W. Douglas Cress

Interactions between E2F1 and SirT1 regulate apoptotic response to DNA damage

The nicotinamide adenine dinucleotide (NAD)-dependent deacetylase Sir2 (silent information regulator 2) regulates gene silencing in yeast and promotes lifespan extension during caloric restriction. The mammalian homologue of Sir2 (SirT1) regulates p53, NF-κB and Forkhead transcription factors, and is implicated in stress response. This report shows that the cell-cycle and apoptosis regulator E2F1 induces SirT1 expression at the transcriptional level. Furthermore, SirT1 binds to E2F1 and inhibits E2F1 activities, forming a negative feedback loop. Knockdown of SirT1 by small interference RNA (siRNA) increases E2F1 transcriptional and apoptotic functions. DNA damage by etoposide causes E2F1-dependent induction of SirT1 expression and knockdown of SirT1 increases sensitivity to etoposide. These results reveal a mutual regulation between E2F1 and SirT1 that affects cellular sensitivity to DNA damage.

Role of Stat3 in Regulating p53 Expression and Function

ABSTRACT Loss of p53 function by mutation is common in cancer. However, most natural p53 mutations occur at a late stage in tumor development, and many clinically detectable cancers have reduced p53 expression but no p53 mutations. It remains to be fully determined what mechanisms disable p53 during malignant initiation and in cancers without mutations that directly affect p53. We show here that oncogenic signaling pathways inhibit the p53 gene transcription rate through a mechanism involving Stat3, which binds to the p53 promoter in vitro and in vivo. Site-specific mutation of a Stat3 DNA-binding site in the p53 promoter partially abrogates Stat3-induced inhibition. Stat3 activity also influences p53 response genes and affects UV-induced cell growth arrest in normal cells. Furthermore, blocking Stat3 in cancer cells up-regulates expression of p53, leading to p53-mediated tumor cell apoptosis. As a point of convergence for many oncogenic signaling pathways, Stat3 is constitutively activated at high frequency in a wide diversity of cancers and is a promising molecular target for cancer therapy. Thus, repression of p53 expression by Stat3 is likely to have an important role in development of tumors, and targeting Stat3 represents a novel therapeutic approach for p53 reactivation in many cancers lacking p53 mutations.

Mcl-1 regulates survival and sensitivity to diverse apoptotic stimuli in human non-small cell lung cancer cells.

Overexpression of anti-apoptotic Bcl-2 family members and deregulation of the pathways that regulate pro-apoptotic family members have been observed in non-small cell lung cancers (NSCLC). Previous reports have identified both Bcl-2 and Bcl-xL proteins as survival factors in lung cancer cells since reductions in these proteins can induce apoptosis and sensitize lung cancer cells to apoptosis induced by chemotherapy agents. Myeloid cell leukemia-1 (Mcl-1), another member of the Bcl-2 family, has been found to be a critical survival factor in hematopoietic cells, yet little data exists for a role of Mcl-1 in human lung cancers. We used NSCLC cell lines to explore how Mcl-1 levels affect lung cancer cell survival and studied tumors from patients to determine expression patterns of Mcl-1. NSCLC cells express abundant Mcl-1 protein and depletion of Mcl-1 levels by antisense Mcl-1 oligonucleotides induces apoptosis in A549 and H1299 lung cancer cells. Reduction in Mcl-1 levels can sensitize lung cancer cells to apoptosis induced by cytotoxic agents as well as by ionizing radiation. Lung cancer cells overexpressing Mcl-1 are less sensitive to apoptosis induced by chemotherapeutic agents, ZD1839 (an inhibitor of EGFR tyrosine kinase) and Bcl-2 or Bcl-xL antisense oligonucleotides. We find that epidermal growth factor (EGF) can enhance Mcl-1 protein levels in an ERK-dependent manner. Signal transduction agents that reduce Mcl-1 levels correlated with their individual ability to induce apoptosis in lung cancer cells. Finally, NSCLC tumors taken directly from patients have elevated levels of Mcl-1 protein compared with normal adjacent lung tissue. Therefore, agents that target Mcl-1 can induce apoptosis and sensitize cells to apoptosis induced by cytotoxic agents. Mcl-1 protein is overexpressed in a subset of human NSCLC and enhanced levels of Mcl-1 may protect lung cancer cells from death induced by a variety of pro-apoptotic stimuli.

Direct repression of the Mcl-1 promoter by E2F1

E2F1 induces apoptosis via both p53-dependent and p53-independent mechanisms. The direct targets in the p53-independent pathway remain enigmatic; however, the induction of this pathway does not require the transactivation domain of E2F1. Using cells that are defective in p53 activation, we show that E2F1 potently represses the expression of Mcl-1 – an anti-apoptotic Bcl-2 family member whose depletion results in apoptosis. We also show that this transcriptional repression is direct and dependent upon E2F1s DNA-binding domain, but does not require the transactivation domain of E2F1. Consistent with this DNA binding requirement of E2F1 to repress Mcl-1, we show that E2F1 binds to the Mcl-1 promoter both in vitro and in vivo, and have identified the DNA element (−143/−117) within this promoter that is required for E2F1 binding and repression. Additionally, cell lines constitutively expressing Mcl-1 are resistant to E2F1-mediated apoptosis – suggesting that Mcl-1 downregulation is a necessary event in the p53-independent apoptotic process. Thus, we identify a p53 family-independent mechanism of E2F1-induced apoptosis in which E2F1 directly represses Mcl-1 expression.

Activation of p27Kip1 Expression by E2F1 A NEGATIVE FEEDBACK MECHANISM

The E2F1 transcription factor is a critical regulator of cell cycle due to its ability to promote S phase entry. However, E2F1 overexpression also sensitizes cells to apoptosis and E2F1-null mice are predisposed to tumor development, suggesting that it also has properties of a growth suppressor. E2F1 transcription function is regulated by interaction with hypophosphorylated pRb. Cdk inhibitors such as p16INK4a and p27Kip1 inhibit pRb phosphorylation by the cyclin D/Cdk4 and cyclin E/Cdk2 complexes, thus keeping E2F1 in an inactive state. We found that E2F1 binds to the p27 promoter in vivo and activates p27 mRNA and protein expression. Depletion of endogenous E2F1 by siRNA causes a reduction in basal p27 expression level. Inhibition of endogenous p27 expression by siRNA increases E2F1 transcriptional activity and permits accelerated cell cycle progression by exogenous E2F1. These observations suggest that induction of p27 acts as a negative feedback mechanism for E2F1 and may also contribute to other functions of E2F1.

Prognostic and Predictive Value of a Malignancy-Risk Gene Signature in Early-Stage Non–Small Cell Lung Cancer

BACKGROUND The malignancy-risk gene signature is composed of numerous proliferative genes and has been applied to predict breast cancer risk. We hypothesized that the malignancy-risk gene signature has prognostic and predictive value for early-stage non-small cell lung cancer (NSCLC) patients. METHODS The ability of the malignancy-risk gene signature to predict overall survival (OS) of early-stage NSCLC patients was tested using a large NSCLC microarray dataset from the Directors Challenge Consortium (n = 442) and two independent NSCLC microarray datasets (n = 117 and 133, for the GSE13213 and GSE14814 datasets, respectively). An overall malignancy-risk score was generated by principal component analysis to determine the prognostic and predictive value of the signature. An interaction model was used to investigate a statistically significant interaction between adjuvant chemotherapy (ACT) and the gene signature. All statistical tests were two-sided. RESULTS The malignancy-risk gene signature was statistically significantly associated with OS (P < .001) of NSCLC patients. Validation with the two independent datasets demonstrated that the malignancy-risk score had prognostic and predictive values: Of patients who did not receive ACT, those with a low malignancy-risk score had increased OS compared with a high malignancy-risk score (P = .007 and .01 for the GSE13212 and GSE14814 datasets, respectively), indicating a prognostic value; and in the GSE14814 dataset, patients receiving ACT survived longer in the high malignancy-risk score group (P = .03), and a statistically significant interaction between ACT and the signature was observed (P = .02). CONCLUSIONS The malignancy-risk gene signature was associated with OS and was a prognostic and predictive indicator. The malignancy-risk gene signature could be useful to improve prediction of OS and to identify those NSCLC patients who will benefit from ACT.

Regulation of E2F1 Function by the Nuclear Corepressor KAP1

KAP1 is a nuclear corepressor with conserved domains for RING finger, B boxes, leucine zipper α helical coiled-coil region, plant homeo domain finger, and bromo domain. The plant homeo domain finger and bromo domain of KAP1 cooperatively function as a transcription repression domain by recruiting the histone deacetylase complex NuRD and histone H3 lysine 9-specific methyltransferase SETDB1. Here we report that KAP1 binds the E2F1 transcription factor in a retinoblastoma protein (pRb)-independent fashion and inhibits E2F1 activity. KAP1 stimulates formation of E2F1-HDAC1 complex and inhibits E2F1 acetylation. Ectopic expression of KAP1 represses E2F1 transcription and apoptosis functions independent of pRb. Depletion of endogenous KAP1 in pRb-deficient Saos2 cells by RNA interference increases E2F1 acetylation level, stimulates E2F1 transcriptional activity, and sensitizes apoptosis response to DNA damage. Therefore, KAP1 contributes to the negative regulation of E2F1 and may serve as a partial backup to prevent E2F1-mediated apoptosis in the absence of pRb.

Histone deacetylase 3 binds to and regulates the multifunctional transcription factor TFII-I.

Histone deacetylase 3 (HDAC3) is one of four members of the human class I histone deacetylases that are implicated in transcriptional repression through deacetylation of acetyllysines in amino-terminal tails of core histones. In an immunoaffinity purification using anti-HDAC3, transcription factor TFII-I copurified with HDAC3. Specificity of the HDAC3-TFII-I interaction was confirmed by coimmunoprecipitation of epitope-tagged proteins, GST pull-down assays, and protein colocalization with indirect immunofluorescence. An anti-TFII-I immunoprecipitate contained histone deacetylase enzymatic activity. Mutational analyses revealed that the carboxyl-terminal of HDAC3 (residues 373–401) and residues 363–606 of TFII-I were required for the HDAC3-TFII-I interaction. Transcriptional activation by TFII-I was severely reduced by overexpression of HDAC3. These results suggest that HDAC3 modulates some of the functions of TFII-I and provides a link between histone deacetylase and a multifunctional transcriptional activator.

TGF-β–inducible microRNA-183 silences tumor-associated natural killer cells

Significance Natural killer (NK) cells are potent tumor-cell killers, but exposure to transforming growth factor beta-1 (TGF-β) abrogates their effectiveness. Here, we show that this suppression is a result of TGF-β induction of microRNA (miR)-183, which binds and represses DNAX activating protein 12 kDa (DAP12), a signal adaptor for lytic function in NK cells. Because introduction of miR-183 alone or its functional blockade in the presence of TGF-β reduced or restored DAP12 levels in NK cells, we define miR-183 as a key factor in TGF-β–mediated immunosuppression. Since DAP12 is required for signaling through multiple NK cytotoxicity receptors and TGF-β is overexpressed by diverse solid malignancies, our data may have significant importance in the development of NK-based cancer immunotherapies. Transforming growth factor β1 (TGF-β), enriched in the tumor microenvironment and broadly immunosuppressive, inhibits natural killer (NK) cell function by yet-unknown mechanisms. Here we show that TGF-β–treated human NK cells exhibit reduced tumor cytolysis and abrogated perforin polarization to the immune synapse. This result was accompanied by loss of surface expression of activating killer Ig-like receptor 2DS4 and NKp44, despite intact cytoplasmic stores of these receptors. Instead, TGF-β depleted DNAX activating protein 12 kDa (DAP12), which is critical for surface NK receptor stabilization and downstream signal transduction. Mechanistic analysis revealed that TGF-β induced microRNA (miR)-183 to repress DAP12 transcription/translation. This pathway was confirmed with luciferase reporter constructs bearing the DAP12 3′ untranslated region as well as in human NK cells by use of sense and antisense miR-183. Moreover, we documented reduced DAP12 expression in tumor-associated NK cells in lung cancer patients, illustrating this pathway to be consistently perturbed in the human tumor microenvironment.

Inhibition of mitogenesis in Balb/c-3T3 cells by Trichostatin A. Multiple alterations in the induction and activation of cyclin-cyclin-dependent kinase complexes.

Trichostatin A (TSA), a global repressor of histone deacetylase activity, inhibits the proliferation of a number of cell types. However, the identification of the mechanisms underlying TSA-mediated growth arrests has remained elusive. In order to resolve in more detail the cellular process modulated during the growth inhibition induced by TSA, we studied the effect of the drug on G0/G1 traverse in mitogen-stimulated quiescent Balb/c-3T3 cells. Cyclin D1 and retinoblastoma proteins were induced following the mitogenic stimulation of both control and TSA-treated cells, and cyclin D1 formed complexes with CDK4 under both conditions. However, cyclin D1-associated kinase was not increased in growth-arrested cells. The lack of cyclin D-associated kinase was paralleled by an accumulation of RB in a hypophosphorylated form, as would be expected. In contrast, p130 became partially phosphorylated, accompanied by a marked increase in p130-dependent E2F DNA binding activity and a partial release of free E2F-4. Despite the presence of E2F complexes not bound to pocket proteins, late G1 E2F-dependent gene expression was not observed. The lack of cyclin D1-associated kinase in TSA-treated cultures was potentially due to high levels of the cyclin-dependent inhibitor p27 kip1 . However, the modulation of p27 kip1 levels by the deacetylase inhibitor cannot be responsible for the induction of the cell cycle arrest, since the growth of murine embryo fibroblasts deficient in both p27 kip1 and p21 cip1 was also inhibited by TSA. These data support a model in which TSA inhibits very early cell cycle traverse, which, in turn, leads to a decrease in cyclin D1-associated kinase activation and a repression of late cell cycle-dependent events. Alterations in early G0/G1 gene expression accompany the TSA-mediated growth arrest.