ENTAMOEBA-HISTOLYTICA STIMULATES THE UNSTABLE TRANSCRIPTION OF C-FOS AND TUMOR-NECROSIS-FACTOR-ALPHA MESSENGER-RNA BY PROTEIN-KINASE-C SIGNAL-TRANSDUCTION IN MACROPHAGES

Macrophages play an important role in the control of and resistance to Entamoeba histolytica (E. histolytica). However, E. histolytica infec tions are characterized by suppression of cell-mediated immunity. To e lucidate the molecular mechanisms whereby amoebae modulate host defenc es, we investigated whether the parasite elicits the 'immediate early' gene c-fos and cytokine tumour necrosis factor-alpha (TNF)-alpha mRNA and determined the signal transduction pathways involved in naive bon e marrow-derived macrophages (BMM). E. histolytica stimulated a rapid and transient expression of c-fos and low levels of TNF-alpha mRNA, wh ereas the non-pathogenic Entamoeba moshkovshii (E. moshkovskii) did no t. Inhibition of the protein kinase C (PKC) pathway with the pharmacol ogical inhibitor H7 and by PKC depletion with phorbol myristate acetat e showed that E. histolytica modulates TNF-alpha and c-fos gene expres sion through a PKC-dependent stimulus-response coupling event. E. hist olytica activated and translocated PKC to the membrane fraction in BMM demonstrating a rapid and direct effect on PKC enzyme activity. Unlik e lipopolysaccharide (LPS), BMM stimulated with E. histolytica had red uced stability of both c-fos and TNF-alpha: mRNA transcripts (> 50%) a nd failed to secrete TNF-alpha protein. BMM treated with amoebic prote ins and stimulated with LPS, or interferon-gamma (IFN-gamma) + LPS, re sulted in a 33% and 50% reduction in TNF-alpha mRNA levels, respective ly. These data argue that although E. histolytica stimulates c-fos and TNF-alpha gene expression through PKC signal transduction, the rapid degradation of the mRNAs, the lack of secreted TNF-alpha protein and t he observed decreased responsiveness to a stimulatory signal may be a novel mechanism whereby the parasite modulates host defence mechanisms .