A CANCER PERSISTENT DNA REPAIR CIRCUIT DRIVEN BY MDM2, MDM4 (MDMX), AND MUTANT P53 FOR RECRUITMENT OF MDC1 AND 53BP1 TO CHROMATIN.

The influence of the metastasis promoting proteins mutant p53 (mtp53) and MDM2 on Cancer Persistent Repair (CPR) to promote cancer cell survival is understudied. Interactions between the DNA repair choice protein 53BP1 and wild type tumor suppressor protein p53 (wtp53) regulates cell cycle control. Cancer cells often express elevated levels of transcriptionally inactive missense mutant p53 (mtp53) that interacts with MDM2 and MDM4/MDMX (herein called MDMX). The ability of mtp53 to maintain a 53BP1 interaction while in the context of interactions with MDM2 and MDMX has not been described. We asked if MDM2 regulates chromatin-based phosphorylation events in the context of mtp53 by comparing the chromatin of T47D breast cancer cells with and without MDM2 in a phospho-peptide stable isotope labeling in cell culture (SILAC) screen. We found reduced phospho-53BP1 chromatin association, which we confirmed by chromatin fractionation and immunofluorescence in multiple breast cancer cell lines. We used the Proximity Ligation Assay (PLA) in breast cancer cell lines and detected 53BP1 in close proximity to mtp53, MDM2, and the DNA repair protein MDC1. Through disruption of the mtp53-MDM2 interaction, by either Nutlin 3a or a mtp53 R273H C-terminal deletion, we uncovered that mtp53 was required for MDM2-53BP1 interaction foci. Our data suggests that mtp53 works with MDM2 and 53BP1 to promote CPR and cell survival.

Gain-of-Function Mutant p53 R273H Interacts with Replicating DNA and PARP1 in Breast Cancer.

Over 80% of triple-negative breast cancers (TNBC) express mutant p53 (mtp53) and some contain oncogenic gain-of-function (GOF) p53. We previously reported that GOF mtp53 R273H upregulates the chromatin association of mini chromosome maintenance (MCM) proteins MCM2-7 and PARP and named this the mtp53-PARP-MCM axis. In this study, we dissected the function and association between mtp53 and PARP using a number of different cell lines, patient-derived xenografts (PDX), tissue microarrays (TMA), and The Cancer Genome Atlas (TCGA) database. Endogenous mtp53 R273H and exogenously expressed R273H and R248W bound to nascent 5-ethynyl-2´-deoxyuridine-labeled replicating DNA. Increased mtp53 R273H enhanced the association of mtp53 and PARP on replicating DNA. Blocking poly-ADP-ribose gylcohydrolase also enhanced this association. Moreover, mtp53 R273H expression enhanced overall MCM2 levels, promoted cell proliferation, and improved the synergistic cytotoxicity of treatment with the alkylating agent temozolomide in combination with the PARP inhibitor (PARPi) talazoparib. Staining of p53 and PARP1 in breast cancer TMAs and comparison with the TCGA database indicated a higher double-positive signal in basal-like breast cancer than in luminal A or luminal B subtypes. Higher PARP1 protein levels and PAR proteins were detected in mtp53 R273H than in wild-type p53-expressing PDX samples. These results indicate that mtp53 R273H and PARP1 interact with replicating DNA and should be considered as dual biomarkers for identifying breast cancers that may respond to combination PARPi treatments. SIGNIFICANCE: p53 gain-of-function mutant 273H and PARP1 interact with replication forks and could serve as potential biomarkers for breast cancer sensitivity to PARP inhibitors. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/3/394/F1.large.jpg.

Identification, validation, and targeting of the mutant p53-PARP-MCM chromatin axis in triple negative breast cancer.

Over 80% of triple negative breast cancers express mutant p53. Mutant p53 often gains oncogenic function suggesting that triple negative breast cancers may be driven by p53 protein type. To determine the chromatin targets of this gain-of-function mutant p53 we used inducible knockdown of endogenous gain-of-function mtp53 in MDA-MB-468 cells in conjunction with stable isotope labeling with amino acids in cell culture and subcellular fractionation. We sequenced over 70,000 total peptides for each corresponding reciprocal data set and were able to identify 3010 unique cytoplasmic fraction proteins and 3403 unique chromatin fraction proteins. The present proteomics experiment corroborated our previous experiment-based results that poly ADP-ribose polymerase has a positive association with mutant p53 on the chromatin. Here, for the first time we report that the heterohexomeric minichromosome maintenance complex that participates in DNA replication initiation ranked as a high mutant p53-chromatin associated pathway. Enrichment analysis identified the minichromosome maintenance members 2-7. To validate this mutant p53- poly ADP-ribose polymerase-minichromosome maintenance functional axis, we experimentally depleted R273H mutant p53 and found a large reduction of the amount of minichromosome maintenance complex proteins on the chromatin. Furthermore a mutant p53-minichromosome maintenance 2 direct interaction was detected. Overexpressed mutant p53, but not wild type p53, showed a protein-protein interaction with minichromosome maintenance 2 and minichromosome maintenance 4. To target the mutant p53- poly ADP-ribose polymerase-minichromosome maintenance axis we treated cells with the poly ADP-ribose polymerase inhibitor talazoparib and the alkylating agent temozolomide and detected synergistic activation of apoptosis only in the presence of mutant p53. Furthermore when minichromosome maintenance 2-7 activity was inhibited the synergistic activation of apoptosis was blocked. This mutant p53- poly ADP-ribose polymerase -minichromosome maintenance axis may be useful for theranostics.

Hot Spot Mutation in TP53 (R248Q) Causes Oncogenic Gain-of-Function Phenotypes in a Breast Cancer Cell Line Derived from an African American patient.

African American (AA) breast cancer patients often have triple negative breast cancer (TNBC) that contains mutations in the TP53 gene. The point mutations at amino acid residues R273 and R248 both result in oncogenic gain-of-function (GOF) phenotypes. Expression of mutant p53 (mtp53) R273H associates with increased cell elasticity, survival under serum deprivation conditions, and increased Poly (ADP ribose) polymerase 1 (PARP1) on the chromatin in the AA-derived TNBC breast cancer cell line MDA-MB-468. We hypothesized that GOF mtp53 R248Q expression could stimulate a similar phenotype in the AA-derived TNBC cell line HCC70. To test this hypothesis we depleted the R248Q protein in the HCC70 cell line using shRNA-mediated knockdown. Using impedance-based real-time analysis we correlated the expression of mtp53 R248Q with increased cell deformability. We also documented that depletion of mtp53 R248Q increased PARP1 in the cytoplasm and decreased PARP1 on the chromatin. We conclude that in the AA-derived TNBC HCC70 cells mtp53 R248Q expression results in a causative tumor associated phenotype. This study supports using the biological markers of high expression of mtp53 R273H or R248Q as additional diagnostics for TNBC resistant subtypes often found in the AA community. Each mtp53 protein must be considered separately and this work adds R248Q to the increasing list of p53 mutations that can be used for diagnostics and drug targeting. Here we report that when R248Q mtp53 proteins are expressed in TNBC, then targeting the gain-of-function pathways may improve treatment efficacy.

Homozygous mdm2 SNP309 cancer cells with compromised transcriptional elongation at p53 target genes are sensitive to induction of p53-independent cell death.

A single nucleotide polymorphism (T to G) in the mdm2 P2 promoter, mdm2 SNP309, leads to MDM2 overexpression promoting chemotherapy resistant cancers. Two mdm2 G/G SNP309 cancer cell lines, MANCA and A875, have compromised wild-type p53 that co-localizes with MDM2 on chromatin. We hypothesized that MDM2 in these cells inhibited transcription initiation at the p53 target genes p21 and puma. Surprisingly, following etoposide treatment transcription initiation occurred at the compromised target genes in MANCA and A875 cells similar to the T/T ML-1 cell line. In all cell lines tested there was equally robust recruitment of total and initiated RNA polymerase II (Pol II). We found that knockdown of MDM2 in G/G cells moderately increased expression of subsets of p53 target genes without increasing p53 stability. Importantly, etoposide and actinomycin D treatments increased histone H3K36 trimethylation in T/T, but not G/G cells, suggesting a G/G correlated inhibition of transcription elongation. We therefore tested a chemotherapeutic agent (8-amino-adenosine) that induces p53-independent cell death for higher clinically relevant cytotoxicity. We demonstrated that T/T and G/G mdm2 SNP309 cells were equally sensitive to 8-amino-adenosine induced cell death. In conclusion for cancer cells overexpressing MDM2, targeting MDM2 may be less effective than inducing p53-independent cell death.

Proteome-wide analysis of mutant p53 targets in breast cancer identifies new levels of gain-of-function that influence PARP, PCNA, and MCM4.

The gain-of-function mutant p53 (mtp53) transcriptome has been studied, but, to date, no detailed analysis of the mtp53-associated proteome has been described. We coupled cell fractionation with stable isotope labeling with amino acids in cell culture (SILAC) and inducible knockdown of endogenous mtp53 to determine the mtp53-driven proteome. Our fractionation data highlight the underappreciated biology that missense mtp53 proteins R273H, R280K, and L194F are tightly associated with chromatin. Using SILAC coupled to tandem MS, we identified that R273H mtp53 expression in MDA-MB-468 breast cancer cells up- and down-regulated multiple proteins and metabolic pathways. Here we provide the data set obtained from sequencing 73,154 peptide pairs that then corresponded to 3,010 proteins detected under reciprocal labeling conditions. Importantly, the high impact regulated targets included the previously identified transcriptionally regulated mevalonate pathway proteins but also identified two new levels of mtp53 protein regulation for nontranscriptional targets. Interestingly, mtp53 depletion profoundly influenced poly(ADP ribose) polymerase 1 (PARP1) localization, with increased cytoplasmic and decreased chromatin-associated protein. An enzymatic PARP shift occurred with high mtp53 expression, resulting in increased poly-ADP-ribosylated proteins in the nucleus. Mtp53 increased the level of proliferating cell nuclear antigen (PCNA) and minichromosome maintenance 4 (MCM4) proteins without changing the amount of pcna and mcm4 transcripts. Pathway enrichment analysis ranked the DNA replication pathway above the cholesterol biosynthesis pathway as a R273H mtp53 activated proteomic target. Knowledge of the proteome diversity driven by mtp53 suggests that DNA replication and repair pathways are major targets of mtp53 and highlights consideration of combination chemotherapeutic strategies targeting cholesterol biosynthesis and PARP inhibition.

Impedimetric detection of mutant p53 biomarker-driven metastatic breast cancers under hyposmotic pressure.

In cancer cells, the oncogenic mutant p53 (mtp53) protein is present at high levels and gain-of-function (GOF) activities with more expression of mtp53 proteins contribute to tumor growth and metastasis. Robust analytical approaches that probe the degree of metastasis of cancer cells in connection with the mtp53 activity will be extremely useful not only for establishing a better cancer prognosis but also understanding the fundamental mechanism of mtp53 oncogenic action. Here we assessed the influence of mtp53 in breast cancers to the mechanical property of breast cancer cells. Recently, ovarian and kidney cancer cell lines have been shown to have higher cellular elasticity as compared to normal cells assessed by monitoring the degree of deformation under hyposmotic pressure. To make fast detection in large scale, the impedance measurement was applied to monitor the swelling ratio of cells with time. The results showed that knockdown of mtp53 leads to decrease in cell swelling. In addition, by means of two types of impedimetric detection systems we consistently detected enhancement of impedance signal in mtp53-expressing breast cancer cells. Based on this observation we hypothesize that highly expressed mtp53 in metastatic mutant breast cancers can promote tumor progression by making cells more deformable and easier to spread out through extracellular matrix. The identification via the electric measurement can be accomplished within 10 minutes. All results in this report suggest that electric probing for the extent of the mtp53 expression of breast cancer cells may serve as a meaningful fingerprint for the cancer diagnostics, and this outcome will also have an important clinical implication for the development of mtp53-based targeting for tumor detection and treatment.

Endogenous human MDM2-C is highly expressed in human cancers and functions as a p53-independent growth activator.

Human cancers over-expressing mdm2, through a T to G variation at a single nucleotide polymorphism at position 309 (mdm2 SNP309), have functionally inactivated p53 that is not effectively degraded. They also have high expression of the alternatively spliced transcript, mdm2-C. Alternatively spliced mdm2 transcripts are expressed in many forms of human cancer and when they are exogenously expressed they transform human cells. However no study to date has detected endogenous MDM2 protein isoforms. Studies with exogenous expression of splice variants have been carried out with mdm2-A and mdm2-B, but the mdm2-C isoform has remained virtually unexplored. We addressed the cellular influence of exogenously expressed MDM2-C, and asked if endogenous MDM2-C protein was present in human cancers. To detect endogenous MDM2-C protein, we created a human MDM2-C antibody to the splice junction epitope of exons four and ten (MDM2 C410) and validated the antibody with in vitro translated full length MDM2 compared to MDM2-C. Interestingly, we discovered that MDM2-C co-migrates with MDM2-FL at approximately 98 kDa. Using the validated C410 antibody, we detected high expression of endogenous MDM2-C in human cancer cell lines and human cancer tissues. In the estrogen receptor positive (ER+) mdm2 G/G SNP309 breast cancer cell line, T47D, we observed an increase in endogenous MDM2-C protein with estrogen treatment. MDM2-C localized to the nucleus and the cytoplasm. We examined the biological activity of MDM2-C by exogenously expressing the protein and observed that MDM2-C did not efficiently target p53 for degradation or reduce p53 transcriptional activity. Exogenous expression of MDM2-C in p53-null human cancer cells increased colony formation, indicating p53-independent tumorigenic properties. Our data indicate a role for MDM2-C that does not require the inhibition of p53 for increasing cancer cell proliferation and survival.

8-Amino-adenosine activates p53-independent cell death of metastatic breast cancers.

8-Amino-adenosine (8-NH(2)-Ado) is a ribose sugar nucleoside analogue that reduces cellular ATP levels and inhibits mRNA synthesis. Estrogen receptor-negative (ER-) metastatic breast cancers often contain mutant p53; therefore, we asked if 8-NH(2)-Ado could kill breast cancer cells without activating the p53-pathway. Regardless of the breast cancer subtype tested or the p53 status of the cells, 8-NH(2)-Ado was more cytotoxic than either gemcitabine or etoposide. 8-NH(2)-Ado treatment inhibited cell proliferation, activated cell death, and did not activate transcription of the p53 target gene p21 or increase protein levels of either p53 or p21. This occurred in the estrogen receptor-positive (ER+) MCF-7 cells that express wild-type p53, the ER+ T47-D cells that express mutant p53, and the ER- MDA-MB-468 cells or MDA-MB-231 cells that both express mutant p53. 8-NH(2)-Ado induced apoptotic death of MCF-7 cells and apoptosis was not inhibited by knockdown of functional p53. Moreover, the pan-caspase inhibitor Z-VAD blocked the 8-NH(2)-Ado-induced MCF-7 cell death. Interestingly, 8-NH(2)-Ado caused the MDA-MB-231 cells to detach from the plate with only limited evidence of apoptotic cell death markers and the cell death was not inhibited by Z-VAD. Inhibition of MDA-MB-231 cell autophagy, by reduction of ATG7 or 3-methyladenine treatment, did not block this 8-NH(2)-Ado-mediated cytotoxicity. Importantly 8-NH(2)-Ado was highly cytotoxic to triple-negative breast cancer cells and worked through a pathway that did not require wild-type p53 for cytoxicity. Therefore, 8-NH(2)-Ado should be considered for the treatment of triple-negative breast cancers that are chemotherapy resistant.

Mutant p53 disrupts mammary tissue architecture via the mevalonate pathway.

p53 is a frequent target for mutation in human tumors, and mutant p53 proteins can actively contribute to tumorigenesis. We employed a three-dimensional culture model in which nonmalignant breast epithelial cells form spheroids reminiscent of acinar structures found in vivo, whereas breast cancer cells display highly disorganized morphology. We found that mutant p53 depletion is sufficient to phenotypically revert breast cancer cells to a more acinar-like morphology. Genome-wide expression analysis identified the mevalonate pathway as significantly upregulated by mutant p53. Statins and sterol biosynthesis intermediates reveal that this pathway is both necessary and sufficient for the phenotypic effects of mutant p53 on breast tissue architecture. Mutant p53 associates with sterol gene promoters at least partly via SREBP transcription factors. Finally, p53 mutation correlates with highly expressed sterol biosynthesis genes in human breast tumors. These findings implicate the mevalonate pathway as a therapeutic target for tumors bearing mutations in p53.

A p53-independent role of Mdm2 in estrogen-mediated activation of breast cancer cell proliferation.

INTRODUCTION: Estrogen receptor positive breast cancers often have high levels of Mdm2. We investigated if estrogen signaling in such breast cancers occurred through an Mdm2 mediated pathway with subsequent inactivation of p53. METHODS: We examined the effect of long-term 17ß-estradiol (E2) treatment (five days) on the p53-Mdm2 pathway in estrogen receptor alpha (ERα) positive breast cancer cell lines that contain wild-type p53 (MCF-7 and ZR75-1). We assessed the influence of estrogen by examining cell proliferation changes, activation of transcription of p53 target genes, p53-chromatin interactions and cell cycle profile changes. To determine the effects of Mdm2 and p53 knockdown on the estrogen-mediated proliferation signals we generated MCF-7 cell lines with inducible shRNA for mdm2 or p53 and monitored their influence on estrogen-mediated outcomes. To further address the p53-independent effect of Mdm2 in ERα positive breast cancer we generated cell lines with inducible shRNA to mdm2 using the mutant p53 expressing cell line T-47D. RESULTS: Estrogen increased the Mdm2 protein level in MCF-7 cells without decreasing the p53 protein level. After estrogen treatment of MCF-7 cells, down-regulation of basal transcription of p53 target genes puma and p21 was observed. Estrogen treatment also down-regulated etoposide activated transcription of puma, but not p21. Mdm2 knockdown in MCF-7 cells increased p21 mRNA and protein, decreased cell growth in 3D matrigel and also decreased estrogen-induced cell proliferation in 2D culture. In contrast, knockdown of p53 had no effect on estrogen-induced cell proliferation. In T-47D cells with mutant p53, the knockdown of Mdm2 decreased estrogen-mediated cell proliferation but did not increase p21 protein. CONCLUSIONS: Estrogen-induced breast cancer cell proliferation required a p53-independent role of Mdm2. The combined influence of genetic and environmental factors on the tumor promoting effects of estrogen implicated Mdm2 as a strong contributor to the bypass of cell cycle checkpoints. The novel finding that p53 was not the key target of Mdm2 in the estrogen activation of cell proliferation could have great benefit for future Mdm2-targeted breast cancer therapies.

Lipid mediators of autophagy in stress-induced premature senescence of endothelial cells.

Our group (Patschan S, Chen J, Gealekman O, Krupincza K, Wang M, Shu L, Shayman JA, Goligorsky MS; Am J Physiol Renal Physiol 294: F100-F109, 2008) previously observed an accumulation of gangliosides coincident with development of cell senescence and demonstrated lysosomal permeabilization in human umbilical vein endothelial cells exposed to glycated collagen I (GC). Therefore, we investigated whether the lysosome-dependent, caspase-independent or type 2-programmed cell death (autophagy) is involved in development of premature senescence of endothelial cells. The cleaved microtubule-associated protein 1 light-chain 3 (LC3), a marker of autophagosome formation, was overexpressed within 24 h of GC treatment; however, by 4-5 days, it was nearly undetectable. Early induction of autophagosomes was associated with their fusion with lysosomes, a phenomenon that later became subverted. Autophagic cell death can be triggered by the products of damaged plasma membrane, sphingolipids, and ceramide. We observed a clustering of membrane rafts shortly after exposure to GC; later, after 24 h, we observed an internalization, accompanied by an increased acid sphingomyelinase activity and accumulation of ceramide. Pharmacological inhibition of autophagy prevented development of premature senescence but did lead to the enhanced rate of apoptosis in human umbilical vein endothelial cells exposed to GC. Pharmacological induction of autophagy resulted in reciprocal changes. These observations appear to represent a mechanistic molecular cascade whereby advanced glycation end products like GC induce sphingomyelinase activity, accumulation of ceramide, clustering, and later internalization of lipid rafts.

Regulation of arginine methylation in endothelial cells: role in premature senescence and apoptosis.

With the recent characterization of enzymes responsible for protein arginine methylation and demonstration that catabolic products of arginine methylation, such as asymmetric dimethylarginine (ADMA), are among the most powerful mechanisms of atherogenesis, developing endothelial dysfunction and cardiovascular complications in a variety of pathologic processes, the need for functional characterization of the methylation-demethylation processes becomes ever more urgent. Therefore, the aims of the present study were to refine the feedback regulation of protein arginine methylation using one of the heavily methylated proteins, an RNA-binding protein Sam68, as a prototype, to elucidate the relations between Sam68 methylation and tyrosine phosphorylation and the role of methylation in RNA binding and subcellular distribution, as well as the cellular consequences of reduced protein methylation. Screening pro-atherogenic substances known to induce endothelial dysfunction showed that ADMA did not affect the level of arginine methylation of Sam68, whereas peroxynitrite was a strong inhibitor of methylation. Advanced glycation-modified collagen I, which accumulates in diabetes and induces formation of peroxynitrite and premature endothelial cell senescence, also inhibited arginine methylation of Sam68. When the level of arginine methylation of Sam68 was pharmacologically reduced, this did not affect its RNA binding or degree of tyrosine phosphorylation, but resulted in the predominantly nuclear hypomethylation pattern. Furthermore, protein hypomethylation resulted in the increased rate of apoptosis and premature senescence. This data may offer an additional explanation for the proapoptotic and senescence-accelerating action of peroxynitrite, a potent inhibitor of protein methylation.