Figure 2

The relationship between bile acid receptor and COVID-19. Both TGR5 (bile acid receptor 1), a membrane receptor, and FXR (bile acid receptor 2), a nuclear receptor, serve as the primary receptors for bile acids. Both receptors participate in affecting the expression of ACE2, which directly controls the entry of SARS-CoV-2 into host cells. SARS-CoV-2 infection/COVID-19 triggers immune system activation, which in some cases can lead to an excessive release of cytokines, causing cytokine storm syndrome and potentially severe complications. Bile acids regulate crucial pathways involved in immune system activation and the development of cytokine storm syndrome triggered by SARS-CoV-2, potentially contributing to the mitigation of COVID-19's clinical manifestations. BA, bile acid; FXR, farnesoid X receptor; TGR5, G-protein-coupled bile acid receptor; ACE2, angiotensin-converting enzyme 2; SARS-CoV-2, severe acute respiratory syndrome coronavirus 2; COVID-19, coronavirus disease 2019; AKT, protein kinase B; IRF3, interferon regulatory factor 3; PKA, protein kinase A; cAMP, cyclic adenosine monophosphate; CREB, cAMP response element-binding protein; IL, interleukin; TNF-α, tumor necrosis factor-alpha; NF-κB, nuclear facto-kappa B; IκBα, NF-κB inhibitor alpha; IFN, interferon; NLRP3, nucleotide-binding oligomerization domain, leucine-rich repeat and pyrin domain-containing 3; T_reg cell, regulatory T cell; AP-1, activator protein 1; SOCS3, suppressor of cytokine signaling 3; STAT3, signal transducer and activator of transcription 3.