Complement protein C1q and adiponectin stimulate Mer tyrosine kinase-dependent engulfment of apoptotic cells through a shared pathway.

The failure to clear apoptotic cells is linked to defects in development and autoimmunity. Complement component C1q is required for efficient engulfment of apoptotic cells (efferocytosis), and C1q deficiency leads to the development of lupus. We recently identified a novel molecular mechanism for C1q-dependent efferocytosis in murine macrophages. C1q elicited the expression of Mer tyrosine kinase (Mer), a receptor that regulates efficient efferocytosis and prevention of autoimmunity. To characterize the C1q-dependent signal transduction mechanism, pathway analysis of the transcriptome from C1q-activated macrophages was performed, and it identified the adiponectin signaling pathway as significantly upregulated with C1q. Adiponectin is structurally homologous to C1q and regulates cellular metabolism via downstream activation of 5'adenosine monophosphate-activated protein kinase (AMPK). Macrophage stimulation with C1q resulted in the activation of AMPK, and silencing of AMPK expression using siRNA-inhibited C1q-dependent efferocytosis. Adiponectin signaling also stimulates activation of nuclear receptors, and inhibition of the nuclear receptor retinoid X receptor abrogated C1q-dependent Mer expression and efferocytosis. Furthermore, adiponectin elicited Mer expression and Mer-dependent efferocytosis in macrophages similar to cells stimulated with C1q. Collectively, our results suggest that C1q and adiponectin share a common signal transduction cascade to promote clearance of apoptotic cells, and identify a novel molecular pathway required for efficient efferocytosis.