Cancer cells' epigenetic composition and predisposition to histone deacetylase inhibitor sensitization

Sp1-mediated epigenetic dysregulation dictates HDAC inhibitor susceptibility of HER2-overexpressing breast cancer

Human epidermal growth factor receptor 2 (HER2/erbB2) is a key driver and therapeutic target for breast cancer. The treatment of HER2-positive breast cancer remains a clinical challenge largely due to the limited understanding of HER2-driving oncogenic signaling and the frequent resistance to simply HER2-targeted therapy. Here, we show that the histone deacetylase inhibitor, trichostatin A (TSA), suppresses HER2-overexpressing breast cancer via upregulation of miR-146a and the resultant repression of its oncogenic targets, interleukin-1 receptor-associated kinase 1 and the chemokine receptor CXCR4. Mechanistically, histone H3K56 acetylation and deacetylation on the MIR146A promoter are catalyzed respectively by the acetyltransferase p300 and histone deacetylase 1 (HDAC1), both of which are recruited to the genomic loci by the transcription factor specificity protein 1 (Sp1). HER2 signaling phosphorylates Sp1 and induces its predominant association with HDAC1, but not p300, leading to histone hypoacetylation and silencing of MIR146A. In addition, the death receptor Fas is similarly downregulated by the aforementioned epigenetic paradigm, indicating its wide involvement in impairing tumor suppressor gene expression. Consequently, TSA synergizes with lapatinib, a tyrosine kinase inhibitor of HER2, to suppress breast cancer in vitro and in rodent models. These findings demonstrate a novel mechanism of HER2-driven carcinogenesis and suggest the applicability of combined HER2 and HDAC targeting in breast cancer therapy.

Histone Deacetylase Inhibitor Induced Radiation Sensitization Effects on Human Cancer Cells after Photon and Hadron Radiation Exposure

Suberoylanilide hydroxamic acid (SAHA) is a histone deacetylase inhibitor, which has been widely utilized throughout the cancer research field. SAHA-induced radiosensitization in normal human fibroblasts AG1522 and lung carcinoma cells A549 were evaluated with a combination of γ-rays, proton, and carbon ion exposure. Growth delay was observed in both cell lines during SAHA treatment; 2 μM SAHA treatment decreased clonogenicity and induced cell cycle block in G1 phase but 0.2 μM SAHA treatment did not show either of them. Low LET (Linear Energy Transfer) irradiated A549 cells showed radiosensitization effects on cell killing in cycling and G1 phase with 0.2 or 2 μM SAHA pretreatment. In contrast, minimal sensitization was observed in normal human cells after low and high LET radiation exposure. The potentially lethal damage repair was not affected by SAHA treatment. SAHA treatment reduced the rate of γ-H2AX foci disappearance and suppressed RAD51 and RPA (Replication Protein A) focus formation. Suppression of DNA double strand break repair by SAHA did not result in the differences of SAHA-induced radiosensitization between human cancer cells and normal cells. In conclusion, our results suggest SAHA treatment will sensitize cancer cells to low and high LET radiation with minimum effects to normal cells.

The epigenetic regulation of Dicer and microRNA biogenesis by Panobinostat

microRNAs (miRs) are small noncoding RNAs that regulate/fine tune many cellular protein networks by targeting mRNAs for either degradation or translational inhibition. Dicer, a type III endoribonuclease, is a critical component in miR biogenesis and is required for mature microRNA production. Abnormal Dicer expression occurs in numerous cancer types and correlates with poor patient prognosis. Recent reports have demonstrated that epigenetic agents, including histone deacetylase inhibitors (HDACi), may regulate Dicer and miR expression. HDACi are a class of epigenetic agents used to treat cancer, viral infections, and inflammatory disorders. However, little is known regarding the epigenetic regulation of miR biogenesis and function. We therefore investigated whether clinically successful HDACi modulated Dicer expression and found that Panobinostat, a clinically approved HDACi, enhanced Dicer expression via posttranscriptional mechanisms. Studies using proteasome inhibitors suggested that Panobinostat regulated the proteasomal degradation of Dicer. Further studies demonstrated that Panobinostat, despite increasing Dicer protein expression, decreased Dicer activity. This suggests that Dicer protein levels do not necessarily correlate with Dicer activity and mature miR levels. Taken together, we present evidence here that Panobinostat posttranscriptionally regulates Dicer/miR biogenesis and suggest Dicer as a potential therapeutic target in cancer.

Anti-GD2 mAb and Vorinostat synergize in the treatment of neuroblastoma

Neuroblastoma (NBL) is a childhood malignancy of the sympathetic nervous system. For high-risk NBL patients, the mortality rate is still over 50%, despite intensive multimodal treatment. Anti-GD2 monoclonal antibody (mAB) in combination with systemic cytokine immunotherapy has shown clinical efficacy in high-risk NBL patients. Targeted therapy using histone deacetylase inhibitors (HDACi) is currently being explored in cancer treatment and already shows promising results. Using our recently developed transplantable TH-MYCN NBL model, we here report that the HDAC inhibitor Vorinostat synergizes with anti-GD2 mAb therapy in reducing NBL tumor growth. Further mechanistic studies uncovered multiple mechanisms for the observed synergy, including Vorinostat-induced specific NBL cell death and upregulation of the tumor antigen GD2 on the cell surface of surviving NBL cells. Moreover, Vorinostat created a permissive tumor microenvironment (TME) for tumor-directed mAb therapy by increasing macrophage effector cells expressing high levels of Fc-receptors (FcR) and decreasing the number and function of myeloid-derived suppressor cells (MDSC). Collectively, these data imply further testing of other epigenetic modulators with immunotherapy and provide a strong basis for clinical testing of anti-GD2 plus Vorinostat combination therapy in NBL patients.

Preclinical antitumor activity of ST7612AA1: a new oral thiol-based histone deacetylase (HDAC) inhibitor

ST7612AA1 (property of Sigma-Tau), a thioacetate-ω (γ-lactam amide) derivative, is a potent, second generation, oral pan-histone deacetylase inhibitor (HDACi). Aim of the study was to assess the efficacy of ST7612AA1 in solid and haematological tumors, and to characterize its mechanism of action. In vitro, ST7612AA1 potently inhibited different class I and class II HDACs, leading to restore the balance of both histone and non-histone protein acetylation. In vivo, it induced significant anti-tumor effects in xenograft models of lung, colon, breast and ovarian carcinomas, leukemia and lymphoma. This was likely due to the modulation of different HDAC substrates and induction of transcriptional changes with respect to several genes involved in key processes, such as cell cycle regulation, DNA damage checkpoints, immune response, cell adhesion and epithelial-to-mesenchymal transition. PK analysis confirmed the pro-drug nature of ST7612AA1, which is rapidly absorbed and converted to ST7464AA1 after a single oral dose in mice. ST7612AA1 was selected from a novel generation of oral HDAC inhibitors. Its high efficacy correlated with its potent and selective inhibitory activity of HDAC and was combined with a favorable pharmacodynamics profile. These aspects support a clinical development of ST7612AA1 towards a broad spectrum of human solid and haematologic malignancies.

HDAC inhibitors and immunotherapy; a double edged sword?

Epigenetic modifications, like histone acetylation, are essential for regulating gene expression within cells. Cancer cells acquire pathological epigenetic modifications resulting in gene expression patterns that facilitate and sustain tumorigenesis. Epigenetic manipulation therefore is emerging as a novel targeted therapy for cancer. Histone Acetylases (HATs) and Histone Deacetylases (HDACs) regulate histone acetylation and hence gene expression. Histone deacetylase (HDAC) inhibitors are well known to affect cancer cell viability and biology and are already in use for the treatment of cancer patients. Immunotherapy can lead to clinical benefit in selected cancer patients, especially in patients with limited disease after tumor debulking. HDAC inhibitors can potentially synergize with immunotherapy by elimination of tumor cells. The direct effects of HDAC inhibitors on immune cell function, however, remain largely unexplored. Initial data have suggested HDAC inhibitors to be predominantly immunosuppressive, but more recent reports have challenged this view. In this review we will discuss the effects of HDAC inhibitors on tumor cells and different immune cell subsets, synergistic interactions and possible mechanisms. Finally, we will address future challenges and potential application of HDAC inhibitors in immunocombination therapy of cancer.

HP1β-dependent recruitment of UBF1 to irradiated chromatin occurs simultaneously with CPDs

Background

The repair of spontaneous and induced DNA lesions is a multistep process. Depending on the type of injury, damaged DNA is recognized by many proteins specifically involved in distinct DNA repair pathways.

Results

We analyzed the DNA-damage response after ultraviolet A (UVA) and γ irradiation of mouse embryonic fibroblasts and focused on upstream binding factor 1 (UBF1), a key protein in the regulation of ribosomal gene transcription. We found that UBF1, but not nucleolar proteins RPA194, TCOF, or fibrillarin, was recruited to UVA-irradiated chromatin concurrently with an increase in heterochromatin protein 1β (HP1β) level. Moreover, Förster Resonance Energy Transfer (FRET) confirmed interaction between UBF1 and HP1β that was dependent on a functional chromo shadow domain of HP1β. Thus, overexpression of HP1β with a deleted chromo shadow domain had a dominant-negative effect on UBF1 recruitment to UVA-damaged chromatin. Transcription factor UBF1 also interacted directly with DNA inside the nucleolus but no interaction of UBF1 and DNA was confirmed outside the nucleolus, where UBF1 recruitment to DNA lesions appeared simultaneously with cyclobutane pyrimidine dimers; this occurrence was cell-cycle-independent.

Conclusions

We propose that the simultaneous presence and interaction of UBF1 and HP1β at DNA lesions is activated by the presence of cyclobutane pyrimidine dimers and mediated by the chromo shadow domain of HP1β. This might have functional significance for nucleotide excision repair.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-8935-7-39) contains supplementary material, which is available to authorized users.

Differentiation therapy for solid tumors

Differentiation therapy for tumors refers to treating malignant tumors via the induction of cell differentiation. The best characterized clinical application of differentiation therapy is the use of all-trans retinoic acid in the treatment of acute promyelocytic leukemia (APL), which markedly improved the outcome of this disease. Unlike the situation with APL, the development of differentiation therapy for solid tumors is far from satisfactory. To date, no differentiation-inducing agents have been demonstrated to exert a curative effect on solid tumors. However, over the past decade progress in understanding the differentiation pathways and the development of differentiation-inducing agents might shed new light on the differentiation therapy for solid tumors.

Histone deacetylase inhibitors selectively target homology dependent DNA repair defective cells and elevate non-homologous endjoining activity

Background

We have previously used the ATAD5-luciferase high-throughput screening assay to identify genotoxic compounds with potential chemotherapeutic capabilities. The successful identification of known genotoxic agents, including the histone deacetylase inhibitor (HDACi) trichostatin A (TSA), confirmed the specificity of the screen since TSA has been widely studied for its ability to cause apoptosis in cancer cells. Because many cancers have acquired mutations in DNA damage checkpoints or repair pathways, we hypothesized that these cancers may be susceptible to treatments that target compensatory pathways. Here, we used a panel of isogenic chicken DT40 B lymphocyte mutant and human cell lines to investigate the ability of TSA to define selective pathways that promote HDACi toxicity.

Results

HDACi induced a DNA damage response and reduced viability in all repair deficient DT40 mutants although ATM-nulls were least affected. The most dramatic sensitivity was observed in mutants lacking the homology dependent repair (HDR) factor BLM or the non-homologous end-joining (NHEJ) and HDR factors, KU/RAD54, suggesting an involvement of either HDR or NHEJ in HDACi-induced cell death. To extend these findings, we measured the frequencies of HDR and NHEJ after HDACi treatment and monitored viability in human cell lines comparably deficient in HDR or NHEJ. Although no difference in HDR frequency was observed between HDACi treated and untreated cells, HDR-defective human cell lines were clearly more sensitive than wild type. Unexpectedly, cells treated with HDACis showed a significantly elevated NHEJ frequency.

Conclusions

HDACi targeting drugs induced significant increases in NHEJ activity in human cell lines but did not alter HDR frequency. Moreover, HDR is required for cellular resistance to HDACi therapy; therefore, NHEJ does not appear to be a critical axis for HDACi resistance. Rather, HDACi compounds induced DNA damage, most likely double strand breaks (DSBs), and HDR proficiency is correlated with cell survival.

Induction of the liver cancer-down-regulated long noncoding RNA uc002mbe.2 mediates trichostatin-induced apoptosis of liver cancer cells

Differential expression of long non-coding RNAs (lncRNAs) plays critical roles in hepatocarcinogenesis. Considerable attention has focused on the antitumor effect of histone deacetylase inhibitor (Trichostatin A, TSA) as well as the coding gene expression-induced apoptosis of cancer cells. However, it is not known whether lncRNA has a role in TSA-induced apoptosis of human hepatocellular carcinoma (HCC) cells. The global expression of lncRNAs and coding genes was analyzed with the Human LncRNA Array V2.0 after 24 h treatment. Expression was verified in cell lines and tissues by quantitative real-time PCR. The data showed that 4.8% (959) of lncRNA and 6.1% (1849) of protein coding gene were significantly differentially expressed. The differential expressions of lncRNA and protein coding genes had distinguishable hierarchical clustering expression profiling pattern. Among these differentially expressed lncRNAs, the greatest change was noted for uc002mbe.2, which had more than 300 folds induction upon TSA treatment. TSA selectively induced uc002mbe.2 in four studied HCC cell lines. Compared with normal human hepatocytes and adjacent noncancerous tissues, uc002mbe.2 expression level was significantly lower in the HCC cell lines and liver cancer tissues. The TSA-induced uc002mbe.2 expression was positively correlated with the apoptotic effect of TSA in HCC cells. In addition, knockdown the expression of uc002mbe.2 significantly reduced TSA-induced apoptosis of Huh7cells. Therefore, TSA-induced apoptosis of HCC cells is uc002mbe.2 dependent and reduced expression of uc002mbe.2 may be associated with liver carcinogenesis.

HDAC inhibitors: roles of DNA damage and repair

Histone deacetylase inhibitors (HDACis) increase gene expression through induction of histone acetylation. However, it remains unclear whether specific gene expression changes determine the apoptotic response following HDACis administration. Herein, we discuss evidence that HDACis trigger in cancer and leukemia cells not only widespread histone acetylation but also actual increases in reactive oxygen species (ROS) and DNA damage that are further increased following treatment with DNA-damaging chemotherapies. While the origins of ROS production are not completely understood, mechanisms, including inflammation and altered antioxidant signaling, have been reported. While the generation of ROS is an explanation, at least in part, for the source of DNA damage observed with HDACi treatment, DNA damage can also be independently induced by changes in the DNA repair activity and chromatin remodeling factors. Recent development of sirtuin inhibitors (SIRTis) has shown that, similar to HDACis, these drugs induce increases in ROS and DNA damage used singly, or in combination with HDACis and other drugs. Thus, induction of apoptosis by HDACis/SIRTis may result through oxidative stress and DNA damage mechanisms in addition to direct activation of apoptosis-inducing genes. Nevertheless, while DNA damage and stress responses could be of interest as markers for clinical responses, they have yet to be validated as markers for responses to HDACi treatment in clinical trials, alone, and in combination.

Epigenetic influences in the aetiology of cancers arising from breast and prostate: a hypothesised transgenerational evolution in chromatin accessibility

Epidemiological studies have consistently supported the notion that environmental and/or dietary factors play a central role in the aetiology of cancers of the breast and prostate. However, for more than five decades investigators have failed to identify a single cause-and-effect factor, which could be implicated; identification of a causative entity would allow the implementation of an intervention strategy in at-risk populations. This suggests a more complex pathoaetiology for these cancer sites, compared to others. When one examines the increases or decreases in incidence of specific cancers amongst migrant populations, it is notable that disease arising in colon or stomach requires one or at most two generations to exhibit a change in incidence to match that of high-incidence regions, whereas for breast or prostate cancer, at least three generations are required. This generational threshold could suggest a requirement for nonmutation-driven epigenetic alterations in the F0/F1 generations (parental/offspring adopting a more westernized lifestyle), which then predisposes the inherited genome of subsequent generations to mutagenic/genotoxic alterations leading to the development of sporadic cancer in these target sites. As such, individual susceptibility to carcinogen insult would not be based per se on polymorphisms in activating/detoxifying/repair enzymes, but on elevated accessibility of crucial target genes (e.g., oncogenes, tumour suppressor genes) or hotspots therein to mutation events. This could be termed a genomic susceptibility organizational structure (SOS). Several exposures including alcohol and heavy metals are epigens (i.e., modifiers of the epigenome), whereas others are mutagenic/genotoxic, for example, heterocyclic aromatic amines; humans are continuously and variously exposed to mixtures of these agents. Within such a transgenerational multistage model of cancer development, determining the interaction between epigenetic modification to generate a genomic SOS and genotoxic insult will facilitate a new level of understanding in the aetiology of cancer.

Chromatin Modulation by Histone Deacetylase Inhibitors: Impact on Cellular Sensitivity to Ionizing Radiation

It is well established that cells are more sensitive to ionizing radiation during the G₂/M phase of the cell cycle when their chromatin is highly compacted. However, highly compacted chromatin is less susceptible to DNA Double Strand Breaks (DSBs) than relaxed chromatin. Therefore, it is now becoming apparent that it is the cell capacity to repair its damaged DNA and refold its chromatin into its original compacted status that primarily affects the overall cellular sensitivity to ionizing radiation. The Histone Deacetylase Inhibitors (HDACIs) are a new class of anticancer agents that relax chromatin structure by increasing the levels of histone acetylation. The effect of HDACIs on normal and cancer cells sensitivity to ionizing radiation differs. Reports have indicated that HDACIs can protect normal cells while simultaneously sensitize cancer cells to ionizing radiation. This difference may stem from the individual characteristic of the normal and cancer cells chromatin structure. This review discusses this possibility and addresses the role of HDACIs in radiation therapy.

Targeted cancer therapy: giving histone deacetylase inhibitors all they need to succeed

Histone deacetylase inhibitors (HDACis) have now emerged as a powerful new class of small-molecule therapeutics acting through the regulation of the acetylation states of histone proteins (a form of epigenetic modulation) and other non-histone protein targets. Over 490 clinical trials have been initiated in the last 10 years, culminating in the approval of two structurally distinct HDACis - SAHA (vorinostat, Zolinza™) and FK228 (romidepsin, Istodax™). However, the current HDACis have serious limitations, including ineffectively low concentrations in solid tumors and cardiac toxicity, which is hindering their progress in the clinic. Herein, we review the primary paradigms being pursued to overcome these hindrances, including HDAC isoform selectivity, localized administration, and targeting cap groups to achieve selective tissue and cell type distribution.

HDAC inhibitors for the treatment of cutaneous T-cell lymphomas

Epigenetic modification by small-molecule histone deacetylase inhibitors (HDAC-Is) has been a promising new antineoplastic approach for various solid and hematological malignancies, particularly for cutaneous T-cell lymphoma (CTCL). Vorinostat, a pan-HDAC-I and, most recently, romidepsin, a bicyclic pan-HDAC-I, have been US FDA approved for treatment of relapsed or refractory CTCL. However, because many patients do not reach the 50% partial response mark and response is not always sustainable, overcoming HDAC-I resistance by adding other agents or finding more selective molecules is an important clinical problem in realizing the full clinical potential of HDAC-Is. In this review, we discuss the molecular basis for HDAC-I function in cancer, the clinical response and side-effect profile experienced by CTCL patients, and the progress made in attempting to identify biomarkers of response and resistance, as well as synergistic combination therapies.