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Lipopolysaccharide induces H1 receptor expression and enhances histamine responsiveness in human coronary artery endothelial cells

Summary Histamine is a well‐recognized modulator of vascular inflammation. We have shown that histamine, acting via H1 receptors (H1R), synergizes lipopolysaccharide (LPS)‐induced production of prostaglandin I2 (PGI2), PGE2 and interleukin‐6 (IL‐6) by endothelial cells. The synergy between histamine and LPS was partly attributed to histamine ‐induced expression of Toll‐like receptor 4 (TLR4). In this study, we examined whether LPS stimulates the H1R expression in human coronary artery endothelial cells (HCAEC) with resultant enhancement of histamine responsiveness. Incubation of HCAEC with LPS (10–1000 ng/ml) resulted in two‐fold to fourfold increases in H1R mRNA expression in a time‐dependent and concentration‐dependent fashion. In contrast, LPS treatment did not affect H2R mRNA expression. The LPS‐induced H1R mRNA expression peaked by 4 hr after LPS treatment and remained elevated above the basal level for 20–24 hr. Flow cytometric and Western blot analyses revealed increased expression of H1R protein in LPS‐treated cells. The specific binding of [3H]pyrilamine to H1R in membrane proteins from LPS‐treated HCAEC was threefold higher than the untreated cells. The LPS‐induced H1R expression was mediated through TLR4 as gene silencing by TLR4‐siRNA and treatment with a TLR4 antagonist inhibited the LPS effect. When HCAEC were pre‐treated with LPS for 24 hr, washed and challenged with histamine, 17‐, 10‐ and 15‐fold increases in PGI2, PGE2 and IL‐6 production, respectively, were noted. Histamine‐induced enhancement of the synthesis of PGI2, PGE2 and IL‐6 by LPS‐primed HCAEC was completely blocked by an H1R antagonist. The results demonstrate that LPS, through TLR4 activation, up‐regulates the expression and function of H1R and amplifies histamine‐induced inflammatory responses in HCAEC.