HISTAMINE RECEPTORS

Molecular Biology of Histamine ReceptorsBoth the histamine H1 and H2receptor belong tothe large family of G-protein coupled receptors(GPCRs). The cDNA encoding a bovine H1receptor protein was cloned in 1991 after anexpression cloning strategy in Xenopusoocytes. 32 The deduced amino acid sequencerevealed a 491 amino acid protein of 56 kDa.Using the cDNA sequence encoding the bovineH1receptor, the cDNA sequences and intronlessgenes encoding the rat, 33 guinea-pig, 34, 35human36-39 and mouse40H1receptor proteinswere cloned soon thereafter. The proteins areslightly different in length, highly homologous anddo not show major differences in pharmacology.Analysis of the 5-flanking region of the human, ratand guinea-pig gene33, 34, 36 resulted in theidentification of several DNA-binding motifs,including potential glucocorticoid responsiveelements. The human H1receptor gene resideson chromosome 3. 41Gantz et al 42 were the first to clone a cDNAencoding a 359 amino acid H2receptor. Usingdegenerate primers based on the knownsequence similarity of various GPCRs, the H2receptor sequence was obtained from caninegastric parietal cDNA by PCR. Soon thereafter,the intronless genes encoding the rat, 43 human, 44guinea-pig45 and mouse46H2receptor werecloned by means of homology screening. As forthe H1receptor, the receptor proteins are slightlydifferent in length, but do not show majorpharmacological differences. Identification of thepromotor region of the human H2receptor generevealed the existence of regulatory transcriptionsites and regions displaying stimulatory andinhibitory effects on gene expression monitored ina luciferase assay. 47 Recent studies haveindicated that the human H2receptor gene resideson chromosome 5. 45 Interestingly, severalpolymorphisms have been found in the human H2receptor gene48and one of the mutations hasbeen linked to schizophrenia. 49Although the genetic information for the H1and H2receptor has been available for some years now,as yet no information on the primary structure ofthe H receptor is known.3Signal Transduction of the HistamineReceptorsThe histamine H1receptor is associated with thephospholipase C-catalyzed formation of inositol1,4,5-triphosphate (IP 3) and 1,2-diacylglycerol(DAG). Histamine induces production of inositolphosphates in several tissues (including brain,airway, intestinal and vascular smooth muscle 10)via a pertussis toxin-insensitive G-protein.Although the G-protein probably belongs to theG q/G11family, the actual nature of the pertussintoxin-insensitive G-protein remains unclear. SinceCa2+is involved in the regulation of many cellularfunctions, the increase of the intracellular Ca2+concentration following H1receptor stimulationcan explain a variety of cellular responses, suchas nitric oxide production, cAMP and cGMPaccumulation and phospholipase A2andphospholipase D activation. 10 Yet, studies with G-protein toxins and in calcium-free mediumindicate that both the H1receptor-mediatedactivation of phospholipase A2and cAMPelevation are also mediated by an unknown,secondary mechanism (G-protein mediated?).The histamine H2receptor is coupled to theadenylate cyclase system in a variety of tissues(e.g. brain, stomach, heart, gastric mucosa,lung). 10 Moreover, cell lines transfected with thecloned H2receptor genes showed an H2receptor-mediated increase of cAMP. 50-52 Althoughcoupling of the H2receptor to adenylate cyclase iswell accepted, some findings argue against auniversal role of cAMP. New signalling pathwayshave recently been described for the H2receptor.In differentiated HL-60 cells and CHO or HEPAcells transfected with the H2receptor cDNA, an H2receptor-mediated increase of the intracellularCa2+concentration and/or IP3levels wasobserved. 52-56 Moreover, in CHO cells expressingthe rat H2 receptor, activation of the H2receptorresulted in an inhibition of the release ofarachidonic acid induced by either constitutivepurinergic receptors or a Ca2+ -ionophore, 51 aswell as an increase in cAMP. These new signaltransduction pathways are both regulated viaunknown, cAMP-independent pathways.The H3receptor is also thought to belong to thesuperfamily of G-protein coupled receptors. Fromboth functional and binding studies an interactionwith a G-protein is suggested. 10 The concept of G-protein involvement is further strengthened by arecent study57showing a pertussis toxin sensitivestimulation of [ 35S]-GTPS binding in rat corticalmembranes. At present, almost nothing is knownabout the intracellular biochemical pathways thatare stimulated via the H3-receptor. Severalstudies failed to show a coupling of H3receptors tointracellular cAMP levels; 10 a not completelyunderstood, negative coupling to phospholipaseC was shown in HGT-1 gastric tumor cells. 58 Acoupling to N-type Ca2+-channels, as shown forother presynaptic receptors, has been reported infunctional studies with heart and duodenalpreparations. 59, 60 The biochemical basis for thiscoupling is also, as yet, unknown.Further DirectionsMany new developments are awaited, particularlyin the field of the H3receptor where both theprimary receptor structure and the signaltransduction pathway(s) are, as yet, unknown.However, new developments are expected in thenext five years. For the H1 and H2receptors,availability of the cDNAs will provide new insightson structure-function relationships of the receptorprotein, receptor regulation, and gene expression,in the years to come.4