[show abstract][hide abstract] ABSTRACT: Cardiac hypertrophy involves the growth of heart muscle cells and is driven by faster protein synthesis which involves increased ribosome biogenesis. However, the signaling pathways that link hypertrophic stimuli to faster ribosome production remain to be identified. Here we have investigated the signaling pathways which promote ribosomal RNA synthesis in cardiomyocytes in response to hypertrophic stimulation. We employed a new non-radioactive labeling approach and show that the hypertrophic agent phenylephrine (PE) stimulates synthesis of 18S rRNA (made by RNA polymerase I) and 5S rRNA (produced by RNA polymerase III) in adult cardiomyocytes. In many settings, rRNA synthesis is driven by rapamycin-sensitive signaling through mammalian target of rapamycin complex 1 (mTORC1). However, the activation of rRNA synthesis by PE is not inhibited by rapamycin, indicating that its regulation involves other signaling pathways. PE stimulates MEK/ERK signaling in these cells. Inhibition of this pathway blocks the ability of PE to activate synthesis of 18S and 5S rRNA. Furthermore, BI-D1870, an inhibitor of the p90(RSK)s, protein kinases which are activated by ERK, blocks PE-activated rRNA synthesis, as did a second p90(RSK) inhibitor, SL0101. BI-D1870 also inhibits the PE-stimulated association of RNA polymerase I with the rRNA promoter. These findings show that signaling via MEK/ERK/p90(RSK), not mTORC1, drives rRNA synthesis in adult cardiomyocytes undergoing hypertrophy. This is important both for our understanding of the mechanisms that control ribosome production and, potentially, for the management of cardiac hypertrophy.
Journal of Molecular and Cellular Cardiology 03/2013; · 5.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: Eukaryotic initiation factor 2B (eIF2B) plays a key role in protein synthesis and in its control. It comprises five different subunits, α-ε, of which eIF2Bε contains the catalytic domain. Formation of the complete complex is crucial for full activity and proper control of eIF2B. Mutations in the genes for eIF2B cause an often severe neurological disorder, "vanishing white matter." eIF2Bγ and eIF2Bε contain homologous and conserved domains with sequence similarity to nucleotidyl transferases (NTs) and acyl transferases and can form a binary complex. The latter contain a hexad repeat that mainly comprises isoleucyl residues (hence termed the "I-patch" region). These data reveal that certain residues in the NT domains of eIF2Bγ/ε, which are highly conserved throughout eukaryotes, play key roles in the interactions between subunits in the eIF2B complex. Our data show that the I-patch regions are important in the interactions between the catalytic eIF2Bγε complex and the other subunits. We also studied the functional effects of vanishing white matter mutations in the NT and I-patch domains. Lastly, our data show that eIF2Bγ promotes the expression of eIF2Bε, providing a mechanism for achieving correct stoichiometry of these eIF2B subunits in the cell.
Journal of Biological Chemistry 01/2012; 287(11):8263-74. · 4.65 Impact Factor
[show abstract][hide abstract] ABSTRACT: Leukoencephalopathy with vanishing white matter (VWM) is a type of leukoencephalopathy with autosomal recessive inheritance. Magnetic resonance imaging (MRI) reveals diffuse leukoencephalopathy with lesions having cerebrospinal fluid (CSF)-like signals. The clinical presentations include progressive cerebellar ataxia, spasticity, and mental decline. The course is chronic progressive with episodes of