Complement component C1q regulates AMPK and ERK1/2 activation in mouse and human macrophages

Abstract

Abstract C1q is the recognition component of the classical complement pathway and a soluble pattern recognition receptor that regulates myeloid cell activation independent of the complement cascades. C1q enhances phagocytosis of antibody opsonized targets and apoptotic cells, and regulates cytokine production in macrophages. C1q deficiency results in autoimmunity, consistent with its function in regulating macrophage activation. The signaling mechanisms leading to C1q-dependent macrophage activation are not fully understood. To investigate the mechanism by which C1q regulates macrophage activation, we utilized a phospho-kinase array to identify targets within the signaling pathway(s). The array identified several targets that were regulated by C1q including AMP-dependent protein kinase α-1 (AMPK) and ERK1/2. Short term stimulation (30 min) with C1q and subsequent addition of immune complexes resulted in enhanced ERK phosphorylation in C1q-treated THP-1 cells and mouse bone marrow derived macrophages (BMDM). These same conditions stimulate enhanced phagocytosis of targets suboptimally opsonized with IgG. Long term stimulation (18 hr) of BMDM with C1q under conditions that lead to enhanced engulfment of apoptotic cells and dampening of proinflammatory TNFα production led to enhanced AMPK activation and inhibition of ERK, consistent with C1q enhancing efferocytosis and dampening TNF production. Silencing AMPK resulted in inhibition of C1q-dependent efferocytosis, however AICAR, the AMPK agonist, was not sufficient for restoring C1q-dependent signaling. These data identify new targets of C1q-dependent signaling which may be important in developing strategies to prevent autoimmune and/or inflammatory diseases.