Autoregulation of McA-RH7777 Hepatoma Cell Proliferation by Histamine H₃ Receptors

Abstract

Previous studies have 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. High l-histidine decarboxylase (HDC) expression and HA synthesis were found in McA-RH7777 cells. Whereas extracellular HA reached submicromolar 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. Reverse transcriptase-polymerase chain reaction 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. High-affinity binding also occurred at cytochrome P450 enzymes. This binding was not inhibited by HA, H₃R agonists, or by a nonimidazole 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 nonimidazole H₃R antagonist, and the HDC inhibitor α-monofluoromethyl histidine increased cell proliferation (up to ∼60%), revealing a H₃R-mediated inhibition by endogenous HA. Moreover, exogenous HA inhibited the increase induced by α-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.