Trypanosoma cruzi: Infection of cultured human endothelial cells alters inositol phosphate synthesis

Infection of cultured endothelial cells with Trypanosoma cruzi alters intracellular Ca²⁺ homeostasis. To help understand the biochemical basis for this phenomenon, we determined the influence of infection on inositol phosphate formation in a broken cell preparation. Inositol phosphates participate in the regulation of cytosolic Ca²⁺. In uninfected endothelial cells, bradykinin guanosine 5′-O-thiophosphate (GTPτS), and calcium all stimulated inositol phosphate (IP1), inositol bisphosphate (IP2), and inositol trisphosphate (IP3) formation within 5 sec of incubation. At longer periods of incubation with GTPτS and bradykinin, formation of IP1 was linear for 30 sec, whereas the rate of IP2 and IP3 generation was maximal at 20 and 5 sec, respectively. Second, infection markedly changed these aspects of inositol phosphate generation. First, unstimulated (basal) levels of IP1 and IP3 were markedly increased over those levels in membranes of uninfected cells. Infection decreased the rate of formation for the three inositol phosphates in response to GTPτS and bradykinin. Finally, infection diminished the magnitude of inositol phosphate synthesis in response to Ca²⁺ for IP1, IP2, and IP3, respectively. Studies on G proteins using cholera and pertussis toxin were carried out to determine if the infection-associated changes in inositol phosphate generation could be attributed to functional changes in these regulatory proteins known to participate in the activation of phospholipase C. Infection markedly decreased the magnitude of cholera and pertussis toxin-dependent ADP ribosylation, as compared to control uninfected cells. Incubation of uninfected endothelial cells with cholera and pertussis toxin also decreased the magnitude of cholera and pertussis toxin ADP ribosylation. Despite the similar effects of infection and toxin treatment on subsequent toxin-catalyzed ADP ribosylation, toxin treatment did not influence inositol phosphate generation. Collectively, these results demonstrate an influence of infection on receptor-dependent and -independent synthesis of inositol phosphates, possibly by an action on phospholipase C. The results help to explain the apparent infection-associated increase in basal Ca²⁺ previously observed and suggest that interference with signal transduction may be a consequence of the presence of the parasite.