Atorvastatin ameliorates arsenic-induced hypertension and enhancement of vascular redox signaling in rats
Introduction
Arsenic is a naturally occurring metalloid that is ubiquitously present in both organic and inorganic forms. People are exposed to elevated levels of inorganic arsenic through contaminated drinking water, food prepared with this water and food crops irrigated with high arsenic water sources. The greatest threat to public health originates from the arsenic contaminated groundwater. People in several countries around the world, including India and Bangladesh, are exposed to arsenic via drinking of contaminated groundwater (Jiang et al., 2013). About 450 million people in the Ganga–Meghna–Brahmaputra region are prone to arsenic poisoning through contaminated groundwater (Roy et al., 2013). Though arsenic-induced cancer has been widely studied, less attention was paid to arsenic-induced vascular diseases. Epidemiological studies showed that chronic arsenic exposure was associated with increased morbidity and mortality from cardiovascular diseases (CVDs), including hypertension, ischemic heart disease, atherosclerosis and peripheral vascular disease (Stea et al., 2014).
Oxidative stress can cause vascular endothelial dysfunction (VED) and development of CVDs (Paravicini and Touyz, 2008). Evidences indicate that arsenicals can generate reactive oxygen species (ROS), especially superoxide anion radical (O2−) and H2O2, in human vascular smooth muscle cells (VSMCs) mainly via NADPH oxidase (Nox) activation and increased p22phox mRNA expression (Lynn et al., 2000). Arsenic increased O2− generation in vascular endothelial cells (ECs; Brachowsky et al., 1999) and rat thoracic aorta (Sharma and Sharma, 2013). Further, environmentally relevant levels of arsenite activated Nox in porcine aortic ECs and produced ROS (Smith et al., 2001). Nox is a family of seven members that transport electrons across biological membranes to reduce O2 to O2− (Bedard and Krause, 2007, Leto et al., 2009). Nox activity in VSMCs is largely dependent upon Nox-4 (Ellmark et al., 2005). Nox-4 is abundantly expressed in both rat and human aortic endothelium and functionally active with p22phox in the ECs (Ago et al., 2004). Nox-4 is unique among the Nox subunits in that it only requires the membrane subunit p22phox for ROS production and appears to be constitutively active (Martyn et al., 2006).
Nitric oxide (NO), which modulates several vascular protective effects such as vasodilation, inhibition of platelet aggregation and prevention of vasospasm, is produced in the endothelium mainly by endothelial nitric oxide synthase (eNOS). On the contrary, NO derived from inducible NOS (iNOS) has vascular cytotoxic effects, which are mediated by its highly reactive oxidation product, peroxynitrite (ONOO−) — a reactive nitrogen species (RNS, Pacher et al., 2007). NO reacts faster with O2− resulting in the production of ONOO− in high enough concentrations to out-compete endogenous levels of superoxide dismutase (SOD; Pacher et al., 2007). Thus, increased O2− production can result in greater scavenging of NO and elevated levels of ONOO−, which can damage lipids, proteins and DNA in a similar fashion as ROS (Szabo, 2003). Arsenic can enhance or reduce NO production, depending on the type of cell, species and dose of arsenical tested (Gurr et al., 2003). Arsenite can suppress relaxation in rat blood vessels by inhibiting eNOS activity in ECs (Lee et al., 2003). In humans, long-term exposure to arsenic-contaminated drinking water reduces NO production in the ECs due to inhibition of eNOS activity (Kumagai and Pi, 2004). Further, it has been reported that arsenic up-regulates iNOS (Prabu and Muthumani, 2012), which may contribute to inflammatory response, increased ROS generation, vascular remodeling, decreased aortic blood flow, attenuation of endothelium-dependent relaxation, EC damage or death (Steed et al., 2010).
Statins are recognized as the most effective drugs for reducing the levels of serum cholesterols and thereby, the incidences of CVDs, mainly by their low density lipoprotein cholesterol (LDL-C)-lowering effect. Besides, they have other beneficial effects as they enhance atherosclerotic plaque stability, inhibit VSMC proliferation and platelet aggregation, reduce vascular inflammation and up-regulate eNOS (Fujita et al., 2007). Further, their antioxidant effects reduce ROS formation, increase O2− elimination, decrease LDL-C oxidation (Suciu, 2009), inhibit the activities of oxidant enzymes such as Nox and myeloperoxidase and up-regulate the activities of antioxidant enzymes such as catalase and paraoxonase (Davignon et al., 2004, Fujita et al., 2007). Statins improve endothelial function by enhancing endothelial NO production, decreasing cholesterol, up-regulating post-transcriptional eNOS mRNA, reducing production of endothelin-1 and diminishing the affinity for angiotensin-I receptors. They were also shown to restore endothelial function by the mechanism independent of cholesterol reduction (Jasinska et al., 2007).
Atorvastatin (ATV), an extensively prescribed statin, attenuates endothelial lipid peroxidation (LPO) and ROS formation by inhibiting Nox activity (Najah et al., 2008). ATV was reported to decrease vascular Nox activity in hypertensive rats (Guimaraes et al., 2013). Statins inhibit ROS production in the rat VSMCs and vasculature by reducing Nox activity, decreasing expression of Nox subunits and inhibiting rac1 GTPase translocation from the cytosolic compartment to the cell membrane (Wassmann et al., 2002, Wassmann et al., 2004). Its anti-atherosclerotic effect is attributed to its antioxidant action that inhibits LDL oxidation (Sezer et al., 2011). Further, statins can protect the endothelial cells against the cytotoxic effect of NO by up-regulating eNOS and simultaneously down-regulating the iNOS activity (Wayman et al., 2003).
Recently we reported that subchronic exposure to arsenic through drinking water caused inflammation and dysfunction in rat aorta and ATV ameliorated this arsenic-induced vascular dysfunction (Kesavan et al., 2014). Considering the importance of these findings, we desired to extend our study by examining the involvement of free radical mechanism in the arsenic-induced vascular dysfunction and the protective effect of ATV. We assumed that ATV could be a therapeutic measure against the oxidative stress-mediated vascular disorders induced in the chronically arsenic-exposed subjects. We evaluated whether ATV can reduce the arsenic-induced rise in blood pressure, an indicator of vascular functional disorder, and dyslipidemia, a metabolic disorder that can predict cardiovascular risk. Further, we sought to understand whether the ameliorative effect could relate to attenuation of disruption in vascular redox homeostasis.
Section snippets
Experimental animals
The study was conducted in the apparently healthy adult male Wistar rats (100–120 g) procured from the Laboratory Animals Resource Section of the Institute. All animals were housed in polypropylene cages with chopped wheat straw as the bedding material. They were maintained under standard management conditions and handled as per the Institute Animal Ethics Guidelines. Rats were given standard rat pellet feed (Amrut Feeds, Pranav Agro Industries Ltd., New Delhi, India) and provided water ad
Normalization of blood pressure with ATV in the arsenic-exposed rats
Fig. 1 depicts the normalization of SBP (Fig. 1A), DBP (Fig. 1B) and MABP (Fig. 1C) with ATV treatment in the arsenic-exposed rats. ATV alone reduced the SBP in the 11th and 13th weeks, DBP in the 11th and 12th weeks, while MABP at the 11th week. Arsenic exposure significantly increased the SBP from the 6th week onwards, but DBP and MABP from the 7th week onwards. In the arsenic-exposed rats, ATV brought back the SBP, DBP as well as MABP to their respective control level from the 11th week
Discussion
We evaluated whether ATV can attenuate the arsenic-induced increase in blood pressure, the most common cardiovascular disorder, and the altered blood lipid profile, a strong predictor of CVD, and also whether it could mechanistically be associated with alterations in NO and ROS signaling. Our findings demonstrate that arsenic produced systemic hypertension and dyslipidemia with concomitant augmentation in nitrosative and oxidative stress in rat aorta. ATV normalized the arsenic-mediated rise in
Conclusion
In summary, arsenic produced systemic hypertension and dyslipidemia associated with nitrosative as well as oxidative stress in rat aorta. Arsenic also reduced the aortic antioxidant status. It appears that ROS-induced redox signaling played a major role in the oxidative mechanism of aortic dysfunction, while RNS-mediated pathway had a minor contribution. ATV regularized the blood pressure and improved the lipid profile. And these ameliorative effects of ATV may relate to improvement in aortic
Declaration of interest
The authors declare that there are no conflicts of interest.
Funding
This work was funded by the Department of Biotechnology (DBT), Ministry of Science and Technology, Government of India, New Delhi (vide order No. BT/PR15292/BRB/10/911/2011 dated 28th August 2012).
Acknowledgments
The Institute Research Fellowship awarded to the first author by the Indian Council of Agricultural Research (ICAR), New Delhi, is gratefully acknowledged. The authors are thankful to the Director, Indian Veterinary Research Institute, for providing necessary facilities.
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