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pubmed:abstractText |
?A-crystallin is a molecular chaperone and an antiapoptotic protein. This study investigated the mechanism of inhibition of apoptosis by human ?A-crystallin and determined if the chaperone activity of ?A-crystallin is required for the antiapoptotic function. ?A-crystallin inhibited chemical-induced apoptosis in Chinese hamster ovary (CHO) cells and HeLa cells by inhibiting activation of caspase-3 and -9. In CHO cells, it inhibited apoptosis induced by the overexpression of human proapoptotic proteins, Bim and Bax. ?A-crystallin inhibited doxorubicin-mediated activation of human procaspase-3 in CHO cells and it activated the PI3K/Akt cell survival pathway by promoting the phosphorylation of PDK1, Akt and phosphatase tensin homologue in HeLa cells. The phosphoinositide 3 kinase (PI3K) activity was increased by ?A-crystallin overexpression but the protein content was unaltered. Downregulation of PI3K by the expression of a dominant-negative mutant or inhibition by LY294002 abrogated the ability of ?A-crystallin to phosphorylate Akt. These antiapoptotic functions of ?A-crystallin were enhanced in a mutant protein (R21A) that shows increased chaperone activity than the wild-type (Wt) protein. Interestingly, a mutant protein (R49A) that shows decreased chaperone activity was far weaker than the Wt protein in its antiapoptotic functions. Together, our study results show that ?A-crystallin inhibits apoptosis by enhancing PI3K activity and inactivating phosphatase tensin homologue and that the antiapoptotic function is directly related to its chaperone activity.
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