Molecular and cell biology of hepatopancreatobiliary disease: Introduction and basic principles

Notch signaling

The Notch pathway plays a significant role in cell fate determination and homeostasis of several adult tissues. Notch receptors and ligands are transmembrane proteins, with four Notch receptors (Notch1–4) and five Notch ligands (Dll1, Dll3, Fll4, Jagged1, and Jagged2) found in mammals. Interactions between receptor and ligand on adjacent cells results in several proteolytic cleavages of the Notch receptor at three distinct sites (S1, S2, and S3), releasing the Notch intracellular domain (NICD). NICD translocates to the nucleus, where it binds CBF1/RBPj, displacing co-repressor complexes, and recruits coactivator molecules, including Mastermind-like (Maml). In the liver, Notch signaling is critical to the development of the intrahepatic biliary system (see Chapter 1 ) and is also crucial in liver regeneration (see Chapter 6 ). This pathway is activated during liver repair and has a clear role in biliary regeneration, although its utility in hepatocyte regeneration is less clear. Notch1 and Jagged1 are both induced by hepatectomy and injury, directing the formation of biliary cells. Alagille syndrome, which is characterized by an autosomal dominant mutation in Notch2 or Jagged2 (encodes a ligand in the Notch pathway), results in malformation of the intrahepatic biliary tree as well as other abnormalities. In liver malignancies, the role of Notch signaling is ambiguous. Notch1 knockout mice develop proliferation and dedifferentiation of endothelial cells with eventual spontaneous development of hepatic angiosarcoma, suggesting a tumor suppressive role. By contrast, constitutive expression of both activated Notch1 and the intracellular domain of Notch2 lead to spontaneous hepatocellular carcinoma (HCC) at 12 months. ^,

In the pancreas, Notch signaling is similarly critical to tissue homeostasis. Activation in pancreatic progenitors prevents specification into either endocrine or exocrine lineages, whereas inhibition of the pathway results in premature differentiation to neurogenin-3 ( Ngn3 ⁺ ) expressing endocrine cells. As in the liver, injury to the adult tissue unveils developmental Notch signaling to permit regeneration following pancreatitis.

Hedgehog

Sonic hedgehog (SHH) is an essential pathway involved in cell growth and tissue patterning. Canonical SHH signal transduction consists of multiple Hedgehog (Hh) ligands, two transmembrane receptors (Patched [PTCH1, PTCH2] and Smoothened [SMO]), and the glioma-associated oncogene homolog (GLI) family of transcription factors (GLI1, GLI2, GLI3). When Hh ligands bind to the receptor Patched, it blocks the inhibitory effect of Patched on the co-receptor Smoothened. Active SMO leads to GLI protein activation with nuclear translocation and activation of target genes. Hedgehog signaling is involved in normal pancreatic and liver development, with mouse models indicating the need for Shh for appropriate organogenesis.

Hh ligands are largely absent in the adult liver but are reactivated in response to liver injury and at times of liver regeneration. In injury, epithelial cells produce Hh ligands that act in a paracrine fashion on mesenchymal-type cells. The pathway is critical to crosstalk among multiple cell types and specifically promotes viability of progenitor cells and activates hepatic stellate cells. Hedgehog signaling plays a role in chronic liver disease with upregulation of PTCH and GLI factors in nonalcoholic fatty liver disease (NAFLD), biliary cirrhosis, and hepatitis B (HBV) and C (HCV). Hedgehog signaling has also been identified in the development of HCC, with correlative data suggesting poorer outcomes in patients with increased SMO. Similarly, in the pancreas, Hedgehog signaling is enhanced early in tumorigenesis and in stromal fibroblasts, with conflicting data suggesting either supportive or restraining roles for enhanced Hedgehog signaling. ^,

Wnt/β-catenin

The Wnt/β-catenin pathway is critical for both pancreatic and liver development and for regeneration. β-Catenin, a cytoplasmic protein, is typically maintained in a phosphorylated state and targeted for degradation by adenomatous polyposis coli (APC) product, casein kinase 1 (CK1), and glycogen synthase kinase-3 (GSK3)-β. Nevertheless, when Wnt proteins, typically glycosylated and acylated moieties, engage the Frizzled (Fz) receptor in the context of a ternary complex, hypophosphorylated β-catenin is released where it can translocate to the nucleus and induce target gene expression.

Similar to Notch, fibroblast growth factor (FGF), and bone morphogenetic protein (BMP) signaling, Wnt signaling is central in the determination of endodermal patterning. Wnt is initially suppressed in early liver development and then activated later, and this pathway is critical to metabolic zonation in the liver. In both hepatectomy and injury models, β-catenin is important for regeneration and the hepatocyte proliferation that occurs. In the pancreas, the absence of β-catenin in animal models leads to disrupted development of the exocrine pancreas. Similarly, gain-of-function experiments show that Wnt pathway activation also results in a hypoplastic pancreas after abnormal acquisition of intestinal features.

Wnt/β-catenin pathways are commonly altered in benign and malignant tumors of the liver. Mutations or deletions of the β-catenin gene, CTNNB1 , or mutations in other genes or aberrant expression of proteins in the pathway, result in cytoplasmic stabilization and nuclear translocation of β-catenin and ultimately dysregulated transcription of cell cycle regulatory proteins. β-Catenin mutated hepatic adenomas account for approximately 10% to 15% of hepatic adenomas and are associated with an increased risk for transformation into HCC (see Chapter 87, Chapter 88, Chapter 89 ). Additionally, abnormal expression of β-catenin is noted in 17% to 40% of HCC and 50% to 75% of hepatoblastoma. In pancreatic cancer, β-catenin stabilization results in the formation of pancreatic tumors, affirming an oncogenic role across the spectrum of HPB disease.