Role of cytoplasmic and secretory phospholipase A2 in intestinal epithelial cell prostaglandin E2 formation.

BACKGROUND: Prostanoid production is dependent on the enzymatic activity of phospholipase A2 enzymes to produce the precursor, arachidonic acid. Two principle phospholipase A2 enzymes play a major role in arachidonic acid production, 85kDa cytoplasmic phospholipase A2 (cPLA2) and 14kDa secretory phospholipase A2 (sPLA2). The purpose of this study was to determine the PLA2 enzyme involved in prostanoid formation in intestinal epithelial cells. METHODS: Employing a human and murine intestinal epithelial cell line, cells were exposed to the stimulants lipopolysaccharide (LPS), interleukin 1beta (IL-1) and calcium ionophore (Ca Ion) in the presence and absence of cPLA2 and sPLA2 inhibitors. The expression of both PLA2 enzymes and prostaglandin E2 (PGE2) formation were determined. RESULTS: Western blotting demonstrated that the cPLA2 enzyme was constitutively expressed in the human cell lines and not evidently increased by exposure to any of the stimulants. In murine cells the cPLA2 enzyme was also constitutively expressed and not induced by the stimulants evaluated. The sPLA2 enzyme was constitutively expressed in both cell lines and appeared to be induced by LPS and IL-1 in human enterocytes but not by Ca Ion. In murine enterocytes sPLA2 was induced by all three stimuli. PGE2 production by the human cell line was increased by LPS, IL-1 and Ca Ion. IL-1 and Ca Ion stimulated PGE2 formation was inhibited by the cPLA2 enzyme inhibitors while LPS stimulated PGE2 production was not inhibited by the cPLA2 inhibitor; but was inhibited by the sPLA2 enzyme inhibitor. Murine epithelial cells increased PGE2 formation in response to IL-1 and Ca Ion, but not LPS and the increased PGE2 was significantly decreased by cPLA2 enzyme inhibitors. CONCLUSIONS: The metabolic pathway of PGE2 formation is variable and the PLA2 enzyme involved in producing PGE2 is dependent on the stimulus and the cell line. In human intestinal epithelial cells, LPS production of PGE2 proceeds through a pathway associated with sPLA2 generated arachidonic acid while IL-1 stimulated PGE2 is produced by arachidonic acid generated by cPLA2. The physiologic significance of the various metabolic pathways of PGE2 formation is unknown.