Nitric oxide (NO) is known to down-regulate drug-metabolizing cytochrome P450 enzymes in an enzyme-selective manner. Ubiquitin–proteasome-dependent and -independent pathways have been reported. Here, we studied the regulation of expression of human CYP51A1, the lanosterol 14α-demethylase required for synthesis of cholesterol and other sterols in mammals, which is found in every kingdom of life. In Huh7 human hepatoma cells, treatment with NO donors caused rapid post-translational down-regulation of CYP51A1 protein. Human NO synthase (NOS)-dependent down-regulation was also observed in cultured human hepatocytes treated with a cytokine mixture and in Huh7 cells expressing human NOS2 under control of a doxycycline-regulated promoter. This down-regulation was partially attenuated by proteasome inhibitors, but only trace levels of ubiquitination could be found. Further studies with inhibitors of other proteolytic pathways suggest a possible role for calpains, especially when the proteasome is inhibited. NO donors also down-regulated CYP51A1 mRNA in Huh7 cells, but to a lesser degree, than the down-regulation of the protein.

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