Apical endocytosis of ricin in MDCK cells is regulated by the cyclooxygenase pathway

Addition of arachidonic acid or stimulation of arachidonic acid production by secretory phospholipase A2 selectively upregulated apical endocytosis of ricin in MDCK cells without affecting basolateral endocytosis. Electron mi croscopic studies revealed that MDCK cells treated with secretory phospholi pase A2 and incubated with horseradish peroxidase had an increased number o f normal appearing peroxidase-labeled endosomes and no sign of membrane ruf fling, Moreover, inhibition of basal arachidonic acid release, either by de creasing the cytosolic phospholipase A(2) activity or the diacylglycerol li pase activity, reduced the rate of apical endocytosis. Furthermore, indomet hacin, an inhibitor of the cyclooxygenase pathway, counteracted the stimula tion of endocytosis seen with both secretory phospholipase A2 and arachidon ic acid, suggesting that formation of eicosanoids such as prostaglandins co uld be essential for the regulation. This idea was supported by the finding that prostaglandin E2, the predominant prostaglandin formed in kidney, als o upregulated ricin uptake, The regulatory effect of the cyclooxygenase pat hway on apical endocytosis of ricin was found to be independent of protein kinases A and C, which are known to selectively control apical clathrin-ind ependent endocytosis in polarized cells.