D-24B Free Communication/Slide Muscle Cellular/Molecular Physiology II
To elucidate the mechanisms controlling ribosome biogenesis during myotube hypertrophy.
L6 myotubes were serum stimulated for 48 hrs.
Serum stimulation resulted in increased total RNA content and myotube hypertrophy (protein/DNA ratio). This increase in total RNA was accompanied by an increased phosphorylation of the tumor suppressor retinoblastoma (Rb). Co-immunoprecipitation analysis of Rb demonstrated that UBF interaction was disrupted by Rb phosphorylation. Serum stimulation resulted in an increase in Cyclin D1 protein expression with a concomitant increase in cdk-4 kinase activity despite no apparent changes in cellular proliferation as determined by DNA content and BrdU labeling. Myotube hypertrophy and RNA accumulation were blocked by Rapamycin treatment, which also prevented the increase in Cyclin D1 protein, cdk-4 kinase activity, Rb phosphorylation, and release of UBF sequestration by Rb.
We propose that the cell cycle regulators cyclin D1, cdk-4 and Rb exert growth control in terminally differentiated myotubes through the regulation of UBF availability and ribosome biogenesis. These results suggest that cyclin D1-dependent cdk-4 kinase activity is regulated through mTOR and is required for ribosome biogenesis and myotube hypertrophy. Supported in part by an ACSM Foundation Graduate Student Research Award (GAN) and NIH AR45617 (KAE).