Abstract P2-09-24: Combination treatment with bromodomain and extra-terminal motif inhibitors in triple-negative breast cancer

Triple-negative breast cancer (TNBC) is one of the most aggressive subtypes of breast cancer. TNBC affects younger women and is characterized by earlier rates of relapse, higher frequency of visceral metastases, and shorter survival outcomes when compared to ER+ or HER2+ disease. Although the disease only represents ˜15% of all breast cancer cases, it accounts for 25% of all breast cancer deaths – with treatment options currently limited to chemotherapy. Development of targeted therapies for TNBC is challenging due to molecular heterogeneity and lack of high-frequency “driver” alterations amenable to therapeutic intervention. Recent studies have demonstrated increased sensitivity of TNBC to the anti-proliferative effects of Bromodomain and Extra-Terminal motif inhibitor (BETi) compared to the other breast cancer subtypes. To determine mechanisms of sensitivity to BETi, we analyzed the effect of a BETi across a panel of TNBC cell line models and identified cell lines that were both sensitive and insensitive to BETi. With the intent of identifying biomarkers of sensitivity, we performed RNA-seq and precision nuclear run-on and sequencing (PRO-seq) on both sensitive and insensitive cell line models and data generated identified significant differences in key growth regulatory and apoptotic signaling pathways, including notable differences in Myc-dependent signaling. Our data suggest potential biomarkers of BETi-sensitivity that may be of value in further pre-clinical studies. Further, our results provide mechanistic rationale for combinations of BETi with select, targeted therapies in a disease that is in need of new therapeutic intervention.

Citation Format: Schafer JM, Lehmann BD, Redman LN, Liu P, Stubbs M, Ruggeri B, Scherle P, Pietenpol JA. Combination treatment with bromodomain and extra-terminal motif inhibitors in triple-negative breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-09-24.

Abstract ES9-1: Adjuvant Treatment of HER2 Positive Breast Cancer: The Next Installment

The treatment of HER2 positive breast cancer radically changed in 2005; at ASCO that year adjuvant trials reported that the addition of trastuzumab consistently showed a major improvement in progression free survival in the treatment of early breast cancer that was HER2 positive. A benefit was seen when trastuzumab was given either concurrently with chemotherapy or sequentially. The trials generally studied one year of trastuzumab although data from Finland showed a benefit for 3 months of treatment. Additional, yet to be reported studies, have explored durations of 6 months and 2 years. Initially there was concern about cardio toxicity and the potential for significant long-term harm, but the rate of cardio toxicity remains low and usually is seen early in the treatment. To further improve outcomes, new agents have been studied in the neo and adjuvant settings. The large ALTTO study randomized patients to either chemotherapy with either a year of trastuzumab, a year of lapatinib (the TKI inhibitor with efficacy in the metastatic arena), the combination of both agents or sequential use of each agent. The lapatinib monotherapy arm was shown to be inferior in an interim analysis and discontinued. Final data is pending although extrapolating from the NEOALTTO study, one may expect heightened activity from dual blockade. The BETH study added bevacizumab to trastuzumab in a 2-arm study trying to combat angiogenesis with HER2 blockade; this study has not reported. Although the same combination studied in the AVEREL study in the advanced setting did not show a benefit, this may not be the case in earlier disease and we need to wait for data. Dual blockade with two antiHER2 agents has been beneficial in the advanced setting and this has led to the APHINITY study, which is comparing chemotherapy with standard trastuzumab or trastuzumab with another antiHER2 antibody, pertuzumab. This study is accruing well and will provide evidence for whether dual blockade with two antibodies is of benefit. It will also provide some clinical trial data for small tumors, as these are eligible for enrollment in this trial. Other new agents, including TDM 1, are being taken into the neoadjuvant setting and will be in adjuvant trials soon. Subcutaneous trastuzumab may provide easier treatment for women over their year of adjuvant therapy. A big question is whether all HER2 tumors are the same or whether we can begin tailoring treatment according to molecular features. Are there some cancers that will be cured with a short course of chemotherapy and trastuzumab while others may need multiple antiHER2 agents combined to acquire the same result? With the collection of tumor samples we can hopefully begin to further understand this. Tailored treatment has the potential to decrease toxicity and improve quality of life issues, provide more rational use of costly resources, and finally develop more intensive therapy for resistant tumors thus improving outcomes overall.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr ES9-1.

Abstract ES2-2: Triple Negative Breast Cancer: Subtypes, Molecular Targets, and Therapeutic Approaches

The term “triple negative breast cancer” (TNBC) is used to classify 10%–20% of all breast cancers that lack estrogen receptor (ER) and progesterone receptor expression as well as amplification of the human epidermal growth factor receptor 2 (HER2)1. Disease heterogeneity and the absence of well-defined molecular targets have made treatment of TNBC challenging. To make significant clinical advances for TNBC patients, integrative and comprehensive genomic and molecular analyses of TNBC are required to understand the complexity of the disease as well as to allow identification of homogeneous subsets and ‘driver pathways’ that can then be therapeutically targeted. In response to this need, we compiled an extensive number of TNBC gene expression (GE) profiles and initiated molecular subtyping of the disease2. We identified two basal-like TNBC subtypes with cell cycle and DDR GE signatures (BL1 and BL2); two mesenchymal subtypes with high expression of genes involved in differentiation and growth factor pathways (M and MSL); an immunomodulatory (IM) type; and a luminal subtype driven by androgen signaling (LAR). Differential GE was used to designate 25 TNBC cell line models representative of these subtypes. Predicted ‘driver’ signaling pathways were pharmacologically targeted in these preclinical models as proof of concept that analysis of distinct GE signatures can inform therapy selection. Representative BL1 and BL2 subtype cell lines preferentially respond to cisplatin. Mesenchymal, basal, and luminal subtype lines with aberrations in PI3K signaling have the greatest sensitivity, in general, to phosphatidylinositol 3-kinase (PI3K) inhibitors. The LAR subtype cell lines express AR and are uniquely sensitive to bicalutamide (AR antagonist). We have also developed “TNBCtype,” a web-based subtyping tool for candidate TNBC tumor samples using our GE metadata and classification methods3. The approaches used and data generated have value for biomarker selection, drug discovery, and clinical trial design that will enable alignment of TNBC patients to appropriate targeted therapies.

1. Dent, R., Trudeau, M., Pritchard, K.I., Hanna, W.M., Kahn, H.K., Sawka, C.A., Lickley, L.A., Rawlinson, E., Sun, P., and Narod, S.A. (2007). Triple-negative breast cancer: clinical features and patterns of recurrence. Clin Cancer Res 13, 4429–4434. 2. Lehmann, B.D., Bauer, J.A., Chen, X., Sanders, M.E., Chakravarthy, A.B., Shyr, Y., and Pietenpol, J.A. (2011). Identification of human triple-negative breast cancer subtypes and preclinical models for selection of targeted therapies. J Clin Invest. 121: 2750–67. 3. Chen, X., Li, J., Gray, W.H., Lehmann, B.D., Bauer, J.A., Shyr, Y., and Pietenpol, J.A. (2012). TNBCtype: A Subtyping Tool for Triple-Negative Breast Cancer. Cancer informatics 11, 147–156.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr ES2-2.

Abstract P3-05-09: The role of TGF-beta receptor type 3 in breast cancer progression

Breast cancer is a diverse group of diseases. Gene expression profiling has identified luminal A, luminal B, Her2-like, Normal breast-like and Basal-like as major classifications of breast cancer diseases. These correlate to the overall patient survival with the basal-like subtype, clinically termed as triple-negative breast cancers (TNBC), demonstrating the poorest overall outcome. The poor patient outcome is primarily due to absence of hormonal receptor targets ER, PR and HER2 which are the major drug-targets for currently available therapies. Considering that TNBC lacks well-defined molecular targets, our group pursued extensive genomic molecular and biological analysis of over 500 tumor tissues from TNBC patients. Ultimately, this led to the discovery of six, TNBC subtypes; among which are Mesenchymal-Stem Like (MSL) and Mesechymal (M). These two subtypes share a unique biological driver, the TGF-beta pathway.

To gain insight into the role of the TGF-β pathway signaling in TNBC we have performed gene expression analysis on all six TNBC subtypes. A major finding was that among all TGF-β pathway-associated genes, TGF-beta receptor type 3 (TBR3) is most differentially expressed in MSL and M tumor subtypes of TNBC. Based on these findings, we are hypothesizing that TBR3 is required for maintenance of tumorigenicity in MSL and M subtypes of the triple-negative breast cancer.

In order to test our hypothesis we have screened 15 TNBC cell lines for both RNA and protein levels of TBR3 and validated that TBR3 is differentially expressed in M and MSL subtypes. Furthermore, we have manipulated TBR3 expression in M and MSL TNBC cell lines and tested their ‘biologies’ using both an in vitro and in vivo model systems. Preliminary results indicate that upon silencing TBR3 expression, cell motility as well as ability to cluster together in 3D culture system is significantly changed. In addition, our xenograft mouse model also demonstrated a significant change in tumor onset and growth.

We are interested in validating the prognostic and functional contribution of TBR3 to the breast cancer progression. The results of this study will provide insight into the potential use of the TGF-β signaling axis for prognostic or therapeutic utility in breast cancer patients.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P3-05-09.

Novel p63 target genes involved in paracrine signaling and keratinocyte differentiation

The transcription factor p63 is required for proper epidermal barrier formation and maintenance. Herein, we used chromatin immunoprecipitation coupled with DNA sequencing to identify novel p63 target genes involved in normal human epidermal keratinocyte (NHEKs) growth and differentiation. We identified over 2000 genomic sites bound by p63, of which 82 were also transcriptionally regulated by p63 in NHEKs. Through the discovery of interleukin-1-α as a p63 target gene, we identified that p63 is a regulator of epithelial–mesenchymal crosstalk. Further, three-dimensional organotypic co-cultures revealed TCF7L1, another novel p63 target gene, as a regulator of epidermal proliferation and differentiation, providing a mechanism by which p63 maintains the proliferative potential of basal epidermal cells. The discovery of new target genes links p63 to diverse signaling pathways required for epidermal development, including regulation of paracrine signaling to proliferative potential. Further mechanistic insight into p63 regulation of epidermal cell growth and differentiation is provided by the identification of a number of novel p63 target genes in this study.

Regulation of epithelial proliferation by TGF-beta

The closely related mammalian TGF-betas (TGF-beta 1, TGF-beta 2 and TGF-beta 3) are potent inhibitors of proliferation of many cell types in vitro. TGF-beta 1 has been demonstrated to be growth inhibitory in vivo for epithelial, endothelial, myeloid and lymphoid cells. Utilizing skin keratinocytes as a model system for studying the mechanism of TGF-beta 1-induced growth inhibition, it has been demonstrated that TGF-beta 1 rapidly inhibits transcription of the c-myc gene. Antisense c-myc oligonucleotides inhibit proliferation of keratinocytes as effectively as does TGF-beta 1, indicating that TGF-beta 1 suppression of c-myc expression is an important component of this growth inhibition. Studies utilizing DNA tumour virus transforming gene constructs have shown that the retinoblastoma gene product, pRb, or a related protein, is needed for TGF-beta 1 suppression of c-myc transcription. Thus, TGF-beta 1 may act through a tumour suppressor gene product, pRb, to suppress transcription of a proto-oncogene, c-myc, and subsequently inhibit cell proliferation.

p63 consensus DNA-binding site: identification, analysis and application into a p63MH algorithm

p53 and p63 belong to a family of sequence-specific transcription factors regulating key cellular processes. Differential composition of the p53 and p63 DNA-binding sites may contribute to distinct functions of these protein homologues. We used SELEX (systematic evolution of ligands by exponential enrichment) methodology to identify nucleic acid ligands for p63. We found that p63 bound preferentially to DNA fragments conforming to the 20 bp sequence 5'-RRRC(A/G)(A/T)GYYYRRRC(A/T)(C/T)GYYY-3'. Relative to the p53 consensus, the p63 consensus DNA-binding site (DBS) was more degenerate, particularly at positions 10 and 11, and was enriched for A/G at position 5 and C/T at position 16 of the consensus. The differences in DNA-binding site preferences between p63 and p53 influenced their ability to activate transcription from select response elements (REs) in cells. A computer algorithm, p63MH, was developed to find candidate p63-binding motifs on input sequences. We identified genes responsive to p63 regulation that contain functional p63 REs. Our results suggest that the sequence composition of REs could be one contributing factor to target gene discrimination between p63 and p53.

p53 and Delta Np63 alpha differentially bind and regulate target genes involved in cell cycle arrest, DNA repair and apoptosis

The mechanism by which the p53 family of proteins coordinately regulates select target genes after various types of cell stress is not well understood. To further define factors that dictate regulation of target genes, we examined the binding of p53, DeltaNp63alpha and RNA polymerase II (pol II) to the regulatory regions of select target genes in primary human epidermal keratinocytes (HEKs) using chromatin immunoprecipitation. In rapidly proliferating cells, we observed constitutive binding of DeltaNp63alpha and varying levels of p53 binding, to consensus sites in target genes involved in cell cycle arrest, DNA repair and apoptosis. Following genotoxic stress, p53 occupancy increased whereas DeltaNp63alpha occupancy decreased at the majority of binding sites examined. Microarray analysis of transcripts isolated from HEKs ectopically expressing p53 and DeltaNp63alpha revealed an inverse regulation of select target genes by the two family members. Collectively, our results suggest that DeltaNp63alpha can function as a repressor of select p53 target genes involved in growth arrest, DNA repair and apoptosis, and that the location of the p53 consensus binding site(s) in a target gene may dictate whether pol II is constitutively bound in proliferating cells.

Cell cycle checkpoint signaling: cell cycle arrest versus apoptosis

Although toxicants may initiate cell damage or stress, the cellular proteins that are involved in control of cell cycle and apoptosis are the final arbiters of cell fate. The biochemical pathways that restrain cell cycle transition and/or induce cell death after stress are known as cell cycle checkpoints. These checkpoints maintain the fidelity of DNA replication, repair, and division. Herein, select cell cycle checkpoint signaling pathways will be discussed and how different components of these pathways are regulated by exogenous and endogenous agents, with focus on the p53 tumor suppressor signaling. The p53 protein is known to play a key role in growth arrest and apoptosis after cell stress, primarily through its ability to regulate the transcription of select downstream target genes in the cell. Further elucidation of the signaling pathways that control growth arrest and apoptosis will continue to provide insights to the complex cellular responses to environmental toxicants.

4-hydroxynonenal induces apoptosis via caspase-3 activation and cytochrome c release

We investigated the mechanism by which 4-hydroxynonenal (HNE), a major aldehydic product of lipid peroxidation, induces apoptosis in tumor cells. Treatment of human colorectal carcinoma (RKO) cells with HNE-induced poly-ADP-ribose-polymerase (PARP) cleavage and DNA fragmentation in a dose- and time-dependent manner. The induction of PARP cleavage and DNA fragmentation paralleled caspase-2, -3, -8, and -9 activation. Pretreatment of cells with an inhibitor of caspase-3, z-DEVD-fmk, or a broad spectrum caspase inhibitor, z-VAD-fmk, abolished caspase activation and subsequent PARP cleavage. Constitutive expression of high levels of Bcl-2 protected cells from HNE-mediated apoptosis. In addition, Bcl-2 overexpression inhibited cytochrome c release from mitochondria and subsequent caspase-2, -3, and -9 activation. These findings demonstrate that HNE triggers apoptotic cell death through a mitochondrion-dependent pathway involving cytochrome c release and caspase activation. Bcl-2 overexpression protected cells from HNE-induced apoptosis through inhibition of cytochrome c release.

A-204197, a new tubulin-binding agent with antimitotic activity in tumor cell lines resistant to known microtubule inhibitors

Drug resistance is a prevalent problem in the treatment of neoplastic disease, and the effectiveness of many clinically useful drugs is limited by the fact that they are substrates for the efflux pump, P-glycoprotein. Because there is a need for new compounds that are effective in treating drug-resistant tumors, we tested A-204197 (4-[4-acetyl-4,5-dihydro-5-(3,4,5-trimethoxyphenyl)-1,3,4-oxadiazol-2-yl]-N,N-dimethylbenzeneamine), a novel oxadiazoline derivative with antiproliferative properties, on cell lines that were either sensitive or resistant to known microtubule inhibitors. Cell lines that were resistant to paclitaxel, vinblastine, or colchicine were equally sensitive to A-204197 (proliferation IC50s ranging from 36 to 48 nM) despite their expression levels of P-glycoprotein. The effect of A-204197 on cell growth was associated with cell cycle arrest in G2-M, increased phosphorylation of select G2-M checkpoint proteins, and apoptosis. In competition-binding assays, A-204197 competed with [3H]-labeled colchicine for binding to tubulin (K(i) = 0.75 microM); however, it did not compete with [3H]-labeled paclitaxel. A-204197 prevented tubulin polymerization in a dose-dependent manner (IC50 = 4.5 microM) in vitro and depolymerized microtubules in a time-dependent manner in cultured cells. These findings indicate A-204197 is a promising new tubulin-binding compound with antimitotic activity that has potential for treating neoplastic diseases with greater efficacy than currently used antimitotic agents.

Kinetics of p53 binding to promoter sites in vivo

Downstream target genes of p53 are thought to mediate its tumor-suppressive activity, but it is unknown whether differential transactivation of these genes is regulated at the level of p53 binding to their promoters. To address this issue, p53 binding in vivo to consensus sites in the p21(Waf1), MDM2, and PIG3 promoters was investigated in cells exposed to adriamycin (ADR) or ionizing radiation as well as in an inducible p53 cell line. p53-DNA complexes were cross-linked in vivo by treating the cells with formaldehyde and processed by chromatin immunoprecipitation-PCR. This methodology allowed for the analysis of relevant p53-DNA complexes by preventing redistribution of cellular components upon collection of cell extracts. Increased p53 binding to the p21(Waf1), MDM2, and PIG3 promoters occurred within 2 h after p53 activation; however, significant increases in PIG3 transcription did not occur until 15 h after p53 binding. Gel shift analyses indicated that p53 had lower affinity for the consensus binding site in the PIG3 promoters compared to its consensus sites in the p21 and MDM2 genes, which suggests that additional factors may be required to stabilize the interaction of p53 with the PIG3 promoter. Further, acetylated p53 (Lys382) was found in chemically cross-linked complexes at all promoter sites examined after treatment of cells with ADR. In summary, the kinetics of p53 binding in vivo to target gene regulatory regions does not uniformly correlate with target gene mRNA expression for the p53 target genes examined. Our results suggest that target genes with low-affinity p53 binding sites may require additional events and will have delayed kinetics of induction compared to those with high-affinity binding sites.

Increased p53 phosphorylation after microtubule disruption is mediated in a microtubule inhibitor- and cell-specific manner

p53 is present at low levels in unstressed cells. Numerous cellular insults, including DNA damage and microtubule disruption, elevate p53 protein levels. Phosphorylation of p53 is proposed to be important for p53 stabilization and activation after genotoxic stress; however, p53 phosphorylation after microtubule disruption has not been analysed. The goal of the current study was to determine if p53 phosphorylation increases after microtubule disruption, and if so, to identify specific p53 residues necessary for microtubule inhibitor-induced phosphorylation. Two dimensional gel analyses demonstrated that the number of p53 phospho-forms in cells increased after treatment with microtubule inhibitors (MTIs) and that the pattern of p53 phosphorylation was distinct from that observed after DNA damage. p53 phosphorylation also varied in a MTI-dependent manner, as Taxol and Vincristine induced more p53 phospho-forms than nocodazole. Further, MTI treatment increased phosphorylation of p53 on serine-15 in epithelial tumor cells. In contrast, serine-15 phosphorylation of p53 did not increase in MTI-treated primary cultures of human fibroblasts. Analysis of ectopically expressed p53 phospho-mutant proteins from Taxol- and nocodazole-treated cells indicated that multiple p53 amino terminal residues, including serine-15 and threonine-18, were required for Taxol-mediated phosphorylation of p53. Taken together, the results of this study demonstrate that distinct p53 phospho-forms are induced by MTI treatment as compared to DNA damage and that p53 phosphorylation is mediated in a MTI- and cell-specific manner. Oncogene (2001) 20, 113 - 124.

Syk: a new player in the field of breast cancer

Breast tumor development and progression are thought to occur through a complex, multistep process, including oncogene activation (eg HER2/neu) and mutation or loss of tumor suppressor genes (eg p53). Determining the function of genetic alterations in breast carcinoma tumorigenesis and metastasis has been the focus of intensive research efforts for several decades. One group of proteins that play a critical role in breast cancer cell signaling pathways are tyrosine kinases. Overexpression of the tyrosine kinase HER2/neu is observed in many human breast cancers and is positively correlated with enhanced tumorigenesis. Recently, another tyrosine kinase, Syk, has been implicated as an important inhibitor of breast cancer cell growth and metastasis. This recent finding was unexpected, since Syk function has been predominantly linked to hematopoietic cell signaling, and is discussed further in this commentary.

p53-dependent expression of PIG3 during proliferation, genotoxic stress, and reversible growth arrest

The p53-inducible gene 3 (PIG3) was recently identified in a screen for genes induced by p53 before the onset of apoptosis. PIG3 shares significant homology with oxidoreductases from several species. In this study, PIG3-specific antibodies were used to analyze cellular PIG3 protein levels under control and genotoxic stress conditions. PIG3 protein was localized to the cytoplasm and induced in primary, non-transformed, and transformed cell cultures after exposure to genotoxic agents. The induction of PIG3 was p53-dependent and occurred with delayed kinetics as compared with other p53 downstream targets, such as p21 and MDM2. Using a p53-inducible cell model system, in which p53-mediated growth arrest is reversible, we found that PIG3 levels were increased during p53-mediated growth arrest. Interestingly, elevated levels of PIG3 were maintained in cells that resumed cycling in the absence of ectopic p53 expression, suggesting that PIG3 is a long-lived reporter, which may be useful for detecting transient activation of p53.

Mechanisms of cell-cycle checkpoints: at the crossroads of carcinogenesis and drug discovery

Human tumors arise from multiple genetic changes that gradually transform growth-limited cells into highly invasive cells that are unresponsive to growth controls. The genetic evolution of normal cells into cancer cells is largely determined by the fidelity of DNA replication, repair, and division. Cell-cycle arrest in response to stress is integral to the maintenance of genomic integrity. The control mechanisms that restrain cell-cycle transition or induce apoptotic signaling pathways after cell stress are known as cell-cycle checkpoints. This review will focus on the mechanisms of cell-cycle checkpoint pathways and how different components of these pathways are frequently altered in the genesis of human tumors. As our knowledge of cell-cycle regulation and checkpoints increases, so will our understanding of how xenobiotic agents can affect these processes to either initiate or inhibit tumorigenesis.

p53 regulation of G(2) checkpoint is retinoblastoma protein dependent

In the present study, we investigated the role of p53 in G(2) checkpoint function by determining the mechanism by which p53 prevents premature exit from G(2) arrest after genotoxic stress. Using three cell model systems, each isogenic, we showed that either ectopic or endogenous p53 sustained a G(2) arrest activated by ionizing radiation or adriamycin. The mechanism was p21 and retinoblastoma protein (pRB) dependent and involved an initial inhibition of cyclin B1-Cdc2 activity and a secondary decrease in cyclin B1 and Cdc2 levels. Abrogation of p21 or pRB function in cells containing wild-type p53 blocked the down-regulation of cyclin B1 and Cdc2 expression and led to an accelerated exit from G(2) after genotoxic stress. Thus, similar to what occurs in p21 and p53 deficiency, pRB loss can uncouple S phase and mitosis after genotoxic stress in tumor cells. These results indicate that similar molecular mechanisms are required for p53 regulation of G(1) and G(2) checkpoints.

K-Ras-independent effects of the farnesyl transferase inhibitor L-744,832 on cyclin B1/Cdc2 kinase activity, G2/M cell cycle progression and apoptosis in human pancreatic ductal adenocarcinoma cells

Pancreatic ductal adenocarcinoma is a highly lethal malignancy that is resistant to traditional cytotoxic therapy. High rates of activating codon 12 K-Ras mutations in this disease have generated considerable interest in the therapeutic application of novel farnesyl transferase inhibitors (FTIs). However, a comprehensive analysis of the effects of FTI treatment on pancreatic cancer cells has not been performed. Treatment of five different human pancreatic cancer cell lines with FTI L-744,832 resulted in inhibition of anchorage-dependent growth, with wide variation in sensitivity among different lines. Effective growth inhibition by L-744,832 correlated with accumulation of cells with a tetraploid (4N) DNA content and high levels of cyclin B1/cdc2 kinase activity, implying cell cycle arrest downstream from the DNA damage-inducible G2/M cell cycle checkpoint. In addition, sensitive cell lines underwent apoptosis as evidenced by changes in nuclear morphology and internucleosomal DNA fragmentation. L-744,832 at a concentration of 1 microM additively enhanced the cytotoxic effect of ionizing radiation, apparently by overriding G2/M checkpoint activation. The effects of FTI treatment on cell growth and cell cycle regulation were associated with changes in posttranslational processing of H-Ras and N-Ras, but not K-Ras. The results confirm the potential therapeutic efficacy of FTI treatment in pancreatic cancer, and suggest that farnesylated proteins other than K-Ras may act as important regulators of G2/M cell cycle kinetics.

Helicobacter pylori strain-specific genotypes and modulation of the gastric epithelial cell cycle

Helicobacter pylori cag+ strains enhance gastric epithelial cell proliferation and attenuate apoptosis in vivo, which may partially explain the increased risk of gastric cancer associated with these strains. The goals of this study were to identify specific H. pylori genes that regulate epithelial cell cycle events and determine whether these effects were dependent upon p53-mediated pathways. AGS gastric epithelial cells were cultured alone or in the presence of 21 clinical H. pylori isolates, H. pylori reference strain 60190, or its isogenic cagA-, picB-, vacA-, or picB-/vacA- derivatives. Coculture of H. pylori with AGS cells significantly decreased cell viability, an effect most prominent with cag+ strains (P < 0.001 versus cag-strains). cag+ strains significantly increased progression of AGS cells from G1 into G2-M at 6 h and enhanced apoptosis by 72 h. Compared with the parental 60190 strain, the picB- mutant attenuated cell cycle progression at 6 h (P < or = 0.05), and decreased apoptosis with enhanced AGS cell viability at 24 h (P < or = 0.04). The vacA- mutant decreased apoptosis and enhanced viability at later (48-72 h) time points (P < or = 0.05). Compared with the wild-type strain, the picB-/vacA- double mutant markedly attenuated apoptosis and increased cell viability at all time points (P < or = 0.05). Furthermore, cocolonization with H. pylori had no significant effect on expression of p53, p21, and MDM2. The diminished AGS cell viability, progression to G2-M, and apoptosis associated with cag+ H. pylori strains were dependent upon expression of vacA and genes within the cag pathogenicity island. These results may explain heterogeneity in levels of gastric epithelial cell proliferation and apoptosis found within H. pyloricolonized mucosa.

Cell cycle checkpoints as therapeutic targets

Most human breast tumors arise from multiple genetic changes which gradually transform differentiated and growth-limited cells into highly invasive cells that are unresponsive to growth controls. The genetic evolution of normal breast cells into cancer cells is largely determined by the fidelity of DNA replication, repair, and division. Cell cycle arrest in response to DNA damage is an important part of the mechanism used to maintain genomic integrity. The control mechanisms that restrain cell cycle transition after DNA damage are known as cell cycle checkpoints. This review will focus on cell cycle checkpoint signaling pathways commonly mutated in human breast tumors and suggest how different components of these checkpoint pathways offer the potential for chemotherapeutic intervention.

Defective G1-S cell cycle checkpoint function sensitizes cells to microtubule inhibitor-induced apoptosis

Defective cell cycle checkpoint function has been linked to enhanced sensitivity of tumor cells to certain genotoxic agents. To determine whether loss of the G1-S checkpoint function would sensitize tumor cells to microtubule inhibitor (MTI)-induced apoptosis, we examined the effect of the MTIs, Taxol and vincristine, on the cell cycle kinetics and survival of two isogenic cell lines, HCT116 p21+/+ and HCT116 p21-/-, which differ only at the p21 locus. p21-deficient cells displayed a dose-dependent, enhanced chemosensitivity to MTIs in both monolayer and soft agar assays as well as in mice xenograft tumors. The increased sensitivity of the p21-deficient cells to MTIs correlated with prolonged cyclin B1/Cdc2 activity and the occurrence of endoreduplication. Furthermore, sensitivity of p53-deficient cells to MTI-induced apoptosis was significantly reduced by induction of ectopic p21 protein. The results suggest that the status of G1-S checkpoint function in tumor cells may be an important determinant in the efficacy of MTIs used clinically.

High affinity insertion/deletion lesion binding by p53. Evidence for a role of the p53 central domain

In addition to binding DNA in a sequence-specific manner, p53 can interact with nucleic acids in a sequence-independent manner. p53 can bind short single-stranded DNA and double-stranded DNA containing nucleotide loops; these diverse associations may be critical for p53 signal transduction. In this study, we analyzed p53 binding to DNA fragments containing insertion/deletion mismatches (IDLs). p53 required an intact central domain and dimerization domain for high affinity complex formation with IDLs. In fact, the C terminus of p53 (amino acids 293-393) was functionally replaceable with a foreign dimerization domain in IDL binding assays. From saturation binding studies we determined that the KD of p53 binding to IDLs was 45 pM as compared with a KD of 31 pM for p53 binding to DNA fragments containing a consensus binding site. Consistent with these dissociation constants, p53-IDL complexes were dissociated with relatively low concentrations of competitor consensus site-containing DNA. Although p53 has a higher affinity for DNA with a consensus site as compared with IDLs, the relative number and availability of each form of DNA in a cell immediately after DNA damage may promote p53 interaction with DNA lesions. Understanding how the sequence-specific and nonspecific DNA binding activities of p53 are integrated will contribute to our knowledge of how signaling cascades are initiated after DNA damage.

Treatment with farnesyl-protein transferase inhibitor induces regression of mammary tumors in transforming growth factor (TGF) alpha and TGF alpha/neu transgenic mice by inhibition of mitogenic activity and induction of apoptosis

Mouse mammary tumor virus-transforming growth factor alpha (MMTV-TGF alpha) and MMTV-TGF alpha/neu transgenic mice develop mammary tumors after a long latency and therefore provide useful model systems for breast cancer with its recognized activation of receptor tyrosine kinase signaling. We used these mice to study the antitumor effect of L-744,832 (FTI), a potent and selective inhibitor of farnesyl-protein transferase, and hence of Ras function. A total of 55 mice were assigned randomly to treatment with FTI or vehicle, and one-half of the mice were crossed over after initial treatment to the opposite group. L-744,832 induced reversible regression of mammary tumors that was paralleled by a decrease in serum levels of TGF alpha secreted by the tumor cells. There was no difference in response to treatment with FTI between MMTV-TGF alpha mice, in which tumorigenesis was accelerated by multiparity or the chemical carcinogen 7,12-dimethylbenzanthracene, and MMTV-TGF alpha/neu mice. The tumor histological type had no impact on FTI sensitivity. For mechanistic analyses, tumor excision biopsies were obtained from 12 mice before and after treatment with L-744,832. In these samples, tumor regression was paralleled biochemically by inhibition of mitogen-activated protein kinase activity and biologically by an increase in G1-phase and decrease in S-phase fractions, as well as induction of apoptosis. These results suggest that the potential clinical use of FTI could be expanded to include cancers harboring activated receptor tyrosine kinases as well as those containing activated Ras.

p21(Waf1/Cip1) inhibition of cyclin E/Cdk2 activity prevents endoreduplication after mitotic spindle disruption

During a normal cell cycle, entry into S phase is dependent on completion of mitosis and subsequent activation of cyclin-dependent kinases (Cdks) in G1. These events are monitored by checkpoint pathways. Recent studies and data presented herein show that after treatment with microtubule inhibitors (MTIs), cells deficient in the Cdk inhibitor p21(Waf1/Cip1) enter S phase with a >/=4N DNA content, a process known as endoreduplication, which results in polyploidy. To determine how p21 prevents MTI-induced endoreduplication, the G1/S and G2/M checkpoint pathways were examined in two isogenic cell systems: HCT116 p21(+/+) and p21(-/-) cells and H1299 cells containing an inducible p21 expression vector (HIp21). Both HCT116 p21(-/-) cells and noninduced HIp21 cells endoreduplicated after MTI treatment. Analysis of G1-phase Cdk activities demonstrated that the induction of p21 inhibited endoreduplication through direct cyclin E/Cdk2 regulation. The kinetics of p21 inhibition of cyclin E/Cdk2 activity and binding to proliferating-cell nuclear antigen in HCT116 p21(+/+) cells paralleled the onset of endoreduplication in HCT116 p21(-/-) cells. In contrast, loss of p21 did not lead to deregulated cyclin D1-dependent kinase activities, nor did p21 directly regulate cyclin B1/Cdc2 activity. Furthermore, we show that MTI-induced endoreduplication in p53-deficient HIp21 cells was due to levels of p21 protein below a threshold required for negative regulation of cyclin E/Cdk2, since ectopic expression of p21 restored cyclin E/Cdk2 regulation and prevented endoreduplication. Based on these findings, we propose that p21 plays an integral role in the checkpoint pathways that restrain normal cells from entering S phase after aberrant mitotic exit due to defects in microtubule dynamics.

Mitotic phosphorylation of Bcl-2 during normal cell cycle progression and Taxol-induced growth arrest

There is increasing evidence that prolonged mitotic arrest initiates apoptosis; however, little is known about the signaling pathways involved. Several studies have associated deregulated Cdc2 activity with apoptosis. Herein, we report that the anti-apoptotic protein, Bcl-2, undergoes cell cycle-dependent phosphorylation during mitosis when there is elevated Cdc2 activity. We found that paclitaxel (Taxol(R)) treatment of epithelial tumor cells induced a prolonged mitotic arrest, elevated levels of mitotic kinase activity, hyperphosphorylation of Bcl-2, and subsequent cell death. The Taxol-induced Bcl-2 phosphorylation was dose-dependent. Furthermore, phosphorylated Bcl-2 remained complexed with Bax in Taxol-treated cells undergoing apoptosis. Immunoprecipitation experiments revealed a Bcl-2-associated kinase capable of phosphorylating histone H1 in vitro. However, the kinase was likely not cyclin B1/Cdc2, since cyclin B1/Cdc2 was not detectable in Bcl-2 immunoprecipitates, nor was recombinant Bcl-2 phosphorylated in vitro by cyclin B1/Cdc2. The results of this study further define a link between mitotic kinase activation and the apoptotic machinery in the cell. However, the role, if any, of prolonged Bcl-2 phosphorylation in Taxol-mediated apoptosis awaits further definition of Bcl-2 mechanism of action. Taxol may increase cellular susceptibility to apoptosis by amplifying the normal downstream events associated with mitotic kinase activation.

Negative regulation of Wee1 expression and Cdc2 phosphorylation during p53-mediated growth arrest and apoptosis

The G2 cell cycle checkpoint protects cells from potentially lethal mitotic entry after DNA damage. This checkpoint involves inhibitory phosphorylation of Cdc2 at the tyrosine-15 (Y15) position, mediated in part by the Wee1 protein kinase. Recent evidence suggests that p53 may accelerate mitotic entry after DNA damage and that the override of the G2 checkpoint may play a role in the induction of apoptosis by p53. To determine the biochemical mechanism by which p53 inactivates the G2 checkpoint, the effects of p53 activation on Wee1 expression, Cdc2-Y15 phosphorylation, and cyclin B1-associated Cdc2 kinase activity were examined. Under conditions of either growth arrest or apoptosis, p53 activation resulted in the down-regulation of Wee1 expression and dephosphorylation of Cdc2. A parallel increase in cyclin B1/Cdc2 kinase activity was observed during p53-mediated apoptosis. Negative regulation of the Wee1 expression and Cdc2 phosphorylation by p53 was also evident in thymus tissue from p53+/+ mice but not from p53-/- mice. Inactivation of the G2 checkpoint may contribute to the tumor suppressor activity of p53.

Induction of cell cycle arrest by the endogenous product of lipid peroxidation, malondialdehyde

We have investigated the effect of the endogenous genotoxin malondialdehyde (MDA) on cell cycle kinetics and the expression and biochemical activity of several cell cycle regulatory proteins. MDA treatment of two human cell lines (RKO and H1299) resulted in a 3- to 6-fold elevation in the levels of the major detectable MDA-DNA adduct, M1G-dR. The increase in M1G-dR was accompanied by irreversible cell cycle arrest, elevation in p53 and p21 protein levels, and inhibition of cyclin E- and cyclin B-associated kinase activities. The decrease in cyclin E- and cyclin B-dependent kinase activities was caused by increased p21 and decreased cdc2 levels, respectively. Comparable levels of p21 induction were observed in RKO (wild-type p53) and H1299 (p53-null) cells. Thus, MDA was able to engage cell cycle checkpoint function in human cell lines when used at concentrations that produce M1G-dR levels of the same magnitude found in human tissues.

Differential cell cycle checkpoint response in normal human keratinocytes and fibroblasts

The incidence of DNA mutation and subsequent risk of transformation in different cell types may depend on cell type-specific variation in position and duration of cell cycle arrest after exposure to DNA-damaging agents. To determine whether cell type-specific checkpoints occur, normal human epidermal keratinocytes (HKs) and human dermal fibroblasts (HFs), isolated from the same tissue, were exposed to genotoxic agents. Following exposure, cell cycle arrest profiles, cell proliferation rates, and select protein levels and activities were analyzed and found to be cell type dependent. After exposure to either gamma-radiation or Adriamycin, HFs arrested primarily in G1, whereas HKs arrested predominantly in G2. The attenuated G1 arrest in the HKs correlated with less p53 protein accumulation, as compared to that observed in G1-arrested HFs. Although gamma-irradiated HFs were unable to reenter the cell cycle, HKs began proliferating 72 h posttreatment. Consistent with the cell cycle profiles observed, cyclin-dependent kinase activities were inhibited for a longer duration in HFs as compared to HKs after gamma-irradiation. The results indicate that cell cycle checkpoint response to genotoxic insult may vary according to cell type within any given tissue. The attenuated G1 arrest observed in HKs may be an important factor in the transforming events leading to skin neoplasia.

The role of transforming growth factor beta in glioma progression

This review examines the apparently paradoxical conversion of transforming growth factor beta's (TGFbeta) regulatory role as a growth inhibitor among normal glial cells to that of a progression factor among glioblastomas (GM). In vitro, TGFbeta functions as an autocrine growth inhibitor of near-diploid gliomas of any grade. In contrast, hyperdiploid glioblastoma multiforme (HD-GM) cultures proliferate in response to TGFbeta, which is mediated by induction of platelet-derived growth factor B chain (PDGF-BB). The dominant hypothesis of TGFbeta's pathogenetic association with malignant transformation has been predicated upon acquisition of resistance to its growth inhibitory effects. However, the lack of obvious correlation with TGFbeta receptor (TbetaR) expression (or loss) between the HD-GM and the TGFbeta-inhibited GM cultures suggests the existence of intrinsically opposed regulatory mechanisms influenced by TGFbeta. The mechanism of conversion might be explained either by the loss of a putative tumor suppressor gene (TSG) which mediates TGFbeta's inhibition of growth or by enhancement of an active oncogenic pathway among the HD-GM. The frequency of mutations within glioma-associated TSG, such as TP53 and RB, suggests that defects in TGFbeta's inhibitory signaling pathway may have analogous effects in the progression to HD-GM, and TGFbeta's conversion to a mitogen. Alternative sites of inactivation which might explain the loss of TGFbeta's inhibitory effect include inactivating mutation/loss of the TbetaR type II, alterations in post-receptor signal transmission or the cyclin/cyclin dependent kinase system which regulates the phosphorylation of pRB. Loss or inactivation of a glial TSG with a consequent failure of inhibition appears to allow TGFbeta's other constitutive effects, such as induction of c-sis, to become functionally dominant. Mechanistically, TGFbeta's conversion from autocrine inhibitor to mitogen promotes 'clonal dominance' by conferring a Darwinian advantage to the hyperdiploid subpopulations through qualitative and quantitative differences in its modulation of PDGF-A and c-sis, with concomitant paracrine inhibition of competing, near-diploid elements.

Cell cycle re-entry following chemically-induced cell cycle synchronization leads to elevated p53 and p21 protein levels

Mimosine (MIM) and aphidicolin (APH) are two agents frequently used in tissue culture-based experiments to achieve cell synchronization at late G1 and S phases. Following MIM or APH treatment of human cancer cell lines, a reversible growth arrest in late G1 and S phases of the cell cycle was correlated with moderate increases in p53 and p21 protein levels. Both p53-dependent and -independent increases in p21 were observed following treatment with either agent. However, a striking increase in p21 protein levels and a continuous elevation in both p53 and p21 protein levels were observed over 48 h after cells re-entered the cell cycle following the chemically-induced synchronization. In addition, the increase in p21 protein levels typically seen following treatment of cells with DNA damaging agents, was enhanced when cells were treated with genotoxic agents following MIM or APH synchronization. These findings suggest that caution should be exercised when interpreting results from experiments using cell synchronization agents, in particular, studies designed to investigate p53- and p21-regulatory pathways.

Differential effects of p53 mutants on the growth of human bronchial epithelial cells

We investigated the effects of five different p53 mutants on the growth of primary cultures of normal human bronchial epithelial (NHBE) cells. The five defective viral pZIP-Neo constructs contained the following mutations at mutational hot-spots found in human cancers: codons 143ala, 175his, 248trp, 249ser, and 273his. NHBE cells were infected with the p53 mutants, wild-type p53, or the pZIP-Neo vector control. The 143ala, 248trp, and 273his mutants, as well as wild-type p53, decreased the colony-forming efficiency and inhibited the growth of NHBE cells. The 175his mutant did not significantly change the growth rates. In NHBE cells from three donors, the 249ser mutant conferred a substantial growth advantage to the NHBE cells in a colony-forming-efficiency assay. In NHBE cells isolated from one donor, the 249ser mutant also produced a significant life span extension. These cells grew rapidly through 80 population doublings and entered an apparent "crisis" in passage 14. Karyotypic analyses of one culture at multiple passages revealed aneuploid populations with alterations of chromosomes 5, 11, and 13; quantitative DNA analysis detected aneuploidy in late passages from that culture and two other primary cultures. These data demonstrated that the codon 249ser mutation could provide a growth advantage to bronchial epithelial cells and suggest that this mutant protein can induce genomic instability.

Apoptosis of late-stage erythroblasts in megaloblastic anemia: association with DNA damage and macrocyte production

An in vitro model of folate-deficient erythropoiesis has been developed using proerythroblasts isolated from the spleens of Friend virus-infected mice fed an amino acid-based, folate-free diet. Control proerythroblasts were obtained from Friend virus-infected mice fed the same diet plus 2 mg folic acid/kg diet. Our previous studies showed that, after 20 to 32 hours of culture in folate-deficient medium with 4 U/mL of erythropoietin, the folate-deficient proerythroblasts underwent apoptosis, whereas control erythroblasts survived and differentiated into reticulocytes over a period of 48 hours. The addition of folic acid or thymidine to the folate-deficient medium prevented the apoptosis of the folate-deficient erythroblasts, thereby implicating decreased thymidylate synthesis as the main cause of apoptosis in the folate-deficient erythroblasts. In the study reported here, we examined intracellular folate levels, uracil misincorporation into DNA, p53 and p21 proteins, and reticulocyte formation in erythroblasts cultured in folate-deficient or control medium. In all experiments, the folate-deficient erythroblasts cultured in folate-deficient medium gave results that varied significantly from folate-deficient erythroblasts cultured in control medium or control erythroblasts cultured in either folate-deficient or control media. Folate-deficient erythroblasts cultured in folate-deficient medium had marked decreases in all coenzyme forms of folate that persisted throughout culture, increased uracil misincorporation into DNA, persistent accumulations of p53 and p21, and decreased reticulocyte production but increased size of individual reticulocytes. A model of folate-deficient erythropoiesis based on apoptosis of late stage erythroblasts is presented. This model provides explanations for the clinical findings in megaloblastic anemia.

Mammalian cells resistant to tumor suppressor genes

Expression of p53 causes growth arrest or apoptosis in many normal and neoplastic cell types, but the relationship between these two effects has remained obscure. To begin to dissect the underlying mechanisms at a genetic level, we have generated mutant cells resistant to the action of wild-type p53. Rat embryo fibroblasts transformed with ras and a temperature-sensitive p53 (tsp53(135val)) gene were chemically mutagenized and selected for growth at a temperature at which p53 adopts a wild-type conformation (31.5 degrees C). Clones that grew exponentially at 31.5 degrees C were selected. Cell fusion experiments demonstrated that the mutations conferring resistance to p53-mediated growth arrest were dominant. The mutagenized clones were resistant not only to p53-mediated growth arrest, but also to the apoptosis induced by E1A in conjunction with p53, and partially resistant to the retinoblastoma tumor suppressor, pRB. The results suggest that a single downstream pathway can control the induction of growth arrest and apoptosis, and that both p53 and RB function through this pathway.

Assignment of the human p27Kip1 gene to 12p13 and its analysis in leukemias

The p27Kip1 (p27) gene encodes an inducible inhibitor of cyclin-dependent kinase activity. Using a murine p27 cDNA as probe, we obtained a human cDNA clone and subsequently used it to isolate a genomic clone of this gene. The coding region of the human p27 gene was contained in two exons. Both the amino acid sequence and intron-exon organization of p27 were similar to those previously found for the related cyclin-dependent kinase inhibitor p21Waf1 (p21). The p27 gene was localized to chromosome band 12p13 by a combination of somatic cell hybrid and fluorescence in situ hybridization analyses. The p27 gene product is thought to control the leukocyte cell cycle and the 12p13 chromosomal band is known to be deleted in leukemias, suggesting that the p27 gene may act as a tumor suppressor gene in leukemias. Although p27 was found to reside in the minimal region of chromosomal loss in hematological malignancies, no mutations of p27 were observed in leukemia samples. Haploinsufficiency of p27 may confer a growth advantage to leukemia cells.

Paradoxical inhibition of solid tumor cell growth by bcl2

The BCL2 gene product has been demonstrated to prevent apoptosis and provide a selective growth advantage to many cell types. We report an unexpected effect of bcl2 expression on the in vitro growth of several solid tumor cell lines. Expression of bcl2 in these cell lines resulted in growth inhibition similar to that seen with p53. In contrast, a COOH-terminal deletion mutant of bcl2 was unable to suppress growth. Thus, the bcl2 protein may exert distinct biological effects in different cell types.

Sequence-specific transcriptional activation is essential for growth suppression by p53

Although several biochemical features of p53 have been described, their relationship to tumor suppression remains uncertain. We have compared the ability of p53-derived proteins to act as sequence-specific transcriptional (SST) activators with their ability to suppress tumor cell growth, using an improved growth-suppression assay. Both naturally occurring and in vitro derived mutations that abrogated the SST activity of p53 lost the ability to suppress tumor cell growth. Additionally, the N- and C-terminal ends of p53 were shown to be functionally replaceable with foreign transactivation and dimerization domains, respectively, with concordant preservation of both SST and tumor-suppressive properties. Only the central region of p53, conferring specific DNA binding, was required to suppress growth by such hybrid proteins. The SST activity of p53 thus appeared to be essential for the protein to function as a tumor suppressor.

WAF1/CIP1 is induced in p53-mediated G1 arrest and apoptosis

The tumor growth suppressor WAF1/CIP1 was recently shown to be induced by p53 and to be a potent inhibitor of cyclin-dependent kinases. In the present studies, we sought to determine the relationship between the expression of WAF1/CIP1 and endogenous regulation of p53 function. WAF1/CIP1 protein was first localized to the nucleus of cells containing wild-type p53 and undergoing G1 arrest. WAF1/CIP1 was induced in wild-type p53-containing cells by exposure to DNA damaging agents, but not in mutant p53-containing cells. The induction of WAF1/CIP1 protein occurred in cells undergoing either p53-associated G1 arrest or apoptosis but not in cells induced to arrest in G1 or to undergo apoptosis through p53-independent mechanisms. DNA damage led to increased levels of WAF1/CIP1 in cyclin E-containing complexes and to an associated decrease in cyclin-dependent kinase activity. These results support the idea that WAF1/CIP1 is a critical downstream effector in the p53-specific pathway of growth control in mammalian cells.

Oncoprotein MDM2 conceals the activation domain of tumour suppressor p53

The tumour-suppressor gene p53 is inactivated in most human malignancies either by missense mutations or by binding to oncogenic proteins. In human soft tissue sarcomas, inactivation apparently results from MDM2 gene amplification. MDM2 is an oncogene product that may function by binding to p53 and inhibiting its ability to activate transcription. Here we show that, when expressed in Saccharomyces cerevisiae, human MDM2 inhibits human p53's ability to stimulate transcription by binding to a region that nearly coincides with the p53 acidic activation domain. The isolated p53 activation domain fused to another DNA-binding protein is also inactivated by MDM2, confirming that MDM2 can inhibit p53 function by concealing the activation domain of p53 from the cellular transcription machinery.

Transforming growth factor beta 1 can induce estrogen-independent tumorigenicity of human breast cancer cells in athymic mice

We have examined the effect of transforming growth factor beta 1 (TGF-beta 1) overexpression in human breast cancer cell tumorigenicity in athymic mice. Estrogen-dependent MCF-7 cells were stably transfected with pSVTGF beta 1. A clone was isolated which overexpressed TGF-beta 1 mRNA and secreted > 10-fold more TGF-beta activity into the tissue culture medium. Similar to the parent line, the MCF-7/TGF-beta 1 cells were relatively insensitive to exogenous TGF-beta 1 and exhibited low levels of TGF-beta receptors. Clonogenicity in soft agarose, doubling time, morphology, and sensitivity to 17 beta-estradiol and the antiestrogen tamoxifen were not altered in the transfected cells. Inoculation s.c. of MCF-7/TGF-beta 1 cells in ovariectomized nude mice resulted in 100% tumor formation which was totally abrogated by i.p. administration of the neutralizing anti-TGF-beta 2G7 IgG2B. The parent cells formed tumors only after estrogen supplementation. By immunohistochemistry, higher levels of TGF-beta 1 protein were detected in MCF-7/TGF-beta 1 tumors than in estrogen-induced parent MCF-7 tumors. Administration of 1 microgram TGF-beta 1 i.p. daily for 3 weeks after tumor cell inoculation transiently supported estrogen-independent growth of parent MCF-7 tumors in castrated nude mice. These data indicate that overexpression of TGF-beta 1 in human breast cancer cells can contribute to their escape from hormone dependence.

Oncogenic forms of p53 inhibit p53-regulated gene expression

Mutant forms of the gene encoding the tumor suppressor p53 are found in numerous human malignancies, but the physiologic function of p53 and the effects of mutations on this function are unknown. The p53 protein binds DNA in a sequence-specific manner and thus may regulate gene transcription. Cotransfection experiments showed that wild-type p53 activated the expression of genes adjacent to a p53 DNA binding site. The level of activation correlated with DNA binding in vitro. Oncogenic forms of p53 lost this activity. Moreover, all mutants inhibited the activity of coexpressed wild-type p53, providing a basis for the selection of such mutants during tumorigenesis.

Transforming growth factor beta 1 regulation of c-myc expression, pRB phosphorylation, and cell cycle progression in keratinocytes

Transforming growth factor beta 1 (TGF-beta 1) is a potent inhibitor of cellular proliferation in a variety of cell types, including skin keratinocytes. TGF-beta 1 suppression of c-myc transcription has been implicated in the mechanism of TGF-beta 1 inhibition of keratinocytes, and evidence suggests that the protein product of the retinoblastoma gene (pRB) is a necessary component in this pathway. Following growth factor stimulation of quiescent keratinocytes, TGF-beta 1 can inhibit cell cycle progression into S phase at any point prior to the G1-S transition but does not inhibit progression through the S phase of the cell cycle. Since pRB is hypophosphorylated during G1 and hyperphosphorylated during S and G2, the G1-S-specific phosphorylation of pRB becomes an attractive target for the growth-inhibitory activities of TGF-beta 1. However, in TGF-beta 1-treated primary human keratinocytes and in a series of human papilloma virus and SV40 immortalized human keratinocyte cell lines, the phosphorylation status of pRB strictly correlated with cell growth. No evidence was found for a direct effect of TGF-beta 1 on the phosphorylation state of pRB in these cells. It was further demonstrated that synthesis of c-myc protein can be rapidly inhibited by TGF-beta 1 addition throughout G1 and S phases, indicating that the phosphorylation state of pRB, at least as it varies during the cell cycle, does not alter the ability of TGF-beta 1 to suppress c-myc expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Definition of a consensus binding site for p53

Recent experiments have suggested that p53 action may be mediated through its interaction with DNA. We have now identified 18 human genomic clones that bind to p53 in vitro. Precise mapping of the binding sequences within these clones revealed a consensus binding site with a striking internal symmetry, consisting of two copies of the 10 base pair motif 5'-PuPuPuC(A/T)(T/A)GPyPyPy-3' separated by 0-13 base pairs. One copy of the motif was insufficient for binding, and subtle alterations of the motif, even when present in multiple copies, resulted in loss of affinity for p53. Mutants of p53, representing each of the four "hot spots" frequently altered in human cancers, failed to bind to the consensus dimer. These results define the DNA sequence elements with which p53 interacts in vitro and which may be important for p53 action in vivo.