Mitochondrial Translocator Protein (TSPO) Function Is Not Essential for Heme Biosynthesis

Area of Science:

• Biochemistry
• Cell Biology
• Mitochondrial Function

Background:

• The function of mammalian translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, remains debated.
• While TSPO has been linked to steroidogenesis and heme biosynthesis, genetic studies have challenged these roles.
• Protoporphyrin IX (PPIX) is considered an endogenous ligand for TSPO, with bacterial TSPO regulating tetrapyrrole metabolism.

Purpose of the Study:

• To investigate the physiological role of TSPO in mammals, specifically its involvement in erythropoiesis, heme levels, PPIX biosynthesis, and mitochondrial bioenergetics.
• To determine if TSPO expression in cancer cells correlates with their ability to convert 5-aminolevulinic acid (ALA) to PPIX and susceptibility to photodynamic therapy.
• To examine the impact of TSPO deficiency on cellular metabolism and porphyrin-related phototoxic cell death.

Main Methods:

• Utilized TSPO knock-out (Tspo(-/-)) mice, primary cells, and various tumor cell lines.
• Assessed erythropoiesis, heme levels, and PPIX biosynthesis in Tspo(-/-) models.
• Measured oxygen consumption rate and mitochondrial membrane potential (ΔΨm) in TSPO-deficient fibroblasts.
• Evaluated phototoxic cell death in response to ALA-induced PPIX accumulation.

Main Results:

• TSPO deficiency did not adversely affect erythropoiesis, heme biosynthesis, or the conversion of ALA to PPIX.
• TSPO expression levels in cancer cells did not correlate with ALA to PPIX conversion efficiency.
• TSPO deficiency in fibroblasts led to decreased oxygen consumption rate and mitochondrial membrane potential, indicating a metabolic shift.
• Porphyrin-mediated phototoxic cell death was not negatively impacted by TSPO deficiency.

Conclusions:

• Mammalian TSPO does not appear to have a critical physiological function in PPIX and heme biosynthesis.
• The presumed roles of TSPO in steroidogenesis and mitochondrial permeability transition require re-evaluation in light of genetic evidence.
• TSPO's function may be more related to cellular metabolic homeostasis than previously thought, particularly in non-erythropoietic cells.