In the present study, besides the higher LPO in DM2 patients, SOD activity was shown to be increased in such individuals. Some studies in patients with DM2 have revealed a decrease in antioxidant defenses and an increase in oxidative damage markers, especially against complications associated with DM2 [26
]. However, similar results to ours were found by Moussa [29
], who observed that SOD, in erythrocytes, showed the highest activity in type 1 and type 2 diabetics and an increase in LPO, assessed by MDA quantification, suggesting an augmented production of RONS in such patients. Savu et al. [30
] also found high total antioxidant capacity and higher concentration of TBARS, in patients with DM2. In this direction, Kimura et al. [31
] found elevated concentrations of a variant of extracellular SOD (EC-SOD) in DM2 patients compared to the controls, with a positive correlation between the levels of EC-SOD and micro- and macrovascular complications.
The presence of oxidative damage, despite the increase in total SOD activity, could be explained by different cellular responses to the OS [10
], due to several factors, being related both to the depletion of antioxidant defenses and to the increased production of RONS, such as the high production of superoxide anion radical (•
), the presence of toxins or even more, the excessive activation of the natural systems of production of these reactive species, such as activation of phagocytic cells in chronic inflammatory diseases, as observed in DM [10
]. Thus, the increase in total SOD activity, observed in the present study, suggests a possible adaptive response, probably due to the increased production of the •
, which would lead to an augmentation in the production of H2
. This mechanism, by its turn, would probably require a higher activity of CAT and GPx. Nevertheless, in this study, no significant differences were detected in the activities of these enzymes in any of the groups (). In this sense, the higher SOD/CAT ratio in the DM2 versus controls, DM2(−)SAH versusC(+)SAH, and DM2LPO versusC(+)SAH may suggest an imbalance between SOD and CAT activities in these groups, which, in turn, could indicate the increase in H2
production. The latter reactive species, when in high concentrations, had been associated with lesions in the pancreatic beta cells, causing disturbance both in cell signaling and gene expression [32
]. However, the present study did not include the quantification of H2
In the absence or inefficiency of defenses against RONS, OS would occur, leading to the activation of cellular mechanisms involved in the stress response, such as NFkB, p38MAPK, and JNK/SAPK, which would stimulate the production of inflammatory cytokines involved in diabetic complications and in pancreatic beta cells dysfunction, thus intensifying defective insulin production [33
]. In this context, increased production of RONS by mitochondria becomes deleterious for cell function, once species such as H2
can cross the mitochondrial membranes, damaging macromolecules in diverse cellular structures [4
]. It is well established that impaired cell glucose metabolism affects mitochondrial function and enhances reactive species production [34
], and this seems to be implicated in the cases of IR and endothelial dysfunction which results in the persistence of the metabolic imbalance observed in DM carriers [5
]. In this scenario, hyperglycemia has been described as a generator of OS [35
]. In accordance with such description, Yang et al. [36
] observed greater serum LPO, using the marker MDA, in hyperglycemic mice, verifying that this increase exacerbated the occurrence of myocardial infarction through NADPH oxidase activation. A study by Marfella et al. [37
] showed that acute hyperglycemia increased the OS, via an increase in the nitrotyrosine levels by a peroxynitrite-independent mechanism, during hyperglycemic clamp, even in individuals without diabetes. These data corroborate the results described after the individual assessment of DM2 carriers in this study, in which two distinct groups were observed, one with elevated LPO (DM2LPO) and poor glycemic control and another with proper glycemic control and LPO similar to that observed for the remaining groups. Interestingly, these results were independent of the presence of SAH. In these patients, the observed correlations between FG and HbA1c with LPO, especially for levels above 5.6
and 6.5%, respectively, are equally indicators of the critical role of the glycemic control in the occurrence of LPO. Thus, it is plausible to suggest that the evident lack of glycemic control, despite the use of pharmaceuticals for this purpose, may promote oxidative stress through NADPH oxidase activation, subsequently increasing •
, which could account, at least partially, for the redox imbalance identified in the case group, as observed by its augmented LPO, the latter followed by increased SOD activity ().
The increased degree of IR, observed in this study from HOMA-IR (), perhaps represents a compensatory mechanism, protecting adipocytes and muscle cells from OS, which is a key element in the pathogenesis of IR in pancreatic beta-cell dysfunction and in hypertension [7
]. Furthermore, it was observed a positive correlation between this marker and TBARS in the DM2LPO group, which is corroborated by studies that have demonstrated the association between IR and OS [38
] and with LPO markers [39
], further reinforcing the connection between glycemic metabolism and OS.
Regarding the transport proteins, high levels of TRF in diabetics and prediabetics, in comparison to the controls (), indicate a possible high concentration in the presence of OS. Furthermore, our findings show a significant difference in the levels of TRF between DM2(+)SAH versus pre-DM and C(+)SAH (). In contrast, these data differ from those observed by Memişoğullari and Bakan [40
], who found that the concentration of TRF was lower in diabetics, especially in those with cardiovascular complications. For CER, an increased concentration was observed only in the DM2(+)SAH group, in relation to the DM2(−)SAH group, with no difference between the other groups (). A study by Vasconcelos et al. (2011) showed a high concentration of CER in hypertensive patients [9
] but found no difference between the C(+)SAH and C(−)SAH groups. A possible explanation for such apparent discrepancies may be the dependency on diverse factors which directly interfere in the metabolic state, such as age, sex, individual dietary profile, time course of the disease, and presence of comorbidities.
In this sense, the groups assessed presented similar patterns for age, body fat distribution, BMI, and lipid profile, as well as proportionality between genders. These are important parameters since some of them can be related to OS, besides being related to several diseases. In older groups (40–69 years), different concentrations for MDA between genders had been already described [41
]. Concerning body fat distribution, the waist circumference (WC) was assessed, being a risk factor for DM2 and cardiovascular diseases (CVD), along with the BMI [42
]. In this context, obesity has been related to OS [38
]. Accordingly, D'Archivio et al. [38
] demonstrated that obese individuals with IR present a profile for the OS similar to that observed in patients with diabetes. Thus, different cardiometabolic parameters are likely synergistically acting, evoking the environment of OS evidenced in DM carriers, as reinforced by the present study. Another substantial factor associated to OS in human populations is smoking. Block et al. [44
] observed that the MDA levels were higher in smoker adults (19–78 years) than in nonsmokers, the same not being observed for another marker, the F2
-Isoprostane. Notwithstanding, smoker subjects were excluded in the present study.
It is worth noting that in 90% of diabetic patients using oral anti-diabetic agents and about 90% of hypertensive diabetics using antihypertensive drugs (), pharmaceuticals with proven antioxidant action, the presence of oxidative damage was still found, so that the possible attenuation of the medication was not sufficient to prevent the occurrence of OS. The probable antioxidant effect of oral antidiabetic agents, especially metformin [45
], and of antihypertensive drugs has already been described [46
]. Metformin analyzed in vitro
, for example, was able to react with •
OH, but not with •
]. Of the other medications used by some study participants, ASA (aspirin) showed a protective effect against cytotoxicity of H2
on endothelial cells of animals in vitro
], with reduced levels of MDA in rabbits [49
]. In addition, statins may attenuate the OS in diabetics [50
]. Furthermore, Evans et al. [51
] demonstrated that insulin therapy improved OS in patients with DM2, probably through changes in the metabolism of free fatty acids. In this study, among the DM2 group, 10.9% were using statins, 27.3% were on insulin therapy, and 32.7% used ASA. Despite this, OS was still detected in these patients, highlighting the complexity of the metabolic pathways determinant of OS in DM, in which the regular use of well-known antioxidant drugs was not capable of inhibiting the establishment of the observed imbalance. Finally, even though the considered sample presents a relatively small size, the occurrence of statistical significance for some of the assessed variables points to the relevance of our findings, as indicators which deserve better attention in further evaluations from larger population studies, especially those related to the redox imbalance participation in determining or maintaining the metabolic alterations observed during the course of DM.