A nucleus-targeted alternately spliced Nix/Bnip3L protein isoform modifies nuclear factor κB (NFκB)-mediated cardiac transcription

Several Bcl2 family proteins are expressed both as mitochondrial-targeted full-length and as cytosolic truncated alternately spliced isoforms. Recombinantly expressed shorter Bcl2 family isoforms can heterotypically bind to and prevent mitochondrial localization of their full-length analogs, thus suppressing their activity by sequestration. This "sponge" role requires 1:1 expression stoichiometry; absent this an alternate role is suggested. Here, RNA sequencing revealed coordinate regulation of BH3-only protein Nix/Bnip3L (Nix) and its alternately spliced soluble form (sNix) in hearts, but relative sNix/Nix expression of ∼1:10. Accordingly, we examined other putative functions of sNix. Although Nix expressed in H9c2 rat myoblasts localized to mitochondria, sNix showed variable cytoplasmic and nuclear distribution. Tumor necrosis factor α (TNFα) induced rapid and complete sNix nucleoplasmic translocation concomitant with nuclear translocation of the p65/RelA subunit of NFκB. sNix co-localized and co-precipitated with p65/RelA after TNFα stimulation; TNFα-induced sNix nuclear translocation did not occur in p65/RelA null murine embryonic fibroblasts. ChIP sequencing of TNFα-stimulated H9c2 cells revealed sNix suppression of p65/RelA binding to a subset of weaker DNA binding sites, accounting for its ability to alter gene expression in cultured cells and in vivo mouse hearts. These findings reveal TNFα-stimulated cytoplasmic-nuclear shuttling of the alternately spliced non-mitochondrial Nix isoform and uncover a role for sNix as a modulator of TNFα/NFκB-stimulated cardiac gene expression. Transcriptional co-regulation of sNix and Nix, combined with sNix posttranslational regulation by TNFα, comprises a previously unknown mechanism for molecular cross-talk between extrinsic death receptor and intrinsic mitochondrial apoptosis pathways.