Histamine Regulates Molecular Clock Oscillations in Human Retinal Pigment Epithelial Cells via H1 Receptors

Eri Morioka, Yuzuki Kanda, Hayato Koizumi, Tsubasa Miyamoto, Masayuki Ikeda

Vertebrate eyes are known to contain circadian clocks, but their regulatory mechanisms remain largely unknown. To address this, we used a cell line from human retinal pigment epithelium (hRPE-YC) with stable coexpression of reporters for molecular clock oscillations (Bmal1-luciferase) and intracellular Ca2+concentrations (YC3.6). We observed concentration-dependent increases in cytosolic Ca2+concentrations after treatment with histamine (1-100 µM) and complete suppression of histamine-induced Ca2+mobilizations by H1histamine receptor (H1R) antagonistd-chlorpheniramine (d-CPA) in hRPE-YC cells. Consistently, real-time RT-PCR assays revealed that H1R showed the highest expression among the four subtypes (H1-H4) of histamine receptors in hRPE-YC cells. Stimulation of hRPE-YC cells with histamine transiently increased nuclear localization of phosphorylated Ca2+/cAMP-response element-binding protein that regulates clock gene transcriptions. Administration of histamine also shifted theBmal1-luciferaserhythms with a type-1 phase-response curve, similar to previous results with carbachol stimulations. Treatment of hRPE-YC cells withd-CPA or with more specific H1R antagonist, ketotifen, blocked the histamine-induced phase shifts. Furthermore, an H2histamine receptor agonist, amthamine, had little effect on theBmal1-luciferaserhythms. Although the function of thein vivohistaminergic system within the eye remains obscure, the present results suggest histaminergic control of the molecular clockviaH1R in retinal pigment epithelial cells. Also, sinced-CPA and ketotifen have been widely used (e.g., to treat allergy and inflammation) in our daily life and thus raise a possible cause for circadian rhythm disorders by improper use of antihistamines.