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.