Received for publication January 8, 2008.
Revised May 9, 2008.
Accepted for publication May 9, 2008.

Autoregulation of McA-RH7777 hepatoma cell proliferation by histamine H₃ receptors

Abstract

Previous studies suggested that histamine (HA) acts as an autocrine growth factor. We have explored the modulation of cell proliferation by HA using McA-RH7777 hepatoma cells. A high L-histidine decarboxylase (HDC) expression and HA synthesis was found in McA-RH7777 cells. Whereas extracellular HA reached sub-micromolar concentrations, intracellular levels were very low, indicating that HA was secreted by the cells. McA-RH7777 cells also express H₃ receptor (H₃R) transcripts and proteins. RT-PCR analysis detected only transcripts for the long isoform. Immunocytochemistry performed with a selective H₃R antibody showed that most cells were immunoreactive. H₃R binding sites (B_max ~ 30 fmol/mg protein) were identified when [¹²⁵I]iodoproxyfan binding was displaced by the agonist imetit. A high affinity binding also occurred at cytochrome P₄₅₀ enzymes. This binding was not inhibited by HA, H₃R agonists or by a non imidazole H₃R antagonist, but was displaced by imidazole H₃R antagonists or by ketoconazole, a imidazole-containing cytochrome inhibitor. HA inhibited proliferation of McA-RH7777 hepatoma cells. The absence of uptake system, its much higher potency at H₃Rs, and its low intracellular levels, suggested that HA interacted with H₃Rs rather than cytochromes. In agreement, both imidazole H₃R antagonists, a non imidazole H₃R antagonist, and the HDC inhibitor -FMH, increased cell proliferation (up to ~ 60 %), revealing a H₃R-mediated inhibition by endogenous HA. Moreover, exogenous HA inhibited the increase induced by -FMH or H₃R antagonists with a nanomolar potency. In conclusion, our findings show that HA regulates proliferation of McA-RH7777 hepatoma cells by interacting with autoinhibitory H₃Rs.

Key words: Autoinhibition, Cell proliferation, Cytochromes, H3 receptor, Hepatoma, Histamine