Disruption of signaling through SEK1 and MKK7 yields differential responses in hypoxic colon cancer cells treated with oxaliplatin.

Transcriptional changes in response to hypoxia are regulated in part through mitogen-activated protein (MAP) kinase signaling to activator protein 1 (AP-1), and thus contribute to resistance of cancer cells to therapy, including platinum compounds. A key role for JNK in pro-apoptotic signaling in hypoxic cells has previously been established. Here we analyze hypoxic signaling through MAPK kinases to AP-1/c-Jun in the HT29 colon adenocarcinoma cell line, and observe activation of stress-activated pathways mediated predominantly by SEK1 and MKK7. In transient transfection assays, introduction of dominant-negative constructs for both MKK7 and SEK1 abolished hypoxia-induced AP-1 activation. Functional studies of the pathway using HT29-derived cell lines stably expressing mutant SEK1 or MKK7 showed impaired activation of Jun NH2-terminal kinase (JNK) and AP-1 in response to hypoxia, more marked in MKK7-deficient than SEK1-deficient cells. Inhibition of SEK1 rendered hypoxic cells more sensitive to oxaliplatin in vitro, whereas the opposite effect was observed in MKK7-deficient cells. The mutant cell lines grown as mouse xenografts were treated with oxaliplatin, bevacizumab, or both. The SEK1-deficient tumors exhibited greater sensitivity to all treatments, whereas MKK7-deficient cells were resistant in vivo, consistent with in vitro observations. These data support a positive contribution of MKK7/JNK to oxaliplatin cytotoxicity and identify SEK1 as a potential target for reversal of hypoxic resistance to oxaliplatin.