Novel cell-specific and dominant negative anti-apoptotic roles of p73 in transformed leukemia cells

New Insights in Thyroid Cancer and p53 Family Proteins

Thyroid cancers are common endocrine malignancies that comprise tumors with different clinical and histological features. Indeed, papillary and follicular thyroid cancers are slow-growing, well-differentiated tumors, whereas anaplastic thyroid cancers are undifferentiated neoplasias that behave much more aggressively. Well-differentiated thyroid carcinomas are efficiently cured by surgery and radioiodine, unlike undifferentiated tumors that fail to uptake radioactive iodine and are usually resistant to chemotherapy. Therefore, novel and more effective therapies for these aggressive neoplasias are urgently needed. Whereas most genetic events underlying the pathogenesis of well-differentiated thyroid cancers have been identified, the molecular mechanisms that generate undifferentiated thyroid carcinomas are still unclear. To date, one of the best-characterized genetic alterations leading to the development of poorly differentiated thyroid tumors is the loss of the p53 tumor suppressor gene. In addition, the existence of a complex network among p53 family members (p63 and p73) and their interactions with other factors that promote thyroid cancer progression has been well documented. In this review, we provide an update on the current knowledge of the role of p53 family proteins in thyroid cancer and their possible use as a therapeutic target for the treatment of the most aggressive variants of this disease.

Highly activated p53 contributes to selectively increased apoptosis of latently HIV-1 infected cells upon treatment of anticancer drugs

Background

Despite the successful inhibition of human immunodeficiency virus type 1 (HIV-1) replication by combination antiretroviral therapy, cells latently infected with HIV-1 remaining in patients are a major obstacle for eradication of HIV-1 infection. The tumor suppressor factor p53 is activated by HIV-1 infection, and restricts HIV-1 replication. However, a therapeutic strategy based on p53 activity has not been considered for elimination of latently infected cells.

Methods

Apoptotic cells were analyzed using flow cytometry with anti-annexin A5-FITC Ab and PI staining upon treatment of anticancer drugs. The expression and activation of p53 and apoptotic molecules in latently HIV-1-infected T cells were compared using Western blot analysis. The role of p53 in the anticancer drug treatment-induced apoptosis of cells latently infected with HIV-1 was determined by knock-down experiment using siRNA against p53.

Results

Upon treatment with 5-fluorouracil (5-FU), apoptosis was increased in latently infected ACH2 cells encoding competent p53 compared with uninfected parent A3.01 cells, while the apoptosis of latently infected p53 null J1.1 cells was less than that of uninfected cells. Treatment with 5-FU increased the levels of cleaved caspase-3 and PARP in ACH2 cells compared with uninfected and latently infected p53 null J1.1 cells. The levels of expression and activation of p53 were higher in both latently infected ACH2 and NCHA2 cells than in uninfected cells. Furthermore, the activation levels of p53 in both cells were further increased upon 5-FU treatment. Consistent with p53 status, apoptosis was markedly increased in ACH2 and NCHA2 cells compared with uninfected and latently infected J1.1 cells upon treatment with other anticancer drugs such as doxorubicin and etoposide. Inhibition of p53 in cells with latent HIV-1 infection diminished apoptosis upon 5-FU treatment.

Conclusion

Evidence described here indicate that when treated with anticancer drugs, apoptosis of cells with latent HIV-1 infection was increased via the p53 activation pathway and may provide information for application of anticancer drugs to selectively eliminate HIV-1 reservoirs.

Differential regulation of TP73 isoforms by 1α,25-dihydroxyvitamin D3 and survivin in human colon and breast carcinomas

We evaluate whether 1,25(OH)(2)D(3) downregulates TP73 variants in colon and breast carcinomas, the role of survivin in this context, and the significance of this network in the clinic. Tumor cells were treated/untreated with 1,25(OH)(2)D(3) and transiently transfected with survivin. Levels of survivin and TP73 variants were evaluated by quantitative RT-PCR and Western blotting. In 75 colon and 60 breast cancer patients, the expressions of survivin and TP73 isoforms were determined. Tumor characteristics were examined in each patient. Survivin protein levels were also evaluated in a subgroup of patients and cell lines. Decrease in survivin and TAp73 transcripts and protein and ΔNp73 mRNA was detected after 1,25(OH)(2)D(3) treatment. Ectopic survivin expression led to an increase in the TAp73, ΔNp73, ΔEx2p73, and ΔEx2-3p73 transcripts. In cancer patients, direct correlations were observed between TP73 variants and survivin levels. 1,25(OH)(2)D(3) negatively regulate survivin and TP73 variants in colon and breast cancer cells. Positive regulation of TP73 isoforms by survivin may exist, which reinforces the possibility that the downregulation of TP73 forms by 1,25(OH)(2)D(3) is survivin-dependent.

TAp73 alpha increases p53 tumor suppressor activity in thyroid cancer cells via the inhibition of Mdm2-mediated degradation

p53 family proteins include p53 tumor suppressor, p63, and p73. Despite the high similarity in structure and function with p53, p63, and p73 function in tumor suppression is still controversial. Here, we show that TAp73alpha, a transcriptionally active p73 isoform, is able to synergize p53 tumor suppressor function in thyroid cancer cells. Indeed, depletion of p73 by small interfering RNA in thyroid cancer cells resulted in a reduced transcriptional activity of p53. Ectopic coexpression of both p53 and TAp73alpha in thyroid cancer cells resulted in increased transcription and tumor suppressor function compared with p53 or TAp73alpha alone, as well as in increased p53 protein levels. The enhancing effect of TAp73alpha on p53 activity is Mdm2 dependent because it is prevented by Mdm2 depletion by small interfering RNA. At least two mechanisms may explain the interference of TAp73alpha with p53 function. First, in thyroid cancer cells, TAp73alpha inhibits the effect of p53 on Mdm2 induction by antagonizing p53 at the Mdm2 promoter level. Second, a TAp73alpha mutant (G264W), which is devoid of DNA binding capability, is still able to increase p53 protein levels by competing with p53 for Mdm2 protein binding. Taken together, these results indicate that in thyroid cancer cells, TAp73alpha is able to increase p53 protein level and function by interfering with Mdm2-mediated p53 degradation. These results may be useful for designing gene therapies aimed at restoring a normal p53 function in thyroid cancer cells.

DNA-damage response network at the crossroads of cell-cycle checkpoints, cellular senescence and apoptosis

Tissue homeostasis requires a carefully-orchestrated balance between cell proliferation, cellular senescence and cell death. Cells proliferate through a cell cycle that is tightly regulated by cyclin-dependent kinase activities. Cellular senescence is a safeguard program limiting the proliferative competence of cells in living organisms. Apoptosis eliminates unwanted cells by the coordinated activity of gene products that regulate and effect cell death. The intimate link between the cell cycle, cellular senescence, apoptosis regulation, cancer development and tumor responses to cancer treatment has become eminently apparent. Extensive research on tumor suppressor genes, oncogenes, the cell cycle and apoptosis regulatory genes has revealed how the DNA damage-sensing and -signaling pathways, referred to as the DNA-damage response network, are tied to cell proliferation, cell-cycle arrest, cellular senescence and apoptosis. DNA-damage responses are complex, involving "sensor" proteins that sense the damage, and transmit signals to "transducer" proteins, which, in turn, convey the signals to numerous "effector" proteins implicated in specific cellular pathways, including DNA repair mechanisms, cell-cycle checkpoints, cellular senescence and apoptosis. The Bcl-2 family of proteins stands among the most crucial regulators of apoptosis and performs vital functions in deciding whether a cell will live or die after cancer chemotherapy and irradiation. In addition, several studies have now revealed that members of the Bcl-2 family also interface with the cell cycle, DNA repair/recombination and cellular senescence, effects that are generally distinct from their function in apoptosis. In this review, we report progress in understanding the molecular networks that regulate cell-cycle checkpoints, cellular senescence and apoptosis after DNA damage, and discuss the influence of some Bcl-2 family members on cell-cycle checkpoint regulation.

Pharmacologic profiling of transcriptional targets deciphers promoter logic

The blueprint for cellular diversity and response to environmental change is encoded in the cis-acting regulatory sequences of most genes. Deciphering this 'cis-regulatory code' requires multivariate data sets that examine how these regions coordinate transcription in response to diverse environmental stimuli and therapeutic treatments. We describe a transcriptional approach that profiles the activation of multiple transcriptional targets against combinatorial arrays of therapeutic and signal transducing agents. Application of this approach demonstrates how cis-element composition and promoter context combine to influence transcription downstream of mitogen-induced signaling networks. Computational dissection of these transcriptional profiles in activated T cells uncovers a novel regulatory synergy between IGF-1 and CD28 costimulation that modulates NF-kappaB and AP1 pathways through signaling cascades sensitive to cyclosporin A and wortmannin. This approach provides a broader view of the hierarchical signal integration governing gene expression and will facilitate a practical design of combinatorial therapeutic strategies for exploiting critical control points in transcriptional regulation.

Survivin regulates the p53 tumor suppressor gene family

Survivin regulates cell division and inhibits apoptosis by blocking caspase activation. The tumor suppressor p53 inhibits cell cycle progression and induces apoptosis. Since Survivin overexpression and loss of wild-type p53 expression/function occur in most cancers, we investigated whether Survivin regulates p53. Stable overexpression of Survivin protects BaF3 cells from Adriamycin-induced apoptosis, while dominant-negative (T34A) and antisense (AS) Survivin accelerate apoptosis. In BaF3 cells and transiently transfected MCF7 breast cancer cells, elevation of total and phospho-Ser15-p53 in response to Adriamycin is blocked by Survivin and enhanced by Survivin disruption. Furthermore, in Adriamycin-treated MCF7 cells, ectopic Survivin decreased p53 mRNA and increased mRNA and protein of the p53 homologues DeltaNp63 and TAp73 and mRNA for DeltaNp73, suggesting that Survivin may differentially regulate p53 family transcription. Concomitant with decreasing p53 mRNA, Survivin decreased Mdm2 mRNA. Survivin disruption by T34A or AS Survivin resulted in reduced Mdm2 protein. The caspase inhibitor, Z-VAD-FMK, blocked the decrease in Mdm2 as well as the increase in p53 resulting from Survivin disruption, indicating that Survivin regulates Mdm2 at the post-translational level. Proteosome inhibition confirmed that reduced p53 protein observed in cells overexpressing Survivin is due to enhanced p53 degradation resulting from Survivin-mediated inhibition of Mdm2 cleavage by caspases. In summary, our results identify regulatory interactions between Survivin and p53 at the mRNA and protein levels, and suggest that the p53 homologues DeltaNp63, TAp73 and DeltaNp73 may also be regulated by Survivin.

Treatment with arsenic trioxide (ATO) and MEK1 inhibitor activates the p73-p53AIP1 apoptotic pathway in leukemia cells

Arsenic trioxide (ATO) induces differentiation and apoptosis of malignant cells in vitro and in vivo and has been used in the treatment of a variety of hematologic malignancies. We found that in NB4 acute promyelocytic and in K562 erythroleukemia cell lines treatment with the MEK1 inhibitors PD98059 and PD184352 greatly enhances apoptotic cell death induced by ATO alone. Combined treatment results in the induction of the p53AIP1 (p53-regulated apoptosis-inducing protein 1) gene in both cell lines. Because NB4 and K562 cell lines carry an inactive p53, we investigated the possible role of p73, a p53 paralogue that has been shown to regulate several p53 target genes including p21, Bax, and p53AIP1. We found that MEK1 inhibitors reduce the levels of dominant-negative (DeltaN) p73 proteins and promote the accumulation of endogenous p73alpha through its transcriptional activation and its tyrosine phosphorylation, resulting in p21 up-regulation and significant inhibition of cell growth. ATO reduces DeltaNp73 levels and promotes a p300-mediated acetylation of endogenous p73, thus favoring cell cycle arrest and apoptosis. Finally, the combined treatment with MEK1 inhibitors and ATO enhances the affinity of phosphoacetylated p73 for the p53AIP1 promoter in vivo, as determined by chromatin immunoprecipitation experiments, leading to p53AIP1 up-regulation and increased apoptosis.

Binding of the C-terminal sterile alpha motif (SAM) domain of human p73 to lipid membranes

The alpha splice variant of p73 (p73alpha), a homologue of the tumor suppressor p53, has close to its C terminus a sterile alpha motif (SAM), SAMp73, that is thought to be involved in protein-protein interactions. Here, we report the lipid binding properties of this domain. Binding was assayed against zwitterionic (phosphatidylcholine) and anionic (phosphatidic acid) lipids and was studied by different biophysical techniques, namely, circular dichroism and fluorescence spectroscopies and differential scanning calorimetry. These techniques unambiguously indicate that SAMp73 binds to lipids. The binding involves protein surface attachment and partial membrane penetration, accompanied by changes in SAMp73 structure.

Targeting combinatorial transcriptional complex assembly at specific modules within the interleukin-2 promoter by the immunosuppressant SB203580

The proximal promoter sequence of the interleukin-2 (IL-2) gene contains a series of composite sites or modules that controls much of its responsiveness to environmental stimuli. The integrated targeting of these modules is therefore a major mode of regulation. This report describes how multiple functional hierarchies, required for the recruitment of the p300 co-activator to the CD28RE/AP1 (TRE) module of the IL-2 promoter, are selectively disrupted in human T-cells by the immunosuppressive and anti-inflammatory actions of the p38 mitogen-activated protein kinase inhibitor (MAPK), SB203580. The molecular hierarchies targeted by SB203580 include the combinatorial interaction of NF-kappaB and CREB at the CD28RE/AP1 element coupled with the subsequent dynamic co-assembly and activation of p300. Several aspects of this targeting are linked to the ability of SB203580 to inhibit p38 MAPK-controlled pathways. Together, these results provide the molecular basis through which the combinatorial structure and context of the composite elements of the IL-2 promoter dictates mitogen responsiveness and drug susceptibility that are quantitatively and qualitatively distinct from the isolated action of single consensus sequences and/or transcriptional motifs.