Introduction
Chronic obstructive pulmonary disease (COPD) is a major incurable global health burden and is currently the 4th largest cause of death in the world (WHO, 2018). Approximately 50% of COPD patients will die from a cardiovascular event (Sin & Man, 2005; TLF, 2017) and consequently the pathobiological mechanisms linking COPD to cardiovascular disease are now an area of intensive research. Each puff of cigarette smoke contains >1016 free radicals per puff, driving oxidative stress and tissue damage (Bartalis, Chan & Wooten, 2007) This oxidative stress and inflammation, have been shown to alter pulmonary blood vessel structure, through driving vascular remodelling, and promoting arterial stiffness and atherosclerosis (Sin, Anthonisen, Soriano & Agusti, 2006). Vascular tone is controlled by vasoactive substances such as NO and prostaglandins, their secretion which can be maintained by circulating oxygen levels (Chan & Vanhoutte, 2013). Under hypoxic conditions like those seen in COPD, dysregulation of this vascular homeostatic balance occurs, due to oxidative damage to the vascular endothelial cells (VECs) leading to impaired NO production, thereby promoting endothelial dysfunction (Chan & Vanhoutte, 2013). NO is a key vasodilator produced by endothelial nitric oxide synthase (eNOS). Under normal physiological conditions, increased sheer stress on the vascular endothelium stimulates mechanosensitive ion channels, triggering a rapid influx of Ca2+ into the cytoplasm of the VECs. This increases eNOS activity via myoendothelial gap junctions, that transmit vasodilatory NO signals to the underlying smooth muscle cells. VECs appear to be sensitive to oxidative damage, which may be the result of the conversion of NO to peroxynitrite (ONOO-) in the presence of the harmful ROS; superoxide (O2-) that ultimately reduces vascular NO bioavailability (Endemann & Schiffrin, 2004; Kolluru, Bir & Kevil, 2012; Tabit, Chung, Hamburg & Vita, 2010). An altered oxidative balance in VECs has been demonstrated to promote cardiovascular manifestations such as atherosclerosis, myocardial infarction (MI) and stroke (Brassington, Selemidis, Bozinovski & Vlahos, 2019; Endemann & Schiffrin, 2004; Kolluru, Bir & Kevil, 2012; Tabit, Chung, Hamburg & Vita, 2010).
Although cardiovascular comorbidities are the largest cause of mortality in COPD, the detrimental effects of CS and its associated oxidative stress on the systemic vasculature remain largely unknown. Given the deleterious role of oxidative stress in COPD, antioxidant treatment may be a viable therapeutic approach to treat the cardiovascular manifestations associated with this disease. We have previously shown that the antioxidant ebselen (2-phenyl-1,2-benzisoselenazol-3(2H)one), an organoselenium glutathione peroxidase (Gpx) mimetic, inhibits CS-induced lung inflammation in mice (Duong, Seow, Bozinovski, Crack, Anderson & Vlahos, 2010). Ebselen treatment may also be an effective therapeutic in chronic diseases such as atherosclerosis, thrombosis, and stroke where oxidative stress and inflammation play a crucial role (Azad & Tomar, 2014; Sarker et al., 2003; Sarma & Mugesh, 2008; Takasago, Peters, Graham, Masayasu & Macrae, 1997). Moreover, studies have shown that Gpx-1 deficient mice have enhanced pulmonary inflammation (Duong, Seow, Bozinovski, Crack, Anderson & Vlahos, 2010), as well as worsened cardiovascular outcomes including a larger infarct volume following ischemic stroke (Crack et al., 2001; Duong, Seow, Bozinovski, Crack, Anderson & Vlahos, 2010), suggesting that Gpx-1 (or compounds which mimic its actions, like ebselen) may exhibit protective effects.
Of interest, Gpx-1 activity has been shown to be elevated in smokers as a potential mechanism to counteract the harmful oxidative stress. However, Gpx-1 has been reported to be severely depleted in the lungs of COPD patients, resulting in an overstated inflammatory response and oxidative burden (Kluchova, Petrasova, Joppa, Dorkova & Tkacova, 2007; Santos et al., 2004; Tkacova, Kluchova, Joppa, Petrasova & Molcanyiova, 2007; Vlahos et al., 2010). A study by Chew et.al has showcased the therapeutic effect of ebselen in Gpx-1 knockout mice, with the study finding that synthetic repletion of Gpx activity in these diabetic mice produced athero-protective effects in vivo (Chew et al., 2010). Exogenous repletion of Gpx-1 with compounds like ebselen may have therapeutic potential in not only treating the pulmonary manifestations of COPD but perhaps its cardiovascular comorbidities.
In the present study we investigated whether chronic CS exposure in a preclinical mouse model of COPD impairs vascular function and whether ebselen can prevent CS-induced vascular dysfunction in mice.