Indirect activation of constitutive androstane receptor in three-dimensionally cultured HepG2 cells

Constitutive androstane receptor (CAR), a member of the nuclear receptor superfamily, is retained as an inactive form phosphorylated at threonine in the cytoplasm of hepatocytes. Upon activation, CAR is dephosphorylated to move into the nucleus and induces the transcription of genes. Thus, nuclear translocation is a key step for CAR activation in hepatocytes. However, this nuclear translocation has not been demonstrated in conventional two-dimensionally-cultured immortalized cell lines such as HepG2, in which CAR spontaneously accumulates in the nucleus. In this study, we showed that treatment with the indirect CAR activator phenobarbital activated transcription of the CYP3A4 gene in three-dimensionally (3D)-cultured HepG2 cells. CAR was retained as its phosphorylated form in the cytoplasm and was translocated to the nucleus in 3D-cultured HepG2 cells in response to treatment with phenobarbital. Moreover, okadaic acid and epidermal growth factor, were found to repress phenobarbital-induced CAR nuclear translocation and subsequent activation of the CYP3A4 gene promoter. These results suggested that 3D-cultured HepG2 cells properly regulated CAR activation as has been observed in hepatocytes.