In a recent study published in Nature, researchers assessed the association between farnesoid X receptor (FXR) inhibition and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection.
As a result of the introduction of medicinal drugs, vaccines, and monoclonal antibodies, coronavirus disease 2019 (COVID-19) management has improved dramatically since the onset of the epidemic. Despite the transformative influence of vaccinations on people with access to them, important global health concerns continue to exist.
Novel SARS-CoV-2 variants continue to appear and are related to high infection rates and significant death rates worldwide. The only preventative medicines available are monoclonal antibodies, which are hindered by the ability of the SARS-CoV-2 spike protein to mutate to evade neutralization. Furthermore, there are no additional approved drugs for COVID-19 pharmacological prophylaxis currently. Consequently, there is an urgent unmet demand for innovative preventive medicines that minimize the risk of severe disease.
About the study
In the present study, researchers identified the FXR as a direct regulator of angiotensin-converting enzyme-2 (ACE2) transcription among respiratory and gastrointestinal tissues impacted by COVID-19.
The team predicted that chenodeoxycholic acid (CDCA) might regulate ACE2 expression via FXR. To test this theory, it was verified that FXR is expressed in vivo in gallbladder cholangiocytes and associated gallbladder cholangiocytes organoids (GCO) in vitro after activation by CDCA therapy. To confirm whether FXR is required for CDCA-induced upregulation of ACE2, the team knocked down FXR among cholangiocyte organoids with short hairpin ribonucleic acid (shRNA), which blocked CDCA-induced upregulation of small heterodimer partner (SHP) and ACE2. To determine if FXR could interact with the ACE2 gene and possibly regulate its transcriptional activity, the team assessed the ACE2 promoter region and noted the existence of an FXR-responsive element.
In vitro, the team examined whether FXR-mediated downregulation of ACE2 could diminish susceptibility to COVID-19. The researchers also subjected airway, gallbladder cholangiocyte, and intestine organoids to physiological amounts of CDCA to imitate the base level of FXR activation noted in vivo. These tissues were then infected with SARS-CoV-2, collected from a patient's nasopharyngeal swab with or without the presence of z-guggulsterone (ZGG) or ursodeoxycholic acid (UDCA). Next, the team determined if the observed decrease in SARS-CoV-2 infection was a direct consequence of FXR-mediated ACE-2 downregulation.
To test whether ACE2 regulation is the sole mechanism through which ZGG and UDCA inhibit SARS-CoV-2 infection, HEK293T cells were modified to over-express ACE2 irrespective of FXR with ZGG or UDCA followed by SARS-CoV-2 infection. The potential effects of UDCA treatment on COVID-19 patient outcomes were also tested by querying the COVID-Hep/SECURELiver registries. These registries provide information on patients with chronic liver disease who also tested COVID-19-positive and those with cholestatic liver diseases who received UDCA.
Utilizing chromatin immunoprecipitation, the team confirmed that active FXR could bind directly to the ACE2 promoter. Significantly, site-specific mutation of the IR-1 region decreased the luciferase signal, revealing the FXR binding site specificity to the ACE2 promoter. In contrast, inhibition of FXR signaling with the FXR antagonist ZGG or UDCA decreased FXR activity, as indicated by lowered levels of SHP, reduced FXR levels on ACE2 promoter, and ACE-2 downregulation at the protein and transcript. Collectively, the findings indicate that FXR directly regulates ACE2 expression in cholangiocytes.
The study also noted that inhibiting FXR signaling with the Food and Drug Administration (FDA)-approved medicine UDCA or ZGG decreases ACE2 expression in many cell types. UDCA or ZGG-mediated inhibition of FXR signaling lowered SARS-CoV-2 infection in all three types of organoids, demonstrating that knocking down FXR with shRNAs reduces ACE2 expression and suppresses viral infection among cholangiocyte organoids regardless of the presence of UDCA/ZGG or CDCA. Thus, after knockdown, neither UDCA nor ZGG affected viral infection.
Without ACE2 regulation, neither UDCA nor ZGG affected viral replication. These findings demonstrate that UDCA and ZGG decrease susceptibility to COVID-19 in several cell types in vitro by regulating ACE2 via FXR. Using multivariable logistic regression, the researchers noted that UDCA exposure was related to a 46% decreased risk of contracting COVID-19. The connection was detected across the COVID-19 spectrum concerning symptomatic disease.
The study findings indicated that FXR could regulate ACE2 expression in different SARS-CoV-2-replicating tissues. Suppression of FXR activity with the therapeutically licensed medication UDCA lowers ACE2 expression and SARS-CoV-2 infection. In addition, the results indicated a possible association between UDCA and favorable clinical outcomes among COVID-19 patients.