Relaxin

Relaxin 1
Identifiers
Symbol	RLN1
Alt. symbols	H1
NCBI gene	6013
HGNC	10026
OMIM	179730
RefSeq	NM_006911
UniProt	P04808
Other data
Locus	Chr. 9 qter-q12
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Relaxin 2
Identifiers
Symbol	RLN2
Alt. symbols	H2, RLXH2, bA12D24.1.1, bA12D24.1.2
NCBI gene	6019
HGNC	10027
OMIM	179740
PDB	6RLX
RefSeq	NM_134441
UniProt	P04090
Other data
Locus	Chr. 9 qter-q12
showSearch for Structures Swiss-model Domains InterPro

Relaxin 3
Identifiers
Symbol	RLN3
Alt. symbols	ZINS4, RXN3, H3
NCBI gene	117579
HGNC	17135
OMIM	606855
RefSeq	NM_080864
UniProt	Q8WXF3
Other data
Locus	Chr. 19 p13.3
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Relaxin is a protein hormone of about 6000 Da^[1] first described in 1926 by Frederick Hisaw.^[2][3]

The relaxin-like peptide family belongs in the insulin superfamily and consists of 7 peptides of high structural but low sequence similarity; relaxin-1 (RLN1), 2 (RLN2) and 3 (RLN3), and the insulin-like (INSL) peptides, INSL3, INSL4, INSL5 and INSL6. The functions of relaxin-3, INSL4, INSL5, and INSL6 remain uncharacterised.^[4]

Synthesis[]

In the female, it is produced by the corpus luteum of the ovary, the breast and, during pregnancy, also by the placenta, chorion, and decidua.

In the male, it is produced in the prostate and is present in human semen.^[5]

Structure[]

Structurally, relaxin is a heterodimer of two peptide chains of 24 and 29 amino acids linked by disulfide bridges, and it appears related to insulin.^[6]

Relaxin is produced from its prohormone, "prorelaxin", by splitting off one additional peptide chain reaction.^{[citation needed]}

Function[]

In humans[]

In females, relaxin is produced mainly by the corpus luteum, in both pregnant and nonpregnant females.^[1] Relaxin levels rise to a peak within approximately 14 days of ovulation, and then decline in the absence of pregnancy, resulting in menstruation.^[7] Relaxin may be involved in the vital process of decidualisation, working alongside steroid hormones to allow the endometrium to prepare for implantation.^[8] During the first trimester of pregnancy, levels rise and additional relaxin is produced by the decidua. Relaxin's peak is reached during the first trimester (14-weeks) and at delivery. Relaxin mediates the hemodynamic changes that occur during pregnancy, such as increased cardiac output, increased renal blood flow, and increased arterial compliance.^{[citation needed]} It also relaxes other pelvic ligaments.^[9] It is believed to soften the pubic symphysis.^{[citation needed]}

In males, relaxin enhances motility of sperm in semen.^[10]

In the cardiovascular system, relaxin works mainly by activating the nitric oxide pathway. Other mechanisms include activation of NFκB leading to vascular endothelial growth factor (VEGF) and matrix metalloproteinases transcription.^[11]

Relaxin has been shown to relax vascular smooth muscle cells and increase nitric oxide production in rat endothelial cells, thus playing a role in regulation of cardiovascular function by dilating systemic resistance arteries.^[11] Relaxin increases the rate and force of cardiac contraction in rat models.^[12] Via upregulation of VEGF, relaxin plays a key role in blood vessel formation (angiogenesis) during pregnancy, tumour development or ischaemic wounds.^[12]

In other animals[]

In animals, relaxin widens the pubic bone and facilitates labor; it also softens the cervix (cervical ripening), and relaxes the uterine musculature.^{[citation needed]} Thus, for a long time, relaxin was looked at as a pregnancy hormone. However, its significance may reach much further. Relaxin may affect collagen metabolism, inhibiting collagen synthesis and enhancing its breakdown by increasing matrix metalloproteinases.^[13] It also enhances angiogenesis and is a potent renal vasodilator.^{[citation needed]}

In the mouse model, relaxin has been found to promote maturation of cardiomyocytes.^[12]

Several animal studies have found relaxin to have a cardioprotective function against ischaemia and reperfusion injury, by reducing cellular damage, via anti-apoptotic and anti-inflammatory effects.^{[citation needed]} Relaxin has been shown to reduce cardiac fibrosis in animal models by inhibiting cardiac fibroblasts secreting collagen and stimulating matrix metalloproteinase.^[12][11]

In the European rabbit (Oryctolagus cuniculus), relaxin is associated with squamous differentiation and is expressed in tracheobronchial epithelial cells as opposed to being involved with reproduction.^[14]

In horses (Equus caballus), relaxin is also an important hormone involved in pregnancy, however, before pregnancy occurs, relaxin is expressed by ovarian structures during the oestrous cycle.^[15] Prior to ovulation, relaxin will be produced by ovarian stromal cells, which will promote secretion of gelatinases and tissue inhibitors of metalloproteinases. These enzymes will then aid the process of ovulation, which will lead to the release of a developed follicle into the fallopian tube.^[15] Furthermore, granular and theca cells in the follicles will express relaxin in increasing levels depending on their size.^[15] During early pregnancy, the preimplantation conceptus will express relaxin, which will promote angiogenesis in the endometrium by up-regulating VEGF.^[15][16] This will allow the endometrium to prepare for implantation. In horses alone, the embryo in the uterus will express relaxin mRNA at least 8 days after ovulation. Then as the conceptus develops expression will increase, which is likely to promote embryo development.^[15]

In addition to relaxin production by the horse embryo, the maternal placenta is the main source of relaxin production, whereas in most animals the main source of relaxin is the corpus luteum.^[15] Placental trophoblast cells produce relaxin, however, the size of the placenta does not determine the level of relaxin production. This is seen because different breeds of horses show different relaxin levels.^[17] From 80 day of gestation onwards, relaxin levels will increase in the mare's serum with levels peaking in late gestation.^[17][18] Moreover, the pattern of relaxin expression will follow the expression of oestrogen, however, there is not yet a known link between these two hormones.^[18] During labour, there is a spike in relaxin 3–4 hours before delivery, which is involved in myometrial relaxation and softening of the pelvic ligaments to aid preparation of the birth canal for the delivery of the horse foetus.^[15][17] Following birth, the levels of relaxin will gradually decrease if the placenta is also delivered, however, if the placenta is retained in the mare then the levels will remain high.^[17] In addition, if the mare undergoes an abortion then the relaxin levels will decline as the placenta ceases to function.^[17]

Receptors[]

Relaxin interacts with the relaxin receptor LGR7 (RXFP1) and LGR8 (RXFP2), which belong to the G protein-coupled receptor superfamily.^[19] They contain a heptahelical transmembrane domain and a large glycosylated ectodomain, distantly related to the receptors for the glycoproteohormones, such as the LH-receptor or FSH-receptor.

Relaxin receptors have been found in the heart, smooth muscle, the connective tissue, and central and autonomous nervous system.^{[citation needed]}

Disorders[]

Women who have been on relaxin treatment during unrelated clinical trials have experienced heavier bleeding during their menstrual cycle, suggesting that relaxin levels could play a role in abnormal uterine bleeding.^[20] However, more research needs to go into this to confirm relaxin as a direct cause.^{[citation needed]}

A lower expression of relaxin has been found amongst women who have endometriosis. The research in this area is limited and more studying of relaxin's contribution could contribute greatly to the understanding of endometriosis.^[20]

Specific disorders related to relaxin have not been heavily described, yet a link to scleroderma and fibromyalgia has also been suggested.^[21]

Pregnancy[]

It is possible that relaxin in the placenta could be a contributing factor to inducing labour in humans and therefore serum relaxin levels during pregnancy have been linked to premature birth.^[20]

Pharmacological targets[]

A recombinant form of human relaxin-2 has been developed as investigational drug RLX030 (serelaxin).^{[citation needed]}

It is suggested that relaxin could be used as a therapeutic target when it comes to gynaecological disorders.^[20]

Evolution[]

Relaxin 1 and relaxin 2 arose from the duplication of a proto-RLN gene between 44.2 and 29.6 million years ago in the last common ancestor of catarrhine primates.^[22] The duplication that led to RLN1 and RLN2 is thought to have been a result of positive selection and convergent evolution at the nucleotide level between the relaxin gene in New World monkeys and the RLN1 gene in apes.^[22] As a result, Old World monkeys, a group that includes the subfamilies colobines and cercopithecines, have lost the RLN1 paralog, but apes have retained both the RLN1 and the RLN2 genes.^[22]

See also[]

• Relaxin family peptide hormones
• Insulin/IGF/Relaxin family
• Relaxin/insulin-like family peptide receptor 1

References[]

External links[]

• Relaxin' at the US National Library of Medicine Medical Subject Headings (MeSH)
• "Relaxin". Human Protein Reference Database. Johns Hopkins University and the Institute of Bioinformatics. Retrieved 2009-05-20.