Transcriptional regulation of Cyclooxygenase-2 gene expression: Novel effects of nonsteroidal anti-inflammatory drugs

Cyclooxygenase-2 (COX-2) gene overexpression is suggested to play important roles in colorectal tumorigenesis. Epidemiological studies revealed that n onsteroidal anti-inflammatory drugs (NSAIDs), such as aspirin and sulindac, which inhibit COX activity, reduce colorectal cancer mortality. Current in vestigations have focused on delineating the molecular mechanisms that regu late COX-2 gene expression and the roles of NSAIDs in cancer chemopreventio n. COX-2 catalyzes the production of prostaglandins (PGs) from arachidonic acid (AA), generated by phospholipases A(2) (PLA(2)s), a family of acyl est erases that cause the release of AA from cellular phospholipids, Pancreatic secretory PLA(2) (sPLA(2)), via its receptor (sPLA(2)R), transcriptionally activates COX-2 gene expression in several cell types, although a specific transcription Factor mediating COX-2 expression has not yet been identifie d. Were, we report that a transcription factor, CCAAT/enhancer-binding prot ein beta (C/EBP beta), plays a critical role in sPLA(2)/IB-induced, recepto r-mediated COX-2 gene expression in MC3T3E1 and NIH3T3 cells. Furthermore, treatment of these cells with NSAIDs in the presence of sPLA(2)/IB appears to potentiate the stimulatory effects on COX-2 mRNA and COX-2 protein expre ssion and a concomitant elevation in PG production. Most significantly, NSA ID treatment appears to drastically suppress the production of cytosolic PL A(2) (cPLA(2)) mRNA. The lack of sPLA(2)IB, sPLA(2)IIA, and sPLA(2)V mRNA e xpression in both NIH3T3 and MC3T3E1 cells suggests that cPLA(2) is the mos t likely enzyme that catalyzes the release of AA, the rate-limiting substra te of COX for the production of PGs. Our results suggest that: (a) sPLA,IB receptor-mediated COX-2 expression is mediated via C/EBP beta; (b) NSAIDs i n the presence of sPLA(2)IB potentiate the stimulatory effects of sPLA(2)IB on COX-2 mRNA expression; and (c) despite the apparent stimulation of COX- 2 expression by NSAIDs, they strikingly deprive COX-2 of its substrate, AA, by suppressing cPLA(2) mRNA expression. Both AA and PGs regulate many vita l biological functions (e.g., motility and invasiveness) that are dysregula ted in most cancer cells, and they have profound effects on cellular differ entiation. Our results raise the possibility that deprivation of COX-2 of i ts substrate by the suppression of cPLA(2) mRNA expression is an additional mechanism used by NSAIDs to inhibit tumorigenesis.