The histamine H 1 receptor (H1R) gene is up-regulated in patients with allergic rhinitis. However, the mechanism and reason underlying this up-regulation are still unknown. Recently, we reported that the H1R expression level is strongly correlated with the severity of allergic symptoms. Therefore, understanding the mechanism of this up-regulation will help to develop new anti-allergic drugs targeted for H1R gene expression. Here we studied the molecular mechanism of H1R up-regulation in HeLa cells that express H1R endogenously in response to histamine and phorbol 12- Histamine is a major chemical mediator of allergic reactions, and its action is mainly mediated by the histamine H 1 receptor (H1R), 2 which is one of the four histamine receptor isoforms. H1R, which is a G protein-coupled receptor, is coupled to the G q protein. The stimulation of H1R activates inositol phospholipid hydrolysis and intracellular Ca 2ϩ mobilization. Typically, the activation of G protein-coupled receptors by their agonists induces down-regulation of the receptors. However, to date, a few reports have demonstrated up-regulation of G proteincoupled receptors by their agonists (1-4). Previously, we found that histamine stimulation up-regulated H1R at both the mRNA and protein level via activation of H1R in HeLa cells that express H1R endogenously (5). Up-regulation of H1R has been observed in patients with allergic rhinitis (6, 7). The strength of H1R signaling depends on the H1R expression level (8). Our recent studies have demonstrated that the level of H1R gene expression is strongly correlated with the severity of allergic symptoms in model rats and patients with pollinosis (9, 10), and compounds that suppress H1R gene up-regulation alleviate allergic symptoms (11-16). These findings suggest that the H1R gene is an allergy-sensitive gene, i.e. its expression level affects the severity of symptoms. Hence, understanding the molecular mechanism of H1R up-regulation may be useful for developing new anti-allergic drugs that target H1R gene expression. However, the mechanism of H1R up-regulation in response to histamine is unknown.Previously, we demonstrated that PKC-dependent signaling is involved in up-regulation of H1R gene expression in HeLa cells (5). PKC consists of at least 11 isoforms, and based on their structures and cofactor requirements, the PKC isoforms are divided into three subgroups; that is, conventional PKC (␣, ␤, and ␥), novel PKC (␦, ⑀, , and ), and atypical PKC ( and /) (17). It has been reported that PKC ␣, ␤, ␦, ⑀, and isoforms are expressed in HeLa cells (18 -20).Coupling of H1R signaling with PKC␣ has been reported in H1R-overexpressing CHO cells (21) and native human epidermal keratinocytes (22). In these cells, however, no up-regulation of H1R gene expression was observed. In addition, NF-B (23), the cAMP response element-binding protein (CREB) (24)), the nuclear factor of activated T-cell (25), and the serumresponsive element (26)