The effect of prolactin on casein kinase II, MAP kinase and PKC in rabbit mammary cells and Nb2 rat lymphoid cells

Mammary gland zinc metabolism: regulation and dysregulation

Zinc (Zn) is required for numerous metabolic processes serving both a structural and catalytic role. The mammary gland has a unique Zn requirement resulting from the need to also transfer an extraordinary amount of Zn into milk (~0.5-1 mg Zn/day) during lactation. Impairments in this process can result in severe Zn deficiency in the nursing offspring which has adverse consequences with respect to growth and development. Moreover, dysregulated mammary gland Zn metabolism has recently been implicated in breast cancer transition, progression and metastasis, thus there is a critical need to understand the molecular mechanisms which underlie these observations. Tight regulation of Zn transporting mechanisms is critical to providing an extraordinary amount of Zn for secretion into milk as well as maintaining optimal cellular function. Expression of numerous Zn transporters has been detected in mammary gland or cultured breast cells; however, understanding the molecular mechanisms which regulate mammary Zn metabolism as well as the etiology and downstream consequences resulting from their dysregulation is largely not understood. In this review, we will summarize the current understanding of the regulation of mammary gland Zn metabolism and its regulation by reproductive hormones, with a discussion of the dysregulation of this process in breast cancer.

Prolactin stimulates the proliferation of normal female cholangiocytes by differential regulation of Ca2+-dependent PKC isoforms

Background

Prolactin promotes proliferation of several cells. Prolactin receptor exists as two isoforms: long and short, which activate different transduction pathways including the Ca²⁺-dependent PKC-signaling. No information exists on the role of prolactin in the regulation of the growth of female cholangiocytes. The rationale for using cholangiocytes from female rats is based on the fact that women are preferentially affected by specific cholangiopathies including primary biliary cirrhosis. We propose to evaluate the role and mechanisms of action by which prolactin regulates the growth of female cholangiocytes.

Results

Normal cholangiocytes express both isoforms (long and short) of prolactin receptors, whose expression increased following BDL. The administration of prolactin to normal female rats increased cholangiocyte proliferation. In purified normal female cholangiocytes, prolactin stimulated cholangiocyte proliferation, which was associated with increased [Ca²⁺]_i levels and PKCβ-I phosphorylation but decreased PKCα phosphorylation. Administration of an anti-prolactin antibody to BDL female rats decreased cholangiocyte proliferation. Normal female cholangiocytes express and secrete prolactin, which was increased in BDL rats. The data show that prolactin stimulates normal cholangiocyte growth by an autocrine mechanism involving phosphorylation of PKCβ-I and dephosphorylation of PKCα.

Conclusion

We suggest that in female rats: (i) prolactin has a trophic effect on the growth of normal cholangiocytes by phosphorylation of PKCβ-I and dephosphorylation of PKCα; and (iii) cholangiocytes express and secrete prolactin, which by an autocrine mechanism participate in regulation of cholangiocyte proliferation. Prolactin may be an important therapeutic approach for the management of cholangiopathies affecting female patients.

Increasing ornithine decarboxylase activity is another way of prolactin preventing methotrexate-induced apoptosis: Crosstalk between ODC and BCL-2

Prolactin has more than 300 separate functions including affecting mammary growth, differentiation, secretion and anti-apoptosis. In the previous studies, prolactin induced Bcl-2 expression to prevent apoptosis and also provoked the activity of ornithine decarboxylase (ODC). Our previous data showed that ODC overexpression upregulates Bcl-2 and prevents tumor necrosis factor alpha (TNF-alpha)- and methotrexate (MTX)-induced apoptosis. Here, we further investigate whether prolactin prevents MTX-induced apoptosis through inducing ODC activity and the relationship between ODC and Bcl-2 upon prolactin stimulation. Prolactin prevented MTX-induced apoptosis in a dose-dependent manner in HL-60 cells. Following prolactin stimulation, ODC enzyme activity also shows an increase in a dose-dependent manner, expressing its maximum level at 3 h, and rapidly declining thereafter. Prolactin-induced ODC activity is completely blocked by a protein kinase C delta (PKCdelta) inhibitor, rottlerin. However, there are no changes in the expressions of ODC mRNA and protein level after prolactin stimulus. It indicates that prolactin may induce ODC activity through the PCKdelta pathway. Besides, Bcl-2 expresses within 1 h of prolactin treatment and this initiating effect of prolactin is not inhibited by alpha-difluoromethylornithine (DFMO). However, Bcl-2 is further enhanced following prolactin stimulation for 4 h and this enhancement is blocked by DFMO. Bcl-2 has no effect on ODC activity and protein levels, but ODC upregulates Bcl-2, which is inhibited by DFMO. Overall, there are two different forms of prolactin effect, it induces Bcl-2 primarily, and following this it stimulates ODC activity. Consequently induced ODC activity further enhances the expression of Bcl-2. The anti-apoptotic effect of prolactin is diminished by DFMO and recovered by putrescine. Obviously, ODC activity is one basis for the anti-apoptotic mechanisms of prolactin. A Bcl-2 inhibitor, HA14-1, together with DFMO, completely blocks the anti-apoptotic effects of prolactin. These results suggest that increasing ODC activity is another way of prolactin preventing MTX-induced apoptosis and that this induction of ODC activity enhances the expression of Bcl-2 strongly enough to bring about the anti-apoptotic function.

The compartmentalization of prolactin signaling in the mouse mammary gland

In mammary epithelial cells, prolactin (PRL) activates at least two signaling pathways: Jak/Stat and nitric oxide (NO). The former induces differentiation as measured by alpha-lactalbumin accumulation, while experiments with sodium nitroprusside (SNP) show that NO inhibits differentiation. In order to resolve this apparent contradiction, the kinetics, inducibility, and cellular localization of NO production and sensitivity in mammary cells were examined. First, mammary cells remained responsive to PRL throughout the incubation with respect to NO production. Second, although desensitization occurred with continuous PRL exposure, recovery began as quickly as 30 min after PRL withdrawal. Since PRL is secreted in pulses in vivo, complete desensitization was not a likely explanation for the cells' escape from NO inhibition. Finally, the cellular site of transduction was examined using the caveolar disrupting agent, methyl-beta-cyclodextrin (MBCD). MBCD inhibited the accumulation of PRL-induced NO but not alpha-lactalbumin. This finding was confirmed by membrane fractionation studies where the PRL-induced NO production occurred primarily in caveolae and PRL-stimulated tyrosine phosphorylation of Stat5, which transcribes the alpha-lactalbumin gene, occurred predominantly in noncaveolar membranes. Finally, endogenous elevations of NO by arginine did not inhibit differentiation. As such, the inhibition seen with SNP appeared to be an artifact of the ubiquitous generation of NO from SNP. Physiologically, PRL induces NO only in caveolae and this restricted distribution does not interfere with differentiation.

Maternal zinc deficiency raises plasma prolactin levels in lactating rats

There is an inverse relation between zinc (Zn) intake and plasma prolactin in men and nonpregnant women. Whether a relation exists in lactating women is unknown, despite the potential consequences of perturbations in prolactin regulation on lactation performance. We examined the effects of low Zn intake on prolactin concentration, the prolactin regulatory pathway in the pituitary gland, and lactation performance in lactating rats. Female rats were fed diets containing 7 (zinc deficient; ZD), 10 (marginally zinc deficient; MZD) or 25 mg Zn/kg (control) from 70 d preconception to lactation d 11. Rats were killed, pituitary glands dissected, and tissues and plasma collected and analyzed for prolactin concentration. Pituitary gland pituitary factor 1 (Pit-1), dopamine 2 receptor (D2R), and prolactin receptor mRNA expression were measured in the pituitary gland. Liver, mammary gland, plasma, and milk Zn were measured. Milk intake of the pups was also recorded. Plasma prolactin concentration was higher in rats fed the ZD (125.9 microg/L) diet compared with control rats (21.7 microg/L). Pituitary gland prolactin concentration was higher in rats fed the ZD diet (69.8 mg/g total protein) compared with controls (29.0 mg/g). Plasma Zn concentration was lower in rats fed the MZD and ZD diets, and mammary gland and milk Zn concentrations were lower in rats fed the ZD diet compared with control rats. Rats fed the ZD diet had lower D2R, prolactin receptor, and Pit-1 mRNA levels, whereas rats fed the MZD diet had lower prolactin receptor and Pit-1 mRNA levels compared with control rats. Milk intake was lower in pups of rats fed the MZD and ZD diets. Our results suggest that marginal Zn nutriture may compromise milk production despite increased prolactin levels. In addition, increased circulating prolactin concentration is not due to altered nursing behavior, but may be due to alterations in the prolactin regulatory pathway in the pituitary gland.

Prolactin regulates adenylyl cyclase and insulin secretion in rat pancreatic islets

A role for prolactin (PRL) in the regulation of adenylyl cyclase (AC), cyclic AMP (cAMP) formation and insulin secretion was studied in isolated rat pancreatic islets cultured for 4 days at 5.5 mM glucose in the absence (control) or presence of PRL (500 ng/ml). In PRL-treated islets, stimulation by glucose (8 mM), carbamylcholine chloride (CCh) and phorbol dibutyrate increased cAMP levels 40, 89, and 151%, respectively, above similarly stimulated control islets without PRL. Moreover, insulin secretion in PRL-treated islets was more than doubled in response to 8 mM glucose plus glucagon-like peptide 1 compared with control islets. PRL also increased protein kinase C (PKC) activity in cultured islets. When islets were cultured at an insulin secretion desensitizing concentration of glucose (11 mM) for 4 days, there was a decrease in forskolin-stimulated cAMP production. However, the presence of PRL with 11 mM glucose prevented the glucose-induced decrease in cAMP production. Insulin secretion in response to 17 mM glucose was also higher (P<0.02) in islets cultured with 11 mM glucose plus PRL compared with islets cultured with 11 mM glucose alone. Islet AC types -III, -V, and -VI mRNA levels increased relative to 18s rRNA following PRL treatment. In contrast, culture at 11 mM glucose decreased relative AC-III, -V and -VI mRNA levels by as much as 50%. Culture with PRL prevented the decrease in AC expression during islet culture with 11 mM glucose, and the mRNA levels remained similar to control islets cultured at 5.5 mM glucose. Thus, PRL not only increased islet AC expression and activity and insulin secretory responsiveness, but also protected islets from chronic glucose-induced inhibition of these beta-cell activation parameters.

The mechanisms by which nitric oxide affects mammary epithelial growth and differentiation

Nitric oxide (NO) enhances prolactin-stimulated DNA synthesis and inhibits prolactin-induced differentiation in mouse mammary epithelium. The molecular pathways used by NO were determined by employing specific inhibitors of the transducers utilized by NO. Inhibitors of the Jun N-terminal kinase (JNK) blocked the effect of NO on DNA synthesis, although this appeared to involve a protein kinase G (PKG)-independent pathway. In contrast, inhibitors of the extracellular signal-regulated kinase (ERK) prevented NO from suppressing alpha-lactalbumin accumulation and this effect was PKG-dependent. NO can also elevate cAMP through the inhibition of phosphodiesterase 3 and cAMP mimicks the actions of NO on both DNA synthesis and differentiation. However, suppression of cAMP levels did not prevent the effects of NO. Therefore, NO uses two separate pathways to affect mammary epithelium: it stimulates growth via JNK and inhibits differentiation through ERK.

Dual role of protein kinase C on mitogen-activated protein kinase activation and human keratinocyte proliferation

Subconfluent normal human keratinocytes exhibit autonomous (autocrine growth factor driven) proliferation and express the specific markers for keratinocyte proliferation K#5 and K#14. In keratinocyte autocrine culture, the exogenously added epidermal growth factor (EGF) has no effect on cell proliferation and mitogen-activated protein kinase (MAPK) activity. PD98059 inhibits MAPK pathway and autocrine keratinocyte proliferation. Staurosporine and Gö6976 strongly inhibit autonomous keratinocyte proliferation. In contrast, Gö6983 (which does not inhibit PKC micro ) inhibits only 20% of autocrine keratinocyte proliferation. Staurosporine inhibits MAPK activity, whereas Gö6976 and Gö6983 strongly increase it. We have concluded that MAPK, PKC micro and probably PKCalpha take part in autocrine keratinocyte proliferation. The effect of Gö6976 and Gö6983 on MAPK activity could be explained by the inhibition of PKC-dependent MAPK-phosphatase expression. The effect of staurosporine could be explained by its paradoxical action (activation) on protein kinase C (PKC) in keratinocytes.

The effect of chemical blockade of PKC with Gö6976 and Gö6983 on proliferation and MAPK activity in IL-6-dependent plasmacytoma cells

Interleukin-6 (IL-6) mediates growth of murine B9 hybridoma/plasmacytoma cells via Ras-dependent mitogen-activated protein kinase (MAPK) pathway. Preincubation of cells with selective protein kinase C (PKC) inhibitors Gö6976 and Gö6983 leads to enhancement of IL-6-induced p44/p42 MAPK activity. The basal p44/p42 MAPK activity is also stimulated in the presence of both inhibitors. On the other hand, Gö6976 completely blocks proliferation, but Gö6983 which does not inhibit PKC(mu) has no effect on the cell growth. These findings suggest that PKC(mu) is required for proliferation and other PKC isoenzymes are involved in regulation of IL-6-dependent growth of B9 cells by negative regulation of the MAPK pathway.

The role of nitric oxide in the biological activity of prolactin in the mouse mammary gland

In mouse mammary epithelial cells, prolactin transiently elevates nitric oxide (NO) to a maximum of 6 nmol/mg protein at 15 min, after which levels fall rapidly. This stimulation can be achieved by as little as 100 ng prolactin/ml and can be mimicked by 100 microg sodium nitroprusside/ml. NO is both necessary and sufficient to mediate the prolactin-induced redistribution of its receptor from internal pools to the cell surface. NO can also enhance DNA synthesis stimulated by submaximal prolactin concentrations (50 ng/ml), but it is not necessary at pharmacological prolactin concentrations (1 microg/ml). In contrast, NO completely inhibits alpha-lactalbumin production. In summary, prolactin transiently elevates NO to enhance DNA synthesis and suppress premature differentiation; thereafter, NO declines, DNA synthesis ceases and differentiation proceeds. This data suggest that NO may mediate some of the effects of prolactin on growth in the mammary gland.

Janus kinase 2 (JAK2) regulates prolactin-mediated chloride transport in mouse mammary epithelial cells through tyrosine phosphorylation of Na+-K+-2Cl- cotransporter

Epithelial chloride (Cl-) transport is achieved by the coordinated action of symporters such as the Na+-K+-2Cl- cotransporter (NKCC1) and chloride channels such as the cystic fibrosis transmembrane conductance regulator (CFTR). As a secretory tissue, mammary epithelial cells are obvious candidates for such mechanisms, but Cl- transport and its hormonal regulation have been poorly delineated in mammary epithelial cells. We determined whether the mammary epithelial cell line, HC11, transports chloride and whether this was regulated by PRL, a hormone known to stimulate ion transport. HC11 cells express both CFTR and NKCC1. Exposure to PRL or PGE1 increased Cl- transport in HC11 cells. This was inhibited by the NKCC1 blocker, furosemide, and by the Cl- channel inhibitor, diphenylamine 2-carboxylate. Dose and time course of PRL action indicate that PRL had maximal effect on Cl- transport at 1 microg/ml and at 10 min of stimulation. Examination of the signaling pathways suggests that the PRL effect on Cl- transport does not involve an increase in [Ca2+]i or MAP kinase activity. RT-PCR analyses indicate that HC11 cells express mRNA for Janus kinase 1 (JAK1), JAK2, and signal transducer and activator of transcription 5 (STAT5) but not for JAK3. PRL treatment of HC11 cells increased phosphorylation of STAT5. The JAK2 inhibitor AG490 blocked phosphorylation of STAT5 and PRL-induced, but not PGE1-induced, Cl- transport. NKCC1, but not CFTR, is tyrosine phosphorylated in HC11 cells. PRL enhanced tyrosine phosphorylation of NKCC1, and this effect was attenuated by the JAK2 inhibitor AG490. These results are the first demonstrations of a role for tyrosine phosphorylation of NKCC1 and of the PRL-JAK2 cascade in the regulation of Cl- transport.

Crucial role of the amino-terminal tyrosine residue 42 and the carboxyl-terminal PEST domain of I kappa B alpha in NF-kappa B activation by an oxidative stress

Activation of transcription factor NF-kappa B involves the signal-dependent degradation of basally phosphorylated inhibitors such as I kappa B alpha. In response to proinflammatory cytokines or mitogens, the transduction machinery has recently been characterized, but the activation mechanism upon oxidative stress remains unknown. In the present work, we provide several lines of evidence that NF-kappa B activation in a T lymphocytic cell line (EL4) by hydrogen peroxide (H2O2) did not involve phosphorylation of the serine residues 32 and 36 in the amino-terminal part of I kappa B alpha. Indeed, mutation of Ser32 and Ser36 blocked IL-1 beta- or PMA-induced NF-kappa B activation, but had no effect on its activation by H2O2. Although I kappa B alpha was phosphorylated upon exposure to H2O2, tyrosine residue 42 and the C-terminal PEST (proline-glutamic acid-serine-threonine) domain played an important role. Indeed, mutation of tyrosine 42 or serine/threonine residues of the PEST domain abolished NF-kappa B activation by H2O2, while it had no effect on activation by IL-1 beta or PMA-ionomycin. This H2O2-inducible phosphorylation was not dependent on I kappa B kinase activation, but could involve casein kinase II, because an inhibitor of this enzyme (5,6-dichloro-1-beta-D-ribofuranosyl-benzimidazole) blocks NF-kappa B activation. H2O2-induced I kappa B alpha phosphorylation was followed by its degradation by calpain proteases or through the proteasome. Taken together, our findings suggest that NF-kappa B activation by H2O2 involves a new mechanism that is totally distinct from those triggered by proinflammatory cytokines or mitogens.

Involvement of tyrosine kinases, Ca2+ and PKC in activation of mitogen-activated protein (MAP) kinase in human polymorphonuclear neutrophils

1. Activation of mitogen-activated protein (MAP) kinase is an early response to a wide variety of stimuli and plays an important role in the regulation of cellular functions. In the present study we investigated the activation of MAP kinase in human polymorphonuclear neutrophils (PMNs). 2. Activity of MAP kinase and protein kinase C (PKC) was measured radiometrically from the rate of phosphorylation of specific peptide substrates. Protein phosphorylation was measured by immunoprecipitation and Western blot analysis. 3. N-Formyl-Met-Leu-Phe (fMLP), phorbol 12-myristate, 13-acetate (PMA) and the Ca2+-ATPase inhibitors thapsigargin (Tg) and cyclopiazonic acid (CPA) increased MAP kinase activity significantly. The tyrosine kinase inhibitors erbstatin and herbimycin A partially inhibited the effects of fMLP and PMA, and completely abolished the effects of both Tg and CPA. The specific PKC inhibitor calphostin C suppressed activation of MAP kinase produced by fMLP and PMA, but had no effect on that produced by Tg and CPA. Tg and CPA were without effect on PKC activity. 4. Immunoprecipitation and Western blot analysis indicated that the 42 and 44 kDa tyrosine-phosphorylated proteins found after stimulation of PMNs were both members of the MAP kinase family. Pretreatment of PMNs with staurosporine, EGTA or erbstatin significantly reduced the tyrosine phosphorylation of MAP kinase(s). 5. These results suggest that in human PMNs, MAP kinase can be stimulated in both a PKC-dependent and a PKC-independent manner. The Ca2+ signal leads to activation of tyrosine kinases, which contribute to the activation of MAP kinase. However, a PMA-sensitive Ca2+-independent pathway also exists. Mobilization of Ca2+ and activation of PKC synergistically induce maximal MAP kinase activation and tyrosine phosphorylation.

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)