Expression and transcriptional regulation of functionally distinct Bmf isoforms in B-chronic lymphocytic leukemia cells.

Bmf is a BH3-only Bcl-2 family member that is normally sequestered to myosin V motors by binding to the dynein light chain 2 (DLC2). Certain damage signals release Bmf, which then binds prosurvival Bcl-2 proteins and triggers apoptosis. Here, two novel isoforms of human Bmf, Bmf-II and Bmf-III, were identified and cloned from cDNA derived from B-chronic lymphocytic leukemia (B-CLL) cells. Bmf-II and Bmf-III were characterized as two splice variants, lacking the BH3 domain but retaining the DLC2 binding domain. Bmf (here called Bmf-I) expression in HeLa cells induced apoptosis and reduced colony formation in contrast to Bmf-II and Bmf-III, which had no effect on apoptosis and instead increased colony formation. While bmf-I mRNA was expressed in many cell types, expression was higher in B lymphoid cells and bmf-II and bmf-III were mainly detected in B-CLL and normal B cells. bmf-I mRNA was upregulated in normal and leukemic B cells, while bmf-III mRNA was downregulated only in B-CLL cells by serum deprivation. We show that Bmf is regulated by transcriptional activation and alternative splicing and conclude that the relative levels of Bmf isoforms may have a role in regulating growth and survival in B cells and leukemic B-CLL cells.