GLYCOSYLPHOSPHATIDYLINOSITOL TOXIN OF PLASMODIUM INDUCES NITRIC-OXIDESYNTHASE EXPRESSION IN MACROPHAGES AND VASCULAR ENDOTHELIAL-CELLS BY A PROTEIN-TYROSINE KINASE-DEPENDENT AND PROTEIN-KINASE C-DEPENDENT SIGNALING PATHWAY

In this study, we demonstrate that glycosylphosphatidylinositol (CPI) is a major toxin of Plasmodium falciparum origin responsible for nitri c oxide (NO) production in host cells, Purified malarial GPI is suffic ient to induce NO release in a time- and dose-dependent manner in macr ophages and vascular endothelial cells, and regulates inducible NO syn thase expression in macrophages, GPI-induced NO production was blocked by the NO synthase-specific inhibitor L-N-monomethylarginine. GPI als o synergizes with IFN-gamma in regulating NO production, The structura lly related molecules dipalmitoylphosphatidylinositol and iM4 glycoino sitolphospholipid from Leishmania mexicana had no such activity, and t he latter antagonized IFN-gamma-induced NO output, CPI activates macro phages by initiating an early onset tyrosine kinase-mediated signaling process, similar to that induced by total parasite extracts, The tyro sine kinase antagonists tyrphostin and genistein inhibited the release of NO by parasite extracts and by CPI, alone or in combination with I FN-gamma, demonstrating the involvement of one or more tyrosine kinase s in the signaling cascade, GPI-induced NO release was also blocked by the protein kinase C inhibitor calphostin C, demonstrating a role for protein kinase C in GPI-mediated cell signaling, and by pyrrolidine d ithiocarbamate, indicating the involvement of the NF-kappa B/c-rel fam ily of transcription factors in cell activation, A neutralizing mAb to malarial GPI inhibited NO production induced by CPI and total malaria l parasite extracts in human vascular endothelial cells and murine mac rophages, indicating that CPI is a necessary agent of parasite origin in parasite-induced NO output, Thus, in contrast to dipalmitoylphospha tidylinositol and glycoinositolphospholipids of Leishmania, malarial C PI initiates a protein tyrosine kinase- and protein kinase C-mediated signal transduction pathway, regulating inducible NO synthase expressi on with the participation of NF-kappa B/c-rel, which leads to macropha ge and vascular endothelial cell activation and downstream production of NO. These events may play a role in the etiology of severe malaria.