Regulation of apolipoprotein A-I gene expression by the histamine H1 receptor: Requirement for NF-κB

Aims: Earlier it had been found by us that apolipoprotein A-I (apo A-I) is suppressed by histamine in HepG2 cells. Histamine has been shown to regulate NF-kappa B activity, though not in hepatocytes. Therefore we examined the role of the histamine receptors and NF-kappa B in histamine-mediated apo A-I gene expression in HepG2 liver cells. Main methods: The effect of histamine on histamine H1 receptor expression, and NF-kappa B p65 and p50 subunits was examined by Western blot. Histamine H1 receptor involvement was examined by loss-of-function (via siRNA) and gain-of-function studies overexpressing the histamine H1 receptor. The requirement for the p65 subunit of NF-kappa B for histamines effect was elucidated by loss-of-function studies (siRNA). Finally, the effect of histamine on NF-kappa B binding to the apo A-I gene promoter was examined by chromatin immunoprecipitation. Key findings: Treatment of HepG2 cells with histamine had no effect on histamine H1 receptor expression. However, treatment with histamine increased NF-kappa B p65 and p50 subunit expression significantly. At low levels, the exogenous histamine H1 receptor plasmid suppressed apo A-I gene promoter activity while addition of higher levels of plasmid DNA actually increased apo A-I gene promoter activity. Inhibition of NF-kappa B activity with SN50 prevented histamine from repressing apo A-I promoter activity as did silencing p65 expression via siRNA. Finally, treatment with histamine increased binding of the p65 subunit of NF-kappa B to the apo A-I gene promoter. Significance: Histamine suppresses apo A-I gene expression in hepatocytes via the histamine H1 receptor by elevating NF-kappa B expression and binding to the apo A-I promoter.