COX-2 and colorectal cancer.

Metabolites of arachidonic acid participate in normal growth responses and in aberrant cellular growth and proliferation, including carcinogenesis. The key step in the conversion of free arachidonic acid to prostaglandins is catalyzed by the cyclooxygenase enzyme (COX). There are two COX enzymes, COX-1 and COX-2. COX-1 is expressed constitutively and is part of normal cell metabolic functions. COX-2, on the other hand, is induced and expressed in neoplastic growths. The connection between COX expression and carcinogenesis was first implicated in studies that demonstrated the efficacy of aspirin and non-steroidal anti-inflammatory drugs to reduce the relative risk of colon cancer and also promote tumor regression in both humans and animal models of colon cancer. Investigation of the molecular basis of these observations showed that high levels of COX-2 protein were present in both human and animal colorectal tumors. A variety of evidence gathered from epidemiological, whole animal, and cellular studies indicate that unregulated COX-2 expression is a rate-limiting step in tumorigenesis and also that the loss of regulation occurs early in carcinogenesis. The interest in the COX-2 enzyme is that specific inhibition of COX-2 could theoretically avoid the gastrointestinal and other complications observed with the use of nonspecific COX inhibitors (most NSAIDs) or COX-1 inhibitors. The mechanisms by which COX-2 inhibitors lead to decreased colon carcinogenesis are not fully understood but they involve an increase not only in COX-2 dependent but also in COX-2 independent mechanisms.