BACKGROUND: Prostaglandin E₂ (PGE₂) and inducible cyclooxygenase (COX)-2, the committed step in PGE₂ synthesis, have been implicated in multiple tumors, as well as bladder cancer. However, regulatory steps in the PGE₂ pathway in addition to COX-2 may have important roles and have been much less studied. Using the BBN-treated mouse model, we investigated mRNA expression of genes governing production and activity of PGE₂ during development of invasive bladder cancer.
METHODS: 5-mo old male mice were divided into BBN-treated or controls groups and 4-5 mice from each group were euthanized after 4-8, 12-16, and 20-23 wks of treatment. Half of each bladder was analyzed by real-time PCR for the gene of interest normalized to GAPDH. Statistical analysis of control versus treated samples was by t-test.
RESULTS: BBN-treated mice showed submucosal inflammation at all times. BBN-treated bladders showed atypia/metaplasia through wk 12, focal CIS at wk 16, and poorly differentiated, invasive cancer by wk 20. BBN treatment increased COX-2 mRNA levels 2 to 3-fold (P<0.05) at all time points. In contrast, COX-1 mRNA, which is supposed to be constitutively expressed, was decreased 40-75% (P<0.01) at 12-23 wks in BBN-treated bladders. BBN increased mRNA expression of the terminal enzymes for PGE₂ biosynthesis, mPGES-1 and mPGES-2, 1.3- to 1.7-fold (P<0.05) only at the earliest time point. Similarly, BBN increased mRNA levels for the PGE₂ receptors, EP2R and EP4R, 2- to 3-fold at 4-8 wks (P<0.01), suggesting increased sensitivity to PGE₂, and levels returned to normal or were decreased at 20-23 wks. Levels of mRNA for cytoplasmic phopholipase A₂ (cPLA₂), which releases substrate for both COXs, were similar in control and treated bladders until wks 20-23 when they decreased 50% (P<0.01) in treated bladders. Expression of 15-prostaglandin dehydrogenase (PGDH), which inactivates PGE₂ and has recently been implicated as a potential tumor suppressor in colon cancer, was reduced 45-55% (P<0.01) in treated bladders at 4-16 wks and by 80% (P<0.001) at 20-23 wks.
CONCLUSIONS: Early in the course of tumor development, there was increased mRNA expression for enzymes that produce PGE₂ and for receptors that respond to PGE₂, as well as decreased mRNA expression for PGDH, which degrades PGE₂, all consistent with increased production or activity of PGE₂. As invasive carcinoma developed, COX-2 mRNA remained elevated and PGDH mRNA remained decreased in BBN-treated bladders, but mRNA expression of other genes in the pathway were decreased in BBN treated relative to control bladders, suggesting some negative feedback. This is the first report that PGDH may play a role in bladder cancer. Differential regulation of components involved in PG synthesis and activity during bladder tumor development suggests that the therapeutic efficacy of targeting the various components will vary with stage of tumor development.