Functional Characterization of the Bcl-2 Splice Variant Beta.

Bcl-2 family proteins are decisive regulators of the intrinsic apoptosis mechanisms. The best known antiapoptotic protein is Bcl-2, which functions via the interaction with proapoptotic proteins due to its association to mitochondrial membranes. Besides the widely described splice variant Bcl-2alpha (26kDa), another 22kDa splice variant Bcl-2beta exists. Bcl-2beta is described as a cytosolic and functionless protein, because of its transmembrane domain deficit.

In this study, we demonstrated for the first time that Bcl-2beta has a function and might act in the linkage between extrinsic and intrinsic pathway of apoptosis and may also play a role in hematopoietic malignancy. By performing binding studies using immuno-coprecipitation (IP) we were able to demonstrate that both splice variants bind to Bad and Bax. Furthermore, Bcl-2beta but not Bcl-2alpha is able to bind to the proapoptotic protein Bid which is localized in the cytoplasma. Bid is known as a linker which transmits an extrinsic apoptotic signal into the intrinsic pathway. Additionally, an overexpression of Bcl-2beta results in an increased chemoresistance to different cytotoxic drugs in-vitro. This effect was stronger than it was seen with the overexpression of Bcl-2alpha. We were also able to show an expression of Bcl-2beta in PBMC in 6 of 12 samples of patients with B-CLL. Bcl-2alpha was expressed in all cases. Using confocal laser scanning microscopy, we also analyzed the sub-cellular localisation of both splice variants.

Conclusion: Various models attempt to describe the mechanisms of Bcl-2 family proteins. It seems that Bcl-2beta is a protein which can function in the cytosole as well as membrane associated in an antiapoptotic manner. Its specific interaction with Bid suggests that Bcl-2beta is the cytosolic counterpart of the Bid induced pathway. The intracellular transformation from Bid to tBid, which links the extrinsic to the intrinsic apoptotic pathway, as well as the proapoptotic property of tBid could be inhibited by Bcl-2beta.