Regional myocardial hybernation detected by low-dose dobutamine stress MRI in minipigs with prolonged left ventricular dyssynchronous contraction

Abstract

Purpose: Neuregulin-1 (NRG-1) is a paracrine growth factor released from endothelial cells in the heart. It stimulates ErbB2 and ErbB4 receptors on cardiomyocytes. Paracrine NRG-1 signalling in the myocardium is activated after exposure to anthracyclines and protects against the development of toxic cardiomyopathy. Conversely, inhibition of NRG-1 signalling after administration of trastuzumab, a monoclonal anti-ErbB2 antibody, exacerbates anthracycline-induced left ventricular (LV) dysfunction. The role of NRG-1 and ErbB activation in other forms of LV dysfunction is still incompletely understood. In this study, we hypothesized that myocardial NRG-1/ErbB signalling is activated during overdrive pacing-induced heart failure. Methods: Nineteen male beagle dogs with pacing-induced heart failure (HF) over 4 to 7 weeks underwent weekly echocardiograms and endomyocardial biopsies. Relative mRNA expression of target genes was measured with real-time RT-PCR. Phosphorylated ErbB2 and ErbB4 were measured by immunoprecipitation and Western blotting after sacrifice in 6 normal dogs, in 6 dogs with moderate HF (4 weeks of pacing) and in 7 dogs with overt congestive HF (7 weeks of pacing). Results: Rapid right ventricular pacing increased LV end-diastolic volume (64±4.3 ml/m2 at baseline and 113±7.2 ml/m2 at week 7, p<0.001) and decreased LV ejection fraction (67±1.6% at baseline and 37±2.6% at week 7, p<0.001). Over the same time course, ventricular NRG-1 mRNA expression increased 4-fold (p<0.01) and expression of a proteolytic enzyme promoting NRG-1 release, ADAM19, increased 2-fold (p<0.05). Left ventricular ErbB2 and ErbB4 receptors showed increasing degrees of overall and site-specific tyrosine phosphorylation over time. ErbB2 receptors underwent truncation exclusively in myocardium of dogs with moderate and severe HF, leading to the formation of a 95 kDa membrane-bound receptor remnant. Conclusions: Pacing-induced cardiomyopathy is accompanied by activation of myocardial NRG-1/ErbB signalling, as shown by increased NRG-1 expression and phosphorylation of ErbB2 and ErbB4 receptors in the LV. As a novel and intriguing observation, we describe the appearance of a truncated form of ErbB2 in the myocardium. In breast tumours, expression of this form is known to be associated with constitutive receptor activity and with poor cancer outcome. Whether, in analogy, ErbB2 truncation in the myocardium can be interpreted as a further manifestation of activated ErbB2 signalling remains to be confirmed, but may have interesting clinical applications.