Elevated plasma cholesterol levels play a dominant role in cardiovascular diseases
] as can be evidenced by the occurrence of atheroma in the aorta of hypercholesterolemic rabbits
]. Accordingly, the AI, which reflects the relationship between the non-HDL cholesterol fraction and the HDL cholesterol fraction and is considered a possible indicator of a predisposition to heart disease
], was directly correlated to the increase in LDL levels. The injury theory of atherosclerosis holds that circulating LDL accumulates at susceptible sites where the oxidation of its protein and lipid components takes place, generating LDLox
]. LDLox, in turn, participates in the inflammatory processes, contributing to lesion progression
]. Our results corroborate this theory, since elevated LDLox levels were found in patients from the high-LDL group. Accordingly, lipid oxidation (TBARS) was also increased in patients from the high-LDL group and we observed a positive correlation between TBARS and LDLox, reinforcing that TBARS levels are at least partially associated to LDLox levels. In contrast to lipid oxidation, protein oxidation seems to start even when LDL levels are considered clinically acceptable, because the protein carbonyl content was increased in the intermediate-LDL group. The delay in the increase of LDL oxidation in the intermediate-LDL group may be related to a protective effect of LDLoxAB that was increased in this group compared to the low-LDL group. This idea is supported by previous studies which demonstrated that even minor modifications of native LDL render it immunogenic and that humoral antibodies are specific for the modifications to apo B
]. These findings indicate that circulating antibodies respond to oxidative modifications in the protein moiety of LDL. In addition, humoral antibodies to modified LDL may redirect its site of degradation, primarily from plasma to the reticuloendothelial cells of the liver from rabbits
]. Thus, LDLoxAB could enhance the removal of LDLox from serum and prevent its entrance into the arterial wall. In agreement, an inverse relationship between LDLoxAB and LDLox levels in healthy subjects is in line with the idea that LDLoxAB has a role in the clearance of LDLox from the circulation
]. Moreover, at low rates of LDL oxidation, it is possible that an immune response might lead to an accelerated macrophage uptake of LDLox and thereby it may play a protective role. When the rate of oxidation is enhanced, however, the immune response might play a pathogenetic role, leading to accelerated macrophage uptake that overwhelms the capacity of the macrophage to handle the ingested LDLox
]. Thus, we can speculate that the rate of oxidation in the intermediate-LDL group is low and LDLoxAB are still able to counteract LDLox, whereas in the high-LDL group this protection did not occur.
SOD activity increased along with the increase in LDL levels among groups and this increase occurred most likely to counteract the superoxide anion overproduction caused by HC
]. The increase in SOD activity, along with the increase in protein carbonyl and LDLoxAB levels, even when LDL levels are considered clinically acceptable; indicate that oxidative stress is an early event in the evolution of hyperlipidemia.
The enzyme TrxR-1 along with protein Trx-1 has been recognized as an essential component for cellular redox control and antioxidant defense
]. In this study, we demonstrated for the first time the behavior of serum TrxR-1 activity during different stages of HC in humans, although an elevation in serum reduced Trx-1 has already been described in patients with HC
]. Additionally, we have recently demonstrated an increase in serum TrxR-1 activity in hypercholesterolemic rabbits
]. However, in the present study, we observed no statistically significant differences in TrxR-1 activity among groups. The thioredoxin system (TrxR-1 and Trx-1) acts as a H2
scavenging system, reducing H2
]. Besides scavenging H2
, TrxR-1 can indirectly maintain the reduced state of many proteins by regenerating oxidized Trx-1 to its reduced form
]. Thus, we can speculate that the lack of changes in TrxR-1 activity in the intermediate-LDL group resulted in the increase of protein oxidation in this group. However, no association was found between TrxR-1 activity and protein carbonyl levels. Despite the unchanged activity of TrxR-1 among groups, we found a weak, but significant correlation between TrxR-1 and SOD activities, which could indicate a role for TrxR-1 in the removal of H2
produced by SOD. Interestingly, in the high-LDL group, SOD activity had a prominent increase over TrxR-1 activity, as measured by the increased SOD/TrxR-1 ratio. This imbalance in SOD/TrxR-1 activities suggests that TrxR1 activity could be lacking to efficiently remove the H2
produced by SOD. Because the SOD/TrxR-1 ratio was positively correlated with TBARS, LDLox and LDLoxAB levels, we can speculate that the imbalance in SOD/TrxR1 ratio could have deleterious effects in hypercholesterolemic subjects due to a peroxide overload. However, this proposal deserves further investigation because glutathione peroxidase and catalase, which are other important enzymes to remove H2
in plasma, were not evaluated in the present study. In fact, an imbalance between SOD and catalase activities has been well documented in various oxidative conditions
], but to the best of our knowledge, this is the first report about an imbalance between SOD and TrxR-1 activity. Because the SOD/TrxR-1 ratio was associated with the AI, we can suggest a possible role of the SOD/TrxR-1 ratio as a marker of cardiovascular events during HC.
Similar to TrxR-1, the behavior of PON1 activity at different stages of HC has not been demonstrated yet. PON1 is believed to be responsible for the antioxidant effects of HDL
] and its activity is inversely associated with the progression of atherosclerosis and the incidence of coronary artery disease
]. In agreement, previous results from our group revealed diminished serum PON1 activity along with atherosclerotic plaque in the aorta of hypercholesterolemic rabbits
]. Moreover, a large prospective study pointed low serum PON1 activity as an independent risk factor for coronary events in men at high risk of coronary heart disease
]. However, we did not found differences in PON1 activity among the three studied groups or significant correlations between LDL levels and PON1 activity. This could be explained by an established balance between PON1 inactivation by oxidant species
] and PON1 elevation in order to provide a protective mechanism against oxidant species and atherosclerotic plaque progression
]. These differences in PON1 activity between previous studies and ours can be due the fact that studies concerning PON1 and cardiovascular diseases usually evaluated the relationship between serum enzyme activity and plaque formation
] or cardiovascular events such as myocardial infarction
]. Here, we evaluated the relationship between PON1 activity and another known predicting factor for cardiovascular diseases, the LDL levels. Moreover, PON1 seems not play a role in early atherosclerosis, although it may play a role in a later stage of cardiovascular diseases
]. Since the atheroma formation was not evaluated in this study, we can speculate that unchanged PON1 activity and the lack of associations between PON1, LDL and LDLox levels are most likely because LDL levels evaluated in this study corresponded to early stages of atherosclerosis.
The involvement of oxidative stress in the initiation and progression of the inflammatory process is well documented
]. Accordingly, we observed a significant correlation between LDLox and hs-CRP, which is an inflammatory marker. Moreover, serum hs-CRP levels were increased in the high-LDL group. Despite the changes in SOD and SOD/TrxR-1 ratio that accompanied the increase in LDL levels, no relationship was found between hs-CRP levels and antioxidant enzymes. This lack of correlation has already been described in the serum of patients with rheumatoid arthritis, although both parameters had been changed by the disease
]. Although the extracellular reduced Trx-1 has proinflammatory effects by potentiating cytokine release from fibroblasts and monocytes
], our results reveal that TrxR-1 seems to have no relationship with the inflammatory response during HC.
In conclusion, the present study demonstrated that some oxidative events initiate even when LDL levels are clinically acceptable. Moreover, hypercholesterolemic patients show an imbalance in SOD/TrxR-1 activities, which may play a role in the oxidative stress, because it is positively associated to LDL oxidation.