XAF1 directs apoptotic switch of p53 signaling through activation of HIPK2 and ZNF313.

X-linked inhibitor of apoptosis (XIAP)-associated factor 1 (XAF1) is a tumor suppressor that is frequently inactivated in many human cancers. However, the molecular mechanism underlying its growth-inhibitory function remains largely unknown. Here, we report that XAF1 forms a positive feedback loop with p53 and acts as a molecular switch in p53-mediated cell-fate decisions favoring apoptosis over cell-cycle arrest. XAF1 binds directly to the N-terminal proline-rich domain of p53 and thus interferes with E3 ubiquitin ligase MDM2 binding and ubiquitination of p53. XAF1 stimulates homeodomain-interacting protein kinase 2 (HIPK2)-mediated Ser-46 phosphorylation of p53 by blocking E3 ubiquitin ligase Siah2 interaction with and ubiquitination of HIPK2. XAF1 also steps up the termination of p53-mediated cell-cycle arrest by activating zinc finger protein 313 (ZNF313), a p21(WAF1)-targeting ubiquitin E3 ligase. XAF1 interacts with p53, Siah2, and ZNF313 through the zinc finger domains 5, 6, and 7, respectively, and truncated XAF1 isoforms preferentially expressed in cancer cells fail to form a feedback loop with p53. Together, this study uncovers a novel role for XAF1 in p53 stress response, adding a new layer of complexity to the mechanisms by which p53 determines cell-fate decisions.

PPARδ promotes oncogenic redirection of TGF-ß1 signaling through the activation of the ABCA1-Cav1 pathway.

TGF-ß1 plays biphasic functions in prostate tumorigenesis, inhibiting cell growth at early stages but promoting malignant progression at later stages. However, the molecular basis for the oncogenic conversion of TGF-ß1 function remains largely undefined. Here, we demonstrate that PPARδ is a direct transcription target of TGF-ß1 and plays a critical role in oncogenic redirection of TGF-ß1 signaling. Blockade of PPARδ induction enhances tumor cell response to TGF-ß1-mediated growth inhibition, while its activation promotes TGF-ß1-induced tumor growth, migration and invasion. PPARδ-mediated switch of TGF-ß1 function is associated with down- and upregulation of Smad and ERK signaling, respectively, and tightly linked to its function to activate ABCA1 cholesterol transporter followed by caveolin-1 (Cav1) induction. Intriguingly, TGF-ß1 activation of the PPARδ-ABCA1-Cav1 pathway facilitates degradation of TGF-ß receptors (TßRs) and attenuates Smad but enhances ERK response to TGF-ß1. Expression of PPARδ and Cav1 is tightly correlated in both prostate tissues and cell lines and significantly higher in cancer vs. normal tissues. Collectively, our study shows that PPARδ is a transcription target of TGF-ß1 and contributes to the oncogenic conversion of TGF-ß1 function through activation of the ABCA1-Cav1-TßR signaling axis.

CAV1/caveolin 1 enhances aerobic glycolysis in colon cancer cells via activation of SLC2A3/GLUT3 transcription.

Although elevated expression of CAV1/caveolin 1 is associated with the malignant progression of various human cancers, the molecular mechanism underlying its oncogenic functions is largely unknown. We found that CAV1 is frequently overexpressed in advanced colorectal tumors due to aberrant promoter CpG site hypomethylation, and its elevation is implicated in enhanced aerobic glycolysis of tumor cells. Depletion of elevated CAV1 downregulates glucose uptake, intracellular ATP level and lactate accumulation, and triggers autophagy through activation of AMPK-TP53/p53 signaling. CAV1 elevation increases glucose uptake and ATP production by stimulating transcription of the glucose transporter SLC2A3/GLUT3 via an HMGA1-binding site within the promoter. Collectively, our study suggests that elevated CAV1 expression may contribute to colorectal tumor progression by providing tumor cells growth and survival advantages under nutritional stress conditions.

Caveolin-1 increases aerobic glycolysis in colorectal cancers by stimulating HMGA1-mediated GLUT3 transcription.

Caveolin-1 (CAV1) acts as a growth suppressor in various human malignancies, but its expression is elevated in many advanced cancers, suggesting the oncogenic switch of its role during tumor progression. To understand the molecular basis for the growth-promoting function of CAV1, we characterized its expression status, differential roles for tumor growth, and effect on glucose metabolism in colorectal cancers. Abnormal elevation of CAV1 was detected in a substantial fraction of primary tumors and cell lines and tightly correlated with promoter CpG sites hypomethylation. Depletion of elevated CAV1 led to AMPK activation followed by a p53-dependent G1 cell-cycle arrest and autophagy, suggesting that elevated CAV1 may contribute to ATP generation. Furthermore, CAV1 depletion downregulated glucose uptake, lactate accumulation, and intracellular ATP level, supporting that aerobic glycolysis is enhanced by CAV1. Consistently, CAV1 was shown to stimulate GLUT3 transcription via an HMGA1-binding site within the GLUT3 promoter. HMGA1 was found to interact with and activate the GLUT3 promoter and CAV1 increased the HMGA1 activity by enhancing its nuclear localization. Ectopic expression of HMGA1 increased glucose uptake, whereas its knockdown caused AMPK activation. In addition, GLUT3 expression was strongly induced by cotransfection of CAV1 and HMGA1, and its overexpression was observed predominantly in tumors harboring high levels of CAV1 and HMGA1. Together, these data show that elevated CAV1 upregulates glucose uptake and ATP production through HMGA1-mediated GLUT3 transcription, suggesting that CAV1 may render tumor cells growth advantages by enhancing aerobic glycolysis.

Epigenetic alteration of PRKCDBP in colorectal cancers and its implication in tumor cell resistance to TNFα-induced apoptosis.

PURPOSE: PRKCDBP is a putative tumor suppressor in which alteration has been observed in several human cancers. We investigated expression and function of PRKCDBP in colorectal cells and tissues to explore its candidacy as a suppressor in colorectal tumorigenesis. EXPERIMENTAL DESIGN: Expression and methylation status of PRKCDBP and its effect on tumor growth were evaluated. Transcriptional regulation by NF-κB signaling was defined by luciferase reporter and chromatin immunoprecipitation assays. RESULTS: PRKCDBP expression was hardly detectable in 29 of 80 (36%) primary tumors and 11 of 19 (58%) cell lines, and its alteration correlated with tumor stage and grade. Promoter hypermethylation was commonly found in cancers. PRKCDBP expression induced the G(1) cell-cycle arrest and increased cellular sensitivity to various apoptotic stresses. PRKCDBP was induced by TNFα, and its level correlated with tumor cell sensitivity to TNFα-induced apoptosis. PRKCDBP induction by TNFα was disrupted by blocking NF-κB signaling while it was enhanced by RelA transfection. The PRKCDBP promoter activity was increased in response to TNFα, and this response was abolished by disruption of a κB site in the promoter. PRKCDBP delayed the formation and growth of xenograft tumors and improved tumor response to TNFα-induced apoptosis. CONCLUSIONS: PRKCDBP is a proapoptotic tumor suppressor which is commonly altered in colorectal cancer by promoter hypermethylation, and its gene transcription is directly activated by NF-κB in response to TNFα. This suggests that PRKCDBP inactivation may contribute to tumor progression by reducing cellular sensitivity to TNFα and other stresses, particularly under chronic inflammatory microenvironment.

Epigenetic inactivation of the NORE1 gene correlates with malignant progression of colorectal tumors.

BACKGROUND: NORE1 (RASSF5) is a newly described member of the RASSF family with Ras effector function. NORE1 expression is frequently inactivated by aberrant promoter hypermethylation in many human cancers, suggesting that NORE1 might be a putative tumor suppressor. However, expression and mutation status of NORE1 and its implication in colorectal tumorigenesis has not been evaluated. METHODS: Expression, mutation, and methylation status of NORE1A and NORE1B in 10 cancer cell lines and 80 primary tumors were characterized by quantitative PCR, SSCP, and bisulfite DNA sequencing analyses. Effect of NORE1A and NORE1B expression on tumor cell growth was evaluated using cell number counting, flow cytometry, and colony formation assays. RESULTS: Expression of NORE1A and NORE1B transcript was easily detectable in all normal colonic epithelial tissues, but substantially decreased in 7 (70%) and 4 (40%) of 10 cancer cell lines and 31 (38.8%) and 25 (31.3%) of 80 primary carcinoma tissues, respectively. Moreover, 46 (57.6%) and 38 (47.5%) of 80 matched tissue sets exhibited tumor-specific reduction of NORE1A and NORE1B, respectively. Abnormal reduction of NORE1 was more commonly observed in advanced stage and high grade tumors compared to early and low grade tumors. While somatic mutations of the gene were not identified, its expression was re-activated in all low expressor cells after treatment with the demethylating agent 5-aza-dC. Bisulfite DNA sequencing analysis of 31 CpG sites within the promoter region demonstrated that abnormal reduction of NORE1A is tightly associated with promoter CpG sites hypermethylation. Moreover, transient expression and siRNA-mediated knockdown assays revealed that both NORE1A and NORE1B decrease cellular growth and colony forming ability of tumor cells and enhance tumor cell response to apoptotic stress. CONCLUSION: Our data indicate that epigenetic inactivation of NORE1 due to aberrant promoter hypermethylation is a frequent event in colorectal tumorigenesis and might be implicated in the malignant progression of colorectal tumors.

Frequent inactivation of hSRBC in ovarian cancers by promoter CpG island hypermethylation.

OBJECTIVE: To explore the implication of human SRBC gene [serum deprivation response factor-related gene product that binds to the c-kinase (hSRBC)] abnormality in ovarian tumorigenesis. DESIGN: Retrospective study. SETTING: Medical center. SAMPLE: Twenty-two epithelial ovarian cancer and six normal ovary tissues. MEASURES: Mutation and altered expression of hSRBC gene. METHODS: hSRBC expression was characterized by polymerase chain reaction (PCR) analysis. Promoter CG dinucleotide (CpG) site methylation was determined using methylation specific PCR and bisulfite sequencing. RESULTS: Expression of hSRBC transcript was easily detectable in all normal tissues we examined, but 50% (two of four) of cancer cell lines and 41% (nine of 22) of primary carcinomas exhibited undetectable or substantially decreased expression. While genomic deletion or somatic mutations of the gene was not identified, its expression was reactivated in tumor cells by 5-aza-2'-deoxycytidine treatment, suggesting epigenetic inactivation of the gene in tumors. Promoter methylation was detected in all nine tumors with low expression but in only one of 13 (7.7%) tumors with normal expression. Bisulfite DNA sequencing analysis of 23 CpG sites within the promoter region revealed that the CpG sites are highly methylated in low-expressing tumors. In addition, promoter CpG sites methylation status showed a tight association with gene expression level. CONCLUSION: Our data demonstrate that epigenetic inactivation of hSRBC due to aberrant promoter hypermethylation is a common event and might be implicated in human ovarian tumorigenesis.

NF-kappaB activates transcription of the RNA-binding factor HuR, via PI3K-AKT signaling, to promote gastric tumorigenesis.

BACKGROUND & AIMS: HuR is a RNA-binding factor whose expression is commonly upregulated in some human tumor types. We explored the molecular mechanism underlying HuR elevation and its role in gastric cancer tumorigenesis. METHODS: HuR expression and subcellular localization were determined by polymerase chain reaction, immunoblot, and immunohistochemical analyses. Its effect on tumor growth was characterized using flow cytometry, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling, and soft agar analyses. Luciferase reporter, chromatin immunoprecipitation, and electrophoretic mobility shift assays were used to measure transcriptional activation by nuclear factor kappaB (NF-kappaB) signaling. RESULTS: Compared with normal gastric tissues, HuR was expressed at higher levels in gastric tumors, particularly in advanced versus early tumors; this increase was associated with enhanced cytoplasmic translocation of HuR. HuR overexpression increased proliferation of tumor cells, activating the G(1) to S transition of the cell cycle, DNA synthesis, and anchorage-independent growth. Small interfering RNA-mediated knockdown of HuR expression reduced tumor cell proliferation and response to apoptotic stimuli. No genetic or epigenetic alterations of HuR were observed in gastric tumor cell lines or primary tumors; overexpression depended on phosphatidylinositol 3-kinase/AKT signaling and NF-kappaB activity. AKT activation increased p65/RelA binding to a putative NF-kappaB binding site in the HuR promoter, the stability of HuR target transcripts, and the cytoplasmic import of HuR. CONCLUSIONS: HuR is a direct transcription target of NF-kappaB; its activation in gastric cancer cell lines depends on phosphatidylinositol 3-kinase/AKT signaling. HuR activation by this pathway has proliferative and antiapoptotic effects on gastric cancer cells.

An ab Initio Study of Intramolecular Hydrogen Bondings in α-Hydroxy Ketomethylene Dipeptide Isostere.

Ab initio calculations of the representative α-hydroxy ketomethylene dipeptide isostere (2S,5S)-5-amino-2-hydroxy-4-oxohexanoic acid (1) are described. All calculations including full geometry optimizations were performed at the MP2/6-31G* level. In the gas phase, 12 low-energy conformers are located by minimizing geometries assembled from stable molecular fragments. Among these conformers, six structurally similar conformers, in which the 2-hydroxyl group forms hydrogen bondings with both the O atom of the 4-carbonyl group in 1,3-fashion and the O atom of 1-carboxylic acid in 1,2-fashion simultaneously, are found to be particularly stable. Thus, the conformational preference of 1 appears to be governed by arrangements and strength of intramolecular hydrogen bondings. To examine conformational natures of 1 in solutions more accurately, we corrected the thermochemical properties and carried out self-consistent reaction field calculations. Going from the gas phase to solutions, the basic features of the conformational preferences in 1 also appear to be maintained in solutions including a highly polar aqueous medium, despite slight changes in the population of each conformer.