David E. Burstein

The role of transcription-dependent priming in nerve growth factor promoted neurite outgrowth

Studies were carried out with clonal PC12 rat pheochromocytoma cells to test and elaborate the “priming” model for the mechanism by which nerve growth factor (NGF) promotes neurite outgrowth. The rate of appearance, initial extent of elongation, and insensitivity to inhibitors of transcription for neurites regenerated by passaged PC12 cells were all found to increase progressively as a function of time of NGF pretreatment. Cells pretreated (or “primed”) with NGF were thus able to rapidly regenerate long processes even when RNA synthesis was blocked. The long-term, progressive nature of neurite outgrowth and priming were not entirely accounted for by cell cycle effects. These observations were thus consistent with a mechanism in which NGF promotes process outgrowth by specifically stimulating cells to synthesize and progressively accumulate substances which are required for neurite production and elongation. This process appeared to require RNA transcription and the accumulation of specific rather than total cell constituents. Priming occurred for cells grown in suspension or in the presence of inhibitors of microtubule assembly. Priming and neurite outgrowth were suppressed at high cell density and were not elicited by exposure to depolarizing agents. Examination of the properties of NGF-promoted neurite outgrowth by cultured neurons suggests that priming may occur during development of the normal nervous system. It is also conceivable that self-priming may be a feature of growth factors in addition to NGF.

Nerve growth factor has both mitogenic and antimitogenic activity

Abstract The present study demonstrates that nerve growth factor (NGF) possesses both antimitogenic and mitogenic activities. To this end, we have employed clonal PC12 rat pheochromocytoma cells and two PC12 variant sublines, U2 and U7. When PC12 cells are exposed to NGF in culture media that are otherwise either permissive (15% serum) or restrictive (1% serum) for proliferation, neuronal differentiation occurs and mitosis ceases. Variant lines of PC12 cells have been selected that continue to proliferate in the presence of NGF in permissive medium but which nevertheless retain NGF receptors and certain NGF responses. In contrast to the parent PC12 cells, when such variants were exposed to NGF in growth-restrictive media, cell proliferation was markedly stimulated. The mitogenic activity of NGF was detectable at 0.1 ng/ml (4 pM) and was maximal at 3 ng/ml (100 pM). Possible contamination of the NGF preparation by epidermal growth factor (EGF) or mitogenic proteolytic enzymes was ruled out by the use of anti-EGF and diisopropylfluoro-phosphate, respectively. These findings show that NGF shares the capacity to stimulate cell division with a variety of other peptide hormones and suggest that the mitogenic activity of NGF could play a role in development of the peripheral nervous system as well as in promotion of in vivo growth of certain neural crest-derived neoplasms.

Synaptophysin: A sensitive and specific marker for ganglion cells in central nervous system neoplasms

Synaptophysin, a 38-kilodalton glycoprotein found in synaptic vesicle membranes, has been shown to be a sensitive marker of neuroendocrine differentiation in non-central nervous system (CNS) tumors. We analyzed the patterns of synaptophysin immunoreactivity in CNS neoplasms in comparison with various normal CNS sites in biopsies. Normal gray matter structures all showed a diffuse punctate granular pattern of neuropil staining without staining of neuronal cell bodies. In contrast, neoplastic ganglion cells in 18 of 18 gangliogliomas/gangliocytomas showed intense immunoreactivity outlinging the borders of the cell bodies. Focal staining was also seen in five of 16 primitive neuroectodermal tumors and in one of three central neurocytomas, but these tumors had a finely granular neuropil pattern of immunoreactivity more like that of normal gray matter than like that of the gangliogliomas. All 35 examples of pure gliomas of various types showed no immunoreactivity. Our data highlight synaptophysin as a sensitive and specific marker of both neuronal lineage and neoplastic character in gangliogliomas.

Dissociation of c-fos from ODC expression and neuronal differentiation in a PC12 subline stably transfected with an inducible N-ras oncogene

In order to develop a model system for investigating the role of ras genes in neuronal differentiation, a construct consisting of a mouse N-ras oncogene linked to a dexamethasone-inducible promoter was devised and transfected into a subline of the PC12 rat pheochromocytoma cell line. Clonal lines were isolated which extended neurite-like processes within one day of exposure to dexamethasone. N-ras had a strong antiproliferative effect on these cells. These effects were reversible after removing dexamethasone. Elevation of mRNA for ornithine decarboxylase (ODC) was detected 6-18 hours after induction of N-ras by dexamethasone. The effects of ras on cell division, differentiation and cell size were analogous, but not identical to the effects of NGF on PC12 cells. One NGF action, induction of c-fos mRNA did not occur in ras-induced cells indicating that c-fos induction is unnecessary for both neurite outgrowth and for subsequent induction of ODC mRNA. The ability of ras to induce ODC, a division promoting enzyme, may also be relevant to the transforming actions of ras oncogenes.

Immunohistochemical detection of XIAP in mesothelium and mesothelial lesions

We examined benign and malignant mesothelial tissue samples for the presence of X-linked inhibitor of apoptosis protein (XIAP), a potent constituent of the inhibitor of apoptosis family of caspase inhibitors. We subjected 55 sections (31 malignant mesotheliomas, 2 well-differentiated peritoneal mesotheliomas, 13 pleural mesothelial hyperplasias, and 9 benign mesothelial tissues) from archival formalin-fixed, paraffin-embedded surgical tissue blocks to citrate-based antigen retrieval and then incubated them with monoclonal anti-XIAP (clone 48, dilution 1:250; BD Biosciences, San Jose, CA) at 4 degrees C for 72 hours and developed them using EnVision-Plus reagents (DAKO, Carpinteria, CA) and diaminobenzidine as the chromogen. Particulate or nonhomogeneous cytoplasmic staining was considered positive. All 9 normal mesothelial samples were negative for XIAP. Of 13 mesothelial hyperplasias, 1 (8%) was weakly positive in fewer than 10% of cells, as was 1 of 2 well-differentiated peritoneal mesotheliomas. Of 31 malignant mesotheliomas, 25 (81%) displayed XIAP positivity. XIAP immunostaining, when strong, allows for distinction of malignant from benign and hyperplastic mesothelial cell populations and is a potentially useful immunodiagnostic marker in small samples and morphologically controversial cases. Elevated expression of XIAP could contribute to tumorigenesis in mesothelioma.

Regulation of protein kinase C activity in neuronal differentiation induced by the N-ras oncogene in PC-12 cells.

Expression of the N-ras oncogene under the control of the glucocorticoid-responsive promoter in the pheochromocytoma cell line UR61, a subline of PC-12 cells, has been used to investigate the differentiation process to neuronal cells triggered by ras oncogenes (I. Guerrero, A. Pellicer, and D. E. Burstein, Biochem. Biophys. Res. Commun. 150:1185-1192, 1988). Using ras-inducible cell lines, we observed that expression of the oncogenic N-ras p21 protein interferes with the ability of phorbol esters to induce downregulation of protein kinase C. This effect was associated with the appearance of immunologically detectable protein kinase C as well as the activity of the enzyme as analyzed either by binding of [3H]phorbol-12,13-dibutyrate in intact cells or by in vitro kinase activity. These results indicate a relationship between ras p21 and protein kinase C in neuronal differentiation in this model system. Comparison to the murine fibroblast system suggests that this relationship may be functional.

Oncogene N-ras mediates selective inhibition of c-fos induction by nerve growth factor and basic fibroblast growth factor in a PC12 cell line.

A cell line was generated from U7 cells (a subline of PC12 rat pheochromocytoma cells) that contains a stably integrated transforming mouse N-ras (Lys-61) gene under the control of the long terminal repeat from mouse mammary tumor virus. Such cells, designated UR61, undergo neuronal differentiation upon exposure to nanomolar concentrations of dexamethasone, as a consequence of expression of the activated N-ras gene (I. Guerrero, A. Pellicer, and D.E. Burstein, Biochem, Biophys. Res. Commun. 150:1185-1192, 1988). Exposure of UR61 cells to either nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) results in a marked induction of c-fos RNA, with kinetics paralleling those of NGF- or bFGF-induced expression of c-fos RNA in PC12 cells. Dexamethasone-induced expression of activated N-ras p21 results in blocking of c-fos RNA induction by NGF or bFGF in a time-dependent manner. Activated N-ras p21-mediated inhibition of c-fos RNA induction in UR61 cells is selective for NGF and bFGF and is not due to selective degradation of c-fos RNA. Normal and transforming N-ras can trans activate the chloramphenicol acetyltransferase gene linked to mouse c-fos regulatory sequences when transient expression assays are performed. Our observations suggest that N-ras p21 selectively interacts with pathways involved in induction of c-fos expression which initiate at the receptors for NGF and bFGF.

Genomic and non-genomic actions of nerve growth factor in development.

Publisher Summary Chemical signals appear to play important roles in development of neural tissue. For example, they may function to spatially and temporally coordinate the differentiation of specific cell groups, to provide tropic signaling and guidance, and to mediate cell-cell communication and feedback. Moreover, because of their potential to function extracellularly, chemical signals could work either in restricted local environments, or, more diffusely, over long distances. Within the last 30 years, a number of macromolecular factors have been identified that can influence the development and differentiation of the nervous system. NGF is presently the most useful model available for studying the actions of chemical factors that regulate neural development. The aim of this chapter is to explore the role of the genome in the action of nerve growth factor.

Mechanisms of the Promotion of Neurite Outgrowth by Nerve Growth Factor

Nerve growth factor (NGF) has a variety of actions on its physiological targets, sympathetic and sensory neurons (Levi-Montalcini and Angeletti, 1968; Green and Shooter, 1980; Thoenen and Barde, 1980). Among the most striking of these actions is the promotion of neurite outgrowth. This chapter will focus on the mechanisms by which NGF causes neuritis to be initiated and maintained.