Inhibition of Caspases by the Small Heat Shock Protein, HspB2Public Deposited
HspB2 was discovered as a chaperone of muscular dystrophy protein kinase (DMPK) and originally named, DMPK binding protein (MKBP) and presently called HspB2. Functionally, MKBP binds to DMPK and stabilizes its kinase activity, thereby accelerating the progression of muscular dystrophy. Although other binding partners of HspB2 have been found, the MKBP-DMPK interaction is only case where the function of HspB2 has been carefully analyzed. As one of ten small heat shock proteins, HspB2 shares high homology to both Hsp27 and αB-crystallin. These and other heat shock proteins have been shown to accelerate breast cancer progression through inhibition of apoptosis. In contrast to Hsp27 and αB-crystallin, the role of HspB2 in apoptosis has not been examined. The work detailed in this dissertation establishes a new anti-apoptotic function of HspB2 in a breast cancer model. Overexpression of HspB2 results in a novel inhibition of apoptosis in breast cancer cell lines shown by nuclear morphology assays and FACS analysis when stimulated with TRAIL or TNFα. In correlation with in vitro experiments, expression of HspB2 confers resistance to xenograft tumor regression following treatment with TRAIL. Inhibition of both upstream and downstream caspases was demonstrated by fluorogenic caspase activity assays and western blotting of TRAIL treated cells expressing HspB2. In contrast, mitochondrial cytochrome c release measured by western blotting and ELISA was not prevented by expression of HspB2. Furthermore, the presence of HspB2 conferred partial protection from apoptosis induced by tBid expression. These experiments aided in defining an anti-apoptotic function for HspB2 in downstream caspase inhibition. These results suggest that HspB2 may associate with caspase-3, subsequently inhibiting apoptosis. In summary, HspB2 expression in breast cancer cells inhibits apoptosis induced by extrinsic pathway stimulants and this anti-apoptotic activity has been mapped to caspases downstream of the mitochondria. These studies describe a novel anti-apoptotic function for HspB2 as an inhibitor of caspases in breast cancer.