Hale, Taben M.
Despite numerous antihypertensive drug treatment options that are currently on the market, a recent analysis of the National Health and Nutrition Examination Survey revealed that of the participants currently undergoing antihypertensive treatment, only 53% had achieved blood pressures less than 140/90 mmHg . Although there may be many reasons for poor control rates, there remains an opportunity, and perhaps a necessity for identifying novel strategies for the treatment of hypertension. Endothelial dysfunction is a common comorbidity of hypertension and it has been shown to significantly increase the risk of future cardiovascular events, including coronary artery disease . As such, improving endothelial function has been proposed as an important goal of antihypertensive therapy. Whether improving endothelial function alone is sufficient to lower blood pressure long-term remains to be determined, but there is certainly evidence to support the benefits to reducing the risk of target organ damage .
Endothelial dysfunction can be characterized by impaired production of nitric oxide, a potent vasodilator that functions in healthy blood vessels to directly relax vascular smooth muscle cells and to oppose the actions of endothelial-derived contracting factors. However, under pathophysiological conditions, decreased nitric oxide bioavailability may result from a number of factors, including reductions in endothelial nitric oxide synthase (eNOS) levels or activity, as well as reductions in eNOS substrate (l-arginine) or cofactor (tetrahydrobiopterin: BH4) . Under conditions in which a suitable substrate is present, but there is low BH4 availability, eNOS can become ‘uncoupled’, whereby superoxide is produced rather than nitric oxide . Uncoupled NOS has been implicated in a variety of cardiovascular disorders including hypertension and atherosclerosis [4,5].
Given the association of endothelial dysfunction and cardiovascular disease, elucidating the pathways involved in eNOS uncoupling and identifying strategies that reverse this effect may reveal novel targets for treatment. To that end, in the present issue of this journal, Kim et al. explore the therapeutic potential of NADPH:quinone oxidoreductase 1 (NQO1) activation in spontaneously hypertensive rats (SHRs). NQO1 is an enzyme that catalyzes the oxidation of NAD(P)H to NAD(P)+ and has been shown to be activated by a variety of substrates, including β-lapachone (βL). The authors previously demonstrated that βL treatment in SHR resulted in increased phosphorylation of aortic eNOS, improved endothelial function and decreased blood pressure . Their present study extends these findings to determine the intermediary steps between NQO1 activation by βL and improved endothelial responses. Their very elegant in-vitro studies showed that βL treatment improves eNOS coupling through LKB1/AMPK-mediated preservation of GTP cyclohydrolase-1 (GTPHC-1). Recent studies have identified GTPCH-1 as an important regulator of vascular function through its role as the rate-limiting step in de-novo BH4 synthesis . Specifically, in-vitro and ex-vivo vascular reactivity studies have very nicely shown the link between GTPCH-1 inhibition, eNOS uncoupling and the resultant reduction in nitric oxide bioavailability and endothelial dysfunction .
Kim et al. further demonstrate that chronic βL reduced blood pressure and improved endothelial-mediated relaxation in aorta concomitant with an increase in the BH4/BH2 ratio. The findings from the in-vitro studies were mirrored in vivo where they demonstrate that chronic βL treatment results in increased phosphorylated-AMPK and LKB1 activation and preservation of GTPCH1 in aorta of SHR. A major conclusion of this study is that blood pressure reduction may be causally related to improved NOS coupling resulting from GTPCH-1 mediated increase in BH4 production. Although enhanced NOS coupling and resultant improvement in endothelial function may contribute to blood pressure lowering, it is also worth exploring the possibility that the antihypertensive effect may also result, at least in part, from other beneficial aspects of the signalling pathway that the authors so clearly described.
NOS uncoupling, most commonly due to decreased levels of BH4 , has been shown to be evident in a variety of experimental models of hypertension [11,12]. For example, Landmesser et al. showed that oxidation of BH4 promoted eNOS uncoupling, associated with an elevation in blood pressure. Moreover, SHR have been shown to express lower levels of BH4 and display impaired endothelial function, as compared with the normotensive Wistar Kyoto rats . Finally, inhibition or loss of the BH4-generating enzyme GTPCH-1 has been shown to result in NOS uncoupling and endothelial dysfunction, associated with an increase in blood pressure . Not surprisingly then, supplementation with BH4 has been investigated as a means to both improve endothelial function and lower blood pressure. Indeed, this treatment has consistently been shown to improve endothelial function both experimentally [14–16] and clinically [17,18]. However, the impact on blood pressure has been less clear. Experimentally, BH4 has been demonstrated to reduce blood pressure , attenuate the development of hypertension  or have no impact on blood pressure [15,16]. Similarly, the clinical impact of BH4 supplementation on hypertension has yielded mixed results [20,21]. In the present  and previous study , Kim et al. demonstrate that NQO1 activation by βL produces sustained reductions in blood pressure and improvement in endothelial function that is associated with prolonged activation of GTPCH-1. Although this leads to increased BH4 production, it may be that this treatment strategy may be more effective than BH4 supplementation because of additional beneficial effects occurring upstream of GTPCH-1, namely reduced NADPH oxidase activity and increased AMPK activity.
In established hypertension, elevated blood pressure is primarily related to increased total peripheral resistance . Structural remodelling, characterized by an increase in the ratio of medial wall thickness to lumen diameter, particularly in the resistance arteries, results in an increase in peripheral resistance, without the additional requirement of increased vascular tone. We and others have shown that drugs that reverse this structurally based vascular resistance are particularly effective in producing long-term and persistent reductions in arterial pressure as well as protection against target organ damage [23–26]. Moreover, small artery structure has been identified as an independent predictor of cardiovascular events . Although the mechanisms that underlie the development of increased vascular structure in hypertension are likely multifactorial, increased oxidative stress has been implicated in playing a major role. Specifically, normalizing oxidative stress has been shown to attenuate angiotensin II-induced vascular remodelling and hypertension , and inhibition of NADPH oxidase activity has been shown to attenuate neointimal formation following balloon injury . The major sources of reactive oxygen species (ROS) in the vascular wall are NADPH oxidases and superoxide. As a result, significant effort is being spent on identifying orally active compounds to inhibit NAPDH oxidase activity . Kim et al. have recently demonstrated that βL activates NQO1 to result in an increase in the ratio of NAD(P) to NAD(P)H and a reduction in NADPH oxidase activity in the kidney of salt-treated rats. In their present study in this issue of the Journal of Hypertension, they also demonstrate a similar increase in the level of NAD+ in the aorta of SHR treated with βL . The resultant decrease in ROS that is likely occurring in the aorta would not only contribute to an improvement in endothelial function but also may have beneficial effects on the vascular structure. Whether reduction in oxidative stress is sufficient to induce a regression of established vascular hypertrophy is not known, but there is evidence that it can protect against future pathological remodelling [28,29].
In addition to improved eNOS coupling and increased aortic NAD+ levels, Kim et al. demonstrate an increase in phosphorylation of AMPK both in endothelial cells in vitro and in aorta in vivo. Increasing AMPK activity has been shown to be protective in a number of cardiovascular diseases including atherosclerosis . Acute treatment with the AMPK activator AICAR was recently shown to lower blood pressure and to relax resistance arteries in SHR . AMPK activation by AICAR has also been shown to reduce vascular smooth muscle cell proliferation in vitro and inhibit neointimal formation following balloon injury [34,35]. Whether AMPK activation by AICAR or by βL induces regression of medial wall thickness in hypertensive rats has not yet been shown. Nonetheless, the protective effects of AMPK on cardiovascular disease progression have been well described [34,35].
Whether restoration of eNOS coupling and improvement of endothelial dysfunction alone is sufficient to induce a long-term reduction in blood pressure remains to be fully determined. However, this  and other recent studies by Kim et al.[8,31] provide intriguing data that suggest that NQO1 activation may be a new target for reducing blood pressure and cardiovascular risk through a variety of mechanisms, in addition to restoration of endothelial function. It will be interesting for future studies to investigate whether βL-induced changes in NADPH oxidase activity, AMPK and BH4 are also observed in the resistance vasculature, and whether NQO1 activation ultimately results in regression of vascular structure in hypertensive rats. Moreover, given the impact of this therapy on AMPK and NADPH oxidase activity, future studies investigating the potential of this treatment on protection against hypertension-induced target organ (e.g. heart, kidney, brain, vasculature) damage are warranted. On the basis of the current lack of success in adequately controlling hypertension in patients , it is important to identify novel treatment strategies for hypertension and related diseases.
T.M.H. is supported by a Beginning Grant in Aid from the American Heart Association (13BGIA14720053).
Conflicts of interest
There are no conflicts of interest.
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