Editorial

Progenitor cell involvement in cirrhotic human liver diseases: from controversy to consensus

Ductular reaction (DR) is observed in virtually all liver diseases in both humans and rodents. Depending on the injury, DR is confined within the periportal area or invades the parenchyma. On severe hepatocellular injury, invasive DR has been proposed to arise for supplying the liver with new hepatocytes. However, experimental data evidenced that DR contribution to hepatocyte repopulation is at the most modest, unless replicative capacity of hepatocytes is abrogated. Herein, we proposed that invasive DR could contribute to operating hepatobiliary junctions on hepatocellular injury. The choline-deficient ethionine-supplemented mouse model of hepatocellular injury and human liver samples were used to evaluate the hepatobiliary junctional role of the invasive form of DR. Choline-deficient ethionine-supplemented–induced DR expanded as biliary epithelium into the lobule and established new junctions with the canaliculi. By contrast, no new ductular-canalicular junctions were observed in mouse models of biliary obstructive injury exhibiting noninvasive DR. Similarly, in humans, an increased number of hepatobiliary junctions were observed in hepatocellular diseases (viral, drug induced, or metabolic) in which DR invaded the lobule but not in biliary diseases (obstruction or cholangitis) in which DR was contained within the portal mesenchyme. In conclusion, our data in rodents and humans support that invasive DR plays a hepatobiliary junctional role to maintain structural continuity between hepatocytes and ducts in disorders affecting hepatocytes.

Chronic liver diseases are characterized by expansion of the small immature cholangiocytes – a mechanism named ductular reaction (DR) – which have the capacity to differentiate into hepatocytes. We investigated the kinetics of this differentiation, as well as analyzing several important features of the newly formed hepatocytes, such as functional maturity, clonal expansion and resistance to stress in mice with long-term liver damage.

We tracked cholangiocytes using osteopontin-iCreER^T2 and hepatocytes with AAV8-TBG-Cre. Mice received carbon tetrachloride (CCl₄) for >24 weeks to induce chronic liver injury. Livers were collected for the analysis of reporter proteins, cell proliferation and death, DNA damage, and nuclear ploidy; hepatocytes were also isolated for RNA sequencing.

During liver injury we observed a transient DR and the differentiation of DR cells into hepatocytes as clones that expanded to occupy 12% of the liver parenchyma by week 8. By lineage tracing, we confirmed that these new hepatocytes derived from cholangiocytes but not from native hepatocytes. They had all the features of mature functional hepatocytes. In contrast to the exhausted native hepatocytes, these newly formed hepatocytes had higher proliferative capability, less apoptosis, a lower proportion of highly polyploid nuclei and were better at eliminating DNA damage.

In chronic liver injury, DR cells differentiate into stress-resistant hepatocytes that repopulate the liver. The process might account for the observed parenchymal reconstitution in livers of patients with advanced-stage hepatitis and could be a target for regenerative purposes.

During chronic liver disease, while native hepatocytes are exhausted and genetically unstable, a subset of cholangiocytes clonally expand to differentiate into young, functional and robust hepatocytes. This cholangiocyte cell population is a promising target for regenerative therapies in patients with chronic liver insufficiency.

Chronic cholestatic liver diseases such as primary biliary cholangitis (PBC) and primary sclerosing cholangitis (PSC) are associated with active hepatic fibrogenesis, and, ultimately, to the development of cirrhosis. However, the precise relationship between cholestasis, in its broad meaning, and liver tissue fibrosis is still poorly defined. Fibrogenesis is currently viewed as a dynamic process that appears strictly related to the extent and duration of parenchymal injury. This relationship is clearly evident in the presence of reiterative hepatocellular necrosis due to viral infection or alcohol abuse. It appears that “pure” intralobular intrahepatic cholestasis secondary to biliary secretory failure of the hepatocyte, in absence of hepatocellular damage, lobular inflammation and bile duct damage and/or proliferation, is not associated with marked and/or progressive liver tissue fibrosis. In contrast, marked and progressive liver tissue fibrosis always follows liver diseases characterized by chronic inflammatory bile duct damage as seen in PBC and PSC or chronic mechanical obstruction of the biliary tree. Overall, the fibrogenic process in these clinical conditions appears to be related to a more complex interaction between immune/inflammatory mechanisms, cytokine networks and the derangement of the homeostasis between epithelial and mesenchymal cells. The elucidation of these mechanisms is indeed crucial for the identification of potential diagnostic and therapeutic targets.

Despite a very slow cell turnover rate in healthy liver, the damaged liver has an amazing capacity to regenerate itself. This regenerative capability of the liver is highly complex and involves, depending on the type and severity of the liver injury, cross talk between numerous resident hepatic cells and recruited bone marrow cells. Two main mechanisms account for the restoration of the liver mass after injury: proliferation of existing individual cell types being mainly hepatocytes (stem cell–independent mechanism) or transdifferentiation from stem cells/hepatic progenitor cells (stem cell–dependent mechanism). Although the two mechanisms are not entirely mutually exclusive, the relative degree of their contribution to liver repopulation has not been addressed adequately. This chapter provides the most recent data regarding the contribution of different cell types and their products as well as numerous signaling pathways, in several models of regenerating liver.

Hepatocellular carcinoma (HCC) accounts for approximately 6% of all new cancer cases diagnosed, and due to its aggressiveness, it is the second most common cause of cancer mortality worldwide. Based on different etiological factors, genetic backgrounds, and longtime development of the disease, HCC is characterized by a high phenotypic and functional heterogeneity. Tumor variability occurs both among patients (intertumoral heterogeneity) and within a single tumor (intratumoral heterogeneity). The intratumoral heterogeneity, in particular the variability of the markers of cancer stem cells (CSC) population may determine specific behavior and prognosis of the tumor. Understanding the cellular mechanisms originating CSC will provide an important hint in the management of HCC. The characterization of the cells of origin of cancer can have significant implication in early diagnosis, in the development of appropriate therapies and in the prevention of relapse. Here, we review recent evidences on the possible cellular origin of CSC that play a role in the heterogeneity of the HCC.