Toll4 (TLR4) expression in cardiac myocytes in normal and failing myocardium

Expression of innate immune response proteins, including IL-1 beta, TNF, an d the cytokine-inducible isoform of nitric oxide synthase (iNOS), have been documented in the hearts of humans and experimental animals with heart fai lure regardless of etiology, although the proximal events leading to their expression are unknown. Noting that expression of a human homologue of Dros ophila Toll, a proximal innate immunity transmembrane signaling protein in the fly, now termed human Toll-like receptor 4 (hTLR4), appeared to be rela tively high in the heart, we examined TLR4 mRNA and protein abundance in is olated cellular constituents of cardiac muscle and in normal and abnormal m urine, rat, and human myocardium. TLR4 expression levels in cardiac myocyte s and in coronary microvascular endothelial cells could be enhanced by eith er LPS or IL-1 beta, an effect inhibited by the oxygen radical scavenger PD TC. Transfection of a constitutively active TLR4 construct, CD4/hTLR4, resu lted in activation of a nuclear factor-kappa B reporter construct, but not of an AP-1 or an iNOS reporter construct, in cardiac myocytes. In normal mu rine, rat, and human myocardium, TLR4 expression was diffuse, and presumabl y cytoplasmic, in cardiac myocytes. However, in remodeling murine myocardiu m remote from sites of ischemic injury and in heart tissue from patients wi th idiopathic dilated cardiomyopathy, focal areas of intense TLR4 staining were observed in juxtaposed regions of 2 or more adjacent myocytes; this st aining was not observed in control myocardium. Increased expression and sig naling by TLR4, and perhaps other Toll homologues, may contribute to the ac tivation of innate immunity in injured myocardium.