INVOLVEMENT OF P21(RACA), PHOSPHOINOSITIDE 3-KINASE, AND VACUOLAR ATPASE IN PHAGOCYTOSIS OF BACTERIA AND ERYTHROCYTES BY ENTAMOEBA-HISTOLYTICA - SUGGESTIVE EVIDENCE FOR COINCIDENTAL EVOLUTION OF AMEBIC INVASIVENESS

Trophozoites of Entamoeba histolytica, the protozoan parasite that cau ses amebic dysentery, phagocytose bacteria in the colonic lumen and er ythrocytes (RBC) in host tissues. Because tissue invasion is an evolut ionary dead end, it is likely that amebic pathogenicity is coincidenta lly selected, i.e., the same methods used to kill bacteria in the colo nic lumen are used by parasites to damage host cells and cause disease . In support of this idea, the amebic lectin and pore-forming peptide are involved in binding and killing, respectively, bacteria and host e pithelial cells. Here amebic phagocytosis of bacteria, RBC, and mucin- coated beads was disrupted by overexpression of E. histolytica p21(rac A-V12), a ras-family protein involved in selection of sites of actin p olymerization, which had been mutated to eliminate its GTPase activity . p21(racAV12) transformants were also defective in capping and cytoki nesis, while pinocytosis of fluorescent dextrans was not affected. Wor tmannin, a fungal inhibitor of phosphoinositide 3-kinase, markedly inh ibited phagocytosis of bacteria, RBC, and mucin-coated beads by wild-t ype amebae. In contrast to p21(racA-V12) overexpression, wortmannin ab olished amebic pinocytosis of dextrans but had no inhibitory effects o n capping. Inhibition of amebic vacuolar acidification by bafilomycin also decreased bacterial and RBC uptake. These results, which demonstr ate similarities between mechanisms of phagocytosis of bacteria and RB C by amebae and macrophages, support the idea of coincidental selectio n of amebic genes encoding proteins that mediate destruction of host c ells.