Regulation of stromal cell cyclooxygenase-2 in the Apc Min/+ mouse model of intestinal tumorigenesis

Abstract

Cyclooxygenase-2 ( Cox-2 ) is expressed predominantly by stromal cells in intestinal adenomas from the Apc ^Min/+ mouse model of familial adenomatous polyposis. We investigated the mechanistic basis of stromal cell Cox-2 expression in Apc ^Min/+ mouse adenomas, as well as Cox-2 expression and activity in histologically normal (HN) Apc ^Min/+ mouse intestine, in order to gain further insights into regulation of Cox-2 as a potential chemoprevention target. Upregulation of Cox-2 in intestinal tumours is not an intrinsic feature of Apc ^Min/+ macrophages as bone marrow-derived Apc ^Min/+ macrophages did not exhibit an abnormality in Cox-2 expression or activity. Intestinal permeability to lactulose or mannitol was similar in Apc ^Min/+ mice and wild-type littermates, implying that macrophage activation by luminal antigen is unlikely to explain stromal cell Cox-2 induction. Moreover, stromal cells exhibited differential expression of Cox-2 and inducible nitric oxide synthase , suggesting ‘alternative’ (M2) rather than ‘classical’ (M1) macrophage activation. Flow cytometric sorting of isolated stromal mononuclear cells (SMNCs), on the basis of M-lysozyme and specific macrophage marker expression, demonstrated that macrophages, neutrophils and non-myelomonocytic cells all contributed to lamina propria prostaglandin (PG) E ₂ synthesis. However, the majority of PGE ₂ synthesis by macrophages was via a Cox-2-dependent pathway compared with predominant Cox-1-derived PGE ₂ production by non-myelomonocytic cells. SMNCs from HN Apc ^Min/+ intestinal mucosa exhibited similar levels of Cox-2 mRNA and protein, but produced more Cox-2-derived PGE ₂ than wild-type cells at 70 days of age. There was an age-dependent decline in PGE ₂ synthesis by Apc ^Min/+ SMNCs, despite tumour progression. These data suggest that other Cox-2-independent factors also control PGE ₂ levels during Apc ^Min/+ mouse intestinal tumorigenesis. Regulation of macrophage Cox-2 expression and other steps in PGE ₂ synthesis (e.g. PGE synthase) are valid targets for novel chemoprevention strategies that could minimize or avoid systemic COX-2 inhibition.