Prolactin activates the interferon-regulated p91 transcription factor and the Jak2 kinase by tyrosine phosphorylation

The effects of prolactin on the immune system, its relationship with the severity of COVID-19, and its potential immunomodulatory therapeutic effect

Prolactin (PRL) is an endocrine hormone secreted by the anterior pituitary gland that has a variety of physiological effects, including milk production, immune system regulation, and anti-inflammatory effects. Elevated levels of PRL have been found in several viral infections, including 2019 coronavirus disease (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV2), a viral pathogen that has recently spread worldwide. PRL production is increased in SARS-CoV2 infection. While PRL can trigger the production of proinflammatory cytokines, it also has several anti-inflammatory effects that can reduce hyperinflammation. The exact mechanism of PRL's contribution to the severity of COVID-19 is unknown. The purpose of this review is to discuss the interaction between PRL and SARS-CoV2 infection and its possible association with the severity of COVID-19.

JAK/STAT pathway and molecular mechanism in bone remodeling

JAK/STAT signaling pathway is involved in many diseases, including autoimmune diseases, which are characterized by a close interconnection between immune and bone system. JAK/STAT pathway is involved in bone homeostasis and plays an important role in proliferation and differentiation of some cell types, including osteoblasts and osteoclasts. Different molecules, such as cytokines, hormones, and growth factors are responsible for the activation of the JAK/STAT pathway, which leads, at the nuclear level, to start DNA transcription of target genes. Bone cells and remodeling process are often influenced by many cytokines, which act as strong stimulators of bone formation and resorption. Our aim, through careful research in literature, has been to provide an overview of the role of the JAK/STAT pathway in bone remodeling and on bone cells, with a focus on cytokines involved in bone turnover through this signal cascade. The JAK/STAT pathway, through the signal cascade activation mediated by the interaction with many cytokines, acts on bone cells and appears to be involved in bone remodeling process. However, many other studies are needed to completely understand the molecular mechanism underlying these bone process.

Lactogenic hormones regulate mammary protein synthesis in bovine mammary epithelial cells via the mTOR and JAK–STAT signal pathways

The expression of CSN3, hormone receptor, the expression of genes regulating the mTOR, JAK–STAT signal pathways, and the relative content of к-casein as well as total casein were determined in the present study to explore the mechanism of the effect of lactogenic hormones on milk-protein synthesis in bovine mammary epithelial cells. The results showed that apoptosis of the cells was increased by inhibitor LY294002, while the expressions of genes encoding PKB, Rheb, PRAS40 and S6K1 in the mTOR signal pathway, JAK2, STAT5A in the JAK–STAT signal pathway, and genes encoding INSR, PRLR, NR3C1 and CSN3 were all downregulated, and the relative contents of κ-casein and total casein were decreased in the mammary epithelial cells compared with those in the control group. Comparatively, the inhibitory effects of AG-490 were more profound than those of LY294002, and the double block using both inhibitors had a greater effect than the single block. The CSN3 gene expression was downregulated and the content of milk casein was decreased by the inhibitors. In addition, the expression of the hormone receptor genes was downregulated. Our results suggest that lactogenic hormones, via their receptors in the membrane, regulated the JAK–STAT and m-TOR signal pathways, and affected cell proliferation and apoptosis, leading to changes in milk-protein synthesis.

Abnormal Mammary Development in 129:STAT1-Null Mice is Stroma-Dependent

Female 129:Stat1-null mice (129S6/SvEvTac-Stat1^tm1Rds homozygous) uniquely develop estrogen-receptor (ER)-positive mammary tumors. Herein we report that the mammary glands (MG) of these mice have altered growth and development with abnormal terminal end buds alongside defective branching morphogenesis and ductal elongation. We also find that the 129:Stat1-null mammary fat pad (MFP) fails to sustain the growth of 129S6/SvEv wild-type and Stat1-null epithelium. These abnormalities are partially reversed by elevated serum progesterone and prolactin whereas transplantation of wild-type bone marrow into 129:Stat1-null mice does not reverse the MG developmental defects. Medium conditioned by 129:Stat1-null epithelium-cleared MFP does not stimulate epithelial proliferation, whereas it is stimulated by medium conditioned by epithelium-cleared MFP from either wild-type or 129:Stat1-null females having elevated progesterone and prolactin. Microarrays and multiplexed cytokine assays reveal that the MG of 129:Stat1-null mice has lower levels of growth factors that have been implicated in normal MG growth and development. Transplanted 129:Stat1-null tumors and their isolated cells also grow slower in 129:Stat1-null MG compared to wild-type recipient MG. These studies demonstrate that growth of normal and neoplastic 129:Stat1-null epithelium is dependent on the hormonal milieu and on factors from the mammary stroma such as cytokines. While the individual or combined effects of these factors remains to be resolved, our data supports the role of STAT1 in maintaining a tumor-suppressive MG microenvironment.

The role of the JAK-STAT pathway in neural stem cells, neural progenitor cells and reactive astrocytes after spinal cord injury

Patients with spinal cord injuries can develop severe neurological damage and dysfunction, which is not only induced by primary but also by secondary injuries. As an evolutionarily conserved pathway of eukaryotes, the JAK-STAT pathway is associated with cell growth, survival, development and differentiation; activation of the JAK-STAT pathway has been previously reported in central nervous system injury. The JAK-STAT pathway is directly associated with neurogenesis and glia scar formation in the injury region. Following injury of the axon, the overexpression and activation of STAT3 is exhibited specifically in protecting neurons. To investigate the role of the JAK-STAT pathway in neuroprotection, we summarized the effect of JAK-STAT pathway in the following three sections: Firstly, the modulation of JAK-STAT pathway in proliferation and differentiation of neural stem cells and neural progenitor cells is discussed; secondly, the time-dependent effect of JAK-STAT pathway in reactive astrocytes to reveal their capability of neuroprotection is revealed and lastly, we focus on how the astrocyte-secretory polypeptides (astrocyte-derived cytokines and trophic factors) accomplish neuroprotection via the JAK-STAT pathway.

Dysregulated STAT1-SOCS1 control of JAK2 promotes mammary luminal progenitor cell survival and drives ERα(+) tumorigenesis

We previously reported that STAT1 expression is frequently abrogated in human estrogen receptor-α-positive (ERα(+)) breast cancers and mice lacking STAT1 spontaneously develop ERα(+) mammary tumors. However, the precise mechanism by which STAT1 suppresses mammary gland tumorigenesis has not been fully elucidated. Here we show that STAT1-deficient mammary epithelial cells (MECs) display persistent prolactin receptor (PrlR) signaling, resulting in activation of JAK2, STAT3 and STAT5A/5B, expansion of CD61(+) luminal progenitor cells and development of ERα(+) mammary tumors. A failure to upregulate SOCS1, a STAT1-induced inhibitor of JAK2, leads to unopposed oncogenic PrlR signaling in STAT1(-/-) MECs. Prophylactic use of a pharmacological JAK2 inhibitor restrains the proportion of luminal progenitors and prevents disease induction. Systemic inhibition of activated JAK2 induces tumor cell death and produces therapeutic regression of pre-existing endocrine-sensitive and refractory mammary tumors. Thus, STAT1 suppresses tumor formation in mammary glands by preventing the natural developmental function of a growth factor signaling pathway from becoming pro-oncogenic. In addition, targeted inhibition of JAK2 may have significant therapeutic potential in controlling ERα(+) breast cancer in humans.

Autocrine prolactin: an emerging market for homegrown (prolactin) despite the imports

Prolactin (PRL) is a peptide hormone that is produced by the pituitary gland and is known to regulate lactogenic differentiation. There is a significant body of evidence that points to autocrine production of prolactin and activation of an autocrine/paracrine signaling pathway to regulate cell proliferation and migration and inhibition of cell death. This perspective highlights the recent study in the October 1, 2012, issue of Genes & Development by Chen and colleagues (pp. 2154-2168) that describes a mechanism for autocrine prolactin production and places the finding in the context of a role for prolactin in breast development and cancer.

Prolactin regulates ZNT2 expression through the JAK2/STAT5 signaling pathway in mammary cells

The zinc transporter ZnT2 (SLC30A2) plays an important role in zinc secretion into milk during lactation. The physiological process of mammary gland secretion is regulated through complex integration of multiple lactogenic hormones. Prolactin plays a primary role in this regulation through the activation of various signaling cascades including Jak2/STAT5, mitogen-activated protein kinase (MAPK), p38, and phosphatidylinositol 3-kinase (PI3K). The precise mechanisms that regulate the transfer of specific nutrients such as zinc into milk are not well understood. Herein we report that prolactin increased ZnT2 abundance transcriptionally in cultured mammary epithelial (HC11) cells. To delineate the responsible mechanisms, we first determined that prolactin-mediated ZnT2 induction was inhibited by pretreatment with the Jak2 inhibitor AG490 but not by the MAPK inhibitor PD-98059. Using a luciferase reporter assay, we demonstrated that ZnT2 promoter activity was increased by prolactin treatment, which was subsequently abolished by expression of a dominant-negative STAT5 construct, implicating the Jak2/STAT5 signaling pathway in the transcriptional regulation of ZnT2. Two putative consensus STAT5 binding sequences in the ZnT2 promoter were identified (GAS1:-674 to -665 and GAS2:-377 to -368). Mutagenesis of the proximal GAS2 element resulted in complete abrogation of PRL-induced ZnT2 promoter activity. The promoter incorporating the distal GAS1 mutation was only able to respond to very high PRL concentrations. Results from both the mutagenesis and gel shift assays indicated that a cooperative relationship exists between GAS1 and GAS2 for PRL-induced activation; however, the proximal GAS2 plays a more critical role in STAT5-mediated signal transduction compared with the GAS1 element. Finally, chromosome immunoprecipition assay further confirmed that prolactin activates STAT5 binding to the ZnT2 promoter in vivo. Taken together, these results illustrate that prolactin regulates the transcription of ZnT2 through activation of the Jak2/STAT5 signaling pathway to assist in providing optimal zinc for secretion into milk during lactation.

Coactivation of janus tyrosine kinase (Jak)1 positively modulates prolactin-Jak2 signaling in breast cancer: recruitment of ERK and signal transducer and activator of transcription (Stat)3 and enhancement of Akt and Stat5a/b pathways

Prolactin (PRL) receptors (PRLRs) have been considered selective activators of Janus tyrosine kinase (Jak)2 but not Jak1, Jak3, or Tyk2. We now report marked PRL-induced tyrosine phosphorylation of Jak1, in addition to Jak2, in a series of human breast cancer cell lines, including T47D, MCF7, and SKBR3. In contrast, PRL did not activate Jak1 in immortalized, noncancerous breast epithelial lines HC11, MCF10A, ME16C, and HBL-100, or in CWR22Rv1 prostate cancer cells or MDA-MB-231 breast cancer cells. However, introduction of exogenous PRLR into MCF10A, ME16C, or MDA-MB-231 cells reconstituted both PRL-Jak1 and PRL-Jak2 signals. In vitro kinase assays verified that PRL stimulated enzymatic activity of Jak1 in T47D cells, and PRL activated Jak1 and Jak2 with indistinguishable time and dose kinetics. Relative Jak2 deficiency did not cause PRLR activation of Jak1, because overexpression of Jak2 did not interfere with PRL activation of Jak1. Instead, PRL activated Jak1 through a Jak2-dependent mechanism, based on disruption of PRL activation of Jak1 after Jak2 suppression by 1) lentiviral delivery of Jak2 short hairpin RNA, 2) adenoviral delivery of dominant-negative Jak2, and 3) AG490 pharmacological inhibition. Finally, suppression of Jak1 by lentiviral delivery of Jak1 short hairpin RNA blocked PRL activation of ERK and signal transducer and activator of transcription (Stat)3 and suppressed PRL activation of Jak2, Stat5a, Stat5b, and Akt, as well as tyrosine phosphorylation of PRLR. The data suggest that PRL activation of Jak1 represents a novel, Jak2-dependent mechanism that may serve as a regulatory switch leading to PRL activation of ERK and Stat3 pathways, while also serving to enhance PRL-induced Stat5a/b and Akt signaling.

Prolactin specifically activates signal transducer and activator of transcription 5b in neuroendocrine dopaminergic neurons

The hypothalamic neuroendocrine dopaminergic (NEDA) neurons are crucial in regulating prolactin secretion from the anterior pituitary. Rising prolactin concentrations stimulate these neurons to secrete dopamine, which acts via the pituitary portal vasculature to inhibit additional prolactin release. Prolactin is known to activate Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathways in other cell types, including neurons. The possible role of JAK-STAT signaling in NEDA neurons has therefore been examined in this study using fetal rat mediobasal hypothalamic cell cultures and an adult rat in vivo preparation. Cultured cells expressing the dopamine synthesizing enzyme tyrosine hydroxylase (TH) responded to prolactin with a time-dependent increase in phospho-STAT5, but not phospho-STAT1 or phospho-STAT3, nuclear labeling. This response was inhibited by the prolactin receptor antagonist Delta1-9-G129R-human prolactin and the JAK inhibitor AG490, but was unaffected by selected serine/threonine kinase inhibitors (H89, KN-93, bisindolymaleimide, or PD98059). Antibodies selective for STAT5a or STAT5b indicated that the response was restricted to STAT5b, with the number of TH cells displaying STAT5b nuclear immunoreactivity rising from less than 10% under basal conditions to approximately 70% after prolactin stimulation. STAT5a nuclear labeling remained unchanged at 6-10% of TH-positive cells. STAT5b selectivity was confirmed in vivo, where the injection of prolactin into bromocriptine-treated rats stimulated a time-dependent increase in STAT5b, but not STAT5a, nuclear staining in the TH-expressing neurons in the arcuate nucleus. These results extend our previous findings with STAT5b-deficient mice and strongly suggest that in NEDA neurons, prolactin signaling via the JAK/STAT pathway is mediated exclusively by STAT5b.

PRL-induced ERalpha gene expression is mediated by Janus kinase 2 (Jak2) while signal transducer and activator of transcription 5b (Stat5b) phosphorylation involves Jak2 and a second tyrosine kinase

In the rat corpus luteum of pregnancy, PRL stimulation of ER expression is a prerequisite for E2 to have any luteotropic effect. Previous work from our laboratory has established that PRL stimulates ERalpha expression at the level of transcription and that the transcription factor Stat5 (signal transducer and activator of transcription 5) mediates this stimulation. Since it is well established that PRL activates Stat5 through the tyrosine kinase, Janus kinase 2 (Jak2), the role of Jak2 in PRL regulation of ERalpha expression was investigated. In primary luteinized granulosa cells, the general tyrosine kinase inhibitors, genistein and AG18, and the Jak2 inhibitor, AG490, prevented PRL stimulation of ERalpha mRNA levels, suggesting that PRL signaling to the ERalpha gene requires Jak2 activity. However, using an antibody that recognizes the tyrosine-phosphorylated forms of both Stat5a and Stat5b (Y694/Y699), it was found that AG490 could inhibit PRL-induced Stat5a phosphorylation only and had little or no effect on Stat5b phosphorylation. These effects of AG490 were confirmed in COS cells overexpressing Stat5b. Also in COS cells, a kinase-negative Jak2 prevented PRL stimulation of ERalpha promoter activity and Stat5b phosphorylation while a constitutively active Jak2 could stimulate both in the absence of PRL. Furthermore, kinase-negative-Jak2, but not AG490, could inhibit Stat5b nuclear translocation and DNA binding. Therefore, it seems that in the presence of AG490, Stat5b remains phosphorylated, is located in the nucleus and capable of binding DNA, but is apparently transcriptionally inactive. These findings suggest that PRL may activate a second tyrosine kinase, other than Jak2, that is capable of phosphorylating Stat5b without inducing transcriptional activity. To investigate whether another signaling pathway is involved, the src kinase inhibitor PP2 and the phosphoinositol-3 kinase inhibitor (PI3K), LY294002, were used. Neither inhibitor alone had any major effect on PRL regulation of ERalpha promoter activity or on PRL-induced Stat5b phosphorylation. However, the combination of AG490 and LY294002 largely prevented PRL-induced Stat5b phosphorylation. These findings indicate that PRL stimulation of ERalpha expression requires Jak2 and also that PRL can induce Stat5b phosphorylation through two tyrosine kinases, Jak2 and one downstream of PI3K. Furthermore, these results suggest that the role of Jak2 in activating Stat5b may be through a mechanism other than simply inducing Stat5b phosphorylation.

DNA binding specificity of different STAT proteins. Comparison of in vitro specificity with natural target sites

STAT transcription factors are expressed in many cell types and bind to similar sequences. However, different STAT gene knock-outs show very distinct phenotypes. To determine whether differences between the binding specificities of STAT proteins account for these effects, we compared the sequences bound by STAT1, STAT5A, STAT5B, and STAT6. One sequence set was selected from random oligonucleotides by recombinant STAT1, STAT5A, or STAT6. For another set including many weak binding sites, we quantified the relative affinities to STAT1, STAT5A, STAT5B, and STAT6. We compared the results to the binding sites in natural STAT target genes identified by others. The experiments confirmed the similar specificity of different STAT proteins. Detailed analysis indicated that STAT5A specificity is more similar to that of STAT6 than that of STAT1, as expected from the evolutionary relationships. The preference of STAT6 for sites in which the half-palindromes (TTC) are separated by four nucleotides (N(4)) was confirmed, but analysis of weak binding sites showed that STAT6 binds fairly well to N(3) sites. As previously reported, STAT1 and STAT5 prefer N(3) sites; however, STAT5A, but not STAT1, weakly binds N(4) sites. None of the STATs bound to half-palindromes. There were no specificity differences between STAT5A and STAT5B.

Role of protein kinases in the prolactin-induced intracellular calcium rise in Chinese hamster ovary cells expressing the prolactin receptor

There is still only limited understanding of the early steps of prolactin signal transduction in target cells. It has been shown that prolactin actions are associated with cell protein phosphorylation, Ca2+ increases, and so on. However, the link between the activation of kinases and calcium influx or intracellular Ca2+ mobilization has not yet been clearly established. Chinese hamster ovary (CHO) cells, stably transfected with the long form of rabbit mammary gland prolactin receptor (PRL-R) cDNA were used for PRL-R signal transduction studies. Spectrofluorimetric techniques were used to measure intracellular calcium ([Ca2+]i) in cell populations with Indo1 as a calcium fluorescent probe. We demonstrate that, although protein kinase C activation (PMA or DiC8) caused a calcium influx in CHO cells, prolactin-induced PKC activation was not responsible for the early effect of prolactin on [Ca2+]i. Activation of protein kinase A (PKA) or protein kinase G did not modify [Ca2+]i and inhibition of PKA pathway did not affect the prolactin response. In the same way, phosphatidylinositol-3 kinaseinhibition had no effect on the prolactin-induced Ca2+ increase. On the other hand, tyrosine kinase inhibitors (herbimycin A, lavendustin A, and genistein) completely blocked the effect of prolactin on [Ca2+]i (influx and release). W7, a calmodulin-antagonist, and a specific inhibitor of calmodulin kinases (KN-62), only blocked prolactin-induced Ca2+ influx but had no significant effect on Ca2+ release. Using pharmacological agents, we present new data concerning the involvement of protein phosphorylations in the early effects of prolactin on ionic channels in CHO cells expressing the long form of PRL-R. Our results suggest that, at least in the very early steps of prolactin signal transduction, serine-threonine phosphorylation does not participate in the prolactin-induced calcium increase. On the other hand, tyrosine phosphorylation is a crucial, very early step, since it controls K+ channel activation, calcium influx, and intracellular calcium mobilization. Calmodulin acts later, since its inhibition only blocks the prolactin-induced Ca2+ influx.

Jak-Stat signal transduction pathway through the eyes of cytokine class II receptor complexes

Cells of the immune system communicate with each other to initiate, establish and maintain immune responses. The communication occurs through cell-to-cell contact or through a variety of intercellular mediators that include cytokines, chemokines, growth factors and hormones. In the case of cytokines, the signal is transmitted from the outside to the inside of a cell through cell surface receptors specific for each cytokine. At this step the signal is also decoded and amplified: ligand binding causes recruitment and/or activation of numerous cytoplasmic proteins. One cytokine can activate a number of signal transduction pathways leading to regulation of a wide array of biological activities. One of these pathways, the Jak-Stat pathway, is briefly reviewed here with respect to the class II cytokine receptors. Signal transduction through receptors for interferons Type I (IFN-alpha, IFN-beta, IFN-omega) and Type II (IFN-gamma), and interleukin 10 (IL-10) is described in detail. In addition, a complex between tissue factor (TF) and coagulation factor VIIa, and two new receptors related to the class II cytokine receptor family are discussed. Oncogene (2000).

A lymphoma growth inhibitor blocks some but not all prolactin-stimulated signaling pathways

Cytokines and hormones activate a network of intracellular signaling pathways to regulate cell division, survival and differentiation. In parallel, a series of growth inhibitory mechanisms critically restrict cell population sizes. For example, mitogens can be opposed in crowded cell cultures through contact-inhibition or by autocrine release of antiproliferative substances. Here, we characterize a small, heat-stable growth inhibitor secreted by a rat T lymphoma line when cultured at high cell density. Short term incubation (<60 min) of prolactin-responsive Nb2 lymphoma cells at high density selectively blocked prolactin stimulation of p42/p44 mitogen-activated protein kinases and transcription factors Stat1 and Stat3 but not prolactin activation of Stat5 or the tyrosine kinase Jak2. The selective effects of cell density on prolactin signaling were reversible. Furthermore, exposure of cells at low density to conditioned media from cells incubated at high density had the same inhibitory effects on prolactin signaling. This selective inhibition of discrete prolactin signals was mimicked by short term preincubation of cells at low density with staurosporine or genistein but not with bis-indoleyl maleimide, cyclic nucleotide analogs, calcium ionophore A23187, or phorbol 12-myristate 13-acetate. A heat-stable, proteinase K-resistant, low molecular weight factor with these characteristics was recovered from high density culture medium. The partially purified inhibitor suppressed Nb2 cell growth with a sigmoidal concentration response consistent with a saturable, receptor-mediated process.

Constitutive activation of the prolactin receptor results in the induction of growth factor-independent proliferation and constitutive activation of signaling molecules

The ability to induce the oncogenic activation of the human prolactin receptor (PRLR) was examined by deleting 178 amino acids of the extracellular ligand-binding domain. Expression of this deletion mutant in the interleukin-3 (IL-3)-dependent murine myeloid cell line 32Dcl3 resulted in the induction of growth factor-independent proliferation. Parental 32Dcl3 cells proliferated only in the presence of exogenous murine IL-3 (mIL-3), while 32Dcl3 cells transfected with the long form of the human PRLR were able to proliferate in response to mIL-3, ovine prolactin, or human PRL. Cells expressing the Delta178 deletion mutant contained numerous phosphotyrosine-containing proteins in the absence of stimulation with either mIL-3 or ovine prolactin. Growth factor stimulation increased the number of proteins phosphorylated and the intensity of phosphorylation. These proteins included constitutively phosphorylated Janus kinase 2, signal transducer and activator of transcription 5, and SHC. Activated extracellular signal-regulated kinases 1 and 2 (ERK1 and ERK2) were observed in unstimulated 32Dcl3 cells expressing the Delta178 mutant. Likewise, transfection of Nb2 cells with the Delta178 deletion mutant induced growth factor-independent proliferation and constitutive activation of Janus kinase 2, ERK1, and ERK2. In addition to the induction of a growth factor-independent state, the expression of the Delta178 deletion mutant also suppressed the apoptosis that occurs when 32Dcl3 cells are cultured in the absence of growth factors such as IL-3. These data suggest that the constitutive activation of the PRLR can be achieved by deletion of the ligand binding domain and that this mutation leads to the oncogenic activation of the receptor as determined by the ability of the receptor to induce growth factor-independent proliferation of factor-dependent hematopoietic cells.

Specific binding of human interferon-gamma to particulates from hemolymph and protocerebrum of tobacco hornworm (Manduca sexta) larvae

Specific binding sites for human interferon-gamma (hIFN-gamma) were found on particulates prepared from the hemolymph and protocerebrum of fifth-instar larvae of the tobacco hornworm, Manduca sexta. A portion of these sites could be solubilized in an active form by the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate (CHAPS). A well-defined specific binding was also associated with hemolymph particulate residue after solubilization by CHAPS. About one-half of [125I]hIFN-gamma binding could be displaced by heparin. The bound hIFN-gamma could be covalently cross-linked to the binding sites using disuccinylamide suberate, and the molecular weight range of these complexes was 200-800 kDa as determined by density gradient sedimentation and gel-exclusion chromatography. Only a small fraction of the hemolymph IFN-gamma binding could be competed by another mammalian cytokine, rat prolactin (rPRL), while there was no sensitivity to rat growth hormone. The small specific rPRL binding found in Manduca hemolymph showed an affinity similar to the prolactin sites found in the liver of pregnant rats. The detergent-insoluble Manduca hIFN-gamma binding was bimodal and similar in affinity distribution to the binding found with human platelet membranes (Kdiss range 0.1-2 nM). The detergent-solubilized IFN-gamma sites were homogenous, with a Kdiss of about 1.5 nM. The IFN-gamma binding sites in Manduca tissues may therefore include molecular species similar to the known invertebrate cytokine receptors and proteoglycan co-receptors.

Fyn associates with Cbl and phosphorylates tyrosine 731 in Cbl, a binding site for phosphatidylinositol 3-kinase

We have investigated the interaction between Cbl and the Src-related tyrosine kinase Fyn. Fyn was observed to be constitutively associated with Cbl in lysates of several different cell types including the interleukin-3-dependent murine myeloid cell line 32Dcl3, and the prolactin-dependent rat thymoma cell line Nb2. Binding studies indicated that Cbl could bind to glutathione S-transferase (GST) fusion proteins encoding the unique, Src homology domain 3 (SH3), and SH2 domains of Fyn, Hck, or Lyn. Fusion proteins encoding either the SH3 or SH2 domains of Fyn bound to Cbl as effectively as the fusion protein encoding the unique, SH3, and SH2 domains of Fyn. The Fyn SH2 domain bound to both tyrosine-phosphorylated and nonphosphorylated Cbl, implying that this interaction might be phosphotyrosine-independent. Binding of the Fyn SH2 domain to Cbl was not disrupted by the addition of phosphotyrosine, phosphoserine, or phosphothreonine. A GST fusion protein encoding the proline-rich region of Cbl bound to Fyn present in a total cell lysate. Far Western blot analysis also indicated that the SH3 domain of Fyn bound preferentially to the proline-rich region of Cbl. The addition of [gamma-32P]ATP to either anti-Cbl immunoprecipitates or anti-Fyn immunoprecipitates resulted in the phosphorylation of both Cbl and Fyn as demonstrated by immunoprecipitation of the phosphorylated proteins with specific antisera. Fyn directly phosphorylated a GST fusion protein containing the C-terminal region of Cbl (GST-CBL-LZIP). In contrast, immunoprecipitated JAK2 was not able to phosphorylate this same region of Cbl. The GST-CBL-LZIP fusion protein contains a binding site for the SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase, which mapped to Tyr731, which is present in the sequence YEAM. Mutation of Tyr731 in GST-CBL-LZIP eliminated binding of the p85 subunit of phosphatidylinositol 3-kinase and substantially reduced the phosphorylation of this fusion protein by Fyn, despite the presence of four other tyrosine residues in this fusion protein. These data are consistent with the hypothesis that Cbl represents a substrate for Src-like kinases that are activated in response to the engagement of cell surface receptors, and that Src-like kinases are responsible for the phosphorylation of a tyrosine residue in Cbl that may regulate activation of phosphatidylinositol 3-kinase.

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)

Leukemia inhibitory factor regulates proopiomelanocortin transcription

Leukemia inhibitory factor (LIF), a pleiotropic cytokine, is expressed in both fetal and adult pituitary tissue, and LIF immunoreactivity is found in functional human pituitary tumors. LIF induces basal, and augments CRH-induced, POMC mRNA and ACTH secretion from AtT20 cells. Therefore, we examined LIF signaling and LIF regulation of POMC expression in AtT20 cells. Immunoneutralization studies demonstrated the dependence of LIF action on both the specific LIF receptor (35% inhibition; p < 0.05) and also the gpl30 affinity converter (41% inhibition; p < 0.05). These antisera also attenuate basal ACTH secretion without added LIF. LIF rapidly induced tyrosyl phosphorylation of both STAT 1 alpha, and STAT beta and also induced phosphorylation of a novel STAT 1 alpha related protein p115. LIF induced POMC transcription (-706/+64) and strikingly potentiated CRH action (up to 18-fold induction). This synergy involved cAMP-dependent pathways, as forskolin action was also potentiated by LIF. Deletion of the major CRH-responsive region in POMC (-323/-166) abolished both CRH and LIF action on POMC transcription. Thus LIF action in pituitary corticotrophs is dependent on LIF receptor heterodimerisation with gpl30 and involves STAT protein tyrosyl phosphorylation. LIF enhances POMC transcription and strongly potentiates the well-documented action of CRH on the POMC gene. These results define a subcellular mechanism for an immuno-neuroendocrine interface between peripheral afferent signals and the HPA axis.

Regulation of mammary differentiation by extracellular matrix involves protein-tyrosine phosphatases

Extracellular matrix and growth factors cooperate to regulate signaling pathways and gene transcription in adherent cells. However, the mechanism of extracellular matrix signaling is poorly defined. In mammary gland, the expression of milk protein genes is controlled by cross-talk between signals derived from the basement membrane protein, laminin, and the lactogenic hormone, prolactin. Signals from basement membrane are transduced by beta1 integrins and are required for prolactin to activate DNA binding of the milk protein gene transcription factor, Stat5. Here we show that basement membrane is necessary for tyrosine phosphorylation of the prolactin receptor and thus directly affects cytokine signaling and differentiation at the level of the plasma membrane. Prolactin does not induce tyrosine phosphorylation of its receptor, Jak2, or Stat5 in nondifferentiated breast epithelia cultured on collagen I, and we show that this is due to a vanadate-sensitive activity that inhibits the prolactin pathway. We suggest that protein-tyrosine phosphatases are novel targets for regulation by extracellular matrix and in mammary cells represent an additional control to the requirement of integrins for milk protein production.

Control of normal mammary epithelial phenotype by integrins

Mammary epithelial cells contact a specialized extracellular matrix in vivo known as the basement membrane. Interactions with extracellular matrix are mediated through integrins. These cell surface receptors are involved with the formation of adhesion complexes, which link the extracellular matrix with the actin-based cytoskeleton, and are also associated with components of growth factor signaling pathways. Differentiation of breast epithelia into lactational cells requires appropriate hormones and integrin-mediated interactions with basement membrane. Integrins may regulate the ability of lactogenic hormones to trigger their intracellular signaling pathways.

Growth hormone stimulates interferon regulatory factor-1 gene expression in the liver

Interferon regulatory factor-1 (IRF-1) is a transcription factor first identified as part of the nuclear response to interferons. IRF-1 has been shown to be activated by many cytokines, including PRL, and has been thought to play a role in PRL-regulated gene expression in several experimental systems, including the Nb2 T lymphoma cell line, where it was first characterized as a PRL-responsive gene. We now find that IRF-1 gene expression is rapidly activated in vivo by both PRL and GH treatment. A single i.p. injection of rat PRL to hypophysectomized female rats caused a transient increase in nascent hepatic nuclear IRF-1 RNA within 15 min of hormone treatment. The rise in IRF-1 transcripts was accompanied by induction of nuclear protein binding to a DNA element from the proximal IRF-1 promoter, as assessed by gel mobility shift assays; this element was shown previously to mediate PRL-activated gene transcription. GH treatment stimulated a greater and more sustained increase in nascent IRF-1 RNA than PRL, leading to accumulation of IRF-1 transcripts for up to 16 h after a single hormone injection. GH also caused a pronounced induction of hepatic nuclear protein binding to the IRF-1 promoter element. Supershift experiments with specific antibodies showed that signal transducer and activator of transcription 1 (STAT1) and to a lesser extent STAT3 were components of the GH-activated protein-DNA complexes. By contrast, these two STATs were not induced in the liver by PRL. Protein binding to the IRF-1 DNA element and IRF-1 gene activation by GH were not blunted by pretreatment with the protein synthesis inhibitor, cycloheximide, indicating that these hormonal effects are primary consequences of GH-activated signal transduction pathways. Our results identify another component of the rapid nuclear response to GH, and support the idea that multiple primary and secondary signaling pathways contribute to the acute actions of GH on gene expression.

Transcriptional inhibition by Stat5. Differential activities at growth-related versus differentiation-specific promoters

Prolactin (PRL) induces transcriptional activation of not only growth-related genes such as interferon regulatory factor-1 (IRF-1) but also differentiation-specific genes such as beta-casein through a signaling cascade consisting of Janus kinases and Stat (signal transducer and activator of transcription) factors. To understand better the role of Stats in PRL signaling, we cloned rat Stat5b from a PRL-responsive T cell line Nb2. A Stat5b-specific peptide antibody was generated. In PRL receptor reconstituted COS cells cotransfected with Stat5b or Stat5a, both Stat5 proteins become tyrosine phosphorylated and bind to the IRF-1 GAS (interferon-gamma activation sequence) element in a PRL-inducible manner. Unexpectedly, both Stat5b and Stat5a inhibit PRL induction of the IRF-1 promoter, but they mediate PRL stimulation of the beta-casein promoter. Stat5-mediated inhibition was observed only at the native IRF-1 promoter and not at the isolated IRF-1 GAS element linked to a heterologous thymidine kinase promoter. Mutational analyses showed that the DNA binding activity of Stat5b is not required, but the carboxyl-terminal transactivation domain is essential for Stat5b to inhibit PRL induction of the IRF-1 promoter. These results suggest that Stat5b mediates inhibition via protein-protein interactions. In contrast, both DNA binding and transactivation domains of Stat5b are required to mediate PRL induction of the beta-casein promoter. Furthermore, a carboxyl-terminal truncated dominant negative Stat5b can reverse Stat5b inhibition at the IRF-1 promoter. These studies suggest that Stat proteins can act as not only positive but also negative regulators of gene transcription. Further, Stat5 can modulate gene expression without binding to DNA but via protein-protein interactions.

Phosphorylation of cbl after stimulation of Nb2 cells with prolactin and its association with phosphatidylinositol 3-kinase

Stimulation of Nb2 cells with PRL results in the rapid phosphorylation of a 120-kDa protein identified as the adapter protein cbl on tyrosine residues. Maximal phosphorylation of cbl occurs at 20 min after PRL stimulation and declines thereafter. Stimulation with as little as 5 nM PRL resulted in the phosphorylation of cbl; increasing the concentration of PRL to 100 nM had only a minimal effect upon the phosphorylation of cbl. The cbl protein appears to be constitutively associated with grb2 and the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase). The constitutive association of cbl with the p85 subunit of PI 3-kinase was observed in Nb2 cells as well as in 32Dcl3 cells transfected with either the rat Nb2 (intermediate) form of the PRL receptor or the long form of the human PRL receptor. A glutathione S-transferase fusion protein encoding the SH3 domain of the p85 subunit of PI 3-kinase bound to cbl in lysates of both unstimulated and PRL-stimulated Nb2 cells; however, neither of the SH2 domains of p85 bound to cbl under the same conditions. PRL stimulation increased the cbl-associated PI kinase activity. The majority of PI kinase activity appeared to be cbl-associated after PRL stimulation. These results suggest that cbl may function as an adapter protein in PRL-mediated signaling events and regulate activation of PI 3-kinase. Our model suggests that the p85 subunit of PI 3-kinase is constitutively associated with cbl through binding of the p85 SH3 domain to a proline-rich sequence in cbl. After the tyrosine phosphorylation of cbl, an SH2 domain(s) of p85 binds to a specific phosphorylation site(s) in cbl, leading to the activation of PI 3-kinase.

Multiple prolactin (PRL) receptor cytoplasmic residues and Stat1 mediate PRL signaling to the interferon regulatory factor-1 promoter

The Nb2 PRL receptor (PRL-R) is known to mediate PRL signaling to the interferon (IFN) regulatory factor-1 (IRF-1) gene via the family of signal transducers and activators of transcription or Stats. To analyze the components of the PRL-R/Stat/IRF-1 signaling pathway, various PRL-R, Stat, and IRF-1-CAT reporter constructs were transiently cotransfected into COS cells. First, mutations in the IFNgamma-activated sequence (GAS), either multimerized or in the context of the 1.7-kb IRF-1 promoter, failed to mediate a PRL response, showing that the IRF-1 GAS is a target of PRL signaling. Next, pairwise alanine substitutions into conserved residues in the proline-rich motif or Box 1 region and two tyrosine mutations, Y308F and Y382F, in the PRL-R intracellular domain all impaired PRL signaling to multimerized GAS or to the 1.7-kb IRF-1 promoter. Furthermore, these PRL-R mutants mediated reduced Stat1 binding to the IRF-1 GAS. Transfection of Stat1 further enhanced PRL signaling to the IRF-1 promoter, suggesting that Stat1 is a positive mediator of PRL action. These studies show that both membrane proximal and distal residues of the PRL-R are involved in signaling to the IRF-1 gene. Further, Stat1 and the GAS element are important for PRL activation of the IRF-1 gene.

JAK2 and STAT5, but not JAK1 and STAT1, are required for prolactin-induced beta-lactoglobulin transcription

Several different Janus kinases (JAKs) and signal transducers and activation of transcription (STATs) have been implicated in mediating the biological responses induced by PRL, based on their ligand-dependent tyrosine phosphorylation and activation. However, these criteria alone do not prove that a particular JAK or STAT is essential for signal transduction. We have used mutant cell lines defective in JAK1, JAK2, or STAT1 to examine their roles in PRL-dependent signaling. JAK2 is absolutely required for PRL-dependent phosphorylation of the receptor, activation of STATs, and induction of beta-lactoglobulin. Wild type, but not kinase-negative JAK2, restores all responses to PRL in JAK2-defective cells, suggesting that JAK2 function, not merely the protein, is required. In contrast, JAK1, which is phosphorylated in response to PRL, is not required for any of these functions. Although STAT1 homodimers do form in response to PRL, no defect in PRL-dependent signaling is apparent when STAT1 is missing, suggesting that STAT5, which is strongly activated in response to PRL, is primarily responsible for driving the expression of PRL-responsive genes.

Prolactin stimulates serine/tyrosine phosphorylation and formation of heterocomplexes of multiple Stat5 isoforms in Nb2 lymphocytes

Transcription factors of the Stat gene family are selectively activated by many hormones and cytokines. Stat5 originally was cloned as a prolactin-stimulated DNA-binding protein, but is also activated by non-lactogenic cytokines in many cell types. The recent identification of two distinct Stat5 genes, which encode a 94-kDa Stat5a and a 92-kDa Stat5b as well as several lower molecular weight isoforms, suggests additional complexity and combinatorial possibilities for transcriptional regulation. We now report a biochemical analysis of prolactin activation of Stat proteins in Nb2 lymphocytes, which was associated with: 1) rapid tyrosine phosphorylation of Stat5a, Stat5b, a COOH-terminally truncated 80-kDa Stat5 form, Stat1alpha, and Stat3; 2) rapid and selective formation of Stat5a/b heterodimers, without involvement of Stat1alpha or Stat3; 3) marked serine, but not threonine phosphorylation of Stat5a and Stat5b; and 4) the appearance of two qualitatively distinct Stat5 protein complexes, which discriminated between oligonucleotides corresponding to the prolactin response elements of the beta-casein and interferon regulatory factor-1 gene promoters. Collectively, our analyses showed that Stat5a and Stat5b respond similarly to prolactin receptor activation, but also suggested that the two genes have evolved unique properties that may contribute to the specificity of receptors that utilize Stat5 signaling proteins.

Prolactin stimulates the JAK2 and focal adhesion kinase pathways in human breast carcinoma T47-D cells

Treatment of T47-D human breast carcinoma cells with recombinant prolactin (rhPRL) induced a concentration- and time-dependent increase in the phosphotyrosine content of JAK2. rhPRL also stimulated JAK2 tyrosine phosphorylation more weakly in three other breast carcinoma lines, MCF-7, ZR-75-1 and MDA-MB-231. Furthermore it stimulated tyrosine phosphorylation of two isoforms of the transcriptional activator STAT5, STAT5a and STAT5b. Surprisingly, rhPRL treatment of T47-D cells also stimulated tyrosine phosphorylation of focal adhesion kinase (FAK), as determined by immunoprecipitation with anti-phosphotyrosine antibody followed by immunoblotting with a specific FAK antibody. The effect of rhPRL was rapid and concentration-dependent, being maximal at 5 ng/ml. At rhPRL concentrations above 25 ng/ml, FAK tyrosine phosphorylation declined but remained above control levels at 100 ng/ml. rhPRL also stimulated paxillin tyrosine phosphorylation in T47-D cells with similar concentration- and time-dependence. In a second human breast carcinoma cell line, MCF-7, rhPRL produced very similar effects on FAK and paxillin tyrosine phosphorylation. These findings identify a new protein tyrosine kinase pathway in the action of the lactogenic hormone rhPRL and represent the first report that a hormone acting through a member of the haemopoietin receptor superfamily can regulate the FAK/paxillin pathway.

Prolactin signal transduction to milk protein genes: carboxy-terminal part of the prolactin receptor and its tyrosine phosphorylation are not obligatory for JAK2 and STAT5 activation

In this study, we have developed several Chinese Hamster ovary (CHO) cell clones stably expressing various deletion mutant forms of the rabbit prolactin receptor (rbPRL-R) to better define the domains of the receptor involved in JAK2 kinase interaction, STAT5 activation, and to assess the role of tyrosine phosphorylation of the PRL-R in signal transduction. We observed that the box 1 region of the receptor was critical for productive interaction with JAK2 and its tyrosine phosphorylation after PRL stimulation. However, this region appeared to require the presence of additional cytoplasmic domain region(s), such as box 2, to exert its complete effect. In addition, we found that a mutant form lacking the 141 C-terminal residues lost the capacity to be tyrosine phosphorylated in response to PRL but remained able to activate JAK2 kinase and STAT5 transcription factor, indicating that it contained the minimal sequence required for STAT5 activation. The absence of tyrosine phosphorylation of this C-terminal rbPRL-R mutant upon PRL stimulation indicated that the phosphorylation of the PRL-R normally occured in the last 141 animo acids (aa) containing three tyrosines and was not absolutely necessary for induction of these early events in PRL signal transduction. Transfectant cell lines expressing wild type (WT) PRL-R and this C-terminal mutant form were able to induce CAT activity upon PRL stimulation when transiently transfected with the ovine-beta-lactoglobulin promoter, containing STAT5 recognition sites, fused to the CAT reporter gene. The comparison between transcriptional activity of these two receptor forms leads to the conclusion that the C-terminal region of the rbPRL-R, containing the physiological sites for tyrosine phosphorylation, is probably responsible for an amplification of the PRL signal to milk protein genes.

Prolactin mediated intracellular signaling in mammary epithelial cells

Prolactin binds to a member of the cytokine receptor superfamily. The cytoplasmic domain of the prolactin receptor (PrlR) displays no enzymatic activity yet prolactin treatment leads to the induction of protein tyrosine phosphorylation. PrlR is associated with JAK2, a protein tyrosine kinase whose activity is stimulated following receptor dimerization. JAK2 subsequently phosphorylates PrlR and other cellular proteins which are recruited to the activated receptor complex. Among the JAK2 substrates is the transcription factor Stat5 whose phosphorylation mediates the transcriptional activation of beta-casein gene expression. In this review we discuss the prolactin induced signaling pathways which mediate differentiation of the mammary gland.

Prolactin as a mitogen in mammary cells

Prolactin (PRL) acts as both a mitogen and a differentiating agent in the breast. The decision to respond to PRL as a mitogen by breast cells depends on the hormonal milieu in which the epithelial cell resides. In addition, PRL's action on the breast is regulated (1) at the level of the hormone itself; (2) at the receptor level; (3) at the level of selection of signaling pathway; and, (4) by combinations of these aspects. The development of cell lines containing only one class of the PRL receptors and showing qualitative differences in response and signaling pathways will help in understanding the pleiotropic nature of PRL action.

Expression of xanthine oxidoreductase in mouse mammary epithelium during pregnancy and lactation: regulation of gene expression by glucocorticoids and prolactin

In the mammary gland of virgin mice, xanthine oxidoreductase (XOR) enzymic activity is barely measurable. a high increase in the levels of the enzyme is observed during the last days of pregnancy and during lactation, and this is parallelled by an elevation in the amounts of the respective protein and transcript. In situ hybridization experiments demonstrate that the XOR mRNA is specifically expressed in the alveolar epithelial cells of the mammary gland. In HC11 cells, a model culture system for normal breast epithelium, the levels of XOR enzymic activity are dose- and time-dependently induced by dexamethasone, and a further synergistic augmentation is observed in the presence of dexamethasone plus prolactin. Increased XOR gene expression is consequent on glucocorticoid receptor activation, as indicated by sensitivity to the specific receptor antagonist RU486. In addition, the phenomenon is likely to involve protein phosphorylation and dephosphorylation events, as suggested by modulation of XOR mRNA by tyrosine kinase and phosphatase inhibitors.

Other kinases can substitute for Jak2 in signal transduction by interferon-gamma

Each cytokine which utilizes the Jak-Stat signal transduction pathway activates a distinct combination of members of the Jak and Stat families. Thus, either the Jaks, the Stats, or both could contribute to the specificity of ligand action. With the use of chimeric receptors involving the interferon gamma receptor (IFN-gammaR) complex as a model system, we demonstrate that Jak2 activation is not an absolute requirement for IFN-gamma signaling. Other members of the Jak family can functionally substitute for Jak2. IFN-gamma can signal through the activation of Jak family members other than Jak2 as measured by Statlalpha homodimerization and major histocompatibility complex class I antigen expression. This indicates that Jaks are interchangeable and indiscriminative in the Jak-Stat signal transduction pathway. The necessity for the activation of one particular kinase during signaling can be overcome by recruiting another kinase to the receptor complex. The results may suggest that the Jaks do not contribute to the specificity of signal transduction in the Jak-Stat pathway to the same degree as Stats.

The SH2 domain-containing tyrosine phosphatase PTP1D is required for interferon alpha/beta-induced gene expression

Interferons (IFNs) induce early response genes by stimulating Janus family (Jak) tyrosine kinases, leading to tyrosine phosphorylation of Stat (signal transducer and activator of transcription) proteins. Previous studies demonstrated that a protein-tyrosine phosphatase (PTP) is required for activation of the ISGF3 transcription complex by IFNalpha/beta, but the specific PTP responsible remained unidentified. We now show that the SH2 domain containing tyrosine phosphatase PTP1D (also designated as SHPTP2, SHPTP3, PTP2C, or Syp) is constitutively associated with the IFNalpha/beta receptor and becomes tyrosine-phosphorylated in response to ligand. Furthermore, transient expression of a phosphatase-inactive mutant or the COOH-terminal SH2 domain of PTP1D causes a dominant negative effect on IFNalpha/beta-induced early response gene expression. These results provide strong evidence that PTP1D functions as a positive regulator of the IFNalpha/beta-induced Jak/Stat signal transduction pathway.

Cloning and characterization of an ovarian-specific protein that associates with the short form of the prolactin receptor

Prolactin (PRL) is essential for progesterone biosynthesis and luteal cell hypertrophy of the rat corpus luteum during pregnancy. Both the long and short form of the PRL receptor have been identified in the corpus luteum of pregnant rat. The long form has been shown to transduce PRL signal in other cells, whereas no information is available on the role of the short form, especially in the corpus luteum. In the present study, we have cloned a rat ovarian-specific phosphoprotein, PRAP (PRL Receptor Associated Protein), which has no significant homology to other known proteins. We have demonstrated that this protein is immunoprecipitated by anti-PRL receptor and anti-phosphotyrosine antibodies. To determine whether PRAP associates with either the long or the short form of the PRL receptor, fusion proteins with glutathione S-transferase containing the cytoplasmic domain of the long or short form of the PRL receptor were produced, purified, and incubated with luteal proteins. Our results indicate that PRAP preferentially binds to the short form of the PRL receptor. Thus, the long form and short forms of the PRL receptor may signal through distinct pathways. These data provide evidence for the involvement of a novel protein in PRL signal transduction and suggest that PRAP may contribute to the luteotropic effects of PRL on the corpus luteum during pregnancy.

Activation of raf-1, MEK, and MAP kinase in prolactin responsive mammary cells

The polypeptide hormone prolactin (Prl), acting through its cell surface receptors, promotes growth and differentiation in normal and malignant breast cells. We demonstrate herein that two Prl-responsive cell lines, NOG-8 normal mouse mammary epithelial and T47D human breast cancer cells, respond to Prl by rapid and transient activation of a series of kinases. Raf-1 was activated within 2-5 min of Prl treatment. This was followed rapidly by activation of MEK (MAP kinase kinase) and MAP kinase activity in these cells. Increased MAP kinase activity was accompanied by tyrosine phosphorylation of both the 42 kDa and 44 kDa isoforms. The tyrosine kinase inhibitors genestein and tyrphostin blocked the increase in MAP kinase activity as well as Prl induced growth of the T47D cells. These results indicate that the Prl receptor, after binding to Prl in mammary cells, activates the raf-MEK-MAP kinase pathway for signal transduction leading to mitogenesis.

Prolactin increases CD4/CD8 cell ratio in thymus-grafted congenitally athymic nude mice

One distinctive effect on T-cell development was analyzed by selectively increasing serum prolactin (PRL) concentration in thymus-grafted congenitally athymic nude mice and by neutralizing PRL in suspension cultures of thymus from 1-day-old neonatal mice. Flow cytometric analysis of single-positive CD4+ and CD8+ cells derived from inguinal lymph nodes revealed a CD4/CD8 cell ratio of 2.2 +/- 0.18 (mean +/- SEM) in thymus-grafted nude mice that is similar to the ratio for immune-competent BALB/c mice (2.0 +/- 0.06). Addition of the pituitary to thymus-grafted nude mice significantly elevated serum PRL (P < 0.005) and increased the CD4/CD8 cell ratio (2.8 +/- 0.12; P < 0.005), demonstrating preferential stimulation of CD4+ cell development. T cells in nude mice receiving sham (submandibular salivary gland) or pituitary grafts alone were below detectable levels. Suspension cultures of neonatal thymus treated with anti-mouse PRL antiserum resulted in 20% and 30% decreases in double-positive CD4+8+ thymocytes and thymocyte viability, respectively. A 10-fold increase in double-negative CD4-8- thymocytes expressing the interleukin 2 receptor alpha chain, CD25, was also observed concurrently. Our findings illustrate an important way in which PRL may participate in two interrelated mechanisms: the regulation of peripheral single-positive cells and the maintenance of thymocyte viability during the double-positive stage of intrathymic differentiation.

Leukemia inhibitory factor (LIF) stimulates proopiomelanocortin (POMC) expression in a corticotroph cell line. Role of STAT pathway

We recently described the expression of leukemia inhibitory factor (LIF) in human fetal and murine corticotrophs. LIF and the related cytokine oncostatin M induced basal, and corticotropin-releasing hormone (CRH) induced proopiomelanocortin (POMC) mRNA and ACTH secretion in AtT20 cells. LIF signaling and regulation of POMC gene transcription were therefore tested. Dexamethasone inhibited both basal- and LIF-induced ACTH secretion (P<0.05) and LIF induction of ACTH was also attenuated by immuneutralization of either the LIF receptor (35%, P<0.05) or the gp130 affinity converter (41%, P<0.05). These antisera also attenuated basal ACTH secretion in the absence of added ligand (P<0.05). To examine intrapituitary LIF signaling, phosphorylation of post-receptor substrates was measured. 1 nM LIF rapidly induced tyrosyl phosphorylation of STAT 1 and STAT 3 proteins, as well as tyrosyl phosphorylation of a 115-kD protein, coimmunoprecipitated with STAT 1. The transfected rat POMC promoter -706/+64, fused to the luciferase reporter gene, was induced by LIF, which exerted strong (18-fold) synergy with CRH. Deletion of the major CRH responsive region in POMC (-323/-166) abolished CRH induction of transcription and severely limited LIF synergy. Although 8 bromo cAMP or forskolin modestly enhanced POMC transcription (2.8-fold), LIF markedly potentiated (7.4-fold) these cAMP activators. These results demonstrate that corticotroph LIF action is receptor mediated and involves activation of STAT signaling pathways. LIF potently synergizes with both CRH and cAMP induction of POMC transcription. This novel intrapituitary signaling mechanism may mediate a neuroimmune pituitary interface.

Targeted disruption of the mouse Stat1 gene results in compromised innate immunity to viral disease

The STAT1 transcription factor is activated in response to many cytokines and growth factors. To study the requirement for STAT1 in vivo, we disrupted the Stat1 gene in embryonic stem (ES) cells and in mice. Stat1(-1-)ES cells were unresponsive to interferon (IFN), but retained responsiveness to leukemia inhibitory factor (LIF) and remained LIF dependent for undifferentiated growth. Stat1(-1-1) animals were born at normal frequencies and displayed no gross developmental defects. However, these animals failed to thrive and were extremely susceptible to viral disease. Cells and tissues from Stat1(-1-) mice were unresponsive to IFN, but remained responsive to all other cytokines tested. Thus, STAT1 appears to be specific for IFN pathways that are essential for viability in the face of otherwise innocuous pathogens.

Interleukin 2 activates STAT5 transcription factor (mammary gland factor) and specific gene expression in T lymphocytes

Although prolactin and interleukin 2 (IL-2) can elicit distinct physiological responses, we have found that their signal pathways share a common signal transducer and activator of transcription, STAT5. STAT5 was originally identified as a mammary gland factor induced by prolactin in lactating breast cells. Here we demonstrate that STAT5 is activated after IL-2 stimulation of two responsive lymphocyte cell lines, Nb2 and YT. Activation of STAT5 is measured both by IL-2-induced tyrosine phosphorylation and by IL-2-induced DNA binding. The STAT5 DNA recognition site is the same as the interferon gamma-activated site (GAS) in the interferon regulatory factor 1 gene. We demonstrate that the GAS element is necessary and sufficient for transcriptional induction by both IL-2 and prolactin in T lymphocytes. These results indicate that the role of STAT5 in the regulation of gene expression is not restricted to mammary cells or to prolactin, but is an integral part of the signal pathway of a critical immunomodulatory cytokine, IL-2.

Stat5 as a target for regulation by extracellular matrix

Transcription of tissue-specific genes in mammary gland requires signals from both prolactin and basement membrane. Here we address the mechanism by which this specialized extracellular matrix regulates transcription. Using mammary cell cultures derived from transgenic mice harboring the ovine beta-lactoglobulin gene, we show that either a basement membrane extract, or purified laminin-1, induced high levels of beta-lactoglobulin synthesis. It is known that prolactin signals through Stat5 (signal transducer and activator of transcription). This transcription factor interacts with gamma-interferon activation site-related motifs within the beta-lactoglobulin promoter, which we show are required for matrix dependence of beta-lactoglobulin expression. The DNA binding activity of Stat5 was present only in extracts of mammary cells cultured on basement membrane, indicating that the activation state of Stat5 is regulated by the type of substratum the cell encounters. Thus, basement membrane controls transcription of milk protein genes through the Stat5-mediated prolactin signaling pathway, providing a molecular explanation for previous studies implicating extracellular matrix in the control of mammary differentiation.

Activation of STAT factors by prolactin, interferon-gamma, growth hormones, and a tyrosine phosphatase inhibitor in rabbit primary mammary epithelial cells

In numerous studies on mammary epithelial cell lines multiple factors, added to the medium or contained in the serum, were required for casein gene expression. It has been shown in these systems that the mammary gland factor (MGF) is implicated in the activation of the beta-casein gene promoter. In the present study, we determined the relationship between known agents that affect casein gene expression and MGF activity using the properties of rabbit primary mammary epithelial cells to respond to PRL alone, when cultured in chemically defined medium. We demonstrate that MGF is rapidly activated by PRL alone or by human growth hormone, a natural ligand of many PRL receptors (PRL-Rs), in the cytoplasm and accumulated in the nucleus. The MGF activation by PRL occurred in the absence of endogenous extracellular matrix, a condition where casein synthesis is known to be markedly reduced. Different inhibitors of protein-tyrosine kinases, which have been shown to reduce casein mRNA synthesis, but not of protein kinase C, decrease the MGF activity. A tyrosine phosphatase inhibitor, sodium pervanadate, induced two GAS-binding complexes related to MGF and STAT1. Our data show that MGF is a latent cytoplasmic factor rapidly activated in mammary epithelial cells, by a mechanism involving a tyrosine kinase and a tyrosine phosphatase.

Interleukin 12 induces tyrosine phosphorylation and activation of STAT4 in human lymphocytes

Interleukin 12 (IL-12) is an important immunoregulatory cytokine whose receptor is a member of the hematopoietin receptor superfamily. We have recently demonstrated that stimulation of human T and natural killer cells with IL-12 induces tyrosine phosphorylation of the Janus family tyrosine kinase JAK2 and Tyk2, implicating these kinases in the immediate biochemical response to IL-12. Recently, transcription factors known as STATs (signal transducers and activators of transcription) have been shown to be tyrosine phosphorylated and activated in response to a number of cytokines that bind hematopoietin receptors and activate JAK kinases. In this report we demonstrate that IL-12 induces tyrosine phosphorylation of a recently identified STAT family member, STAT4, and show that STAT4 expression is regulated by T-cell activation. Furthermore, we show that IL-12 stimulates formation of a DNA-binding complex that recognizes a DNA sequence previously shown to bind STAT proteins and that this complex contains STAT4. These data, and the recent demonstration of JAK phosphorylation by IL-12, identify a rapid signal-transduction pathway likely to mediate IL-12-induced gene expression.

Interleukin-3, erythropoietin, and prolactin activate a STAT5-like factor in lymphoid cells

Interleukin-3 (IL-3)-, erythropoietin (EPO)-, and prolactin (PRL)-induced signal transduction via the JAK/STAT pathway was studied in the IL-3-dependent BAF-3 lymphoid cell line. Transfected cells expressing either the long form of the PRL receptor or the EPO receptor were used. We demonstrated that IL-3, EPO, and PRL activated a transcription factor related to the mammary transcription factor STAT5 but not to STAT1, -2, -3, or -4 as opposed to interferon gamma (IFN gamma) which activated STAT1 in the same cells. Similarly, PRL and EPO activated a STAT5-like factor (STAT5-L) in the rat Nb2 and the human UT7 cells expressing endogenous PRL and EPO receptors, respectively. The hematopoietic STAT5-L activated by IL-3, EPO, or PRL was identified as a 97-kDa tyrosine-phosphorylated protein. These results confer to STAT5 a much broader role than previously suggested.

The amino-terminal portion of the JAK2 protein kinase is necessary for binding and phosphorylation of the granulocyte-macrophage colony-stimulating factor receptor beta c chain

The binding of granulocyte-macrophage colony stimulating factor (GM-CSF) to its receptor stimulates JAK2 protein kinase activation, protein phosphorylation, and JAK2 association with the beta c chain of the GM-CSF receptor. To better understand how different domains of the JAK2 function to regulate association and phosphorylation of the beta c receptor, the minimal portion of the beta c receptor necessary for JAK2 binding has been determined. Using glutathione S-transferase (GST) fusion proteins expressing different portions of the membrane-proximal domain of the beta c chain, we demonstrate that JAK2 binds to amino acids 458-495, but showed little binding to fusion proteins containing amino acids 483-559, 483-530, or 458-484. The GST-beta c 458-495 bound equally well to the wild type (WT) JAK2, a carboxyl-terminal deletion of JAK2 removing the protein kinase domain (amino acids 1000-1129), and a deletion of the kinase-like domain (amino acids 523-746). However, an amino-terminal JAK2 deletion (amino acids 2-239) markedly reduced binding to this GST-beta c. Far Western blotting demonstrated that a GST fusion protein containing amino acids 1-294 of JAK2, but not fusion proteins containing amino acids 295-522, 523-746, or 747-1127, bound GST-beta c 458-559. When the JAK2 WT and deletions were transiently expressed along with the alpha and beta c subunits of the GM-CSF receptor and the cells were treated with GM-CSF, the following results were obtained: 1) WT JAK2 phosphorylated the beta c subunit in a GM-CSF-dependent manner, 2) the kinase-like domain deletion phosphorylated the beta c subunit, and 3) both the kinase domain deletion and the amino-terminal deletion failed to stimulate phosphorylation of the beta c subunit. Therefore, phosphorylation of the beta c subunit requires the binding of JAK2 through its amino terminus.