Abstract 218: Ischemic Induction of Total and p300 HAT Activity is Associated with Cardioprotection in a Mouse MI Model
Introduction: The nuclear histone acetyltransferase (HAT) p300 is a critical regulator of cardiac growth and MEF-2 and GATA-4-dependent gene expression. We have shown that protein levels of p300 are induced by myocardial ischemia, and that overexpression of p300 exerts protection against ischemia both in vitro and in vivo. How this protection is exerted remains unknown. The aim of the present study was to investigate the relationship between myocardial ischemia and p300 HAT activity during ischemic injury in mice.
Methods: Mice with cardiac-targeted p300 transgene (TG) and wt littermates (WT) underwent coronary occlusion or sham operation for 0, 1, 2, 3, or 8d. p300 protein and mRNA were measured by Western blot and realtime PCR. Total cellular and p300-immunoprecipitatable HAT activity were determined by colorimetric assay and normalized to total protein. Myocardial area at risk was measured by Evans Blue staining. All values are reported as mean ± SD.
Results: We confirmed that despite reduced ischemic damage in TG mice, myocardial area at risk was the same as in WT (42 ± 1.9 vs 42 ± 0.8% of left ventricle, WT vs TG, n = 5). In WT, p300 mRNA increased by 1.5- and 1.7-fold at 1 and 2d respectively, in ischemic vs remote non-ischemic tissue (p < 0.01). p300 protein levels were also increased (2.0- and 2.5-fold at 3 and 8d; p < 0.001). Total cell HAT activity was similar in TG and WT at baseline. In both TG and WT mice, ischemia induced total cellular HAT activity by 1.5- and 2.1-fold vs sham operation after 3 and 8d, respectively (all p < 0.05). Acetylation of histone-3 (Ac-H3), a nuclear p300 target protein, increased by 1.5- and 2.0-fold at 3 and 8d in WT; fold increase in TG mice was similar (p < 0.01). However TG mice had higher total Ac-H3 than WT at all time points (1.5-fold of WT at 0, 3 and 8d, p < 0.01). Consistent with this, nuclear-localized p300-specific HAT activity rose in both WT and TG mice (1.5–3.3 fold, n = 6, p < 0.001), but was 1.9–3.0-fold higher in TG mice at each time point.
Conclusions: Both HAT activity (total and p300-specific) and histone acetylation increase significantly during myocardial ischemia. Increased p300 protein in transgenic mice further augments both p300 HAT activity and Ac-H3, possibly contributing to its cardioprotective role during acute ischemia in vivo.