C yclooxygenase-2 (COX-2) expression is normally tightly regulated. However, constitutive overexpression plays a key role in colon carcinogenesis. To understand the molecular nature of enhanced COX-2 expression detected in colon cancer, we examined the ability of the AU-rich element–containing (ARE-containing) 3′ untranslated region (3′UTR) of COX-2 mRNA to regulate rapid mRNA decay in human colon cancer cells. In tumor cells displaying enhanced growth and tumorigenicity that is correlated with elevated COX-2, vascular endothelial growth factor (VEGF), and IL-8 protein levels, the corresponding mRNAs were transcribed constitutively and turned over slowly. The observed mRNA stabilization is owing to defective recognition of class II-type AREs present within the COX-2, VEGF, and IL-8 3′UTRs; c-myc mRNA, containing a class I ARE decayed rapidly in the same cells. Correlating with cellular defects in mRNA stability, the RNA-binding of trans-acting cellular factors was altered. In particular, we found that the RNA-stability factor HuR binds to the COX-2 ARE, and overexpression of HuR, as detected in tumors, results in elevated expression of COX-2, VEGF, and IL-8. These findings demonstrate the functional significance rapid mRNA decay plays in controlling gene expression and show that dysregulation of these trans-acting factors can lead to overexpression of COX-2 and other angiogenic proteins, as detected in neoplasia.