Mammotroph autoregulation: intracellular fate of internalized prolactin

Prolactin and pain of endometriosis

Women experience chronic pain more often than men with some pain conditions being specific to women while others are more prevalent in women. Prolactin, a neuropeptide hormone with higher serum levels in women, has recently been demonstrated in preclinical studies to sensitize nociceptive sensory neurons in a sexually dimorphic manner. Dysregulation of prolactin and prolactin receptors may be responsible for increased pain especially in female predominant conditions such as migraine, fibromyalgia, and pelvic pain. In this review, we focus on the role of prolactin in endometriosis, a condition characterized by pelvic pain and infertility that affects a large proportion of women during their reproductive age. We discuss the symptoms and pathology of endometriosis and discuss how different sources of prolactin secretion may contribute to this disease. We highlight our current understanding of prolactin-mediated mechanisms of nociceptor sensitization in females and how this mechanism may apply to endometriosis. Lastly, we report the results of a systematic review of clinical studies conducted by searching the PubMed and EMBASE databases to identify association between endometriosis and blood levels of prolactin. The results of this search strongly indicate that serum prolactin levels are increased in patients with endometriosis and support the possibility that high levels of prolactin may promote pelvic pain in these patients and increase vulnerability to other comorbid pain conditions likely by dysregulating prolactin receptor expression. Targeting of prolactin and prolactin receptors may improve management of pain associated with endometriosis.

Traffic of endogenous, transduced, and endocytosed prolactin in rabbit lacrimal acinar cells

The rabbit lacrimal gland undergoes an immunophysiological transformation during pregnancy, reminiscent of that of the mammary gland as it prepares to deliver secretory IgA into the nascent fluid product. The contents of TGF-beta and prolactin (PRL) within ductal epithelial cells increase, and their primary localizations shift from the apical to the basal cytoplasm, suggesting a transformation from exocrine to paracrine secretion. Studies with ex vivo acinar cell models demonstrated that elevated PRL suppresses traffic of secretory proteins into the regulated exocrine apparatus and directs them into a novel, induced, regulated paracrine apparatus [Wang, Y., Chiu, C.T., Nakamura, T., Walker, A.M., Petridou, B., Trousdale M.D., Hamm-Alvarez S.F., Schechter J.E., Mircheff A.K., 2007. Elevated prolactin redirects secretory vesicle traffic in rabbit lacrimal acinar cells. Am. J. Physiol. Endocrinol. Metab. 292, E1122-E1134]. However, it was not clear whether PRL itself entered the induced paracrine apparatus. In the present study, confocal immunofluorescence microscopy revealed that natively expressed PRL and over-expressed PRL co-localized with PRL receptors (PRLR); rab11, a marker for the recycling endosome; gamma-adaptin, a marker for the Golgi complex and trans-Golgi network; and rab7, a marker for the autophagic lysosomal apparatus. Natively expressed, over-expressed, and endocytosed PRL also co-localized with rab4 and rab5A, markers for the early endosome, and with rab3D, a marker for regulated exocrine secretory vesicles. Endocytosed PRL was stored in intact form and released in response to stimulation with carbachol. Subcellular fractionation analysis detected relative excesses of PRL over PRLR in fractions that contained fragments of the recycling endosome and fractions that contained both secretory vesicle fragments and prelysosomal and autolysosomal fragments. EM-gold microscopy demonstrated PRL within small vesicles, consistent with endosomes or secondary lysosomes, and in large vesicles, consistent with regulated secretory vesicles. The secretory vesicles were preponderantly localized in the apical cytoplasm of control cells, and in the basal cytoplasm of PRL over-expressing cells. These results indicate that when lacrimal epithelial cells synthesize PRL, and when they endocytose it from their ambient medium, they traffic it both into the endosomes that constitute the constitutive transcytotic paracrine apparatus and also into regulated secretory vesicles, which are associated with the exocrine apparatus at low PRL levels and with the induced paracrine apparatus at high PRL levels.

Development of a prolactin receptor-targeting fusion toxin using a prolactin antagonist and a recombinant form of Pseudomonas exotoxin A

Human prolactin (hPRL) promotes the proliferation and differentiation of mammary epithelial cells during mammary gland development and has been linked to breast tumor development. The receptor for hPRL (hPRL-R) is elevated in a majority of human breast tumors, suggesting the overexpression of hPRL-R makes cancer cells highly sensitive to the mitogenic and anti-apoptotic activity of hPRL. These findings provide the rationale for the development of hPRL-R targeting breast cancer therapeutics. Previously, an hPRL-R antagonist, G129R, was developed that competitively binds to the hPRL-R resulting in growth inhibition and the induction of apoptosis in certain types of breast cancer cells. To further increase the potency of G129R, we fused G129R to a truncated form of Pseudomonas exotoxin A (PE(40)) that lacks the cell recognition domain of the toxin but retains the domains necessary for PE(40)_ to translocate into the cytosol and inhibit protein synthesis. We postulated that the fusion of G129R with PE(40)-KDEL would (1) deliver the recombinant toxin to breast cancer cells where hPRL-R is overexpressed; (2) block hPRL signaling via its G129R moiety; and (3) inhibit protein synthesis via its PE(40)-KDEL moiety. We demonstrate that the fusion toxin can competitively bind to hPRL-Rs on T-47D human breast cancer cells and inhibit STAT5 phosphorylation induced by hPRL. In addition, we show that G129R-PE(40)-KDEL is selectively cytotoxic to breast cancer cell lines expressing the hPRL-R and that cell death is associated with the inhibition of protein synthesis and does not involve caspase mediated apoptosis.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Autoregulation of the rat prolactin gene in lactotrophs

The autoregulation of prolactin (PRL) secretion in the rat has been demonstrated at both the hypothalamus and the pituitary levels. Studies on the direct negative feedback effect of PRL in the lactotrophs have concentrated on the acute effect on PRL secretion which does not involve change in PRL synthesis. In this study, we have developed a cotransfection assay in somatolactotrophs where we examine the effect of PRL on the transcription of its own gene. We found that oPRL, at physiological concentrations, exerts a strong and specific inhibition of the rPRL gene transcription in PRL-deficient GC cells. This effect is mediated by both the intermediate and the long forms of PRL receptor. The inhibition was also reproduced in GH3 cells, which secretes PRL, by adding exogenous oPRL in the presence of anti-rat PRL antiserum to neutralize endogenous rPRL. Cellular specificity was demonstrated by testing this regulation in non-pituitary cell types where no modulation of the PRL promoter reporter gene could be elicited by PRL, even with cotransfection with the Pit-1 expression vector. Finally, deletions of the rPRL promoter indicate that the full inhibitory effect of PRL requires the same regulatory domains (proximal and distal) that have been described for the other PRL gene regulators. These results strongly suggest the existence of the extra-short loop regulation of the rat PRL at the transcriptional level.

Cytoplasmic and nuclear cytokine receptor complexes

Much of our understanding on how hormones and cytokines transmit their message into the cell is based on the receptor activation at the plasma membrane. Many experimental in vitro models have established the paradigm for cytokine action based upon such activation of their cell surface receptor. The signaling from the plasma membrane activated cytokine receptor is driven to the nucleus by a rapid ricochet of protein phosphorylation, ultimately integrated as a differentiative, proliferative, or transcriptional message. The Janus kinase (JAK)--signal transducers and activators of transcription (STAT) pathway that was first thought to be cytokine receptor specific now appears to be activated by other noncytokine receptors. Also, evidence is accumulating showing that cytokines modulate the signal transduction machinery of the tyrosine kinase receptors and that of the heterotrimeric guanosine triphosphate (GTP)-binding protein-coupled receptors. Thus cytokine receptor signaling has become much more complex than originally hypothesized, challenging the established model of specificity of the action of a given cytokine. This review is focused on another level of complexity emerging within cytokine receptor superfamily signaling. Over the past 10 years, data from different laboratories have shown that cytokines and their receptors localize to intracellular compartments including the nucleus, and, in some cases, biological responses have been correlated with this unexpected location, raising the possibility that cytokines act as their own messenger through inter-actions with nuclear proteins. Thus, the interplay between cytokine receptor engagement and cellular signaling turns out to be more dynamic than originally suspected. The mechanisms and regulations of intracellular translocation of the cytokines, their receptors, and their signaling proteins are discussed in the context that such compartmentalization provides some of the specificity of the responses mediated by each cytokine.

Prolactin as an autocrine/paracrine factor in breast tissue

The neuroendocrine hormone prolactin (PRL) stimulates breast growth and differentiation during puberty, pregnancy, and lactation. Despite extensive and convincing data indicating that PRL significantly contributes to the pathogenesis and progression of rodent mammary carcinoma, parallel observations for human breast cancer have not been concordant. In particular, the therapeutic alteration of somatolactogenic hormone levels has not consistently altered the course of human breast cancer. Recent data, however, suggest that extra-pituitary tissues are capable of elaborating PRL; indeed, the observation of sustained serum levels of PRL in post-hypophysectomy patients supports this hypothesis. Proof of an autocrine/paracrine loop for PRL within normal and malignant human breast tissues requires that the following three criteria be met: (1) PRL must be synthesized and secreted within mammary tissues; (2) the receptor for PRL (PRLR) must be present within these tissues; and, (3) proliferative responses to autocrine/paracrine PRL must be demonstrated. These criteria have now been fulfilled in several laboratories. With the demonstration of a PRL autocrine/paracrine loop in mammary glands, the basis for the ineffective treatment of human breast cancer by prior endocrine-based anti-somatolactogenic therapies is evident. These findings provide the precedent for novel therapeutic strategies aimed at interrupting the stimulation of breast cancer growth by PRL at both endocrine and autocrine/paracrine levels.

Role of prolactin in the in vitro development of interleukin-2-driven anti-tumoural lymphokine-activated killer cells

Exogenous prolactin (PRL) has been shown to synergize with low-dose interleukin-2 (IL-2) and induce the proliferation and lymphokine-activated killer (LAK) maturation of natural killer (NK) cells. PRL itself can also generate LAK activity. Here we show that its local production occurs during, and is necessary for, LAK development. IL-2-stimulated peripheral blood mononuclear cells (PBMC) and purified NK cells were exposed to anti-human (h)PRL antiserum, and residual LAK activity was measured on day 7 against the promyelocytic leukaemia cell line HL-60. Inhibition of LAK activity was much more evident in PBMC compared with NK cell cultures (47% decrease. P - 0.013 and 18.5% decrease. P = 0.048, respectively). Up-modulation of a 32S-methionine-labelled 27,000 MW protein was detected in the lysates and supernatants of IL-2-stimulated PBMC immunoprecipitated with an anti-PRL antiserum. By contrast, the cytoplasmic PRL immunoreactivity observed in freshly isolated NK cells and in IL-2-stimulated, but not unstimulated, NK cell cultures was not associated with PRL gene activation, and can thus be referred to internalized PRL. Preferential re-uptake of externally derived PRL by IL-2-stimulated NK cells was also indicated by up-modulation of the PRL receptor. These data, as a whole, indicate that the PRL promotion of LAK differentiation is mainly mediated by paracrine secretion, with a minor contribution from internalized PRL.

Nuclear translocation of prolactin: collaboration of tyrosine kinase and protein kinase C activation in rat Nb2 node lymphoma cells

Recent evidence has suggested that prolactin (PRL), internalized by lactogen-dependent Nb2 lymphoma cells, is actively translocated to the nucleus where it binds to PRL receptors. Moreover, the mitogenic action of PRL in these cells has been separately linked to protein tyrosyl phosphorylation and activation of protein kinase C (PKC). Therefore, the coupling of PRL internalization and nuclear translocation to the activation of these signal transduction pathways was investigated. Results from control experiments indicated that 30% of internalized and 5% total cell-associated 125I-rat PRL could be recovered within nuclei obtained from Nb2 cells previously incubated with the radiolabel for 3 h at 37 degrees C. Furthermore, internalized PRL was found to be intact and not associated with any carrier proteins. Addition of tyrosine kinase (TK) antagonists, genistein or tyrphostin, significantly reduced cell surface binding, internalization, and nuclear translocation of 125I-rat PRL. In contrast, neither the level of cell-associated nor internalized hormone differed between cells treated with the PKC antagonists, staurosporine or calphostin C, and control cultures. Instead, PKC inhibition significantly reduced nuclear PRL translocation. The inhibitory effects of the TK and PKC antagonists on PRL internalization and nuclear translocation in intact Nb2 cells were verified by immunofluorescence microscopy in parallel experiments. In other experiments, each of the kinase inhibitors blocked PRL-stimulated Nb2 cell proliferation in a concentration-dependent manner. It is concluded that activated TK and PKC collaborate in the process of PRL internalization and translocation to the nucleus. TK activation may participate in PRL receptor binding or hormone internalization while activation of PKC appears to be required for its nuclear targeting. Since TK and PKC activation are required for lactogen-stimulated Nb2 cell proliferation, we suggest that a component of the mitogenic pathway in these cells is a direct nuclear interaction of PRL.

Molecular and structural analysis of nuclear localizing anti-DNA lupus antibodies

To determine the structure of three nuclear localizing lupus anti-DNA immunoglobulins (Igs) and to search for clues to mechanisms of cellular and/or nuclear access, their H- and L-chain variable region sequences were determined and subjected to three-dimensional modeling. Although the results indicate heterogeneity in their primary structures, the H chains are encoded by 3 members of the J558 VH gene family with a common tertiary conformation that is not shared by a J558-encoded nonnuclear localizing anti-DNA control Ig. Furthermore, at least two of the Igs share a conformational motif in the H-chain CDR3, and all three Igs contain multiple positively charged amino acids in their CDRs, resembling nuclear localization signals that direct protein nuclear import. Notably, each VH and VK gene is also found recurrently among previously described autoantibodies. Molecular analysis further indicates that both germline-encoded and significantly mutated V genes can generate nuclear localizing anti-DNA Ig.

Internalization and nuclear localization of peptide hormones

The presence of neuropeptide receptors on the plasma membrane is well accepted, as is its internalization and down-regulation. The analysis of the fate of these peptides within their target-cells is difficult. Endogenous peptides or administered native peptides are visualized in these cells using immunocytology after cryoultramicrotomy. Labelled peptides can be injected and their internalization kinetics studied using ultrastructural autoradiography. The pituitary gland is a suitable model for the study of the neuropeptide mechanism, with the lactotroph function being taken as an example in the present case. Prolactin (PRL) release depends on two main neuropeptides: thyrotropin-releasing hormone (TRH) and somatostatin (SS). The TRH immunoreactivity obtained from endogenous as well as injected material was restricted to the plasma membrane, secretory granules, cytoplasmic matrix and nucleus. The internalization kinetics of exogenous native TRH showed an increase of immunoreactive material in all compartments including the nucleus. The endogenous SSs (SS14 and SS28) were detected in the same subcellular lactotroph compartments. Injection of 125I-SS showed a rapid binding of SS at the plasma membrane level before internalization. For 60 min of in vivo uptake, 125I-SS28, the large SS molecule, was detected in the cytoplasm only, while 125I-SS14 was found in the nuclear matrix. In vitro 125I-SS28 was restricted to the nuclear membrane. Under physiological conditions the endogenous neuropeptides were visualized in the nucleus, but after injection of labelled peptides only small molecules were found in the nucleus. The significance of the presence of these neuropeptides is discussed.

Identification of prolactin receptors in hepatic nuclei

Prolactin is a trophic hormone which may act directly at the hepatocyte nucleus. In this study, specific prolactin binding sites were sought in purified rat liver nuclei. Saturable and specific, high affinity 125I-prolactin binding sites were demonstrated to be on or within the nucleus. Prolactin binding was competitively inhibited by rat and ovine prolactins but not by rat growth hormone. Using immunogold electron microscopy, we detected prolactin receptors throughout the nucleus, in association with heterochromatin. Furthermore, endogenous immunoreactive prolactin was demonstrated to be within hepatic nuclei. We conclude that rat liver nuclei possess prolactin binding sites which likely participate in hormone-directed growth processes.

Regulation of interleukin 2-driven T-lymphocyte proliferation by prolactin

The requirement for prolactin in interleukin 2-driven T-cell proliferation was evaluated. Addition of an anti-prolactin antiserum resulted in the specific inhibition of T-cell proliferation in a time- and dose-dependent manner. Synthesis of prolactin and its mRNA, however, did not occur during interleukin 2 stimulation. Instead, previously internalized prolactin, presumably from fetal bovine serum, appears to serve as the source of prolactin under serum-free conditions. A 7-fold increase in a prolactin receptor occurred as a function of cell cycle progression; accumulation of a 1.6-kilobase prolactin receptor mRNA increased approximately 2-fold. Interleukin 2 stimulation induced the translocation of prolactin into the nucleus and prolactin receptor to the nuclear periphery. These data indicate that extracellular prolactin is requisite for T-cell proliferation and suggest that the effects of prolactin are exerted in the nucleus.

Medium flow rate modulates autocrine-paracrine feedback of GH and PRL release by perifused GH3 cells

We previously documented both the spontaneous acceleration of growth hormone (GH) and prolactin (PRL) production by GH3 cells during perifusion and the suppression of their production during plate culture. We here present the role played by medium flow itself in this differential behavior. Increasing rates of perifusion flow (pump rates of 1 to 5 ml/h, equivalent to chamber flow rates of 0.19 to 1.3 microliters.min-1.mm-2 of cross-sectional area) were associated with enhanced GH and PRL secretion. Flow rate-dependent basal hormone secretion rates were established quickly and were stable for the first 10 to 14 h of perifusion. The previously documented independent, spontaneous, and continuously accelerating production of both hormones that followed during the subsequent 40 (PRL) to 60 (GH) h of perifusion was also shown to be flow-rate related. Any time the rate of medium flow was changed within an experiment, the rate of hormone secretion was modulated. However, that modulation did not interrupt ongoing flow-associated acceleration of hormone production once the latter had begun. In addition, GH3 cell product(s) from one cell column reversibly inhibited secretion from cells in a downstream column. The inhibition did not occur when cells in the downstream column had been exposed to trypsin. Other work had suggested that neither GH, PRL, insulinlike growth factor-I, leucine, nor nutrient exhaustion were responsible for the effect. These data are consistent with autocrine-paracrine feedback regulation of GH3 cells by a secretory product(s). Feedback would thus provide a mechanism to effect flow-rate-dependent modulation of GH and PRL release, and to explain accelerating hormone production during perifusion.

Nyctohemeral rhythms of gonadal, thyroid and pineal function in the hyperprolactinemic male rat

In young adult male rats bearing a donor anterior pituitary gland grafted for 3 weeks under a kidney capsule, serum prolactin (PRL) concentrations were elevated and exhibited a rhythm with the highest values in the light phase. Serum PRL in control animals did not exhibit a significant rhythm. Eutopic pituitary PRL content, manifesting a biphasic (12-hr) rhythm with crests during the day and night in controls, exhibited a similar pattern in grafted rats though an overall reduction in pituitary PRL content was seen in the grafted animals. Neither the normal biphasic serum testosterone rhythm nor the normal 24-hr rhythm (nocturnal surge) of pineal N-acetyltransferase activity and melatonin content were altered in the hyperprolactinemic rats. Serum thyroxine (T4) and triiodothyronine (T3) and their free indices (FT4I, FT3I) and serum thyrotropin (TSH) were highest during the day in controls and grafted rats and a 12-hr rhythmic component was detected in data for these variables. In the grafted animals, the 12-hr component was reflected in an additional peak at night detectable by testing of means. The overall serum T4, FT4I, and TSH levels were lower in grafted rats though overall T3 and FT3I levels did not differ between grafted and controls. T3 uptake (T3U) values were similar between controls and grafted rats, in both cases exhibiting a fall during the night. Changes in serum thyronines could not be explained by changes in serum binding as assessed by the T3U, and thus may represent changes in thyroidal secretion of T4.(ABSTRACT TRUNCATED AT 250 WORDS)

Translocation of epidermal growth factor to the hepatocyte nucleus during rat liver regeneration

To determine the fate of exogenous epidermal growth factor (EGF) in regenerating liver, 125I-labeled EGF was injected into rat portal veins at various times after 70% hepatectomy. Epidermal growth factor was taken up by the liver remnant at all time points studied (0, 4, 8, 16, and 36 h), but at 8 h after hepatectomy a large quantity was retained by the liver and EGF degradation products appearing in the bile decreased markedly. Electron microscopic autoradiography of the regenerating livers 1 h after injection of 125I-EGF demonstrated that 27% of the grains were associated with hepatocyte nuclei compared to 0.5% in shamoperated controls. There was also a concomitant decrease in grains associated with the lysosomal compartment. Nuclei isolated from regenerating livers exposed to 125I-EGF also demonstrated a three-fold increase in radioactivity compared to nuclei from control livers. Nearly 70% of nuclear radioactivity was precipitable with a specific antibody to EGF, and a small fraction appeared to be part of a high molecular weight complex. These data support the hypothesis that during the pre-S phase of liver regeneration, EGF is translocated to the nucleus rather than to lysosomes, and may participate in the initiation of deoxyribonucleic acid synthesis or alteration of gene expression.