Oxidative stress plays a prominent role in the pathophysiology of MS. However, different studies of oxidative stress in pwMS have shown equivocal results so far. In this paper, we have analysed the values of antioxidant agents (SOD and GR activity, and TAC levels) and of products of lipid and DNA degradation (4-HNE and 8 OH-dG), finding differences in pwMS compared to healthy controls, except for GR activity. Our results indicate that pwMS have an increase in the activation of certain antioxidant systems, such as SOD, while others, such as TAC and the levels of degradation products of oxidative stress 4-HNE and 8 OH-DG, are lower than the control group. This is probably because the increase in oxidative stress in pwMS leads an excessive consumption of antioxidant components and, secondarily, a decrease in serum degradation products. In pwMS, the TAC levels were linked to age, time of evolution and a change in the speed of processing, while 4-HNE was associated with the time of evolution and CI.
Organisms have a series of defense mechanisms against pro-oxidant products, the most important of which are antioxidant enzymes21. SOD is part of the physiological response to oxidative stress17 and its activity is increased in active demyelinating lesions in comparison to normal-appearing white matter and control groups16,22. Our data, like those from previous studies, show increased activity in pwMS23–25. Not all results are homogenous, since other authors have observed decreased values in pwMS26. Ljubisavljevic et al. found greater SOD activity in patients with a lesser lesion load in T2 and suggest that this could be negatively related to EDSS27. In longitudinal studies the results are variable; some have observed a decrease in the relapse phase with a subsequent increase28, while Obradovic et al. observed that SOD activity did not change after the relapse, but that there was greater basal activity associated with a better prognosis24. As for the DMT, the effect of starting modifying drugs was studied with discordant results12,29.
Glutathione (GSH) is the most abundant antioxidant at the intracellular level and is fundamental for maintaining an appropriate balance at the cellular level. Its metabolism is involved in the development of various neurodegenerative diseases30. GR is a protein that triggers the regeneration of GHS. Prior studies are few and far between and show mixed results, in part due to the difference in the type of biochemical analysis. Jensen et al. measured the activity in erythrocytes, granulocytes and lymphocytes, finding a downward trend in pwMS without reaching significance31,32, while other groups have found elevated activity in CSF and serum of pwMS33,34. In our case, we did not observe differences between the two groups.
Another way of measuring the antioxidant response is via the TAC, a parameter that incorporates not just the intrinsic defense mechanisms of the organism, but also the compounds that we ingest as part of our diet and that have an antioxidant effect35. Its levels are modified by increased consumption in situations of greater oxidative stress or by increased intake of antioxidant foods. In our sample, its levels are lower in pwMS, results that are consistent with other published papers33,36, in which lower TAC levels have actually been observed in patients with higher EDSS23,36–38. Nonetheless, the results are heterogenous among studies39, such as the study carried out by Khajenobar et al. in which higher levels were observed in pwMS, both in terms of serum and CSF40. We obtained higher values in pwMS with greater age and more time of evolution, without this being correlated with other parameters, such as EDSS. This could indicate lower consumption over time due to a less effective response to oxidative stress. This disparity in the findings may be influenced in part by diet and the consumption of lipoic acid, among other things, since diet can be a confusing factor that has not been measured in the majority of studies, nor in ours41.
Reactive species derived from oxidative stress generally have a very short half-life21, so their detection in serum is very limited, making the analysis of products derived from oxidation of great interest. Specifically, oxidative damage to membrane lipids generates highly reactive aldehydes, including 4-HNE. This component is very important in cell signalling, with effects in transduction, differentiation and apoptosis. It plays a prominent role in neurodegenerative diseases and has been detected in the brain tissue of patients with Alzheimer’s disease42, MS 16and in CSF in the case of amyotrophic lateral sclerosis43. This is the first study in which 4-HNE has been measured in the serum of pwMS, revealing levels that are significantly inferior to those of the control groups. What is more, and very interestingly, we found greater levels of 4-HNE in patients with less time of evolution, which could mean greater lipid degradation in the earlier stages of the disease. On another note, a commonly used marker of nucleotide degradation is 8OH-dG, which is formed from the damage caused by hydroxyl radicals to deoxyguanosine. Studies of pwMS in which both are measured are scarce.
We obtained lower levels of 8OH-dG in pwMS, an unexpected finding considering that elevated levels of 8OH-dG have previously been observed in neuropathological studies compared to normal-appearing white matter and control groups. In other studies, serum determination of 8OH-dG has not produced homogeneous results, with both higher and33 lower levels in pwMS40. One explanation of the apparently contradictory findings of lower levels in the degradation products, both of 8OH-dG and of 4-HNE, could be that greater antioxidant activity in response to an increase in the oxidative stress would, in a secondary manner, produce a decrease in peripheral levels of these degradation products16. In fact, Tasset et al. observed a decline in 8OH-dG after NTZ was started, which would associate starting DMT with increased activation of the antioxidant mechanisms29.
The evaluation of symptoms associated with MS, such as CI, fatigue or depression, and their relationship with oxidative stress has been explored in very few articles, so our study provides new information on the role of oxidative stress and its potential as a biomarker of these symptoms. CI is very common among pwMS; it can be present from the early stages of the disease and affects up to 70% of patients. The most commonly affected areas are processing speed and learning and visual memory, although deficits may also appear in executive function, long-term memory or attention44. Oxidative stress could play a part in the development of cognitive problems45 and could even be a therapeutic target46, but thus far, the studies in pwMS are scarce and have not yielded significant results. Naseri et al. assessed cognitive function using the MACFIMS battery and various parameters relating to oxidative stress, without finding differences except in the levels of glutathione peroxidase 147. In our case, we used a brief cognitive test, such as the SDMT, which fundamentally evaluates processing speed. The results obtained are of great interest, given that the levels of 4-HNE and TAC have been associated with the presence of CI and, furthermore, with a correlation with the test score. The TAC levels are lower in pwMS with a higher score in the SDMT, which probably means an increase in oxidative stress in pwMS and cognitive problems, increased compensatory antioxidant activity and higher consumption of the antioxidant components. 4-HNE levels are lower in people with MS with lower scores, a finding consistent with the TAC data, since an increase in antioxidant activity would translate into a decrease in degradation products. These results indicate that oxidative stress could be involved in the development of cognitive problems, with further studies required to evaluate its potential as a biomarker.
Depression has a high prevalence in MS, with a rate of 50% throughout the lifetime of pwMS48. An increase in oxidative stress parameters has been observed therein, and therapeutic trials have even been carried out with antioxidant agents for their treatment49. Another of the most common symptoms in MS is fatigue, affecting up to 80–90% of patients at any given time in their evolution. This is defined as a lack of physical and/or mental energy perceived by the individual that interferes with their habitual activities50,51. In the CNS, the presence of circulating proinflammatory cytokines promotes dysfunction in glutamate reuptake and a decrease in dopamine, both related to the development of fatigue and both capable of inducing an increase in oxidative stress through different pathways.52. Despite the fact that MS, depression and fatigue share pathophysiological mechanisms associated with oxidative stress, the studies carried out thus far are scarce and no differences have been found in fatigued or depressed patients12,53,54, which is consistent with the observations of our study. Only Katarina et al. observed differences when analysing the levels of uric acid, finding a negative correlation with sadness in the BDI and daily activities in the MFIS55.
Up until now, the link between stress and treatment with DMT whose use has been introduced in recent years, such as OCR, AZM or CLA, had not been evaluated. Despite the differences in terms of efficacy, no different results have been observed based on the DMT, not even in patients being treated with DMF, to whom a special effect of oxidative stress is attributed by way of the Nrf2 pathway being activated56.
This study has various limitations, including a sample with a limited n. What is more, it is a cross-sectional study that does not allow us to infer causality regarding the evolution of oxidative stress and its link to clinical, disease progression and radiological factors. On the other hand, it would have been interesting to also know the levels of vitamin D, ferritin and uric acid in the control groups. Nevertheless, the data obtained are relevant and can help to better understand the role of oxidative stress in MS.
In conclusion, oxidative stress plays a prominent role not only in the pathophysiology of MS but also in other symptoms, such as fatigue or CI. Our results show an increase in antioxidant activity in pwMS that could have an influence on lower levels of products derived from it. The pwMS with cognitive problems have different oxidative stress profile than pwMS without CI, with higher TAC levels and lower values of lipid degradation products (4-HNE) being observed. We do not know if this relationship is a cause or a consequence, but it does represent progress in understanding its pathophysiology. Despite the high prevalence of CI in pwMS, therapeutic possibilities are scarce and its evaluation in daily clinical practice is limited. There are no published studies in which CI is linked to factors of oxidative stress, which is why our results could help to advance the development of a biomarker associated with CI and in treatments aiming to intervene in oxidative stress.