Abstract 746: A Cyclin D2-Rb Pathway Regulates Cardiac Myocyte Size and RNA Polymerase III After Biomechanical Stress in Adult Myocardium
Normally, cell cycle progression is tightly coupled to the accumulation of cell mass (cell growth). The molecular processes that regulate hypertrophic growth and the mechanisms whereby proliferation and cell growth are coupled are poorly understood. We have identified Cyclin D2 (CycD2), a G1 cyclin implicated in mediating S phase entry, as a potential regulator of hypertrophic growth in adult post mitotic myocardium. To examine the role of CycD2 and its downstream targets, we subjected mice deficient for CycD2 and Rb to mechanical stress. Hypertrophic growth in response to transverse aortic constriction (TAC) was attenuated in CycD2 null compared to wildtype mice (5.45 ± 0.29 mg/g CycD2+/+ versus 4.81 ± 0.14 mg/g CycD2−/−; P<0.05). Blocking the increase in CycD2 with siRNA in response to hypertrophic agonists prevented phosphorylation and inactivation of the CycD2-target Rb in vitro. When mice deficient for Rb (CRbL/L) were subjected to TAC, hypertrophic growth was potentiated (5.23±0.3 mg/g CRb+/+ versus 6.44±0.5 mg/g CRbL/L; P<0.05). Hypertrophic growth requires protein synthesis and transcription of tRNA genes by RNA pol III, which increases with hypertrophic signals. This load-induced increase in RNA pol III activity is augmented in Rb-deficient hearts, consistent with the established role of Rb as a direct repressor of the Brf-1 subunit of RNA pol III-specific transcription factor B (TFIIIB). We performed co-immunoprecipitation studies which demonstrate that Rb binds to Brf-1 in adult myocardium under basal conditions, but this association is disrupted in response to a hypertrophic stimulus. These investigations identify an essential role for the CycD2-Rb pathway as a governor of cardiac myocyte enlargement in response to biomechanical stress and, more fundamentally, as a regulator of the load-induced activation of RNA pol III.