Mimicry and inhibition of nerve growth factor effects: interactions of staurosporine, forskolin, and K252a in PC12 cells and normal rat chromaffin cells in vitro

Neuronal differentiation by analogs of staurosporine

RGC-5 cells are transformed cells that express several surface markers characteristic of neuronal precursor cells, but resemble glial cells morphologically and divide in culture. When treated with the apoptosis-inducing agent staurosporine, RGC-5 cells assume a neuronal morphology, extend neurites, stop dividing, and express ion channels without acute signs of apoptosis. This differentiation with staurosporine is similar to what has been described for certain other neuronal cell lines, and occurs by a mechanism not yet understood. Inhibition of several kinases known to be inhibited by staurosporine fails to differentiate RGC-5 cells, and examination of the kinome associated with staurosporine-dependent differentiation has been unhelpful so far. To better understand the mechanism of staurosporine-mediated differentiation of neuronal precursor cells, we studied the effects of the following structurally similar molecules on differentiation of neuronal and non-neuronal cell lines, comparing them to staurosporine: 9,12-epoxy-1H-diindolo[1,2,3-fg:3',2',1'-kl]pyrrolo[3,4-i][1,6]benzodiazocine-10-carboxylic acid, 2,3,9,10,11,12-hexahydro-10-hydroxy-9-methyl-1-oxo-, methyl ester, (9S,10R,12R)-(K252a), (5R,6S,8S)-6-hydroxy-5-methyl-13-oxo-6,7,8,13,14,15-hexahydro-5H-16-oxa-4b,8a,14-triaza-5,8-methanodibenzo[b,h]cycloocta[jkl]cyclopenta[e]-as-indacene-6-carboxylic acid (K252b), staurosporine aglycone (K252c), 7-hydroxystaurosporine (UCN-01), and 4'-N-benzoylstaurosporine (PKC-412). Morphological differentiation, indicated by neurite extension and somal rounding, was quantitatively assessed with NeuronJ. We found that the critical structural component for differentiation in RGC-5 cells is a basic amine adjacent to an accessible methoxy group at the 3' carbon. Given that UCN-01 and similar compounds are potent anti-cancer drugs, examination of molecules that share similar structural features may yield insights into the design of other drugs for differentiation.

A system for analyzing the event timing profile of single cells by using a model of neurite maturation in PC12D cells

Nerve growth factor (NGF)-induced neurite maturation in PC12D cells involves neuritogenesis and neurite outgrowth. Actions of compounds affecting the neurite maturation are sometimes invisible behind the individually variable events in nature even in the clonal population. In this study, we designed a time-lapse imaging system to determine the timing of each event in individual PC12D cells. To examine the system, we analyzed the effect of staurosporine on the neurite maturation in PC12D cells. By using the system, we obtained four event timing data sets (stimulation by NGF with and without staurosporine at the concentrations of 0.01, 0.1, and 1 microM). A permutation test of these data sets revealed that staurosporine caused an early induction of neurite outgrowth during neurite maturation in PC12D cells. These results suggest that the time-lapse imaging system to determine the timing of each event in individual cells can provide a novel insight into features of a cell mass by single-cell analysis and is expected to be utilized for profiling of compounds that can serve as candidate drugs.

Foliar trichome- and aquaporin-aided water uptake in a drought-resistant epiphyte Tillandsia ionantha Planchon

The atmospheric epiphyte Tillandsia ionantha is capable of surviving drought stress for 6 months or more without any exogenous water supply via an as of yet to be determined mechanism. When plants were soaked in water for 3 h, leaves absorbed a remarkably large amount of water (30-40% on the basis of fresh weight), exhibiting a bimodal absorption pattern. Radiolabeled water was taken up by the leaves by capillary action of the epidermal trichomes within 1 min (phase 1) and then transported intracellularly to leaf tissues over 3 h (phase 2). The removal of epidermal trichome wings from leaves as well as rinsing leaves with water significantly lowered the extracellular accumulation of water on leaf surfaces. The intracellular transport of water was inhibited by mercuric chloride, implicating the involvement of a water channel aquaporin in second-phase water absorption. Four cDNA clones (TiPIP1a, TiPIP1b, TiPIP1c, and TiPIP2a) homologous to PIP family aquaporins were isolated from the leaves, and RT-PCR showed that soaking plants in water stimulated the expression of TiPIP2a mRNA, suggesting the reinforcement in ability to rapidly absorb a large amount of water. The expression of TiPIP2a complementary RNA in Xenopus oocytes enhanced permeability, and treatment with inhibitors suggested that the water channel activity of TiPIP2a protein was regulated by phosphorylation. Thus, the high water uptake capability of T. ionantha leaves surviving drought is attributable to a bimodal trichome- and aquaporin-aided water uptake system based on rapid physical collection of water and subsequent, sustained chemical absorption.

Activation of silent synapses with sustained but not decremental long-term potentiation

Silent synapses display no excitatory post-synaptic currents (EPSCs) at resting potentials, but can conduct at depolarized potentials. In the hippocampal CA1 region of young animals, conversion of silent synapses to functional synapses occurs rapidly after pairing post-synaptic depolarization with 1Hz pre-synaptic stimulation, a protocol that also induces long-term potentiation (LTP). LTP appears to have a decremental phase and a sustained phase. Many studies have shown that decremental LTP can be pharmacologically isolated from sustained LTP, suggesting that they represent two distinct forms, rather than "phases" of LTP that are expressed simultaneously through different mechanisms. We investigated whether silent synapse activation (SSA) is associated specifically with the expression of sustained or decremental LTP. We found that under control conditions, in which sustained and decremental LTP were induced, SSA was observed. However, under conditions in which only decremental LTP was expressed (in the presence of a protein kinase antagonist), SSA did not occur. We conclude that SSA is associated with the expression of sustained LTP, not decremental LTP, and requires protein kinase activation. These findings support the hypothesis that decremental and sustained LTP are expressed through different mechanisms.

Transforming growth factor-beta1 regulation of growth zone chondrocytes is mediated by multiple interacting pathways

Transforming growth factor beta 1 (TGF-beta1) affects growth plate chondrocytes through Smad-mediated mechanisms and has been shown to increase protein kinase C (PKC). This study determined if PKC mediates the physiological response of rat costochondral growth zone (GC) chondrocytes to TGF-beta1; if the physiological response occurs via type II or type III TGF-beta receptors, and, if so, which receptor mediates the increase in PKC; and the signal transduction pathways involved. Treatment of confluent GC cells with TGF-beta1 stimulated [(3)H]thymidine and [(35)S]sulfate incorporation as well as alkaline phosphatase (ALPase) and PKC specific activities. Inhibition of PKC with chelerythrine, staurosporine, or H-7 caused a dose-dependent decrease in these parameters, indicating that PKC signaling was involved. TGF-beta1-dependent PKC and the physiological response of GC cells to TGF-beta1 was reversed by anti-type II TGF-beta receptor antibody and soluble type II TGF-beta receptor, showing that TGF-beta1 mediates these effects through the type II receptor. The increase in [3H]thymidine incorporation and ALPase specific activity were also regulated by protein kinase A (PKA) signaling, since the effects of TGF-beta1 were partially blocked by the PKA inhibitor H-8. The mechanism of TGF-beta1 activation of PKC is through phospholipase A(2) (PLA(2)) and not through phospholipase C (PLC). Arachidonic acid increased PKC in control cultures and was additive with TGF-beta1. Prostanoids are required, as indomethacin blocked the effect of TGF-beta1, and Cox-1, but not Cox-2, is involved. TGF-beta1 stimulates prostaglandin E(2) (PGE(2)) production and exogenous PGE(2) stimulates PKC, but not as much as TGF-beta1, suggesting that PGE(2) is not sufficient for all of the prostaglandin effect. In contrast, TGF-beta1 was not regulated by diacylglycerol; neither dioctanoylglycerol (DOG) nor inhibition of diacylglycerol kinase with R59022 had an effect. G-proteins mediate TGF-beta1 signaling at different levels in the cascade. TGF-beta1-dependent increases in PGE(2) levels and PKC were augmented by the G protein activator GTP gamma S, whereas inhibition of G-protein activity via GDP beta S, pertussis toxin, or cholera toxin blocked stimulation of PKC by TGF-beta1, indicating that both G(i) and G(s) are involved. Inhibition of PKA with H-8 partially blocked TGF-beta1-dependent PKC, suggesting that PKA inhibition on the physiological response was via PKA regulation of PKC signaling. This indicates that multiple interacting signaling pathways are involved: TGF-beta1 stimulates PLA(2) and prostaglandin release via the action of Cox-1 on arachidonic acid. PGE(2) activates the EP2 receptor, leading to G-protein-dependent activation of PKA. PKA signaling results in increased PKC activity and PKC signaling regulates proliferation, differentiation, and matrix synthesis.

Inhibition of carbachol stimulated acid secretion by interleukin 1beta in rabbit parietal cells requires protein kinase C

BACKGROUND

Interleukin 1beta (IL-1beta) is a potent inhibitor of gastric acid secretion. Regulatory actions at several levels have previously been demonstrated, including direct inhibition of parietal cell acid secretion. Although IL-1beta may activate several intracellular signalling pathways, the mechanisms responsible for inhibition of carbachol stimulated acid secretion have not been determined.

AIMS

To investigate the roles of protein kinase C (PKC) and the sphingomyelinase signalling pathways in the regulation of acid secretion by IL-1beta.

METHODS

Rabbit parietal cells were obtained by collagenase-EDTA digestion and centrifugal elutriation. Acid secretion stimulated by carbachol and A23187 (to mimic elevations in intracellular calcium) was assessed by 14C aminopyrine uptake in response to IL-1beta, PKC, and sphingomyelinase manipulation.

RESULTS

IL-1beta inhibited carbachol and A23187 stimulated acid secretion in a dose dependent manner. The inhibitory actions were completely reversed by each of three different PKC inhibitors, staurosporine, H-7, and chelerythrine, as well as by PKC depletion with high dose phorbol ester pretreatment. IL-1beta did not downregulate parietal cell muscarinic receptor. IL-1beta significantly increased membrane PKC activity. Activation of the sphingomyelinase/ceramide pathway had no effect on basal or stimulated acid secretion. The inhibitory action of IL-1beta was independent of protein kinase A and protein kinase G activity.

CONCLUSIONS

IL-1beta directly inhibits parietal cell carbachol stimulated acid secretion. This action occurs distal to muscarinic receptor activation and elevations in intracellular calcium and requires PKC.

Vasopressin-induced neurotrophism in cultured neurons of the cerebral cortex: dependency on calcium signaling and protein kinase C activity

Neuronal process outgrowth has been postulated to be one of the fundamental steps involved in neuronal development. To test whether vasopressin can influence neuronal development by acting on the outgrowth of neuronal processes, we determined the neurotrophic action of the memory-enhancing peptide, vasopressin, in neurons derived from the cerebral cortex, a site of integrative cognitive function and long-term memory. Exposure to V(1) receptor agonist significantly increased multiple features of nerve cell morphology, including neurite length, number of branches, branch length, number of branch bifurcation points and number of microspikes. The dose-response profile of V(1) receptor agonist-induced neurotrophism exhibited a biphasic function, with lower concentrations inducing a significant increase while higher concentrations generally induced no significant effect. The neurotrophic effect of V(1) receptor activation did not require growth factors present in serum. Analysis of the regional selectivity of the vasopressin-induced neurotrophic effect revealed significant V(1) receptor agonist-induced neurotrophism in occipital and parietal neurons, whereas frontal and temporal neurons were unresponsive. Results of experiments to determine the mechanism of vasopressin-induced neurotrophism demonstrated that vasopressin-induced neurotrophism is dependent on V(1)a receptor activation, requires L-type calcium channel activation and activation of both pathways of the phosphatidylinositol signaling cascade, inositol trisphosphate and protein kinase C. These studies are the first to describe a functional cellular response for vasopressin in the cerebral cortex. The findings are discussed with respect to their implications for understanding the role of vasopressin-induced neurotrophism, the associated signaling pathways required for this response, and the ability of vasopressin to enhance memory function.

Staurosporine-induced neuronal death: multiple mechanisms and methodological implications

To examine whether multiple pathways of cell death exist in sympathetic neurons, we studied the cell death pathway induced by staurosporine (STS) in sympathetic neurons and compared it with the well-characterized NGF deprivation-induced death pathway. Increasing concentrations of STS were found to induce sympathetic neuronal death with different biochemical and morphological characteristics. One hundred nM STS induced metabolic changes, loss of cytochrome c, and caspase-dependent morphological degeneration which closely resembled the apoptotic death induced by NGF deprivation. In contrast, sympathetic neurons treated with 1 microM STS showed no loss of cytochrome c but exhibited extensive, caspase-independent, chromatin changes that were not TUNEL positive. One microM STS-treated sympathetic neurons had greatly reduced metabolic activities and became committed to die rapidly, yet maintained soma structure and appeared viable by other criteria even up to 48 h after STS treatment, illustrating the need to assess cell death by multiple criteria. Lastly, in contrast to the cell death-inducing activities of 100 nM STS or 1 microM STS, very low concentrations of STS (1 nM STS) inhibited sympathetic neuronal death by acting either at or prior to c-jun phosphorylation in the NGF deprivation-induced PCD pathway.

Odorants induce the phosphorylation of the cAMP response element binding protein in olfactory receptor neurons

Although odorants are known to activate olfactory receptor neurons through cAMP, the long-term effects of odorant detection are not known. Our recent findings indicate that there is also a delayed and sustained cAMP response, with kinetics sufficient to mediate long-term cellular responses. This cAMP response is mediated by cGMP through activation of adenylyl cyclase by protein kinase G (PKG). Therefore, we investigated the ability of odorants to regulate gene expression in rat olfactory epithelium. The cAMP-responsive binding protein (CREB) is a well-characterized transcription factor regulated by cAMP. We examined CREB activity in rat olfactory epithelium and olfactory receptor neurons (ORNs) after stimulation with odorants. Odorants increased levels of phosphorylated CREB in olfactory epithelium in vivo, and this increase was localized to ORNs in vitro. Incubation with 8-bromo-cGMP or sodium nitroprusside, a guanylyl cyclase activator, also increased phosphorylated CREB. In vitro, cAMP-dependent protein kinase phosphorylated CREB. In contrast, PKG failed to phosphorylate CREB directly in vitro. Our results demonstrate that the delayed odorant-induced cAMP signal activates CREB, which in turn may modulate gene expression in ORNs. In addition, cGMP indirectly affects CREB activation. This effect of cGMP on CREB activity through cAMP provides another mechanism for the modulation of CREB.

Secretion of micronemal proteins is associated with toxoplasma invasion of host cells

Toxoplasma gondii is an obligate intracellular parasite that actively invades a wide variety of vertebrate cells, although the basis of its pervasive cell invasion is not completely understood. Here, we demonstrate, using several independent assays, that Toxoplasma invasion of host cells is tightly coupled to the release of proteins stored within apical secretory granules called micronemes. Both microneme secretion and cell invasion were highly temperature dependent, and partial depletion of microneme resulted in a transient loss of infectivity. Chelation of parasite intracellular calcium strongly inhibited both microneme release and invasion of host cells, and this effect was partially reversed by raising intracellular calcium using the ionophore A23187. We also provide evidence that a staurosporine-sensitive kinase activity regulates microneme discharge and is required for parasite invasion of host cells. Additionally, we demonstrate that, during apical attachment to the host cell, the micronemal protein MIC2 is released at the junction between the parasite and the host cell. During invasion, MIC2 is successively translocated towards the posterior end of the parasite and is shed before entry of the parasite into the vacuole. Furthermore, we show that the full-length cellular form of MIC2, but not the proteolytically modified secreted form of MIC2, binds specifically to host cells. Collectively, these observations strongly imply that micronemal proteins play a role in Toxoplasma invasion of host cells.

Cellular defects and altered gene expression in PC12 cells stably expressing mutant huntingtin

Expanded polyglutamine tracts cause huntingtin and other proteins to accumulate and aggregate in neuronal nuclei. Whether the intranuclear aggregation or localization of a polyglutamine protein initiates cellular pathology remains controversial. We established stably transfected pheochromocytoma PC12 cells that express the N-terminal fragment of huntingtin containing 20 (20Q) or 150 (150Q) glutamine residues. The 150Q protein is predominantly present in the nuclei, whereas the 20Q protein is distributed throughout the cytoplasm. Electron microscopic examination confirmed that most of the 150Q protein is diffuse in the nucleus with very few microscopic aggregates observed. Compared with parental PC12 cells and cells expressing 20Q, cells expressing 150Q display abnormal morphology, lack normal neurite development, die more rapidly, and are more susceptible to apoptotic stimulation. The extent of these cellular defects in 150Q cells is correlated with the expression level of the 150Q protein. Differential display PCR and expression studies show that cells expressing 150Q have altered expression of multiple genes, including those that are important for neurite outgrowth. Our study suggests that mutant huntingtin in the nucleus is able to induce multiple cellular defects by interfering with gene expression even in the absence of aggregation.

Modulation of luminol chemiluminescence of fMet-Leu-Phe-stimulated neutrophils by affecting dephosphorylation and the metabolism of phosphatidic acid

This paper is addressed to study how PKC-mediated effects and phosphatidic acid interact together in activation of NADPH-oxidase in formyl-methionyl-leucyl-phenylalanine (fMet-Leu-Phe) stimulated neutrophils as detected by luminol chemiluminescence. The early luminescence response in fMet-Leu-Phe-stimulated cells (up to 5 min after stimulation) depends mainly on reactive oxygen species generated extracellularly, whereas all later events are caused by oxidation of luminol inside the cells. The two protein phosphatase inhibitors, okadaic acid and calyculin A, dramatically increased the late luminescence of cells. This enhancement was totally inhibited by the phospholipase D modulator butanol, while the protein kinase C (PKC) inhibitor bisindolylmaleimide I was insensitive. The early luminescence response of the cells was slightly inhibited by both protein phosphatase inhibitors and depended on protein kinase C as well as on phospholipase D activities. Propranolol, an inhibitor of phosphatidate phosphohydrolase, enhanced all parts of luminescence response of fMet-Leu-Phe-stimulated neutrophils at concentrations up to 2.5 x 10(-5) mol/L. While the late luminescence response of propranolol-treated cells was not inhibited by the PKC inhibitor bisindolylmaleimide I, the first response depended on protein kinase C. The inhibitor of diacylglycerol kinase R59949 enhanced the luminescence signal only during the first 4 min in fMet-Leu-Phe-stimulated cells. Only diacylglycerols derived from phospholipase C, such as 1-stearoyl-2-arachidonoyl-sn-glycerol, were able to initiate an oxidative burst in cells. Saturated diacylglycerols (e.g. 1,2-dipalmitoyl-sn-glycerol or 1,2-distearoyl-sn-glycerol) did not yield any luminol chemiluminescence, although they were incorporated into the plasma membrane, as evidenced by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. Our results demonstrate that phosphatidic acid produced by phospholipase D is responsible for NADPH-oxidase activity in fMet-Leu-Phe-stimulated neutrophils over the entire measuring time, whereas PKC-mediated processes are only involved during the first 5 min.

Gö 6976 is a potent inhibitor of neurotrophin-receptor intrinsic tyrosine kinase

We report here that addition of the protein kinase C inhibitor Gö 6976 blocked neurotrophin-induced signaling and autophosphorylation of neurotrophin-specific tyrosine kinase (Trk) receptors, either Trk B in cortical neurons or Trk A in GT1-1-trk9 cells. The effect of Gö 6976 on Trk autophosphorylation was not inhibited by 100 microM orthovanadate, suggesting that the block was not due to the activation of tyrosine phosphatases. Moreover, addition of 10-100 nM concentrations of Gö 6976 inhibited either Trk B or Trk A intrinsic kinase activity in cell-free assays. Gö 6976 also blocked the ability of brain-derived neurotrophic factor to promote cortical neuronal survival and the ability of nerve growth factor to promote PC12 cell survival and differentiation. These results suggest that Gö 6976, besides its known inhibitory effects on lipid- and calcium-dependent isoforms of protein kinase C, can also inhibit neurotrophin signaling by directly inhibiting the intrinsic Trk.

Neurotrophin-induced activation of casein kinase 2 in rat hippocampal slices

Casein kinase 2 is present in the brain, including the hippocampus. It is associated with long-term potentiation and is known to be involved in phosphorylation of proteins potentially important for neuroplasticity, but regulation of its activity in neuronal cells is not yet known. In the present work, it was found that brain-derived neurotrophic factor and neurotrophin-4 control the activity of casein kinase 2 in hippocampal slices of adult rat. It is shown that: (i) treatment of slices for 4 h with the neurotrophins results in a five-fold increase in the activity of cytosolic casein kinase 2; (ii) this effect does not require protein synthesis. In addition, using calcium chelators, phospholipase inhibitors and protein kinase inhibitors, evidence is provided that: (i) neurotrophin-induced activation of casein kinase 2 is dependent on the availability of intracellular calcium due to stimulation of phospholipase C; (ii) both a tyrosine kinase(s) and a serine/threonine kinase(s) convey the signal of calcium. Since there is now accumulating evidence for involvement of brain-derived neurotrophic factor, intracellular calcium, tyrosine kinases and serine/threonine kinases in the regulation of synaptic plasticity, it is suggested that the signalling cascade detected here might contribute to control of synaptic strength in the hippocampus.

Characterization of a nerve growth factor-inducible cellular activity that enhances herpes simplex virus type 1 gene expression and replication of an ICP0 null mutant in cells of neural lineage

Herpes simplex virus type 1 (HSV-1) ICP0 is required for efficient viral gene expression during lytic infection, especially at low multiplicities. A series of cellular activities that can substitute for ICP0 has been identified, suggesting that when the activity of ICP0 is limiting, these activities can substitute for ICP0 to activate viral gene expression. The cellular activities may be especially important during reactivation of HSV from neuronal latency when viral gene expression is initiated in the absence of prior viral protein synthesis. Consistent with this hypothesis, we have identified an inducible activity in cells of neural lineage (PC12) that can complement the low-multiplicity growth phenotype of an ICP0 null mutant, n212. Pretreatment of PC12 cells with nerve growth factor (NGF) or fibroblast growth factor (FGF) prior to infection produced a 10- to 20-fold increase in the 24-h yield of n212 but only a 2- to 4-fold increase in the yield of wild-type virus relative to mock treatment. Slot blot analysis of nuclear DNA isolated from infected cells treated or mock treated with NGF indicated that NGF treatment does not significantly affect viral entry. The NGF-induced activity in PC12 cells was expressed transiently, with peak complementing activity observed when cells were treated with NGF 12 h prior to infection. Addition of NGF 3 h after infection had little effect on virus yield. The NGF-induced cellular activity was inhibited by pretreatment of PC12 cells with kinase inhibitors that have high specificity for kinases involved in NGF/FGF-dependent signal transduction. RNase protection assays demonstrated that the NGF-inducible PC12 cell activity, like that of ICP0, functions to increase the level of viral mRNA during low-multiplicity infection. These results suggest that activation of viral transcription by ICP0 and transcriptional activation of cellular genes by NGF and FGF utilize common signal transduction pathways in PC12 cells.

A unique allogenic model of metastatic pheochromocytoma: PC12 rat pheochromocytoma xenografts to nude mice and establishment of metastases-derived PC12 variants

Local invasion and metastatic spread to distant sites are major causes of death in patients with malignant pheochromocytoma. Since appropriate in vivo models do not exist, little is known about the underlying mechanisms of tumor growth and invasion. We, therefore, developed an animal model of malignant pheochromocytoma and established organotropic metastatic variants of PC12 rat pheochromocytoma cells. PC12 cells were established as xenografts to BALB/c NCR-NU mice. Subsequent to development of tumors or metastases, primary cultures from local tumors, metastases to lymph nodes, lungs and liver were established. These were subcultured in vitro and reinjected for up to five successive in vivo/in vitro cycles. Xenografted PC12 cells grew tumors with a doubling time of 6.78 +/- 0.58 days during log phase of tumor growth, killing hosts within 5-12 weeks depending on the experimental conditions. Tumors reproducibly metastasized to lymph nodes and the lung. Spontaneous metastases to the liver were not observed, but were achieved by intrasplenic injection of parent PC12 cells. In vitro, the metastatic cell lines displayed striking differences in morphology, overall growth patterns and nutritional requirements as well as binding to purified extracellular matrix proteins compared to the parent cell line. In vivo, the metastatic variants showed marked enhancement of metastatic ability. This is the first report of PC12 rat pheochromocytoma cells to exhibit the malignant phenotype in vivo. We also established variant PC12 cell lines that preferentially metastasized to specific sites and that had acquired different in vitro behavior and ability to metastasize. This unique model system should be useful for further studies relating to the invasion and metastases of pheochromocytoma and may prove valuable for investigations of novel antineoplastic therapies in vitro and in vivo.

G-protein activation, intracellular Ca2+ mobilization and phosphorylation studies of membrane currents induced by AlF4- in Xenopus oocytes

We have examined the electrophysiological responses induced by aluminium fluoride (AlF4-) and carbachol in Xenopus oocytes. Application of AlF4- induced Ca(2+)-dependent oscillatory and smooth Cl- currents. Pre-treatment of oocytes with microinjected guanosine 5'-O-(2-thiodiphosphate) diminished the currents, indicating that the effect of AlF4- occurred through G-protein activation. Confocal imaging of intracellular Ca2+ clearly demonstrated that AlF4- could increase the internal Ca2+ concentration in oocytes in the absence of external Ca2+. A protein kinase (PK) activator (4-beta-phorbol 12,13-dibutyrate) decreased the AlF4(-)-induced membrane currents, whereas a PK inhibitor (staurosporine) caused an increase. On the other hand, the protein phosphatase inhibitor (okadaic acid) showed little effect. Although the effects of the phosphorylating/dephosphorylating agents on the carbachol-induced currents were qualitatively similar to the case of AlF4-, some quantitative differences was noted. The results are discussed in terms of the signaling pathways involving muscarinic receptors and G-protein(s) in Xenopus oocytes.

Protein kinase inhibitors block neurite outgrowth from explants of goldfish retina

A role for protein phosphorylation in the process of neurite outgrowth has been inferred from many studies of the effects of protein kinase inhibitors and activators on cultured neurotumor cells and primary neuronal cells from developing brain or ganglia. Here we re-examine this issue, using a culture system derived from a fully differentiated neuronal system undergoing axonal regeneration--the explanted goldfish retina following optic nerve crush. Of the relatively non-selective protein kinase inhibitors employed, H7, staurosporine and K252a were found to block neurite outgrowth, whereas HA1004 had no effect, a result which appears to rule out a critical role for protein kinase A. The more selective protein kinase C inhibitors, sphingosine, calphostin C and Ro-31-8220 were all inhibitory, as was prolonged treatment with phorbol ester and the protein phosphatase inhibitor okadaic acid. These results are in support of a role for protein kinase C in axonal regrowth.

Antiproliferative effect of suramin on primary cultures of human pheochromocytomas and rat PC12 pheochromocytoma cells

BACKGROUND

Suramin inhibits growth of neural crest-derived cells and is used to treat adrenocortical cancer and neuroblastoma in clinical trials. The antiproliferative effect of suramin was evaluated in primary cultures of human pheochromocytoma and the PC12 rat pheochromocytoma cell line in vitro and in vivo.

METHODS

Human pheochromocytoma and PC12 rat pheochromocytoma cells were grown in medium supplemented with suramin at concentrations of 1-1,000 micrograms/ml (1.43-1.43 mM) for up to five generations. Suramin did not induce neuronal differentiation, but inhibited growth of cultured human pheochromocytoma cells with IC50 (inhibitory concentration at which a 50% reduction of proliferation is observed) of 50-250 micrograms/ml. Also, suramin inhibited proliferation of PC12 cells with IC50 of 228 micrograms/ml after 5 days and 161 micrograms/ml at 10 days of treatment. Colony formation assays demonstrated these effects to be cytotoxic rather than cytostatic. Thus when reproductive integrity of PC12 cells was taken into account, IC50 was calculated with 118 micrograms/ml and 129 micrograms/ml, respectively. In vivo experiments were performed with subcutaneously xenotransplanted PC12 cells (BALB/c NCR-NU mice). Suramin did not alter tumorigenicity and did not inhibit local tumor growth.

RESULTS

These data determine for the first time an antiproliferative effect of suramin in pheochromocytoma cells. Suramin is cytotoxic to pheochromocytoma cells in vitro at levels that are clinically achievable.

CONCLUSIONS

Suramin may have potential as an antiproliferative drug in nonresectable pheochromocytoma.

A factor with a zinc- and phorbol ester-binding domain is necessary for endosome fusion

An inhibitory effect of several zinc chelators on endosome fusion reconstituted in an in vitro system has been recently reported (A. Aballay et al., 1995, Biochem. J. 312, 919-923). The factor that requires zinc for its activity is still unknown. Since the regulatory domain of protein kinase C (PKC) contains cysteine-rich motifs which coordinate zinc, we suspected that PKC or a PKC-like protein might be that factor. To test this hypothesis, we studied the effect of calphostin C, a specific inhibitor of PKC that interacts with the cysteine-rich motif, and PMA (phorbol 12-myristate 13-acetate), an activator of several PKC isoforms that bind to the same region, on endosome fusion. Calphostin C inhibited endosome fusion in a zinc-regulated manner, whereas PMA enhanced endosome fusion. Moreover, fusion was strongly stimulated when both PMA and zinc were added together to zinc-depleted fusion reactions. Inhibitors of the catalytic domain of PKC had no effect on the assay suggesting that the kinase activity is not required. In contrast, a glutathione S-transferase fusion protein containing a cysteine-rich region of the regulatory domain of PKCgamma inhibited endosome fusion in a PMA-dependent manner. Western blot analysis demonstrated the presence of proteins containing PKC-like cysteine-rich regions that are released from endosomal fractions by zinc chelators. These results indicate that the previously proposed zinc-dependent factor required for endosome fusion could be either a PKC isoform or a protein containing the phorbol ester-binding domain of PKC.

Specific activation of a c-Jun NH2-terminal kinase isoform and induction of neurite outgrowth in PC-12 cells by staurosporine

Staurosporine, a protein kinase inhibitor, is known to mimic the effect of nerve growth factor (NGF) in promoting neurite outgrowth. To elucidate the mechanism by which staurosporine induces neurite outgrowth in PC-12 cells, we performed an in-gel kinase assay using myelin basic protein as a substrate, and found that staurosporine induced the activation of a kinase with an apparent molecular mass of 57 kDa. The dose of staurosporine required to activate this kinase was consistent with that required to induce neurite outgrowth. Interestingly, the staurosporine-activated kinase was immunoprecipitated by anti-c-Jun NH2-terminal kinase (JNK) isoforms antibody, but not by anti-JNK1-specific antibody or anti-ERK1 antibody, raising the possibility that this kinase is a novel JNK isoform. The substrate specificity of the kinase was distinct from those of osmotic shock-activated JNKs and NGF-activated ERK1. The kinase phosphorylates transcription factors including c-Jun, Elk-1, and ATF2, as well as myelin basic protein, suggesting that it plays a role in gene induction. Furthermore, staurosporine induced immediate-early genes including Nur77 and fos, but not jun. The activation of the staurosporine-activated kinase, as well as the induction of neurite outgrowth, did not require Ras function, while Ras was required for the activation of ERKs and neurite outgrowth induced by NGF. Taken together, these results indicate staurosporine specifically activates a JNK isoform, which may contribute to biological activities including neurite outgrowth.

Essential role of adenosine triphosphate in activation of 17beta-hydroxysteroid dehydrogenase in the rat Leydig cell

The forskolin-induced steroidogenic block of testosterone production residing beyond pregnenolone synthesis in rat Leydig cells was localized to the level of the 17beta-hydroxysteroid dehydrogenase (17betaHSD) reaction in this study. The use of forskolin analogs that discriminate between the diterpene's inhibitory effect on the glucose transporter(s) (1,9-dideoxyforskolin) and its activation of adenylate cyclase (6-aminoethyl carbamyl forskolin) revealed that the block is related to inhibition of glucose transporter(s). 1,9-Dideoxyforskolin, but not 6-aminoethyl carbamyl forskolin, caused a significant inhibition of basal and hCG-stimulated testosterone production with accumulation of androstenedione. Glucose-deficient media produced the same metabolic block in the absence of forskolin, with a significant reduction in 17betaHSD activity and increases in the apparent Km for androstenedione. In contrast, metabolic steps before testosterone formation were not affected. Glucose-induced 17betaHSD activation was mimicked by the addition of ATP or GTP in glucose-deficient media, but not by nonhydrolyzable triphosphate analogs or NADPH. A decrease in 17betaHSD activity caused by KT-5720, a specific inhibitor of protein kinase A and the calmodulin antagonist W-7, indicates that the ATP requirement may be related to the participation of protein kinases in the activation of 17betaHSD. ATP levels derived from alternative (nonglycolytic) pathways are adequate to support basal and hormone-stimulated enzymatic activities in the metabolism of cholesterol to androstenedione. However, the integrity of the glucose transport system with subsequent ATP generation is required for activation of 17betaHSD in the final step of androgen biosynthesis. In conclusion, the conversion of androstenedione to testosterone requires the contribution of the glycolytic pathway to meet ATP requirements for 17betaHSD activity.

Purification and characterization of a p13suc1-bound serine/threonine kinase that is expressed in mature rat brain

We previously reported that the histone-H1 kinase activity bound to p13suc1 increased dramatically during development of the rat brain. In the present work, an in situ kinase assay in an SDS/polyacrylamide gel that contained substrate proteins was employed to characterize the enzyme. Two major proteins of 45 kDa and 100 kDa were found to have p13suc1-bound histone-H1 kinase activity. The former (p45) exhibited strong activity towards histone H1 and had weak autophosphorylation activity, whereas the latter (p100) acted on myelin basic protein or histone H1, and underwent autophosphorylation. p45 was further purified from the nuclear-enriched fraction of rat brain to near homogeneity through sequential column chromatographies. The purified enzyme retained its ability to bind specifically to p13suc1, which suggests that this binding does not require a cofactor. The immunochemical and enzymatic properties of p45 revealed that it differs from Cdk that are known to bind to p13suc1 with high affinity. However, in vitro p45 acted on the peptide motif that is conserved among substrates for cyclin-dependent kinases (Cdk) and mitogen-activated protein kinases, which implies that this protein might belong to the large family of proline-directed kinases. The evidence obtained in this study suggest that p45 is a nuclear p13suc1-bound kinase that has unique functions in the mature brain.

Inhibition of neuronal cyclin-dependent kinase-5 by staurosporine and purine analogs is independent of activation by Munc-18

Neuronal cdk5 can phosphorylate certain lys-ser-pro (KSP) motifs of neurofilaments and tau protein in the nervous system. We have immunoprecipitated the cdk5 from rat brain using a polyclonal antibody raised against the C-terminus of cdk5. The immunoprecipitate has phosphorylated a KSPXK peptide analog of NF-H, as well as histone H1 and a bacterially expressed rat NF-H protein. The kinase activity was inhibited by staurosporine, isopentanyladenine and olomoucine in a dose dependent manner. Kinetic studies indicated Ki values of 39 nM, 38 microM and 8 microM, respectively for staurosporine, isopentanyladenine and olomoucine. The inhibition by staurosporine was non-competitive with respect to phosphoryl acceptor acceptor substrates. Western blot analysis of the immunoprecipitate showed both cdk5 and p67 (Munc-18), a putative regulator molecule of the kinase. Addition of p67 fusion protein enhanced the kinase activity of the immunoprecipitate by 60% above the basal activity. P67 elevated Ki values for both staurosporine and olomoucine. The degree of inhibition at high concentrations of these inhibitors was unaltered by exogenous p67 indicating a lack of competitive interactions with p67. The high affinity of staurosporine for cdk5 suggests that cdk5 may be one of the targets for the neurotropic effect of staurosporine.

Plasmalopsychosine of human brain mimics the effect of nerve growth factor by activating its receptor kinase and mitogen-activated protein kinase in PC12 cells. Induction of neurite outgrowth and prevention of apoptosis

Plasmalopsychosine, a characteristic fatty aldehyde conjugate of beta-galactosylsphingosine (psychosine) found in brain white matter, enhances p140trk (Trk A) phosphorylation and mitogen-activated protein kinase (MAPK) activity and as a consequence induces neurite outgrowth in PC12 cells. The effect of plasmalopsychosine on neurite outgrowth and its prolonged activation of MAPK was similar to that of nerve growth factor (NGF), and the effect was specific to neuronal cells. Plasmalopsychosine was not capable of competing with cold chase-stable, high affinity binding of NGF to Trk A, indicating that plasmalopsychosine and NGF differ in terms of Trk A-activating mechanism. Tyrosine kinase inhibitors K-252a and staurosporine, known to inhibit the neurotrophic effect of NGF, also inhibited these effects of plasmalopsychosine, suggesting that plasmalopsychosine and NGF share a common signaling cascade. Plasmalopsychosine prevents apoptosis of PC12 cells caused by serum deprivation, indicating that it has "neurotrophic factor-like" activity. Taken together, these findings indicate that plasmalopsychosine may play an important role in development and maintenance of the vertebrate nervous system.

Modulation of serotonin binding sites in Spisula solidissima oocytes by phorbol ester

In Spisula solidissima oocytes, serotonin (5-hydroxytryptamine, 5-HT)-dependent meiosis reinitiation is mediated via specific 5-HT membrane binding sites. This oocyte response is inhibited by the phorbol ester TPA. To assess whether the inhibitory effect of TPA was due to alteration of oocyte membrane binding sites, we studied their characteristics after TPA treatment. [3H]-5HT binding assays revealed that TPA decreased the affinity and, after prolonged treatment, increased the number of oocyte binding sites. Moreover, inhibitory actions of TPA on 5-HT-induced meiosis reinitiation paralleled its inhibitory effects on 5-HT binding site affinity. The inhibitory actions in biological assays were restricted to TPA (an inactive analog of TPA, TPA-met was inefficient) and were completely reversed by staurosporine. Our data thus suggest an inhibitory role for protein kinase C on oocyte 5-HT binding sites under physiological conditions.

Hierarchical analysis of the nerve growth factor-dependent and nerve growth factor-independent differentiation signaling pathways in PC12 cells with protein kinase inhibitors

The effects of a series of protein kinase inhibitors on nerve growth factor (NGF)-dependent and NGF-independent neurite outgrowth in PC12 cells have established an ordered relationship among those protein kinases sensitive to down regulation by bryostatin, stimulation by staurosporine, inhibition by sphingosine, or inhibition by 6-thioguanine (6-TG). Quantitation of the biphasic staurosporine effects on NGF-induced neurite outgrowth (Hashimoto and Hagino: J Neurochem 53:1675-1685, 1989) gave an IC50 of 2-4 nM for inhibition and an EC50 of 15-20 nM for induction of neurite extension. Both sphingosine and 6-TG inhibited neurite outgrowth induced by staurosporine and basic fibroblast derived growth factor (bFGF), as well as by NGF; therefore, sphingosine- and 6-TG-sensitive protein kinase steps occur after the convergence of the NGF, bFGF, and staurosporine signal pathways. Down regulation of protein kinase C by bryostatin chronic treatment, which inhibits NGF- and bFGF-induced neuritogenesis (Singh et al.: Biochemistry 33:542-551, 1994), did not inhibit the staurosporine-induced neurite outgrowth. Thus, the bryostatin-sensitive protein kinase C must occur subsequent to the convergence of the bFGF and NGF pathways, but before (or parallel to) staurosporine initiation of neurite outgrowth. In contrast, low concentrations of phorbol myristoyl acetate (PMA) or bryostatin, which activate protein kinase C activity, enhanced the staurosporine- or NGF-induced neurite extension. These data indicate that stimulation of one or more protein kinase C isozymes can synergistically interact with the signaling pathway to increase the rate of neuritogenesis. Inhibition by 5-7.5 nM staurosporine acted rapidly to arrest and decrease development of neurites up to 24 hr after NGF treatment, as did K252a and NGF polyclonal antibody addition. Our cellular data support the concept that staurosporine acts to inhibit the NGF receptor Trk (Nye et al.: Mol Biol Cell 3:677-686, 1992), but that downstream steps can be activated by the higher concentration of staurosporine to bypass Trk and lead to neurite generation. Effects of staurosporine, 6-TG, and sphingosine on c-fos gene induction with or without NGF were not correlated with the generation of neurites. The sequence of protein kinases sensitive to these effectors appears to be in the order (but not consecutive) bryostatin, staurosporine, sphingosine, and 6-TG.

Cyclic AMP-independent up-regulation of the human serotonin transporter by staurosporine in choriocarcinoma cells

Treatment of confluent cultures of JAR human placental choriocarcinoma cells with staurosporine caused a marked stimulation of serotonin transport activity in these cells. The stimulatory effect was noticeable at nanomolar concentrations of staurosporine, and a treatment time of > 4 h was required for staurosporine to elicit the effect. At 40 nM and with a treatment time of 16 h, the stimulation of the transport activity was 3.5-6.0-fold. None of the several other protein kinase inhibitors tested had similar effect except KT 5720, a protein kinase A inhibitor, which showed a small but significant (approximately 1.4-fold) stimulatory effect at a concentration of 5 microM. Blockade of RNA synthesis and protein synthesis in the cells prevented completely the stimulation of the transport activity induced by staurosporine. The stimulation was observed not only in intact cells but also in plasma membrane vesicles prepared from staurosporine-treated cells. The stimulation was accompanied by a 5-7-fold increase in the steady state levels of the transporter-specific mRNAs, by a 7-fold increase in the maximal velocity of the transport process, and by a 6-fold increase in the transporter density in the plasma membrane. Even though both staurosporine and cholera toxin had similar effects on the serotonin transport activity in these cells, the effect was not additive when the cells were treated with both reagents together. While treatment of the cells with cholera toxin markedly elevated intracellular levels of cAMP, staurosporine did not have any effect on the cellular levels of this cyclic nucleotide. It is concluded that staurosporine up-regulates the serotonin transport activity in JAR cells by increasing the steady state levels of the serotonin transporter mRNA and by the consequent increase in the transporter density in the plasma membrane and that the process involves a cAMP-independent signaling pathway.

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.

Staurosporine affects calcium homeostasis in cultured bovine adrenal chromaffin cells

These studies show that the potent, non-specific, protein kinase inhibitor, staurosporine, disrupts Ca2+ homeostasis in cultured bovine adrenal chromaffin cells. Staurosporine treatment reduces basal and A23187-stimulated catecholamine release from chromaffin cells, but does not inhibit activated Ca2+ influx. Furthermore, pretreatment with staurosporine also reduces Ca(2+)-stimulated catecholamine release from digitonin-permeabilized cells (t1/2, 40.6 min; IC50, 66.0 nm). However, staurosporine does not inhibit the rise in intracellular Ca2+ ([Ca2+]i) in response to nicotine stimulation as measured by fura-2 photometry. These studies demonstrate that staurosporine interferes with the secretory process at some step at or after the rise in [Ca2+]i in adrenal chromaffin cells. Examination of the effects of staurosporine on 45Ca2+ movement shows that staurosporine produces a slowly developing basal 45Ca2+ accumulation; after 30 min no significant change is observed, but by 120 min, 45Ca2+ accumulation is increased by 29.5%. Thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (tBHQ), inhibitors of Ca(2+) ATPases, were used to determine whether staurosporine induced 45Ca2+ accumulation results from sequestration of 45Ca2+ within intracellular stores. While thapsigargin has no significant effect, concomitant treatment with tBHQ prevents the increase in 45Ca2+ uptake associated with staurosporine treatment. Therefore, the tBHQ-sensitive Ca2+ store, but not the thapsigargin/inositol 1,4,5-triphosphate-sensitive Ca2+ store, appears to be staurosporine-sensitive. Overall, these studies indicate that staurosporine reduces catecholamine release by interfering with Ca2+ homeostasis. Furthermore, this work suggests that a staurosporine-sensitive phosphoprotein(s) is involved with the regulation of Ca2+ homeostasis in bovine adrenal chromaffin cells.

Agonist-induced desensitization of histamine H1 receptor-mediated inositol phospholipid hydrolysis in human umbilical vein endothelial cells

1. The regulation of histamine-induced [3H]-inositol phosphate formation was studied in human cultured umbilical vein endothelial cells (HUVEC). 2. Histamine (EC50 4.8 microM) produced a 12.7 fold increase in [3H]-inositol phosphate formation over basal levels. Prior exposure to 0.1 mM histamine (2 h) produced a 78% reduction in the response to subsequent histamine (0.1 mM) challenge. The IC50 for this histamine-induced desensitization was 0.9 microM. 3. The inositol phosphate response to histamine (0.1 mM) was inhibited by phorbol dibutyrate (IC50 40 nM; maximal reduction 64%). This effect was antagonized by both staurosporine (100 nM) and Ro 31-8220 (10 microM). However, the histamine-induced desensitization of the H1-receptor-mediated inositol phosphate response was insensitive to the protein kinase inhibitors, staurosporine, Ro 31-8220, K252a and KN62. 4. Prior exposure to sodium nitroprusside (100 microM), forskolin (10 microM) or dibutyryl cyclic AMP (1 mM) had no effect upon histamine-induced [3H]-inositol phosphate formation. 5. NaF (20 mM) and thrombin (EC50 0.4 u ml-1) also induced inositol phosphate formation in HUVEC. Histamine pretreatment (0.1 mM, 10-120 min) failed to modify the inositol phosphate response to a subsequent NaF or thrombin challenge. 6. We conclude that the desensitization of histamine H1-receptor-mediated [3H]-inositol phosphate formation occurs at the level of the receptor and involves a mechanism independent of activation of protein kinase A, G, or C, or calcium calmodulin-dependent protein kinase II.

Staurosporine, K-252a, and K-252b stabilize calcium homeostasis and promote survival of CNS neurons in the absence of glucose

Staurosporine, K-252a, and the 9-carboxylic related compound K-252b are low-molecular-weight alkaloids from microbial origin that at high concentrations are kinase inhibitors and can antagonize the effects of neuronal growth factors. Paradoxically, we have found that very low concentrations of these agents (10 fM-10 nM) prolong the survival of hippocampal, septal, and cortical neurons deprived of glucose. These agents did not prevent the depletion of ATP caused by glucose deprivation. The large elevation of intracellular calcium levels that normally mediates glucose deprivation-induced damage was attenuated by staurosporine, K-252a, and K-252b. Western blot analysis using antiphosphotyrosine antibody showed that staurosporine and the K-252 compounds (10-100 pM) stimulated tyrosine phosphorylation of several different proteins. The tyrosine kinase inhibitor genistein significantly reduced the protective effect of staurosporine and the K-252 compounds, indicating that tyrosine phosphorylation was required for neuroprotection by these compounds. Taken together, the data demonstrate that low concentrations of staurosporine and the K-252 compounds can stabilize calcium homeostasis, possibly by a mechanism involving activation of receptor tyrosine kinase transduction pathways.

Staurosporine induces the outgrowth of neurites from the dorsal root ganglion of the chick embryo and PC12D cells

Staurosporine, a potent inhibitor of protein kinases, caused the rapid outgrowth of neurites from cultured dorsal root ganglia of chick embryos and from PC12D cells, a subline of PC12 cells. Treatment of dorsal root ganglia with 1 to 20 nM staurosporine resulted in the extensive outgrowth of neurites that were indistinguishable from those induced by NGF, as assessed by phase-contrast microscopy, electron microscopy and cytochemical staining of actin and tubulin. However, neurites generated from the ganglia in response to the higher concentrations of staurosporine (40-100 nM) seemed to have different characteristics, possibly as a result of the inhibition of cell migration from ganglia. The sequential changes in morphology of PC12D cells in response to staurosporine and to NGF were revealed by staining of actin. Ruffling membranes emerged at the margins of PC12D cells within 4 min after the addition of staurosporine or of NGF. From 10 min to 24 h after the addition of either compound, the ruffles were transformed into several projections that became growing neurites. The formation of ruffles and the outgrowth of neurites were both apparent at a concentration of staurosporine of 10 nM. The neurites that emerged from PC12D cells in response to staurosporine and in response to NGF were indistinguishable under the phase-contrast microscope and after staining of actin and tubulin. However, staurosporine never promoted survival of PC12D cells in serum-free conditions as that promoted by NGF. The observations indicate that staurosporine at nanomolar concentrations may reproduce the neurogenic changes that induced by NGF in primed neuronal cells, although it can not mimic the action of NGF that supports survival of neurons.

Staurosporine-induced reduction of secretory function in cultured bovine adrenal chromaffin cells

Staurosporine, a potent inhibitor of protein kinases, has been used to investigate the involvement of protein kinases in cellular processes such as secretory function and differentiation. We have been examining the effects of staurosporine on secretory function under the same conditions it induces dramatic changes in cell morphology in cultured bovine adrenal chromaffin cells. Our results show that treatment with 100 nM staurosporine reduces catecholamine release stimulated by 56 mM KCl, 10 microM nicotine, and 2 mM BaCl2 in a time-dependent manner (t1/2s, 42, 32, and 31 min, respectively). However, we demonstrate that the time-dependent effects on secretory function are not the direct result of staurosporine-induced changes in cell morphology. The effects of staurosporine on secretion stimulated by KCl, nicotine, and BaCl2 are concentration-dependent (IC50s, 6.3, 29.3, and 34.9 nM, respectively). Staurosporine pretreatment does not inhibit activated 45Ca2+ influx, but does reduce catecholamine release stimulated directly by Ca2+ from permeabilized cells. Furthermore, staurosporine also inhibits basal release with time- and concentration-dependencies (IC50, 9.3 nM and t1/2, 21 min) similar to those found for stimulated release. These results suggest that prolonged staurosporine pretreatment may result in the depletion/alteration of a component essential for the more terminal steps of the secretory process.

Staurosporine activates a 60,000 M(r) protein kinase in bovine chromaffin cells that phosphorylates myelin basic protein in vitro

Bovine chromaffin cells contain a family of renaturable protein kinases. One of these, a 60,000 M(r) kinase (PK60) that phosphorylated myelin basic protein in vitro, was activated fourfold when cells were treated with the protein kinase inhibitor staurosporine. Because staurosporine inhibits protein kinase C, the role of this kinase in the regulation of PK60 activity was investigated. Fifty nanomolar staurosporine produced half-maximal inhibition of protein kinase C activity in chromaffin cells, whereas approximately 225 nM staurosporine was required to induce half-maximal activation of PK60. Other protein kinase C inhibitors, H-7 and K-252a, did not mimic the effect of staurosporine on PK60 activity. Chromaffin cells have three protein kinase C isoforms: alpha, epsilon, and zeta. Prolonged treatment with phorbol esters depleted the cells of protein kinase C alpha and epsilon, but not zeta. Neither activation nor depletion of protein kinase C affected the basal activity of PK60. Moreover, staurosporine activated PK60 in cells depleted of protein kinase C alpha and epsilon; thus, staurosporine appeared to activate PK60 by a mechanism that does not require these protein kinase C isoforms. Incubation of cell extracts with staurosporine in vitro did not activate PK60. Incubation of these extracts with adenosine 5'-O-(3-thiotriphosphate), however, caused a twofold activation of PK60. Although this suggests that PK60 activity is regulated by phosphorylation, the mechanism by which staurosporine activates PK60 is not known. Staurosporine has been reported to promote neurite outgrowth from chromaffin cells. The role of PK60 in mediating the effects of staurosporine on chromaffin cell function remains to be determined.

K-252 compounds: modulators of neurotrophin signal transduction

K-252 compounds, which share a common polyaromatic aglycon structure, are rather general and potent inhibitors of various protein kinases, including protein kinase C and tyrosine-specific protein kinases, and possibly act by interfering at or near the ATP binding site. However, chemical modifications in their sugar moiety can result in high specificity of the inhibitory action and, furthermore, can induce other stimulatory and inhibitory effects on nerve cells. These compounds are of particular interest because, in intact cells, they inhibit the actions of NGF and other neurotrophins without diminishing comparable actions of other growth factors. This effect seems to reflect a direct inhibitory action on trk neurotrophin receptor proteins. At concentrations lower than those necessary to inhibit neurotrophin actions, K-252a and K-252b have been shown to potentiate the stimulatory effects of NT-3 on different neurons in culture and on PC12 cells. The structural requirements for this effect seem to be different from those for the inhibition of neurotrophin actions. These findings raise the possibility of development of compounds of high selectivity, able to inhibit or potentiate the transduction mechanisms of individual neurotrophins, and identify K-252a and K-252b as lead compounds for the development of such selective molecules. Specific inhibitors and stimulators of neurotrophins would be valuable tools to investigate biological functions of the neurotrophins in vitro and in vivo. Furthermore, it is possible that, in the future, highly selective drugs with agonistic or antagonistic actions on neurotrophin mechanisms could become therapeutically useful in the treatment of neurological disease and injury.

Time-dependent inhibition of stimulated adrenal catecholamine release by staurosporine

Staurosporine, a potent inhibitor of protein kinases, is used to study the involvement of protein kinases in cellular processes. In the present studies, the effect of prolonged staurosporine treatment on catecholamine secretion in cultured bovine adrenal chromaffin cells was examined. Staurosporine inhibits catecholamine release stimulated by 10 microM nicotine, depolarizing concentrations of potassium (56 mM KCl), and 2 mM BaCl2. The effects of staurosporine on KCl-stimulated release are time dependent, with a half-time of approximately 50 min and a maximal inhibition at 2 h. Our results indicate that activation of a staurosporine-sensitive protein kinase is not directly involved in the stimulus-secretion coupling process. This does not rule out the possibility that Ca2+/phospholipid-dependent protein kinase or other protein kinases may acutely modulate release. However, these results suggest that a protein(s), which is phosphorylated by a staurosporine-sensitive protein kinase(s), is required to maintain the integrity of the stimulus-secretion coupling process.

K-252b selectively potentiates cellular actions and trk tyrosine phosphorylation mediated by neurotrophin-3

K-252b, a protein kinase inhibitor, has been shown earlier to inhibit nerve growth factor actions on cholinergic neurons of the basal forebrain. In the present study, K-252b was found to prevent trophic actions of two other neurotrophins, brain-derived neurotrophic factor, and neurotrophin-3, on central cholinergic and dopaminergic neurons, peripheral sensory neurons, and PC12 pheochromocytoma cells, when used at greater than 2 microM concentration. Comparable actions of nonneurotrophin growth factors were not affected. Surprisingly, at 0.1-100 nM, K-252b selectively enhanced the trophic action of neurotrophin-3 on central cholinergic neurons, peripheral sensory neurons, and PC12 cells. In PC12 cells, K-252b potentiated the neurotrophin-3-induced tyrosine phosphorylation of trk, a protein kinase responsible for transmitting neurotrophin signals. Of the three structurally related nerve growth factor inhibitors, K-252a, K-252b, and staurosporine, only the first two also mediated neurotrophin-3 potentiation. These findings indicate that K-252b generally and selectively potentiates the neurotrophic action of neurotrophin-3 and suggest that this action involves trk-type neurotrophin receptors.

Staurosporine clamps cytosolic free Ca2+ concentrations of human neutrophils

The present studies indicate that 50 nM-10 microM-staurosporine increased cytosolic free Ca2+ concentrations ([Ca2+]i) of fura-2-loaded neutrophils in a non-linear manner. The rise in [Ca2+]i was rapid, reaching a plateau (e.g. to 0.4 microM with 1 microM-staurosporine) within 30 s, and was maintained for more than 20 min. Pretreating cells with pertussis toxin had no effect on this reaction. The elevation of [Ca2+]i was insensitive to extracellular Ca2+ concentrations and was due entirely to mobilization of intracellular Ca2+ stores. Mn(2+)-quench studies confirmed the absence of Ca2+ influx. No Ca2+ efflux occurred in staurosporine-treated cells. In combination studies, staurosporine potentiated Ca2+ influx induced by N-formylmethionyl-leucyl-phenylalanine (FMLP) and did not block Ca2+ efflux associated with peptide stimulation of neutrophils. Studies with permeabilized cells showed that staurosporine did not directly release intracellular Ca2+ stores, nor did it affect the sequestration of Ca2+ by a Ca2+/ATPase pump. A radioligand-binding assay failed to detect changes in the level of inositol 1,4,5-trisphosphate in neutrophils incubated with less than or equal to 1 microM-staurosporine, but in cells treated with 10 microM-staurosporine the assay recorded a transient increase in this second messenger similar to that induced by FMLP. Finally, lysozyme, but not beta-glucuronidase, was released from staurosporine-treated cells. The present results suggest that staurosporine increased [Ca2+]i by indirectly mobilizing internal Ca2+ stores. Staurosporine suppression of Ca2+ efflux and generation of a persistent signal may account for the maintained elevation of [Ca2+]i.

Thrombin causes neurite retraction in neuronal cells through activation of cell surface receptors

The mechanism by which thrombin induces neurite retraction was studied in NB2a mouse neuroblastoma cells. The rapid effect of thrombin (completed within minutes) appears to involve an interaction between its anion-binding exosite and the thrombin receptor. Structural alterations of this site increase the EC50 for thrombin-mediated retraction, and a hirudin C-terminal peptide that blocks this site inhibits the response. The thrombin effect was mimicked by a 14 amino acid peptide starting with Ser-42, at the proposed cleavage site of the human thrombin receptor. The protein kinase inhibitors staurosporine and H-7 blocked thrombin-induced retraction. It is therefore proposed that thrombin-mediated neurite retraction is caused by cleavage-induced activation of the thrombin receptor and involves stimulation of a protein kinase(s).

6-Methylmercaptopurine riboside is a potent and selective inhibitor of nerve growth factor-activated protein kinase N

Protein kinase N (PKN) is a soluble, apparently novel serine protein kinase that is activated by nerve growth factor (NGF) and other agents in PC12 pheochromocytoma cells as well as in several nonneuronal cell lines. Purine analogs, such as 6-thioguanine and 2-aminopurine, have been found to inhibit PKN in vitro. When applied to intact cells, these compounds suppress certain biological responses to NGF, but not others, a findings suggesting the presence of multiple pathways in the NGF mechanism. We report here that 6-methylmercaptopurine riboside (6-MMPR) inhibits NGF-stimulated PKN activity in vitro with an apparent Ki of approximately 5 nM. This is approximately 1,000-fold lower than the Ki of the most potent purine inhibitor of PKN. Compounds similar to 6-MMPR, but lacking the methyl or riboside groups, were much less potent as PKN inhibitors. A survey of six additional purified protein kinases shows no inhibitory effect of 6-MMPR, thus indicating a good degree of specificity of this compound for PKN. In contrast to NGF-stimulated PKN, a PKN-like activity stimulated in PC12 cells in response to activation of cyclic AMP-dependent protein kinase was nearly insensitive to 6-MMPR. Application of 6-MMPR to intact PC12 cells resulted in blockade of several responses to NGF (neurite regeneration and ornithine decarboxylase induction) but not of several others (rapid enhancement of tyrosine hydroxylase phosphorylation and PKN activation). These findings suggest that 6-MMPR is a potent and selective agent for characterizing PKN in vitro and for assessing its potential role in the multiple pathways of the NGF mechanism of action.

K-252b is a selective and nontoxic inhibitor of nerve growth factor action on cultured brain neurons

K-252b is a kinase inhibitor structurally related to K-252a, which is known to abolish selectively the effects of nerve growth factor (NGF) on PC12 cells and PNS neurons. We tested whether K-252b, K-252a, and staurosporine, another related compound, are effective and selective inhibitors of NGF actions on CNS neurons. All three compounds, at appropriate concentrations, completely and selectively prevented the NGF-mediated activity increase of the cholinergic marker enzyme choline acetyltransferase in cultures of rat basal forebrain cells. The stimulatory effects of basic fibroblast growth factor and insulin on choline acetyltransferase in these cultures and on dopamine uptake in cultures of dissociated ventral mesencephalon were not affected. No signs of toxicity were observed in cultures treated with K-252b. In contrast, K-252a and staurosporine, at concentrations required to block the NGF actions on cholinergic cells, were cytotoxic and produced cell loss. In addition, K-252a, at higher concentrations and in the absence of growth factors, increased cell numbers. Our study suggests that K-252b is a selective and nontoxic inhibitor of NGF actions in the brain and may become a useful tool to study these actions in vivo.