Cyclooxygenase-2 (COX-2) is up-regulated in many
cancers and is a rate-limiting step in colon
carcinogenesis. Nonsteroidal antiinflammatory drugs, which inhibit COX-2, prevent
colon cancer and cause apoptosis. The mechanism for this response is not clear, but it might result from an accumulation of the substrate,
arachidonic acid, an absence of a
prostaglandin product, or diversion of the substrate into another pathway. We found that
colon adenocarcinomas overexpress another
arachidonic acid-utilizing
enzyme,
fatty acid-
CoA ligase (FACL) 4, in addition to COX-2. Exogenous
arachidonic acid caused apoptosis in
colon cancer and other cell lines, as did
triacsin C, a FACL inhibitor. In addition,
indomethacin and
sulindac significantly enhanced the apoptosis-inducing effect of
triacsin C. These findings suggested that unesterified
arachidonic acid in cells is a signal for induction of apoptosis. To test this hypothesis, we engineered cells with inducible overexpression of COX-2 and FACL4 as "sinks" for unesterified
arachidonic acid. Activation of the enzymatic sinks blocked apoptosis, and the reduction of cell death was inversely correlated with the cellular level of
arachidonic acid. Inhibition of the COX-2 component by nonsteroidal antiinflammatory drugs restored the apoptotic response. Cell death caused by exposure to
tumor necrosis factor alpha or to
calcium ionophore also was prevented by removal of unesterified
arachidonic acid. We conclude that the cellular level of unesterified
arachidonic acid is a general mechanism by which apoptosis is regulated and that COX-2 and FACL4 promote
carcinogenesis by lowering this level.