BaeR protein from Salmonella enterica serovar Paratyphi A induces inflammatory response in murine and human cell lines

BaeR is the response regulator of the two-component system, BaeSR, found in Escherichia coli (E. coli) and Salmonella. Several biological functions of BaeR, related to multidrug efflux and bacterial virulence, have been described. Herein, we report a putative function of BaeR during inflammatory response of the host by using BaeR protein of Salmonella enterica Paratyphi A (S. Paratyphi A) origin overexpressed in E. coli, and RAW 264.7 and THP-1 cells as in vitro models. BaeR (3 μg/ml) upregulated iNOS mRNA expression in both cell lines, and induced significant production of NO. Greater than ten-fold (TNF-α), 24-fold (IL-1β) and 156-fold (IL-6) increases in mRNA expression levels were observed in THP-1 cells treated with BaeR, compared to untreated controls. Furthermore, an eight-fold (IL-1β), 12-fold (IL-6) and 41-fold (TNF-α) higher protein concentrations were observed in RAW 264.7 cells stimulated with BaeR, compared to control cells. Immunoblot analysis showed BaeR-induced phosphorylation of the MAPKs (ERK 1/2, JNK and p38 MAPK) in RAW 264.7 cells. Pharmacological inhibition of the three MAPKs using specific inhibitors resulted in significant reduction of BaeR-induced NO production and iNOS mRNA expression by inhibitors of JNK and p38 MAPK. Also, all inhibitors of the MAPKs significantly attenuated BaeR-induced IL-1β, IL-6 and TNF-α at both transcript and protein levels with different degrees of inhibition. Taken together, our data suggest that BaeR is a putative inducer of inflammatory response and the MAPKs are involved in the process.