Apoptosis induces Bcl-XS and cleaved Bcl-XL in chronic lymphocytic leukaemia

The Bcl-X gene has both pro-survival, Bcl-XL, and pro-apoptotic, Bcl-XS, gene products, which are produced by alternative splicing. The function of these proteins has previously been characterised in cell lines, often by transfecting expression constructs, and primary cell systems capable of dynamically regulating Bcl-XL and Bcl-XS have not been described. Such a system is potentially important to allow testing of agents that promote apoptosis by increasing the amount of Bcl-XS at the expense of Bcl-XL. In this report we characterise Bcl-X gene products in primary human leukaemic B-cells in culture conditions associated with survival and apoptosis. We found that Bcl-XS was induced in spontaneous and drug-induced apoptosis and that apoptosis induced in cells cultured on mouse fibroblasts expressing CD40 ligand with IL-4 (CD154/IL-4), a condition mimicking the tissue microenvironment, additionally produced expression of cleavage products of Bcl-XL. Both Bcl-XS and Bcl-XL were produced in a caspase dependent manner. We tested emetine, an agent previously reported to increase Bcl-XS but found that it did not have this effect in primary human B-cells. Therefore, there are two mechanisms—cleavage of Bcl-XL and production of Bcl-XS—by which Bcl-X gene products could enhance apoptosis in CLL but neither appeared to have a primary role in inducing leukaemic cell death.

Research highlights
► Bcl-X gene has both pro-survival, Bcl-XL, and pro-apoptotic, Bcl-XS, gene products.
► No well-characterised primary cell systems to test drugs operating through this mechanism exist.
► Bcl-XS and cleaved Bcl-XL were induced in a caspase dependent manner.
► Cleavage of Bcl-XL and production of Bcl-XS both enhance apoptosis.
► Emetine induced apoptosis does not depend on Bcl-XS in primary human leukaemic B-cells.