Transforming growth factor-beta differentially inhibits MyD88-dependent, but not TRAM- and TRIF-dependent, lipopolysaccharide-induced TLR4 signaling.
Transforming growth factor-beta1 (TGF-beta1) is a multifunctional, potent anti-inflammatory cytokine produced by many cell types that regulates cell proliferation, apoptosis, and immune responses. Toll-like receptors (TLRs) recognize various pathogen-associated molecular patterns and are therefore a pivotal component of the innate immune system. In this study we show that TGF-beta1 blocks the NF-kappaB activation and cytokine release that is stimulated by ligands for TLRs 2, 4, and 5. We further show that TGF-beta1 can specifically interfere with TLR2, -4, or -5 ligand-induced responses involving the adaptor molecule MyD88 (myeloid differentiation factor 88) but not the TRAM/TRIF signaling pathway by decreasing MyD88 protein levels in a dose- and time-dependent manner without altering its mRNA expression. The proteasome inhibitor epoxomicin abolished the MyD88 degradation induced by TGF-beta1. Furthermore, TGF-beta1 resulted in ubiquitination of MyD88 protein, suggesting that TGF-beta1 facilitates ubiquitination and proteasomal degradation of MyD88 and thereby attenuates MyD88-dependent signaling by decreasing cellular levels of MyD88 protein. These findings importantly contribute to our understanding of molecular mechanisms mediating anti-inflammatory modulation of immune responses by TGF-beta1.[1]References
- Transforming growth factor-beta differentially inhibits MyD88-dependent, but not TRAM- and TRIF-dependent, lipopolysaccharide-induced TLR4 signaling. Naiki, Y., Michelsen, K.S., Zhang, W., Chen, S., Doherty, T.M., Arditi, M. J. Biol. Chem. (2005) [Pubmed]
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