Effects of prolactin on steroid production by human luteal cells in vitro

Completely humanizing prolactin rescues infertility in prolactin knockout mice and leads to human prolactin expression in extrapituitary mouse tissues

A variety of fundamental differences have evolved in the physiology of the human and rodent prolactin (PRL) systems. The PRL gene in humans and other primates contains an alternative promoter, 5.8 kbp upstream of the pituitary transcription start site, which drives expression of PRL in "extrapituitary" tissues, where PRL is believed to exert local, or paracrine, actions. Several of these extrapituitary PRL tissues serve a reproductive function (eg, mammary gland, decidua, prostate, etc), consistent with the hypothesis that local PRL production may be involved in, and required for, normal reproductive physiology in primates. Rodent research models have generated significant findings regarding the role of PRL in reproduction. Specifically, disruption (knockout) of either the PRL gene or its receptor causes profound female reproductive defects at several levels (ovaries, preimplantation endometrium, mammary glands). However, the rodent PRL gene differs significantly from the human, most notably lacking the alternative promoter. Understanding of the physiological regulation and function of extrapituitary PRL has been limited by the absence of a readily accessible experimental model, because the rodent PRL gene does not contain the alternative promoter. To overcome these limitations, we have generated mice that have been "humanized" with regard to the structural gene and tissue expression of PRL. Here, we present the characterization of these animals, demonstrating that the human PRL transgene is responsive to known physiological regulators both in vitro and in vivo. More importantly, the expression of the human PRL transgene is able to rescue the reproductive defects observed in mouse PRL knockout (mPRL(-)) females, validating their usefulness in studying the function or regulation of this hormone in a manner that is relevant to human physiology.

Prolactin secretion patterns: basic mechanisms and clinical implications for reproduction

Prolactin (PRL) is one of the most versatile hormones in the mammalian body affecting reproductive, sexual, metabolic, immune, and other functions. It is therefore not surprising that the neural control of PRL secretion is complex, involving the coordinated actions of several hypothalamic nuclei. A plethora of experimental data exists on the hypothalamic control of hormone secretion under various physiological stimuli. There have been even mathematical models and computer studies published, which help to understand the complex hypothalamic–pituitary network. Nevertheless, the putative role of PRL for human reproduction still has to be clarified. Here, we review data on the underlying mechanisms controlling PRL secretion using both experimental and mathematical approaches. These investigations primarily focus on rhythmic secretion in rats during early pregnancy or pseudopregnancy, and they point to the important role of oxytocin as a crucial PRL-releasing factor. Recent data on human studies and their theoretical and clinical implications are reviewed as well. In particular, studies demonstrating a sustained PRL surge after sexual climax in males and females are presented, indicating possible implications for both sexual satiation and reproductive functions. Taking these data together, there is evidence for the hypothesis that the PRL surge induced by sexual activity, together with the altered PRL rhythmic pattern, is important for successful initialization of pregnancy not only in rodents but also possibly in humans. However, further investigations are needed to clarify such a role in humans.

Luteolysis: a neuroendocrine-mediated event

In many nonprimate mammalian species, cyclical regression of the corpus luteum (luteolysis) is caused by the episodic pulsatile secretion of uterine PGF2alpha, which acts either locally on the corpus luteum by a countercurrent mechanism or, in some species, via the systemic circulation. Hysterectomy in these nonprimate species causes maintenance of the corpora lutea, whereas in primates, removal of the uterus does not influence the cyclical regression of the corpus luteum. In several nonprimate species, the episodic pattern of uterine PGF2alpha secretion appears to be controlled indirectly by the ovarian steroid hormones estradiol-17beta and progesterone. It is proposed that, toward the end of the luteal phase, loss of progesterone action occurs both centrally in the hypothalamus and in the uterus due to the catalytic reduction (downregulation) of progesterone receptors by progesterone. Loss of progesterone action may permit the return of estrogen action, both centrally in the hypothalamus and peripherally in the uterus. Return of central estrogen action appears to cause the hypothalamic oxytocin pulse generator to alter its frequency and produce a series of intermittent episodes of oxytocin secretion. In the uterus, returning estrogen action concomitantly upregulates endometrial oxytocin receptors. The interaction of neurohypophysial oxytocin with oxytocin receptors in the endometrium evokes the secretion of luteolytic pulses of uterine PGF2alpha. Thus the uterus can be regarded as a transducer that converts intermittent neural signals from the hypothalamus, in the form of episodic oxytocin secretion, into luteolytic pulses of uterine PGF2alpha. In ruminants, portions of a finite store of luteal oxytocin are released synchronously by uterine PGF2alpha pulses. Luteal oxytocin in ruminants may thus serve to amplify neural oxytocin signals that are transduced by the uterus into pulses of PGF2alpha. Whether such amplification of episodic PGF2alpha pulses by luteal oxytocin is a necessary requirement for luteolysis in ruminants remains to be determined. Recently, oxytocin has been reported to be produced by the endometrium and myometrium of the sow, mare, and rat. It is possible that uterine production of oxytocin may act as a supplemental source of oxytocin during luteolysis in these species. In primates, oxytocin and its receptor and PGF2alpha and its receptor have been identified in the corpus luteum and/or ovary. Therefore, it is possible that oxytocin signals of ovarian and/or neural origin may be transduced locally at the ovarian level, thus explaining why luteolysis and ovarian cyclicity can proceed in the absence of the uterus in primates. However, it remains to be established whether the intraovarian process of luteolysis is mediated by arachidonic acid and/or its metabolite PGF2alpha and whether the central oxytocin pulse generator identified in nonprimate species plays a mediatory role during luteolysis in primates. Regardless of the mechanism, intraovarian luteolysis in primates (progesterone withdrawal) appears to be the primary stimulus for the subsequent production of endometrial prostaglandins associated with menstruation. In contrast, luteolysis in nonprimate species appears to depend on the prior production of endometrial prostaglandins. In primates, uterine prostaglandin production may reflect a vestigial mechanism that has been retained during evolution from an earlier dependence on uterine prostaglandin production for luteolysis.

Prolactin inhibits the steroidogenesis in midfollicular phase human granulosa cells cultured in a chemically defined medium

In vitro studies were conducted on prolactin (PRL) effects on human granulosa cell steroidogenesis. Cells derived from healthy midfollicular phase follicles were cultured in a chemically defined medium supplemented with androstenedione (delta 4 A) 10(-7) M. Cultures treated with follicle-stimulating hormone (FSH) showed a dose-dependent increase of estradiol (E2) and progesterone (P) secretion. The authors demonstrated that PRL (greater than or equal to 10 ng/ml) inhibits basal as well as FSH (10 ng/ml)-stimulated E2 and P secretion. This PRL effect was overcome only by FSH maximal stimulating doses (100 ng/ml). These results suggest a direct inhibitory effect of PRL on granulosa cell steroidogenesis acting as a negative modulator of FSH action. These effects might be related to the ovarian dysfunction observed in hyperprolactinemia.

Human corpus luteum: immunocytochemical evidence for presence of prolactin

Six human corpora lutea (day 17-25) of the menstrual cycle and 4 ovarian stromal tissues from 7 cycling women were examined for the presence of the hormone, prolactin, by immunohistochemistry using the indirect peroxidase-antiperoxidase method. After mounting tissue sections of 4 micron, endogenous peroxidases were removed with hydrogen peroxide and the sections were incubated for 1 h at room temperature followed by 16 h at 4 degrees C with a highly specific antisera for human prolactin, nonimmunized normal rabbit serum for a control reaction, or antiserum preadsorbed with excess human prolactin for specificity determination. Following the reaction with the second antibody (goat antirabbit IgG) for 1 h at room temperature, prolactin was localized using peroxidase anti-peroxidase and 3.3'-diaminobenzidine as the chromogen. Prolactin was present and could be localized in the luteal cells of all 6 corpora lutea, but not in any of the ovarian stroma studied. Human adenohypophysis served as a positive tissue control for prolactin immunopositive staining. The localization of immunoreactive prolactin in the corpus luteum demonstrates directly the presence of this hormone in the human ovary, adding further evidence for its role in luteal function.

Serum gonadotrophin and sex steroid hormone levels during mid-follicular and mid-luteal phases in hyperprolactinaemic women with regular menstrual cycles

Hyperprolactinaemia was found in 15 of 135 infertile patients with regular menstrual cycles, biphasic basal body temperature record and no galactorrhoea. In those 15 women, mean serum prolactin levels during the mid-follicular and mid-luteal phases of the menstrual cycle were 29.8 (SEM 1.8) ng/ml and 29.5 (SEM 1.3) ng/ml, respectively. Although serum FSH and LH levels were similar in normal and hyperprolactinaemic women, serum oestradiol level during the mid-follicular phase was subnormal in hyperprolactinaemic women (P less than 0.05). In contrast, serum oestradiol and progesterone levels during the mid-luteal phase and luteal phase length were similar in normoprolactinaemic and hyperprolactinaemic groups. The results suggest that hyperprolactinaemia is associated with defects of follicle development as measured by oestradiol production during the mid-follicular phase, but not with corpus luteum function as measured by progesterone production during the mid-luteal phase, and luteal phase length.

Metoclopramide-induced hyperprolactinaemia: effects on corpus luteum function, endometrial steroid receptor concentrations and 17 beta-hydroxysteroid dehydrogenase activity

Induced hyperprolactinaemia impairs ovarian follicular development, especially during the recruitment period. The consequences of hyperprolactinaemia during the luteal phase alone on corpus luteum function have not been characterized, nor have the actions of excessive circulating PRL on the endometrium. In this study, postovulatory 5-d administration of metoclopramide (MC) increased serum concentrations of PRL and decreased those of pregnenolone and progesterone indicating inhibition of steroidogenesis in the corpus luteum. This effect may partly explain the relatively common failure of implantation in association with induced ovulation using regimens leading to transient hyperprolactinaemia. In contrast to this, MC-induced hyperprolactinaemia during the mid-follicular (4 d) or early luteal phase of the cycle did not alter the concentrations of cytosol or nuclear oestrogen and progestin receptors or the activity of 17 beta-hydroxysteroid dehydrogenase in endometrial tissue. Thus, transiently elevated circulating PRL does not seem to have direct effects on female sex steroid receptors or their function in the proliferative or secretory endometrium.

Effects of prolactin on steroidogenesis by human luteinized granulosa cells

The role of prolactin (PRL) in the regulation of human ovarian function is poorly understood, and reported findings are often contradictory. We have evaluated the effects of several doses of PRL on the secretion of progesterone (P) and estradiol (E2) by human granulosa cells from preovulatory follicles of gonadotropin-stimulated women. The effects of PRL were also evaluated when androstenedione (delta 4 A) was added to the cultured cells. Our findings show that the human granulosa lutein cells retain their ability to secrete large amounts of P and lesser quantities of E2 for a prolonged period in culture. E2, but not P, secretion was stimulated by the presence of delta 4 A. PRL in low doses (25 and 50 ng/ml) did not alter the secretion of either E2 or P. In contrast, high doses of PRL (100 and 1000 ng/ml) resulted in a significant suppression of E2 secretion. P secretion was inhibited only by the highest dose (1000 ng/ml) of PRL. We conclude that steroidogenesis by the granulosa lutein cells of preovulatory human follicles is probably not influenced by the quantities of PRL that are normally present in the blood or follicular fluid. Higher doses, however, may suppress production of E2 and P.