Although previous studies have implicated elevated arginase activity and expression in atherogenesis (reviewed in 
), results of the present investigation demonstrate that an “atherogenic diet” can result in more widespread alterations in arginase activity and arginine metabolism than has previously been recognized. Major new findings of this study include demonstrations that the HC diet led to markedly elevated circulating arginase activity and reduced arginine bioavailability in both wild-type and apoE−/−
mice, and these changes occurred also in the apoE−/−
mice on the high fat diet. So far as we are aware, effects of high fat or high cholesterol diets on plasma arginase and global arginine bioavailability have not been evaluated in any previous studies. The elevation of plasma arginase activity correlated closely with increased plasma ALT levels, indicating that damage of the liver, which contains the greatest amount of arginase in the body, was primarily responsible for the elevated plasma arginase. If plasma arginase were derived significantly from tissues other than liver, it is probable that plasma values for ALT and arginase in apoE−/−
mice on the HC diet—the only condition in which arginase activity was significantly elevated in multiple tissues—would not correlate with plasma values for ALT and arginase in apoE−/−
mice on the HF diet. With the exception of a single outlier, however, the correlation was strong across all diet groups (). Increased plasma arginase following liver injury has been well-documented by other investigators 
. Although reductions in global arginine bioavailability in this study were consistently associated with elevations in plasma arginase activity rather than with increases in tissue arginase activity, this does not rule out the possibility that increased tissue arginase activity may additionally result in localized reductions in arginine availability that may impact vascular or tissue function.
Reductions in global arginine bioavailability are associated with cardiovascular complications and have been shown to be an independent risk factor for morbidity and mortality in humans 
. In sickle cell patients, reduced global arginine bioavailability is associated with pulmonary arterial hypertension 
, which leads to right ventricular hypertrophy that likely contributes to the increased mortality in this patient population. In non-sickle cell patients, reductions in global arginine bioavailability are associated with development of obstructive atherosclerotic coronary artery disease and increased incidence of major adverse cardiovascular events 
. Taken together, these findings raise the possibility that diet-induced reductions in global arginine bioavailability alone may increase risk for developing some types of cardiovascular/endothelial dysfunction that may not be readily apparent except perhaps at longer time periods of dietary treatment, even in animal models that do not develop significant atherosclerotic plaques within a few months (e.g., mice fed HF diets).
As noted previously, mice fed the HC diet exhibit an accelerated rate of atherosclerotic development and hepatic inflammation that is correlated with increased plasma cholesterol. Interestingly, the HF diet did not raise plasma cholesterol levels but did result in liver injury in the apoE−/− mice, as indicated by the elevated plasma arginase and ALT activities, but not in the C57BL/6J strain. In contrast to the HC diet, however, the HF diet did not lead to increased arginase activity in tissues of apoE−/− mice.
Although both C57BL/6J and apoE−/−
mice maintained on the HC diet, which results in accelerated plaque development 
, exhibited increases in plasma arginase and reductions in global arginine bioavailability, the apoE−/−
mice fed the HC diet were the only animals with increases in tissue arginase activity and expression. Although C57BL/6J mice on the HC diet also had cholesterol levels that were significantly increased, previous studies have shown that the HC diet does not induce significant atheroma formation within the two-month time frame used in this study 
. It is therefore of particular interest that apoE−/−
mice fed the same percentage cholesterol as the HC diet used in our study had reduced plaque development when the apoE−/−
mutation was expressed in a mouse strain that completely lacks expression of arginase II 
. The fact that multiple tissues in the apoE−/−
mice fed the HC diet had increased arginase activity and expression suggests that an atherogenic diet, via increased arginase expression, may have a broader impact on vascular and organ function than has previously been appreciated.
The fact that tissue arginase was elevated only in apoE−/−
mice on the HC diet raises the possibility that the mechanism(s) underlying increased arginase expression also may be part of the inflammatory process leading to atherosclerosis. It is known that arginase is increased in response to a variety of inflammatory stimuli 
, but the precise stimuli involved in arginase induction under atherogenic conditions have not been identified. Since activation of liver X receptors by cholesterol metabolites has been shown to induce expression of the arginase II gene 
, it is possible that cholesterol metabolites may themselves play a direct role in inducing arginase II expression in tissues of apoE−/−
mice fed the HC diet.
In summary, these findings demonstrate for the first time that increased plasma arginase activity and widespread tissue expression of arginase II–together with reductions in global arginine bioavailability–are associated with a diet that induces atherogenesis in apoE−/− mice. Further studies are warranted to elucidate the possible roles of increased circulating arginase, reduced global arginine bioavailability, and elevated tissue arginase activity in vascular dysfunction, as well as to identify the specific cells in tissues in which arginase expression was increased and to elucidate their impact on vascular function.