REGULATION OF RDNA TRANSCRIPTION DURING ENDOTHELIN-1-INDUCED HYPERTROPHY OF NEONATAL CARDIOMYOCYTES - HYPERPHOSPHORYLATION OF UPSTREAM BINDING-FACTOR, AN RDNA TRANSCRIPTION FACTOR

Treatment of cultured neonatal cardiomyocytes with endothelin-1 and ph orbol 12-myristate 13-acetate (PMA) results in cardiomyocyte hypertrop hy. However, the signal transduction pathways involved in this process are poorly understood. Because increased ribosome biogenesis is a req uisite for hypertrophy, we sought to (1) confirm the hypothesis that t hese two hypertrophic agents did indeed induce rRNA synthesis and (2) examine the mechanism through which this induction was accomplished. I n this study, hypertrophy of contraction-arrested neonatal cardiomyocy tes induced by treatment with either endothelin-l or PMA was associate d with increased rDNA transcription. Western blots demonstrated that t he enhanced rates of rDNA transcription were not mediated by increased amounts of either RNA polymerase I or upstream binding factor (UBF), an rDNA transcription factor. However, immunoprecipitation of [P-32]or thophosphate-labeled UBF from hypertrophying neonatal cardiomyocytes s uggested that the increased rate of rDNA transcription may be due to t he hyperphosphorylation of UBF, which would increase the activity of U BF. The increase in UBF phosphorylation occurred within 3 to 6 hours a fter exposure to either agent, was maximal at 12 hours, and was sustai ned for at least the first 24 hours of exposure. Phosphoamino acid ana lysis of UBF immunoprecipitated from control and treated cardiomyocyte s demonstrated that UBF was phosphorylated exclusively on serine resid ues. Our previous studies have shown that the cellular UBF content inc reased in adrenergic- and contraction-induced models of cardiac hypert rophy. This study with endothelin-l and PMA demonstrates that the modu lation of UBF phosphor ylation is an additional pathway by which ribos ome biogenesis may be regulated in neonatal cardiomyocytes. These resu lts support the hypothesis chat UBF is an important regulatory factor during the initiation and maintenance of the accelerated rate of rDNA transcription observed during neonatal cardiomyocyte hypertrophy media ted by both phorbol esters and endothelin-1.