Mitochondrial Translocator Protein (TSPO)-Dependent Bioenergetic Remodeling and Cell Viability in Response to Tetrapyrrole Compounds.

Mitochondria are central to cellular energy production and are strongly affected by metabolic and oxidative stress. The 18-kDa translocator protein (TSPO), located mainly in the outer mitochondrial membrane, has been linked to mitochondrial respiration, redox balance and cell survival, and can also bind endogenous tetrapyrroles such as protoporphyrin IX (PpIX). This study asked whether changes in tetrapyrrole/heme metabolism occur together with mitochondrial bioenergetic remodeling and changes in the TSPO–VDAC system, and whether loss of TSPO affects the cellular response to PpIX stress. Two public proteomic datasets were analysed. The first compared cysteine-supplemented and cysteine-depleted conditions (PXD082345), while the second compared PpIX-rich and low-PpIX regions of human glioma tissue (PXD039170). Under cysteine excess, oxidative phosphorylation and fatty-acid metabolism were strongly reduced, together with several respiratory-chain, TCA-cycle and fatty-acid-oxidation proteins. In contrast, TSPO and VDAC1–3 increased. Heme-related proteins also changed, with FECH decreasing and ALAS1 and HMOX1 increasing, consistent with altered heme/tetrapyrrole homeostasis. In the PpIX-rich glioma dataset, FECH and CPOX were reduced and HMOX1 increased, while TSPO and VDAC1–3 tended to decrease. The opposite direction of TSPO and VDAC in the two datasets suggests that their response to tetrapyrrole and mitochondrial stress is likely to depend on the biological context. To examine whether TSPO itself may influence the response to PpIX, cell viability was also assessed in WT and TSPO-KO SH-SY5Y cells. In a single preliminary experiment, PpIX reduced the MTT signal in both genotypes, with TSPO-KO cells showing consistently lower relative MTT values than WT cells across the tested PpIX concentrations. Because this experiment was not independently replicated, the finding should be considered preliminary. Overall, the results support a close relationship between tetrapyrrole homeostasis, mitochondrial bioenergetic remodeling and the TSPO–VDAC stress response, while the direct role of TSPO in driving these changes remains to be established.