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

Putative intraovarian regulators

Ovarian folliculogenesis is an exponential process marked by dramatic proliferation and differentiation of the somatic and germ cell elements of the follicle. Although the central roles of gonadotropins and of gonadal steroids in this explosive agenda are well accepted, the variable fate of follicles afforded comparable gonadotropic stimulation suggests the existence of additional intraovarian modulatory systems. Accordingly, consideration must be given to another set (or sets) of regulatory principles that may provide these missing modulatory loops. The work of multiple contributors favors the hypothesis that these modulatory loops are comprised of a host of peptidergic principles which engagein situin the modulation of ovarian growth and function. In its capacity as an intraovarian regulator, a given agent may be acting independently of, as an amplifier of, as an attenuator of, or even as a mediator of, gonadotropin action. Together, gonadotropins, steroids, and locally derived peptidergic principles form a triad that modulates the growth and differentiation of ovarian follicles (Figure 1).

BMP and FGF regulate the development of EGF-responsive neural progenitor cells

Temporal changes in progenitor cell responses to extrinsic signals play an important role in development, but little is known about the mechanisms that determine how these changes occur. In the rodent CNS, expression of epidermal growth factor receptors (EGFRs) increases during embryonic development, conferring mitotic responsiveness to EGF among multipotent stem cells. Here we show that cell-cell signaling controls this change. Whereas EGF-responsive stem cells develop on schedule in explant and aggregate cultures of embryonic cortex, co-culture with younger cortical cells delays their development. Exogenous BMP4 mimics the effect of younger cells, reversibly inhibiting changes in EGFR expression and responsiveness. Moreover, blocking endogenous BMP receptors in progenitors with a virus transducing dnBMPR1B accelerates changes in EGFR signaling. This involves a non-cell-autonomous mechanism, suggesting that BMP negatively regulates signal(s) that promote the development of EGF-responsive stem cells. FGF2 is a good candidate for such a signal, as we find that it antagonizes the inhibitory effects of younger cortical cells and exogenous BMP4. These findings suggest that a balance between antagonistic extrinsic signals regulates temporal changes in an intrinsic property of neural progenitor cells.

Nerve growth factor treatment prevents the increase in superoxide produced by epidermal growth factor in PC12 cells

Stimulation of pheochromocytoma (PC12) cells with the mitogen epidermal growth factor (EGF) produced a rapid and robust accumulation of intracellular reactive oxygen species (ROS), an accumulation which, in other systems, has been shown to be essential for mitogenesis. Brief pretreatment of the cells with nerve growth factor (NGF) suppressed the EGF-mediated ROS increase. EGF failed to produce elevations in ROS in a PC12 variant stably expressing a dominant-negative p21(ras) construct (PC12-N17) or in cells pretreated with the MEK inhibitor PD098059. NGF failed to suppress the increase in ROS in the PC12 variant nnr5, which lacks p140(trk) receptors. The suppression of the increase in ROS by NGF was restored in nnr5 cells stably expressing p140(trk) (nnr5-trk), but NGF failed to prevent the increase in ROS in nnr cells expressing mutant p140(trk) receptors that lack binding sites for Shc and phospholipase Cgamma. Among several inhibitors of superoxide-generating enzymes, only the lipoxygenase inhibitor, nordihydroguaiaretic acid reduced EGF-mediated ROS accumulation. The inhibitory action of NGF on ROS production was mimicked by the nitric oxide donor, sodium nitroprusside, and was blocked by an inhibitor of nitric-oxide synthetase, L-nitroarginine methyl ester. These results suggest a novel mechanism for the rapid interruption of mitogenic signaling by the neurotrophin NGF.

Transcriptional down-regulation of epidermal growth factor receptors by nerve growth factor treatment of PC12 cells

Treatment of PC12 cells with nerve growth factor leads to a decrease in the number of epidermal growth factor receptors on the cell membrane. The mRNA for the epidermal growth factor receptor decreases in a comparable fashion. This decrease appears due to a decrease in the transcription of the epidermal growth factor receptor gene because first, there is no difference in the stability of the epidermal growth factor receptor mRNA, second, newly transcribed epidermal growth factor receptor mRNA is decreased in nerve growth factor-differentiated cells, and third, constructs containing the promoter region of the epidermal growth factor receptor gene are transcribed much less readily in nerve growth factor-differentiated cells than in untreated cells. The decreases in mRNA are not seen in the p140(trk)-deficient variant PC12nnr5 cells nor in cells containing either dominant-negative Ras or dominant-negative Src. Treatment with nerve growth factor also increases the cellular content of GCF2, a putative transcription factor inhibitory for the transcription of the epidermal growth factor receptor gene. The increase in GCF2, like the decrease in the epidermal growth factor receptor mRNA, is not seen in PC12nnr5 cells nor in cells expressing either dominant-negative Ras or dominant-negative Src. The results suggest that nerve growth factor-induced down-regulation of the epidermal growth factor receptor is under transcriptional control, is p140(trk)-, Ras-, and Src-dependent, and may involve transcriptional repression by GCF2.

Effects of epidermal growth factor on the [3H]-thymidine uptake in the SK-N-SH and SH-SY5Y human neuroblastoma cell lines

The studies on the factors that regulate the biology of the neuroblastoma cell lines may offer important information on the development of tissues and organs that derive from the neural crest. In the present paper we study the action of epidermal growth factor (EGF) on two human neuroblastoma cell lines: SK-N-SH which is composed at least of two cellular phenotypes (neuroblastic and melanocytic/glial cells), and its pure neuroblastic subclone SH-SY5Y. The results show that EGF (10 ng/ml) significantly stimulates the incorporation of [3H]-thymidine in the SK-N-SH cells only in the presence of fetal bovine serum (FBS) (control = 58,285 +/- 9327 cpm; EGF = 75,523 +/- 4457, p < 0.05). Such effect is not observed in the presence of a chemical defined medium, that is, in the absence of FBS (control = 100,997 +/- 4375; EGF = 95,268 +/- 4683; NS) In the SH-SY5Y cells the EGF does not modify the incorporation of [3H]-thymidine either in the presence of 10% of BFS (control = 113,838 +/- 6978; EGF = 119,434 +/- 9441; NS) or in its absence (control = 46,197 +/- 3335; EGF = 44,472 +/- 3493; NS). The results here reported suggest that: a) EGF may affect the proliferation of cells derived from a primary human neuroblastoma; b) this is evident by the EGF-induced increase of [3H]-thymidine incorporation in SK-N-SH cells; c) it is required the presence of other growth factors, present in the FBS, for the mitogenic action to be accomplished; d) since the pure neuroblastic SH-SY5Y cell line are refractory to the EGF, the effects observed in SK-N-SH cells probably occur on the melanocytic/glial cell subpopulation.

Down-regulation of epidermal growth factor receptors by nerve growth factor in PC12 cells is p140(trk)-, Ras-, and Src-dependent

Nerve growth factor (NGF) treatment causes a profound down-regulation of epidermal growth factor receptors during the differentiation of PC12 cells. This process is characterized by a progressive decrease in epidermal growth factor (EGF) receptor level measured by 125I-EGF binding, tyrosine phosphorylation, and Western blotting. Treatment of the cells with NGF for 5 days produces a 95% reduction in the amount of [35S]methionine-labeled EGF receptors. This down-regulation does not occur in PC12nnr5 cells, which lack the p140(trk) NGF receptor. However, in PC12nnr5 cells stably transfected with p140(trk), the NGF-induced heterologous down-regulation of EGF receptors is reconstituted in part. NGF-induced heterologous down-regulation, but not EGF-induced homologous down-regulation of EGF receptors, is blocked in Ras- and Src-dominant-negative PC12 cells. Treatment with either pituitary adenylate cyclase-activating peptide (PACAP) or staurosporine stimulates neurite outgrowth in PC12 cell variants, but neither induces down-regulation of EGF receptors. NGF treatment of PC12 cells in suspension induces down-regulation of EGF receptors in the absence of neurite outgrowth. These results strongly suggest a p140(trk)-, Ras- and Src-dependent mechanism of NGF-induced down-regulation of EGF receptors and separate this process from NGF-induced neurite outgrowth in PC12 cells.

Epidermal-growth-factor-dependent activation of the src-family kinases

The precise role of src-type kinases as signal transducers has been under intensive investigation but only in a few instances has their role been revealed in any detail. Thus, src, fyn and yes are activated upon stimulation by platelet-derived growth factor or colony-stimulating factor in cells expressing high levels of these receptors. Activation of src-family kinases by other receptor tyrosine kinases such as the epidermal-growth-factor (EGF) receptor has not been directly demonstrated. In this report, we demonstrate EGF-dependent activation of src-family tyrosine kinases in NIH3T3 cells overexpressing the human EGF receptor. Activation is rapid (< 1 min) and persistent (up to 16 h). Furthermore, we show a correlation between the level of EGF receptor expressed and the degree of src-family kinase activation. We show that src-family kinase activity is also activated by addition of EGF to PC12 cells, which endogenously express relatively high levels of EGF receptor. Most strikingly, we show that A431 cells, which endogenously express very high levels of EGF receptor, show 10-fold elevated src-family kinase activity as compared to DHER14 cells, and that this activity is constitutive. This activity is completely blocked by AG1478, a specific inhibitor of the EGF-receptor tyrosine kinase activity, pointing to a direct link between overexpression of the EGF receptor and enhanced src-family kinase activity. Our findings suggest that EGF-dependent src-family kinase activity is detectable only when the levels of EGF receptor reach a specific level. Additionally, high levels of EGF receptor, as in A431 cells, may contribute to the elevated activation of src-family kinases. Sustained src-family kinase activation, similar to that seen in v-src-transformed cells, may play a role in tumorogenesis and tumor maintenance.

Epidermal growth factor stimulates substrate-selective protein-tyrosine-phosphatase activity

This study investigates the regulation of protein-tyrosine-phosphatase (PTPase; EC 3.1.3.48) activity by epidermal growth factor (EGF). Cytosol from EGF-treated A-431 human epidermoid carcinoma cells was used as a source of PTPase activity, and tyrosine-phosphorylated ErbB2, EGF receptor, phospholipase C-gamma 1, and the Ras GTPase-activating protein were used as substrates to monitor PTPase activity. EGF stimulated PTPase activity that was selective toward these substrates, as it dephosphorylated ErbB2 and the EGF receptor, but not phospholipase C-gamma 1 and the Ras GTPase-activating protein. EGF stimulated PTPase activity in several cell lines, regardless of EGF receptor number, and the activity was localized in the cytosol. The dephosphorylation activity in vitro was dependent on the presence of reducing agents and was inhibited by orthovanadate. Agonists such as phorbol 12-myristate 13-acetate, isoproterenol, or ATP were unable to stimulate PTPase activity. Physiological relevance is indicated by experiments showing that EGF treatment of a human mammary cancer cell line rapidly induced the dephosphorylation of ErbB2.

Cross talk among tyrosine kinase receptors in PC12 cells: desensitization of mitogenic epidermal growth factor receptors by the neurotrophic factors, nerve growth factor and basic fibroblast growth factor

We have studied the effects of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) on epidermal growth factor (EGF) binding to PC12 cells. We show that NGF and bFGF rapidly induce a reduction in 125I-EGF binding to PC12 cells in a dose-dependent manner. This decrease amounts to 50% for NGF and 35% for bFGF. Both factors appear to act through a protein kinase C(PKC)-independent pathway, because their effect persists in PKC-downregulated PC12 cells. Scatchard analysis indicates that NGF and bFGF decrease the number of high affinity EGF binding sites. In addition to their effect on EGF binding, NGF and bFGF activate in intact PC12 cells one or several serine/threonine kinases leading to EGF receptor threonine phosphorylation. Using an in vitro phosphorylation system, we show that NGF- or bFGF-activated extracellular regulated kinase 1 (ERK1) is able to phosphorylate a kinase-deficient EGF receptor. Phosphoamino acid analysis indicates that this phosphorylation occurs mainly on threonine residues. Furthermore, two comparable phosphopeptides are observed in the EGF receptor, phosphorylated either in vivo after NGF treatment or in a cell-free system by NGF-activated ERK1. Finally, a good correlation was found between the time courses of ERK1 activation and 125I-EGF binding inhibition after NGF or bFGF treatment. In conclusion, in PC12 cells the NGF- and bFGF-stimulated ERK1 appears to be involved in the induction of the threonine phosphorylation of the EGF receptor and the decrease in the number of high affinity EGF binding sites.

Neuronal cell cultures: a tool for investigations in developmental neurobiology

The aim of this review is to describe environmental requirements for survival of neuronal cells in culture, and secondly to survey the complex interplay between hormones, neurotrophic factors, transport- and extracellular matrix- proteins, which characterize the developmental program of differentiating neurons. An overall reconsideration of the literature in this vast field is above the limits of the present paper; since progress and refinement in the techniques of neuronal cell cultures have paralleled the advancement in Developmental Neurobiology, we will run instead through the main steps which form the conceptual framework of neuronal cell cultures.

Intraovarian regulation: peptidergic signaling systems

Intraovarian regulation, an evolving field, is now at a crossroad. Although a number of putative intraovarian regulators appear to be of import to ovarian physiology, none has thus far been demonstrated to be indispensable to in vivo ovarian function. That notwithstanding, it is already clear that optimal gonadotropin hormonal action is highly contingent upon the input of tissue-based regulatory principles. It is with a strong sense of excitement that future work in this evolving area is anticipated.