The reported increased long chain fatty acid oxidation LC

The reported increased long-chain fatty Calcium Colorimetric Assay Kit oxidation (LC-FAO) in those cells can certainly aid OXPHOS, but studies interrogating the functional importance of LC-FAO to support M(IL-4) phenotypes have yielded conflicting results (Van den Bossche et al., 2017). The need for increased LC-FAO in M(IL-4) macrophages was first suggested by Vats et al. (2006) in a study using 50 μM etomoxir, an inhibitor of carnitine palmitoyl transferase 1 (CPT1). This mitochondrial membrane enzyme, together with CPT2, facilitates the transport of LC-FAs into the mitochondrial matrix for subsequent oxidation (Figure 1). Follow-up studies with etomoxir concentrations ranging from 10 to 100 μM observed no effect on IL-4-induced activation of mouse and human macrophages, and thus the field awaited genetic models to resolve the debate surrounding whether LC-FAO is obligatory for alternative macrophage activation or merely associated with it (Van den Bossche et al., 2017). Nomura et al. (2016) took the first step and applied CPT2-deficient macrophages to demonstrate that M(IL-4) activation does not require LC-FAO. While this suggested off-target effects of etomoxir, these results could not rule out that CPT1 may have functions independent of LC-FAO. Therefore, experiments with CPT1-deficient macrophages were still needed to unequivocally clarify its role in M(IL-4) cells. Employing distinct genetic and pharmacological approaches, Divakaruni et al. now demonstrate that macrophages still acquire their IL-4-elicited phenotype in the absence of CPT1a (considered the main isoform in leukocytes) or CPT2, and highlight that the increased LC-FAO in M(IL-4) cells is less crucial than previously considered. They also observe that a high concentration of etomoxir (200 μM) has distinct off-target effects, including suppression of OXPHOS through inhibition of electron transport chain complex I and by blocking adenine nucleotide translocase (ANT). In contrast to previous studies that applied the ATP synthase inhibitor oligomycin to block OXPHOS (Tan et al., 2015, Van den Bossche et al., 2016, Vats et al., 2006), the current paper surprisingly observed that OXPHOS is dispensable for M(IL-4) activation. Instead, depletion of the intracellular CoA pool in the presence of high etomoxir concentrations was identified as the reason for suppressed M(IL-4) activation, revealing CoA homeostasis as a new regulator of macrophage responses. This is an area ripe for further investigations, and the main open question arising from those findings is why CoA is critical for M(IL-4) cells. The authors speculate that sequestration of CoA in etomoxiryl-CoA (the active form of etomoxir) may limit the availability of acetyl-CoA, which can alter the epigenetic landscape of the cell. Interestingly, succinyl-CoA and malonyl-CoA appear to be affected and, given their crucial role in regulating macrophage responses, future research should explore the mechanisms by which CoA may regulate macrophage fates. Another key question to be addressed is whether the effects of etomoxir on CoA homeostasis are specific for M(IL-4) cells, or whether other macrophage responses or T cells are similarly affected. Could disrupted CoA levels in inflammatory macrophages be another explanation for why they cannot be reprogrammed into M(IL-4) macrophages, as reported earlier (Van den Bossche et al., 2016)?