Identification of novel RARβ2 transcript variants with short 5′-UTRs in normal and cancerous breast epithelial cells

Abstract

Functional significance of RARβ2 as a putative tumor suppressor gene has been studied in breast cancer and other tumors. The long 5′-untranslated region (5′-UTR) of its transcript with multiple open-reading frames (uORFs) is considered as a regulatory unit for translation. Here, for the first time we identified RARβ2 transcript variants with short 5′-UTRs in both normal and malignant breast epithelial cells. The 5′-RACE analysis of RARβ2 mRNA in these cells demonstrated the existence of short RARβ2 transcript variants that are identical to the sequence of known RARβ2, but lack all the uORFs present in the full-length 5′-UTR. By RT–PCR analysis, we found that the expression of both transcripts with short and full-length 5′-UTR is mediated by retinoic acid, while cellular sensitivity is preferentially correlated to upregulation of short RARβ2 transcript variants in response to retinoic acid. The transfection and in vitro translation assay indicated that the short 5′-UTR has no inhibitory effects on translation, while the presence of full-length 5′-UTR inhibited translation by 60%. In addition, no promoter activity was detectable in RARβ2 full-length 5′-UTR region. Our data suggest that the RARβ2 transcript variants with short 5′-UTR may serve as major transcripts for RARβ2 protein translation as well as potential targets for retinoids in breast cancer prevention and therapy studies.