NF-kappaB as an integrator of diverse signaling pathways: the heart of myocardial signaling?

ABSTRACT

NF-kappaB is a pleiotropic transcription factor implicated in the regulation of diverse biological phenomena, including apoptosis, cell survival, cell growth, cell division, innate immunity, cellular differentiation, and the cellular responses to stress, hypoxia, stretch and ischemia. In the heart, NF-kappaB has been shown to be activated in atherosclerosis, myocarditis, in association with angina, during transplant rejection, after ischemia/reperfusion, in congestive heart failure, dilated cardiomyopathy, after ischemic and pharmacological preconditioning, heat shock, burn trauma, and in hypertrophy of isolated cardiomyocytes. Regulation of NF-kappaB is complicated; in addition to being activated by canonical cytokine-mediated pathways, NF-kappaB is activated by many of the signal transduction cascades associated with the development of cardiac hypertrophy and response to oxidative stress. Many of these signaling cascades activate NF-kappaB by activating the IkappaB kinase (IKK) complex a major component of the canonical pathway. These signaling interactions occur largely via signaling crosstalk involving the mitogen-activated protein kinase/extracellular signalregulated kinase kinases (MEKKs) that are components of mitogen activated protein kinase (MAPK) signaling pathways. Additionally, there are other signaling factors that act more directly to activate NF-kappaB via IkappaB or by direct phosphorylation of NF-kappaB subunits. Finally, there are combinatorial interactions at the level of the promoter between NF-kappaB, its coactivators, and other transcription factors, several of which are activated by MAPK and cytokine signaling pathways. Thus, in addition to being a major mediator of cytokine effects in the heart, NF-kappaB is positioned as a signaling integrator. As such, NF-kappaB functions as a key regulator of cardiac gene expression programs downstream of multiple signal transduction cascades in a variety of physiological and pathophysiological states. We show that genetic blockade of NF-kappaB reduces infarct size in the murine heart after ischemia/reperfusion (I/R), implicating NF-kappaB as a major determinant of cell death after I/R. These results support the concept that NF-kappaB may be an important therapeutic target for specific cardiovascular diseases.