Molecular cloning and characterization of six novel isoforms of human Bim, a member of the proapoptotic Bcl‐2 family¹

Abstract

Bim protein is one of the BH3-only proteins, members of the Bcl-2 family that have only one of the Bcl-2 homology regions, BH3. BH3-only proteins are essential initiators of apoptotic cell death. Thus far, three isoforms of Bim have been reported, i.e. Bim(EL), Bim(L) and Bim(S). Here we report the cloning and characterization of six novel isoforms of human Bim, designated as Bimalpha1, alpha2, and beta1-beta4, which are generated by alternative splicing. Unlike the three known isoforms, none of these novel isoforms contained a C-terminal hydrophobic region. Among the novel isoforms, only Bimalpha1 and alpha2 contained a BH3 domain and were proapoptotic, although less potent than the classical isoforms. These two isoforms localized, at least in part, in mitochondria when transiently expressed in HeLa cells as a green fluorescent protein-fused form. These results suggest that the BH3 domain is necessary for induction of apoptosis and mitochondrial localization but not sufficient for the full proapoptotic activity. While the classical isoforms were always predominantly expressed in transformed cells, expression profiles of bim isoforms were highly variable among normal tissues at least in humans, suggesting a tissue-specific transcriptional regulation of bim.

The splicing factor SRSF1 regulates apoptosis and proliferation to promote mammary epithelial cell transformation

“…BIM is alternatively spliced to generate multiple isoforms with different apoptotic potential 31,32 , e.g., BIM EL, L, S, and β2. SRSF1 overexpression promoted the inclusion of a novel alternative 3’ exon, generating two new isoforms: BIM γ1 and γ2 (Fig.…”

“…The γ1 and γ2 mRNAs lack exons 2 and 3 (Supplementary Fig. 5), which encode the BH3 domain of BIM; this domain binds anti-apoptotic Bcl-2 family members, and is necessary for induction of apoptosis by BIM 32 . Thus, the changes in BIM AS we observed are consistent with the delay in luminal apoptosis promoted by SRSF1.…”

“…BIM belongs to the Bcl-2 family, and promotes apoptosis by inhibiting anti-apoptotic Bcl-2 family members 31 . Several isoforms have been characterized, with varying apoptotic activity, BIM S being the most potent 31,32,40 . The new γ1 and γ2 isoforms are not expected to promote apoptosis, as they lack exons coding for the BH3 domain and the C-terminal hydrophobic region, similar to the β2 isoform 32 .…”

“…Several isoforms have been characterized, with varying apoptotic activity, BIM S being the most potent 31,32,40 . The new γ1 and γ2 isoforms are not expected to promote apoptosis, as they lack exons coding for the BH3 domain and the C-terminal hydrophobic region, similar to the β2 isoform 32 . Overexpression of the γ1 isoform in control cells increased acinar size and decreased apoptosis.…”

A Single Nucleotide Polymorphism in cBIM Is Associated with a Slower Achievement of Major Molecular Response in Chronic Myeloid Leukaemia Treated with Imatinib

“…Alternatively, these haplotypes could be related to polymorphisms in spliced elements like introns or in consensus splicing sequences leading to an accumulation of inactive or less active isoforms. Several inactive isoforms have previously been described for BIM [27], [28] and were found in the patients and cell lines studied (Data not shown and supporting figure S1a). Thus, the quantitative analysis of mRNA for each of these isoforms could provide information on cellular ratios.…”