Peroxisomes serve as important centers for cellular redox metabolism and communication. However, fundamental gaps remain in our understanding of how the peroxisomal redox equilibrium is maintained. In particular, very little is known about the function of the nonenzymatic antioxidant glutathione in the peroxisome interior and how the glutathione antioxidant system balances with peroxisomal protein thiols. So far, only one human peroxisomal glutathione-consuming enzyme has been identified: glutathione S-transferase 1 kappa (GSTK1). To study the role of this enzyme in peroxisomal glutathione regulation and function, a GSTK1-deficient HEK-293 cell line was generated and fluorescent redox sensors were used to monitor the intraperoxisomal GSSG/GSH and NAD⁺/NADH redox couples and NADPH levels. We provide evidence that ablation of GSTK1 does not change the basal intraperoxisomal redox state but significantly extends the recovery period of the peroxisomal glutathione redox sensor po-roGFP2 upon treatment of the cells with thiol-specific oxidants. Given that this delay (i) can be rescued by reintroduction of GSTK1, but not its S16A active site mutant, and (ii) is not observed with a glutaredoxin-tagged version of po-roGFP2, our findings demonstrate that GSTK1 contains GSH-dependent disulfide bond oxidoreductase activity.

Pexophagy, the selective degradation of peroxisomes, is essential for removing excess or dysfunctional peroxisomes, and its dysregulation has been linked to various diseases. Although optineurin (OPTN), an autophagy receptor involved in mitophagy, aggrephagy, and xenophagy, has also been implicated in pexophagy in HEK-293 cells, the underlying mechanisms remain unclear. Using proximity labeling, we identified PEX14, a peroxisomal membrane protein, as a neighboring partner of OPTN. Microscopy analyses revealed that clustering of peroxisomes with OPTN is a key feature of OPTN-mediated pexophagy. Biochemical studies demonstrated that PEX14 and OPTN interact through their coiled-coil and ubiquitin-binding domains, respectively. Further analyses showed that the C-terminal half of overexpressed OPTN triggers pexophagy, likely by oligomerizing with endogenous OPTN. The colocalization of PEX14–OPTN complexes with LC3, together with the suppression of OPTN-mediated peroxisome degradation by bafilomycin A1, supports a model in which PEX14 acts as a docking site for OPTN on the peroxisomal membrane, enabling the recruitment of the autophagic machinery for OPTN-mediated pexophagy.

Prostate cancer (PCa) is associated with disruptions in cellular redox balance. Given the intricate role of peroxisomes in redox metabolism, we conducted comprehensive proteomics analyses to compare peroxisomal and redox protein profiles between benign (RWPE-1) and malignant (22Rv1, LNCaP, and PC3) prostate cell lines. Our analyses revealed significant enrichment of the “peroxisome” pathway among proteins notably upregulated in androgen receptor (AR)-positive cell lines. In addition, catalase (CAT) activity was consistently higher in these malignant cell lines compared to RWPE-1, which contrasts with previous studies reporting lower CAT levels and increased H₂O₂ levels in PCa tissues compared to adjacent normal tissues. To mimic this clinical scenario, we used RNA interference to knock down CAT expression. Our results show that reduced CAT levels enhanced 22Rv1 and LNCaP cell proliferation. R1881-induced activation of AR, a key driver of PCa, increased expression of the H₂O₂-producing peroxisomal β-oxidation enzymes acyl-coenzyme A oxidase 1 and 3, reduced CAT expression and activity, and elevated peroxisomal H₂O₂ levels. Considering these changes and other antioxidant enzyme profile alterations, we propose that enhanced AR activity in PCa reduces CAT function, leading to increased peroxisomal H₂O₂ levels that trigger adaptive stress responses to promote cell survival, growth, and proliferation.

Peroxisomes are functionally specialized organelles that harbor multiple hydrogen peroxide (H₂O₂)-producing and -degrading enzymes. Given that this oxidant functions as a major redox signaling agent, peroxisomes have the intrinsic ability to mediate and modulate H₂O₂-driven processes, including autophagy. However, it remains unclear whether changes in peroxisomal H₂O₂ (po-H₂O₂) emission impact the autophagic process and to which extent peroxisomes with a disturbed H₂O₂ metabolism are selectively eliminated through a process called “pexophagy”. To address these issues, we generated and validated HEK-293 and HeLa pexophagy reporter cell lines in which the production of po-H₂O₂ can be modulated. We demonstrate that (i) po-H₂O₂ can oxidatively modify multiple selective autophagy receptors and core autophagy proteins, (ii) neither modest nor robust levels of po-H₂O₂ emission act as a prime determinant of pexophagy, and (iii) high levels of po-H₂O₂ impair autophagic flux by oxidative inhibition of enzymes involved in LC3II formation. Unexpectedly, our analyses also revealed that the autophagy receptor optineurin can be recruited to peroxisomes, thereby triggering pexophagy. In summary, these findings lend support to the idea that, during cellular and organismal aging, peroxisomes with enhanced H₂O₂ release can escape pexophagy and downregulate autophagic activity, thereby perpetuating the accumulation of damaged and toxic cellular debris.

Cell culture conditions highly influence cell metabolism in vitro . This is relevant for preclinical assays, for which fibroblasts are an interesting cell model, with applications in regenerative medicine, diagnostics and therapeutic development for personalized medicine as well as in the validation of ingredients for cosmetics. Given these cells’ short lifespan in culture, we aimed to identify the best cell culture conditions and promising markers to study mitochondrial health and stress in Normal Human Dermal Fibroblasts (NHDF). We tested the effect of reducing glucose concentration in the cell medium from high glucose (HGm) to a more physiological level (LGm), or its complete removal and replacement by galactose (OXPHOSm), always in the presence of glutamine and pyruvate. We have demonstrated that only with OXPHOSm it was possible to observe the selective inhibition of mitochondrial ATP production. This reliance on mitochondrial ATP was accompanied by changes in oxygen consumption rate (OCR) and extracellular acidification rate (ECAR), oxidation of citric acid cycle substrates, fatty acids, lactate and other substrates, mitochondrial network extension and polarization and changes in several key transcripts related to energy metabolism. We also evaluated the relevance of galactose, glutamine and pyruvate for OXPHOS stimulation, by comparing OCR and ECAR in the presence or absence of these substrates. Galactose and pyruvate seem to be important, but redundant, to promote OXPHOS, whereas glutamine was essential. We concluded that LGm does not promote significant metabolic changes but the short-term adaptation to OXPHOSm is ideal for studying mitochondrial health and stress in NHDF. Author Contributions: CC, SAP, SLCP and IMS performed experiments. TCO and PJO designed research and acquired funding. JS, and OB analyzed data. CC and TCO analyzed data and wrote the paper. All authors contributed to the final version of the manuscript.