Abstract 442: Down-regulation of MET/RON receptor tyrosine kinases in colon cancer cells under chronic hypoxia as a mechanism for resistance towards targeted therapy

Hypoxia is a pathogenic factor observed in solid tumors, which plays a critical role in tumor malignancy and drug resistance. The MET and RON receptor tyrosine kinases belong to a unique proto-oncogene family and contribute to tumorigenic progression and malignant phenotypes in various epithelial cancers. To address their pathogenic significance under chronic hypoxic condition (1% O2, 5% CO2 for 5 days), functional analysis of MET and RON were conducted using colon cancer HCT116 and HT29 cells as models. Contrary to previous reports, we consistently observed that levels of MET and RON were significantly decreased over a period of 5 days under chronic hypoxic condition as documented by Western blot and cell-surface fluorescent analysis. The reduction was also accompanied with decreased in both spontaneous and ligand (HGF/MSP)-induced tyrosine phosphorylation manners, which affected the downstream signaling cascades. Moreover, we observed that hypoxia-induced HIF-1α expression (known as a positive regulator for both receptors) is unable to sustain MET or RON expression, suggesting that the receptor down-regulation is mediated through a HIF-1α-independent mechanism. RT-PCR analysis revealed that diminished MET and RON expression occurs at the mRNA transcription level. However, increase in protein degradation also contributed to the observed reduction. Functional studies further demonstrated that chronic hypoxia facilitates colon cancer cells to develop a unique pathogenic phenotype. This phenotype was featured by reduced cell migration, decreased cell proliferation, and lack of S phase in cell cycle profile. Tumor cells were also insensitive in response towards HGF or MSP-induced cell movement and replication. These cancer cells were resistant to cytotoxic effects induced by MET-RON duel small molecule inhibitors such as PHA665752 and compound I. As shown by cytotoxicity assay, the IC50 values of these inhibitors in cell survival were significantly increased in cancer cells under chronic hypoxic treatment compared to normoxic cells. In conclusion, our results demonstrate that under chronic hypoxia colon cancer cells developed a pathogenic phenotype characterized by diminished expression of MET and RON. Such a change contributes to the insensitivity of tumor cells towards cytotoxic activities of specifically targeted drugs. Thus, diminished expression of MET and RON under chronic hypoxia is a potential mechanism that affects the effectiveness of targeted therapy in the treatment colon cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 442.