Competition through dimerization between anti-apoptotic and pro-apoptotic Hax-1 proteins [Signal Transduction]

December 17th, 2013 by Koontz, J., Kontrogianni-Konstantopoulos, A.

Studies on Hax-1 have mainly focused on variant (v) 1, demonstrating its anti-apoptotic properties. However, HAX1 is heavily spliced, generating structurally distinct isoforms. We sought to characterize the Hax-1 isoforms expressed in rat heart before and after insult. We confirmed the presence of at least four Hax-1 transcripts in healthy rat cardiac muscle. These exhibited differential expression before and after induction of myocardial infarction, with v2 being up-regulated 12-fold at the transcript level, and 1.5-fold at the protein level, post-insult. Contrary to anti-apoptotic rat and human v1, overexpression of rat v2 or human v4 (the human homologue of rat v2) in epithelial cells exacerbated cell death by 30% following H2O2 treatment, compared to control vector. Co-expression of rat v1 and v2 or human v1 and v4 neutralized the protective effects of rat and human v1, and the pro-apoptotic effects of rat v2 and human v4 by modulating cytochrome C release. This is, at least partly, mediated by the ability of Hax-1 proteins to form homotypic and heterotypic dimers with binding affinities ranging from ~3.8 nM for v1 dimers to ~97 nM for v1/v2 dimers. The minimal binding region supporting these interactions lies between amino acids 97-278, which are shared by nearly all Hax-1 proteins, indicating that additional factors regulate the preferential formation of Hax-1 homo- or hetero-dimers. Our studies are the first to show that Hax-1 is a family of anti- and pro-apoptotic regulators that may modulate cell survival and death through homo- or hetero-dimerization.