Hepatic angiotensin-converting enzyme 2 expression in metabolic dysfunction-associated steatotic liver disease and in patients with fatal COVID-19

Abstract

BACKGROUND

Metabolic dysfunction-associated steatotic liver disease (MASLD), characterised by hepatic lipid accumulation, causes inflammation and oxidative stress accompanied by cell damage and fibrosis. Liver injury (LI) is also frequently reported in patients hospitalised with coronavirus disease 2019 (COVID-19), while pre-existing MASLD increases the risk of LI and the development of COVID-19-associated cholangiopathy. Mechanisms of injury at the cellular level remain unclear, but it may be significant that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which causes COVID-19, uses angiotensin-converting expression enzyme 2 (ACE2), a key regulator of the ‘anti-inflammatory’ arm of the renin-angiotensin system, for viral attachment and host cell invasion.

AIM

To determine if hepatic ACE2 levels are altered during progression of MASLD and in patients who died with severe COVID-19.

METHODS

ACE2 protein levels and localisation, and histological fibrosis and lipid droplet accumulation as markers of MASLD were determined in formalin-fixed liver tissue sections across the MASLD pathological spectrum (isolated hepatocellular steatosis, metabolic dysfunction-associated steatohepatitis (MASH) +/- fibrosis, end-stage cirrhosis) and in post-mortem tissues from patients who had died with severe COVID-19, using ACE2 immunohistochemistry and haematoxylin and eosin and picrosirius red staining of total collagen and lipid droplet areas, followed by quantification using machine learning-based image pixel classifiers.

RESULTS

ACE2 staining is primarily intracellular and concentrated in the cytoplasm of centrilobular hepatocytes and apical membranes of bile duct cholangiocytes. Strikingly, ACE2 protein levels are elevated in non-fibrotic MASH compared to healthy controls but not in the progression to MASH with fibrosis and in cirrhosis. ACE2 protein levels and histological fibrosis are not associated, but ACE2 and liver lipid droplet content are significantly correlated across the MASLD spectrum. Hepatic ACE2 levels are also increased in COVID-19 patients, especially those showing evidence of LI, but are not correlated with the presence of SARS-CoV-2 virus in the liver. However, there is a clear association between the hepatic lipid droplet content and the presence of the virus, suggesting a possible functional link.

CONCLUSION

Hepatic ACE2 levels were elevated in nonfibrotic MASH and COVID-19 patients with LI, while lipid accumulation may promote intra-hepatic SARS-CoV-2 replication, accelerating MASLD progression and COVID-19-mediated liver damage.

Keywords: Metabolic dysfunction-associated steatotic liver disease; Angiotensin-converting enzyme 2; Immunohistochemistry; COVID-19; COVID-19-associated cholangiopathy

Core Tip: There has been much recent interest in angiotensin-converting expression enzyme 2 (ACE2) as the fulcrum of the ‘anti-inflammatory’ renin-angiotensin system pathway because severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the cause of coronavirus disease 2019 (COVID-19), targets ACE2 for viral attachment and host cell invasion. Previously ACE2 mRNA has been measured in metabolic dysfunction-associated steatotic liver disease (MASLD) and COVID-19 infection but, uniquely, we used immunohistochemistry, alongside measurement of fibrosis and lipid, to show that ACE2 protein levels and hepatic lipid content are correlated across the MASLD pathophysiological spectrum and in COVID-19 patients showing evidence of liver injury. Hepatic lipids are also associated with the presence of SARS-CoV-2 virus in the liver suggesting a possible functional link.