Effects of Constant Activation of MEK1/2 and STAT3 Pathways on the Priming of Liver Regeneration
Introduction
- Liver Regeneration
Some species, including worms, insects and amphibians, have the ability to rebuild a complex structure of entire body parts[1] , but the regenerative capacity of humans and large mammals is very limited. However, organs or body parts from these large mammals possess quite good regeneration ability. Liver has a particularly prominent regenerative capacity rapidly triggered upon injury or resection[2] . Liver regeneration is a process of compensatory hyperplasia after mass loss, in which the remaining lobes increase in size as a consequence of cell proliferation, with the goal of replacing lost functional mass[3] . Two-thirds partial hepatectomy (2/3 PHx) in rodents has been widely used as in-vivo model to study liver regeneration since its introduction[4] . Liver regeneration is triggered by surgical removal of 2/3 of the liver, with many molecules and signaling pathways involved[5] , for example, cytokines and growth factors can activate NF-kappa;B and signal transducer and activator of transcription 3 (STAT3), and transcriptional factors of activator protein 1 (AP-1) and CCAAT/enhancer binding protein (C/EBP) beta; thus initiating a cascade of gene expression and eventually lead to cell proliferation[6] . Liver regeneration can be also triggered by injury, and the other valuable animal models to study liver regeneration after liver injury induced by several hepatotoxicants such as CCl4.
- Nuclear receptors and Farnesoid X Receptor (FXR)
The nuclear receptor superfamily is one of the largest family of transcription factor that can be activated by ligands. Nuclear receptors regulate cell growth and differentiation by establishing a connection between signaling molecules and transcriptional responses, acting as master regulators for genes involved in metabolic regulations[7]. In contrast to hormones, ligands for the nuclear receptors originate from metabolites, which comprise lipophilic small molecules such as steroids, free fatty acids, active forms of vitamin A and vitamin D, etc.[8].
Like other transcription regulators, nuclear receptors exhibit a modular structure. A typical nuclear receptor is characterized with a hyper-variable and ligand-independent AF-1 transactivation domain in the N-terminal region(A/B), a highly conserved DNA binding domain(DBD) or region C to recognize specific DNA sequences, and a ligand-binding and dimerization domain(region E/F) containing a ligand-dependent AF-2 transactivation domain within the C-terminal portion.[9-11] To act as a transcription factor to regulate downstream target genes, nuclear receptors require to bind as monomers, homodimers, or heterodimers with retinoid X receptor(RXR) to the hormone response elements (HREs) within their target gene. The dimerization is mediated by a strong dimerization interface present in the LBD[12, 13], and the E/F domain helps ligand binding, dimerization and activation.
FIG.1. Schematic diagram of a nuclear receptor. [10]
FXR belongs to nuclear receptor family, and is highly expressed in liver, intestine kidney and adrenal gland. FXR regulates the transcription of genes involved in bile acid synthesis, absorption, and transportation, thereby maintains bile acids homeostasis. It has been shown that deficiency of FXR in mice results in the dysregulation of bile acid homeostasis with accompanying chaotic metabolic homeostasis[14] . FXR inhibits bile acid synthesis through a feedback mechanism involving small heterodimer partner (SHP) in the liver. After being activated by its ligand of BA, FXR induces the transcription of a negative regulator of nuclear receptor SHP, which competes the binding to Cyp7a1 with the transcription factor of liver-related homolog-1 (LRH-1) and thus inhibit the rate-limiting enzyme Cyp7a1 in the biosynthesis of bile acid[15] . At the same time, SHP interacts with hepatocyte nuclear factor (HNF) 4alpha; to inhibit its transcriptional activity, resulting in the repression of sterol 12-alpha;-hydroxylase (CYP 8B1) expression in bile acid synthesis[16] . Moreover, FXR also regulates the expression of various BA transporters, including sodium taurocholate co-transporting polypeptide (NTCP), bile salt export pump (BSEP), and ileal BA transporters (IBABP), and OST-beta;, etc.[17]. Dysregulation of FXR can also lead to liver diseases such as nonalcoholic fatty liver disease (NAFLD), liver carcinogenesis and diabetes[18, 19].
In addition to a key metabolic regulator, FXR has much broader functions such as promoting hepatic regeneration[20]. Activated-FXR specifically up-regulates the ERK signaling pathway, which targets a variety of anti-apoptotic regulators, including Bcl-2 family, caspases, or apoptotic inhibitors, to suppress liver cell apoptosis[21]. The role of FXR in preventing hepatocytes from apoptosis is consistent with its role in promoting liver repair. After liver injury, BA-activated FXR in the liver mediates the negative feedback loop by inhibiting bile acid synthesis genes of Cyp7a1, Cyp8b1, Cyp27a1, as well as bile acid uptake transporter NTCP[22], which contributes to a rapid modulation of the bile acid pool size, composition and compartmentalization, thus prevent hepatocyte from apoptosis and necrosis caused by increased bile acid concentration. In the intestine, FXR induces OSTalpha; and OSTbeta; to reduce the accumulation of bile acids in intestinal cells[23, 24]. FXR activation in the intestine also induces fibroblast growth factor 15 (Fgf15, human ortholog FGF19), inhibiting the synthesis of Cyp7a1 and bile acid synthesis in the liver. At the same time, FXR promotes hepatocyte proliferation for proper liver repair through a direct activation of the transcription of Forkhead box m1b (Foxm1b) , which is a key regulator of hepatic cell cycle progression[25]. Moreover, FXR rewires mitochondrial metabolism to fuel liver growth as a result of activation of the pyruvate dehydrogenase lipoamide kinase isozyme 4 (PDK4)[26]. In summary, FXR promotes liver regeneration directly, in addition to its independent roles through regulating the BA homeostasis during the liver regeneration process[27] .
- Bile Acid and Its Receptors
BAs are synthesized in the liver, stored in the gall bladder, and secreted into the intestinal tract postprandially to help the digestion and absorption of fats and fat-soluble vitamins in the small intestine. Most of them (95%) are reabsorbed and transported back to the liver through the portal vein, which is known as enterohepatic circulation. Bile acids are the final products of cholesterol catabolism and play a critical role in cholesterol metabolism and energy homeostasis in the body and intestinal absorption of nutrients[28] . Cholesterol is an important component of intimal structure in the body, and its metabolic disturbance would cause a series of metabolic diseases such as atherosclerosis and hyperlipoproteinemia. Cholesterol cannot be decomposed directly in the body, while bile acids form the major excretory route, using cholesterol as a raw material to convert it into bile acids through a series of enzymatic catalytic reactions. Approximately 30% of cholesterol metabolism is completed by BA synthesis[29, 30] . Additionally, BAs increase the solubility of cholesterol in bile by the micellar solubilization, enabling cholesterol and some free cholesterol synthesized in the liver to move from the hepatocyte to the intestinal lumen, ultimately leading to elimination via the fecal route[31] .
