TNFAIP2 increases macrophage response to CSF-1 through multiple effects on CSF-1 receptor

The cytokine CSF-1 and its tyrosine kinase-encoding receptor CSF1R are important for the development and proliferation/survival of most tissue macrophages. We recently identified TNF-α-induced protein 2 (TNFAIP2) as a unique cellular regulator of CSF1R activation: TNFAIP2 increased the response of macrophages to CSF-1, which might be explained at least in part by the induction of CSF1R clustering by TNFAIP2, since CSF1R clusters accelerate CSF-1-induced CSF1R dimerization/activation. Here, we report an enhanced trafficking of CSF1R to the cell surface as an additional mechanism by which TNFAIP2 increases macrophage response to CSF-1. The mutation in the phosphatidylinositol 4,5-bisphosphate (PIP2)-binding site of TNFAIP2 or CSF1R, or the deprivation of cellular PIP2 reduced the enhanced CSF1R trafficking by TNFAIP2, indicating the importance of PIP2-mediated membrane localization of TNFAIP2 and CSF1R. Mechanistically, a small GTPase RalA and the exocyst complex involved in vesicle trafficking were required for the enhanced CSF1R trafficking by TNFAIP2. Interestingly, the RalA-exocyst complex cascade was also required for the enhanced CSF1R clustering by TNFAIP2. Our results suggest that TNFAIP2 enhances intracellular trafficking and cluster formation of CSF1R through PIP2, RalA, and the exocyst complex, thereby increasing the macrophage response to CSF-1. Our results also suggest that TNFAIP2 regulates other receptor tyrosine kinases.