Histone deacetylase inhibitors suppress mutant p53 transcription via histone deacetylase 8

HDAC8: a multifaceted target for therapeutic interventions

Histone deacetylase 8 (HDAC8) is a class I histone deacetylase implicated as a therapeutic target in various diseases, including cancer, X-linked intellectual disability, and parasitic infections. It is a structurally well-characterized enzyme that also deacetylates nonhistone proteins. In cancer, HDAC8 is a major 'epigenetic player' that is linked to deregulated expression or interaction with transcription factors critical to tumorigenesis. In the parasite Schistosoma mansoni and in viral infections, HDAC8 is a novel target to subdue infection. The current challenge remains in the development of potent selective inhibitors that would specifically target HDAC8 with fewer adverse effects compared with pan-HDAC inhibitors. Here, we review HDAC8 as a drug target and discuss inhibitors with respect to their structural features and therapeutic interventions.

Preclinical activity of combined HDAC and KDM1A inhibition in glioblastoma

BACKGROUND

Glioblastoma (GBM) is the most common and aggressive form of brain cancer. Our previous studies demonstrated that combined inhibition of HDAC and KDM1A increases apoptotic cell death in vitro. However, whether this combination also increases death of the glioma stem cell (GSC) population or has an effect in vivo is yet to be determined. Therefore, we evaluated the translational potential of combined HDAC and KDM1A inhibition on patient-derived GSCs and xenograft GBM mouse models. We also investigated the changes in transcriptional programing induced by the combination in an effort to understand the induced molecular mechanisms of GBM cell death.

METHODS

Patient-derived GSCs were treated with the combination of vorinostat, a pan-HDAC inhibitor, and tranylcypromine, a KDM1A inhibitor, and viability was measured. To characterize transcriptional profiles associated with cell death, we used RNA-Seq and validated gene changes by RT-qPCR and protein expression via Western blot. Apoptosis was measured using DNA fragmentation assays. Orthotopic xenograft studies were conducted to evaluate the effects of the combination on tumorigenesis and to validate gene changes in vivo.

RESULTS

The combination of vorinostat and tranylcypromine reduced GSC viability and displayed efficacy in the U87 xenograft model. Additionally, the combination led to changes in apoptosis-related genes, particularly TP53 and TP73 in vitro and in vivo.

CONCLUSIONS

These data support targeting HDACs and KDM1A in combination as a strategy for GBM and identifies TP53 and TP73 as being altered in response to treatment.

Phase I study of pazopanib and vorinostat: a therapeutic approach for inhibiting mutant p53-mediated angiogenesis and facilitating mutant p53 degradation

BACKGROUND

We carried out a phase I trial of the vascular endothelial growth factor inhibitor pazopanib and the histone deacetylase inhibitor vorinostat to determine the safety and efficacy. Because these agents are known to target factors activated by TP53 mutation and facilitate mutant p53 degradation, a subgroup analysis may be interesting in patients with TP53 mutant malignancies.

PATIENTS AND METHODS

Patients with advanced solid tumors (n = 78) were enrolled following a 3 + 3 design, with dose expansion for those with responsive tumors. Hotspot TP53 mutations were tested when tumor specimens were available.

RESULTS

Adverse events of ≥grade 3 included thrombocytopenia, neutropenia, fatigue, hypertension, diarrhea and vomiting. Overall, the treatment produced stable disease for at least 6 months or partial response (SD ≥6 months/PR) in 19% of the patients, median progression-free survival (PFS) of 2.2 months, and median overall survival (OS) of 8.9 months. In patients with detected hotspot TP53 mutant advanced solid tumors (n = 11), the treatment led to a 45% rate of SD ≥6 months/PR (1 PR and 3 SD ≥6 months), median PFS of 3.5 months, and median OS of 12.7 months, compared favorably with the results for patients with undetected hotspot TP53 mutations (n = 25): 16% (1 PR and 3 SD ≥6 months, P = 0.096), 2.0 months (P = 0.042), and 7.4 months (P = 0.1), respectively.

CONCLUSION

The recommended phase II dosage was oral pazopanib at 600 mg daily plus oral vorinostat at 300 mg daily. The preliminary evidence supports further evaluation of the combination in cancer patients with mutated TP53, especially in those with metastatic sarcoma or metastatic colorectal cancer.

CLINICAL TRIAL REGISTRATION

www.clinicaltrials.gov, NCT01339871.

Mechanism of N-Acylthiourea-mediated activation of human histone deacetylase 8 (HDAC8) at molecular and cellular levels

We reported previously that an N-acylthiourea derivative (TM-2-51) serves as a potent and isozyme-selective activator for human histone deacetylase 8 (HDAC8). To probe the molecular mechanism of the enzyme activation, we performed a detailed account of the steady-state kinetics, thermodynamics, molecular modeling, and cell biology studies. The steady-state kinetic data revealed that TM-2-51 binds to HDAC8 at two sites in a positive cooperative manner. Isothermal titration calorimetric and molecular modeling data conformed to the two-site binding model of the enzyme-activator complex. We evaluated the efficacy of TM-2-51 on SH-SY5Y and BE(2)-C neuroblastoma cells, wherein the HDAC8 expression has been correlated with cellular malignancy. Whereas TM-2-51 selectively induced cell growth inhibition and apoptosis in SH-SY5Y cells, it showed no such effects in BE(2)-C cells, and this discriminatory feature appears to be encoded in the p53 genotype of the above cells. Our mechanistic and cellular studies on HDAC8 activation have the potential to provide insight into the development of novel anticancer drugs.

Histone deacetylases and atherosclerosis

Atherosclerosis is the most common pathological process that leads to cardiovascular diseases, a disease of large- and medium-sized arteries that is characterized by a formation of atherosclerotic plaques consisting of necrotic cores, calcified regions, accumulated modified lipids, smooth muscle cells (SMCs), endothelial cells, leukocytes, and foam cells. Recently, the question about how to suppress the occurrence of atherosclerosis and alleviate the progress of cardiovascular disease becomes the hot topic. Accumulating evidence suggests that histone deacetylases(HDACs) play crucial roles in arteriosclerosis. This review summarizes the effect of HDACs and HDAC inhibitors(HDACi) on the progress of atherosclerosis.

The histone deacetylase inhibitor, Vorinostat, represses hypoxia inducible factor 1 alpha expression through translational inhibition

Hypoxia inducible factor 1α (HIF-1α) is a master regulator of tumor angiogenesis being one of the major targets for cancer therapy. Previous studies have shown that Histone Deacetylase Inhibitors (HDACi) block tumor angiogenesis through the inhibition of HIF-1α expression. As such, Vorinostat (Suberoylanilide Hydroxamic Acid/SAHA) and Romidepsin, two HDACis, were recently approved by the Food and Drug Administration (FDA) for the treatment of cutaneous T cell lymphoma. Although HDACis have been shown to affect HIF-1α expression by modulating its interactions with the Hsp70/Hsp90 chaperone axis or its acetylation status, the molecular mechanisms by which HDACis inhibit HIF-1α expression need to be further characterized. Here, we report that the FDA-approved HDACi Vorinostat/SAHA inhibits HIF-1α expression in liver cancer-derived cell lines, by a new mechanism independent of p53, prolyl-hydroxylases, autophagy and proteasome degradation. We found that SAHA or silencing of HDAC9 mechanism of action is due to inhibition of HIF-1α translation, which in turn, is mediated by the eukaryotic translation initiation factor - eIF3G. We also highlighted that HIF-1α translation is dramatically inhibited when SAHA is combined with eIF3H silencing. Taken together, we show that HDAC activity regulates HIF-1α translation, with HDACis such as SAHA representing a potential novel approach for the treatment of hepatocellular carcinoma.

Arsenic trioxide reactivates proteasome-dependent degradation of mutant p53 protein in cancer cells in part via enhanced expression of Pirh2 E3 ligase

The p53 gene is mutated in more than 50% of human tumors. Mutant p53 exerts an oncogenic function and is often highly expressed in cancer cells due to evasion of proteasome-dependent degradation. Thus, reactivating proteasome-dependent degradation of mutant p53 protein is an attractive strategy for cancer management. Previously, we found that arsenic trioxide (ATO), a drug for acute promyelocytic leukemia, degrades mutant p53 protein through a proteasome pathway. However, it remains unclear what is the E3 ligase that targets mutant p53 for degradation. In current study, we sought to identify an E3 ligase necessary for ATO-mediated degradation of mutant p53. We found that ATO induces expression of Pirh2 E3 ligase at the transcriptional level. We also found that knockdown of Pirh2 inhibits, whereas ectopic expression of Pirh2 enhances, ATO-induced degradation of mutant p53 protein. Furthermore, we found that Pirh2 E3 ligase physically interacts with and targets mutant p53 for polyubiquitination and subsequently proteasomal degradation. Interestingly, we found that ATO cooperates with HSP90 or HDAC inhibitor to promote mutant p53 degradation and growth suppression in tumor cells. Together, these data suggest that ATO promotes mutant p53 degradation in part via induction of the Pirh2-dependent proteasome pathway.

Gain-of-function of mutant p53: mutant p53 enhances cancer progression by inhibiting KLF17 expression in invasive breast carcinoma cells

Kruppel-like-factor 17 (KLF17) is a negative regulator of metastasis and epithelial-mesenchymal-transition (EMT). However, its expression is downregulated in metastatic breast cancer that contains p53 mutations. Here, we show that mutant-p53 plays a key role to suppress KLF17 and thereby enhances cancer progression, which defines novel gain-of-function (GOF) of mutant-p53. Mutant-p53 interacts with KLF17 and antagonizes KLF17 mediated EMT genes transcription. Depletion of KLF17 promotes cell viability, decreases apoptosis and induces drug resistance in metastatic breast cancer cells. KLF17 suppresses cell migration and invasion by decreasing CD44, PAI-1 and Cyclin-D1 expressions. Taken together, our results show that KLF17 is important for the suppression of metastasis and could be a potential therapeutic target during chemotherapy.

Ginsenoside Rg3 Inhibits Constitutive Activation of NF-κB Signaling in Human Breast Cancer (MDA-MB-231) Cells: ERK and Akt as Potential Upstream Targets

Ginsenoside Rg3, one of the major ingredients of heat-processed ginseng, has been reported to inhibit the growth of various cancer cells. We previously reported that Rg3 inhibited the proliferation and induced apoptosis of breast cancer (MDA-MB-231) cells. In the present study, we have explored the mechanism underlying the anti-proliferative and proapoptotic effects of Rg3 in MDA-MB-231 cells, which have constitutively activated NF-κB and the mutant form of p53. Rg3 inhibited DNA binding and transcriptional activity of NF-κB and these effects were attributable to its suppression of IKKβ activity, degradation of IκBα and subsequent nuclear translocation of the p65 subunit of NF-κB. Similarly, the constitutive activation of ERK and Akt through phosphorylation was gradually reduced in MDA-MB-231 cells treated with Rg3. The pharmacological inhibitors of these kinases both U0126 (MEK1/2 inhibitor) and LY294002 (PI3K inhibitor) abrogated the NF-κB DNA binding activity in MDA-MB-231 cells. In addition, Rg3 treatment lowered the levels of the mutant p53 in concentration- and time-dependent manners. Rg3 also increased the association between p53 and its negative regulator Mdm2 in MDA-MB-231 cells. These findings suggest that Rg3 induced apoptosis in MDA-MB-231 cells, which is mediated by blocking NF-κB signaling via inactivation of ERK and Akt as well as destabilization of mutant p53.

Mutant p53 in cancer: new functions and therapeutic opportunities

Many different types of cancer show a high incidence of TP53 mutations, leading to the expression of mutant p53 proteins. There is growing evidence that these mutant p53s have both lost wild-type p53 tumor suppressor activity and gained functions that help to contribute to malignant progression. Understanding the functions of mutant p53 will help in the development of new therapeutic approaches that may be useful in a broad range of cancer types.

Ribophorin II regulates breast tumor initiation and metastasis through the functional suppression of GSK3β

Mutant p53 (mtp53) gain of function (GOF) contributes to various aspects of tumor progression including cancer stem cell (CSC) property acquisition. A key factor of GOF is stabilization and accumulation of mtp53. However, the precise molecular mechanism of the mtp53 oncogenic activity remains unclear. Here, we show that ribophorin II (RPN2) regulates CSC properties through the stabilization of mtp53 (R280K and del126-133) in breast cancer. RPN2 stabilized mtp53 by inactivation of glycogen synthase kinase-3β (GSK3β) which suppresses Snail, a master regulator of epithelial to mesenchymal transition. RPN2 knockdown promoted GSK3β-mediated suppression of heat shock proteins that are essential for mtp53 stabilization. Furthermore, our study reveals that high expression of RPN2 and concomitant accumulation of mtp53 were associated with cancer tissues in a small cohort of metastatic breast cancer patients. These findings elucidate a molecular mechanism for mtp53 stabilization and suggest that RPN2 could be a promising target for anti-CSC therapy.

DEC1 coordinates with HDAC8 to differentially regulate TAp73 and ΔNp73 expression

P73, a member of the p53 family, plays a critical role in neural development and tumorigenesis. Due to the usage of two different promoters, p73 is expressed as two major isoforms, TAp73 and ΔNp73, often with opposing functions. Here, we reported that transcriptional factor DEC1, a target of the p53 family, exerts a distinct control of TAp73 and ΔNp73 expression. In particular, we showed that DEC1 was able to increase TAp73 expression via transcriptional activation of the TAp73 promoter. By contrast, Np73 transcription was inhibited by DEC1 via transcriptional repression of the ΔNp73 promoter. To further explore the underlying mechanism, we showed that DEC1 was unable to increase TAp73 expression in the absence of HDAC8, suggesting that HDAC8 is required for DEC1 to enhance TAp73 expression. Furthermore, we found that DEC1 was able to interact with HDAC8 and recruit HDAC8 to the TAp73, but not the ΔNp73, promoter. Together, our data provide evidence that DEC1 and HDAC8 in differentially regulate TAp73 and ΔNp73 expression, suggesting that this regulation may lay a foundation for a therapeutic strategy to enhance the chemosensitivity of tumor cells.

Cooperation of p53 mutations with other oncogenic alterations in cancer

Following the initial findings suggesting a pro-oncogenic role for p53 point mutants, more than 30 years of research have unveiled the critical role exerted by these mutants in human cancer. A growing body of evidence, including mouse models and clinical data, has clearly demonstrated a connection between mutant p53 and the development of aggressive and metastatic tumors. Even if the molecular mechanisms underlying mutant p53 activities are still the object of intense scrutiny, it seems evident that full activation of its oncogenic role requires the functional interaction with other oncogenic alterations. p53 point mutants, with their pleiotropic effects, simultaneously activating several mechanisms of aggressiveness, are engaged in multiple cross-talk with a variety of other cancer-related processes, thus depicting a complex molecular landscape for the mutant p53 network. In this chapter revealing evidence illustrating different ways through which this cooperation may be achieved will be discussed. Considering the proposed role for mutant p53 as a driver of cancer aggressiveness, disarming mutant p53 function by uncoupling the cooperation with other oncogenic alterations, stands out as an exciting possibility for the development of novel anti-cancer therapies.

The up-regulation of histone deacetylase 8 promotes proliferation and inhibits apoptosis in hepatocellular carcinoma

BACKGROUND

Histone deacetylase 8 (HDAC8), a member of class I HDACs, has been reported to be involved in transcriptional regulation, cell cycle progression, and developmental events, and several studies have shown that HDAC8 plays a critical role in tumorigenesis. However, the expression level and the potential role of HDAC8 in hepatocellular carcinoma (HCC) remain unclear.

AIM

The purpose of this study was to investigate protein expression of HDAC8 in HCC tissues and the effects of HDAC8 knockdown on the proliferation and apoptosis of liver cancer cells, and to explore the possible mechanisms.

METHODS

First, we used quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR), immunohistochemical staining, and western blot to examine the mRNA and protein expression of HDAC8 in HCC cell lines and tissues. Then, we assessed the correlation between clinicopathological parameters and the protein expression of HDAC8. Furthermore, we employed the interfering RNA method to explore the potential role of HDAC8 in HCC progression in vitro.

RESULTS

Our results showed that expression of HDAC8 was significantly up-regulated both in HCC cell lines and tumor tissues compared to human normal liver cell line LO2 and corresponding non-tumor tissues. Moreover, we found that HDAC8 knockdown could dramatically inhibit HCC cell proliferation and enhance the apoptosis rate in vitro. Western blot revealed that intrinsic apoptotic pathway proteins, including BAX, BAD, and BAK, were elevated after HDAC8 knockdown. The cleavage of caspase-3 and PARP, which are downstream of intrinsic apoptotic pathway, were also enhanced. In addition, suppression of HDAC8 also elevated the expression of p53 and acetylation of p53 at Lys382, whereas the acetylation of p53 at Lys373 did not change.

CONCLUSIONS

Our study revealed that HDAC8 was overexpressed in HCC. HDAC8 knockdown suppresses tumor growth and enhances apoptosis in HCC via elevating the expression of p53 and acetylation of p53 at Lys382. HDAC8 might serve as a potential therapeutic target in HCC.

Disarming mutant p53 oncogenic function

In the last decade intensive research has confirmed the long standing hypothesis that some p53 point mutants acquire novel activities able to cooperate with oncogenic mechanisms. Particular attention has attracted the ability of several such mutants to actively promote the development of aggressive and metastatic tumors in vivo. This knowledge opens a new dimension on rational therapy design, suggesting novel strategies based on pharmacological manipulation of those neomorphic activities. P53 point mutants have several characteristics that make them attractive targets for anti-cancer therapies. Remarkably, mutant p53 has been found predominantly in tumor cells and may act pleiotropically by interfering with a variety of cellular processes. Therefore, drugs targeting mutant p53 may selectively affect tumor cells, inactivating simultaneously several mechanisms of tumor promotion. Moreover, the high frequency of missense mutations on the p53 gene suggests that interfering with mutant p53 function may provide a valuable approach for the development of efficient therapies able to target a wide range of tumor types.

Exome sequencing identifies a novel EP300 frame shift mutation in a patient with features that overlap Cornelia de Lange syndrome

Rubinstein-Taybi syndrome (RTS) and Cornelia de Lange syndrome (CdLS) are genetically heterogeneous multiple anomalies syndromes, each having a distinctive facial gestalt. Two genes (CREBBP and EP300) are known to cause RTS, and five (NIPBL, SMC1A, SMC3, RAD21, and HDAC8) have been associated with CdLS. A diagnosis of RTS or CdLS is molecularly confirmed in only 65% of clinically identified cases, suggesting that additional causative genes exist for both conditions. In addition, although EP300 and CREBBP encode homologous proteins and perform similar functions, only eight EP300 positive RTS patients have been reported, suggesting that patients with EP300 mutations might be escaping clinical recognition. We report on a child with multiple congenital abnormalities and intellectual disability whose facial features and complex phenotype resemble CdLS. However, no mutations in CdLS-related genes were identified. Rather, a novel EP300 mutation was found on whole exome sequencing. Possible links between EP300 and genes causing CdLS are evident in the literature. Both EP300 and HDAC8 are involved in the regulation of TP53 transcriptional activity. In addition, p300 and other chromatin associated proteins, including NIPBL, SMCA1, and SMC3, have been found at enhancer regions in different cell types. It is therefore possible that EP300 and CdLS-related genes are involved in additional shared pathways, producing overlapping phenotypes. As whole exome sequencing becomes more widely utilized, the diverse phenotypes associated with EP300 mutations should be better understood. In the meantime, testing for EP300 mutations in those with features of CdLS may be warranted.

The effects of a histone deacetylase inhibitor on biological behavior of diffuse large B-cell lymphoma cell lines and insights into the underlying mechanisms

Background

Epigenetic control using histone deacetylase (HDAC) inhibitors is a promising therapy for lymphomas. Insights into the anti-proliferative effects of HDAC inhibitors on diffuse large B-cell lymphoma (DLBCL) and further understanding of the underlying mechanisms, which remain unclear to date, are of great importance.

Methods

Three DLBCL cell lines (DoHH2, LY1 and LY8) were used to define the potential epigenetic targets for Trichostatin A (TSA)-mediated anti-proliferative effects via CCK-8 assay. Cell cycle distribution and apoptosis were detected by flow cytometry. We further investigated the underlying molecular mechanisms by examining expression levels of relevant proteins using western blot analysis.

Results

TSA treatment inhibited the growth of all three DLBCL cell lines and enhanced cell cycle arrest and apoptosis. Molecular analysis revealed upregulated acetylation of histone H3, α-tubulin and p53, and dephosphorylation of pAkt with altered expression of its main downstream effectors (p21, p27, cyclin D1 and Bcl-2). HDAC profiling revealed that all three cell lines had varying HDAC1–6 expression levels, with the highest expression of all six isoforms, in DoHH2 cells, which displayed the highest sensitivity to TSA.

Conclusion

Our results demonstrated that the HDAC inhibitor TSA inhibited DLBCL cell growth, and that cell lines with higher expression of HDACs tended to be more sensitive to TSA. Our data also suggested that inhibition of pAkt and activation of p53 pathway are the main molecular events involved in inhibitory effects of TSA.

TAp73 protein stability is controlled by histone deacetylase 1 via regulation of Hsp90 chaperone function

Histone deacetylases (HDACs) play important roles in fundamental cellular processes, and HDAC inhibitors are emerging as promising cancer therapeutics. p73, a member of the p53 family, plays a critical role in tumor suppression and neural development. Interestingly, p73 produces two classes of proteins with opposing functions: the full-length TAp73 and the N-terminally truncated ΔNp73. In the current study, we sought to characterize the potential regulation of p73 by HDACs and found that histone deacetylase 1 (HDAC1) is a key regulator of TAp73 protein stability. Specifically, we showed that HDAC1 inhibition by HDAC inhibitors or by siRNA shortened the half-life of TAp73 protein and subsequently decreased TAp73 expression under normal and DNA damage-induced conditions. Mechanistically, we found that HDAC1 knockdown resulted in hyperacetylation and inactivation of heat shock protein 90, which disrupted the interaction between heat shock protein 90 and TAp73 and thus promoted the proteasomal degradation of TAp73. Functionally, we found that down-regulation of TAp73 was required for the enhanced cell migration mediated by HDAC1 knockdown. Together, we uncover a novel regulation of TAp73 protein stability by HDAC1-heat shock protein 90 chaperone complex, and our data suggest that TAp73 is a critical downstream mediator of HDAC1-regulated cell migration.

p53 mutations in cancer

In the past fifteen years, it has become apparent that tumour-associated p53 mutations can provoke activities that are different to those resulting from simply loss of wild-type tumour-suppressing p53 function. Many of these mutant p53 proteins acquire oncogenic properties that enable them to promote invasion, metastasis, proliferation and cell survival. Here we highlight some of the emerging molecular mechanisms through which mutant p53 proteins can exert these oncogenic functions.

HDAC inhibitor-based therapies: can we interpret the code?

Abnormal epigenetic control is a common early event in tumour progression, and aberrant acetylation in particular has been implicated in tumourigenesis. One of the most promising approaches towards drugs that modulate epigenetic processes has been seen in the development of inhibitors of histone deacetylases (HDACs). HDACs regulate the acetylation of histones in nucleosomes, which mediates changes in chromatin conformation, leading to regulation of gene expression. HDACs also regulate the acetylation status of a variety of other non-histone substrates, including key tumour suppressor proteins and oncogenes. Histone deacetylase inhibitors (HDIs) are potent anti-proliferative agents which modulate acetylation by targeting histone deacetylases. Interest is increasing in HDI-based therapies and so far, two HDIs, vorinostat (SAHA) and romidepsin (FK228), have been approved for treating cutaneous T-cell lymphoma (CTCL). Others are undergoing clinical trials. Treatment with HDIs prompts tumour cells to undergo apoptosis, and cell-based studies have shown a number of other outcomes to result from HDI treatment, including cell-cycle arrest, cell differentiation, anti-angiogenesis and autophagy. However, our understanding of the key pathways through which HDAC inhibitors affect tumour cell growth remains incomplete, which has hampered progress in identifying malignancies other than CTCL which are likely to respond to HDI treatment.

The rebel angel: mutant p53 as the driving oncogene in breast cancer

Breast cancer is the most frequent invasive tumor diagnosed in women, causing over 400 000 deaths yearly worldwide. Like other tumors, it is a disease with a complex, heterogeneous genetic and biochemical background. No single genomic or metabolic condition can be regarded as decisive for its formation and progression. However, a few key players can be pointed out and among them is the TP53 tumor suppressor gene, commonly mutated in breast cancer. In particular, TP53 mutations are exceptionally frequent and apparently among the key driving factors in triple negative breast cancer -the most aggressive breast cancer subgroup-whose management still represents a clinical challenge. The majority of TP53 mutations result in the substitution of single aminoacids in the central region of the p53 protein, generating a spectrum of variants ('mutant p53s', for short). These mutants lose the normal p53 oncosuppressive functions to various extents but can also acquire oncogenic properties by gain-of-function mechanisms. This review discusses the molecular processes translating gene mutations to the pathologic consequences of mutant p53 tumorigenic activity, reconciling cell and animal models with clinical outcomes in breast cancer. Existing and speculative therapeutic methods targeting mutant p53 are also discussed, taking into account the overlap of mutant and wild-type p53 regulatory mechanisms and the crosstalk between mutant p53 and other oncogenic pathways in breast cancer. The studies described here concern breast cancer models and patients-unless it is indicated otherwise and justified by the importance of data obtained in other models.

Histone deacetylases (HDACs) as mediators of resistance to apoptosis in melanoma and as targets for combination therapy with selective BRAF inhibitors

HDACs are viewed as enzymes used by cancer cells to inhibit tumor suppressor mechanisms. In particular, we discuss their role as suppressors of apoptosis in melanoma cells and as mediators of resistance to selective BRAF inhibitors. Synergistic increases in apoptosis are seen when pan-HDAC inhibitors are combined with selective BRAF inhibitors. Moreover, cell lines from patients with acquired resistance to Vemurafenib undergo PLX4720 induced apoptosis when combined with pan-HDAC inhibitors. The mechanisms of upregulation of HDACs and the mechanisms involved in HDACi reversal of resistance to apoptosis are as yet poorly understood.