Abstract
BackgroundArachidonic acid metabolite, generated by cyclooxygenase (COX), is implicated in the colorectal cancer (CRC) pathogenesis. Inhibiting COX may therefore have anti-carcinogenic effects. Results from use of non-steroidal anti-inflammatory drugs inhibiting only COX have been conflicting. It has been postulated that this might result from the shunting of arachidonic acid metabolism to the 5-lipoxygenase (5-LOX) pathway. Cancer cell viability is promoted by 5-LOX through several mechanisms that are similar to those of cyclooxygenase-2 (COX-2). Expression of 5-LOX is upregulated in colorectal adenoma and cancer. The aim of this study was to investigate the shunting of arachidonic acid metabolism to the 5-LOX pathway by cyclooxygenase inhibition and to determine if this process antagonizes the anti-cancer effect in colorectal cancer cells.MethodsThree colorectal cancer cell lines (HCA7, HT-29 & LoVo) expressing 5-LOX and different levels of COX-2 expression were used. The effects of aspirin (a non-selective COX inhibitor) and rofecoxib (COX-2 selective) on prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) secretion were quantified by ELISA. Proliferation and viability were studied by quantifying double-stranded DNA (dsDNA) content and metabolic activity. Apoptosis was determined by annexin V and propidium iodide staining using confocal microscopy, and caspase-3/7 activity by fluorescent substrate assay.ResultsCOX inhibitors suppressed PGE2 production but enhanced LTB4 secretion in COX-2 expressing cell lines (P <0.001). The level of COX-2 expression in colorectal cancer cells did not significantly influence the anti-proliferative and pro-apoptotic effects of COX inhibitors due to the shunting mechanism.ConclusionsThis study provides evidence of shunting between COX and 5-LOX pathways in the presence of unilateral inhibition, and may explain the conflicting anti-carcinogenic effects reported with use of COX inhibitors.
Highlights
Arachidonic acid metabolite, generated by cyclooxygenase (COX), is implicated in the colorectal cancer (CRC) pathogenesis
Eicosanoid production prostaglandin E2 (PGE2) and leukotriene B4 (LTB4) secretion were quantified by enzyme-linked immunosorbent assay (ELISA) (Cayman Chemicals, Tallinn, Estonia) as previously described [32]
HT29 cells express an inactive isoform, and LoVo cells do not express COX-2; PGE2 release was minimal from these cells
Summary
Arachidonic acid metabolite, generated by cyclooxygenase (COX), is implicated in the colorectal cancer (CRC) pathogenesis. The aim of this study was to investigate the shunting of arachidonic acid metabolism to the 5-LOX pathway by cyclooxygenase inhibition and to determine if this process antagonizes the anti-cancer effect in colorectal cancer cells. COX and 5-lipoxygenase (5-LOX) are the key enzymes involved in the generation of prostaglandins and leukotrienes respectively from this precursor. These were originally identified as playing important roles in the modulation of inflammation. The former is constitutively expressed in most tissues, whereas the latter is an immediate-to-early response gene [11] It is undetectable in most normal tissues, but is upregulated in colorectal neoplasms and their precursor lesions, [12] in which levels of downstream prostaglandin E2 (PGE2) are elevated [13]. It was suggested that arachidonic acid might be shunted from one pathway to the other when a particular pathway is inhibited in the cellular processes of cancer [22] and inflammation [23]
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