Normal Liver Progenitor Cells in Culture

Abstract

The stem cell nature of normal liver progenitor cells (LPCs) is addressed by studies of normal LPCs in culture. Several questions are addressed such as: What are the patterns of proliferation, lineage commitment, differentiated gene expression, plasticity, and responses to epigenetic and environmental signals? Early studies were interpreted to show that propagable LPCs were derived from dedifferentiated or retrodifferentiated mature liver cells. The recognition that oval cells seen in hepatocarcinogenesis models in rats had characteristics of LPCs suggested that these cells might actually be LPCs or descendants of LPCs. A comparison of more than 30 publications over three decades reporting explants; clonal lines; fresh isolates or strains of cells from noncarcinogen-exposed normal mouse, rat, pig, and human liver; or embryonic tissues indicates that small, immature LPCs, which have the plasticity to mature into ductal cells or hepatocytes, can be obtained from embryonic and fetal tissues, as well as adult liver. A wide variation in the methods of isolation, culture media, feeder layers, growth factors, and substrata used to study putative LPCs in vitro makes comparisons of results from different laboratories difficult. Although the liver is endodermally derived, putative coexpression of primitive hematopoietic and hepatocytic markers is consistent with LPCs in hepatic as well as in blood-forming tissues. In addition to bile duct and hepatocytic differentiation, LPCs have been reported to express markers of pancreatic and endothelial cells in vitro, and to differentiate into bile ducts, hepatocytes, pancreatic islet and acinar epithelial cells, intestinal epithelial cells, and cardiac myocytes after transplantation in vivo. Culture of LPCs on STO embryonic fibroblast feeder layers maintains primitive phenotypes, but requirements of feeder layers appear not to be absolute and are poorly understood. Emerging trends suggest HGF, Flt-3 ligand, SCF, EGF, and DMSO promote hepatocyte differentiation of LPCs, and that transforming growth factorβ, Na+-butyrate, and culture on Matrigel promote biliary differentiation; however, exceptions have been reported. Critical studies on proliferation kinetics have not convincingly shown self-renewal and asymmetric cell division expected of tissue stem cells, but long-term doublings (up to 150 generations) without spontaneous transformation suggest considerable growth potential. The source of LPCs in normal liver remains unknown and controversial. LPCs may be derived from a liver tissue progenitor cell located in the duct or periductal tissue; from retrodifferentiation of more mature hepatocytes; from bone marrow-derived cells, which circulate through the liver; or from bone marrow remnants of intrahepatic embryonic development. Given the lack of well-defined markers for LPCs, incomplete knowledge of their growth characteristics and regulation signals, their apparent heterogeneity, their apparent plasticity, the possibilities of transdifferentiation or retrodifferentiation of other cells to LPCs, or fusion of LPCs with other cells, as well as their potential for tumorigenesis, much research needs to be conducted to understand what LPCs are and how to use them.