Mitogenic signaling pathways of growth factors can be distinguished by the involvement of pertussis toxin-sensitive guanosine triphosphate-binding protein and of protein kinase C

Synthesis and structure–activity relationships of thioflavone derivatives as specific inhibitors of the ERK-MAP kinase signaling pathway

Condensation of nitrobenzaldehydes 3 and alpha-[o-(p-methoxybenzylthio)benzoyl] sulfoxide 4 gave alpha-sulfinyl enones 5. Treatment of 5 with formic acid caused cyclization followed by debenzylation to afford 3-(methylsulfinyl)thioflavanones 6. Double-bond formation with elimination of methanesulfenic acid was performed by refluxing 6 in benzene, and, finally, the nitro group of 2-phenyl-4H-1-benzothiopyran-4-one (thioflavones) 7 was reduced with tin in tetrafluoroboric acid. Various 2'-aminothioflavones 8 thus prepared were evaluated for their inhibitory effects on the ERK-MAP kinase pathway. In a cell-based assay, 2-(2'-amino-3'-methoxyphenyl)-4H-1-benzothiopyran-4-one (8b) showed a more potent inhibitory effect than the corresponding oxygen compound (PD98059, 1) on the Raf-induced activation of the ERK-MAP kinase pathway as well as cell proliferation. Furthermore, compound 8b selectively and potently inhibited the proliferation of tumor cells in which the ERK-MAP kinase pathway is constitutively activated.

G-protein expression and intimal hyperplasia after arterial injury: a role for Galpha(i) proteins

PURPOSE

Guanine nucleotide binding protein (G-protein) coupled receptors are involved in smooth muscle cell proliferation, but the role of G-proteins in arterial intimal hyperplasia has not been defined. This study examines the expression of G-proteins in the developing intimal hyperplasia after balloon injury of the rat carotid artery and specifically tests the hypothesis that the pertussis toxin sensitive G(i) G-protein subunit plays a role in the initiation of intimal hyperplasia.

METHODS

In vitro responses to serum stimulation (10% fetal bovine serum) were examined in the presence and absence of pertussis toxin (PTx). After a standard balloon injury in male Sprague-Dawley rats, the expression of G-protein subunits (alpha(o), alpha(i), alpha(q), alpha(s), and betagamma) was determined by means of Western blotting in the first 28 days. Thereafter, a second set of animals was allocated to control and PTx-treated (a Galpha(i) inhibitor; 500 ng/mL in an externally applied 30% pluronic gel) groups. Smooth muscle cell proliferation was estimated by means of thymidine analogue 5-bromo-2' deoxyuridine incorporation 2 days after injury, and vessel dimensions were determined by means of videomorphometry 14 days after injury.

RESULTS

There was inhibition of DNA synthesis and smooth muscle cell proliferation in response to serum with an IC(50) of 100 ng/mL. Three days after balloon injury, there was an increase in Galpha(i3) expression, which decreased at days 7, 14, and 28, compared with the uninjured carotid. Galpha(q) expression increased in a time-dependent manner. There was a marked time-dependent increase in Gbetagamma in the 28 days. Galpha(i2) and Galpha(s) isoforms (45 and 52 kDa) did not change significantly with time. There was no major change in Galpha(i1) and Galpha(o) in the study period. At 14 days, PTx treatment reduced intimal hyperplasia by 52% (63 +/- 4 microm vs. 30 +/- 5 microm, control vs. PTx; P <.001). Medial smooth muscle cell proliferation at day 2 was decreased in the PTx group, compared with that in the gel-coated group (15% +/- 2% and 26% +/- 3%; P = .02).

CONCLUSION

After balloon injury, there is a time-dependent increase in G-protein expression, which is subunit specific. Activation of PTx sensitive G-proteins (Galpha(i)) is involved during the initiation of intimal hyperplasia after arterial injury, and their inhibition results in a decrease in early medial cell proliferation. This acute interruption of G(i) signaling produces a long-term decrease in intimal hyperplasia.

Biological activities of novel lipid mediator sphingosine 1-phosphate in rat hepatic stellate cells

Sphingosine 1-phosphate (S-1-P), a lipid mediator shown to be a ligand for aortic G protein-coupled receptor [corrected] (AGRs), endothelial differentiation gene (EDG)1, EDG3, and AGR16/EDG5, is stored in platelets and released on their activation. Platelet consumption occurs in acute liver injury. Hepatic stellate cells (HSCs) play an important role in wound healing. Effects of S-1-P on HSCs were investigated. S-1-P enhanced proliferation of culture-activated HSCs. The mitogenic effect was pertussis toxin sensitive, mitogen-activated protein kinase dependent, and more prominent at lower cell density. S-1-P increased contraction of collagen lattices containing HSCs, irrespective of activation state, in a C3 exotoxin-sensitive manner. mRNAs of EDG1 and AGR16, but not of EDG3, were detected in HSCs. In HSC activation, EDG1 mRNA levels were downregulated, whereas AGR16 mRNA levels were unchanged. Considering that HSCs are capable of production of extracellular matrices and modulation of blood flow in sinusoids, our results suggest that S-1-P may play a role in wound healing process in the liver.

Diverse expression of G-proteins in human sarcoma cell lines with different osteogenic potential: evidence for the involvement of Gi2 in cell proliferation

Previously, it has been shown that the GTP-binding protein Gi2 is implicated in cellular growth [1,2] and differentiation [2,3]. In the present paper we demonstrate that this is also the case for human sarcoma cells. Six human osteosarcoma and three soft tissue sarcoma clonal cell lines were analyzed for levels of G-protein mRNA and polypeptide expression and effector enzyme (i.e., adenylate cyclase and phospholipase C) activation, which were all compared with individual growth rates. Unexpectedly, it appeared that the various strains exhibited large inter-individual variations in G-protein expression and signaling system activation. However, cell doubling time in the exponential phase of growth was inversely correlated (r = 0.71, P < 0.05) to immunodetected levels of intrinsic Gi2 alpha. Furthermore, cells stably transfected with a retroviral (pZipNeo(SV)X) construct containing the activating or inactivating Gi2 alpha-R179E or Gi2 alpha-G204A point mutations consistently reduced or enhanced individual cell strain doubling time, respectively. It appeared that other parameters investigated, including cellular alkaline phosphatase and monoclonal antibody epitope binding, both being markers of the proliferating osteoblast, did not correlate with cell doubling times.

Cell type-specific modulation of cell growth by transforming growth factor beta 1 does not correlate with mitogen-activated protein kinase activation

Transforming growth factor beta 1 (TGF-beta 1) is a multifunctional cytokine that positively or negatively regulates the proliferation of various types of cells. In this study we have examined whether or not the activation of the mitogen-activated protein (MAP) kinases is involved in the transduction of cell growth modulation signals of TGF-beta 1, as MAP kinase activity is known to be closely associated with cell cycle progression. Although TGF-beta 1 stimulated the growth of quiescent Balb 3T3 and Swiss 3T3 cells, it failed to detectably stimulate the tyrosine phosphorylation and activation of the 41- and 43-kDa MAP kinases at any time point up to the reinitiation of DNA replication. TGF-beta 1 also failed to stimulate the expression of the c-fos gene. Furthermore, TGF-beta 1 synergistically enhanced the mitogenic action of epidermal growth factor (EGF) without affecting EGF-induced MAP kinase activation in these fibroblasts, and it inhibited the EGF-stimulated proliferation of mouse keratinocytes (PAM212) without inhibiting EGF-induced MAP kinase activation. Thus, the ability of TGF-beta 1 to modulate cell proliferation is apparently not associated with the activation of MAP kinases. In this respect, TGF-beta 1 is clearly distinct from the majority, if not all, of peptide growth factors, such as platelet-derived growth factor and EGF, whose ability to modulate cell proliferation is closely associated with the activation of MAP kinases. These results also suggest that the activation of MAP kinases is not an absolute requirement for growth factor-stimulated mitogenesis.

Epidermal growth factor induces Egr-1 messenger RNA and protein in mouse osteoblastic cells

The nuclear signaling events activated when epidermal growth factor (EGF) interacts with osteoblasts to produce effects on growth and differentiation are not clearly understood, and may include induction of immediate early genes such as Egr-1, a zinc finger transcription factor. In the present study, Northern analyses were performed to define the effects of EGF on the expression of Egr-1 mRNA in MC3T3-E1 mouse osteoblastic cells. Following treatment of quiescent, subconfluent MC3T3-E1 cells with 0.1-100 ng/ml EGF for various periods, maximal induction of Egr-1 mRNA occurred when cells were treated for 30-60 minutes with 1-10 ng/ml EGF. Inhibition of protein kinase C activity by pretreatment with 1 microM chelerythrine chloride or by prolonged stimulation with 50 ng/ml tetradecanoyl phorbol acetate (TPA) partially diminished the induction of Egr-1 by EGF. Using an immunohistochemical approach, 10 ng/ml EGF was observed to induce Egr-1 protein within 30-60 minutes and this induction was localized to the nucleus. These observations indicate that EGF induces Egr-1 mRNA and protein via protein kinase C and other signaling pathways, and that Egr-1 may be part of the regulatory network mediating the actions of EGF on growth and differentiation of osteoblasts.

Down-regulation and differential restoration of cAMP responses upon transient phorbol ester treatment of primary osteoblastic cells

We studied cAMP responses induced by parathyroid hormone (PTH), prostaglandin E2 (PGE2) and forskolin in foetal rat calvariae-derived osteoblastic cells after 24 h treatment with a protein kinase C (PKC) activating phorbol ester. After this treatment, meant to down-regulate PKC activity, all tested cAMP responses were attenuated and were indeed accompanied by a decline in PKC activity. PTH receptor affinity was not altered and PTH receptor number was only slightly lowered after 24 h phorbol ester treatment. These results indicate that modulation of the cAMP responses by 24 h PMA treatment was mainly caused by a general impairment of adenylyl cyclase activity. Removal of the phorbol ester and subsequent culture for 2 days rendered the cells hyper-responsive to PTH: the PTH-induced cAMP response was 2 to 3 times higher than in control cells. Again no change in binding affinity of the PTH receptor was observed and receptor number was just 10% lower than in control cells. The PGE2- and forskolin-induced cAMP responses were not higher than normal. So, transient phorbol ester treatment leads to a differential, agonist-dependent restoration of the cAMP signalling system.

Signal transduction pathways involved in tumour necrosis factor secretion by Plasmodium falciparum-stimulated human monocytes

Tumour necrosis factor (TNF) plays a pivotal role in the induction of cerebral complications during Plasmodium falciparum malaria. TNF secretion by macrophages can be induced by lipopolysaccharide (LPS) and by P. falciparum antigens, but it is unclear whether similar mechanisms control the monokine expression in both cases. The signal transduction pathway by which parasite antigens induce TNF secretion remains to be established. The results reported here, using various inhibitors of second messenger pathways, clearly demonstrate that the signal transduction leading to TNF secretion is mediated partly through protein kinase C and calmodulin-dependent protein kinase activation. Furthermore, this signal seems to be differentially regulated after LPS or parasite stimulation, since cyclo-oxygenase inhibition by indomethacin resulted in twofold more TNF production enhancement with LPS stimulation than with parasite stimulation. The nature of the receptor involved in the parasite induced-macrophage stimulation remains obscure. However, the results discussed here indicate that parasite antigens stimulate multiple signal transduction pathways via G protein. Identification of the different pathways involved in these receptor-mediated events may be invaluable in the development of specific inhibitors against TNF over-production during cerebral malaria.

Differential regulation of acidic and basic fibroblast growth factor gene expression in fibroblast growth factor-treated rat aortic smooth muscle cells

The acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF) proteins are potent vascular smooth muscle cell (SMC) mitogens that are expressed by endothelial cells and SMCs in vivo. Overexpression of these proteins in transfected cell lines can result in autocrine transformation; therefore, the precise control of fibroblast growth factor gene expression in the vessel wall may be an important mechanism regulating vascular cell growth. In the present study, we demonstrate that bFGF can induce bFGF mRNA expression, but not aFGF mRNA expression, in serum-starved rat aortic SMCs. bFGF autoinduction is maximal at 4 hours, requires de novo RNA and protein synthesis, and is mediated predominantly by a protein kinase C-dependent signaling pathway. Furthermore, aFGF treatment of rat SMCs also increases bFGF mRNA and protein expression; however, aFGF mRNA levels are only slightly modulated. These results suggest that the local release of aFGF or bFGF within the vessel wall could promote a prolonged period of elevated bFGF synthesis. This, in turn, could be of importance in the SMC hyperplasia that occurs in response to vascular injury and during atherosclerotic plaque formation.

Effect of epidermal growth factor on mouse sperm acrosome reaction induced by zona pellucida

PROBLEM

The effect of epidermal growth factor (EGF) on the acid-solubilized zona pellucida (ZP)-induced acrosome reaction was investigated in mouse sperm.

METHOD

Mouse epididymal sperm were capacitated in modified Krebs-Ringer bicarbonate buffer (m-KRB) for 120 min and further treated with acid-solubilized ZP(4 zona/microliters) for an additional 60 min to induce the acrosome reaction. The chlortetracycline fluorescence assay was used to monitor the acrosome reaction. The acrosome reacted sperm featured the acrosome reacted (AR) pattern, which demonstrates the lack of fluorescence on the head and bright midpiece.

RESULTS

EGF caused an early increase in the AR pattern in response to the acid-solubilized ZP in a dose-dependent manner. The EGF-dependent stimulation of the ZP-induced acrosome reaction was inhibited by an inhibitor of protein tyrosine kinases, genistein or activators of Ca++ and phospholipid-dependent protein kinase (protein kinase C). Furthermore, the stimulatory effect of EGF was not attenuated when sperm were capacitated in the presence of islet-activating protein, an inactivator of inhibitory guanine nucleotide-binding regulatory protein (Gi protein).

CONCLUSION

EGF stimulates the ZP-induced acrosome reaction in a manner that is independent of the Gi protein. The EGF action is regulated by protein tyrosine kinase and protein kinase C.

Establishment and characterization of a human gastric scirrhous carcinoma cell line in serum-free chemically defined medium

We have established a human gastric scirrhous carcinoma cell line (designated as HSC-43) in a serum-free chemically defined medium (CDM) without any polypeptide growth factor, from a primary tumor of a 56-year-old male patient. HSC-43 cells grew in vitro in adherence with a population doubling time of 55 hr, and had the cytological properties of mucinous epithelial tumor cells. Cytogenetic analysis of the cells revealed pseudotetraploidy, with structural abnormalities of deletion at chromosome Iq25 and with 3 marker chromosomes. Some cells had retained features of signet-ring cells and caused fibroblastic proliferation when transplanted into athymic nude mice. The possible involvement of transforming growth factor-alpha (TGF-alpha), and its receptor, the epidermal-growth-factor receptor (EGFR), on the growth of HSC-43 cells was studied. Synthesis and secretion of TGF-alpha by HSC-43 cells were confirmed by biological assay and enzyme-linked immunosorbent assay. Radioreceptor analysis showed the presence of receptors for EGF in HSC-43 cells. Proliferation of HSC-43 cells was inhibited by antibodies against TGF-alpha and/or the EGFR. However, neither TGF-alpha nor epidermal growth factor (EGF) was effective in stimulating the cell growth of HSC-43 cells, irrespective of the cell density when supplemented exogenously. Our data suggest that TGF-alpha and EGFR play a role in the autocrine growth of HSC-43 cells. This may be another example of growth regulation of gastric carcinoma.

Mitogen-induced tyrosine phosphorylation of 41 kDa and 43 kDa proteins. Potential role in integrating multiple mitogenic signalling pathways

We have examined the possible involvement of pertussis toxin (PT)-sensitive GTP-binding protein and protein kinase C (PKC) in mitogen-induced tyrosine phosphorylation of the 41 kDa and 43 kDa cytosol proteins using PT-pretreated (inactivation of PT-sensitive GTP-binding protein) or phorbol 12-myristate 13-acetate (PMA)-pretreated (depletion of PKC) mouse fibroblasts. The effects of the inactivation of PT-sensitive GTP-binding protein and the depletion of PKC on mitogen-stimulated tyrosine phosphorylation of the proteins were similar and varied significantly and systematically in response to growth factors. The important finding was that such inhibitory effects of PT-sensitive GTP-binding protein inactivation and PKC depletion on protein tyrosine phosphorylation induced by each mitogen always correlated well with their inhibitory effects on each mitogen-stimulated DNA synthesis. Although the extent of platelet-derived-growth-factor-induced phosphorylation of the proteins was decreased to approx. 50% in PT- and PMA-pretreated cells compared with native cells, protein phosphorylation itself was not affected and occurred at identical sites on each protein in native, PT- and PMA-pretreated cells. These results suggest that: (1) 41 kDa and 43 kDa proteins are located downstream of PT-sensitive GTP-binding protein and PKC in the mitogenic signalling pathways of growth factors, (2) protein phosphorylation occurs via a cascade of events which includes the activation of the receptor tyrosine kinases, PKC and other unidentified kinase(s) which directly participate(s) in the phosphorylation of the 41 kDa and 43 kDa proteins, and (3) their phosphorylation may play an important role in integrating multiple mitogenic signalling pathways.

Phospholipids regulate growth and function of MDCK cells in hormonally defined serum free medium

The effects of the simple phospholipids phosphatidic acid (PA) and lysophosphatidic acid (LPA) on the growth and function of Madin Darby Canine Kidney (MDCK) cells has been studied. We observed that PA and LPA not only stimulated the growth of MDCK cells (at 20 microM), but also stimulated the growth of normal rabbit kidney cells in serum free medium (albeit at a lower dosage of 5 microM). Evidence was obtained that PA interacts synergistically with insulin so as to elicit a growth stimulatory effect. Recently, extracellular PA and LPA were proposed to stimulate mitogenesis in several types of animal cells by binding to particular sites on the plasma membrane which are coupled to signaling mechanisms such as adenylate cyclase via a pertussis toxin sensitive, inhibitory guanosine triphosphate binding protein (Gi protein) (15). However, even when the pertussis toxin dosage was increased to 50 ng/ml, LPA still had a dramatic growth stimulatory effect on MDCK cells. In the absence of LPA pertussis toxin was slightly growth stimulatory to MDCK cells. Phospholipids such as PA and LPA have been observed to prevent prostaglandin-induced increases in adenylate cyclase activity in other cell types via their effects on such a pertussis toxin sensitive Gi protein. If PA and LPA act on MDCK cells in this manner, then these phospholipids may possibly prevent the effect of PGE1 on the growth of normal MDCK cells. However PGE1 was still growth stimulatory to normal MDCK cells. The effects of PA on PGE1 independent variants of MDCK cells, which have elevated intracellular cyclic AMP levels (22), were also examined. In the presence of PA, PGE1 remained growth inhibitory, rather than growth stimulatory to the PGE1 independent cells. However, the PA dosage required to elicit an optimal growth response (5 microM) was dramatically reduced, as compared with normal MDCK cells (20 microM). This altered dosage requirement could be explained by the elevated intracellular cyclic AMP levels in the PGE1 independent variants. Like PGE1 and 8-bromocyclic AMP, PA and LPA also significantly increased the initial rate of Rb+ uptake by confluent monolayers of MDCK cells. The increase in the initial rate of Rb+ uptake could be explained by an increase in the ouabain-sensitive component of Rb+ uptake. An increase in the initial rate of ouabain-insensitive Rb+ uptake was also observed in LPA treated MDCK cell cultures.

Protein kinase C initially inhibits the induction of meiotic cell division in Xenopus oocytes

We have used one activator and two inhibitors of protein kinase C (PKC) to examine the role of this enzyme in the induction of meiotic cell division. At 1 U/ml, phosphatidylcholine-specific phospholipase C increases DAG, alters intracellular pH and inhibits the induction of meiosis by insulin or progesterone. However, when added about 1.6 h after progesterone, the enzyme speeds the induction of cell division. Microinjection of inhibitor peptide (19-36) of PKC has little effect on progesterone action but stimulates the induction of meiosis by insulin. When the inhibitor peptide is injected about 2h after insulin addition, the peptide inhibits. A second PKC inhibitor, staurosporine, decreases PKC-dependent intracellular pH and in vitro oocyte PKC activity. At similar concentrations, staurosporine stimulates insulin or progesterone action, but, when added after about 2 h, the drug inhibits induction by insulin. We conclude that PKC is initially inhibitory to the induction of meiotic cell division but then may become synergistic.

Hepatocyte growth factor rapidly induces the tyrosine phosphorylation of 41-kDa and 43-kDa proteins in mouse keratinocytes

We have examined the hepatocyte growth factor (HGF)-mediated changes in protein-tyrosine phosphorylation in mouse keratinocytes (PAM-212) and canine kidney epithelial cells (MDCK). In PAM-212 cells HGF and epidermal growth factor, both of which stimulated the DNA synthesis, rapidly induced the tyrosine phosphorylation of two 41-kDa and two 43-kDa proteins: increased tyrosine phosphorylation of those proteins has been commonly observed when quiescent fibroblasts are stimulated with a variety of mitogenic agents. In contrast, HGF did not stimulate the DNA synthesis but induced cell dissociation in MDCK cells; under this condition, increased tyrosine phosphorylation of the 41-kDa and 43-kDa protein was not observed. A possible role of the increased tyrosine phosphorylation of 41-kDa and 43-kDa protein in the signaling pathway of HGF is discussed.

Pertussis toxin provides evidence for two independent signalling pathways leading to the activation of the nerve growth factor gene

Increased expression of the nerve growth factor (NGF) gene may be obtained by treating L929 fibroblasts with serum, phorbol 12-myristate 13-acetate (PMA), or 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3). The possible involvement of GTP-binding proteins (G proteins) in these regulatory events was monitored by exposing the cells to pertussis toxin (PT), a compound known to inactivate several types of G proteins by ADP ribosylation. Measurements of the pool of NGF mRNA by Northern blot analysis, and quantification of the factor secreted by the cells with a double-site ELISA assay, indicate that pretreatment with PT decreases by about 60% the effect of serum on the levels of NGF transcript and secreted factor. This effect is accompanied by a corresponding decrease of the expression of c-fos gene, which takes place soon after the addition of serum to the cells. In contrast, PT had no effect on the basal level of NGF mRNA found in cells maintained in serum-free medium or in cells stimulated with PMA or 1,25-(OH)2D3. These results indicate that some serum factor(s) acts via plasma membrane receptors able to interact with PT-sensitive G proteins to modulate NGF gene expression. In contrast, 1,25-(OH)2D3 appears to mediate its action through a different signalling pathway, which is likely to require its cytosolic receptor, and is independent of PT-sensitive G protein and c-fos induction.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential recovery of protein kinase C-alpha and -epsilon isozymes after long-term phorbol ester treatment in rat renal mesangial cells

Glomerular mesangial cells have been shown to express two protein kinase C (PKC) isozymes, PKC-alpha and PKC-epsilon. Upon long-term treatment with phorbol ester PKC-alpha is depleted faster than PKC-epsilon. Here we demonstrate that removal of phorbol ester results in a differential recovery of PKC-alpha and -epsilon isozymes. Whereas PKC-epsilon starts to recover within 1h, PKC-alpha does not begin to recover before 4 h after removal of phorbol ester. These data suggest a differential rate of protein synthesis of PKC-alpha and -epsilon. In parallel to the recovery of PKC isozymes mesangial cells also regained their functional responsiveness, i.e., stimulation of prostaglandin synthesis and feedback inhibition of angiotensin II-stimulated InsP3 formation.

A mutant alpha subunit of Gi2 induces neoplastic transformation of Rat-1 cells

In a recently discovered class of oncogenes, GTPase-inhibiting mutations constitutively activate alpha subunits of signal-transducing guanine nucleotide-binding proteins (G proteins). Somatic mutations in a subclass of endocrine tumors are found in the arginine-179 codon of the alpha subunit of Gi2 (alpha i2), creating the putative gip2 oncogene. We have tested the ability of gip2 to mediate neoplastic transformation of Rat-1 and NIH 3T3 fibroblasts in tissue culture. Expression of a mutant alpha i2 cDNA encoding cysteine in place of arginine-179 (alpha i2-R179C) caused Rat-1 cells to grow to a higher density in monolayer culture, to lose anchorage dependence, and to form tumors when injected subcutaneously into nude mice. In contrast, expression of alpha i2-R179C failed to alter growth or tumorigenicity of NIH 3T3 cells. We conclude that gip2 is an oncogene, by the criterion that it induces neoplastic transformation of Rat-1 cells. Failure of gip2 to transform NIH 3T3 cells is in keeping with clinical indications that gip2 is a tissue-selective oncogene.