The biological and clinical significance of HER family members and cancer stem cells in colorectal cancer and response to therapeutic interventions

Despite the approval of the anti-epidermal growth factor receptor (EGFR) monoclonal antibodies (mAbs) cetuximab and panitumumab for the treatment of colorectal cancer patients, there is currently no reliable predictive marker for response to therapy. EGFR is the prototype of human epidermal growth factor receptor (HER) family, which includes three additional members namely HER-2, HER-3 and HER-4. The aim of this study was to investigate the sensitivity of a panel of human colorectal cancer cell lines to treatment with afatinib, an irreversible pan-HER blocker, the EGFR tyrosine kinase inhibitors (TKls) gefitinib and erlotinib, anti-EGFR mAb ICR62 and cytotoxic drugs and to determine whether there was any association between the expression levels of HER family members, the putative cancer stem cell (CSC) markers CD44 and CD133 and ABC transporters (e.g. p-glycoprotein) and response to these agents. The expression pattern and clinical significance of these markers in tumour specimens from 86 Dukes' C and D colorectal cancer patients was determined. Overexpression of the HER family members were found to be uncommon in this panel of human colorectal cancer cell lines, except for EGFR overexpression in DiFi cells. Of the HER inhibitors, afatinib was the most effective agent for inhibiting the growth in vitro of human colorectal cancer cell lines and the expression of HER -4 alone and the co-expression of all the HER family members were associated with response to treatment with afatinib. Acquired resistance of the EGFR overexpressing colorectal cancer cells to treatment with afatinib, ICR62 or gefitinib was accompanied by the up-regulation of EGFR. However, tumour cells that acquired resistance to anti-EGFR mAb ICR62 remained sensitive to treatment with afatinib and vice versa, suggesting that mechanisms underlying the antitumor activity of ICR62 and afatinib are different. The expression ofEGFR, HER-2, HER-3, HER-4 and co-expression of all the HER family members was found in 42.5%, 77%, 52%, 91 % and 18% of the tumour specimens from 86 Dukes' C and D colorectal cancer patients respectively. While all the patients expressed COX-2 and Ki-67, the expression ofCD44 and CD133 was found in 58.6% and 81.9% of the cases, respectively. The expression of EGFR and low expression of Ki-67 were found to be significantly associated with poor disease free survival. The co-expression of HER family members in colorectal cancer patients provides a rationale for investigating the pattern, prognostic significance and predictive value of the HER family members for response to therapy with anti-EGFR mAbs. Further investigations in vivo on the therapeutic potential of pan-HER blockers, such as afatinib, in colorectal cancer patients whose tumours co-express more than one member of the HER family should also be considered