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In response to environmental stimuli, macrophages change their nutrient consumption and undergo an early metabolic adaptation that progressively shapes their polarization state. During the transient, early phase of pro-inflammatory macrophage activation, an increase in tricarboxylic acid (TCA) cycle activity has been reported, but the relative contribution of branched-chain amino acid (BCAA) leucine remains to be determined. Here, we show that glucose but not glutamine is a major contributor of the increase in TCA cycle metabolites during early macrophage activation in humans. We then show that, although uptake of BCAAs is not altered, their transamination by BCAT1 is increased following 8 h lipopolysaccharide (LPS) stimulation. Of note, leucine is not metabolized to integrate into the TCA cycle in basal or stimulated human macrophages. Surprisingly, the pharmacological inhibition of BCAT1 reduced glucose-derived itaconate, α-ketoglutarate and 2-hydroxyglutarate levels without affecting succinate and citrate levels, indicating a partial inhibition of the TCA cycle. This indirect effect is associated with NRF2 (also known as NFE2L2) activation and anti-oxidant responses. These results suggest a moonlighting role of BCAT1 through redox-mediated control of mitochondrial function during early macrophage activation., Competing Interests: Competing interestsA.E.P. is an employee of Ergon Pharmaceuticals Ltd. The remaining authors declare no competing financial interests., (© 2020. Published by The Company of Biologists Ltd.)

In response to environmental stimuli, macrophages change their nutrient consumption and undergo an early metabolic adaptation that progressively shapes their polarization state. During the transient, early phase of pro-inflammatory macrophage activation, an increase in tricarboxylic acid (TCA) cycle activity has been reported, but the relative contribution of branched-chain amino acid (BCAA) leucine remains to be determined. Here, we show that glucose but not glutamine is a major contributor of the increase in TCA cycle metabolites during early macrophage activation in humans.We then show that, although uptake of BCAAs is not altered, their transamination by BCAT1 is increased following 8 h lipopolysaccharide (LPS) stimulation. Of note, leucine is not metabolized to integrate into the TCA cycle in basal or stimulated human macrophages. Surprisingly, the pharmacological inhibition of BCAT1 reduced glucose-derived itaconate, α-ketoglutarate and 2-hydroxyglutarate levels without affecting succinate and citrate levels, indicating a partial inhibition of the TCA cycle. This indirect effect is associated with NRF2 (also known as NFE2L2) activation and antioxidant responses. These results suggest a moonlighting role of BCAT1 through redox-mediated control of mitochondrial function during early macrophage activation. [ABSTRACT FROM AUTHOR]

Oncogenic KRas activates mitochondrial fission through Erk-mediated phosphorylation of the mitochondrial fission GTPase Drp1. Drp1 deletion inhibits tumorigenesis of KRas-driven pancreatic cancer, but the role of mitochondrial dynamics in other Ras-driven malignancies is poorly defined. Here we show that in vitro and in vivo growth of KRas-driven lung adenocarcinoma is unaffected by deletion of Drp1 but is inhibited by deletion of Opa1, the GTPase that regulates inner membrane fusion and proper cristae morphology. Mechanistically, Opa1 knockout disrupts cristae morphology and inhibits electron transport chain (ETC) assembly and activity, which inhibits tumor cell proliferation through loss of NAD+ regeneration. Simultaneous inactivation of Drp1 and Opa1 restores cristae morphology, ETC activity, and cell proliferation indicating that mitochondrial fission activity drives ETC dysfunction induced by Opa1 knockout. Our results support a model in which mitochondrial fission events disrupt cristae structure, and tumor cells with hyperactive fission activity require Opa1 activity to maintain ETC function. [Display omitted] • KRas-mutant lung adenocarcinoma requires Opa1, but not Drp1, in vitro and in vivo • Acute Opa1 deletion inhibits LUAD by disrupting ETC function, not fusion or apoptosis • Mitochondrial fission unopposed by Opa1 inhibits ETC assembly and NAD+ regeneration Sessions et al. find that KRas-mutant lung adenocarcinoma requires the mitochondrial fusion GTPase Opa1 to maintain cristae structure and ETC function. Opa1 deletion leads to mitochondrial fission-mediated ETC disassembly and inhibition of NAD+ regeneration required for oxidative metabolism. Drp1 deletion does not inhibit LUAD development, but it rescues effects of Opa1 deletion. [ABSTRACT FROM AUTHOR]