Nonsteroidal anti-inflammatory drugs (
NSAIDs) are antitumorigenic in humans as well as in animal models of intestinal
neoplasia, such as the
adenomatous polyposis coli (Min/+) (Apc(Min/+)) mouse.
NSAIDs inhibit
cyclooxygenase (COX)
isozymes, which are responsible for the committed step in
prostaglandin biosynthesis, and this has been considered the primary mechanism by which
NSAIDs exert their antitumorigenic effects. However, mounting evidence suggests the existence of COX-independent mechanisms. In the present study, we attempted to clarify this issue by treating Apc(Min/+) mice bearing established
tumors with
NSAIDs (
piroxicam and
sulindac, 0.5 and 0.6 mg/mouse/day, respectively) for 6 days and concomitantly bypassing COX inhibition by treatment with the E
prostaglandin (EP) receptor agonists 16,16-dimethyl-prostaglandin E(2) (
PGE(2)) and 17-phenyl-trinor-PGE(2) (10 microg each, three times daily) administered via gavage and/or i.p. routes. Treatment with
piroxicam and
sulindac resulted in 95% and 52% fewer
tumors, respectively, and a higher ratio of apoptosis:mitosis in
tumors from
sulindac-treated mice as compared with controls. These effects were attenuated by concomitant EP receptor agonist treatment, suggesting
PGE(2) is important in the maintenance of
tumor integrity. Immunological sequestration of
PGE(2) with an anti-PGE(2)
monoclonal antibody likewise resulted in 33% fewer
tumors in Apc(Min/+) mice relative to untreated controls, additionally substantiating a role for
PGE(2) in
tumorigenesis. The EP receptor subtype EP1 mediates the effects of
PGE(2) by increasing intracellular
calcium levels ([Ca(2+)](i)), whereas antagonism of EP1 has been shown to attenuate
tumorigenesis in Apc(Min/+) mice. We demonstrate that [Ca(2+)](i) is significantly elevated in
tumors of Apc(Min/+) mice relative to the adjacent normal-appearing mucosa. Furthermore, treatment with
piroxicam results in significantly lower [Ca(2+)](i) in
tumors, and this effect is attenuated by concomitant treatment with the EP1/EP3 receptor agonist 17-phenyl-trinor-PGE(2). Overall, our results suggest that
NSAIDs exert their antitumorigenic effects, in part, via interference with
PGE(2) biosynthesis, and these effects may be mediated through changes in intracellular
calcium levels.