Mechanisms of NFATc3 activation by increased superoxide and reduced hydrogen peroxide in pulmonary arterial smooth muscle.

We recently demonstrated increased superoxide (O2(·-)) and decreased H2O2 levels in pulmonary arteries of chronic hypoxia-exposed wild-type and normoxic superoxide dismutase 1 (SOD1) knockout mice. We also showed that this reciprocal change in O2(·-) and H2O2 is associated with elevated activity of nuclear factor of activated T cells isoform c3 (NFATc3) in pulmonary arterial smooth muscle cells (PASMC). This suggests that an imbalance in reactive oxygen species levels is required for NFATc3 activation. However, how such imbalance activates NFATc3 is unknown. This study evaluated the importance of O2(·-) and H2O2 in the regulation of NFATc3 activity. We tested the hypothesis that an increase in O2(·-) enhances actin cytoskeleton dynamics and a decrease in H2O2 enhances intracellular Ca(2+) concentration, contributing to NFATc3 nuclear import and activation in PASMC. We demonstrate that, in PASMC, endothelin-1 increases O2(·-) while decreasing H2O2 production through the decrease in SOD1 activity without affecting SOD protein levels. We further demonstrate that O2(·-) promotes, while H2O2 inhibits, NFATc3 activation in PASMC. Additionally, increased O2(·-)-to-H2O2 ratio activates NFATc3, even in the absence of a Gq protein-coupled receptor agonist. Furthermore, O2(·-)-dependent actin polymerization and low intracellular H2O2 concentration-dependent increases in intracellular Ca(2+) concentration contribute to NFATc3 activation. Together, these studies define important and novel regulatory mechanisms of NFATc3 activation in PASMC by reactive oxygen species.

NFAT is required for spontaneous pulmonary hypertension in superoxide dismutase 1 knockout mice.

Elevated reactive oxygen species are implicated in pulmonary hypertension (PH). Superoxide dismutase (SOD) limits superoxide bioavailability, and decreased SOD activity is associated with PH. A decrease in SOD activity is expected to increase superoxide and reduce hydrogen peroxide levels. Such an imbalance of superoxide/hydrogen peroxide has been implicated as a mediator of nuclear factor of activated T cells (NFAT) activation in epidermal cells. We have shown that NFATc3 is required for chronic hypoxia-induced PH. However, it is unknown whether NFATc3 is activated in the pulmonary circulation in a mouse model of decreased SOD1 activity and whether this leads to PH. Therefore, we hypothesized that an elevated pulmonary arterial superoxide/hydrogen peroxide ratio activates NFATc3, leading to PH. We found that SOD1 knockout (KO) mice have elevated pulmonary arterial wall superoxide and decreased hydrogen peroxide levels compared with wild-type (WT) littermates. Right ventricular systolic pressure (RVSP) was elevated in SOD1 KO and was associated with pulmonary arterial remodeling. Vasoreactivity to endothelin-1 was also greater in SOD1 KO vs. WT mice. NFAT activity and NFATc3 nuclear localization were increased in pulmonary arteries from SOD1 KO vs. WT mice. Administration of A-285222 (selective NFAT inhibitor) decreased RVSP, arterial wall thickness, vasoreactivity, and NFAT activity in SOD1 KO mice to WT levels. The SOD mimetic, tempol, also reduced NFAT activity, NFATc3 nuclear localization, and RVSP to WT levels. These findings suggest that an elevated superoxide/hydrogen peroxide ratio activates NFAT in pulmonary arteries, which induces vascular remodeling and increases vascular reactivity leading to PH.