Abstract 1949: The Protein Kinase MAP4K4 Is Activated in Failing Human Hearts and Mediates Cardiomyocyte Apoptosis in Experimental Models, in vitro and in vivo
Mitogen-activated protein kinases (MAPKs) have been implicated in cardiac muscle hypertrophy, apoptosis, and heart failure, but relatively little is known of the proximal MAP4Ks that might couple cardiac stress signals to downstream effector kinases. Previously we have shown that epitope-tagged MAP4K4 (hematopoietic progenitor kinase/germinal center kinase-like kinase, HGK) was activated by diverse apoptotic signals in cultured cardiomyocytes and transgenic mouse hearts. Here, we report results for the activation of MAP4K4 in the hearts of patients undergoing therapeutic transplantation, and for the functional consequences of disrupting endogenous MAP4K4 by RNA interference. Compared with 10 control donor hearts obtained at overlapping ages, MAP4K4 activity increased in all subjects with dilated (4.5-fold; N = 13), hypertrophic (4-fold; N = 5), ischemic (2.5-fold; N = 6) and adriamycin-induced cardiomyopathy (3-fold, N = 2; p < 0.001 for all four groups). Activity of the MAP4K4 target, TGFbeta-activated kinase-1 (TAK1/MAP3K7), and the executioner caspase, caspase-3, increased concordantly in all subjects. In mouse myocardium, each of four biological signals for cardiomyocyte apoptosis induced the kinase activity of MAP4K4: biomechanical stress, ischemia/reperfusion injury, and gain-of-function mutations for TNFalpha and Gq. Analogous results were seen in cultured cardiomyocytes, using oxidative stress, ceramide and the cardiotoxic anti-cancer drug doxorubicin. Transgenic expression of MAP4K4 in myocardium produced little or no baseline phenotype, but markedly sensitized the myocardium to apoptosis caused by Gq. In culture, we proved that MAP4K4 induces apoptosis via TAK1, JNK, and the mitochondrial death pathway. As predicted, ceramide-induced and hydrogen peroxide-induced apoptosis was suppressed using RNA interference to silence expression of MAP4K4. Thus, MAP4K4 is activated in response to many specific cardiac death signals, and mediates cardiac apoptosis in vitro and in vivo. The prevalent activation of MAP4K4 in failing human hearts and the pro-death actions of the kinase suggest that MAP4K4 is a possible therapeutic target in the treatment of apoptotic cardiomyopathies.