Hydrogen peroxide (H₂O₂), a non-radical reactive oxygen species generated during many (patho)physiological conditions, is currently universally recognized as an important mediator of redox-regulated processes. Depending on its spatiotemporal accumulation profile, this molecule may act as a signaling messenger or cause oxidative damage. The focus of this review is to comprehensively evaluate the evidence that peroxisomes, organelles best known for their role in cellular lipid metabolism, also serve as hubs in the H₂O₂ signaling network. We first briefly introduce the basic concepts of how H₂O₂ can drive cellular signaling events. Next, we outline the peroxisomal enzyme systems involved in H₂O₂ metabolism in mammals and reflect on how this oxidant can permeate across the organellar membrane. In addition, we provide an up-to-date overview of molecular targets and biological processes that can be affected by changes in peroxisomal H₂O₂ metabolism. Where possible, emphasis is placed on the molecular mechanisms and factors involved. From the data presented, it is clear that there are still numerous gaps in our knowledge. Therefore, gaining more insight into how peroxisomes are integrated in the cellular H₂O₂ signaling network is of key importance to unravel the precise role of peroxisomal H₂O₂ production and scavenging in normal and pathological conditions.

Ever since the first characterization of peroxisomes, a central theme has been their involvement in cellular hydrogen peroxide (H ₂ O ₂ ) metabolism. While the reputation of H ₂ O ₂ drastically changed from an exclusively toxic molecule to a signaling messenger, the regulatory role of peroxisomes in these signaling events is still largely underappreciated. This is mainly because the number of known protein targets of peroxisome-derived H ₂ O ₂ is rather limited and testing of specific targets is predominantly based on knowledge previously gathered in related fields of research. To gain a broader and more systematic insight into the role of peroxisomes in redox signaling, an unbiased approach is urgently needed. To accomplish this goal, we have combined a previously developed cell system in which peroxisomal H ₂ O ₂ production can be modulated with a yeast AP-1-like-based sulfenome mining strategy to inventory protein thiol targets of peroxisome-derived H ₂ O ₂ in different subcellular compartments. Using this unbiased approach, we were able to identify specific and common targets of peroxisome-derived and exogenous H ₂ O ₂ in peroxisomes, the cytosol, and mitochondria. We also observed that the sulfenylation kinetics profiles of key targets belonging to different protein families can vary considerably. In addition, we obtained compelling but indirect evidence that peroxisome-derived H ₂ O ₂ may oxidize at least some of its targets through a redox relay mechanism. In conclusion, given that sulfenic acids function as key intermediates in H ₂ O ₂ signaling, the findings presented in this study provide initial but critical insight into how peroxisomes may be integrated in the cellular H ₂ O ₂ signaling network. Highlights: YAP1C-trapping is a robust tool to assess the peroxisomal H ₂ O ₂ -dependent sulfenome Exogenous and peroxisome-derived H ₂ O ₂ have both common and distinct targets ANXA2, PRDX1, and SKP1 are major targets of peroxisome-derived H ₂ O ₂ The sulfenylation kinetics profiles of key redox-active proteins vary considerably Production of H ₂ O ₂ inside peroxisomes directly impacts the mitochondrial sulfenome

As befits their name, peroxisomes play a central role in cellular hydrogen peroxide (H₂O₂) metabolism. In addition, it is increasingly appreciated that changes in peroxisomal H₂O₂ metabolism can actively contribute to the initiation and progression of redox stress-related human pathologies such as cancer, diabetes, and neurodegeneration. However, to function as a subcellular hub for H₂O₂ metabolism and signaling, peroxisomes must exchange this oxidant with their environment. After providing some basic background information on the concepts and importance of reduction-oxidation (redox) regulation in living cells, this chapter introduces the reader to what is known about peroxisomal H₂O₂ metabolism and signaling, with a focus on human health and disease. In addition, it reviews research carried out to better understand how H₂O₂ permeates the peroxisomal membrane. Furthermore, it examines the human peroxisomal membrane proteome in search of proteins with peroxiporin-like properties, thereby focusing on the long-standing question if we already hold the key to unlocking this yet unsolved mystery. Finally, the chapter concludes with a brief reflection on potential research directions in this fascinating but challenging field. <!----> <!----> <!----> <!---->