Maladaptive regeneration - the reawakening of developmental pathways in NASH and fibrosis

Abstract

With the rapid expansion of the obesity epidemic, nonalcoholic fatty liver disease is now the most common chronic liver disease, with almost 25% global prevalence. Nonalcoholic fatty liver disease ranges in severity from simple steatosis, a benign 'pre-disease' state, to the liver injury and inflammation that characterize nonalcoholic steatohepatitis (NASH), which in turn predisposes individuals to liver fibrosis. Fibrosis is the major determinant of clinical outcomes in patients with NASH and is associated with increased risks of cirrhosis and hepatocellular carcinoma. NASH has no approved therapies, and liver fibrosis shows poor response to existing pharmacotherapy, in part due to an incomplete understanding of the underlying pathophysiology. Patient and mouse data have shown that NASH is associated with the activation of developmental pathways: Notch, Hedgehog and Hippo-YAP-TAZ. Although these evolutionarily conserved fundamental signals are known to determine liver morphogenesis during development, new data have shown a coordinated and causal role for these pathways in the liver injury response, which becomes maladaptive during obesity-associated chronic liver disease. In this Review, we discuss the aetiology of this reactivation of developmental pathways and review the cell-autonomous and cell-non-autonomous mechanisms by which developmental pathways influence disease progression. Finally, we discuss the potential prognostic and therapeutic implications of these data for NASH and liver fibrosis.

Fig. 1 |. Overview of developmental pathways in mammalian cells.

a | Notch signalling. Canonical Notch signalling is activated by the ligand-to-receptor interaction between two neighbouring cells. Upon ligand binding, Notch receptor undergoes sequential cleavages by a disintegrin and metalloproteinase (ADAM) protease and γ-secretase, leading to the release and nuclear translocation of the Notch intracellular domain (NICD). NICD interacts with immunoglobulin-κJ region (RBPJ) and Mastermind (MAM) to initiate the transcription of downstream targets such as the HES and HEY family of genes. Notch receptor can also induce non-canonical signalling by regulating β-catenin protein degradation. b | Hippo–YAP–TAZ signalling. In the Hippo-off state, the inactive STK3/4–LATS1/2 cascade is unable to phosphorylate YAP and TAZ. Unphosphorylated stable YAP and TAZ translocate into the nucleus and bind to the transcription factor TEAD family to regulate transcription. c | Hedgehog signalling. Hedgehog (HH) ligand binds to the cell-surface receptor Patched (PTCH), releasing its inhibition of Smoothened (SMO). Active SMO prevents the phosphorylation and cytoplasmic sequestration of GLI protein by kinases including protein kinase A (PKA), glycogen synthase kinase 3β (GSK3β) and casein kinase 1 (CK1). Stabilized GLI enters the nucleus and promotes Hedgehog target gene expression. d | WNT–β-catenin signalling. Binding of the WNT ligand to its receptor Frizzled and co-receptor low-density lipoprotein receptor-related protein 5 or 6 (LPR5/6) recruits the destruction complex, consisting of axin, adenomatous polyposis coli (APC), and the kinases GSK3β and CK1α, to the cell membrane via Dishevelled (DVL). This coordinated effort prevents the destruction complex from phosphorylating β-catenin, which facilitates its degradation. Stabilized β-catenin translocates to the nucleus, binds to the T cell factor (TCF) transcription factor family and promotes the expression of WNT target genes.

Fig. 2 |. A network of developmental signalling controls lineage commitment during mouse liver development.

Liver development is regulated by a highly controlled network of pathways in a spatiotemporal manner. Near the portal vein, portal fibroblast-derived Jagged1 activates NOTCH2 on hepatoblasts to induce biliary differentiation, in conjunction with transforming growth factor-β (TGFβ) and YAP–TAZ, by transactivation of biliary lineage-defining factors, including SOX9, hepatocyte nuclear factor 1β (HNF1β) and HNF6. In the absence of these signals, increased HNF1α and HNF4α promote a hepatocyte lineage. In parallel, responding to WNT signals near the central vein, β-catenin activates pericentral genes to establish normal liver zonation.

Fig. 3 |. Cellular reprogramming during mouse liver regeneration.

In response to acute injury, hepatocytes proliferate to maintain liver functional capacity. However, when the insult persists, Notch and YAP signalling pathways are activated in a subset of hepatocytes, resulting in ‘biphenotypic’ cells with the expression of biliary and progenitor markers such as SOX9 and osteopontin (OPN). Biphenotypic cells can dedifferentiate into progenitor-like cells as a strategy to evade the insult, with the potential to re-differentiate back into hepatocytes upon injury cessation. Alternatively, biphenotypic cells can also transdifferentiate into cholangiocytes to cope with injury-induced cholestasis. When injury persists, ductular reaction or oval cell response is induced, originated mainly from cholangiocytes, which can contribute to liver regeneration when hepatocyte proliferative capacity is severely compromised. In the absence of hepatoblast Notch and hepatocyte nuclear factor 6 (HNF6), transforming growth factor-β (TGFβ) can functionally substitute Notch signalling to form a functional biliary system.

Fig. 4 |. Pathogenesis of nasH and fibrosis in mouse models.

As a result of chronic substrate overload, some steatotic hepatocytes undergo cell death due to sustained lipotoxicity and endoplasmic reticulum (ER) stress, which releases factors such as damage-associated molecular patterns (DAMPs) and Hedgehog (HH) ligands, which activate resident Kupffer cells and hepatic stellate cells (HSCs), resulting in inflammatory and fibrogenic responses. Other hepatocytes become reprogrammed by Notch, HH and YAP–TAZ to initiate a maladaptive response that induces hepatocyte secretion of fibrogenic factors such as osteopontin (OPN) and HH ligands to activate HSCs. This process creates a regenerative niche that aggravates fibrosis as well as compensatory proliferation to restore loss of mass. A by-product of this proliferative response is genomic instability, which might contribute to tumorigenesis. HCC, hepatocellular carcinoma; IHH, Indian Hedgehog; NASH, nonalcoholic steatohepatitis; NICD, Notch intracellular domain.

Fig. 5 |. The role of developmental pathways in liver cancer.

In the injured and fibrotic liver, hepatocyte developmental pathway activity results in proliferation and cellular transformation. Notch activation induces a range of cell-cycle genes, the autocrine mitogen Igf2, and biliary and progenitor-associated genes (that is, Sox9 and Spp1) that regulate the tumour microenvironment (TME) and cellular plasticity. Enhanced cell–cell contact and extracellular matrix (ECM) stiffness as well as a variety of nutrient factors, such as bile acids and cholesterol, increase YAP–TAZ stability, nuclear translocation, and transcription factor TEAD activity to augment proliferation and influence cellular metabolism and the TME to favour tumorigenesis, and might drive further Notch activity. In parallel, β-catenin activation, in part due to driver mutations in Ctnnb1, induces a T cell factor (TCF)-dependent proliferative response, alters hepatocyte metabolism via regulation of glutamine synthetase (encoded by Glul) and peroxisome proliferator-activated receptor-α (encoded by Ppara), and promotes tumoural T cell exclusion. HSC, hepatic stellate cell; NICD, Notch intracellular domain; RBPJ, immunoglobulin-κJ region.