Inositol pyrophosphates mediate the DNA-PK/ATM-p53 cell death pathway by regulating CK2 phosphorylation of Tti1/Tel2.

The apoptotic actions of p53 require its phosphorylation by a family of phosphoinositide-3-kinase-related-kinases (PIKKs), which include DNA-PKcs and ATM. These kinases are stabilized by the TTT (Tel2, Tti1, Tti2) cochaperone family, whose actions are mediated by CK2 phosphorylation. The inositol pyrophosphates, such as 5-diphosphoinositol pentakisphosphate (IP7), are generated by a family of inositol hexakisphosphate kinases (IP6Ks), of which IP6K2 has been implicated in p53-associated cell death. In the present study we report an apoptotic signaling cascade linking CK2, TTT, the PIKKs, and p53. We demonstrate that IP7, formed by IP6K2, binds CK2 to enhance its phosphorylation of the TTT complex, thereby stabilizing DNA-PKcs and ATM. This process stimulates p53 phosphorylation at serine 15 to activate the cell death program in human cancer cells and in murine B cells.

Cystathionine γ-lyase deficiency mediates neurodegeneration in Huntington's disease.

Huntington's disease is an autosomal dominant disease associated with a mutation in the gene encoding huntingtin (Htt) leading to expanded polyglutamine repeats of mutant Htt (mHtt) that elicit oxidative stress, neurotoxicity, and motor and behavioural changes. Huntington's disease is characterized by highly selective and profound damage to the corpus striatum, which regulates motor function. Striatal selectivity of Huntington's disease may reflect the striatally selective small G protein Rhes binding to mHtt and enhancing its neurotoxicity. Specific molecular mechanisms by which mHtt elicits neurodegeneration have been hard to determine. Here we show a major depletion of cystathionine γ-lyase (CSE), the biosynthetic enzyme for cysteine, in Huntington's disease tissues, which may mediate Huntington's disease pathophysiology. The defect occurs at the transcriptional level and seems to reflect influences of mHtt on specificity protein 1, a transcriptional activator for CSE. Consistent with the notion of loss of CSE as a pathogenic mechanism, supplementation with cysteine reverses abnormalities in cultures of Huntington's disease tissues and in intact mouse models of Huntington's disease, suggesting therapeutic potential.

Inositol polyphosphate multikinase is a transcriptional coactivator required for immediate early gene induction.

Profound induction of immediate early genes (IEGs) by neural activation is a critical determinant for plasticity in the brain, but intervening molecular signals are not well characterized. We demonstrate that inositol polyphosphate multikinase (IPMK) acts noncatalytically as a transcriptional coactivator to mediate induction of numerous IEGs. IEG induction by electroconvulsive stimulation is virtually abolished in the brains of IPMK-deleted mice, which also display deficits in spatial memory. Neural activity stimulates binding of IPMK to the histone acetyltransferase CBP and enhances its recruitment to IEG promoters. Interestingly, IPMK regulation of CBP recruitment and IEG induction does not require its catalytic activities. Dominant-negative constructs, which prevent IPMK-CBP binding, substantially decrease IEG induction. As IPMK is ubiquitously expressed, its epigenetic regulation of IEGs may influence diverse nonneural and neural biologic processes.

Inositol polyphosphate multikinase is a coactivator of p53-mediated transcription and cell death.

The tumor suppressor protein p53 is a critical stress response transcription factor that induces the expression of genes leading to cell cycle arrest, apoptosis, and tumor suppression. We found that mammalian inositol polyphosphate multikinase (IPMK) stimulated p53-mediated transcription by binding to p53 and enhancing its acetylation by the acetyltransferase p300 independently of its inositol phosphate and lipid kinase activities. Genetic or RNA interference (RNAi)-mediated knockdown of IPMK resulted in decreased activation of p53, decreased recruitment of p53 and p300 to target gene promoters, abrogated transcription of p53 target genes, and enhanced cell viability. Additionally, blocking the IPMK-p53 interaction decreased the extent of p53-mediated transcription. These results suggest that IPMK acts as a transcriptional coactivator for p53 and that it is an integral part of the p53 transcriptional complex facilitating cell death.

Sulfhydration mediates neuroprotective actions of parkin.

Increases in S-nitrosylation and inactivation of the neuroprotective ubiquitin E3 ligase, parkin, in the brains of patients with Parkinson's disease are thought to be pathogenic and suggest a possible mechanism linking parkin to sporadic Parkinson's disease. Here we demonstrate that physiologic modification of parkin by hydrogen sulfide, termed sulfhydration, enhances its catalytic activity. Sulfhydration sites are identified by mass spectrometry analysis and are investigated by site-directed mutagenesis. Parkin sulfhydration is markedly depleted in the brains of patients with Parkinson's disease, suggesting that this loss may be pathologic. This implies that hydrogen sulfide donors may be therapeutic.

Hydrogen sulfide: a gasotransmitter of clinical relevance.

Though the existence of hydrogen sulfide (H2S) in biological tissues has been known for over 300 years, it is the most recently appreciated of the gasotransmitters as a physiologic messenger molecule. The enzymes cystathionine γ-lyase (CSE) and cystathionine ß-synthase (CBS) had long been speculated to generate H2S, and inhibitors of these enzymes had been employed to characterize influences of H2S in various organs. Definitive evidence that H2S is a physiologic regulator came with the development of mice with targeted deletion of CSE and CBS. Best characterized is the role of H2S, formed by CSE, as an endothelial derived relaxing factor that normally regulates blood pressure by acting through ATP-sensitive potassium channels. H2S participates in various phases of the inflammatory process, predominantly exerting anti-inflammatory actions. Currently, the most advanced efforts to develop therapeutic agents involve the combination of H2S donors with non-steroidal anti-inflammatory drugs (NSAIDs). The H2S releasing moiety provides cytoprotection to gastric mucosa normally adversely affected by NSAIDs while the combination of H2S and inhibition of prostaglandin synthesis may afford synergistic anti-inflammatory influences.

p53-mediated apoptosis requires inositol hexakisphosphate kinase-2.

Inositol pyrophosphates have been implicated in numerous biological processes. Inositol hexakisphosphate kinase-2 (IP6K2), which generates the inositol pyrophosphate, diphosphoinositol pentakisphosphate (IP7), influences apoptotic cell death. The tumor suppressor p53 responds to genotoxic stress by engaging a transcriptional program leading to cell-cycle arrest or apoptosis. We demonstrate that IP6K2 is required for p53-mediated apoptosis and modulates the outcome of the p53 response. Gene disruption of IP6K2 in colorectal cancer cells selectively impairs p53-mediated apoptosis, instead favoring cell-cycle arrest. IP6K2 acts by binding directly to p53 and decreasing expression of proarrest gene targets such as the cyclin-dependent kinase inhibitor p21.

Effect of ancillary ligands on the DNA interaction of [Cr(diimine)3]3+ complexes containing the intercalating dipyridophenazine ligand.

The synthesis of photoluminescent Cr(III) complexes of the type [Cr(diimine)(2)(DPPZ)](3+) are described, where DPPZ is the intercalating dipyridophenazine ligand, and diimine corresponds to the ancillary ligands bpy, phen, DMP, and TMP (where bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, DMP = 5,6-dimethyl-1,10-phenanthroline, and TMP = 3,4,7,8-tetramethyl-1,10-phenanthroline). For TMP, DMP, and phen as ancillary ligands, the complexes have also been resolved into their Lambda and Delta optical isomers. A comparison of the photophysical and electrochemical properties reveal similar (2)E(g) --> (4)A(2g) (O(h)) emission wavelengths and lifetimes, and a variation of 110 mV in the (2)E(g) excited state oxidizing power. A detailed investigation has been undertaken of ancillary ligand effects on the DNA binding of these complexes with a range of polynucleotides. For all four complexes, emission is quenched by the addition of calf thymus B-DNA, with the emission lifetime data yielding bimolecular quenching rate constants close to the diffusion controlled limit. Equilibrium dialysis studies have established a general predilection for AT base binding sites, while companion experiments with added distamycin (a selective minor groove binder) provide evidence for a minor groove binding preference. For the case of [Cr(TMP)(2)(DPPZ)](3+), concomitant equilibrium dialysis and circular dichroism measurements have demonstrated very strong enantioselective binding by the Lambda optical isomer. The thermodynamics of DNA binding have also been explored via isothermal titration calorimetry (ITC). The ITC data establish that the primary binding mode for all four Cr(III) complexes is entropically driven, a result that is attributed to the highly favorable free energy contribution associated with the hydrophobic transfer of the Cr(III) complexes from solution into the DNA binding site.