Prolactin activates Ras via signaling proteins SHC, growth factor receptor bound 2, and son of sevenless

The Human Intermediate Prolactin Receptor I-tail Contributes Breast Oncogenesis by Targeting Ras/MAPK Pathway

Prolactin and its receptor (PRLr) in humans are significantly involved in breast cancer pathogenesis. The intermediate form of human PRLr (hPRLrI) is produced by alternative splicing and has a novel 13 amino acid tail ("I-tail") gain. hPRLrI induces significant proliferation and anchorage-independent growth of normal mammary epithelia in vitro when coexpressed with the long form hPRLr (hPRLrL). hPRLrL and hPRLrI coexpression is necessary to induce the transformation of mammary epithelia in vivo. The I-tail is associated with the ubiquitin-like protein neural precursor cell expressed developmentally downregulated protein 8. Treatment with the neural precursor cell expressed developmentally downregulated protein 8-activating enzyme inhibitor pevonedistat resulted in increased hPRLrL and the death of breast cancer cells. The goal of this study was to determine the function of the hPRLrI I-tail in hPRLrL/hPRLrI-mediated mammary transformation. hPRLrL/hPRLrI and hPRLrL/hPRLrIΔ13 (I-tail removal mutant) were delivered to MCF10AT cells. Cell proliferation was decreased when hPRLrI I-tail was removed. I-tail deletion decreased anchorage-independent growth and attenuated cell migration. The I-tail was involved in Ras/MAPK signaling but not PI3K/Akt signaling pathway as shown by western blot. I-tail removal resulted in decreased hPRLrI stability. RNA-sequencing data revealed that I-tail removal resulted in differential gene expression induced by prolactin. Ingenuity Pathway Analysis revealed that the activity of ERK was attenuated. Treatment of breast cancer cells with ERK1/2 inhibitor ulixertinib resulted in decreased colony-forming ability and less proliferation. These studies suggest that the hPRLrI I-tail contributed to breast oncogenesis and may be a promising target for the development of new breast cancer therapies.

Breast Cancer and Prolactin - New Mechanisms and Models

The pathogenesis of breast cancer is driven by multiple hormones and growth factors. One of these, prolactin (PRL), contributes to both mammary differentiation and oncogenesis, and yet the basis for these disparate effects has remained unclear. The focus of this review is to examine and place into context 2 recent studies that have provided insight into the roles of PRL receptors and PRL in tumorigenesis and tumor progression. One study provides novel evidence for opposing actions of PRL in the breast being mediated in part by differential PRL receptor (PRLr) isoform utilization. Briefly, homomeric complexes of the long isoform of the PRLr (PRLrL-PRLrL) promotes mammary differentiation, while heteromeric complexes of the intermediate and long PRLr (PRLrI-PRLrL) isoforms trigger mammary oncogenesis. Another study describes an immunodeficient, prolactin-humanized mouse model, NSG-Pro, that facilitates growth of PRL receptor-expressing patient-derived breast cancer xenografts. Evidence obtained with this model supports the interactions of physiological levels of PRL with estrogen and ERBB2 gene networks, the modulatory effects of PRL on drug responsiveness, and the pro-metastatic effects of PRL on breast cancer. This recent progress provides novel concepts, mechanisms and experimental models expected to renew interest in harnessing/exploiting PRLr signaling for therapeutic effects in breast cancer.

A Spontaneous Aggressive ERα+ Mammary Tumor Model Is Driven by Kras Activation

The NRL-PRL murine model, defined by mammary-selective transgenic rat prolactin ligand rPrl expression, establishes spontaneous ER+ mammary tumors in nulliparous females, mimicking the association between elevated prolactin (PRL) and risk for development of ER+ breast cancer in postmenopausal women. Whole-genome and exome sequencing in a discovery cohort (n = 5) of end-stage tumors revealed canonical activating mutations and copy number amplifications of Kras. The frequent mutations in this pathway were validated in an extension cohort, identifying activating Ras alterations in 79% of tumors (23 of 29). Transcriptome analyses over the course of oncogenesis revealed marked alterations associated with Ras activity in established tumors compared with preneoplastic tissues; in cell-intrinsic processes associated with mitosis, cell adhesion, and invasion; as well as in the surrounding tumor environment. These genomic analyses suggest that PRL induces a selective bottleneck for spontaneous Ras-driven tumors that may model a subset of aggressive clinical ER+ breast cancers.

Changes in prolactin receptor homodimer availability may cause late feathering in chickens

Chicken early (EF) and late feathering (LF) are sex-linked phenotypes conferred by wild-type k⁺ and dominant K alleles on chromosome Z, respectively. Besides prolactin (PRL) receptor (PRLR) and sperm flagellar 2 (SPEF2) genes, the K allele contains a fusion gene in which partially duplicated PRLR (dPRLR) and SPEF2 (dSPEF2) genes are linked in a tail-to-tail manner. The causative dPRLR gene encodes a C-terminal truncated receptor. LF chickens have short or no primaries at hatching; however, their feather growth rate is higher than that of EF chickens. This study aimed to elucidate the molecular basis of the K allele's biphasic effect on feather development. By 3'RACE and RT-PCR analyses, we demonstrated that dSPEF2 gene transcription occurred beyond all coding exons of the dPRLR gene on the opposite strand and that dPRLR mRNA was less abundant than PRLR mRNA. In addition, a 5'UTR splice variant (SPV) of PRL receptor mRNAs was increased in LF chickens. In vitro expression analysis of 5'UTR linked to the luciferase reporter gene revealed higher translation efficiency of SPV. RT-qPCR showed that the dPRLR mRNA level was higher in embryos; conversely, SPV was higher in hatched chickens, as was dSPEF2 mRNA. These findings suggest that the K allele inhibits feather development at the fetal stage by expressing dPRLR to attenuate PRLR function and promotes feather growth after hatching by increasing PRLR through dSPEF2 mRNA expression. Increased SPV may cause greater feather growth than that in EF chickens by increasing the availability of PRLR homodimers and enhancing PRL signaling.

Crosstalk between STAT5 activation and PI3K/AKT functions in normal and transformed mammary epithelial cells

Janus kinases (JAKs) and signal transducers and activators of transcription (STATs) have been shown to function downstream of several peptide hormones and cytokines that are required for postnatal development and secretory function of the mammary gland. As part of an extended network, these signal transducers can engage in crosstalk with other pathways to facilitate synergistic, and sometimes antagonistic, actions of different growth factors. Specifically, signaling through the JAK2/STAT5 cascade has been demonstrated to be indispensable for the specification, proliferation, differentiation, and survival of secretory mammary epithelial cells. Following a concise description of major cellular programs in mammary gland development and the role of growth factors that rely on JAK/STAT signaling to orchestrate these programs, this review highlights the significance of active STAT5 and its crosstalk with the PI3 kinase and AKT1 for mediating the proliferation of alveolar progenitors and survival of their functionally differentiated descendants in the mammary gland. Based on its ability to provide self-sufficiency in growth signals that are also capable of overriding intrinsic cell death programs, persistently active STAT5 can serve as a potent oncoprotein that contributes to the genesis of breast cancer. Recent experimental evidence demonstrated that, similar to normal developmental programs, oncogenic functions of STAT5 rely on molecular crosstalk with PI3K/AKT signaling for the initiation, and in some instances the progression, of breast cancer. The multitude by which STATs can interact with individual mediators of the PI3K/AKT signaling cascade may provide novel avenues for targeting signaling nodes within molecular networks that are crucial for the survival of cancer cells.

Life stage differences in mammary gland gene expression profile in non-human primates

Breast cancer (BC) is the most common malignancy of women in the developed world. To better understand its pathogenesis, knowledge of normal breast development is crucial, as BC is the result of disregulation of physiologic processes. The aim of this study was to investigate the impact of reproductive life stages on the transcriptional profile of the mammary gland in a primate model. Comparative transcriptomic analyses were carried out using breast tissues from 28 female cynomolgus macaques (Macaca fascicularis) at the following life stages: prepubertal (n = 5), adolescent (n = 4), adult luteal (n = 5), pregnant (n = 6), lactating (n = 3), and postmenopausal (n = 5). Mammary gland RNA was hybridized to Affymetrix GeneChip(®) Rhesus Macaque Genome Arrays. Differential gene expression was analyzed using ANOVA and cluster analysis. Hierarchical cluster analysis revealed distinct separation of life stage groups. More than 2,225 differentially expressed mRNAs were identified. Gene families or pathways that changed across life stages included those related to estrogen and androgen (ESR1, PGR, TFF1, GREB1, AR, 17HSDB2, 17HSDB7, STS, HSD11B1, AKR1C4), prolactin (PRLR, ELF5, STAT5, CSN1S1), insulin-like growth factor signaling (IGF1, IGFBP1, IGFBP5), extracellular matrix (POSTN, TGFB1, COL5A2, COL12A1, FOXC1, LAMC1, PDGFRA, TGFB2), and differentiation (CD24, CD29, CD44, CD61, ALDH1, BRCA1, FOXA1, POSTN, DICER1, LIG4, KLF4, NOTCH2, RIF1, BMPR1A, TGFB2). Pregnancy and lactation displayed distinct patterns of gene expression. ESR1 and IGF1 were significantly higher in the adolescent compared to the adult animals, whereas differentiation pathways were overrepresented in adult animals and pregnancy-associated life stages. Few individual genes were distinctly different in postmenopausal animals. Our data demonstrate characteristic patterns of gene expression during breast development. Several of the pathways activated during pubertal development have been implicated in cancer development and metastasis, supporting the idea that other developmental markers may have application as biomarkers for BC.

Prolactin regulation of the prostate gland: a female player in a male game

Prolactin is best known for its actions on the mammary gland. However, circulating prolactin is also detected in males and its receptor (PRLR) is expressed in the prostate, suggesting that the prostate is a target of prolactin. Germline knockout of prolactin or its receptor has failed to reveal a key role for prolactin signaling in mouse prostate physiology. However, several studies involving rodent models and human prostate cell lines and specimens have supported the contribution of the canonical PRLR-Jak2-Stat5a/b pathway to prostate cancer tumorigenesis and progression. Increased expression of prolactin in the prostate itself (rather than changes in circulating prolactin levels) and crosstalk with androgen receptor (AR) signaling are potential mechanisms for increased Stat5a/b signaling in prostate cancer. In the mouse prostate, prolactin overexpression results in disorganized expansion of the basal/stem cell compartment, which has been proposed to house putative prostate tumor-initiating cells. These findings provide new insight into the molecular and cellular targets by which locally produced prolactin could contribute to prostate cancer initiation and progression. A number of pharmacological inhibitors targeting various levels of the PRLR-Jak2-Stat5a/b pathway have been developed and are entering clinical trials for advanced prostate cancer.

Prolactin-stimulated activation of ERK1/2 mitogen-activated protein kinases is controlled by PI3-kinase/Rac/PAK signaling pathway in breast cancer cells

There is strong evidence that deregulation of prolactin (PRL) signaling contributes to pathogenesis and chemoresistance of breast cancer. Therefore, understanding cross-talk between distinct signal transduction pathways triggered by activation of the prolactin receptor (PRL-R), is essential for elucidating the pathogenesis of metastatic breast cancer. In this study, we applied a sequential inhibitory analysis of various signaling intermediates to examine the hierarchy of protein interactions within the PRL signaling network and to evaluate the relative contributions of multiple signaling branches downstream of PRL-R to the activation of the extracellular signal-regulated kinases ERK1 and ERK2 in T47D and MCF-7 human breast cancer cells. Quantitative measurements of the phosphorylation/activation patterns of proteins showed that PRL simultaneously activated Src family kinases (SFKs) and the JAK/STAT, phosphoinositide-3 (PI3)-kinase/Akt and MAPK signaling pathways. The specific blockade or siRNA-mediated suppression of SFK/FAK, JAK2/STAT5, PI3-kinase/PDK1/Akt, Rac/PAK or Ras regulatory circuits revealed that (1) the PI3-kinase/Akt pathway is required for activation of the MAPK/ERK signaling cascade upon PRL stimulation; (2) PI3-kinase-mediated activation of the c-Raf-MEK1/2-ERK1/2 cascade occurs independent of signaling dowstream of STATs, Akt and PKC, but requires JAK2, SFKs and FAK activities; (3) activated PRL-R mainly utilizes the PI3-kinase-dependent Rac/PAK pathway rather than the canonical Shc/Grb2/SOS/Ras route to initiate and sustain ERK1/2 signaling. By interconnecting diverse signaling pathways PLR may enhance proliferation, survival, migration and invasiveness of breast cancer cells.

Morphologic transformation of human breast epithelial cells MCF-10A: dependence on an oxidative microenvironment and estrogen/epidermal growth factor receptors

Background

MCF-10A, immortalized but non-transformed human breast epithelial cells, are widely used in research examining carcinogenesis. The studies presented here were initiated with the observation that MCF-10A cells left in continuous culture for prolonged periods without re-feeding were prone to the development of transformed foci. We hypothesized that the depletion of labile culture components led to the onset of processes culminating in the observed cell transformation. The purpose of this study was to define the factors which promoted transformation of this cell line.

Results

Changes in levels of phenol red (PHR), hydrocortisone (HC), and epidermal growth factor (EGF) with or without estrogen treatment indicated that both oxidative stress- and estrogen receptor alpha (ERα)-mediated pathways contribute to cell transformation. Gene array and Western blotting analyses of cells maintained in our laboratory and of those from other sources documented detectable ERα and ERbeta (ERβ) in this ERα-negative cataloged cell line. Results also indicate the possibility of a direct association of EGF receptor (EGFR) and ERα in these cells as well as the formation and high induction of a novel ternary complex that includes ERβ (ERα/ERβ/EGFR) in cells grown under conditions facilitating transformation.

Conclusions

Our studies resulted in the development of a growth protocol where the effects of chronic, physiologically relevant alterations in the microenvironment on cellular transformation were examined. From our results, we were able to propose a model of transformation within the MCF-10A cell line in which oxidative stress, ER and EGFR play essential roles. Overall, our work indicates that the immediate microenvironment of cells exerts powerful growth cues which ultimately determine their transformation potential.

PIKE-A is required for prolactin-mediated STAT5a activation in mammary gland development

PI 3-kinase enhancer A (PIKE-A) is critical for the activation of Akt signalling, and has an essential function in promoting cancer cell survival. However, its physiological functions are poorly understood. Here, we show that PIKE-A directly associates with both signal transducer and activator of transcription 5a (STAT5a) and prolactin (PRL) receptor, which is essential for PRL-provoked STAT5a activation and the subsequent gene transcription. Depletion of PIKE-A in HC11 epithelial cells diminished PRL-induced STAT5 activation and cyclin D1 expression, resulting in profoundly impaired cell proliferation in vitro. To confirm the function of PIKE-A in PRL signalling in vivo, we generated PIKE knockout (PIKE-/-) mice. PIKE-/- mice displayed a severe lactation defect that was characterized by enhanced apoptosis and impaired proliferation of mammary epithelial cells. At parturition, STAT5 activation and cyclin D1 expression were substantially reduced in the mammary epithelium of PIKE-/- mice. The defective mammary gland development in PIKE-/- mice was rescued by overexpression of a mammary-specific cyclin D1 transgene. These data establish a critical function for PIKE-A in mediating PRL functions.

Prolactin and estradiol utilize distinct mechanisms to increase serine-118 phosphorylation and decrease levels of estrogen receptor alpha in T47D breast cancer cells

Potential interactions between prolactin (PRL) and estradiol (E2) in breast cancer cells were explored by examining the effect of PRL on estrogen receptor (ER) serine-118 phosphorylation, ER down-regulation, and E2-stimulated cell proliferation. Both E2 and PRL resulted in prolonged ERalpha serine-118 phosphorylation, but used different signaling pathways to achieve this end. Both hormones also decreased the amount of ERalpha, but the mechanisms were different: for E2, the decrease was rapid and resulted from proteasomic degradation, whereas for PRL the decrease was slow and resulted from an effect on levels of ERalpha mRNA. PRL alone had no effect on cell number, but enhanced the increase in number in response to E2. These results are the first to demonstrate similar effects of PRL and E2 on parameters considered key to E2's effects. This suggests heretofore unrecognized and potentially important interactions between these two hormones in the natural history of breast cancer.

Prolactin activates mitogen-activated protein kinase signaling and corticotropin releasing hormone transcription in rat hypothalamic neurons

Prolactin (PRL) modulates maternal behavior and mediates hypothalamic pituitary adrenal axis inhibition during lactation via PRL receptors in the brain. To identify mechanisms mediating these effects, we examined the effects of PRL on signaling and CRH transcription in hypothalamic neurons in vivo and in vitro. Western blot of hypothalamic proteins from rats receiving intracerebroventricular PRL injection revealed increases in phosphorylation of the MAPK and ERK. Double-staining immunohistochemistry demonstrated phosphorylated ERK localization in parvocellular CRH neurons as well as magnocellular vasopressin and oxytocin neurons of the hypothalamic paraventricular (PVN) and supraoptic nuclei. PRL also induced ERK phosphorylation in vitro in the hypothalamic cell line, 4B, which expresses PRL receptors, and in primary hypothalamic neuronal cultures. Using reporter gene assays in 4B cells, or quantitative RT-PCR for primary transcript in hypothalamic cell cultures, PRL potentiated forskolin-stimulated CRH transcription through activation of the ERK/MAPK pathway. The effect of PRL in hypothalamic cell cultures was unaffected by tetrodotoxin, suggesting a direct effect on CRH neurons. The data show that PRL activates the ERK/MAPK pathway and facilitates CRH transcription in CRH neurons, suggesting that the inhibitory effect of PRL on hypothalamo-pituitary-adrenal axis activity reported in vivo is indirect and probably mediated through modulation of afferent pathways to the PVN. In addition, the prominent stimulatory action of PRL on the ERK/MAPK pathway in the hypothalamic PVN and supraoptic nucleus is likely to mediate neuroplasticity of the neuroendocrine system during lactation.

Spatial signaling networks converge at the adaptor protein Shc

Endothelial cells, which are located at the interface between the blood and the vessel wall, respond dynamically to a variety of stimuli initiating signaling cascades that regulate cardiovascular development, physiology and pathology. These inputs include soluble factors that bind to their receptors, integrin-matrix interactions, cell-cell contacts and mechanical forces due to the flowing blood. While these stimuli can mediate unique downstream signals, it is well-accepted that signaling pathways are highly interwoven into complex signaling networks with several levels of cross-talk, integration and coordination. Recent studies suggest that several signaling networks coalesce at the adaptor protein Shc.

Prolactin regulation of mammary gland development

Mammary morphogenesis is orchestrated with other reproductive events by pituitary-driven changes to the systemic hormone environment, initiating the formation of a mammary ductal network during puberty and the addition of secretory alveoli during pregnancy. Prolactin is the major driver of development during pregnancy via regulation of ovarian progesterone production (in many species) and direct effects on mammary epithelial cells (in all species). Together these hormones regulate two aspects of development that are the subject of intense interest: (1) a genomic regulatory network that integrates many additional spatial and temporal cues to control gene expression and (2), the activity of a stem and progenitor cell hierarchy. Amalgamation of these two aspects will increase our understanding of cell proliferation and differentiation within the mammary gland, with clear application to our attempts to control breast cancer. Here we focus on providing an over-view of prolactin action during development of the model murine mammary gland.

Jak2/Stat5 signaling in mammogenesis, breast cancer initiation and progression

During normal mammary gland development, the tyrosine kinase Jak2 and its main substrate, the signal transducer and activator of transcription-5 (Stat5), are critical for the growth and differentiation of alveolar progenitors as well as the survival of secretory mammary epithelial cells. Genetic studies in mouse models support a role for the Stat5 transcription factor as a proto-oncogene in mammary tumor initiation. On the other hand, the analysis of nuclear Stat5 in human breast malignancies suggests a role of the Jak2/Stat5 pathway in the restriction of the metastatic potential of neoplastic mammary epithelial cells. Following an overview on the function of the Jak2/Stat5 pathway during normal mammary gland development, this review discusses recently published observations on human breast cancers as well as experimental evidence from genetically engineered mice that propose a dual role of Jak2/Stat5 signaling in breast cancer initiation and progression. Future studies to further test the concept of contrasting effects of Jak2/Stat5 pathway on breast cancer initiation and metastatic progression are proposed.

The Janus Kinase 2 Is Required for Expression and Nuclear Accumulation of Cyclin D1 in Proliferating Mammary Epithelial Cells

Using a conditional knockout approach, we previously demonstrated that the Janus kinase 2 (Jak2) is crucial for prolactin (PRL) signaling and normal mammary gland development. PRL is suggested to synchronously activate multiple signaling cascades that emerge on the PRL receptor (PRLR). This study demonstrates that Jak2 is essential for the activation of the signal transducer and activator of transcription 5 (Stat5) and expression of Cish (cytokine-inducible SH2-containing protein), a Stat5-responsive negative regulator of Jak/Stat signaling. However, Jak2 is dispensable for the PRL-induced activation of c-Src, focal adhesion kinase, and the MAPK pathway. Despite activation of these kinases that are commonly associated with proliferative responses, the ablation of Jak2 reduces the multiplication of immortalized mammary epithelial cells (MECs). Our studies show that signaling through Jak2 controls not only the transcriptional activation of the Cyclin D1 gene, but, more importantly, it regulates the accumulation of the Cyclin D1 protein in the nucleus by altering the activity of signal transducers that mediate the phosphorylation and subsequent nuclear export of Cyclin D1. In particular, the levels of activated Akt (protein kinase B) and inactive glycogen synthase kinase-3β (i.e. a kinase that regulates the nuclear export and degradation of Cyclin D1) are reduced in MECs lacking Jak2. The proliferation of Jak2-deficient MECs can be rescued by expressing of a mutant form of Cyclin D1 that cannot be phosphorylated by glycogen synthase kinase-3β and therefore constitutively resides in the nucleus. Besides discriminating Jak2-dependent and Jak2-independent signaling events emerging from the PRLR, our observations provide a possible mechanism for phenotypic similarities between Cyclin D1 knockouts and females lacking individual members of the PRLR signaling cascade, in particular the PRLR, Jak2, and Stat5.

Ligand-Independent Dimerization of the Human Prolactin Receptor Isoforms: Functional Implications

Prolactin (PRL) contributes to the growth of normal and malignant breast tissues. PRL initiates signaling by engaging the PRL receptor (PRLr), a transmembrane (TM) receptor belonging to the cytokine receptor family. The accepted view has been that PRL activates the PRLr by inducing dimerization of the receptor, but recent reports show ligand-independent dimerization of other cytokine receptors. Using coimmunoprecipitation assays, we have confirmed ligand-independent dimerization of the PRLr in T47D breast cancer and HepG2 liver carcinoma cells. In addition, mammalian cells transfected with differentially epitope-tagged isoforms of the PRLr indicated that long, intermediate, and DeltaS1 PRLrs dimerized in a ligand-independent manner. To determine the domain(s) involved in PRLr ligand-independent dimerization, we generated PRLr constructs as follows: (1) the TM-ICD, which consisted of the TM domain and the intracellular domain (ICD) but lacked the extracellular domain (ECD), and (2) the ECD-TM, which consisted of the TM domain and the ECD but lacked the ICD. These constructs dimerized in a ligand-independent manner in mammalian cells, implicating a significant role for the TM domain in this process. These truncated PRLrs were functionally inert alone or in combination in cells lacking the PRLr. However, when introduced into cells containing endogenous PRLr, the ECD-TM inhibited human PRLr signaling, whereas the TM-ICD potentiated human PRLr signaling. These studies indicate that the ECD-TM and the TM-ICD are capable of modulating PRLr function. We also demonstrated an endogenous TM-ICD in T47D cells, suggesting that these findings are relevant to PRL-signaling pathways in breast cancer.

Prolactin signalling in porcine theca cells: the involvement of protein kinases and phosphatases

The hypothesis that protein kinase C (PKC) and tyrosine kinases, as well as serine-threonine and tyrosine phosphatases, are involved in prolactin (PRL) signalling in theca cells harvested from porcine follicles was tested. Theca cells were incubated with PRL for 24 h to stimulate progesterone (P4) production. In addition, treatments included inhibitors of PKC and tyrosine kinases, as well as serine-threonine phosphatase inhibitor and tyrosine phosphatase inhibitor. Prolactin significantly stimulated P4 production by theca cells and all inhibitors suppressed the PRL-stimulated P4 production. After incubation with PRL for 2, 5, 10 or 20 min, theca cells were homogenized and cytosolic and membrane fractions were obtained. This was followed by determination of PKC activity in partially purified subcellular fractions by measuring the transfer of 32P from [gamma-32P] adenosine triphosphatase (ATP) to histone III-S. In unstimulated porcine theca cells the major proportion of PKC activity was present in the cytosol. Incubation of cells with PRL resulted in a rapid, time-dependent increase in the amount of PKC activity in the membrane fraction. Protein kinase C activity in the membrane fraction was maximal after 10 min of cells' exposure to PRL. Protein kinase C activation was assessed also by measuring the specific association of 3H-phorbol dibutyrate (3H-PDBu) with theca cells after treatment with PRL. Prolactin significantly increased 3H-PDBu-specific binding in theca cells. In contrast to PKC, total inositol phosphate accumulation was not affected by PRL in the current study. In summary, PRL stimulated P4 production by porcine theca cells derived from large follicles. The results of the study were consistent with the hypothesis that PKC is one of the intracellular mediators of PRL action in porcine theca cells. Protein kinase C activation does not appear to occur through the action of phosphatidylinositol-dependent phospholipase C. Moreover, the involvement of tyrosine kinases, as well as tyrosine and serine-threonine phosphatases, in PRL signalling in the examined cells is suggested.

Stat5 promotes homotypic adhesion and inhibits invasive characteristics of human breast cancer cells

Signal transducer and activator of transcription-5 (Stat5) mediates prolactin (PRL)-induced differentiation and growth of breast epithelial cells. We have recently identified active Stat5 as a tumor marker of favorable prognosis in human breast cancer, and determined that Stat5 activation is lost during metastatic progression. Here we provide novel evidence for an invasion-suppressive role of Stat5 in human breast cancer. Activation of Stat5 by PRL in human breast cancer lines was associated with increased surface levels of the invasion-suppressive adhesion molecule E-cadherin in vitro and in xenotransplant tumors in vivo. Inducible E-cadherin was blocked by dominant-negative (Dn) Stat5 or Dn-Jak2, but not by Dn-Stat3. Further experimental data indicated a role of Stat5 as a coordinate regulator of additional invasion-related characteristics of human breast cancer cells, including cell surface association of beta-catenin, homotypic cell clustering, invasion through Matrigel, cell migration, and matrix metalloproteinase activity. A role of Stat5 as a suppressor of breast cancer invasion and metastatic progression provides a biological mechanism to explain the favorable prognosis associated with active Stat5 in human breast cancer.

Impaired alveologenesis and maintenance of secretory mammary epithelial cells in Jak2 conditional knockout mice

Jak2 is a hormone-receptor-coupled kinase that mediates the tyrosine phosphorylation and activation of signal transducers and activators of transcription (Stat). The biological relevance of Jak2-Stat signaling in hormone-responsive adult tissues is difficult to investigate since Jak2 deficiency leads to embryonic lethality. We generated Jak2 conditional knockout mice to study essential functions of Jak2 during mammary gland development. The mouse mammary tumor virus-Cre-mediated excision of the first coding exon resulted in a Jak2 null mutation that uncouples signaling from the prolactin receptor (PRL-R) to its downstream mediator Stat5 in the presence of normal and supraphysiological levels of PRL. Jak2-deficient females were unable to lactate as a result of impaired alveologenesis. Unlike Stat5a knockouts, multiple gestation cycles could not reverse the Jak2-deficient phenotype, suggesting that neither other components of the PRL-R signaling cascade nor other growth factors and their signal transducers were able to compensate for the loss of Jak2 function to activate Stat5 in vivo. A comparative analysis of Jak2-deficient mammary glands with transplants from Stat5a/b knockouts revealed that Jak2 deficiency also impairs the pregnancy-induced branching morphogenesis. Jak2 conditional mutants therefore resemble PRL-R knockouts more closely, which suggested that Jak2 deficiency might affect additional PRL-R downstream mediators other than Stat5a and Stat5b. To address whether Jak2 is required for the maintenance of PRL-responsive, differentiating alveolar cells, we utilized a transgenic strain that expresses Cre recombinase under regulatory elements of the whey acidic protein gene (Wap). The Wap-Cre-mediated excision of Jak2 resulted in a negative selection of differentiated alveolar cells, suggesting that Jak2 is required not only for the proliferation and differentiation of alveolar cells but also for their maintenance during lactation.

Src mediates prolactin-dependent proliferation of T47D and MCF7 cells via the activation of focal adhesion kinase/Erk1/2 and phosphatidylinositol 3-kinase pathways

Prolactin (PRL) stimulates breast cancer cell proliferation; however, the involvement of PRL-activated signaling molecules in cell proliferation is not fully established. Here we studied the role of c-Src on PRL-stimulated proliferation of T47D and MCF7 breast cancer cells. We initially observed that PRL-dependent activation of focal adhesion kinase (Fak), Erk1/2, and cell proliferation was mediated by c-Src in T47D cells, because expression of a dominant-negative form of c-Src (SrcDM, K295A/Y527F) blocked the PRL-dependent effects. The Src inhibitor PP1 abrogated PRL-dependent in vivo activation of Fak, Erk1/2, p70S6K, and Akt and the proliferation of T47D and MCF7 cells; Janus kinase 2 (Jak2) activation was not affected. However, in vitro, Fak and Jak2 kinases were not directly inhibited by PP1, demonstrating the effect of PP1 on c-Src kinase as an upstream activator of Fak. Expression of Fak mutant Y397F abrogated PRL-dependent activation of Fak, Erk1/2, and thymidine incorporation, but had no effect on p70S6K and Akt kinases. MAPK kinase 1/2 (Mek1/2) inhibitor PD184352 blocked PRL-induced stimulation of Erk1/2 and cell proliferation; however, p70S6K and Akt activation were unaffected. The phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 abolished cell proliferation and activation of p70S6K and Akt; however, PRL-dependent activation of Erk1/2 was not modified. Moreover, we show that both c-Src/PI3K and c-Src/Fak/Erk1/2 pathways are involved in the up-regulation of c-myc and cyclin d1 expression mediated by PRL. The previous findings suggest the existence of two PRL-dependent signaling cascades, initiated by the c-Src-mediated activation of Fak/Erk1/2 and PI3K pathways that, subsequently, control the expression of c-Myc and cyclin D1 and the proliferation of T47D and MCF7 breast cancer cells.

Mechanisms of corticosteroid resistance in rheumatoid arthritis: a putative role for the corticosteroid receptor beta isoform

Corticosteroids (CSs) have potent immunosuppressive effects and are commonly used to treat a range of immunological and inflammatory diseases such as rheumatoid arthritis (RA). These effects are mediated by the ability of CSs to modulate gene expression. CSs act by binding to the CS receptor (CR), which exists as alpha and beta isoforms. Only CRalpha binds CS. CRbeta functions as an endogenous inhibitor of CS and is expressed in several tissues. The CS/CRalpha complex binds to the glucocorticosteroid response element in the nucleus and also interferes with AP-1 and NF-kappaB binding. Thus, CSs inhibit the transcription of AP-1 and NF-kappaB inducible genes, such as interleukin (IL)-2, IL-6, IL-8, IL-1beta, and tumor necrosis factor (TNF) alpha, as well as T-cell proliferation. In clinical practice, a proportion of RA patients do not respond adequately to CS therapy. On this basis, RA patients can be divided on clinical grounds and on the ability of CSs to inhibit concanavalin A (conA)-induced peripheral blood T-cell proliferation in vitro into CS-sensitive (SS) and CS-resistant (SR) subgroups. The in vitro defined SS and SR subgroups correlate with the clinical responses to CS therapy. The mechanisms of the SR in RA patients remain unknown but may include the following: dysregulation of CRalpha function, alterations in the intracellular signaling mechanisms and/or utilization of various other cellular activation pathways, perturbations of the cytokine milieu, and inhibition of lipocortin. In SR subjects, CSs fail to significantly inhibit conA-induced IL-2 and IL-4 secretion and LPS-induced IL-8, IL-1beta secretion in vitro. CS therapy fails to reduce the circulating levels of IL-8, IL-1beta, and TNFalpha in SR RA patients. Peripheral blood mononuclear cells (PBMCs) from SR significantly overexpress activated NF-kappaB and IkappaBalpha. In vitro CSs fail to significantly inhibit conA-induced NF-kappaB activation in PBMCs from SR RA patients. Our preliminary observations show enhanced CRbeta expression by PBMCs from SR RA patients. It is most likely that other molecular mechanisms such as enhanced AP-1 expression are involved, and we currently are investigating such possibilities.

PRL signal transduction in the epithelial compartment of rat prostate maintained as long-term organ cultures in vitro

Using long-term organ cultures of rat prostate tissue explants, we previously demonstrated that PRL both stimulates proliferation and acts as an androgen-independent suppressor of apoptosis in prostate epithelial cells, leading to epithelial hyperplasia. In this work we delineate intracellular signaling molecules activated by PRL in prostate tissue to identify candidate signaling proteins that are responsible for maintaining survival and proliferation of prostate epithelium in androgen-deprived growth environment. We now show that signal transducer and activator of transcription-5a (Stat5a) and Stat5b become tyrosine phosphorylated in response to PRL stimulation in rat prostate using prostate organ culture as an experimental model. Stat5 was translocated to the nuclei of epithelial cells of prostate tissue as demonstrated by immunohistochemistry. Furthermore, EMSA showed PRL-inducible binding of Stat5a homodimers and Stat5a/5b heterodimers to the PRL response element of the beta-casein gene promoter. Signaling molecules Stat3, Stat1, MAPK, or protein kinase B, which can be activated by PRL in other target cells, were not activated by PRL in prostate tissue. Furthermore, we show that Stat5a and Stat5b are continuously phosphorylated in rat prostate in vivo, although they are expressed to varying degree in separate lobes of rat prostate. Collectively, our results suggest that PRL signaling in rat prostate tissue is primarily transduced via Stat5a and Stat5b. The Stat5 pathway represents one candidate signaling mechanism, used by PRL and possibly other growth factors and cytokines, that supports the viability of prostate epithelial cells during long-term androgen deprivation.

Signaling via Shc family adapter proteins

The adapter protein Shc was initially identified as an SH2 containing proto-oncogene involved in growth factor signaling. Since then a number of studies in multiple systems have implicated a role for Shc in signaling via many different types of receptors, such as growth factor receptors, antigen receptors, cytokine receptors, G-protein coupled receptors, hormone receptors and integrins. In addition to the ubiquitous ShcA, two other shc gene products, ShcB and ShcC, which are predominantly expressed in neuronal cells, have also been identified. ShcA knockout mice are embryonic lethal and have clearly suggested an important role for ShcA in vivo. Based on dominant negative studies and mouse embryos deficient in ShcA, a clear role for Shc in leading to mitogen activated protein kinase (MAPK) activation has been established. However MAPK activation may not be the sole function of Shc proteins. Although Shc has also been linked to other signaling events such as c-Myc activation and cell survival, the mechanistic understanding of these signaling events remains poorly characterized. Given the apparently central role that Shc plays signaling via many receptors, delineating the precise mechanism(s) of Shc-mediated signaling may be critical to our understanding of the effects mediated through these receptors.

Prolactin induction of insulin gene transcription: roles of glucose and signal transducer and activator of transcription 5

GH and PRL stimulate insulin production in pancreatic beta-cells through induction of insulin gene transcription. The transcriptional effects of GH are mediated through the binding of signal transducer and activator of transcription-5 (STAT5) to a consensus recognition sequence (TTCnnnGAA) in the rat insulin-1 promoter. In this study we demonstrate that PRL also induces the binding of STAT5 proteins to the rat insulin-1 STAT5 motif. However, the magnitude of binding of STAT5 nuclear proteins, as assessed by electrophoretic mobility shift assays, was only 1/30th that of the binding of the same STAT5 proteins to the beta-casein STAT5 site. The differences in the affinities of the rat insulin-1 and beta-casein STAT5 motifs are explained in part by differences in promoter sequences flanking the STAT5 sites. To assess the importance of the STAT motif in PRL induction of insulin gene transcription, we deleted the STAT5 consensus sequence in the rat insulin 1 promoter, cloned the truncated promoter upstream of the luciferase reporter gene, and transfected the construct into rat insulinoma (INS-1) cells. The transcriptional activity of this construct was compared with that of the wild-type promoter. Although deletion of the STAT5 site in the promoter reduced the basal luciferase activity, the response to PRL was unaffected. PRL also induced transcription of constructs containing the wild-type human insulin promoter or the rat insulin-2 promoter, which contain no classic STAT5 sequences. The transcriptional effect of PRL was manifest even when cells were incubated in glucose-free medium, indicating that the action of the hormone is not mediated solely through changes in glucose uptake or glucose metabolism. To identify PRL-responsive regions of the rat and human insulin promoters, we constructed a series of promoter truncations and assessed their responsiveness to PRL. A PRL-responsive region of the rat insulin-1 promoter was localized between nucleotides -165 and -109. A PRL-responsive region of the human insulin promoter was localized between nucleotides -346 and -250. Additional regions of the human and rat insulin-1 promoters were required for PRL induction of a heterologous, minimal thymidine kinase promoter, suggesting that there are multiple PRL-responsive elements in the insulin genes. These observations suggest a glucose- and STAT5-independent pathway by which PRL may induce insulin gene transcription.

Prolactin regulation of apoptosis-associated gene expression in T cells

Evidence accumulated over the last two decades indicates important actions for prolactin (PRL) in regulation of several functions of the immune system. That PRL can serve to facilitate immune cell proliferation is well established. In addition, PRL appears to play a salient role in the genesis and/or potentiation of certain autoimmune diseases. Recent evidence from several laboratories has extended the spectrum of PRL actions in immunological systems to include regulation of lymphocyte pool size through the process of apoptosis. Experimental results obtained using lactogen-dependent rat pre-T cell lines, the Nb2 lymphoma, have demonstrated that PRL suppresses cell death mechanisms activated by cytokine/hormone deprivation and cytotoxic drugs such as glucocorticoids. In this paper, we review results from studies conducted to investigate the mechanism(s) underlying PRL-regulated apoptosis suppression. Effects of the hormone on expression of apoptosis-associated genes of the Bcl-2 family as well as the protooncogene pim-1 in proliferating Nb2 sublines and in cells exposed to apoptotic stimuli are presented. It is concluded that PRL-mediated apoptosis suppression in immune cells reflects a complex interaction among several gene products.

Role of cyclophilins in somatolactogenic action

Prolactin (PRL) and growth hormone (GH) are members of the somatolactogenic hormone family, the pleiotropic actions of which are necessary for vertebrate growth and mammary differentiation. The basis for the specific function of these hormones has remained uncertain; however, their action is associated with internalization and translocation into the nucleus. A yeast two-hybrid screen identified an interaction between PRL and cyclophilin B (CypB), a peptidyl prolyl isomerase (PPI) found in the endoplasmic reticulum (ER), extracellular space, and nucleus. The interaction between CypB and PRL/GH was confirmed in vitro and in vivo through the use of recombinant proteins and coimmunoprecipitation studies. The exogenous addition of CypB potentiated the proliferation of PRL- and GH-dependent cell lines 18- and 40-fold, respectively. The potentiation of PRL action by CypB was accompanied by a dramatic increase in the nuclear retrotranslocation of PRL. Immunogold electron microscopy has revealed this retrotransport to occur via a vesicular pathway. A CypB mutant, termed CypB-NT, was generated that lacked the putative wild-type N-terminal nuclear localization sequence. Although CypB-NT demonstrated levels of PRL binding and PPI activity equivalent to wild-type CypB, it was incapable of mediating the nuclear retrotranslocation of PRL or enhancing PRL-driven proliferation. These studies reveal CypB as an important chaperone facilitating the nuclear retrotransport and action of the somatolactogenic hormone family.

Translocation and action of polypeptide hormones within the nucleus. Relevance to lactogenic transduction

The action of polypeptide hormones at the cell surface as mediated by transmembrane receptors is well recognised. However, a growing body of evidence also indicates that such hormones are also translocated and act directly within the cell nucleus. This chapter will overview what is known of the action of one such example, namely prolactin (PRL), from its classic action at the cell surface, to its novel function within the nucleus.

Constitutive tyrosine phosphorylation of ErbB-2 via Jak2 by autocrine secretion of prolactin in human breast cancer

Overexpression of the oncogene for ErbB-2 is an unfavorable prognostic marker in human breast cancer. Its oncogenic potential appears to depend on the state of tyrosine phosphorylation. However, the mechanisms by which ErbB-2 is constitutively tyrosine-phosphorylated in human breast cancer are poorly understood. We now show that human breast carcinoma samples with ErbB-2 overexpression have higher proliferative and metastatic activity in the presence of autocrine secretion of prolactin (PRL). By using a neutralizing antibody or dominant negative (DN) strategies or specific inhibitors, we also show that activation of Janus kinase Jak2 by autocrine secretion of PRL is one of the significant components of constitutive tyrosine phosphorylation of ErbB-2, its association with Grb2 and activation of mitogen-activated protein (MAP) kinase in human breast cancer cell lines that overexpress ErbB-2. Furthermore, the neutralizing anti-PRL antibody or erbB-2 antisense oligonucleotide or DN Jak2 or Jak2 inhibitor or DNRas or MAP kinase kinase inhibitor inhibits the proliferation of both untreated and PRL-treated cells. Our results indicate that autocrine secretion of PRL stimulates tyrosine phosphorylation of ErbB-2 by Jak2, provides docking sites for Grb2 and stimulates Ras-MAP kinase cascade, thereby causing unrestricted cellular proliferation. The identification of this novel cross-talk between ErbB-2 and the autocrine growth stimulatory loop for PRL may provide new targets for therapeutic and preventive intervention of human breast cancer.

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.

Prolactin (PRL)-PRL receptor system increases cell proliferation involving JNK (c-Jun amino terminal kinase) and AP-1 activation: inhibition by glucocorticoids

PRL receptor (PRLR) signal transduction supports PRL-induced growth/differentiation processes. While PRL is known to activate Jak2-Stat5 (signal transducer and activator of transcription 5) signaling pathway, the mechanism by which cell proliferation is stimulated is less known. We show that PRL induces proliferation of bovine mammary gland epithelial cells and AP-1 site activation. Using PRLR mutants and the PRLR short form, we have found that both homodimerization of PRLR wild type and the integrity of box-1 and C-distal tyrosine of PRLR intracellular domain are needed in PRL-induced proliferation and AP-1 activation. The effect of PRL has been assayed in the presence of dexamethasone (Dex), insulin, and alone. We found that Dex negatively regulates PRL-induced proliferation and AP-1 site activation. We demonstrate that PRL exerts activation of AP-1 transcriptional complex, and the mechanism by which this activation is produced is also studied. We show that PRL induces an increase in the c-Jun content of AP-1 transcriptional complexes. The PRL-induced c-Jun of AP-1 transcriptional complex diminishes in the presence of Dex in a dose-dependent manner. Dex inhibition was reversed by the higher concentration of PRL added to cells. Despite the fact that the regulation of the AP-1 site is complex, we found that PRL activates the c-Jun amino terminal kinase (JNK), while glucocorticoid prevents this JNK activation. These data support a regulation of cellular growth by PRL-PRLR system by increasing AP-1 transcriptional complex activity via JNK activation. JNK activation can be repressed by glucocorticoid in a DNA-binding-independent manner.

Association of 2',5'-oligoadenylate synthetase with the prolactin (PRL) receptor: alteration in PRL-inducible stat1 (signal transducer and activator of transcription 1) signaling to the IRF-1 (interferon-regulatory factor 1) promoter

The PRL receptor (PRL-R) signals through the Janus tyrosine kinases (JAK) and other non-JAK tyrosine kinases, some of which are preassociated with the PRL-R. To clone PRL-R interacting proteins, the intracellular domain (ICD) of the long form of the PRL-R was used in a yeast two-hybrid screen of a human B cell cDNA library. One PRL-R interacting protein was identified as the 42-kDa form of the enzyme 2',5'-oligoadenylate synthetase (OAS). The in vivo interactions in yeast were further confirmed by an in vitro interaction assay and by coimmunoprecipitation in transfected mammalian cells. Functionally, OAS reduced the basal activity of two types of promoters in transiently transfected COS-1 cells. In the presence of PRL, OAS inhibited PRL induction of the immediate early IRF-1 (interferon-regulatory factor 1) promoter, but not PRL induction of the differentiation-specific beta-casein promoter, suggesting that OAS exerts specific effects on immediate early gene promoters. The inhibitory effects of OAS were accompanied by a reduction in PRL-inducible Stat1 (signal transducer and activator of transcription 1) DNA binding activity at the IRF-1 GAS (interferon-gamma-activated sequence) element. These results demonstrate a novel interaction of OAS with the PRL-R and suggest a role for OAS in modulating Stat1-mediated signaling to an immediate early gene promoter. Although previously characterized as a regulator of ribonuclease (RNase) L antiviral responses, OAS may have additional effects on cytokine receptor signal transduction pathways.

Stimulation of c-Src by prolactin is independent of Jak2

Interaction of prolactin (PRL) with its receptor (PRLR) leads to activation of Jak and Src family tyrosine kinases. The PRL/growth hormone/cytokine receptor family conserves a proline-rich sequence in the cytoplasmic juxtamembrane region (Box 1) required for association and subsequent activation of Jaks. In the present work, we studied the mechanisms underlying c-Src kinase activation by PRL and the role that Jak2 plays in this process. PRL addition to chicken embryo fibroblasts (CEF) expressing the rat PRLR long form resulted in activation of c-Src and Jak2 and in tyrosine phosphorylation of the receptor. Receptor phosphorylation was due to associated Jak2, since in cells expressing either a Box 1 mutated PRLR (PRLR(4P-A)), which is unable to interact with Jak2, or a kinase-domain-deleted Jak2 (Jak2Deltak), PRL did not stimulate receptor phosphorylation. Interestingly, addition of PRL to cells expressing PRLR(4P-A) resulted in an activation of c-Src equivalent to that observed with the wild-type receptor. These findings indicate that PRL-mediated stimulation of c-Src was independent of Jak2 activation and of receptor phosphorylation. Our results suggest that PRL-activated Src could send signals to downstream cellular targets independently of Jak2.

Differential effects of prolactin and src/abl kinases on the nuclear translocation of STAT5B and STAT5A

In this study, DNA binding and tyrosine phosphorylation of STAT5A and STAT5B were compared with their subcellular localization determined using indirect immunofluorescence microscopy. Following prolactin activation, both STAT5A and STAT5B were rapidly translocated into the nucleus and displayed a detergent-resistant, punctate nuclear staining pattern. Similar to prolactin induction, src activation resulted in tyrosine phosphorylation and DNA binding of both STAT5A and STAT5B. However, nuclear translocation of only STAT5B but not STAT5A was observed. This selective nuclear translocation appears to be mediated via the carboxyl-terminal sequences in STAT5B. Furthermore, overexpression of a dominant negative kinase-inactive mutant of JAK2 prevented prolactin-induced tyrosine phosphorylation and nuclear translocation of STAT5A and STAT5B but did not block src kinase activation and nuclear translocation of STAT5B. In co-transfection assays, prolactin-mediated activation but not src kinase-mediated activation of STAT5B resulted in the induction of a beta-casein promoter-driven reporter construct. These results suggest that STAT5 activation by src may occur by a mechanism distinct from that employed in cytokine activation of the JAK/STAT pathway, resulting in the selective nuclear translocation of STAT5B.

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.

Prolactin-independent modulation of the beta-casein response element by Erk2 MAP kinase

The MAP kinases have been suggested to play a role in intracellular signalling by PRL. A reporter gene construct, PRE3-CAT, which manifests PRL responsiveness through a Stat5-binding site (PRE), was induced by PRL in CHO cells expressing the PRL-R. A fusion protein (Gal4-Stat5(695)), containing the C-terminal domain of Stat5a (amino acids 695-794) linked to the DNA-binding domain of Gal4 (Gal4 DBD), strongly activated transcription of a luciferase reporter gene. Therefore, the Stat5 C-terminus, which contains a potential MAP kinase phosphorylation site, exhibits a modular transactivating function. A kinase-defective mutant of Erk2 (iMAPK) caused a dose-dependent suppression of PRL-stimulated PRE3-CAT, and also inhibited the induction of PRE3-CAT by Jak2 over-expression. Correspondingly, over-expression of the MAP kinase activator v-Src increased the PRL-stimulated level of PRE3-CAT. Gal4-Stat5(695) activity was not modulated by PRL or Jak2, consistent with the absence of the relevant tyrosine phosphorylation site at residue 694. Gal4-Stat5(695) was not inhibited by iMAPK, indicating that the C-terminal transactivation region of Stat5a is not sensitive to direct modulation of a MAP kinase pathway. These results suggest that alteration of Erk2 activity by growth factors may modulate PRL-induced gene expression by a mechanism upstream of Stat5.

Phase II study of RC-160 (vapreotide), an octapeptide analogue of somatostatin, in the treatment of metastatic breast cancer

RC-160 (octastatin/vapreotide) is a potent octapeptide analogue of somatostatin with growth inhibitory activity in experimental tumours in vitro and in vivo, including breast cancer. We evaluated the efficacy and tolerability of high-dose RC-160, 3 mg day(-1) on week 1 increased to 4.5 mg day(-1) for weeks 2-4 and subsequently 6 mg day(-1) until the end of treatment, administered by continuous subcutaneous infusion in the management of 14 women with previously treated metastatic breast cancer. The age range was 37-80 years (median 58.5 years) and performance status 0-2. The treatment was well tolerated with no dose reductions being required. No grade 3 or 4 toxicities were seen. Abscess formation developed at the infusion site in eight patients and erythema and discomfort was seen in a further three patients. A significant reduction in IGF-I levels occurred by day 7 and was maintained throughout the treatment. The lowest dose of RC-160 produced the maximal IGF-I response. Although there was no reduction in prolactin levels in patients whose baseline levels were normal, elevated prolactin levels found in three patients fell to within the normal range 7 days after commencing RC-160 treatment. A small but significant rise in fasting blood glucose levels was also recorded, the highest level on treatment being 7.6 mmol l(-1). No objective tumour responses were observed, all patients showing disease progression within 3 months of commencing treatment. These findings demonstrate that high-dose RC-160, administered as a continuous subcutaneous infusion, can reduce serum levels of the breast growth factors IGF-I and prolactin but is ineffective in the management of metastatic breast cancer. Encouraging preclinical anti-tumour activity and the favourable toxicity profile in patients suggest the merit of future studies combining RC-160 with anti-oestrogen, cytotoxic and anti-angiogenic agents.

Prolactin receptor signaling: shared components with the T-cell antigen receptor in Nb2 lymphoma cells

Previously, we reported that activation of the human prolactin receptor (PRLR) produced a protein phosphorylation pattern strikingly similar to that provoked by Concanavalin A (Con A), an activator of the T-cell antigen receptor (TCR). These results suggested that certain signaling components of the TCR may be shared by the activated PRLR. Additional studies here assessed the levels of TCR expression following PRLR stimulation and the effect of TCR activation on PRL-stimulated proliferation in lactogen-dependent pre-T Nb2-11 lymphoma cells. The results indicated that the TCR was expressed on the surface of approx 4% of exponentially proliferating and prolactin- (PRL) treated cells. In contrast, approx 45% of quiescent cells, cultured in the absence of PRL for 24 h, expressed the TCR at the cell surface, suggesting that lactogen withdrawal may up-regulate TCR cell-surface expression. Moreover, TCR activation with anti-CD3 antibodies attenuated PRL-stimulated Nb2-11 cell proliferation in a concentration-dependent manner. In other experiments, immunoprecipitation and immunoblotting of Nb2-11 lysates revealed that activation of the PRLR resulted in rapid tyrosyl phosphorylation of ZAP-70, a critical TCR-associated tyrosine kinase. In addition, ZAP-70 was found to associate transiently with the putative guanine nucleotide exchange factor and substrate, Vav, in PRL-treated cells. ZAP-70 was also found to associate constitutively with the PRLR; PRL stimulation provoked the transient recruitment of Vav to the complex. These observations suggest that PRL signaling reflects the transient formation of a PRLR-ZAP-70-Vav complex and its immunomodulatory actions involve diverse interactions that affect TCR expression and signaling mechanisms.

T cell activation through the CD43 molecule leads to Vav tyrosine phosphorylation and mitogen-activated protein kinase pathway activation

CD43, the most abundant membrane protein of T lymphocytes, is able to initiate signals that lead to Ca2+ mobilization and interleukin-2 production, yet the molecular events involved in signal transduction pathway of the CD43 molecule are only beginning to be understood. We have shown recently that cross-linking CD43 on the cell surface of human T lymphocytes with the anti-CD43 monoclonal antibody L10 leads to CD43-Fyn kinase interactions and to Fyn phosphorylation on tyrosine residues. This interaction seems to be mediated by the SH3 domain of Fyn and a proline-rich sequence located in the cytoplasmic domain of CD43. Here we show that CD43-specific activation of human T lymphocytes induced tyrosine phosphorylation of the adaptor protein Shc and of the guanine exchange factor Vav, as well as the formation of a macromolecular complex that comprises Shc, GRB2, and Vav. CD43 ligation resulted in enhanced formation of Vav.SLP-76 complexes and in the activation and nuclear translocation of ERK2. Cross-linking of the CD43 molecule in 3T3-CD43(+) cells induced luciferase activity from a construct under the control of the Fos serum responsive element. Altogether, these data suggest that the mitogen-activated protein kinase pathway is involved in CD43-dependent interleukin-2 gene expression.

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)

Stoichiometric structure-function analysis of the prolactin receptor signaling domain by receptor chimeras

The intracellular domain of the prolactin (PRL) receptor (PRLr) is required for PRL-induced signaling and proliferation. To identify and test the functional stoichiometry of those PRLr motifs required for transduction and growth, chimeras consisting of the extracellular domain of either the alpha or beta subunit of human granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor (GM-CSFr) and the intracellular domain of the rat PRLr were synthesized. Because the high-affinity binding of GM-CSF results from the specific pairing of one alpha- and one beta-GM-CSFr, use of GM-CSFr/PRLr chimera enabled targeted dimerization of the PRLr intracellular domain. To that end, the extracellular domains of the alpha- and beta-GM-CSFr were conjugated to one of the following mutations: (i) PRLr C-terminal truncations, termed alpha278, alpha294, alpha300, alpha322, or beta322; (ii) PRLr tyrosine replacements, termed Y309F, Y382F, or Y309+382F; or, (iii) PRLr wild-type short, intermediate, or long isoforms. These chimeras were cotransfected into the cytokine-responsive Ba/F3 line, and their expression was confirmed by ligand binding and Northern and Western blot analyses. Data from these studies revealed that heterodimeric complexes of the wild type with C-terminal truncation mutants of the PRLr intracellular domain were incapable of ligand-induced signaling or proliferation. Replacement of any single tyrosine residue (Y309F or Y382F) in the dimerized PRLr complex resulted in a moderate reduction of receptor-associated Jak2 activation and proliferation. In contrast, trans replacement of these residues (i.e., alphaY309F and betaY382F) markedly reduced ligand-driven Jak2 activation and proliferation, while cis replacement of both tyrosine residues in a single intracellular domain (i.e., alphaY309+382F) produced an inactive signaling complex. Analysis of these GM-CSFr-PRLr complexes revealed equivalent levels of Jak2 in association with the mutant receptor chains, suggesting that the tyrosine residues at 309 and 382 do not contribute to Jak association, but instead to its activation. Heterodimeric pairings of the intracellular domains from the known PRLr receptor isoforms (short-intermediate, short-long, and intermediate-long) also yielded inactive receptor complexes. These data demonstrate that the tyrosine residues at 309 and 382, as well as additional residues within the C terminus of the dimerized PRLr complex, contribute to PRL-driven signaling and proliferation. Furthermore, these findings indicate a functional requirement for the pairing of Y309 and Y382 in trans within the dimerized receptor complex.

Prolactin decreases epidermal growth factor receptor kinase activity via a phosphorylation-dependent mechanism

Previously, we have shown that prolactin inhibits epidermal growth factor (EGF)-induced mitogenesis in mouse mammary epithelial cells without altering the response to other growth promoting agents. This effect has been associated with reduced EGF-induced EGF receptor (EGFR) tyrosine phosphorylation, Grb-2 association, and Ras activation. Our current hypothesis is that prolactin induces an alteration in EGFR kinase activity via a phosphorylation-dependent mechanism. To test this hypothesis, we treated normal murine mammary gland cells with or without 100 ng/ml prolactin. EGFR isolated by wheat germ agglutinin affinity chromatography from nontreated cells exhibited substantial ligand-induced phosphorylation, and EGFR isolated from prolactin-treated cells displayed minimal EGF-induced EGFR phosphorylation, as well as decreased kinase activity toward exogenous substrates. The observed decrease in ligand-induced EGFR phosphorylation could not be attributed to either differential amounts of EGFR, decreased EGF binding affinity, or the presence of a phosphotyrosine phosphatase or ATPase. EGFR isolated from prolactin-treated cells exhibited increased phosphorylation on threonine. Removal of this phosphorylation with alkaline phosphatase restored EGFR kinase activity to levels observed in nontreated cells. Therefore, these results suggest that prolactin antagonizes EGF signaling by increasing EGFR threonine phosphorylation and decreasing EGF-induced EGFR tyrosine phosphorylation.

Activation of the Jak2-Stat5 signaling pathway in Nb2 lymphoma cells by an anti-apoptotic agent, aurintricarboxylic acid

Biological effects of many hormones and cytokines are mediated through receptor-associated Jak tyrosine kinases and cytoplasmic Stat transcription factors, including critical physiological processes such as immunity, reproduction, and cell growth and differentiation. Pharmaceuticals that control Jak-Stat pathways are therefore of considerable interest. Here we demonstrate that a single Jak-Stat pathway can be activated by aurintricarboxylic acid (ATA), a negatively charged triphenylmethane derivative (475 Da) with anti-apoptotic properties. In prolactin (PRL)-dependent Nb2 lymphocytes, ATA sustained cell growth in the absence of hormone and mimicked rapid PRL-induced tyrosine phosphorylation of Jak2 and activation of Stat5a and Stat5b with tyrosine phosphorylation, heterodimerization, DNA binding, and induction of the Stat5-regulated pim-1 protooncogene. ATA also mimicked PRL activation of serine kinases ERK1 and ERK2. However, unlike PRL, ATA did not regulate Stat1 or Stat3. ATA also did not affect Jak3, which is activated in these cells by interleukin-2 family cytokines. Although the mechanism and specificity by which ATA activates Jak2, Stat5, and ERKs in Nb2 cells are still unclear, the present study demonstrates that certain hormone or cytokine effects on Jak-Stat pathways can be discretely imitated by a low molecular weight, non-peptide pharmaceutical. The results are also consistent with Stat5 involvement in lymphocyte growth and survival.

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.

Dominant negative and cooperative effects of mutant forms of prolactin receptor

In addition to a long form of 591 amino acids (aa), two other forms of PRL receptor (PRLR), differing in the length of their cytoplasmic domains, have been identified in the rat. The Nb2 form, lacking 198 aa in the cytoplasmic domain, is able to transmit a lactogenic signal similar to the long form, whereas the short form of 291 aa is inactive. The ability of PRL to activate the promoter of the beta-casein gene or the lactogenic hormone responsive element fused to the luciferase reporter was assessed in Chinese hamster ovary cells or 293 fibroblasts transiently transfected with PRLR cDNAs. The function of the short form was examined after cotransfection of both the long and short forms. These results clearly show that the short form acts as a dominant negative inhibitor through the formation of inactive heterodimers, resulting in an inhibition of Janus kinase 2 (JAK2) activation. The present study also investigates the possible participation of cytoplasmic receptors in the signal transduction pathway, using cotransfection experiments and a new approach that selectively determines the contribution of cytoplasmic receptors in the process of signal transduction. We cotransfected Chinese hamster ovary cells with two cDNA constructs: a cytoplasmic (soluble) form of the receptor with a deleted signal peptide (delta-19), which is unable to bind PRL, and a functionally inactive receptor mutant (lacking box 1), which is anchored in the plasma membrane and able to bind PRL. This approach has allowed us to show that delta-19, lacking expression at the plasma membrane, can transduce the hormonal message, at least to a limited extent (up to 30% of wild type efficiency), providing that association/activation occurs with a PRL-PRLR complex initiated at the cell surface level; box 1 of the cytoplasmic form is necessary to rescue this partial transcriptional activity of the inactive mutant. This partial recovery is also parallel to the partial activation of JAK2, indicating that the signal transduction pathway implicated JAK2. Our results provide evidence that heterodimerization of receptors can be implicated either in the positive or in negative activation of gene transcription.

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, epidermal growth factor or transforming growth factor-alpha activate a mammary cell-specific enhancer in mouse mammary tumor virus-long terminal repeat

Mammary specific expression of elevated levels of mouse mammary tumor virus (MMTV) contributes to mammary carcinogenesis. Mechanisms which regulate provirus expression have not been completely defined. Using a MMTV-long repeat terminal (MMTV-LRT) directed chloramphenicol-acetyltransferase (CAT) reporter gene system and a human breast cancer cell line T47D, we demonstrate that prolactin (PRL), epidermal growth factor (EGF), or transforming growth factor-alpha (TGF-alpha) act on a mammary cell-specific enhancer at the extreme 5' end of the MMTV-LTR involving sequences -1094 through -858. PRL and either EGF or TGF-alpha exert concerted roles in this activation of these sequences. In contrast, using a plasmid construct lacking this mammary cell-specific enhancer, EGF or TGF-alpha, but not PRL, act synergistically with progesterone to induce CAT activity, indicating that the action of PRL on regulatory elements of the MMTV-LTR is restricted to this mammary cell-specific enhancer involving sequences -1094 through -858. A mobility shift assay was used to demonstrate that PRL, EGF or TGF-alpha induce nuclear factors (MP4, MAF, and MGF) which bind directly to this mammary cell-specific enhancer element.

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.

Role of Bag-1 in the survival and proliferation of the cytokine-dependent lymphocyte lines, Ba/F3 and Nb2

The expression and function of the newly identified Bcl-2- and Raf-1- binding protein, Bag-1, during the cytokine-regulated growth of B and T cell lines was examined. Immunoblot analysis of lysates from the interleukin-3 (IL-3)-dependent B cell line Ba/F3, and the PRL-dependent T cell line Nb2, revealed that variations in Bag-1 levels paralleled alterations in cellular proliferation, viability, and apoptosis induced by the presence or absence of growth factor. To test whether up-regulation of Bag-1 levels altered cellular survival and proliferation, Ba/F3 cells were transfected with a Bag-1 expression construct. The overexpression of Bag-1 in transfected Ba/F3 cells induced an IL-3-independent state. Such transfectants demonstrated sustained viability and proliferation, with minimal apoptosis, in the complete absence of exogenous IL-3. Bag-1 expression was also compared in glucocorticoid-sensitive Nb2 cells and a PRL-independent, glucocorticoid-resistant subline, SFJCD1, during culture of these lines in dexamethasone (Dex). Bag-1 levels were profoundly decreased by the addition of Dex to Nb2 cells, precedent to the onset of apoptotic cell death. In contrast, Dex treatment or PRL withdrawal had no effect on levels of Bag-1 within the SFJCD1 line. These findings establish that the overexpression of Bag-1 in the appropriate cellular context promotes cellular survival and growth, events that may result from the juxtaposition of this protein with mitogenic and antiapoptotic signaling pathways.