Prostaglandins and the regulation of tumor growth

Abstract

Increased expression of inducible cyclooxygenase (COX-2) is associated with a wide variety of tumors. In addition, inhibitors of COX have shown a great deal of promise in vitro and in animal models as potential antitumor therapies. COX enzymes use the substrate arachidonic acid to produce prostaglandin (PG)H2, the precursor to all the prostanoids. Therefore, the release of individual prostanoids depends on the abundance and functional coupling to individual PG synthase isoenzymes. Colony stimulating factors (CSFs) are also potential antitumor agents via their ability to augment the immune response. When COX-2 is expressed, the CSF, granulocyte macrophage (GM)-CSF, and granulocyte (G)-CSF are exquisitely sensitive to endogenous PGs. In addition, the ability of COX-2 to suppress GM-CSF release is mediated via traditional IP/EP prostanoid receptors linked to cAMP-dependent pathways. Therefore, inhibition of COX-2 in tumors may have the important side effect of enhancing the immune response. Recently, novel signaling pathways for PG derivatives have been discovered; in particular the PGD2 dehydration product 15-deoxy-delta(12,14)-(15d)-PGJ2 was identified as a ligand for the nuclear receptor/transcription factor, peroxisome proliferator-activated receptor (PPAR)-gamma. PPARgamma is present at high levels in a number of tumors, and is also present in endothelial cells. 15d-PGJ2 as well as other nonprostanoid PPARgamma ligands are antitumor, and antiangiogenic, by dramatically inhibiting the growth of tumor cells and endothelial cells by either causing terminal differentiation, and/or by inducing apoptosis. We have recently found that, in addition to IP and EP ligands generated by COX-2, PPARgamma ligands similarly inhibit GM-CSF release. Effecting individual prostanoid pathways at the level of COX expression, profile of PG products produced or selective PG receptor activation may produce novel therapies, either dependent or independent of CSF release, to target cancers.