Inhibition of a new AXL isoform, AXL3, induces apoptosis of mantle cell lymphoma cells.

Mantle cell lymphoma (MCL) is an aggressive B-cell non-Hodgkin lymphoma having a poor overall survival that is in need for the development of new therapeutics. In this study, we report the identification and expression of a new isoform splice variant of the tyrosine kinase receptor AXL in MCL cells. This new AXL isoform, called AXL3, lacks the ligand-binding domain of the commonly described AXL splice variants and is constitutively activated in MCL cells. Interestingly, functional characterization of AXL3, using CRISPR inhibition, revealed that only the knock down of this isoform leads to apoptosis of MCL cells. Importantly, pharmacological inhibition of AXL activity resulted in a significant decrease in the activation of well-known proproliferative and survival pathways activated in MCL cells (ie, ß-catenin, Ak strain transforming, and NF-κB). Therapeutically, preclinical studies using a xenograft mouse model of MCL indicated that bemcentinib is more effective than ibrutinib in reducing the tumor burden and to increase the overall survival. Our study highlights the importance of a previously unidentified AXL splice variant in cancer and the potential of bemcentinib as a targeted therapy for MCL.

Gene Methylation and Silencing of WIF1 Is a Frequent Genetic Abnormality in Mantle Cell Lymphoma.

We have previously shown that the Wnt canonical pathway (WCP) is constitutively active in most cases of mantle cell lymphoma (MCL). Here, we aimed to elucidate the mechanisms underlying this biochemical deregulation. We hypothesized that gene methylation/silencing of WIF1 (Wnt inhibitory factor-1), a physiologic inhibitor of WCP, contributes to the deregulation of WCP and promotes cell growth in MCL. In support of this hypothesis, we found that the expression of WIF1 was detectable in none of the 4 MCL cell lines, and in only 2 of 5 tumors (40%) examined. Using methylation-specific PCR, we found evidence of gene methylation of WIF1 in 4 of 5 cell lines (80%) and in 24 of 29 (82%) tumors. The addition of the demethylation agent 5-aza-2'-deoxycytidine to Mino and JeKo-1, two WIF1-negative cell lines, restored the expression of WIF1 mRNA in these cells. Gene transfection of WIF1 into JeKo-1 and Mino cells significantly reduced cell growth, and this finding correlated with substantial downregulations of various proteins in WCP, such as ß-catenin and pGSK-3ß. In conclusion, our results support the concept that gene methylation/silencing of WIF1 is a frequent event in MCL, and this abnormality contributes to the aberrant activation of WCP. These results have provided further evidence that aberrant Wnt signaling is pathogenetically important in MCL and it may represent a potential therapeutic target.

Endoplasmic Reticulum Calcium Pumps and Tumor Cell Differentiation.

Endoplasmic reticulum (ER) calcium homeostasis plays an essential role in cellular calcium signaling, intra-ER protein chaperoning and maturation, as well as in the interaction of the ER with other organelles. Calcium is accumulated in the ER by sarco/endoplasmic reticulum calcium ATPases (SERCA enzymes) that generate by active, ATP-dependent transport, a several thousand-fold calcium ion concentration gradient between the cytosol (low nanomolar) and the ER lumen (high micromolar). SERCA enzymes are coded by three genes that by alternative splicing give rise to several isoforms, which can display isoform-specific calcium transport characteristics. SERCA expression levels and isoenzyme composition vary according to cell type, and this constitutes a mechanism whereby ER calcium homeostasis is adapted to the signaling and metabolic needs of the cell, depending on its phenotype, its state of activation and differentiation. As reviewed here, in several normal epithelial cell types including bronchial, mammary, gastric, colonic and choroid plexus epithelium, as well as in mature cells of hematopoietic origin such as pumps are simultaneously expressed, whereas in corresponding tumors and leukemias SERCA3 expression is selectively down-regulated. SERCA3 expression is restored during the pharmacologically induced differentiation of various cancer and leukemia cell types. SERCA3 is a useful marker for the study of cell differentiation, and the loss of SERCA3 expression constitutes a previously unrecognized example of the remodeling of calcium homeostasis in tumors.

Sarco/endoplasmic reticulum calcium ATPase 3 (SERCA3) expression in gastrointestinal stromal tumours.

Accurate characterisation of gastrointestinal stromal tumours (GIST) is important for prognosis and the choice of targeted therapies. Histologically the diagnosis relies on positive immunostaining of tumours for KIT (CD117) and DOG1. Here we report that GISTs also abundantly express the type 3 Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA3). SERCA enzymes transport calcium ions from the cytosol into the endoplasmic reticulum and play an important role in regulating the intensity and the periodicity of calcium-induced cell activation. GISTs from various localisations, histological and molecular subtypes or risk categories were intensely immunopositive for SERCA3 with the exception of PDGFRA-mutated cases where expression was high or moderate. Strong SERCA3 expression was observed also in normal and hyperplastic interstitial cells of Cajal. Decreased SERCA3 expression in GIST was exceptionally observed in a zonal pattern, where CD117 staining was similarly decreased, reflecting clonal heterogeneity. In contrast to GIST, SERCA3 immunostaining of spindle cell tumours and other gastrointestinal tumours resembling GIST was negative or weak. In conclusion, SERCA3 immunohistochemistry may be useful for the diagnosis of GIST with high confidence, when used as a third marker in parallel with KIT and DOG1. Moreover, SERCA3 immunopositivity may be particularly helpful in cases with negative or weak KIT or DOG1 staining, a situation that may be encountered de novo, or during the spontaneous or therapy-induced clonal evolution of GIST.

CD37 is a safe chimeric antigen receptor target to treat acute myeloid leukemia.

Acute myeloid leukemia (AML) is characterized by the accumulation of immature myeloid cells in the bone marrow and the peripheral blood. Nearly half of the AML patients relapse after standard induction therapy, and new forms of therapy are urgently needed. Chimeric antigen receptor (CAR) T therapy has so far not been successful in AML due to lack of efficacy and safety. Indeed, the most attractive antigen targets are stem cell markers such as CD33 or CD123. We demonstrate that CD37, a mature B cell marker, is expressed in AML samples, and its presence correlates with the European LeukemiaNet (ELN) 2017 risk stratification. We repurpose the anti-lymphoma CD37CAR for the treatment of AML and show that CD37CAR T cells specifically kill AML cells, secrete proinflammatory cytokines, and control cancer progression in vivo. Importantly, CD37CAR T cells display no toxicity toward hematopoietic stem cells. Thus, CD37 is a promising and safe CAR T cell AML target.

Efficient CAR T cell targeting of the CA125 extracellular repeat domain of MUC16.

BACKGROUND: Ovarian cancer (OC) is the leading cause of death from gynecologic malignancies in the Western world. Contributing factors include a high frequency of late-stage diagnosis, the development of chemoresistance, and the evasion of host immune responses. Currently, debulking surgery and platinum-based chemotherapy are the treatment cornerstones, although recurrence is common. As the clinical efficacy of immune checkpoint blockade is low, new immunotherapeutic strategies are needed. Chimeric antigen receptor (CAR) T cell therapy empowers patients' own T cells to fight and eradicate cancer, and has been tested against various targets in OC. A promising candidate is the MUC16 ectodomain. This ectodomain remains on the cell surface after cleavage of cancer antigen 125 (CA125), the domain distal from the membrane, which is currently used as a serum biomarker for OC. CA125 itself has not been tested as a possible CAR target. In this study, we examined the suitability of the CA125 as a target for CAR T cell therapy. METHODS: We tested a series of antibodies raised against the CA125 extracellular repeat domain of MUC16 and adapted them to the CAR format. Comparisons between these candidates, and against an existing CAR targeting the MUC16 ectodomain, identified K101 as having high potency and specificity. The K101CAR was subjected to further biochemical and functional tests, including examination of the effect of soluble CA125 on its activity. Finally, we used cell lines and advanced orthotopic patient-derived xenograft (PDX) models to validate, in vivo, the efficiency of our K101CAR construct. RESULTS: We observed a high efficacy of K101CAR T cells against cell lines and patient-derived tumors, in vitro and in vivo. We also demonstrated that K101CAR functionality was not impaired by the soluble antigen. Finally, in direct comparisons, K101CAR, which targets the CA125 extracellular repeat domains, was shown to have similar efficacy to the previously validated 4H11CAR, which targets the MUC16 ectodomain. CONCLUSIONS: Our in vitro and in vivo results, including PDX studies, demonstrate that the CA125 domain of MUC16 represents an excellent target for treating MUC16-positive malignancies.

Gene methylation and silencing of SOCS3 in mantle cell lymphoma.

The significance of loss of SOCS3, a negative regulator of signalling pathways including those of STAT3 and NF-κB, was examined in mantle cell lymphoma (MCL). The protein expression and gene methylation status of SOCS3 were detected using immunohistochemistry/Western blots and methylation-specific polymerase chain reaction, respectively. To evaluate its functional importance, SOCS3 was restored in two SOCS3-negative MCL cell lines using a lentiviral vector. Loss of SOCS3 protein expression was found in 3/4 MCL cell lines and 18/33 (54.5%) tumours. SOCS3 was found consistently methylated in cell lines (3/4) and tumours (7/7) negative for SOCS3, and was unmethylated in all SOCS3-positive cell line (1/1) and tumours (5/5) examined. Treatment of all three SOCS3-negative cell lines with 2'-deoxy-5-azacytidine restored SOCS3 expression. SOCS3 is biologically important in MCL, as lentiviral transfer of SOCS3 in SOCS3-negative cell lines increased their apoptotic activity, downregulated nuclear factor (NF)-κB-p65, cyclin D1 (CCND1), BCL2 and BCL-XL (BCL2L1), and substantially dampened interleukin 10-induced STAT3 activation. In 19 patients aged ≤ 69 years at time of diagnosis, we found that those that carried SOCS3-negative tumours showed a trend toward a worse outcome (P = 0.1, log-rank).

Constitutive activation of metalloproteinase ADAM10 in mantle cell lymphoma promotes cell growth and activates the TNFα/NFκB pathway.

One of the main functions of A Disintegrin and Metalloproteinase 10 (ADAM10) is to regulate the bioavailability of adhesion molecules and ligands to various cellular-signaling receptors. Constitutive activation of ADAM10 has been implicated in the pathogenesis of several types of solid tumors. In this study, we found that mantle cell lymphoma (MCL) cell lines and all 12 patient samples examined expressed the active/mature form of ADAM10. In contrast, PBMCs from healthy donors (n = 5) were negative. Using immunohistochemistry, ADAM10 was readily detectable in 20 of 23 (87%) MCL tumors, but absent in 5 reactive tonsils. Knockdown of ADAM10 using short interfering RNA (siRNA) in MCL cells significantly induced growth inhibition and cell-cycle arrest, and these changes were correlated with down-regulation of cyclin D1, up-regulation of p21(waf1), and significant reductions in the TNFα production/transcriptional activity of NFκBp65. The addition of recombinant ADAM10 to MCL cells led to the opposite biologic effects. Lastly, down-regulation of ADAM10 using siRNA enhanced the growth-suppressing effects mediated by the proteasome inhibitors MG132 and bortezomib. We conclude that constitutive activation of ADAM10 contributes to the growth of MCL and therefore inhibition of ADAM10 may be a useful strategy to enhance the response of MCL to other therapeutic agents.

Fusion tyrosine kinase NPM-ALK Deregulates MSH2 and suppresses DNA mismatch repair function novel insights into a potent oncoprotein.

The fusion tyrosine kinase NPM-ALK is central to the pathogenesis of ALK-positive anaplastic large cell lymphoma (ALK(+)ALCL). We recently identified that MSH2, a key DNA mismatch repair (MMR) protein integral to the suppression of tumorigenesis, is an NPM-ALK-interacting protein. In this study, we found in vitro evidence that enforced expression of NPM-ALK in HEK293 cells suppressed MMR function. Correlating with these findings, six of nine ALK(+)ALCL tumors displayed evidence of microsatellite instability, as opposed to none of the eight normal DNA control samples (P = 0.007, Student's t-test). Using co-immunoprecipitation, we found that increasing levels of NPM-ALK expression in HEK293 cells resulted in decreased levels of MSH6 bound to MSH2, whereas MSH2·NPM-ALK binding was increased. The NPM-ALK·MSH2 interaction was dependent on the activation/autophosphorylation of NPM-ALK, and the Y191 residue of NPM-ALK was a crucial site for this interaction and NPM-ALK-mediated MMR suppression. MSH2 was found to be tyrosine phosphorylated in the presence of NPM-ALK. Finally, NPM-ALK impeded the expected DNA damage-induced translocation of MSH2 out of the cytoplasm. To conclude, our data support a model in which the suppression of MMR by NPM-ALK is attributed to its ability to interfere with normal MSH2 biochemistry and function.

Dihydroorotate dehydrogenase inhibition acts synergistically with tyrosine kinase inhibitors to induce apoptosis of mantle cell lymphoma cells.

Mantle cell lymphoma (MCL) is a non-Hodgkin lymphoma that remains incurable with the treatment options available today. In the present study, we have identified the dihydroorotate dehydrogenase (DHODH), an essential enzyme for the de novo biosynthesis of pyrimidine-based nucleotides, to be overexpressed in MCL in comparison to healthy peripheral blood mononuclear cells (PBMC). In vitro inhibition of the DHODH activity using a newly developed DHODH inhibitor, namely (R)-HZ05, can induce MCL cell death in the nanomolar range independently than the P53 status of the investigated cell lines. Moreover, the combination of (R)-HZ05 with tyrosine kinase inhibitor shows the synergistic activity on cell death. Pre-clinical investigation on the efficacy of (R)-HZ05 shows that it can be prolonged animal lifespan similar to ibrutinib. (R)-HZ05 use in combination with tyrosine kinase inhibitor demonstrated a superior efficacy on tumor burden reduction and survival than either drug alone. We have demonstrated that the depletion of the pyrimidine nucleotide pool, using DHODH inhibitor, represents a new therapeutic strategy that may benefit MCL patients.

The tyrosine 343 residue of nucleophosmin (NPM)-anaplastic lymphoma kinase (ALK) is important for its interaction with SHP1, a cytoplasmic tyrosine phosphatase with tumor suppressor functions.

The cytoplasmic tyrosine phosphatase SHP1 has been shown to inhibit the oncogenic fusion protein nucleophosmin (NPM)-anaplastic lymphoma kinase (ALK), and loss of SHP1 contributes to NPM-ALK-mediated tumorigenesis. In this study, we aimed to further understand how SHP1 interacts and regulates NPM-ALK. We employed an in vitro model in which GP293 cells were transfected with various combinations of NPM-ALK (or mutants) and SHP1 (or mutants) expression vectors. We found that SHP1 co-immunoprecipitated with NPM-ALK, but not the enzymatically inactive NPM-ALK(K210R) mutant, or the mutant in which all three functionally important tyrosine residues (namely, Tyr(338), Tyr(342), and Tyr(343)) in the kinase activation loop (KAL) of ALK were mutated. Interestingly, whereas mutation of Tyr(338) or Tyr(342) did not result in any substantial change in the NPM-ALK/SHP1 binding (assessed by co-immunoprecipitation), mutation of Tyr(343) abrogated this interaction. Furthermore, the NPM-ALK/SHP1 binding was readily detectable when each of the remaining 8 tyrosine residues known to be phosphorylated were mutated. Although the expression of SHP1 effectively reduced the level of tyrosine phosphorylation of NPM-ALK, it did not affect that of the NPM-ALK(Y343F) mutant. In soft agar clonogenic assay, SHP1 expression significantly reduced the tumorigenicity of NPM-ALK but not that of NPM-ALK(Y343F). In conclusion, we identified Tyr(343) of NPM-ALK as the crucial site for mediating the NPM-ALK/SHP1 interaction. Our results also support the notion that the tumor suppressor effects of SHP1 on NPM-ALK are dependent on its ability to bind to this oncogenic protein.

ß-catenin is constitutively active and increases STAT3 expression/activation in anaplastic lymphoma kinase-positive anaplastic large cell lymphoma.

BACKGROUND: The role of ß-catenin in cancer has been most studied in tumors of epithelial cell origin. The functional status and biological significance of this protein in anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma is unknown. DESIGN AND METHODS: ALK-positive anaplastic large cell lymphoma cell lines and patients' tumor samples were examined for status of ß-catenin expression and signaling. The subcellular localization of ß-catenin was assessed using immunohistochemistry, sub-cellular fractionation and confocal microscopy, while its transcriptional activity was studied using the TOPFlash/FOPFlash luciferase reporter assay. To examine the biological significance of ß-catenin, short interfering RNA was used to knock-down its expression; the resulting biological effects were studied using trypan-blue exclusion and MTS assay, and the impact on its various downstream targets was assessed using quantitative real-time polymerase chain reaction and western blots. RESULTS: ß-catenin was transcriptionally active in three of three ALK-positive anaplastic large cell lymphoma cell lines, and this finding correlates with the nuclear localization of ß-catenin in these cells and the neoplastic cells identified in most of the patients' tumor samples. ß-catenin is biologically significant in ALK-positive anaplastic large cell lymphoma, since down-regulation of ß-catenin resulted in a significant reduction in their cell growth. Down-regulation of ß-catenin led to a marked reduction in both the total protein level and the activated/phosphorylated form of STAT3, another signaling protein previously shown to be important in the pathogenesis of ALK-positive anaplastic large cell lymphoma. In contrast to some of the oncogenic tyrosine kinases, modulation of nucleophosmin-anaplastic lymphoma kinase expression did not result in any detectable change in the protein level, nuclear localization or tyrosine phosphorylation of ß-catenin; however, inhibition of nucleophosmin-anaplastic lymphoma kinase expression significantly down-regulated the transcriptional activity of ß-catenin. CONCLUSIONS: ß-catenin signaling is constitutively active in ALK-positive anaplastic large cell lymphoma and represents a previously unknown mechanism by which the high levels of STAT3 expression and activation in these tumors are sustained. Our results suggest that the interaction between oncogenic tyrosine kinases and various cell signaling proteins may be more complex than previously believed.

IL-21 contributes to JAK3/STAT3 activation and promotes cell growth in ALK-positive anaplastic large cell lymphoma.

Interleukin (IL)-21 has been reported to both stimulate cell growth and promote survival in benign lymphoid cells and several types of hematopoietic neoplasms. It induces JAK3/STAT3 signaling, a biologically important cellular pathway activated in most cases of anaplastic lymphoma kinase (ALK)-expressing anaplastic large cell lymphoma (ALK(+)ALCL). Therefore, we hypothesize that IL-21 may contribute to JAK3/STAT3 activation and cell growth in ALK(+)ALCL. By reverse transcription-PCR, we found consistent expression of IL-21 receptor (IL-21R) in all ALK(+)ALCL cell lines and frozen tumors examined. IL-21 was also consistently expressed in ALK(+)ALCL tumors, although its mRNA was detectable in only one of three cell lines tested. By immunohistochemistry, we examined 10 paraffin-embedded ALK(+)ALCL tumors; all cases were positive for both IL-21 and IL-21R in these neoplastic cells. IL-21 signaling is biologically significant in ALK(+)ALCL since the addition of recombinant IL-21 enhanced the activation of JAK3/STAT3 and significantly increased cell growth in ALK(+)ALCL cell lines. However, small interfering RNA down-regulation of IL-21R significantly decreased both STAT3 activation and cell growth. IL-21R expression is not linked to nucleophosmin-ALK since forced expression of nucleophosmin-ALK and small interfering RNA down-regulation of nucleophosmin-ALK did not significantly change the expression of either IL-21R or IL-21. Our findings thus support the enhancement of JAK3/STAT3 activation and cell growth in ALK(+)ALCL via IL-21 signaling. These results further support the concept that constitutive activation of STAT3 in these tumors is multifactorial.

Constitutive activation of the Wnt canonical pathway in mantle cell lymphoma.

Aberrations of the Wnt canonical pathway (WCP) are known to contribute to the pathogenesis of various types of cancer. We hypothesize that these defects may exist in mantle cell lymphoma (MCL). Both the upstream and downstream aspects of WCP were examined in MCL cell lines and tumors. Using WCP-specific oligonucleotide arrays, we found that MCL highly and consistently expressed Wnt3 and Wnt10. beta-catenin, a transcriptional factor that is a downstream target of WCP, is localized to the nucleus and transcriptionally active in all 3 MCL cell lines examined. By immunohistochemistry, 33 (52%) of 64 MCL tumors showed nuclear localization of beta-catenin, which significantly correlated with the expression of the phosphorylated/inactive form of GSK3beta (p-GSK3beta; P = .011, Fisher). GSK3beta inactivation is directly linked to WCP stimulation, since addition of recombinant sFRP proteins (a naturally occurring decoy for the Wnt receptors) resulted in a significant decrease in p-GSK3beta. Down-regulation of DvL-2 (an upstream signaling protein in WCP) by siRNA or selective inhibition of beta-catenin using quercetin significantly decreased cell growth in MCL cell lines. To conclude, WCP is constitutively activated in a subset of MCL and it appears to promote tumorigenesis in MCL.

Calreticulin signals upstream of calcineurin and MEF2C in a critical Ca(2+)-dependent signaling cascade.

We uncovered a new pathway of interplay between calreticulin and myocyte-enhancer factor (MEF) 2C, a cardiac-specific transcription factor. We establish that calreticulin works upstream of calcineurin and MEF2C in a Ca(2+)-dependent signal transduction cascade that links the endoplasmic reticulum and the nucleus during cardiac development. In the absence of calreticulin, translocation of MEF2C to the nucleus is compromised. This defect is reversed by calreticulin itself or by a constitutively active form of calcineurin. Furthermore, we show that expression of the calreticulin gene itself is regulated by MEF2C in vitro and in vivo and that, in turn, increased expression of calreticulin affects MEF2C transcriptional activity. The present findings provide a clear molecular explanation for the embryonic lethality observed in calreticulin-deficient mice and emphasize the importance of calreticulin in the early stages of cardiac development. Our study illustrates the existence of a positive feedback mechanism that ensures an adequate supply of releasable Ca(2+) is maintained within the cell for activation of calcineurin and, subsequently, for proper functioning of MEF2C.

Stabilization of ß-catenin upon B-cell receptor signaling promotes NF-kB target genes transcription in mantle cell lymphoma.

B-cell receptor (BCR) signaling pathways and interactions with the tumor microenvironment account for mantle cell lymphoma (MCL) cells survival in lymphoid organs. In several MCL cases, the WNT/ß-catenin canonical pathway is activated and ß-catenin accumulates into the nucleus. As both BCR and ß-catenin are important mediators of cell survival and interaction with the microenvironment, we investigated the crosstalk between BCR and WNT/ß-catenin signaling and analyzed their impact on cellular homeostasis as well as their targeting by specific inhibitors. ß-catenin was detected in all leukemic MCL samples and its level of expression rapidly increased upon BCR stimulation. This stabilization was hampered by the BCR-pathway inhibitor Ibrutinib, supporting ß-catenin as an effector of the BCR signaling. In parallel, MCL cells as compared with normal B cells expressed elevated levels of WNT16, a NF-κB target gene. Its expression increased further upon BCR stimulation to participate to the stabilization of ß-catenin. Upon BCR stimulation, ß-catenin translocated into the nucleus but did not induce a Wnt-like transcriptional response, i.e., TCF/LEF dependent. ß-catenin rather participated to the regulation of NF-κB transcriptional targets, such as IL6, IL8, and IL1. Oligo pull down and chromatin immunoprecipitation experiments demonstrated that ß-catenin is part of a protein complex that binds the NF-κB DNA consensus sequence, strengthening the idea of an association between the two proteins. An inhibitor targeting ß-catenin transcriptional interactions hindered both NF-κB DNA recruitment and induced primary MCL cells apoptosis. Thus, ß-catenin likely represents another player through which BCR signaling impacts on MCL cell survival.

A DHODH inhibitor increases p53 synthesis and enhances tumor cell killing by p53 degradation blockage.

The development of non-genotoxic therapies that activate wild-type p53 in tumors is of great interest since the discovery of p53 as a tumor suppressor. Here we report the identification of over 100 small-molecules activating p53 in cells. We elucidate the mechanism of action of a chiral tetrahydroindazole (HZ00), and through target deconvolution, we deduce that its active enantiomer (R)-HZ00, inhibits dihydroorotate dehydrogenase (DHODH). The chiral specificity of HZ05, a more potent analog, is revealed by the crystal structure of the (R)-HZ05/DHODH complex. Twelve other DHODH inhibitor chemotypes are detailed among the p53 activators, which identifies DHODH as a frequent target for structurally diverse compounds. We observe that HZ compounds accumulate cancer cells in S-phase, increase p53 synthesis, and synergize with an inhibitor of p53 degradation to reduce tumor growth in vivo. We, therefore, propose a strategy to promote cancer cell killing by p53 instead of its reversible cell cycle arresting effect.

Publisher Correction: A DHODH inhibitor increases p53 synthesis and enhances tumor cell killing by p53 degradation blockage.

The original PDF version of this Article listed the authors as "Marcus J.G.W. Ladds," where it should have read "Marcus J. G. W. Ladds, Ingeborg M. M. van Leeuwen, Catherine J. Drummond et al.#".Also in the PDF version, it was incorrectly stated that "Correspondence and requests for materials should be addressed to S. Lín.", instead of the correct "Correspondence and requests for materials should be addressed to S. Laín."This has been corrected in the PDF version of the Article. The HTML version was correct from the time of publication.