BCAT1 affects mitochondrial metabolism independently of leucine transamination in activated human macrophages

Abstract

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 remain 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 BCAA uptake is not altered, their transamination by BCAT1 is increased following 8h lipopolysaccharide (LPS) stimulation. Of note, leucine is not metabolized to integrate the TCA cycle in neither basal nor 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 TCA cycle. This indirect effect is associated with NRF2 activation and anti-oxidant responses. These results suggest a moonlighting role of BCAT1 through redox-mediated control of mitochondrial function during early macrophage activation.

Targeting Amino Acid Metabolic Vulnerabilities in Myeloid Malignancies

“…ERG-240 is a leucine analog that has been identified as a novel potent BCAT1 inhibitor ( 30 ). ERG-240 treatment reduces oxygen consumption, glycolysis, and itaconate levels in myeloid cells ( 30 , 31 ). 2-hydroygluterate (R-2HG) is a naturally occurring enantiomer with structural similarity to α KG and thus inhibits the unidirectional α KG-dependent BCAT1 activity ( 32 ).…”

Metabolic Regulation of Antiviral Immune Responses in Macrophages

“…The loss of BCAT1 resulted in reduced itaconate and IRG1 levels due to decreased glycolytic activity (120,121). Interestingly, this occurs independent to its native function by driving NRF2mediated antioxidant response and glucose-derived TCA metabolite production (120). This further demonstrates the complicated networks of pathways integral to driving effector function.…”

“…Intestinal macrophages with a selective knockout of BCAA transporter CD98hc were found to be more prone to apoptosis (119). Further, the cytosolic BCAA aminotransferase (BCAT1) is upregulated during antibacterial responses to provide additional fuel to mount an immune response (120,121). Interestingly, this is independent of its role in BCAA catabolism as leucine levels were unchanged, suggesting a possible moonlighting role for BCAT1 (120).…”

“…Further, the cytosolic BCAA aminotransferase (BCAT1) is upregulated during antibacterial responses to provide additional fuel to mount an immune response (120,121). Interestingly, this is independent of its role in BCAA catabolism as leucine levels were unchanged, suggesting a possible moonlighting role for BCAT1 (120). This assortment of energetic pathways to support cellular activation and function provides the cell with some level of flexibility in tissue microenvironments with varied levels of nutrients available and/or under conditions of stress such as infections where nutrients are rapidly consumed.…”

“…leucine, valine, isoleucine) are also used to maintain the inflammatory state, with increased uptake and expression of BCAT1 (121,188). The loss of BCAT1 resulted in reduced itaconate and IRG1 levels due to decreased glycolytic activity (120,121). Interestingly, this occurs independent to its native function by driving NRF2mediated antioxidant response and glucose-derived TCA metabolite production (120).…”