Differential regulations of chemosensitivity in oral squamous cell carcinoma cells under hypoxic conditions

Proc Amer Assoc Cancer Res, Volume 47, 2006 1254 Tissue hypoxia is now recognized to contribute to the biologically aggressive tumor phenotypes and emergence of therapeutic resistance. Hypoxia-inducible factor-1 (HIF-1), the key transcription factor under hypoxic conditions, controls a variety of genes and result in these malignant phenotypes, but the functional mechanisms of HIF-1 and its related hypoxia-inducible genes in oral squamous cell carcinomas (OSCC) remain unclear. In this study, we tried to clarify the roles of these hypoxia-inducible genes in hypoxia-induced resistance to anti-cancer chemotherapy in OSCC. We first evaluated the sensitivities of OSCC cell lines, HSC-2 and Ca9-22, to CDDP treatments under normoxia (21% pO2) and hypoxia (1% pO2) by MTT assay. Interestingly, 24- or 48-hour continuous exposure as well as pre-treatment of hypoxia for 24 hours conferred CDDP-resistance in HSC-2 but did not in Ca9-22. To characterize the differences in these two cell lines, we compared protein expression levels of HIF-1 heterodimeric subunits, HIF-1α and Arnt. The immunoblotting revealed that hypoxic treatment (1% pO2 for 8 hours) drastically increased HIF-1α protein in HSC-2 alone, although it was hardly detectable in the cells as well as Ca9-22 cells under normoxic condition. Expression level of Arnt was relatively high in HSC-2 to that in Ca9-22, but the levels in both cell lines were not influenced by hypoxic treatment. Hypoxia-inducible genes were analyzed in both cell lines by using CodeLink Expression Bioarray System, UniSet Human 20K I Bioarray (19881 probes). The comprehensive gene expression analyses demonstrated that 24-hour hypoxic treatment up-regulated 2638 and 1173 genes, and down-regulated 2881 and 2404 genes in HSC-2 and Ca9-22, respectively. Among them, 2130 genes were specifically up-regulated in HSC-2, while 2332 genes were down-regulated. We found that they included genes which are well known as sensitivity determinants of CDDP, such as VEGF, HSPB, IL6, MLH1, and MSH2. These results suggest that our selected genes may play some important roles in hypoxia-induced CDDP-resistance. Functional analyses of these novel hypoxia-inducible genes are now going on.