Hypoxia-dependent reactive oxygen species signaling in the pulmonary circulation: focus on ion channels

Abstract

Significance: An acute lack of oxygen in the lung causes hypoxic pulmonary vasoconstriction, which optimizes gas exchange. In contrast, chronic hypoxia triggers a pathological vascular remodeling causing pulmonary hypertension, and ischemia can cause vascular damage culminating in lung edema.

Recent advances: Regulation of ion channel expression and gating by cellular redox state is a widely accepted mechanism; however, it remains a matter of debate whether an increase or a decrease in reactive oxygen species (ROS) occurs under hypoxic conditions. Ion channel redox regulation has been described in detail for some ion channels, such as Kv channels or TRPC6. However, in general, information on ion channel redox regulation remains scant.

Critical issues and future directions: In addition to the debate of increased versus decreased ROS production during hypoxia, we aim here at describing and deciphering why different oxidants, under different conditions, can cause both activation and inhibition of channel activity. While the upstream pathways affecting channel gating are often well described, we need a better understanding of redox protein modifications to be able to determine the complexity of ion channel redox regulation. Against this background, we summarize the current knowledge on hypoxia-induced ROS-mediated ion channel signaling in the pulmonary circulation.

FIG. 1.

Schematic illustration of the effects of alveolar hypoxia on the pulmonary circulation. The effects of alveolar hypoxia on the pulmonary circulation can be divided in three phases: (i) the acute (30 sec-20 min), (ii) the sustained phase (20min-hours), and (iii) the chronic phase (days-weeks). Within seconds, acute hypoxia leads to hypoxic pulmonary vasoconstriction (HPV), matching blood perfusion to alveolar ventilation. Under conditions of generalized sustained and chronic alveolar hypoxia, this vasoconstriction is morphologically fixed by media hypertrophy (vascular remodeling) inducing pulmonary hypertension (PH).

FIG. 2.

Opposing models of the effect of acute hypoxia on ROS production and Kv channel regulation. With regard to the effect of acute hypoxia on PASMC depolarization, two models are discussed: The first model (right side) favors the closure of Kv-channels mediated by a reduced ROS release (most likely by mitochondria) that activates vasoconstriction. The second model proposes that an increase of ROS from mitochondria and/or NADPH oxidases triggers such events via DAG-mediated activation of TRPC6, subsequent influx of Na⁺ and Ca²⁺, and inhibition of Kv channels by Na⁺. DAG, diacylglycerol; DAGK, diacylglycerol kinase; EC, extracellular; E_m, membrane potential; IC, intracellular; Kv, voltage-gated K⁺ channels; PASMC, pulmonary arterial smooth muscle cells; ROS, reactive oxygen species; TRPC6, transient receptor potential channel 6; VOCC, voltage-operated Ca²⁺ channel.

FIG. 3.

Proposed redox regulation of Ca²⁺-activated K⁺ channels. Not much is known about the redox regulation of K_Ca channels. Small mitochondrial depolarization causes elevated ROS production and activates transient K_Ca currents. In contrast, large mitochondrial depolarization reduces ROS and inhibits transient K_Ca currents. Hypoxia was shown to reduce K_Ca channel activity, but the detailed effects of hypoxia on K_Ca channels still remain largely unresolved. Oxidizing agents induce up- or down-regulation of BK_Ca channel activity depending on the experimental condition. Cysteine oxidation decreases the currents of large conductance Ca²⁺-activated K⁺ channels, whereas methionine oxidation increases currents. Redox regulation of K_Ca channels most likely depends on the concentration of ROS or RNS, the oxidant/species, and the cell type. ?=Effects of hypoxia on K_Ca channels are largely unresolved. EC, extracellular; IC, intracellular; K_Ca, Ca²⁺-activated K⁺ channel; RNS, reactive nitrogen species.

FIG. 4.

Proposed redox regulation of VOCCs. VOCCs are activated when PASMC depolarization reaches a certain threshold (excitation–contraction coupling), but they are also redox sensitive. Cysteine residues in the pore-forming α₁-subunit are the molecular targets for ROS, and both activation and inhibition of channel activity by oxidation have been described. Oxidants affect VOCC activity, expression, trafficking, open time, and open probability. Oxidation of SH groups by ROS decreases cardiac L-type Ca²⁺-currents, whereas oxidation of SH groups by other oxidizing agents (DTNB) causes stimulation of Ca²⁺-currents. The effect of oxidizing agents on VOCCs depends on the species and the mode of action. GSH inhibits the current, and cellular GSH levels are known to be reduced during hypoxia. S-nitrosylation of extracellular SH groups of the L-type Ca²⁺ channel increases currents, whereas S-nitrosylation in the α1-subunit decreases currents. Again, opposing findings might be explained by concentration- and species-dependent effects of ROS or RNS. DTNB, Ellman's reagent [5,5′-dithiobis-(2-nitrobenzoic acid)]; GSH, glutathione; SH, sulfhydryl group.

FIG. 5.

Speculative redox regulation of TRPC3/4 and TRPC1 containing channels. TRPC3/4 are regulated by ROS, and TRPC3/4 containing channels can be activated in the presence of oxidants. ROS induce disruption of cholesterol-rich lipid rafts and membrane cholesterol oxidation, which has been suggested to activate TRPC3/4 containing channels. TRPC1 was shown to play an important role during vascular remodeling in chronic hypoxia-induced PH. Similar mechanisms as for TRPC3/4 might apply for the redox regulation of TRPC1. Although an activation of TRPC3/4 and TRPC1 (similar to TRPC6) by PLC and PLC-mediated hydrolysis of membrane-bound PIP cannot be excluded, the mechanism of oxidative stress-mediated TRPC3 activation does not involve PIP hydrolysis. The role of PLC in TRPC1 activation has not yet been addressed. GSSG, glutathione disulfide.

FIG. 6.

Hypothesized role of TRPC6 in lung ischemia-reperfusion injury. In an animal model of LIRE, opening of TRPC6 in pulmonary vascular endothelial cells and subsequent Ca²⁺ influx was triggered by endothelial Nox2-derived production of superoxide, activation of phospholipase C-γ, inhibition of DAG kinase (DAGK), accumulation of DAG, and DAG-mediated activation of TRPC6. In this model, H₂O₂ re-enters the cell (extracellular loop), activates PLCγ, and inactivates DAGK. H₂O₂, hydrogen peroxide; LIRE, lung ischemia–reperfusion-induced edema; Nox2, NADPH oxidase 2; PIP₂, phosphatidylinositol 4,5-bisphosphate; PLCγ, phospholipase C-γ.