Heat shock protein 20 (HSPB6) regulates apoptosis in human hepatocellular carcinoma cells: Direct association with Bax

A small heat shock protein (HSP), HSP20 (HSPB6) is ubiquitously expressed in various tissues and has several functions. We previously reported that the expression of HSP20 protein in human hepatocellular carcinoma (HCC) cells is inversely proportional to the progression of HCC. In addition, we showed that HSP20 is associated with phosphoinositide 3-kinase (PI3K) and inhibits the proliferation of HCC cells via suppression of the AKT signaling pathway. However, the relationship between HSP20 and apoptosis in HCC has not yet been elucidated. To clarify whether HSP20 is implicated in the apoptosis of HCC cells, in the present study, we examined the effect of HSP20 on caspases, the central regulators of apoptosis, using human HCC-derived HuH7 cells that are transfected with wild-type human HSP20 (HSP20-overexpressing cells). The cleavage of caspase-3 and caspase-7 in HSP20-overexpressing cells was enhanced compared with the empty vector-transfected cells (control cells). In addition, the cleavage of nuclear poly (ADP-ribose) polymerase (PARP) in HSP20-overexpressing cells was also strengthened. We further investigated the direct targets of HSP20 focusing on Bcl-2 family proteins in the HSP20-overexpressing cells. HSP20 proteins in the cells were coimmunoprecipitated with Bax. On the contrary, Bad, Bcl-2 and Bcl-xL were not coimmunoprecipitated with HSP20. These findings strongly suggest that HSP20 directly associates with Bax and stimulates caspase cascade in human HCC cells.