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Aim: 1,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) inhibits cell growth and induces apoptosis in numerous types of tumors. We aimed to examine the mRNA and protein expression of 1,25(OH)(2)D(3)-inactivating CYP24A1 and mRNA expression of the activating CYP27B1 enzymes, as well as that of vitamin D receptor (VDR), in hepatocellular carcinoma (HCC) cell cultures in response to 1,25(OH)(2)D(3) administration., Materials and Methods: Increasing amounts of 1,25(OH)(2)D(3) (0.256-10 nM) were added to cultures of HepG2, Huh-Neo, Hep3B, Huh5-15 human HCC cell lines and cells then incubated for various time periods (30 min-28 h). The mRNA expression was analyzed by real time reverse transcription-polymerase chain reaction (RT-PCR). CYP24A1 protein in HepG2 cells was detected by immuncytochemistry., Results: CYP24A1 mRNA expression significantly (p<0.0001) increased in response to 1,25(OH)(2)D(3) administration in two cell lines: in HepG2 cells, the CYP24A1 mRNA level exhibited 5,300-fold elevation, reaching a maximum value at 8 h; in Huh-Neo cells, the increase was 152-fold that of the baseline value, with the maximum being reached at 14 h. There was no significant change in Hep3B and Huh5-15 cell lines, nor was there any change in CYP27B1 and VDR gene expression in any cell cultures. Immuncytochemistry in HepG2 cells proved that gene activation was followed by CYP24A1 protein synthesis., Conclusion: Our novel data indicate that administration of 1,25(OH)(2)D(3) results in a marked increase of CYP24A1 mRNA expression in some, but not all, human HCC lines in vitro. These differences could be dependent upon the origin of the tumor cells.