Abstract 998: Bmx Non-Receptor Tyrosine Kinase Signaling is Necessary for Pharmacologic Protection of Ischemic Myocardium
Previous studies have demonstrated that pharmacologic agents, including nitric oxide (NO) donors, reduce myocardial infarct size in numerous mammalian species if given prior to the ischemic insult. Recent reports suggested altered expression of Bmx tyrosine kinase in the heart following protective interventions, although the relevance to preventing cell death remains unknown. Bmx is a member of the Tec family of non-receptor tyrosine kinases which have an established role in growth and differentiation in lymphocytes. To evaluate the role of this protein in the myocardium, we used mice with the first coding exon of the bmx gene replaced by lacZ (Bmx KO mice) which exhibit loss of functional Bmx mRNA and protein. Cardiac function and morphology was ostensibly normal at baseline in Bmx KO mice as compared to strain-matched balb/c controls and the KO mice did not exhibit altered lifespan. Ischemic injury, induced by 30 min ligation of the LAD followed by 24 hr reperfusion, induced up-regulation of Bmx in WT hearts. Infarct size in these animals was 28 ± 6% of the risk region as determined by post-mortem tetrazolium staining. Bmx KO mice subjected to this same injury had nearly identical infarct sizes (27 ± 4%, p = NS vs. WT control), demonstrating that loss of Bmx does not affect the basal susceptibility of the myocardium to acute ischemic injury. Administration of the NO donor DETA/NO (4 x 0.1mg/kg, i.v.) to WT mice reduced infarct size by ~50% (to 15 ± 3%, p = 0.01 vs. WT no treatment) in agreement with previous studies. However, infarct size was not reduced in the hearts of Bmx KO mice following treatment with the same dose of NO donor (infarct size = 26 ± 4%, p = NS vs. KO no treatment; p = 0.02 vs. WT + DETA/NO), demonstrating that loss of Bmx impairs the ability of the myocardium to respond to this protective stimulus. Biochemical studies on hearts from NO donor-treated mice show that phosphorylation of Akt, which is activated in WT mice, is absent in the KO myocardium. Up-regulation of the anti-apoptotic protein Bcl-2, previously shown to be involved in NO donor-induced protection, is also compromised in the Bmx KO mice. These studies provide the first assessment of Bmx activity in the heart using genetic tools and demonstrate a novel role for this kinase in protection of ischemic myocardium.