Thioredoxin-dependent redox regulation of p53-mediated p21 activation

Metalloregulation of the tumor suppressor protein p53: zinc mediates the renaturation of p53 after exposure to metal chelators in vitro and in intact cells

The tumor suppressor p53 is a transcription factor which binds DNA through a structurally complex domain stabilized by a zinc atom. Zinc chelation disrupts the architecture of this domain, inducing the protein to adopt an immunological phenotype identical to that of many mutant forms of p53. In this report, we used 65Zn to show that incorporation of zinc within the protein was required for folding in the 'wild-type' conformation capable of specific DNA-binding. Using a cellular assay, we show that addition of extracellular zinc at concentrations within the physiological range (5 microM) was required for renaturation and reactivation of wild-type p53. Among other divalent metals tested (Cd2+, Cu2+, Co2+, Fe2+ and Ni2+), only Co2+ at 125 microM had a similar effect. Recombinant metallothionein (MT), a metal chelator protein, was found to modulate p53 conformation in vitro. In cultured cells, overexpression of MT by transfection could modulate p53 transcriptional activity. Taken together, these results suggest that zinc binding plays a regulatory role in the control of p53 folding and DNA-binding activity.

Going APE over ref-1

The DNA base excision repair (BER) pathway is responsible for the repair of cellular alkylation and oxidative DNA damage. A crucial and the second step in the BER pathway involves the cleavage of baseless sites in DNA by an AP endonuclease. The major AP endonuclease in mammalian cells is Ape1/ref-1. Ape1/ref-1 is a multifunctional protein that is not only responsible for repair of AP sites, but also functions as a reduction-oxidation (redox) factor maintaining transcription factors in an active reduced state. Ape1/ref-1 has been shown to stimulate the DNA binding activity of numerous transcription factors that are involved in cancer promotion and progression such as Fos, Jun, NF(B, PAX, HIF-1(, HLF and p53. Ape1/ref-1 has also been implicated in the activation of bioreductive drugs which require reduction in order to be active and has been shown to interact with a subunit of the Ku antigen to act as a negative regulator of the parathyroid hormone promoter, as well as part of the HREBP transcription factor complex. Ape1/ref-1 levels have been found to be elevated in a number of cancers such as ovarian, cervical, prostate, rhabdomyosarcomas and germ cell tumors and correlated with the radiosensitivity of cervical cancers. In this review, we have attempted to try and assimilated as much data concerning Ape1/ref-1 and incorporate the rapidly growing information on Ape1/ref-1 in a wide variety of functions and systems.

p53 protein oxidation in cultured cells in response to pyrrolidine dithiocarbamate: a novel method for relating the amount of p53 oxidation in vivo to the regulation of p53-responsive genes

A novel method was developed to determine the oxidation status of proteins in cultured cells. Methoxy-polyethylene glycol-maleimide MW 2000 (MAL-PEG) was used to covalently tag p53 protein that was oxidized at cysteine residues in cultured cells. Treatment of MCF7 breast cancer cells with pyrrolidine dithiocarbamate (PDTC), a metal chelator, resulted in a minimum of 25% oxidation of p53. The oxidized p53 had an average of one cysteine residue oxidized per p53 protein molecule. The effect of PDTC treatment on downstream components of the p53 signal-transduction pathway was tested. PDTC treatment prevented actinomycin D-mediated up-regulation of two p53 effector gene products, murine double minute clone 2 oncoprotein and p21(WAF1/CIP1) (where WAF1 corresponds to wild-type p53-activated fragment 1 and CIP1 corresponds to cyclin-dependent kinase-interacting protein 1). Actinomycin D treatment led to accumulation of p53 protein in the nucleus. However, when cells were simultaneously treated with PDTC and actinomycin D, p53 accumulated in both the nucleus and the cytoplasm. The data indicate that an average of one cysteine residue per p53 protein molecule is highly sensitive to oxidation and that p53 can be efficiently oxidized by PDTC in cultured cells. PDTC-mediated oxidation of p53 correlates with altered p53 subcellular localization and reduced activation of p53 downstream effector genes. The novel method for detecting protein oxidation detailed in the present study may be used to determine the oxidation status of specific proteins in cells.

Oxidized LDL induces an oxidative stress and activates the tumor suppressor p53 in MRC5 human fibroblasts

It is now well established that oxidized LDL (OxLDL) is involved in the progression of the atheromatous plaque via several mechanisms, including its cytotoxicity toward the arterial wall. Our study demonstrates that a 4-h incubation of cultured human fibroblasts with 25-75 microg/ml OxLDL induced a dose-dependent increase in the intracellular levels of reactive oxygen species (ROS) and lipid peroxidation end products (TBARS). This effect was markedly prevented by the antioxidant vitamin E. The lipid extract of OxLDL partially reproduced the action of the LDL particle itself. Concomitantly, OxLDL enhanced the DNA binding activity of p53 measured by electrophoretic mobility shift assay, and the intracellular protein level of p53 determined by immunoblot analysis. Cycloheximide prevented the OxLDL-induced augmentation in both p53 binding activity and intracellular level. Again, the lipid extract of OxLDL reproduced the effect of OxLDL on p53 binding activity, whereas vitamin E prevented it. These results indicate that OxLDL initiates an intracellular oxidative stress by means of its lipid peroxidation products, leading to the activation of the tumour suppressor p53 by enhancement of p53 protein synthesis. This effect might be related to the cytotoxic effect of OxLDL since the activation of p53 is known to lead to cell cycle arrest, necrosis or apoptosis.

The mitochondrial thioredoxin system

Eukaryotic organisms from yeast to human possess a mitochondrial thioredoxin system composed of thioredoxin and thioredoxin reductase, similar to the cytosolic thioredoxin system that exists in the same cells. Yeast and mammalian mitochondrial thioredoxins are monomers of approximately 12 kDa and contain the typical conserved active site WCGPC. However, there are important differences between yeast and mammalian mitochondrial thioredoxin reductases that resemble the differences between their cytosolic counterparts. Mammalian mitochondrial thioredoxin reductase is a selenoprotein that forms a homodimer of 55 kDa/subunit; while yeast mitochondrial thioredoxin reductase is a homodimer of 37 kDa/subunit and does not contain selenocysteine. A function of the mitochondrial thioredoxin system is as electron donor for a mitochondrial peroxiredoxin, an enzyme that detoxifies the hydrogen peroxide generated by the mitochondrial metabolism. Experiments with yeast mutants lacking both the mitochondrial thioredoxin system as well as the mitochondrial peroxiredoxin system suggest an important role for mitochondrial thioredoxin, thioredoxin reductase, and peroxiredoxin in the protection against oxidative stress.