Cyclooxygenase 2 promotes cell survival by stimulation of dynein light chain expression and inhibition of neuronal nitric oxide synthase activity

Cyclooxygenase 2 (COX-2) inhibits nerve growth factor (NGF) withdrawal apoptosis in differentiated PC12 cells. The inhibition of apoptosis by COX-2 was concomitant with prevention of caspase 3 activation. To understand how COX-2 prevents apoptosis, we used cDNA expression arrays to determine whether COX-2 regulates differential expression of apoptosis-related genes. The expression of dynein light chain (DLC) (also known as protein inhibitor of neuronal nitric oxide synthase [PIN]) was significantly stimulated in PC12 cells overexpressing COX-2. The COX-2-dependent stimulation of DLC expression was, at least in part, mediated by prostaglandin E(2). Overexpression of DLC also inhibited NGF withdrawal apoptosis in differentiated PC12 cells. Stimulation of DLC expression resulted in an increased association of DLC/PIN with neuronal nitric oxide synthase (nNOS), thereby reducing nNOS activity. Furthermore, nNOS expression and activity were significantly increased in differentiated PC12 cells after NGF withdrawal. This increased nNOS activity as well as increased nNOS dimer after NGF withdrawal were inhibited by COX-2 or DLC/PIN overexpression. An nNOS inhibitor or a membrane-permeable superoxide dismutase (SOD) mimetic protected differentiated PC12 cells from NGF withdrawal apoptosis. In contrast, NO donors induced apoptosis in differentiated PC12 cells and potentiated apoptosis induced by NGF withdrawal. The protective effects of COX-2 on apoptosis induced by NGF withdrawal were also overcome by NO donors. These findings suggest that COX-2 promotes cell survival by a mechanism linking increased expression of prosurvival genes coupled to inhibition of NO- and superoxide-mediated apoptosis.     

Involvement of tetrahydrobiopterin in trophic effect of erythropoietin on PC12 cells.

Tetrahydrobiopterin (BH(4)) synthesis is reported to be stimulated by nerve growth factor (NGF) in PC12 cells, suggesting involvement of BH(4) in the trophic effect of NGF. We have recently reported that erythropoietin (EPO) and BH(4) enhance survival of PC12 cells. In the present study, we investigated involvement of BH(4) in the trophic effect of EPO on PC12 cells. Cellular BH(4) content was increased by EPO (10(-10) to 10(-8) M) in a dose- and time-related manner. EPO (10(-10) to 10(-8) M) increased the viable cell number of PC12 cells. In addition to EPO, BH(4) (1, 3, and 10 microM) increased the viable cell number of PC12 cells. Administration of 0.3 mM 2,4-diamino-6-hydroxypyrimidine, an inhibitor of BH(4) synthesis, blunted EPO-induced increases in BH(4) content and the viable cell number of PC12 cells. These results taken together suggest that BH(4) is involved in the trophic effects of EPO on PC12 cells.     

c-Myc functionally cooperates with Bax to induce apoptosis

c-Myc promotes apoptosis by destabilizing mitochondrial integrity, leading to the release of proapoptotic effectors including holocytochrome c. Candidate mediators of c-Myc in this process are the proapoptotic members of the Bcl-2 family. We show here that fibroblasts lacking Bak remain susceptible to c-Myc-induced apoptosis whereas bax-deficient fibroblasts are resistant. However, despite this requirement for Bax, c-Myc activation exerts no detectable effects on Bax expression, localization, or conformation. Moreover, susceptibility to c-Myc-induced apoptosis can be restored in bax-deficient cells by ectopic expression of Bax or by microinjection of a peptide comprising a minimal BH3 domain. Microinjection of BH3 peptide also restores sensitivity to c-Myc-induced apoptosis in p53-deficient primary fibroblasts that are otherwise resistant. By contrast, there is no synergy between BH3 peptide and c-Myc in fibroblasts deficient in both Bax and Bak. We conclude that c-Myc triggers a proapoptotic mitochondrial destabilizing activity that cooperates with proapoptotic members of the Bcl-2 family.     

A novel apoptotic cascade mediated by CDK4 in rat pheochromocytoma PC12 cells.

Apoptosis induced by serum withdrawal in pheochromocytoma PC12 cells is promoted by overexpression of cyclin-dependent kinase 4 (CDK4). We compared CDK4-promoted apoptosis with that induced by serum withdrawal alone in PC12 cells. Protein synthesis inhibitors did not prevent apoptosis in parental cells, but prevented the promotion of apoptosis by CDK4 overexpression. Nerve growth factor, basic-fibroblast growth factor, and Bcl-2 proteins protected both parental and CDK4-overexpressing cells from apoptosis. However, insulin-like growth factor-I and Bcl-X(L) protein only partially inhibited apoptosis in the CDK4-overexpressing cells. Bcl-2 or Bcl-X(L) had no significant effect on CDK4 kinase activity in both cell lines. These results suggest a novel CDK4-mediated apoptotic cascade which is normally restrained, but which is activated by CDK4 overexpression. This apoptotic cascade should eventually converge with the cascade induced by serum withdrawal in normal PC12 cells. We discuss the interactions among these apoptotic cascades and the points where anti-apoptotic agents act.     

Effects of Erythropoietin on Neuronal Activity

Recently, erythropoietin (EPO) receptors and synthesis of EPO have been identified in the brain. To clarify the effects of EPO on neuronal cells, we investigated the effects of EPO on Ca2+ uptake, intracellular Ca2+ concentration, membrane potential, cell survival, release and biosynthesis of dopamine, and nitric oxide (NO) production in differentiated PC12 cells, which possess EPO receptors. EPO (10(-12)-10(-10) M) increased 45Ca2+ uptake and intracellular Ca2+ concentration in PC12 cells in a dose-related manner; these increases were inhibited by nicardipine (1 microM) or anti-EPO antibody (1:100 dilution). EPO induced membrane depolarization in PC12 cells. After a 5-day culture without serum and nerve growth factor (NGF), viable cell number decreased to 50% of that of the control cells cultured with serum and NGF. EPO (10(-13)-10(-10) M) increased the number of viable cells cultured without serum and NGF; this increase was blunted by nicardipine or anti-EPO antibody. Incubation with EPO (10(-13)-10(-10) M) stimulated mitogen-activated protein kinase activity in PC12 cells. EPO (10(-13)-10(-10) M) increased dopamine release from PC12 cells and tyrosine hydroxylase activity; these increases were sensitive to nicardipine or anti-EPO antibody. Following a 4-h incubation with EPO (10(-14)-10(-10) M), NO production was increased, which was blunted by nicardipine and anti-EPO antibody. In contrast, maximal NO synthase activity was not changed by EPO. These results suggest that EPO stimulates neuronal function and viability via activation of Ca2+ channels.     

Activation of c-Ha-Ras by nitric oxide modulates survival responsiveness in neuronal PC12 cells

p21(c-Ha-Ras) (Ras) can be activated by the guanine nucleotide exchange factor mSOS1 or by S-nitrosylation of cysteine 118 via nitric oxide (NO). To determine whether these two Ras-activating mechanisms modulate distinct biological effects, a NO-nonresponsive Ras mutant (Ras(C118S)) was stably expressed in the PC12 cells, a cell line that generates NO upon nerve growth factor treatment. We report here that Ras(C118S) functions indistinguishably from wild type Ras in activating and maintaining the mSOS1- and Raf-1-dependent mitogen-activated protein kinase cascade necessary for neuronal differentiation. However, continuous (>5 days) exposure to nerve growth factor reveals that, in contrast to parental or wild-type Ras-overexpressing PC12 cells, Ras(C118S)-expressing PC12 cells cannot sustain the basal interaction between Ras and phosphatidylinositol 3-kinase. This results in spontaneous apoptosis of these cells despite the presence of nerve growth factor and serum. Thus unique downstream effector interactions and biological outcomes can be differentially modulated by distinct modes of Ras activation.     

Select Alterations in Protein Kinases and Phosphatases During Apoptosis of Differentiated PC12 Cells

Abstract: The involvement of cell cycle-regulatory proteins in apoptosis of neuronally differentiated PC12 cells induced by the removal of nerve growth factor and serum was examined. Three major findings are presented. (1) Cdc2 kinase protein levels increased fivefold in apoptotic PC12 cells by day 3 of serum and nerve growth factor deprivation. Histone H1 kinase activity was increased significantly in p13^suc1 precipitates of apoptotic PC12 cells, which was due to increased activation and/or expression of cdc2 kinase. (2) The protein levels of cyclin-dependent kinase 4, cyclin D, and proliferating cell nuclear antigen that are normally expressed in the cell cycle were increased during neuronal PC12 cell apoptosis. (3) The levels of the catalytic subunit, but not the regulatory subunit of the calcium/calmodulin-dependent protein phosphatase 2B, decreased significantly concomitant with a significant decrease in protein phosphatase 2B activity early in the apoptotic process. Protein phosphatase 2A activity decreased slightly but significantly after 3 days of serum and nerve growth factor deprivation, and no alterations in protein phosphatase 1 were observed during the apoptotic process. These data demonstrate that certain cell cycle-regulatory proteins are inappropriately expressed and that alterations in specific phosphorylation events, as indicated by the increase in histone H1 kinase activity and the decrease in protein phosphatase 2B activity, are most likely occurring during apoptosis of PC12 cells. These observations support the hypothesis that apoptosis may be due in part to a nondividing cell's uncoordinated attempt to reenter and progress through the cell cycle.     

Sepiapterin attenuates 1-methyl-4-phenylpyridinium-induced apoptosis in neuroblastoma cells transfected with neuronal NOS: role of tetrahydrobiopterin, nitric oxide, and proteasome activation

In this study, we investigated the molecular mechanism of toxicity of 1-methyl-4-phenylpyridinium (MPP+), an ultimate toxic metabolite of a mitochondrial neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, that causes parkinsonism in experimental animals and humans. Using wild-type and human neuronal nitric oxide synthase (nNOS) stably transfected neuroblastoma cells (SH-SY5Y), we showed that nNOS overexpression in SH-SY5Y cells greatly enhanced proteasome activity and mitigated MPP+-induced apoptosis. During MPP+-induced oxidative stress, intracellular BH4 levels decreased, resulting in nNOS "uncoupling" (i.e., switching from nitric oxide to superoxide generation). Increasing the intracellular BH4 levels by sepiapterin supplementation restored the nNOS activity, inhibited superoxide formation, increased proteasome activity, decreased protein ubiquitination, and attenuated apoptosis in MPP+-treated cells. Implications of BH4 depletion in dopaminergic cells and sepiapterin supplementation to augment the striatal nNOS activity in the pathogenesis mechanism and treatment of Parkinson disease are discussed.     

NBS1, the Nijmegen breakage syndrome gene product, regulates neuronal proliferation and differentiation

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder, characterized by progressive microcephaly, growth retardation, immunodeficiency, and pre-disposition to tumor formation. To investigate the functions of the NBS gene product, NBS1, on neurons, PC12 cells overexpressing NBS1 and related mutants and primary cortical neuronal culture were used in the present study. Small interfering RNA (siRNA) was applied to repress the expression of endogenous Nbs1 in PC12 cells and primary cortical neurons. We demonstrated that overexpression of NBS1 increases cellular proliferation and decreases the apoptosis of PC12 cells in serum withdrawal and ionizing irradiation, through the activation of phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathway. Overexpression of NBS1 also decreases neurite elongation on PC12 cells under nerve growth factor stimulation. Transfection of NBS1-overexpressing PC12 cells with a dominant negative Akt mutant attenuates the neuroprotection and cellular proliferation effects of NBS1 while having no effect on neurite elongation. PC12 cells overexpressing NBS657del5 and NBS653 mutants, in which the major NBS1 protein in cells are truncated proteins, have decreased cellular proliferation, increased cell death, and decreased neurite elongation compared with those of control PC12 cells. Repression of Nbs1 by siRNA decreases the PI 3-kinase activity and Akt phosphorylation levels, and induces neurite elongation in PC12 cells even without nerve growth factor stimulation. Repression of Nbs1 by siRNA in primary cortical neurons also increased neurite elongation, but increased neuronal death. We conclude that NBS1 can regulate neuronal proliferation and neuroprotection via PI 3-kinase/Akt pathway while regulating neuronal differentiation in a different pathway. Excessive accumulation of truncated protein secondary to 657del5 mutation may be detrimental to neurons, leading to defective neuronal proliferation and differentiation.     

Proliferative inhibition by dominant-negative Ras rescues naive and neuronally differentiated PC12 cells from apoptotic death.

We have used the nerve growth factor (NGF)-responsive PC12 cell line as a model to examine the role of cell cycle progression in apoptotic neuronal cell death triggered by withdrawal of trophic support. Because p21 Ras plays a key role in mitogenic signaling, we tested whether interference with the activity of this protein would affect cell cycle progression and thereby apoptotic death after trophic factor deprivation. For this purpose, we exploited PC12 cells transfected with an inducible form of dominant-inhibitory Ras. In contrast to non-transfected and uninduced cells, which continue to synthesize DNA when deprived of trophic support, PC12 cells induced to express dominant-inhibitory Ras showed little thymidine incorporation. When non-transfected and uninduced cells were deprived of trophic support, these underwent rapid apoptotic death that could be prevented by NGF. However, cells in which dominant-inhibitory Ras was induced and which were consequently quiescent did not die upon withdrawal of trophic support and showed long-term survival in the absence of NGF or other trophic factors. Moreover, induction of dominant-inhibitory Ras also rescued non-dividing, neuronally differentiated PC12 cells from death caused by NGF withdrawal. These findings suggest a relationship between proliferative capacity and neuronal apoptosis and raise the hypothesis that following withdrawal of trophic support, neurons undergo an unsuccessful and fatal attempt to re-enter the cell cycle.     

Butyrate- but not TGFbeta1-induced apoptosis of colorectal adenoma cells is associated with increased expression of the differentiation markers E-cadherin and alkaline phosphatase

Sodium butyrate and transforming growth factor beta (TGFbeta1) are growth inhibitory to colonic adenoma cell lines. Butyrate induces apoptosis, whereas in some adenoma cell lines, TGFbeta1 can be growth inhibitory without apoptosis. In this report, we show that the adenoma cell line PC/BH/C1 undergoes apoptosis in response to TGFbeta1. Butyrate induced cell death is preceded by the induction of two markers of colonic differentiation--alkaline phosphatase (ALP) activity and E-cadherin protein expression. However, TGFbeta1-induced apoptosis was not accompanied by induction of these differentiation markers. It is possible that the apoptosis induced by TGFbeta1 in the adenoma cell line PC/BH/C1 is due to conflicting signals, as downregulation of c-myc protein in response to TGFbeta1 occurs only slowly in this cell line. Development of resistance to TGFbeta1 in colonic tumours may involve two separate stages--resistance to growth inhibition and resistance to TGFbeta1-induced apoptosis. Our results indicate that sodium butyrate induces apoptosis via differentiation, but TGFbeta1 induces apoptosis by a differentiation-independent mechanism. As for butyrate, the induction of E-cadherin expression is a potentially important chemopreventative action, since increased E-cadherin expression has been correlated with decreased metastatic potential. This is the first report of induction of E-cadherin by a naturally occurring factor in the diet. Butyrate may reduce tumour growth and invasion, not only as a result of the induction of apoptosis, but also through increased expression of E-cadherin.     

Ornithine decarboxylase is a mediator of c-Myc-induced apoptosis.

c-Myc plays a central role in the regulation of cell cycle progression, differentiation, and apoptosis. However, the proteins which mediate c-Myc function(s) remain to be determined. Enforced c-myc expression rapidly induces apoptosis in interleukin-3 (IL-3)-dependent 32D.3 murine myeloid cells following IL-3 withdrawal, and this is associated with the constitutive, growth factor-independent expression of ornithine decarboxylase (ODC), a rate-limiting enzyme of polyamine biosynthesis. Here we have examined the role of ODC in c-Myc-induced apoptosis. Enforced expression of ODC, like c-myc, is sufficient to induce accelerated death following IL-3 withdrawal. ODC induced cell death in a dose-dependent fashion, and alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ODC enzyme activity, effectively blocked ODC-induced cell death. ODC-induced cell death was due to the induction of apoptosis. We also demonstrate that ODC is a mediator of c-Myc-induced apoptosis. 32D.3-derived c-myc clones have augmented levels of ODC enzyme activity, and their rates of death were also a function of their ODC enzyme levels. Importantly, the rates of death of c-myc clones were inhibited by treatment with DFMO. These findings demonstrate that ODC is an important mediator of c-Myc-induced apoptosis and suggest that ODC mediates other c-Myc functions.     

Peroxynitrite-Induced Cytotoxicity in PC12 Cells: Evidence for an Apoptotic Mechanism Differentially Modulated by Neurotrophic Factors

Abstract: Peroxynitrite is a powerful oxidant formed by the near-diffusion-limited reaction of nitric oxide with superoxide. Large doses of peroxynitrite (>2 m M ) resulted in rapid cell swelling and necrosis of undifferentiated PC12 cells. However, brief exposure to lower concentrations of peroxynitrite (EC₅₀ = 850 µ M ) initially (3–4 h) caused minimal damage to low-density cultures. By 8 h, cytoplasmic shrinkage with nuclear condensation and fragmentation became increasingly evident. After 24 h, 36% of peroxynitrite-treated cells demonstrated these features associated with apoptosis. In addition, 46% of peroxynitrite-treated cells demonstrated DNA fragmentation (by terminal-deoxynucleotide transferase-mediated dUTP-digoxigenin nick end-labeling) after 7 h, which was inhibited by posttreatment with the endonuclease inhibitor aurintricarboxylic acid. Serum starvation also resulted in apoptosis in control cells (23%), the percentage of which was not altered significantly by peroxynitrite treatment. Although peroxynitrite is known to be toxic to cells, the present study provides a first indication that peroxynitrite induces apoptosis. Furthermore, pretreatment of cells with nerve growth factor or insulin, but not epidermal growth factor, was protective against peroxynitrite-induced apoptosis. However, both acidic and basic fibroblast growth factors greatly increased peroxynitrite-initiated apoptosis, to 63 and 70%, respectively. Thus, specific trophic factors demonstrate differential regulation of peroxynitrite-induced apoptosis in vitro.     

v-Crk, an effector of the nerve growth factor signaling pathway, delays apoptotic cell death in neurotrophin-deprived PC12 cells

v-Crk is a member of a class of SH2 and SH3-containing adaptor proteins that have been implicated in regulating the TrkA receptor tyrosine kinase and potentiating Nerve Growth Factor (NGF)-mediated neurite outgrowth in pheochromocytoma (PC12) cells (Hempstead et al, Mol. Cell Biol. 14: 1964 - 1971). Given the fact that NGF induces both differentiation and survival by binding to TrkA, we examined the rate of apoptotic cell death elicited by NGF-withdrawal in native, v-Crk, and TrkA-expressing PC12 cells. While more than 50% of native PC12 cells underwent apoptosis within 48 h of NGF withdrawal, the v-Crk and TrkA-expressing cells were much more resistant to apoptosis under these conditions, whereby approximately 70 and 95%, respectively, of the cells were alive. The ability of v-Crk to delay apoptosis required prior NGF-dependent differentiation, since naive undifferentiated v-Crk expressing PC12 cells or cells that express v-Crk mutants that are defective in NGF signaling were not protected from apoptosis during growth factor withdrawal. Moreover, addition of 50 ng/ml EGF to serum and NGF deprived v-Crk expressing cells, which also causes neurite outgrowth, promoted complete and long-term survival, although such EGF replacement had no neurotrophic effect on wild-type PC12 cells or PC12 cells overexpressing Human Bcl-2. These experiments suggest that v-Crk potentiation of a receptor tyrosine kinase under conditions of growth factor deprivation is essential for preventing apoptosis. However, unlike native PC12 cells, neither v-Crk or TrkA-expressing PC12 cells exhibited a G1 arrest when incubated for 2 weeks in NGF. Thus, v-Crk and TrkA may protect NGF deprived PC12 by preventing cell cycle arrest and hence an aborted entry into a defective cell cycle. Moreover, during NGF-withdrawal, v-CrkPC12 cells exhibited down regulation in MAP kinase and JNK activities while in native cells, these activities increased within 6 - 8 h after NGF deprivation. Thus, unlike v-Crk-mediated augmentation of differentiation, sustained activation of MAP kinase may not be required for v-Crk-induced cell survival.     

Divergent stress responses to IL-1beta, nitric oxide, and tunicamycin by chondrocytes

As the only cell in cartilage responsible for matrix synthesis, the chondrocyte's viability is crucial to healthy tissue. It must tolerate stresses from both mechanical and cellular sources. This study examines the endoplasmic reticulum (ER) stress response in chondrocytes after exposure to IL-1beta, nitric oxide, or tunicamycin in order to determine whether this form of stress causes cell death. Cultures of the immortalized human juvenile costal chondrocyte cell line, C-28/I2, were treated with IL-1beta, S-nitroso-N-acetylpenicillamine (SNAP), and tunicamycin. Increasing intracellular nitric oxide levels by SNAP treatment or inhibiting protein folding in the ER lumen by tunicamycin induced the ER stress response as evidenced by increased protein and gene expression of GADD153 as well as PERK and eIF2-alpha phosphorylation, and resulted in apoptosis. IL-1beta treatment induced PERK and eIF2-alpha phosphorylation, but not GADD153 expression or apoptosis. The ER stress signaling pathway of IL-1beta involved iNOS because blocking its expression, inhibited ER stress gene expression. Therefore, inducing the ER stress response in chondrocytes results in divergent responses depending on the agent used. Even though IL-1beta, a common proinflammatory cytokine, induces the ER stress response, it is not proapoptotic to chondrocytes. On the other hand, exposure to high levels of intracellular nitric oxide induce chondrocyte apoptosis as part of the ER stress response.     

Induction of Nitric Oxide Synthase and Nitric Oxide-Mediated Apoptosis in Neuronal PC12 Cells After Stimulation with Tumor Necrosis FActor-α/Lipopolysaccharide

Abstract: Exposure of neuronal PC12 cells, differentiated by nerve growth factor, to tumor necrosis factor-α (TNF-α) and bacterial lipopolysaccharide (LPS) resulted in de novo synthesis of inducible nitric oxide synthase (iNOS) mRNA and protein with an increase up to 24 h. Brain NOS expression was unaffected. The induction of iNOS in differntiated PC12 cells was associated with cell death characterized by features of apoptosis, The NOS inhibitors N -monomethylarginine, aminoguanidine, and 2-amino-5,6-dihydro-6-methyl-4 H -1,3-thiazine HCl prevented TNF-α/LPS-induced cell death and DNA fragmentation, suggesting that the TNF-α/LPS-induced cell death is mediated by iNOS-derived NO. This hypothesis is supported by the finding that addition of l -arginine, which serves as a precursor and limiting factor of enzyme-derived NO production, potentiated TNF-α/LPS-induced loss of viability.     

c-Myc-induced apoptosis in fibroblasts is inhibited by specific cytokines.

We have investigated the mechanism by which deregulated expression of c-Myc induces death by apoptosis in serum-deprived fibroblasts. We demonstrate that Myc-induced apoptosis in low serum is inhibited by a restricted group of cytokines, principally the insulin-like growth factors and PDGF. Cytokine-mediated protection from apoptosis is not linked to the cytokines' abilities to promote growth. Protection from apoptosis is evident in the post-commitment (mitogen-independent) S/G2/M phases of the cell cycle and also in cells that are profoundly blocked in cell cycle progression by drugs. Moreover, IGF-I inhibition of apoptosis occurs in the absence of protein synthesis, and so does not require immediate early gene expression. We conclude that c-Myc-induced apoptosis does not result from a conflict of growth signals but appears to be a normal physiological aspect of c-Myc function whose execution is regulated by the availability of survival factors. We discuss the possible implications of these findings for models of mammalian cell growth in vivo.