The molecular chaperone αA-crystallin enhances lens epithelial cell growth and resistance to UVA stress

alpha A-Crystallin (alpha A) 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 alpha A, but in differentiated fiber cells, alpha A is the most abundant soluble protein. The lenses of alpha A-knock-out mice develop opacities at an early age, implying a critical role for alpha A in the maintenance of fiber cell transparency. However, the function of alpha-crystallin in the lens epithelium is unknown. To investigate the physiological function of alpha A in lens epithelial cells, we used the following two systems: alpha A knock-out (alpha A(-/-)) mouse lens epithelial cells and human lens epithelial cells that overexpress cuk The growth rate of alpha A(-/-) 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 alpha A(-/-) cells in the G(2)/M phases. Exposure of mouse lens epithelial cells to physiological levels of WA resulted in an increase in the number of apoptotic cells in the cultures. Four hours after irradiation the fraction of apoptotic cells in the alpha A(-/-) cultures was increased 40-fold over wild type. In cells lacking alpha A, WA exposure modified F-actin, but actin was protected in cells expressing alpha A Stably transfected cell lines overexpressing human alpha A were generated by transfecting extended life span human lens epithelial cells with the mammalian expression vector construct pCI-neo alpha A Cells overexpressing alpha A were resistant to WA stress, as determined by clonogenic survival. alpha A remained cytoplasmic after exposure to either WA or thermal stress indicating that, unlike other sHSPs, the protective effect of alpha A was not associated with its relocalization to the nucleus. These results indicate that alpha A has important cellular functions in the lens over and above its well characterized role in refraction.