Abstract 1531: Programmed Death by Hypoxia-acidosis Involves Bnip3- and ERK-dependent Activation of Histone Acetyltransferase (HAT) and Genomic Reprogramming
Hypoxia-acidosis causes programmed cell death by a Bnip3-dependent pathway. Multiple signaling pathways are activated by hypoxia-acidosis and Bnip3 death involves increased mitochondrial membrane permeability and communication between mitochondria and the cytoplasm. A feature of this death pathway is pronounced genomic DNA degradation. In this study we tested the hypothesis that Bnip3 modulates chromatin structure making it more accessible to transcription factors and DNases. Histone acetylation is associated with open chromatin and active gene transcription, histone deacetylation is associated with gene silencing. We found that combined hypoxia-acidosis but not hypoxia or acidosis separately was associated with a highly significant increase in acetylation level of histones H3 and H4 (H3: 7.3 ±1.2; H4: 8.1± 2.1 relative to untreated/aerobic). Hyperacetylation correlated closely with acidosis, accumulated Bnip3, and progressive cell death; it was initiated when intracellular pH fell below 7.0 and continued to increase with decreasing pH. The degree of histone acetylation is determined by the balance between acetyltransferase (HAT) and deacetyltransferase (HDAC) activities, therefore we measured these activities. We found that HAT activity increased 2.8±0.8 fold during hypoxia-acidosis and no change in HDAC activity. To confirm that these changes were related to Bnip3, cardiac myocytes were transfected with a specific siRNA. When siRNA-treated myocytes were subjected to hypoxia-acidosis the changes of histone acetylation (p<0.05) and HAT activity were eliminated, confirming that hyperacetylation was part of the Bnip3 pathway. Investigations of potential signaling molecules upstream of histone acetylation revealed that hyperacetylation was blocked by inhibiting ERK but not p38. Hyperacetylation correlated with increased phosphorylation of the histone acetyltransferase protein CREB and this was also prevented by Bnip3 siRNA. These activities were not observed with staurosporine apoptosis. We conclude that Bnip3 death pathway involves cross-talk with MAPK signaling pathways that culminates in enhanced HAT activity, global hyperacetylation of histones and increased accessibility of modulating factors to the genome.