A Dominant Role of Toll-Like Receptor 4 in the Signaling of Apoptosis in Bacteria-Faced Macrophages

Abstract

Conserved bacterial components potently activate host immune cells through transmembrane Toll-like receptors (TLRs), which trigger a protective immune response but also may signal apoptosis. In this study, we investigated the roles of TLR2 and TLR4 as inducers of apoptosis in Yersinia enterocolitica-infected macrophages. Yersiniae suppress activation of the antiapoptotic NF-κB signaling pathway in host cells by inhibiting inhibitory κB kinase-β. This leads to macrophage apoptosis under infection conditions. Experiments with mouse macrophages deficient for TLR2, TLR4, or both receptors showed that, although yersiniae could activate signaling through both TLR2 and TLR4, loss of TLR4 solely diminished Yersinia-induced apoptosis. This suggests implication of TLR4, but not of TLR2, as a proapoptotic signal transducer in Yersinia-conferred cell death. In the same manner, agonist-specific activation of TLR4 efficiently mediated macrophage apoptosis in the presence of the proteasome inhibitor MG-132, an effect that was less pronounced for activation through TLR2. Furthermore, the extended stimulation of overexpressed TLR4 elicited cellular death in epithelial cells. A dominant-negative mutant of Fas-associated death domain protein could suppress TLR4-mediated cell death, which indicates that TLR4 may signal apoptosis through a Fas-associated death domain protein-dependent pathway. Together, these data show that TLR4 could act as a potent inducer of apoptosis in macrophages that encounter a bacterial pathogen.

IFN Regulatory Factor 3-Dependent Induction of Type I IFNs by Intracellular Bacteria Is Mediated by a TLR- and Nod2-Independent Mechanism

“…The slight reduction in IFN-␤ mRNA expression observed in TRIF-deficient cells is in sharp contrast to the complete lack of IFN-␤ synthesis and Stat1 tyrosine phosphorylation caused by TRIF deficiency after TLR3 stimulation and by TRIF or TRAM deficiency after TLR4 stimulation (35,36). Besides supporting the notion that IFN-␤ production and IFN-␤-mediated death sensitization are TLR-independent events, our analyses of infected macrophages show that the mechanism of death induction by L. monocytogenes differs from that used by Yersinia enterolytica, Bacillus anthracis, or the sipB mutant of Salmonella typhimurium, which in all cases requires signaling through TLR4 (69,70).…”

Toll-like receptors and their role in carcinogenesis and anti-tumor treatment

“…A few samples of primary blast lines obtained from patients with AML also responded to the additive effects of LPS and IFN-α, indicating that sensitivity to TLR4-mediated inducible apoptosis is found in a fraction of AML samples with no correlation with FAB phenotypes [74]. It has been observed that cells stimulated with TLR ligands udergo programmed cell death after NF-κB activation by Fas-associated death domain protein (FADD) and subsequent caspase recruitment [75][76][77][78]. This phenomenon reflects the regulatory capacity of TLR signaling protecting the host from excessive proliferation of immunecompetent cells being activated during inflammatory reactions, and may be implicated in an anti-tumor response enhanced with TLR stimulation.…”

The Proteasome Pathway Destabilizes Yersinia Outer Protein E and Represses Its Antihost Cell Activities

“…To overexpress ubiquitin for the YopE and ubiquitin IP experiments, HEK293 cells were seeded in six-well cell culture plates and transfected with a human cDNA construct encoding an octameric tandem fusion of hemagglutinin-ubiquitin (35) or empty control vector. Transfections were conducted by the calcium-phosphate transfection method, as described (36). The ubiquitin expression vector was provided by M. Treier (European Molecular Biology Laboratory, Heidelberg, Germany).…”