Histone deacetylases (HDACs) regulate transcription and specific cellular functions, such as tumor suppression by p53, and are frequently altered in cancer. Inhibitors of HDACs (HDACIs) possess antitumor activity and are well tolerated, supporting the idea that their use might develop as a specific strategy for cancer treatment. The molecular basis for their selective antitumor activity is, however, unknown. We investigated the effects of HDACIs on leukemias expressing the PML-RAR or AML1-ETO oncoproteins, known to initiate leukemogenesis through deregulation of HDACs. Here we report that: (i) HDACIs induce apoptosis of leukemic blasts, although oncogene expression is not sufficient to confer HDACI sensitivity to normal cells; (ii) apoptosis is p53 independent and depends, both in vitro and in vivo, upon activation of the death receptor pathway (TRAIL and Fas signaling pathways); (iii) TRAIL, DR5, FasL and Fas are upregulated by HDACIs in the leukemic cells, but not in normal hematopoietic progenitors. These results show that sensitivity to HDACIs in leukemias is a property of the fully transformed phenotype and depends on activation of a specific death pathway.

Surgery remains the most effective treatment for ovarian stromal tumors and, whenever feasible, for relapsing disease. Platinum-based chemotherapy is currently used in metastatic or recurrent tumors.

The p53 tumor suppressor belongs to a family of proteins that sense multiple cellular inputs to regulate cell proliferation, apoptosis, and differentiation. Whether and how these functions of p53 intersect with the activity of extracellular growth factors is not understood. Here, we report that key cellular responses to TGF-beta signals rely on p53 family members. During Xenopus embryonic development, p53 promotes the activation of multiple TGF-beta target genes. Moreover, mesoderm differentiation is inhibited in p53-depleted embryos. In mammalian cells, the full transcriptional activation of the CDK inhibitor p21(WAF1) by TGF-beta requires p53. p53-deficient cells display an impaired cytostatic response to TGF-beta signals. Smad and p53 protein complexes converge on separate cis binding elements on a target promoter and synergistically activate TGF-beta induced transcription. p53 can physically interact in vivo with Smad2 in a TGF-beta-dependent fashion. The results unveil a previously unrecognized link between two primary tumor suppressor pathways in vertebrates.

DNA damage leads to a halt in proliferation owing to apoptosis or senescence, which prevents transmission of DNA alterations. This cellular response depends on the tumor suppressor p53 and functions as a powerful barrier to tumor development. Adult stem cells are resistant to DNA damage-induced apoptosis or senescence, however, and how they execute this response and suppress tumorigenesis is unknown. We show that irradiation of hematopoietic and mammary stem cells up-regulates the cell cycle inhibitor p21, a known target of p53, which prevents p53 activation and inhibits p53 basal activity, impeding apoptosis and leading to cell cycle entry and symmetric self-renewing divisions. p21 also activates DNA repair, limiting DNA damage accumulation and self-renewal exhaustion. Stem cells with moderate DNA damage and diminished self-renewal persist after irradiation, however. These findings suggest that stem cells have evolved a unique, p21-dependent response to DNA damage that leads to their immediate expansion and limits their long-term survival.A dult stem cells (SCs) are thought to be resistant to DNA damage (DD)-induced apoptosis or senescence owing to the activation of unique pro-survival and DD repair (DDR) responses (1-3). Genetic alterations that decrease DNA repair activities lead to increased DD and reduced self-renewal in SCs, suggesting that DDR is critical to preservation of SC function (1, 4, 5). DDR decreases during physiological aging, a phenomenon correlated with the accumulation of endogenous DD and decreased selfrenewal in aged SCs (6-9).In differentiated cells, DD triggers a checkpoint response that leads to apoptosis or senescence and depends on activation of the tumor suppressor p53 (10). This is considered a powerful tumor-suppressor mechanism, as demonstrated by the finding that p53 is invariably inactivated in spontaneous tumors (11). After irradiation, p53 is up-regulated in populations enriched for hematopoietic, hair follicle bulge, and colon SCs (5,(12)(13)(14)(15). Whether this is critical for activation of the DDR response and maintenance of self-renewal, why p53 induction does not result in SC apoptosis or senescence, and how tumor suppression is executed in SCs remain unclear, however. Indirect evidence indicates that the cell cycle inhibitor p21, a downstream effector of p53, might be involved in DD processing in SCs. In the absence of p21, SCs exhaust prematurely (16) and after a low radiation dose display reduced reconstitution capacity (17). Here we report our studies on the role of p53 and p21 in DD processing of highly purified hematopoietic SCs (HSCs) and mammary SCs (MaSCs).Results X-Rays Induce p53-Independent Up-Regulation of p21 in HSCs. To investigate p21 and p53 regulation in HSCs after DD, we treated mice with X-rays using the maximal sublethal dose of 5.5 Gy (SI Appendix, Fig. S1). After 6 h, mice were killed, and bone marrow (BM) mononuclear cells (MNCs) were FACS-sorted to obtain highly purified populations of HSCs [long-term reconstituting HSCs (LT-HSCs), Lin . In a pa...

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Mutations of p53 are remarkably rare in acute promyelocytic leukemias (APLs). Here, we demonstrate that the APL-associated fusion proteins PML-RAR and PLZF-RAR directly inhibit p53, allowing leukemic blasts to evade p53-dependent cancer surveillance pathways. PML-RAR causes deacetylation and degradation of p53, resulting in repression of p53 transcriptional activity, and protection from p53-dependent responses to genotoxic stress. These phenomena are dependent on the expression of wild-type PML, acting as a bridge between p53 and PML-RAR. Recruitment of histone deacetylase (HDAC) to p53 and inhibition of p53 activity were abrogated by conditions that either inactivate HDACs or trigger HDAC release from the fusion protein, implicating recruitment of HDAC by PML-RAR as the mechanism underlying p53 inhibition.

Leukemia-associated fusion proteins establish aberrant transcriptional programs, which result in the block of hematopoietic differentiation, a prominent feature of the leukemic phenotype. The dissection of the mechanisms of deregulated transcription by leukemia fusion proteins is therefore critical for the design of tailored antileukemic strategies, aimed at reestablishing the differentiation program of leukemic cells. The acute promyelocytic leukemia (APL)-associated fusion protein PML-retinoic acid receptor (RAR) behaves as an aberrant transcriptional repressor, due to its ability to induce chromatin modifications (histone deacetylation and DNA methylation) and silencing of PML-RAR target genes. Here, we indicate that the ultimate result of PML-RAR action is to impose a heterochromatin-like structure on its target genes, thereby establishing a permanent transcriptional silencing. This effect is mediated by the previously described association of PML-RAR with chromatin-modifying enzymes (histone deacetylases and DNA methyltransferases) and by recruitment of the histone methyltransferase SUV39H1, responsible for trimethylation of lysine 9 of histone H3.

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