Evidence for selection of virulent sub-populations of Entamoeba histolytica by cholesterol

Effect of phosphatidylcholine-cholesterol liposomes on Entamoeba histolytica virulence

Trophozoites of Entamoeba histolytica HM-1:IMSS become less virulent after long-term maintenance in axenic cultures. The factors responsible for the loss of virulence during in vitro cultivation remain unclear. However, it is known that in vitro cultivation of amoeba in culture medium supplemented with cholesterol restores their virulence. In this study, we analyzed the effect of adding phosphatidylcholine-cholesterol (PC-Chol) liposomes to the culture medium and evaluated the effect of this lipid on various biochemical and biological functions of E. histolytica HM-1:IMSS in terms of its virulence. The addition of PC-Chol liposomes to the culture medium maintained the virulence of these parasites against hamster liver at the same level as the original virulent E. histolytica strain, even though these amoebae were maintained without passage through hamster liver for 18 months. The trophozoites also showed increased endocytosis, erythrophagocytosis, and carbohydrate residue expression on the amoebic surface. Protease activities were also modified by the presence of cholesterol in the culture medium. These findings indicate the capacity of cholesterol to preserve amoeba virulence and provide an alternative method for the maintenance of virulent E. histolytica trophozoites without the need for in vivo procedures.

Cloning and partial characterization of Entamoeba histolytica PTPases

Reversible protein tyrosine phosphorylation is an essential signal transduction mechanism that regulates cell growth, differentiation, mobility, metabolism, and survival. Two genes coding for protein tyrosine phophatases, designed EhPTPA and EhPTPB, were cloned from Entamoeba histolytica. EhPTPA and EhPTPB proteins showed amino acid sequence identity of 37%, both EhPTPases showed similarity with Dictyostelium discoideum and vertebrate trasmembranal PTPases. mRNA levels of EhPTPA gene are up-regulated in trophozoites recovered after 96h of liver abscess development in the hamster model. EhPTPA protein expressed as a glutathione S-transferase fusion protein (GST::EhPTPA) showed enzymatic activity with p-nitrophenylphosphate as a substrate and was inhibited by PTPase inhibitors vanadate and molybdate. GST::EhPTPA protein selectively dephosphorylates a 130kDa phosphotyrosine-containing protein in trophozoite cell lysates. EhPTPA gene codifies for a 43kDa native protein. Up-regulation of EhPTPA expression suggests that EhPTPA may play an important role in the adaptive response of trophozoites during amoebic liver abscess development.

Oxido-reductive functions of Entamoeba histolytica in relation to virulence

Entamoeba histolytica can reduce nitro-blue tetrazolium (NBT) in Hank's balanced salt solution to almost the same extent as in Eagle's minimal medium. Further, this reduction was stimulated only to a minor degree by glucose, pyruvate and DL-serine, substrates known to support respiratory activity (O2 uptake) in E. histolytica. However, both NADH and NADPH increased NBT reduction several-fold, the effect being greater with NADPH. A sizeable proportion of this endogenous dye-reducing capability (in Hank's solution) was associated with low-speed sediments obtainable from amoebic homogenates, which also shared the bulk of 125I labelling (when the homogenates were prepared after surface labelling with Na 125I). Conversion of the dye to formazan was strongly inhibited by -SH blocking agents, but was not influenced by rotenone and antimycin A. The activity was also inhibited by H2O2, but stimulated by catalase. Superoxide dismutase only slightly curtailed NBT reduction in intact cells, but inhibited it in homogenates in a concentration-dependent manner to a maximal extent of 33%. Almost the same degree of curtailment of this activity was induced by anaerobic conditions. Both concanavalin A (Con A) and phorbol myristate acetate stimulated the activity in intact cells, though the effect of Con A was nullified by alpha-methyl mannoside. Both NBT-reducing capability and alcohol dehydrogenase activities were higher in the more virulent IP:106 strain, and they increased with time in cultures of NIH:200 in a cholesterol-enriched environment.