Abstract
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.