Epidermal growth factor potentiates cyclic AMP accumulation in A-431 cells

Mechanisms of vascular angiotensin II surface receptor regulation by epidermal growth factor

We investigated mechanisms by which epidermal growth factor (EGF) reduces angiotensin II (AngII) surface receptor density and stimulated actions in vascular smooth muscle cells (VSMC). EGF downregulated specific AngII radioligand binding in intact cultured rat aortic smooth muscle cells but not in cell membranes and also inhibited AngII-stimulated contractions of aortic segments. Inhibitors of cAMP-dependent kinases, PI-3 kinase, MAP kinase, cyclooxygenase, and calmodulin did not prevent EGF-mediated downregulation of AngII receptor binding, whereas the EGF receptor kinase inhibitor AG1478 did. Total cell AngII AT1a receptor protein content of EGF-treated and untreated cells, measured by immunoblotting, did not differ. Actinomycin D or cytochalasin D, which interacts with the cytoskeleton, but not the protein synthesis inhibitor cycloheximide, prevented EGF from downregulating AngII receptor binding. Consistently, EGF inhibited AngII-stimulated formation of inositol phosphates in the presence of cycloheximide but not in the presence of actinomycin D or cytochalasin D. In conclusion, EGF needs an intact signal transduction pathway to downregulate AngII surface receptor binding, possibly by altering cellular location of the receptors.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Tyrosine kinase effects on adrenoceptor-stimulated cyclic AMP accumulation in preoptic area and hypothalamus of female rats: modulation by estradiol

These studies examined the functional interactions between adrenergic G-protein coupled receptors and protein tyrosine kinases in the preoptic area and hypothalamus, brain regions that regulate reproductive function in female rats, and evaluated whether in vivo treatment with estradiol for 2 days modulates the cross-talk between these two signaling pathways. In hypothalamic slices genistein, a general tyrosine kinase inhibitor, enhances norepinephrine-stimulated cAMP synthesis independent of estradiol treatment. Genistein appears to act by increasing beta-adrenoceptor signaling. At high norepinephrine concentrations, estradiol potentiates genistein enhancement of the cAMP response in hypothalamic slices. This interaction between estradiol and genistein appears to involve modification of alpha(2)-adrenoceptor signaling mechanisms. In preoptic area slices, genistein enhancement of norepinephrine-stimulated cAMP synthesis is only observed in estradiol-treated rats. In this brain region, genistein enhances cAMP accumulation by modifying alpha(1)- and/or alpha(2)-adrenoceptor rather than beta-adrenoceptor signaling. Genistein amplification of norepinephrine-stimulated cAMP synthesis is not mediated by interactions with estrogen receptors, or by regulation of adenylyl cyclase or phosphodiesterase activities. At the concentration used, genistein inhibits tyrosine phosphorylation in slices from both brain regions. Daidzein, an inactive analogue of genistein, fails to enhance the norepinephrine-stimulated cAMP response in either brain region independent of hormone treatment. These results suggest that protein tyrosine kinases regulate adrenergic responses in the hypothalamus and preoptic area. Moreover, the functional interaction between adrenergic G-protein coupled receptor signaling and protein tyrosine kinases is modified in a brain region and receptor subtype specific manner by estradiol.

Epidermal growth factor regulates glucose metabolism through lactate dehydrogenase A messenger ribonucleic acid expression in cultured porcine Sertoli cells

In a previous work, we reported that lactate dehydrogenase A4 (LDH A4) activity is a key step in the stimulatory effect of epidermal growth factor (EGF) on lactate production in cultured Sertoli cells. Here, we further investigated the regulatory mechanisms involved in EGF action on LDH A mRNA expression. Steady-state levels of LDH A mRNA analyzed by Northern blot hybridization were induced to 2. 9-fold in response to a 36-h incubation with EGF (ED(50) = 4 ng/ml, 0.63 x 10(-9) M). Whether EGF-induced increases of LDH A mRNA levels are the result of increased transcription and/or altered mRNA stability was investigated. The decay curves for the 1.5-kilobase LDH A mRNA transcript in Sertoli cells were not different in the absence or presence of EGF, suggesting that EGF did not affect LDH A mRNA stability. Inhibitors of protein synthesis (cycloheximide) and RNA synthesis (actinomycin D, and 5,6-dichloro-1-beta-ribofuranosyl benzimidazole) completely abrogated the EGF-induced LDH A mRNA expression, indicating that EGF increased LDH A mRNA levels through a transcriptional mechanism, which probably involves protein synthesis. Finally, the partial inhibitory effect of a protein kinase C (PKC) inhibitor, bisindolylmaleimide, on EGF-stimulated LDH A mRNA supports a partial involvement of PKC in the action of the growth factor. Since EGF is produced in Sertoli and in germ cells, its action is probably exerted in a context of a local control. As EGF also regulates other parameters involved in glucose metabolism, its effect on LDH A might be viewed in a general context related to the control of energy metabolism by the growth factor in the testicular cells.

Vanadate stimulation of adenylyl cyclase: an index of tyrosine kinase vascular effects

BACKGROUND

Beyond their mitogenic effects, hormones such as insulin, which activate receptor tyrosine kinases, regulate vascular tone. Further, we have demonstrated that receptor tyrosine kinase activation enhances adenylyl cyclase activation, a prominent mechanism that mediates vasodilation. However, whether tyrosine kinase-mediated human vascular responses parallel tyrosine kinase-mediated cellular effects on adenylyl cyclase activity is unknown.

METHODS AND RESULTS

To assess tyrosine kinase-mediated vascular responses, vascular sensitivity to insulin was assessed with the dorsal hand vein linear variable differential transformer technique. Insulin infusion resulted in a dose-dependent relaxation in all subjects. Cellular responses were assessed by means of the insulinomimetic vanadate-mediated sensitization of vascular adenylyl cyclase activity. Vanadate stimulated a tyrosine kinase-dependent enhancement of adenylyl cyclase function in human and rat aortic vascular smooth muscle cells, human lymphocytes, and human aortic endothelial cells. Further, maximal insulin-mediated vasodilation was significantly positively correlated with maximal vanadate-mediated enhancement of human lymphocyte adenylyl cyclase activity.

CONCLUSION

Insulin-mediated vasodilation is positively correlated with vanadate-mediated enhancement of adenylyl cyclase activity. Vanadate-mediated enhancement of adenylyl cyclase activity in lymphocytes may represent an index of tyrosine kinase-mediated vascular effects.

The adaptive intestinal response to massive enterectomy is preserved in c-SRC-deficient mice

BACKGROUND/PURPOSE

The Src family of protein tyrosine kinases has been implicated in the downstream mitogenic signaling of several ligands including epidermal growth factor (EGF). Because EGF likely plays a role in adaptation after massive small bowel resection (SBR), we tested the hypothesis that c-src is required for this important response.

METHODS

A 50% proximal SBR or sham operation (bowel transection or reanastomosis alone) was performed on c-src-deficient (n = 14) or wild-type (C57bl/6) mice (n = 20). The ileum was harvested on postoperative day 3 and adaptive parameters determined as changes in ileal wet weight, protein and DNA content, proliferation index, villus height, and crypt depth. Comparisons were done using analysis of variance (ANOVA), and a Pvalue less than .05 was considered significant. Values are presented as mean +/- SEM.

RESULTS

The activity of c-src was increased in the ileum of wild-type mice after SBR but remained unchanged in c-src-deficient mice. Despite this lack of increase, adaptation occurred after SBR in the c-src-deficient mice as demonstrated by increased ileal wet weight, protein and DNA content, proliferation index, villus height, and crypt depth similar to wild-type mice.

CONCLUSIONS

The adaptive response of the intestine to massive SBR is preserved despite reduced activity of the c-src protein. The mitogenic signaling that characterizes intestinal adaptation and is associated with receptor activation by EGF or other growth factors probably occurs by mechanisms independent of c-src protein tyrosine kinase.

Epidermal growth factor-induced heterologous desensitization of the luteinizing hormone/choriogonadotropin receptor in a cell-free membrane preparation is associated with the tyrosine phosphorylation of the epidermal growth factor receptor

Epidermal growth factor (EGF) attenuated hCG-stimulated adenylyl cyclase activity in rat luteal and follicular membranes. H7, an equipotent serine/threonine protein kinase inhibitor of cAMP-dependent protein kinases, cGMP-dependent protein kinases, and lipid-dependent protein kinase C, did not effect the ability of EGF to decrease hCG-responsive adenylyl cyclase activity, suggesting that a serine/threonine phosphorylation event catalyzed by these kinases was not critically involved in EGF-induced desensitization. Likewise, pertussis toxin-catalyzed ADP-ribosylation of a 40-kDa luteal membrane protein, which exhibited immunoreactivity with an antibody against Gi alpha, did not hinder the ability of EGF to attenuate hCG-stimulated adenylyl cyclase activity, indicating that Gi did not mediate EGF-induced desensitization. Rather, EGF-induced heterologous desensitization of LH/CG receptor in ovarian membranes was closely associated with the specific and prominent tyrosine phosphorylation of the 170-kDa EGF receptor. Both EGF-stimulated autophosphorylation of EGF receptor and EGF-induced LH/CG receptor desensitization were attenuated by genistein, a tyrosine kinase inhibitor. These results suggest that tyrosine phosphorylation of the 170-kDa EGF receptor is a necessary component of the signaling pathway in EGF-induced heterologous desensitization of the LH/CG receptor.

Selective modulation of protein kinase A and protein kinase C activities in epidermal growth factor (EGF)-stimulated MCF-7 breast cancer cells

In human MCF-7 breast cancer cells, both protein kinase A (PKA) and different members of the protein kinase C (PKC) family are stimulated upon binding of epidermal growth factor (EGF) to cell surface receptors. Selective stimulation of calcium-dependent PKCs with 10(-6) to 10(-9) M Thymeleatoxin significantly increased the proliferation rate of MCF-7 cells over 5 days in culture. This stimulation was blocked by the PKC antagonist Chelerythrine. In contrast, selective activation of PKA by addition of 1 mM dibutyryl cyclic AMP (dBcAMP) did not affect the proliferation rate of MCF-7 cells. Similarly, activation of the adenylate cyclase by 1 microM Forskolin and inhibition of PKA by the cyclic AMP analogue Rp-cAMPS did not modulate the proliferation rate of these cells. Activation of PKC stimulated the expression of the immediate early gene c-fos but c-myc expression was not significantly enhanced. On the other hand, PKA activation increased both c-myc and c-fos expression in MCF-7 cells. These results suggest that PKA activation and c-myc expression are not obligatory for proliferation of MCF-7 cells.

Modulation of cyclic AMP levels in a clonal neural cell line by inhibitors of tyrosine phosphorylation

The convergence of tyrosine kinase and cyclic AMP (cAMP) signal transduction pathways was investigated in the HT4.7 neural cell line with inhibitors of tyrosine kinases and tyrosine phosphatases. The protein tyrosine kinase inhibitor genistein inhibited isoproterenol-stimulated cAMP production by 40-60% in whole cells, with no effect on basal cAMP levels. In both whole cells and membranes, genistein also inhibited cAMP produced in response to direct stimulation of adenylyl cyclase with forskolin. However, in the absence of phosphodiesterase inhibitors, genistein presentation resulted in an increase in cAMP levels. Genistein inhibited phosphodiesterase activity by 80-85%, indicating that tyrosine phosphorylation stimulates both cAMP synthesis and degradation. The decrease in cAMP levels by genistein was not merely competitive inhibition of adenylyl cyclase with respect to ATP, since the Km of adenylyl cyclase for ATP remained essentially the same in either the presence or the absence of genistein. Another tyrosine kinase inhibitor, herbimycin A, which inhibits by a different mechanism than genistein, also decreased forskolin-stimulated cAMP in whole cells. As would be expected for the involvement of tyrosine phosphorylation in the control of cAMP production, inhibition of tyrosine phosphatases by vandate increased forskolin-stimulated cAMP production. These results suggest that cAMP production can be regulated by tyrosine phosphorylation, and the simultaneous activation of both cAMP synthesis and degradation may serve to alter the duration of cAMP elevation.

Protein kinase C potentiation of the tyrosine kinase inhibitor-stimulated cyclic GMP production in rat pinealocytes

Inhibition of tyrosine kinase activities elevates cyclic GMP (cGMP) levels in rat pinealocytes. Since protein kinase C (PKC) and intracellular Ca2+ both interact with the agonist-stimulated cGMP accumulation, in this study their interactions with the tyrosine kinase inhibitor-mediated cGMP response were investigated. Two tyrosine kinase inhibitors, genistein and tyrphostin B42, increased basal cGMP accumulation concentration dose-dependently. This increase in cGMP accumulation was potentiated by 4 beta-phorbol 12-myristate 13-acetate (PMA), an activator of PKC, and blocked by calphostin C, a specific PKC inhibitor. The tyrosine kinase inhibitors had no effect on the in vitro or PMA-mediated translocation of PKC activity. However, when the phosphodiesterase was inhibited by isobutylmethylxanthine (IBMX), neither the tyrosine kinase inhibitors alone nor in combination with PMA had an effect on cGMP accumulation, suggesting that phosphodiesterase is a probable site of action of the inhibitors. In comparison, elevation of intracellular Ca2+ by BayK 8644, ionomycin, or KCl inhibited the genistein- or tyrphostin B42-mediated increase in cGMP accumulation. This inhibition persisted in the presence of IBMX and was partly reversed by a Ca2+/calmodulin inhibitor. These results suggest that PKC modulates the rate of cGMP degradation through signalling pathways involving tyrosine phosphorylation. However, the inhibitory effect of the Ca(2+)-elevating agents on the tyrosine kinase inhibitor-stimulated cGMP accumulation appears to be independent of phosphodiesterase inhibition.

Modulatory action of epidermal growth factor on differentiated human granulosa lutein cells: cross-talk between ligand activated receptors for EGF and gonadotropin

A number of local regulatory factors including polypeptide growth factors like epidermal growth factor (EGF) have been suggested to play an active role within the human ovary. In order to understand the physiology of EGFs action, it is essential to demonstrate and characterize the receptors for this growth factor on ovarian cells which was the aim of this study. We demonstrate using [125I]EGF that specific high affinity sites with Ka for this ligand reaching 2.2 x 10(-9) M for growing cultures of human granulosa-lutein cells and 0.13 x 10(-9) M for the membrane fraction prepared from these cells. Additionally we have identified a 170 kD protein as the EGF receptor with the help of affinity cross linking and immunoblotting procedures. Furthermore, we observed that a pretreatment of granulosa lutein cells with EGF for a short duration (0-30 min) leads to a dose- and time dependent upregulation of the LH-receptor-coupled adenylate cyclase activity. A maximal effect (159 +/- 12% increase compared with untreated cells, P < 0.001, n = 4) was reached at 10-15 min with 10-20 ng/ml EGF. Specific inhibition of the receptor tyrosine kinase activity abolished the observed EGF-induced sensitization of the cyclase activity. Differentiation of granulosa cells in vivo is a prerequisite for ovulation and later transformation into highly differentiated lutein cells, a process depending on the presence of ligands that elevate cAMP production. The observed modulation of the adenylate cyclase by EGF could be a regulatory component for the differentiated status of the granulosa cells during different phases of the cycle.

The cooperative interaction of two different signaling pathways in response to bufalin induces apoptosis in human leukemia U937 cells

Bufalin, an active principle of Chinese medicine, chan'su, induced typical apoptosis in human leukemia U937 cells. When U937 cells were treated with 10(-8) M bufalin in the absence of serum, mitogen-activated protein (MAP) kinase activity was markedly increased 6 h after the start of treatment and elevated so for 12 h. Prior to the activation of MAP kinase, increased activities of Ras, Raf-1, and MAP kinase kinase were found, but these enzymes were transiently activated by the treatment with bufalin. These results suggest that the signal was transmitted sequentially from Ras, Raf-1, and MAP kinase kinase to MAP kinase. In association with this signal transduction, the concentration of cAMP in the cells decreased markedly, suggesting that Raf-1 was also activated by a decrease in the extent of phosphorylation by protein kinase A. In fact, pretreatment of U937 cells with forskolin and 3-isobutyl-1-methylxanthine, which are known to increase the concentration of cAMP in the cells, and subsequent treatment with bufalin resulted in a decrease in both Raf-1 activity and DNA fragmentation. To confirm the participation of MAP kinase in the apoptotic process, antisense cDNA for MAP kinase kinase 1 was expressed in U937 cells. The transformants were significantly resistant to both DNA fragmentation and cell death in response to bufalin. Our findings suggest that a pathway with the persistent activation of MAP kinase in U937 cells in response to bufalin is at least one of the signal transduction pathways involved in the induction of apoptosis.

Platelet-derived growth factor stimulates protein kinase A through a mitogen-activated protein kinase-dependent pathway in human arterial smooth muscle cells

The abilities of platelet-derived growth factor (PDGF) and insulin-like growth factor (IGF-I) to regulate cAMP metabolism and mitogen-activated protein kinase (MAP kinase) activity were compared in human arterial smooth muscle cells (hSMC). PDGF-BB stimulated cAMP accumulation up to 150-fold in a concentration-dependent manner (EC50 approximately 0.7 nM). The peak of cAMP formation and cAMP-dependent protein kinase (PKA) activity occurred approximately 5 min after the addition of PDGF and rapidly declined thereafter. Incubating cells with PDGF and 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor) enhanced the accumulation of cAMP and PKA activity by an additional 2.5-3-fold, whereas IBMX alone was essentially without effect. The PDGF-stimulated increase in cAMP was prevented by addition of the cyclooxygenase inhibitor indomethacin, consistent with release of prostaglandins stimulating cAMP. PDGF, but not IGF-I, stimulated MAPK activity, cytosolic phospholipase A2 (cPLA2) phosphorylation, and cAMP synthesis which indicated a key role for MAP kinase in the activation of cPLA2. Further, PDGF stimulated the rapid release of arachidonic acid and synthesis of prostaglandin E2 (PGE2) which could be inhibited by a cPLA2 inhibitor (AACOCF3). Calcium mobilization was required for PDGF-induced arachidonic acid release and PGE2 synthesis but not for MAPK activation, whereas PKC was required for PGE2-mediated activation of PKA. In summary, these results demonstrated that PDGF increases cAMP formation and PKA activity through a MAP kinase-mediated activation of cPLA2, arachidonic acid release, and PGE2 synthesis in human arterial smooth muscle cells.

Epidermal growth factor (EGF) receptor localization in cultured human granulosa lutein cells and the stimulation of progesterone production by EGF and transforming growth factor-alpha (TGF-alpha)

PURPOSE

The purpose of this study was to investigate the expression of EGF receptor (EGF-R) in human granulosa cells undergoing luteinization and progesterone production by these cells in response to EGF an TGF-alpha alone or in combination with luteinizing hormone (LH). Granulosa cells were obtained from IF patients following oocyte retrieval 34 to 36 hr post-hCG injection. EGF receptor was localized in cells by means of immunoperoxidase staining using a polyclonal primary antibody directed against the human EGF-R. To assess progesterone production, cells were seeded overnight, washed, and cultured with the growth factors +/- LH. Medium and treatments were changed every 24 hr for 3 days.

RESULTS

Specific EGF-R staining was observed in the cultured cells compared to those incubated with antibody that was preabsorbed with a 10-fold excess of EGF. Basal progesterone accumulation per 24-hr period was stimulated dose dependently on each day of culture, by both EGF (up to 3.5-fold at 5 or 50 ng/ml) and TGF-alpha (up to 4-fold at 50 ng/ml). The addition of LH alone also stimulated progesterone accumulation daily, and this effect was further enhanced dose dependently by cotreatment with EGF or TGF-alpha. Furthermore, tyrphostin, an EGF-R-specific tyrosine kinase inhibitor, inhibited both basal and growth factor-stimulated progesterone production.

CONCLUSION

These data suggest an EGF receptor-mediated physiological role for EGF and TGF-alpha in human luteal function involving an autocrine and/or a paracrine stimulation of progesterone production.

Role of heterotrimeric G-proteins in epidermal growth factor signalling

Since in 1986 it was reported that a pertussis toxin-sensitive substrate was involved in the Ca2+ signal induced by epidermal growth factor (EGF) in rat hepatocytes, much evidence accumulated to implicate heterotrimeric G-proteins in EGF action. EGF can also induce a cyclic AMP signal, but while the generation of a Ca2+ signal appears to be quite general in EGF action, the increase in cyclic AMP occurs only in few cell types. In non-transformed cell types these effects appear to involve G-proteins. EGF not only induces cell proliferation but also interacts with hormones in the short-term control of cell function in quiescent cells. Most of the known interactions are on cyclic AMP mediated hormone effects, and in many cases, the interaction between EGF and hormones involves G-proteins. Here we review the evidence accumulated in recent years that implicate G-proteins in EGF action. An understanding of the mechanisms involved may reveal new mechanisms of G-protein regulation and will contribute to our knowledge of EGF function and signal transduction.

Localization of epidermal growth factor (EGF) receptor in the rat corpus luteum, and EGF and transforming growth factor-alpha stimulation of luteal cell steroidogenesis in vitro

Epidermal growth factor (EGF) and transforming growth factor-alpha (TGF-alpha) have potent mitogenic effects on granulosa and theca cells. However, their effects on steroidogenesis by these cells is controversial, and there is limited information regarding their effects on luteal cell steroidogenesis. The present study investigated the cellular distribution of the EGF receptor (EGF-R) in the rat corpus luteum (CL) by immunocytochemical staining, and the effects of EGF and TGF-alpha on progesterone and 20 alpha-dihydroprogesterone (20 alpha-OH-P) production in cultures of luteal cells. Using a primary antibody directed against the human EGF-R peptide, specific EGF-R staining was obtained in the CL. Both small and large luteal cells had EGF-R staining. In initial cell culture experiments, treatment of freshly isolated luteal cells with EGF or TGF-alpha (0.5-50 ng/ml) for 24 h had no effect on progesterone and 20 alpha-OH-P accumulation. Addition of LH (250 ng/ml) alone caused a 3.5-fold increase in both progestins, but co-treatment with EGF or TGF-alpha produced no further enhancement of progestin accumulation. However, when cells were seeded overnight and the attached cells were washed prior to growth factor treatment for 3 days with media change every 24 h, both EGF and TGF-alpha caused dose-dependent increases in progesterone accumulation/24 h period (up to 2-fold at 50 ng/ml growth factor) on days 1 and 2 but not day 3 of treatment. 20 alpha-OH-P accumulation was similarly stimulated (up to 2.5-fold) by EGF and TGF-alpha under these conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

EGF modulates phosphoinositide levels in ovarian granulosa cells stimulated by luteinizing hormone

Hamster granulosa cells were exposed to epidermal growth factor (EGF) and luteinizing hormone (LH) to study cross-talk between second messenger pathways involving tyrosine kinase, cAMP, and phosphoinositides. Granulosa cells from ovarian preovulatory follicles of PMSG-primed hamsters were incubated with various additives in serum-free medium. LH, but not EGF, stimulated inositol phosphate (IP) accumulation; however, when combined with LH, EGF inhibited IP accumulation in a manner that was concentration dependent for both LH and EGF. The inhibitory effects of EGF were significantly reduced by the tyrosine kinase inhibitor genistein and by pertussis toxin suggesting a role for tyrosine kinase and an inhibitory G-protein (Gi) in this system. EGF stimulated an increase in cAMP, but it does not appear to modulate LH-stimulated IP levels via cAMP.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Evidence for involvement of a GTP-binding protein in activation of Ca2+ influx by epidermal growth factor in A431 cells: effects of fluoride and bacterial toxins

Aluminium fluoride (AlF4-), a G protein activator, was used to study a possible role of G protein in the control of the pathways for Ca2+ influx through plasma membrane of human carcinoma A431 cells. Fluorimetric measurements with the Ca2+ indicator Indo-1 have shown that addition of fluoride induces an increase in concentration of cytosolic free calcium ([Ca2+]in) due to both release of Ca2+ from intracellular stores and Ca2+ influx from the extracellular medium. The cells stimulated by fluoride became unresponsive to subsequent addition of epidermal growth factor (EGF), histamine and bradykinin. The Ca2+ signal induced by fluoride as well as one induced by EGF was inhibited by the pretreatment of cells with protein kinase C activator, phorbol myristate acetate (PMA). The pretreatment of the cells with pertussis toxin produced no effect on EGF-induced calcium response. In contrast, the pretreatment with cholera toxin (CTX) increased the basal level of [Ca2+]in and abolished the effect of EGF. The effects of CTX could not be reproduced by treating the cells with forskolin or IBMX, agents known to elevate cAMP content in the cell. Patch clamp experiments have shown that fluoride increases the activity of Ca(2+)-permeable channels identical to those activated by EGF from the extracellular side of the membrane [Mozhayeva et al. (1991) J. Membr. Biol. 124, 113-126]. The results obtained suggest the involvement of GTP-binding protein in signal transduction from the EGF receptor to Ca(2+)-permeable channel of plasma membrane in A431 cells.

Phospholipase D treatment enhances gonadotrophin receptor-coupled adenylate cyclase activity in isolated bovine luteal cells

LH-stimulated adenylate cyclase activity in membrane preparations of bovine luteal cells could be enhanced by treating the cells with either phospholipase D or its hydrolysis product, phosphatidic acid. Similar augmentary effects were also produced following treatment of the cells with EGF. Moreover, EGF could stimulate the formation of [3H]phosphatidic acid in [3H]myristic acid preloaded cells, suggesting that EGF is able to activate cellular phospholipase D. Also, PMA was able to increase the phosphatidic acid formation with a parallel increase in the adenylate cyclase activity. We propose, therefore, that phosphatidic acid may act as an intracellular second messenger linking EGF-mediated activation of phospholipase D with the sensitization of LH receptor-coupled adenylate cyclase signalling system.

Epidermal growth factor produces inotropic and chronotropic effects in rat hearts by increasing cyclic AMP accumulation

Previously we have shown that epidermal growth factor (EGF) stimulates cardiac adenylyl cyclase and increases cAMP accumulation in the rat heart (Nair et al., Biochem. J. 264, 563-571, 1989). Moreover, we have shown that the stimulation of adenylyl cyclase by EGF in heart is mediated via activation of the stimulatory GTP binding regulatory protein Gs alpha (Nair et al., J. Biol. Chem. 265, 21317-21322, 1990). Since cAMP increases the beating rate of hearts, studies were performed to investigate the effects of EGF on mechanical function of the heart and the role of cAMP in mediating the cardiac effects of EGF. In isolated perfused rat hearts EGF (15 nM) decreased perfusion pressure, increased ventricular contractility and heart rate in a manner similar to that observed with the beta-adrenergic receptor agonist isoproterenol (10 nM). In the presence of the adenosine A1 receptor agonist (-)-N6-(R-phenylisopropyl)-adenosine (PIA, 100 nM) which via activation of the inhibitory GTP binding protein Gi inhibits adenylyl cyclase, the effects of EGF on cAMP accumulation in the heart were markedly attenuated. PIA also decreased the ability of EGF and isoproterenol to alter cardiac contractility and beating rate. However, PIA did not attenuate the increase in heart rate and contractility induced by the alpha-adrenergic agonist phenylephrine which does not stimulate cAMP accumulation in the heart. These data suggest that EGF alters cardiac function by increasing cellular cAMP accumulation.

The cell biology of blastocyst development

Preimplantation development encompasses the "free"-living period of mammalian embryogenesis, which culminates in the formation of a fluid-filled structure, the blastocyst. Cavitation (blastocyst formation) is accompanied by the expression of a novel set of gene products that contribute directly to the attainment of cell polarity with the trophectoderm, which is both the first epithelium of development and the outer cell layer encircling the inner cell mass of the blastocyst. Several of these gene products have been identified and include the tight junction (ZO-1), Na/K-ATPase (alpha and beta subunits), uvomorulin, gap junction (connexin43), and growth factors such as transforming growth factor-alpha (TGF-alpha) and epidermal growth factor (EGF). This review will examine the role(s) of each of these gene products during the onset and progression of blastocyst formation. The trophectodermal tight junctional permeability seal regulates the leakage of blastocoel fluid and also assists in the maintenance of a polarized Na/K-ATPase distribution to the basolateral plasma membrane domain of the mural trophectoderm. The polarized distribution of the Na/K-ATPase plays an integral role in the establishment of a trans-trophectoderm Na+ gradient, which drives the osmotic accumulation of water across the epithelium into the nascent blastocoelic cavity. The cell adhesion provided by uvomorulin is necessary for the establishment of the tight junctional seal, as well as the maintenance of the polarized Na/K-ATPase distribution. Growth factors such as TGF-alpha and EGF stimulate an increase in the rate of blastocoel expansion, which could, in part, be mediated by secondary messengers that result in an increase in Na/K-ATPase activity. Insight into the mechanism of cavitation has, therefore, directly linked blastocyst formation to trophectoderm cell differentiation, which arises through fundamental cell biological processes that are directly involved in the attainment of epithelial cell polarity.

Overexpression of pp60c-src is associated with altered regulation of adenylyl cyclase

The ability of activators of the beta-adrenergic receptor to elevate intracellular cAMP levels in murine fibroblasts is enhanced upon overexpression of avian c-src [Bushman et al. (1990) Proc. natn. Acad. Sci. U.S.A. 87, 7462-7466]. To investigate the molecular basis for this effect, we prepared particulate fractions from control and pp60c-src overexpressing C3H10T1/2 fibroblasts and assessed the relative abilities of several activators of the beta-adrenergic receptor-Gs-adenylyl cyclase (AC) signal transduction pathway to stimulate the enzymatic response. Two- to three-fold increases in both the sensitivity and maximum responsiveness of AC to the beta-adrenergic agonist isoproterenol were consistently observed in fractions prepared from the c-src overexpressing cells. Interestingly, the AC response to two agents believed to act directly at the level of the G protein were either enhanced (NaF) or unaffected (GTP gamma S) by c-src overexpression. Finally, overexpression of c-src was associated with a reduced ability of both Mn2+ and forskolin to activate AC directly. These results suggest that overexpression of wild type c-src may affect two distinct steps in the regulation of AC exerting a positive effect at the level of Gs activation and a negative effect on AC itself. As no differences in the relative number or affinity of beta-adrenergic receptors, or in the level of AC, Gs alpha or G beta, were detected between control cells and those overexpressing c-src, we propose that pp60c-src overexpression results in a modification of one or more components in this signal transduction pathway.

Epidermal growth factor potentiates the transmitter-induced stimulation of C-AMP and inositol phosphates in human pigment epithelial cells in culture

Salbutamol, isoproterenol and dopamine stimulate C-AMP production in human retinal pigment epithelium (RPE) cells by activation of beta 2-type receptors. Epidermal growth factor (EGF) in contrast does not alter basal levels of C-AMP but elevates in an apparently dose-dependent manner the isoproterenol-induced stimulation of C-AMP. EGF also potentiates the forskolin-induced stimulation of C-AMP but has no effect on the elevation of C-AMP caused by NECA (5'-[N-ethyl]-carboxamido adenosine), an adenosine A2-receptor agonist. EGF, isoproterenol and NECA have no effect on basal levels of inositol phosphates (InsPs) in human RPE cells, but EGF specifically elevates the carbachol-induced stimulation of InsPs. The carbachol effect on InsPs is attenuated by the phorbol ester PMA (4 beta-phorbol 12 myrisate 13-acetate). PMA did not, however, affect the stimulation of C-AMP caused by isoproterenol. The interaction of EGF and C-AMP is further demonstrated in experiments where the incorporation of [3H]thymidine into RPE cells was studied, as an index for proliferation. EGF stimulates RPE cell proliferation while isoproterenol and dibutyryl C-AMP nullify the EGF effect. Dibutyryl C-AMP has a negative effect on RPE cell proliferation while isoproterenol is ineffective. The data presented here suggest that after stimulation of EGF receptors, tyrosine-kinase-activated products can influence secondary messenger products produced from activation of beta 2-type (linked with C-AMP formation) and muscarinic (linked with InsPs production) receptors in RPE cells. We could find no evidence of an interaction between receptors associated with C-AMP and InsPs/diacylglycerol production.

Changes in cAMP phosphodiesterase activity and cAMP concentration during mouse preimplantation development

Cyclic nucleotide phosphodiesterase (PDE) activity and cAMP amounts were measured in mouse preimplantation embryos at the 1-cell, 2-cell, 8-cell/morula, and mid-blastocyst stages. PDE activity remained constant between the 1-cell and 2-cell stages. It decreased by the 8-cell stage and continued to decrease by the mid blastocyst stage to about 14% of the 1- and 2-cell values. By contrast, cAMP amounts remained essentially constant at 0.05 fmole/embryo (0.3 microM) from the 1-cell to the blastocyst stage and increased to 0.175 fmole in the fully expanded blastocyst that was close to hatching. Measurements of embryo volume indicated that intracellular volume remained essentially constant up to the blastocyst stage. The morphological changes in cell shape that accompany differentiation of the trophectoderm and that are coupled with blastocoel expansion decreased the intracellular volume. This decrease resulted in an increase in the cAMP concentration to about 0.4 microM by the mid-blastocyst stage. Previous studies indicate that either cAMP or TGF-alpha/EGF can stimulate the rate of blastocoel expansion. Although TGF-alpha/EGF can elevate cAMP levels in other cell types, TGF-alpha, at a concentration that maximally stimulates the rate of blastocoel expansion, did not elevate cAMP in blastocysts. Thus, it was unlikely that elevation of cAMP is the mechanism by which TGF-alpha stimulates the rate of blastocoel expansion.

Effects of epidermal growth factor on glycolysis in A431 cells

A431 cells were treated with epidermal growth factor (EGF) to study the mechanism by which this factor accelerates the glycolytic flux. After EGF treatment, fructose-2,6-bisphosphate (Fru-2,6-P2) levels rose up to 2-fold. This change correlated with an increase in phosphofructokinase-2 activity, which was not due to a change in the transcription or translation of the enzyme, neither in the amount of enzyme. PK-C does not appear to be involved in the signalling mechanism since EGF was equally potent in PK-C depleted cells than in control cells. The increase in Fru-2,6-P2 levels was lower and more transient in cells treated with EGF in a calcium-free medium than in the presence of the cation, and it was restored by the addition of calcium to the medium. These results suggest a possible role for calcium-mediated pathways in the control of Fru-2,6-P2 levels in A431 cells.

Epidermal growth factor stimulates cAMP accumulation in cultured rat cardiac myocytes

We have previously shown that epidermal growth factor (EGF) augments cAMP accumulation in the heart and stimulates cardiac adenylyl cyclase via a G protein mediated mechanism (Nair et al., 1989). More recently, employing an antibody against the carboxy-terminus decapeptide of Gs alpha, we have demonstrated that Gs alpha mediates the effects of EGF on cardiac adenylyl cyclase (Nair et al., 1990). Since the heart comprises of a variety of cell types, the purpose of the studies presented here was to determine whether or not the effects of EGF on adenylyl cyclase were mediated in cardiac myocytes or noncardiomyocytes. Therefore, cultures of ventricular cardiomyocytes and noncardiomyocytes from neonatal rat hearts were established and characterized. Apart from the differences in cellular morphology, cardiomyocytes but not the noncardiomyocytes employed in our studies expressed the alpha- and beta-myosin heavy chain (MHC) mRNA and the beta-MHC protein. Additionally, as described previously, treatment of cardiomyocytes with thyroid hormone increased alpha-MHC mRNA and decreased the expression of beta-MHC mRNA, indicating that the cardiomyocytes employed in our studies were responding in a physiologically relevant manner. EGF in a time-dependent manner increased cAMP accumulation in the cardiomyocytes but not in noncardiomyocytes. Maximum and half-maximum effects were observed at 100 nM and 2 nM concentrations of EGF, respectively. As determined by the presence of immunoreactive EGF receptors and tyrosine phosphorylation of the 170 kDa protein in membranes of cardiomyocytes and noncardiomyocytes, both the cell populations contained functional EGF receptors. Therefore, the differential effects of EGF on cAMP accumulation in the two cell populations appear to be due to differential coupling of the EGF receptors to the adenylyl cyclase system rather than the absence of EGF receptors in noncardiomyocytes. Consistent with our previous findings in isolated membranes and perfused rat hearts, EGF-elicited increase in cAMP accumulation in cardiomyocytes did not involve activation of beta-adrenoreceptors and was abolished by prior treatment of cells with cholera toxin. Overall, our findings demonstrate that EGF-elicited increase in cAMP accumulation in the heart is the reflection of changes in cAMP content of cardiomyocytes and not noncardiomyocytes.

Mitogenic effects of fibroblast growth factors in cultured fibroblasts. Interaction with the G-protein-mediated signaling pathways

We have shown that FGF (basic or acidic) is mitogenic for quiescent hamster lung fibroblasts (CCL39 line). It is active alone but is much more efficient in synergistic combinations with G-protein-activating agents. When used alone, FGF appears to exert its mitogenic effects without involving any of the major G-protein-mediated signaling pathways. It causes no significant hydrolysis of phosphoinositides, it does not alter the activity of adenylate cyclase, and its mitogenicity is insensitive to pertussis toxin. It therefore seems likely that all pleiotropic actions of FGF are primarily mediated by the intrinsic protein tyrosine kinase of its receptors. However, FGF, acting through its receptor tyrosine kinase, and thrombin, acting through G-protein-coupled receptors, induce a common set of early responses detected within seconds or minutes at the level of membranes, cytoplasm, and nuclei. Typical examples of early responses are activation of Na/H antiporter and Na/K/Cl cotransporter, phosphorylation of ribosomal protein S6, and increased transcription of early-immediate genes (c-fos, c-jun, and c-myc). Not only various classes of growth factors acting via distinct transducing mechanisms activate common targets, but also their synergistic effects on reinitiation of DNA synthesis is reflected on the early responses. How does the coordination of these signaling events take place? A partial answer to this question is illustrated in Figure 6 in which "switch kinases" play the role of integrators of multiple extracellular signals. Raf and, perhaps more convincingly, MAP kinases that are activated by dual phosphorylation on tyrosine and threonine residues are potential good candidates for this integration. This hypothetical scheme could therefore explain, in part, the coordination and the synergy commonly observed in the mitogenic response. The synergy could be generated at the level of MAP kinases simply by dual activating phosphorylations. With the recent cloning of MAP kinases, these questions will be more easily addressed. Another important gap that will have to be filled in future studies is the identification of all the members of the kinase cascade. When used in synergistic combinations with G-protein-activating agents, FGF does exert in contrast some effects on the G-protein-mediated pathways. It potentiates the G-protein-mediated activations of both PIP2-PLC and adenylate cyclase.(ABSTRACT TRUNCATED AT 400 WORDS)

Photosensitizing action of furocoumarins on membrane components and consequent intracellular events

The photodamage induced in membrane components by furocoumarins is reviewed. The oxygen-dependent photoreactions between furocoumarins and cell membrane constituents lead mainly to lipid peroxidation and the formation of cross-linking in ghost proteins, whereas the oxygen-independent photoreactions lead essentially to a C4 cycloaddition between the furocoumarin and the unsaturated fatty acids. In the latter, cycloadducts are formed between the 3,4 double bond of the furocoumarin and the olefinic double bond of the unsaturated fatty acid. The stereochemical structures of these cycloadducts and the reaction mechanism of the cycloaddition are discussed. Finally, the modulation of several membrane systems by furocoumarins and the consequent intracellular events are reviewed.