EGF triggers neuronal differentiation of PC12 cells that overexpress the EGF receptor

Inhibition of MAP kinase kinase blocks the differentiation of PC-12 cells induced by nerve growth factor

The mitogen-activated protein kinase (MAP kinase) pathway is thought to play an important role in the actions of neurotrophins. A small molecule inhibitor of the upstream kinase activator of MAP kinase, MAP kinase kinase (MEK) was examined for its effect on the cellular action of nerve growth factor (NGF) in PC-12 pheochromocytoma cells. PD98059 selectively blocks the activity of MEK, inhibiting both the phosphorylation and activation of MAP kinases in vitro. Pretreatment of PC-12 cells with the compound completely blocked the 4-fold increase in MAP kinase activity produced by NGF. Half-maximal inhibition was observed at 2 microM PD98059, with maximal effects at 10-100 microM. The tyrosine phosphorylation of immunoprecipitated MAP kinase was also completely blocked by the compound. In contrast, the compound was without effect on NGF-dependent tyrosine phosphorylation of the pp140trk receptor or its substrate Shc and did not block NGF-dependent activation of phosphatidylinositol 3'-kinase. However, PD98059 completely blocked NGF-induced neurite formation in these cells without altering cell viability. These data indicate that the MAP kinase pathway is absolutely required for NGF-induced neuronal differentiation in PC-12 cells.

The GTPase-activating protein of Ras suppresses platelet-derived growth factor beta receptor signaling by silencing phospholipase C-gamma 1

The beta receptor for platelet-derived growth factor (beta PDGFR) is activated by binding of PDGF and undergoes phosphorylation at multiple tyrosine residues. The tyrosine-phosphorylated receptor associates with numerous SH2-domain-containing proteins which include phospholipase C-gamma 1 (PLC gamma), the GTPase-activating protein of Ras (GAP), the p85 subunit of phosphatidylinositol 3 kinase (PI3K), the phosphotyrosine phosphatase Syp, and several other proteins. Our previous studies indicated that PI3K and PLC gamma were required for relay of the mitogenic signal of beta PDGFR, whereas GAP and Syp did not appear to be required for this response. In this study, we further investigated the role of GAP and Syp in mitogenic signaling by beta PDGFR. Focusing on the PLC gamma-dependent branch of beta PDGFR signaling, we constructed a series of mutant beta PDGFRs that contained the binding sites for pairs of the receptor-associated proteins: PLC gamma and PI3K, PLC gamma and GAP, or PLC gamma and Syp. Characterization of these mutants showed that while all receptors were catalytically active and bound similar amounts of PLC gamma, they differed dramatically in their ability to initiate DNA synthesis. This signaling deficiency related to an inability to efficiently tyrosine phosphorylate and activate PLC gamma. Surprisingly, the crippled receptor was the one that recruited PLC gamma and GAP. Thus, GAP functions to suppress signal relay by the beta PDGFR, and it does so by silencing PLC gamma. These findings demonstrate that the biological response to PDGF depends not only on the ability of the beta PDGFR to recruit signal relay enzymes but also on the blend of these receptor-associated proteins.

K252a-potentiation of EGF-induced neurite outgrowth from PC12 cells is not mimicked or blocked by other protein kinase activators or inhibitors

Epidermal growth factor (EGF) has recently been shown to cause certain strains of PC12 cells to extend short neurites. This EGF-induced differentiation of PC12 was found to be potentiated by the protein kinase inhibitor, K252a, in that PC12 cells treated with both EGF and K252a extended long branched neurites similar to those induced by nerve growth factor (NGF). As reported here no other protein kinase inhibitor or activator mimicked or blocked the effect of K252a on EGF-induced PC12 differentiation. Cyclic adenosine 3',5'-monophosphate (cAMP) also potentiated EGF-induced neurite outgrowth from PC12 cells, but the mechanism of this potentiation was different from that of K252a. Cells that had been exposed to EGF and then stripped of their neurons extended neurites again when retreated with EGF in the absence of RNA synthesis or when treated with NGF in the absence of RNA synthesis. Thus EGF can prime PC12 cells for either EGF or for NGF, a finding that further suggests that EGF and NGF use similar signaling pathways to induced neuronal differentiation of PC12.

Staurosporine causes epidermal growth factor to induce differentiation in PC12 cells via receptor up-regulation

Although they all utilize tyrosine kinase receptors and activate signaling pathways characterized by a similar set of phosphoproteins, epidermal growth factor (EGF) promotes only cell division while fibroblast growth factor (FGF) and nerve growth factor (NGF) can induce division followed by differentiation in PC12 cells. EGF, in contrast to NGF and FGF, cannot maintain the sustained phosphorylation and activation of mitogen-activated protein (MAP) kinase kinase and MAP kinases, which may account for the difference in phenotypic response. The pretreatment of PC12 cells with staurosporine, a protein kinase inhibitor, causes a substantial increase in both receptor and MAP kinase phosphorylation that results in a differentiative response (neurite proliferation). However, neurites begin to disappear after 3 days, despite the continual presence of EGF, and are largely gone after 5 days, which is not the case with NGF and FGF. Thus, the effect of staurosporine is not permanent. Northern and Western blots indicate that the staurosporine response mainly results from a substantial up-regulation in EGF receptor synthesis, thus providing a much stronger cell surface signal and supporting the view that quantitative rather than qualitative differences distinguish the EGF versus NGF/FGF signaling pathways in these cells.

Inactivation of p42 MAP kinase by protein phosphatase 2A and a protein tyrosine phosphatase, but not CL100, in various cell lines

BACKGROUND

Mitogen-activated protein (MAP) kinase is central to a signal transduction pathway that triggers cell proliferation or differentiation. Activation of the p42mapk isoform requires its phosphorylation at two residues, Thr 183 and Tyr 185, and this phosphorylation is catalysed by MAP kinase kinase (MAPKK). Relatively little is known, however, about the enzymes that dephosphorylate these residues, thereby inactivating the pathway. Recently, the CL100 phosphatase has been shown to inactivate p42mapk in vitro by dephosphorylating Thr 183 and Tyr 185 at similar rates. CL100, the product of an immediate early gene, is synthesized within one hour of stimulating cells with growth factors or exposure to oxidative stress or heat shock. Incubation of NIH 3T3 fibroblasts with cycloheximide prevents both synthesis of CL100 and inactivation of p42mapk after stimulation with serum.

RESULTS

Depleting cells of CL100 and preventing its induction using cycloheximide stopped the inactivation of p42mapk in Swiss 3T3 fibroblasts following stimulation with epidermal growth factor (EGF), but had no effect on the rapid inactivation of p42mapk in response to EGF in adipose (3T3-L1) or chromaffin (PC12) cells or in response to platelet-derived growth factor (PDGF) in endothelial (PAE) cells. Moreover, maximal induction of CL100 mRNA and a CL100-like activity did not trigger inactivation of p42mapk, which was sustained at a high level after stimulation of PC12 cells with nerve growth factor, PAE cells with serum, or Swiss 3T3 cells with PDGF. Dephosphorylation of Tyr 185 but not Thr 183 of p42mapk was suppressed by vanadate in EGF-stimulated PC12 cells; dephosphorylation of Thr 183, by contrast, was elicited by a vanadate-insensitive activity. Protein phosphatase-2A was the only vanadate-insensitive phosphatase acting on Thr 183 of p42mapk or on MAPKK to be detected in PC12 cell extracts. Phosphorylation of Thr 183 also inhibited the dephosphorylation of Tyr 185 in vitro by the major vanadate-sensitive Tyr 185-specific phosphatase, explaining why dephosphorylation of Thr 183 is rate-limiting for p42mapk inactivation in PC12 cells after stimulation with EGF.

CONCLUSIONS

The rapid inactivation of p42mapk initiated five minutes after stimulation of endothelial, adipose and chromaffin cells with growth factor is not catalysed by CL100, but rather by protein phosphatase 2A and by a protein tyrosine phosphatase distinct from CL100. Induction of CL100 is not accompanied by the inactivation of p42mapk in a number of situations.

The B cell antigen receptor of class IgD induces a stronger and more prolonged protein tyrosine phosphorylation than that of class IgM

Most mature B lymphocytes coexpress two classes of antigen receptor, immunoglobulin (Ig)M and IgD. The differences in the signal transduction from the two receptors are still a matter of controversy. We have analyzed B cell lines expressing IgM or IgD antigen receptors with the same antigen specificity. Cross-linking of these receptors with either antigen, or class-specific antibodies, results in the activation of protein tyrosine kinases and the phosphorylation of the same substrate proteins. The kinetic and the intensity of phosphorylation, however, was quite different between the two receptors when they were cross-linked by antigen. In membrane IgM-expressing cells, the substrate phosphorylation reached a maximum after 1 minute and diminished after 60 minutes whereas, in the membrane IgD-expressing cells, the substrate phosphorylation increased further over time, reached its maximum at 60 minutes, and persisted longer than 240 minutes after exposure to antigen. As a result, the intensity of protein tyrosine phosphorylation induced by cross-linking of membrane IgD was stronger than that induced by membrane IgM. Studies of chimeric receptors demonstrate that only the membrane-proximal C domain and/or the transmembrane part of membrane-bound IgD molecule is required for the long-lasting substrate phosphorylation. Together, these data suggest that the signal emission from the two receptors is controlled differently.

Specificity of receptor tyrosine kinase signaling: transient versus sustained extracellular signal-regulated kinase activation

A number of different intracellular signaling pathways have been shown to be activated by receptor tyrosine kinases. These activation events include the phosphoinositide 3-kinase, 70 kDa S6 kinase, mitogen-activated protein kinase (MAPK), phospholipase C-gamma, and the Jak/STAT pathways. The precise role of each of these pathways in cell signaling remains to be resolved, but studies on the differentiation of mammalian PC12 cells in tissue culture and the genetics of cell fate determination in Drosophila and Caenorhabditis suggest that the extracellular signal-regulated kinase (ERK-regulated) MAPK pathway may be sufficient for these cellular responses. Experiments with PC12 cells also suggest that the duration of ERK activation is critical for cell signaling decisions.

Function of B-cell antigen receptor of different classes

Most mature B lymphocytes co-express two classes of antigen receptor, IgM and IgD. The differences in the signal transduction from the 2 receptors are still a matter of controversy. We have analysed B-cell lines expressing IgM or IgD antigen receptors with the same antigen specificity. Cross-linking of these receptors with either antigen or class-specific antibodies results in the activation of protein tyrosine kinases and the phosphorylation of the same substrate proteins. The kinetics and intensity of phosphorylation, however, were quite different between the 2 receptors when they were cross-linked by antigen. In membrane IgM-expressing cells, the substrate phosphorylation reached a maximum already after 1 min and diminished after 60 min whereas in the membrane IgD-expressing cells, the substrate phosphorylation increases further over time, reached its maximum at 60 min and persisted longer than 240 min after exposure to antigen. Recently prolonged signaling has been found to be responsible for signaling differences between tyrosine kinase receptors using otherwise similar signaling routes. Thus, the duration of a signal may be an important biological feature of signal transducing cascades.

A novel kinase cascade triggered by stress and heat shock that stimulates MAPKAP kinase-2 and phosphorylation of the small heat shock proteins

MAPK-activated protein kinase-2 (MAPKAP kinase-2) is activated in vitro by the p42 and p44 isoforms of MAPK (p42/p44MAPK). In several cell lines, however, MAPKAP kinase-2 is activated by sodium arsenite, heat shock, or osmotic stress and not by agonists that activate p42/p44MAPK. We have identified a MAPK-like enzyme that acts as a MAPKAP kinase-2 reactivating kinase (RK). RK is recognized by an antiserum raised against a Xenopus MAPK (Mpk2), which is most similar to HOG1 from S. cerevisiae. We also identified a RK kinase (RKK) on the basis of its ability to activate either RK or a GST-Mpk2 fusion protein. The RKK, RK, and MAPKAP kinase-2 constitute a new stress-activated signal transduction pathway in vertebrates that is distinct from the classical MAPK cascade.

Identification of a latent MAP kinase kinase kinase in PC12 cells as B-raf

A latent MAP kinase kinase kinase activity previously detected after chromatography of PC12 cell extracts on Mono Q [(1992) FEBS Lett. 314, 461-465] has been identified as B-Raf by immunoblotting and immunoprecipitation experiments and by its specific activation with B-Raf antibodies. B-Raf is phosphorylated after stimulation of PC12 cells with nerve growth factor or epidermal growth factor, but this is not accompanied by activation of the Mono Q-purified enzyme whether assayed in the absence or presence of GTP-Ras.

PC12 cells overexpressing the insulin receptor undergo insulin-dependent neuronal differentiation

BACKGROUND

Stimulation of phaeochromocytoma PC12 cells by nerve growth factor leads to growth arrest and neuronal differentiation, whereas insulin induces various metabolic responses such as metabolism of glucose and lipids. Moreover, both insulin and epidermal growth factor stimulate the proliferation of PC12 cells. In spite of their different biological effects, nerve growth factor, insulin and epidermal growth factor induce very similar early responses in PC12 cells. Stimulation with nerve growth factor leads to the sustained activation and nuclear translocation of mitogen-activated protein (MAP) kinase. By contrast, both insulin and epidermal growth factor induce the transient activation of MAP kinase, without pronounced nuclear translocation of the enzyme. We have investigated whether the differential activation of signaling pathway components can account for the distinct cellular responses to these different growth factors.

RESULTS

By overexpressing insulin receptors in PC12 cells, we observed insulin-dependent neurite outgrowth, similar to that induced by nerve growth factor in both non-transfected and overexpressing cells. Overexpression of insulin receptors in PC12 cells leads to a more pronounced, but similar pattern of insulin-induced tyrosine-phosphorylated proteins in PC12 cells, including enhanced recruitment of Grb2/Sos into a complex with either Shc or IRS1. MAP kinase activation in response to insulin stimulation of cells overexpressing the insulin receptor is similar to MAP kinase activation in response to NGF stimulation of parental or overexpressing PC12 cells: the activation is prolonged and nuclear translocation of the enzyme occurs.

CONCLUSION

The differential subcellular localization and duration of MAP kinase activation induced by insulin and NGF may explain the difference in the biological actions of these two factors on PC12 cells. Our results show that the strength of the signal generated by a receptor with tyrosine kinase activity can influence the downstream signaling pathway, leading to cell differentiation instead of cell proliferation.