REGULATION OF RIBOSOMAL DNA-TRANSCRIPTION DURING CONTRACTION-INDUCED HYPERTROPHY OF NEONATAL CARDIOMYOCYTES

Cardiac hypertrophy requires protein accumulation, This results largel y horn an increased capacity for protein synthesis, which in turn is t he result of an elevated rate of ribosome biogenesis. The process of r ibosome formation is regulated at the level of transcription of the ri bosomal RNA genes. In this study, we examined the amounts and activiti es of various components of the ribosomal DNA transcription apparatus in contraction-arrested neonatal cardiomyocytes and in spontaneously c ontracting cardiomyocytes that hypertrophy. Nuclear run-on assays demo nstrated that spontaneously contracting cardiomyocytes supported a a-f old increased rate of ribosomal DNA transcription, However, enzy matic assay of total solubilized RNA polymerase I and Western blots demonst rated that contraction-induced increases in ribosomal RNA synthesis we re not accompanied by increased activity or amounts of RNA polymerase I. In contrast, accelerated ribosome biogenesis was accompanied by an increased amount of the ribosomal DNA transcription factor, UBF. Immun oprecipitation of [P-32]orthophosphate-labeled UBF from hypertrophying , neonatal cardiomyocytes indicated that the accumulated UBF protein w as phosphorylated and thus, in the active form. UBF mRNA levels began to increase within 3-6 h of the initiation of contraction and preceded the elevation in rDNA transcription. Nuclear run-on assays demonstrat ed increased rates of transcription of the UBF gene. Transfection of c himeric reporter constructs containing deletions of the 5'-flanking re gion of the UBF gene revealed the presence of contraction response ele ments between -1189 and -665 relative to the putative start of transcr iption, These results are consistent with the hypothesis that UBF is a n important factor in the regulation of rDNA transcription during cont raction-mediated neonatal cardiomyocyte hypertrophy.