Abstract
Long-chain fatty acid (LCFA) oxidation has been shown to play an important role in interleukin-4 (IL-4)-mediated macrophage polarization (M(IL-4)). However, many of these conclusions are based on the inhibition of carnitine palmitoyltransferase-1 with high concentrations of etomoxir that far exceed what is required to inhibit enzyme activity (EC90 < 3 μM). We employ genetic and pharmacologic models to demonstrate that LCFA oxidation is largely dispensable for IL-4-driven polarization. Unexpectedly, high concentrations of etomoxir retained the ability to disrupt M(IL-4) polarization in the absence of Cpt1a or Cpt2 expression. Although excess etomoxir inhibits the adenine nucleotide translocase, oxidative phosphorylation is surprisingly dispensable for M(IL-4). Instead, the block in polarization was traced to depletion of intracellular free coenzyme A (CoA), likely resulting from conversion of the pro-drug etomoxir into active etomoxiryl CoA. These studies help explain the effect(s) of excess etomoxir on immune cells and reveal an unappreciated role for CoA metabolism in macrophage polarization.
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•Alternative macrophage activation by IL-4 does not require fatty acid oxidation•Etomoxir concentrations ≥10 μM inhibit the adenine nucleotide translocase•OXPHOS is dispensable for many markers of IL-4-activated macrophages (M(IL-4))•200 μM etomoxir blocks M(IL-4) by perturbing intracellular coenzyme A homeostasis
The CPT-1 inhibitor etomoxir has been used to suggest long-chain fatty acid (LCFA) oxidation is necessary for alternative macrophage activation. Divakaruni and colleagues now show that LCFA oxidation is dispensable. They demonstrate multiple off-target effects of etomoxir and show that depletion of coenzyme A by etomoxir blocks M(IL-4) differentiation.