Advances in the mechanism of liver injury caused by COVID-19

ZHENG Li-ting, LIU Shan-shan, XIE You-cheng, CHEN Yu-chun, ZHANG Jiu-cong,YU Xiao-hui✉

1.Gansu University of Traditional Chinese Medicine, Lanzhou 730000, China

2.Department of Gastroenterology, The 940 Hospital of Joint Logistic Support Force of PLA, Lanzhou 730050, China

Keywords:

ABSTRACT In addition to the performance of the respiratory system, COVID-19 can also have different degrees of abnormal liver function, and even liver injury and liver failure may occur in serious cases.In addition, infection with novel coronavirus will aggravate the liver damage of patients with chronic liver disease, resulting in a significant increase in severe morbidity and mortality.Based on this, this paper briefly reviews the mechanism of liver injury caused by COVID-19 and its impact on patients with chronic liver disease, in order to provide a theoretical basis for the prevention and treatment of COVID-19.

1.Introduction

COVID-19 (coronavirus disease 2019) first discovered in 2019 is an acute infectious disease caused by severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) [1].According to WHO statistics, as of October 28, 2022, there have been 630 million confirmed cases and 6.57 million deaths worldwide[2].Although most patients infected with SARS-CoV-2 have no symptoms or mild symptoms, there are still a certain number of severe COVID-19 patients who can develop acute respiratory distress syndrome(acute respiratory distress syndrome,ARDS) within tens of days,accompanied by multiple organ failure or even death [3].In addition to respiratory symptoms, SARS-CoV-2 infection can also involve heart, liver, brain, kidney and other extrapulmonary organs [4].Among them, liver injury is relatively common, which can be characterized by asymptomatic elevation of liver transaminase to varying degrees of liver injury.In a retrospective study involving 1935 inpatients with COVID-19, 53.2% of patients had abnormal transaminase and 20.5% of patients had liver injury [5].In addition,the incidence of SARS-CoV-2 infection in patients with chronic liver diseases such as viral hepatitis, liver cirrhosis and liver cancer is greatly increased due to impaired immune function.These complications and potential risk factors seriously interfere with the prevention and treatment of COVID-19.Even after rehabilitation,COVID-19 will still affect the normal life of patients because of its sequelae [6].

Therefore, this paper reviews the pathogenesis of liver injury in COVID-19 and the influence of SARS-CoV-2 infection on patients with chronic liver disease.

2.Clinical characteristics of liver injury in patients with COVID-19

The clinical manifestations of COVID-19 patients are not only respiratory symptoms, but also asymptomatic or non-specific symptoms, such as fever, fatigue, diarrhea, nausea and vomiting.However, early liver injury may have similar manifestations, so the correlation between them is difficult to distinguish [7].In addition,some studies have reported skin pigmentation in patients with COVID-19, which may be due to abnormal liver function, decreased ability to inactivate estrogen, at the same time, due to abnormal liver function, decreased ability to metabolize iron, the increase of iron content in the blood will also lead to skin color deepening [8].From the demographic distribution of infection, elderly COVID-19 patients have a higher risk of liver injury [9].The incidence of liver injury in males (63.4%) is much higher than that in females (36.6%).Severe and severe liver injuries are more common than mild patients[10].And the average hospitalization time of patients with abnormal liver function is longer than that of patients with normal liver function, and abnormal liver function may exist for a long time even after the patient is cured and discharged [11].

It is reported that the proportion of COVID-19 patients with abnormal liver function at admission is 37.2% ～ 76.3%, and the proportion of liver injury is 21.5% ～ 45.7% [10].Most of the patients showed mild elevation of glutamic oxaloacetic transaminase(aspartate aminotransferase,AST) and glutamic pyruvic transaminase(alanine aminotransferase,ALT), and also increased total total bilirubin (TBIL) [12].The increase of TBIL in severe and critically ill patients was more obvious, but there were no obvious symptoms of jaundice.In addition, the level of albumin could also be slightly decreased, but there was no significant change in prothrombin[13].Glutamyltransferase (gamma-glutamyl transferase, GGT) and alkaline phosphatase (alkaline phosphatase,ALP) increased by 21.1%and 6.1%, of which the proportion of patients with severe liver injury whose GGT level exceeded 3 times the normal upper limit during hospitalization could be as high as 58.1% [14].Hypoalbuminemia and abnormal GGT or AST levels were significant independent risk factors for COVID-19-related death [15].

As the liver plays a key role in the synthesis of coagulation factors and albumin, liver dysfunction may lead to multiple system damage,such as multiple organ failure, coagulation dysfunction and ARDS[16].In addition, the liver is the main metabolic and detoxifying organ of the human body.even when the liver function is slightly damaged,the therapeutic efficacy and safety of antiviral drugs may be changed because of the decrease of liver metabolic capacity.Therefore, it is very important to understand the mechanism of COVID-19-related liver injury in more detail.

3.Mechanism of COVID-19 liver injury

Liver injury in patients with COVID-19 is mainly caused by direct liver injury caused by SARS-CoV-2, drug-induced liver injury, hypoxia-ischemia and vascular endothelial damage, immune imbalance and cytokine storm, while chronic liver disease can also aggravate COVID-19-related liver injury.

3.1 Direct injury

SARS-CoV-2 has an affinity for whole body organs, and SARSCoV-2RNA expression can be detected in the liver and many other extrapulmonary organs [17].SARS-CoV-2 enters the cells by binding to angiotensin converting enzyme 2 receptor (angiotensinconvertingenzyme2receptor,ACE2).Priola et al.[18] RNA sequencing analysis of single cells in the liver showed that the expression of ACE2 was the highest in bile duct cells, followed by hepatic sinusoid endothelial cells (liversinusoidalendothelial cells,LSECs)and hepatocytes.Therefore, part of the liver injury caused by COVID-19 may be due to the direct binding of SARS-CoV-2 to the receptors on bile duct cells, resulting in bile duct cell injury and bile acid excretion dysfunction.Although the expression level of ACE2 in liver is very low, the distribution of SARS-CoV-2 receptor is not consistent with that of organ infection.Wang et al.[19] performed autopsy on 2 dead patients with elevated transaminase.Transmission electron microscope, immunohistochemistry and pathological study showed that there were a large number of SARS-CoV-2 virus particles in the cytoplasm of hepatocytes, which indicated that SARS-CoV-2 could enter and replicate in hepatocytes.The infected hepatocytes showed enlargement of mitochondria, dilation of endoplasmic reticulum and decrease of glycogen granules.These data suggest that the liver is a potential target for possible SARSCoV-2.

3.2 Drug-induced liver injury

As no specific drug has been developed to effectively treat SARS-CoV-2, antipyretic, antiviral, Chinese herbal medicine,immunosuppressant and other drugs used by patients during infection may directly or indirectly lead to drug-induced liver injury.In addition, long-term medication for underlying liver disease may interact with the above-mentioned drugs for the treatment of COVID-19 and may also increase the risk of liver injury.According to the latest report from the European Association for liver Research,lopinavir / ritonavir, colchicine, azithromycin, hydroxychloroquine and ivermectin are no longer recommended for the treatment of SARS-CoV-2 infection [20].Lopinavir / ritonavir, as an anti-HIV virus protease inhibitor, has been shown to have anti-SARS-CoV-2 activity.In a retrospective study of 417 COVID-19 patients, it was found that Lopinavir / ritonavir increased the risk of liver injury by four times [21].

According to reports, through the statistics of drug use in patients with COVID-19 after admission, it was found that the proportion of Lopinavir / ritonavir, glucocorticoid and thymic polypeptide was higher in patients with abnormal liver function, and long-term use of medium or high dose of glucocorticoid ( 10 mg/ prednisone or the same dose) was significantly correlated with severe COVID-19 patients [22].In addition, some studies have found that the use of glucocorticoids may activate hepatitis virus, because glucocorticoids can activate hepatitis B virus replication by directly stimulating HBV genome sequence [23].Therefore, when using drugs to treat COVIDat 19:00, drugs with potential liver injury should be evaluated and used with caution in clinic.Once abnormal liver transaminase occurs in patients taking hepatotoxic drugs, drug-induced liver injury should be identified or excluded first.

3.3 Ischemia and hypoxia and vascular endothelial injury

Studies have shown that more than 40% of COVID-19 patients with hypoxemia need mechanical ventilation, and the proportion of critically ill patients who need oxygen therapy can be as high as 71.1% [24].The liver is an important organ responsible for digestion and metabolism, which is extremely sensitive to hypoxia, while patients with COVID-19 have varying degrees of hypoxia.In severe cases, it can also be complicated with systemic inflammatory response syndrome, respiratory distress syndrome and multiple organ failure, which will aggravate liver tissue hypoxia.In addition, hypoxia, as an important regulator of ACE2, can upregulate its expression in hepatocytes under hypoxia conditions,while the increased expression of ACE2 in hepatocytes and bile duct endothelial cells may promote the entry of SARS-CoV-2 into the liver and aggravate liver injury [25].

The pathological sign of ischemic liver injury is central lobular necrosis, which is usually characterized by a significant increase of serum transaminase in a short time.Under the condition of insufficient blood perfusion and hypoxia, insufficient oxygen and lipid accumulation in hepatocytes can lead to cell death,mitochondrial damage, reactive oxygen species and their oxidation products are significantly increased, which can further promote the release of a variety of inflammatory factors by activating transcription factors sensitive to oxidation products.Aggravate liver injury [26].At the same time, Kupffer cells can produce cytokines sensitive to ischemia, which aggravate the inflammatory response by activating neutrophils.In addition, microcirculatory dysfunction caused by LSEC injury can further aggravate coagulation dysfunction and thrombosis in patients [27].Although macrophages are also involved in the process of liver injury and can lead to coagulation dysfunction, it is usually a short and insignificant process in COVID19-related liver injury and generally does not develop into obvious acute liver injury [28].

In addition, a prominent feature of SARS-CoV-2 infection is vascular endothelial damage, so some researchers believe that COVID-19 is a vascular disease [29].Von Willebrand factor antigen(von Willebrand factor antigen,VWF), as a marker of endothelial injury, is related to the severity of COVID-19.When VWF exceeds 423%, a higher mortality rate can be observed.In addition, soluble thrombomodulin, angiopoietin-2 and e-selectin were also increased in critically ill patients [30].Other studies have reported that D-dimer and fibrinogen are higher in COVID-19, and the increase of D-dimer is positively correlated with severe COVID-19 and high mortality[31].

3.4 Immune imbalance and cytokine storm

Some mild COVID-19 patients may deteriorate rapidly and enter a state of multiple organ failure, which is related to the storm of cytokines caused by excessive immune response.Cytokine storm refers to the process of abnormal increase of inflammation-related cytokines caused by excessive activation of the immune system after severe stimulation such as infection, leading to severe tissue damage and even organ failure [32].This process can not only lead to lung damage, but also affect other organs such as the liver, heart and kidneys.Current studies have shown that cytokine storm is an important node in the transition of COVID-19 from mild to severe and critically ill, and is an important cause of death of patients.A variety of inflammatory cytokines can be detected in COVID-19 patients, including IL-1 β, IL-2, IL-6, IL-10, IFN- γ, TNF- α,IP-10, MCP-1 and so on.These cytokines are related to the severity of the disease [33].In addition, it is worth noting that cytokine storms may directly lead to immune cell death, tissue injury and respiratory arrest.Studies have found that significant changes have taken place in immune organs and cells in some severe COVID-19 patients, such as spleen atrophy and necrosis, lymph node necrosis,renal hemorrhage, hepatomegaly, central nervous system neuronal degeneration and so on [34].In addition, there is a correlation between lymphopenia and liver injury in patients with COVID-19.The increase of IL-6 and IL-10 and the decrease of CD4+T lymphocytes are independent risk factors for severe liver injury [35].

4.COVID 19 and chronic liver disease

Chronic liver disease increases the risk of SARS-CoV-2 infection and death.However, the severity and mortality of SARS-CoV-2 infection are not consistent with each liver disease.SARS-CoV-2 infection and mortality increased significantly in patients with alcoholic liver disease, metabolic fatty liver disease, liver cirrhosis and hepatocellular carcinoma, but there was no significant difference in SARS-CoV-2 infection and mortality between patients with viral hepatitis and autoimmune liver disease.

4.1 Alcoholic liver disease

Affected by epidemic control and home isolation, global alcohol consumption has increased significantly, and the increase in alcohol consumption is often accompanied by an increase in the number of hospitals [36].according to statistics from 257 hospitals in Japan during the epidemic, the hospitalization rate per 1 000 people due to alcoholic liver disease (alcohol-relatedliverdisease,ALD) or pancreatitis is 1.22 times higher than before the epidemic.At the same time, it also increased patients′ risk of exposure to SARSCoV-2, and in an analysis of data involving 155 countries, alcohol intake was significantly associated with an increased risk of COVID-19 infection [37].A multicenter study in the United States shows that ALD, decompensated liver cirrhosis and hepatocellular carcinoma are independent risk factors for higher overall mortality in patients with COVID-19 [38].

The effects of ALD on COVID-19 patients mainly include the following three aspects: first, alcohol directly affects the function and activity of immune cells, destroys the innate immune system and adaptive immune system, weakens the body′s defense against infection, and increases the probability of SARS-CoV-2 infection.Second, long-term drinking increases the risk of ARDS, which may be related to alveolar epithelial dysfunction in chronic alcohol abusers [39].Third, patients with a history of alcoholism usually have other complications, including metabolic syndrome, which is an independent prognostic factor for COVID-19 [40].

4.2 Metabolic fatty liver disease

It has been reported that patients with metabolism-related fatty liver disease (Metabolicas sociated fatty liver disease,MAFLD) are more likely to develop abnormal liver function, higher risk of disease progression and longer time for virus shedding than patients with non-metabolic fatty liver disease after infection with MFLD [41].A multicenter cohort study of 280 patients with COVID-19 showed that ALT and AST levels in SARS-CoV-2-infected MAFLD patients were significantly higher than those in non-MAFLD patients, suggesting that MAFLD patients are more likely to develop liver injury after SARSCoV-2 infection [42].In addition, progressive cholestasis and related sclerosing cholangitis are common complications of chronic MAFLD patients after SARS-CoV-2 infection [43].The risk of severe COVID-19 in patients with MAFLD was 2.6-5 times higher than that in patients without MAFLD, and patients with MAFLD at admission and elevated serum IL-6 levels had a higher risk of severe COVID-19 [44].And studies have found that the more visceral adipose tissue accumulates in patients with COVID-19, the higher the risk of entering ICU treatment.That is, for every 1cm increase in the abdominal circumference of the patient, the possibility of ICU treatment and mechanical ventilation increased 1.13 times and 1.25 times respectively[45].

However, genetic susceptibility to MAFLD and liver fat accumulation do not increase susceptibility to severe COVID-19 patients, while inflammatory states associated with MAFLD and cytokine storms caused by imbalances in the immune system may be the reasons for a significant increase in infection risk and entry into ICU in MAFLD patients [46].Recent studies have shown that SARSCoV-2 can aggravate tissue metabolic disorders in obese,diabetic, elderly and men by damaging insulin / IGF signaling pathways in liver, lung, adipose tissue and pancreatic cells [47].Overall, NAFLD increases the risk of developing severe COVID-19 and the possibility of a poor prognosis.

4.3 Viral hepatitis

Viral hepatitis is an infectious disease characterized by inflammation and necrosis of the liver caused by different viruses.In the case of persistent virus infection, liver inflammation will progress to liver cirrhosis and liver cancer, causing great damage to human health.The liver function indexes (AST, ALT, ALP, GGT,LDH, TBIL) in patients with HepatitisBvirus,HBV and SARSCoV-2 co-infection were higher than those in patients infected with SARS-CoV2 alone [48].Compared with HBV-negative patients infected with SARS-CoV-2, HBV-positive patients had more serious monocytopenia and thrombocytopenia, and relatively insufficient in lipid metabolism and albumin production [49].In addition, compared with patients with SARS-CoV-2 infection only, patients with coinfection had higher levels of D-dimer and IL6 [50], suggesting that inflammation may promote liver injury after SARS-CoV-2 infection.Contrary to the above results, two large retrospective cohort studies of 2073 and 5936 patients with COVID-19 found that current or previous HBV infection was not associated with a higher incidence of liver injury and mortality, while abnormal direct bilirubin and AST levels at admission were independent predictors of COVID-19 mortality [51-52].In addition, the detection of HBV-related markers in patients with co-infection showed that there was no significant change in the quantitative levels of HBeAg/Ab, HBsAg/Ab and HBV-DNA during SARS-CoV-2 infection, indicating that coinfection did not cause seroconversion or reactivation of chronic hepatitis B.nor did it increase the severity of the disease or length of stay [53].

In a study of 90 cases of hepatitis A (hepatitisAvirus,HAV) and hepatitis E (hepatitisEvirus,HEV) complicated with SARS-CoV-2 infection, it was found that there was no significant correlation between SARS-CoV-2 and serum antibody levels of hepatitis An and E, indicating that HAV and HEV did not increase the risk of SARSCoV2 infection and liver injury [54].In addition, another cohort study of 126 patients with hepatitis C virus (HepatitisCvirus,HCV)diagnosed with COVID-19 found that the proportion of severe patients with active HCV was significantly higher than that of patients with inactive HCV, and patients with active HCV infection spent significantly longer hours in hospital and intensive care units because of COVID-19, and a higher proportion of patients needed mechanical ventilation.In addition, they found that HCV viral load was a significant independent risk factor for liver damage and allcause death [55].

4.4 Autoimmune liver disease

Autoimmune liver disease (AILD) refers to liver inflammatory lesions caused by autoimmune reaction, including primary sclerosing cholangitis (primary sclerosing cholangitis,PSC), primary cholangitis (primary biliary cholangitis,PBC), autoimmune hepatitis(autoimmune hepatitis,AIH) and overlap syndrome characterized by any of the two diseases mentioned above.Most patients with AILD need lifelong immunosuppressive therapy to delay the progression of liver cirrhosis and liver failure.Zecher et al reported that the positive rate of AILD patients infected with COVID-19 was 2.2% [56].From the clinical characteristics, after COVID-19 infection, the incidence of gastrointestinal symptoms in AIH patients was higher than that in other patients with chronic liver disease, but there was no significant difference in the incidence of respiratory symptoms and other symptoms [57].

In two large retrospective analyses of AIH patients infected with SARS-CoV-2, the presence or absence of AIH had no effect on the severity of COVID-19, ICU hospitalization or mortality.And continuous immunosuppression in patients with AIH reduces the risk of new liver injury during COVID-19.However, COVID-19 can also cause AIH recurrence, but the proportion of these patients is very small (3.6%).AIH recurrence may be related to the reduced dose of immunosuppressive drugs [58].Other studies have also reported that reducing the use of immunosuppressive drugs in AIH patients during COVID-19 may increase the risk of disease recurrence, and that COVID-19 may trigger severe AIH even after remission of infection[59].Therefore, during the epidemic period, AIH patients should not stop immunosuppressive therapy, and it is necessary to carry out continuous and regular follow-up of COVID-19 patients with AIH.

4.5 Liver cirrhosis

During COVID-19, a number of reports have shown that the SARS-CoV-2 infection rate in cirrhotic patients is lower than that in non-cirrhotic patients, but compared with patients without SARSCoV-2 infection, infected patients are 4.1 times more likely to need mechanical ventilation treatment, and 30-day mortality is 3.5 times higher [60-62].Decompensation in patients with liver cirrhosis after infection with SARS-CoV-2 is also common and is significantly associated with an increased risk of death.A study of 152cirrhotic patients with SARSCoV-2 infection found that 39 patients (25%)entered decompensation and 24 died, accounting for 51.1% of the total deaths [63].In addition, liver function grade, Child-Pugh grade,FIB-4 score and MELD score were also closely related to the risk of death.For example, the mortality rate of patients with liver cirrhosis complicated with SARS-CoV-2 infection increased from 23.9% in Child-PughA class to 63.0% in Child-PughC class [64].

In addition, some studies have found that the expression of ACE2 and angiotensin II is increased in patients with liver cirrhosis.in this pathological condition, SARS-CoV-2 binds more ACE2 on the cell surface, and ACE2 converts angiotensin II into angiotensin 1-7(Ang1-7), resulting in cytokine activation leading to hepatocyte necrosis or apoptosis, which aggravates liver injury in patients [65].In general, liver cirrhosis increases the risk of severe morbidity and death of COVID-19, while COVID-19 also aggravates liver injury in patients with liver cirrhosis.Therefore, it is necessary to carry out more rigorous clinical detection and treatment of cirrhotic patients infected with SARS-CoV2.

4.6 Liver cancer

The risk of SARS-CoV-2 infection in patients with liver cancer is higher than that in non-cancer patients, and the prognosis is poor,and there may be a higher mortality rate for high-risk patients with tumor, especially those with NRS2002 score 3 and advanced tumor stage [66].In a large study of liver cancer patients infected with SARS-CoV-2, 52 patients died within 30 days after infection,of which 82.7% were related to SARS-CoV-2 infection [67].The risk of death in cancer patients infected with SARS-CoV-2 was mainly affected by age, sex and complications, but not by whether they had received radiotherapy, targeted therapy, immunotherapy or hormone therapy in the past 4 weeks [68].In addition, in a followup study of patients with liver cancer, it was found that the long follow-up interval caused by COVID-19 pandemic may reduce the overall effective rate of patients, and when the follow-up interval is more than 95 days, the prognosis of patients is even worse [69].This should remind patients with liver cancer, especially the older patients with other complications, are more likely to develop low immune function, so it is necessary to strengthen the monitoring of relevant clinical indicators after SARS-CoV-2 infection, and take targeted treatment methods according to the specific conditions of patients, in addition, follow-up studies should be carried out to further evaluate the impact of liver cancer on the efficacy and outcome of COVID-19 treatment.

5.Summary and prospect

At present, with the continuous variation of the virus, its strong transmission, atypical clinical symptoms and strong immune avoidance have become the biggest obstacles to the development of effective drugs against SARS-CoV-2.China is a big country with liver disease, and many patients may be infected with SARS-CoV-2 on the basis of chronic liver disease, so we should strengthen the biochemical monitoring of liver function in patients with COVID-19 and give hepatoprotective treatment on the basis of active treatment of COVID-19, so as to reduce liver injury and prevent the occurrence of poor prognosis.In addition, the mechanism of SARSCoV-2-related liver injury and how SARS-CoV-2 affects chronic liver disease is not completely clear, so future research should pay more attention to the relationship between SARS-CoV-2 and liver to provide help for the life and health of the general public.

Author contribution

Zheng Liting: participate in the main writing and revision of the article.Liu Shanshan, Chen Yuchun, Xie Youcheng: participate in data collection, collation and analysis.Zhang Jiucong, Yu Xiaohui:participate in topic selection, conception and design.Declaration of conflict of interest

All authors declare that there is no conflict of interest.