Mutant P53-expressing Cell Lines Research Articles

Differential effect of plakoglobin in restoring the tumor suppressor activities of p53-R273H vs. p53-R175H mutants.

The six most common missense mutations in the DNA binding domain of p53 are known as "hot spots" and include two of the most frequently occurring p53 mutations (p53-R175H and p53-R273H). p53 stability and function are regulated by various post-translational modifications such as phosphorylation, acetylation, sumoylation, methylation, and interactions with other proteins including plakoglobin. Previously, using various carcinoma cell lines we showed that plakoglobin interacted with wild-type and several endogenous p53 mutants (e.g., R280K, R273H, S241F, S215R, R175H) and restored their tumor suppressor activities in vitro. Since mutant p53 function is both mutant-specific and cell context-dependent, we sought herein, to determine if plakoglobin tumor suppressive effects on exogenously expressed p53-R273H and p53-R175H mutants are similarly maintained under the same genetic background using the p53-null and plakoglobin-deficient H1299 cell line. Functional assays were performed to assess colony formation, migration, and invasion while immunoblotting and qPCR were used to examine the subcellular distribution and expression of specific proteins and genes that are typically regulated by or regulate p53 function and are altered in mutant p53-expressing cell lines and tumors. We show that though, plakoglobin interacted with both p53-R273H and p53-R175H mutants, it had a differential effect on the transcription and subcellular distribution of their gene targets and their overall oncogenic properties in vitro. Notably, we found that plakoglobin's tumor suppressive effects were significantly stronger in p53-R175H expressing cells compared to p53-R273H cells. Together, our results indicate that exploring plakoglobin interactions with p53-R175H may be useful for the development of cancer therapeutics focused on the restoration of p53 function.

Abstract 5288: Development of lead mutant-p53 reactivating peptides towards clinical application

Abstract p53 is the most extensively studied tumor suppressor. It serves as a major barrier against malignant transformation and the p53 pathway is inactivated in almost all human tumors. A large percentage of tumors inactivate p53 by point mutations in the DNA binding domain (DBD) of the protein. Most mutations destabilize p53 protein folding, causing its partial denaturation at physiological temperature. Thus a high proportion of human tumors overexpress a potential potent tumor suppressor in a non-functional, improperly folded form. Previously, we reported the use of phage display technology to select for peptides that reactivate mutant p53 by stabilizing a wt protein conformation, taking advantage of a unique combination of selection steps (Tal P, at al Oncotarget. 2017 Jan 3;8(1):164-178). We now describe further progress in the development of one lead peptide, pCAP-250, which contains 9 amino acids and a myristic fatty acid at its N-terminus. The pCAP-250 sequence bears 100% homology to RAD9, a known p53 interacting protein. We show physical interaction between pCAP-250 and the p53DBD. Characterization of this interaction by NMR reveals a significant chemical shift of amino acids located in the p53DBD L1 loop and Helix-2, two regions critical for p53 transcriptional activity and interaction with DNA. We also report the optimization of our lead sequence by rational design to yield more potent derivatives with higher efficacy in cancer cell elimination and longer in vivo half-life. RNA-seq analysis shows that our lead peptides induce robust activation of p53 target genes. Furthermore, we report that in addition to p53 activation, pCAP-250 also has a p53-independent anti-cancer activity. Overall, our lead peptides cause specific cell death in a wide spectrum of mutant p53 expressing cell lines, with an IC50 of 0.1-12µM, while having no effect on non-transformed cells at these concentrations. The pharmacokinetic profile of pCAP-250 and its derivatives shows that they are completely stable in human serum and have plasma half-life of 1.5-10h in vivo when administered IV or subcutaneously, presumably due to interaction with albumin through the fatty acid moiety. Remarkably, our lead peptides elicit remarkable regression of very aggressive tumors in several preclinical mouse models of renal, colon, pancreatic and ovarian cancer. In vivo efficacy can be achieved through several routes of administration, including intra-tumor injection as well as systemic subcutaneous continuous slow release and subcutaneous bi-daily injections. Thus, such peptides might serve as novel therapeutic agents for p53-mutated human cancer. Citation Format: Perry Tal, Moshe Oren, Orna Palgi, Varda Rotter, KWANGIL JEONG. Development of lead mutant-p53 reactivating peptides towards clinical application [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5288.

Abstract P1-02-05: A genome-wide functional genomics screen reveals unique co-driver mutations of mutant TP53 promoting cellular heterogeneity during breast cancer progression

Abstract Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer that lacks three major drug-targetable receptors, ER, PR, and HER2. TNBC patients have much worse 5-year survival rates (60%) in contrast to 90% for other breast cancer subtypes and display highly heterogeneous molecular profiles, cellular phenotypes, and drug responses, which poses major challenges in patient treatments. The tumor suppressor gene TP53 is mutated in 30% of breast tumors overall and but highly prevalent (~80%) in TNBC. Unlike mutations in other tumor suppressor genes that are predominantly loss-of-function deletions or truncations, TP53 mutations occur mostly as >100 different missense mutations within the DNA binding domain, implying that the mutant proteins may exert both loss-of-function activities and gain of distinct neomorphic functions, thus contributing to phenotypic heterogeneity of TNBC. When we characterized systematically a panel of MCF10A cell lines expressing 10 most prevalent missense mutant p53 proteins, the cell lines indeed displayed highly diverse neomorphic cellular phenotypes distinct from those of p53-knockdown cells. To investigate molecular mechanisms underlying the heterogeneity, we then performed RNA-Seq and pathway analysis and identified the key pathways, such as the Hippo/YAP pathway, that were dysregulated correlatively with phenotypic aggressiveness of the mutant p53 cell lines. In addition, ChIP-Seq analysis revealed that promoter binding capacity and preference of mutant p53 proteins associated with more aggressive phenotypes were more severely affected, especially for the genes in the dysregulated pathways identified from RNA-Seq analysis. These demonstrated collectively that different missense p53 mutations lead to heterogeneous phenotypes by exerting distinct neomorphic molecular functions. Further, given that TP53 mutations by themselves cannot drive full cancer progression, these imply that cells with different p53 missense mutations need distinct sets of additional “co-driver” mutations and alteration of cellular programs specific to each mutation for full cancer progression, representing potential molecular targets for personalized therapies. Supporting this hypothesis, when we performed genome-wide in vitro CRISPR screens in search of co-driver mutations specific to different p53 mutations, a unique set of hits was identified for each mutant p53-expressing cell lines. However, in in vivo mouse xenograft models, even the cells expressing aggressive p53 mutants such R273C failed to develop tumors upon transducing gene-deleting CRISPR gRNA libraries at high MOI. Based on reasoning that development of tumor requires mutations in both tumor suppressors and oncogenes, we then performed CRISPR screens on the p53-R273C cells overexpressing MYC, a known oncogene for TNBC, and observed tumor formation within 9 weeks, only after the CRISPR library transduction. By next-generation sequencing of the gRNA cassettes amplified from the tumors, >20 novel co-driver candidates in addition to known tumor suppressors such as NF2 and PTEN were identified. Interestingly, ARAF, a proto-oncogene, was one of the top candidates found in multiple tumors, and the targeted sequencing confirmed out-of-frame deletions resulting in truncated proteins with only the N-term Ras-binding domain. We are currently validating the functional relevance of these findings in conjunction with the dysregulated pathways identified from RNA-Seq and ChIP-Seq analysis. Taken together, our integrated approach of utilizing phenotyping, multi-omics bioinformatics analysis, and screening has revealed the molecular mechanisms underlying phenotypic and molecular heterogeneity and potential molecular targets of TNBC. Citation Format: Dustin Grief, Anasuya Pal, Laura Gonzalez-Malerva, Seron Eaton, Chenxi Xu, Grant Christensen, Joy Blain, Nicholas Mellor, Jason Steel, Chitrak Gupta, Ellen Streitwieser, Abhishek Singharoy, Jin Park, Joshua LaBaer. A genome-wide functional genomics screen reveals unique co-driver mutations of mutant TP53 promoting cellular heterogeneity during breast cancer progression [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-02-05.

Abstract P5-09-01: Using isogenic model systems to determine mechanisms regulating mutant p53 protein stability in breast cancer cells

Abstract Two of the most common events in triple-negative breast cancers (TNBC) are mutation of the tumor suppressor gene TP53 and development of aneuploidy. This is often the case in aggressive and molecularly heterogeneous cancer subtypes such as TNBC, where ~80% of cases harbor a mutation in TP53. In addition to losing wild-type (WT) tumor-suppressive function, mutant p53 proteins are proposed to acquire gain-of-function (GOF) activity, leading to novel oncogenic phenotypes. Mutant p53 proteins can accumulate to high levels in tumors, and this stabilization is thought to be a pre-requisite for mutant p53 to exert its GOF effects. Mechanisms underlying the accumulation of mutant p53 are not well understood. To study mutant p53 GOF mechanisms and phenotypes, we used CRISPR/Cas9-mediated genome editing and developed two isogenic TNBC cell line models (one non-transformed and one tumor-derived) that express the most frequently occurring p53 missense mutations (R175H and R273H), are deficient for functional p53 protein (null), or retain the wild-type (WT) protein. In these engineered models, endogenous p53 expression is regulated by the native p53 promoter, thus providing a controlled system for rigorous functional experimentation across different p53 states. Through functional genomics analyses comparing isogenic TNBC cell lines, which initially differed only by the TP53 genotype, we have evaluated the relationship between mutant p53 and development of aneuploidy and determined that development of aneuploidy and not TP53 genotype contributed to several previously reported mutant p53 GOF phenotypes in vitro and in vivo1. Immunoblotting, immunofluorescence, and immunohistochemistry experiments revealed that mutant p53-expressing cell lines display a range of p53 protein levels, with several maintaining low levels of p53, similar to the cell lines which contain WT p53. In addition, mutant p53 levels correlate with aneuploidy, but not mean nuclear area or whole-genome doubling status, and protein levels increase over time in cell lines that become aneuploid. Using our various clonal cell lines that exhibit differing p53 protein stability, we are currently determining if p53 stability is dysregulated in aneuploid cell lines after copy number gain or loss of genes encoding critical p53 regulators, leading to the accumulation of the mutant protein. Analysis of cancer cell line genetic dependencies and human tumor data from The Cancer Genome Atlas (TCGA) has identified genetic and clinical correlates with increased mutant p53 protein levels. Utilizing biochemical and bioinformatic approaches, we aim to uncover mechanisms for increased mutant p53 stability and identify novel regulators of both WT and mutant p53 proteins. The dissection of mechanisms that contribute to stabilization of p53 protein will not only give insight into the regulation and tumor suppressive function of WT p53, but it has the potential for clinical translation in human cancers that have high-frequency p53 mutation and high levels of mutant p53 protein, as targeting mutant p53 through pharmacologic inhibition or antibody-based therapies is an active area of investigation. 1Article in press in Nature Communications Citation Format: Lindsay Redman-Rivera, Hailing Jin, Brian D Lehmann, Jennifer A Pietenpol. Using isogenic model systems to determine mechanisms regulating mutant p53 protein stability in breast cancer cells [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-09-01.

Abstract 3908: miR-3132 induces TRAIL and cell death in mutant p53-expressing cancer cells

Abstract Tumor suppressor p53, guardian of the genome, is frequently mutated or functionally dysregulated in more than 50% of human tumors. p53 mutation is a later event in tumorigenesis and a number of p53 mutants have “Gain of Function” (GOF) properties that have been shown to promote invasive and more aggressive phenotypes in cancer cells. Mutant p53 has been an attractive and promising therapeutic target for advanced stages of tumors. Further understanding the GOF properties of mutant p53 may help with design of novel strategies to target mutant p53 and/or key GOF pathways. We developed a novel functional high throughput screening (HTS) assay to identify miRNAs that selectively target GOF in mutant p53-expressing cell lines. Of the 2754 miRNA mimics screened, we identified 56 miRNAs that selectively reduced cell viability in TP53 R175H (mutant) cells. The current study characterizes the role of our lead mimic, miR-3132 in mutant p53 cells. We found that expression of miR-3132 is lost in cancer cells and that miR-3132 expression is correlated to p53 status, with mutant p53 cells expressing lower levels of miR-3132 than those with wild-type p53. Restoring expression of miR-3132 via mimics induces anti-proliferative and pro-apoptotic effects in a broad panel of colorectal (CRC), breast and lung cancer cell lines. In addition, miR-3132 induces TRAIL-dependent cell death in mutant p53-expressing cells. All cell lines show potent upregulation of cleaved caspases -8, -9 and -3, at 72 hrs post transfection with miR-3132. DNA damaging agents and serum starvation endogenously increase mature miR-3132 levels in wild-type p53 cells. However, this increase is blunted in mutant p53 cells, indicating that p53 could have a role in modulating either the transcriptional or post-transcriptional levels of miR-3132. In summary, we have identified miR-3132 as a TRAIL and cell death-inducing miRNA whose regulation is dependent on p53. Our ongoing work aims at finding the key targets of miR-3132 that explain the cellular response, TRAIL induction and subsequent cell death. Our long-term goal is to develop miR-3132 as a novel single or combinatorial anti-cancer agent for mutant p53 cancer cells. Citation Format: Amriti R. Lulla, Margret B. Einarson, Yan Zhou, Avital Lev, David T. Dicker, Wafik S. El-Deiry. miR-3132 induces TRAIL and cell death in mutant p53-expressing cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3908.

Mutant p53 gain of function underlies high expression levels of colorectal cancer stem cells markers

Emerging notion in carcinogenesis ascribes tumor initiation and aggressiveness to cancer stem cells (CSCs). Specifically, colorectal cancer (CRC) development was shown to be compatible with CSCs hypothesis. Mutations in p53 are highly frequent in CRC, and are known to facilitate tumor development and aggressiveness. Yet, the link between mutant p53 and colorectal CSCs is not well-established. In the present study, we set to examine whether oncogenic mutant p53 proteins may augment colorectal CSCs phenotype. By genetic manipulation of mutant p53 in several cellular systems, we demonstrated that mutant p53 enhances colorectal tumorigenesis. Moreover, mutant p53-expressing cell lines harbor larger sub-populations of cells highly expressing the known colorectal CSCs markers: CD44, Lgr5, and ALDH. This elevated expression is mediated by mutant p53 binding to CD44, Lgr5, and ALDH1A1 promoter sequences. Furthermore, ALDH1 was found to be involved in mutant p53-dependent chemotherapy resistance. Finally, analysis of ALDH1 and CD44 in human CRC biopsies indicated a positive correlation between their expression and the presence of oncogenic p53 missense mutations. These findings suggest novel insights pertaining the mechanism by which mutant p53 enhances CRC development, which involves the expansion of CSCs sub-populations within CRC tumors, and underscore the importance of targeting these sub-populations for CRC therapy.

MiR-6883 Family miRNAs Target CDK4/6 to Induce G1 Phase Cell-Cycle Arrest in Colon Cancer Cells.

CDK4/6 targeting is a promising therapeutic strategy under development for various tumor types. In this study, we used computational methods and The Cancer Genome Atlas dataset analysis to identify novel miRNAs that target CDK4/6 and exhibit potential for therapeutic development in colorectal cancer. The 3'UTR of CDK4/6 mRNAs are targeted by a family of miRNAs, which includes miR-6883-5p, miR-149*, miR-6785-5p, and miR-4728-5p. Ectopic expression of miR-6883-5p or miR-149* downregulated CDK4 and CDK6 levels in human colorectal cancer cells. RNA-seq analysis revealed an inverse relationship between the expression of CDK4/6 and miR-149* and intronic miRNA-6883-5p encoding the clock gene PER1 in colorectal cancer patient samples. Restoring expression of miR-6883-5p and miR-149* blocked cell growth leading to G0-G1 phase cell-cycle arrest and apoptosis in colorectal cancer cells. CDK4/6 targeting by miR-6883-5p and miR-149* could only partially explain the observed antiproliferative effects. Notably, both miRNAs synergized with the frontline colorectal cancer chemotherapy drug irinotecan. Further, they resensitized mutant p53-expressing cell lines resistant to 5-fluorouracil. Taken together, our results established the foundations of a candidate miRNA-based theranostic strategy to improve colorectal cancer management. Cancer Res; 77(24); 6902-13. ©2017 AACR.

Abstract 5455: Novel miRNA-based therapeutic approach to selectively target mutant p53 in cancer

Abstract Tumor suppressor p53, guardian of the genome, is frequently mutated or functionally dysregulated in more than 50% of human tumors. p53 mutation is a later event in tumorigenesis and a number of p53 mutants have “Gain of Function” (GOF) properties that have been shown to promote invasive and more aggressive phenotypes in cancer cells. Mutant p53 has been an attractive and promising therapeutic target for advanced stages of tumors. Yet, mutant p53 has proved to be one of the most undruggable targets. Thus, there is continued interest in understanding the GOF properties of mutant p53 and in designing novel strategies to target mutant p53 and/or key GOF pathways. Mature MicroRNAs (miRNAs) are ~22 nt endogenous, non-coding RNA sequences that bind to 3’UTR of their target genes and inhibit their translation. miRNA mimics are emerging therapeutics and attractive tools for mapping pathway networks. We developed a novel functional high throughput screening (HTS) assay to identify miRNAs that selectively target mutant p53-expressing cell lines. The HTS was performed in isogenic TP53+/+ (wild-type), TP53-/- (null) and TP53 R175H (mutant) HCT-116 colorectal cancer cell lines. Cell viability was used as the HTS read-out of our functional screen. Of 2754 miRNA mimics screened, we identified 56 miRNAs that selectively target TP53 R175H (mutant) cells. Our ongoing work is directed to further validating and identifying which of these miRNA mimics selectively induce apoptosis in mutant p53-expressing cells. We are also using reporter-based assays to identify a fraction of the 56 miRNAs that can bind to the p53 3’UTR. Our long-term goal is to selectively target mutant p53-expressing cells using miRNA mimics as single agents or in combination with FDA-approved or experimental therapeutics. Our strategy is may provide a unique opportunity for development of targeted therapy for TP53 mutant tumors. Citation Format: Amriti R. Lulla, Margret B. Einarson, Yan Zhou, David T. Dicker, Wafik S. El-Deiry. Novel miRNA-based therapeutic approach to selectively target mutant p53 in cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5455. doi:10.1158/1538-7445.AM2017-5455

Gain-of-function mutant p53 downregulates miR-223 contributing to chemoresistance of cultured tumor cells

Mutant p53 proteins are expressed at high frequency in human tumors and are associated with poor clinical prognosis and resistance to chemotherapeutic treatments. Here we show that mutant p53 proteins downregulate micro-RNA (miR)-223 expression in breast and colon cancer cell lines. Mutant p53 binds the miR-223 promoter and reduces its transcriptional activity. This requires the transcriptional repressor ZEB-1. We found that miR-223 exogenous expression sensitizes breast and colon cancer cell lines expressing mutant p53 to treatment with DNA-damaging drugs. Among the putative miR-223 targets, we focused on stathmin-1 (STMN-1), an oncoprotein known to confer resistance to chemotherapeutic drugs associated with poor clinical prognosis. Mutant p53 silencing or miR-223 exogenous expression lowers the levels of STMN-1 and knockdown of STMN-1 by small interfering RNA increases cell death of mutant p53-expressing cell lines. On the basis of these findings, we propose that one of the pathways affected by mutant p53 to increase cellular resistance to chemotherapeutic agents involves miR-223 downregulation and the consequent upregulation of STMN-1.

Mutant p53 mediates survival of breast cancer cells

Background:p53 is the most commonly mutated tumour-suppressor gene in human cancers. Unlike other tumour-suppressor genes, most p53 cancer mutations are missense mutations within the core domain, leading to the expression of a full-length mutant p53 protein. Accumulating evidence has indicated that p53 cancer mutants not only lose tumour suppression activity but also gain new oncogenic activities to promote tumourigenesis.Methods:The endogenous mutant p53 function in human breast cancer cells was studied using RNA interference (RNAi). Gene knockdown was confirmed by quantitative PCR and western blotting. Apoptosis was evaluated by morphological changes of cells, their PARP cleavage and annexin V staining.Results:We show that cancer-associated p53 missense mutants are required for the survival of breast cancer cells. Inhibition of endogenous mutant p53 by RNAi led to massive apoptosis in two mutant p53-expressing cell lines, T47D and MDA-MB-468, but not in the wild-type p53-expressing cells, MCF-7 and MCF-10A. Reconstitution of an RNAi-insensitive mutant p53 in MDA-MB-468 cells completely abolished the apoptotic effects after silencing of endogenous mutant p53, suggesting the specific survival effects of mutant p53. The apoptotic effect induced by mutant p53 ablation, however, is independent of p63 or p73 function.Conclusion:These findings provide clear evidence of a pro-survival ‘gain-of-function’ property of a subset of p53 cancer mutants in breast cancer cells.

Promising combination therapies with gemcitabine

Because treatment regimens for breast cancer commonly include gemcitabine, we evaluated two promising combinations in preclinical studies: gemcitabine (Gemzar; Eli Lilly and Company, Indianapolis, IN) with either ionizing radiation or docetaxel (Taxotere; Aventis Pharmaceuticals, Inc, Parsippany, NJ). In breast cancer cell lines that expressed either wild-type p53 (MCF-7) or mutant p53 (MCF-7/Adr), sensitivity to the cytotoxic effects of gemcitabine during a 24-hour incubation was similar (IC 50 values 80 and 60 nmol/L in MCF-7 and MCF-7/Adr, respectively). Both cell lines were well radiosensitized by gemcitabine at the corresponding IC 50, with radiation enhancement ratios of 1.6 to 1.7. Although the MCF-7 cells accumulated nearly twice as much gemcitabine triphosphate compared with the MCF-7/Adr cells, a similar reduction in 2′-deoxyadenosine 5′-triphosphate pools was observed. While the number of dying cells, as measured by sub-G1 DNA content or S-phase cells unable to replicate DNA, differed between the wild-type p53 or mutant p53-expressing cell lines, neither parameter correlated with radiosensitization. Docetaxel was a more potent cytotoxic agent than gemcitabine in MCF-7 cells (IC 50 = 1 nmol/L). Strong synergistic cytotoxicity was observed in cells treated with gemcitabine (24 hours) followed by docetaxel (24 hours) or the reverse sequence. However, simultaneous addition of the two drugs was antagonistic. To determine whether synergy with radiation or docetaxel was mediated by increased DNA damage, DNA double-strand breaks (double-strand breaks) were measured by immunostaining for phosphorylated H2AX. Ionizing radiation produced more double-strand breaks than gemcitabine alone, while no significant double-strand breaks formed with docetaxel alone. The addition of docetaxel or ionizing radiation to gemcitabine-treated cells did not increase H2AX foci formation. These results show that the combination of gemcitabine with ionizing radiation or docetaxel produces strong, schedule-dependent synergy in breast cancer cells that is not mediated through increasing DNA double-strand breaks.

Differential activation of the Fas/CD95 pathway by Ad-p53 in human gliomas

Adenoviral p53 gene transfer (Ad-p53) induces apoptosis in glioma cells expressing mutant p53, but fails in cells with wild-type p53. Endogenously, gliomas express varied levels of Fas/CD95, yet constitutively high levels of Fas/CD95 ligand. Because the mechanism behind the differential apoptotic response to Ad-p53 infection remains elusive, we examined how the Fas/CD95 pathway is involved in U87MG (wt-p53), D54 (wt-p53), U251MG (mutant-p53), and U373MG (mutant-p53) glioma cell lines. Ad-p53 infection did not alter the levels of Fas/CD95 ligand in either wild-type or mutant p53-expressing cell lines. In contrast, Ad-p53 infection led to an approximately 3-fold increase in Fas/CD95 mRNA expression in mutant p53-bearing cell lines but not in their wild-type (wt) counterparts, as assessed in an RNase protection assay. Fas/CD95 mRNA induction appeared to be regulated at the transcriptional level because Ad-p53 infection resulted in up to a 4-fold increase in Fas/CD95 promoter reporter activity. Subsequently, flow cytometric analysis revealed a 2- to 4-fold increase in surface Fas/CD95 expression following Ad-p53 infection in mutant-p53-containing cell lines. Use of the protein transport inhibitor Brefeldin A significantly inhibited Ad-p53-induced surface Fas/CD95 expression, but only partially inhibited apoptosis in mutant-p53 cell lines. These results suggest that p53 regulates Fas/CD95 expression at the transcriptional level and through protein trafficking in mutant-p53 cell lines. Fluorogenic activity assays demonstrated that induction of caspase-8 activity following Ad-p53 infection correlated with increases in Fas/CD95 expression. Incubating cells with a caspase-8-specific inhibitor Ac-IETD-CHO prior to Ad-p53 infection inhibited caspase-8 activity and apoptosis. Together, our results suggest that regulation of the Fas/CD95 pathway is partly responsible for Ad-p53-induced apoptosis in glioma cells, which depends on the p53 status of the involved cells. Additionally, the inability of Ad-p53 to activate the Fas/CD95 pathway in wt-p53 glioma cells coincides with their apoptotic-resistant phenotype. Further elucidation of the nature of this resistance could ultimately augment the efficacy of Ad-p53 gene therapy.

Transcriptional upregulation of the insulin-like growth factor binding protein IGFBP-3 by sodium butyrate increases IGF-independent apoptosis in human colonic adenoma-derived epithelial cells.

Sodium butyrate (NaBt) and the pro-apoptotic IGFBP-3 protein, expressed at the top of the normal colonic crypt, have both been implicated in the regulation of apoptosis in colonic epithelial cells. Recent studies in human breast and hepatic cell lines have shown that NaBt can transcriptionally upregulate IGFBP-3 expression. However, the role of butyrate in the regulation of IGFBP-3 expression in the colon is less clear, with reports of both up- and downregulation of the IGFBP-3 protein in colorectal cancer cell lines. In this study we have shown that the level of IGFBP-3 protein expression in colonic epithelial cells correlates with the p53 status of the cells; wildtype p53 cells secrete higher levels of IGFBP3 protein than mutant p53 cell lines. Data presented shows that, when treated with a dose of NaBt that induced significant apoptosis (4 mM for 48 h), there was an upregulation of IGFBP-3 protein in both wildtype and mutant p53 expressing cell lines. The NaBt-induced increase in secreted IGFBP-3 protein was associated with transcriptional upregulation of the IGFBP-3 gene. Using a transfected derivative of the S/RG/C2 adenoma-derived cell line, which stably expressed exogenous IGFBP-3 protein at levels equivalent to that secreted by the 4 mM NaBt-treated parental line (1-3 ng/10(6) cells), we have shown a >2-fold increase in the sensitivity of the cells to NaBt-induced apoptosis when compared with the vector control and parental cell lines. Furthermore, inhibition of the secreted IGFBP-3 protein, by addition of neutralizing antibodies, resulted in a significant decrease in NaBt-induced apoptosis. These data suggest that IGFBP-3 may act as a positive regulator of NaBt-induced apoptosis in colonic epithelial cells, and represents a potentially important mechanism whereby the sensitivity of colonic epithelial cells to NaBt-induced apoptosis can be increased.