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To the Editor:

We read with great interest the article by Meng et al.1 regarding the anti-epithelial-mesenchymal transition (EMT) and antitumor metastasis effects of a histamine receptor ligand, clobenpropit, on human cholangiocarcinoma (CCA) both in vitro and in vivo. In this well-performed study the authors provided evidence that clobenpropit significantly decreased CCA proliferation by way of a Ca2+-dependent pathway and altered morphological development and invasion in vitro, which effects became greater after H3 histamine receptors (H3HRs) knockdown or overexpression of H4 histamine receptors (H4HRs). The authors hence attributed these effects of clobenpropit to primary H4HR-mediated effects.

We agree with the authors that clobenpropit is a specific H3 antagonist / H4 agonist and appreciate the efforts that the authors made to exclude the involvement of H3HRs in vitro. However, some statements mentioned by the authors may be inaccurate and the in vivo evidence may be insufficient to some extent, which made their conclusion less persuasive.

The authors demonstrated in the Discussion that clobenpropit stimulated H4HRs at a much higher affinity compared with H3HRs according to one reference (see reference 28 in the article). On the contrary, according to the comprehensive information from the International Union of Pharmacology (IUPHAR) receptor database,2 the affinity of clobenpropit for human H3HRs (pKi value: 8.4-9.4) is much higher than that for human H4HRs (pKi value: 7.4-8.3). Furthermore, the authors showed that, after H3HR knockdown, clobenpropit decreased CCA proliferation to a greater extent than in cells with H3HR expression, which we believe should not be interpreted as that H3HRs were not involved but logically indicates that H3HRs did play certain roles in CCA proliferation in this in vitro system. Decreased expression of H3HRs will obviously result in loss of a blocking effect of their antagonism. Therefore, the blocking effect of clobenpropit on H3HRs should not be neglected, especially in vivo, when the endogenous natural agonist, histamine, widely exists. However, the in vivo experiments of this study did not exclude the possible involvement of H3HRs.

Therefore, we feel that more evidence (obtained based on either pharmacological methods or transgenic animal models) should be provided before any conclusion can be drawn about which receptors (H3HRs or H4HRs) are really involved in suppressing human CCA progression.

References

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  • 1
    Meng F, Han Y, Staloch D, Francis T, Stokes A, Francis H. The H4 histamine receptor agonist, clobenpropit, suppresses human cholangiocarcinoma progression by disruption of epithelial mesenchymal transition and tumor metastasis. HEPATOLOGY 2011; 54: 1718-1728.
  • 2
    Affinity parameters for clobenpropit in IUPHAR receptor database: www.iuphar-db.org/DATABASE/LigandDisplayForward?tab=biology&ligandId=1223

Gong-Hao He M.D., Ph.D.*, Gui-Li Xu Ph.D.*, Wen-Ke Cai M.D., Ph.D.*, Wei Xia M.Sc.*, * Kunming General Hospital of Chengdu Military Region, Kunming, Yunnan, China.