Mitochondrial Translocator Protein (TSPO) Is a Regulator of Fatty Acid Metabolism

Translocator protein (TSPO), first identified as a secondary target of benzodiazepines and termed the ‘peripheral’ benzodiazepine receptor (PBR), is a highly conserved protein in the outer mitochondrial membrane. Despite intensive research for the past 25 years, the exact function of TSPO remains elusive. High TSPO expression seen in steroidogenic mitochondria, and induction of steroid hormone production by TSPO binding drugs, led to a proposed function for TSPO in mitochondrial cholesterol import, the rate‐limiting step in steroidogenesis. However, this prevalent model was recently refuted based on Tspo gene deletion studies in vivo and in vitro that indicated no deficits in steroidogenesis. Seeking a new functional definition for TSPO, we found that high TSPO expression is not exclusive to steroidogenic cells, but also to other cell types active in lipid metabolism such as liver and adipose tissues. We therefore hypothesized that TSPO plays an important role in mitochondrial fatty acid metabolism. Using steroidogenic MA‐10 Leydig cells that express very high levels of TSPO as the model, we generated CRISPR/Cas9‐mediated TSPO knockout MA‐10 clones (MA‐10: Tspo Δ/Δ) and examined changes to core mitochondrial functions after TSPO deletion. MA‐10: Tspo Δ/Δ cells showed a shift in substrate utilization for energy production from glucose to fatty acids with significantly higher mitochondrial fatty acid oxidation (FAO), and increased reactive oxygen species (ROS) production. Overexpression of TSPO using a lentiviral system reversely reduced expression of FAO genes in MA‐10 cells. Extending this in vitro observation to in vivo tissues in global TSPO knockout ( Tspo −/− ) mice, we observed that loss of TSPO in adrenal glands (a major steroidogenic tissue), and liver (the chief metabolic compartment), both significantly upregulated FAO genes compared to the control floxed ( Tspo fl/fl ) tissues. These results demonstrate the first experimental evidence that TSPO affects mitochondrial energy homeostasis through controlling FAO at the level of outer mitochondrial membrane, possibly through regulating activity of carnitine palmitoyltransferase I (CPT1). Therapeutic modulation of TSPO function using available TSPO binding drugs could be beneficial for increasing energy expenditure through FAO in lipid metabolic disorders. Support or Funding Information Startup funds from Cornell University