Absence of Nicotinamide Nucleotide Transhydrogenase in C57BL/6J Mice Exacerbates Experimental Atherosclerosis

BACKGROUND

Mitochondrial reactive oxygen species (ROS) contribute to inflammation and vascular remodeling during atherosclerotic plaque formation. C57BL/6N (6N) and C57BL/6J (6J) mice display distinct mitochondrial redox balance due to the absence of nicotinamide nucleotide transhydrogenase (NNT) in 6J mice. We hypothesize that differential NNT expression between these animals alters plaque development.

METHODS

6N and 6J mice were treated with AAV8-PCSK9 (adeno-associated virus serotype 8/proprotein convertase subtilisin/kexin type 9) virus leading to hypercholesterolemia, increased low-density lipoprotein, and atherosclerosis in mice fed a high-fat diet (HFD). Mice were co-treated with the mitochondria-targeted superoxide dismutase mimetic MitoTEMPO to assess the contribution of mitochondrial ROS to atherosclerosis.

RESULTS

Baseline and HFD-induced vascular superoxide is increased in 6J compared to 6N mice. MitoTEMPO diminished superoxide in both groups demonstrating differential production of mitochondrial ROS among these strains. PCSK9 treatment and HFD led to similar increases in plasma lipids in both 6N and 6J mice. However, 6J animals displayed significantly higher levels of plaque formation. MitoTEMPO reduced plasma lipids but did not affect plaque formation in 6N mice. In contrast, MitoTEMPO surprisingly increased plaque formation in 6J mice.

CONCLUSION

These data indicate that loss of NNT increases vascular ROS production and exacerbates atherosclerotic plaque development.

Abstract P216: Nicotinamide Nucleotide Transhydrogenase Regulates Mitochondrial Redox Balance and Endothelial Function in Response to Angiotensin II

Hypertension (HTN) involves increased peripheral resistance to blood flow mediated by changes in vascular tone. Endothelial dysfunction is an initiating step in HTN and mitochondrial reactive oxygen species (ROS) contribute to endothelial dysfunction. Nicotinamide nucleotide transhydrogenase (NNT) is a critical enzyme in the regulation of mitochondrial NADPH levels and can impact mitochondrial redox status. We have demonstrated that the loss of NNT in C57Bl/6J mice is associated with a more severe hypertensive phenotype in response to Ang II characterized by mitochondrial and vascular dysfunction driven by vascular ROS production in vivo and in vitro . Little is known regarding the role of NNT human endothelial cells, but based on previous studies we hypothesize that the loss of NNT promotes mitochondrial and cellular ROS production and disrupts normal endothelial • NO function. In human aortic endothelial cells (HAEC) treatment with 500nM Ang II for two hours significantly increased NNT expression (225 + 1 3%) and glutathione peroxidase 2 (Gpx2) activity (17.81 + 0 .75 vs 29.06 + 2 .53 nmol/min/ug protein) consistent with NNT’s role in driving the mitochondrial antioxidant systems. Conversely, siRNA knockdown of NNT inhibited Gpx2 activity (18.32 + 1.17 vs 13.16 + 0 .7 nmol/min/ug protein) and increased mitochondrial superoxide production (0.12 + 0 .01 vs 0.17 + 0 .01 nmol/mg protein) in response to Ang II. These changes in the redox balance of the mitochondria contribute to mitochondrial dysfunction where the loss of NNT led to a marked decrease in the respiratory control ratio (RCR) of the mitochondria (6.42 + 0 .17 vs 3.83 + 0 .1) of Ang II treated cells. Ultimately, this shift to a pro-oxidative mitochondrial redox phenotype in these cells conspired to inhibit Ang II-induced • NO production (35.60 + 1 .28 vs 30.10 + 1 .24 pmol/mg protein). Our data indicate that the absence of NNT expression increases the steady state levels of mitochondrial ROS through increased ROS production and decreased antioxidant activity, which is contributes to reduced • NO production and endothelial dysfunction. These studies highlight NNT as a novel regulator of mitochondrial redox tone that may critically impact the ability of vascular ROS to contribute to the development of HTN.

Abstract 591: Absence of Nicotinamide Nucleotide Transhydrogenase Exacerbates Atherosclerosis in Mice

Oxidant stress contributes to endothelial cell injury and inflammation that are hallmarks of early stage atherosclerosis. Emerging evidence implicates mitochondrial reactive oxygen species (ROS) as important contributors to this oxidant stress and differences in mitochondrial function may augment this process. We have shown that variation in mitochondrial function and ROS production associated with ethnicity contributes to endothelial and vascular dysfunction. To model these distinct mitochondrial redox phenotypes we used C57Bl/6N (6N) and C57Bl/6J (6J) mice that also display unique mitochondrial functional properties due to the differential expression nicotinamide nucleotide transhydrogenase (NNT). Mice were treated with adeno-associated virus encoding a gain-of-function form of proprotein convertase subtilisin/kexin type 9 (PCSK9) that leads to hypercholesterolemia, increased LDL levels, and atherosclerosis in mice. PCSK9 treatment and 8 weeks of high fat diet led to increases in plasma lipids in both 6N and 6J mice. However, 6J animals displayed significantly higher levels of fat deposition in the vasculature and increased plaque size in the carotid sinus. 6N mice co-treated with the mitochondria targeted superoxide dismutase mimetic MitoTEMPO for the final 4 weeks of the experiment displayed reduced plasma lipids, but no impact on fat deposition or plaque size was observed. In contrast, MitoTEMPO increased vascular fat deposition and plaque size in 6J mice consistent with a more severe atherosclerotic phenotype. Increased mitochondrial ROS was confirmed by demonstrating elevated vascular superoxide in 6J versus 6N animals and that this difference is exacerbated on high fat diet. MitoTEMPO diminished vascular superoxide production to near baseline levels in both groups, yet increased plaque size and fat deposition in 6J mice suggests a role for hydrogen peroxide in this process. These data indicate that loss of NNT and changes in mitochondrial function increase vascular ROS production and exacerbate lipid deposition and plaque development in the early stages of atherosclerosis.