The endoplasmic reticulum (ER) is a multifunctional organelle responsible for the synthesis and folding of proteins as well as calcium storage and signaling. Perturbations of ER function cause ER stress leading to the unfolded protein response (UPR), which includes inhibition of protein synthesis, protein refolding and clearance of misfolded proteins. The UPR aims at restoring cellular homeostasis, however, prolonged ER stress can trigger apoptosis. ER stress-induced apoptosis has been implicated in the pathogenesis of various diseases such as brain ischemia/reperfusion, neurodegeneration, diabetes and, most recently, myocardial infarction and heart failure. Initial events leading to UPR and apoptosis in the heart include protein oxidation and disturbed calcium handling upon ischemia/reperfusion, and forced protein synthesis during cardiac hypertrophy. While XBP-1 and ATF6-mediated induction of ER chaperones seems to protect the heart from ischemia/reperfusion injury, the PERK/ATF4/CHOP branch of the UPR might transmit proapoptotic signals. The precise mechanism of ER stress-induced cardiomyocyte apoptosis remains elusive, however, recent data suggest that the mitochondrial apoptotic machinery is recruited through the upregulation of Puma, a proapoptotic member of the Bcl-2 family. Importantly, suppression of Puma activity prevented both ER stress and ischemia/reperfusion-induced cardiomyocyte loss, highlighting the ER stress pathways as potential therapeutic targets in cardiovascular diseases.

Background-Puma (p53-upregulated modulator of apoptosis) is a proapoptotic Bcl-2 family protein that serves as a general sensor in response to pathological apoptotic stimuli. In previous work, we demonstrated that puma ablation protects the heart from reperfusion injury in a Langendorff setting. Consistent with this, downregulation of Puma in isolated cardiac myocytes prevented apoptosis induced by different proapoptotic agents. Here, we extended our research to investigate the role of Puma, a downstream mediator of p53, in the development of heart failure using Puma Ϫ/Ϫ mice. Methods and Results-Mice underwent transverse aortic constriction, and the characteristics of cardiac remodeling were analyzed by echocardiography, histology, and gene expression at multiple time points after surgery. Four weeks after the operation, puma deletion attenuated pressure overload-induced apoptosis and fibrosis; however, it did not affect hypertrophy and angiogenesis and maintained functional performance (fractional shortening, 39% versus 25.2% in Puma Ϫ/Ϫ versus WT mice, respectively). Even at 12 weeks after transverse aortic constriction, Puma Ϫ/Ϫ mice displayed only slightly reduced contractility. In addition, transverse aortic constriction induced puma expression in a partially p53-dependent manner. To corroborate these findings, we studied another heart failure model in which heart-specific mdm4 deletion leads to p53 activation and dilated cardiomyopathy. In these mice, Puma was upregulated and its deletion rescued the cardiomyopathy phenotype. Conclusions-Our data indicate that Puma might be a critical component of the apoptotic signaling pathways that contribute to ventricular remodeling and heart failure. Therefore, Puma inactivation may serve as a preferential target to prevent heart failure induced by cellular stress. (Circulation. 2011;124:31-39.)Key Words: apoptosis Ⅲ hypertrophy Ⅲ heart failure Ⅲ remodeling I t is estimated that at least 5 million people in the United States suffer from chronic heart failure, which is associated with progressive myocardial dysfunction accompanied by cardiac remodeling. 1 Recent studies identified the p53 molecular pathways underlying cardiac remodeling during pathophysiological stimulation such as pressure overload. [2][3][4][5][6] p53 is a tumor suppressor protein with proapoptotic and antiproliferative activities achieved through transcriptional activation of a large number of target genes, including different proapoptotic proteins such as Puma (p53-upregulated modulator of apoptosis), Noxa, and Bax. 7 Editorial see p 7 Clinical Perspective on p 39Puma, which contains only 1 Bcl-2-like homology domain (BH3-only), is a unique member of the Bcl-2 family because it integrates and implements most signals mediated by different apoptosis inducers, including doxorubicin, hypoxia/reoxygenation, kinase inhibitors, phorbol esters, cytokines, growth factor deprivation, and gamma irradiation. 8 Similar to other BH3-only proteins, Puma serves as a proximal signaling molecule that transduces de...

Phosphoinositide 3-kinase (PI3K) activation and synthesis of phosphatidylinositol-3,4-bisphosphate (PI-3,4-P 2 ) and phosphatidylinositol-3,4,5-trisphosphate (PI-3,4,5-P 3 ) lipids mediate growth factor signaling that leads to cell proliferation, migration, and survival. PI3K-dependent activation of Akt is critical for myoblast differentiation induced by serum withdrawal, suggesting that in these cells PI3K signaling is activated in an unconventional manner. Here we investigate the mechanisms by which PI3K signaling and Akt are regulated during myogenesis. We report that PI-3,4-P 2 and PI-3,4,5-P 3 accumulated in the plasma membranes of serum-starved 3T3-L6 myoblasts due to de novo synthesis and increased lipid stability. Surprisingly, only newly synthesized lipids were capable of activating Akt. Knockdown of the lipid phosphatase PTEN moderately increased PI3K lipids but significantly increased Akt phosphorylation and promoted myoblast differentiation. Knockdown of the lipid phosphatase Ship2, on the other hand, dramatically increased the steady-state levels of PI-3,4,5-P 3 but did not affect Akt phosphorylation and increased apoptotic cell death. Together, these results reveal the existence of two distinct pools of PI3K lipids in differentiating 3T3-L6 myoblasts: a pool of nascent lipids that is mainly dephosphorylated by PTEN and is capable of activating Akt and promoting myoblast differentiation and a stable pool that is dephosphorylated by Ship2 and is unable to activate Akt.