OVEREXPRESSION OF THE TRANSCRIPTION FACTOR UBF1 IS SUFFICIENT TO INCREASE RIBOSOMAL DNA-TRANSCRIPTION IN NEONATAL CARDIOMYOCYTES - IMPLICATIONS FOR CARDIAC-HYPERTROPHY

The accelerated protein accumulation characteristic of cardiomyocyte h ypertrophy results from increased cellular protein synthetic capacity (elevated ribosome content). The rate limiting step in ribosome accumu lation is transcription of the rRNA genes. During neonatal cardiomyocy te hypertrophy induced by norepinephrine or spontaneous contraction, c hanges in the expression of a ribosomal DNA transcription factor, UBF, correlated with increased rates of ribosome biogenesis. We hypothesiz ed that elevated expression of UBF was part of the mechanism by which these hypertrophic stimuli effected increases in the rate of transcrip tion from the rDNA promoter, In this study, we have examined directly the effect of overexpressing UBF on rDNA transcription in neonatal car diomyocytes in culture, In control experiments, a novel reporter const ruct for rDNA transcription (pSMECAT) showed similar increases in acti vity in response to hypertrophic stimuli (10(-4) M phenylephrine, 10(- 7) M endothelin, and spontaneous contraction) as did the endogenous rR NA genes. When contraction-arrested cardiomyocytes were cotransfected with pSMECAT and increasing amounts of a UBF1 expression vector; a dos e-dependent (3-5 fold) increase in rDNA transcription was observed, We stern blot analysis confirmed that the overexpressed, FLAG-tagged UBF accumulated in the cardiomyocyte nuclei. The observation that overexpr ession of UBF1 is sufficient to increase rDNA transcription in neonata l cardiomyocytes provides evidence in support of the hypothesis that t he regulation of UBF is a key component of the increased ribosome biog enesis and protein accumulation associated with cardiomyocyte hypertro phy.