Is carnosine effective to alleviate lung injury: a systematic review

Introduction

Carnosine, discovered at the beginning of 20th century by V.S. Gulewitsch, is a natural dipeptide that consisting of β-alanine and l-histidine [1]. Carnosine, which is mainly found in muscles, has different characteristics. Many studies have shown that carnosine has antioxidant effects by scavenging reactive oxygen and nitrogen species and chelating metals [2], [3], [4]. Carnosine also exhibits pH-buffering activity via suppressing lipid peroxidation [2, 4]. Studies have also shown that carnosine can provide membrane protection by inhibiting lipid peroxidation and lowering malondialdehyde (MDA) levels [2].

There are many studies that observing the effects of carnosine on wound healing. Roberts et al., who investigated the effects of carnosine on wound healing, demonstrated that carnosine improved wound healing when given as a part of an enteral formula [5]. In another study examining the effects of carnosine on lung injury was indicated that carnosine can accelerate lung injury induced by irradiation [6].

Both acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) are important clinical syndromes and refer almost similar pathophysiological processes [7]. ALI is a serious clinical problem associated with pulmonary oedema and severe hypoxemia, and it is closely related with high morbidity and mortality rate [8]. ARDS, which is a severe form of ALI, is acute respiratory failure caused by non-cardiogenic pulmonary oedema that is defined by bilateral infiltration of lungs on radiology and severe hypoxemia. ARDS is also a serious morbidity and mortality burden similar with ALI [9, 10]. There are several factors that lead to ALI and/or ARDS, including pneumonia, sepsis, acute pancreatitis, trauma, inhalation of gastric contents, and intrinsic or extrinsic oxidants [11, 12]. Both ALI and ARDS are characterized by diffuse alveolar damage (DAD), pulmonary oedema, neutrophil infiltration of lungs, and pulmonary surfactant dysfunction [11].

When the lung is injured, regardless of the cause of ALI/ARDS, inflammatory processes are activated [8, 10]. In the pathogenesis of ALI, neutrophils, which play a main role in formation of microvascular and the lung tissue injury, macrophages, and proinflammatory mediators such as tumor necrosis factor-α (TNF-α) and nitric oxide (NO) are activated [8]. On the other hand, in the pathogenesis of ARDS, inflammation can lead to DAD, which is a result of alveolar epithelial barrier dysfunction, endothelial dysfunction, and protein-rich alveolar oedema [9, 10]. As a result of these changes, hypoxemia and acute respiratory failure are observed when pulmonary oedema accumulates in the interstitium and alveoli [10].

In the light of all these knowledge, we aimed to examine the effects of carnosine on lung injury. To our knowledge, this is the first systematic review that investigates effects of carnosine on lung injury. We think that this paper can be beneficial for developing novel treatments for pulmonary diseases.

Methods

Analysis of the data

The titles and abstracts were screened to be sure that included studies had the true scope of this systematic review. For further assessment, the full text of the papers were taken. All sections of each papers (abstract, results, tables and figures) were reviewed independently for eligibility and data extraction. Studies not relevant to the topic, reviews, abstracts, meetings, and hypothesis were removed. Publications that met the inclusion criteria were mentioned in Figure 1.

Figure 1:

Flow diagram of studies included in the systematic review.

Results

As shown in Figure 1, we screened totally 162 publications and then excluded 20 duplicate articles. Of the 142 remaining publications, 133 of them were excluded; 68 of them were irrelevant, six of them were not in English, 49 of them were review, and 10 of them were meeting abstracts, editorial, conference paper, case report and hypothesis. The remaining nine articles were used in the final analysis of this systematic review (Table 1).

Risk of bias analysis

Bias analysis of each study were given with risk of bias summary and risk of bias graph (Figure 2). The risk of selection, performance, detection, attrition and reporting bias of the studies were analyzed according to the Cochrane Handbook [14].

Figure 2:

A. Risk of bias summary of the included studies for the systematic review. B. Risk of bias graph summary of the included studies for the systematic review.

Discussion

In this systematic review, we demonstrated that carnosine has beneficial effects on improving lung tissue injury by suppressing proinflammatory cytokines, certain chemokines, oxidative agents, and apoptosis.

In a study by carried out by Lamb et al. demonstrated that patients with ARDS had higher levels of oxidant agents in their BAL compared to healthy controls [24]. In addition, Bunnell et al. found that ARDS patients had lower levels of glutathione (GSH) in the alveolar fluid [25]. In addition to ARDS, also patients with IPF exhibited lower levels of GSH in their epithelial lining fluid [26].

Several research showed that carnosine has important antioxidant activities. Boldyrev et al. indicated that carnosine suppressed the lipid peroxidation towards the end of 20th century [27]. In another study, Caruso et al. found that carnosine reduced intracellular levels of superoxide anions in a murine model that used RAW 264.7 macrophages by suppressing TNF-α and IL-6 mRNAs, and enhancing antiinflammatory mediators such as IL-4, and IL-10 [28]. In another study of Caruso et al. demonstrated that carnosine inhibited total NO levels in a murine model induced by LPS and IFN-γ [29]. More recently, the effects of carnosine for modulation of antiinflammatory and proinflammatory roles on RAW 264.7 macrophages were evaluated and found that carnosine reduced levels of MDA, expression of prooxidant enzymes, IL-1β and IL-6 and expression of antioxidant enzymes [30].

Carnosine also plays another important role against virus infections, too. Rothan et al. examined the antiviral activities of carnosine on Dengue virus and Zika virus in Huh7 cells, which are human liver cells. At the end of the investigation, they found that exogenous carnosine significantly suppressed viral infection by inhibiting virus entry to the host cells and viral genome replication [31].

Influenza A virus (IAV) is a common causative agent of pneumonia-related death worldwide and it can cause a significant increase in mortality, especially during pandemics. After the IAV infection, increase in the activation of NF-Kβ, releasing of proinflammatory cytokines and chemokines are observed. These responses to the viral infection can contribute to lung tissue damage by inducing TNF-α and iNOS, which underlie the IVA-related alveolar injury [32].

As we all know, we live extraordinary times worldwide since December 2019 because of the COVID-19 (SARS-CoV-2), which is a member of the genus Betacoronavirus [33]. In the COVID-19 infection, cytokine storm is one of the most encountered and most serious problem. Several cytokines and chemokines, including TNF-α, TGF-β, IL-1β, IL-8, macrophage inflammatory protein-1α (MIP-1α) are observed in this cytokine storm. However, IL-6 is one of the most major cytokine involved in this cytokine storm and it plays a central role. It was shown that cytokine storm is associated with severe lung injury by impairing the integrity of endothelial and epithelial cells, thus causing vascular permeability and pulmonary oedema, neutrophil and macrophage infiltration to the lungs, and hypoxia [34].

Previously, we mentioned many features of carnosine before. We think that carnosine may also be beneficial in the treatment of COVID-19. Firstly, one of the most important activities of carnosine is inhibition of angiotensin converting enzyme activity, which plays a central role in COVID-19 infection. Nakagawa et al. showed that carnosine inhibited ACE activity in vitro even at lower doses than those normally found in muscles and neuronal tissues [35, 36]. Secondly, carnosine is also important as a result of chelate formation with metal ions as mentioned before. PZ, which is a chelate with carnosine and zinc, can boost the inhibition of proinflammatory processes closely related with COVID-19 [33, 35]. Thirdly and finally, carnosine can decrease oxidative stress by suppressing levels of NO and iNOS, which are worsening viral infections [33].

As a result, in this comprehensive systematic review, we investigated the activity of carnosine in lung injury. It must be emphasized that included studies in this review are animal models or in vitro cell samples. Unfortunately, no human studies are available to ensure correctness of these results. Further studies, especially carried out in human, are needed to confirm these effects of carnosine.

Strength and limitation of included studies

There are certain strengths in our study. One of them is that our study includes animal and in vitro studies, which can control in terms of physiological conditions. Another advantage of the study is that the animal studies have a control group, which provides comparison and reduced systematic bias. Finally, the duration of included studies was found to be appropriate.

The study also has certain disadvantages. The major limitation is that lack of human studies in this systematic review. Human studies are the most suitable data to confirm mechanisms of such molecules. The effects of carnosine in human studies are changeable, thus further studies are needed to investigate exact effects in humans.

Conclusion

All of the articles included in the systematic review demonstrated that carnosine has positive effects to alleviate lung injury. However, the effects of carnosine are still unclear. Thus, human studies are needed to elucidate the action mechanisms of carnosine.