Protein phosphorylation

During ischemia and reperfusion PKC translocation occurs with concomitant increase in its phosphorylation. Translocation of a and e PKC isoforms from the cytosol to membrane and nuclei and PKC8 from membrane to cytosol has been observed during ischemia in perfused rat heart models.130 However, with brief episodes of ischemia and reperfusion, PKC5 translocates to the sarcolemma.131. The role of the PKCs in lessening ischemia and reperfusion injury has been addressed by several studies. Translocation of PKCe occurs during the ischemic component of the preconditioning cycles132 and targeted deletion of the protein kinase C epsilon gene abolished the infarct size reduction induced by preconditioning in perfused mouse hearts.133 Furthermore, hearts from transgenic animals overexpressing PKCe are more tolerant to ischemia.134

The role of PKC5 in the context of ischemia and reperfusion is somewhat controversial. Activation of PKC5 by pharmacological agents (opioids, diazoxide) is shown to increase resistance of the heart to ischemic stress.135,136 Overexpression of active PKC5 in cells or hearts (e.g pretreated with thyroxine) resulted in protection against ischemic injury through a negative regulation of ischemia induced p38 MAPK activation.137, 138 Conversely, ischemic preconditioning exaggerated cardiac damage in PKC5 null-mice.139 Furthermore, pharmacological activation of PKC5 resulted in phosphorylation of HSF1 and overexpression of Hsp72 and induced protection against ischemia and reperfusion injury.140 However, activation of PKC5 by specific peptide activators increased ischemic damage in perfused rat hearts and cardiomyocytes.141

2.3.3. The Rho signaling

Rho signaling is activated by receptors with intrinsic enzymatic activity or GPCR (G-protein coupled receptors), as well as by non receptor stimuli. Activation of small G proteins of the Rho family (Rac and Cdc42) activates MKK4/7 and MKK3/6 followed by sequential activation of JNK1/2 and p38 MAPKu/p respectively. Important end-effectors including transcription factors and proteins are activated through this cascade; c-jun and ATF-2 (by JNK), Elk-1, ATF-2, MEF-2 and CREB (by p38 MAPK), heat shock proteins -Hsp27 (by p38 MAPK), alpha-B crystallin (by p38 MAPK) and several other cellular substrates. Figure 19. This signaling facilitates apoptosis through various targets.129 p38 MAPK directly phosphorylates Bcl-2 inactivating its anti-apoptotic effects, increases p53 protein levels with subsequent increased expression and mitochondrial translocation of Bax and induces caspase-3 cleavage. However, the antiapoptotic chaperone alphaB crystallin is also phosphorylated and activated by p38 MAPK.142

JNKs induce apoptosis through transcription dependent mechanisms that involve c-jun activation and the induction of pro-apoptotic genes. JNKs phosphorylate Bcl-2 and inactivate it, while activate the pro-apoptotic Bad, Bim and Bmf. JNKs can directly activate Bid independently of the caspase-8. Furthermore, JNKs have a direct action on mitochondria and induce cytochrome c release.143 Figure 19.

p38 MAPK is activated at the time of ischemia followed by a further activation during reperfusion. JNKs are not activated by ischemia alone but only after ischemia and reperfusion. Activation of these kinases can occur by reactive oxygen or reactive nitrogen species. The role of p38 MAPK and JNKs as pro- vs anti-apoptotic signals in the context of ischemia and reperfusion is a matter of controversy. However, there is some consensus that sustained activation of p38 MAPK or JNKs during prolonged ischemia and reperfusion increases cell death and apoptosis while transient activation seems to be protective. In cell based models of simulated ischemia, activation of p38 MAPK was detrimental while blockade with SB203580 (presumed to be p38 MAPK inhibitor) dur-





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