Leukotriene B4 type 2 receptor (Leukotriene_B4_typ-2_rcpt)
Type: Family

G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions, including various autocrine, paracrine and endocrine processes. They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups . The term clan can be used to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence . The currently known clan members include rhodopsin-like GPCRs (Class A, GPCRA), secretin-like GPCRs (Class B, GPCRB), metabotropic glutamate receptor family (Class C, GPCRC), fungal mating pheromone receptors (Class D, GPCRD), cAMP receptors (Class E, GPCRE) and frizzled/smoothened (Class F, GPCRF) . GPCRs are major drug targets, and are consequently the subject of considerable research interest. It has been reported that the repertoire of GPCRs for endogenous ligands consists of approximately 400 receptors in humans and mice . Most GPCRs are identified on the basis of their DNA sequences, rather than the ligand they bind, those that are unmatched to known natural ligands are designated by as orphan GPCRs, or unclassified GPCRs .

The rhodopsin-like GPCRs (GPCRA) represent a widespread protein family that includes hormone, neurotransmitter and light receptors, all of which transduce extracellular signals through interaction with guanine nucleotide-binding (G) proteins. Although their activating ligands vary widely in structure and character, the amino acid sequences of the receptors are very similar and are believed to adopt a common structural framework comprising 7 transmembrane (TM) helices .

Leukotrienes (LT) are potent lipid mediators derived from arachidonic acid metabolism. They can be divided into two classes, based on the presence or absence of a cysteinyl group. Leukotriene B4 (LTB4) does not contain such a group, whereas LTC4, LTD4, LTE4 and LTF4 are cysteinyl leukotrienes.

LTB4 is one of the most effective chemoattractant mediators known, and is produced predominantly by neutrophils and macrophages. It is involved in a number of events, including: stimulation of leukocyte migration from the bloodstream; activation of neutrophils; inflammatory pain; host defence against infection; increased interleukin production and transcription . It is found in elevated concentrations in a number of inflammatory and allergic conditions, such as asthma, psoriasis, rheumatoid arthritis and inflammatory bowel disease, and has been implicated in the pathogenesis of these diseases .

Binding sites for LTB4 have been observed in membrane preparations from leukocytes, macrophages and spleen. Two receptors for LTB4 have since been cloned (BLT1 and BLT2); both are members of the rhodopsin-like G-protein-coupled receptor superfamily .

The leukotriene B4 type 2 receptor gene (BLT2) has been located in both the human and mouse genomes, and is found in close proximity to BLT1 in both species . The receptor is expressed in most human tissues, with highest levels in the liver, spleen, ovary and leukocytes . Binding of LTB4 to the receptor produces increased levels of inositol trisphosphate and calcium, inhibition of forskolin-stimulated adenylyl cyclase activity and chemotaxis . These effects may be accomplished by coupling to G-proteins of the Gq, Gi and Gz classes .