Increased cyclooxygenase-2 (cox-2) expression and activity in a murine model of metastatic breast cancer

Elevated prostaglandin E-2 (PGE(2)) production is a common feature of human malignancies. This activity has often been attributed to increased metabol ic activity of the cyclooxygenase enzymes, although a direct comparison of these 2 parameters i.e., prostaglandin production and cox protein expressio n, is rarely performed in the same malignant tissue. Using a murine model o f metastatic breast cancer, we show that PGE2 levels are positively correla ted with increased tumorigenic and metastatic potential. Because prostaglan din synthesis is a product of 2 isoforms of the cyclooxygenase enzyme, we e xamined the expression and activity of both isoforms. All tumor cell lines examined, regardless of phenotype, express both cox-1 and cox-2 proteins in vitro. In contrast to the uniform cox-2 expression in vitro, only tumors r esulting from the transplantation of metastatic cell lines express cox-2 in vivo. Cox-1 is detected in both metastatic and nonmetastatic tumors. Thus, this is the first evidence that, in the tumor milieu, cox-2 expression can be regulated differently in metastatic vs. nonmetastatic lesions. Examinat ion of PGE2 synthesis in vitro reveals that nearly complete inhibition of p rostaglandin synthesis occurs in the presence of either indomethacin, which inhibits both isoforms, or NS398, which is selective for the cox-2 isoform . Thus, even though cell lines express both isoforms, the majority of the p rostaglandin synthesis stems from the activity of the inducible, cox-2 isof orm. Likewise, cell growth is inhibited by both indomethacin and NS398 in a dose-dependent manner, albeit at higher drug concentrations than required to ablate PGE2 synthesis. Despite the inhibition of prostaglandin synthesis , the cox-2 enzyme levels (protein and mRNA) were increased by either indom ethacin or NS398. (C) 2001 Wiley-Liss, Inc.