The effects of smoking on endothelial cell function

Cigarette smoking is an important risk factor for cardiovascular disease which adversly affects the vasculature involving both coronary and peripheral arteries. The vasoregulatory function of endothelial cells (ECs) (as indicated by reduced endothelial-dependent vasodilatation (EDV) in vivo) is one of the earliest to be affected. Nitric oxide (NO) is primarily responsible for vasoregulatory and homeostatic functions of ECs. The exact mechanism(s) for smoking-mediated NO dysfunction and its effect on EC-derived homeostatic factors are unclear. The majority of earlier in vitro studies have used either crude cigarette smoke extract or one isolated component of cigarette smoke to examine the effects of smoking on endothelial cells. These models were not physiological. Furthermore, ECs from different vascular beds may show distinct reactions to different pathophysiological stimuli. In the first phase of the project, EDV in vivo was correlated with the NO biosynthetic pathway in vitro. Flow-mediated EDV of the brachial artery was measured, and human umbilical vein ECs (HUVECs) were exposed to serum from the same individuals, followed by in vitro measurement of basal (12 hour)- as well as stimulated-NO production, endothelial nitric oxide synthase (eNOS) expression and eNOS activity. A significant correlation was found between in vivo EDV and stimulated-NO levels in vitro Smokers showed significantly decreased EDV as well as decreased NO levels and eNOS activity but increased eNOS expression in vitro. In vitro, NO levels positively correlated with eNOS activity but negatively correlated with eNOS expression indicating that the mechanism(s), which decreased NO levels in smokers, also increased the eNOS expression. However, the activity of the latter was abnormal. It was also observed that in vivo as well as in vitro NO biosynthesis was similarly dysfunctional in light ( one pack/day) smokers, suggesting that exposure to relatively small amounts of smoke initiated unknown cellular mechanism(s) to overwhelm the NO biosynthesis. Additionally, HUVECs exposed to smokers' serum showed decreased fibrinolytic and antithrombotic activity in culture. The second phase of the project was performed using human coronary artery ECs (HCAECs) in order to extend the previous findings to an arterial EC line. In addition, specific free radical scavengers were used to investigate the role, as well as the source of, free radicals in the present model. Smokers' serum induced similar alterations in NO biosynthesis in HCAECs as in HUVECs. It was found that reactive oxygen species (ROS), specifically hydrogen peroxide, was responsible (at least in part) for up-regulation of eNOS. However, this upregulation did not translate to increased activity possibly due to a direct effect of other ROS such as superoxide and peroxynitrite on eNOS. In addition, it was demonstrated that endogenous free radical sources such as uncoupled eNOS, the mitochondrial electron transport chain and xanthine oxidase were involved in NO dysfunction in HCAECs exposed to smokers' sera. The present project adopted a novel approach to investigate the mechanism of reduced EDV and NO biosynthesis in smokers. Data from this project confirm that reduced EDV in smokers is associated with dysfunctional NO biosynthesis and ROS play a central role in this dysfunction. Additionally, these data suggest that the ROS in smoking may arise from both exogenous and endogenous sources and may have a paradoxical effect on eNOS protein expression and activity. Finally, smoking also appears to decease the availability of EC-derived fibrinolytic and anti-thrombotic factors.