Increased expression of UBF is a critical determinant for rRNA synthesis and hypertrophic growth of cardiac myocytes

Abstract

Recent evidence suggests that increased translational efficiency of existing ribosomes alone is insufficient to account for the hypertrophic growth of cardiomyocytes and that synthesis of new functional ribosomes must occur. The rate-limiting step in ribosome accumulation is the transcription of the ribosomal 45S genes (rDNA) by RNA polymerase I. Our previous studies have demonstrated that increases in the expression of the rDNA transcription factor UBF correlated with hypertrophy of neonatal cardiomyocytes. These studies expand this observation to examine directly the hypothesis that increased UBF levels are an essential requirement for the initiation of cardiac hypertrophy. We demonstrate that the introduction of UBF antisense RNA into myocytes, using adenovirus approaches, efficiently inhibits UBF accumulation during induction of cardiomyocyte hypertrophy. Moreover, this approach results in a significant reduction in rDNA transcription, rRNA levels, and protein accumulation, which are all the hallmarks of cardiac growth. Furthermore, UBF antisense RNA expression did not alter re-expression of the fetal gene program, which confirmed that the effect was specific for transcription by RNA polymerase I. These findings demonstrate that an increase in rRNA synthesis is required for hypertrophy of cardiomyocytes and also implicate UBF as a major regulatory factor in this process. Approaches that target UBF activity may be of therapeutic use in the regression of pathophysiological cardiac hypertrophy.

mTOR-Dependent Regulation of Ribosomal Gene Transcription Requires S6K1 and Is Mediated by Phosphorylation of the Carboxy-Terminal Activation Domain of the Nucleolar Transcription Factor UBF†

“…The rDNA transcription reporter pSMECAT, control reporter pSMECAT-7, UBF expression vector pcDNA3-UBF1-FLAG, and UBF antisense expression vectors have been described in detail previously (2,17,21). The actin riboprobe vector Act-387, used to generate [ 32 P]UTP-labeled internal standard for the in vitro transcription experiments was a gift from D. Autelitano (Cryptome Research Pty.…”

“…Serum-starved cells were then treated for the times indicated with 10% FBS in the presence or absence of rapamycin (final concentration of 20 nM). Neonatal cardiomyocytes were isolated from the ventricles of 1-day-old Sprague-Dawley rat pups as described previously (2,21). Myocytes were plated at 1,250 cells/mm 2 in modified Eagle's medium (MEM) containing 10% newborn calf serum and 0.1 mM bromodeoxyuridine and then allowed to attach overnight.…”

Adenoviral-Directed Expression of the Type 1A Angiotensin Receptor Promotes Cardiomyocyte Hypertrophy via Transactivation of the Epidermal Growth Factor Receptor

“…Large-scale amplification and purification of AdNHA-AT 1 virus was performed as described. 21,23 The titer of AdNHA-AT 1 viral stock used was 1ϫ10 8 PFU/L. Dominant/negative Ras (pCMV5-N17Ras) was obtained from Dr N. …”

The von Hippel-Lindau Protein pVHL Inhibits Ribosome Biogenesis and Protein Synthesis

“…For example, loss of two major tumor suppressor gene products, the retinoblastoma protein (pRB) and p53, stimulates ribosome biogenesis in cancer cells (6). pRB is a suppressor of rRNA synthesis (7,8) and p53 inhibits ribosome biogenesis through repressing the transcription of both RNA polymerase I and III (9). Notably, phosphatase and tensin homolog deleted in chromosome 10 (PTEN) also represses polymerase I transcription and MYC increases polymerase I activity and transcription of polymerase II and III genes (10).…”