Isolation and functional analysis of mouse UbA52 gene and its relevance to diabetic nephropathy.

In delineating the mechanism(s) of diabetic nephropathy various novel genes have been isolated, whereas others remain to be discovered. We identified several up-regulated genes in the kidneys of diabetic newborn mice. Among them was UbA52, a ubiquitin ribosomal fusion protein. Its mRNA expression in the kidney was proportional to blood glucose levels. By in situ hybridization and immunohistochemistry, UbA52 was exclusively localized to renal tubules, and its expression was markedly increased in diabetic mice. The up-regulated UbA52 mRNA and protein expression were also observed in Madin-Darby canine kidney cells, a tubular cell line, treated with 30 mm glucose in both cell lysates and ribosomal fractions. To explore the mechanism(s) of its increased expression, UbA52 genomic DNA was isolated. A transcription start site at -22 bp from the initiation codon was identified and confirmed by primer extension analysis. The UbA52 promoter region included glucose response-related E-box sequences and stress response elements (STRE). Unlike in humans, mouse UbA52 gene had no introns in the coding or 5'-ATG-flanking regions. To identify the DNA segment with maximal promoter activity, deletion constructs were prepared using a pSEAP vector system and transfected into COS7 kidney cells. Maximal activity was confined to -198 to +68 bp, which included E-boxes and STRE motifs. A dose-dependent increase in the promoter activity was observed in cells exposed to high glucose. Mutations in the first E-box (CAGCTG --> TGGCTG) or STRE (CCCCT --> CATCT) resulted in a decrease in the SEAP activity under high glucose ambience. Given the presence of glucose-responsive motifs in the promoter region and decrease in the SEAP activity in E-box mutants in the presence of glucose, these data suggest that UbA52, a ribosomal fusion protein, may be relevant in the pathogenesis of diabetic nephropathy.