Positive role of mitogen-activated protein kinase phosphatase-1 in regulating the anti-mycobacterial immune responses

Abstract

Tuberculosis remains one of the major epidemic threats to human. It is caused by Mycobacterium tuberculosis (Mtb). This pathogen is recognized by monocytes/macrophages resulting in the release of specific cytokines for eliciting immune responses to control the infection. Of these, tumour necrosis factor-alpha (TNF-α) plays a key role in establishing granulomas to restrict Mtb dissemination. We previously showed that using Bacillus Calmette Guerin (BCG) as a model, mycobacteria induce TNF-α through the activation of double-stranded RNA-dependent protein kinase (PKR) and mitogen-activated protein kinases (MAPK). To control overactivation of MAPK following Gram-negative bacteria infection, MAPK phosphatase-1 (MKP-1) has been postulated to exert negative regulation on MAPK activation. Therefore, we hypothesize MKP-1 modulates mycobacterium-stimulated immune responses. To examine whether BCG can induce MKP-1, primary human blood monocytes (PBMo) were infected with BCG and MKP-1 expression was assayed by quantitative RT-PCR and Western blot. Our results showed that BCG induced the expression of MKP-1 in a time-dependent manner. This MKP-1 expression was found to be positively controlled by extracellular signal-regulated kinase-1 and -2 (ERK1/2) and p38 MAPK. To investigate the role of MKP-1 in regulating the BCG-induced responses, the cellular level of MKP-1 protein was knocked down by MKP-1-specific siRNA. Consequently, we demonstrated the level of TNF-α expression induced by BCG in PBMo was reduced, but its induction by lipopolysaccharide was further augmented. Moreover, the phosphorylation levels of BCG-activated ERK1/2 and p38 MAPK were decreased by the MKP-1 knockdown. Taken together, our results indicated that apart from its well-known negative role in regulating LPS-induced responses, MKP-1 acts as a positive regulator in modulating the BCG-induced activation of ERK1/2 and p38 MAPK, and consequently the induced expression of TNF-α. These results provide new insights into the regulation of proinflammatory cytokines in the pathogenesis of mycobacterial infections.