Aquaporin-8 (AQP8) allows the bidirectional transport of water and hydrogen peroxide across biological membranes. Depending on its concentration, H 2 O 2 exerts opposite roles, amplifying growth factor signaling in physiological conditions, but causing severe cell damage when in excess. Thus, H 2 O 2 permeability is likely to be tightly controlled in living cells. Aims: In this study, we investigated whether and how the transport of H 2 O 2 through plasma membrane AQP8 is regulated, particularly during cell stress. Results: We show that diverse cellular stress conditions, including heat, hypoxia, and ER stress, reversibly inhibit the permeability of AQP8 to H 2 O 2 and water. Preventing the accumulation of intracellular reactive oxygen species (ROS) during stress counteracts AQP8 blockade. Once inhibition is established, AQP8-dependent transport can be rescued by reducing agents. Neither H 2 O 2 nor water transport is impaired in stressed cells expressing a mutant AQP8, in which cysteine 53 had been replaced by serine. Cells expressing this mutant are more resistant to stress-, drug-, and radiation-induced growth arrest and death. Innovation and Conclusion: The control of AQP8-mediated H 2 O 2 transport provides a novel mechanism to regulate cell signaling and survival during stress. Antioxid.

Hydrogen peroxide (H2O2) is an essential second intracellular messenger. To reach its targets in the cytosol, H2O2 must cross a membrane, a feat that requires aquaporins (AQP) endowed with ‘peroxiporin’ activity (AQP3, AQP8, AQP9). Here, we exploit different organelle-targeted H2O2-sensitive probes to show that also AQP11 efficiently conduits H2O2. Unlike other peroxiporins, AQP11 is localized in the endoplasmic reticulum (ER), accumulating partly in mitochondrial-associated ER membranes (MAM). Its downregulation severely perturbs the flux of H2O2 through the ER, but not through the mitochondrial or plasma membranes. These properties make AQP11 a potential regulator of ER redox homeostasis and signaling.

H 2 O 2 produced by extracellular NADPH oxidases regulates tyrosine kinase signaling inhibiting phosphatases. How does it cross the membrane to reach its cytosolic targets? Silencing aquaporin-8 (AQP8), but not AQP3 or AQP4, inhibited H 2 O 2 entry into HeLa cells. Re-expression of AQP8 with silencing-resistant vectors rescued H 2 O 2 transport, whereas a C173A-AQP8 mutant failed to do so. Lowering AQP8 levels affected H 2 O 2 entry into the endoplasmic reticulum, but not into mitochondria. AQP8 silencing also inhibited the H 2 O 2 spikes and phosphorylation of downstream proteins induced by epidermal growth factor. These observations lead to the hypothesis that H 2 O 2 does not freely diffuse across the plasma membrane and AQP8 and other H 2 O 2 transporters are potential targets for manipulating key signaling pathways in cancer and degenerative diseases.