Description
Activation of Pim1 kinase confers powerful cardioprotective properties to cells stressed by myocardial infarction or ischemia reperfusion (I/R) injury. Evidence suggests that Pim1 acts in part by protecting mitochondrial structural integrity. Recently, Dynamin related protein 1 (Drp1) has been indicated as a mediator of mitochondrial morphological changes during ischemia reperfusion and apoptotic stresses. Inhibition of Drp1 has been shown to protect mitochondrial membrane potential, reduce cytochrome c release and inhibit Bax mediated mitochondrial outer membrane permeablization and apoptotic cell death. In this study we examine the mechanistic role of Drp1 in mediating cardiac I/R injury from the perspective of a known cardioprotective kinase, Pim1. Simulated ischemia/reperfusion (sI/R) induced mitochondrial fragmentation in neonatal rat cardiac myocytes (NRCMs). Interestingly, acute Drp1 accumulation to the mitochondria was observed in vitro and in vivo following reperfusion but not during mitochondrial fragmentation. Furthermore, small molecule inhibition of Drp1 was found to protect against sI/R induced DNA damage only when added following reperfusion when Drp1 mitochondrial accumulation was greatest. Similarly, inhibition of Pim1 activity by adenoviral overexpression of Pim1 dominant negative protein (PimDN) in NRCMs or transgenic mouse hearts resulted in increased Drp1 localization to the mitochondria. Interestingly, PimDN expressing NRCMs did not show perturbed mitochondrial morphology. Overexpressing Pim1 in NRCMs corroborated these findings by reducing Drp1 localization to the mitochondria. PimDN expression also promoted the dephosphorylation of the Serine 637 residue of Drp1, known to induce Drp1 accumulation to the mitochondria and increase sensitivity to apoptotic stimuli. Together these data suggests that Pim1 activity reduces proapoptotic actions of Drp1 in response to I/R injury. Furthermore, these data support the claim that Drp1 potentiates apoptotic signaling independent of its role in mitochondrial fragmentation.
