Post-transcriptional control of cyclooxygenase-2 gene expression - The role of the 3 '-untranslated region

The cyclooxygenase (COX)-2 enzyme is responsible for increased prostaglandi n formation in inflammatory states and is the major target of nonsteroidal anti-inflammatory drugs. Normally COX-2 expression is tightly regulated, ho wever, constitutive overexpression plays a hey role in colon carcinogenesis . To understand the mechanisms controlling COX-2 expression, we examined th e ability of the 3'-untranslated region of the COX-2 mRNA to regulate post- transcriptional events. When fused to a reporter gene, the 3'-untranslated region mediated rapid mRNA decay (t(1/2) = 30 min), which was comparable to endogenous COX-2 mRNA turnover in serum-induced fibroblasts treated with a ctinomycin D or dexamethasone. Deletion analysis demonstrated that a conser ved 116-nucleotide AU-rich sequence element (ARE) mediated mRNA degradation . In transiently transfected cells, this region inhibited protein synthesis approximately 3-fold. However, this inhibition did not occur through chang es in mRNA stability since mRNA half-life and steady-state mRNA levels were unchanged. RNA mobility shift assays demonstrated a complex of cytoplasmic proteins that bound specifically to the ARE, and UV cross-linking studies identified proteins ranging from 90 to 35 kDa. Fractionation of the cytosol showed differential association of ARE-binding proteins to polysomes and S 130 fractions. We propose that these factors influence expression at a post -transcriptional step and, if dysregulated, may increase COX-2 protein as d etected in colon cancer.