The TORC1-activated Proteins, p70S6K and GRB10, Regulate IL-4 Signaling and M2 Macrophage Polarization by Modulating Phosphorylation of Insulin Receptor Substrate-2

Abstract

Edited by Luke O'NeillLung M2 macrophages are regulators of airway inflammation, associated with poor lung function in allergic asthma. Previously, we demonstrated that IL-4-induced M2 gene expression correlated with tyrosine phosphorylation of the insulin receptor substrate-2 (IRS-2) in macrophages. We hypothesized that negative regulation of IRS-2 activity after IL-4 stimulation is dependent upon serine phosphorylation of IRS-2. Herein, we describe an inverse relationship between tyrosine phosphorylation (Tyr(P)) and serine phosphorylation (Ser(P)) of IRS-2 after IL-4 stimulation. Inhibiting serine phosphatase activity increased Ser(P)-IRS-2 and decreased Tyr(P)-IRS-2 leading to reduced M2 gene expression (CD200R, CCL22, MMP12, and TGM2). We found that inhibition of p70S6K, downstream of TORC1, resulted in diminished Ser(P)-IRS-2 and prolonged Tyr(P)-IRS-2 as well. Inhibition of p70S6K increased expression of CD200R and CCL22 indicating that p70S6K negatively regulates some, but not all, human M2 genes. Knocking down GRB10, another negative regulatory protein downstream of TORC1, enhanced both Tyr(P)-IRS-2 and increased expression of all four M2 genes. Furthermore, GRB10 associated with IRS-2, NEDD4.2 (an E3-ubiquitin ligase), IL-4R␣, and ␥C after IL-4 stimulation. Both IL-4R␣ and ␥C were ubiquitinated after 30 min of IL-4 treatment, suggesting that GRB10 may regulate degradation of the IL-4 receptor-signaling complex through interactions with NEDD4.2. Taken together, these data highlight two novel regulatory proteins that could be therapeutically manipulated to limit IL-4-induced IRS-2 signaling and polarization of M2 macrophages in allergic inflammation.Both genetic and environmental factors influence the onset and exacerbation of allergic asthma, which remains one of the most costly, non-communicable diseases of the human population. Allergic inflammation is characterized by airway hyperreactivity, type 2 cytokines (IL-4, -5, and -13), IgE, and the overproduction of mucus in the lung (1-3). Macrophages present at the site of allergic inflammation can be polarized by Th2-cytokines to become "alternatively activated" or M2 macrophages. In addition, the abundance of M2 macrophages in the lungs of asthmatics correlates with symptom severity (4 -6). As M2 macrophage polarization is dependent upon IL-4 stimulation, this cytokine and its signaling pathways represent excellent targets for asthma therapies.Our previous work has identified IL-4 signaling through the type I IL-4 receptor complex and activation of insulin receptor substrate 2 (IRS-2) 2 as important for the degree of polarization of M2 macrophages (7). IL-4 binds first to the IL-4R␣ chain with high affinity followed by recruitment of the common ␥ chain (IL-2R␥ or ␥C) to create a functional type I receptor. Assembly of this receptor complex leads to recruitment and phosphorylation of the JAK1 and -3 proteins followed by robust, yet transient, tyrosine phosphorylation of IRS-2 and STAT6. Phosphorylated STAT6 homodimerizes and translocates to the nucleus, ...

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