Delayed and sustained activation of p42/p44 mitogen-activated protein kinase induced by proteasome inhibitors through p21(ras) in PC12 cells

Prolonged treatment with the proteasome inhibitor MG-132 induces apoptosis in PC12 rat pheochromocytoma cells

Rat pheochromocytoma (PC12) cells were treated with the proteasome inhibitor MG-132 and morphological changes were recorded. Initially, neuronal differentiation was induced but after 24 h signs of morphological deterioration became apparent. We performed nuclear staining, flow cytometry and WST-1 assay then analyzed signal transduction pathways involving Akt, p38 MAPK (Mitogen-Activated Protein Kinase), JNK (c-Jun N-terminal Kinase), c-Jun and caspase-3. Stress signaling via p38, JNK and c-Jun was active even after 24 h of MG-132 treatment, while the survival-mediating Akt phosphorylation declined and the executor of apoptosis (caspase-3) was activated by that time and apoptosis was also observable. We examined subcellular localization of stress signaling components, applied kinase inhibitors and dominant negative H-Ras mutant-expressing PC12 cells in order to decipher connections of stress-mediating pathways. Our results are suggestive of that treatment with the proteasome inhibitor MG-132 has a biphasic nature in PC12 cells. Initially, it induces neuronal differentiation but prolonged treatments lead to apoptosis.

Oxytocin in the regulation of social behaviours in medial amygdala-lesioned mice via the inhibition of the extracellular signal-regulated kinase signalling pathway

The neuropeptide oxytocin (OXT) has been implicated in the pathophysiology of behavioural deficits among patients with autism spectrum disorder (ASD). However, the molecular mechanisms underlying its role in ASD remain unclear. In the present study, a murine model with ASD-like phenotypes was induced by intra-medial amygdala injection of N-methyl-d-aspartate, and it was used to investigate the role of OXT in behaviour regulation. Behavioural tests were performed to verify the ASD-like phenotypes of N-methyl-d-aspartate-treated mice, and the results showed that mice with bilateral medial amygdala lesions presented significant behavioural deficits, including impaired learning and memory and increased anxiety and depression. We also observed a notably decreased level of OXT in both the plasma and the hypothalamus of medial amygdala-lesioned mice, and the extracellular signal-regulated kinase (ERK) was activated. Further studies demonstrated that the administration of OXT alleviated ASD-like symptoms and significantly inhibited phosphorylation of ERK; the inhibitory effect was similar to that of U0126, an ERK signalling inhibitor. In addition, OXT administration modulated the expression of downstream proteins of the ERK signalling pathway, such as cyclic adenosine monophosphate response element binding and c-fos. Taken together, our data indicate that OXT plays an important role in ameliorating behavioural deficits in an ASD-like mouse model, which was mediated by inhibiting the ERK signalling pathway and its downstream proteins.

Neurotrophic natural products: chemistry and biology

Neurodegenerative diseases and spinal cord injury affect approximately 50 million people worldwide, bringing the total healthcare cost to over 600 billion dollars per year. Nervous system growth factors, that is, neurotrophins, are a potential solution to these disorders, since they could promote nerve regeneration. An average of 500 publications per year attests to the significance of neurotrophins in biomedical sciences and underlines their potential for therapeutic applications. Nonetheless, the poor pharmacokinetic profile of neurotrophins severely restricts their clinical use. On the other hand, small molecules that modulate neurotrophic activity offer a promising therapeutic approach against neurological disorders. Nature has provided an impressive array of natural products that have potent neurotrophic activities. This Review highlights the current synthetic strategies toward these compounds and summarizes their ability to induce neuronal growth and rehabilitation. It is anticipated that neurotrophic natural products could be used not only as starting points in drug design but also as tools to study the next frontier in biomedical sciences: the brain activity map project.

Partial rescue of geldanamycin-induced TrkA depletion by a proteasome inhibitor in PC12 cells

In this work we tried to identify mechanisms that could explain how chemical inhibition of heat-shock protein 90 reduces nerve growth factor signaling in rat pheochromocytoma PC12 cells. Geldanamycin is an antibiotic originally discovered based on its ability to bind heat-shock protein 90. This interaction can lead to the disruption of heat-shock protein 90-containing multimolecular complexes. It can also induce the inhibition or even degradation of partner proteins dissociated from the 90 kDa chaperone and, eventually, can cause apoptosis, for instance, in PC12 cells. Before the onset of initial apoptotic events, however, a marked decrease in the activity of extracellular signal-regulated kinases ERK 1/2 and protein kinase B/Akt can be observed together with reduced expression of the high affinity nerve growth factor receptor, tropomyosine-related kinase, TrkA, in this cell type. The proteasome inhibitor MG-132 can effectively counteract the geldanamycin-induced reduction of TrkA expression and it can render TrkA and ERK1/2 phosphorylation but not that of protein kinase B/Akt by nerve growth factor again inducible. We have found altered intracellular distribution of TrkA in geldanamycin-treated and proteasome-inhibited PC12 cells that may, at least from the viewpoint of protein localization explain why nerve growth factor remains without effect on protein kinase B/Akt. The lack of protein kinase B/Akt stimulation by nerve growth factor in turn reveals why nerve growth factor treatment cannot save PC12 cells from geldanamycin-induced programmed cell death. Our observations can help to better understand the mechanism of action of geldanamycin, a compound with strong human therapeutical potential.

Neuroprotective effects of MAPK/ERK1/2 and calpain inhibitors on lactacystin-induced cell damage in primary cortical neurons

The dysfunction of the proteasome system is implicated in the pathomechanism of several chronic neurodegenerative diseases. Lactacystin (LC), an irreversible proteasome inhibitor, induces cell death in primary cortical neurons, however, the molecular mechanisms of its neurotoxic action has been only partially unraveled. In this study we aimed to elucidate an involvement of the key enzymatic pathways responsible for LC-induced neuronal cell death. Incubation of primary cortical neurons with LC (0.25-50 μg/ml) evoked neuronal cell death in concentration- and time-dependent manner. Lactacystin (2.5 μg/ml; 6.6μM) enhanced caspase-3 activity, but caspase-3 inhibitor, Ac-DEVD-CHO did not attenuate the LC-evoked cell damage. Western blot analysis showed a time-dependent, prolonged activation of MAPK/ERK1/2 pathway after LC exposure. Moreover, inhibitors of MAPK/ERK1/2 signaling, U0126 and PD98052 attenuated the LC-evoked cell death. We also found that LC-treatment resulted in the induction of calpains and calpain inhibitors (MDL28170 and calpeptin) protected neurons against the LC-induced cell damage. Neuroprotective action of MAPK/ERK1/2 and calpain inhibitors were connected with attenuation of LC-induced DNA fragmentation measured by Hoechst 33342 staining and TUNEL assay. However, only MAPK/ERK1/2 but not calpain inhibitors, attenuated the LC-induced AIF (apoptosis inducing factor) release. Further studies showed no synergy between neuroprotective effects of MAPK/ERK1/2 and calpain inhibitors given in combination when compared to their effects alone. The obtained data provided evidence for neuroprotective potency of MAPK/ERK1/2 and calpain, but not caspase-3 inhibition against the neurotoxic effects of LC in primary cortical neurons and give rationale for using these inhibitors in the treatment of neurodegenerative diseases connected with proteasome dysfunction.

Mitogen-activated protein kinase signaling is involved in suramin-induced neurite outgrowth in a neuronal cell line

Suramin is a well-known antitrypanosomal drug and a novel experimental agent for the treatment of several cancers. Previous study showed that suramin is an activator of extracellular signal-regulated kinase (ERK1/2) signaling in several cell lines including Chinese hamster ovary cells, although the physiological relevance of this activation remains uncertain. Here, it was shown that suramin enhances neurite outgrowth concomitant with activation of ERK1/2 in Neuro-2a cells, a neuronal cell line. These neurite outgrowth and ERK1/2 activation were significantly inhibited by PD98059, an inhibitor of mitogen-activated protein kinase kinase, as well as by activation of endogenous adenosine A2A receptors. The suramin-induced phosphorylation of ERK1/2 was also inhibited by inhibitors of Src family kinases. This attenuation of ERK1/2 activity was accompanied by a significant decrease in suramin-induced neurite outgrowth. These results suggest that suramin activates the Src/ERK1/2 signaling pathway that induces neurite outgrowth, both of which are negatively regulated by cAMP produced in response to activation of endogenous adenosine A2A receptors.

Honokiol-induced neurite outgrowth promotion depends on activation of extracellular signal-regulated kinases (ERK1/2)

We have found that honokiol [4-allyl-2-(3-allyl-4-hydroxy-phenyl)-phenol] can promote neurite outgrowth and mobilize intracellular Ca2+ store in primary cultured rat cortical neurons. In this study, we examined the effects of honokiol on extracellular signal-regulated kinases (ERK1/2) and Akt, and their possible relationship to neurite outgrowth and Ca2+ mobilization. Honokiol-induced neurite outgrowth in the cultured rat cortical neurons was significantly reduced by PD98059, a mitogen-activated protein kinase kinase (MAPKK, MAPK/ERK kinase MEK, direct upstream of ERK1/2) inhibitor, but not by LY294002, a phosphoinositide 3-kinase (PI3K, upstream of Akt) inhibitor. Honokiol also significantly enhanced the phosphorylation of ERK1/2 in a concentration-dependent manner, whereas the effect of honokiol on Akt phosphorylation was characterized by transient enhancement in 10 min and lasting inhibition after 30 min. The phosphorylation of ERK1/2 enhanced by honokiol was inhibited by PD98059 as well as by KN93, a Ca2+/calmodulin-dependent kinase II (CaMK II) inhibitor. Moreover, the products of the phosphoinositide specific phospholipase C (PLC)-derived inositol 1,4,5-triphosphate (IP3) and 1,2-diacylglycerol (DAG) were measured after honokiol treatment. Together with our previous findings, these results suggest that the signal transduction from PLC, IP3, Ca2+, and CaMK II to ERK1/2 is involved in honokiol-induced neurite outgrowth.

Oxidative neuronal injury. The dark side of ERK1/2

The extracellular signal regulated protein kinases (ERK1/2) are essential for normal development and functional plasticity of the central nervous system. However, a growing number of recent studies in models of cerebral ischemia, brain trauma and neurodegenerative diseases implicate a detrimental role for ERK1/2 signaling during oxidative neuronal injury. Neurons undergoing oxidative stress-related injuries typically display a biphasic or sustained pattern of ERK1/2 activation. A variety of potential targets of reactive oxygen species and reactive nitrogen species could contribute to ERK1/2 activation. These include cell surface receptors, G proteins, upstream kinases, protein phosphatases and proteasome components, each of which could be direct or indirect targets of reactive oxygen or nitrogen species, thereby modulating the duration and magnitude of ERK1/2 activation. Neuronal oxidative stress also appears to influence the subcellular trafficking and/or localization of activated ERK1/2. Differences in compartmentalization of phosphorylated ERK1/2 have been observed in diseased or injured human neurons and in their respective animal and cell culture model systems. We propose that differential accessibility of ERK1/2 to downstream targets, which is dictated by the persistent activation of ERK1/2 within distinct subcellular compartments, underlies the neurotoxic responses that are driven by this kinase.

AP-1-independent sensitization to oxidative stress-induced apoptosis by proteasome inhibitors

Hydrogen peroxide (H(2)O(2)) induces apoptosis of mesangial cells via c-Jun N-terminal kinase (JNK)-activator protein-1 (AP-1) and extracellular signal-regulated kinase (ERK)-AP-1 pathways. We recently found that subtoxic doses of proteasome inhibitors, MG132 and lactacystin, dramatically enhanced H(2)O(2)-induced apoptosis in mesangial cells. In this report, we examined molecular mechanisms involved in this phenomenon, especially focusing on AP-1 pathways. Reporter assays showed that MG132 induced activation of AP-1. However, pharmacological inhibitors of AP-1, retinoic acid, and curcumin, did not suppress the proapoptotic effect of MG132. Suppression of JNK-AP-1 by transfection with either a dominant-negative mutant of JNK or a dominant-negative mutant of c-Jun did not attenuate the apoptosis enhancement by MG132. Similarly, suppression of ERK-AP-1 by PD98059 or dominant-negative mutants of ERK did not affect the apoptosis-promoting effect of MG132. Interestingly, pretreatment with MG132 did not enhance activation of AP-1 by H(2)O(2). These data suggested a novel, AP-1-independent promotion of apoptosis by proteasome inhibitors.

Retinoic acid-induced neuritogenesis of human neuroblastoma SH-SY5Y cells is ERK independent and PKC dependent

Retinoic acid (RA), an active metabolite of vitamin A, is a natural morphogen involved in development and differentiation of the nervous system. To elucidate signaling mechanisms involved in RA-induced neuritogenesis, we used human neuroblastoma SH-SY5Y cells, an established in vitro model for studying RA action, to examine the role of extracellular signal-regulated kinase (ERK) 1 and 2 in RA-induced neuritogenesis and cell survival. From immunoblotting experiments, we observed that RA induced delayed but persistent ERK1 and ERK2 phosphorylation (until 96 hr) that was reduced significantly by the specific mitogen-activated protein kinase (MAPK)/ERK kinase (MEK) inhibitor U0126. For the subsequent studies we chose 24 hr as the reference time. Inhibition of ERK activation did not affect RA-induced neuritogenesis (percentage of neurite-bearing cells and neurite length) but significantly reduced cell survival. In addition, we analyzed the signaling pathway that mediates ERK activation. Our results suggest that RA-induced ERK phosphorylation does not follow the classic Raf kinase-dependent pathway. Protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI 3-K) are possible alternative kinases involved in the ERK signaling pathway. In fact, in the presence of the specific PKC inhibitor GF 109203X, or the specific PI 3-K inhibitor wortmannin, we observed a significant dose-dependent reduction in ERK phosphorylation. RA-induced neuritogenesis and cell survival were reduced by GF 109203X in a concentration-dependent manner. These results suggest that rather than ERK1 and ERK2, it is PKC that plays an important role during early phases of RA-induced neuritogenesis.

Proteasome Mediates Dopaminergic Neuronal Degeneration, and Its Inhibition Causes α-Synuclein Inclusions

Parkinson's disease is characterized by dopaminergic neuronal death and the presence of Lewy bodies. alpha-Synuclein is a major component of Lewy bodies, but the process of its accumulation and its relationship to dopaminergic neuronal death has not been resolved. Although the pathogenesis has not been clarified, mitochondrial complex I is suppressed, and caspase-3 is activated in the affected midbrain. Here we report that a combination of 1-methyl-4-phenylpyridinium ion (MPP(+)) or rotenone and proteasome inhibition causes the appearance of alpha-synuclein-positive inclusion bodies. Unexpectedly, however, proteasome inhibition blocked MPP(+)- or rotenone-induced dopaminergic neuronal death. MPP(+) elevated proteasome activity, dephosphorylated mitogen-activating protein kinase (MAPK), and activated caspase-3. Proteasome inhibition reversed the MAPK dephosphorylation and blocked caspase-3 activation; the neuroprotection was blocked by a p42 and p44 MAPK kinase inhibitor. Thus, the proteasome plays an important role in both inclusion body formation and dopaminergic neuronal death but these processes form opposite sides on the proteasome regulation in this model.

Proteasome Inhibitors Induce Inhibitory κB (IκB) Kinase Activation, IκBα Degradation, and Nuclear Factor κB Activation in HT-29 Cells

The transcription factor nuclear factor kappaB (NF-kappaB) is activated and seems to promote oncogenesis in certain cancers. A major mechanism of NF-kappaB activation in cells involves cytoplasm-to-nucleus translocation of this transcription factor after hydrolysis of the cytoplasmic inhibitor inhibitory kappaB (IkappaB) by the 26S proteasome. Because selective proteasome inhibitors have been shown to block IkappaB degradation; consequently, NF-kappaB activation in a variety of cellular systems, proteasome inhibitors were proposed as potential therapeutic agents for the treatment of cancer. However, under certain conditions, IkappaB degradation and NF-kappaB activation are not mediated by the proteasome system. We investigated how proteasome inhibitors affected NF-kappaB activation in the intestinal epithelial cancer cell line HT-29, which has been documented to have an atypical NF-kappaB regulation. Treatment of cells with the selective proteasome inhibitors carbobenzoxy-L-leucyl-L-leucyl-L-norvalinal (MG-115), carbobenzoxy-L-leucyl-L-leucyl-L-leucinal (MG-132), or lactacystin induced NF-kappaB activation as indicated by both an increase in NF-kappaB DNA binding and transcriptional activity. This increase in NF-kappaB activation caused by proteasome inhibitors was accompanied by an increase in IkappaB kinase activation and a degradation of IkappaBalpha but not IkappaBbeta. Furthermore, proteasome inhibitors induced the expression of NF-kappaB target genes. In summary, these results demonstrate a unique effect of proteasome inhibitors on the IkappaB-NF-kappaB systems in HT-29 cells, in which proteasome inhibitors activate rather than deactivate the NF-kappaB system. We conclude that the use of proteasome inhibitors to block NF-kappaB activation in cancer cells may not always be a viable approach.

Role of reactive oxygen species in extracellular signal-regulated protein kinase phosphorylation and 6-hydroxydopamine cytotoxicity

A number of reports indicate the potential for redox signalling via extracellular signal-regulated protein kinases (ERK) during neuronal injury. We have previously found that sustained ERK activation contributes to toxicity elicited by 6-hydroxydopamine (6-OHDA) in the B65 neuronal cell line. To determine whether reactive oxygen species (ROS) play a role in mediating ERK activation and 6-OHDA toxicity, we examined the effects of catalase, superoxide dismutase (SOD1), and metalloporphyrin antioxidants ('SOD mimetics') on 6-OHDA-treated cells. We found that catalase and metalloporphyrin antioxidants not only conferred protection against 6-OHDA but also inhibited development of sustained ERK phosphorylation in both differentiated and undifferentiated B65 cells. However, exogenously added SOD1 and heat-inactivated catalase had no effect on either toxicity or sustained ERK phosphorylation. This correlation between antioxidant protection and inhibition of 6-OHDA-induced sustained ERK phosphorylation suggests that redox regulation of ERK signalling cascades may contribute to neuronal toxicity.

Immunosuppressant FK506 induces neurite outgrowth in PC12 mutant cells with impaired NGF-promoted neuritogenesis via a novel MAP kinase signaling pathway

We obtained a drug-hypersensitive PC12 mutant cell (PC12m3), in which neurite outgrowth was strongly stimulated by various drugs such as FK506, calcimycin and cAMP, under the condition of NGF treatment. The frequency of neurite outgrowth stimulated by FK506 was approximately 40 times greater than by NGF alone. The effects of FK506 on neurite outgrowth in PC12m3 cells were inhibited by rapamycin, an FK506 antagonist, and by calcimycin, a calcium ionophore. PC12m3 cells had a strong NGF-induced MAP kinase activity, the same as PC12 parental cells. However, FK506-induced MAP kinase activity was detected only in PC12m3 cells. The activation of MAP kinase by FK506 in PC12m3 cells was markedly inhibited by rapamicin and calcimycin. FK506-induced MAP kinase activity was also inhibited by MAP kinase inhibitor U0126. These results demonstrate that drug-hypersensitive PC12m3 cells have a novel FK506-induced MAP kinase pathway for neuritogenesis.

Sustained activation of N-WASP through phosphorylation is essential for neurite extension

Neurite extension is a key process for constructing neuronal circuits during development and remodeling of the nervous system. Here we show that Src family tyrosine kinases and proteasome degradation signals synergistically regulate N-WASP in neurite extension. Src family kinases activate N-WASP through tyrosine phosphorylation, which induces Arp2/3 complex-mediated actin polymerization. Tyrosine phosphorylation of N-WASP also initiates its degradation through ubiquitination. When neurite growth is stimulated in culture, degradation of N-WASP is markedly inhibited, leading to accumulation of the phosphorylated N-WASP. On the other hand, under culture conditions that inhibit neurite extension, but favor proliferation, the phosphorylated N-WASP is degraded rapidly. Collectively, neurite extension is regulated by the balance of N-WASP phosphorylation (activation) and degradation (inactivation), which are induced by tyrosine phosphorylation.

Repeated, intermittent treatment with amphetamine induces neurite outgrowth in rat pheochromocytoma cells (PC12 cells)

Repeated, intermittent treatment with amphetamine (AMPH) leads to long-term neurobiological adaptations in rat brain including an increased number and branching of dendritic spines. This effect depends upon several different cell types in the intact brain. Here we demonstrate that repeated, intermittent AMPH treatment induces neurite outgrowth in cultured PC12 cells without the requirement for integrated synaptic pathways. PC12 cells were treated with 1 micro M AMPH for 5 min a day, for 5 days. After 10 days of withdrawal, there was an increase in the percentage of cells with neurites ( approximately 30%) and the length of neurites as well as an increase in the level of GAP-43 and neurofilament-M. Neurite outgrowth was enhanced as withdrawal time was increased. Neurite outgrowth was much greater following repeated, intermittent treatment with AMPH compared to continuous or single treatment with AMPH. Pretreatment with cocaine, a monoamine transporter blocker, inhibited the AMPH-mediated increase in neurite outgrowth. Neither NGF antibody nor DA receptor antagonists blocked AMPH-induced neurite outgrowth, demonstrating that AMPH-induced neurite outgrowth is not dependent on endogenous NGF release or DA receptors. Thus we have demonstrated that repeated, intermittent treatment with AMPH has a neurotrophic effect in PC12 cells. The effect requires the action of AMPH on the norepinephrine transporter, and shares characteristics in its development with other forms of sensitization but does not require an intact neuroanatomy.

Nerve growth factor uses Ras/ERK and phosphatidylinositol 3-kinase cascades to up-regulate the N-methyl-D-aspartate receptor 1 promoter

We reported previously that nerve growth factor (NGF) up-regulates activity of the N-methyl-D-aspartate receptor 1 (NR1) promoter. We have explored the pathways and nuclear targets of NGF signaling in regulating the NR1 promoter. PD98059 and wortmannin, but not rapamycin, significantly attenuated NGF-induced transcriptional activity from an NR1 promoter-luciferase construct. Coexpressing constitutively active forms of Ras, Raf, or MAPK/ERK kinase 1 (MEK1) increased promoter activity dramatically. The MEK1-induced increase was largely prevented by mutations of the tandem GC boxes in the promoter. Promoter activity was also increased significantly by coexpressed GC box-binding proteins (Sp1, 3, or 4) in nonstimulated PC12 cells. Either an extracellular signal-regulated kinase-1 (ERK1)- or Sp1-specific antibody coprecipitated Sp1 with ERKs, and the coprecipitation was enhanced significantly by NGF treatment of PC12 cells. ERK2 also incorporated radioactivity of [gamma(32)P]ATP into recombinant Sp1. However, ERK2-treated Sp1 and PC12 nuclear extracts or nuclear extracts from NGF-treated cells exhibited reduced binding to the promoter or a consensus GC box. Our results suggest that NGF utilizes both the Ras/ERK and phosphatidylinositol 3-kinase pathways to up-regulate NR1 promoter activity and that Sp1 is a novel substrate of NGF-activated ERKs. NGF-increased NR1 promoter activity may involve a complicated mechanism of Sp1 phosphorylation and possible transcription factor exchange.

Unexpected transcriptional induction of monocyte chemoattractant protein 1 by proteasome inhibition: involvement of the c-Jun N-terminal kinase-activator protein 1 pathway

Proteasome inhibitors, the well-known inhibitors of NF-kappaB, are recently considered therapeutic agents for inflammation. However, the anti-inflammatory properties of these agents have not been fully evaluated. In this report we describe a novel effect of proteasome inhibitors on the expression of monocyte chemoattractant protein 1 (MCP-1) in mesangial cells. We found that proteasome inhibitor MG132 dose-dependently induced expression of MCP-1 at the transcriptional level. The stimulatory effect was similarly observed with other proteasome inhibitors (proteasome inhibitor 1 and lactacystin) and in other cell types (NRK fibroblasts). The 5'-flanking region of the MCP-1 gene contains multiple AP-1 sites. To explore the mechanisms involved, we examined the effects of proteasome inhibition on the AP-1 pathway. Northern blot analysis showed that MG132 rapidly induced the expression of c-jun, but not c-fos. Immunoblot analysis showed that MG132 prevented degradation of c-Jun protein. Kinase assay revealed that c-Jun N-terminal kinase (JNK) was rapidly activated by MG132. Consistent with these results, a reporter assay showed that AP-1 activity was up-regulated after treatment with MG132. Curcumin, a pharmacological inhibitor of the JNK-AP-1 pathway, abrogated the induction of MCP-1 by MG132. Similarly, stable transfection with a dominant-negative mutant of c-Jun attenuated both MG132-induced activation of AP-1 and expression of MCP-1. The transcriptional activation by proteasome inhibitors was observed not only in MCP-1, but also in other AP-1-dependent genes, including stromelysin and mitogen-activated protein kinase phosphatase 1. These data revealed that proteasome inhibition triggered the expression of MCP-1 and other genes via the multistep induction of the JNK-c-Jun/AP-1 pathway.

CD4(+)CD8(dim) T lymphocytes exhibit enhanced cytokine expression, proliferation and cytotoxic activity in response to HCMV and HIV-1 antigens

CD4(+)CD8(dim) T cells represent a minor subset of the total CD3(+) T cell population in peripheral blood. Although transient and persistent expansions of these cells have been reported in both healthy and diseased individuals, the functional properties of the CD4(+)CD8(dim) population are largely unknown. In this study, we examined antigen-specific cytokine and proliferative responses of the CD4(+)CD8(dim) subset. In whole blood cultures stimulated with the viral antigens HCMV and HIV-1, a significant fraction of the CD4(+)CD8(dim) subset exhibited cytokine expression and proliferation in response to antigen activation. Typically, the CD4(+)CD8(dim) population contained two- to eightfold higher frequencies of antigen-specific cytokine producing cells than the CD4(+)CD8(-) population. Phenotypic analysis of the cytokine expressing CD4(+)CD8(dim) population indicated that these cells are memory T cells, with a high frequency of this population expressing the cytotoxic markers CD56 and perforin. Furthermore, the CD4(+)CD8(dim) cytokine responses to CMV were shown to be MHC class II dependent. Significantly, purified CD4(+)CD8(dim) T cells were found to possess higher CMV-specific cytotoxic activity than purified CD4(+)CD8(-) T cells in a standard (51)Cr-release CTL assay. Thus, CD4(+)CD8(dim) T cells appear to be MHC class II dependent, are capable of cytolytic effector activity, and are highly enriched within the CD4(+) cell populations specific for HCMV and HIV-1.

Sustained extracellular signal-regulated kinase activation by 6-hydroxydopamine: implications for Parkinson's disease

Although the toxin 6-hydroxydopamine (6-OHDA) is utilized extensively in animal models of Parkinson's disease, the underlying mechanism of its toxic effects on dopaminergic neurons is not completely understood. We examined the effects of 6-OHDA on the CNS-derived tyrosine hydroxylase expressing B65 cell line, with particular attention to the regulation of the extracellular signal-regulated protein kinases (ERK). 6-OHDA elicited a dose-dependent cytotoxicity in B65 cells. Toxic doses of 6-OHDA also elicited a biphasic pattern of ERK phosphorylation with a prominent sustained phase, a pattern that differed from that observed with hydrogen peroxide (H(2)O(2)) treatment. 6-OHDA-elicited ERK phosphorylation was blocked by PD98059, an inhibitor of the upstream mitogen activated protein kinase kinase (MEK) that phosphorylates and activates ERK. PD98059 also conferred protection against 6-OHDA cytotoxicity, but did not affect H(2)O(2) toxicity in B65 cells. These results suggest that ERK activation plays a direct mechanistic role in 6-OHDA toxicity, rather than representing a protective compensatory response, and raise the possibility that abnormal patterns of ERK activation may contribute to dopaminergic neuronal cell death.

Influence of proteasome and redox state on heat shock-induced activation of stress kinases, AP-1 and HSF

We studied the pattern of activation of stress kinases and of transcription factors activator protein-1 (AP-1) and heat shock factor (HSF) in FAO cells by combining two treatments, i.e. heating (42 degrees C for 1 h) and proteasome inhibition, each known to cause cellular heat shock response. The co-treatment heat shock (HS) and proteasome inhibitor (a peptidyl aldehyde or lactacystin) showed cumulative effects on the intensity and duration of activation of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) at the end of the HS period and during recovery. Similarly, the thiol-reducing agents N-(2-mercaptoethyl)-1,3-diaminopropane and dithiothreitol strongly activated both JNK and p38 MAPK in cells undergoing HS. AP-1 DNA binding activity in response to proteasome inhibitors was so strong that it shadowed the stimulatory effect of HS in the combined treatment, but lactacystin, which is the most potent and specific proteasome inhibitor, decreased the binding late during recovery from HS. Thiol-reducing agents prevented AP-1 DNA binding induced by HS. The combined HS/proteasome inhibitors or HS/thiol-reducing agents treatments cooperatively activated HSF DNA binding. Expression of collagenase I and hsp 70 mRNAs reflects the different behavior of AP-1 and HSF transcription factors in cells exposed to HS and proteasome inhibition. The data seem to indicate that JNK and p38 MAPK activations are not necessarily coupled to DNA binding of AP-1, which can be either increased or inhibited when these kinases are activated. AP-1 and HSF show opposite patterns of response to HS in the presence of proteasome inhibitors or reducing agents.

N1E-115 mouse neuroblastoma cells express MT1 melatonin receptors and produce neurites in response to melatonin

Melatonin, a pineal hormone that induces sleep, has become a popular over-the-counter drug. The cellular effects of melatonin, however, are only beginning to be studied. We have recently shown that stimulation of the MT1 melatonin receptor induces rapid and dramatic cytoskeletal rearrangements in transformed non-neuronal cells (Witt-Enderby et al., Cell. Motil. Cytoskel. 46 (2000) 28). These cytoskeletal changes result in the formation of structures that closely resemble neurites. In this work, we show that the N1E-115 mouse neuroblastoma cell line rapidly responds to melatonin stimulation and forms neurites within 24 h. We also demonstrate that these cells readily bind 2-[125I]iodomelatonin at levels consistent with what is noted for native tissues (B(max)=3.43+/-1.56 fmol/mg protein; K(d)=240 pM). Western analysis shows that these cells possess and express melatonin receptors of the MT1 subtype. Treatment with pertussis toxin eliminates neurite formation whereas treatment with the MT2 subtype-specific activator, BMNEP, does not induce neurite formation. We have previously shown that increases in MEK 1/2 and ERK 1/2 phosphorylation are correlated with the shape changes in transformed CHO cells. Western analysis of the MEK/ERK signaling pathway in N1E-115 cells shows that this pathway is most likely maximally and constitutively stimulated. This may account for the spontaneous production of neurites noted for this cell line after long culture periods. The results of this work show that melatonin receptor stimulation in a neuronal cell type results in the formation of neurites and that the receptors responsible for melatonin-induced neurite formation in N1E-115 cells are most likely of the MT1 subtype.

Melatonin induction of filamentous structures in non-neuronal cells that is dependent on expression of the human mt1 melatonin receptor

Melatonin has gained recent popularity as a treatment for insomnia and other sleep disorders; however, its cellular effects are unknown. We report the effects of melatonin on the cellular morphology of Chinese hamster ovary (CHO) cells transformed to express the human melatonin receptors, mt1 and MT2. Our results show that melatonin exerts a strong influence on cellular shape and cytoskeletal organization in a receptor-dependent and possibly subtype-selective manner. The cell shape change that we see after a 5-h treatment of these non-neuronal cells with a pharmacological concentration of melatonin consists of the formation of long filamentous outgrowths that are reminiscent of the neurite processes produced by differentiating nerve cells. This morphological change occurs exclusively in cells expressing the mt1 receptor. We find that the microtubule and microfilament organization within these outgrowths is similar to that of neurites. Microtubules are required for the shape change to occur as Colcemid added in combination with melatonin completely blocks outgrowth formation. We demonstrate that the number of cells showing the altered cell shape is dependent on melatonin concentration, constant exposure to melatonin and that outgrowth frequencies increase when protein kinase A (PKA) is inhibited. Concomitant melatonin-dependent increases in MEK 1/2 and ERK 1/2 phosphorylation are noted in mt1-CHO cells only. The production of filamentous outgrowths is dependent on the translation of new protein but not the transcription of new mRNA. Outgrowth number is not controlled by centrosomes but is instead controlled by the polymerization state of the actin cytoskeleton. The results of this work show that the organization of the cytoskeleton is affected by processes specifically mediated or regulated by the mt1 receptor and may represent a novel alternative mechanism for the stimulation of process formation.