Down-regulation of Rad51 and decreased homologous recombination in hypoxic cancer cells

Abstract

There is an emerging concept that acquired genetic instability in cancer cells can arise from the dysregulation of critical DNA repair pathways due to cell stresses such as inflammation and hypoxia. Here we report that hypoxia specifically down-regulates the expression of RAD51, a key mediator of homologous recombination in mammalian cells. Decreased levels of Rad51 were observed in multiple cancer cell types during hypoxic exposure and were not associated with the cell cycle profile or with expression of hypoxia-inducible factor. Analyses of RAD51 gene promoter activity, as well as mRNA and protein stability, indicate that the hypoxiamediated regulation of this gene occurs via transcriptional repression. Decreased expression of Rad51 was also observed to persist in posthypoxic cells for as long as 48 h following reoxygenation. Correspondingly, we found reduced levels of homologous recombination in both hypoxic and posthypoxic cells, suggesting that the hypoxia-associated reduction in Rad51 expression has functional consequences for DNA repair. In addition, hypoxia-mediated down-regulation of Rad51 was confirmed in vivo via immunofluorescent image analysis of experimental tumors in mice. Based on these findings, we propose a novel mechanism of genetic instability in the tumor microenvironment mediated by hypoxia-induced suppression of the homologous recombination pathway in cancer cells. The aberrant regulation of Rad51 expression may also create heterogeneity in the DNA damage response among cells within tumors, with implications for the response to cancer therapies.Solid tumors constitute a unique tissue type, characterized by hypoxia, low pH, and nutrient deprivation (45). Although decreased oxygen tension is potentially toxic to normal human cells, cancer cells acquire genetic and adaptive changes allowing them to survive and proliferate in a hypoxic microenvironment. Intratumoral hypoxia induces profound alterations in numerous physiological processes, including altered glucose metabolism, up-regulated angiogenesis, increased invasive capacity, and dysregulation of apoptotic programs (37).From a clinical standpoint, many studies have established hypoxia as an independent and adverse prognostic variable in patients with head and neck, cervical, or soft tissue (sarcoma) tumors (3,26). With regard to the extent of hypoxia observed in tumors, it has been proposed that cells within hypoxic regions of solid tumors often derive almost all metabolic energy requirements from up-regulated glycolytic pathways. This phenomenon has been referred to as the Pasteur effect (34) and provides a partial physiologic explanation for the viability of tumor cells exposed to severe hypoxia within the tumor microenvironment. Polarographic needle electrode studies used to measure oxygen tension directly in cancer patients have revealed that a significant proportion of breast carcinomas (up to 40%) contain regions of severely decreased oxygen tension (0 to 2.5 mm Hg, compared to the normal tissue range of 24 to 66 mm Hg) while still ...