Histamine is a ubiquitous messenger molecule released from mast cells, enterochromaffin-like cells and neurons. Its various actions are mediated by histamine receptors H1, H2, H3 and H4 (1). The bovine histamine H1 receptor (HRH1) was the first HRH1 gene to be cloned, soon to be followed by other species, including human HRH1. The human HRH1 gene is located on chromosome 3p25 and encodes for a 487 amino acid G protein-coupled receptor (GPCR) with a long third intracellular loop (IL3) (1). The HRH1 predominantly couples to Gα_q/11 proteins, leading to the activation of phospholipase C (PLC) and subsequent release of the second messengers inositol trisphosphate (IP₃) and diacylglycerol (DAG) followed by the activation of protein kinase C (PKC) and the release of [Ca²⁺]_i (1). HRH1 is expressed in a wide variety of tissues including the gastrointestinal tract, the central nervous system, the immunological system, and the cardiovascular and genitourinary systems (2–5).

HRH1 is involved in a wide array of processes including thermal regulation, memory and learning, and control of the sleep-wake cycle, food intake, and emotional and aggressive behaviors. HRH1 is also involved in the pathological process of allergy, including allergic rhinitis, atopic dermatitis, anaphylaxis and asthma, and also has a role in autoimmune diseases and malignancy (2–7). Histamine is involved in cell proliferation, and key events in tumor development and progression have been extensively investigated (8,9). It has been found that H1R stimulates proliferation of breast carcinoma, melanoma and astrocytoma tumor cells (10–12). In this context, the investigation of H1R and its antagonists support a clear rationale for future supportive anticancer therapies. However, a comprehensive investigation of whether HRH1 is involved in the formation of various tumors has not yet been carried out.

In the present study, we identified HRH1 genes from the human, chimpanzee, macaque, orangutan, dog, cow, horse, mouse, rat, opossum, chicken, Xenopus tropicalis, zebrafish and fugu by comparative genomic analyses. A search for conserved transcription factor-binding sites within the promoter regions of the human HRH1 gene was then carried out. The expression data, functional relevant single nucleotide polymorphisms (SNPs) and comparative proteomic analyses were conducted. Furthermore, meta-analysis of the prognostic value of HRH1 genes in various types of cancers was also performed.

A large number of molecules involved in cell proliferation and key events in tumor development and progression have been extensively investigated including histamine (31–33). Histamine exerts its functions through binding to G protein-associated histamine H1, H2, H3, and H4 receptors (HRH1, HRH2, HRH3 and HRH4), resulting in the activation of different signal transduction pathways (31–33). In addition to the human HRH1, in the present study, we identified other HRH1 genes from other 13 vertebrate genomes and found that HRH1 exists in all types of vertebrates including the fish, amphibians, birds and mammals. All the identified HRH1 genes were encoding for a GPCR protein with a long third intracellular loop (IL3). The phylogenetic tree showed that HRH1 is separated for the order: fish, amphibians, birds and mammals. Primate HRH1s are almost the same and are clustered together. From the alignment and phylogenetic tree, mammalian HRH1s are conserved among the vertebrate genomes, suggesting that the function of HRH1 may have played an important physiological role for all vertebrates in the long process of evolution. Moreover, this process was under purifying selection, in accordance with the fact that multiple biological functions have been ascribed to this protein, including its function in central and peripheral tissues.

Histamine is an endogenous biogenic amine widely distributed throughout the organism and is long known to be a pleiotropic mediator in different (patho)-physiological conditions (1). Accordingly, by EST sequence search, one of its receptors, the HRH1 gene is also expressed in various tissues and cells including stomach, liver, heart, brain, placenta, embryonic tissue, kidney, vascular, intestine, esophagus, lung, adrenal gland, mouth, connective tissue, cervix, ovary, trachea, testis, uterus and skin. The same results were confirmed by the investigation of available microarray experiments and ‘virtual northern blot’, which showed a predominant expression of HRH1 in the bone marrow, whole blood, lymph node, thymus, brain, cerebellum, retina, spinal cord, heart, smooth muscle, skeletal muscle, small intestine, colon, adipocyte, kidney, liver, lung, pancreas, thyroid salivary gland, skin, ovary, uterus, placenta, prostate and testis. Available SNPs (2,455) were identified in the human HRH1 gene. We identified 88 SNPs including 4 available alleles disrupting an existing exonic splicing enhancer and 84 SNPs causing a missense mutation, which may impact the effect of histamine on the HRH1 protein. However, the effects of these SNPs on the physiological and pathological functions of HRH1 warrant further investigation.

Notably, most malignant cell lines and experimental tumors contain a high concentration of endogenous histamine, which can be released to the extracellular media via a paracrine or autocrine pathway. Moreover, histamine itself may be a crucial mediator in cancer development and progression by regulating diverse biological responses related to tumor growth including angiogenesis, cell invasion, migration, differentiation, apoptosis and modulation of the immune response (31–33). As one receptor of histamine, HRH1 expression in normal tissues vs. cancer samples has not been well studied. Following a search in the PrognoScan database, human HRH1 was also found to be expressed in bladder cancer, blood cancer, brain cancer, breast cancer, colorectal cancer, eye cancer, head and neck cancer, lung cancer, ovarian cancer, skin cancer and soft tissue cancer tissues. The expression of HRH1 in these tumors may explain the effects of histamine on cancer behaviors. Out of 153, 23 tests showed an association between microarray expression in the HRH1 gene and cancer prognosis (bladder cancer 2/4, blood cancer 2/18, brain cancer 1/9, breast cancer 6/50, colorectal cancer 1/18, eye cancer 1/2, head and neck cancer 0/2, lung cancer 6/29, ovarian cancer 2/18, prostate cancer 0/1, skin cancer 0/2 and soft tissue cancer 2/2) with a 5% level of significance. This suggests that the expression of HRH1 is related to the prognosis of many types of cancers including hematological and solid cancers. The mechanism of HRH1 involved in the process of these tumors warrants further investigation. It is important to note that the relationship between the expression of HRH1 and prognosis varied in different cancers, even in the same cancer from different databases. This implies that the function of HRH1 in these tumors may be multidimensional, not just as a tumor suppressor or oncogene.

The GR, GCNF, STAT5A, MIF-1, HSF2 and c-Myb regulatory transcription factor binding sites were identified in the HRH1 gene upstream (promoter) region. GRs are nuclear hormone receptors of the NR3C class, which also includes mineralocorticoid, progesterone and androgen receptors. GRs are ubiquitously expressed and mediate a vast array of physiological functions. Glucocorticoids are widely used for the treatment of lymphoid malignancy due to their marked effects on cell cycle progression and apoptosis (34). Glucocorticoids are able to alter signaling in key survival pathways and this can result in reversible growth arrest or cell death in certain cell types (34–36). STAT5A is a member of the STAT family of transcription factors. It is activated by, and mediates the responses of many cell ligands, such as IL2, IL3, IL7, GM-CSF, erythropoietin, thrombopoietin and different growth hormones. Activation of this protein in many types of cancers has been shown to be essential for tumorigenesis (37,38). C-Myb encodes a transcription factor that regulates the expression of numerous genes during cell cycle progression. c-Myb is a cellular proto-oncogene, which has the ability to regulate the expression of cell cycle genes and is involved in cell proliferation and carcinogenesis (39,40). These tumor-related transcriptional factors may be involved in the effect of HRH1 in tumors.