Molecular characterisation, regulation and evolutionary analysis of uroplakin 1B: a tetraspanin family member

Abstract

Uroplakin 1B (UPKIB) is an integral structural protein interacting with uroplakins 1A, 2 and 3 to form hexameric plaques along the bladder lumen in the asymmetric unit membrane of urothelial umbrella cells in humans and other mammals. UPKIB mRNA expression is deregulated in transitional cell carcinomas (TCCs), however the mechanisms of regulation of UPKIB have not been established. Using genome databases, a Xenopus UPKIB homologue was identified. Maximum Parsimony and BAMBE (Bayesian Analysis in Molecular Biology and Evolution) data support a close evolutionary relationship between mammalian and amphibian UPKIB mRNA. Using Unigene, UPKIB human expressed sequence tags were identified in tissues including brain, skeletal muscle and liver, suggesting the relatively widespread distribution of this membrane protein. The UPKIB genomic structure was also deduced using genome databases. Contig AC083800, identified in a high throughput genomic sequence database, spanned UPKIB and 9 exons and 8 introns were defined. A 67bp 5' untranslated region was identified using 5' rapid amplification of cDNA ends. This product was sequenced and a putative UPKIB promoter and transcription start site was deduced. Contig AC083800 spanned the transcription start site and putative promoter. Transcription factor binding motif prediction programs detected no TATA box, but did predict a CCAAT box and several binding motifs including 4 Sp-1 sites and a NFKB site. A weak CpG island was identified within a 0.5kb region including the putative promoter, exon 1 and intron 1, which was 54% GC rich with CpG:GpC ratio of 0.46, containing 15 CpG dinucleotides. Seven TCC cell lines and five peripheral blood lymphocyte samples were analysed for UPKIB expression using RT-PCR and two cell lines expressed UPKIB transcripts. Eleven CpG sites in the putative promoter were investigated for methylation using bisulfite modification analysis in normal PBL, TCC cell lines and patient TCC samples. An inverse correlation was established in TCC cell lines between UPKIB mRNA expression and degree of methylation. 5-Aza-2'deoxycytidine induced UPKIB mRNA expression in T24 cells, previously observed not to express UPKIB. Sequence analysis of patient samples revealed more complex CpG methylation patterns, reflecting tumour heterogeneity. In summary, the uroplakin 1B gene has been characterised and one mechanism of regulation of gene expression involves methylation.