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Oncogenic KRAS and BRAF Differentially Regulate Hypoxia-Inducible Factor-1 and -2 in Colon Cancer

¹ Gastrointestinal Unit and ² Cancer Center, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts and ³ Department of Cell Biology, Faculty of Medicine and ⁴ Center for Advanced Molecular Medicine, Fukuoka University, Fukuoka, Japan

Requests for reprints: Daniel C. Chung, Gastrointestinal Unit, Massachusetts General Hospital, Harvard Medical School, 50 Blossom Street, Boston, MA 02114. Phone: 617-726-8687; Fax: 617-643-0195; E-mail: dchung{at}partners.org.

KRAS and BRAF mutations are frequently observed in human colon cancers. These mutations occur in a mutually exclusive manner, and each is associated with distinctive biological features. We showed previously that K-ras can interact with hypoxia to activate multiple signaling pathways. Many hypoxic responses are mediated by hypoxia-inducible factor (HIF)-1 and HIF-2, and we sought to define the roles of mutant KRAS and BRAF in the induction of HIF-1 and HIF-2 in colon cancer cells. Ectopic expression of mutant K-ras in Caco2 cells enhanced the hypoxic induction of only HIF-1, whereas mutant BRAF enhanced both HIF-1 and HIF-2. Knockout or knockdown of mutant KRAS in DLD-1 and HCT116 cells impaired the hypoxic induction of only HIF-1. HIF-1 mRNA levels were comparable in cells with and without a KRAS mutation. However, the rate of HIF-1 protein synthesis was higher in cells with a KRAS mutation, and this was suppressed by the phosphoinositide 3-kinase inhibitor LY294002. In contrast, knockdown of mutant BRAF in HT29 cells suppressed both HIF-1 and HIF-2. Although BRAF regulated mRNA levels of both HIF-1 and HIF-2, knockdown of BRAF or treatment with the MEK inhibitor PD98059 impaired the translation of only HIF-2. Our data reveal that oncogenic KRAS and BRAF mutations differentially regulate the hypoxic induction of HIF-1 and HIF-2 in colon cancer, and this may potentially contribute to the phenotypic differences of KRAS and BRAF mutations in colon tumors. [Cancer Res 2009;69(21):8499–506]

Key Words: KRAS • BRAF • HIF