Using a global DNA methylation profiling strategy, we find that Cox-1
is epigenetically regulated in pancreatic cancers, and is not expressed in most normal pancreatic duct cells. Furthermore, as epigenetic silencing of Cox-2
can also occur during pancreatic cancer development 16
, some pancreatic cancers evolve to lack both cyclooxygenases. Since stromal cells adjacent to infiltrating pancreatic cancers such as fibroblasts, endothelial cells 44
and inflammatory cells express high levels of Cox-1
, these cells are a likely source of prostaglandins for pancreatic cancer cells deficient in cyclooxygenases. Although pancreatic cancer cells lacking cyclooxygenases do not produce PGE2
, they still proliferate in response to PGE2
. Previous studies have identified cancers lacking Cox-2 expression, but to our knowledge no prior studies have recognized that cancer cells could be deficient in both Cox-1 and Cox-2.
Until recently, COX-1
had been reported to be the constitutive, and COX-2
, the inducible cyclooxygenase. It is now clear that many tissues lack Cox-1 expression and based on our gene expression and immunohistochemical data it appears that pancreatic duct cells rely on Cox-2 rather than Cox-1 to produce prostaglandins 6, 45
The lack of expression of Cox-1 in pancreatic duct may help explain epidemiological studies demonstrating that aspirin, which is primarily a Cox-1 inhibitor, does not prevent the development of pancreatic cancer, but does reduce colon cancer incidence and mortality(16).
We hypothesized that the lack of Cox expression in some pancreatic cancer cells may render them dependent on exogenous PGE2
such as from stromal fibroblasts. Prostaglandins are secreted by most cells, and act as autocrine- and paracrine-signaling molecules requiring controlled release, uptake, and metabolism to initiate and terminate signaling 7
. The main efflux transporter of PGE2
is the multidrug resistance–associated protein (Mrp)-4 which exports PGE2
to the extracellular milieu) 46, 47
. Exported PGE2
can bind to transmembrane prostaglandin receptors. The other main transmembrane prostaglandin transporter (PGT) carriesPGE2
into the cytoplasm. Interestingly, Mrp4 is overexpressed in colorectal and othercancers 48
. In model systems, MRP4 knockout results in a pronounced reduction in extracellular PGE2
, and Mrp4is inhibited by certain nonsteroidal anti-inflammatory drugs 46, 49
. We find that stromal fibroblasts supply PGE2
to pancreatic cancer cells and blocking Mrp4-dependent PGE2
excretion from fibroblasts reduces the proliferation of pancreatic cancer cells lacking cyclooxygenases. Thus, inhibition of Mrp4 may represent a useful treatment strategy for pancreatic cancer cells deficient in cyclooxygenases. Inhibiting Mrp4 could be a more effective strategy for targeting the Cox pathway in pancreatic cancers since Cox inhibitors will not have any direct effect on pancreatic cancer cells lacking Cox expression. Cox inhibitors could still target stromal fibroblast Cox expression but since fibroblasts express both Cox-1 and Cox-2, non-selective Cox inhibitors would be required, with greater potential for systemic toxicity. The provision of prostaglandins by stromal fibroblasts may partly explain why Cox-2 inhibitors have not been shown to be effective in treating pancreatic cancers 50
. It is not known if inhibiting PGE2
production by blocking Mrp4 would be a more targeted and therapeutically safer approach to blocking prostaglandin E2 effects. An alternative approach to inhibiting the cancer promoting effects of PGE2 would be to inhibit PGE2 receptors 51–53
, but since there are 4 PGE2 receptors, EP1, EP2, EP3 and EP4, this might require blocking multiple receptors.
In summary, we find evidence for epigenetic regulation of Cox-1 in pancreatic cancers and demonstrate that most pancreatic cancers lack Cox-1 expression. Indeed, some pancreatic cancers are devoid of either Cox-1 or Cox-2 expression rendering dependent on exogenous sources of prostaglandins. Inhibiting the efflux of prostaglandins from stromal fibroblasts by blocking the prostaglandin transporter, MRP4, inhibits the proliferation of pancreatic cancer cells lacking cyclooxygenases.