Ribosomal biogenesis induction by high glucose requires activation of upstream binding factor in kidney glomerular epithelial cells

Abstract

Diabetes promotes protein synthesis to induce kidney hypertrophy and increase renal matrix proteins. Increased capacity for mRNA translation by way of ribosomal biogenesis facilitates sustained stimulation of protein synthesis. We tested the hypothesis that high glucose induces ribosomal biogenesis as indicated by an increase in rRNA synthesis in the setting of augmented protein synthesis. High glucose (30 mM) increased global protein synthesis, expression of matrix proteins, laminin γ1and fibronectin, and rDNA transcription in glomerular epithelial cells (GECs) compared with 5 mM glucose. High glucose induced Ser388phosphorylation of upstream binding factor (UBF), an rDNA transcription factor, along with increased phosphorylation of Erk and p70S6 kinase. Inactivation of Erk and p70S6 kinase either by their respective chemical inhibitors or by expression of their inactive mutant constructs blocked high-glucose-induced UBF phosphorylation. High glucose reduced nuclear content of p19ARF and promoted dissolution of inactive UBF-p19ARF complex. High glucose also promoted association of UBF with RPA194, a subunit of RNA polymerase I. Inhibition of Erk, p70S6 kinase, and UBF1 by transfecting GECs with their respective inactive mutants abolished laminin γ1synthesis, protein synthesis, and rDNA transcription. Renal cortex from type 1 diabetic rats and type 2 diabetic db/ db mice showed increased phosphorylation of UBF, Erk, and p70S6 kinase coinciding with renal hypertrophy and onset of matrix accumulation. Our data suggest that augmented ribosome biogenesis occurs in an UBF-dependent manner during increased protein synthesis induced by high glucose in the GECs that correlates with UBF activation and renal hypertrophy in rodents with type 1 and type 2 diabetes.