Reversible regulation of the retinoblastoma protein/E2F-1 pathway during "reverse cardiac remodelling" after ventricular unloading

Abstract

Background: Cyclin D1, the retinoblastoma (Rb) protein, and the E2F transcription factors are involved in the pathogenesis of cardiac hypertrophy. Cyclin D1/cdk4 complexes, by phosphorylation, inactivate Rb, thereby abrogating its growth-inhibitory effect. Ventricular unloading is associated with reversible regulation of numerous cardiomyocyte molecular systems and decreased hypertrophy. Accordingly, the hypothesis whether the Rb/E2F-1 pathway is altered by ventricular unloading was tested, and correlations with the cyclin D1 protein expression and cardiomyocyte diameters were explored.

Methods: In 21 paired myocardial samples (before and after unloading) from patients with congestive heart failure (CHF), cyclin D1, phosphorylated Rb (pRb), its homologues p107 and p130 (pocket proteins), and E2F-1 were immunohistochemically investigated and morphometrically quantified. Cardiomyocyte diameters were morphometrically determined.

Results: Cyclin D1 and the proteins of the Rb/E2F-1 pathway were significantly increased during CHF compared with controls and were significantly decreased after unloading. Cyclin D1, pRb, and p130 protein expression correlated significantly with cardiomyocyte diameters. A significant positive correlation was noted between the pocket proteins, E2F-1, and cyclin D1.

Conclusion: Increased protein expression of phosphorylated (inactivated) Rb and the pocket proteins is associated with cardiomyocyte hypertrophy in CHF. Rb inactivation might be explained by phosphorylation by increased numbers of cyclin D1/cdk4 complexes associated with cardiomyocyte hypertrophy. However, ventricular unloading can reversibly regulate this process. These data underscore the importance of cell cycle regulatory proteins in the pathogenesis of CHF-associated (maladaptive) cardiomyocyte hypertrophy and might offer novel clues for pharmacologic approaches of congestive heart failure.