The Molecular Chaperone αA-Crystallin Enhances Lens Epithelial Cell Growth and Resistance to UVA Stress

Abstract αA-Crystallin (αA) is a member of the small heat shock protein (sHSP) family and has the ability to prevent denatured proteins from aggregating in vitro. Lens epithelial cells express relatively low levels of αA, but in differentiated fiber cells, αA is the most abundant soluble protein. The lenses of αA-knock-out mice develop opacities at an early age, implying a critical role for αA in the maintenance of fiber cell transparency. However, the function of α-crystallin in the lens epithelium is unknown. To investigate the physiological function of αA in lens epithelial cells, we used the following two systems: αA knock-out (αA(−/−)) mouse lens epithelial cells and human lens epithelial cells that overexpress αA. The growth rate of αA(−/−) mouse lens epithelial cells was reduced by 50% compared with wild type cells. Cell cycle kinetics, measured by fluorescence-activated cell sorter analysis of propidium iodide-stained cells, indicated a relative deficiency of αA(−/−) cells in the G2/M phases. Exposure of mouse lens epithelial cells to physiological levels of UVA resulted in an increase in the number of apoptotic cells in the cultures. Four hours after irradiation the fraction of apoptotic cells in the αA(−/−) cultures was increased 40-fold over wild type. In cells lacking αA, UVA exposure modified F-actin, but actin was protected in cells expressing αA. Stably transfected cell lines overexpressing human αA were generated by transfecting extended life span human lens epithelial cells with the mammalian expression vector construct pCI-neoαA. Cells overexpressing αA were resistant to UVA stress, as determined by clonogenic survival. αA remained cytoplasmic after exposure to either UVA or thermal stress indicating that, unlike other sHSPs, the protective effect of αA was not associated with its relocalization to the nucleus. These results indicate that αA has important cellular functions in the lens over and above its well characterized role in refraction.