Expression of αB-crystallin overrides the anti-apoptotic activity of XIAP

Although crystallins are major structural proteins in the lens, -crystallins perform non-lens functions, and B-crystallin has been shown to act as an anti-apoptotic mediator in various cells. The present study was undertaken to examine whether B-crystallin expressed in human malignant glioma cells exerts anti-apoptotic acitivity. In addition, we sought to elucidate the mechanism underlying any observed anti-apoptotic function of B-crystallin in these cells. Three glioma cell lines, U373MG, U118MG, and T98G, were used. We observed that only the U373MG cell line expresses B-crystallin, whereas the other 2 glioma cell lines, U118MG and T98G, demonstrated no endogenous expression of B-crystallin. We next observed that the silencing of B-crystallin sensitized U373MG cells to suberoylanilide hydroxamic acid (SAHA)induced apoptosis and that B-crystallin associates with caspase-3 and XIAP. Because XIAP is the most potent suppressor of mammalian apoptosis through the direct binding with caspases, we assessed whether XIAP also plays an anti-apoptotic role in SAHA-induced apoptosis in B-crystallin-expressing U373MG cells. Of note, the silencing of XIAP did not alter the amount of cell death induced by SAHA, indicating that XIAP does not exert an anti-apoptotic activity in U373MG cells. We then determined whether the ectopic expression of B-crystallin in glioma cells caused a loss of the anti-apoptotic activity of XIAP. Accordingly, we established 2 B-crystallin over-expressing glioma cell lines, U118MG and T98G, and found that the silencing of XIAP did not sensitize these cells to SAHA-induced apoptosis. These findings suggest that B-crystallin expressed in glioma cells overrides the anti-apoptotic activity exerted by XIAP.