Hepatic cancer stem cells may arise from adult ductal progenitors

Abstract

Cancer stem cells (CSCs) are defined as cells within tumors that can self-renew and differentiate into heterogeneous lineages of cancerous cells. The origin of CSCs is not well understood. Recent evidence suggests that CSCs in hepatocellular carcinoma could be generated via oncogenic transformation and partial differentiation of adult hepatic ductal progenitor cells.

Hepatocellular carcinoma (HCC) is the fifth most common primary malignancy worldwide and the third most common cause of cancer-related death.¹ In most cases it develops after chronic inflammation, liver fibrosis, or cirrhosis, which are induced by hepatitis B or C virus (HBV and HCV) infection, alcohol consumption or drug abuse, or metabolic disease-mediated liver damage. Liver damage may arise from various physiologic or non-physiologic stimuli, including nutritional signals leading to fatty liver formation, elevated levels of toxic substances such as bile acids or drug metabolites that directly cause hepatocyte death, or intrinsic events that cause genome instability. HCC is essentially a non-curable disease, as current therapeutic approaches do not seem to be effective in shrinking cancerous lesions or in preventing cancer formation.²

Resistance to cytotoxic chemotherapeutics and increased tumor invasiveness has been attributed to the existence of a unique population of tumor cells with stemness properties, the so-called cancer stem cells (CSCs).

Cancer stem cells are defined by features of self-renewal capacity and multilineage differentiation potential, and can be efficiently propagated in xenografts, where they recapitulate with high fidelity the heterogeneity and pathological features of the original tumor.³ Cancer stem cells can thus establish phenotypic and functional heterogeneity of cells inherent to the primary tumor and contribute to the hierarchical organization of cells within tumors by fueling their own renewal and the parallel generation of more differentiated cancer cells. Over the past decade the cancer stem cell concept has attracted major attraction as it provides explanations for several poorly understood clinical phenomena including differential tumor invasiveness, metastatic potential, resistance to chemotherapy, and tumor growth recurrence after treatment.

The cancer stem cell concept emerged at the forefront of cancer research with the identification of molecular markers that allow the isolation of CSCs. Such markers, including Cd133, Cd44, Aldh1, EpCAM and Sox9, have been used to define CSC populations in hepatocellular carcinoma and their presence correlates with tumor invasiveness and poor prognosis after treatment. Interestingly, several of these markers readily identify a subset of cells in the ductal epithelium of the liver that have been recently identified as bona fide hepatic progenitor cells and can differentiate to hepatocytes under specific conditions of liver damage.⁴ These findings raised the possibility that hepatic CSCs might be derived directly from normal adult progenitor cells, but retain self-renewal and differentiation properties and acquire cancer cell properties as they go through the oncogenic transformation process. Alternatively, they may arise via reprogramming of terminally committed hepatocytes and gain “stemness” characteristics by the induction of specific stem cell-specific regulators. The above 2 possibilities are depicted in Figure 1.

Figure 1.

Origin of cancer stem cells in hepatocellular carcinoma. Left panel: During the oncogenic transformation process some hepatocytes may dedifferentiate and acquire stem cell properties. Right panel: Activated adult hepatic progenitor cells located in the biliary epithelium partially differentiate to a hepatic lineage and undergo oncogenic transformation. HCC, hepatocellular carcinoma.

The pathway assuming hepatocyte dedifferentiation is supported by recent studies, which demonstrated that hepatocytes possess a high level of plasticity and are capable of undergoing reprogramming toward the biliary epithelial cell lineage or toward cells bearing progenitor properties following liver damage or intrahepatic cholangiocarcinoma formation.^5,6 Thus, one can assume that under specific injury conditions, reprogramming of cancerous hepatocytes to cells with stem cell properties could give rise to CSCs. In line with this scenario is the recent identification of bona fide HCC progenitor cells (HcPCs) in injury-induced mouse HCC models, probably generated by damage-induced dedifferentiation of mature hepatocytes.⁷

In contrast, our recent study demonstrated that hepatic CSCs could also be derived from normal hepatic progenitors located in the ductal epithelium of the liver.⁸ The generation of hepatic CSCs from ductal progenitors was observed in liver-specific Setd8-KO (Setd8, best known as PR-SET7) mice, which spontaneously develop hepatocellular carcinoma. PR-SET7 is an H4K20 monomethyltransferase that is essential for maintaining genome integrity. Inactivation of PR-SET7 in hepatocytes led to genome instability and cell duplication-dependent necrosis. Initial cell death triggered inflammation and fibrosis as a result of the activation of hepatic stellate cells. In parallel, a local regenerative process was induced, whereby neighboring hepatocytes entered the cell cycle and accelerated the cell death process. Prolonged inflammation and cell death resulted in elevated production of IL-6 and reactive oxygen species (ROS), which triggered STAT3 activation and oncogenic transformation of the newly generated hepatocytes. Because PR-SET7-deficient hepatocytes are destined to undergo necrotic cell death after the first cell division all newly generated hepatocytes were derived from ductal progenitors, which started to proliferate early during the process. The livers of PR-SET7-KO mice were repopulated by hepatocytes expressing CSC markers, such as Sox9, CD133, EpCAM, and Cd44. Their potential to self-renew in culture and give rise to tumors containing stem cell marker-expressing cells and more differentiated cells in xenografts demonstrates that they correspond to cancer stem cells.

The significance of ductal progenitors in liver regenerative processes has recently been questioned, since their contribution to regeneration of the liver cell mass after different forms of injury was negligible.^9,10 These studies established that old hepatocytes serve as the major, if not exclusive, source of liver regeneration, regardless of the type of injury.^9,10 However, in other conditions when hepatocytes cannot proliferate, ductal progenitors represent the only source of “new hepatocytes.” In liver-specific PR-SET7-KO mice, elimination of existing hepatocytes and the activation of ductal progenitor cells initiated regenerative cycles coupled with inflammation and oncogenic transformation. As a result, HCC in the liver-specific PR-SET7-KO mice is composed mainly of CSC subtypes. Therefore these animals represent a unique model for studying CSC biogenesis and for testing drugs that specifically target hepatic cancer stem cells.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Funding

Work in the authors’ laboratory is supported by European Research Council Advanced Investigator Grant (ERC-2011-AdG294464) and the Greek Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF), Thales (Thales 656).