Phorbol esters specifically enhance the cytolytic activity of Entamoeba histolytica

Abstract

Entamoeba histolytica causes invasive amebiasis by lysis of host tissue and inflammatory cells. The in vitro cytolysis of target Chinese hamster ovary (CHO) cells by axenic E. histolytica trophozoites (strain HM1:IMSS) is a calcium- and phospholipase A-dependent event initiated by the binding to the target cell of the galactose-inhibitable surface lectin of the parasite. We utilized phorbol esters as a probe to determine whether an amebic protein kinase C has a role in the cytolytic event. The addition of phorbol 12-myristate 13-acetate (PMA) at 10(-6) or 10(-7) M resulted in a greater than twofold enhancement of amebic killing of target CHO cells over 30 min (P less than 0.01). Prior exposure of only the amebae, but not the CHO cells, to PMA produced a similar effect (P less than 0.01). The inactive analog 4-alpha-phorbol had no effect on amebic killing of CHO cells. The PMA-mediated enhancement of amebic cytolysis persisted for up to 60 min after a 5-min exposure; however, after a 30-min exposure to PMA (10(-6) M) there was no augmentation of amebic killing of CHO cells. PMA (10(-6) M) did not promote adherence of parasites to CHO cells but did enhance amebic cytolysis of previously adherent target cells (P less than 0.01). Sphingosine, a specific inhibitor of protein kinase C, abolished both the PMA-stimulated and the basal cytolytic activity of E. histolytica. PMA enhanced CHO cell cytolysis by the less virulent wild-type strain H-303:NIH (P less than or equal to 0.02) but did not augment the activity of the less virulent strain H-200:NIH or two avirulent clones of HM1 (L6 and C919). In summary, these experiments with the phorbol esters and sphingosine as probes to modulate the activity of protein kinase C indicate participation of a parasite protein kinase C in the cytolytic activity of virulent, axenic E. histolytica trophozoites and thus in the pathogenesis of amebiasis.