Effects of 12-0-Tetradecanoylphorbol-13-acetate (TPA) on rat pheochromocytoma (PC12) cells: interactions with epidermal growth factor and nerve growth factor

38-Amino acid form of pituitary adenylate cyclase activating peptide induces process outgrowth in human neuroblastoma cells

Permanent cell lines from human neuroblastoma, a sympathoadrenal malignancy, are known to exhibit a more neuronal phenotype characterized by outgrowth of long processes in response to multiple second messenger analogs. In this report we demonstrate that the 38-amino acid form of a peptide homologous to vasoactive intestinal peptide (VIP), pituitary adenylate cyclase activating peptide (PACAP), as well as the 27-amino acid form of PACAP, induce NB-OK human neuroblastoma cells to extrude cellular processes within 5 hr of treatment with either peptide at 10(-8) M. Treatment of NB-OK cells with PACAP38 or PACAP27 at 10(-8) M for 1 hr also elevates cAMP content greater than 100-fold and inositol lipid turnover 11- to 12-fold. VIP acutely induces process outgrowth and elevates intracellular second messenger levels in NB-OK cells only at higher concentrations, 10(-6) M or greater. In contrast to the equipotency of PACAP27 and PACAP38 in stimulating the outgrowth of processes observed after 5 hr of treatment, PACAP38 is much more potent than PACAP27 when NB-OK cells are scored for process outgrowth after 72 hr of treatment. Correlating with the extended time course over which morphologic changes are seen with PACAP38, cAMP levels remain elevated for a more prolonged time span during treatment with PACAP38 than PACAP27. After 72 hr of treatment with PACAP38 versus treatment with PACAP27, cAMP levels are elevated 10-fold versus 3-fold, respectively. PACAP38 at 10(-8) M also induces process outgrowth in two additional human neuroblastoma lines tested, SMS-KAN and LA-N-1, whereas PACAP27 and VIP at the same concentration are less effective.(ABSTRACT TRUNCATED AT 250 WORDS)

Posttranscriptional regulation of GAP-43 gene expression in PC12 cells through protein kinase C-dependent stabilization of the mRNA

We have previously shown that nerve growth factor (NGF) selectively stabilizes the GAP-43 mRNA in PC12 cells. To study the cellular mechanisms for this post-transcriptional control and to determine the contribution of mRNA stability to GAP-43 gene expression, we examined the effects of several agents that affect PC12 cell differentiation on the level of induction and rate of degradation of the GAP-43 mRNA. The NGF-mediated increase in GAP-43 mRNA levels and neurite outgrowth was mimicked by the phorbol ester TPA, but not by dibutyryl cAMP or the calcium ionophore A12783. Downregulation of protein kinase C (PKC) by high doses of phorbol esters or selective PKC inhibitors prevented the induction of this mRNA by NGF, suggesting that NGF and TPA act through a common PKC-dependent pathway. In mRNA decay studies, phorbol esters caused a selective 6-fold increase in the half-life of the GAP-43 mRNA, which accounts for most of the induction of this mRNA by TPA. The phorbol ester-induced stabilization of GAP-43 mRNA was blocked by the protein kinase inhibitor polymyxin B and was partially inhibited by dexamethasone, an agent that blocks GAP-43 expression and neuronal differentiation in PC12 cells. In contrast, the rates of degradation and the levels of the GAP-43 mRNA in control and TPA-treated cells were not affected by cycloheximide treatment. Thus, changes in GAP-43 mRNA turnover do not appear to require continuous protein synthesis. In conclusion, these data suggest that PKC activity regulates the levels of the GAP-43 mRNA in PC12 cells through a novel, translation-independent mRNA stabilization mechanism.

Synergistic effects of nerve growth factor and phorbol 12-myristate 13-acetate on rapid motility and process formation in PC12 cells: the role of laminin

A126-1B2 cells, a PKA (cAMP-dependent protein kinase)-deficient variant of PC12 cells, but not parental PC12 cells, form processes within 15-30 min of exposure to both nerve growth factor (NGF) and activators of protein kinase C when grown on tissue culture plastic (Glowacka and Wagner, J Neurosci Res 25: 453-462, 1990). Time-lapse microscopy has demonstrated that these processes are formed by a novel mechanism, i.e., rapid movement of the cell body away from a point of attachment, which morphologically resembles a growth cone. These "fast" neurites are attached to the substratum at a number of points, which display membrane activity in the form of active ruffling and the extension of filopodia and membrane pleats. Thus, these processes are formed by a mechanism distinct from that used by PC12 and other neuronal cells to form processes in culture. Wild-type PC12 cells also migrate and form fast neurites in response to a combination of NGF and phorbol 12-myristate 13-acetate (PMA), when they are grown in conditioned media or plates, suggesting that a secreted factor that can bind to the substratum is essential for the rapid formation of these neurites. Similarly, wild-type PC12 cells grown on a laminin-coated substratum also migrate and form "fast neurites" in response to a combination of NGF and PMA. This rapid migration is attenuated by an anti-alpha 1, beta 1-integrin antisera, implicating a laminin-integrin interaction; and it is inhibited by alpha-lactalbumin, suggesting an involvement of a beta 1,4 galactosyltransferase in the response. The formation of fast neurites is not dependent on concurrent protein synthesis, but it is inhibited by lithium, cytochalasin D, and methylthioadenosine or pretreatment of cells with NGF. Thus PC12 cells grown on the appropriate substrate have the ability to migrate rapidly and thereby form neuron-like processes within minutes of exposure to NGF and PMA. This morphological response to a combination of agents may provide an alternative means by which nerve cells form connections. Alternatively, it may reflect a mechanism that facilitates cellular migration during developmental processes.

Ethanol enhances growth factor-induced neurite formation in PC12 cells

Ethanol can injure the nervous system by disturbing the growth of neural processes. PC12 cells, which form neurites in response to nerve growth factor (NGF), fibroblast growth factor (FGF), and cAMP analogues, were used to study mechanisms by which ethanol alters process outgrowth. Ethanol potentiated NGF-induced neurite outgrowth in cells cultured on different substrata and in serum-containing or defined medium. Ethanol did not increase NGF receptor binding or internalization of NGF. Neurite outgrowth induced by basic FGF was also increased by ethanol but outgrowth induced by forskolin was not. Ethanol potentiated NGF-induced expression of Thy-1, but not of neural cell adhesion molecule (N-CAM), indicating that some, but not all actions of NGF are enhanced by ethanol. In some brain regions, chronic exposure to ethanol increases the growth of dendrites. This has been explained as a compensatory response of surviving neurons to the loss of neighboring cells, and not as a direct effect of ethanol. The present findings suggest that, in some cells, ethanol directly promotes growth factor-mediated neurite formation. This could harm the nervous system by disturbing the balanced development and organization of synapses.

Acute regulation of the epidermal growth factor receptor in response to nerve growth factor

PC12 cells possess specific receptors for both nerve growth factor and epidermal growth factor, and by an unknown mechanism, nerve growth factor is able to attenuate the propagation of a mitogenic response to epidermal growth factor. The differentiation response of PC12 cells to nerve growth factor, therefore, predominates over the proliferative response to epidermal growth factor. We have observed that the addition of nerve growth factor to PC12 cells rapidly produces a decrease in surface 125I-epidermal growth factor binding capacity. Unlike previously described nerve growth factor effects on 125I-epidermal growth factor binding capacity, which required several days of nerve growth factor exposure, the decreases we report occur within minutes of nerve growth factor addition: A 50% decrease in 125I-epidermal growth factor binding capacity is evident at 10 min. This rapid nerve growth factor response is concentration dependent; inhibition of 125I-epidermal growth factor binding is detectable at nerve growth factor levels as low as 0.2 ng/ml and is maximal at approximately 50 ng/ml, consistent with known ranges of biological activity. No demonstrable differences in the rate of epidermal growth factor receptor synthesis or degradation were observed in cells acutely exposed to nerve growth factor. Scatchard analysis revealed that acute nerve growth factor treatment decreased the number of both high- and low-affinity 125I-epidermal growth factor binding sites, while the receptor affinity remained unchanged. We have also investigated the involvement of various potential intracellular mediators of nerve growth factor action and of known intracellular modulatory systems of the epidermal growth factor receptor for their capacity to participate in this nerve growth factor activity.

NGF-induction of the metalloproteinase-transin/stromelysin in PC12 cells: involvement of multiple protein kinases

In previous work, we found that nerve growth factor (NGF) induced expression of the mRNA transcript encoding the metalloproteinase transin/stromelysin in PC12 cells. Transin was found, moreover, to be a "late" gene product whose expression correlated with neurites extension. In this study, various aspects of the NGF intracellular signaling pathway in PC12 cells are investigated. We show that the protein kinase inhibitor staurosporine, but not various other kinase inhibitors, specifically blocked the NGF induction of transin. Preliminary characterization of this staurosporine-sensitive kinase suggest that it does not correspond to a tyrosine kinase, nor various serine kinases, and that it is involved both at the transcriptional and posttranscriptional levels of transin gene regulation. In contrast to these effects of staurosporine, various activators of protein kinases C and A augmented the NGF induction of transin. Similar effects of these kinase inhibitors and activators were also observed with the expression of various immediate-early genes that have been proposed to mediate the transcriptional effects of NGF, including c-fos and c-jun. These data suggest, therefore, that the NGF induction of transin mRNA expression involves multiple protein kinases acting at a number of postreceptor regulatory steps in the NGF signaling pathway.

Rapid phosphorylation of cellular proteins during differentiation of neuroblastoma cells induced by recombinant human interferon-gamma

Cells of two human neuroblastoma lines, GOTO and KP-N-RT-LN, differentiated in response to treatment with 1,000 IU/ml of recombinant human interferon-gamma (rIFN-gamma), but did not respond to rIFN-alpha 2 or IFN-beta. Treatment with rIFN-gamma rapidly increased the phosphorylation of several cell proteins; in particular, there was enhanced phosphorylation of a 61-kD protein within 1 min after treatment. This enhancement was not observed in variant sublines of GOTO cells, which did not differentiate when treated with rIFN-gamma. These findings suggest that when neuroblastoma cells are induced to differentiate by treatment with rIFN-gamma, phosphorylation of the 61-kD protein plays an important role.

Nerve growth factor and fibroblast growth factor regulate neurite outgrowth and gene expression in PC12 cells via both protein kinase C- and cAMP-independent mechanisms

Nerve growth factor (NGF), acidic fibroblast growth factor (aFGF), and basic fibroblast growth factor (bFGF) promote the survival and differentiation of a variety of peripheral and central neurons. The signal transduction mechanisms that mediate the actions of these factors in neuronal cells are not well understood. We examined the effect of a deficiency in protein kinase C (PKC) and/or cAMP second messenger systems on the actions of NGF, aFGF, and bFGF in the pheochromocytoma (PC12) cell line. Activation of PKC was not required for NGF, aFGF, and bFGF to maximally induce ornithine decarboxylase (ODC), transcription of the early response genes, d2 and d5, or neurite outgrowth. In a PC12 cell mutant that is deficient in cAMP responsiveness (A126-1B2), all three growth factors maximally induced the transcription of d5 and neurite outgrowth, but aFGF and bFGF did not induce significant increases in ODC. NGF and aFGF maximally induced the transcription of d2 in A126-1B2 cells, but bFGF-induced d2 transcription was attenuated. NGF, aFGF, and bFGF maximally induced neurite outgrowth and d5 transcription in A126 cells that were made deficient in PKC. The d2 transcriptional response was substantially reduced in cells deficient in both PKC and cAMP responsiveness. These observations lead us to conclude that (a) cAMP- and PKC-dependent events are, at least in part, causally linked to NGF, aFGF, and bFGF induction of both ODC and transcription of d2 and may control functionally redundant pathways; (b) NGF, aFGF, and bFGF can elicit neurite outgrowth and increase transcription of d2 and d5 in PC12 cells via mechanisms that are independent of both PKC and cAMP; (c) NGF, aFGF, and bFGF can induce ODC in the absence of PKC; and (d) aFGF and bFGF require cAMP responsiveness to induce ODC in PC12 cells.

Nerve growth factor and phorbol esters increase the number of choline acetyltransferase-positive cells in two morphologically distinct classes of basal forebrain neurons in primary cultures

Nerve growth factor (NGF) has been shown to be active in the CNS as a neurotrophic agent. Cholinergic neurons of the basal forebrain are one cell type in the CNS which have been identified as a target for NGF. When dissociated cell cultures from the basal forebrain were treated for 7 days with NGF (20 ng/100 microliters), the number of choline acetyltransferase (ChAT)-immunopositive cells was increased from 30 +/- 6 to 58 +/- 3. Cholinergic cells taken from the basal forebrain exhibit 3 different morphologies: stellate, pyramidal, and bipolar. The NGF treatment was found to increase the number of stellate cells from 7 +/- 2 to 23 +/- 2 and the number of pyramidal cells from 14 +/- 2 to 26 +/- 2, but had no effect on the number of bipolar cells. The activation of protein kinase C by phorbol 12-myristate, 13-acetate (TPA) also increased the number of ChAT-positive cells in a dose-dependent manner. A maximal increase was observed with 10 ng/ml of TPA which increased the number of positive cells from a basal level of 21 +/- 4 to 42 +/- 4. As was the case with NGF, only the stellate and pyramidal cells were affected by the phorbol ester treatment. In co-addition experiments, the cultures were treated with 10 ng/100 of NGF and 10 ng/ml of TPA, with the result that there was no further increase in the number of immunopositive cells over the NGF controls. These results suggest that the mechanisms by which NGF and TPA increase the number of ChAT-positive cells are interactive at some point. The effect of TPA at the higher doses of NGF was distinctly different. When cells were treated with 20 ng/100 microliters of NGF and 0.05-50 ng/ml of TPA, the NGF response was down-regulated to the level of the vehicle-treated controls.

The mode of action of nerve growth factor in PC12 cells

This review deals with the mechanism of nerve growth factor action. In view of the many and diversified effects of this growth factor, and since it could utilize different mechanism(s) in distinct types of cells, we have confined our analysis to the best characterized and more extensively studied target, the clonal cell line PC12. When exposed to NGF in vitro, these neoplastic cells recapitulate the last major steps of neuronal differentiation, i.e., the commitment to become a neuron and the acquisition of the neuronal phenotype. This is characterized by electrically excitable neurites, a display of a highly organized cytoskeleton, and the specific chemical and molecular neuronal properties. These effects are elicited upon the interaction of NGF with a receptor whose gene has been cloned and whose kinetic properties are now relatively well characterized. It is not yet clear, on the contrary, if and which of the several potential second messengers (cAMP, Ca, or phosphoinositides) that undergo marked fluctuations following NGF binding, transduce and amplify the NGF message. Among both the early and late effects of NGF is the modulation of expression of several genes. Some of the products of these genes are mainly restricted to nerve cells and others appear to play a crucial role in regulating the proper assembly of cytoskeletal elements. It is hypothesized that this complex array of chemical, molecular, and ultrastructural changes is triggered by NGF, not through activation of a single pathway, but more likely via combinatorial processes whereby several intracellular signals interplay before the irreversible commitment of becoming a neuron is undertaken.

Nerve growth factor stimulates the phosphorylation of a 250 kDa cytoskeletal protein in cell-free extracts of PC12 cells

Nerve growth factor (NGF) rapidly stimulates the phosphorylation of a 250 kDa cytoskeletally-associated protein (pp250) by a protein kinase which is also associated with structural elements of the cell. We have solubilized these proteins and demonstrated that NGF-stimulated phosphorylation can be observed in cell free extracts of cytoskeletons from NGF-treated PC12 cells. The pp250 substrate and the 250-kinase were solubilized from PC12 cytoskeletons by treatment with 2 M urea. Phosphorylation of pp250 was maximally stimulated following treatment of the cells for 5 min with NGF. This effect was transient, diminishing with longer exposure of the cells to hormone. The 250-kinase preferred Mn2+ over Mg2+ and was inhibited by both Na+ and K+. The phosphorylation of pp250 was not affected by Ca2+. Upon fractionation of the urea-soluble cytoskeletal proteins by gel filtration, the 250-kinase eluted in two peaks; one peak of enzyme activity coeluting with the pp250 substrate, and a second peak of enzyme activity eluting with an apparent Mr of approximately 60 kDa. Treatment of the PC12 cells with the phorbol ester TPA also stimulated the phosphorylation of pp250, although this effect was not as great as that produced by NGF. This cell free system should be a valuable tool in the investigation of the mechanisms of NGF action.

Heterologous regulation of EGF receptors in fibroblastic cells

Platelet-derived growth factor (PDGF) increases the mitogenic activity of epidermal growth factor (EGF) in several cells lines, including BALB/C-3T3. PDGF-treated BALB/C-3T3 cells manifest a reduced capacity to bind 125I-labeled EGF due to a loss of high affinity EGF receptors. Cholera toxin potentiates the ability of PDGF to both decrease EGF binding and initiate mitogenesis. Whether PDGF increases EGF sensitivity via its effects on EGF receptors is not known and requires a more complete understanding of the mechanism by which PDGF decreases EGF binding. 12-O-tetradecanoylphorbol 13-acetate (TPA) also reduces EGF binding in BALB/C-3T3 and other cells, presumably by activating protein kinase C and, consequently, inducing the phosphorylation of EGF receptors at threonine-654. PDGF indirectly activates protein kinase C, and EGF receptors in PDGF-treated WI-38 cells are phosphorylated at threonine-654. Thus, the effects of PDGF on EGF binding may also be mediated by protein kinase C. We investigated this hypothesis by comparing the actions of PDGF and TPA on EGF binding in density-arrested BALB/C-3T3 cells. Both PDGF and TPA caused a rapid, transient, cycloheximide-independent loss of 125I-EGF binding capacity. The actions of both agents were potentiated by cholera toxin. However, whereas TPA allowed EGF binding to recover, PDGF induced a secondary and cycloheximide-dependent loss of binding capacity. Most importantly, PDGF effectively reduced binding in cells refractory to TPA and devoid of detectable protein kinase C activity. These findings indicate that PDGF decreases EGF binding by a mechanism that involves protein synthesis and is distinct from that of TPA.

Cholinergic neurons of fetal rat telencephalon in aggregating cell culture respond to NGF as well as to protein kinase C-activating tumor promoters

Serum-free aggregating cell cultures of fetal rat telencephalon treated with the potent tumor promoter phorbol 12-myristate 13-acetate (PMA) showed a dose-dependent, persistent stimulation of the enzymes choline acetyltransferase (ChAT), glutamic acid decarboxylase and glutamine synthetase. After elimination of the proliferating cells by treatment of the cultures with Ara-C (0.4 microM) only the cholinergic marker enzyme, ChAT, could be stimulated by tumor promoters. The non-promoting phorbol ester, 4 alpha-phorbol 12,13-didecanoate proved to be inactive in these cultures, whereas the potent non-phorbol tumor promoter, mezerein, produced an even greater stimulatory effect than PMA. Since PMA and mezerein are potent and specific activators of protein kinase C, the present results suggest a role for this second messenger in the development of cholinergic telencephalon neurons. Stimulation of ChAT required prolonged exposure (48 h) of the cultures to PMA and the responsiveness of the cholinergic neurons to the tumor promoters decreased with progressive cellular maturation. The cholinergic telencephalon neurons showed the same pattern of responsiveness for tumor promoters as for nerve growth factor (NGF). However, the combined treatment with NGF and either PMA or mezerein produced an additive stimulatory effect, suggesting somewhat different mechanisms of action.

Nerve growth factor rapidly induces c-fos mRNA in PC12 rat pheochromocytoma cells

The nerve growth factor (NGF)-mediated increase in c-fos gene expression in the rat pheochromocytoma PC12 cell line has been investigated. NGF treatment of PC12 cells results in an increased level of c-fos mRNA within 15 min. An approximately 100-fold increase in the level of c-fos mRNA occurs 30-45 min after exposure to NGF and the c-fos mRNA concentration returns to its basal level 2 hr after NGF treatment. Thus, the half-life of this RNA transcript is extremely short. In the presence of cycloheximide, the c-fos gene is superinduced and the increased level of c-fos mRNA persists for at least 24 hr. The induction of c-fos gene expression was further studied by utilizing a monoclonal antibody (mAb-192) that is directed against the NGF receptor but does not compete with NGF for binding to the receptor. Treatment of the cells with mAb-192 inhibits the NGF-stimulated elevation of c-fos mRNA, suggesting that the antibody may interfere with the receptor's ability to generate the signal required to stimulate the transcription of this gene. NGF is not the only agent capable of inducing c-fos gene expression in these cells; epidermal growth factor, the tumor promoter phorbol 12-myristate 13-acetate, and the calcium ionophore A23187, agents that induce the c-fos gene in other cell lines, are also effective in PC12 cells. The mRNA for the nuclear protein fos is rapidly induced by NGF and other agents to which PC12 cells respond. This supports the hypothesis that the fos gene product may play a role in signal transduction.

Platelet-derived growth factor modulates epidermal growth factor receptors by a mechanism distinct from that of phorbol esters

Platelet-derived growth factor (PDGF) and phorbol 12-myristate 13-acetate (PMA) decrease high affinity binding of 125I-labeled epidermal growth factor (EGF) and potentiate mitogenesis in BALB/c 3T3 cells, and both have been shown to induce the phosphorylation of the EGF receptor at threonine residues. These similarities suggest that the actions of PDGF on EGF binding may be mediated by protein kinase C, the cellular effector of PMA. We show that in density-arrested BALB/c 3T3 cells PDGF and PMA induce a rapid, transient, cycloheximide-independent loss of EGF binding activity. As has been previously shown for PDGF, the ability of PMA to reduce EGF binding was enhanced by cholera toxin, a potent activator of adenylate cyclase. In contrast to PMA, however, PDGF induced a further reduction in EGF binding that was strictly dependent upon continued protein synthesis. Furthermore, PDGF effectively reduced EGF binding in cells refractory to PMA. Cells desensitized to PMA, presumably due to the loss of protein kinase C activity, also remained mitogenically responsive to PDGF. These data suggest that the mechanism by which PDGF modulates EGF binding differs from that of PMA and thus, at least in part, is independent of protein kinase C.

Protein kinase C as a component of a nerve growth factor-sensitive phosphorylation system in PC12 cells

The treatment of PC12 cells with either nerve growth factor or phorbol 12-myristate 13-acetate caused a decrease in the phosphorylation of a soluble 100-kDa protein (Nsp100). After treatment with nerve growth factor, the activity of Ca2+/phospholipid-dependent protein kinase (protein kinase C) in the cytosol was increased. When the cytosol from untreated PC12 cells was preincubated with purified protein kinase C and its cofactors, the phosphorylation of Nsp100 was decreased. The preincubation of cytosol from nerve growth factor-treated PC12 cells with protein kinase C did not decrease Nsp100 phosphorylation further. Moreover, preincubation of partially purified Nsp100 kinase with protein kinase C decreased its ability to phosphorylate Nsp100. These results suggest that the binding of nerve growth factor to its receptor on PC12 cells causes an increase in the activity of protein kinase C in the cytosol and phosphorylation of Nsp100 kinase, which in turn lowers its ability to phosphorylate Nsp100.

Two receptor classes for epidermal growth factor on pheochromocytoma cells, distinguishable by temperature, lectins, and tumor promoters

Rat pheochromocytoma cells (clone PC12) display cell surface receptors for both nerve growth factor (NGF) and epidermal growth factor (EGF) and therefore provide a useful model system with which to study the role of these receptors in the regulation of proliferation and differentiation. In this paper PC12 cells are demonstrated to possess two classes of EGF receptors, a high-affinity class with 7,600 sites per cell and an apparent dissociation constant (Kd) of 0.05 nM, and a low-affinity class with 62,000 sites per cell and a Kd of 14.1 nM. These findings are contrary to literature data (Huff et al., 1981; Vale and Shooter, 1983) but can be explained in part by differences in experimental conditions. Binding studies at 37 degrees C compared with room temperature demonstrated similar affinities of both classes, but during prolonged incubation at 37 degrees C, the binding capacities of both classes decreased. Furthermore the high-affinity class was sensitive to lectins, such as concanavalin A (Con A), and to the tumor-promoting phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Both compounds caused a decrease of the affinity of the high-affinity class without affecting the low-affinity class. At high concentrations of Con A or TPA, a decrease of the apparent number of binding sites of the low-affinity class was also observed. The similarities between the characteristics of EGF binding and NGF binding in PC12 cells are striking and will be discussed.

Role of phorbol diesters in in vitro murine megakaryocyte colony formation

In vitro megakaryocyte differentiation is regulated by two activities: a megakaryocyte colony-stimulating activity (Mk-CSA), which is required for proliferation, and an auxiliary factor, megakaryocyte potentiating activity, which plays a role in later differentiation events. Tumor-promoting phorbol esters alter many cellular differentiation-related events. Thus, it was hypothesized that phorbol esters may bring about megakaryocyte differentiation in vitro. 4 beta-Phorbol 12-myristate 13-acetate (PMA), when co-cultured with a source of Mk-CSA, stimulated a threefold increase in colony numbers. Co-culture of PMA and megakaryocyte potentiator activity did not stimulate colony formation, thus eliminating any action of PMA as an Mk-CSA. The direct effect of PMA on the formation of megakaryocyte colonies was established by (a) the function of PMA as a megakaryocyte potentiator in serum-free experiments, (b) the ability of PMA to stimulate megakaryocyte colony formation using bone marrow cells depleted of populations known to produce potentiating activity, (c) the inability of bone marrow adherent cells previously treated with phorbol, 12,13-dibutyrate (PDBu) to augment megakaryocyte colony formation, and (d) the ability of PMA to induce the growth of immature megakaryocytes into large single megakaryocytes. Structure:activity experiments resulted in equivalent activities for PMA and PDBu, whereas the nontumor promoter phorbol 12,13-diacetate and phorbol itself lacked activity. The observations in this study indicate that phorbol esters can bring about megakaryocyte differentiation, and during colony formation, can induce effects identical to those brought about by biological sources of megakaryocyte potentiator activity.

Changes in inducibility of ornithine decarboxylase activity in differentiating human neuroblastoma cells

Human SH-SY5Y neuroblastoma cells could be induced to differentiate morphologically and biochemically in the presence of 12-O-tetradecanoylphorbol-13-acetate (TPA), retinoic acid (RA), or a combination of these two substances. The phenotypical changes induced by these substances differed, but one effect of both was an inhibition of the cell growth. Addition of TPA or RA to non-treated cells had no effect on the activation of ornithine decarboxylase (ODC, EC 4.1.1.17.), while a change to fresh medium stimulated the ODC to maximum activity after 4-6 h. The activity was not altered by the presence of RA in the fresh medium, but TPA partially inhibited the medium-stimulated ODC activity. Cells treated for 4 or 8 days with TPA or a combination of TPA and RA had a low ODC activity which could not be induced by fresh medium. However, RA-treated (and thus growth-inhibited) cells still responded to a change of medium by exhibiting an ODC activity of the same magnitude and duration as in medium-stimulated control cells. The results seem to suggest that the growth inhibition induced by TPA and RA, respectively, is mediated by different mechanisms.

Phorbol ester induces desensitization of adenylate cyclase and phosphorylation of the beta-adrenergic receptor in turkey erythrocytes

Incubation of turkey erythrocytes with the phorbol ester phorbol 12-myristate 13-acetate (PMA) results in a dose- and time-dependent desensitization of isoproterenol-stimulated adenylate cyclase activity. Compared to controls, membranes from PMA-treated cells have an isoproterenol-stimulated adenylate cyclase activity that is decreased 20%-40%, with little effect on forskolin or fluoride activation of adenylate cyclase. No change in beta-adrenergic receptor number is observed after PMA treatment, indicating that the major effect of PMA is to uncouple receptor interactions with Ns, the stimulatory guanine nucleotide regulatory protein of adenylate cyclase. Purification of beta-adrenergic receptors from 32Pi-labeled turkey erythrocytes, incubated in the presence or absence of PMA, indicates that the phorbol ester is capable of inducing a 3-fold increase in phosphorylation of the beta-adrenergic receptor. The PMA effect is similar to the phosphorylation of the beta-adrenergic receptor during isoproterenol- and dibutyryl cAMP-induced desensitization of adenylate cyclase in turkey erythrocytes. The findings indicate that decreased receptor-Ns coupling is correlated with receptor phosphorylation and that phorbol esters can influence the responsiveness of hormone-sensitive adenylate cyclase in certain cell types.

[Epidermal growth factor: structure, location, phosphorylation and regulation of its receptor]

Epidermal growth factor (EGF) is a Mr 6045 polypeptide first characterized for its ability to stimulate mitogenesis in epidermal and epithelial cells. The first step in the action of the growth factor is its binding to specific, high affinity membrane receptors. These receptors have been studied in a number of tissues and cell culture lines. The level of EGF receptors is modulated by many agents. EGF down-regulates its receptor. In addition, the number of EGF receptors is decreased by other growth factors (platelet-derived growth factor; transforming growth factor), by many tumor promoters and by viral transformation. Several hormones also can regulate EGF binding in its target tissues.

The neurite-promoting effect of fibroblast growth factor on PC12 cells

Treatment of PC12 cells with fibroblast growth factor(s) from either brain or pituitary caused neurite outgrowth comparable to that produced by nerve growth factor. The neurite outgrowth was preceded by a substantial rise in the activity of ornithine decarboxylase.

Ionic responses and growth stimulation induced by nerve growth factor and epidermal growth factor in rat pheochromocytoma (PC12) cells

Rat pheochromocytoma cells (clone PC12) respond to nerve growth factor (NGF) by the acquirement of a phenotype resembling neuronal cells. In an earlier study we showed that NGF causes an increase in Na+,K+ pump activity, as monitored by ouabain-sensitive Rb+ influx. Here we show that addition of epidermal growth factor (EGF) to PC12 cells resulted in a stimulation of Na+,K+ pump activity as well. The increase of Na+,K+ pump activity by NGF or EGF was due to increased Na+ influx. This increased Na+ influx was sensitive to amiloride, an inhibitor of Na+,H+ exchange. Furthermore, no changes in membrane potential were observed upon addition of NGF or EGF. Amiloride-sensitive Na+,H+ exchange in PC12 cells was demonstrated by H+ efflux measurements and the effects of weak acids on Na+ influx. These observations suggest that both NGF and EGF activate an amiloride-sensitive, electroneutral Na+,H+ exchange mechanism in PC12 cells. These findings were surprising in view of the opposite ultimate biological effects of NGF and EGF, e.g., growth arrest vs. growth stimulation. However, within 24 h after addition, NGF was found to stimulate growth of PC12 cells, comparable to EGF. In the presence of amiloride, this stimulated growth by NGF and EGF was abolished. In contrast, amiloride did not affect NGF-induced neurite outgrowth of PC12 cells. From these observations it is concluded that in PC12 cells: (a) NGF has an initial growth stimulating effect; (b) neurite outgrowth is independent of increased amiloride-sensitive Na+ influx; and (c) growth stimulation by NGF and EGF is associated with increased amiloride-sensitive Na+ influx.