Zinc homeostasis and functions of zinc in the brain

The brain barrier system, i.e., the blood-brain and blood-cerebrospinal fluid barriers, is important for zinc homeostasis in the brain. Zinc is supplied to the brain via both barriers. A large portion of zinc serves as zinc metalloproteins in neurons and glial cells. Approximately 10% of the total zinc in the brain, probably ionic zinc, exists in the synaptic vesicles, and may serve as an endogenous neuromodulator in synaptic neurotransmission. The turnover of zinc in the brain is much slower than in peripheral tissues such as the liver. However, dietary zinc deprivation affects zinc homeostasis in the brain. Vesicular zinc-enriched regions, e.g., the hippocampus, are responsive to dietary zinc deprivation, which causes brain dysfunctions such as learning impairment and olfactory dysfunction. Olfactory recognition is reversibly disturbed by the chelation of zinc released from amygdalar neuron terminals. On the other hand, the susceptibility to epileptic seizures, which may decrease vesicular zinc, is also enhanced by zinc deficiency. Therefore, zinc homeostasis in the brain is closely related to neuronal activity. Even in adult animals and probably adult humans, adequate zinc supply is important for brain functions and prevention of neurological diseases.     

Expression and phosphorylation of a MARCKS-like protein in gastric chief cells: Further evidence for modulation of pepsinogen secretion by interaction of Ca2+/calmodulin with protein kinase C

In gastric chief cells, agents that activate protein kinase C (PKC) stimulate pepsinogen secretion and phosphorylation of an acidic 72-kDa protein. The isoelectric point and molecular mass of this protein are similar to those for a common PKC substrate; the MARCKS (for Myristoylated Alanine-Rich C Kinase Substrate) protein. We examined expression and phosphorylation of the MARCKS-like protein in a nearly homogeneous suspension of chief cells from guinea pig stomach. Western blotting of fractions from chief cell lysates with a specific MARCKS antibody resulted in staining of a myristoylated 72-kDa protein (pp72), associated predominantly with the membrane fraction. Using permeabilized chief cells. we examined the effect of PKC activation (with the phorbol ester PMA), in the presence of basal (100 nM) or elevated cellular calcium (1 μM), on pepsinogen secretion and phosphorylation of the 72-kDa MARCKS-like protein. Secretion was increased 2.3-, 2.6-, and 4.5-fold by incubation with 100 nM PMA, 1 μM calcium, and PMA plus calcium, respectively. A PKC inhibitor (1 μM CGP 41 251) abolished PMA-induced secretion, but did not alter calcium-induced secretion. This indicates that calcium-induced secretion is independent of PKC activation. Chief cell proteins were labeled with ³²P-orthophosphate and phosphorylation of pp72 was detected by autoradiography of 2-dimensional polyacrylamide gels. In the presence of basal calcium PMA (100 nM) caused a > two-fold increase in phosphorylation of pp72. Without PMA, calcium did not alter phosphorylation of pp72. However, 1 μM calcium caused an approx. 50% attenuation of PMA-induced phosphorylation of pp72. Experiments with a MARCKS “phosphorylation/calmodulin binding domain peptide” indicated that calcium/calmodulin inhibits phosphorylation of pp72 by binding to the phosphorylation/calmodulin binding domain and not by inhibiting PKC activity. These observations support the hypothesis that, in gastric chief cells, interplay between calcium/calmodulin binding and phosphorylation of a common domain on the 72-kDa MARCKS-like protein plays a role in modulating pepsinogen secretion. J. Cell. Biochem. 64:514–523. © 1997 Wiley-Liss, Inc.     

Molecular determinants for the differential coupling of Galpha(16) to the melatonin MT1, MT2 and Xenopus Mel1c receptors

The pineal neurohormone melatonin modulates a variety of physiological processes through different receptors. It has recently been reported that the cloned melatonin receptors (MT1, MT2 and Mel1c) exhibit differential abilities to stimulate phospholipase C (PLC) via G(16). Here we examined the molecular basis of such differences in melatonin receptor signaling. Coexpression of MT1 or MT2 with the alpha subunit of G(16) (Galpha(16) ) allowed COS-7 cells to accumulate inositol phosphates in response to 2-iodomelatonin. In contrast, Mel1c did not activate Galpha(16) even though its expression was demonstrated by radioligand binding and agonist-induced inhibition of adenylyl cyclase. As Mel1c possesses an exceptionally large C-terminal tail, we further asked if this structural feature prevented productive coupling to Galpha(16). Eleven chimeric melatonin or mutant receptors were constructed by swapping all or part of the C-terminal tail between MT1, MT2 and Mel1c. All chimeras were fully capable of binding 2-[(125) I]iodomelatonin and inhibiting adenylyl cyclase. Chimeras containing the full-length Mel1c tail were incapable of activating Galpha(16), while those that contained the complete C-terminal region of either MT1 or MT2 stimulated PLC. Incorporation of the extra portion of the C-terminal tail of Mel1c to either MT1 or MT2 completely abolished the chimeras' ability to stimulate PLC via Galpha(16). In contrast, truncation of the C-terminal tail of Mel1c allowed interaction with Galpha(16). Our results suggest that Galpha(16) can discern structural differences amid the three melatonin receptors and provide evidence for functional distinction of Mel1c from MT1 and MT2 receptors.     

Role of guanine nucleotide regulatory protein in polyphosphoinositide degradation and activation of phagocytic leukocytes by chemoattractants

Leukocyte activation by chemoattractants provides an important model to study the biochemical mechanisms of stimulus-response coupling in these cells. Well-defined chemotactic factors induce readily quantifiable responses in phagocytic leukocytes. These include directed migration and the production and release of toxic substances including oxygen radicals and lysosomal enzymes. The development of radiolabeled synthetic oligopeptides with potent chemotactic activity allowed the demonstration of chemoattractant receptors on polymorphonuclear leukocytes (PMNs) as well as macrophages. In membrane preparations from these cells, these receptors exist in high- and low-affinity states which are regulated by guanosine di- and triphosphates. This suggested that chemoattractant receptors interact with guanine nucleotide regulatory proteins (N or G proteins). Although chemoattractants elicit a rapid but transient increase in intracellular cAMP levels, they neither stimulate nor inhibit membrane-bound adenylate cyclase, suggesting a novel role for N proteins in certain receptor-transduction mechanisms. Stimulation of phagocytes by chemoattractants is also associated with a rapid increase in cytosolic Ca2+ concentrations ([ Ca2+]i) which appears to result from the production of inositol 1,4,5-triphosphate (IP3) as a consequence of the diesteric cleavage of phosphatidylinositol 4,5-bisphosphate (PIP2). Treatment of phagocytes with pertussis toxin (PT), which ADP-ribosylates and thereby inactivates certain N proteins, abolishes the cells' responsiveness to chemoattractants. More direct evidence for a role of a PT-sensitive N protein in leukocyte activation was provided by the demonstration that chemoattractants stimulate the hydrolysis of PIP2 in PMN membranes only in the presence of GTP. This receptor-mediated hydrolysis of PIP2 is not observed in plasma membranes prepared from PT-treated PMNs. Therefore, these studies suggest that occupancy of chemoattractant receptors activates a PT-sensitive N protein. The activated N protein shifts the Ca2+ requirement for phospholipase C activity from supraphysiological levels to ambient cytosolic Ca2+ concentrations. Cleavage of PIP2 results in the formation of the second messenger molecules, IP3 and 1,2-diacylglycerol, which can initiate cellular activation. These messengers also seem to activate responses which feed back to attenuate receptor stimulation of phospholipase.     

Effects of the Ca ionophore A23187 on zinc-induced apoptosis in C6 glioma cells

Zinc ions are essential, but at elevated concentrations, they also have toxic effects on mammalian cells. Zinc plays a crucial role in cell proliferation and differentiation and it even protects cells against apoptosis caused by various reagents. On the other hand, zinc at high concentrations causes cell death that was characterized as apoptotic by internucleosomal DNA fragmentation, formation of apoptotic bodies, and breakdown of the mitochondrial membrane potential. In the present work, a clone of rat C6 glioma cells that was resistant to toxic effects of ZnCl₂ up to 250 μM was employed to study the effect of the ionophore A23187 on zinc-induced apoptosis. Neither 150 μM Zn²⁺ nor 100 nM A23187 alone caused apoptosis as measured by internucleosomal DNA fragmentation. However, combined exposure of C6 cells to 100 nM A23187 and 150 μM Zn²⁺ for 48 h was effective in inducing apoptosis. Because the so-called calcium ionophore A23187 is not specific for Ca²⁺ ions but also transports Zn²⁺ with high selectivity over Ca²⁺, we investigated whether this substance promoted the uptake of Zn²⁺ ions into C6 cells. Employing the zinc-specific fluorescence probe Zinquin, we observed that the very low concentration of 1.9 nM A23187 significantly and rapidly raised the intracellular mobile Zn²⁺ content. Analysis by atomic absorption spectroscopy revealed that incubation with 1.9 nM A23187 caused a doubling of the total intracellular zinc level within 60 min. We conclude that the apoptosis evoked by the combined action of Zn²⁺ and A23187 was the result of enhanced Zn²⁺ influx evoked by the ionophore, resulting in higher intracellular zinc levels.     

Cadmium-induced apoptosis in C6 glioma cells: Mediation by caspase 9-activation

The induction of apoptotic cell death by cadmium was investigated in eight mammalian cell lines. Great differences in the cytotoxicity of cadmium were found with different cell lines: Rat C6 glioma cells turned out to be most sensitive with an IC₅₀-value of 0.7 M, while human A549 adenocarcinoma cells were relatively resistant with an IC₅₀-value of 164 M CdCl₂. The mode of cadmium-induced cellular death was identified to involve apoptotic DNA fragmentation in three cell lines, i.e., in C6 glioma cells, E367 neuroblastoma cells and NIH3T3 fibroblasts. In C6 glioma cells, this process was investigated in detail. Internucleosomal DNA-fragmentation occurred 40 h after application of CdCl₂ and was concentration-dependent between 1–100 M CdCl₂, followed by a decrease at higher concentrations due to necrotic processes. Apoptotic chromatin-condensation and nuclear fragmentation was observed 48 h after application of 2.5 M CdCl₂. Furthermore, cadmium (1 M, 48 h) caused a breakdown of the mitochondrial membrane potential as shown by the decline in mitochondrial uptake of rhodamine 123. Also, we found an activation of caspase 9, a protease known to be activated in apoptotic processes following mitochondrial damage. Besides Cd²⁺, other toxic heavy metal ions (Hg²⁺, Pb²⁺, Ni²⁺, Fe²⁺, CrO₄ ^2–, Cu²⁺ or Co²⁺) did not induce apoptotic DNA fragmentation in C6 cells. The only exception was Zn²⁺ which caused apotosis at high concentrations (>150 M) whereas it protected against cadmium-induced apoptosis at low concentrations (10–50 M).     

Zinc metabolism and homeostasis: The application of tracer techniques to human zinc physiology

Tracer kinetic techniques based on zinc stable isotopes have a vital role in advancing knowledge of human zinc physiology and homeostasis. These techniques have demonstrated the complexity of zinc metabolism, and have been critical to estimating the size and interrelationships of those pools of zinc that exchange rapidly with zinc in plasma and which are likely to be especially important for zinc dependent biology. This paper presents findings from recent research linking a steady state compartmental model with non-steady state post-prandial sampling from the intestine, utilizing a combination of intestinal intubation/perfusion and stable isotope tracer kinetic techniques. The gastrointestinal tract has a central role in maintaining whole body zinc homeostasis. While the fractional absorption of zinc from a meal depends on the quantity of exogenous zinc and on such dietary factors as phytic acid, the fractional absorption does not appear to be dependent on the size of the rapidly exchanging pool of the host. In contrast, the quantity of endogenous zinc excreted via the intestine is positively correlated with both the amount of absorbed zinc and the zinc `status' of the host, and thus this process has an equally critical role in maintaining zinc homeostasis. The observed alterations in zinc metabolism in some disease states can be understood in the context of known homeostatic processes. In other conditions, however, such alterations as inflammation-associated hyperzincuria and zinc redistribution, the links between homeostatic perturbation and cellular biology are yet to be explained. Thus the challenge remains for research at the whole body level to carefully characterize zinc distribution and exchange under diverse circumstances, while research at the cellular level must elucidate the regulatory processes and the factors to which they respond.     

Dual coupling of the cloned 5-HT1A receptor to both adenylyl cyclase and phospholipase C is mediated via the same Gi protein.

The coloned 5-HT_1A receptor, stably expressed in HeLa cells, has been shown to mediate the effects of 5-hydroxytryptamine (5-HT) to inhibit cAMP formation and to stimulate the hydrolysis of phosphatidylinositol. Both responses were found to be pertussis toxin sensitive. We have examined these two responses in membranes derived from these cells and show that the 5-HT_1A receptor can directly regulate the activity of adenylyl cyclase and phospholipase C in response to agonist. In order to examine whether the same or distinct guanine nucleotide-binding regulatory protein(s) (G protein) are involved in these two signal transduction pathways, we used anti-peptide antibodies recognizing the -subunits of G_i1, G_i2, G_i3 as specific tools, since these pertussis toxin substrates are expressed in HeLa cells. These antibodies have previously been shown to prevent receptor-G protein coupling by binding to the regions of G proteins which are putatively involved in interaction with receptors. Our results indicate that the G_i proteins, but preferentially G₃, mediate the effects of 5-HT both to inhibit adenylyl cyclase and to stimulate phospholipase C. These findings demonstrate that the same receptor interacting with the same C protein can regulate several distinct effector molecules.     

Evidence that phosphatidylcholine-specific phospholipase C is a key molecule mediating insulin-induced enhancement of gene expression from human cytomegalovirus promoter in CHO cells

The signal transduction from insulin to its receptors and Ras has been extensively studied, while little has been reported beyond these steps. We found that the expression of human interleukin 6 gene under the control of immediate early gene promoter of human cytomegalovirus was enhanced by insulin sitmulation in Chinese hamster ovary cells. The induction effect of insulin was not significantly affected by inhibitors or activators of conventional protein kinase C, cAMP dependent protein kinase and phosphoinositide -3 kinase, however, pre-incubation of the cells with D609, a specific inhibitors of phosphatidylcholine-specific phospholipase C completely abolished the induction effect. These results clearly demonstrate that phosphatidylcholine-specific phospholipase C is a key molecule mediating insulin-induced enhancement of hIL-6 expression from the human cytomegalovirus promoter in Chinese hamster ovary cells and strongly suggest that it plays an important role in the insulin signaling pathways.Abbreviations CHO – Chinese hamster ovary; hCMV promoter – immediate early gene promoter of human cytomegalovirus; hIL-6 – human interleukin 6; PC-PLC-phosphatidylcholine-specific phospholipase C; PI-3 kinase – phosphoinositide 3 kinase; PKA – cAMP dependent protein kinase; PKC – protein kinase C.     

Levodopa modulating effects of inducible nitric oxide synthase and reactive oxygen species in glioma cells

Neurological injury and Parkinson disease (PD) are often associated with the increase of nitric oxide (NO) and free radicals from resident glial cells in the brain. In vitro, exposure to L-3-4-dihydroxyphenylalanine (L-DOPA), one of the main therapeutic agents for the treatment of PD, can lead to neurotoxicity. In this study, lipopolysaccharide (LPS) and interferon-gamma (IFN-g) were used to stimulate C6 glioma cells in the presence of varying concentrations of L-DOPA (1 microM-1 mM). The results indicated a slight augmentation of NO(2)(-) production at low concentrations of L-DOPA (<100 microM) and complete inhibition of NO(2)(-) at higher concentrations (500 microM, 1 mM), (p < 0.001). Western blot analysis corroborated that L-DOPA effects on iNOS was at the level of its protein expression. Total reactive oxygen species (ROS) were detected using 2', 7'-dichlorofluorescein diacetate fluorescence dye (2', 7'-DCFC) and there was an increase of intensity with the increasing concentrations of L-DOPA. Furthermore, large amounts of superoxide (O(2)(-)) and hydrogen peroxide (H(2)O(2)) were generated from the autoxidation of L-DOPA. C6 cells contain high levels of catalase, with inadequate levels of superoxide dismutase (SOD); therefore, there was an accumulation of O(2)(-), tantamount to elevation in 2'7'-DCFC intensity. Simultaneous accumulation of O(2)(-) and NO(2)(-) would propel formation of peroxynitrite (ONOO-). SOD completely attenuated the autoxidation of L-DOPA and significantly reversed the inhibitory effects on iNOS at high concentrations. The data obtained confirmed that the observed effects on iNOS were not due to the activation of the D(1) or beta1 adrenergic receptors by L-DOPA. It was concluded from this study that L-DOPA contributed to the modulation of iNOS and to the increase of O(2)(-) production in the stimulated glioma cells in vitro.     

Coordinated Regulation of Radioadaptive Response by Protein Kinase C and p38 Mitogen-Activated Protein Kinase

Eukaryotic cells are known to have an inducible or adaptive response that enhances radioresistance after a low priming dose of radiation. This radioadaptive response seems to present a novel cellular defense mechanism. However, its molecular processing and signaling mechanisms are largely unknown. Here, we studied the role of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) in the expression of radioadaptive response in cultured mouse cells. Protein immunoblot analysis using isoform-specific antibodies showed an immediate activation of PKC-alpha upon X-irradiation as indicated by a translocation from cytosol to membrane. A low priming dose caused a prolonged translocation, while a nonadaptive high dose dramatically downregulated the total PKC level. Low-dose X-rays also activated the p38 MAPK. The activation of p38 MAPK and resistance to chromosome aberration formation were blocked by SB203580, an inhibitor of p38 MAPK, and Calphostin C, an inhibitor of PKC. Furthermore, it was demonstrated that p38 MAPK was physically associated with delta1 isoform of phospholipase C (PLC-delta1), which hydrolyzed phosphatidylinositol bisphosphate into diacylglycerol, an activator of PKC, and that SB203580 also blocked the activation of PKC-alpha. These results indicate the presence of a novel mechanism for coordinated regulation of adaptive response to low-dose X-rays by a nexus of PKC-alpha/p38 MAPK/PLC-delta1 circuitry feedback signaling pathway with its breakage operated by downregulation of labile PKC-alpha at high doses or excess stimuli.     

Tubulin stimulates adenylyl cyclase activity in C6 glioma cells by bypassing the beta-adrenergic receptor: a potential mechanism of G protein activation

While the cytoskeleton is known to play several roles in the biology of the cell, one role, which has been revealed only recently, is that of a participant in the signal transduction process. Tubulin binds specifically to the alpha subunits of Gs (stimulatory GTP-binding regulatory protein of adenylyl cyclase), Gi1 (inhibitory protein of adenylyl cyclase), and Gq and transactivates those molecules through direct transfer of GTP. The relevance of this transactivation process to G proteins which are normally activated by a neurotransmitter-occupied receptor is the subject of this study. C6 glioma cells, made permeable with saponin, retained tight coupling between Gs and the beta-adrenergic receptor. Although 5-guanylylimidodiphosphate (GppNHp) was incapable of activating Gs (and subsequently, adenylyl cyclase) in the absence of agonist, tubulin with GppNHp bound (tubulin-GppNHp) activated adenylyl cyclase with an EC(50) of 30 nM. Desensitization of beta-adrenergic receptors by isoproterenol exposure had no effect on the ability of tubulin-GppNHp to activate Gs and adenylyl cyclase. When the photoaffinity GTP analog, azidoanilido GTP (AAGTP; P3(4-azidoanilido)-P1-5'-GTP), was added to C6 membranes or permeable C6 cells, it was only weakly incorporated by G alpha s in the absence of isoproterenol. When the same concentration of dimeric tubulin with AAGTP bound was introduced, AAGTP was transferred from tubulin to G alpha s, activating the latter species. Similar 'preferential' activation of G alpha s by tubulin-AAGTP versus the free nucleotide was seen using purified components. Thus, membrane-associated tubulin may serve to activate G alpha s, independent of signals not normally coupled to that protein. Tubulin may act as an agent to link a variety of membrane-associated signalling systems.     

Coordinated effects of electromagnetic field exposure on erythropoietin-induced activities of phosphatidylinositol-phospholipase C and phosphatidylinositol 3-kinase

Initial studies with the erythropoietin-sensitive human hematopoietic cell line, TF1, demonstrated both multifarious effects of pulsed electromagnetic field (EMF) exposure on lipid signal transduction and antiproliferative effects of EMF. Stimulation of TF1 cells with erythropoietin resulted in increased phosphatidylinositol 3-kinase activity within 2 min. Addition of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, produced a decrease in cell proliferation as measured by accumulation of cells in the G₀/G₁ phase of the cell cycle and suppression of erythropoietin-induced DNA synthesis. Similar effects on cell proliferation were seen under EMF treatment. Phosphatidylinositol 3-kinase activity in erythropoietin-stimulated TF1 cells, measured in whole-cell extracts, increased 34% within 2 min and remained above basal levels for at least 20 min. EMF decreased erythropoietin-stimulated phosphatidylinositol 3-kinase activity to lower than basal levels. Additionally, translocation of the 85-kDa regulatory subunit (p85) of phosphatidylinositol 3-kinase to the membrane was prevented by EMF. Phosphatidylinositol-specific phospholipase C was activated, as reflected by increases in diacylglycerol and inositol trisphosphate at 15–60 s after EMF treatment. These results provide the first evidence of subtle coordinated changes by EMF associated with loss of phosphatidylinositol 3-kinase activity, inhibition of the translocation of p85 to the membrane, and activation of phosphatidylinositol-phospholipase C.     

Effects of insulin,pertussis toxin and cholera toxin on protein synthesis and diacylglycerol production in 3T3 fibroblasts: Evidence for a G-protein mediated activation of phospholipase C in the insulin signal mechanism

The rapid increase in protein synthesis that occurs on addition of insulin (1 mU/ml) to stepped-down 3T3 cells was blocked by pre-incubation of the cells with pertussis toxin. Cholera toxin on the other hand stimulated protein synthesis and this effect was insensitive to actinomycin D and inhibited by pro-treatment of the cells with phorbol dibutyrate to deplete cell protein kinase C. Insulin was found to cause a rapid and transient increase in diacylglycerol (DAG) synthesis. The insulin-induced increase in diacylglycerol was blocked by pertussis toxin. Exogenous DAG (10 M) stimulated protein synthesis within 1 hour. The results suggest that insuIin stimulates ribosomal activity through a signal mechanism that involves a G-protein mediated activation of phospholipase C to increase DAG levels.     

Down-regulation by elicitors of phosphatidylcholine-hydrolyzing phospholipase C and up-regulation of phospholipase A in plant cells

Phosphatidylcholine, labeled by two fluorescent fatty acids, was fed to cultured plant cells (Petrosilenum crispum, L.; VBI-0, Nicotiana benthiana, L.) and fluorescent diacylglycerol (DAG) was the major metabolite. When a glycoprotein elicitor, derived from Phytophthora sojae, was applied to the parsley cells and the small protein cryptogein from Phytophthora cryptogea was applied to the tobacco cells, these signal substances strongly and rapidly decreased the pool of fluorescent diacylglycerol and weakly increased the pool of free fluorescent fatty acid and of fluorescent lysophosphatidylcholine. The cells responded in a very similar way to the application of mastoparan, a wasp venom peptide. As phosphatidic acid was only a very minor fluorescent metabolite DAG is hypothesized to arise by the action of a phosphatidylcholine-hydrolyzing phospholipase C which was down-regulated by elicitors. Up-regulation of a phospholipase A by elicitors is also suggested by these results. This is the first evidence for phosphatidylcholine-hydrolyzing phospholipase C in plant signal transduction.     

Sequential phosphorylation of SLP-76 at tyrosine 173 is required for activation of T and mast cells

Cooperatively assembled signalling complexes, nucleated by adaptor proteins, integrate information from surface receptors to determine cellular outcomes. In T and mast cells, antigen receptor signalling is nucleated by three adaptors: SLP-76, Gads and LAT. Three well-characterized SLP-76 tyrosine phosphorylation sites recruit key components, including a Tec-family tyrosine kinase, Itk. We identified a fourth, evolutionarily conserved SLP-76 phosphorylation site, Y173, which was phosphorylated upon T-cell receptor stimulation in primary murine and Jurkat T cells. Y173 was required for antigen receptor-induced phosphorylation of phospholipase C-γ1 (PLC-γ1) in both T and mast cells, and for consequent downstream events, including activation of the IL-2 promoter in T cells, and degranulation and IL-6 production in mast cells. In intact cells, Y173 phosphorylation depended on three, ZAP-70-targeted tyrosines at the N-terminus of SLP-76 that recruit and activate Itk, a kinase that selectively phosphorylated Y173 in vitro. These data suggest a sequential mechanism whereby ZAP-70-dependent priming of SLP-76 at three N-terminal sites triggers reciprocal regulatory interactions between Itk and SLP-76, which are ultimately required to couple active Itk to its substrate, PLC-γ1.     

Specifying protein kinase C functions in melanoma

The protein kinase C (PKC) family of serine/threonine protein kinases is a heterogeneous group of enzymes receiving and integrating signals involved in both normal melanocyte biology and melanoma pathology. Alterations in PKC enzyme expression and activation contribute to the malignant phenotype of melanoma in both oncogenic and tumor suppressive roles. Delineating the diverse and often context-dependent functions of PKC enzymes in melanocyte/melanoma biology is key to capitalize on these kinases as drug targets. This review summarizes several of the diverse functions of PKC in melanocyte and melanoma biology with a focus on PKC enzyme regulation and function.     

Zinc induces apoptosis that can be suppressed by lanthanum in C6 rat glioma cells

Zinc ions have both essential and toxic effects on mammalian cells. Here we report the ability of zinc to act as an inducer of apoptosis in C6 rat glioma cells. Incubation with 150 to 300 microM ZnCl2 caused cell death that was characterized as apoptotic by internucleosomal DNA fragmentation, formation of apoptotic bodies, nuclear fragmentation and breakdown of the mitochondrial membrane potential. On the other hand, zinc deprivation by the membrane permeable chelator TPEN [N,N,N',N',-tetrakis (2-pyridyl-methyl)-ethylenediamine] also induced programmed death in this cell line, indicating the existence of intracellular zinc levels below and above which apoptosis is induced. Zinc-induced apoptosis in C6 cells was independent of major signaling pathways (protein kinase C, mitogen activated protein kinase and guanylate cyclase) and protein synthesis, but was increased by facilitating zinc uptake with the ionophore pyrithione. Lanthanum(III)chloride was also able to increase the net zinc uptake, but nevertheless apoptotic features and zinc toxicity were reduced. Remarkably, lanthanum suppressed the zinc-induced breakdown of the mitochondrial membrane potential. We conclude that in C6 cells lanthanum acts in two different ways, as a promoter of net zinc uptake and as a suppressor of zinc-induced apoptosis.