Contribution of p38 mitogen-activated protein kinase isotypes to cardiac physiology

• Main Author: Sarafraz Shekary, Negin
• Other Authors: Clark, James ; Marber, Michael Stephen
• Published: King's College London (University of London) 2010
• Subjects: 612.1
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.684405

The isoform-specific functions of the four isoforms of p38 mitogen-activated protein kinase (p38 α-, β-, γ- and δ- MAPK) in the adult heart are largely unknown, partly due to the lack of isotype-specific tools to manipulate or measure p38 MAPK isoform activity. Improved understanding of p38 MAPK isoform regulation will benefit development of selective pharmacological inhibitors and move towards eliminating the potential drawbacks of chronic systematic inhibition of this important kinase. This Thesis describes our investigation of endogenous expression of p38 MAPK in the murine heart; the functional contributions of endogenously expressed p38 MAPK in response to clinically-relevant stimuli such as such as pharmacological preconditioning, ischaemia and reperfusion, and pro- inflammatory cytokines (central in the progression of heart failure, such as TNF-α, IL-1) and osmotic stress; and the involvement of p38 and  MAPK isoforms in left ventricular (LV) remodelling in response to in vivo models of cardiac hypertrophy and following myocardial infarction (MI). Using commercially available isoform-specific antibodies we have demonstrated that all p38 MAPK isoforms are expressed in the murine heart with p38α and  being the most abundant. p38β and  MAPK expressed at lower levels. The transcripts for all the p38 MAPK isoforms were detected. Immunocytochemistry of isolated cardiac myocytes demonstrated a diverse localization of p38 MAPK isoforms which suggest different functions. In isolated perfused hearts, we showed that mice lacking the β isoform (p38β KO) are refractory to pharmacological preconditioning by the carbon monoxide-releasing molecule, CORM-3. Our data demonstrate that CORM-3 pre-treatment followed by a 5 min washout of hearts prior to an in vitro ischaemia-reperfusion results in decreased infarct size and preserved LV function. With respect to p38 and δ isoforms, we observed a significant reduction in left ventricular developed pressure in response to sorbitol (osmotic stressor) in wild type (WT) hearts which was significantly ameliorated in p38 knockout (p38 KO) hearts. This was accompanied by a reduction in the level of p38 MAPK phosphorylation in transgenic mice compared with the WT. A comparable response was observed between WT and p38 KO mice in response to the other stimuli. The potential roles of p38 and δ MAPK were examined in a model of isoproterenol (ISO)-induced cardiac hypertrophy. Our studies revealed no significant differences between the WT and the transgenic phenotypes in response to hypertrophic stimuli. Infusion of ISO resulted in comparable LV remodelling, as assessed by echocardiography. In addition, no differences were observed in the cardiac function (assessed by pressure volume analysis) between the two genotypes. These finding suggest that p38γ and δ MAPK are unlikely to be involved in geometric remodelling of hypertrophy. Investigating the possible contribution of p38γ and δ MAPK in post-MI remodelling in vivo (using a permanent left anterior descending artery ligation model) revealed no apparent difference between WT and p38 KO mice. Echocardiographic and pressure-volume analysis showed comparable LV dilatation in WT and p38δ KO mice. Our data confirmed that p38α MAPK is the dominant isoform in the murine myocardium and is activated in response to ischaemia, ischaemia reperfusion and a number of pro-inflammatory cytokines. We propose that p38β MAPK is implicated in pharmacological preconditioning whereas p38γ and δ isoforms appear to be important in the myocardial response to osmotic stress. p38γ and δ isoforms also seem to be implicated in LV remodelling and somehow contribute to functional changes during cardiac hypertrophy and following-MI.