Involvement of FrzA/sFRP-1 and the Wnt/Frizzled Pathway in Ischemic Preconditioning
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Phosphorylation and subsequent inactivation of glycogen synthase kinase (GSK)-3β via the Akt/PI3-Kinase pathway during ischemic preconditioning (PC) has been shown to be cardioprotective. As FrzA/sFRP-1, a secreted antagonist of the Wnt/Frizzled pathway, is expressed in the heart and is able to decrease the phosphorylation of GSK-3β in vitro on vascular cells, we examined its effect during PC using transgenic mouse overexpressing FrzA in cardiomyocytes (α-MHC promoter) under a conditional transgene expression approach (tet-off system). Overexpression of FrzA inhibited the increase in GSK-3β phosphorylation as well as protein kinase C (PKC) epsilon activation in transgenic mice after PC as compared with littermates. Phospho-Akt (P-Akt), phospho-JNK, or the cytoplasmic β-catenin levels were not modified, phospho-p38 (P-p38) was slightly increased in transgenic mice after PC as compared with littermates. FrzA transgenic mice displayed a larger infarct size and a greater worsening of cardiac function compared with littermates. All these differences were reversed by the addition of doxycycline. This study demonstrates for the first time that disruption of a β-catenin independent Wnt/Frizzled pathway induces the activation of GSK-3β and reverses the benefit of preconditioning.