Protein kinase D1 links IRAK1 and TRAF6 in the MyD88-dependent Toll-like receptor signaling pathway. (INM9P.457)

Abstract

Abstract The initial proinflammatory responses to Saccharopolyspora rectivirgula (SR), which causes farmers' lung disease, is dependent on Toll-like receptor 2 (TLR2) and MyD88, and MyD88 is absolutely required for SR-induced protein kinase D1 (PKD1) activation. We found that SR-mediated proinflammatory responses and neutrophil infiltration in the lungs are ablated in mice treated with PKD inhibitor or lacking the PKD1 gene. However, the mechanisms by which PKD1 is activated and functions to regulate TLR signaling are incompletely understood. In the present study, we found that upon TLR ligand stimulation, the receptor signaling complex composed of MyD88, IRAK4, IRAK1, TRAF6, and PKD1 is formed in membrane lipid rafts and that IRAK4 and IRAK1, but not TRAF6, are required for recruitment of PKD1 to the receptor complex and for activation by TLR ligands. In addition, MyD88-dependent translocation of TRAF6 into lipid rafts, ubiquitination of TRAF6, degradation of IRAK1, and activation of TAK1 are dependent on PKD1. Our findings indicate that PKD1 is a downstream effector of IRAK1, but an upstream modulator of TRAF6. Together with the known ability of PKD1 to shuttle between membrane lipid rafts and the cytosol, our results further support the hypothesis that PKD1 functions as a molecular shuttle that brings TRAF6 to IRAK1 in membrane lipid rafts and then aids in translocation of the IRAK1/TRAF6 complex to the cytosol, where TRAF6 interacts with its downstream effectors.