The possibility that ingestion of alcoholic beverages might be associated with cardioprotective effects was first appreciated by consideration of epidemiologic data indicating that long-term, regular consumption of red wine at low to moderate levels (1 to 3 drinks per day for months to years) correlated with decreases in the incidence of coronary artery disease as well as improved survival in patients suffering myocardial infarctions (
Das and Ursini, 2002;
Korthuis, 2004). Although the antioxidant properties of red wine constituents were originally thought to largely explain its cardioprotective effects, subsequent epidemiologic evidence has indicated that consumption of white wine, beer, or spirits is also beneficial (
Das and Ursini, 2002;
Korthuis, 2004;
Shigematsu et al., 2003). The latter observations pointed to the importance of alcohol
per se in the beneficial actions of alcohol intake on cardiovascular mortality. While subsequent studies established that the cardioprotective effects of moderate alcohol consumption could be partially explained by effects on plasma lipids, platelet function, and fibrinolytic activity, alcohol intake maintains a significant association with reduced cardiovascular mortality even after controlling for lipoprotein and hemostatic factors. More recent work indicates that alcohol administration 24 hrs prior to I/R (referred to as antecedent alcohol, equivalent to alcohol preconditioning, but with trace residual alcohol levels during I/R) induces the development of an anti-inflammatory phenotype in postcapillary venules such that these vessels fail to express P-selectin or support leukocyte adhesion and emigration in postischemic tissues (
Dayton et al., 2005;
Gaskin et al., 2007;
Kamada et al., 2004;
Wang et al., 2007;
Yamaguchi et al., 2007). In addition, alcohol consumption at low to moderate levels is associated with reduced C-reactive protein levels, a plasma marker of inflammation, and decreased production of inflammatory cytokines such as interleukin-6 (
Albert et al., 2003;
Das and Ursini, 2002;
Korthuis, 2004;
Korthuis, 2006). Given the critical importance of infiltrating leukocytes in the pathogenesis of atherosclerosis and I/R injury, these new observations provide novel insight regarding the mechanisms whereby alcohol ingestion reduces the likelihood and extent of I/R injury in individuals at risk for cardiovascular disease.
The anti-adhesive effects of antecedent alcohol intake on postischemic leukosequestration has been extensively studied using intravital microscopic approaches, which allow direct quantification of leukocyte rolling, stationary adhesion, and emigration in single postcapillary venules in real time. Yamaguchi and coworkers (
Yamaguchi et al., 2007) were the first to demonstrate that acute intubation of a low-moderate dose of alcohol in mice (>0.5 g/kg; ~300 μl vol.) completely prevents adhesive interactions between circulating leukocytes and the endothelium from developing in tissues exposed to subsequent I/R. The temporal expression of this protected or preconditioned state induced by alcohol ingestion was biphasic. The initial phase (acute alcohol preconditioning) develops rapidly, involves activation of pre-existing effector molecules, and is short-lived, with peak anti-inflammatory effects (50% reduction in leukocyte adhesion) occurring 2 to 3 hours after intake, and then disappearing by 4 hours after administration. The second or delayed phase of ischemic tolerance induced by alcohol emerges 12 to 24 hours after alcohol intake, is longer-lived (24 hrs or more), requires the expression of new gene products, and is notable for its magnitude of protection, completely preventing postischemic leukocyte rolling and adhesion (
Kamada et al., 2004;
Korthuis, 2006;
Yamaguchi et al., 2007). In these studies, plasma alcohol levels peaked at 45mg/dL within 30 minutes of gastric instillation by gavage and returned to control levels within 60 minutes of ingestion. This observation indicates that neither the acute phase of protection that arises within 1 to 3 hours of alcohol ingestion nor the late phase that reemerges 12 to 24 hours later are due to direct effects of alcohol. Indeed, alcohol must be largely absent before the cardioprotective effects of the alcohol become apparent.
Because late phase alcohol preconditioning exerted much more powerful and long-lived effects, most subsequent studies have focused on elucidation of the mechanistic underpinnings for this latter phase. As a result of this work, it has become apparent that the factors that contribute to the development of tolerance to ischemia can be conceptually compartmentalized into: 1) triggers that inaugurate entrance into the anti-inflammatory phenotype, 2) downstream signal transducers that are activated by the initiating factors and induce an increase in activity or expression of 3) end-effectors that mediate the anti-adhesive effects noted during I/R 24 hrs after ingestion (
Kamada et al., 2004;
Korthuis, 2006).
Early work in this area focused primarily on uncovering the initiators of alcohol preconditioning, as this may provide important clues towards the development of rational therapeutic interventions that could be used in lieu of alcohol to induce similar cardioprotective effects, but avoid potential untoward physiologic and social effects associated with alcohol consumption. The well-known effect of alcohol to inhibit nucleoside transporters in cell membranes (thereby limiting adenosine reuptake into cells) led to the postulate that adenosine might serve as a trigger for entrance into the anti-inflammatory state induced by adenosine. Pharmacologic inhibitor studies support this concept and indicated that adenosine A2 receptor occupancy is required for this process (
Yamaguchi et al., 2007). Uncovering the role of adenosine A2 receptors was an important mechanistic distinction as the downstream signaling elements activated by adenosine A1/A3 versus A2 receptors are very different. Indeed, adenylyl cyclase or protein protein kinase A blockade, but not PI3 kinase inhibitors, prevents the late phase of alcohol preconditioning (
Kamada et al., 2004;
Korthuis, 2006). On the other hand, administration of adenosine A2 receptor agonists, cell-permeant cyclic AMP analogs, or adenylyl cyclase activators (e.g., isoproterenol, forskolin) mimics the postischemic anti-inflammatory effects of late-phase alcohol preconditioning.
Because alcohol enhances both basal and flow-stimulated NOS activity and NO production in vivo and in cultured endothelial cells, it has been suggested that production of this gaseous monoxide during the period of alcohol exposure may also serve as an important triggering element for the late phase of alcohol preconditioning (
Wallerath et al., 2003;
Yamaguchi et al., 2002;
Yamaguchi et al., 2007). Support for this concept is derived from four lines of evidence. First, administration of NOS antagonists just prior to, but not 1 hour after, alcohol administration on day 1 abolishes the anti-adhesive effects of late alcohol preconditioning on day 2 (
Yamaguchi et al., 2002;
Yamaguchi et al., 2003;
Yamaguchi et al., 2007). The latter observation supports the concept that NO plays an important role in promoting the development of the anti-inflammatory phenotype that becomes apparent 18 to 24 hours later. Second, plasma levels of nitrite/nitrate, a marker for NO production, are increased during the period of alcohol exposure (
Yamaguchi et al., 2007). Third, tissues pretreated with NO donors in lieu of alcohol develop an antiinflammatory phenotype 24 hours after administration (
Yamaguchi et al., 2002;
Yamaguchi et al., 2007). Finally, the anti-inflammatory phenotype induced by alcohol does not appear in mice that are genetically deficient in eNOS (
Yamaguchi et al., 2007). This last finding not only provides the fourth line of evidence supporting a role for NO as a triggering element in alcohol preconditioning, but also indicates that the eNOS isoform is essential for the development of the anti-inflammatory phenotype in response to alcohol. Interestingly, NO donors remain effective as preconditioning stimuli in eNOS-deficient mice, indicating that downstream mediators of preconditioning remain effective in these mice.
As mentioned in an earlier section of this workshop summary, the factors responsible for increasing eNOS activity in late alcohol preconditioning (or any form of preconditioning for that matter) are largely unknown; however, it now appears that this effect is triggered by an adenosine A2 receptor dependent mechanism (
Yamaguchi et al., 2002;
Yamaguchi et al., 2007). This notion is supported by the observations that ligation of adenosine A2 receptors increases the activity of cAMP-dependent kinase (PKA), which in turn activates eNOS by phosphorylating Ser-1177. Moreover, adenosine stimulates l-arginine transport and NO biosynthesis by activation of A
2 receptors on human umbilical vein endothelial cells. More recent work has established that AMP kinase activation may also play a critical role as an upstream signaling step in eNOS-dependent preconditioning by alcohol (
Gaskin et al., 2007).
Another well-known effect of alcohol is to increase the generation of reactive oxygen species, including superoxide and the hydroxyethyl radical (
Das and Vasudevan, 2007). Although they are generally considered to exert deleterious effects in biologic systems, it is becoming increasingly apparent that reactive oxygen species may participate in a number of normal physiologic phenomena by serving as second messengers in transmembrane signaling processes. Indeed, administration of a cell-permeant superoxide dismutase (SOD) mimetic, the porphyrin MnTBAP, coincident with alcohol prevents the postischemic antiadhesive effects that become apparent 24 hours after ingestion of the alcohol (
Yamaguchi et al., 2003). Moreover, exposing postcapillary venules to a superoxide generating system (hypoxanthine/xanthine oxidase) 24 hours prior to I/R mimicked the antiadhesive effects produced by antecedent alcohol exposure. Additional support for the concept that oxidants may participate in triggering the development of the anti-inflammatory phenotype in response to antecedent alcohol ingestion is derived from studies directed at elucidating their source of production. Inhibition of either xanthine oxidase or NADPH oxidase alone attenuated the anti-adhesive effects of alcohol preconditioning by 50 percent, whereas concomitant inhibition of both oxidant-producing enzymes effectively prevented the development of the protected phenotype (
Yamaguchi et al., 2003). The latter studies indicate that xanthine oxidase and NADPH oxidase are important enzymatic sources of the reactive oxygen species that trigger entrance into the anti-inflammatory phenotype displayed by postcapillary venules exposed to alcohol 24 hours prior to I/R. These observations have been extended to stroke models where antecedent alcohol has been shown to reduce I/R-induced delayed neuronal death, neuronal and dendritic degeneration, oxidative DNA damage, glial cell activation and neutrophil infiltration. These beneficial actions of alcohol preconditioning were prevented by treatment with a SOD mimetic or an NADPH oxidase inhibitor (
Wallerath et al., 2003).
As noted earlier, there is evidence implicating NO, formed secondary to adenosine A
2-receptor-dependent activation of endothelial NOS, in the beneficial actions of antecedent alcohol ingestion. This raises the possibility that NO produced during the period of alcohol preconditioning initiates the protective effects of late alcohol-PC by a mechanism that involves its interaction with xanthine oxidase- and/or NAD(P)H oxidase-derived oxidants. This is an important issue because isoform-selective PKC translocation and activation appears to play a critical role as an obligatory downstream signaling element in late alcohol preconditioning (
Dayton et al., 2005). However, NO and NO-releasing agents reversibly inactivate PKC. On the other hand, peroxynitrite, which is formed by the interaction of NO with superoxide, not only induces PKC activation, but has been implicated as a trigger for the beneficial actions of other forms of preconditioning including that induced by antecedent exposure to brief ischemia. Taken together, these observations suggest that reactive NO species formed secondary NO/superoxide interactions may be required to induce the development of alcohol preconditioning (
Dayton et al., 2005;
Yamaguchi et al., 2003).
Although the mediators of late phase alcohol preconditioning are unclear, the time course required for its development suggests that the appearance of the protected phenotype 18 to 24 hours after alcohol ingestion requires the formation of new gene products capable of producing anti-inflammatory agents. In this regard, it is tempting to speculate that alcohol might enhance the expression of cyclooxygenase-2, inducible NOS (iNOS), or heme oxygenase-1 (HO-1), which in turn generate prostacyclin and other eicosanoids, NO, and carbon monoxide, respectively, all of which produce robust antiadhesive effects in postcapillary venules. In addition to carbon monoxide, HO-1 also generates the powerful antioxidants bilirubin and biliverdin, which may act to prevent the formation of oxidant-dependent chemoattractants during I/R. It is also possible that alcohol may exploit the anti-adhesive properties of adenosine as a mediator of the preconditioned state secondary to enhanced production or decreased salvaging (via activation of 5′-nucleotidase or inhibition of adenosine kinase, respectively) of the nucleoside in postischemic tissues. The fact that an anti-inflammatory phenotype does not become apparent in mice treated with a pharmacologic HO inhibitor during reperfusion and fails to develop in HO-1 knockout mice suggests that this protective protein may serve as a major effector.