Side Population Cells From an Immortalized Human Liver Epithelial Cell Line Exhibit Hepatic Stem-Like Cell Properties

The existence of hepatic stem cells in human livers is controversial. We investigated whether the side population (SP) cells derived from an immortalized human liver epithelial cell line THLE-5b possess the properties of hepatic stem-like cells. SP cells derived from THLE-5b were isolated using flow cytometry and were assayed for the expression of phenotypic markers by reverse transcription polymerase chain reaction and immunostaining. THLE-5b SP cells retained the capacity to generate both SP and non-SP cells, showed a capacity for self-renewal, and were more efficient in colony formation than non-SP cells. Neither the SP nor the non-SP cells formed tumors when transplanted into athymic nude mice or severe combined immunodeficient mice. The expression level of stem cell-associated markers such as an ATP-binding cassette membrane transporter, epithelial cell adhesion molecule, c-kit, Thy-1, and octomer binding transcription factor 4 was higher in SP cells than in non-SP cells. When cultivated as rotation-mediated aggregates, the expression of liver-specific genes including tryptophan oxygenase and CYP3A4 was up-regulated in SP cells, suggesting that THLE-5b SP cells have the ability to differentiate into a hepatocyte phenotype. One of the clonal cell lines derived from the SP cells expressed stem cell-associated markers. These results indicate that SP cells derived from THLE-5b possess hepatic stem-like cell properties and suggest that THLE-5b can be used as a model of normal human liver progenitor or stem cell line.