The complexity of signaling in host–pathogen interactions revealed by the Toxoplasma gondii-dependent modulation of JNK phosphorylation

The inhibition of apoptosis by Toxoplasma gondii is governed by its modulation of several signaling cascades including the NFκappaB and JNK pathways. This is evident in the dysregulation of JNK activation following treatment with UV and TNFα, both apoptogenic stimuli. Infection-mediated interference with the JNK cascade was found to be highly reproducible in HeLa cells. In light of emerging evidence regarding cross talk between the JNK and NFκB cascades, we examined the impact of infection in wild type and RelA/p65−/− mouse embryonic fibroblasts (MEF). Remarkably, parasite infection failed to significantly impact both UV and TNFα-mediated JNK phosphorylation in both cell lines suggesting a cell type specific effect. Furthermore siRNA-mediated knockdown of RelA/p65 failed to impact the parasite mediated effects on stimulus dependent activation of JNK in HeLa cells. Finally, the infection mediated suppression of JNK phosphorylation in HeLa cells did not result in decreased JNK kinase activity. Rather, the reduced levels of phospho-JNK in infected cells correlated with increased phosphatase activity noted by the partial rescue of the phenotype following treatment with okadaic acid. Taken together the results indicate that manipulation of the JNK pathway does not involve NFκB and is furthermore not a central component of the parasite enforced block of apoptosis. It further highlights the complexity of these systems and the danger of extrapolating results both within and across pathogen–host cell systems based on limited studies.