Effect of various protein kinase inhibitors on the induction of milk protein gene expression by prolactin

Genistein Directly Represses the Phosphorylation of STAT5 in Lactating Mammary Epithelial Cells

Genistein is a soy isoflavone and shows various physiological activities, such as affinities for estrogen receptors (ERs) and inhibitory effects on the epidermal growth factor receptor (EGFR) pathway. A previous study reported that genistein downregulates milk production ability in mammary epithelial cells (MECs) while decreasing the phosphorylation of STAT5. The ER and EGFR pathways indirectly regulate STAT5. In this study, the repressing mechanism of genistein against the phosphorylation of STAT5 was investigated using a culture model of mouse MECs with milk production ability. The results revealed that genistein did not influence the behavior of ERα and ERβ, whereas genistein immediately repressed the phosphorylation of ERK1/2. However, the decrease in phosphorylated STAT5 occurred independent of the phosphorylation of EGFR. Genistein repressed new phosphorylation of STAT5 by prolactin without influencing the phosphorylation of JAK2. In conclusion, this study indicates that genistein directly inhibits the phosphorylation of STAT5 in lactating MECs.

Prolactin signaling in porcine adrenocortical cells: involvement of protein kinases

Prolactin (PRL) was found to have a stimulatory effect on adrenal steroidogenesis in vivo and in vitro in several species including pigs. PRL signal transduction pathways, however, in adrenocortical cells are poorly recognized. Therefore, the goal of this paper is to ascertain the involvement of protein kinase C (PKC) and tyrosine kinases in PRL signaling in porcine adrenal cortex. Adrenals were harvested from locally slaughtered mature gilts. Cortical cells were dispersed by sequential treatment with collagenase. The cells were seeded into 24-well culture plates at a density of 3 x 10(5)/mL. Cells were incubated with or without PRL (500 ng/mL), ACTH (5 nM--a positive control), tyrosine kinase inhibitor--genistein (1; 2.5 or 5 microM), PKC inhibitor--sphingosine (20-1000 nM) and PKC activators--diacylglycerol (DiC8; 10-100 microM) and phorbol ester (PMA; 1-1000 nM). All incubations were performed for 8 h (95% air and 5% CO(2), 37 degrees C). PRL and ACTH (P < 0.05) increased cortisol and androstenedione (A(4)) secretion. DiC8 and PMA mimicked the stimulatory effect of PRL. Sphingosine (P < 0.05) suppressed basal and PRL-stimulated steroid secretion. Genistein inhibited (P < 0.05) PRL-stimulated cortisol secretion and enhanced (P < 0.05) basal and PRL-stimulated A(4) secretion. Moreover, PKC activation was assessed by measuring the specific association of [3H]phorbol dibutyrate ([3H]PDBu) with adrenocortical cells after treatment with PRL or ionomycin (a positive control). PRL (within 2-3 min) and ionomycin (within 2-5 min) increased (P < 0.05) specific binding of [3H]PDBu to the porcine adrenocortical cells. In addition, PRL did not augment the cortisol and A(4) secretion by PKC-deficient adrenocortical cells. In conclusion, presented results support the hypothesis that PKC and tyrosine kinases are involved in PRL signaling in adrenocortical cells in pigs. Moreover, activation of PKC is associated with the increased secretion of cortisol and A(4).

The mechanism of growth-promoting effects of prolactin in embryogenesis--links to growth factors

The polypeptide hormone prolactin (PRL) has been implicated in the regulation of embryonic growth and development, but the control mechanisms involved in the effects of the hormone are poorly understood. Several investigators suggested that there may be a possible link between the effects of PRL and insulin-like growth factors (IGFs). Recent studies have also shown that ligand-induced activation of PRL receptors leads to tyrosine phosphorylation of multiple intracellular proteins, and tyrosine kinase activation takes place in mediating the mitogenic action of PRL. In order to determine whether IGFs are involved in mediating the growth-promoting effect of PRL, rat embryos were culture in vitro for 48 h in whole rat serum and serum depleted of low molecular weight molecules (30 kD retenate) supplemented with rat PRL in the presence and absence of antisera against rat PRL, IGF I and IGF II. To investigate the effects of inhibiting the signal transduction of the PRL receptors, the embryos were preincubated for 2 h in retenate in the presence of tyrosine kinase inhibitors, tyrphostin 47 and genistein, then rat PRL was added to the culture medium. Embryos cultured in retenate showed severe growth retardation, and the addition of rat PRL caused significant increase in growth and development of the embryos suggesting that embryos may be able to utilize maternally derived PRL during organogenesis. The presence of antiserum against rat PRL abolished the PRL-induced increase in development and antibodies against IGF I and II had a similar effect, suggesting that IGFs may be involved in the effect of the hormone. The 2-hour preincubation with genistein and tyrphostin also abolished the PRL-induced increase in development. These results indicate that functional PRL receptors are present in rat embryos at this stage which may play an important role in the control of growth and development and this may be linked to growth factors and their receptors.

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

Prolactin induces milk protein gene expression in rabbit primary mammary cells without any concomitant cell multiplication. Prolactin or other lactogenic hormones is the major inducer of cell division in the rat lymphoid Nb2 cells. In Nb2 cells, prolactin also rapidly induces the expression of the c-myc gene, and beta-actin and stathmin gene expression is induced more slowly. The possible involvement of casein kinase II (CKII), mitogen-activated protein kinase (MAPK) and protein kinase C (PKC) in these process is not well known. The present work was undertaken to evaluate the effect of prolactin on these protein kinases and to determine the possible involvement of these enzymes in the activity of several genes under the control of the hormone. In rabbit mammary cells, prolactin did not alter CKII activity but did transiently stimulate MAP kinase activity. Prolactin also stimulated Ca(2+)-independent PKC. This effect was visible after 10 min and was maintained for at least 24 hr. Staurosporine, an inhibitor of PKC and of several tyrosine kinases altered Ca(2+)-independent PKC only moderately. In contrast, GF 109203X, a potent and specific inhibitor of PKC, abrogated almost all PKC activity. Staurosporine, but not GF 109203X, prevented the induction of the casein gene by prolactin. In Nb2 cells, prolactin induced a slow stimulation of CKII activity. The hormone did not induce MAP kinase activity. Prolactin stimulated Ca(2+)-independent PKC over periods of 24 hr. GF 109203X, but not staurosporine, inhibited PKC activity, whereas staurosporine but not GF 109203X, inhibited the induction of Nb2 cell multiplication and the accumulation of c-myc, beta-actin and stathmin mRNAs. From these data, it can be concluded that (1) the stimulation of CKII by prolactin in Nb2 cells is concomitant with cell multiplication: (2) MAPK stimulation is not necessary for prolactin to induce Nb2 cell multiplication; and (3) PKC is stimulated in mammary and Nb2 cells, but this stimulation is not required for prolactin to stimulate casein, c-myc, beta-actin and stathmin gene expression and Nb2 cell division.

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

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

The effects of various kinase and phosphatase inhibitors on the transmission of the prolactin and extracellular matrix signals to rabbit alpha S1-casein and transferrin genes

In all species, milk protein genes are specifically expressed in the mammary gland under the control of lactogenic hormones and extracellular matrix. In rabbit, casein gene expression is induced by prolactin alone and this induction is amplified by extracellular matrix. Transferrin gene expression is induced by extracellular matrix in the absence of hormones. The transduction mechanisms of prolactin and extracellular matrix to milk protein genes is only partly known. The present study has been undertaken to determine if protein kinases and phosphatases are involved in these mechanisms. Rabbit primary mammary cells were cultured in three different conditions (i) directly on floating collagen I, (ii) on plastic after a trypsinization to remove endogenous extracellular matrix, and (iii) on floating collagen I after a trypsinization to restore a functional extracellular matrix. In these culture conditions, prolactin and several protein kinase and phosphatase inhibitors were added to the medium. The expression of alpha S1-casein and transferrin genes was evaluated using Northern blotting analysis. In cells cultured directly on collagen I, staurosporine, quercetin and 6-dimethylaminopurine strongly inhibited prolactin action of alpha S1-casein gene whereas herbimycin A was only partly inhibitory. An erbstatin analogue, tyrosine phosphate, 1(5 isoquinolylsulphonyl) 2-methylpiperazine and GF 109 203 X did not alter prolactin action. The inhibitors which inhibited prolactin action when cells were directly cultured on collagen I were also those which prevented the induction of alpha S1-casein gene expression when cells were cultured on plastic in the absence of extracellular matrix. The induction of transferrin gene by the extracellular matrix was inhibited slightly by quercetin. Okadaic acid, phenylarsine oxide and sodium pervanadate which inhibit Ser/Thr and Tyr phosphatase inhibitors were unable to mimic prolactin action on alpha S1-casein gene expression. On the contrary, these inhibitors prevented prolactin action. These data suggest that a cascade including protein kinases and phosphatases for Ser/Thr and Tyr phosphate is involved in the transduction of the prolactin message from its receptor to casein genes. The signal delivered to the mammary cells by the extracellular matrix is quite different, possibly involving another cascade of protein kinases.

The rabbit mammary gland prolactin receptor is tyrosine-phosphorylated in response to prolactin in vivo and in vitro

We report the first in vivo study demonstrating tyrosine phosphorylation of mammary gland proteins including the prolactin receptor, in response to the injection of prolactin. Immunoblotting of mammary gland membrane extracts revealed that subunits of 200, 130, 115, 100, 90, 70, and 45 kDa display increased tyrosine phosphorylation within 5 min of prolactin administration. The 100-kDa component was identified as the full-length prolactin receptor by a variety of means including immunoprecipitation and immunoblotting with monoclonal (U5, 917, 110, and 82) and polyclonal (46) antibodies to the prolactin receptor. Maximal receptor phosphorylation was seen within 1 min of hormone injection, and to obtain a strong response it was necessary to deprive rabbits of their endogenous prolactin for 36 h. Rapid tyrosine phosphorylation of the full-length receptor was verified by its demonstration in Chinese hamster ovary cells stably transfected with rabbit prolactin receptor cDNA. Both in vivo and in vitro, the phosphorylation signal was transient, being markedly reduced within 10 min of exposure to prolactin. Tyrosine-phosphorylated receptor was shown to be associated with JAK 2 by immunoblotting of receptor immunoprecipitated from transfected Chinese hamster ovary cells with polyclonal 46. A 48-kDa ATP-binding protein was also shown to be associated with the mammary gland receptor by U5 or polyclonal 46 immunoprecipitation of receptor complexes following covalent labeling with [alpha-32P]azido-ATP. Our demonstration of prolactin receptor tyrosine phosphorylation raises the possibility of signaling pathways regulated by receptor/SH2 protein interaction, which would facilitate prolactin specific responses. The fact that a period of hormone deprivation is needed for significant hormone triggered receptor phosphorylation indicates that the mammary gland receptor exists in a largely desensitized state in vivo, analogous to the related growth hormone receptor.

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

The prolactin (PRL) receptor is a member of the family of cytokine receptors that lack intrinsic tyrosine kinase activity but contain two conserved cysteines in their N-terminal regions and a WSXWS motif adjacent to their transmembrane domains. In a manner similar to the interferons (IFNs), exposure of cells to PRL results in tyrosine phosphorylation of several cellular proteins and the rapid transcriptional induction of the IFN regulatory factor 1 gene. In this communication, we demonstrate that treatment of rat Nb2 lymphoma cells with PRL activates a latent protein factor so that it binds to an enhancer in the IFN regulatory factor 1 gene. This enhancer has been shown to be required for IFN-gamma-activated expression of this gene. PRL-induced assembly of the DNA binding complex, PRL-stimulated factor, required tyrosine phosphorylation. PRL-stimulated factor contained at least one protein that was antigenically similar to the p91 transcription factor, a component of several transcription complexes required for cytokine-activated gene expression. PRL not only induced the tyrosine phosphorylation of p91 but also induced tyrosine phosphorylation of Jak2, a tyrosine kinase required for IFN-gamma-activated gene expression. These results provide evidence for a signaling mechanism, some of whose components are shared by both PRL and IFN-gamma receptors, that results in the expression of early response genes.

Receptor to nucleus signaling by prolactin and interleukin 2 via activation of latent DNA-binding factors

The mechanism of action of prolactin (PRL), a lactogenic and immunoregulatory hormone, has remained undetermined despite its critical role in development. This study identifies a DNA-binding factor induced by PRL that appears to mediate a signal from the cell surface receptor to specific gene expression in the nucleus. PRL stimulates the proliferation of Nb2 T-lymphoma cells and activates transcription of the interferon-regulatory factor 1 (IRF-1) gene. Within minutes of PRL stimulation, a PRL-induced factor (PRLIF) is activated and binds to a target site in the promoter of the IRF-1 gene. The PRLIF-binding site contains an inverted GAAA repeat that is also functional in the hormone-responsive beta-casein gene. The PRL-receptor complex signals tyrosine phosphorylation of JAK2, a nonreceptor tyrosine kinase, which may lead to activation of PRLIF. T-cell proliferation and transcriptional activation of the IRF-1 gene is also induced by the cytokine interleukin 2 (IL-2). This report demonstrates the rapid activation of an IL-2 nuclear-activated factor that recognizes the same GAAA inverted repeat in the IRF-1 promoter. PRLIF and IL-2 nuclear-activated factor are newly identified factors that appear to serve fundamental roles in the signal transduction pathways of PRL and IL-2, respectively, leading to the transcriptional regulation of responsive genes.

Purification and characterization of casein kinase II from lactating rabbit mammary gland

1. Highly purified 200 kDa casein kinase II from rabbit lactating mammary gland (MG-CK II) was obtained by means of a new purification procedure consisting of one phosphocellulose and three Monó Q steps. 2. Its Km for ATP was 2.22 microM and 0.57 mg/ml and 0.13 mg/ml for partially dephosphorylated casein and phosvitin respectively. Stathmine was also suitable as substrate. 2-aminopurine and 6-dimethylaminopurine inhibited efficiently MG-CK II (Ki = 5 and 1 mM respectively). 3. MG-CK II autophosphorylated on its alpha-, alpha'- and beta-subunits. The beta-subunit autophosphorylation was enhanced in presence of exogenous substrate. Its modulation was highly dependent on ATP concentration. 4. The effects of basic compounds which affected dramatically the phosphorylation of dephosphorylated casein in presence of various ATP concentrations were reported.