Lack of association between the hepatocyte growth factor receptor, c-met, and anti-apoptic action of bag-1 in endothelial cells

Abstract

Bag-1 is a novel multifunctional protein. It was identified based on its ability to bind the anti-apoptotic protein, bcl-2, and also reported to interact with the heat shock protein 70 kDa (Hsp70). Thus, bag-1 may modulate apoptosis and the chaperone activity. More interestingly, bag-1 can bind to several growth factor receptors or steroid hormone receptors and regulate their function and signaling. The receptor of hepatocyte growth factor (HGF), c-met, associated with bag-1 in a study measuring immunoprecipitation in endothelial cells, we decided to investigate the contribution of bag-1 to the anti-apoptotic action of HGF. Endogenous expression of bag-1 in endothelial cells was confirmed mainly in the cytosol fraction. The treatment of human recombinant HGF (rHGF) increased tyrosine kinase and ERK phosphorylation, whereas over-expression of bag-1 had no effect on this phosphorylation. In DNA synthesis as assessed by thymidine incorporation, over-expression of bag-1 also did not induce any additional increase. In contrast, in an assay of cell death as assessed by caspase activity and lactate dehydrogenase release, over-expression of bag-1 alone attenuated serum-free and tumor necrosis factor-α -induced cell death in endothelial cells. No synergistic effect was observed between bag-1 and rHGF. To further study the association of HGF and bag-1, we examined the effect of a deletion mutant of the bag-1 C-terminal region (CTR), because bag-1 CTR is necessary to bind to c-met. Unexpectedly, over-expression of bag-1 CTR also attenuated the endothelial cell death, similar to rHGF. Taken together, these results indicate that over-expression of bag-1 has an anti-apoptotic effect on endothelial cells independent of HGF signaling.