DARPP-32 is a novel regulator of M2-macrophage polarization: Sustains STAT6 phosphorylation by directly binding and inhibiting tyrosine phosphatase SHP-1.

Identifying novel factors driving M2-macrophage polarization may help understand immune modulation in cancers. We identified DARPP-32, a protein phosphatase inhibitor, to be significantly upregulated in FBXW7 knock out THP-1 spontaneously polarized to M2 macrophages. DARPP-32 levels remarkably increased in macrophages treated with different M2-inducers while it copiously decreased upon treatment with M1 inducer Lipopolysaccharide/IFN-γ. Intriguingly, mere DARPP-32 over-expression robustly promoted M2-macrophage polarization in different macrophage cell lines and murine primary bone marrow derived macrophages, even without any external stimulation. Conversely, DARPP-32 depletion hampered the IL-4 induced M2-polarization. Strikingly, DARPP-32 overexpression also dampened the classical M1 activation profile of macrophages in response to LPS, skewing their function towards more anti-inflammatory M2-like phenotype. Mechanistically, Thr34-phosphorylated DARPP-32 positively regulates JAK1-STAT6 signalling by directly binding and inhibiting protein tyrosine phosphatase SHP-1, a key regulator of immune cell signalling and macrophage activation. In line with increased DARPP-32 levels in M2-polarized macrophages, which are often tumor-associated macrophages, we also observed markedly increased DARPP-32 levels in tumor-infiltrating macrophages within primary TNBC tumors. We also observed an increase in DARPP-32 levels in macrophages co-cultured with tumor cells. This is first comprehensive study that demonstrate a novel function of DARPP-32 as a driver/regulator of macrophage polarization towards anti-inflammatory M2-phenotype.