TSPO deficiency reveals a novel role for porphyrins in regulating white adipose tissue lipid metabolism

Abstract

The translocator protein (TSPO) is a mitochondrial outer membrane protein with high expression in cells specialized for lipid metabolism, including white adipose tissue (WAT), where its physiological function remains poorly defined. Our previous work demonstrated a role for TSPO in mitochondrial fatty acid oxidation, prompting us to investigate its contribution to adipocyte lipid homeostasis under metabolic stress. We employed a combination of in vivo and in vitro approaches using global Tspo knockout (Tspo-/-) mice, primary adipocyte cultures, and pharmacological TSPO-binding drugs to examine TSPO function. We show that Tspo-/- mice exhibited increased WAT mass and adipocyte hypertrophy following high-fat diet feeding, with no changes in caloric intake. These changes were accompanied by suppression of key lipolytic genes, reduced circulating NEFA and glycerol levels, and altered expression of fatty acid oxidation genes. Lipidomic profiling showed no genotype-dependent changes, indicating impaired mobilization rather than altered lipid composition. In vitro, TSPO deficiency enhanced lipid accumulation during adipocyte diberentiation and impaired expression of lipolytic genes. TSPO-binding drugs phenocopied this response in TSPO-expressing cells. Mechanistically, we identify protoporphyrin IX (PPIX), an endogenous TSPO ligand, as a suppressor of adipogenic and lipolytic programs. PPIX levels increase during adipogenesis, and its accrual inhibits both lipid accumulation and lipolytic response, whereas hemin does not elicit these ebects. Our findings identify TSPO as a regulator of adipocyte lipid metabolism through a previously unrecognized TSPO-PPIX axis that influences lipid mobilization in adipocytes. This mechanism provides insight into TSPOs role in metabolic adaptation and highlights its potential as a therapeutic target in obesity-associated adipose dysfunction.