p42/mitogen-activated protein kinase as a converging target for different growth factor signaling pathways: use of pertussis toxin as a discrimination factor.

Mitogen-activated protein (MAP) kinase is a 42-kDa serine/threonine-specific protein kinase that requires phosphorylation on both tyrosine and threonine residues for activity. This enzyme is rapidly and transiently activated in quiescent cells after addition of various agonists, including insulin, epidermal growth factor, platelet-derived growth factor, and phorbol esters. We show here that addition of the growth factors thrombin or basic fibroblast growth factor to CCL39 fibroblasts rapidly induces tyrosine phosphorylation of the p42 MAP kinase protein and concomitantly stimulates MAP kinase enzymatic activity. To elucidate the signaling pathways utilized in this activation, we took advantage of the sensitivity of CCL39 cells to the toxin of bordetella pertussis, which ADP-ribosylates two Gi proteins in this cell system. We show that pretreatment of cells with the toxin inhibited thrombin stimulation of MAP kinase by greater than 75% but had no detectable effect on the stimulation induced by basic fibroblast growth factor. We also demonstrate that these two growth factors that synergize for mitogenicity are able to cooperate in activation of MAP kinase and that this synergism is partially sensitive to pertussis toxin. Finally, we describe a 44-kDa protein, the tyrosine phosphorylation of which appears to be coregulated with p42 MAP kinase. We conclude that p42 MAP kinase (and the pp44 protein) are at or are downstream from a point of convergence of two different receptor-induced signaling pathways and might well play a key role in integrating those signals.     

Inhibition of oxytocin-stimulated phosphoinositide turnover in rat myometrium by pertussis and cholera toxins may involve protein kinase a activation

Both pertussis and cholera toxins inhibit oxytocin-stimulated phosphoinositide turnover in rat myometrium. The actions of pertussis and cholera toxins as well as those of CPTcAMP are reversed by H-8, an inhibitor of protein kinase A. H-8 does not have a major effect on cAMP elevation by the toxins in the presence of oxytocin. The results suggest that the stimulation by oxytocin of phosphoinositide turnover does not involve direct obligatory coupling to a pertussis toxin-sensitive GTP-binding protein. Rather, indirect effects on protein kinase A activation may contribute to the inhibitory effects of both cholera and pertussis toxins. This study suggests that caution must be exercised in interpreting inhibition of phosphoinositide turnover by pertussis toxin in whole cell experiments as indicative of direct involvement of a toxin-sensitive GTP-binding protein.     

Mastoparan, a novel mitogen for Swiss 3T3 cells, stimulates pertussis toxin-sensitive arachidonic acid release without inositol phosphate accumulation

Mastoparan, a basic tetradecapeptide isolated from wasp venom, is a novel mitogen for Swiss 3T3 cells. This peptide induced DNA synthesis in synergy with insulin in a concentration-dependent manner; half-maximum and maximum responses were achieved at 14 and 17 microM, respectively. Mastoparan also stimulated DNA synthesis in the presence of other growth promoting factors including bombesin, insulin-like growth factor-1, and platelet-derived growth factor. The synergistic mitogenic stimulation by mastoparan can be dissociated from activation of phospholipase C. Mastoparan did not stimulate phosphoinositide breakdown, Ca2+ mobilization or protein kinase C-mediated phosphorylation of a major cellular substrate or transmodulation of the epidermal growth factor receptor. In contrast, mastoparan stimulated arachidonic acid release, prostaglandin E2 production, and enhanced cAMP accumulation in the presence of forskolin. These responses were inhibited by prior treatment with pertussis toxin. Hence, mastoparan stimulates arachidonic acid release via a pertussis toxin-sensitive G protein in Swiss 3T3 cells. Arachidonic acid, like mastoparan, stimulated DNA synthesis in the presence of insulin. The ability of mastoparan to stimulate mitogenesis was reduced by pertussis toxin treatment. These results demonstrate, for the first time, that mastoparan stimulates reinitiation of DNA synthesis in Swiss 3T3 cells and indicate that this peptide may be a useful probe to elucidate signal transduction mechanisms in mitogenesis.     

B-Raf-dependent regulation of the MEK-1/mitogen-activated protein kinase pathway in PC12 cells and regulation by cyclic AMP.

Growth factor receptor tyrosine kinase regulation of the sequential phosphorylation reactions leading to mitogen-activated protein (MAP) kinase activation in PC12 cells has been investigated. In response to epidermal growth factor, nerve growth factor, and platelet-derived growth factor, B-Raf and Raf-1 are activated, phosphorylate recombinant kinase-inactive MEK-1, and activate wild-type MEK-1. MEK-1 is the dual-specificity protein kinase that selectively phosphorylates MAP kinase on tyrosine and threonine, resulting in MAP kinase activation. B-Raf and Raf-1 are growth factor-regulated Raf family members which regulate MEK-1 and MAP kinase activity in PC12 cells. Protein kinase A activation in response to elevated cyclic AMP (cAMP) levels inhibited B-Raf and Raf-1 stimulation in response to growth factors. Ras.GTP loading in response to epidermal growth factor, nerve growth factor, or platelet-derived growth factor was unaffected by protein kinase A activation. Even though elevated cAMP levels inhibited Raf activation, the growth factor activation of MEK-1 and MAP kinase was unaffected in PC12 cells. The results demonstrate that tyrosine kinase receptor activation of MEK-1 and MAP kinase in PC12 cells is regulated by B-Raf and Raf-1, whose activation is inhibited by protein kinase A, and MEK activators, whose activation is independent of cAMP regulation.     

Tyrosine phosphorylation in human neutrophil

Protein tyrosine phosphorylation in human neutrophils was examined by immunoblotting with antibodies specific for phosphotyrosine. The addition of the human hormone granulocyte-macrophage colony stimulating factor to human neutrophils caused an increase in the tyrosine phosphorylation levels of several proteins. The increases in at least two of these proteins having molecular masses of 40 kDa (p40) and 54 kDa (p54) were rapid and were inhibited in pertussis toxin treated cells. The newly synthesized tyrosine kinase inhibitor ST 638 inhibited the increases in the levels of the tyrosine phosphorylation in p92, p78, p54 and p40 proteins. The epidermal growth factor receptor tyrosine kinase inhibitors were less effective. The addition of the chemotactic factor fMet-Leu-Phe to human neutrophils also caused an increase in tyrosine phosphorylation in some of these proteins. The pattern of the fMet-Leu-Phe-induced tyrosine phosphorylation was different from that produced by GM-CSF. The increases were also inhibited by ST 638. In addition, ST 638 inhibited superoxide production but not actin polymerization in control and GM-CSF-treated cells stimulated with fMet-Leu-Phe. Moreover, the active but not inactive phorbol esters increase the tyrosine phosphorylation only in the 40 kDa protein. These results suggest several points: (a) some of the responses produced by GM-CSF and fMet-Leu-Phe are mediated through tyrosine phosphorylation, (b) the GM-CSF receptor is coupled to a pertussis toxin sensitive G-protein, (c) the 40 kDa protein is probably the Gi alpha 2, and (d) the 78 or the 92 kDa protein is most likely the receptor for GM-CSF, which indicates that the receptor may have a tyrosine kinase domain.     

Regulation of transferrin receptor cycling by protein kinase C is independent of receptor phosphorylation at serine 24 in Swiss 3T3 fibroblasts

Treatment of Swiss 3T3 fibroblasts with tumor-promoting phorbol diester or with platelet-derived growth factor caused the phosphorylation of the transferrin receptor by protein kinase C (Ca2+/phospholipid-dependent enzyme) at serine 24 and increased the cell surface expression of the transferrin receptor. The hypothesis that the regulation of transferrin receptor cycling by protein kinase C is causally related to the phosphorylation of the receptor at serine 24 was critically tested. Site-directed mutagenesis of the human transferrin receptor cDNA was used to substitute serine 24 with threonine or alanine residues in order to create phosphorylation defective receptors. Wild-type and mutated transferrin receptors were expressed in Swiss 3T3 fibroblasts using the retrovirus vector pZipNeoSV (X). These receptors were functionally active and caused the receptor-mediated endocytosis of diferric transferrin. Incubation of the fibroblasts with phorbol diester caused the phosphorylation of the wild-type (Ser-24) human transferrin receptor, but this treatment did not result in the phosphorylation of the mutated (Ala-24 and Thr-24) receptors. The cycling of the phosphorylation defective receptors was regulated by phorbol diester and platelet-derived growth factor in a manner similar to that observed for the wild-type receptor. We conclude that the regulation of transferrin receptor cycling by protein kinase C is independent of receptor phosphorylation at serine 24 in Swiss 3T3 fibroblasts.     

Tethering of the platelet-derived growth factor beta receptor to G-protein-coupled receptors. A novel platform for integrative signaling by these receptor classes in mammalian cells

Here we provide evidence to show that the platelet-derived growth factor beta receptor is tethered to endogenous G-protein-coupled receptor(s) in human embryonic kidney 293 cells. The tethered receptor complex provides a platform on which receptor tyrosine kinase and G-protein-coupled receptor signals can be integrated to produce more efficient stimulation of the p42/p44 mitogen-activated protein kinase pathway. This was based on several lines of evidence. First, we have shown that pertussis toxin (which uncouples G-protein-coupled receptors from inhibitory G-proteins) reduced the platelet-derived growth factor stimulation of p42/p44 mitogen-activated protein kinase. Second, transfection of cells with inhibitory G-protein alpha subunit increased the activation of p42/p44 mitogen-activated protein kinase by platelet-derived growth factor. Third, platelet-derived growth factor stimulated the tyrosine phosphorylation of the inhibitory G-protein alpha subunit, which was blocked by the platelet-derived growth factor kinase inhibitor, tyrphostin AG 1296. We have also shown that the platelet-derived growth factor beta receptor forms a tethered complex with Myc-tagged endothelial differentiation gene 1 (a G-protein-coupled receptor whose agonist is sphingosine 1-phosphate) in cells co-transfected with these receptors. This facilitates platelet-derived growth factor-stimulated tyrosine phosphorylation of the inhibitory G-protein alpha subunit and increases p42/p44 mitogen-activated protein kinase activation. In addition, we found that G-protein-coupled receptor kinase 2 and beta-arrestin I can associate with the platelet-derived growth factor beta receptor. These proteins play an important role in regulating endocytosis of G-protein-coupled receptor signal complexes, which is required for activation of p42/p44 mitogen-activated protein kinase. Thus, platelet-derived growth factor beta receptor signaling may be initiated by G-protein-coupled receptor kinase 2/beta-arrestin I that has been recruited to the platelet-derived growth factor beta receptor by its tethering to a G-protein-coupled receptor(s). These results provide a model that may account for the co-mitogenic effect of certain G-protein-coupled receptor agonists with platelet-derived growth factor on DNA synthesis.     

Regulatory factors that determine growth and phenotype of normal human melanocytes

Normal human melanocytes, unlike malignant melanoma cells, required at least three growth-promoting agents, i.e., phorbol ester for protein kinase C activation and the growth factors basic fibroblast growth factor (bFGF) and insulin, for growth in chemically defined W489 medium. Cell growth was further stimulated by addition of agents that increase intracellular levels of cyclic adenosine 3',5'-monophosphate (cAMP) to the medium. Among these agents, the pituitary hormones alpha-melanocyte-stimulating hormone (alpha-MSH) and follicle-stimulating hormone were the most potent, whereas bacterial toxins, including cholera, tetanus, and pertussis toxin and their subunits either were less mitogenic or gave variable results depending on the culture tested. Medium containing phorbol ester PMA, growth factors bFGF and insulin (or insulin-like growth factor-I), and synthetic alpha-MSH supported melanocyte growth for more than 5 months with doubling times between 5 and 8 days. Two copper-binding proteins, ceruloplasmin and tyrosinase, were mitogenic when added to medium and ceruloplasmic induced a long bi- to tripolar-shape of cells. Addition of 1 mM dibutyryl cAMP to the medium led to the formation of dendrites in all cells, with an average of 28 extensions per cell. Although cell growth was inhibited by dibutyryl cAMP, cells were not terminally differentiated and continued to proliferate. Dendritic melanocytes showed a 2.2-fold increase in activity of the tyrosine kinase pp60c-src. The induction of dendritic processes in melanocytes by dibutyryl cAMP or sodium butyrate was reversible and appears to reflect the expression of the mature melanocytic phenotype in situ.     

Insulin and platelet-derived growth factor stimulate phosphorylation of the c-raf product at serine and threonine residues in intact cells

We have examined the phosphorylation of the serine threonine kinase, the product of c-raf proto-oncogene in response to insulin or platelet-derived growth factor in intact cells. Both insulin and platelet-derived growth factor stimulated phosphorylation of the c-raf protein about 2- to 3-fold. The phosphorylation occurred exclusively on serine and threonine residues; phosphotyrosine was not detected. In immune-complex kinase assays, treatment with insulin, and platelet-derived growth factor increased autophosphorylation of the c-raf kinase, suggesting activation of its kinase activity. To investigate whether the phosphorylation of the c-raf protein in intact cells results from an autophosphorylation event or from the phosphorylation by other cellular kinase(s), we replaced lysine 375 in the putative ATP-binding domain of the c-raf protein with alanine using oligonucleotide site-directed mutagenesis and expressed the mutated protein in NIH3T3 cells. The substitution resulted in the inactivation of the serine/threonine-specific autophosphorylation in immune-complex kinase assays. In intact cells, however, although phosphorylation of the mutant protein in response to insulin and platelet-derived growth factor occurred to a lesser extent than that of the wild-type protein, the phosphopeptide maps were indistinguishable. These results suggest that serine threonine phosphorylation might be responsible for the activation of c-raf kinase upon treatment of cells with insulin and platelet-derived growth factor, and most of the phosphate associated with the c-raf protein results from its phosphorylation by as yet uncharacterized cellular serine/threonine kinase(s).     

Dissociation of mitogen-activated protein kinase activation from p125 focal adhesion kinase tyrosine phosphorylation in Swiss 3T3 cells stimulated by bombesin, lysophosphatidic acid, and platelet-derived growth factor.

The experiments presented here were designed to examine the contribution of p125 focal adhesion kinase (p125FAK) tyrosine phosphorylation to the activation of the mitogen-activated protein kinase cascade induced by bombesin, lysophosphatidic acid (LPA), and platelet-derived growth factor (PDGF) in Swiss 3T3 cells. We found that tyrosine phosphorylation of p125FAK in response to these growth factors is completely abolished in cells treated with cytochalasin D or in cells that were suspended in serum-free medium for 30 min. In marked contrast, the activation of p42mapk by these factors was independent of the integrity of the actin cytoskeleton and of the interaction of the cells with the extracellular matrix. The protein kinase C inhibitor GF 109203X and down-regulation of protein kinase C by prolonged pretreatment of cells with phorbol esters blocked bombesin-stimulated activation of p42mapk, p90rsk, and MAPK kinase-1 but did not prevent bombesin-induced tyrosine phosphorylation of p125FAK. Furthermore, LPA-induced p42mapk activation involved a pertussis toxin-sensitive guanylate nucleotide-binding protein, whereas tyrosine phosphorylation of p125FAK in response to LPA was not prevented by pretreatment with pertussis toxin. Finally, PDGF induced maximum p42mapk activation at concentrations (30 ng/ml) that failed to induce tyrosine phosphorylation of p125FAK. Thus, our results demonstrate that p42mapk activation in response to bombesin, LPA, and PDGF can be dissociated from p125FAK tyrosine phosphorylation in Swiss 3T3 cells.     

Foetal calf serum enhances cholera toxin-catalysed ADP-ribosylation of the pertussis toxin-sensitive guanine nucleotide binding protein, Gi2, in rat glioma C6BU1 cells

The major G-protein of rat glioma C6BU1 cells corresponds immunologically to Gi2. In the absence of guanine nucleotides, this protein is shown to be a substrate for ADP-ribosylation catalysed by both cholera and pertussis toxins. Under these conditions, a receptor for a growth factor, which has previously been shown to be activated by foetal calf serum, modulated the effects of both cholera and pertussis toxins on the G-protein. These ligand-mediated alterations of cholera and pertussis toxin-catalysed ADP ribosylation demonstrate that, in this system, the growth factor receptor interacts functionally with Gi2.     

Growth factor-stimulated protein phosphorylation in 3T3-L1 cells. Evidence for protein kinase C-dependent and -independent pathways

Exposure of serum-deprived 3T3-L1 fibroblasts to phorbol 12-myristate 13-acetate (PMA), synthetic diacylglycerols, platelet-derived growth factor (PDGF), or pituitary fibroblast growth factor (FGF) resulted in stimulated phosphorylation of an acidic, multicomponent, soluble protein of Mr 80,000. Phosphorylation of this protein was promoted to a lesser extent by epidermal growth factor; however, neither insulin nor dibutyryl cAMP was effective. Phosphoamino acid analysis and peptide mapping of the Mr 80,000 32P-protein after exposure of fibroblasts to PDGF revealed identical patterns to those obtained with PMA or diacylglycerols. In contrast to the Mr 80,000 protein, proteins of Mr 22,000 (and pI 4.4) and Mr 31,000 were also phosphorylated in response to insulin as well as to PMA, diacylglycerols, epidermal growth factor, PDGF, and FGF in these cells. Similar findings were noted in fully differentiated 3T3-L1 adipocytes. Preincubation of the cells with high concentrations of active phorbol esters abolished specific [3H]phorbol 12,13-dibutyrate binding, protein kinase C activity, and immunoreactivity and also prevented stimulated phosphorylation of the Mr 80,000 protein by PMA, diacylglycerols, PDGF, or FGF, supporting the contention that this effect was mediated through protein kinase C. The stimulated phosphorylation of the Mr 22,000 and 31,000 proteins in response to PMA was also abolished by such pretreatment. In contrast, the ability of insulin, PDGF, and FGF to promote phosphorylation of the Mr 22,000 and 31,000 proteins was unaffected in the protein kinase C-deficient cells. We conclude that PDGF and FGF may exert some of their effects on these cells through at least two distinct pathways of protein phosphorylation, phorbol ester-like (P) activation of protein kinase C, and an insulin-like (I) pathway exemplified by phosphorylation of the Mr 22,000 and 31,000 proteins.     

cAMP inhibits CSF-1-stimulated tyrosine phosphorylation but augments CSF-1R-mediated macrophage differentiation and ERK activation

Macrophage colony stimulating factor (M-CSF) or CSF-1 controls the development of the macrophage lineage through its receptor tyrosine kinase, c-Fms. cAMP has been shown to influence proliferation and differentiation in many cell types, including macrophages. In addition, modulation of cellular ERK activity often occurs when cAMP levels are raised. We have shown previously that agents that increase cellular cAMP inhibited CSF-1-dependent proliferation in murine bone marrow-derived macrophages (BMM) which was associated with an enhanced extracellular signal-regulated kinase (ERK) activity. We report here that increasing cAMP levels, by addition of either 8-bromo cAMP (8BrcAMP) or prostaglandin E(1) (PGE1), can induce macrophage differentiation in M1 myeloid cells engineered to express the CSF-1 receptor (M1/WT cells) and can potentiate CSF-1-induced differentiation in the same cells. The enhanced CSF-1-dependent differentiation induced by raising cAMP levels correlated with enhanced ERK activity. Thus, elevated cAMP can promote either CSF-1-induced differentiation or inhibit CSF-1-induced proliferation depending on the cellular context. The mitogen-activated protein kinase/extracellular signal-related protein kinase kinase (MEK) inhibitor, PD98059, inhibited both the cAMP- and the CSF-1R-dependent macrophage differentiation of M1/WT cells suggesting that ERK activity might be important for differentiation in the M1/WT cells. Surprisingly, addition of 8BrcAMP or PGE1 to either CSF-1-treated M1/WT or BMM cells suppressed the CSF-1R-dependent tyrosine phosphorylation of cellular substrates, including that of the CSF-1R itself. It appears that there are at least two CSF-1-dependent pathway(s), one MEK/ERK dependent pathway and another controlling the bulk of the tyrosine phosphorylation, and that cAMP can modulate signalling through both of these pathways.     

The effect of retinoic acid on protein phosphorylation in mouse melanoma cells

Vitamin A inhibits growth and increases the activity of cAMP-dependent protein kinase in B16 mouse melanoma cells. In this report we show that retinoic acid (RA) treatment of intact cells alters their subsequent in vitro protein phosphorylation, but we could not demonstrate any changes in in vivo protein phosphorylation. A 48-h treatment with RA results in a concentration-dependent decrease of protein phosphorylation of a 95K molecular weight (MW) protein in both supernatant and particulate fractions. The phosphorylation of this protein does not appear to be regulated by cAMP. Proteins at 92K and 82K MW in the supernatant fraction are increased in phosphorylation. The former (but not the latter) is regulated by cAMP. In the particulate fraction a variety of proteins 12K-68K MW are increased in phosphorylation, as the cells are treated with increasing amounts of RA. The phosphorylation of most of these proteins is regulated by cAMP. Another inhibitor of B16 cell growth, melanocyte-stimulating hormone (MSH) also alters protein phosphorylation. At short incubation periods (1 h), this hormone stimulates phosphorylation of a number of proteins (17-40K MW), while in longer incubation periods (48 h) phosphorylation is inhibited. All of these phosphorylations appear to be regulated by cAMP. We attempted to repeat these observations using intact-cell phosphorylation with 32PO4. In two experiments we saw small changes in the phosphorylation of proteins. In most experiments, however, we could find no change in the phosphoproteins. Further experiments have led us to question the in vivo phosphorylation, since treatment of the cells with MSH, cholera toxin, or db-cAMP also did not affect intact-cell protein phosphorylation. We have previously documented that under these latter conditions cAMP levels are greatly elevated and cAMP-dependent protein kinase is activated. The in vitro phosphorylation results suggests that in RA-treated cells, kinase activities and/or protein substrate levels are changing. However, the physiological significance of the particular MW phosphoproteins changes we have described must await resolution of the in vivo phosphorylation data.     

Tannic acid, a potent inhibitor of epidermal growth factor receptor tyrosine kinase

Increasing evidence supports the hypothesis that tannic acid, a plant polyphenol, exerts anticarcinogenic activity in chemically induced cancers. In the present study, tannic acid was found to strongly inhibit tyrosine kinase activity of epidermal growth factor receptor (EGFr) in vitro (IC50 = 323 nM). In contrast, the inhibition by tannic acid of p60(c-src) tyrosine kinase (IC50 = 14 microM) and insulin receptor tyrosine kinase (IC50 = 5 microM) was much weaker. The inhibition of EGFr tyrosine kinase by tannic acid was competitive with respect to ATP and non-competitive with respect to peptide substrate. In cultured cells, growth factor-induced tyrosine phosphorylation of growth factor receptors, including EGFr, platelet-derived growth factor receptor, and basic fibroblast growth factor receptor, was inhibited by tannic acid. No inhibition of insulin-induced tyrosine phosphorylation of insulin receptor and insulin-receptor substrate-1 was observed. EGF-stimulated growth of HepG2 cells was inhibited in the presence of tannic acid. The inhibition of serine/threonine-specific protein kinases, including cAMP-dependent protein kinase, protein kinase C and mitogen-activated protein kinase, by tannic acid was only detected at relatively high concentration, IC50 being 3, 325 and 142 microM respectively. The molecular modeling study suggested that tannic acid could be docked into the ATP binding pockets of either EGFr or insulin receptor. These results demonstrate that tannic acid is an in vitro potent inhibitor of EGFr tyrosine kinase.     

Regulation of the phosphatidylinositol 3-kinase,Akt/protein kinase B,FRAP/mammalian target of rapamycin,and ribosomal S6 kinase 1 signaling pathways by thyroid-stimulating hormone (TSH) and stimulating type TSH receptor antibodies in the thyroid gland

Thyroid-stimulating hormone (TSH) regulates the growth and differentiation of thyrocytes by activating the TSH receptor (TSHR). This study investigated the roles of the phosphatidylinositol 3-kinase (PI3K), PDK1, FRAP/mammalian target of rapamycin, and ribosomal S6 kinase 1 (S6K1) signaling mechanism by which TSH and the stimulating type TSHR antibodies regulate thyrocyte proliferation and the follicle activities in vitro and in vivo. The TSHR immunoprecipitates exhibited PI3K activity, which was higher in the cells treated with either TSH or 8-bromo-cAMP. TSH and cAMP increased the tyrosine phosphorylation of TSHR and the association between TSHR and the p85alpha regulatory subunit of PI3K. TSH induced a redistribution of PDK1 from the cytoplasm to the plasma membrane in the cells in a PI3K- and protein kinase A-dependent manner. TSH induced the PDK1-dependent phosphorylation of S6K1 but did not induce Akt/protein kinase B phosphorylation. The TSH-induced S6K1 phosphorylation was inhibited by a dominant negative p85alpha regulatory subunit or by the PI3K inhibitors wortmannin and LY294002. Rapamycin inhibited the phosphorylation of S6K1 in the cells treated with either TSH or 8-bromo-cAMP. The stimulating type TSHR antibodies from patients with Graves disease also induced S6K1 activation, whereas the blocking type TSHR antibodies from patients with primary myxedema inhibited TSH- but not the insulin-induced phosphorylation of S6K1. In addition, rapamycin treatment in vivo inhibited the TSH-stimulated thyroid follicle hyperplasia and follicle activity. These findings suggest an interaction between TSHR and PI3K, which is stimulated by TSH and cAMP and might involve the downstream S6K1 but not Akt/protein kinase B. This pathway may play a role in the TSH/stimulating type TSH receptor antibody-mediated thyrocyte proliferation in vitro and in the response to TSH in vivo.     

Phorbol 12-myristate 13-acetate inhibits granulocyte-macrophage colony stimulating factor-induced protein tyrosine phosphorylation in a human factor-dependent hematopoietic cell line.

The human myeloid cell line MO7 requires either granulocyte-macrophage colony stimulating factor (GM-CSF) or interleukin 3 (IL-3) for proliferation. We have previously shown that both GM-CSF and IL-3 transiently induce tyrosine phosphorylation of a number of proteins, including two cytosolic proteins, p93 and p70, which are maximally phosphorylated 5-15 min after addition of growth factor to factor-deprived cells. GM-CSF-induced proliferation of MO7 cells was found to be inhibited by two activators of protein kinase C, phorbol 12-myristate 13-acetate (PMA) and bryostatin-1. PMA did not affect surface expression or affinity of the GM-CSF receptor but significantly inhibited GM-CSF- or IL-3-induced tyrosine phosphorylation of p93 and p70. In contrast, PMA augmented GM-CSF-induced tyrosine phosphorylation of another protein, p42. Pretreatment of cells with sodium orthovanadate to inhibit protein tyrosine phosphatases (PTPase) partially reversed the inhibitory effects of PMA. These results suggest that one aspect of GM-CSF and IL-3 signal transduction, protein tyrosine phosphorylation, can be inhibited by a mechanism which does not involve receptor down-regulation, and may involve either receptor down-regulation, and may involve either inhibition of a receptor-activated tyrosine kinase, activation of a protein tyrosine phosphatase, or both. This mechanism could be important in exerting control of proliferation of some types of hematopoietic cells.     

Protein-tyrosine kinases regulate the phosphorylation, protein interactions, subcellular distribution, and activity of p21ras GTPase-activating protein.

The p21ras GTPase-activating protein (GAP) down-regulates p21ras by stimulating its intrinsic GTPase activity. GAP is found predominantly as a monomer in the cytosol of normal cells. However, in cells expressing an activated cytoplasmic protein-tyrosine kinase, p60v-src, or stimulated with epidermal growth factor, GAP becomes phosphorylated on tyrosine and serine and forms distinct complexes with two phosphoproteins of 62 and 190 kDa (p62 and p190). In v-src-transformed Rat-2 cells, a minor fraction of GAP associates with the highly tyrosine phosphorylated p62 to form a complex that is localized at the plasma membrane and in the cytosol. In contrast, the majority of GAP enters a distinct complex with p190 that is exclusively cytosolic and contains predominantly phosphoserine. Epidermal growth factor stimulation also induces a marked conversion of monomeric GAP to higher-molecular-weight species in rat fibroblasts. The GAP-p190 complex is dependent on phosphorylation and shows reduced GAP activity. These results indicate that protein-tyrosine kinases induce GAP to form multiple heteromeric complexes, which are strong candidates for regulators or targets of p21ras.     

Na+/K+/Cl- cotransporter activates MAP-kinase cascade downstream to protein kinase C, and upstream to MEK

In this study, we demonstrated that the specific inhibitors of the Na+/K+/Cl- cotransporter (NKCC1), bumetanide and furosemide, inhibited extracellular regulated kinase (ERK) phosphorylation in Balb/c 3T3 fibroblasts, stimulated with a variety of mitogens. In addition to fibroblast growth factor (FGF) shown before, the various mitogens tested in the present study (endothelial growth factor (EGF), platelet-derived growth factor (PDGF), insulin, thrombin, and the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA)). Enter, the Ras/Raf/MEK/ERK cascade via different growth factors receptors and through one of the two main routes. The results of the present study provide evidence that have led us to conclude that the target protein which is controlled by the Na+/K+/Cl- cotransporter, is downstream of tyrosine kinase receptors, as well as of the G-protein-coupled receptor (GPCR). Several additional lines of evidence supported the above conclusion: (i) furosemide inhibits phosphorylation of MAPK kinase (MEK) induced by receptor tyrosine kinase (RTK) ligands, such as PDGF, FGF, and EGF. (ii) Furosemide also inhibited ERK phosphorylation, induced by thrombin, a GPCR. (iii) Furosemide inhibited MEK and ERK phosphorylation even when ERK phosphorylation was induced by direct activation of protein kinase C (PKC) by TPA, which bypasses early steps of the mitogenic cascade. In addition, we found that furosemide did not affect PKC phosphorylation induced directly by TPA. Taken together, the results of the present study indicate that the signal transduction protein, controlled by the Na+/K+/Cl- cotransporter, must be downstream of the PKC, and at/or upstream to MEK in the Ras/Raf/MEK/ERK cascade.