Biological roles of growth hormone/prolactin from an evolutionary perspective

Abstract

Although growth hormone (GH) and prolactin (PRL) are usually recognized as pituitary hormones, their expression is not restricted to the adenohypophysis and can also be found in extra-pituitary tissues including placenta. Furthermore, GH, PRL, and their receptors structurally belong to the cytokine family of proteins, and indeed they have remarkable pleiotropic effects. In this review, we analyzed the biological roles of GH/PRL from an evolutionary perspective. We have recognized that the biological significance of GH/PRL can be summarized as follows: cytokines (metabokines) that regulate the shift of nutrients and even of whole bodies to live in the most appropriate environment(s) for conducting growth and reproduction. In this sense, the common keyword of the two metabokines is “shift” for environmental adaptation. Considering that these metabokines flexibly changed their biological roles, GH/PRL may have played important roles during vertebrate evolution.

Introduction

Growth hormone (GH) is named as a hormone for somatic growth, and prolactin (PRL) for lactation. Since both hormones are mainly expressed in the adenohypophysis, they are generally recognized as anterior pituitary hormones. However, because of their naming, other biological roles of GH/PRL, especially their metabolic effects, are often underestimated.

In fact, both GH and PRL possess unique characteristics compared with other pituitary hormones in at least three points as described below.

First, GH, PRL and their homodimeric receptors structurally belong to the class I cytokine family rather than classical hormone systems (peptides, amines, steroids, and their receptors), and have remarkably versatile biological actions like other cytokines. This versatility may underscore their diverse biological roles throughout evolutionary stages.

Second, both GH and PRL genes, in most species, consist of multiple paralogs. They are expressed in a variety of tissues, and their protein products act as metabokines (i.e., cytokines involved in metabolic regulation). We speculate that, in both GH and PRL, one of the multiple paralogous genes came under the control of a pituitary-specific enhancer during the course of early vertebrate evolution, and as a result, both cytokines behave as “pituitary hormones” (see the subsequent sections).

Third, unlike other anterior pituitary hormones, no peripheral endocrine organ is regulated by GH or PRL. Instead, they directly exert their biological effects in a variety of peripheral tissues.

In the case of GH, it is originally a metabokine that adapts to starvation. Indeed, GH switches the cells’ energy source from carbohydrate to fatty acids in a famine state. However, because IGF-I, a potent anabolic factor, is also induced by GH, concurrent effects of GH and IGF-I (i.e., a catabolic and an anabolic factor, respectively) make the essential role of GH unclear.

The commonly known effects of GH/PRL are usually summarized as shown in Table 1. Nevertheless, it is difficult to understand their essential biological roles if they are only recognized as pituitary hormones. In this review, we try to elucidate the innate roles of GH/PRL as metabokines by unwinding their diverse effects during each step of evolution.

Table 1

The effects of GH and PRL from a classical view

GH

1. somatic growth (mainly via IGF-I)

2. metabolic effects

1) anabolic effect on protein metabolism (partly via IGF-I)

2) carbohydrate metabolism (anti-insulin effect)

3) lipid metabolism (lipolysis)

4) mineral-electrolyte metabolism (renal sodium reabsorption)

5) bone metabolism (both anabolic and catabolic)

PRL

1. mammotropic effects

1) development of the mammary gland

2) lactogenic effect

2. extra-mammary effects

1) regulation of gonadal function

maintenance of placental function during pregnancy

suppression of HPG* axis during pregnancy and lactation

spermatogenesis

2) regulation of mineral-electrolyte metabolism

osmoregulation, water-sodium metabolism

(mainly in fish, amphibian)

3) immune regulation

4) angiogenesis

5) behavioral effects

maternal behavior

excursion, water drive, migration

*HPG: hypothalamo-pituitary-gonadal

Origin of GH/PRL family-related genes

An ancient gene/protein of GH/PRL can be found in one of the most primitive chordates amphioxus (Branchiostoma japonicum, Branchiostoma floriae) [1]. It is highly likely that GH/PRL genes diverged from a common ancestor gene at a very early stage of vertebrate evolution [2]. The origin of GH receptor (GHR)/PRL receptor (PRLR)-like protein is also reported in amphioxus (Genbank accession number: AJP08966.1, XP_035698293.1) [1]. These proteins/receptors may be possible prototypes of vertebrate GH/PRL family members, although their amino acid sequences are not highly homologous to those of vertebrates. On the other hand, in Petromyzon marinus, one of the most primitive jawless vertebrates, GH (XP_032826436.1, XP_032835209.1) and PRL (UXR72814.1) proteins are independently recognized, as well as their receptors (QHX35240.1, XP_032826437.1) [3-5]. In addition, POU1F1 (Pit-1) gene is found to be expressed in amphioxus Hatschek’s pit [6], an ancestor of the anterior pituitary gland where GH-like protein is expressed [1], suggesting the origin of the link between GH and POU1F1. Therefore, it is speculated that some of the ancestral GH/PRL genes came under the control of pituitary-specific transcription factors including POU1F1, during early vertebrate evolution, and started to be expressed as “pituitary hormones.”

Interestingly, the origin of the transcription factor STAT5, a representative intracellular mediator of GH/PRL, is presumed to be much older than GHR/PRLR [7]. Possible ancestor proteins are already found in very primitive organisms like cnidaria including Hydra (XP_028415351.1, XP_046843891.1), and functional STAT5a/b proteins (AmphiSTATa, AmphiSTATb) are reported in chordate amphioxus.

Origin of the pituitary GH/PRL-producing cells

In mammals, GH/PRL are expressed not only in the anterior pituitary but in multiple tissues, such as neural, reproductive, and lymphoid [8, 9]. Since their receptors are also expressed almost ubiquitously, GH/PRL are acting in an autocrine/paracrine fashion similar to other cytokines. However, GH/PRL in the circulating blood are known to be undetectable in patients with panhypopituitarism. This indicates that pituitary-derived (i.e., POU1F1-dependent) GH/PRL play dominant roles in circulation as an endocrine system. As mentioned above, the origin of POU1F1-expressing cells is found in amphioxus Hatschek’s pit, where the glycoprotein hormone thyrostimulin, an ancestor of TSH, is expressed along with GH/PRL [1, 10]. Furthermore, the expression of TSH, GH, and PRL are under the control of POU1F1 in non-mammalian vertebrates like zebrafish [11]. TSH has a single POU1F1-dependent promoter, whereas GH/PRL have multiple promoters/enhancers, each one of them working in a POU1F1-dependent manner. Since POU1F1 is a potent homeobox-type transcription factor, we speculate that, although GH/PRL are expressed in the pituitary as well as extra-pituitary tissues like other cytokines, the POU1F1 driven enhancers allows for them to be considered pituitary hormones in POU1F1 lineage cells [12, 13]. Indeed, GH-, PRL-, and TSH-producing cells have similar characteristics. In humans, lactotroph cells (and some somatotropic tumor cells) are physiologically responsive to TRH stimulation like thyrotrophs (indicating a paradoxical response) [14]. Moreover, a substantial portion of somatotroph tumor cells (GHoma) are known to express PRL, and a small subset of GHoma is shown to express TSH as well as GH [15, 16].

Regulation and function of GH in mammals

Regulation of pituitary GH expression

Pituitary GH gene expression is regulated by a variety of extracellular hormones/factors. We examined short-term regulation of GH gene transcription using the MtT/S rat somatotroph cell line [17]. We found that GH-releasing hormone (GHRH) stimulates, whereas somatostatin (SRIF) inhibits the transcriptional activity of the rat GH gene [18], similar to the effects of these factors on GH secretion. In addition to these hypothalamic factors, several nuclear receptor-related hormones such as glucocorticoid, estrogen, and thyroid hormone are found to influence GH gene transcription [19]. IGF-I was shown to have a negative effect on the transcriptional activity of the GH gene, suggesting the presence of long negative feedback regulation [20, 21].

GHRH is not only crucial for GH regulation but is also necessary for somatotroph development. In the anterior pituitary, corticotroph and thyrotroph cells are only slightly reduced in CRH and TRH knockout mice, respectively [22, 23]. Gonadotropin expression also recovers following short-term GnRH treatment in most patients with hypogonadotropic hypogonadism [24]. These results suggest that the development of the pituitary corticotroph, thyrotroph, and gonadotroph cells is not profoundly dependent on the corresponding hypothalamic factors. In contrast, somatotroph cells seem to be much more dependent on hypothalamic GHRH, because pituitary volume is shown to be markedly reduced in GHRH or GHRH receptor knockout mice [25] as well as in humans [26], indicating the importance of GHRH in somatotroph development.

Regarding GHRH expression in the hypothalamus, Potter et al. found that the homeobox transcription factor Gsh-1 (Genomic screen homeobox-1) (encoded by Gsx gene)-null mice are occasionally dwarf and lack hypothalamic GHRH expression [27]. Therefore, we examined the transcriptional regulation of the GHRH gene in JEG3 cell line in vitro, and confirmed the important role of Gsh-1 in GHRH expression [28]. Furthermore, regarding the short-term regulation of the GHRH gene, we identified the nuclear factor of activated T cells (NFAT) as a transcriptional factor for neuronal excitation-dependent GHRH expression [29].

GHRH is also involved in metabolism. GHRH neurons are located in the arcuate nucleus of the hypothalamus, where a variety of nutrient signals are sensed. Among them, one of the major known stimuli for GHRH expression in human is starvation [30]. In addition, ghrelin, a hunger hormone expressed mainly in the stomach [31], is a potent GH secretagogue partly via GHRH. Altogether, GH is considered to have dual roles: somatotropic effects mainly via IGF-I, and effects as a metabokine by adapting to starvation. Indeed, not only upstream factors are involved in metabolism, GH itself induces fat specific protein 27 (FSP27) via the JAK2/STAT5 pathway in adipocytes [32]. FSP27-induced lipolysis provides free fatty acids (FFAs) as an energy source during starvation [33]. FFAs are also known to induce insulin resistance and reduce glucose utilization to maintain plasma glucose levels during starvation. This suggest that factors downstream of GH affect metabolism.

Physiological roles of ghrelin

Ghrelin is a peptide hormone conserved from bony fish to human. In mammals, ghrelin is expressed mainly in the stomach during starvation [31, 34]. The peptide stimulates appetite [35], as well as facilitates the secretion of hunger-related hormones like GH and ACTH [34]. It consists of 28 amino acids, in which the octanoyl group (C8 medium chain fatty acid) is bound to the 3rd amino acid serine. This octanoyl group is critical for ghrelin’s activity. Ghrelin O-acyltransferase (GOAT) is an enzyme in attaching octanoyl fatty acid to ghrelin (octanoylation), and indeed phenotypes of GOAT knockout mice present a very similar phenotype to ghrelin-null mice [36, 37]. Although the mechanism and physiological significance of ghrelin’s octanoylation is not completely understood, we speculate the following mechanism. In animal cells, β-oxidation which is the essential catabolic process in breaking fatty acids into acetyl-CoA to generate energy occurs both in the peroxisome and mitochondria: β-oxidation from long to medium chain (until C6–C8) in peroxisome, and subsequently from medium chain to acetyl-CoA in mitochondria [38]. Based on this information, intracellular accumulation of ~C8 medium acyl-CoA during the peroxisome-mitochondria transition can be a lipid-mediated hunger signal (i.e., enhanced β-oxidation), with resultant synthesis of octanoyl-ghrelin, an active form of ghrelin. This hypothesis is shown in Fig. 1.

Fig. 1

Formation of octanoyl-ghrelin and its physiological significance (hypothesis)

Abbreviation: GOAT, ghrelin O-acyltransferase.

Regulation of GH-IGF-I system

Molecular mechanisms of the signaling pathway of GH/PRL action are quite similar, both exerting their effects via class 1 cytokine receptor/JAK/STAT pathways. The main difference is the intracellular mediator: PRL and GH use STAT5a and STAT5b, respectively. GH-stimulated GHR dimerization activates the JAK/STAT5b pathway and induces IGF-I gene transcription among many other genes. As mentioned in the previous section, GH emerged as a metabokine in response to starvation in vertebrates, whereas IGF-I is a growth factor playing important roles even in primitive invertebrates such as C. elegans [39]. IGF-I is an anabolic hormone during nutrient-rich condition whereas GH itself plays critical roles during starvation. The fact that transcription of an anabolic hormone (IGF-I) gene came under the control of a catabolic hormone (GH) during evolution is quite curious. Furthermore, the molecular mechanism(s) whereby GH regulates IGF-I gene transcription has long been a matter of debate. IGF-I is one of the numerous GH/STAT5b-target genes, in which STAT5-response element(s) is usually found in most. On the other hand, the IGF-I gene does not have a STAT5-response element. The human IGF-I gene has two alternative proximal promoters (P1, P2) [40], and our group examined the putative binding site(s) for phosphorylated STAT5b, without success (unpublished data). Other studies reported putative response elements, not on the promoters but on the distal enhancer (77 kb upstream from exon 1) or intron of the IGF-I gene (intron 2 between exon 1 and exon 2) [41, 42]. More recently, STAT5b was found to induce IGF-I gene promoter through interaction with other transcription factors like hepatocyte nuclear factors (HNFs) (HNF1α, HNF3γ) [43, 44]. Indeed, our co-expression study using HNF1α and STAT5b in human hepatocytes in vitro showed a combined positive effect of the two factors on the IGF gene promoter (unpublished data, Fig. 2). These results may explain the mechanism whereby GH exerts potent stimulatory effect on IGF-I gene transcription in the liver rather than in other peripheral tissues. The origin of the liver arouse from hepatic cecum in amphioxus in which the GH-IGF-I axis was established [1], and then developed as an organ in jawless vertebrates [45, 46]. Collectively, it seems that the link between GH/JAK/STAT5b and HNFs/IGF-I system in the liver has allowed for the development of the unique GH-IGF-I axis. Using this system, GH has started to behave as a growth hormone by dominating the transcriptional regulation of the IGF-I gene in the liver.

Fig. 2

The effects of STAT5b and HNF1α co-expression on the transcriptional activity of human IGF-I P2 promoter. A: Structure of human IGF-I gene. P1 and P2 designate the two alternative promoters [40]. B: The effects of STAT5b and HNF1α co-expression on the transcriptional activity of human IGF-I P2 promoter. HuH7 human hepatocyte cells were transfected with human IGF-P2 promoter (~1.8 kb)-luciferase plasmid with or without human HNF1α expression vector, and also with or without constitutive mouse Stat5b expression vector by lipofection. After 48 hours, cells were harvested, and the promoter activity was determined by a luciferase assay. *p < 0.05.

Essential roles of GH action

As mentioned above, it is apparently difficult to comprehend the two opposing facets of GH action, i.e., GH has two opposite aspects: a starvation hormone (catabolic), and that as a hormone for somatic growth (anabolic). The link between GH and IGF-I is also known to change (or shift) depending on nutritional conditions [47, 48]. The conflicting biological roles of GH can be understood if we recognize GH as a metabokine able to shift and/or switch nutrients to adapt to famished or satiated environments. One of the essential biological roles of GH is to facilitate cellular uptake of amino acids [49-51]. This effect is suitable for gluconeogenesis during starvation (i.e., supply of glucogenic amino acids for glucose production) in the liver to maintain blood glucose level. GH also shifts the nutrient source from glucose to FFA by stimulating intracellular lipolysis and supplying FFAs, which in turn induce insulin resistance. Since GH is known to not facilitate IGF-I gene expression during malnutrition [47, 48], intracellular amino acids are not used for protein anabolism. In contrast, GH stimulates IGF-I expression in nutrient-rich conditions, and the combination of GH and IGF-I enhances somatic growth by simultaneous occurrence of amino acid uptake (GH) and protein anabolism [IGF-I and mammalian target of rapamycin (mTOR) pathway] (Fig. 3). The fate of incorporated amino acids is switchable, enabling GH to act as a catabolic hormone during starvation, or as an anabolic hormone during satiety. In other words, GH is a metabokine regulating nutrient shift depending on nutritional environments.

Fig. 3

Biological effects of GH/IGF-I in either nutrient-poor (starved) or rich (satiated) condition

Bold letters/lines show active, whereas shaded letters/lines show inactive, states.

Abbreviation: a.a., amino acids; G6P, glucose-6-phosphatase; mTOR, mammalian target of rapamycin.

Biological roles of GH-V/human placental lactogen (hPL)

The human GH gene cluster, located on chromosome 17, consists of 5 family genes encoding the corresponding proteins (GH-N, GH-V, hPL-A, hPL-B, hPL-L), likely due to repeated local gene duplication [52]. Whereas GH-N is expressed mainly in the anterior pituitary, others are expressed in the placenta. GH-V, which is structurally almost indistinguishable from GH-N, is expessed in the fetal trophoblast, is secreted into the maternal circulation, and is known to induce insulin resistance [53]. GH-V also stimulates hepatic IGF-I production, which in turn exerts negative feedback effect on pituitary GH-N expression. Thus, in mid- and late pregnancy, a large amount of GH-V secreted from the fetus takes over maternal GH-N by shutting off its pituitary expression [54]. This system is physiologically reasonable, because GH-V-mediated increase in IGF-I supports placental development and function [55]. During pregnancy, unlike pituitary GH-N, placental GH-V is not suppressed by high IGF-I and induces lipolysis and FFA-mediated insulin resistance. As a result, carbohydrate shift from mother to fetus occurs. Direct positive effect of GH on amino acid shift to fetus in the placenta by increasing transporter expression is also reported [56].

Human placental lactogen (hPL), also known as chorionic somatomammotropin (CS), is located in the same gene locus as GH-N/GH-V, and is regarded as a metabokine facilitating the nutrient shift from mother to fetus. There are three hPL paralogs (hPL-A–C). Among the three paralogs, hPL-A and hPL-B are abundantly expressed in the fetal trophoblast of the placenta during pregnancy. They then enter the maternal circulation in a high concentration (~8 μg /mL in late pregnancy) [57] and cause various biological effects possibly through both GHR and PRLR, playing the role of metabokine.

Regulation and function of PRL

Regulation of pituitary PRL expression

Although multiple paralogs of PRL-related genes are identified in non-primate vertebrates, especially in rodents [58], interestingly, only one PRL gene is identified in primates including humans. This is in contrast to the fact that, as mentioned above, human GH has multiple paralogs forming a cluster within the same gene locus. Human PRL gene is located on chromosome 6, and a POU1F1-dependent promoter defines pituitary-specific PRL expression. Because most of the PRL in the systemic circulation is derived from the adenohypophysis, PRL is typically recognized as a pituitary hormone.

However, the PRL gene has an alternative promoter that allows for transcription from a different exon in a variety of tissues such as brain, skin, immune system, adipose tissue, and placenta [59, 60]. This extra-pituitary and POU1F1-independent PRL expression is observed universally in all vertebrate species. Therefore, it is speculated that, like GH, PRL is originally a metabokine that is expressed in various tissues, and one of the splice variants of PRL controlled by a potent and POU1F1-specific promoter makes PRL “a pituitary hormone.”

Function of pituitary PRL

In mammals, the major roles of plasma PRL are mammary gland development and milk production [61], nursing behavior, and inhibition of hypothalamic gonadotropic function during pregnancy and lactation. Milk is the only energy/mineral source of infants for somatic growth. Since calcium (Ca) and amino acids for milk protein like casein-Ca are important among others, PRL facilitates the expression of amino acid transporters such as L-type amino acid transporter 1 (LAT1), sodium-coupled neutral amino acid transporter 2 (SNAT2) [62, 63], and Ca transporters in the intestine, mammary gland, and placenta [64, 65]. In this context, PRL acts as a metabokine regulating the shift of nutrients from mother to fetus/infants. In addition, PRLR is known to be expressed in β-cells of pancreatic islets, and indeed PRL stimulates insulin secretion to compensate for the insulin resistance caused by GH/hPL during pregnancy, preventing gestational diabetes mellitus (GDM). Mice lacking PRLR in β-cells are shown to develop GDM [66]. Thus, PRL may at least partly be involved in the nutrient shift during pregnancy by regulating glucose metabolism at the whole body level.

PRL is known to affect maternal behavior. Indeed, hyperprolactinemia during pregnancy and lactation is associated with increased nursing behavior [67]. The hypothalamus is known to be a target of PRL, and PRL receptors are found in various parts of hypothalamic nuclei [67]. Thus, in corporation with other factors such as oxytocin, PRL is thought to be involved in nutritional shift from mother to infants at the behavioral level (i.e., maternal care) as well as in milk production itself.

PRL also inhibits gonadal function at the hypothalamic level, although the precise mechanism(s) is not completely understood. Kisspeptin plays a central role in the regulation of reproduction, and kisspeptin neurons in the hypothalamus express PRL and leptin receptors [68, 69]. Recent works suggest that plasma PRL directly inhibits the function of kisspeptin neurons [70, 71], thereby suppressing LH secretion. However, the molecular event(s) behind the inhibition of kisspeptin gene expression by PRL-activated JAK/STAT5 system has not been clarified, and awaits further investigation.

Regulation of extra-pituitary PRL system

Because circulating PRL appears to have little or no role except perinatal periods, PRL is often regarded as a dispensable hormone, especially in males. Nevertheless, virtually all vertebrates have at least one PRL gene. From our knowledge, in humans, no case of a homozygote PRL-null patient has been reported, although patients harboring heterologous loss-of-function mutation with alactogenesis is documented [72]. In animal models, PRL- or PRLR-null mice show profound reproductive defects [65]. These facts suggest the indispensable role of PRL in mammalian reproduction.

PRL is expressed in a variety of tissues other than the anterior pituitary in a POU1F1- independent manner [73], and recent studies focus on the crucial role of placental PRL among others in reproduction. PRL is expressed in the mammalian placenta, and is shown to be necessary for implantation of fertilized egg(s) in humans, and for maintenance of corpus luteum during pregnancy in rodents [74, 75]. As mentioned before, the copy number of the PRL gene is extremely variable among species; one copy in humans, more than ten copies in cattle, and twenty or more copies in rodents, likely caused by local gene duplication [58]. Although the reason for the marked quantitative difference among species is not clear, finely tuned expression of multiple PRL genes might be necessary for the maintenance of both the structural complexity and pleiotropic function of placenta in these multifetal, animals. Indeed, placental PRL and its 16 kDa fragment are known to be involved in angiogenesis and anti-angiogenesis, respectively [76], and thereby involved in the regulation of infrastructure formation for nutrient shift in the placenta. Pathologically, the fragments of pituitary-derived PRL formed in the placenta are shown to be responsible for peripartum cardiomyopathy in humans [77]. Altogether, the available data suggest that PRL locally expressed in the placenta may be playing a crucial role in vascular formation, supporting the nutrient shift between fetus and mother.

In addition, PRL is known to be involved in the immunological tolerance of the fetus in the placenta and amnionic fluid [78]. This is reasonable because JAK/STAT is originally the main signaling pathway in immune cells. It is possible that PRL might exert immunomodulatory effects via interaction between STAT5b and NF-κB [79].

Essential biological roles of PRL action

PRL is usually recognized as a hormone for mammalian lactation. Taxonomically, this has been shown not to be the case, as the PRL and GH genes are preserved from early vertebrates and amphioxus [1]. Indeed, PRL has a variety of pleiotropic functions; freshwater acclimation in fish [80], metamorphosis and water drive (Triturus viridescens) in amphibians [81-83], osmoregulation in some non-mammalian vertebrates [84], and lactation/reproduction in mammals (Table 1). This apparently unrelated array of functions makes it difficult to understand the essential biological positioning of PRL.

PRL is known to be involved in adaptation to fresh water by osmoregulation in some non-mammalian vertebrates, i.e., regulation of the shift of water and minerals at the gills, skin, and kidneys. This suggests that, in these animals, the plasma levels of PRL determine the preference of living circumstances, and/or elicit the shift of the animal’s own body (excursion, migration, water drive [81-83, 85]). The effect may also be related to the drive for selecting the place of reproduction (i.e., the link between PRL and reproduction in some non-mammalian vertebrates). In mammals that are living on land and nurturing offsprings within their body, the whole body shift for selection of living/reproductive circumstance is no longer necessary. Instead, PRL plays important roles in the placenta by regulating the mineral shift for osmoregulation of amnion [86] and angiogenesis supporting the nutrient shift for fetal growth, and in the mammary gland by facilitating nutrient supply for milk production. PRL is also involved in the metabolic shift of cholesterol by supporting progesterone production in the corpus luteum during pregnancy by regulating scavenger receptor type B1 (SR-B1) cholesterol transporter gene expression [87]. From the standpoint of nutrient shift, many, if not all, of these versatile effects of PRL can be explained by some common target proteins. For example, sodium chloride cotransporter (NCC) for water and mineral shifts [84, 88], transient receptor potential vanilloid (TRPV) channels for calcium [89], and aquaporins for water [90, 91], throughout vertebrates. Furthermore, some specific factors like aquaporins and claudin are the target of PRL both in fish ionocytes and in mammalian breast cells [64, 91, 92]. Overall, the essential role of PRL as a metabokine throughout vertebrate evolution seems to be the regulation of nutrient shift for growth/survival, and that of water shift for osmoregulation.

Concluding remarks

We have reviewed the roles of GH/PRL from the standpoint of chordate/vertebrate evolution beyond their roles as pituitary hormones. Based on the available data reviewed in this article, the biological significance of GH/PRL may be summarized as follows: GH/PRL are metabokines regulating the shift of nutrients (and even of whole bodies) to adapt and thrive in the most appropriate environment conductive for growth and reproduction. Both metabokines mediate ion/water shift for osmoregulation, and seawater/freshwater acclimation by facilitating shift themselves in fishes and in most primitive vertebrate such as sea lamprey [93]. GH/PRL play important roles during metamorphosis/living on land in amphibians, and PRL may be involved in migration in birds [94, 95]. In mammals, GH acts as a metabokine in the context of nutritional adaptation, shifting amino acids for gluconeogenesis during starvation, and shifting them to protein synthesis for somatic growth during satiated states. PRL mediates nutritional shift by delivering nutrients from mother to fetus in the placenta and for milk production in the mammary gland. Overall, the common keyword of the two metabokines is “shift”, as summarized in Figs. 4 and 5. Because of the remarkable flexibility of these metabokines both in temporal and quantitative (including copy number of genes) aspects, vertebrates may have been able to accomplish individual as well as species preservation by adapting to the changes in diverse environments. In this sense, GH/PRL may also be regarded as “metabokines for sustainability,” playing important roles during vertebrate evolution.

Fig. 4

GH as a “shift” hormone/metabokine to adapt to different environments

Fig. 5

PRL as a “shift” hormone/metabokine to adapt to different environments

Author Contributions

YI, DC, and TA wrote the manuscript. MN was involved in revising the manuscript.

Acknowledgements

None.

Disclosure

None.

References

• 1   Li  M,  Gao  Z,  Ji  D,  Zhang  S (2014) Functional characterization of GH-like homolog in amphioxus reveals an ancient origin of GH/GH receptor system. Endocrinology 155: 4818–4830.
• 2   Daza  DO,  Larhammar  D (2018) Evolution of the growth hormone, prolactin, prolactin 2 and somatolactin family. Gen Comp Endocrinol 264: 94–112.
• 3   Kawauchi  H,  Suzuki  K,  Yamazaki  T,  Moriyama  S,  Nozaki  M, et al. (2002) Identification of growth hormone in the sea lamprey, an extant representative of a group of the most ancient vertebrates. Endocrinology 143: 4916–4921.
• 4   Gong  N,  Ferreira-Martins  D,  McCormick  SD,  Sheridan  MA (2020) Divergent genes encoding the putative receptors for growth hormone and prolactin in sea lamprey display distinct patterns of expression. Sci Rep 10: 1674.
• 5   Gong  N,  Ferreira-Martins  D,  Norstog  JL,  McCormick  SD,  Sheridan  MA (2022) Discovery of prolactin-like in lamprey: role in osmoregulation and new insight into the evolution of the growth hormone/prolactin family. Proc Natl Acad Sci USA 119: e2212196119.
• 6   Candiani  S,  Holland  ND,  Oliveri  D,  Parodi  M,  Pestarino  M (2008) Expression of the amphioxus Pit-1 gene (AmphiPOU1F1/Pit-1) exclusively in the developing preoral organ, a putative homolog of the vertebrate adenohypophysis. Brain Res Bull 75: 324–330.
• 7   Cao  Y,  Fang  T,  Fan  M,  Wang  L,  Lv  C, et al. (2021) Functional characterization of STATa/b genes encoding transcription factors from Branchiostoma belcheri. Dev Comp Immunol 114: 103838.
• 8   Harvey  S,  Azumaya  Y,  Hull  KL (2000) Pituitary and extrapituitary growth hormone: Pit-1 dependence? Can J Physiol Pharmacol 78: 1013–1028.
• 9   Marano  RJ,  Ben-Jonathan  N (2014) Minireview: Extrapituitary prolactin: an update on the distribution, regulation, and functions. Mol Endocrinol 28: 622–633.
• 10   Wang  P,  Liu  S,  Yang  Q,  Liu  Z,  Zhang  S (2018) Functional characterization of thyrostimulin in amphioxus suggests an ancestral origin of the TH Signaling pathway. Endocrinology 159: 3536–3548.
• 11   Nica  G,  Herzog  W,  Sonntag  C,  Hammerschmidt  M (2004) Zebrafish pit1 mutants lack three pituitary cell types and develop severe dwarfism. Mol Endocrinol 18: 1196–1209.
• 12   Ho  Y,  Liebhaber  SA,  Cooke  NE (2011) The role of the hGH locus control region in somatotrope restriction of hGH-N gene expression. Mol Endocrinol 25: 877–884.
• 13   Enwright JF  3rd,  Kawecki-Crook  MA,  Voss  TC,  Schaufele  F,  Day  RN (2003) A PIT-1 homeodomain mutant blocks the intranuclear recruitment of the CCAAT/enhancer binding protein alpha required for prolactin gene transcription. Mol Endocrinol 17: 209–222.
• 14   Okinaga  H,  Takano  K,  Hayashi  S,  Yasufuku-Takano  J,  Teramoto  A, et al. (2005) Mechanisms of TRH-induced GH release (paradoxical response) in human somatotroph adenoma cells. Endocr J 52: 763–767.
• 15   Nyquist  P,  Laws  ER Jr,  Elliott  E (1994) Novel features of tumors that secrete both growth hormone and prolactin in acromegaly. Neurosurgery 35: 179–183.
• 16   Mori  R,  Inoshita  N,  Takahashi-Fujigasaki  J,  Joki  T,  Nishioka  H, et al. (2013) Clinicopathological features of growth hormone-producing pituitary adenomas in 242 acromegaly patients: classification according to hormone production and cytokeratin distribution. ISRN Endocrinol 2013: 723432.
• 17   Inoue  K,  Hattori  M,  Sakai  T,  Inukai  S,  Fujimoto  N, et al. (1990) Establishment of a series of pituitary clonal cell lines differing in morphology, hormone secretion, and response to estrogen. Endocrinology 126: 2313–2320.
• 18   Morishita  M,  Iwasaki  Y,  Onishi  A,  Asai  M,  Mutsuga  N, et al. (2003) The effects of GH-releasing hormone/somatostatin on the 5'-promoter activity of the GH gene in vitro. J Mol Endocrinol 31: 441–448.
• 19   Iwasaki  Y,  Morishita  M,  Asai  M,  Onishi  A,  Yoshida  M, et al. (2004) Effects of hormones targeting nuclear receptors on transcriptional regulation of the growth hormone gene in the MtT/S rat somatotrope cell line. Neuroendocrinology 79: 229–236.
• 20   Yamashita  S,  Melmed  S (1986) Insulin-like growth factor I action on rat anterior pituitary cells: suppression of growth hormone secretion and messenger ribonucleic acid levels. Endocrinology 118: 176–182.
• 21   Niiori-Onishi  A,  Iwasaki  Y,  Mutsuga  N,  Oiso  Y,  Inoue  K, et al. (1999) Molecular mechanisms of the negative effect of insulin-like growth factor-I on growth hormone gene expression in MtT/S somatotroph cells. Endocrinology 140: 344–349.
• 22   Muglia  L,  Jacobson  L,  Dikkes  P,  Majzoub  JA (1995) Corticotropin-releasing hormone deficiency reveals major fetal but not adult glucocorticoid need. Nature 373: 427–432.
• 23   Shibusawa  N,  Yamada  M,  Hirato  J,  Monden  T,  Satoh  T, et al. (2000) Requirement of thyrotropin-releasing hormone for the postnatal functions of pituitary thyrotrophs: ontogeny study of congenital tertiary hypothyroidism in mice. Mol Endocrinol 14: 137–146.
• 24   Tomita  M (1980) Consecutive administrative of synthetic LRH in the evaluation of gonadotrophin reserve in children. Acta Endocrinol (Copenh) 94: 289–296.
• 25   Alba  M,  Fintini  D,  Bowers  CY,  Parlow  AF,  Salvatori  R (2005) Effects of long-term treatment with growth hormone-releasing peptide-2 in the GHRH knockout mouse. Am J Physiol Endocrinol Metab 289: E762–E767.
• 26   Shohreh  R,  Sherafat-Kazemzadeh  R,  Jee  YH,  Blitz  A,  Salvatori  R (2011) A novel frame shift mutation in the GHRH receptor gene in familial isolated GH deficiency: early occurrence of anterior pituitary hypoplasia. J Clin Endocrinol Metab 96: 2982–2986.
• 27   Li  H,  Zeitler  PS,  Valerius  MT,  Small  K,  Potter  SS (1996) Gsh-1, an orphan Hox gene, is required for normal pituitary development. EMBO J 15: 714–724.
• 28   Mutsuga  N,  Iwasaki  Y,  Morishita  M,  Nomura  A,  Yamamori  E, et al. (2001) Homeobox protein Gsh-1-dependent regulation of the rat GHRH gene promoter. Mol Endocrinol 15: 2149–2156.
• 29   Asai  M,  Iwasaki  Y,  Yoshida  M,  Mutsuga-Nakayama  N,  Arima  H, et al. (2004) Nuclear factor of activated T cells (NFAT) is involved in the depolarization-induced activation of growth hormone-releasing hormone gene transcription in vitro. Mol Endocrinol 18: 3011–3019.
• 30   Hartman  ML,  Veldhuis  JD,  Johnson  ML,  Lee  MM,  Alberti  KG, et al. (1992) Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab 74: 757–765.
• 31   Kojima  M,  Hosoda  H,  Date  Y,  Nakazato  M,  Matsuo  H, et al. (1999) Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature 402: 656–660.
• 32   Sharma  R,  Luong  Q,  Sharma  VM,  Harberson  M,  Harper  B, et al. (2018) Growth hormone controls lipolysis by regulation of FSP27 expression. J Endocrinol 239: 289–301.
• 33   Sakharova  AA,  Horowitz  JF,  Surya  S,  Goldenberg  N,  Harber  MP, et al. (2008) Role of growth hormone in regulating lipolysis, proteolysis, and hepatic glucose production during fasting. J Clin Endocrinol Metab 93: 2755–2759.
• 34   Nunez-Salces  M,  Li  H,  Feinle-Bisset  C,  Young  RL,  Page  AJ (2021) The regulation of gastric ghrelin secretion. Acta Physiol (Oxf) 231: e13588.
• 35   Nakazato  M,  Murakami  N,  Date  Y,  Kojima  M,  Matsuo  H, et al. (2001) A role for ghrelin in the central regulation of feeding. Nature 409: 194–198.
• 36   Kouno  T,  Akiyama  N,  Ito  T,  Okuda  T,  Nanchi  I, et al. (2016) Ghrelin O-acyltransferase knockout mice show resistance to obesity when fed high-sucrose diet. J Endocrinol 228: 115–125.
• 37   Kang  K,  Zmuda  E,  Sleeman  MW (2011) Physiological role of ghrelin as revealed by the ghrelin and GOAT knockout mice. Peptides 32: 2236–2241.
• 38   Rottensteiner  H,  Theodoulou  FL (2006) The ins and outs of peroxisomes: co-ordination of membrane transport and peroxisomal metabolism. Biochim Biophys Acta 1763: 1527-1540.
• 39   Barbieri  M,  Bonafè  M,  Franceschi  C,  Paolisso  G (2003) Insulin/IGF-I-signaling pathway: an evolutionarily conserved mechanism of longevity from yeast to humans. Am J Physiol Endocrinol Metab 285: E1064–E1071.
• 40   Jansen  E,  Steenbergh  PH,  van Schaik  FM,  Sussenbach  JS (1992) The human IGF-I gene contains two cell type-specifically regulated promoters. Biochem Biophys Res Commun 187: 1219–1226.
• 41   Chia  DJ,  Ono  M,  Woelfle  J,  Schlesinger-Massart  M,  Jiang  H, et al. (2006) Characterization of distinct Stat5b binding sites that mediate growth hormone-stimulated IGF-I gene transcription. J Biol Chem 281: 3190–3197.
• 42   Varco-Merth  B,  Mirza  K,  Alzhanov  DT,  Chia  DJ,  Rotwein  P (2012) Biochemical characterization of diverse Stat5b-binding enhancers that mediate growth hormone-activated insulin-like growth factor-I gene transcription. PLoS One 7: e50278.
• 43   Metón  I,  Boot  EP,  Sussenbach  JS,  Steenbergh  PH (1999) Growth hormone induces insulin-like growth factor-I gene transcription by a synergistic action of STAT5 and HNF-1α. FEBS Lett 444: 155–159.
• 44   Eleswarapu  S,  Ge  X,  Wang  Y,  Yu  J,  Jiang  H (2009) Growth hormone-activated STAT5 may indirectly stimulate IGF-I gene transcription through HNF-3γ. Mol Endocrinol 23: 2026–2037.
• 45   Wang  Y,  Zhang  S (2011) Identification and expression of liver-specific genes after LPS challenge in amphioxus: the hepatic cecum as liver-like organ and “pre-hepatic” acute phase response. Funct Integr Genomics 11: 111–118.
• 46   Muramatsu  B,  Suzuki  DG,  Suzuki  M,  Higashiyama  H (2024) Gross anatomy of the pacific hagfish, Eptatretus burgeri, with special reference to the coelomic viscera. Anat Rec (Hoboken) 307: 155–171.
• 47   Yamamoto  M,  Iguchi  G,  Fukuoka  H,  Suda  K,  Bando  H, et al. (2013) SIRT1 regulates adaptive response of the growth hormone-insulin-like growth factor-I axis under fasting conditions in liver. Proc Natl Acad Sci USA 110: 14948–14953.
• 48   Caputo  M,  Pigni  S,  Agosti  E,  Daffara  T,  Ferrero  A, et al. (2021) Regulation of GH and GH signaling by nutrients. Cells 10: 1376.
• 49   Turner  MR,  Reeds  PJ,  Munday  KA (1976) Action of growth hormone in vitro on the net uptake and incorporation in protein of amino acids in muscle from intact rabbits given protein-deficient diets. Br J Nutr 35: 1–10.
• 50   Pacitti  AJ,  Inoue  Y,  Plumley  DA,  Copeland  EM,  Souba  WW (1992) Growth hormone regulates amino acid transport in human and rat liver. Ann Surg 216: 353–361.
• 51   Pupim  LB,  Flakoll  PJ,  Yu  C,  Ikizler  TA (2005) Recombinant human growth hormone improves muscle amino acid uptake and whole-body protein metabolism in chronic hemodialysis patients. Am J Clin Nutr 82: 1235–1243.
• 52   Baumann  GP (2009) Growth hormone isoforms. Growth Horm IGF Res 19: 333–340.
• 53   Liao  S,  Vickers  MH,  Stanley  JL,  Ponnampalam  AP,  Baker  PN, et al. (2016) The placental variant of human growth hormone reduces maternal insulin sensitivity in a dose-dependent manner in C57BL/6J Mice. Endocrinology 157: 1175–1186.
• 54   Alsat  E,  Guibourdenche  J,  Couturier  A,  Evain-Brion  D (1998) Physiological role of human placental growth hormone. Mol Cell Endocrinol 140: 121–127.
• 55   Forbes  K,  Westwood  M,  Baker  PN,  Aplin  JD (2008) Insulin-like growth factor I and II regulate the life cycle of trophoblast in the developing human placenta. Am J Physiol Cell Physiol 294: C1313–C1322.
• 56   Tung  E,  Roberts  CT,  Heinemann  GK,  De Blasio  MJ,  Kind  KL, et al. (2012) Increased placental nutrient transporter expression at midgestation after maternal growth hormone treatment in pigs: a placental mechanism for increased fetal growth. Biol Reprod 87: 126.
• 57   Cattini  PA,  Jin  Y,  Jarmasz  JS,  Noorjahan  N,  Bock  ME (2020) Obesity and regulation of human placental lactogen production in pregnancy. J Neuroendocrinol 32: e12859.
• 58   Soares  MJ,  Alam  SM,  Duckworth  ML,  Horseman  ND,  Konno  T, et al. (2007) A standardized nomenclature for the mouse and rat prolactin superfamilies. Mamm Genome 18: 154–156.
• 59   Gellersen  B,  Kempf  R,  Telgmann  R,  DiMattia  GE (1994) Nonpituitary human prolactin gene transcription is independent of Pit-1 and differentially controlled in lymphocytes and in endometrial stroma. Mol Endocrinol 8: 356–373.
• 60   Featherstone  K,  White  MR,  Davis  JR (2012) The prolactin gene: a paradigm of tissue-specific gene regulation with complex temporal transcription dynamics. J Neuroendocrinol 24: 977–990.
• 61   Linzell  JL,  Peaker  M,  Taylor  JC (1975) The effects of prolactin and oxytocin on milk secretion and on the permeability of the mammary epithelium in the rabbit. J Physiol 253: 547–563.
• 62   Velázquez-Villegas  LA,  López-Barradas  AM,  Torres  N,  Hernández-Pando  R,  León-Contreras  JC, et al. (2015) Prolactin and the dietary protein/carbohydrate ratio regulate the expression of SNAT2 amino acid transporter in the mammary gland during lactation. Biochim Biophys Acta 1848: 1157–1164.
• 63   Zhou  J,  Jiang  M,  Shi  Y,  Song  S,  Hou  X, et al. (2020) Prolactin regulates LAT1 expression via STAT5 (signal transducer and activator of transcription 5) signaling in mammary epithelial cells of dairy cows. J Dairy Sci 103: 6627–6634.
• 64   Anantamongkol  U,  Takemura  H,  Suthiphongchai  T,  Krishnamra  N,  Horio  Y (2007) Regulation of Ca2+ mobilization by prolactin in mammary gland cells: possible role of secretory pathway Ca2+-ATPase type 2. Biochem Biophys Res Commun 352: 537–542.
• 65   Wongdee  K,  Charoenphandhu  N (2013) Regulation of epithelial calcium transport by prolactin: from fish to mammals. Gen Comp Endocrinol 181: 235–240.
• 66   Banerjee  RR,  Cyphert  HA,  Walker  EM,  Chakravarthy  H,  Peiris  H, et al. (2016) Gestational diabetes mellitus from inactivation of prolactin receptor and MafB in islet beta-cells. Diabetes 65: 2331–2341.
• 67   Grattan  DR (2002) Behavioural significance of prolactin signalling in the central nervous system during pregnancy and lactation. Reproduction 123: 497–506.
• 68   Backholer  K,  Smith  JT,  Rao  A,  Pereira  A,  Iqbal  J, et al. (2010) Kisspeptin cells in the ewe brain respond to leptin and communicate with neuropeptide Y and proopiomelanocortin cells. Endocrinology 151: 2233–2243.
• 69   Kokay  IC,  Petersen  SL,  Grattan  DR (2011) Identification of prolactin-sensitive GABA and kisspeptin neurons in regions of the rat hypothalamus involved in the control of fertility. Endocrinology 152: 526–535.
• 70   Araujo-Lopes  R,  Crampton  JR,  Aquino  NS,  Miranda  RM,  Kokay  IC, et al. (2014) Prolactin regulates kisspeptin neurons in the arcuate nucleus to suppress LH secretion in female rats. Endocrinology 155: 1010–1020.
• 71   Brown  RSE,  Khant Aung  Z,  Phillipps  HR,  Barad  Z,  Lein  HJ, et al. (2019) Acute suppression of LH secretion by prolactin in female mice is mediated by kisspeptin neurons in the arcuate nucleus. Endocrinology 160: 1323–1332.
• 72   Moriwaki  M,  Welt  CK (2021) PRL Mutation causing alactogenesis: insights into prolactin structure and function relationships. J Clin Endocrinol Metab 106: e3021–e3026.
• 73   Christensen  HR,  Murawsky  MK,  Horseman  ND,  Willson  TA,  Gregerson  KA (2013) Completely humanizing prolactin rescues infertility in prolactin knockout mice and leads to human prolactin expression in extrapituitary mouse tissues. Endocrinology 154: 4777–4789.
• 74   Grosdemouge  I,  Bachelot  A,  Lucas  A,  Baran  N,  Kelly  PA, et al. (2003) Effects of deletion of the prolactin receptor on ovarian gene expression. Reprod Biol Endocrinol 1: 12.
• 75   Garzia  E,  Borgato  S,  Cozzi  V,  Doi  P,  Bulfamante  G, et al. (2004) Lack of expression of endometrial prolactin in early implantation failure: a pilot study. Hum Reprod 19: 1911–1916.
• 76   Struman  I,  Bentzien  F,  Lee  H,  Mainfroid  V,  D’Angelo  G, et al. (1999) Opposing actions of intact and N-terminal fragments of the human prolactin/growth hormone family members on angiogenesis: an efficient mechanism for the regulation of angiogenesis. Proc Natl Acad Sci USA 96: 1246–1251.
• 77   Ricke-Hoch  M,  Pfeffer  TJ,  Hilfiker-Kleiner  D (2020) Peripartum cardiomyopathy: basic mechanisms and hope for new therapies. Cardiovasc Res 116: 520–531.
• 78   Flores-Espinosa  P,  Méndez  I,  Irles  C,  Olmos-Ortiz  A,  Helguera-Repetto  C, et al. (2023) Immunomodulatory role of decidual prolactin on the human fetal membranes and placenta. Front Immunol 14: 1212736.
• 79   Yu-Lee  LY (2002) Prolactin modulation of immune and inflammatory responses. Recent Prog Horm Res 57: 435–455.
• 80   Malintha  GHT,  Woo  DW,  Celino-Brady  FT,  Seale  AP (2023) Temperature modulates the osmosensitivity of tilapia prolactin cells. Sci Rep 13: 20217.
• 81   Chadwick  CS (1941) Further observations on the water drive in Triturus viridescens. II Induction of the water drive with the lactogenic hormone. J Exp Zool 86: 175–187.
• 82   Meier  AH,  Garcia  LE,  Joseph  MM (1971) Corticosterone phases a circadian water drive response to prolactin in the spotted newt, Notopthalmus viridescens. Biol Bull 141: 331–336.
• 83   Dey  S,  Karar  A (2023) Regulatory effects of prolactin in amphibians. South Asian J Exp Biol 13: 307–327.
• 84   Breves  JP,  McCormick  SD,  Karlstrom  RO (2014) Prolactin and teleost ionocytes: new insights into cellular and molecular targets of prolactin in vertebrate epithelia. Gen Comp Endocrinol 203: 21–28.
• 85   Yada  T,  Tsuruta  T,  Sakano  H,  Yamamoto  S,  Abe  N, et al. (2010) Changes in prolactin mRNA levels during downstream migration of the amphidromous teleost, ayu Plecoglossus altivelis. Gen Comp Endocrinol 167: 261–267.
• 86   Raabe  MA, McCoshen, JA (1986) Epithelial regulation of prolactin effect on amniotic permeability. Am J Obstet Gynecol 154: 130–134.
• 87   Stocco  C,  Callegari  E,  Gibori  G (2001) Opposite effect of prolactin and prostaglandin F(2 alpha) on the expression of luteal genes as revealed by rat cDNA expression array. Endocrinology 142: 4158–4161.
• 88   Breves  JP,  Serizier  SB,  Goffin  V,  McCormick  SD,  Karlstrom  RO (2013) Prolactin regulates transcription of the ion uptake Na+/Cl– cotransporter (ncc) gene in zebrafish gill. Mol Cell Endocrinol 369: 98–106.
• 89   Ajibade  DV,  Dhawan  P,  Fechner  AJ,  Meyer  MB,  Pike  JW, et al. (2010) Evidence for a role of prolactin in calcium homeostasis: regulation of intestinal transient receptor potential vanilloid type 6, intestinal calcium absorption, and the 25-hydroxyvitamin D(3) 1alpha hydroxylase gene by prolactin. Endocrinology 151: 2974–2984.
• 90   Breves  JP,  Inokuchi  M,  Yamaguchi  Y,  Seale  AP,  Hunt  BL, et al. (2016) Hormonal regulation of aquaporin 3: opposing actions of prolactin and cortisol in tilapia gill. J Endocrinol 230: 325–337.
• 91   Kaihoko  Y,  Tsugami  Y,  Suzuki  N,  Suzuki  T,  Nishimura  T, et al. (2020) Distinct expression patterns of aquaporin 3 and 5 in ductal and alveolar epithelial cells in mouse mammary glands before and after parturition. Cell Tissue Res 380: 513–526.
• 92   Kobayashi  K,  Tsugami  Y,  Matsunaga  K,  Oyama  S,  Kuki  C, et al. (2016) Prolactin and glucocorticoid signaling induces lactation-specific tight junctions concurrent with β-casein expression in mammary epithelial cells. Biochim Biophys Acta 1863: 2006–2016.
• 93   Ferreira-Martins  D,  Walton  E,  Karlstrom  RO,  Sheridan  MA,  McCormick  SD (2023) The GH/IGF axis in the sea lamprey during metamorphosis and seawater acclimation. Mol Cell Endocrinol 571: 111937.
• 94   Holberton  RL,  Boswell  T,  Hunter  MJ (2008) Circulating prolactin and corticosterone concentrations during the development of migratory condition in the Dark-eyed Junco, Junco hyemalis. Gen Comp Endocrinol 155: 641–649.
• 95   Chakraborty  A,  Saha  I (2021) Regulatory effects of prolactin on breeding and migratory behaviours in birds. South Asian J Exp Biol 11: 337–344.