Objective: Overload-induced heart failure is associated with myocyte apoptosis induced by unknown mechanisms. Wnt genes encode secreted signaling molecules that bind to frizzled receptors and stabilize cytosolic beta-catenin which is translocated into the nucleus, acts as transcriptional activator and imparts an apoptosis resistant phenotype. This signaling pathway is antagonized by secreted frizzled related proteins (sFRPs) which modulate apoptosis susceptibility in cell culture models. On the basis of these considerations, the present investigation compares myocardial mRNA expression of sFRPs and the level of soluble beta-catenin in tissue samples from nonfailing and failing hearts.
Methods: Nonischemic transmural samples from human failing left ventricles and from nonfailing donor ventricles were used in the present study. The mRNA concentration of the Wnt-antagonists sFRP 1-4 were determined by quantitative reverse transcription polymerase chain reaction (RT-PCR). The myocardial localization of sFRP 3 and 4 expression was investigated using in situ RT-PCR. The pool of soluble beta-catenin was quantified by Western blot analysis of protein extracts.
Results: The mRNA levels of proapoptotic sFRPs 3 and 4 but not of sFRP 1 and 2 were elevated in failing ventricles compared to donor hearts. There was no significant difference between patients suffering from a dilated cardiomyopathy or a coronary heart disease. sFRPs 3 and 4 were expressed in cardiomyocytes and their expression correlated with the mRNA expression of the proapoptotic Fas/Fas-antagonist ratio, but inversely with the mRNA levels of the antiapoptotic bcl-xL. The size of the pool of 0.1% Triton soluble beta-catenin tended to decrease in myocardial samples with high sFRP 3 and 4 expression levels.
Conclusions: The results support the hypothesis that in failing human myocardium the Wnt/beta-catenin pathway is attenuated by enhanced expression of two endogenous Wnt-antagonists. This might contribute to an apoptosis susceptible phenotype of overloaded human myocardium.