Effect of hypoxia on the expression of αB-crystallin in head and neck squamous cell carcinoma

Background: The presence of hypoxia in head and neck squamous cell carcinoma (HNSCC) is associated with therapeutic resistance and increased risk of metastasis formation. aB-crystallin (HspB5) is a small heat shock protein, which is also associated with metastasis formation in HNSCC. In this study, we investigated whether aB-crystallin protein expression is increased in hypoxic areas of HNSCC biopsies and analyzed whether hypoxia induces aB-crystallin expression in vitro and in this way may confer hypoxic cell survival. Methods: In 38 HNSCC biopsies, the overlap between immunohistochemically stained aB-crystallin and pimonidazole-adducts (hypoxiamarker) was determined. Moreover, expression levels of aB-crystallin were analyzed in HNSCC cell lines under hypoxia and reoxygenation conditions and after exposure to reactive oxygen species (ROS) and the ROS scavenger N-acetylcysteine (NAC). siRNA-mediated knockdown was used to determine the influence of aB-crystallin on cell survival under hypoxic conditions. Results: In all biopsies aB-crystallin was more abundantly present in hypoxic areas than in normoxic areas. Remarkably, hypoxia decreased aB-crystallin mRNA expression in the HNSCC cell lines. Only after reoxygenation, a condition that stimulates ROS formation, aB-crystallin expression was increased. aB-crystallin mRNA levels were also increased by extracellular ROS, and NAC abolished the reoxygenation-induced aB-crystallin upregulation. Moreover, it was found that decreased aB-crystallin levels reduced cell survival under hypoxic conditions. Conclusions: We provide the first evidence that hypoxia stimulates upregulation of aB-crystallin in HNSCC. This upregulation was not caused by the low oxygen pressure, but more likely by ROS formation. The higher expression of aB-crystallin may lead to prolonged survival of these cells under hypoxic conditions.