The effect of prolactin on murine in vitro fertilization and embryo development

Prolactin from Pluripotency to Central Nervous System Development

Prolactin (PRL) is a versatile hormone that exerts more than 300 functions in vertebrates, mainly associated with physiological effects in adult animals. Although the process that regulates early development is poorly understood, evidence suggests a role of PRL in the early embryonic development regarding pluripotency and nervous system development. Thus, PRL could be a crucial regulator in oocyte preimplantation and maturation as well as during diapause, a reversible state of blastocyst development arrest that shares metabolic, transcriptomic, and proteomic similarities with pluripotent stem cells in the naïve state. Thus, we analyzed the role of the hormone during those processes, which involve the regulation of its receptor and several signaling cascades (Jak/Mapk, Jak/Stat, and PI3k/Akt), resulting in either a plethora of physiological actions or their dysregulation, a factor in developmental disorders. Finally, we propose models to improve the knowledge on PRL function during early development.

Prolactin: the new biology of an old hormone

Prolactin (PRL) is a paradoxical hormone. Historically known as the pituitary hormone of lactation, it has had attributed to it more than 300 separate actions, which can be correlated to the quasi-ubiquitous distribution of its receptor. Meanwhile, PRL-related knockout models have mainly highlighted its irreplaceable role in functions of lactation and reproduction, which suggests that most of its other reported target tissues are presumably modulated by, rather than strictly dependent on, PRL. The multiplicity of PRL actions in animals is in direct opposition to the paucity of arguments that suggest its involvement in human pathophysiology other than effects on reproduction. Although many experimental data argue for a role of PRL in the progression of some tumors, such as breast and prostate cancers, drugs lowering circulating PRL levels are ineffective. This observation opens new avenues for research into the understanding of whether local production of PRL is involved in tumor growth and, if so, how extrapituitary PRL synthesis is regulated. Finally, the physiological relevance of PRL variants, such as the antiangiogenic 16K-like PRL fragments, needs to be elucidated. This review is aimed at critically discussing how these recent findings have renewed the manner in which PRL should be considered as a multifunctional hormone.

Prolactin (PRL) and its receptor: actions, signal transduction pathways and phenotypes observed in PRL receptor knockout mice

PRL is an anterior pituitary hormone that, along with GH and PLs, forms a family of hormones that probably resulted from the duplication of an ancestral gene. The PRLR is also a member of a larger family, known as the cytokine class-1 receptor superfamily, which currently has more than 20 different members. PRLRs or binding sites are widely distributed throughout the body. In fact, it is difficult to find a tissue that does not express any PRLR mRNA or protein. In agreement with this wide distribution of receptors is the fact that now more than 300 separate actions of PRL have been reported in various vertebrates, including effects on water and salt balance, growth and development, endocrinology and metabolism, brain and behavior, reproduction, and immune regulation and protection. Clearly, a large proportion of these actions are directly or indirectly associated with the process of reproduction, including many behavioral effects. PRL is also becoming well known as an important regulator of immune function. A number of disease states, including the growth of different forms of cancer as well as various autoimmune diseases, appear to be related to an overproduction of PRL, which may act in an endocrine, autocrine, or paracrine manner, or via an increased sensitivity to the hormone. The first step in the mechanism of action of PRL is the binding to a cell surface receptor. The ligand binds in a two-step process in which site 1 on PRL binds to one receptor molecule, after which a second receptor molecule binds to site 2 on the hormone, forming a homodimer consisting of one molecule of PRL and two molecules of receptor. The PRLR contains no intrinsic tyrosine kinase cytoplasmic domain but associates with a cytoplasmic tyrosine kinase, JAK2. Dimerization of the receptor induces tyrosine phosphorylation and activation of the JAK kinase followed by phosphorylation of the receptor. Other receptor-associated kinases of the Src family have also been shown to be activated by PRL. One major pathway of signaling involves phosphorylation of cytoplasmic State proteins, which themselves dimerize and translocate to nucleus and bind to specific promoter elements on PRL-responsive genes. In addition, the Ras/Raf/MAP kinase pathway is also activated by PRL and may be involved in the proliferative effects of the hormone. Finally, a number of other potential mediators have been identified, including IRS-1, PI-3 kinase, SHP-2, PLC gamma, PKC, and intracellular Ca2+. The technique of gene targeting in mice has been used to develop the first experimental model in which the effect of the complete absence of any lactogen or PRL-mediated effects can be studied. Heterozygous (+/-) females show almost complete failure to lactate after the first, but not subsequent, pregnancies. Homozygous (-/-) females are infertile due to multiple reproductive abnormalities, including ovulation of premeiotic oocytes, reduced fertilization of oocytes, reduced preimplantation oocyte development, lack of embryo implantation, and the absence of pseudopregnancy. Twenty per cent of the homozygous males showed delayed fertility. Other phenotypes, including effects on the immune system and bone, are currently being examined. It is clear that there are multiple actions associated with PRL. It will be important to correlate known effects with local production of PRL to differentiate classic endocrine from autocrine/paracrine effects. The fact that extrapituitary PRL can, under some circumstances, compensate for pituitary PRL raises the interesting possibility that there may be effects of PRL other than those originally observed in hypophysectomized rats. The PRLR knockout mouse model should be an interesting system by which to look for effects activated only by PRL or other lactogenic hormones. On the other hand, many of the effects reported in this review may be shared with other hormones, cytokines, or growth factors and thus will be more difficult to study. (ABSTRACT TRUNCATED)

Null mutation of the prolactin receptor gene produces multiple reproductive defects in the mouse

Mice carrying a germ-line null mutation of the prolactin receptor gene have been produced by gene targeting in embryonic stem cells. Heterozygous females showed almost complete failure of lactation attributable to greatly reduced mammary gland development after their first, but not subsequent, pregnancies. Homozygous females were sterile owing to a complete failure of embryonic implantation. Moreover, they presented multiple reproductive abnormalities, including irregular cycles, reduced fertilization rates, defective preimplantation embryonic development, and lack of pseudopregnancy. Half of the homozygous males were infertile or showed reduced fertility. This work establishes the prolactin receptor as a key regulator of mammalian reproduction, and provides the first total ablation model to further study the role of the prolactin receptor and its ligands.

Effects of induced hyperprolactinemia on in vitro fertilization cycles

OBJECTIVE

To investigate the effect of induced endogenous hyperprolactinemia on the luteinization process, as expressed by the shift in the P:E2 ratio after hCG injection in IVF cycles.

PATIENTS AND INTERVENTIONS

Serum PRL, E2, and P levels were measured in 49 IVF patients (leuprolide acetate and hMG protocol) on the day of hCG injection. Estradiol and P also were measured on the day after hCG. Serum P:E2 ratios were calculated for two groups of patients; group I (control): PRL < or = 20 ng/mL (conversion factor to SI unit, 1.00); group II (hyperprolactinemia): PRL > 20 ng/mL. Estradiol and P also were measured in follicular fluid (FF) and the gamete performance was compared between groups.

RESULTS

Data analysis showed no significant differences in the mean +/- SD serum peak E2 (pg/mL; conversion factor to SI unit, 3.671) between groups: group I, 1,769 +/- 843; group II, 2,333 +/- 1,194; the mean FF E2 (pg/mL) group I, 351 +/- 221; group II, 370 +/- 186; or the mean FF P (ng/mL; conversion factor to SI unit, 3.180) group I, 8,357 +/- 3,127; group II, 11,354 +/- 12,888. No significant differences were found between groups in the P:E2 ratios on days 1 or 2: group I, 78 +/- 48 and 209 +/- 137; group II, 70 +/- 47 and 224 +/- 197, respectively. The magnitude of the P shift also showed no significant difference between the two groups; the mean +/- SD shift in the P level was 2.9 +/- 2.2 for group I, and 4.3 +/- 5.1 for group II. The serum PRL level had no effect on the fertilization rate (60% for group I and 70% for group II) or on the pregnancy rate (17% for group I and 23% for group II).

CONCLUSION

These findings suggest that mild endogenous hyperprolactinemia induced by ovarian stimulation does not affect granulosa cell luteinization and gamete performance in humans.