Dual silencing of IGF-I receptor and EGF receptor in colorectal cancer cells is associated with decreased proliferation and enhanced apoptosis

Overexpression and/or activation of tyrosine kinase receptors are common features of colorectal cancer (CRC). Using the human CRC cell lines DLD-1 and Caco-2, we evaluated the role of the IGF-I receptor (IGF-IR) and EGF receptor (EGFR) in cellular functions of these cells. We used the small interfering RNA (siRNA) technology to specifically down-regulate IGF-IR and EGFR expression. Knockdown of IGF-IR and EGFR resulted in inhibition of cell proliferation of DLD-1 and Caco-2 cells. An increased rate of apoptosis was associated with siRNA-mediated silencing of IGF-IR and EGFR as assessed by activation of caspase-3/-7. The combined knockdown of both the EGFR and the IGF-IR decreased cell proliferation and induced cell apoptosis more effectively than did silencing of either receptor alone. Comparable effects on cell proliferation and apoptosis were observed after single and combinational treatment cells by the IGF-IR tyrosine kinase inhibitor NVP-AEW541 and/or the EGFR tyrosine kinase inhibitor erlotinib. Combined IGF-IR and EGFR silencing by either siRNAs or tyrosine kinase inhibitors diminished the phosphorylation of down-stream signaling pathways AKT and ERK1/2 more effectively than did the single receptor knockdown. Single IGF-IR knockdown inhibited IGF-I-dependent phosphorylation of AKT, but had no impact on IGF-I- or EGF-dependent phosphorylation of ERK1/2 indicating a role of the EGFR for ligand-dependent ERK1/2 phosphorylation. The present data demonstrate that the inhibition of the IGF-IR transduction cascade augments the anti-poliferative and pro-apoptotic effects of EGFR inhibition in CRC cells. A clinical application of combination therapy targeting both the EGFR and IGF-IR could be a promising therapeutic strategy.