Bombesin and epidermal growth factor stimulate the mitogen-activated protein kinase through different pathways in Swiss 3T3 cells

Staurosporine aglycone bilaterally regulates ERK1/2 phosphorylation in rat pulmonary arterial smooth muscle cells

Staurosporine, a protein kinase C (PKC) inhibitor, has been reported to regulate the phosphorylation of ERK1/2 in several cell lines. It is still unknown, however, whether its derivative staurosporine aglycone (SA) has the same effect on ERK1/2 activation. In this study, we investigated the effect of SA on ERK1/2 activity in rat pulmonary arteries and pulmonary arterial smooth muscle cells (PASMCs). The pulmonary arteries and PASMCs were treated with SA at different time points and concentrations, and the activation of ERK1/2 was analyzed by Western blotting. The results showed that SA at nanomolar concentrations suppressed ERK1/2 phosphorylation through the PKC pathway alone, but SA at 30 micromol/L for 2 h enhanced the phosphorylation of ERK1/2. The activation of ERK1/2 was inhibited by the MAPK/ERK kinase inhibitor PD98059 or the protein kinase A (PKA) activator isoproterenol. Together, these results suggest that SA has a strong dual regulating effect on ERK1/2 through the PKC and (or) PKA pathways in rat PASMCs.

Activation of extracellular signal-regulated kinase mediates bombesin-induced mitogenic responses in prostate cancer cells

Bombesin and its mammalian homologue gastrin-releasing peptide have been shown to be highly expressed and secreted by neuroendocrine cells in prostate cancer, and are thought to be related to the carcinogenesis and progression of this disease. We found, in this study, bombesin specifically induced mitogen-activated protein (MAP) kinase activation as shown by increased extracellular regulated kinase (ERK) phosphorylation and epidermal growth factor (EGF) receptor transactivation in prostate cancer cells, which express functional gastrin-releasing peptide receptor. The transactivation of EGF receptor was required for bombesin-induced ERK phosphorylation. Furthermore, non-receptor tyrosine kinase Src and cellular Ca2+ were shown to be involved in bombesin-induced EGF receptor transactivation and ERK phosphorylation. Inhibition of either EGF receptor transactivation or ERK activation blocked bombesin-induced DNA synthesis in these cells. Taken together, these data suggest bombesin may act as a mitogen in prostate cancer by activating MAP kinase pathway via EGFR transactivation.

Extracellular signal-regulated kinases and contractile responses in ovine adult and fetal cerebral arteries

Accumulating evidence suggests that extracellular signal-regulated kinases (ERK1/2) play a key role in regulating vascular tone. To test the hypotheses that ERK1/2 modulate cerebral artery agonist-induced contraction, and that this changes with developmental age, we measured both total and phosphorylated ERK1/2 in adult and fetal ovine cerebral arteries. In middle cerebral arteries (MCA) we also examined tension and [Ca2+]i responses to phenylephrine (PHE), in the absence and presence of the ERK1/2 inhibitor U-0126 and the mitogen-activated protein kinase kinase (MAPKK or MEK) inhibitor PD-98059. In the fetus, but not adult, U-0126 potentiated PHE-induced contraction. In both age groups, inhibition by U-0126, but not PD-98059, decreased the PHE-induced [Ca2+]i increase; in fact for adult, this eliminated any significant [Ca2+]i increase. In turn in the adult, but not fetus, protein kinase C (PKC) inhibition by staurosporine (3 x 10(-8) M) prior to ERK1/2 inhibition by U-0126 (10(-5) M) prevented this elimination of [Ca2+]i increase. In adult and fetal cerebral arteries basal total ERK1/2 levels were similar. However, in fetal arteries the basal phosphorylated ERK1/2 levels were significantly less than in adult. In fetal, but not adult, cerebral arteries, 10(-6)-10(-4) M PHE increased ERK1/2 phosphorylation in a concentration- and time-dependent manner. The ERK1/2 inhibitor U-0126, but not the MEK inhibitor PD-98059, lowered basal activated ERK1/2 levels in vessels of both age groups. These results suggest that basal levels of phosphorylated ERK1/2 play an important role in suppressing Ca2+ sensitivity, perhaps by PKC inhibition. The developmental increase in cerebral artery basal phosphorylated ERK levels from fetus to adult, suggests a transition in the regulation of contraction from Ca2+ sensitivity in the fetal arteries to Ca2+ concentration in the adult vessels.

Determination of endogenous extracellular signal-regulated protein kinase by microchip capillary electrophoresis

The application of microchip capillary electrophoresis (CE) to the assay of extracellular signal-regulated protein kinase (ERK) is presented. In this assay, ERK catalyzes the transfer of gamma-phosphate from adenosine 5(')-triphosphate to the threonine residue of a fluorescently labeled nonapeptide (APRTPGGRR), and the phosphorylated and nonphosphorylated peptides were detected by fluorescence. The phosphorylated and nonphosphorylated peptides and the internal standard were separated within 20s, and the increase in magnitude of the phosphorylated peptide peak was monitored to assess ERK activity. ERK reactions were prepared off-chip and analyzed on a single-lane glass microchip fabricated by standard methods. It was demonstrated that microchip CE could be used to measure endogenous amounts of ERK by spiking known concentrations of recombinant ERK2 into the lysates of serum-starved human umbilical vein endothelial cells (HUVEC) and recovering between 90 and 100% for all samples. Endogenous ERK activity was determined by microchip where HUVEC were stimulated with 500pM vascular endothelial growth factor (VEGF) at different times before cell lysis. The results showed a transient VEGF-mediated ERK activation that peaked at 10min, which was consistent with previous reports using conventional techniques. The microchip assay provided a rapid, accurate, and precise alternative to conventional methods of determining endogenous ERK activity.

Role of the different mitogen-activated protein kinase subfamilies in the stimulation of dog and human thyroid epithelial cell proliferation by cyclic adenosine 5'-monophosphate and growth factors

We have investigated the role of the different classes of MAPKs, i.e. ERKs, c-Jun N-terminal kinases (JNKs), and p38 MAPK in the proliferation of dog and human thyroid epithelial cells (thyrocytes) in primary cultures. In these cells, TSH, acting through cAMP, epidermal growth factor, hepatocyte growth factor (HGF), and phorbol 12-myristate 13-acetate induce DNA synthesis. With the exception of HGF, all of these factors require the presence of insulin for mitogenic effects to be expressed. We found that TSH and forskolin are without effect on the phosphorylation and activity of the different classes of MAPKs. In contrast, all the cAMP-independent growth factors, whereas without effect on the phosphorylation and activity of JNKs and p38 MAPK, stimulated the ERKs. This effect was strong and sustained in response to HGF, epidermal growth factor and 12-myristate 13-acetate but weak and transient in response to insulin. Moreover, whereas in stimulated cells DNA synthesis was inhibited by PD 098059, an inhibitor of MAPK kinase 1 and consequently of ERKs, it was not modified by SB 203580, an inhibitor of p38 MAPK. Taken together, these data 1) exclude a role of JNKs and p38 MAPK in the proliferation of dog and human thyrocytes; 2) suggest that the mitogenic action of the cAMP-independent agents requires a strong and sustained activation of both ERKs and phosphatidylinositol 3-kinase/protein kinase B as realized by HGF alone or by the other agents together with insulin; and 3) show that TSH and cAMP do not activate ERKs but that the weak activation of ERKs by insulin is nevertheless necessary for DNA synthesis to occur.

EGF receptor function is required in late G(1) for cell cycle progression induced by bombesin and bradykinin

We examined the role of epidermal growth factor (EGF) receptor (EGFR) tyrosine kinase activation in G protein-coupled receptor (GPCR) agonist-induced mitogenesis in Swiss 3T3 and Rat-1 cells. Addition of EGFR tyrosine kinase inhibitors (e.g., tyrphostin AG-1478) abrogated bombesin-induced extracellular signal-regulated kinase (ERK) activation in Rat-1 cells but not in Swiss 3T3 cells, indicating the importance of cell context in determining the role of EGFR in ERK activation. In striking contrast, treatment with tyrphostin AG-1478 markedly (~70%) inhibited DNA synthesis induced by bombesin in both Swiss 3T3 and Rat-1 cells. Similar inhibition of bombesin-induced DNA synthesis in Swiss 3T3 cells was obtained using four structurally different inhibitors of EGFR tyrosine kinase. Furthermore, kinetic analysis indicates that EGFR function is necessary for bombesin-induced mitogenesis in mid-late G(1) in both Swiss 3T3 and Rat-1 cells. Our results indicate that EGFR kinase activity is necessary in mid-late G(1) for promoting the accumulation of cyclins D1 and E and implicate EGFR function in the coupling of GPCR signaling to the activation of the cell cycle.

Activator protein-1 transcription factor mediates bombesin-stimulated cyclooxygenase-2 expression in intestinal epithelial cells

Colorectal carcinogenesis is a complex, multistep process involving genetic alterations and progressive changes in signaling pathways regulating intestinal epithelial cell proliferation, differentiation, and apoptosis. Although cyclooxygenase-2 (COX-2), gastrin-releasing peptide (GRP), and its receptor, GRP-R, are not normally expressed by the epithelial cells lining the human colon, the levels of all three proteins are aberrantly overexpressed in premalignant adenomatous polyps and colorectal carcinomas of humans. Overexpression of these proteins is associated with altered epithelial cell growth, adhesion, and tumor cell invasiveness, both in vitro and in vivo; however, a mechanistic link between GRP-R-mediated signaling pathways and increased COX-2 overexpression has not been established. We report that bombesin, a homolog of GRP, potently stimulates the expression of COX-2 mRNA and protein as well as the release of prostaglandin E(2) from a rat intestinal epithelial cell line engineered to express GRP-R. Bombesin stimulation of COX-2 expression requires an increase in [Ca(2+)](i), activation of extracellular signal-regulated kinase (ERK)-1 and -2 and p38(MAPK), and increased activation and expression of the transcription factors Elk-1, ATF-2, c-Fos, and c-Jun. These data suggest that the expression of GRP-R in intestinal epithelial cells may play a role in carcinogenesis by stimulating COX-2 overexpression through an activator protein-1-dependent pathway.

Proinsulin C-peptide rapidly stimulates mitogen-activated protein kinases in Swiss 3T3 fibroblasts: requirement of protein kinase C, phosphoinositide 3-kinase and pertussis toxin-sensitive G-protein

It has been demonstrated that proinsulin C-peptide possesses several biological activities and that its specific binding sites are present on the surface of cell membranes. However, the molecular and cellular mechanisms of C-peptide actions are poorly known. In the present study we examined the possible involvement of the mitogen-activated protein kinase (MAPK) pathway in C-peptide effects. C-peptide induced the phosphorylation of MAPK [p44 extracellular signal-regulated kinase 1 (ERK1) and p42 ERK2] in Swiss 3T3 and 3T3-F442A fibroblasts but not in 3T3-L1 fibroblasts and some other cell lines such as L(6)E(9) muscle cells. In Swiss 3T3 cells, C-peptide-induced phosphorylation of MAPK was dependent on time and concentration, being maximal at 1 min and at 1 nM C-peptide and was accompanied by an increase in MAPK activity and MAPK kinase (MEK) phosphorylation. The MAPK phosphorylation by C-peptide was abolished by treatment with pertussis toxin (PTX) and also with a MEK inhibitor, PD 98059. In addition, MAPK phosphorylation was attenuated by treatment with a phosphoinositide 3-kinase (PI-3K) inhibitor, wortmannin, and with a protein kinase C (PKC) inhibitor, GF109203X, and by down-regulation of PKC by prolonged treatment with PMA. Similar effects of the inhibitors and PTX were found on the MAPK phosphorylation induced by neuropeptide Y. These results suggest that C-peptide activates MAPK through a putative G(i)/G(o)-linked receptor for C-peptide and through PI-3K-dependent and PKC-dependent pathways.

Stimulation of mitogen-activated protein kinase by gonadotropin-releasing hormone in human granulosa-luteal cells

The present study investigated the activation of mitogen-activated protein kinases (MAPKs) by a GnRH agonist (GnRHa) in human granulosa-luteal cells (hGLCs). The phosphorylation state of p44 and p42 MAPK was examined using antibodies that distinguish phospho-p44/42 MAPK (Thr(202)/Tyr(204)) from total p44/42 MAPK (activated plus inactivated). Activation of MAPK by GnRHa was observed within 5 min and was sustained for 60 min after treatment. GnRHa stimulated MAPK activation in a dose-dependent manner, with maximum stimulation (6.7-fold over basal levels) at 10(-7) M. Pretreatment with a protein kinase C (PKC) inhibitor, GF109203X, completely blocked GnRHa-induced MAPK activation. In addition, pretreatment with a PKC activator, phorbol-12-myristate 13-acetate, potentiated GnRH-induced MAPK activation. These results indicate that GnRHa stimulates MAPK activation through a PKC-dependent pathway in hGLCs, possibly coupled to G(q)alpha protein. MAPK activation was also observed in response to 8-bromo-cAMP or cholera toxin, but not pertussis toxin. Forskolin (50 microM) substantially stimulated a rapid cAMP accumulation, whereas GnRHa (10(-7) M) or pertussis toxin (100 mg/ml) did not affect basal intracellular cAMP levels. Cotreatment of GnRHa (10(-7) M) did not attenuate forskolin- or hCG-stimulated cAMP accumulation. These results suggest that the GnRH receptor is probably not coupled to G(s)alpha or G(i)alpha in hGLCs. Finally, GnRHa (10(-7) M) stimulated a significant increase in Elk-1 phosphorylation and c-fos messenger RNA expression, as revealed by an in vitro kinase assay and Northern blot analysis, respectively. These results clearly demonstrate that GnRH activates the MAPK cascade through a PKC-dependent pathway in the human ovary.

Insulin selectively stimulates nuclear phosphoinositide-specific phospholipase C (PI-PLC) beta1 activity through a mitogen-activated protein (MAP) kinase-dependent serine phosphorylation

Using NIH 3T3 cells, we have investigated nuclear phosphoinositide metabolism in response to insulin, a molecule which acts as a proliferating factor for this cell line and which is known as a powerful activator of the mitogen-activated protein (MAP) kinase pathway. Insulin stimulated inositol lipid metabolism in the nucleus, as demonstrated by measurement of the diacylglycerol mass produced in vivo and by in vitro nuclear phosphoinositide-specific phospholipase C (PI-PLC) activity assay. Despite the fact that nuclei of NIH 3T3 cells contained all of the four isozymes of the beta family of PI-PLC (i.e. beta1, beta2, beta3, and beta4), insulin only activated the beta1 isoform. Insulin also induced nuclear translocation of MAP kinase, as demonstrated by Western blotting analysis, enzyme activity assays, and immunofluorescence staining, and this translocation was blocked by the specific MAP kinase kinase inhibitor PD98059. By means of both a monoclonal antibody recognizing phosphoserine and in vivo labeling with [(32)P]orthophosphate, we ascertained that nuclear PI-PLC-beta1 (and in particular the b subtype) was phosphorylated on serine residues in response to insulin. Both phosphorylation and activation of nuclear PI-PLC-beta1 were substantially reduced by PD98059. Our results conclusively demonstrate that activation of nuclear PI-PLC-beta1 strictly depends on its phosphorylation which is mediated through the MAP kinase pathway.

Activation of the luteinizing hormone beta promoter by gonadotropin-releasing hormone requires c-Jun NH2-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by gonadotropin-releasing hormone (GnRH) in the gonadotrope cell line LbetaT2 was investigated. Treatment with gonadotropin-releasing hormone agonist (GnRHa) activates extracellular signal-regulated kinase (ERK) and c-Jun NH(2)-terminal kinase (JNK). Activation of ERK by GnRHa occurred within 5 min, and declined thereafter, whereas activation of JNK by GnRHa occurred with a different time frame, i.e. it was detectable at 5 min, reached a plateau at 30 min, and declined thereafter. GnRHa-induced ERK activation was dependent on protein kinase C or extracellular and intracellular Ca(2+), whereas GnRHa-induced JNK activation was not dependent on protein kinase C or on extracellular or intracellular Ca(2+). To determine whether a mitogen-activated protein kinase family cascade regulates rat luteinizing hormone beta (LHbeta) promoter activity, we transfected the rat LHbeta (-156 to +7)-luciferase construct into LbetaT2 cells. GnRH activated the rat LHbeta promoter activity in a time-dependent manner. Neither treatment with a mitogen-activated protein kinase/ERK kinase (MEK) inhibitor, PD98059, nor cotransfection with a catalytically inactive form of a mitogen-activated protein kinase construct inhibited the induction of the rat LHbeta promoter by GnRH. Furthermore, cotransfection with a dominant negative Ets had no effect on the response of the rat LHbeta promoter to GnRH. On the other hand, cotransfection with either dominant negative JNK or dominant negative c-Jun significantly inhibited the induction of the rat LHbeta promoter by GnRH. In addition, GnRH did not induce either the rat LHbeta promoter activity in LbetaT2 cells transfected stably with dominant negative c-Jun. These results suggest that GnRHa differentially activates ERK and JNK, and a JNK cascade is necessary to elicit the rat LHbeta promoter activity in a c-Jun-dependent mechanism in LbetaT2 cells.

Rat CXC chemokine GRO/CINC-1 paradoxically stimulates the growth of gastric epithelial cells

BACKGROUND

CXC chemokines such as interleukin (IL)-8 are neutrophil chemoattractants, the levels of which increase in Helicobacter pylori-infected gastric mucosa. Many investigators have focused on the chemotactic aspects of IL-8: however, CXC chemokines are also reported to have angiogenic activity and to serve as remodelling factors. Rat GRO/CINC-1 is a rodent counterpart of human GROalpha, a member of the family of CXC chemokines. Gastric mucosa infected with H. pylori is in a state of hyperproliferation, with increases in the amounts of growth factors such as hepatocyte growth factor (HGF).

AIM

To investigate whether rat GRO/CINC-1 had growth-stimulating activity for gastric epithelial cells.

METHODS

The rat gastric epithelial cell line RGM-1 was incubated in serum-free medium for 12 h to adjust the cell cycle to the G0 phase, and GRO/CINC-1 was then added for 24 h. The total cell number was determined by fluorogenic analysis after propidium iodide staining, and cell proliferation was assessed by measuring 5-bromo-2'-deoxyuridine (BrdU) incorporation. The activity of p42/p44 mitogen-activated protein kinase (MAPK) was measured 5-20 min after the start of GRO/CINC-1 exposure.

RESULTS

Cultures treated with GRO/CINC-1 showed a significant increase in cell number and BrdU incorporation in a concentration-dependent fashion. The MAPK activity increased within 5 min after GRO/CINC-1 application and returned to the control level at 20 min.

CONCLUSION

The growth-stimulatory effect of GRO/CINC-1 on rat gastric epithelial cells suggests a dual function of this chemokine: proinflammatory action and induction of epithelial proliferation.

Prolactin-releasing peptide activation of the prolactin promoter is differentially mediated by extracellular signal-regulated protein kinase and c-Jun N-terminal protein kinase

Regulation of the mitogen-activated protein kinase (MAPK) family by prolactin-releasing peptide (PrRP) in both GH3 rat pituitary tumor cells and primary cultures of rat anterior pituitary cells was investigated. PrRP rapidly and transiently activated extracellular signal-regulated protein kinase (ERK) in both types of cells. Both pertussis toxin, which inactivates G(i)/G(o) proteins, and exogenous expression of a peptide derived from the carboxyl terminus of the beta-adrenergic receptor kinase I, which specifically blocks signaling mediated by the betagamma subunits of G proteins, completely blocked the PrRP-induced ERK activation, suggesting the involvement of G(i)/G(o) proteins in the PrRP-induced ERK activation. Down-regulation of cellular protein kinase C did not significantly inhibit the PrRP-induced ERK activation, suggesting that a protein kinase C-independent pathway is mainly involved. PrRP-induced ERK activation was not dependent on either extracellular Ca(2+) or intracellular Ca(2+). However, the ERK cascade was not the only route by which PrRP communicated with the nucleus. JNK was also shown to be significantly activated in response to PrRP. JNK activation in response to PrRP was slower than ERK activation. Moreover, to determine whether a MAPK family cascade regulates rat prolactin (rPRL) promoter activity, we transfected the intact rPRL promoter ligated to the firefly luciferase reporter gene into GH3 cells. PrRP activated the rPRL promoter activity in a time-dependent manner. Co-transfection with a catalytically inactive form of a MAPK construct or a dominant negative JNK, partially but significantly inhibited the induction of the rPRL promoter by PrRP. Furthermore, co-transfection with a dominant negative Ets completely abolished the response of the rPRL promoter to PrRP. These results suggest that PrRP differentially activates ERK and JNK, and both cascades are necessary to elicit rPRL promoter activity in an Ets-dependent mechanism.

Activation of protein kinase C is required for the stable attachment of adherent platelets to collagen but is not needed for the initial rapid adhesion under flow conditions

We have investigated the role of protein kinase C (PKC) in the initial events of alpha(2)beta(1)-integrin-mediated platelet adhesion to collagen under flow conditions. Although adhesion caused activation of PKC, as evidenced by pleckstrin phosphorylation, the PKC inhibitors GF 109203X and Gö 6976 had no effect on adhesion, even though they prevented pleckstrin phosphorylation. The initial kinetics and extent of platelet adhesion to collagen (<5 seconds) and tyrosine phosphorylation of p125(FAK) and p72(syk) were not influenced by the PKC inhibitors, whereas adhesion to polylysine was prevented. These results indicate that adhesion to collagen and polylysine involve different mechanisms and requirements for PKC activation. Pretreatment with GF 109203X destabilized collagen-adherent platelets, accelerating their detachment, which was associated with tyrosine dephosphorylation of p125(FAK). Thus, although PKC activation was not required for rapid platelet adhesion to collagen, it appears to play an important role in stabilizing the attachment of adherent platelets to collagen. We also examined the effect of PKC activation by the phorbol ester phorbol 12-myristate 13-acetate (PMA) on platelet adhesion to collagen. PMA at 100 nmol/L strongly potentiated adhesion and tyrosine phosphorylation of p125(FAK) and p72(syk) and activated beta(1)-integrins, as determined by increased exposure of the 15/7 epitope. The PMA-stimulated adhesion was partially blocked by an anti-alpha(2)beta(1) antibody, was completely inhibited by GF 109203X, and was not correlated with the extent of pleckstrin phosphorylation. Therefore, strong PKC activation may lead to inside-out signaling, enhancing the role of beta(1)-integrins in adhesion. Pleckstrin phosphorylation does not appear to be involved in the initial phase of basic or PMA-stimulated adhesion but may help stabilize the adherent platelets.

Modulation of interleukin-6 by beta2-adrenoceptor in endotoxin-stimulated renal macrophage cells

BACKGROUND

Activation of the cAMP signaling pathway by means of beta2-adrenoceptor agonists has been shown to up-regulate interleukin-6 (IL-6) gene expression and to stimulate IL-6 production in macrophage cells. However, whether beta2-adrenoceptor activation can also modify the rate of IL-6 production in macrophage cells activated by the bacterial endotoxins has not yet been determined. Using renal resident macrophage cells treated with endotoxin, lipopolysaccharide (LPS), and beta2-adrenoceptor agonist, terbutaline, we investigated the role of cAMP pathway, tumor necrosis factor (TNF)-alpha and mitogen-activated protein kinase (MAPK) pathway (p42/p44) in regulating IL-6 production.

METHODS

IL-6 protein, mRNA, and promoter activity were measured in these cells exposed to LPS (1 microg/ml) and/or terbutaline (10(-9) to 10(-6) M). Furthermore, the time course effects of terbutaline on cAMP, MAPK (p42/p44), and TNF-alpha release were evaluated in the cells.

RESULTS

Terbutaline at high concentrations (10(-6) M) significantly up-regulated IL-6 by approximately 25% (P<0.05), whereas at a lower concentration (10(-8) M), it down-regulated IL-6 production by 42% (P<0.05). Terbutaline (10(-8) and 10(-6) M) caused a concentration- and time-dependent stimulation of cAMP (P<0.05) and TNF production (P<0.05) and a time-dependent decrease in MAPK activity (P<0.05). Following the addition of a cAMP inhibitor, IL-6 promoter activity was correlated with TNF-alpha levels and MAPK activity.

CONCLUSIONS

A biphasic effect of beta2-adrenoceptor agonist on IL-6 production in renal resident macrophage cells became apparent when LPS was exposed to the cells. The terbutaline-induced down-regulation of IL-6 gene production was mediated by an inhibitory effect of terbutaline on TNF-alpha, which was exerted through the MAPK and cAMP pathways, whereas the up-regulation appeared to be due to a direct action of intracellular cAMP.

Involvement of intermediary metabolites in the pathway of extracellular Ca2+-induced mitogen-activated protein kinase activation in human fibroblasts

Human fibroblasts in culture will grow in serum-free medium containing serum replacement factors, but without protein growth factors, as long as the Ca2+ level is 1.0-2.0 mM. When the Ca2+ is reduced to 0.1 mM, the cells stop cycling, but they can be reinduced to cycle by raising the Ca2+ level to 1.0 mM Ca2+ or to higher concentrations that result in activation of mitogen-activated protein kinase (MAPK). We now report that exposure of human fibroblasts to extracellular Ca2+ increased the level of inositol (1,4,5)-trisphosphate in the cytoplasm and caused a transient rise in the concentration of intracellular free Ca2+. Ca2+-induced MAPK activation was partly abolished by treatment of the cells with pertussis toxin. It was also decreased by treatment of cells with thapsigargin, which depletes intracellular Ca2+ stores; with phorbol 12-myristyl 13-acetate (PMA), which down-regulates protein kinase C (PKC); with the calmodulin antagonists N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide HCl (W-7), and calmidazolium (24571); as well as with lanthanum, a Ca2+ channel inhibitor. Ca2+ stimulation did not result in phosphorylation of the c-raf-1 protein. Our results suggest that extracellular Ca2+ stimulates MAPK activation through a pathway(s) involving a pertussis toxin-sensitive G protein, phospholipase C, intracellular free Ca2+, calmodulin, and PKC.

Effects of bombesin on methadone-induced apoptosis of human lung cancer cells

The therapeutic opioid methadone, used to treat cancer pain and opioid addiction, is also a potent inducer of apoptosis in human lung cancer cells, thereby inhibiting their growth. However, in contrast to its central nervous system (CNS) actions, this effect appears to be mediated through a non-opioid mechanism involving bombesin, an autocrine growth-stimulatory factor that plays a central role in the early events of pulmonary carcinogenesis. Exposure of 'variant' small cell lung carcinoma (SCLC) and non-SCLC cells, which secrete low concentrations (< 0.01 pmol/mg protein) of bombesin, to nanomolar concentrations of methadone resulted in increased levels of mitogen-activated protein (MAP) kinase phosphatases and inactivation of MAP kinase, suppression of the bcl-2 protein, and induction of apoptosis. These effects of methadone were reversed by the addition of bombesin to the culture medium, at concentrations of < 1 microM, and 'classic' SCLC cells, which secrete high concentrations of bioactive bombesin (> 6 pmol/mg protein), were found not to respond to methadone. Thus, methadone's effectiveness is dependent upon the concentration of bioactive bombesin secreted by lung cancer cells. Methadone treatment suggests a novel therapeutic approach for patients presenting 'variant' SCLC and non-SCLC morphologies, since they respond less to conventional therapy.

A2B adenosine and P2Y2 receptors stimulate mitogen-activated protein kinase in human embryonic kidney-293 cells. cross-talk between cyclic AMP and protein kinase c pathways

Mitogen-activated protein kinase (MAPK) cascades underlie long-term mitogenic, morphogenic, and secretory activities of purinergic receptors. In HEK-293 cells, N-ethylcarboxamidoadenosine (NECA) activates endogenous A2BARs that signal through Gs and Gq/11. UTP activates P2Y2 receptors and signals only through Gq/11. The MAPK isoforms, extracellular-signal regulated kinase 1/2 (ERK), are activated by NECA and UTP. H-89 blocks ERK activation by forskolin, but weakly affects the response to NECA or UTP. ERK activation by NECA or UTP is unaffected by a tyrosine kinase inhibitor (genistein), attenuated by a phospholipase C inhibitor (U73122), and is abolished by a MEK inhibitor (PD098059) or dominant negative Ras. Inhibition of protein kinase C (PKC) by GF 109203X failed to block ERK activation by NECA or UTP, however, another PKC inhibitor, Ro 31-8220, which unlike GF 109203X, can block the zeta-isoform, and prevents UTP- but not NECA-induced ERK activation. In the presence of forskolin, Ro 31-8220 loses its ability to block UTP-stimulated ERK activation. PKA has opposing effects on B-Raf and c-Raf-1, both of which are found in HEK-293 cells. The data are explained by a model in which ERK activity is modulated by differential effects of PKC zeta and PKA on Raf isoforms.

Mitogen-activated protein kinase cascade is involved in endothelin-1-induced rat puerperal uterine contraction

The regulation of mitogen-activated protein (MAP) kinase by endothelin-1 (ET-1) in cultured rat puerperal uterine myometrial cells was investigated. ET-1 caused the rapid stimulation of MAP kinase activity. ET-1-induced MAP kinase activation is neither extracellular Ca2+- nor intracellular Ca2+-dependent. ET-1 stimulation also led to an increase in phosphorylation of son-of-sevenless (SOS), and transfection of dominant negative SOS attenuated the ET-1-induced MAP kinase activity. Phorbol-12-myristate 13-acetate (PMA) also induced the MAP kinase activity, but pretreatment of the cultured cells with PMA, to down-regulate protein kinase C (PKC), did not abolish the activation of MAP kinase by ET-1. In addition, down-regulation of PKC had no effect on ET-1-induced SOS phosphorylation. Pertussis toxin, which inactivates Gi/Go proteins, blocked the ET-1-induced MAP kinase activation but not the PMA-induced MAP kinase activation. The results suggested that MAP kinase is acutely activated by ET-1 through a pertussis toxin-sensitive G protein and SOS, not through the PMA-sensitive PKC. In addition, although reverse-transcriptase PCR assays detected messenger RNA for both ET- 1 receptor subtypes in cultured rat puerperal uterine myometrial cells, ET-1-induced MAP kinase activity and uterine contraction were blocked by treatment with BQ485, an antagonist selective for an ET type A receptor (but not by BQ788, an ET type B receptor antagonist). Ritodrine, which is known to relax uterine muscle contraction, attenuated ET-1-induced MAP kinase activity. We further examined the role of MAP kinase pathway in uterine contraction using an inhibitor of MEK activity, PD098059. This inhibitor completely inhibited the ET-1-induced MAP kinase activation and partially, but significantly, inhibited the ET-1-induced uterine contraction. These results indicate that ET-1-induced MAP kinase signaling cascade may play an important role in the ET-1-induced uterine contraction.

Differential role of insulin receptor substrate (IRS)-1 and IRS-2 in L6 skeletal muscle cells expressing the Arg1152 --> Gln insulin receptor

In L6 muscle cells expressing the Arg1152 --> Gln insulin receptor (Mut), basal tyrosine phosphorylation of insulin receptor substrate (IRS)-1 was increased by 35% compared with wild-type cells (WT). Upon exposure to insulin, IRS-1 phosphorylation increased by 12-fold in both the Mut and WT cells. IRS-2 was constitutively phosphorylated in Mut cells and not further phosphorylated by insulin. The maximal phosphorylation of IRS-2 in basal Mut cells was paralleled by a 4-fold increased binding of the kinase regulatory loop binding domain of IRS-2 to the Arg1152 --> Gln receptor. Grb2 and phosphatidylinositol 3-kinase association to IRS-1 and IRS-2 reflected the phosphorylation levels of the two IRSs. Mitogen-activated protein kinase activation and [3H]thymidine incorporation closely correlated with IRS-1 phosphorylation in Mut and WT cells, while glycogen synthesis and synthase activity correlated with IRS-2 phosphorylation. The Arg1152 --> Gln mutant did not signal Shc phosphorylation or Shc-Grb2 association in intact L6 cells, while binding Shc in a yeast two-hybrid system and phosphorylating Shc in vitro. Thus, IRS-2 appears to mediate insulin regulation of glucose storage in Mut cells, while insulin-stimulated mitogenesis correlates with the activation of the IRS-1/mitogen-activated protein kinase pathway in these cells. IRS-1 and Shc-mediated mitogenesis may be redundant in muscle cells.

Apolipoprotein A-I stimulates human placental lactogen release by activation of MAP kinase

Apolipoprotein A-I (apo A-I) stimulates human placental lactogen (hPL) release via protein kinase C (PKC)-dependent pathways. Since PKC has been shown to activate the MAP kinase cascade in other cell types, we examined the effect of two inhibitors of the MAP kinase cascade on apo A-I-induced hPL secretion and the effect of apo A-I on MAP kinase activity in human trophoblast cells. Apigenin (10 microM) and PD98059 (100 microM) inhibited apo A-I-induced hPL release by 94 and 73%, respectively. Moreover, apo A-I activated MAP kinase in a time- and dose-dependent manner. Activation of PKC by phorbol myristate acetate (PMA) stimulated MAP kinase activity, and down-regulation of PKC completely prevented apo A-I-stimulation of MAP kinase activity. Taken together, these results strongly suggest that activation of MAP kinase is involved in the intracellular mechanism of apo A-I-induced hPL release.

Comparison of the signal transduction pathways activated by gastrin in enterochromaffin-like and parietal cells

BACKGROUND & AIMS

Gastrin stimulates acid secretion from parietal cells and histamine release from enterochromaffin-like (ECL) cells through identical gastrin receptors. However, gastrin has been shown to have a trophic effect only on ECL cells. The aim of this study was to compare gastrin-induced signal transduction pathways in the ECL and parietal cells of Mastomys natalensis, an African rodent.

METHODS

Both ECL and parietal cells were isolated from the gastric mucosa of M. natalensis, and intracellular signal transduction events in response to gastrin were investigated.

RESULTS

Gastrin elicited histamine release from ECL cells and acid secretion from parietal cells in association with enhanced inositol phospholipid turnover. Although gastrin increased [3H]thymidine incorporation into ECL cells, it had no effect on parietal cells. Moreover, tyrosine phosphorylation and activation of mitogen-activated protein (MAP) kinase as well as c-fos and c-jun gene expression were augmented only in ECL cells. In addition, gastrin increased the formation of guanosine triphosphate-Ras with a simultaneous decrease in guanosine diphosphate-Ras levels in ECL but not in parietal cells.

CONCLUSIONS

Although gastrin receptors are present in both ECL and parietal cells, they activate the Ras-MAP kinase pathway only in ECL cells.

Activation of mitogen-activated protein kinases/extracellular signal-regulated kinases in human hepatocellular carcinoma

Mitogen-activated protein kinase/extracellular signal-regulated protein kinase (MAPK/ERK) is a key molecule in intracellular signal transducing pathways that transport extracellular stimuli from cell surface to nuclei. MAPK/ERK has been revealed to be involved in the physiological proliferation of mammalian cells and also to potentiate them to transform. However, its role in the outgrowth of human hepatocellular carcinoma (HCC) has yet to be clarified. Therefore, in this study, we investigated the activation of MAPK/ERK and its associated gene expression in HCC. MAPK/ERK was activated in 15 of 26 cases of HCC we examined (58%), and its activity level was significantly higher in HCC than in the adjacent non-cancerous lesions. Besides, MAPK/ERK activation in HCC was positively correlated with protein expression of transcription factor c-Fos. Furthermore, in 25 of 26 cases of HCC which genomic DNA was available, 22 cases without genomic DNA amplification exhibited positive correlation, not only between protein expression of c-Fos and cyclin D1, but also between MAPK/ERK activation and cyclin D1 expression. Concerning the relationship between MAPK/ERK activation and the clinicohistopathological features of HCC, the tumor (HCC) versus non-tumor (non-cancerous counterpart) ratio (T/N) of MAPK/ERK activity was positively correlated with tumor size, but neither with the stage of HCC nor the degree of differentiation of HCC. In conclusion, these findings suggest that MAPK/ERK activation in human HCC may play an important role in multistep hepatocarcinogenesis, especially in the progression of HCC; at least in part, through cyclin D1 up-regulation primarily induced by MAPK/ERK via c-Fos.

Bombesin stimulates cholecystokinin secretion through mitogen-activated protein-kinase-dependent and -independent mechanisms in the enteroendocrine STC-1 cell line

Bombesin has been reported to stimulate cholecystokinin (CCK) secretion from rat duodeno-jejunal I-cells. Bombesin was shown to activate mitogen-activated protein kinases (MAPKs) in cell types such as Swiss 3T3 fibroblasts and rat pancreatic acinar cells. No information is available on whether MAPK is activated in intestinal endocrine cells upon bombesin stimulation. This was studied by using the CCK-producing enteroendocrine cell line STC-1. Bombesin stimulated markedly and transiently both p42(MAPK) and p44(MAPK), with a maximum at 2 min, and a decrease to basal levels within 10 min. As expected, bombesin stimulated MAPK kinase 1 (MEK-1) activity. Activation of protein kinase C (PKC) with PMA also stimulated p42(MAPK), p44(MAPK) and MEK-1. Treatment of cells with PD 098059 (at 10 microM or 30 microM), which selectively inhibits MEK phosphorylation, blocked bombesin-induced p42(MAPK) and p44(MAPK) activation for at least 90 min. However, PD 098059 inhibited bombesin- and PMA-stimulated CCK secretion during the first 15 min, but failed to significantly reduce CCK release at later times. Inhibition of PKC with staurosporine, or PKC down-regulation by prolonged treatment with PMA, both drastically decreased MEK-1, p42(MAPK) and p44(MAPK) activation upon bombesin stimulation. Additionally, PKC activation appeared to be required for both MAPK-dependent (early) and -independent (late) CCK responses to bombesin. It is concluded that the early CCK secretory response of STC-1 cells to bombesin involves MAPK pathway activation through a PKC-dependent mechanism, whereas the late phase of bombesin-induced CCK secretion, that also requires PKC, appears to result from a MAPK-independent process.

[D-Arg1,D-Phe5,D-Trp7,9,Leu11]Substance P acts as a biased agonist toward neuropeptide and chemokine receptors

Substance P derivatives are potential therapeutic compounds for the treatment of small cell lung cancer and can cause apoptosis in small cell lung cancer cells in culture. These peptides act as broad spectrum neuropeptide antagonists, blocking calcium mobilization induced by gastrin-releasing peptide, bradykinin, cholecystokinin, and other neuropeptides. We show that [D-Arg1,D-Phe5,D-Trp7,9, Leu11]substance P has unique agonist activities in addition to this described antagonist function. At doses that block calcium mobilization by neuropeptides, this peptide causes activation of c-Jun N-terminal kinase and cytoskeletal changes in Swiss 3T3 fibroblasts and stimulates migration and calcium flux in human neutrophils. Activation of c-Jun N-terminal kinase is dependent on the expression of the gastrin-releasing peptide receptor in rat 1A fibroblasts, demonstrating that the responses to the peptide are receptor-mediated. We hypothesize that [D-Arg1,D-Phe5,D-Trp7,9, Leu11]substance P acts as a biased agonist on neuropeptide and related receptors, activating certain guanine nucleotide-binding proteins through the receptor, but not others.

Extracellular sphingosine 1-phosphate stimulates formation of ethanolamine from phosphatidylethanolamine: modulation of sphingosine 1-phosphate-induced mitogenesis by ethanolamine

In this work, we determined the effects of sphingosine 1-phosphate (S1P) on phospholipase D (PLD)-mediated hydrolysis of phosphatidylethanolamine (PtdEtn), and evaluated the effects of the water-soluble product ethanolamine on S1P-induced DNA synthesis in NIH 3T3 cells. In [14C]ethanolamine-labelled cells, S1P (0.5-5 microM) stimulated PLD-mediated hydrolysis of PtdEtn 1.5-2.1-fold. Down-regulation of protein kinase C by chronic (24 h) treatment of cells with 300 nM PMA, or pretreatments (10 min) with the cell-permeant calcium chelator 1,2-bis-(O-aminophenoxy)-ethane-N,N, N',N'-tetra-acetic acid tetra-acetoxymethyl ester led to the inhibition of S1P-induced PtdEtn hydrolysis. S1P alone was a weak inducer of DNA synthesis, but its effects were enhanced by phosphocholine (PCho), insulin, ATP or PMA. Ethanolamine (5-100 microM) did not modify the mitogenic effect of S1P alone, whereas at 50-100 microM concentrations it actually enhanced the mitogenic effect of PCho via a mitogen-activated protein (MAP) kinase-independent mechanism. In contrast, 5-20 microM concentrations of ethanolamine, which correspond to normal blood ethanolamine levels in humans, strongly inhibited DNA synthesis induced by S1P plus PCho via a MAP kinase-dependent mechanism; importantly, less or no inhibition was observed with 50-100 microM concentrations of ethanolamine. At 5-50 microM concentrations, ethanolamine also inhibited the synergistic mitogenic effects of both S1P plus insulin (22-27% inhibition) and PCho plus ATP (45-73% inhibition) but not those of S1P plus PMA or S1P plus ATP. The results indicate that S1P stimulates PLD-mediated hydrolysis of PtdEtn by a mechanism that may involve a regulatory protein kinase C isoform. Increased formation of ethanolamine by PLD-mediated PtdEtn hydrolysis or by other means may be required for maximal stimulation of DNA synthesis by S1P in the presence of insulin, and particularly PCho.

Activation and signal transduction via mitogen-activated protein (MAP) kinases in T lymphocytes

The various mitogen-activated protein (MAP) kinases have central roles in the signalling pathways of T lymphocytes. Their activation is uniquely dependent on dual phosphorylation of a serine/threonine and a tyrosine residue and is regulated by several levels of kinases in parallel cascades. In addition, both the MAP kinases and their upstream, activating kinases are regulated by several phosphatases. Although each of the MAP kinases have many cytoplasmic substrates, their ability to translocate to the nucleus means that they can transmit signals from the cytoplasm directly to transcription factors, which are sometimes nuclear bound. The MAP kinase cascades are activated in T lymphocytes by a variety of different external stimuli. They play an important role in transducing both the signal from T cell receptor and costimulatory molecules, on the T cell surface, and are able to regulate several of the transcription factors controlling the expression of critical genes, including that for IL-2. This review examines how the activation of several MAP kinases is regulated, their role in signal transduction initiated by a variety of stimuli, and how this may lead to different cellular responses.

Involvement of protein kinase C in the activation of extracellular signal-regulated kinase 1/2 by UVC irradiation

UVC irradiation activates mitogen-activated protein kinases (MAPKs), including ERK, JNK, and P38. This study examined the role of protein kinase C (PKC) in the regulation of UVC-stimulated MAPKs activation. Either the depletion of PKC by prolonged treatment of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) or the inhibition of PKC by a selective PKC inhibitor, UCN-01-ME, attenuated UVC-activation of ERK1/2, keeping the activation of JNK1/2 intact. However, K252a, a non-selective PKC inhibitor, inhibited the activation of both ERK1/2 and JNK1/2 by UVC. In three isoforms of PKC (alpha, delta, epsilon) examined, PKC epsilon shows the most evident translocation, a temporal association with cell membrane, upon the UVC irradiation of NIH 3T3 cells. These results suggest that PKC is acting in the UVC-dependent activation of ERK1/2, and PKC epsilon is one of the PKC isozymes playing such a role.

Bombesin and zinc enhance the synergistic mitogenic effects of insulin and phosphocholine by a MAP kinase-dependent mechanism in Swiss 3T3 cells

Simultaneous treatment of serum-starved (24 h) Swiss 3T3 cells with insulin (500 nM) and phosphocholine (PCho) (0.25-1 mM) resulted in synergistic stimulation of DNA synthesis via a mitogen activated protein (MAP) kinase-independent rapamycin-sensitive mechanism. Co-treatment of cells with bombesin (10 nM) or zinc (25 microM) enhanced the combined mitogenic effects of insulin and PCho 2-3-fold; however, in the presence of bombesin or zinc the combined effects of insulin and PCho were not inhibited by rapamycin. The potentiating effects of bombesin and zinc on insulin plus PCho-induced DNA synthesis were accompanied by large stimulation of p42 MAP kinase activity. The results indicate that in Swiss 3T3 cell cultures, synergistic stimulation of DNA synthesis by extracellular insulin and PCho via a p42 MAP kinase-dependent mechanism requires the presence of other growth regulatory agents, such as bombesin or zinc.

Bombesin, vasopressin, endothelin, bradykinin, and platelet-derived growth factor rapidly activate protein kinase D through a protein kinase C-dependent signal transduction pathway

Protein kinase D (PKD) is a serine/threonine protein kinase that is activated by phorbol esters via protein kinase C in intact cells. To assess the physiological significance of this putative pathway, we examined the regulation of PKD in living cells by mitogenic regulatory peptides and by platelet-derived growth factors (PDGF). Our results demonstrate that bombesin rapidly induces PKD activation in Swiss 3T3 cells, as shown by autophosphorylation and syntide-2 phosphorylation assays. Maximum PKD activation (14-fold above base-line levels) was obtained 90 s after bombesin stimulation. Bombesin also induced PKD activation in Rat-1 cells stably transfected with the bombesin/gastrin releasing peptide (GRP) receptor and in COS-7 cells transiently co-transfected with PKD and bombesin/GRP receptor expression constructs. No inducible kinase activity was demonstrated when COS-7 cells were transfected with a kinase-deficient PKD mutant. Bombesin-mediated PKD activation was prevented by treatment of Swiss 3T3 cells with the protein kinase C inhibitors GF 1092030X and Ro 31-8220. In contrast, these compounds did not inhibit PKD activity when added directly in vitro. Vasopressin, endothelin, and bradykinin also activated PKD in Swiss 3T3 cells through a PKC-dependent pathway. Platelet-derived growth factor-stimulated PKD activation in Swiss 3T3 cells and in porcine aortic endothelial cells stably transfected with PDGF-beta receptors. Treatment with GF 1092030X or Ro 31-8220 inhibited PKD activation induced by PDGF. Thus, our results indicate that PKD is activated by multiple signaling peptides through a protein kinase C-dependent signal transduction pathway in a variety of cell types.

Role of mitogen-activated protein kinase pathway in prostaglandin F2alpha-induced rat puerperal uterine contraction

In this study, prostaglandin (PG) F2alpha was found to activate mitogen-activated protein (MAP) kinase and MAP kinase kinase (MEK) in cultured rat puerperal uterine myometrial cells. PGF2alpha stimulation also led to an increase in phosphorylation of raf-1, son of sevenless (SOS), and Shc. Furthermore, we examined the mechanism by which PGF2alpha induced MAP kinase phosphorylation. Both pertussis toxin (10 ng/ml), which inactivates Gi/Go proteins, and expression of a peptide derived from the carboxyl terminus of the beta-adrenergic receptor kinase 1 (betaARK1), which specifically blocks signaling mediated by the betagamma subunits of G proteins, blocked the PGF2alpha-induced activation of MAP kinase. Ritodrine (1 microM), which is known to relax uterine muscle contraction, attenuated PGF2alpha-induced tyrosine phosphorylation of MAP kinase. Moreover, to examine the role of MAP kinase pathway in uterine contraction, an inhibitor of MEK activity, PD098059, was used. Although MEK inhibitor had no effect on PGF2alpha-induced calcium mobilization, this inhibitor partially inhibited PGF2alpha-induced uterine contraction. These results provide evidence that PGF2alpha stimulates the MAP kinase signaling pathway in cultured rat puerperal uterine myometrial cells through Gbetagamma protein, suggesting that this new pathway may play an important role in the biological action of PGF2alpha on these cells.

Leptin induces mitogen-activated protein kinase-dependent proliferation of C3H10T1/2 cells

Leptin, secreted by adipocytes, regulates satiety and energy expenditure. Several forms of leptin receptors produced by alternative mRNA splicing are found in many tissues, including the hypothalamus, liver, lung, kidney, hematopoietic cells, and gonads, suggesting that leptin exerts effects in these tissues. In accordance with the distribution of leptin receptors, there is accumulating evidence that leptin plays various roles in reproduction, hematopoiesis, and the immune systems in addition to the regulation of food intake and energy expenditure. In the present study, we examined the in vitro effects of leptin on proliferation of a mouse embryonic cell line, C3H10T1/2, and its mechanism of action. Leptin caused a dose- and time-dependent increase in mitogen-activated protein kinase (MAPK) activity that was accompanied by an increase in C3H10T1/2 cell number. The MAPK kinase-1-specific inhibitor PD98059 completely blocked the increases in both MAPK activity and cell proliferation caused by leptin. These findings indicate that leptin stimulates the proliferation of C3H10T1/2 cells via the MAPK cascade.

Mechanism of mitogen-activated protein kinase activation by gonadotropin-releasing hormone in the pituitary of alphaT3-1 cell line: differential roles of calcium and protein kinase C

The mechanism of mitogen-activated protein kinase (MAPK, ERK) stimulation by the GnRH analog [D-Trp6]GnRH (GnRH-a) was investigated in the gonadotroph-derived alphaT3-1 cell line. GnRH-a as well as the protein kinase C (PKC) activator 12-O-tetradecanoyl phorbol-13-acetate (TPA) stimulated a sustained response of MAPK activity, whereas epidermal growth factor (EGF) stimulated a transient response. MAPK kinase (MEK) is also activated by GnRH-a, but in a transient manner. GnRH-a and TPA apparently activated mainly the MAPK isoform ERK1, as revealed by Mono-Q fast protein liquid chromatography followed by Western blotting as well as by gel kinase assay. GnRH-a and TPA stimulated the tyrosine phosphorylation of several proteins, and this effect as well as the stimulation of MAPK activity were inhibited by the PKC inhibitor GF 109203X. Similarly, down-regulation of TPA-sensitive PKC subspecies nearly abolished the effect of GnRH-a and TPA on MAPK activity. Furthermore, the protein tyrosine kinase (PTK) inhibitor genistein inhibited protein tyrosine phosphorylation and reduced GnRH-a-stimulated MAPK activity by 50%, suggesting the participation of genistein-sensitive and insensitive pathways in GnRH-a action. Although Ca2+ ionophores have only a marginal stimulatory effect, the removal of Ca2+ markedly reduced MAPK activation by GnRH-a and TPA, but had no effect on GnRH-a and TPA stimulation of protein tyrosine phosphorylation. Interestingly, the removal of Ca2+ also partly inhibited the activation of MAPK by EGF and vanadate/H2O2. Thus, a calcium-dependent component(s) downstream of PKC and PTK might also participate in MAPK activation. Elevation of cAMP by forskolin exerted partial inhibition on EGF, but not on TPA or GnRH-a action, suggesting that MEK activators other than Raf-1 might be involved in GnRH action. We conclude that Ca2+, PTK, and PKC participate in the activation of MAPK by GnRH-a, with Ca2+ being necessary downstream to PKC and PTK.

Effect of growth factors on the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase in Rat-1 fibroblasts

The activation of glycolytic flux is a biochemical characteristic of growing cells. Several reports have demonstrated the role of fructose 2,6-bisphosphate in this process. In this paper we show that the levels of 6-phosphofructo-2-kinase/fructose-2, 6-bisphosphatase (6PF2K/Fru-2,6-P2ase) mRNA are modulated in response to serum and growth factors and this effect is due to regulation of its transcription rate. The modulation of the expression of this enzyme by growth factors differs according their mitogenic effect; both lysophosphatidic acid and epidermal growth factor, when added alone, increased the mRNA levels, but endothelin had no effect. Furthermore, cAMP, which acts as an antimitogenic signal in Rat-1 fibroblasts, produced a decrease in 6PF2K/Fru-2, 6-P2ase mRNA and inhibited the effects of lysophosphatidic acid and epidermal growth factor on 6PF2K/Fru-2,6-P2ase expression. PD 098059, a specific inhibitor of the activation of the mitogen-activated protein kinase, was able to prevent the effect of EGF on 6PF2K/Fru-2, 6-P2ase gene expression. These results imply that activation of mitogen-activated protein kinase is required for the stimulation of the transcription of 6PF2K/Fru-2,6-P2ase by EGF.

[D-Arg1,D-Trp5,7,9,Leu11]Substance P coordinately and reversibly inhibits bombesin- and vasopressin-induced signal transduction pathways in Swiss 3T3 cells

The novel substance P (SP) analogue, [D-Arg1,D-Trp5,7,9,Leu11]SP like [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP inhibited DNA synthesis induced by bombesin, vasopressin, and bradykinin, but did not interfere with the mitogenic response induced by other growth factors or pharmacological agents in Swiss 3T3 cells. [D-Arg1,D-Trp5, 7,9,Leu11]SP reversibly inhibited bombesin-induced DNA synthesis, causing a 6-fold greater rightward shift in the bombesin dose response than [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP at identical concentrations (10 microM). We found that the new, more potent, SP analogue coordinately and reversibly inhibited bombesin-induced Ca2+ mobilization and protein kinase C (PKC) and mitogen-activated protein (MAP) kinase activation. The dose-response curves for bombesin-induced Ca2+ mobilization and MAP kinase activation were similarly displaced (51- and 40-fold, respectively) by [D-Arg1, D-Trp5,7,9,Leu11]SP. In addition, [D-Arg1,D-Trp5,7,9,Leu11]SP reversibly inhibited bombesin-induced tyrosine phosphorylation of Mr 110,000-130,000 and 70,000-80,000 bands as well as p125 focal adhesion kinase. [D-Arg1,D-Trp5,7,9,Leu11]SP also reversibly and coordinately inhibited vasopressin-induced Ca2+ mobilization, PKC stimulation, MAP kinase activation, tyrosine phosphorylation, and DNA synthesis in Swiss 3T3 cells. Surprisingly, deletion of the terminal Leu of [D-Arg1,D-Phe5,D-Trp7,9,Leu11]SP to yield [D-Arg1, D-Phe5,D-Trp7,9]SP1-10 resulted in a selective loss of inhibitory activity of this analogue against bombesin- but not vasopressin-stimulated DNA synthesis, Ca2+ mobilization, and MAP kinase activation. Collectively, these results suggest that SP analogues act at the receptor level to coordinately and reversibly antagonize bombesin- or vasopressin-induced signal transduction in Swiss 3T3 cells.

Reduced requirement of mitogen-activated protein kinase (MAPK) activity for entry into the S phase of the cell cycle in Swiss 3T3 fibroblasts stimulated by bombesin and insulin

Bombesin induced a marked and persistent activation of the mitogen-activated protein kinase kinase-1 (MEK-1), p42(mapk) and p90(rsk) in Swiss 3T3 cells by a pathway that was independent of p74(raf-1) but dependent on the activity of protein kinase C. Pretreatment of the cells with a specific inhibitor of MEK-1, PD 098059, markedly reduced the early and abolished the sustained phase of bombesin-induced p42(mapk) activation. In addition, PD 098059 prevented bombesin-induced DNA synthesis and progression of the cells through the cell cycle, indicating that the mitogenic effect of bombesin is dependent on the activation of p42(mapk). However, in the presence of insulin, which neither stimulated p42(mapk) activation nor DNA synthesis on its own in Swiss 3T3 cells, bombesin potently stimulated DNA synthesis even at concentrations of PD 098059 (15 microM) that completely abolished the mitogenic effect of bombesin alone. Furthermore, Swiss 3T3 cells stably transfected with interfering mutants of MEK-1 showed a marked decrease in the mitogenic effect of bombesin. In contrast, the combination of bombesin and insulin strongly stimulated DNA synthesis in these cells to levels comparable with that obtained in the wild type cells. Thus, our data demonstrate that insulin dramatically reduced the requirement for the mitogen-activated protein kinase pathway for reinitiation of DNA synthesis in bombesin-treated Swiss 3T3 cells and consequently indicate that the contribution of the mitogen-activated protein kinase cascade to mitogenesis depends on the combination of extracellular signals that are used to stimulate these cells.

Protein kinase C-independent activation of mitogen-activated protein kinase by epidermal growth factor in skin fibroblasts

In this study, we demonstrated that epidermal growth factor (EGF) stimulated the phosphorylation of myelin basic protein (MBP), a mitogen-activated protein kinase (MAPK) substrate, in crude extracts of human dermal fibroblasts. Moreover, using a selective protein kinase C inhibitor, GF 109203X (3-[1-[3-(dimethylamino)propyl]-1 H-indol-3-yl]-4 (1 H-indol-3-yl)-1 H-pyrrole-2,5-dione monohydrochloride), we observed that protein kinase C was partially involved in the total MBP phosphorylation. To determine the role of protein kinase C in the MBP phosphorylation, we separated, using fast protein liquid chromatography, the proteins present in the fibroblast crude extracts; we thus detected two distinct MBP kinase activities. The first one was stimulated by EGF and corresponded to p42mapk and p44mapk isoforms; this stimulation was not modified by GF 109203X. The second MBP kinase activity was not stimulated by EGF and was due to two protein kinase C isoforms reacting with an anti-protein kinase C zeta antibody. These results show that, in human dermal fibroblasts, EGF stimulates p42mapk and p44mapk isoforms in a protein kinase C-independent manner.

Sphingosylphosphorylcholine activation of mitogen-activated protein kinase in Swiss 3T3 cells requires protein kinase C and a pertussis toxin-sensitive G protein

Sphingosylphosphorylcholine (SPC) is a potent mitogen for Swiss 3T3 cells, but the signaling mechanisms involved are poorly characterized. Here, we report that addition of SPC induces a rapid and transient activation of p42 mitogen-activated protein kinase (p42MAPK) in these cells. SPC-induced p42MAPK activation peaked at 5 min and was undetectable after 30 min of incubation with SPC. The effect of SPC on p42MAPK activation was comparable to that induced by bombesin and platelet-derived growth factor. As SPC strongly induced phosphorylation of the major protein kinase C (PKC) substrate 80K/MARCKS in either intact or permeabilized cells, we examined whether PKC could be involved in SPC-induced p42MAPK activation. Here, we demonstrate that p42MAPK activation by SPC was dependent on PKC activity as shown by inhibition of PKC with the bisindolymaleimide GF 109203X or down-regulation of PKC by prolonged treatment of Swiss 3T3 cells with phorbol esters. Activation of both PKC and p42MAPK by SPC was markedly inhibited by treatment with pertussis toxin, implicating a G proteins(s) of the Gi/G(o) subfamily in the action of SPC. SPC-induced rapid activation of a downstream target of p42MAPK, p90 ribosomal S6 kinase (p90rsk), also required PKC and a pertussis toxin-sensitive G protein. In addition, SPC-induced mitogenesis was dependent on a Gi protein in Swiss 3T3 cells. SPC also induced p42MAPK activation and DNA synthesis in secondary cultures of mouse embryo fibroblasts through a pertussis toxin-sensitive pathway. As G proteins link many cell surface receptors to effector proteins, we hypothesize, therefore, that SPC could bind to a receptor that mediates at least some of its biological effects in Swiss 3T3 cells and mouse embryo fibroblasts.

Extracellular Ca2+ stimulates the activation of mitogen-activated protein kinase and cell growth in human fibroblasts

In serum-free medium containing serum replacements but totally lacking in protein growth factors, diploid human fibroblasts remained quiescent if the extracellular Ca2+ concentration was only 0.1 mM. However, when the Ca2+ concentration in this medium was increased to 1 mM, the cells replicated as rapidly as they do in medium supplemented with protein growth factors. When quiescent cells in medium with only 0.1 mM Ca2+ were exposed to 1 or 10 mM Ca2+ or 100 ng/ml epidermal growth factor (EGF), the 42 kDa and 44 kDa forms of mitogen-activated protein kinase (MAPK) were rapidly activated, as demonstrated by a characteristic electrophoretic mobility shift of these proteins and by their enhanced ability to phosphorylate myelin basic protein (MBP). Analysis of fractions from Mono Q anion-exchange chromatography of lysates of cells exposed to 10 mM Ca2+ or 100 ng/ml EGF revealed a peak of MBP phosphorylation activity that was coeluted with p42 and p44 MAPK as shown by immunoblot analysis. Activation of MAPK by extracellular Ca2+ was dose-dependent and biphasic, with a peak of activation at 5-10 min after exposure, followed by a period of sustained activation of MAPK at a lower level. This pattern has been shown [Vouret-Craviari, Van Obberghen-Schilling, Scimeca, Van Obberghen and Pouysségur (1993) Biochem J. 289, 209-214] to correlate with the re-entry of mammalian cells into the cell cycle.

Mitogen-activated protein kinase kinase inhibition does not block the stimulation of glucose utilization by insulin

Insulin stimulates the activity of mitogen-activated protein kinase (MAPK) via its upstream activator, MAPK kinase (MEK), a dual specificity kinase that phosphorylates MAPK on threonine and tyrosine. The potential role of MAPK activation in insulin action was investigated with the specific MEK inhibitor PD98059. Insulin stimulation of MAPK activity in 3T3-L1 adipocytes (2.7-fold) and L6 myotubes (1.4-fold) was completely abolished by pretreatment of cells with the MEK inhibitor, as was the phosphorylation of MAPK and pp90Rsk, and the transcriptional activation of c-fos. Insulin receptor autophosphorylation on tyrosine residues and activation of phosphatidylinositol 3'-kinase were unaffected. Pretreatment of cells with PD98059 had no effect on basal and insulin-stimulated glucose uptake, lipogenesis, and glycogen synthesis. Glycogen synthase activity in extracts from 3T3-L1 adipocytes and L6 myotubes was increased 3-fold and 1.7-fold, respectively, by insulin. Pretreatment with 10 microM PD98059 was without effect. Similarly, the 2-fold activation of protein phosphatase 1 by insulin was insensitive to PD98059. These results indicate that stimulation of the MAPK pathway by insulin is not required for many of the metabolic activities of the hormone in cultured fat and muscle cells.

A synthetic inhibitor of the mitogen-activated protein kinase cascade

Treatment of cells with a variety of growth factors triggers a phosphorylation cascade that leads to activation of mitogen-activated protein kinases (MAPKs, also called extracellular signal-regulated kinases, or ERKs). We have identified a synthetic inhibitor of the MAPK pathway. PD 098059 [2-(2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one] selectively inhibited the MAPK-activating enzyme, MAPK/ERK kinase (MEK), without significant inhibitory activity of MAPK itself. Inhibition of MEK by PD 098059 prevented activation of MAPK and subsequent phosphorylation of MAPK substrates both in vitro and in intact cells. Moreover, PD 098059 inhibited stimulation of cell growth and reversed the phenotype of ras-transformed BALB 3T3 mouse fibroblasts and rat kidney cells. These results indicate that the MAPK pathway is essential for growth and maintenance of the ras-transformed phenotype. Further, PD 098059 is an invaluable tool that will help elucidate the role of the MAPK cascade in a variety of biological settings.

Activation of MAP kinase by the LHRH receptor through a dual mechanism involving protein kinase C and a pertussis toxin-sensitive G protein

The LHRH receptor in alpha T3-1 gonadotrope cells was shown to bring about a marked and sustained activation of MAP kinase. This response was prevented by protein kinase C inhibition or down-regulation and could be partially mimicked by phorbol ester. Additional evidence for inhibition of this response by pertussis toxin and partial mimicry by mastoparan (in a pertussis toxin-sensitive manner) provides the first evidence for Gi/Go-mediated signal transduction by the LHRH receptor. This dual mechanism of MAP kinase activation appears to be exceptional amongst the G protein-linked receptors that have been investigated.

Nerve growth factor stimulates a novel protein kinase in PC-12 cells that phosphorylates and activates mitogen-activated protein kinase kinase (MEK)

Activation of mitogen-activated protein kinase (MAP kinase) plays an important role in the cellular effects of nerve growth factor (NGF). Although the precise pathway by which NGF activates MAP kinase is not clear, several enzymes have been identified that may form a linear phosphorylation cascade, in which MAP kinase is activated by MAP kinase kinase (MEK). A key enzyme that links the ras-GTP complex to MEK is widely believed to be the raf kinase. However, immunoprecipitation experiments in PC-12 cells revealed that raf is not the major NGF-dependent MEK kinase [Zheng, Ohmichi, Saltiel and Guan (1994) Biochemistry 33, 5595-5599]. We have identified a protein kinase from PC-12 cells that catalyses both the phosphorylation and activation of MEK. This activity is stimulated 3-fold in cells treated with NGF. The partial purification on FPLC and characterization of this MEK kinase indicate that it is distinct from raf, MEK, MAP kinase and other previously described NGF-stimulated protein kinases. The activity of this enzyme is unaffected by direct addition to the assay of heparin, staurosporine, K252A and the heat-stable cyclic AMP-dependent kinase peptide inhibitor, but is slightly inhibited by NaF and calcium ions. Comparison of its behaviour on gel permeation and sucrose-density gradients indicates a molecular mass in the region of 50,000 Da. Moreover, isoelectric focusing of the enzyme revealed a pI of approx. 7.3. The kinase activity is specific for ATP as substrate with a Km of 11 microM, and requires Mg2+ as a cofactor. Analysis of the activation of this enzyme in PC-12 cells transfected with a dominant inhibitory mutant of p21ras suggests that this MEK kinase resides downstream of ras in the MAP kinase activation pathway. Moreover, site-directed mutation of the residues on MEK that are phosphorylated by raf does not completely abrogate phosphorylation by the MEK kinase, suggesting that this enzyme may share some phosphorylation sites with raf, but also phosphorylates MEK on other sites.

Differential modulation of bombesin-stimulated phospholipase C beta and mitogen-activated protein kinase activity by [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P

Mitogenic stimulation of Swiss 3T3 fibroblasts with bombesin results in receptor-mediated activation of a complex array of effectors, including phospholipase C beta and mitogen-activated protein (MAP) kinase. Incubation of Swiss 3T3 fibroblasts with the 11-amino acid [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide inhibited bombesin-stimulated cell proliferation and phospholipase C beta activation even at high bombesin concentrations. The peptide did not inhibit the activation of phospholipase C beta by a GTPase-deficient form of the Gq-like protein, G16, indicating that the peptide does not inhibit phospholipase C beta and is acting at a point upstream of the activated form of the G protein alpha subunit. The peptide inhibited MAP kinase activation at low bombesin concentrations, but unlike phospholipase C beta, this inhibition could be overcome with 30 nM bombesin. In control Swiss 3T3 cells, bombesin did not measurably activate Ras or Raf-1 above basal levels. Following incubation of the cells with the [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide, 50 nM bombesin activated Raf-1 4-6-fold over basal levels. Platelet-derived growth factor-stimulated activities of PLC, Ras, Raf-1, and MAP kinase were unaltered after incubation of Swiss 3T3 cells with the [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide, as was platelet-derived growth factor-stimulated growth of the Swiss 3T3 cells. Thus, the peptide behaves as an antagonist that differentially inhibited phospholipase C beta and MAP kinase signal transduction pathways. The growth arrest observed with the peptide indicates that the bombesin-stimulated activation of MAP kinase is not sufficient to support mitogenesis in Swiss 3T3 cells.

Protein-tyrosine-phosphatase SHPTP2 is a required positive effector for insulin downstream signaling

SHPTP2 is a ubiquitously expressed tyrosine-specific protein phosphatase that contains two amino-terminal Src homology 2 (SH2) domains responsible for its association with tyrosine-phosphorylated proteins. In this study, expression of dominant interfering mutants of SHPTP2 was found to inhibit insulin stimulation of c-fos reporter gene expression and activation of the 42-kDa (Erk2) and 44-kDa (Erk1) mitogen-activated protein kinases. Cotransfection of dominant interfering SHPTP2 mutants with v-Ras or Grb2 indicated that SHPTP2 regulated insulin signaling either upstream of or in parallel to Ras function. Furthermore, phosphotyrosine blotting and immunoprecipitation identified the 125-kDa focal adhesion kinase (pp125FAK) as a substrate for insulin-dependent tyrosine dephosphorylation. These data demonstrate that SHPTP2 functions as a positive regulator of insulin action and that insulin signaling results in the dephosphorylation of tyrosine-phosphorylated pp125FAK.

Regulation of endothelin-1- and lysophosphatidic acid-stimulated tyrosine phosphorylation of focal adhesion kinase (pp125fak) in Rat-1 fibroblasts

The characteristics of protein tyrosine phosphorylation were examined in Rat-1 fibroblasts in response to endothelin-1 (ET-1) and 1-oleoyl-lysophosphatidic acid (LPA). Both agonists stimulated the biphasic tyrosine phosphorylation of at least three major proteins of approx. 120 kDa (pp116, pp120 and pp130) and two of 80 kDa (pp80 and pp70). Immunoprecipitation experiments indicated that the pp120 protein corresponded to the recently described focal adhesion protein kinase pp125fak. Phorbol 12-myristate 13-acetate, alone or in combination with the calcium ionophore A23187, also stimulated the phosphorylation of pp125fak but to a smaller extent than LPA or ET-1. Removal of both extracellular and intracellular Ca2+ did not significantly reduce LPA- and ET-1-stimulated tyrosine phosphorylation of pp125fak. In cells where protein kinase C activity was down-regulated or inhibited, ET-1-stimulated tyrosine phosphorylation of pp125fak was reduced to a greater extent than phosphorylation in response to LPA. In addition, ET-1-stimulated tyrosine phosphorylation of pp80 was decreased by 50-70% in response to protein kinase C inhibition at both 2 and 60 min whereas LPA-stimulated tyrosine phosphorylation of this protein was only reduced at 2 min. Pretreatment with pertussis toxin reduced the tyrosine phosphorylation of pp42 and pp44 forms of mitogen-activated protein kinase in response to both ET-1 and LPA but reduced the tyrosine phosphorylation of pp125fak only in response to LPA. These results indicate agonist-specific differences in the regulation of pathways mediating the tyrosine phosphorylation of pp125fak and other target proteins.