Diabetes Mellitus (DM) is a very common chronic metabolic disease characterized by the absence or ineffectiveness of insulin. According to the International Diabetes Federation, nearly 500 million people in the world have this disease. The number of patients with DM is estimated to approach 600 million in 2030. The money spent on DM and its related diseases is over 750 billion dollars and this amount is continually increasing.1 One of the reasons why DM is such an important health problem is the serious complications it has caused over the years. As a result of these complications, many organs or functions are adversely affected, especially the kidneys, liver, nervous system and cardiovascular system.2–4

One of the functions negatively affected by DM is the sense of hearing. It has been reported that sudden or gradual hearing loss can develop at varying rates in DM.4,5 According to researches, DM causes hearing loss by affecting the cochlea, vestibulocochlear nerve and the temporal bone.6,7 Oxidative stress and inflammation are the mechanisms held responsible here. In DM patients, hyperglycemia leads to an increase in the polyol pathway, aldose reductase activity and advanced glycation end-products production. As a result, increased inflammation and Reactive Oxygen Species (ROS) cause vascular endothelial damage, neuropathy in the vestibulocochlear nerve, and loss of outer vibratile hair cells.6,8–10 In many studies based on these data, the effect of antioxidant molecules on hearing loss due to DM has been tested and successful results have been reported.6,9,11

Astaxanthin (AST) is a member of the carotenoid family, whose well-known members are vitamin C and vitamin E. AST is a molecule that is not synthesized in the human body, and it must be taken from the outside. AST can be found in some natural sources otherwise it can be synthesized chemically in the laboratory environment.1 Sea creatures are the most important natural sources of AST. The United States Food and Drug Administration (FDA) approved AST as a nutritional supplement in 1999.12 In fact, studies have reported that AST can exert an antioxidant effect up to 100 times more than other carotenoids.13 This superiority is due to its molecular structure. AST has effective ROS scavenging effects, especially singlet oxygen. Studies have shown that AST effectively reduces oxidative damage, lipid peroxidation, advanced glycation end-products development and has strong antioxidant effects.13–16

In this study, the aim was to investigate the effects of AST, which has strong antioxidant effects, on hearing loss due to DM. For this purpose, an experimental streptozotocin-induced DM model was conducted. Following the development of DM, rodents were applied with AST, and its protective effects against hearing loss were investigated. In order to reveal the effects of AST, biochemical parameters that may be indicators of oxidation and inflammation, as well as hearing tests were used.

The main objective of this study is to investigate the effects of AST, an antioxidant molecule, on hearing loss in diabetic rats. DM not only shortens the life expectancy with the chronic complications it causes, but also negatively affects the quality of life. Studies on this subject have shown that hearing problems are more common in diabetic individuals than in those who do not have this disease, and they start at an earlier age.17 It has been demonstrated that hearing loss in patients with DM is associated with glycemic control, and the risk is higher in people with poor control.18 The reason for the increased risk is the damage of high glucose levels to the auditory pathways and cochlear structures, especially the hair cells.19 Hyperglycemia leads to an increase in advanced glycation end-products and ROS in tissues. This causes oxidation and inflammation. As a result, these events damage the structures involved in hearing and cause deterioration in their functions.8,10,20 Tissues involved in hearing functions can secrete antioxidant enzymes, including GPx, SOD and CAT.21 GPx converts glutathione, which is involved in H2O2 detoxification, from its oxidized form to its reduced form. SOD catalyzes singlet oxygen and CAT catalyzes hydrogen peroxide. DM causes excessive ROS release in these tissues and a relative insufficiency of the mentioned antioxidant enzyme levels. As a result, ROS that cannot be adequately removed cause oxidative damage, lipid peroxidation, inflammation, and consequently cellular damage and hearing loss.8–11

In this study, ABR and DPOAE hearing tests were applied to diabetic rats in order to evaluate their auditory functions. Both tests applied can successfully reveal the functional results of changes in the inner ear in humans and animals.22 ABR reflects the electrical activities of the auditory nerve and its brainstem connections and is mostly used to evaluate the neural aspect of hearing loss.9 DPOAE is a test that provides the means to evaluate the hearing loss due to outer hair cells damage in the cochlea with the measurement made in the outer ear canal.6,22,23 As a result of the measurements made with this test, there are studies reporting that DM causes hearing loss, especially against high-frequency sounds, and in some other studies, it has been said that hearing loss is valid for all frequencies. In this study, we observed significant deterioration in hearing functions in both ABR and DPOAE tests in the DM group. While no difference was observed between the groups at day 0 in the ABR test, ABR thresholds levels were gradually increasing with the effect of DM on days 30 and 60. In the DPOAE test, the 30th and 60th day measurements in the DM group were found to be significantly lower than the 0th day measurements. This occurred at both low and high frequencies. These results, in line with the literature, show that DM damages auditory pathways and cochlear structures. In the DM+AST group, hearing impairments observed in the DM group were not observed. In the DM+AST group, unlike the DM group, the measurements on Day 30 and day 60 were not different from the measurements on day 0. This is due to the protective effect of AST. AST was able to reverse the effect of DM in the auditory pathways and cochlea.

Although this study is the first in the literature to investigate the effects of AST on hearing loss due to DM, the effect of this molecule has been tried and found successful in many different conditions due to DM. Conditions in which AST is tried include retinopathy due to DM, cataracts, nephropathy, and neuropathy.14,24,25 The common point reached in these studies is that AST has beneficial effects by inhibiting oxidation and inflammation. AST is known as a powerful antioxidant with no side effects. AST quenching singlet oxygen, scavenging radicals prevent lipid peroxidation and suppress oxidative stress-related gene expression.26 AST inhibits the inflammatory response by suppressing the release of pro-inflammatory cytokines, including TNF-α, IL-1β and IL-6.25,27

Lately, the protective effects of AST against cisplatin-induced autotoxicity have been reported in various studies. Gu and his colleagues reported alleviated cisplatin-induced autotoxicity and oxidation-related pathologies with AST.28 Two more researchers were shown similar results with AST against cisplatin-induced autotoxicity.29,30 The researchers were pointed out that reduced oxidative stress and cellular damage with AST applications were responsible for the protective effects. One of these studies, Kınal, and colleagues,29 remarked that AST’s protective effects specifically against high-frequency hearing loss only at higher doses (40 mg/kg). Although the reason for autotoxicity was different, the present study results were in accordance with the literature data. The present study differed from Kınal and his colleagues with the applied AST dose (80 mg/kg), tested against high and low-frequency hearing loss. The results of this study reveal the protective effects of AST at higher doses.

In this study, while the levels of Gpx, SOD, CAT and MDA were analyzed in blood and cochlear tissue samples in order to reveal the antioxidant and anti-inflammatory effects of AST, the levels of pro-inflammatory cytokines such as TNF-α, IL-1β and IL-6 were only measured in cochlear tissue samples. MDA is a product of lipid peroxidation and is considered an important marker of oxidative stress.6,31 In this study, while the MDA level increased in the DM group in blood and cochlear tissue samples, this increase was not observed in the DM+AST group. This result, which can be interpreted as being successful in preventing lipid peroxidation, is probably due to the fact that AST increases the level of antioxidant enzymes. Because Gpx, SOD and CAT levels were found to be significantly higher in the DM+AST group compared to DM. These enzymes are an important indicator of the body’s antioxidant capacity.14,32 In this study, AST significantly increased the levels of all three enzymes. This ensures that the ROS scavenging ability remains at a high level, even in a situation such as DM that causes intense oxidation. AST also caused a decrease in the levels of pro-inflammatory cytokines such as TNF-α, IL-1β and IL-6, which were increased with DM in cochlear tissue. Considering that these cytokines increase oxidative damage and induce apoptosis, decreasing the levels of these substances by AST plays an important role in protecting auditory cells.

As a result, when the data obtained is assessed, AST showed a protective effect against hearing disorders due to DM. AST demonstrated this beneficial effect by increasing antioxidant capacity and suppressing pro-inflammatory cytokines. Even though the effects of AST were investigated in a diabetic experimental animal model, there is a need to evaluate its therapeutic effects on humans. If proven successful in treating hearing-loss symptoms, AST can be suggested and prescribed to diabetic patients as an adjunct therapy with its antioxidant effects.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

The authors declare no conflicts of interest.