Cytoplasmic phospholipase A2 alpha overexpression in stromal cells is correlated with angiogenesis in human colorectal cancer

In colorectal cancer, cyclooxygenase-2 (COX-2) overexpression in stromal cells induces angiogenesis through EP2 prostaglandin E2 receptor signaling. Cytoplasmic phospholipase A2 (PLA2) alpha preferentially hydrolyses arachidonic acid, which is the limiting substrate for prostaglandin production, from membrane phospholipids. We therefore investigated a possible relationship between cytoplasmic PLA2 and COX-2 overexpression in stromal cells, angiogenesis and microsatellite instability in 48 human colorectal adenocarcinomas. Cytoplasmic PLA2 and COX-2 expression in stromal cells and vascular endothelial growth factor (VEGF) expression in tumor cells were evaluated by immunohistochemistry. Microvessel density was assessed in 10 x 400 fields after CD31 staining. Microsatellite instability was evaluated by PCR and immunohistochemistry. A total of 16 tumors had microsatellite instability. We found an overexpression of cytoplasmic PLA2 in superficial stromal cells. These cells corresponded to fibroblasts and myofibroblasts. There was an association between the number of cytoplasmic PLA2 and COX-2-expressing cells (P=0.006). Cytoplasmic PLA2-positive stromal cells usually also expressed COX-2. A high number of cytoplasmic PLA2-positive stromal cells was correlated with a high microvessel density (P=0.002), a strong VEGF (P=0.01) and the absence of microsatellite instability (P=0.001). The coordinate overexpression of cytoplasmic PLA2 and COX-2 in stromal cells could lead to an important prostaglandin production. These results suggest that cytoplasmic PLA2 overexpression in these cells regulates COX-induced angiogenesis probably by providing arachidonic acid, which is the limiting factor for prostaglandin production. The lower number of cytoplasmic PLA2-positive stromal cells in carcinomas with microsatellite instability could be related to their lower microvessel density and VEGF expression.