Three regions within the mature mouse and rat uteroplacental site possess trophoblast cells: the labyrinth zone, the junctional zone, and the uterine mesometrial compartment [Fig. 2]. PRL family members are expressed in cell type-, location-, and temporal-specific profiles 36. In rodents, three different trophoblast cell lineages are capable of expressing members of the PRL family: trophoblast giant cells, spongiotrophoblast cells, and invasive trophoblast cells 343738. Each cell type expresses a unique subset of PRL family members. Locations of hormone/cytokine production influence post-translational processing and temporal and spatial access to biological targets. Trophoblast giant cells form the early interface with the maternal environment. Once the chorioallantoic placenta is established trophoblast giant cells localize to the junctional zone and during the last week of gestation this same cell type arises within the labyrinth zone. The labyrinth zone of the chorioallantoic placenta is principally involved in bidirectional nutrient/waste transport between maternal and fetal compartments, functions assumed by syncytial trophoblast cells 39. The transient development of trophoblast giant cells within the labyrinth zone may facilitate hormone/cytokine delivery into the fetal compartment. Transcriptional control of trophoblast giant cell-specific gene expression has been investigated using the Rcho-1 trophoblast cell line 4142. Spongiotrophoblast cells arise just prior to midgestation, are prominent cellular constituents of the junctional zone, and a major source of most PRL family members throughout the latter half of gestation. Secretory products of spongiotrophoblast cells are particularly abundant in maternal circulation. An in vitro cell culture system for studying spongiotrophoblast cell expression of PRL family members has been established 42. During the last week of gestation, a population of trophoblast cells exits the chorioallantoic placenta. These invasive trophoblast cells are more prominent in the rat where they invade deep into the uterine mesometrial compartment forming intimate relationships with spiral arteries, producing a unique subset of PRL family members 3738. The invasive trophoblast cells have first access to blood-borne nutrients, toxic compounds, signaling molecules, and cells entering the uterine environment. In vitro systems for investigating invasive trophoblast cells have not been described. The two remaining lineages of rodent trophoblast cells, glycogen cells and syncytial trophoblast cells, do not express members of the rodent PRL family. However, mouse syncytial trophoblast cells, which share transport properties with human syncytial trophoblast cells, do possess the transcriptional machinery needed to activate transgenic DNA constructs containing the locus control region of the human GH gene family 43. The conservation suggests an interesting analogous relationship between syncytial trophoblast cells of humans and rodents. Promoter regions for the mouse and rat PL-II genes have been identified that direct placental-specific expression in transgenic mice 14444546. At least one member of the PRL family has been shown to exhibit allele-specific expression patterns. These experiments were performed in deer mice, Peromyscus 47. Peromyscus PL-Iv expression is restricted to the paternal allele and to spongiotrophoblast cells of the chorioallantoic placenta. Interestingly, spongiotrophoblast cell development is regulated by a maternally imprinted transcription factor, Mash2 48, and is abnormally affected by disruptions in genomic imprinting, as seen in placentas from cloned mice 49 and following interspecies breeding 5051. At present, it is not known whether genomic imprinting is a feature shared among other members of the Peromyscus PRL family or between PRL and GH family loci from other species; nor is the biological significance of the allele-specific expression pattern appreciated.

Three trophoblast cell types are associated with placentation in ruminants: i) uninucleate trophoblast cells, ii) binucleate trophoblast cells, and iii) heterokaryons formed by the fusion of binucleate trophoblast cells with endometrial epithelial cells. Uninucleate trophoblast cells serve as precursors for binucleate cell formation. Binucleate trophoblast cells and trophoblast-endometrial heterokaryons of sheep and cattle produce PLs 152. PRP-I shows a similar expression pattern in the bovine placenta 53. Expression of PLs and PRP-I begins around the time of the initial embryo-uterine interactions 1753. The cellular localization and ontogeny of placental expression for other members of the bovine PRL family have not been reported. Ovine placental GH is expressed in each of the above cell populations during a restricted period of gestation (days 35 to 70), and is controlled by mechanisms distinct from the regulation of GH production by the anterior pituitary 6. A new bovine trophoblast cell differentiation culture system capable of binucleate cell formation and PL synthesis offers great promise for future in vitro analyses of PRL and GH family gene expression 5455.

Transcripts for members of the GH family (GH-V, CS-A, CS-B, and CS-L) have each been detected in the human placenta 223. CS-A and CS-B genes encode for identical mature secreted proteins, collectively referred to as CS, and are regulated similarly 223. Although, CS-L transcripts have been identified, they likely contribute to low levels of translated products 2. Two principal trophoblastic structures exist within the human placenta: villous and extravillous trophoblast cells. Syncytial trophoblast cells of the villous portion of the chorioallantoic placenta are a major source of GH family members, as are extravillous invasive trophoblast 22356. The latter cell population is analogous with the invasive trophoblast of the rodent placenta and merits additional investigation regarding their expression of GH family cytokines. Transcription of GH family genes is regulated by a locus control region located 23 kb upstream of the cluster 4357. Four members of the rhesus monkey GH family (CS-1, CS-2, CS-3, and GH-V), possessing extensive homology with human GH family members, are expressed in the monkey chorioallantoic placenta 58.

The mammary gland and corpus luteum represent two classic biological targets of a subset of the PRL family 77787980818283. The physiological responses of these targets to PRL family members are mediated by PRL-Rs. Ligands activating PRL-Rs during pregnancy arise from maternal (lactotroph and decidual: PRL), trophoblast (PL-Is, PL-Iv, PL-II), and fetal (lactotroph: PRL) tissues (Fig. 3). Production of these ligands is orchestrated in a precise temporal pattern that ensures the presence of activators of the PRL-R signaling system throughout gestation 363848586 (Fig. 4). The PRL-R agonists stimulate mammary epithelial cell growth and differentiation and help maintain corpus luteum integrity and progesterone and relaxin biosynthesis. Phenotypes of PRL^-/- and PRL-R^-/- null mutant mouse models support the importance of the PRL-R signaling pathway in mammary gland and luteal biology 87. Investigations using gene-targeting approaches are inherently more complex for studying the biology of the multiple PL genes expressed in trophoblast cells. In addition to the mammary gland and corpus luteum, evidence has arisen for PRL and the PLs modulating pregnancy-dependent changes in brain function, pancreas biology, and the immune system 8889909192. Local production of PRL by decidual cells may also contribute to decidual cell survival 9394. The existence of multiple ligands activating the PRL-R signaling pathway provides for complementarity and specialization in ligand regulation, availability, and possibly action 87.

Insights into intracellular events subsequent to PRL-R activation by members of the placental PRL family are minimal. It is assumed that PLs activate signal transduction pathways downstream of the PRL-R similar to those activated by PRL. Beyond the abilities of members of the PL-I subfamily to stimulate the Jak-Signal Transducer and Activator of Transcription (STAT) pathway in Nb2 lymphoma cells 9596 and cultured luteinized granulosa cells 80, information on intracellular signaling pathways triggered by classical members of the placental PRL family is lacking. It seems likely that at least some aspects of PRL signaling are mimicked by PLs; however, whether these uteroplacental ligands reproduce the full spectrum of PRL action or elicit unique responses is unknown. The autocrine/paracrine actions of PRL within the decidua appear to be mediated by activation of classic PRL ligand-PRL-R signal transduction, including stimulation of Jak2-STAT5 and phosphatidylinositol 3-kinase/Akt signaling pathways 9394.

PLs have been postulated to affect maternal (corpus luteum function, uterine gland development, intermediary metabolism, and mammary gland development) and fetal physiological processes 7. The general implication is that PLs contribute to normal physiological adjustments occurring during pregnancy; promoting corpus luteum function, uterine and mammary gland development, and facilitating nutrient delivery and utilization by the fetus. These predictions have not always been supported by empirical observation. The absence of supportive evidence may at times be attributed to experimental design or may instead reflect a more subtle and potentially unique role of PLs in the biology of pregnancy (see below). Some of the most impressive physiological data relates to the local actions of PLs on uterine histiotrophic nutrient delivery, which is of paramount importance in ruminants 34. PL binds to endometrial gland epithelia and is capable of increasing uterine gland density and uterine milk protein expression 9798. These in vivo actions of PL are dependent upon the hormonal milieu and require exposure to progesterone and interferon tau. Ovine placental GH likely contributes to the pregnancy-dependent functional changes in uterine gland function 9798 and may also influence fetal tissues prior to the ontogeny of fetal GH production. A point of consideration is warranted regarding comparisons of the biology of PLs in sheep and cattle. Ovine and bovine PLs have undergone significant sequence divergence, especially when compared to the minimal divergence for their respective PRLs and GHs 99. These structural changes have contributed to some differences in regulation and bioavailability and potentially in activities of the PLs from each species.

Ruminant PLs offer a unique mode of action. It has been long known that PLs of ruminants interact with both PRL and GH receptors 7952. Early evidence suggested that ruminant PLs were dual-acting agonists for both PRL-R and GH-R signaling pathways and there was some suggestion of a unique receptor system for ruminant PLs. Many of these early experiments were performed with receptors isolated from non-ruminant species. It is now known that ruminant PLs effectively signal through PRL-R homodimers, PRL-R/GH-R heterodimers, and in the absence of PRL-R may act as a GH-R antagonist [9; Fig. 3]. Some of the GH-like effects of PLs may be mediated by their interaction with PRL-R/GH-R heterodimers. Ruminant PLs activate Jak/STAT and mitogen activated protein kinase (MAPK) signaling pathways 79100. The nature of PL signal transduction appears to differ depending upon whether PRL-R homodimers or PRL-R/GH-R heterodimers are activated. Ligand-receptor heterodimer interaction results in prolonged STAT3 activation, which may culminate in distinct cellular responses 100. Existence of a separate ruminant PL receptor remains elusive. At this juncture, ovine placental GH is thought to act via mechanisms identical to GH produced by the anterior pituitary 67.

Classical PRL-R and GH-R mediated actions in primates include those stimulated by decidual PRL and placental members of the GH family. Human decidual PRL may accumulate in the decidual extracellular matrix due to its association with heparin containing molecules 69 and its targets include, intrauterine (e.g. modulatory effects on uterine gland development and function, angiogenesis, trophoblast cell development and function, and immune regulation), amniotic, and possibly fetal tissues 2325. There is an intriguing parallel between the intrauterine effects of decidual PRL and those of ruminant PLs on histiotrophic nutrient delivery. Similarly, CSs and GH-V have been implicated in the modulation of maternal physiology and directly or indirectly as regulators of fetal growth and development, as suggested for ruminant PLs 723.

Genetic deletions impacting CS and GH-V have been described and offer insight into the biology of members of the placental GH family 223101102103. In several instances the deficiencies of CS and GH-V had no demonstrable impact on the progression of pregnancy 101102; however, a patient with CS and GH-V deficiencies exhibiting pregnancy-associated pathologies (including preeclampsia and intrauterine growth restriction) has been described 103. Pathologies in the latter clinical presentation may or may not be related to the absence of CS and GH-V proteins. Although, the anecdotal nature of the clinical findings makes it difficult to make absolute conclusions about the biology of the placental GH family, they suggest that the CS and GH-V genes may be dispensable in some pregnancies.

The basis for much of the speculation concerning CS and GH-V biology relates to their close structural similarity with pituitary-derived GH (GH-N) and the nature of their interactions with PRL and GH receptors 2723. Unlike rodent and ruminant GHs, primate GH-Ns function as potent agonists for both PRL-R and GH-R signaling. In comparison to GH-N, CSs are equipotent activators of PRL-R signaling and weak GH-R agonists, whereas GH-V is a more potent GH-R agonist, and a less potent PRL-R agonist. Introduction of these subtle functional variants into maternal and fetal circulation during pregnancy would seem to be an effective means of modulating PRL-R and GH-R activation and thus maternal and fetal physiology. Based on PRL-R and GH-R expression profiles, PRL- or CS-activated PRL-R signaling is probably more significant in the fetus than is GH-R signaling. Some evidence exists for a unique CS receptor present in human fetal tissues; however, the molecular structure of the receptor has not been reported 23. Signaling via PRL-R/GH-R heterodimers has not been described for members of the primate GH family but would seem to be a means of expanding the diversity of signals emanating from the ligands. PRL-R and GH-R signal transduction cascades activated by PRL and GH family ligands arising at the maternal-fetal interface have not been extensively investigated but are likely similar to those activated by anterior pituitary-derived PRL and GH and involve the Jak/STAT pathway among other signaling pathways.