To ensure normal development and tissue homeostasis, dead cells must be removed by phagocytes through a process known as efferocytosis. This is a highly energetic process, requiring extensive actin polymerization to engulf large apoptotic cell corpses. A new study shows that efferocytosis is associated with a coordinated programme of expression of membrane transport proteins of the solute carrier (SLC) family, which mediate glucose uptake for enhanced glycolysis and lactate release for establishment of an anti-inflammatory environment.

RNA sequencing of phagocytes undergoing efferocytosis identified numerous changes in multiple transcriptional programmes. As expected, there was decreased expression of pro-inflammatory genes, increased expression of actin rearrangement genes and increased expression of anti-inflammatory genes, but there was also upregulation of glycolysis-associated genes and downregulation of genes required for oxidative phosphorylation (OXPHOS), fatty acid oxidation (FAO) and cholesterol synthesis. A notable change was in 33 genes encoding SLC proteins; 19 SLCs were upregulated (such as those involved in carbohydrate metabolism) and 14 SLCs were downregulated (such as those involved in OXPHOS and FAO). By contrast, macrophages undergoing antibody-mediated phagocytosis did not show expression changes in the same SLCs.

Credit: S. Bradbrook/Springer Nature Limited

One SLC family member that was strongly upregulated during efferocytosis was SLC2A1 (also known as GLUT1), which mediates the transport of glucose into cells from the extracellular space. Treatment with an SLC2A1 inhibitor or knockdown of Slc2a1 expression reduced corpse uptake by phagocytes in vitro and in vivo. In a mouse model of atherosclerosis (Ldlr–/– mice), mice with a myeloid cell-specific Slc2a1 deletion showed a build-up of necrotic material in aortic roots when fed a high-fat diet, suggesting defective corpse clearance.

The role of SLC2A1 as a glucose transporter was shown to be required for efferocytosis, as switching cells to a glucose-free medium or pre-treating phagocytes with the non-metabolizable glucose analogue 2-deoxyglucose decreased efferocytosis. Consistent with the gene expression data, Seahorse analysis of cell metabolism showed increased aerobic glycolysis and concurrent suppression of OXPHOS in efferocytic phagocytes. Importantly, SLC2A1-mediated glucose uptake and induction of glycolysis was shown to contribute to actin polymerization during efferocytosis.

Efferocytosis involves three stages: ‘smell’, when factors released by apoptotic cells are sensed; ‘taste’, when phagocyte–apoptotic cell contact is established; and ‘ingestion’, when corpses are engulfed and processed. Morioka et al. showed that distinct steps of SLC2A1-dependent aerobic glycolysis were regulated by these different stages of efferocytosis. Apoptotic cell supernatant induced upregulation of Sgk1 (which phosphorylates and promotes SLC2A1 plasma membrane expression); the find-me signal ATP also upregulated Sgk1; and the binding (without internalization) of apoptotic targets or phosphatidylserine liposomes to phagocytes induced Slc2a1 expression.

Following corpse ingestion by phagocytes, there was an increase in expression of Slc16a1, which is a transporter for the glycolytic by-products lactate and pyruvate. Knockdown of Slc16a1 expression or treatment with an SLC16A1 inhibitor reduced apoptotic cell uptake, with a concomitant reduction in lactate release and an accumulation of intracellular lactate. This reduction in lactate release was associated with an impaired ability to promote an anti-inflammatory M2-like macrophage phenotype.

knockdown of Slc2a1 expression reduced corpse uptake by phagocytes

So, coordinated regulation of select SLCs during efferocytosis provides the necessary metabolic programme for anti-inflammatory corpse clearance.