Entamoeba histolytica stimulates the unstable transcription of c-fos and tumour necrosis factor-alpha mRNA 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 infections are characterized by suppression of cell-mediated immunity. To elucidate the molecular mechanisms whereby amoebae modulate host defences, 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 bone marrow-derived macrophages (BMM). E. histolytica stimulated a rapid and transient expression of c-fos and low levels of TNF-alpha mRNA, whereas the non-pathogenic Entamoeba moshkovshii (E. moshkovskii) did not. Inhibition of the protein kinase C (PKC) pathway with the pharmacological inhibitor H7 and by PKC depletion with phorbol myristate acetate showed that E. histolytica modulates TNF-alpha and c-fos gene expression through a PKC-dependent stimulus-response coupling event. E. histolytica activated and translocated PKC to the membrane fraction in BMM demonstrating a rapid and direct effect on PKC enzyme activity. Unlike lipopolysaccharide (LPS), BMM stimulated with E. histolytica had reduced stability of both c-fos and TNF-alpha mRNA transcripts (> 50%) and failed to secrete TNF-alpha protein. BMM treated with amoebic proteins and stimulated with LPS, or interferon-gamma (IFN-gamma)+LPS, resulted in a 33% and 50% reduction in TNF-alpha mRNA levels, respectively. 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 the observed decreased responsiveness to a stimulatory signal may be a novel mechanism whereby the parasite modulates host defence mechanisms.