Increased retinoic acid receptor-β₄ correlates in vivo with reduced retinoic acid receptor-β₂ in esophageal squamous cell carcinoma

Different retinoic acid receptor-β (RAR-β) isoforms seem to have contrasting biological effects in human carcinogenesis. Both in vitro and in vivo data indicate that RAR-β₂ expression is frequently lost or reduced (and transfecting RAR-β₂ suppresses growth and promotes apoptosis) in various cancer cells and tissues, whereas RAR-β₄ expression is increased in several cancer cell lines. To clarify the effects of different RAR-β isoforms in esophageal carcinogenesis, we used real-time quantitative reverse transcription-PCR to assess in vivo RAR-β mRNA levels in specimens of normal and malignant human esophageal tissue, comparing these levels with each other and the expressions of other genes. RAR- β₂ mRNA expression was significantly reduced (i.e., lower in cancer than normal tissue) in 67% (18 of 27, P = 0.001) and RAR- β₄ mRNA was increased in 52% (14 of 27, P = 0.054) of our esophageal cancer cases. The expressions of RAR-β₁, chicken ovalbumin upstream promoter-transcription factor-I (COUP-TFI), COUP-TFII, and peroxisome proliferator-activated receptor-γ (PPAR-γ) mRNA were reduced, whereas epidermal growth factor receptor and cyclin D1 expressions were increased in tumor compared with in normal tissues. Reduced RAR-β₂ expression correlated with increased RAR- β₄ expression (P = 0.002) and with the suppression of COUP-TFI and COUP-TFII (P = 0.050 and 0.023, respectively) in tumor samples. These are the first in vivo expression patterns of RAR-β₂ and RAR-β₄ reported in humans or animals and support the in vitro data on these isoforms and their contrasting biological effects in human carcinogenesis.