Guanine nucleotide biosynthesis is regulated by the cellular p53 concentration

Nucleotide biosynthesis links glutathione metabolism to ferroptosis sensitivity

The tumor suppressor protein p53 inhibits ferroptosis by reducing the consumption of glutathione in nucleotide biosynthesis.

Nucleotide synthesis is a metabolically demanding process essential for DNA replication and other processes in the cell. Several anticancer drugs that inhibit nucleotide metabolism induce apoptosis. How inhibition of nucleotide metabolism impacts non-apoptotic cell death is less clear. Here, we report that inhibition of nucleotide metabolism by the p53 pathway is sufficient to suppress the non-apoptotic cell death process of ferroptosis. Mechanistically, stabilization of wild-type p53 and induction of the p53 target gene CDKN1A (p21) leads to decreased expression of the ribonucleotide reductase (RNR) subunits RRM1 and RRM2. RNR is the rate-limiting enzyme of de novo nucleotide synthesis that reduces ribonucleotides to deoxyribonucleotides in a glutathione-dependent manner. Direct inhibition of RNR results in conservation of intracellular glutathione, limiting the accumulation of toxic lipid peroxides and preventing the onset of ferroptosis in response to cystine deprivation. These results support a mechanism linking p53-dependent regulation of nucleotide metabolism to non-apoptotic cell death.

Integrative omics analysis of p53-dependent regulation of metabolism

Accumulated evidence in the last decade implies that regulation of metabolism by p53 represents a reviving mechanism vital to prevent tumorigenesis. To gain a more in-depth understanding of metabolic regulation by baseline levels of p53, we employed both metabolomics and transcriptomics analysis with human colon cancer cell-line HCT116 depleted of p53. Metabolomics analyses with UPLC/quadrupole time-of-flight mass spectrometry identified 283 significantly changed metabolites including 138 important metabolites. Transcriptomics analysis with microarray revealed 1317 differentially expressed genes. By integrated analysis of both omics data, we found nucleotides metabolism and sulfur-related metabolism are of great importance. Our study provided a pilot comprehensive view of the metabolism regulated by p53 and suggests several potential p53 targets in metabolism for further study.

Fine-tuning of the cellular signaling pathways by intracellular GTP levels

It has become increasingly evident that among purine nucleotides, guanine based nucleotides specially guanosine-5'-triphosphate (GTP) serve as an important and independent regulatory factors for development and diverse cellular functions such as differentiation, metabolism, proliferation and survival in multiple tissues. In this brief review, it has been provided selective outline related to delicate regulation of signaling pathways by guanosine based nucleotides as intracellular signaling molecules. Although the exact mode of action of theses nucleotides in many biological processes and signaling pathways is still elusive, it has become well clear that intracellular guanosine based nucleotides content rather than adenosine based nucleotides could modulate the intensity and duration of signaling which ultimately impact on cell's fate. It opens an entirely new perspective for developing new and potential therapeutic strategies to combat diseases like cancer, hypoxia, etc.

Mammary stem cells: expansion and animal productivity

Identification and characterization of mammary stem cells and progenitor cells from dairy animals is important in the understanding of mammogenesis, tissue turnover, lactation persistency and regenerative therapy. It has been realized by many investigators that altered lactation, long dry periods (non-milking period between two consecutive lactation cycles), abrupt cessation of lactation (common in water buffaloes) and disease conditions like mastitis, greatly reduce milk yield thus render huge financial losses within the dairy sector. Cellular manipulation of specialized cell types within the mammary gland, called mammary stem cells (MaSCs)/progenitor cells, might provide potential solutions to these problems and may improve milk production. In addition, MaSCs/progenitor cells could be used in regenerative therapy against tissue damage caused by mastitis. This review discusses methods of MaSC/progenitor cell manipulation and their mechanisms in bovine and caprine animals. Author believes that intervention of MaSCs/progenitor cells could lessen the huge financial losses to the dairy industry globally.

The mortal strand hypothesis: non-random chromosome inheritance and the biased segregation of damaged DNA

If a eukaryotic cell is to reproduce, it must duplicate its genetic information in the form of DNA, and faithfully segregate that information during a complex process of cell division. During this division process, the resulting cells inherit one, and only one, copy of each chromosome. Over thirty years ago, it was predicted that the segregation of sister chromosomes could occur non-randomly, such that a daughter cell would preferentially inherit one of the two sister chromosomes according to some characteristic of that chromosome's template DNA strand. Although this prediction has been confirmed in studies of various cell-types, we know little of both the mechanism by which the asymmetric inheritance occurs and the significance it has to cells. In this essay, we propose a new model of non-random chromosome segregation-the mortal strand hypothesis-and discuss tests of the model that will provide insight into the molecular choreography of this intriguing phenomenon.

ATP-binding cassette G-subfamily transporter 2 regulates cell cycle progression and asymmetric division in mouse cardiac side population progenitor cells

RATIONALE

After cardiac injury, cardiac progenitor cells are acutely reduced and are replenished in part by regulated self-renewal and proliferation, which occurs through symmetric and asymmetric cellular division. Understanding the molecular cues controlling progenitor cell self-renewal and lineage commitment is critical for harnessing these cells for therapeutic regeneration. We previously have found that the cell surface ATP-binding cassette G-subfamily transporter 2 (Abcg2) influences the proliferation of cardiac side population (CSP) progenitor cells, but through unclear mechanisms.

OBJECTIVE

To determine the role of Abcg2 on cell cycle progression and mode of division in mouse CSP cells.

METHODS AND RESULTS

Herein, using CSP cells isolated from wild-type and Abcg2 knockout mice, we found that Abcg2 regulates G1-S cell cycle transition by fluorescence ubiquitination cell cycle indicators, cell cycle-focused gene expression arrays, and confocal live-cell fluorescent microscopy. Moreover, we found that modulation of cell cycle results in transition from symmetric to asymmetric cellular division in CSP cells lacking Abcg2.

CONCLUSIONS

Abcg2 modulates CSP cell cycle progression and asymmetric cell division, establishing a mechanistic link between this surface transporter and cardiac progenitor cell function. Greater understanding of progenitor cell biology and, in particular, the regulation of resident progenitor cell homeostasis is vital for guiding the future development of cell-based therapies for cardiac regeneration.

In vitro expansion of the mammary stem/progenitor cell population by xanthosine treatment

BACKGROUND

Mammary stem cells are critical for growth and maintenance of the mammary gland and therefore are of considerable interest for improving productivity and efficiency of dairy animals. Xanthosine treatment has been demonstrated to promote expansion of putative mammary stem cells in vivo, and hepatic and hair follicle stem cells in vitro. In the latter, xanthosine promoted the symmetrical division of hepatic and hair follicle stem cells. The objective of this study was to determine if treating primary cultures of bovine mammary epithelial cells (MEC) with xanthosine increases the stem/progenitor cell population by promoting symmetrical division of mammary stem cells.

RESULTS

In vitro treatment with xanthosine increased the population of MEC during the exponential phase of cell growth, reducing the doubling time from 86 h in control cultures to 60 h in xanthosine-treated cultures. The bromodeoxyuridine (BrdU) labeling index and the proportion of MEC in S-phase both were increased by xanthosine treatment, indicating that increased cell accretion was due to increased cell proliferation. Analysis of daughter-pairs indicated that xanthosine promoted a shift from asymmetric to symmetric cell division. Moreover, the 30 % increase in symmetric cell division was concomitant with an increase in the proportion of MEC that were positive for a putative stem cell marker (FNDC3B) and a trend toward increased telomerase activity. These results suggest that xanthosine treatment in vitro can increase cell proliferation, promote symmetric cell division and enhance stem/progenitor cell activity.

CONCLUSIONS

Xanthosine treatment increased the proliferation rate of bovine MEC in vitro. This was likely to be mediated by an increase in the proportion of stem/progenitor cells in the MEC population due to promotion of symmetrical stem cell division by xanthosine.

Culture environment-induced pluripotency of SACK-expanded tissue stem cells

Previous efforts to improve the efficiency of cellular reprogramming for the generation of induced pluripotent stem cells (iPSCs) have focused mainly on transcription factors and small molecule combinations. Here, we report the results of our focus instead on the phenotype of the cells targeted for reprogramming. We find that adult mouse pancreatic tissue stem cells derived by the method of suppression of asymmetric cell kinetics (SACK) acquire increased potency simply by culture under conditions for the production and maintenance of pluripotent stem cells. Moreover, supplementation with the SACK agent xanthine, which promotes symmetric self-renewal, significantly increases the efficiency and degree of acquisition of pluripotency properties. In transplantation analyses, clonal reprogrammed pancreatic stem cells produce slow-growing tumors with tissue derivative of all three embryonic germ layers. This acquisition of pluripotency, without transduction with exogenous transcription factors, supports the concept that tissue stem cells are predisposed to cellular reprogramming, particularly when symmetrically self-renewing.

A resource for discovering specific and universal biomarkers for distributed stem cells

Specific and universal biomarkers for distributed stem cells (DSCs) have been elusive. A major barrier to discovery of such ideal DSC biomarkers is difficulty in obtaining DSCs in sufficient quantity and purity. To solve this problem, we used cell lines genetically engineered for conditional asymmetric self-renewal, the defining DSC property. In gene microarray analyses, we identified 85 genes whose expression is tightly asymmetric self-renewal associated (ASRA). The ASRA gene signature prescribed DSCs to undergo asymmetric self-renewal to a greater extent than committed progenitor cells, embryonic stem cells, or induced pluripotent stem cells. This delineation has several significant implications. These include: 1) providing experimental evidence that DSCs in vivo undergo asymmetric self-renewal as individual cells; 2) providing an explanation why earlier attempts to define a common gene expression signature for DSCs were unsuccessful; and 3) predicting that some ASRA proteins may be ideal biomarkers for DSCs. Indeed, two ASRA proteins, CXCR6 and BTG2, and two other related self-renewal pattern associated (SRPA) proteins identified in this gene resource, LGR5 and H2A.Z, display unique asymmetric patterns of expression that have a high potential for universal and specific DSC identification.

Linking the p53 tumour suppressor pathway to somatic cell reprogramming

Reprogramming somatic cells to induced pluripotent stem (iPS) cells has been accomplished by expressing pluripotency factors and oncogenes, but the low frequency and tendency to induce malignant transformation compromise the clinical utility of this powerful approach. We address both issues by investigating the mechanisms limiting reprogramming efficiency in somatic cells. Here we show that reprogramming factors can activate the p53 (also known as Trp53 in mice, TP53 in humans) pathway. Reducing signalling to p53 by expressing a mutated version of one of its negative regulators, by deleting or knocking down p53 or its target gene, p21 (also known as Cdkn1a), or by antagonizing reprogramming-induced apoptosis in mouse fibroblasts increases reprogramming efficiency. Notably, decreasing p53 protein levels enabled fibroblasts to give rise to iPS cells capable of generating germline-transmitting chimaeric mice using only Oct4 (also known as Pou5f1) and Sox2. Furthermore, silencing of p53 significantly increased the reprogramming efficiency of human somatic cells. These results provide insights into reprogramming mechanisms and suggest new routes to more efficient reprogramming while minimizing the use of oncogenes.

In vivo expansion of the mammary stem/ progenitor cell population by xanthosine infusion

Mammary stem cells provide for growth and maintenance of the mammary gland and are therefore of considerable interest as determinants of productivity and efficiency of dairy animals and as targets of carcinogenesis in humans. Xanthosine treatment was previously shown to promote expansion of hepatic stem cells in vitro. The objective of this study was to determine if in vivo treatment with xanthosine can increase the mammary stem cell population. Xanthosine was infused into the right mammary glands of four female Holstein calves for 5 consecutive days. Immediately after each xanthosine treatment, calves were injected intravenously with 5-bromo-2-deoxyuridine (BrdU). Forty days after the final treatment, calves were euthanized and mammary tissue harvested. BrdU-label retaining epithelial cells (LREC) were detected immunohistochemically and quantified. Retention of BrdU was used as a marker for putative bovine mammary stem cells. Infusion of xanthosine into the bovine mammary gland significantly increased the number of LREC in treated glands compared to contralateral control glands (P < 0.05). LREC averaged 0.4% of epithelial cells in control glands and 0.8% in xanthosine-treated glands. The increase in LREC in xanthosine-treated glands was supported by a concomitant increase in telomerase activity (P < 0.01) and a correlation between LREC and telomerase (P < 0.05; r (2) = 0.7). Data indicate that in vivo treatment with xanthosine can be used to increase the number of mammary stem cells. This is the first demonstration of an in vivo treatment to increase the endogenous population of mammary stem cells, with utility for biomedical research and dairy management.

Microglia instruct subventricular zone neurogenesis

Microglia are increasingly implicated as a source of non-neural regulation of postnatal neurogenesis and neuronal development. To evaluate better the contributions of microglia to neural stem cells (NSCs) of the subventricular neuraxis, we employed an adherent culture system that models the continuing proliferation and differentiation of the dissociated neuropoietic subventricular tissues. In this model, neuropoietic cells retain the ability to self-renew and form multipotent neurospheres, but progressively lose the ability to generate committed neuroblasts with continued culture. Neurogenesis in highly expanded NSCs can be rescued by coculture with microglial cells or microglia-conditioned medium, indicating that microglia provide secreted factor(s) essential for neurogenesis, but not NSC maintenance, self-renewal, or propagation. Our findings suggest an instructive role for microglial cells in contributing to postnatal neurogenesis in the largest neurogenic niche of the mammalian brain.

Biological Principles for Ex Vivo Adult Stem Cell Expansion

Adult stem cells (ASCs) are the engines that drive the renewal of adult mammalian tissues. They divide continuously, throughout life, to produce new progeny cells that undergo a robust development program of differentiation and maturation to replace older expired tissue cells. The same cell turnover program may function to provide limited repair and regeneration of adult tissues in some cases. The regenerative potential of ASCs drives the current intense interest in adapting them for applications in cell replacement therapy. However, research to explore this potential has been blunted by an unyielding biological problem. ASCs have proven highly refractory to expansion of their numbers and long-term propagation in culture. A review of reported strategies to overcome this problem reveals that many studies focus on traditional cell culture factors that may not apply to ASCs and overlook a special property of ASCs that may be universally critical for successful expansion, asymmetric cell kinetics (ACK). This property is reflected by the different kinetics fate of the two sister cells resulting from an ASC division: one cell remains an ASC and keeps the potential to divide for the entire life span of the tissue, while the other cell's progeny eventually differentiates and undergoes terminal division arrest. This unique property of ASCs may prove to be the obligatory factor that must be breached by any method that will succeed in accomplishing routine expansion of ASCs of diverse tissue origin.

Synthesis of some monocyclic analogues of mycophenolic acid via the Johnson ortho ester Claisen rearrangement

The synthesis of some monocyclic analogues of mycophenolic acid in which the lactone ring has been eliminated, leaving the aromatic ring intact and the same oxygenated substituents flanking the hexenoic acid side chain with an (E)-geometry at the double bond, has been accomplished via the Johnson ortho ester Claisen rearrangement. The synthetic methodology reported here allows the preparation of mycophenolic acid analogues bearing alkyl substituents at the alpha- and beta-positions on the side chain.

Immortal DNA strand cosegregation requires p53/IMPDH-dependent asymmetric self-renewal associated with adult stem cells

Because they are long-lived and cycle continuously, adult stem cells (ASCs) are predicted as the most common precursor for cancers in adult mammalian tissues. Two unique attributes have been proposed to restrict the carcinogenic potential of ASCs. These are asymmetric self-renewal that limits their number and immortal DNA strand cosegregation that limits their accumulation of mutations due to DNA replication errors. Until recently, the molecular basis and regulation of these important ASC-specific functions were unknown. We developed engineered cultured cells that exhibit asymmetric self-renewal and immortal DNA strand cosegregation. These model cells were used to show that both ASC-specific functions are regulated by the p53 cancer gene. Previously, we proposed that IMP dehydrogenase (IMPDH) was an essential factor for p53-dependent asymmetric self-renewal. We now confirm this proposal and provide quantitative evidence that asymmetric self-renewal is acutely sensitive to even modest changes in IMPDH expression. These analyses reveal that immortal DNA strand cosegregation is also regulated by IMPDH and confirm the original implicit precept that immortal DNA strand cosegregation is specific to cells undergoing asymmetric self-renewal (i.e., ASCs). With IMPDH being the rate-determining enzyme for guanine ribonucleotide (rGNP) biosynthesis, its requirement implicates rGNPs as important regulators of ASC asymmetric self-renewal and immortal DNA strand cosegregation. An in silico analysis of global gene expression data from human cancer cell lines underscored the importance of p53-IMPDH-rGNP regulation for normal tissue cell kinetics, providing further support for the concept that ASCs are key targets for adult tissue carcinogenesis.

Potential role of mycophenolate mofetil in the management of neuroblastoma patients

In human neuroblastoma cell lines (LAN5, SHEP and IMR32), mycophenolic acid (MPA) at concentrations (10(-7)-10(-6) M) readily attainable during immunosuppressive therapy with mycophenolate mofetil (Cellcept), induces guanine nucleotide depletion leading to cell cycle arrest and apoptosis through a p53 mediated pathway (up-regulation of p53, p21 and bax and down-regulation of bcl-2 and survivin). MPA-induced apoptosis is also associated to a marked decrease of p27 protein. In the same cell lines MPA, at lower concentrations (50 nM), corresponding to the plasma levels of the active free drug during Cellcept therapy, induces differentiation toward the neuronal phenotype by causing a partial chronic guanine nucleotide depletion. MPA-induced differentiation is not associated to p27 accumulation as occurs using retinoic acid. At a fixed concentration of MPA a higher percentage of apoptotic or differentiated cells is obtained when non dialysed serum substitutes for the dialysed one, due to the higher hypoxanthine concentration in the former (about 10 microM) leading to competition on HPRT-mediated salvage of guanine. At hypoxanthine or oxypurinol concentrations higher than 1 microM (up to 100 microM) no further enhancement of MPA effects was obtained, in agreement with the recently described safety of the allopurinol-mycophenolate mofetil combination in the treatment of hyperuricemia of kidney transplant recipients. The apoptotic effects of MPA do not appear to be significantly increased by the UDP-glucuronosyltransferase inhibitor niflumic acid.

Role of human nucleoside transporters in the cellular uptake of two inhibitors of IMP dehydrogenase, tiazofurin and benzamide riboside

Benzamide riboside (BR) and tiazofurin (TR) are converted to analogs of NAD that inhibit IMP dehydrogenase (IMPDH), resulting in cellular depletion of GTP and dGTP and inhibition of proliferation. The current work was undertaken to identify the human nucleoside transporters involved in cellular uptake of BR and TR and to evaluate their role in cytotoxicity. Transportability was examined in Xenopus laevis oocytes and Saccharomyces cerevisiae that produced individual recombinant human concentrative nucleoside transporter (CNT) and equilibrative nucleoside transporter (ENT) types (hENT1, hENT2, hCNT1, hCNT2, or hCNT3). TR was a better permeant than BR with a rank order of transportability in oocytes of hCNT3 >> hENT1 > hENT2 > hCNT2 >> hCNT1. The concentration dependence of inhibition of [(3)H]uridine transport in S. cerevisiae by TR exhibited lower K(i) values than BR: hCNT3 (5.4 versus 226 microM), hENT2 (16 versus 271 microM), hENT1 (57 versus 168 microM), and hCNT1 (221 versus 220 microM). In cytotoxicity experiments, BR was more cytotoxic than TR to cells that were either nucleoside transport-defective or -competent, and transport-competent cells were more sensitive to both drugs. Exposure to nitrobenzylmercaptopurine ribonucleoside conferred resistance to BR and TR cytotoxicity to hENT1-containing CEM cells, thereby demonstrating the importance of transport capacity for manifestation of cytoxicity. A breast cancer cell line with mutant p53 exhibited 9-fold higher sensitivity to BR than the otherwise similar cell line with wild-type p53, suggesting that cells with mutant p53 may be potential targets for IMPDH inhibitors. Further studies are warranted to determine whether this finding can be generalized to other cell types.

Guanine nucleotide depletion triggers cell cycle arrest and apoptosis in human neuroblastoma cell lines

Mycophenolic acid (MPA) specifically inhibits inosine-5'-monophosphate dehydrogenase, the first committed step toward GMP biosynthesis. In its morpholinoethyl ester pro-drug form it is one of the most promising immunosuppressive drugs recently developed. The aim of the present study was to investigate the in vitro effects of MPA, at concentrations readily attainable during immunosuppressive therapy, on 3 human neuroblastoma cell lines (LAN5, SHEP and IMR32). Mycophenolic acid (0.1-10 microM) caused a decrease of intracellular levels of guanine nucleotides, a G(1) arrest and a time- and dose-dependent death by apoptosis. These effects, associated with an up-regulation of p53, p21 and bax, a shuttling of p53 protein into the nucleus and a down-regulation of bcl-2, survivin and p27 protein, were reversed by the simultaneous addition of guanine or guanosine and were more evident using nondialysed serum containing hypoxanthine. These results suggest that in neuroblastoma cell lines clinically attainable concentrations of mycophenolic acid deplete guanine nucleotide pools triggering G(1) arrest and apoptosis through p53-mediated pathways, indicating a potential role of its morpholinoethyl ester pro-drug in the management of patients with neuroectodermal tumors.

p53-independent induction of p21(waf1/cip1) contributes to the activation of caspases in GTP-depletion-induced apoptosis of insulin-secreting cells

We investigated the role of some key regulators of cell cycle in the activation of caspases during apoptosis of insulin-secreting cells after sustained depletion of GTP by a specific inosine 5'-monophosphate dehydrogenase inhibitor, mycophenolic acid (MPA). p21(Waf1/Cip1) was significantly increased following MPA treatment, an event closely correlated with the time course of caspase activation under the same conditions. MPA-induced p21(Waf1/Cip1) was not mediated by p53, since p53 mass was gradually reduced over time of MPA treatment. The increment of p21(Waf1/Cip1) by MPA was further enhanced in the presence of a pan-caspase inhibitor, indicating that the increased p21(Waf1/Cip1) may occur prior to caspase activation. This notion of association of p21(Waf1/Cip1) accumulation with caspase activation and apoptosis was substantiated by using mimosine, a selective p21(Waf1/Cip1) inducer independent of p53. Mimosine, like MPA, also increased p21(Waf1/Cip1), promoted apoptosis and simultaneously increased the activity of caspases. Furthermore, knocking down of p21(Waf1/Cip1) transfection of siRNA duplex inhibited caspase activation and apoptosis due to GTP depletion. In contrast to p21(Waf1/Cip1), a reduction in p27(Kip1) occurred in MPA-treated cells. These results indicate that p21(Waf1/Cip1) may act as an upstream signal to block mitogenesis and activate caspases which in turn contribute to induction of apoptosis.

Clonal expansion of adult rat hepatic stem cell lines by suppression of asymmetric cell kinetics (SACK)

Adult stem cells have potential use for several biomedical applications, including cell replacement therapy, gene therapy, and tissue engineering. However, such applications have been limited due to difficulties encountered in expanding functional adult stem cells. We have developed a new approach to the problem of adult stem cell expansion based on the suppression of asymmetric cell kinetics (SACK). We postulated that asymmetric cell kinetics, required for adult stem cell function, were a major barrier to their expansion in culture. As such, conversion of adult stem cells from asymmetric cell kinetics to symmetric cell kinetics would promote their exponential expansion and longterm propagation in culture. The purine nucleoside xanthosine (Xs), which promotes guanine ribonucleotide biosynthesis, can be used to reversibly convert cells from asymmetric cell kinetics to symmetric cell kinetics. We used Xs supplementation to derive clonal epithelial cell lines from adult rat liver that have properties of adult hepatic stem cells. The properties of two Xs-derived cell lines, Lig-8 and Lig-13, are described in detail and compared to properties of adult rat hepatic cell lines derived without Xs supplementation. The Xs-derived cell lines exhibit Xs-dependent asymmetric cell kinetics and Xs-dependent expression of mature hepatic differentiation markers. Interestingly, Lig-8 cells produce progeny with properties consistent with hepatocyte differentiation, while Lig-13 progeny cells have properties consistent with bile duct epithelium differentiation. A stable adult cholangiocyte stem cell line has not been previously described. Consistent with the principles of their derivation, the SACK-derived hepatic cell lines exhibit neither senescence nor tumorigenic properties, and their differentiation properties are stable after longterm culture. These characteristics of SACK-derived stem cell lines underscore asymmetric cell kinetics as an essential adult stem cell property with potential to be the basis for a general approach to expansion and propagation of diverse adult stem cells.

Asymmetric cell kinetics genes: the key to expansion of adult stem cells in culture

A singular challenge in stem cell research today is the expansion and propagation of functional adult stem cells. Unlike embryonic stem cells, which are immortal in culture, adult stem cells are notorious for the difficulty encountered when attempts are made to expand them in culture. One overlooked reason for this difficulty may be the inherent asymmetric cell kinetics of stem cells in postnatal somatic tissues. Senescence is the expected fate of a culture whose growth depends on adult stem cells that divide with asymmetric cell kinetics. Therefore, the bioengineering of strategies to expand adult stem cells in culture requires knowledge of cellular mechanisms that control asymmetric cell kinetics. The properties of several genes recently implicated to function in a cellular pathway(s) that regulates asymmetric cell kinetics are discussed. Understanding the function of these genes in asymmetric cell kinetics mechanisms may be the key that unlocks the adult stem cell expansion problem.

Cytotoxicity and cellular differentiation activity of methylenebis(phosphonate) analogs of tiazofurin and mycophenolic acid adenine dinucleotide in human cancer cell lines

Mycophenolic acid (MPA) is a fungally-derived inhibitor of inosine 5'-monophosphate dehydrogenase (IMPDH). MPA binds IMPDH at the nicotinamide sub-site of the NAD cofactor binding domain leaving the adenosine sub-site empty. In order to improve the binding affinity we synthesized MPA analogs by linking adenosine 5'-methylenebis(phosphonate) with mycophenolic alcohols containing 2-, 4-, and 6-carbon atoms in their aliphatic side chain. Adenine dinucleotide analogs of tiazofurin, selenazofurin and benzamide riboside were synthesized as P1, P2-disubstituted pyrophosphates. Cytotoxicity of each analog was examined in human colon adenocarcinoma HT-29 and erythroleukemia K562 cells, and induction of differentiation in K562 cells by these agents was determined. Mycophenolic acid is currently used as an immunosuppressant but its anticancer action is limited by inactivation due to rapid glucuronidation. The new analogs show resistance to metabolism to inactive species and exhibit enhanced cytotoxicity in tumor cell lines, and therefore could be useful as anticancer agents.

Activation of caspase-2 mediates the apoptosis induced by GTP-depletion in insulin-secreting (HIT-T15) cells

This study investigated the possible involvement of a specific caspase(s) (a family of aspartate-specific cysteine proteases) in programmed cell death of islet beta-cells due to sustained GTP depletion. Treatment (up to 48 h) with 3 microg/ml mycophenolic acid (MPA), which specifically depletes intracellular guanine nucleotides, reduced cell-cycle progression from G1 phase into S and G2/M phases (as assessed by flow cytometry) and, subsequently, induced apoptosis of HIT-15 cells (transformed pancreatic beta-cells). The latter was accompanied by a marked increase of caspase-2 activity (+343%) and moderate activation of caspase-9 (+150%) and caspase-3 (+145%). Importantly, only caspase-2 activation preceded induction of apoptosis. There was no change in activity of caspase-1, -4, -5, -6, and -8. Release of the mitochondrial protein cytochrome c into cytosol was also observed at a late stage. Cotreatment of cells with a permeable pan-caspase inhibitor (Z-VAD-FMK) blocked GTP depletion-induced cell death in a dose-dependent manner. A specific caspase-2 inhibitor (Z-VDVAD-FMK), but not a caspase-3 inhibitor (DEVD-CHO), was also capable of restoring cell viability. Interestingly, activation of caspase-2 leads to caspase-3 activation because the caspase-2 inhibitor abrogated caspase-3 activity. Our results indicate that, while activation of multiple caspases are involved in the execution phase of GTP depletion-induced apoptosis, caspase-2 appears to play the major role in the initiation of this program. This study revealed a novel, caspase-2 mediated form of apoptosis that may be consequent to impaired mitogenesis.

Novel mycophenolic adenine bis(phosphonate) analogues as potential differentiation agents against human leukemia

Novel mycophenolic adenine dinucleotide (MAD) analogues have been prepared as potential inhibitors of inosine monophosphate dehydrogenase (IMPDH). MAD analogues resemble nicotinamide adenine dinucleotide binding at the cofactor binding domain of IMPDH; however, they cannot participate in hydride transfer and therefore inhibit the enzyme. The methylenebis(phosphonate) analogues C2-MAD and C4-MAD were obtained by coupling 2',3'-O-isopropylideneadenosine 5'-methylenebis(phosphonate) (22) with mycophenolic alcohols 20 and 21 in the presence of diisopropylcarbodiimide followed by deprotection. C2-MAD was also prepared by coupling of mycophenolic methylenebis(phosphonate) derivative 30 with 2',3'-O-isopropylideneadenosine. Compound 30 was conveniently synthesized by the treatment of benzyl-protected mycophenolic alcohol 27 with a commercially available methylenebis(phosphonic dichloride) under Yoshikawa's reaction conditions. C2-MAD and C4-MAD were found to inhibit the growth of K562 cells (IC(50) = 0.7 microM and IC(50) = 0.1 microM, respectively) as potently as mycophenolic acid (IC(50) = 0.3 microM). In addition, C2-MAD and C4-MAD triggered vigorous differentiation of K562 cells an order of magnitude more potently than tiazofurin, and MAD analogues were resistant to glucuronidation in vitro. These results show that C2-MAD and C4-MAD may be of therapeutic interest in the treatment of human leukemias.

Acivicin induces apoptosis independently of gamma-glutamyltranspeptidase activity

Inhibition of cellular gamma-glutamyltranspeptidase (GGT) enzyme activity by its specific inhibitor acivicin is frequently used in studies aimed at demonstrating the physiological role of this enzyme. However, because acivicin is a glutamine antagonist, it also inhibits many other glutamine-dependent enzymes involved in purine and pyrimidine biosynthesis. The objective of the present work is to determine whether acivicin exhibits apoptotic properties and the significance of GGT activity level in the response to acivicin treatment. We compared acivicin (0-150 microM) effect on V79 cell lines expressing or not expressing human GGT. Apoptosis was assayed by annexin-V staining, cell cycle analysis, and caspase activation using flow cytometry. We found that acivicin causes a dose- and time-dependent apoptosis in the GGT-negative V79 cell line as well as in its GGT-positive counterpart line. This is the evidence that acivicin induces apoptosis in V79 cell independently of their GGT activity level.

Inosine-5'-monophosphate dehydrogenase is required for mitogenic competence of transformed pancreatic beta cells

The relation of inosine-5'-monophosphate dehydrogenase (IMPDH; the rate-limiting enzyme in GTP synthesis) to mitogenesis was studied by enzymatic assay, immunoblots, and RT-PCR in several dissimilar transformed pancreatic ss-cell lines, using intact cells. Both of the two isoforms of IMPDH (constitutive type 1 and inducible type 2) were identified using RT-PCR in transformed beta cells or in intact islets. IMPDH 2 messenger RNA (mRNA) and IMPDH protein were both regulated reciprocally by changes in levels of their end-products. Flux through IMPDH was greatest in rapidly growing cells, due mostly to increased uptake of precursor. Glucose (but not 3-0-methylglucose, L-glucose, or fructose) further augmented substrate uptake and also increased IMPDH enzymatic activity after either 4 or 21 h of stimulation. Serum or ketoisocaproate also increased IMPDH activity (but not uptake). Two selective IMPDH inhibitors (mycophenolic acid and mizoribine) reduced IMPDH activity in all cell lines, and, with virtually identical concentration-response curves, inhibited DNA synthesis (assessed as bromodeoxyuridine incorporation) in response to glucose, serum, or ketoisocaproate. Inhibition of DNA synthesis was reversible, completely prevented by repletion of cellular guanine (but not adenine) nucleotides, and could not be attributed to toxic effects. Despite the fact that modulation of IMPDH expression by guanine nucleotides was readily detectable, glucose and/or serum failed to alter IMPDH mRNA or protein, indicating that their effects on IMPDH activity were largely at the enzyme level. Precursors of guanine nucleotides failed, by themselves, to induce mitogenesis. Thus, adequate IMPDH activity (and thereby, availability of GTP) is a critical requirement for beta-cell proliferation. Although it is unlikely that further increases in GTP can, by themselves, initiate DNA synthesis, such increments may be needed to sustain mitogenesis.

PKB/Akt interacts with inosine-5' monophosphate dehydrogenase through its pleckstrin homology domain

The pleckstrin homology (PH) domain of the protooncogenic serine/threonine protein kinase PKB/Akt can bind phosphoinositides. A yeast-based two-hybrid system was employed which identified inosine-5' monophosphate dehydrogenase (IMPDH) type II as specifically interacting with PKB/Akts PH domain. IMPDH catalyzes the rate-limiting step of de novo guanosine-triphosphate (GTP) biosynthesis. Using purified fusion proteins, PKB/Akts PH domain and IMPDH associated in vitro and this association moderately activated IMPDH. Purified PKB/Akt also associated with IMPDH in vitro. We could specifically pull-down PKB/Akt or IMPDH from mammalian cell lysates using glutathione-S-transferase (GST)-IMPDH or GST-PH domain fusion proteins, respectively. Additionally, PKB/Akt and IMPDH could be co-immunoprecipitated from COS cell lysates and active PKB/Akt could phosphorylate IMPDH in vitro. These results implicate PKB/Akt in the regulation of GTP biosynthesis through its interaction with IMPDH, which is involved in providing the GTP pool used by signal transducing G-proteins.

An ATP/ADP-dependent molecular switch regulates the stability of p53-DNA complexes

Interaction with DNA is essential for the tumor suppressor functions of p53. We now show, for the first time, that the interaction of p53 with DNA can be stabilized by small molecules, such as ADP and dADP. Our results also indicate an ATP/ADP molecular switch mechanism which determines the off-on states for p53-DNA binding. This ATP/ADP molecular switch requires dimer-dimer interaction of the p53 tetramer. Dissociation of p53-DNA complexes by ATP is independent of ATP hydrolysis. Low-level ATPase activity is nonetheless associated with ATP-p53 interaction and may serve to regenerate ADP-p53, thus recycling the high-affinity DNA binding form of p53. The ATP/ADP regulatory mechanism applies to two distinct types of p53 interaction with DNA, namely, sequence-specific DNA binding (via the core domain of the p53 protein) and binding to sites of DNA damage (via the C-terminal domain). Further studies indicate that ADP not only stabilizes p53-DNA complexes but also renders the complexes susceptible to dissociation by specific p53 binding proteins. We propose a model in which the DNA binding functions of p53 are regulated by an ATP/ADP molecular switch, and we suggest that this mechanism may function during the cellular response to DNA damage.

Inosine monophosphate dehydrogenase: A molecular switch integrating pleiotropic GTP-dependent beta-cell functions

Studies of pancreatic islet function in the pathogenesis of type 2 diabetes mellitus have tended to focus on the short-term control of insulin secretion. However, the long-term control of beta-cell mass is also relevant to diabetes, since this parameter is reduced substantially even in non-insulin-dependent diabetes in humans. In animal models of type 2 diabetes, the normal balance between beta-cell proliferation and programmed cell death is perturbed. We take the perspective in this overview that inosine monophosphate dehydrogenase (IMPDH; EC 1.1.1. 205) may represent a previously neglected molecular integrator or sensor that exerts both functional (secretory) and anatomical (proliferative) effects within beta-cells. These properties reflect the fact that IMPDH is a rate-limiting enzyme in the new synthesis of the purine guanosine triphosphate (GTP), which modulates both exocytotic insulin secretion and DNA synthesis, as well as a number of other critical cellular functions within the beta-cell. Alterations in the expression or activity of IMPDH may be central to beta-cell replication, cell cycle progression, differentiation, and maintenance of adequate islet mass, effects that are probably mediated both by GTP directly, and indirectly via low molecular mass GTPases. If GTP becomes depleted, a hierarchy of beta-cell functions becomes progressively paralyzed, until eventually the effete cell is removed via apoptosis.

Multiple murine double minute gene 2 (MDM2) proteins are induced by ultraviolet light

The mdm2 (murine double minute 2) oncogene encodes several proteins, the largest of which (p90) binds to and inactivates the p53 tumor suppressor protein. Multiple MDM2 proteins have been detected in tumors and in cell lines expressing high levels of mdm2 mRNAs. Here we show that one of these proteins (p76) is expressed, along with p90, in wild-type and p53-null mouse embryo fibroblasts, indicating that it may have an important physiological role in normal cells. Expression of this protein is induced, as is that of p90, by UV light in a p53-dependent manner. The p76 protein is synthesized via translational initiation at AUG codon 50 and thus lacks the N terminus of p90 and does not bind p53. In cells, p90 and p76 can be synthesized from mdm2 mRNAs transcribed from both the P1 (constitutive) and P2 (p53-responsive) promoters. Site-directed mutagenesis reveals that these RNAs give rise to p76 via internal initiation of translation. In addition, mdm2 mRNAs lacking exon 3 give rise to p76 exclusively, and such mRNAs are induced by p53 in response to UV light. These data indicate that p76 may be an important product of the mdm2 gene and a downstream effector of p53.

Inosine-5'-monophosphate dehydrogenase: regulation of expression and role in cellular proliferation and T lymphocyte activation

Guanine nucleotide synthesis is essential for the maintenance of normal cell growth and function, as well as for cellular transformation and immune responses. The expression of two genes encoding human inosine-5'-monophosphate dehyrogenase (IMPDH) type I and type II results in the translation of catalytically indistinguishable enzymes that control the rate-limiting step in the de novo synthesis of guanine nucleotides. Cellular IMPDH activity is increased more than 10-fold in activated peripheral blood T lymphocytes and is attributable to the increased expression of both the type I and type II enzymes. In contrast, abrogation of cellular IMPDH activity by selective inhibitors prevents T lymphocyte activation and establishes a requirement for elevated IMPDH activity in T lymphocytic responses. In order to assess the molecular mechanisms governing the expression of the IMPDH type I and type II genes in resting and activated peripheral blood T lymphocytes, we have cloned the human IMPDH type I and type II genes and characterized their genomic organization and their respective 5'-flanking regions. Both genes contain 14 highly conserved exons that vary in size from 49 to 207 base pairs. However, the intron structures are completely divergent, resulting in disparities in gene length (18 kilobases for type I and 5.8 kilobases for type II). In addition, the 5'-regulatory sequences are highly divergent; expression of the IMPDH type I gene is controlled by three distinct promoters in a tissue specific manner while the type II gene is regulated by a single promoter and closely flanked in the 5' region by a gene of unknown function. The conservation of the IMPDH type I and type II coding sequence in the presence of highly divergent 5'-regulatory sequences points to a multifactorial control of enzyme expression and suggests that tissue-specific and/or developmentally specific regulation of expression may be important. Delineation of these regulatory mechanisms will aid in the elucidation of the signaling events that ultimately lead to the synthesis of guanine nucleotides required for cellular entry into S phase and the initiation of DNA replication.

Prolonged depletion of guanosine triphosphate induces death of insulin-secreting cells by apoptosis

Inhibitors of IMP dehydrogenase, such as mycophenolic acid (MPA) and mizoribine, which deplete cellular GTP, are used clinically as immunosuppressive drugs. The prolonged effect of such agents on insulin-secreting beta-cells (HIT-T15 and INS-1) was investigated. Both MPA and mizoribine inhibited mitogenesis, as reflected by [3H]thymidine incorporation. Cell number, DNA and protein contents, and cell (metabolic) viability were decreased by about 30%, 60%, and 80% after treatment of HIT cells with clinically relevant concentrations (e.g. 1 microg/ml) of MPA for 1, 2, and 4 days, respectively. Mizoribine (48 h) similarly induced the death of HIT cells. INS-1 cells also were damaged by prolonged MPA treatment. MPA-treated HIT cells displayed a strong and localized staining with a DNA-binding dye (propidium iodide), suggesting condensation and fragmentation of DNA, which were confirmed by detection of DNA laddering in multiples of about 180 bp. DNA fragmentation was observed after 24-h MPA treatment and was dose dependent (29%, 49%, and 70% of cells were affected after 48-h exposure to 1, 3, and 10 microg/ml MPA, respectively). Examination of MPA-treated cells by electron microscopy revealed typical signs of apoptosis: condensed and marginated chromatin, apoptotic bodies, cytosolic vacuolization, and loss of microvilli. MPA-induced cell death was almost totally prevented by supplementation with guanosine, but not with adenosine or deoxyguanosine, indicating a specific effect of GTP depletion. An inhibitor of protein isoprenylation (lovastatin, 10-100 microM for 2-3 days) induced cell death and DNA degradation similar to those induced by sustained GTP depletion, suggesting a mediatory role of posttranslationally modified GTP-binding proteins. Indeed, impeding the function of G proteins of the Rho family (via glucosylation using Clostridium difficile toxin B), although not itself inducing apoptosis, potentiated cell death induced by MPA or lovastatin. These findings indicate that prolonged depletion of GTP induces beta-cell death compatible with apoptosis; this probably involves a direct impairment of GTP-dependent RNA-primed DNA synthesis, but also appears to be modulated by small GTP-binding proteins. Treatment of intact adult rat islets (the beta-cells of which replicate slowly) induced a modest, but definite, death by apoptosis over 1- to 3-day periods. Thus, more prolonged use of the new generation of immunosuppressive agents exemplified by MPA might have deleterious effects on the survival of islet or pancreas grafts.

Cellular responses in mouse leukemia L1210 cells made resistant to deoxyadenosine

Recent studies have implicated nucleotides in diverse and unexpected functions related to p53 levels, p53-dependent G0/G1 cell cycle arrest, and the role of dATP in the activation of the caspase-induced apoptosis. Using deoxyadenosine-resistant L1210 cells (ED2 and Y8) that had ribonucleotide reductase that was not sensitive to inhibition by dATP and also exhibited other metabolic alterations, the properties of these cells with respect to the role(s) of nucleotides in these functions were explored. In the ED2 and Y8 cells that did not express p53 protein, the pools of UTP, CTP, ATP, and GTP were markedly decreased. The decreased cellular levels of UTP and CTP did not result in these cells being more sensitive to either PALA or acivicin. The ED2 and Y8 cells did not block in G0/G1 in response to PALA treatment even though the basal cellular concentrations of UTP and CTP were reduced 50 to 80%. While it has been shown that dATP in combination with cytochrome c is involved in the apoptotic pathway, the concentration of exogenous deoxyadenosine required to induce apoptosis in the parental L1210 cells was far in excess of the concentration required to inhibit cell growth. Deoxyadenosine did not cause an increase in apoptosis in the deoxyadenosine-resistant Y8 cells. These data suggest that the new roles ascribed to nucleotides may be specific for the particular cell type under very specific conditions.

Coexpression of p53 and tissue transglutaminase genes in human normal and pathologic adrenal tissues

The presence of p53 and tissue transglutaminase (tTG) gene expressions was investigated in human normal and pathologic adrenal tissues with two aims (1) to determine the tissue content of p53 protein, its messenger ribonucleic acid (mRNA) and, especially, tTG mRNA which has not been previously reported and (2) to study possible differences in the coexpression of p53 and tTG in various adrenal disorders. Using Northern blot analysis, p53 and tTG mRNAs were detected in each adrenal tissue examined including 5 normal human adrenals, 6 aldosterone-producing adenomas, 3 Cushing's adenomas, 1 primary nodular adrenocortical hyperplasia causing Cushing's syndrome in an infant, 12 non-hyperfunctioning adrenocortical adenomas, and 4 adrenocortical carcinomas. The results showed a significant positive correlation between these two mRNAs in all adrenal tissues except adrenocortical carcinomas. Compared to normal adrenals, high p53 mRNA levels were observed in aldosterone-producing and Cushing's adenomas and, most markedly, in a tissue from a primary nodular adrenocortical hyperplasia. Also, Cushing's adenomas had significantly higher tTG mRNA contents. Immunohistochemistry for wild-type and mutant p53 protein showed numerous p53 positive cells with a strong nuclear staining in a tissue from a primary nodular adrenocortical hyperplasia, whereas the p53 positive cells were absent, except those with a faint nuclear staining, in all other adrenal tissues. However, all adrenal tissues showed detectable p53 contents by the more sensitive method of luminometric immunoassay (LIA). Using this method, aldosterone-producing adenomas exhibited significantly higher p53 contents than normal adrenal tissues. These observations may support potentially important roles for p53 and tTG in adrenal pathophysiology, especially in mechanisms which influence the evolution and/or progression of aldosterone-producing and Cushing's adenomas and, most probably, hyperplasias.

Synthesis of a methylenebis(phosphonate) analogue of mycophenolic adenine dinucleotide: a glucuronidation-resistant MAD analogue of NAD

Mycophenolic alcohol (MPAlc), obtained by reduction of the carboxylic group of mycophenolic acid (MPA), was coupled with 2',3'-O-isopropylideneadenosine 5'-methylenebis(phosphonate) (4) in the presence of diisopropylcarbodiimide (DIC) to give P1-(2',3'-O-isopropylideneadenosin-5'-yl)-P2-(mycophenolic alcohol-6'-yl)methylenebis(phosphonate) (8) in 32% yield. Deisopropy-lidenation of 8 with CF3COOH/H2O afforded the methylenebis(phosphonate) analogue 3 of mycophenolic adenine dinucleotide (MAD). Compound 3, beta-methylene-MAD, was found to be a potent inhibitor of inosine monophosphate dehydrogenase (IMPDH) type II (Ki = 0.3 microM) as well as an inhibitor of growth of K562 cells (IC50 = 1.5 microM). In contrast to MPA and mycophenolic alcohol, beta-methylene-MAD was not converted into the glucuronide when incubated with uridine 5'-diphosphoglucuronyltransferase.

Inosine-5'-monophosphate dehydrogenase is a rate-determining factor for p53-dependent growth regulation

We have proposed that reduced activity of inosine-5'-monophosphate dehydrogenase (IMPD; IMP:NAD oxidoreductase, EC 1.2.1.14), the rate-limiting enzyme for guanine nucleotide biosynthesis, in response to wild-type p53 expression, is essential for p53-dependent growth suppression. A gene transfer strategy was used to demonstrate that under physiological conditions constitutive IMPD expression prevents p53-dependent growth suppression. In these studies, expression of bax and waf1, genes implicated in p53-dependent growth suppression in response to DNA damage, remains elevated in response to p53. These findings indicate that under physiological conditions IMPD is a rate-determining factor for p53-dependent growth regulation. In addition, they suggest that the impd gene may be epistatic to bax and waf1 in growth suppression. Because of the role of IMPD in the production and balance of GTP and ATP, essential nucleotides for signal transduction, these results suggest that p53 controls cell division signals by regulating purine ribonucleotide metabolism.

Novel nicotinamide adenine dinucleotide analogues as potential anticancer agents: quest for specific inhibition of inosine monophosphate dehydrogenase

Synthetic nicotinamide adenine dinucleotide (NAD) analogues containing 5-beta-D-ribofuranosylnicotinamide (C-NAD), 6-beta-D-ribofuranosylpicolinamide (C-PAD), 3-beta-D-ribofuranosylbenzamide (BAD), and 2-beta-D-ribofuranosylthiazole-4-carboxamide (TAD) in place of the nicotinamide riboside moiety are described and evaluated as potential inhibitors of inosine monophosphate dehydrogenase (IMPDH). TAD and BAD showed potent inhibitory activity against the enzyme in the form of pyrophosphates, as well as metabolically stable methylene- and difluoromethylenebis(phosphonate)s. Fluorination at the C2' (ribo and arabino configuration) and C3' (ribo) of the adenosine moiety of TAD afforded analogues highly potent against IMPDH, but weakly active against alcohol dehydrogenase. With the exception of the methylenebis(phosphonate) analogue of TAD compounds containing a methylene bridge were poor inhibitors of growth of K562 cells. On the other hand, NAD analogues containing difluoromethylene linkage were highly effective in inhibition of K562 cell growth, as well as potent inducers of K562 cell differentiation. Such compounds, therefore, may be of potential therapeutic interest.

A significant decrease of the transcriptional activity of p53 mutants deriving from human functional adrenal tumors

Recently, our laboratory has found a high incidence (77%) of p53 gene mutations in human functional adrenal tumors. Furthermore, the majority of mutant sites were assembled at codons 100, 102, and 249. These mutation sites are not common, and there have been no studies addressing whether or not these mutants points or mutant styles cause the p53 protein to lose function. It has been well known that p53 is a transcription factor. To examine the transcriptional activities of these mutant p53 genes from patients with functional adrenal tumors, we constructed p53 expression plasmids from tumors and paired adjacent normal adrenal gland tissues, using a transient co-transfection assay with a reporter gene in H358 cells. Wild-type p53 from normal adrenal gland tissues specifically trans-activates the expression of a chloramphenicol acetyltransferase (CAT) reporter gene in H358 cells. Three mutant p53 proteins (at codons 100, 102, and 249, respectively) from tumors showed a >90% loss of transcriptional activity. One mutant at codon 68, other than at hot spots, remained at approximately 65% transcriptional activity. An immunoprecipitation assay showed that the mutant proteins of codon 68 and codon 102 could respond to the three monoclonal antibodies (PAbDO-1, PAb1620, and PAb421), indicating that there were no obvious changes in the antigenicity of the proteins. However, the mutant protein of codon 249 could not respond to the carboxy-terminus-specific antibody PAb421 and conformation-specific antibody PAb1620, indicating that there were some obvious changes in the conformation of the mutant proteins. The mutant protein of codon 100 could not be detected by immunoprecipitation assay but could be analyzed by Western blot. In a further study using a DNA-binding assay, it was shown that the loss of transcriptional activity was caused by the loss of DNA-binding ability. These results show that the p53 mutants, derived from functional adrenal tumors, actually lost DNA-binding ability and decreased the transcriptional activity. However, the role of the mutant protein in the tumorigenesis of functional adrenal tumors requires further investigation.

A reversible, p53-dependent G0/G1 cell cycle arrest induced by ribonucleotide depletion in the absence of detectable DNA damage

Cells with a functional p53 pathway undergo a G0/G1 arrest or apoptosis when treated with gamma radiation or many chemotherapeutic drugs. It has been proposed that DNA damage is the exclusive signal that triggers the arrest response. However, we found that certain ribonucleotide biosynthesis inhibitors caused a p53-dependent G0 or early G1 arrest in the absence of replicative DNA synthesis or detectable DNA damage in normal human fibroblasts. CTP, GTP, or UTP depletion alone was sufficient to induce arrest. In contrast to the p53-dependent response to DNA damage, characterized by long-term arrest and irregular cellular morphologies, the antimetabolite-induced arrest was highly reversible and cellular morphologies remained relatively normal. Both arrest responses correlated with prolonged induction of p53 and the Cdk inhibitor P21(WAF1/CIP1/SDI1) and with dephosphorylation of pRb. Thus, we propose that p53 can serve as a metabolite sensor activated by depletion of ribonucleotides or products or processes dependent on ribonucleotides. Accordingly, p53 may play a role in inducing a quiescence-like arrest state in response to nutrient challenge and a senescence-like arrest state in response to DNA damage. These results have important implications for the mechanisms by which p53 prevents the emergence of genetic variants and for developing more effective approaches to chemotherapy based on genotype.

Retrovirally mediated wild-type p53 restores S-phase modulation without inducing WAF1 mRNA in breast carcinoma cells containing mutant p53

The mechanism of negative growth regulation by the nuclear phosphoprotein p53 in breast cancer cells may rely on its role as a transcriptional activator of cell cycle-related genes. We have tested this hypothesis using retrovirally transduced wild-type (wt) p53 in breast cancer cell lines containing homozygously endogenous mutant (mt) p53. Restoring the expression of wt p53, the percentage of cells in S phase was reduced, G1/S transition was slowed, and progression through S was restrained. The fraction of cells with a flattened "Cdk-minus" phenotype increased 5- to 10-fold. High constitutive mRNA expression of the cyclin-Cdk inhibitor WAF1 in MDAMB231 cells was not induced upon restored wt p53 expression suggesting a p53-independent pathway in the regulation of WAF1 mRNA expression. Wt p53 acted trans-dominantly in the presence of accumulating mt p53 and installed a modulation of G1/S transition and S phase progression independent of WAF1 expression.

Radiation induced G1-block and p53 status in six human cell lines

Considerable attention has recently been focused on the fact that the tumor suppressor protein p53 is involved in the cellular response to radiation. In its wild-type form the protein appears to control a cell cycle checkpoint, preventing entry into S-phase following DNA damage. A number of authors observed a radiation induced G1-block in cells expressing wild-type p53, but not in p53 mutant cells. We obtained similar results with four human tumour cell lines as well as two strains of human fibroblasts, whose p53 status was ascertained at the protein as well as DNA levels. In addition to cell cycle delays in exponentially growing cell cultures, we have studied the possible role of the p53 in the transition from quiescence to active proliferation. Cells were irradiated after 6 days of serum-starvation and labelled with BrdU at different times after addition of fresh medium. Entry into S-phase was found to be delayed by several hours in the p53 wild-type cells, but no such effect was observed in the p53 mutants. Where a delay occurred, it was roughly proportional to the X-ray dose. Although it remains to be clarified, whether the cells were delayed only in G1 or also in G0, it is interesting to note that entry into S-phase can be delayed by irradiation in a quiescent state immediately before serum-stimulation, provided the cells are wild-type with respect to p53. Certain differences in the cell cycle response of transformed and untransformed cells were noted.

Apoptosis overview emphasizing the role of oxidative stress, DNA damage and signal-transduction pathways

Apoptosis (programmed cell death) is a central protective response to excess oxidative damage (especially DNA damage), and is also essential to embryogenesis, morphogenesis and normal immune function. An understanding of the cellular events leading to apoptosis is important for the design of new chemotherapeutic agents directed against the types of leukemias and lymphomas that are resistant to currently used chemotherapeutic protocols. We present here a review of the characteristic features of apoptosis, the cell types and situations in which it occurs, the types of oxidative stress that induce apoptosis, the signal-transduction pathways that either induce or prevent apoptosis, the biologic significance of apoptosis, the role of apoptosis in cancer, and an evaluation of the methodologies used to identify apoptotic cells. Two accompanying articles, demonstrating classic apoptosis and non-classic apoptosis in the same Epstein-Barr virus-transformed lymphoid cell line, are used to illustrate the value of employing multiple criteria to determine the type of cell death occurring in a given experimental system. Aspects of apoptosis and programmed cell death that are not covered in this review include histochemistry, details of cell deletion processes in the sculpting of tissues and organs in embryogenesis and morphogenesis, and the specific pathways leading to apoptosis in specific cell types. The readers should refer to the excellent books and reviews on the morphology, biochemistry and molecular biology of apoptosis already published on these topics. Emphasis is placed, in this review, on a proposed common pathway of apoptosis that may be relevant to all cell types.