Abstract A62: p38MAPK implication in cetuximab response.

Cetuximab is used in colorectal cancer (CRC), as targeted therapy against the Epithelial Growth Factor receptor (EGFR), in association with chemotherapy (5-FU and irinotecan). Its binding inhibits signalling pathways downstream to the receptor leading to a decrease in proliferation and surviving. In KRAS mutated CRC patients, cetuximab is ineffective, and half of KRAS Wild Type (WT) patients does not respond to cetuximab either. Thus, a better knowledge of cetuximab mechanism of action will help to improve response rate. We have previously shown that activation of the MAP Kinase p38 (p38MAPK) induces irinotecan resistance in vitro and in vivo. The aim of our project is to determine, in KRAS WT colorectal cells, if p38MAPK is involved as well in the cetuximab effect. Experiments were done on two KRAS WT CRC cell lines which respond differently to cetuximab: Caco2 cells (30% of survival inhibition, p53 mutated) and DiFi cells (80% of survival inhibition, p53 Wild Type). Cytotoxic experiments combining cetuximab treatment and inhibition of p38MAPK by a pharmacological inhibitor (SB202190) were performed. We assessed apoptosis and inhibition of proliferation by FACS analysis of cell cycle and DNA synthesis. In addition, Erk, Bim and p27 were analysed by Western Blot. Our results showed that inhibition of p38MAPK by SB202190 enhances cetuximab cytotoxic effect in Caco2 cells but impairs it in DiFi cells. We also observed that inhibition of p38MAPK decreases cetuximab induced apoptosis (from 40% of cells in subG1 to 15%) and anti-proliferative effect (from 10% EdU positive cells to 40%) in DiFi cells. The prevention of cell death by SB202190 in cetuximab treated DiFi cells could be explained by Erk pathway activation and the decrease of p27 and Bim respectively involved in cellular proliferation and mitochondrial apoptosis. We have shown that genetic inhibition of p53 also prevents cetuximab cytotoxicity in DiFi cells and we observed a synergistic effect with p38MAPK inhibition leading to higher impairment of cetuximab effect. Our results have shown the same inhibiting effect of SB202190 on the cytotoxic effect of two tyrosine-kinase inhibitors targeting EGFR (lapatinib and erlotinib) in DiFi cells indicating that p38MAPK implication is linked to inhibition of EGFR kinase activity not to cetuximab only. We have shown that p38MAPK is involved in response to the inhibition of EGFR activity. However, it seems to have different role according to the cell line used and genetic background (p53 status). Citation Information: Mol Cancer Ther 2013;12(11 Suppl):A62. Citation Format: Laetitia Marzi, Eve Combes, Nadia Vezzio-Vie, Gaelle Thomas, Christel Larbouret, Laetitia Linares, Clara Montagut, Mar Iglesias, Vincent Denis, Maguy Del Rio, Pierre Martineau, Celine Gongora. p38MAPK implication in cetuximab response. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2013 Oct 19-23; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(11 Suppl):Abstract nr A62.