Sodium orthovanadate enhances proliferation of progenitor cells in the adult rat subventricular zone after focal cerebral ischemia

A Study of Vanadate Group Substitution into Nanosized Hydroxyapatite Doped with Eu3+ Ions as a Potential Tissue Replacement Material

In this study, nanosized vanadate-substituted hydroxyapatites doped with 1 mol% and 2 mol% Eu3+ ions were obtained via the precipitation method. To evaluate the structure and morphology of the obtained compounds, the XRPD (X-ray powder diffraction) technique, Rietveld refinement, SEM-EDS (scanning electron microscopy-energy-dispersive spectrometry) and TEM (transmission electron microscopy) techniques as well as FTIR (Fourier transform infrared) spectroscopy were performed. Moreover, the chemical formula was confirmed using the ICP-OES (Inductively coupled plasma optical emission spectroscopy spectroscopy). The calculated average grain size for powders was in the range of 25 to 90 nm. The luminescence properties of vanadium-substituted hydroxyapatite were evaluated by recording emission spectra and excitation spectra as well as luminescence kinetics. The crucial step of this research was the evaluation of the biocompatibility of the synthesized nanomaterials. Therefore, the obtained compounds were tested toward sheep red blood cells and normal human dermal fibroblast to confirm the nontoxicity and biocompatibility of new nanosized Eu3+ ion-doped vanadate-hydroxyapatite. Moreover, the final step of the research allowed us to determine the time dependent ion release to the simulated body fluid environment. The study confirmed cytocompatibility of vanadium hydroxyapatite doped with Eu3+ ions.

Neuroprotective effects of a protein tyrosine phosphatase inhibitor against hippocampal excitotoxic injury

Neuronal excitotoxicity is the neuronal cell death arising from prolonged exposure to glutamate and the associated excessive influx of ions into the cell. Sodium orthovanadate (Na₃VO₄,) competitively inhibits the protein tyrosine phosphatases that affect intracellular protein phosphorylation. No study has examined the role of protein tyrosine phosphatases in kainic acid (KA)-induced excitotoxic injury using sodium orthovanadate. Thus, the present study was conducted to determine the neuroprotective effects of sodium orthovanadate on KA-induced neuronal death in organotypic hippocampal slice culture. We also performed an in vivo electrophysiology study in Sprague-Dawley rats to observe the function of surviving cells after sodium orthovanadate treatment in KA-induced excitotoxicity. Rats were anaesthetized with sodium pentobarbital and KA was injected unilaterally in CA3 of the hippocampus by microinjection-cannula. Neuronal cell death, as assessed by propidium iodide uptake, was reduced by 10 and 25 μM sodium orthovanadate treatment (24 and 48 h) compared with the KA-only group. Sodium orthovanadate enhanced survival signals by increasing levels of phospho-Akt and superoxide dismutase. In addition, sodium orthovanadate treatment reduced calcineurin level for neuronal protection, which regulates activation of cellular calcium caused by KA-induced injury. In vivo results showed that sodium orthovanadate treatment elicited resistance to KA-induced behavior seizures and significantly reduced the duration of epileptiform discharges. In addition, sodium orthovanadate treatment (25 mM) significantly prevented the increase in power spectra induced by KA injection. These results suggest that sodium orthovanadate decreases the acute effects of KA, thereby inducing neuroprotective effects with reduced reactive oxygen species and cellular Ca²⁺. Thus, sodium orthovanadate may protect hippocampal neurons against excitotoxicity, and surviving neurons may function to reduce seizures.

Involvement of BDNF/ERK signaling in spontaneous recovery from trimethyltin-induced hippocampal neurotoxicity in mice

Trimethyltin (TMT) toxicity causes histopathological damage in the hippocampus and induces seizure behaviors in mice. The lesions and symptoms recover spontaneously over time; however, little is known about the precise mechanisms underlying this recovery from TMT toxicity. We investigated changes in the brain-derived neurotrophic factor/extracellular signal-regulated kinases (BDNF/ERK) signaling pathways in the mouse hippocampus following TMT toxicity. Mice (7 weeks old, C57BL/6) administered TMT (2.6 mg/kg intraperitoneally) showed acute and severe neurodegeneration with increased TUNEL-positive cells in the dentate gyrus (DG) of the hippocampus. The mRNA and protein levels of BDNF in the hippocampus were elevated by TMT treatment. Immunohistochemical analysis showed that TMT treatment markedly increased phosphorylated ERK1/2 expression in the mouse hippocampus 1-4 days after TMT treatment, although the intensity of ERK immunoreactivity in mossy fiber decreased at 1-8 days post-treatment. In addition, ERK-immunopositive cells were localized predominantly in doublecortin-positive immature progenitor neurons in the DG. In primary cultured immature hippocampal neurons (4 days in vitro), BDNF treatment alleviated TMT-induced neurotoxicity, via activation of the ERK signaling pathway. Thus, we suggest that BDNF/ERK signaling pathways may be associated with cell differentiation and survival of immature progenitor neurons, and will eventually lead to spontaneous recovery in TMT-induced hippocampal neurodegeneration.

Extracellular matrix and the neural stem cell niche

Basal lamina is present in many stem cell niches, but we still have a poor understanding of the role of this and other extracellular matrix (ECM) components. Here, we review current knowledge regarding ECM expression and function in the neural stem cell niche, focusing on the subependymal zone of the adult CNS. An increasing complexity of ECM molecules has been described, and a number of receptors expressed on the stem cells identified. Experiments perturbing the niche using genetics or cytotoxic ablation of the rapidly dividing precursors, or using explant culture models to examine specific growth factors, have been influential in showing how changes in these ECM receptors might regulate neural stem cell behavior. However the role of changes in the matrix itself remains to be determined. The answers will be important, as they will point to the molecules required to engineer niches ex-vivo so as to provide tools for regenerative neuroscience.

Protein tyrosine phosphatase (PTP) inhibition enhances chromosomal stability after genotoxic stress: decreased chromosomal instability (CIN) at the expense of enhanced genomic instability (GIN)?

Inappropriate survival signaling after DNA damage may facilitate clonal expansion of genetically compromised cells, and it is known that protein tyrosine phosphatase (PTP) inhibitors activate key survival pathways. In this study we employed the genotoxicant, hexavalent chromium [Cr(VI)], which is a well-documented carcinogen of occupational and environmental concern. Cr(VI) induces a complex array of DNA damage, including DNA double strand breaks (DSBs). We recently reported that PTP inhibition bypassed cell cycle arrest and abrogated Cr(VI)-induced clonogenic lethality. Notably, PTP inhibition resulted in an increase in forward mutations at the HPRT locus, supporting the hypothesis that PTP inhibition in the presence of DNA damage may lead to genomic instability (GIN), via cell cycle checkpoint bypass. The aim of the present study was to determine the effect of PTP inhibition on DNA DSB formation and chromosomal integrity after Cr(VI) exposure. Diploid human lung fibroblasts were treated with Cr(VI) in the presence or absence of the PTP inhibitor, sodium orthovanadate, for up to 24h, and cells were analyzed for DNA DSBs and chromosomal damage. Cr(VI) treatment induced a rapid increase in DNA DSBs, and a significant increase in total chromosomal damage (chromatid breaks and gaps) after 24h. In sharp contrast, PTP inhibition abrogated both DNA DSBs and chromosomal damage after Cr(VI) treatment. In summary, PTP inhibition in the face of Cr(VI) genotoxic stress decreases chromosomal instability (CIN) but increases mutagenesis, which we postulate to be a result of error-prone DNA repair.

Polo-like kinase 1 enhances survival and mutagenesis after genotoxic stress in normal cells through cell cycle checkpoint bypass

Polo-like kinase 1 (Plk1) is a key regulator of mitosis. Aberrant Plk1 activity is found in tumors, but little is known regarding its role in the DNA damage response of normal cells and its potential contribution to the early stages of carcinogenesis. Inappropriate survival signaling after DNA damage may facilitate clonal expansion of genetically compromised cells, and it is known that protein tyrosine phosphatase (PTP) inhibitors activate key survival pathways. In this study, we employed hexavalent chromium [Cr(VI)], a well-documented genotoxicant, to investigate the mechanism by which survival pathway activation could lead to loss of checkpoint control via a mechanism involving Plk1. We recently reported that PTP inhibition enhances clonogenic survival and mutagenesis after Cr(VI) exposure by overriding Cr-induced growth arrest. Here, we report that checkpoint bypass, facilitated by PTP inhibition, was associated with decreased Cdk1 Tyr15 phosphorylation, as well as increased Plk1 activity and nuclear localization. Plk1 was necessary for increased survival after PTP inhibition and Cr(VI) exposure in normal human fibroblasts via enhanced mitotic progression. In addition, pharmacological inhibition of Plk1 abolished the PTP inhibitor-induced bypass of the G(2)/M checkpoint. Notably, Plk1 overexpression increased survival and mutagenesis after Cr(VI) exposure in wild-type Saccharomyces cerevisiae. Taken together, our data indicate that Plk1 activation and nuclear localization are necessary for PTP-regulated mitotic progression after DNA damage. Our studies highlight a role for Plk1 in the loss of checkpoint control, increased survival and mutagenesis after genotoxic exposure in normal cells, which in turn may lead to genomic instability and carcinogenesis.

AKT1 mediates bypass of the G1/S checkpoint after genotoxic stress in normal human cells

Certain forms of hexavalent chromium [Cr(VI)] are human carcinogens. Our recent work has shown that a broad range protein tyrosine phosphatase (PTP) inhibitor, sodium orthovanadate (SOV), abrogated both Cr(VI)-induced growth arrest and clonogenic lethality. Notably, SOV enhanced Cr(VI) mutation frequency, ostensibly through forced survival of genetically damaged cells. In the present study, co-treatment with this PTP inhibitor bypassed the Cr(VI)-induced G(1)/S checkpoint arrest in diploid human lung fibroblasts (HLF). Moreover, the PTP inhibitor abrogated the Cr(VI)-induced decrease in the expression of key effectors of the G(1)/S checkpoint [Cyclin D1, phospho Ser 807/811 Rb (pRB), p27]. Cr(VI)-induced G(1) arrest was associated with the cytoplasmic appearance of pRb and the nuclear localization of p27, both of which were reversed by the PTP inhibitor. The PTP inhibitor's reversal of G(1)/S checkpoint effector localization after Cr exposure was found to be Akt1-dependent, as this was abrogated by transfection with either akt1 siRNA or an Akt1-kinase dead plasmid. Furthermore, Akt1 activation alone was sufficient to induce G(1)/S checkpoint bypass and to prevent Cr(VI)-induced changes in pRb and p27 localization. In conclusion, this work establishes Akt1 activation to be both sufficient to bypass the Cr(VI)-induced G(1)/S checkpoint, as well as necessary for the observed PTP inhibitor effects on key mediators of the G(1)/S transition. The potential for Akt to bypass G(1)/S checkpoint arrest in the face of genotoxic damage could increase genomic instability, which is a hallmark of neoplastic progression.

Cardioprotection by vanadium compounds targeting Akt-mediated signaling

Treatment with inorganic and organic compounds of vanadium has been shown to exert a wide range of cardioprotective effects in myocardial ischemia/reperfusion-induced injury, myocardial hypertrophy, hypertension, and vascular diseases. Furthermore, administration of vanadium compounds improves cardiac performance and smooth muscle cell contractility and modulates blood pressure in various models of hypertension. Like other vanadium compounds, we documented bis(1-oxy-2-pyridinethiolato) oxovanadium (IV) [VO(OPT)] as a potent cardioprotective agent to elicit cardiac functional recovery in myocardial infarction and pressure overload-induced hypertrophy. Vanadium compounds activate Akt signaling through inhibition of protein tyrosine phosphatases, thereby eliciting cardioprotection in myocardial ischemia/reperfusion-induced injury and myocardial hypertrophy. Vanadium compounds also promote cardiac functional recovery by stimulation of glucose transport in diabetic heart. We here discuss the current understanding of mechanisms underlying vanadium compound-induced cardioprotection and propose a novel therapeutic strategy targeting for Akt signaling to rescue cardiomyocytes from heart failure.

Enhanced clonogenic survival induced by protein tyrosine phosphatase (PTP) inhibition after Cr(VI) exposure is mediated by c-Raf and Ras activity

Our recent studies showed that maintenance of protein tyrosine phosphorylation by PTP inhibition enhanced cell growth, clonogenic survival, and mutagenesis after a single low-level Cr(VI) exposure, thereby suggesting that tyrosine phosphorylation-dependent signaling may govern inappropriate survival in human lung fibroblasts (HLFs). Our goal is to identify specific phospho-tyrosine regulator(s)/ downstream effectors involved in enhanced survival after Cr(VI) exposure and PTP inhibition. Phosphotyrosine profiling array showed that PTP inhibition following Cr(VI) exposure increased tyrosine phosphorylation of specific proteins, such as FGR and ABL, which are upstream regulators of both Erk and Akt pathways. To explore the roles of these pathways in the PTP-induced increase in clonogenic survival after Cr(VI) exposure, we examined the effect of combined Akt1 and Erk1/2 knockdown via siRNA technology. Akt1 and/or Erk1/2 silencing had no effect on the PTP inhibitor-induced increase in survival following Cr(VI) exposure, suggesting the presence of non-Akt/non-Erk-mediated survival signaling. Interestingly, geldanamycin, an HSP90 inhibitor and non-specific Raf inhibitor, abrogated the PTP inhibitor-mediated increase in survival following Cr(VI) exposure and abolished the expression/activity of c-Raf and activity of Mek. These findings prompted us to explore upstream regulators of Erk, i.e., Ras, c-Raf and Mek for their potential roles in clonogenic survival. GW5074, a specific c-Raf kinase inhibitor did not alter the effect of the PTP inhibitor but decreased Cr(VI)-mediated clonogenic lethality, potentially though Mek hyperactivation. A genetic approach with a c/a Mek1 mutant also showed that Mek activity was not directly associated with the PTP inhibitor effect. Finally, a genetic approach with d/n or c/a Ras and c-Raf mutants, showed that Ras and c-Raf activities play a substantive role in enhancing clonogenic survival by PTP inhibition following Cr(VI) insult. In conclusion, these studies highlight a novel pro-survival mechanism for clonogenic survival in the face of genotoxic stress in the presence of PTP inhibition via an Erk/Mek-independent and Ras/c-Raf-dependent regulation in normal human lung fibroblasts.

Pathophysiological relevance of forkhead transcription factors in brain ischemia

Forkhead box transcription factor, class 0 (FOXO) is a mammalian homologue of DAF-16, which is known to regulate the lifespan of Caenorhabditis elegans and includes subfamiies of forkhead transcription factors such as FOXO1 (FKHR). FOXO3 (FKHRL1), FOXO4 (AFX) and FOXO6. All these FOXO members are expressed in the brain with different spatial patterns. FOXO1 is phosphorylated on three sites (Thr-24, Ser-256 and Ser-319) in phosphatidylinositol 3-kinase (PI3-K)/Akt-dependenr manner, thereby inhibiting apoptosis signals. We here documented dephosphorylation of FOXO1, FOXO3 and FOXO4 following transient forebrain ischemia with its concomitant translocation into the nucleus in neurons in the gerbil and mouse brains. The dephosphorylarion of FOXO1 following brain ischemia is in part mediated by constirutively active calcineurin in the mouse hippocampus. The activation of FOXOs preceded delayed neuronal death in the vulnerable hippocampal regions following ischemic brain injury. The FOXOl activation is accompanied by an increase in DNA binding activity for FOXO1-responsive element on the Fas ligand promoter. Thus, downstream targets induced by FOXOl include Fas ligand and Bcl-2-interacting mediator of cell death (Bim) in the brain ischemia. Accumulating evidence documented how FOXO activation is involved in the mechanisms of ischemic cell death. In this chapter, we document the activation mechanism of FOXO factors following brain ischemia and deline their downstream targets underlying neuronal death. The pathophysiological relevance of crosstalk between FOXOs and calcineurmn pathways is also discussed. Finally, we propose therapeutic perspectives to rescue neurons from delayed neuronal death by promoting the Akt signaling. Vanadium compounds, protein tyrosine phosphatase inhibitor, up-regulates Akt activity in the brain and thereby rescues neurons from delayed neuronal death by inhibiting FOXO-dependent and -independent death signals in neurons.

Bypass of hexavalent chromium-induced growth arrest by a protein tyrosine phosphatase inhibitor: enhanced survival and mutagenesis

Although the consequences of genotoxic injury include cell cycle arrest and apoptosis, cell survival responses after genotoxic injury can produce intrinsic death-resistance and contribute to the development of a transformed phenotype. Protein tyrosine phosphatases (PTPs) are integral components of key survival pathways, and are responsible for their inactivation, while PTP inhibition is often associated with enhanced cell proliferation. Our aim was to elucidate signaling events that modulate cell survival after genotoxin exposure. Diploid human lung fibroblasts (HLF) were treated with Cr(VI) (as Na(2)CrO(4)), the soluble oxyanionic dissolution product of certain particulate chromates, which are well-documented human respiratory carcinogens. In vitro soluble Cr(VI) induces a wide spectrum of DNA damage, in both the presence and absence of a broad-range PTP inhibitor, sodium orthovanadate (SOV). Notably, SOV abrogated Cr(VI)-induced clonogenic lethality. The enhanced survival of Cr(VI)-exposed cells after SOV treatment was predominantly due to a bypass of cell cycle arrest, as there was no effect of the PTP inhibitor on Cr-induced apoptosis. Moreover, the SOV effect was not due to decreased Cr uptake as evidenced by unchanged Cr-DNA adduct burden. Additionally, the bypass of Cr-induced growth arrest by SOV was accompanied by a decrease in Cr(VI)-induced expression of cell cycle inhibiting genes, and an increase in Cr(VI)-induced expression of cell cycle promoting genes. Importantly, SOV resulted in an increase in forward mutations at the HPRT locus, supporting the hypothesis that PTP inhibition in the presence of certain types of DNA damage may lead to increased genomic instability, via bypass of cell cycle checkpoints.

Bis(1-oxy-2-pyridinethiolato)oxovanadium(IV) enhances neurogenesis via phosphatidylinositol 3-kinase/Akt and extracellular signal regulated kinase activation in the hippocampal subgranular zone after mouse focal cerebral ischemia

Although neurogenesis in the hippocampus is critical for improvement of depressive behaviors and cognitive functions in neurodegeneration disorders, there is no therapeutic agent available to promote neurogenesis in adult brain following brain ischemic injury. Here we found that i.p. administration of bis(1-oxy-2-pyridinethiolato)oxovanadium(IV) [VO(OPT)], which stimulates phosphatidylinositol 3-kinase (PI3K)/Akt and extracellular signal regulated kinase (ERK) pathways, markedly enhanced brain ischemia-induced neurogenesis in the subgranular zone (SGZ) of the mouse hippocampus. VO(OPT) treatment enhanced not only the number of proliferating cells but also migration of neuroblasts. VO(OPT)-induced neurogenesis was associated with Akt and ERK activation in neural precursors in the SGZ. Likewise, VO(OPT)-induced neurogenesis was blocked by both PI3K/Akt and mitogen-activated protein kinase/extracellular signal regulated kinase kinase (MEK)/ERK inhibitors. VO(OPT) treatment rescued decreased phosphorylation of glycogen synthesis kinase 3beta (GSK-3beta) at Ser-9. Finally, amelioration of cognitive dysfunction seen following brain ischemia was positively correlated with VO(OPT)-induced neurogenesis. Taken together, VO(OPT) is a potential therapeutic agent that enhances ischemia-induced neurogenesis through PI3K/Akt and ERK activation, thereby improving memory and cognitive deficits following brain ischemia.

Neuroprotective effects of sodium orthovanadate after hypoxic-ischemic brain injury in neonatal rats

Sodium orthovanadate (SOV), a competitive inhibitor of protein tyrosine phosphatases, is neuroprotective in adult animals following an ischemic event. The present study evaluated whether SOV might be protective in a rat pup hypoxic-ischemic (HI) model. Seven-day-old rat pups had the right carotid artery permanently ligated followed by 140 min of hypoxia (8% oxygen). SOV 1.15, 2.3, 4.6, 9.2 or 18.4 mg/kg and vehicle were administered by i.p. injection at 5 min after reoxygenation. Brain damage was evaluated by weight loss of the right hemisphere at 22 days after hypoxia and by gross and microscopic morphology. SOV lowered blood glucose at doses of 1.15, 2.3 and 4.6 mg/kg and induced toxic effects at 9.2mg/kg. The doses of 2.3 and 4.6 mg/kg of SOV significantly reduced brain weight loss (p<0.05), but treatment with 1.15 or 9.2mg/kg did not. SOV 4.6 mg/kg also improved the histopathologic score and diminished the HI induced reduction of Akt and ERK-1/2 phosphorylation in the cortex (p<0.05) and increased the density of BrdU-positive cells in the subventricular zone (p<0.01). In conclusion, SOV has neuroprotective effects in the neonatal rat HI model partially mediated by activating Akt and ERK-1/2 pathways.