## 4 Discussion
Similar to the results about the dependence of NF-kB, Nrf2, and HO-1 expressions on age, here, data show that in the MetS subgroup with individuals with IMT \(\geq\) 0.90 mm, these genes were under-regulated, achieving the levels of the control group or even lower, leading to the establishment of statistical significance between the MetS subgroups with different CIMT (Figure 3). HO-1 is an enzyme induced by Nrf2, playing a critical role in antioxidant defence. It is considered beneficial due to its protective effect against oxidative damage [15]. HO-1 has been described as a key regulator of adaptive cellular response to oxidative stress and low-grade inflammation. It plays a role in cellular homeostasis through the strong antioxidant activity of bilirubin and the anti-inflammatory effect of carbon monoxide [34; 35]. In humans, HO-1 deficiency has been known to cause oxidative stress and endothelial cell injury [14]. In brief, upregulation of the HO-1 protein may represent an attempt to minimize endothelial injury [13]. The difference between the MetS subgroups (MetS with age \(<\) 55 and MetS with age \(\geq\) 55) was not significant, indicating that these parameters do not vary with age. Similarly, MDA, ADMA, and HO-1 plasma levels were significantly higher in the MetS subgroups based on IMT as compared to the controls and did not differ between the two MetS subgroups divided according to their CIMT (Figure 5). It has been proven that increased MDA levels as a marker of oxidative stress and lipid peroxidation and ADMA as an endogenous competitive inhibitor of nitric oxide synthase may lead to reduced eNOS activity and NO levels, contributing to ED in diabetes mellitus [36]. Asymmetric dimethylarginine may activate NF-B, which is responsible for the expression of endothelin-1 and other proinflammatory mediators, such as TNF-alpha, interleukin 6 (IL-6), free radicals, chemokines, and adhesives molecules, as well as inducible NO synthase (iNOS), contributing to endothelial injury [10; 16]. We showed that increased NF-kB expression corresponds to elevated plasma ADMA and MDA levels. In addition, the increased plasma ADMA concentrations were found to cause an increase in superoxide (O\({}^{2-}\)) production. Superoxide can combine with NO to form peroxynitrite [37], a radical with potentially deleterious effects on the vascular wall, which further contributes to the exacerbation of endothelial injury and the increase of IMT. The activation of the Nrf2/HO-1 signalling pathway plays a critical role in the antioxidant defence of endothelial cells in response to various inflammatory and oxidative stimuli [38]. Increased plasma HO-1 levels can inhibit oxidative stress and low-grade inflammation, thus improving endothelial function by enhancing NO levels [39]. It has been reported that the increased HO-1 and heme degradation products can improve endothelial function, at least in part, by compensating for the loss of NO bioavailability due to elevated plasma ADMA levels [39]. Our data indicate increased CIMT and elevated plasma ADMA and MDA levels, which are markers of oxidative stress in individuals with MetS. This is in accordance with other investigations [7; 8; 9; 40]. Increased ADMA levels, a marker of ED, and increased CIMT are among the early manifestations of atherosclerosis [41; 42]. Our data show that plasma HO-1 and CIMT are elevated in people with MetS, which corresponds with the findings of other authors [17]. High plasma HO-1 levels may reflect an increased oxidative stress condition and may be aimed at protecting against carotid atherosclerosis progression [18]. The imbalanced production of ROS and/or the damaged antioxidant defence system at an advanced age are probably the cause of the earlier development of morphological changes in the endothelium, registered by the increase of CIMT and the risk of late clinical complications and incidents [18].