The Small GTPase Arf6 Is Essential for the Tram/Trif Pathway in TLR4 Signaling

Background: The small GTPase Arf6 affects LPS-induced cytokine secretion. Results: Arf6 regulates transport of the TLR4 adaptor protein Tram and internalization of LPS. Conclusion: Arf6 plays an essential role in regulating the Tram/Trif-dependent TLR4 pathway. Significance: Knowing how TLR4 is modulated is crucial for understanding innate immunity. Recognition of lipopolysaccharides (LPS) by Toll-like receptor 4 (TLR4) at the plasma membrane triggers NF-B activation through recruitment of the adaptor proteins Mal and MyD88. Endocytosis of the activated TLR4 allows recruitment of the adaptors Tram and Trif, leading to activation of the transcription factor IRF3 and interferon production. The small GTPase ADP-ribosylation factor 6 (Arf6) was shown to regulate the plasma membrane association of Mal. Here we demonstrate that inhibition of Arf6 also markedly reduced LPS-induced cytokine production in Mal(-/-) mouse macrophages. In this article, we focus on a novel role for Arf6 in the MyD88-independent TLR4 pathway. MyD88-independent IRF3 activation and IRF3-dependent gene transcription were strictly dependent on Arf6. Arf6 was involved in transport of Tram to the endocytic recycling compartment and internalization of LPS, possibly explaining its requirement for LPS-induced IRF3 activation. Together, these results show a critical role for Arf6 in regulating Tram/Trif-dependent TLR4 signaling.