Publication Date

2012-03-07

Availability

Open access

Embargo Period

2012-03-07

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PHD)

Department

Molecular and Cellular Pharmacology (Medicine)

Date of Defense

2012-02-10

First Committee Member

Keith A. Webster

Second Committee Member

Sandra Lemmon

Third Committee Member

Pedro Salas

Fourth Committee Member

Miguel Perez-Pinzon

Fifth Committee Member

Xupei Huang

Abstract

The c-Jun N-terminal kinase-1 (JNK-1) is rapidly induced by reactive oxygen species generated when the heart is reperfused during acute myocardial infarction (AMI). However, the role of JNK-1 activation in reperfusion injury has remained controversial because different groups have reported both protective and injurious consequences. My thesis work has resolved much of this controversy. I have demonstrated that JNK-1 switches from a pro-survival role when the period of ischemia is brief to become injurious when the period of ischemia is extended. My results further show that this switch in function is driven by dynamic changes in the metabolic state of the heart that take place during progressive ischemia. These results were confirmed genetically and pharmacologically in an in vivo model of AMI. I found that brief ischemia of 5 minute duration caused minimal apoptosis and infarction in wild type mice. However the same brief ischemia when implemented while simultaneously blocking JNK-1 activity using the selective inhibitor SP600125 or by genetic ablation of the JNK-1 gene caused massive apoptosis, infarction and negative remodeling that resulted in severe dilated cardiomyopathy. Conversely when ischemia was extended to 20 minutes or more, mice suffered severe ischemia-reperfusion injury, extensive infarction and negative remodeling and these effects were ameliorated by inhibition of JNK-1 again either pharmacologically or genetically. I investigate the mechanisms of the JNK-1 switch in function in vivo and in cultured cardiac myocytes. In vitro there was a comparable switch in the function of JNK-1 from protective when glucose and ATP levels were maintained during hypoxia to injurious when reoxygenation followed glucose and ATP depletion. Both apoptotic and necrotic death pathways were affected and responded reciprocally to JNK-1 inhibitors under the two bioenergetic conditions. I identified differential glucose/ATP-dependent regulation of Akt and Erk, the major components of the RISK signaling by JNK-1 as part and perhaps the entire mechanism for the JNK-1 switch. JNK-1 differentially regulated Akt phosphorylation of the regulatory site Thr450 and thereby regulated the catalytic Thr308 site in a glucose/ATP-dependent manner. JNK-1 was essential for full activity of the Raf1-MEK-Erk pathway in the glucose/ATP-sustained condition in vitro. My studies define a novel role for JNK-1 as a conditional survival kinase that protects the heart against brief but not protracted ischemia. During the ischemia phase of AMI the myocardial tissue becomes hypoxic and there is progressive acidosis. Hypoxia triggers the switch from oxidative phosphorylation to glycolysis for ATP generation and this generates excess lactic acid that is not cleared because of blocked blood flow. Therefore tissue pH falls rapidly and hypoxia-acidosis becomes an obligatory marker of severe extended myocardial ischemia. Our group has reported that the ubiquitous death protein Bnip3 is induced in an inactive form by hypoxia and the death functions are only activated by concurrent acidosis. I used a novel model of hypoxia with concurrent intracellular acidosis to investigate the roles of hypoxia and acidosis in activating Bnip3. My results support our earlier studies and confirm that intracellular pH is required to activate the death functions of Bnip3 in this model. In these experiments intracellular acidosis was induced by treating HL-1 atrial myocyte cultures with the macrolide antibiotic and selective vacuolar ATPase inhibitor bafilomycin A (BafA). BafA causes acidosis by inhibiting lysosomal proton transport and blocking proton removal from the sarcoplasm. I demonstrated that enhanced acidosis caused by BafA treatment activated Bnip3 and significantly augmented Bnip3-dependnet cell death. SiRNA studies confirmed the requirement for Bnip3 in this model. These findings confirm in a separate model that Bnip3 only provides death signals when hypoxia is combined with acidosis.

Keywords

JNK; AMI; ischemia-reperfusion; Bnip3; acidosis

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