Activation of synaptic NMDA receptors induces membrane insertion of new AMPA receptors and LTP in cultured hippocampal neurons

Long-term potentiation (LTP) of excitatory transmission in the hippocampus likely contributes to learning and memory. The mechanisms underlying LTP at these synapses are not well understood, although phosphorylation and redistribution of AMPA receptors may be responsible for this form of synaptic plasticity. We show here that miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons reliably demonstrate LTP when postsynaptic NMDA receptors are briefly stimulated with glycine. LTP of these synapses is accompanied by a rapid insertion of native AMPA receptors and by increased clustering of AMPA receptors at the surface of dendritic membranes. Both LTP and glycine-facilitated AMPA receptor insertion are blocked by intracellular tetanus toxin (TeTx), providing evidence that AMPA receptors are inserted into excitatory synapses via a SNARE-dependent exocytosis during LTP.

Nonredundant roles of the mPer1 and mPer2 genes in the mammalian circadian clock

Mice carrying a null mutation in the Period 1 (mPer1) gene were generated using embryonic stem cell technology. Homozygous mPer1 mutants display a shorter circadian period with reduced precision and stability. Mice deficient in both mPer1 and mPer2 do not express circadian rhythms. While mPER2 regulates clock gene expression at the transcriptional level, mPER1 is dispensable for the rhythmic RNA expression of mPer1 and mPer2 and may instead regulate mPER2 at a posttranscriptional level. Studies of clock-controlled genes (CCGs) reveal a complex pattern of regulation by mPER1 and mPER2, suggesting independent controls by the two proteins over some output pathways. Genes encoding key enzymes in heme biosynthesis are under circadian control and are regulated by mPER1 and mPER2. Together, our studies show that mPER1 and mPER2 have distinct and complementary roles in the mouse clock mechanism.

Efficient activation of aromatic C-H bonds for addition to C-C multiple bonds

Efficient electrophilic metalation of aromatic C-H bonds leading to new C-C bond formation through regio- and stereoselective addition to alkynes and alkenes has been realized by a catalytic amount (0.02 to 5 mole percent) of palladium(II) or platinum(II) compounds in a mixed solvent containing trifluoroacetic acid at room temperature. Various arenes undergo unexpected selective trans hydroarylation to terminal or internal C&cjs0812;C bonds inter- and intramolecularly with high efficiency (up to a turnover number of 4500 for palladium), especially for electron-rich arenes, giving thermodynamically unfavorable cis-alkenes, and the oxygen- and nitrogen-containing heterocycles. The simplicity, generality, and efficiency of this process should be very attractive to the possible industrial application for the functionalization of arenes.

Structure of PAK1 in an autoinhibited conformation reveals a multistage activation switch

The p21-activated kinases (PAKs), stimulated by binding with GTP-liganded forms of Cdc42 or Rac, modulate cytoskeletal actin assembly and activate MAP-kinase pathways. The 2.3 A resolution crystal structure of a complex between the N-terminal autoregulatory fragment and the C-terminal kinase domain of PAK1 shows that GTPase binding will trigger a series of conformational changes, beginning with disruption of a PAK1 dimer and ending with rearrangement of the kinase active site into a catalytically competent state. An inhibitory switch (IS) domain, which overlaps the GTPase binding region of PAK1, positions a polypeptide segment across the kinase cleft. GTPase binding will refold part of the IS domain and unfold the rest. A related switch has been seen in the Wiskott-Aldrich syndrome protein (WASP).

Human beta-defensins

The last decade led to the discovery and characterization of several human beta-defensins. Analysis of genomic information indicates that the number of beta-defensin-like molecules encoded by the human genome may number in the tens. Growing interest in beta-defensins steadily enhances our knowledge about various aspects of their gene location, expression patterns and the transcription factors involved in their regulation in vivo. The hallmark property of beta-defensins, their antimicrobial activity, is clearly only the tip of the iceberg in the extensive network of inter-relations within the immune system in which these peptides function. Structural studies of beta-defensins provide the molecular basis for a better understanding of their properties, functions and their potential for practical applications. In this review, we present some recent advances in the studies of human beta-defensins, with an emphasis on possible correlations between their structural and functional properties.

Perinatal lethality with kidney and pancreas defects in mice with a targetted Pkd1 mutation

PKD1 is the most common site for mutations in human autosomal dominant polycystic kidney disease (ADPKD). ADPKD is characterized by progressive replacement of kidney tissue by epithelial cysts and eventual renal failure. Hepatic and pancreatic cysts are also common. The PKD1 protein, polycystin, is a cell-surface protein of unknown function that is widely expressed in epithelia and in vascular smooth muscle and myocardium. None of the genetic forms of murine polycystic disease map to the murine Pkd1 locus. We introduced into mice by homologous recombination a Pkd1 truncation mutation, Pkd1-, that mimics a mutation found in ADPKD. Pkd1- heterozygotes have no discernible phenotype, whereas homozygotes die during the perinatal period with massively enlarged cystic kidneys, pancreatic ductal cysts and pulmonary hypoplasia. Renal cyst formation begins at embryonic day 15.5 (E15.5) in proximal tubules and progresses rapidly to replace the entire renal parenchyma. The timing of cyst formation indicates that full-length polycystin is required for normal morphogenesis during elongation and maturation of tubular structures in the kidney and pancreas.

The p35/Cdk5 kinase is a neuron-specific Rac effector that inhibits Pak1 activity

Cyclin-dependent kinase 5 (Cdk5) and its neuron-specific regulator p35 are essential for neuronal migration and for the laminar configuration of the cerebral cortex. In addition, p35/Cdk5 kinase concentrates at the leading edges of axonal growth cones and regulates neurite outgrowth in cortical neurons in culture. The Rho family of small GTPases is implicated in a range of cellular functions, including cell migration and neurite outgrowth. Here we show that the p35/Cdk5 kinase co-localizes with Rac in neuronal growth cones. Furthermore, p35 associates directly with Rac in a GTP-dependent manner. Another Rac effector, Pak1 kinase, is also present in the Rac-p35/Cdk5 complexes and co-localizes with p35/Cdk5 and Rac at neuronal peripheries. The active p35/Cdk5 kinase causes Pak1 hyperphosphorylation in a Rac-dependent manner, which results in down-regulation of Pak1 kinase activity. Because the Rho family of GTPases and the Pak kinases are implicated in actin polymerization, the modification of Pak1, imposed by the p35/Cdk5 kinase, is likely to have an impact on the dynamics of the reorganization of the actin cytoskeleton in neurons, thus promoting neuronal migration and neurite outgrowth.

CAKbeta/Pyk2 kinase is a signaling link for induction of long-term potentiation in CA1 hippocampus

Long-term potentiation (LTP) is an activity-dependent enhancement of synaptic efficacy, considered a model of learning and memory. The biochemical cascade producing LTP requires activation of Src, which upregulates the function of NMDA receptors (NMDARs), but how Src becomes activated is unknown. Here, we show that the focal adhesion kinase CAKbeta/Pyk2 upregulated NMDAR function by activating Src in CA1 hippocampal neurons. Induction of LTP was prevented by blocking CAKbeta/Pyk2, and administering CAKbeta/Pyk2 intracellularly mimicked and occluded LTP. Tyrosine phosphorylation of CAKbeta/Pyk2 and its association with Src was increased by stimulation that produced LTP. Finally, CAKbeta/Pyk2-stimulated enhancement of synaptic AMPA responses was prevented by blocking NMDARS, chelating intracellular Ca(2+), or blocking Src. Thus, activating CAKbeta/Pyk2 is required for inducing LTP and may depend upon downstream activation of Src to upregulate NMDA receptors.

Conditional expression of fibroblast growth factor-7 in the developing and mature lung

Effects of fibroblast growth factor-7 (FGF-7) on lung morphogenesis, respiratory epithelial cell differentiation, and proliferation were assessed in transgenic mice in which the human FGF-7 cDNA was controlled by a conditional promoter under the direction of regulatory elements from either the human surfactant protein-C (SP-C) or rat Clara cell secretory protein (ccsp) genes. Expression of FGF-7 was induced in respiratory epithelial cells of the fetal lung by administration of doxycycline to the dam. Prenatally, doxycycline induced FGF-7 mRNA in respiratory epithelial cells in both Sp-c and Ccsp transgenic lines, increasing lung size and causing cystadenomatoid malformation. Postnatally, mice bearing both Ccsp-rtta and (Teto)(7)-cmv-fgf-7 transgenes survived, and lung morphology was normal. Induction of FGF-7 expression by doxycycline in the Ccsp-rtta x (Teto)(7)-cmv-fgf-7 mice caused marked epithelial cell proliferation, adenomatous hyperplasia, and pulmonary infiltration with mononuclear cells. Epithelial cell hyperplasia caused by FGF-7 was largely resolved after removal of doxycycline. Surfactant proteins, TTF-1, and aquaporin 5 expression were conditionally induced by doxycycline. The Sp-c-rtta and Ccsp-rtta activator mice provide models in which expression is conditionally controlled in respiratory epithelial cells in the developing and mature lung, altering lung morphogenesis, differentiation, and proliferation.

Activation of Pak by membrane localization mediated by an SH3 domain from the adaptor protein Nck

BACKGROUND

The adaptor protein Nck consists of three Src homology 3 (SH3) domains followed by one SH2 domain. Like the Grb2 adaptor protein, which is known to couple receptor tyrosine kinases to the small GTPase Ras, Nck is presumed to bind to tyrosine-phosphorylated proteins using its SH2 domain and to downstream effector proteins using its SH3 domain. Little is known, however, about the specific biological function of Nck. The Pak family of serine/threonine kinases are known to be activated by binding to the GTP-bound form of Cdc42 or Rac1, which are small GTPases of the Rho family that are involved in regulating the organization of the actin cytoskeleton.

RESULTS

We present evidence that Nck can mediate the relocalization and subsequent activation of the Pak1 kinases. We show that Nck associates in vivo with Pak using the second of its three SH3 domains, and that localization of this individual Nck SH3 domain, or of Pak kinase itself, to the membrane results in activation of Pak and stimulation of downstream mitogen activated protein kinase cascades. Activation of downstream signaling by the membrane-localized Nck SH3 domain is blocked by a kinase-inactive mutant form of Pak1.

CONCLUSION

These results demonstrate that localization of Pak1 to the membrane in the absence of other signals is sufficient for its activation, and imply that the Nck adaptor protein could function to link changes in tyrosine phosphorylation of cellular proteins to the Cdc42/Pak signaling pathway.

Site-specific incorporation of a phosphotyrosine mimetic reveals a role for tyrosine phosphorylation of SHP-2 in cell signaling

The regulation of protein tyrosine phosphatase (PTPase) SHP-2 is proposed to involve tyrosine phosphorylation on two tail tyrosine residues. Using "expressed protein ligation", nonhydrolyzable phosphotyrosine analogs were introduced at known phosphorylation sites in SHP-2. Biochemical analysis suggests that a phosphonate at Tyr542 interacts intramolecularly with the N-terminal SH2 domain to relieve basal inhibition of the PTPase, whereas a phosphonate at Tyr-580 stimulates the PTPase activity by interaction with the C-terminal SH2 domain. Microinjection experiments indicate that a single phosphorylation of Tyr-542 of SHP-2 is sufficient to activate the MAP kinase pathway in living cells. These studies support a novel mechanism explaining how tyrosine phosphorylation of a PTPase is important in signal transduction.

Suppressed miR-424 expression via upregulation of target gene Chk1 contributes to the progression of cervical cancer

MicroRNAs (miRNAs) act as important gene regulators in human genomes and their aberrant expression links to many malignancies. We previously identified a different characteristic miRNA expression profile in cervical cancer from that in cervical normal tissues, including the downregulated miR-424. However, the role and mechanism of miR-424 in cervical cancer still remain unknown. Here, we focused on identifying the tumor-suppressive function and clinical significance of miR-424 and exploring the mechanistic relevance by characterizing its target. We showed a significantly decreased expression of miR-424 in 147 cervical cancer tissues versus 74 cervical normal tissues by performing quantitative RT-PCR. In 147 cervical cancer tissue samples, low-level expression of miR-424 was positively correlated with poor tumor differentiation, advanced clinical stage, lymph node metastasis and other poor prognostic clinicopathological parameters. Further in vitro observations showed that enforced expression of miR-424 inhibited cell growth by both enhancing apoptosis and blocking G1/S transition, and suppressed cell migration and invasion in two human cervical cancer cell lines, SiHa and CaSki, implying that miR-424 functions as a tumor suppressor in the progression of cervical cancer. Interestingly, overexpression of miR-424 inhibited the expression of protein checkpoint kinase 1 (Chk1) and phosphorylated Chk1 (p-Chk1) at residues Ser345 and decreased the activity of luciferase-reporter containing the 3'-untranslated region (UTR) of Chk1 with predicted miR-424-binding site. Moreover, miR-424 expression levels were inversely correlated with Chk1 and p-Chk1 protein levels in both cervical cancer and normal tissues. Furthermore, RNAi-mediated knockdown of Chk1 decreased matrix metalloproteinase 9 expression and phenocopied the tumor suppressive effects of miR-424 in cell models. Taken together, our results identify a crucial tumor suppressive role of miR-424 in the progression of cervical cancer at least partly via upreglating the expression of Chk1 and p-Chk1, and suggest that miR-424 might be a candidate of prognostic predictor or an anticancer therapeutic target for cervical cancer patients.

Subsets of midbrain dopaminergic neurons in monkeys are distinguished by different levels of mRNA for the dopamine transporter: comparison with the mRNA for the D2 receptor, tyrosine hydroxylase and calbindin immunoreactivity

The midbrain dopamine system can be divided into two groups of cells based on chemical characteristics and connectivity. The dorsal tier neurons, which include the dorsal pars compacta and the ventral tegmental area, are calbindin-positive, and project to the shell of the nucleus accumbens. The ventral tier neurons are calbindin-negative and project to the sensorimotor striatum. This study examined the distribution of the mRNAs for the dopamine transporter molecule (DAT) and the D2 receptor in the midbrain of monkeys by using in situ hybridization. The distribution patterns were compared to that of tyrosine hydroxylase and calbindin immunohistochemistry. The results show that high levels of hybridization for DAT and the D2 receptor mRNA are found in the ventral tier, calbindin-negative neurons and relatively low levels are found in the dorsal, calbindin-positive tier. Within the dorsal tier, the dorsal substantia nigra pars compacta has the least amount of both messages. These results show that in monkeys, the ventral tegmental area and the dorsal pars compacta form a dorsal continuum of dopamine neurons which express lower levels of mRNA for DAT and D2 receptor than the ventral tier. DAT has been shown to be involved in the selective neurotoxicity of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Different levels of DAT mRNA and calbindin may explain the differential effects of MPTP neurotoxicity.

Arginine vasotocin a key hormone in fish physiology and behaviour: a review with insights from mammalian models

The arginine vasotocin (AVT) neuroendocrine system clearly provides integrative regulation of many aspects of fish physiology and behaviour, including circadian and seasonal biology, responses to stress, metabolism, reproduction, cardiovascular function, and osmoregulation. These are all considered here providing an important context for the design of experiments and interpretation of results for investigations of specific aspects of AVT function. Salt and water balance is a consistent function from fish to mammals and is examined in more detail. Both AVT and AVP secretion is sensitive to hyperosmotic stimuli and associated cellular dehydration, while hypovolaemia would appear less important. AVT and AVP both mediate renal water conservation, though actions involve different receptors and precise targets in fish (V1) and mammals (V2). The actions of AVT to promote gill NaCl extrusion in fish are conserved in the AVP-induced natriuresis in mammalian kidney to support restoration of plasma osmolality. The AVT/AVP regulatory mechanisms involve both altered neurohypophysial peptide secretion and changes in target-tissue receptor expression/modulation of action. Both mechanisms importantly afford integration with the actions of other related hormone systems.

Pharmacodynamic comparisons of levofloxacin, ciprofloxacin, and ampicillin against Streptococcus pneumoniae in an in vitro model of infection

The increasing frequency of penicillin-resistant pneumococcus continues to be of concern throughout the world. Newer fluoroquinolone antibiotics, such as levofloxacin, have shown enhanced in vitro activity against Streptococcus pneumoniae. In this study, the bactericidal characteristics and pharmacodynamic profiles of levofloxacin, ciprofloxacin, and ampicillin against four isolates of S. pneumoniae were compared by using an in vitro model of infection. Standard antibiotic dosing regimens which simulated the pharmacokinetic profile observed in humans were used. Control and treatment models were sampled for bacterial CFU per milliliter over the duration of each 24- or 48-h experiment. In addition, treatment models were sampled for MIC determinations and drug concentration. Regrowth of all isolates as well as an increase in MICs throughout the study period was observed in the ciprofloxacin experiments. A limited amount of regrowth was noted during levofloxacin therapy for one isolate; however, no change in MIC was detected for any isolate. Ampicillin showed rapid and sustained bactericidal activity against all isolates. In this study, ratios of effective fluoroquinolone area under the concentration-time curve (AUC):MIC values ranged from 30 to 55. Levofloxacin, owing to its larger AUC0-24 values, has excellent and sustained activity against different pneumococcal strains superior to that of ciprofloxacin.

Comparison of Pkd1-targeted mutants reveals that loss of polycystin-1 causes cystogenesis and bone defects

A high level of polycystin-1 expression is detected in kidneys of all patients with autosomal dominant polycystic kidney disease (ADPKD). Mice that overexpress polycystin-1 also develop renal cysts. Whether overexpression of polycystin-1 is necessary for cyst formation is still unclear. Here, we report the generation of a targeted mouse mutant with a null mutation in Pkd1 and its phenotypic characterization in comparison with the del34 mutants that carry a 'truncation mutation' in Pkd1. We show that null homozygotes develop the same, but more aggressive, renal and pancreatic cystic disease as del34/del34. Moreover, we report that both homozygous mutants develop polyhydramnios, hydrops fetalis, spina bifida occulta and osteochondrodysplasia. Heterozygotes also develop adult-onset pancreatic disease. We show further that del34 homozygotes continue to produce mutant polycystin-1, thereby providing a possible explanation for increased immunoreactive polycystin-1 in ADPKD cyst epithelia in the context of the two-hit model. Our data demonstrate for the first time that loss of polycystin-1 leads to cyst formation and defective skeletogenesis, and indicate that polycystin-1 is critical in both epithelium and chondrocyte development.

Fibroblast growth factor-10. A second candidate stromal to epithelial cell andromedin in prostate

Fibroblast growth factor (FGF)-10, a homologue of FGF-7, is expressed significantly in normal rat prostate tissue, well differentiated rat prostate tumors with an epithelial and stromal compartment and only in derived prostate stromal cells in culture. Similar to FGF-7, recombinant rat FGF-10 was a specific mitogen for prostate epithelial cells. In contrast to FGF-7 which is widely expressed among stromal cells in tissues, the expression of FGF-10 correlated with the presence of stromal cells of muscle origin. Radioreceptor binding assays and covalent cross-linking analysis revealed that FGF-10 binds with an affinity equal to FGF-7 to resident epithelial cell receptor, FGFR2IIIb, but unlike FGF-7 also binds the IIIb splice variant of FGFR1. Analysis of mRNA expression by RNase protection revealed that, similar to FGF-7, the expression of FGF-10 was responsive to androgen in stromal cells from normal prostate and non-malignant differentiated tumors. Although FGF-10 cDNA exhibits a signal sequence for secretion, cultured stromal cells exhibit strictly a cell-associated FGF-10 antigen that correlates with an alternately translated intracellular isoform. FGF-10 requires 1.4 times higher NaCl for elution from immobilized heparin than does FGF-7 and binds to four times the number of sites on the pericellular matrix of epithelial cells. The results show that prostate stromal cell-derived FGF-10, like FGF-7, exhibits the properties of an andromedin which may indirectly mediate control of epithelial cell growth and function by androgen. Although FGF-10 and FGF-7 bind and activate the same resident epithelial cell receptor (FGFR2IIIb), differences in cell type of origin, compartmentation by alternate translation, the affinity for FGFR1IIIb, and access to FGFR by differential interaction with pericellular matrix heparan sulfate suggest they may play both independent and compensatory roles in prostate homeostasis.

Association of genetic variation in FTO with risk of obesity and type 2 diabetes with data from 96,551 East and South Asians

AIMS/HYPOTHESIS

FTO harbours the strongest known obesity-susceptibility locus in Europeans. While there is growing evidence for a role for FTO in obesity risk in Asians, its association with type 2 diabetes, independently of BMI, remains inconsistent. To test whether there is an association of the FTO locus with obesity and type 2 diabetes, we conducted a meta-analysis of 32 populations including 96,551 East and South Asians.

METHODS

All studies published on the association between FTO-rs9939609 (or proxy [r (2) > 0.98]) and BMI, obesity or type 2 diabetes in East or South Asians were invited. Each study group analysed their data according to a standardised analysis plan. Association with type 2 diabetes was also adjusted for BMI. Random-effects meta-analyses were performed to pool all effect sizes.

RESULTS

The FTO-rs9939609 minor allele increased risk of obesity by 1.25-fold/allele (p = 9.0 × 10(-19)), overweight by 1.13-fold/allele (p = 1.0 × 10(-11)) and type 2 diabetes by 1.15-fold/allele (p = 5.5 × 10(-8)). The association with type 2 diabetes was attenuated after adjustment for BMI (OR 1.10-fold/allele, p = 6.6 × 10(-5)). The FTO-rs9939609 minor allele increased BMI by 0.26 kg/m(2) per allele (p = 2.8 × 10(-17)), WHR by 0.003/allele (p = 1.2 × 10(-6)), and body fat percentage by 0.31%/allele (p = 0.0005). Associations were similar using dominant models. While the minor allele is less common in East Asians (12-20%) than South Asians (30-33%), the effect of FTO variation on obesity-related traits and type 2 diabetes was similar in the two populations.

CONCLUSIONS/INTERPRETATION

FTO is associated with increased risk of obesity and type 2 diabetes, with effect sizes similar in East and South Asians and similar to those observed in Europeans. Furthermore, FTO is also associated with type 2 diabetes independently of BMI.

The neuronal repressor REST/NRSF is an essential regulator in medulloblastoma cells

Medulloblastoma is the most malignant pediatric brain tumor. It is believed to originate from the undifferentiated external granule layer cells in the cerebellum, but the mechanism of tumorigenesis remains unknown. Here we studied three types of human medulloblastoma cells that express markers corresponding to different levels of neuronal differentiation. They expressed the neuronal repressor element 1 (RE1) silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF; refs. 7-10) at very high levels compared with either neuronal progenitor NTera2 (NT2) cells or fully differentiated human neuron teratocarcinoma (hNT cells). To counter the effect of REST/NRSF, we used a recombinant transcription factor, REST-VP16, constructed by replacing repressor domains of REST/NRSF with the activation domain of viral protein (VP16). Transient expression of REST-VP16 in medulloblastoma cells was able to compete with the endogenous REST/NRSF for DNA binding and stimulate neuronal promoters. High-efficiency expression of REST-VP16 mediated by adenovirus vectors (Ad.REST-VP16) in medulloblastoma cells was able to counter REST/NRSF-mediated repression of neuronal promoters, stimulate expression of endogenous neuronal genes and trigger apoptosis through the activation of caspase cascades. Furthermore, intratumoral injection of Ad.REST-VP16 in established medulloblastoma tumors in nude mice inhibited their growth. Therefore, REST/NRSF may serve as a new target for therapeutic interventions for medulloblastoma through agents such as REST-VP16.

Inhibition of MDM2 by hsp90 contributes to mutant p53 stabilization

Stabilization and overexpression are hallmarks of mutant p53 found in nearly 50% of human tumors. Mutations in the conformation-sensitive core domain of p53 often lead to association with molecular chaperones such as hsp70 and hsp90. Inhibition of hsp90 function accelerates mutant p53 degradation. We recently found that expression of p53 core domain mutants inhibits MDM2 degradation, suggesting that mutant p53 can modulate MDM2 functions. In this report, we show that mutant p53 mediates formation of MDM2-p53-hsp90 complexes. Release of MDM2 from the p53-hsp90 complex after DNA damage restores MDM2 but not p53 turnover, whereas dissociation of hsp90 by geldanamycin increases the degradation of both MDM2 and mutant p53. Mutant p53 degradation after hsp90 inhibition requires MDM2 expression. The interaction between MDM2 and hsp90 is disrupted by the 2A10 antibody, which recognizes a site on MDM2 important for binding to alternative reading frame (ARF). Expression of mutant p53 prevents MDM2 from binding ARF and accumulating in the nucleolus in an hsp90-dependent fashion. These results suggest that hsp90 recruited by mutant p53 conceals the ARF-binding site on MDM2 and inhibits its ubiquitin-protein isopeptide ligase function, resulting in the stabilization of both mutant p53 and MDM2.