Inhibition of cyclooxygenase: a novel approach to cancer prevention

An expanding body of evidence indicates that downregulation of the cyclooxygenases (Cox-1 and Cox-2) will be an important strategy for preventing cancer because cyclooxygenases catalyze the formation of prostaglandins (PGs), and PGs have multiple effects that favor tumorigenesis. PGs also are more abundant in cancers than in the normal tissues from which cancers arise. Overexpression of Cox-2 in epithelial cells inhibits apoptosis and increases the invasiveness of tumor cells; inhibitors of Cox (e.g., NSAIDS) are chemopreventive; and tumorigenesis is inhibited in Cox-2 knockout mice. We focus in this review on strategies to selectively inhibit and downregulate the Cox-2 isoform. This is important because simultaneous inhibition of Cox-1 (constitutively expressed) and Cox-2 (inducible isoform), which is achieved with classical NSAIDs, interferes with the housekeeping functions of Cox-1 and thereby causes serious side effects, such as peptic ulcer disease. Simultaneous inhibition of Cox-1 and Cox-2 hence is not a realistic approach for chemoprevention in individuals at low to moderate risk for cancer. On the other hand, it appears possible to avoid many NSAID-dependent side effects by selective inhibition of Cox-2, which is also the isoform that is upregulated in benign and malignant tumors. Through understanding the biochemistry of these enzymes and the regulation of Cox-1 and Cox-2 gene expression, we review how Cox-2 can be regulated selectively as a target for chemopreventive therapy. We also discuss the potential importance and advantages of a multifaceted approach to diminishing the function of Cox-2 (i.e., combining inhibitors of enzyme function with inhibitors of gene expression).