Neohormone systems as exciting targets for drug development

Endocrine-disrupting chemicals: Mainstream recognition of health effects and implications for the practicing internist

Rapidly advancing evidence documents that a broad array of synthetic chemicals found ubiquitously in the environment contribute to disease and disability across the lifespan. Although the early literature focused on early life exposures, endocrine-disrupting chemicals (EDCs) are now understood to contribute substantially to chronic disease in adulthood, especially metabolic, cardiovascular, and reproductive consequences as well as endocrine cancers. The contribution to mortality is substantial, with over 90,000 deaths annually and at least $39 billion/year in lost economic productivity in the United States (US) due to exposure to certain phthalates that are used as plasticizers in food packaging. Importantly, exposures are disproportionately high in low-income and minoritized populations, driving disparities in these conditions. Though non-Hispanic Blacks and Mexican Americans comprise 12.6% and 13.5% of the US population, they bear 16.5% and 14.6% of the disease burden due to EDCs, respectively. Many of these exposures can be modified through safe and simple behavioral changes supported by proactive government action to both limit known hazardous exposures and to proactively screen new industrial chemicals prior to their use. Routine healthcare maintenance should include guidance to reduce EDC exposures, and a recent report by the Institute of Medicine suggests that testing be conducted, particularly in populations heavily exposed to perfluoroalkyl substances-chemicals used in nonstick coatings as well as oil- and water-resistant clothing.

Physiology and evolution of the INSL3/RXFP2 hormone/receptor system in higher vertebrates

Although the insulin-like peptide hormone INSL3 and its cognate receptor RXFP2 (relaxin-family peptide receptor 2) have existed throughout chordate evolution, their physiological diversification appears to be linked closely with mammalian emergence and radiation. In contrast, they have been lost in birds and reptiles. Both hormone and receptor are expressed from autosomal genes which have maintained their synteny across vertebrate evolution. Whereas the INSL3 gene comprises only two exons closely linked to the JAK3 gene, RXFP2 is normally encoded by 18 exons. Both genes, however, are subject to alternative splicing to yield a variety of possibly inactive or antagonistic molecules. In mammals, the INSL3-RXFP2 dyad has maintained a probably primitive association with gametogenesis, seen also in fish, whereby INSL3 promotes the survival, growth and differentiation of male germ cells in the testis and follicle development in the ovary. In addition, however, the INSL3/RXFP2 system has adopted a typical 'neohormone' profile, essential for the promotion of internal fertilisation and viviparity; fetal INSL3 is essential for the first phase of testicular descent into a scrotum, and also appears to be associated with male phenotype, in particular horn and skeletal growth. Circulating INSL3 is produced exclusively by the mature testicular Leydig cells in male mammals and acts as a potent biomarker for testis development during fetal and pubertal development as well as in ageing. As such it can be used also to monitor seasonally breeding animals as well as to investigate environmental or lifestyle conditions affecting development. Nevertheless, most information about INSL3 and RXFP2 comes from a very limited selection of species; it will be especially useful to gain further information from a more diverse range of animals, especially those whose evolution has led them to express unusual reproductive phenotypes.

Relaxin and the 'Milky Way': The lactocrine hypothesis and maternal programming of development

Maternal effects on early postnatal development in mammals are mediated, in part, by milk-borne bioactive factors transmitted from mother to nursing offspring. The term 'lactocrine' was coined to describe this mode of signaling. Relaxin (RLX), one of a family of neohormones found in mammals, is detectable in milk from multiple species. In the pig, evidence of bioactive proRLX in colostrum/milk, immunoreactive RLX in the circulation of nursed neonates, and RLX receptor expression in RLX-sensitive neonatal female reproductive tract tissues, established RLX as a prototypical lactocrine-active factor. Observations provided the foundation for the lactocrine hypothesis for maternal programming of postnatal development. Studies designed to test the lactocrine hypothesis provided insights into both short-term effects of milk-borne bioactive factors in the neonate, and long-term consequences of maternal lactocrine programming of endometrial function and fecundity in adults. Thus, RLX led to the 'Milky Way'.

Neohormones in milk

Neohormone systems evolved specifically to regulate those mammalian traits, such as internal fertilization, pregnancy and lactation, which have proved to be central to the success, environmental independence, and adaptability of mammals as a vertebrate group. Neohormones such as oxytocin or relaxin are not only involved in the regulation of mammary gland development and function, but are also significant components of milk itself. Particularly for the latter hormone, it has been shown for the pig that relaxin in the first milk is taken up by the gastrointestinal tract of the offspring, enters the neonatal circulation and can have specific physiological and epigenetic effects on target organs such as the female reproductive system. Nevertheless, there are large gaps in our knowledge and understanding of such lactocrine systems especially in regard to other neohormones, species, and neonatal organ systems.

Relaxin-like peptides in male reproduction - a human perspective

The relaxin family of peptide hormones and their cognate GPCRs are becoming physiologically well-characterized in the cardiovascular system and particularly in female reproductive processes. Much less is known about the physiology and pharmacology of these peptides in male reproduction, particularly as regards humans. H2-relaxin is involved in prostate function and growth, while insulin-like peptide 3 (INSL3) is a major product of the testicular Leydig cells and, in the adult, appears to modulate steroidogenesis and germ cell survival. In the fetus, INSL3 is a key hormone expressed shortly after sex determination and is responsible for the first transabdominal phase of testicular descent. Importantly, INSL3 is becoming a very useful constitutive biomarker reflecting both fetal and post-natal development. Nothing is known about roles for INSL4 in male reproduction and only very little about relaxin-3, which is mostly considered as a brain peptide, or INSL5. The former is expressed at very low levels in the testes, but has no known physiology there, whereas the INSL5 knockout mouse does exhibit a testicular phenotype with mild effects on spermatogenesis, probably due to a disruption of glucose homeostasis. INSL6 is a major product of male germ cells, although it is relatively unexplored with regard to its physiology or pharmacology, except that in mice disruption of the INSL6 gene leads to a disruption of spermatogenesis. Clinically, relaxin analogues may be useful in the control of prostate cancer, and both relaxin and INSL3 have been considered as sperm adjuvants for in vitro fertilization.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Insulin-Like Factor 3 and the HPG Axis in the Male

The hypothalamic-pituitary-gonadal (HPG) axis comprises pulsatile GnRH from the hypothalamus impacting on the anterior pituitary to induce expression and release of both LH and FSH into the circulation. These in turn stimulate receptors on testicular Leydig and Sertoli cells, respectively, to promote steroidogenesis and spermatogenesis. Both Leydig and Sertoli cells exhibit negative feedback to the pituitary and/or hypothalamus via their products testosterone and inhibin B, respectively, thereby allowing tight regulation of the HPG axis. In particular, LH exerts both acute control on Leydig cells by influencing steroidogenic enzyme activity, as well as chronic control by impacting on Leydig cell differentiation and gene expression. Insulin-like peptide 3 (INSL3) represents an additional and different endpoint of the HPG axis. This Leydig cell hormone interacts with specific receptors, called RXFP2, on Leydig cells themselves to modulate steroidogenesis, and on male germ cells, probably to synergize with androgen-dependent Sertoli cell products to support spermatogenesis. Unlike testosterone, INSL3 is not acutely regulated by the HPG axis, but is a constitutive product of Leydig cells, which reflects their number and/or differentiation status and their ability therefore to produce various factors including steroids, together this is referred to as Leydig cell functional capacity. Because INSL3 is not subject to the acute episodic fluctuations inherent in the HPG axis itself, it serves as an excellent marker for Leydig cell differentiation and functional capacity, as in puberty, or in monitoring the treatment of hypogonadal patients, and at the same time buffering the HPG output.

Relaxin induces matrix-metalloproteinases-9 and -13 via RXFP1: induction of MMP-9 involves the PI3K, ERK, Akt and PKC-ζ pathways

We determined the precise role of relaxin family peptide (RXFP) receptors-1 and -2 in the regulation of MMP-9 and -13 by relaxin, and delineated the signaling cascade that contributes to relaxin's modulation of MMP-9 in fibrocartilaginous cells. Relaxin treatment of cells in which RXFP1 was silenced resulted in diminished induction of MMP-9 and -13 by relaxin, whereas overexpression of RXFP1 potentiated the relaxin-induced expression of these proteinases. Suppression or overexpression of RXFP2 resulted in no changes in the relaxin-induced MMP-9 and -13. Studies using chemical inhibitors and siRNAs to signaling molecules showed that PI3K, Akt, ERK and PKC-ζ and the transcription factors Elk-1, c-fos and, to a lesser extent, NF-κB are involved in relaxin's induction of MMP-9. Our findings provide the first characterization of signaling cascade involved in the regulation of any MMP by relaxin and offer mechanistic insights on how relaxin likely mediates extracellular matrix turnover.

Human testicular insulin-like factor 3: in relation to development, reproductive hormones and andrological disorders

Knockout of the gene encoding insulin-like factor 3 (INSL3) results in cryptorchidism in mice due to disruption of the transabdominal phase of testicular descent. This finding was essential for understanding the complete course of testis descensus, and wound up years of speculations regarding the endocrine regulation of this process. INSL3 is, along with testosterone, a major secretory product of testicular Leydig cells. In addition to its crucial function in testicular descent, INSL3 is suggested to play a paracrine role in germ cell survival and an endocrine role in bone metabolism. INSL3 is produced in human prenatal and neonatal, and in adult Leydig cells to various extents, and is in a developmental context regulated like testosterone, with production during second trimester, an early postnatal peak and increasing secretion during puberty, resulting in high adult serum levels. INSL3 production is entirely dependent on the state of Leydig cell differentiation, and is stimulated by the long-term trophic effects mediated by luteinizing hormone (LH). Once differentiated, Leydig cells apparently express INSL3 in a constitutive manner, and the hormone is thereby insensitive to the acute, steroidogenic effects of LH, which for example is an important factor in the regulation of testosterone. Clinically, serum INSL3 levels can turn out to be a usable tool to monitor basal Leydig cell function in patients with various disorders affecting Leydig cell function. According to animal studies, foetal INSL3 production is, directly or indirectly, sensitive to oestrogenic or anti-androgenic compounds. This provides important insight into the mechanism by which maternal exposure to endocrine disrupters can result in cryptorchidism in the next generation. Conclusively, INSL3 is an interesting testicular hormone with potential clinical value as a marker for Leydig cell function. It should be considered on a par with testosterone in the evaluation of testicular function and the consequences of Leydig cell dysfunction.

INSL3 in the ruminant: a powerful indicator of gender- and genetic-specific feto-maternal dialogue

The hormone Insulin-like peptide 3 (INSL3) is a major secretory product of the Leydig cells from both fetal and adult testes. Consequently, it is a major gender-specific circulating hormone in the male fetus, where it is responsible for the first phase of testicular descent, and in the adult male. In most female mammals, circulating levels are very low, corresponding to only a small production of INSL3 by the mature ovaries. Female ruminants are exceptional in exhibiting high INSL3 gene expression by the thecal cells of antral follicles and by the corpora lutea. We have developed a specific and sensitive immunoassay to measure ruminant INSL3 and show that, corresponding to the high ovarian gene expression, non-pregnant adult female sheep and cows have up to four times the levels observed in other female mammals. Significantly, this level declines during mid-pregnancy in cows carrying a female fetus, in which INSL3 is undetectable. However, in cows carrying a male fetus, circulating maternal INSL3 becomes elevated further, presumably due to the transplacental transfer of fetal INSL3 into the maternal circulation. Within male fetal blood, INSL3 is high in mid-pregnancy (day 153) corresponding to the first transabdominal phase of testicular descent, and shows a marked dependence on paternal genetics, with pure bred or hybrid male fetuses of Bos taurus (Angus) paternal genome having 30% higher INSL3 levels than those of Bos indicus (Brahman) paternity. Thus INSL3 provides the first example of a gender-specific fetal hormone with the potential to influence both placental and maternal physiology.

Relaxin signalling in primary cultures of human myometrial cells

In myometrium of pigs and rats, though not humans, relaxin appears to mediate an inhibition of spontaneous and oxytocin-induced contractility, presumably acting through a G-protein coupled receptor (RXFP1) to generate cAMP. In humans, circulating relaxin is highest in the first trimester, including the time of implantation, when transitory uterine quiescence could help a blastocyst to implant. We investigated whether relaxin can activate adenylate cyclase in primary human myometrial cells from non-pregnant tissue, and we show that relaxin is able to stimulate the generation of cAMP in a manner, which is dependent upon a tyrosine phosphorylation activity, as in the endometrium. We identified transcripts for the relaxin receptor RXFP1 as full-length variants, though a minor splice variant missing exon 2 was also present in low amounts. These cells also express transcripts encoding RXFP2, the receptor for the closely related hormone, INSL3. Although able to respond to relaxin at high concentrations, this receptor does not appear to function by contributing to the cAMP production in human myometrial cells, nor does INSL3 act as a functional agonist or antagonist of relaxin action. In conclusion, the inability of relaxin to inhibit contractility in human myometrial cells would appear to be due to events downstream of simple cAMP generation.

Cooperative binding of insulin-like Peptide 3 to a dimeric relaxin family peptide receptor 2

Insulin-like peptide 3 (INSL3) binds to a G protein-coupled receptor (GPCR) called relaxin family peptide receptor 2 (RXFP2). RXFP2 belongs to the leucine-rich repeat-containing subgroup (LGR) of class A GPCRs. Negative cooperativity has recently been demonstrated in other members of the LGR subgroup. In this work, the kinetics of INSL3 binding to HEK293 cells stably transfected with RXFP2 (HEK293-RXFP2) have been investigated in detail to study whether negative cooperativity occurs and whether this receptor functions as a dimer. Our results show that negative cooperativity is present and that INSL3-RXFP2 binding shows both similarities and differences with insulin binding to the insulin receptor. A dose-response curve for the negative cooperativity of INSL3 binding had a reverse bell shape reminiscent of that seen for the negative cooperativity of insulin binding to its receptor. This suggests that binding of INSL3 may happen in a trans rather than in a cis way in a receptor dimer. Bioluminescence resonance energy transfer (BRET(2)) experiments confirmed that RXFP2 forms constitutive homodimers. Heterodimerization between RXFP2 and RXFP1 was also observed.

Diverse signalling mechanisms used by relaxin in natural cells and tissues: the evolution of a "neohormone"

The small peptide hormone relaxin is a member of a rapidly evolving family of hormones and growth factors, whose mode of action appears to be particularly adapted to purely mammalian physiology. It is representative of a new category of hormones, referred to as neohormones, which appear to have evolved specifically to accommodate the needs of viviparity, lactation and wound repair. The mechanism of receptor signalling has also evolved in this family, with older members using receptor tyrosine kinases and new members such as relaxin adopting 7-transmembrane G-protein coupled receptors. Although relaxin primarily generates cAMP as second messenger, studies of relaxin signalling show that this does not conform to a classic G-protein dependent activation of adenylate cyclase: it requires additional cytoplasmic components, it can involve further coupling to PI3-kinase and PKCzeta and it is absolutely dependent on a tyrosine kinase activity linked closely to the relaxin receptor. Relaxin may also independently activate glucocorticoid receptors. This diversity of signalling leads to a broad range of possible downstream transcriptional effects. Finally, in tissues where relaxin is known to be effective, there is often also local relaxin induction, amplifying the effects of the endocrine hormone.

Relaxin family peptide receptors--former orphans reunite with their parent ligands to activate multiple signalling pathways

The relaxin family peptides, although structurally closely related to insulin, act on a group of four G protein-coupled receptors now known as Relaxin Family Peptide (RXFP) Receptors. The leucine-rich repeat containing RXFP1 and RXFP2 and the small peptide-like RXFP3 and RXFP4 are the physiological targets for relaxin, insulin-like (INSL) peptide 3, relaxin-3 and INSL5, respectively. RXFP1 and RXFP2 have at least two binding sites--a high-affinity site in the leucine-rich repeat region of the ectodomain and a lower-affinity site in an exoloop of the transmembrane region. Although they respond to peptides that are structurally similar, RXFP3 and RXFP4 demonstrate distinct binding properties with relaxin-3 being the only peptide that can recognize these receptors in addition to RXFP1. Activation of RXFP1 or RXFP2 causes increased cAMP and the initial response for both receptors is the resultant of Gs-mediated activation and G(oB)-mediated inhibition of adenylate cyclase. With RXFP1, an additional delayed increase in cAMP involves betagamma subunits released from G(i3). In contrast, RXFP3 and RXFP4 inhibit adenylate cyclase and RXFP3 causes ERK1/2 phosphorylation. Drugs acting at RXFP1 have potential for the treatment of diseases involving tissue fibrosis such as cardiac and renal failure, asthma and scleroderma and may also be useful to facilitate embryo implantation. Activators of RXFP2 may be useful to treat cryptorchidism and infertility and inhibitors have potential as contraceptives. Studies of the distribution and function of RXFP3 suggest that it is a potential target for anti-anxiety and anti-obesity drugs.