Introduction
Cholestatic liver disease (CLD) manifests as a multitude of etiological heterogeneous hepatobiliary disorders, mainly including primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC), in adults. CLD, which is also called cholestasis, describes a range of conditions caused by the accumulation of bile acids in the liver, resulting in hepatocellular necrosis, apoptosis, progressive fibrosis, and even end-stage liver disease[1].
Accumulated evidence has proved that ursodeoxycholic acid (UDCA) can decrease the progression of PBC[2]; however, approximately one-third of patients do not respond to UDCA treatment[3]. In 2016, obeticholic acid (OCA), a farnesoid X receptor (FXR) agonist, was found could decrease serum alkaline phosphatase (AP) level in CLD patient, was approved by the FDA for UDCA nonresponders[4, 5]. In spite of this, long-term follow-up is needed to confirm the safety and effectiveness of this novel treatment. At present, there is no clinical evidence that there is any medical therapy can alter the course of PSC. Current treatment regimens focus on symptom management and treatment of cholangitis[6]. Effective drugs for PBC and PSC are still limited and new treatment strategies are urgently needed.
Increasing evidence shows that human cholestasis is closely related to the disorder of microbiome composition[7, 8], increased intestinal permeability, enhanced translocation of pathogenic bacteria and bacterial toxins, such as lipopolysaccharide (LPS) into the liver[9, 10]. Inflammasomes and proinflammatory cytokines are then activated due to these microbe-derived products, which are recognized by the innate immune system via pathogen recognition receptors (TLRs and NLRs)[11].When persistent liver inflammation is unresolved, the proinflammatory milieu can play a detrimental role in parenchymal and nonparenchymal liver cells resulting in fibrosis and ultimately loss of function[12].
Macrophages, which are composed of resident tissue macrophages and monocyte-derived recruited cells, can differentiate into either classically activated macrophages (a pro-inflammatory phenotype, also called M1 polarity) or alternatively activated macrophages (M2 polarity) which express anti-inflammatory cytokines[13]. Recent studies have demonstrated that the intestinal microbiome leads to the process of cholestasis-mediated cell death and inflammation by activating the mechanisms of the inflammasome in macrophages[14]. Additionally, clinical studies have suggested that in cholestasis patients, the recruitment of monocytes and macrophages in diseased liver is significantly increased[15]. The expression of various monocyte chemotactic proteins, such as monocyte chemoattractant protein (MCP)-1 is significantly increased in the livers of patients with cholestasis[16]. In parallel to these findings, C–C chemokine receptor type-2 (CCR-2) expressed by liver macrophages is accompanied by increasing macrophage numbers in the livers of cholestasis patients[17].
Importantly, macrophages in CLD patients are more susceptible to stimulation signals such as endotoxin (for example, LPS), which have also been shown to be increased in cholestasis patients[18]. In line with the above findings, the expression and activity of TLR4 (the primary receptor of LPS) are upregulated on the monocytes of CLD patients, leading to LPS hyperreactivity and increased production of proinflammatory cytokines [i.e., interleukin (IL)-1β, IL-6, and IL-8][19]. It is reported that macrophage-derived IL-1β is a key cytokine activating hepatocyte nuclear factor κB (NFκb), which is the master inflammation regulator. Activation of NFκb can also interfere with FXR and liver X receptor (LXR) signaling, which results in transcriptional suppression of bile and sterol transporters, finally culminating in cholestasis[20]. The above evidence indicates that hepatic macrophages, namely Kupffer cells (KCs), exert a pivotal role during the development of CLD.
Traditional Chinese medicines (TCMs) have been demonstrated to be an important source for potential drug discovery[21]. Tectorigenin (TEC), a plant isoflavone, has attracted much attention due to its multiple activities such as antiproliferation, antiinflammatory and antioxidant effects. We and others have demonstrated that TEC can inhibit macrophage activation (M1 polarity) in vivo and in vitro[22, 23]. However, the underlying molecular mechanisms still require further investigation. It is noteworthy that TEC has been widely reported to provide protective functions in the liver[24-26]. This evidence prompted us to hypothesize that TEC could alleviate CLD by suppressing hepatic macrophage recruitment and activation. Thus, in the present study, we investigated whether TEC intervention could improve the development of CLD in the ANIT-induced and DDC-induced mouse models. We also investigated the molecular mechanism by which TEC regulates the polarization of bone marrow-derived macrophages (BMDMs) and primary mouse KCs.