Arachidonic acid, phospholipase A2, and colorectal carcinogenesis
Phospholipase A2 (PLA2) plays an important role in metabolizing arachidonic acid (AA) from arachidonyl-phospholipids (AA-PL). In cancer cells, however, the rate of AA metabolism increases rapidly, which in turn signals for hyperproliferation, tumorigenesis, angiogenesis, and invasiveness. Therefore, understanding the roles of the various PLA2 isotypes, and the mechanisms that facilitate AA-PL metabolism are important. It is also important to know if PLA2 interacts with other signaling molecules to coordinate AA-PL metabolism. In the current investigation, an attempt was made to examine the short term (4 hours) effect of AA on differential gene expression in the colorectal adenocarcinoma cells, HT-29. Using microarray tools, I analyzed HT-29 cells, cultured in serum depleted and enriched media, to delineate differential gene expression inherent to AA metabolism. It was observed that more than 6800 genes were differentially regulated by a minimum of two fold including rab oncogenes, phospholipases, and kinases. To examine the possible roles for the various PLA2's, HT-29 cells were treated individually with (a) arachidonyl-trifluoromethyl-ketone (AACOCF 3), an inhibitor of calcium dependent PLA2 (cPLA2), which produced differential effects on 491 transcripts (b) bromoenol-lactone (BEL), an inhibitor of calcium independent PLA2 (iPLA2), which affected 259 transcripts and finally (c) aristolochic acid (ArA), an inhibitor of secreted PLA2 (sPLA2), which influenced the expression of 1849 different transcripts including cPLA2. These results suggest that AA metabolism in HT-29 cells is accomplished through the interplay of several different phenomena that includes transport, phosphorylation, and phospholipases. The microarray data was further confirmed by biochemical analysis of AA-PL metabolism. Results suggest that AACOCF3 produced a four fold increase of AA retention in the membrane PL's. On the other hand, BEL did not result in the dramatic increase of AA retention like AACOCF3 . Most importantly ArA, an inhibitor of sPLA2, increased the level of AA-PL but only after a threshold of sPLA2 inhibition was attained. Simultaneously, all three treatments generated a shift of AA partitioning from phosphatidylethanolamine (PE) to phosphatidylinositol (PI) in a dose dependent manner indicating the potential participation of an alternate signaling cascade. Altogether it can be postulated that the hypermetabolism of AA is initiated through signals under the regulation of a possible sPLA 2 receptor. Binding of this receptor induces trafficking mechanisms (rab proteins), enzyme activation and transcriptional regulation machinery (kinases), and phospholipid metabolism entities (PLA2's), which are coordinated and controlled through this cellular interplay. ^
Biology, Molecular|Biology, Cell|Health Sciences, Oncology
Jones, Raymond T, "Arachidonic acid, phospholipase A2, and colorectal carcinogenesis" (2005). ETD Collection for University of Texas, El Paso. AAI3167936.