The rLrp of Mycobacterium tuberculosis inhibits proinflammatory cytokine production and downregulates APC function in mouse macrophages via a TLR2-mediated PI3K/Akt pathway activation-dependent mechanism

Abstract

We demonstrate that Mycobacterium tuberculosis recombinant leucine-responsive regulatory protein (rLrp) inhibits lipopolysaccharide (LPS)-induced tumor necrosis factor alpha (TNF-α), interleukin-6, and interleukin-12 production and blocks the nuclear translocation of subunits of the nuclear-receptor transcription factor NF-κB (Nuclear factor-kappa B). Moreover, rLrp attenuated LPS-induced DNA binding and NF-κB transcriptional activity, which was accompanied by the degradation of inhibitory IκBα and a consequent decrease in the nuclear translocation of the NF-κB p65 subunit. RLrp interfered with the LPS-induced clustering of TNF receptor-associated factor 6 and with interleukin-1 receptor-associated kinase 1 binding to TAK1. Furthermore, rLrp did not attenuate proinflammatory cytokines or the expression of CD86 and major histocompatibility complex class-II induced by interferon-gamma in the macrophages of Toll-like receptor 2 deletion (TLR2) mice and in protein kinase b (Akt)-depleted mouse cells, indicating that the inhibitory effects of rLrp were dependent on TLR2-mediated activation of the phosphatidylinositol 3-OH kinase (PI3K)/Akt pathway. RLrp could also activate the PI3K/Akt pathway by stimulating the rapid phosphorylation of PI3K, Akt, and glycogen synthase kinase 3 beta in macrophages. In addition, 19 amino acid residues in the N-terminus of rLrp were determined to be important and required for the inhibitory effects mediated by TLR2. The inhibitory function of these 19 amino acids of rLrp raises the possibility that mimetic inhibitory peptides could be used to restrict innate immune responses in situations in which prolonged TLR signaling has deleterious effects. Our study offers new insight into the inhibitory mechanisms by which the TLR2-mediated PI3K/Akt pathway ensures the transient expression of potent inflammatory mediators.

Innate immunity in tuberculosis: host defense vs pathogen evasion

“…83 The Mtb recombinant leucine-responsive regulatory protein (rLrp) inhibits pro-inflammatory cytokine production and downregulates macrophage antigen presentation via TLR2mediated activation of the PI3K/AKT pathway. 84 Previous research from our lab revealed that Mtb-secreted proteins such as PtpA and Mce3E modulate TLR signaling by targeting the downstream molecules involved in the NF-κB and MAPK pathways. 85,86 Interestingly, a recent study demonstrated that G protein-coupled receptor 160 (GPR160) regulates mycobacteria entry into macrophages by activating MAPK/ERK signaling, which suggests a crosstalk between the GPCR and TLR signaling pathways during mycobacterial infection.…”

GSK3: A Kinase Balancing Promotion and Resolution of Inflammation

“…For instance, Akt-mediated GSK3 inactivation following stimulation of TLR2, 4, 5, or 9 with appropriate agonists (lipoteichoic acid, LPS or synthetic E. coli lipid A, flagellin, and human CpG, respectively) significantly suppressed pro-inflammatory cytokine secretion and induced (TLR2-dependent) IL-10 production in human monocytes in a CREB-and CREB-binding protein (CBP)-dependent manner [90]. In LPS-treated murine macrophage-like RAW264.7 cells and primary murine macrophages, preceding TLR2 stimulation by recombinant leucine-responsive regulatory protein preincubation results in PI3K/Akt activation, GSK3β-Ser9 phosphorylation, reduced NF-κB activity/nuclear translocation, and suppression of pro-inflammatory IL-6 and -12 expression [160]. In LPS-challenged human monocytes, it was shown that Akt-dependent GSK3 inactivation may be supported by additional mTORC2-dependent activation of Akt as well as (mTORC1-dependent) activation of GSK3β-Ser9-targeting S6K [161].…”

Mycobacterium tuberculosis-triggered Hippo pathway orchestrates CXCL1/2 expression to modulate host immune responses

“…The modulation of the intricate host signaling networks majorly governs the net observable responses. In this regard, it has been reported that alongside immunologically active pathways, many early development-specific signaling cascades are reprogrammed to furnish infection-defined defensive measures1213141516171819. One such pathway which is garnering attention is Hippo signaling pathway.…”