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The ability of cancer cells to survive and grow under hypoxic conditions has been known for decades, but the mechanisms remain poorly understood. Under certain conditions, cancer cells undergo changes in their bioenergetic profile to favor mitochondrial respiration by activating the peroxisome proliferator-activated receptor gamma (PGC-1alpha) and upregulating mitochondrial biogenesis. In this study, we hypothesized that augmented mitochondrial biogenesis plays a critical role for cancer cells to survive hypoxia. Consistent with this hypothesis, both hypoxic human hepatocellular carcinoma (HCC) tumors and HCC cell lines subjected to hypoxia increase mitochondrial biogenesis. Silencing of PGC-1alpha in hypoxic HCC cell lines halts their proliferation. Mechanistic investigations in vitro indicated that intracellular High Mobility Group Box (HMGB)-1 protein, a nuclear protein overexpressed in HCC, is essential for the process. Silencing of HMGB1 in hypoxic HCC cell lines resulted in a significant decrease in PGC-1-alpha activation and mitochondrial biogenesis. Without HMGB1, hypoxic HCC cells had significantly reduced ATP production and decreased cellular proliferation and increased apoptosis. In a diethynitrosamine (DEN)-induced murine model of HCC, genetic blocking of HMGB1 in the hypoxic tumors resulted in a significant decrease in tumor growth. Tumors lacking HMGB1 had a significant reduction in mitochondrial biogenesis and a significant increase in mitochondrial dysfunction. Further in vitro mechanistic experiments indicated that, during hypoxia, HMGB1 translocates from the nucleus to the cytoplasm and binds to cytoplasmic Toll-like receptor (TLR)-9. This binding leads to the activation of p38 and subsequent phosphorylation of PGC-1alpha with resultant upregulation of mitochondrial biogenesis.
The findings suggest that during hypoxia HMGB1 upregulates mitochondrial biogenesis in HCC cancer cells promoting tumor survival and proliferation.