SIGNAL-TRANSDUCTION IN MACROPHAGES BY GLYCOSYLPHOSPHATIDYLINOSITOLS OF PLASMODIUM, TRYPANOSOMA, AND LEISHMANIA - ACTIVATION OF PROTEIN-TYROSINE KINASES AND PROTEIN-KINASE-C BY INOSITOLGLYCAN AND DIACYLGLYCEROLMOIETIES

The perturbation of various glycosylphosphatidylinositol (GPI)-anchore d surface proteins imparts profound regulatory signals to macrophages, lymphocytes and other cell types. The specific contribution of the GP I moieties to these events however is unclear. This study demonstrates that purified GPIs of Plasmodium falciparum, Trypanosoma brucei, and Leishmania mexicana origin are sufficient to initiate signal transduct ion when added alone to host cells as chemically defined agonists. GPI s (10 nM-1 mu M) induce rapid activation of the protein tyrosine kinas e (PTK) p59(hck) in macrophages. The minimal structural requirement fo r PTK activation is the evolutionarily conserved core glycan sequence Man alpha 1-2Man alpha 1 -6Man alpha 1-4GlcN1-6myo-inositol. GPI-assoc iated diacylglycerols independently activate the calcium-independent e psilon isoform of protein kinase C. Both signals collaborate in regula ting the downstream NF-kappa B/rel-dependent gene expression of interl eukin 1 alpha, tumor necrosis factor (TNF) alpha, and inducible NO syn thase. The alkacylglycerol-containing iM4 GIPL of L. mexicana, however , is unable to activate protein kinase C and inhibits TNF expression i n response to other agonists, establishing signaling specificity among structurally distinct GPIs. GPI alone appears sufficient to mimic the activities of malaria parasite extracts in the signaling pathway lead ing to TNF expression. A mAb to GPI blocks TNF induction by parasite e xtracts indicating that GPI is a necessary agent in this response. As protozoal GPIs are closely related to their mammalian counterparts, th e data indicate that GPIs do indeed constitute a novel outside-in sign aling system, acting as both agonists and second messenger substrates, and imparting at least two separate signals through the structurally distinct glycan and fatty acid domains. These activities may underlie aspects of pathology and immune regulation in protozoal infections.