines and find that, like pancreatic, HER-3 is coexpressed with EGFR in the most erlotinib-sensitive cell lines but not in erlotinib-insensitive cell lines. These studies suggest that HER-3 could be used as a biomarker to select patients who are most likely to respond to erlotinib therapy.