Lysophosphatidic acid receptor EDG-4 (LPA_rcpt_EDG4)
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 .

Lysophospholipids (LPs), such as lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P) and sphingosylphosphorylcholine (SPC), have long been known to act as signalling molecules in addition to their roles as intermediates in membrane biosynthesis . They have roles in the regulation of cell growth, differentiation, apoptosis and development, and have been implicated in a wide range of pathophysiological conditions, including: blood clotting, corneal wounding, subarachinoid haemorrhage, inflammation and colitis . A number of G protein-coupled receptors bind members of the lysophopholipid family - these include: the cannabinoid receptors; platelet activating factor receptor; OGR1, an SPC receptor identified in ovarian cancer cell lines; PSP24, an orphan receptor that has been proposed to bind LPA; and at least 8 closely related receptors, the EDG family, that bind LPA and S1P .

LPA is found in all cell types in small quantities (associated with membrane biosynthesis) but is produced in significant quantities by some cellular sources, accounting for the levels of LPA in serum. LPA is also found in elevated levels in ovarian cancer ascites, and acts to stimulate proliferation and promote survival of the cancer cells . The effects of LPA on the proliferation and morphology of a number of other cell types have been well documented . However, identification of the mechanisms by which these effects are accomplished has been complicated by a number of factors, such as: a lack of antagonists, difficulty in ligand-binding experiments and the responsiveness of many cell types to LPA . The G protein-coupled receptors EDG-2, EDG-4 and EDG-7 have now been identified as high affinity receptors for LPA. It has been suggested that these receptors should now be referred to as lpA1, lpA2 and lpA3 respectively .

EDG-4 is expressed at high levels in the testis and peripheral blood leukocytes of humans, and the testis, kidney and embryonic brain in mouse. Lower levels of expression are found in human pancreas, spleen, thymus and prostate, and mouse heart, lung, spleen, thymus, stomach and brain . Variant forms of the receptor are also expressed in cancer cells . Binding of LPA to EDG-4 results in increased calcium levels, inhibition of adenyly cylase, activation of MAP kinases and cell rounding, through coupling to Gi/o, Gq/11 and G12/13 proteins .