Inhibition of protein phosphatase activity induces p53-dependent apoptosis in the absence of p53 transactivation

p53/FBXL20 axis negatively regulates the protein stability of PR55α, a regulatory subunit of PP2A Ser/Thr phosphatase

We have previously reported an important role of PR55α, a regulatory subunit of PP2A Ser/Thr phosphatase, in the support of critical oncogenic pathways required for oncogenesis and the malignant phenotype of pancreatic cancer. The studies in this report reveal a novel mechanism by which the p53 tumor suppressor inhibits the protein-stability of PR55α via FBXL20, a p53-target gene that serves as a substrate recognition component of the SCF (Skp1_Cullin1_F-box) E3 ubiquitin ligase complex that promotes proteasomal degradation of its targeted proteins. Our studies show that inactivation of p53 by siRNA-knockdown, gene-deletion, HPV-E6-mediated degradation, or expression of the loss-of-function mutant p53R175H results in increased PR55α protein stability, which is accompanied by reduced protein expression of FBXL20 and decreased ubiquitination of PR55α. Subsequent studies demonstrate that knockdown of FBXL20 by siRNA mimics p53 deficiency, reducing PR55α ubiquitination and increasing PR55α protein stability. Functional tests indicate that ectopic p53R175H or PR55α expression results in an increase of c-Myc protein stability with concomitant dephosphorylation of c-Myc-T58, which is a PR55α substrate, whose phosphorylation otherwise promotes c-Myc degradation. A significant increase in anchorage-independent proliferation is also observed in normal human pancreatic cells expressing p53R175H or, to a greater extent, overexpressing PR55α. Consistent with the common loss of p53 function in pancreatic cancer, FBXL20 mRNA expression is significantly lower in pancreatic cancer tissues compared to pancreatic normal tissues and low FBXL20 levels correlate with poor patient survival. Collectively, these studies delineate a novel mechanism by which the p53/FBXL20 axis negatively regulates PR55α protein stability.

Dying to Survive-The p53 Paradox

The p53 tumour suppressor is best known for its canonical role as "guardian of the genome", activating cell cycle arrest and DNA repair in response to DNA damage which, if irreparable or sustained, triggers activation of cell death. However, despite an enormous amount of work identifying the breadth of the gene regulatory networks activated directly and indirectly in response to p53 activation, how p53 activation results in different cell fates in response to different stress signals in homeostasis and in response to p53 activating anti-cancer treatments remains relatively poorly understood. This is likely due to the complex interaction between cell death mechanisms in which p53 has been activated, their neighbouring stressed or unstressed cells and the local stromal and immune microenvironment in which they reside. In this review, we evaluate our understanding of the burgeoning number of cell death pathways affected by p53 activation and how these may paradoxically suppress cell death to ensure tissue integrity and organismal survival. We also discuss how these functions may be advantageous to tumours that maintain wild-type p53, the understanding of which may provide novel opportunity to enhance treatment efficacy.

Protein phosphatase 1 in tumorigenesis: is it worth a closer look?

Cancer cells take advantage of signaling cascades to meet their requirements for sustained growth and survival. Cell signaling is tightly controlled by reversible protein phosphorylation mechanisms, which require the counterbalanced action of protein kinases and protein phosphatases. Imbalances on this system are associated with cancer development and progression. Protein phosphatase 1 (PP1) is one of the most relevant protein phosphatases in eukaryotic cells. Despite the widely recognized involvement of PP1 in key biological processes, both in health and disease, its relevance in cancer has been largely neglected. Here, we provide compelling evidence that support major roles for PP1 in tumorigenesis.

PR55α regulatory subunit of PP2A inhibits the MOB1/LATS cascade and activates YAP in pancreatic cancer cells

PP2A holoenzyme complexes are responsible for the majority of Ser/Thr phosphatase activities in human cells. Each PP2A consists of a catalytic subunit (C), a scaffold subunit (A), and a regulatory subunit (B). While the A and C subunits each exists only in two highly conserved isoforms, a large number of B subunits share no homology, which determines PP2A substrate specificity and cellular localization. It is anticipated that different PP2A holoenzymes play distinct roles in cellular signaling networks, whereas PP2A has only generally been defined as a putative tumor suppressor, which is mostly based on the loss-of-function studies using pharmacological or biological inhibitors for the highly conserved A or C subunit of PP2A. Recent studies of specific pathways indicate that some PP2A complexes also possess tumor-promoting functions. We have previously reported an essential role of PR55α, a PP2A regulatory subunit, in the support of oncogenic phenotypes, including in vivo tumorigenicity/metastasis of pancreatic cancer cells. In this report, we have elucidated a novel role of PR55α-regulated PP2A in the activation of YAP oncoprotein, whose function is required for anchorage-independent growth during oncogenesis of solid tumors. Our data show two lines of YAP regulation by PR55α: (1) PR55α inhibits the MOB1-triggered autoactivation of LATS1/2 kinases, the core member of the Hippo pathway that inhibits YAP by inducing its proteasomal degradation and cytoplasmic retention and (2) PR55α directly interacts with and regulates YAP itself. Accordingly, PR55α is essential for YAP-promoted gene transcriptions, as well as for anchorage-independent growth, in which YAP plays a key role. In summary, current findings demonstrate a novel YAP activation mechanism based on the PR55α-regulated PP2A phosphatase.

The protective mechanism of schisandrin A in d-galactosamine-induced acute liver injury through activation of autophagy

CONTEXT

The principal bioactive lignan of Schisandra chinensis fructus, commonly used for traditional Chinese medicine, is schisandrin A. Schisandrin A has been widely reported as being very effective for the treatment of liver disease. However, the mechanisms of its protective effects in liver remain unclear.

OBJECTIVE

To explore the hepatoprotective mechanisms of schisandrin A.

MATERIALS AND METHODS

d-Galactosamine (d-GalN)-induced liver injury in mice was used as a model. Schisandrin A was examined for its protective mechanisms using hematoxylin-eosin (HE) staining, enzyme-linked immunosorbent assay (ELISA), western blotting and real-time PCR (RT-PCR).

RESULTS

Aspartate amino-transferase (AST) and alanine transaminase (ALT) levels in the schisandrin A group were significantly decreased (p < 0.01) compared with those in the d-GalN-treated group. HE results showed that the pathological changes in hepatic tissue seen in the d-GalN-treated were reduced in the schisandrin A/d-GalN-treated group, with the morphological characteristics being close to those of the control (untreated) group. Western blotting results showed that schisandrin A can activate autophagy flux and inhibit progression of apoptosis. The immune function of the schisandrin A-pretreated group was assayed by flow cytometry. It was found that the mechanism may involve activated autophagy flux, inhibited apoptosis, and improved immunity in response to liver damage.

CONCLUSION

Our results show that the hepatoprotective mechanisms of schisandrin A may include activation of autophagy flux and inhibition of apoptosis. These results provide pharmacological evidence supporting its future clinical application.

The natural compound cantharidin induces cancer cell death through inhibition of heat shock protein 70 (HSP70) and Bcl-2-associated athanogene domain 3 (BAG3) expression by blocking heat shock factor 1 (HSF1) binding to promoters

Heat shock factor 1 (HSF1) enhances the survival of cancer cells under various stresses. The knock-out of HSF1 impairs cancer formation and progression, suggesting that HSF1 is a promising therapeutic target. To identify inhibitors of HSF1 activity, we performed cell-based screening with a library of marketed and experimental drugs and identified cantharidin as an HSF1 inhibitor. Cantharidin is a potent antitumor agent from traditional Chinese medicine. Cantharidin inhibited heat shock-induced luciferase activity with an IC50 of 4.2 μm. In contrast, cantharidin did not inhibit NF-κB luciferase reporter activity, demonstrating that cantharidin is not a general transcription inhibitor. When the HCT-116 colorectal cancer cells were exposed to heat shock in the presence of cantharidin, the induction of HSF1 downstream target proteins, such as HSP70 and BAG3 (Bcl-2-associated athanogene domain 3), was suppressed. HSP70 and its co-chaperone BAG3 have been reported to protect cells from apoptosis by stabilizing anti-apoptotic Bcl-2 family proteins. As expected, treating HCT-116 cancer cells with cantharidin significantly decreased the amounts of BCL-2, BCL-xL, and MCL-1 protein and induced apoptotic cell death. Chromatin immunoprecipitation analysis showed that cantharidin inhibited the binding of HSF1 to the HSP70 promoter and subsequently blocked HSF1-dependent p-TEFb recruitment. Therefore, the p-TEFb-dependent phosphorylation of the C-terminal domain of RNA polymerase II was blocked, arresting transcription at the elongation step. Protein phosphatase 2A inhibition with PP2CA siRNA or okadaic acid did not block HSF1 activity, suggesting that cantharidin inhibits HSF1 in a protein phosphatase 2A-independent manner. We show for the first time that cantharidin inhibits HSF1 transcriptional activity.

Mitochondrial p53 mediates a transcription-independent regulation of cell respiration and interacts with the mitochondrial F₁F0-ATP synthase

We and others previously reported that endogenous p53 can be located at mitochondria in the absence of stress, suggesting that p53 has a role in the normal physiology of this organelle. The aim of this study was to characterize in unstressed cells the intramitochondrial localization of p53 and identify new partners and functions of p53 in mitochondria. We find that the intramitochondrial pool of p53 is located in the intermembrane space and the matrix. Of note, unstressed HCT116 p53(+/+) cells simultaneously show increased O₂ consumption and decreased mitochondrial superoxide production compared with their p53-null counterpart. This data was confirmed by stable H1299 cell lines expressing low levels of p53 specifically targeted to the matrix. Using immunoprecipitation and mass spectrometry, we identified the oligomycin sensitivity-conferring protein (OSCP), a subunit of the F₁F₀-ATP synthase complex, as a new partner of endogenous p53, specifically interacting with p53 localized in the matrix. Interestingly, this interaction seems implicated in mitochondrial p53 localization. Moreover, p53 localized in the matrix promotes the assembly of F₁F₀-ATP synthase. Taking into account that deregulations of mitochondrial respiration and reactive oxygen species production are tightly linked to cancer development, we suggest that mitochondrial p53 may be an important regulator of normal mitochondrial and cellular physiology, potentially exerting tumor suppression activity inside mitochondria.

ERK1/2 signaling plays an important role in topoisomerase II poison-induced G2/M checkpoint activation

Topo II poisons, which target topoisomerase II (topo II) to generate enzyme mediated DNA damage, have been commonly used for anti-cancer treatment. While clinical evidence demonstrate a capability of topo II poisons in inducing apoptosis in cancer cells, accumulating evidence also show that topo II poison treatment frequently results in cell cycle arrest in cancer cells, which was associated with subsequent resistance to these treatments. Results in this report indicate that treatment of MCF-7 and T47D breast cancer cells with topo II poisons resulted in an increased phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2) and an subsequent induction of G2/M cell cycle arrest. Furthermore, inhibition of ERK1/2 activation using specific inhibitors markedly attenuated the topo II poison-induced G2/M arrest and diminished the topo II poison-induced activation of ATR and Chk1 kinases. Moreover, decreased expression of ATR by specific shRNA diminished topo II poison-induced G2/M arrest but had no effect on topo II poison-induced ERK1/2 activation. In contrast, inhibition of ERK1/2 signaling had little, if any, effect on topo II poison-induced ATM activation. In addition, ATM inhibition by either incubation of cells with ATM specific inhibitor or transfection of cells with ATM specific siRNA did not block topo II poison-induced G2/M arrest. Ultimately, inhibition of ERK1/2 signaling greatly enhanced topo II poison-induced apoptosis. These results implicate a critical role for ERK1/2 signaling in the activation of G2/M checkpoint response following topo II poison treatment, which protects cells from topo II poison-induced apoptosis.

Myrtenal ameliorates diethylnitrosamine-induced hepatocarcinogenesis through the activation of tumor suppressor protein p53 and regulation of lysosomal and mitochondrial enzymes

Myrtenal is a novel class of compound belongs to monoterpenes found predominantly in mint, pepper, etc., and it was shown to have excellent pharmacological activities against many diseases among which cancer is imperative. Hepatocellular carcinoma is a primary malignancy of the hepatocytes, which rapidly leads to death in short periods. The aim of this study was to investigate the possible therapeutic efficiency of myrtenal against diethylnitrosamine-induced experimental hepatocarcinogenesis by analyzing the key enzymes of carbohydrate metabolism, lysosomal and mitochondrial TCA cycle enzymes, and also the possible role of tumor suppressor protein p53, and scanning electron microscopic studies. The results revealed that myrtenal significantly ameliorated the altered enzymes of carbohydrate metabolism, lysosomal and mitochondrial enzymes, and interestingly the tumor suppressor protein p53 was found to be significantly accumulated in myrtenal-treated animals, which inevitably confirms that myrtenal has a prominent role in preventing the liver cancer during treatment. Furthermore, the antineoplastic property was well evidenced by the mRNA expression of p53 protein by the reverse-transcriptase polymerase chain reaction and immunoblot analysis. The observed anticancer property of myrtenal may be due to the involvement and expression of p53 and influence in the mitochondrial and lysosomal membrane integrity and also interference in the gluconeogenesis process of cancer cells. Our results suggest that myrtenal is very efficient and useful compound in the treatment of liver cancer in future.

GENE EXPRESSION PROFILING OF HUMAN LIVER CARCINOMA (HepG2) CELLS EXPOSED TO THE MARINE TOXIN OKADAIC ACID

The marine toxin, okadaic acid (OA) is produced by dinoflagellates of the genera Prorocentrum and Dinophysis and is the causative agent of the syndrome known as diarrheic shellfish poisoning (DSP). In addition, OA acts as both a tumor promoter, attributed to OA-induced inhibition of protein phosphatases as well as an inducer of apoptosis. To better understand the potentially divergent toxicological profile of OA, the concentration dependent cytotoxicity and alterations in gene expression on the human liver tumor cell line HepG2 upon OA exposure were determined using RNA microarrays, DNA fragmentation, and cell proliferation assays as well as determinations of cell detachment and cell death in different concentrations of OA. mRNA expression was quantified for approximately 15,000 genes. Cell attachment and proliferation were both negatively correlated with OA concentration. Detached cells displayed necrotic DNA signatures but apoptosis also was broadly observed. Data suggest that OA has a concentration dependent effect on cell cycle, which might explain the divergent effects that at low concentration OA stimulates genes involved in the cell cycle and at high concentrations it stimulates apoptosis.

Okadaic acid induces apoptosis through the PKR, NF-κB and caspase pathway in human osteoblastic osteosarcoma MG63 cells

Okadaic acid (OA) is the major component of diarrheic shellfish poisoning toxins and a potent inhibitor of protein phosphatase 1 and 2A. However, the underlying regulatory mechanisms involved in OA-induced cell death are not well understood. In the present study, we examined the effects of OA on apoptosis of MG63 cells by characterizing apoptotic morphological changes of the cells and DNA fragmentation. The roles of double-stranded RNA-dependent protein kinase (PKR), nuclear factor-κB (NF-κB) and caspase in OA-mediated apoptosis in MG63 cells were also examined. Results showed that OA induced cytotoxicity and apoptosis in MG63 cells at IC50 of 75 nM. A functional PKR pathway is required to induce apoptosis in response to OA treatment. Blockade of NF-κB by ammonium pyrrolidinedithiocarbamate (PDTC) resulted in down-regulation of apoptosis. The caspase-3 and caspase-8 inhibitors blocked apoptosis in MG63 cells. In conclusion, our results imply that OA can induce MG63 cell apoptosis through the PKR, NF-κB and caspase pathway.

Effect of Lecithin on d-Galactosamine Induced Hepatotoxicity Through Mitochondrial Pathway Involving Bcl-2 and Bax

Twenty four Wistar strain albino rats were used for the investigations. Lecithin 50 and 100 mg/kg b wt was administered for 1 week by oral route. Liver damage was induced by intra peritoneal administration of 400 mg/kg b wt d-galactosamine on the last day. At the end of the study animals were sacrificed and liver enzyme levels, histopathology, mitochondrial integrity, expression of p53, Bax and Bcl-2 mRNA levels were studied. Increases in the liver enzyme levels by d-GalN were significantly inhibited by pretreatment with lecithin. Histopathological observation further confirmed the hepatoprotective effect of lecithin. In addition, the disruption of mitochondrial membrane, up regulation of Bax and down regulation of Bcl-2 mRNA levels in the liver of d-GalN intoxicated rats were effectively prevented by pretreatment with lecithin. The results of the present study validate our conviction that d-GalN causes hepatic damage via mitochondrial pathway involving Bax and Bcl-2.

Intra-uterine growth restriction is associated with increased apoptosis and altered expression of proteins in the p53 pathway in villous trophoblast

Intrauterine growth restriction (IUGR) affects 3-8% of pregnancies and is associated with altered cell turnover in the villous trophoblast, an essential functional cell type of the human placenta. The intrinsic pathway of apoptosis, particularly p53, is important in regulating placental cell turnover in response to damage. We hypothesised that expression of proteins in the p53 pathway in placental tissue would be altered in IUGR. Expression of constituents of the p53 pathway was assessed using real-time PCR, Western blotting and immunohistochemistry. p53 mRNA and protein expression was increased in IUGR, which localised to the syncytiotrophoblast. Similar changes were noted in p21 and Bax expression. There was no change in the expression of Mdm2, Bak and Bcl-2. The association between altered trophoblast cell turnover in IUGR and increased p53 expression is reminiscent of that following exposure to hypoxia. These observations provide further insight into the potential pathogenesis of IUGR. Further research is required to elicit the role and interactions of p53 and its place in the pathogenesis of IUGR.

Okadaic acid and dinophysis toxin 2 have differential toxicological effects in hepatic cell lines inducing cell cycle arrest, at G0/G1 or G2/M with aberrant mitosis depending on the cell line

Okadaic acid is one of the toxins responsible for the human intoxication known as diarrhetic shellfish poisoning, which appears after the consumption of contaminated shellfish. The main diarrhetic shellfish poisoning toxins are okadaic acid, dinophysistoxin-1, -2, and -3. In vivo, after intraperitoneal injection, dinophysistoxin-2 is approximately 40% less toxic than okadaic acid in mice. The cytotoxic and genotoxic effect of okadaic acid varies very significantly in different cell lines, so similar responses could be expected for dinophysistoxin-2. In order to determine whether this was the case, we studied the effect of okadaic acid and dinophysistoxin-2 in two hepatic cell lines (HepG2 and Clone 9). The cytotoxicity of these toxins, as well as their effects on the cell cycle and its regulation on both cell lines, were determined. Okadaic acid and dinophysistoxin-2 resulted to be equipotent in clone 9 cultures, while okadaic acid was more potent than dinophysistoxin-2 in HepG2 cell cultures. Both toxins had opposite effects on the cell cycle; they arrested the cell cycle of clone 9 cells in G2/M inducing aberrant mitosis while arresting the cell cycle of HepG2 in G0/G1. When the effect of the toxins on p53 subcellular distribution was studied, p53 was detected in the nuclei of both cell types. The effect of the toxins on the gene expression of cyclins and cyclin-dependent kinases was different for both cell lines. The toxins induced an increase in gene expression of cyclins A, B, and D in clone 9 cells while they induced a decrease in cyclins A and B in HepG2 cells. They also induced a decrease in cyclin-dependent kinase 1 in HepG2 cells.

Involment of p53, Bax, and Bcl-2 pathway in microcystins-induced apoptosis in rat testis

It has been reported that microcystins (MCs) could accumulate in the gonads of mammals and MCs exposure exerts obvious toxic effects on male reproductive system of mammals. We have comfirmedthat MCs could accumulate and induce apoptosis in rat testis. The p53, Bax, and Bcl-2 protein play important roles in mitochondria-dependent apoptotic pathway, and this study aimed to investigate whether the p53, Bax, and Bcl-2 pathway is involved in microcystins-induced apoptosis in rat testis and discussed the possible mechanisms. Our results show that MCs led to persistent increase of transcriptional and protein level of P53 and Bax expression but led to decrease of Bcl-2 expression, resulting in an increased ratio of Bax to Bcl-2, which might contribute to apoptotic cell death of rat testis following MCs treatment. The increased ratio of expression of Bax to that of Bcl-2 induced by MCs suggests their important role in MCs-induced apoptosis in rat testis tissue.

Protective Role of Catechin on d-Galactosamine Induced Hepatotoxicity Through a p53 Dependent Pathway

Objective of this study was to obtain a better understanding of the mechanism responsible for the d-galactosamine (d-GalN) induced hepatotoxicity and to study the effect of catechin against d-GalN induced hepatotoxicity. Catechin 50 and 100 mg/kg b.wt was administered for 1 week by oral route. Liver damage was induced by intra-peritoneal administration of 400 mg/kg b.wt d-galactosamine on the last day of catechin treatment. At the end of treatment all animals were killed and liver enzyme levels were estimated. Dissected hepatic samples were used for histopathology, RNA isolation, expression studies of Bax, Bcl-2 and p53 mRNA levels and mitochondrial membrane potential studies. We found that increases in the liver enzyme activity and decrease in antioxidant enzyme activity by d-GalN were significantly restricted by oral pretreatment with catechin. Disruption of mitochondrial membrane potential, up regulation of p53, Bax and down regulation of Bcl-2 mRNA levels in the liver of d-GalN intoxicated rats were effectively prevented by pretreatment with catechin.

p53 Plays an important role in cell fate determination after exposure to microcystin-LR

BACKGROUND

Microcystin-LR, a cyclic heptapeptide, possesses the ability to inhibit the serine/threonine protein phosphatases PP1 and PP2A and, consequently, exhibits acute hepatocytotoxicity. Moreover, microcystin-LR induces cellular proliferation, resulting in tumor-promoting activity in hepatocytes. However, mechanisms that regulate the balance between cell death and proliferation after microcystin-LR treatment remain unclear.

OBJECTIVE

We examined the contribution of the transcription factor p53, as well as that of the hepatic uptake transporter for microcystin-LR, organic anion transporting polypeptide 1B3 (OATP1B3), to the cellular response to microcystin-LR exposure.

METHODS

We analyzed intracellular signaling responses to microcystin-LR by immunoblotting and real-time reverse-transcriptase polymerase chain reaction techniques using HEK293 human embryonic kidney cells stably transfected with SLCO1B3 (HEK293-OATP1B3). In addition, we analyzed the effect of attenuation of p53 function, via the p53 inhibitor pifithrin-alpha, and knockdown of p53 mRNA on the cytotoxicity of microcystin-LR using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay.

RESULTS

Microcystin-LR induced the phosphorylation and accumulation of p53 in HEK293-OATP1B3 cells, which resulted in up-regulation of the expression of p53 transcript targets, including p21 and seven in absentia homolog 1 (siah-1). In addition, microcystin-LR activated Akt signaling through the phosphorylation of Akt and glycogen synthase kinase 3beta. Although Akt signaling was activated, the accumulation of p53 led cells to apoptosis after treatment with 50 nM microcystin-LR for 24 hr. Both pharmacological inhibition of transcription factor activity of p53 by pifithrin-alpha and knockdown of p53 with small hairpin RNA attenuated the susceptibility of HEK293-OATP1B3 cells to microcystin-LR.

CONCLUSIONS

This study demonstrates the importance of p53 in the regulation of cell fate after exposure to microcystin-LR. Our results suggest that, under conditions of p53 inactivation (including p53 mutation), chronic exposure to low doses of microcystin-LR may lead to cell proliferation through activation of Akt signaling. Results of this study may contribute to the development of chemoprevention and chemotherapeutic approaches to microcystin-LR poisoning.

The p53 family and programmed cell death

The p53 tumor suppressor continues to hold distinction as the most frequently mutated gene in human cancer. The ability of p53 to induce programmed cell death, or apoptosis, of cells exposed to environmental or oncogenic stress constitutes a major pathway whereby p53 exerts its tumor suppressor function. In the past decade, we have discovered that p53 is not alone in its mission to destroy damaged or aberrantly proliferating cells: it has two homologs, p63 and p73, that in various cellular contexts and stresses contribute to this process. In this review, the mechanisms whereby p53, and in some cases p63 and p73, induce apoptosis are discussed. Other reviews have focused more extensively on the contribution of individual p53-regulated genes to apoptosis induction by this protein, whereas in this review, we focus more on those factors that mediate the decision between growth arrest and apoptosis by p53, p63 and p73, and on the post-translational modifications and protein-protein interactions that influence this decision.

Identification of a protective role for protein phosphatase 1cgamma1 against oxidative stress-induced vascular smooth muscle cell apoptosis

The development of therapeutic strategies to inhibit reactive oxygen species (ROS)-mediated damage in blood vessels has been limited by a lack of specific targets for intervention. Targeting ROS-mediated events in the vessel wall is of interest, because ROS play important roles throughout atherogenesis. In early atherosclerosis, ROS stimulate vascular smooth muscle cell (VSMC) growth, whereas in late stages of lesion development, ROS induce VSMC apoptosis, causing atherosclerotic plaque instability. To identify putative protective genes against oxidative stress, mouse aortic VSMC were infected with a retroviral human heart cDNA expression library, and apoptosis was induced in virus-infected cells by 2,3-dimethoxy-1,4-naphthoquinone (DMNQ) treatment. A total of 17 different, complete cDNAs were identified from the DMNQ-resistant VSMC clones by PCR amplification and sequencing. The cDNA encoding PP1cgamma1 (catalytic subunit of protein phosphatase 1) was present in several independent DMNQ-resistant VSMC clones. DMNQ increased mitochondrial ROS production, caspase-3/7 activity, DNA fragmentation, and decreased mitochondrial transmembrane potential in VSMC while decreasing PP1cgamma1 activity and expression. Depletion of PP1cgamma1 expression by short hairpin RNA significantly enhanced basal as well as DMNQ-induced VSMC apoptosis. PP1cgamma1 overexpression abrogated DMNQ-induced JNK1 activity, p53 Ser(15) phosphorylation, and Bax expression and protected VSMC against DMNQ-induced apoptosis. In addition, PP1cgamma1 overexpression attenuated DMNQ-induced caspase-3/7 activation and DNA fragmentation. Inhibition of p53 protein expression using small interfering RNA abrogated DMNQ-induced Bax expression and significantly attenuated VSMC apoptosis. Together, these data indicate that PP1cgamma1 overexpression promotes VSMC survival by interfering with JNK1 and p53 phosphorylation cascades involved in apoptosis.

Potent cytotoxicity of the phosphatase inhibitor microcystin LR and microcystin analogues in OATP1B1- and OATP1B3-expressing HeLa cells

Microcystins are a family of cyclic peptides that are potent inhibitors of the protein phosphatase families PP1 and PP2A. Only three human proteins are thought to be able to mediate the hepatic uptake of microcystins (the organic anion-transporting polypeptides OATP1B1, OATP1B3, and OATP1A2), and the predominant hepatic expression of these transporters accounts for the liver-specific toxicity of microcystins. A significant obstacle in the study of microcystins as anticancer drugs is the requirement of specific transport proteins for cellular uptake. We report that OATP1B3 mRNA is up-regulated in non-small cell lung cancer tumors in comparison with normal control tissues. This finding led to the exploration of microcystins as potential anticancer agents. We have developed a HeLa cell model with functional OATP1B1 and OATP1B3 activity. Transiently transfected HeLa cells are over 1,000-fold more sensitive to microcystin LR than the vector-transfected control cells, showing that transporter expression imparts marked selectivity for microcystin cytotoxicity. In addition, microcystin analogues showed variable cytotoxicities in the OATP1B1- and OATP1B3-transfected cells, including two analogues with IC(50) values <1 nmol/L. Cytotoxicity of microcystin analogues seems to correlate to the inhibition of PP2A in these cells and induces rapid cell death as seen by chromatin condensation and cell fragmentation. These studies show that microcystin-induced phosphatase inhibition results in potent cytotoxicity when microcystin compounds can gain intracellular access and are a potent novel class of therapeutic agents for tumors expressing these uptake proteins.

Differential expression of genes associated with cell proliferation and apoptosis induced by okadaic acid during the transformation process of BALB/c 3T3 cells

Okadaic acid (OA) is a tumor promoter in two-stage carcinogenesis experiments. Nevertheless, the effects of OA on cell transformation, cell proliferation and apoptosis vary widely, and the molecular events underlying these effects of OA are not well understood. In the present study, we examined the promoting activity and the associated effects on cell growth and apoptosis mediated by OA in BALB/c 3T3 cells, and evaluated alterations of gene transcriptional expression by microarray analysis. The promoting activity of OA was estimated by a two-stage transformation assay, in which cells were treated first with a low dose of the initiator N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and then with OA for 14 days. It showed that OA, at concentrations of 7.8-31.3 ng/ml, enhanced the transformation of MNNG-treated cells. In the promotion phase, cells exposed to OA (7.8 ng/ml) grew slowly for the first 2 days and subsequently died. As determined by Hoechst 33342 fluorescent dye and Annexin-V/PI dual-colored flow cytometry, OA induced morphologically apoptotic cells and increased the percentage of early apoptotic cells. The gene expression profile induced by OA at five time points in the promotion phase was determined by use of a specific mouse toxicological microarray containing 1796 clones, and a total of 177 differentially expressed genes were identified. By gene ontology analysis, 31 of these were determined to be functionally involved with cell growth and/or maintenance. In this group, numerous genes associated with the cell proliferation and cell cycle progression were down-regulated at early and/or middle time points. Among these was a subset of genes associated with apoptosis, in which Bnip3, Cycs, Casp3 and Bag1 genes are involved in the mitochondrial pathway of apoptosis. Ier3, Mdm2 and Bnip3 genes may be p53 targets. Furthermore, real-time PCR confirmed the expression changes of five genes selected at random from the differentially expressed genes. We conclude that OA induces cell growth inhibition and apoptosis in the two-stage, MNNG-initiated transformation of BALB/c 3T3 cells. The results of gene expression profile analysis imply that multiple molecular pathways are involved in OA-induced proliferation inhibition and apoptosis. Mitochondrial and p53-associated apoptotic pathways also may contribute to OA-induced apoptosis.

Leucine carboxyl methyltransferase-1 is necessary for normal progression through mitosis in mammalian cells

Protein phosphatase 2A (PP2A) is a multifunctional phosphatase that plays important roles in many cellular processes including regulation of cell cycle and apoptosis. Because PP2A is involved in so many diverse processes, it is highly regulated by both non-covalent and covalent mechanisms that are still being defined. In this study we have investigated the importance of leucine carboxyl methyltransferase-1 (LCMT-1) for PP2A methylation and cell function. We show that reduction of LCMT-1 protein levels by small hairpin RNAs causes up to a 70% reduction in PP2A methylation in HeLa cells, indicating that LCMT-1 is the major mammalian PP2A methyltransferase. In addition, LCMT-1 knockdown reduced the formation of PP2A heterotrimers containing the Balpha regulatory subunit and, in a subset of the cells, induced apoptosis, characterized by caspase activation, nuclear condensation/fragmentation, and membrane blebbing. Knockdown of the PP2A Balpha regulatory subunit induced a similar amount of apoptosis, suggesting that LCMT-1 induces apoptosis in part by disrupting the formation of PP2A(BalphaAC) heterotrimers. Treatment with a pan-caspase inhibitor partially rescued cells from apoptosis induced by LCMT-1 or Balpha knockdown. LCMT-1 knockdown cells and Balpha knockdown cells were more sensitive to the spindle-targeting drug nocodazole, suggesting that LCMT-1 and Balpha are important for spindle checkpoint. Treatment of LCMT-1 and Balpha knockdown cells with thymidine dramatically reduced cell death, presumably by blocking progression through mitosis. Consistent with these results, homozygous gene trap knock-out of LCMT-1 in mice resulted in embryonic lethality. Collectively, our results indicate that LCMT-1 is important for normal progression through mitosis and cell survival and is essential for embryonic development in mice.

An endogenous serine/threonine protein phosphatase inhibitor, G-substrate, reduces vulnerability in models of Parkinson's disease

Relative neuronal vulnerability is a universal yet poorly understood feature of neurodegenerative diseases. In Parkinson's disease, dopaminergic (DA) neurons in the substantia nigra (SN) (A9) are particularly vulnerable, whereas adjacent DA neurons within the ventral tegmental area (A10) are essentially spared. Our previous laser capture microdissection and microarray study (Chung et al., 2005) demonstrated that molecular differences between these DA neurons may underlie their differential vulnerability. Here we show that G-substrate, an endogenous inhibitor of Ser/Thr protein phosphatases, exhibits higher expression in A10 compared with A9 DA neurons in both rodent and human midbrain. Overexpression of G-substrate protected dopaminergic BE(2)-M17 cells against toxins, including 6-OHDA and MG-132 (carbobenzoxy-L-leucyl- L-leucyl-L-leucinal), whereas RNA interference (RNAi)-mediated knockdown of endogenous G-substrate increased their vulnerability to these toxins. G-substrate reduced 6-OHDA-mediated protein phosphatase 2A (PP2A) activation in vitro and increased phosphorylated levels of PP2A targets including Akt, glycogen synthase kinase 3beta, and extracellular signal-regulated kinase 2 but not p38. RNAi to Akt diminished the protective effect of G-substrate against 6-OHDA. In vivo, lentiviral delivery of G-substrate to the rat SN increased baseline levels of phosphorylated Akt and protected A9 DA neurons from 6-OHDA-induced toxicity. These results suggest that inherent differences in the levels of G-substrate contribute to the differential vulnerability of DA neurons and that enhancing G-substrate levels may be a neuroprotective strategy for the vulnerable A9 (SN) DA neurons in Parkinson's disease.

Proenkephalin assists stress-activated apoptosis through transcriptional repression of NF-kappaB- and p53-regulated gene targets

The respective pro- and antiapoptotic functions of the transcription factors p53 and nuclear factor kappaB (NF-kappaB), and their potential impact on tumorigenesis and response to tumor therapy are well recognized. The capacity of the RelA(p65) subunit of NF-kappaB to specify a pro-apoptotic outcome in response to some stimuli is less well recognized, but needs to be understood if rational manipulation of the NF-kappaB pathway is to be deployed in cancer therapy. In this report, we provide evidence that the growth-responsive nuclear protein, proenkephalin (Penk), is required, in part, for apoptosis induction, in response to activation or overexpression of p53 and RelA(p65). We describe UV-C-inducible physical associations between endogenous Penk and p53 and RelA(p65) in mammalian cell lines. Depletion of Penk by RNA interference (RNAi) substantially preserves viable cell number following exposure to UV-C irradiation or hydrogen peroxide and confers transient protection in cells exposed to the genotoxin etoposide. In virally transformed and human tumor cell lines, overexpression of nuclear Penk with overabundant or activated p53, or RelA(p65) even in the absence of p53, enhances apoptosis to the point of synergy. We have further shown that Penk depletion by RNAi substantially derepresses transcription of a range of antiapoptotic gene targets previously implicated in repression-mediated apoptosis induction by NF-kappaB and p53. Physical association of endogenous Penk with the transcriptional co-repressor histone deacetylase suggests that it may be a component of a transcriptional repression complex that contributes to a pro-apoptotic outcome, following activation of the NF-kappaB and p53 pathways, and could therefore help to facilitate an antitumor response to a broad range of agents.

Transforming Activities of JC Virus Early Proteins

Polyomaviruses, as their name indicates, are viruses capable of inducing a variety of tumors in vivo. Members of this family, including the human JC and BK viruses (JCV, BKV), and the better characterized mouse polyomavirus and simian virus 40 (SV40), are small DNA viruses that commandeer a cell's molecular machinery to reproduce themselves. Studies of these virus-host interactions have greatly enhanced our understanding of a wide range of phenomena from cellular processes (e.g., DNA replication and transcription) to viral oncogenesis. The current chapter will focus upon the five known JCV early proteins and the contributions each makes to the oncogenic process (transformation) when expressed in cultured cells. Where appropriate, gaps in our understanding of JCV protein function will be supplanted with information obtained from the study of SV40 and BKV.

Modulation of hippocampal calcium signalling and plasticity by serine/threonine protein phosphatases

Kinases and phosphatases act antagonistically to maintain physiological phosphorylation/dephosphorylation at numerous intracellular sites critical for neuronal signalling. In this study, it was found that inhibition of serine/threonine phosphatases by exposure of hippocampal slices to okadaic acid (OA) or cantharidin (CA; 100 nmol/L) for 2 h resulted in reduced basal synaptic transmission and blocked the induction of synaptic plasticity in the form of long-term potentiation as determined by electrophysiological analysis. Fura-2 Ca(2+) imaging revealed a bidirectional modulation of N-methyl-D-aspartate (NMDA) -mediated Ca(2+) responses and reduced KCl-mediated Ca(2+) responses in neonatal cultured hippocampal neurons after phosphatase inhibition. While OA inhibited NMDA-induced Ca(2+) influx both acutely and after incubation, CA-enhanced receptor-mediated Ca(2+) signalling at low concentrations (1 nmol/L) but reduced NMDA and KCl-mediated Ca(2+) responses at higher concentrations (100 nmol/L). Changes in Ca(2+) signalling were accompanied by increased phosphorylation of cytoskeletal proteins tau and neurofilament and the NMDA receptor subunit NR1 in selective treatments. Incubation with OA (100 nmol/L) also led to the disruption of the microtubule network. This study highlights novel signalling effects of prolonged inhibition of protein phosphatases and suggests reduced post-synaptic signalling as a major mechanism for basal synaptic transmission and long-term potentiation impairments.

Expression and function of protein phosphatase PP2A in malignant testicular germ cell tumours

Testicular germ cell tumours (TGCT) represent the most common malignancy in young males. We reported previously that two prototype members of the mitogen-activated protein kinase (MAPK) family, the MAPK ERK kinase (MEK) and extracellular signal-regulated kinase (ERK), are inactive in malignant testicular germ cells and become active after drug stimulation, leading to apoptosis of tumour cells. In this study, we asked whether the protein phosphatase PP2A, a known inhibitor of the MEK-ERK pathway, participates in the proliferation and/or apoptosis of primary TGCT (n = 48) as well as two TGCT cell lines (NTERA and NCCIT). Quantitative RT-PCR, immunohistochemistry, western blot analyses and phosphatase assay indicate that primary TGCT as well as TGCT cell lines express PP2A and that PP2A is active in TGCT cell lines. The inhibition of PP2A by application of two PP2A inhibitors, cantharidic acid (CA) and okadaic acid (OA), results in a significant increase in caspase-3-mediated apoptosis of TGCT cell lines. Thereby, PP2A inhibition was accompanied by phosphorylation and activation of MEK and ERK. Functional assays using the MEK inhibitor PD98059 demonstrated that the phosphorylation of MEK and ERK was required for the induction of caspase-3-mediated apoptosis of malignant germ cells. Thus, our data suggest that inhibition of PP2A mediates its apoptosis-inducing effect on TGCT through activation of the MEK-ERK signalling pathway that leads to caspase-3-mediated apoptosis of tumour cells. In addition our results support previous observations that PP2A exerts an anti-apoptotic effect on malignant tumour cells.

Protein serine/threonine phosphatase-1 dephosphorylates p53 at Ser-15 and Ser-37 to modulate its transcriptional and apoptotic activities

We have previously demonstrated that the serine/threonine protein phosphatase-1 (PP-1) plays an important role in promoting cell survival. However, the molecular mechanisms by which PP-1 promotes survival remain largely unknown. In the present study, we provide evidence to show that PP-1 can directly dephosphorylate a master regulator of apoptosis, p53, to negatively modulate its transcriptional and apoptotic activities, and thus to promote cell survival. As a transcriptional factor, the function of p53 can be greatly regulated by phosphorylation and dephosphorylation. While the kinases responsible for phosphorylation of the 17 serine/threonine sites have been identified, the dephosphorylation of these sites remains largely unknown. In the present study, we demonstrate that PP-1 can dephosphorylate p53 at Ser-15 and Ser-37 through co-immunoprecipitation, in vitro and in vivo dephosphorylation assays, overexpression and silence of the gene encoding the catalytic subunit for PP-1. We further show that mutations mimicking constitutive dephosphorylation or phosphorylation of p53 at these sites attenuate or enhance its transcriptional activity, respectively. As a result of the changed p53 activity, expression of the downstream apoptosis-related genes such as bcl-2 and bax is accordingly altered and the apoptotic events are either largely abrogated or enhanced. Thus, our results demonstrate that PP-1 directly dephosphorylates p53, and dephosphorylation of p53 has as important impact on its functions as phosphorylation does. In addition, our results reveal that one of the molecular mechanisms by which PP-1 promotes cell survival is to dephosphorylate p53, and thus negatively regulate p53-dependent death pathway.

Inhibition of PP2A, but not PP5, mediates p53 activation by low levels of okadaic acid in rat liver epithelial cells

The microbial toxin okadaic acid (OA) specifically inhibits PPP-type ser/thr protein phosphatases. OA is an established tumor promoter with numerous cellular effects that include p53-mediated cell cycle arrest. In T51B rat liver epithelial cells, a model useful for tumor promotion studies, p53 activation is induced by tumor-promoting (low nanomolar) concentrations of OA. Two phosphatases sensitive to these concentrations of OA, PP2A and protein phosphatase 5 (PP5), have been implicated as negative regulators of p53. In this study we examined the respective roles of these phosphatases in p53 activation in non-neoplastic T51B cells. Increases in p53 activity were deduced from levels of p21 (cip1) and/or the rat orthologue of mdm2, two p53-regulated gene products whose induction was blocked by siRNA-mediated knockdown of p53. As observed with 10 nM OA, both phospho-ser15-p53 levels and p53 activity were increased by 10 microM fostriecin or SV40 small t-antigen. Both of these treatments selectively inhibit PP2A but not PP5. siRNA-mediated knockdown of PP2A, but not PP5, also increased p53 activity. Finally, adenoviral-mediated over-expression of an OA-resistant form of PP5 did not prevent increased phospho-ser15-p53, p53 protein, or p53 activity caused by 10 nM OA. Together these results indicate that PP5 blockade is not responsible for OA-induced p53 activation and G1 arrest in T51B cells. In contrast, specific blockade of PP2A mimics p53-related responses to OA in T51B cells, suggesting that PP2A is the target for this response to OA.

Altered expression of p53, Bcl-2 and Bax induced by microcystin-LR in vivo and in vitro

It has been reported that MC-LR could induce apoptosis in a variety of cell types. Although the induction of oxidative stress and mitochondrial alteration played critical role in MC-LR induced apoptosis, but the exact mechanisms of MC-LR induced apoptosis are still unknown. In spite of extensive studies on MC-LR mediated cell damages, there is little information on the protein expression of p53 and Bcl-2, Bax in vivo and in vitro, which are vital regulator of apoptosis in response to a variety of stimuli. The present study was undertaken to determine the expression level of p53 and Bcl-2, Bax in cultured hepatocytes and rat liver tissues. The results show that MC-LR can increase the expression of p53 and Bax significantly both in vivo and in vitro, however, MC-LR can only decrease the expression of Bcl-2 significantly in vitro and there is no difference observed in vivo. It can be concluded that the expression of p53, Bcl-2 and Bax are involved in the regulation of MC-LR induced apoptosis.

BRCA1-mediated G2/M cell cycle arrest requires ERK1/2 kinase activation

Germline mutations in the BRCA1 gene are associated with an increased susceptibility to the development of breast and ovarian cancers. Evidence suggests that BRCA1 protein plays a key role in mediating DNA damage-induced checkpoint responses. Several studies have shown that ectopic expression of BRCA1 in human cells can trigger cellular responses similar to those induced by DNA damage, including G2/M cell cycle arrest and apoptosis. While the effects of ectopic BRCA1 expression on the G2/M transition and apoptosis have been extensively studied, the factors that dictate the balance between these two responses remain poorly understood. We have recently shown that ectopic expression of BRCA1 in MCF-7 human breast cancer cells resulted in activation of extracellular signal-regulated protein kinase 1 and 2 (ERK1/2) and G2/M cell cycle arrest. Furthermore, inhibition of BRCA1-induced ERK1/2 activation using mitogen-activated protein kinase kinase 1 and 2 (MEK1/2)-specific inhibitors resulted in increased apoptosis, suggesting a potential role of ERK1/2 kinases in BRCA1-mediated G2/M checkpoint response. In this study, we assessed the role of ERK1/2 kinases in the regulation of BRCA1-mediated G2/M cell cycle arrest. Results indicate that BRCA1-induced G2/M cell cycle arrest and ERK1/2 activation correlate with changes in the level and/or activity of several key regulators of the G2/M checkpoint, including activation of Chk1 and Wee1 kinases, induction of 14-3-3, and down-regulation of Cdc25C. Furthermore, inhibition of ERK1/2 kinases using MEK1/2-specific inhibitors results in a marked attenuation of the BRCA1-induced G2/M arrest. Biochemical studies established that ERK1/2 inhibition abolished the effects of BRCA1 on components of the G2/M checkpoint, including regulation of Cdc25C expression and activation of Wee1 and Chk1 kinases. These results implicate a critical role of ERK1/2 signaling in the regulation of BRCA1 function on controlling the G2/M checkpoint responses.

Asiatic acid induces apoptosis in SK-MEL-2 human melanoma cells

Asiatic acid (AA) is a pentacyclic triterpene found in Centella asiatica. In the present study, the mechanism of anticancer effect of AA on skin cancer was investigated. AA decreased viability and induced apoptosis in human melanoma SK-MEL-2 cells in a time- and dose-dependent manner. AA also markedly increased intracellular reactive oxygen species (ROS) level and enhanced the expression of Bax but not Bcl-2 protein in the cells. In addition, AA-induced activation of caspase-3 activity in a dose-dependent manner. Pretreatment with Trolox, an antioxidant, significantly blocked the induction of Bax and activation of caspase-3 in AA-treated cells. Furthermore, Ac-DEVD-CHO, a specific caspase-3 inhibitor, and Trolox prevented the AA-induced apoptosis. AA did not elevate p53 nuclear protein levels that are present in a mutant form in SK-MEL-2 cells. These results suggest that AA-induced apoptosis may be mediated through generation of ROS, alteration of Bax/Bcl-2 ratio and activation of caspase-3, but p53-independent. These results further suggest that AA may be a good candidate for the therapeutic intervention of human skin cancer.

p53 is a regulator of macrophage differentiation

While it is well accepted that p53 plays a role in apoptosis, less is known as to its involvement in cell differentiation. Here we show that wild-type p53 facilitates IL-6-dependent macrophage differentiation. Treatment of M1/2 cells expressing the temperature-sensitive p53 143 (Val to Ala) mutant, at the wild-type conformation, facilitated the appearance of mature macrophages that exhibited phagocytic activity. Enhancement of differentiation by the p53 143 (Val to Ala) in the wild-type conformation was coupled with the inhibition of apoptosis induction by this protein. In agreement with previous studies, we found that p53 levels were reduced during p53-dependent macrophage differentiation. This occurred when p53 levels before IL-6 stimuli were high. Interestingly, the p53 143 (Val to Ala) protein, at the mutant conformation, enhanced macrophage differentiation, as did the wild-type conformation, whereas the p53 273 (Arg to His) core mutant exerted an inhibitory effect on this pathway. The transcription-deficient p53 molecules, p53 (22-23) and p53 22,23,143, could not induce p53-dependent differentiation. Moreover, the p53 (22-23) protein inhibited the p53-independent differentiation pathway. Interestingly, the p53 (22-23) protein not only blocked IL-6-mediated differentiation, but also induced significant apoptotic cell death, upon IL-6 stimulation. Taken together, our data show that wild-type p53 enhances macrophage differentiation, while various p53 mutant types exert different effects on this differentiation pathway.

Inhibitors of protein phosphatases 1 and 2A differentially prevent intrinsic and extrinsic apoptosis pathways

Inhibitors of serine/threonine protein phosphatases can inhibit apoptosis. We investigated which protein phosphatases are critical for this protection using calyculin A, okadaic acid, and tautomycin. All three phosphatase inhibitors prevented anisomycin-induced apoptosis in leukemia cell models. In vitro, calyculin A does not discriminate between PP1 and PP2A, while okadaic acid and tautomycin are more selective for PP2A and PP1, respectively. Increased phosphorylation of endogenous marker proteins was used to define concentrations that inhibited each phosphatase in cells. Concentrations of each inhibitor that prevented anisomycin-induced apoptosis correlated with inhibition of PP2A. The inhibitors prevented Bax translocation to mitochondria, indicating inhibition upstream of mitochondria. Tautomycin and calyculin A, but not okadaic acid, also prevented apoptosis induced through the CD95/Fas death receptor, and this protection correlated with inhibition of PP1. The inhibitors prevented Fas receptor oligomerization, FADD recruitment, and caspase 8 activation. The differential effects of PP1 and PP2A in protection from death receptor and mitochondrial-mediated pathways of death, respectively, may help one to define critical steps in each pathway, and regulatory roles for serine/threonine phosphatases in apoptosis.

p53's believe it or not: lessons on transcription-independent death

The requirement for an intact p53 signaling pathway to sense tumor-promoting DNA damage is evident from over 20 years of molecular, cellular, and whole animal studies. Without a doubt, p53's major contribution in maintaining the genomic integrity of multicellular organisms is through transcriptional regulation of genes required for cell cycle arrest, DNA repair, and apoptosis. Nonetheless, evidence is mounting that p53 has an extranuclear role in the cytoplasm to induce apoptosis, perhaps coupled to its transcriptional effects, or conceivably at instances when transcription is not optimal or possible. This phenomenon, transcription-independent p53-induced apoptosis (TIPA), has been described for almost 10 years, yet little is known about mechanisms by which p53 can directly engage the apoptotic cascade in the absence of transcription. Here we will explore what is currently known about TIPA learned from various p53 mutants and truncations, along with discussing several proposed mechanisms.

Mitochondrial p53 levels parallel total p53 levels independent of stress response in human colorectal carcinoma and glioblastoma cells

p53 can eliminate damaged cells through the induction of mitochondria-mediated apoptosis. Recent observations have provided strong evidence that a fraction of total p53 translocates to mitochondria specifically in response to a death stimulus. Unexpectedly, mutant p53, which is expressed at much higher levels than wild type in unstressed cells, is apparently always present at the mitochondria, independent of apoptotic signal. This prompted us to ask whether cell lines with intact p53-dependent apoptosis and cell cycle arrest pathways exist in which the mitochondrial localization of wild-type p53, like that of mutant, is independent of a death stimulus and instead, correlates with the total p53 levels. Here, we document that human HCT116 colorectal carcinoma cells treated with adriamycin or 5-fluorouracil (5FU) can accumulate total p53 to equally high levels, and mitochondrial p53 to proportionate levels, although only 5FU treatment provoked p53-dependent apoptosis. Along the same line, HCT116 derivatives with increased basal p53 levels, and glioblastoma cells with a doxycycline-inducible p53, also revealed proportionate mitochondrial p53 levels, and even unstressed HCT116 cells had some p53 located at the mitochondria. Finally, mitochondrial and total p53 showed distinct post-translational modifications. Thus, cell lines exist in which the mitochondrial p53 levels parallel total levels independent of apoptosis.

A modified p53 enhances apoptosis in sarcoma cell lines mediated by doxorubicin

Mdm2 is frequently overexpressed in sarcoma cells and may contribute to drug resistance by increasing p53 degradation. We investigated the induction of apoptosis in sarcoma cells via adenovirus-mediated gene transfer of wild-type p53 and two modified p53 genes, p53 14/19 and p53 22/23, whose protein products are resistant to Mdm2-mediated degradation. We found that adenovirus-wt p53 (Ad-wt p53) induces significant apoptosis in HT1080 fibrosarcoma cells expressing low levels of Mdm2, but fails to induce apoptosis in SJSA osteosarcoma cells expressing high levels of Mdm2. In contrast, Ad-p53 14/19 induces significant apoptosis in both cell lines. Interestingly, Ad-p53 22/23, a vector encoding a transcription-defective p53 mutant, causes limited apoptosis in both cell lines. We demonstrate that doxorubicin induces phosphorylation of both wt p53 and p53 14/19 protein at multiple sites. We tested the efficacy of doxorubicin and cisplatin with either Ad-wt p53, Ad-p53 22/23 or Ad-p53 14/19. SJSA cells, although harbouring endogenous wt p53, did not undergo significant apoptosis following doxorubicin or cisplatin exposure alone or combined with Ad-wt p53. In contrast, doxorubicin or cisplatin plus Ad-p53 14/19 induced significant apoptosis. Gene transfer of p53 14/19 in combination with the administration of doxorubicin or cisplatin is a potential therapeutic approach for cancers expressing high levels of Mdm2.

Role and regulation of p53 in depolarization-induced neuronal death

The tumor suppressor gene p53 is a potent transcriptional regulator for genes involved in many cellular activities including cell cycle arrest and apoptosis. In this study, we examined the role of p53 in neuronal death induced by the sodium channel modulator veratridine. We also analyzed the involvement of Ca2+, mitochondria and reactive oxygen species in p53 activation. Exposure of hippocampal neurons to veratridine (0.3-100 microM) resulted in a dose-dependent neuronal death, measured 24 h after treatment. p53-Like immunoreactivity, undetectable in neurons under control conditions, was observed in about 25% of neurons, 7 h after veratridine exposure. Treatments that modified the alkaloid-induced Ca2+ influx including tetrodotoxin or Ca2+ removal, prevented either veratridine-induced cell death or p53 immunoreactivity. Mitochondria were involved in veratridine-induced cell death, as the alkaloid collapsed inner transmembrane mitochondrial potential in a Ca2+ influx dependent manner. Treatments of neuronal cultures with the permeability transitory pore blockers cyclosporin A and bongkrekic acid prevented veratridine-induced p53 immunoreactivity and neuronal death, placing mitochondria upstream of veratridine-induced p53 immunoreactivity. Reactive oxygen species also participated in veratridine-induced neurotoxicity and p53 activation. Antisense knockdown of p53 resulted in a significant increase in neuronal survival after veratridine treatment. This protective effect was maintained on N-methyl-D-aspartate or ischemia-induced death but not on staurosporine cytotoxicity. These results together suggest that p53-expression is involved in veratridine-induced neuronal death and that p53 might be a link between toxic stimuli of different types and neuronal death.

Phosphorylation of p53 at serine 37 is important for transcriptional activity and regulation in response to DNA damage

The p53 tumor suppressor protein plays a critical role in mediating cellular response to stress. Upon DNA damage, post-translational modifications stabilize and activate this nuclear phosphoprotein. To determine the effect of phosphorylation site mutants in the context of the whole p53 protein, we performed reporter assays in p53 and MDM2 knockout mouse embryonic fibroblasts transfected with full-length p53 constructs. We show that mutation of S37 causes a decrease in p53 transcriptional activity compared to wild-type p53. Our data further suggest that the dephosphorylation of p53 at S37 is a regulated event involving protein phosphatase 2A (PP2A). Coimmunoprecipitation and immunofluorescence microscopy studies demonstrate that PP2A and p53 associate with one another in vivo following gamma-irradiation. Consistent with these observations, phosphorylated S37 accumulates in cell extracts prepared from gamma-irradiated Molt-4 cells in the presence of okadaic acid. Furthermore, in vitro phosphatase assays show that PP2A dephosphorylates p53 at S37. These results suggest that dephosphorylation of p53 at S37 plays a role in the transcriptional regulation of the p53 protein in response to DNA damage.

Microcystin-LR and okadaic acid-induced cellular effects: a dualistic response

Microcystins, potent heptapeptide hepatotoxins produced by certain bloom-forming cyanobacteria, are strong protein phosphatase inhibitors. They covalently bind the serine/threonine protein phosphatases 1 and 2A (PP1 and PP2A), thereby influencing regulation of cellular protein phosphorylation. The paralytic shellfish poison, okadaic acid, is also a potent inhibitor of these PPs. Inhibition of PP1 and PP2A has a dualistic effect on cells exposed to okadaic acid or microcystin-LR, with both apoptosis and increased cellular proliferation being reported. This review summarises the existing data on the molecular effects of microcystin-LR inhibition of PP1 and PP2A both in vivo and in vitro, and where possible, compares this to the action of okadaic acid.

Iron deprivation induces apoptosis independently of p53 in human and murine tumour cells

Iron deprivation induces apoptosis in some sensitive cultured tumour cells, while other cells are resistant. In order to elucidate the mechanisms involved in apoptosis induction by iron deprivation, we studied the expression of p53 and the expression of selected p53-regulated genes. To discriminate between changes coupled only with iron deprivation and changes involved in apoptosis induction by iron deprivation, we compared the expression of the genes in sensitive (human Raji, mouse 38C13) versus resistant (human HeLa, mouse EL4) cells under iron deprivation. Iron deprivation was achieved by incubation in a defined iron-free medium. The level of p53 mRNA decreased significantly under iron deprivation in sensitive cells, but it did not change in resistant cells. On the contrary, the level of the p53 protein under iron deprivation was slightly increased in sensitive cells while it was not changed in resistant cells. The activity of p53 was assessed by the expression of selected p53-regulated targets, i.e. p21(WAF1/CIP1) gene, mdm2, bcl-2 and bax. We did not detect any relevant change in mRNA levels as well as in protein levels of these genes under iron deprivation with the exception of p21(WAF1/CIP1). We detected a significant increase in the level of p21 mRNA in both (sensitive and resistant) mouse cell lines tested, however, we did not find any change in both (sensitive and resistant) human cell lines. Moreover, the p21(WAF1/CIP1) protein was accumulated in mouse-sensitive 38C13 cells under iron deprivation while all other cell lines tested, including human-sensitive cell line Raji, did not show any accumulation of p21(WAF1/CIP1) protein. It seems that the p21(WAF1/CIP1) mRNA, as well as protein accumulation, is not specifically coupled with apoptosis induction by iron deprivation and that it is rather cell-line specific. Taken together, we suggest that iron deprivation induces apoptosis at least in some cell types independently of the p53 pathway.

Irod/Ian5: an inhibitor of gamma-radiation- and okadaic acid-induced apoptosis

Protein phosphatase-directed toxins such as okadaic acid (OA) are general apoptosis inducers. We show that a protein (inhibitor of radiation- and OA-induced apoptosis, Irod/Ian5), belonging to the family of immune-associated nucleotide binding proteins, protected Jurkat T-cells against OA- and gamma-radiation-induced apoptosis. Unlike previously described antiapoptotic proteins Irod/Ian5 did not protect against anti-Fas, tumor necrosis factor-alpha, staurosporine, UV-light, or a number of chemotherapeutic drugs. Irod antagonized a calmodulin-dependent protein kinase II-dependent step upstream of activation of caspase 3. Irod has predicted GTP-binding, coiled-coil, and membrane binding domains. Irod localized to the centrosomal/Golgi/endoplasmic reticulum compartment. Deletion of either the C-terminal membrane binding domain or the N-terminal GTP-binding domain did not affect the antiapoptotic function of Irod, nor the centrosomal localization. The middle part of Irod, containing the coiled-coil domain, was therefore responsible for centrosomal anchoring and resistance toward death. Being widely expressed and able to protect also nonimmune cells, the function of Irod may not be limited to the immune system. The function and localization of Irod indicate that the centrosome and calmodulin-dependent protein kinase II may have important roles in apoptosis signaling.

Effect of microcystin-LR and cyanobacterial extract from Polish reservoir of drinking water on cell cycle progression, mitotic spindle, and apoptosis in CHO-K1 cells

Microcystin-LR is a cyanobacterial toxin possessing a potent tumor-promoting activity mediated through inhibition of protein phosphatases PP1 and PP2A. Because these enzymes are involved in fundamental cell processes, we decided to examine the influence of microcystin-LR on cell cycle progression, onset of anaphase, segregation of chromosomes by the mitotic spindle, and apoptosis in Chinese hamster ovary (CHO-K1) cells. Cells were incubated with 25, 50, and 100 microM of pure microcystin-LR and a cyanobacterial extract for 14, 18, and 22 h. Giemsa staining of cells treated with these toxins revealed a dose- and time-dependent increase of mitotic indices, accumulation of abnormal G(2)/M figures with hypercondensed chromosomes, abnormal anaphases with defective chromosome separation, and polyploid cells. Because spindle checkpoint is a fundamental regulatory mechanism that assures the onset of anaphase and subsequent exit from mitosis, we examined the spindle organization in microcystin-treated cells. The majority of the mitotic cells showed monopolar and multipolar mitotic spindles (multiple asters). Microtubule bundles were present in interphase cells. Our results indicate that microcystin-LR induces apoptosis and necrosis in a dose- and time-dependent manner and that the frequency of dead cells cells is positively correlated with the frequency of polyploid cells.

Inhibition of nuclear protein phosphatase activity in mouse hepatocytes by the cyanobacterial toxin microcystin-LR

Microcystin-LR (MCLR) is a cyanobacterial hepatotoxin and protein phosphatase inhibitor that contaminates water reservoirs worldwide. MCLR localizes to the cytosol of hepatocytes, however, immunohistochemical studies indicate that it accumulates in the nucleus. MCLR toxicosis is associated with decreased hepatic protein phosphatase activity, but effects in nuclear protein phosphatase activity have not been investigated. Balb/c mice were given lethal (100 microg/kg) or sublethal (12, 23 and 45 microg/kg) i.p. doses of MCLR and hepatic nuclear extracts were analyzed for protein phosphatase 1 and 2A activity. There was profound inhibition of nuclear protein phosphatase activity within 50 min of lethal dosing, however an inhibition was not detected with sublethal doses. MCLR immunohistochemistry revealed widespread lobular staining in the lethal group and centrilobular staining in the sublethal groups. At the cellular level there was nuclear and cytoplasmic staining of equal intensity. As an indicator of nuclear protein phosphatase activity, the phosphorylation of p53, a nuclear phosphoprotein and known substrate for protein phosphatases 1 and 2A, was evaluated. Balb/c mice were treated with sublethal doses of MCLR or saline vehicle after induction of hepatic p53 by the DNA damaging agent diethylnitrosamine (DEN). P53 was immunoprecipitated and probed with phosphoserine specific antibodies by Western blotting. There was greater phosphoserine reactivity of p53 protein in animals treated with MCLR relative to saline treated controls, consistent with increased phosphorylation of serine sites. It is concluded that an interaction of this toxin with nuclear protein phosphatases occurs within 50 min of lethal dosing, which leads to a profound inhibition of enzymatic activity. Even sublethal doses of MCLR that do not result in significant inhibition of activity in bulk nuclei, result in detectable changes in phosphorylation of p53.

Caspase inhibition shifts neuroepithelioma cell response to okadaic acid from apoptosis to an apoptotic-like form of death

We have previously shown that the protein phosphatase inhibitor okadaic acid (OA) induces caspase-3 activation and apoptosis in CHP-100 human neuroepithelioma cells. Herein we provide a more general picture of the effects brought about by OA in this system, also investigating whether caspase activation is necessary for apoptosis induction. We report that incubation for 24 h with 10 nM OA induced a large fraction of the cell population to undergo premature chromosome condensation (PCC) or mitotic arrest, but not apoptosis. The former two effects were also observed after cell treatment with 20 nM OA; however, at this concentration, typical apoptotic cells were also detected, characterized by pycnotic and fragmented nuclei. Occurrence of the above-mentioned apoptotic figures turned extensive at 100 nM OA. The pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD.fmk, 100 microM) fully prevented apoptosis induced by 20 nM OA, increasing PCC incidence. Conversely, 100 nM OA induced an apoptotic-like phenotype, even in the presence of Z-VAD.fmk: in this case, however, nuclei, albeit pycnotic, displayed morphological characteristics distinct from those of typical apoptotic cells; moreover, as assessed by flow cytometry, they were largely unfragmented. The reported OA effects occurred in a setting in which neither p53 nor p21(Cip1/Waf1) was upregulated, thus ruling out a role for these proteins in apoptosis induction. On the other hand, apoptotic doses of OA induced a shift of the retinoblastoma gene product to the hypophosphorylated state and its downregulation by a caspase-dependent mechanism.