E2F-7 couples DNA damage-dependent transcription with the DNA repair process

The cellular response to DNA damage, mediated by the DNA repair process, is essential in maintaining the integrity and stability of the genome. E2F-7 is an atypical member of the E2F family with a role in negatively regulating transcription and cell cycle progression under DNA damage. Surprisingly, we found that E2F-7 makes a transcription-independent contribution to the DNA repair process, which involves E2F-7 locating to and binding damaged DNA. Further, E2F-7 recruits CtBP and HDAC to the damaged DNA, altering the local chromatin environment of the DNA lesion. Importantly, the E2F-7 gene is a target for somatic mutation in human cancer and tumor-derived mutant alleles encode proteins with compromised transcription and DNA repair properties. Our results establish that E2F-7 participates in 2 closely linked processes, allowing it to directly couple the expression of genes involved in the DNA damage response with the DNA repair machinery, which has relevance in human malignancy.

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.

Actin nucleators in the nucleus: an emerging theme

Actin is an integral component of the cytoskeleton, forming a plethora of macromolecular structures that mediate various cellular functions. The formation of such structures relies on the ability of actin monomers to associate into polymers, and this process is regulated by actin nucleation factors. These factors use monomeric actin pools at specific cellular locations, thereby permitting rapid actin filament formation when required. It has now been established that actin is also present in the nucleus, where it is implicated in chromatin remodelling and the regulation of eukaryotic gene transcription. Notably, the presence of typical actin filaments in the nucleus has not been demonstrated directly. However, studies in recent years have provided evidence for the nuclear localisation of actin nucleation factors that promote cytoplasmic actin polymerisation. Their localisation to the nucleus suggests that these proteins mediate collaboration between the cytoskeleton and the nucleus, which might be dependent on their ability to promote actin polymerisation. The nature of this cooperation remains enigmatic and it will be important to elucidate the physiological relevance of the link between cytoskeletal actin networks and nuclear events. This Commentary explores the current evidence for the nuclear roles of actin nucleation factors. Furthermore, the implication of actin-associated proteins in relaying exogenous signals to the nucleus, particularly in response to cellular stress, will be considered.

T-ARG-eting E2F-1 growth control

Comment on: Cho EC, et al. EMBO J 2012; 31:1785-97.

HDAC inhibitors in cancer biology: emerging mechanisms and clinical applications

Reversible acetylation mediated by histone deacetylases (HDACs) influences a broad repertoire of physiological processes, many of which are aberrantly controlled in tumor cells. As HDAC inhibition prompts tumor cells to enter apoptosis, small-molecule HDAC inhibitors have been developed as a new class of mechanism-based anti-cancer agent, many of which have entered clinical trials. Although the clinical picture is evolving and the precise utility of HDAC inhibitors remains to be determined, it is noteworthy that certain tumor types undergo a favorable response, in particular hematological malignancies. Vorinostat and romidepsin have been approved for treating cutaneous T-cell lymphoma in patients with progressive, persistent or recurrent disease. Here, we discuss developments in our understanding of molecular events that underlie the anti-cancer effects of HDAC inhibitors and relate this information to the emerging clinical picture for the application of these inhibitors in the treatment of cancer.

Actin nucleation by a transcription co-factor that links cytoskeletal events with the p53 response

Despite its obvious importance in tumorigenesis, little information is available on the mechanisms that integrate cell motility and invasion with nuclear events.  Tumor suppressor p53 is a DNA damage responsive transcription factor which initiates a checkpoint response culminating in cell cycle arrest or apoptosis. JMY is a transcription co-factor that functions in the nucleus during the p53 response. By forming a DNA damage-dependent complex with the p300 co-activator and the Mdm2 oncoprotein, JMY takes on a significant role in regulating the p53 response.  Here, we discuss recent studies describing an unexpected cytoplasmic role of JMY in regulating cell motility and invasion.  Control of cadherin expression and actin nucleation allows JMY to influence cell motility and invasion, contrasting with its nuclear role as a p53 co-factor which drives the response to DNA damage.  JMY therefore connects cell motility and invasion with the p53 response, and its aberrant regulation is likely to significantly contribute to tumorigenesis. How these findings might relate to JMY's role as a transcription co-factor are discussed, as well as the mechanisms through which JMY integrates cytoskeletal events and cellular motility with the DNA damage response.

Biomarkers for predicting clinical responses to HDAC inhibitors

Post-translational modifications of histone and non-histone proteins by acetylation are known to play a key role in tumourigenesis. Pharmacological manipulation of acetylation has been possible with the identification of small molecule inhibitors of histone deacetylases (HDAC), the enzymes responsible for deacetylating lysine residues. An explosion of drug discovery efforts in recent years has led to the development of an extensive group of HDAC inhibitors, many of which have been shown pre-clinically to have potent anti-tumour activity. Clinical trials using these agents are now underway, with Vorinostat (suberoylanilide hydroxamic acid) having been approved by the FDA for treating cutaneous T-cell lymphoma (CTCL) in patients with progressive, persistent or recurrent disease. This review discusses how biomarkers are being identified and used to expand our knowledge of the mechanisms by which HDAC inhibitors exhibit their anti-cancer effects. In the longer term, biomarkers will provide a means towards achieving patient stratification in tumour types that will respond favourably to HDAC inhibitors.

p53 methylation--the Arg-ument is clear

p53 Activity is of critical importance in suppressing human cancer formation, highlighted by the fact that the majority of human tumors have defective or inactive p53. In normal cells, p53 is held at low levels in a latent form and, following DNA damage, is stabilized which usually results in apoptosis or cell cycle arrest. Most functions of p53 can be ascribed to its role as a sequence specific transcription factor, and an extensive repertoire of downstream responsive genes have been identified. p53 activity is influenced by post-translational modifications, including phosphorylation and lysine methylation. Most recently, arginine methylation mediated by PRMT5, has been identified as an additional and important p53 modification. DNA damage induced p53 arginine methylation impacts on the biochemical properties and functional outcome of the p53 response.

p53 ubiquitination by Mdm2: a never ending tail?

p53 function is of critical importance in suppressing human cancer formation, highlighted by the fact that the majority of human tumors harbor compromised p53 activity. In normal cells, p53 is held at low levels in a latent form and cellular stress results in the rapid stabilization of p53. Mdm2 mediates ubiquitin-dependent degradation of p53 which plays a key role in maintaining cellular p53 levels. Ubiquitination was, until recently, considered a straightforward system involved in p53 degradation, but recent work has demonstrated how ubiquitination can alter p53 activity, not stability. In this review we summarize current understanding on p53 ubiquitination by Mdm2 with a particular focus on how the balance between protein levels and other post-translational modifications will direct the p53 response.

Genome-wide loss-of-function screen reveals an important role for the proteasome in HDAC inhibitor-induced apoptosis

Aberrant acetylation has been strongly linked to tumorigenesis, and the modulation of acetylation through targeting histone deacetylases (HDACs) is gathering increasing pace as a viable therapeutic strategy. A genome-wide loss-of-function screen identified HR23B, which shuttles ubiquitinated cargo proteins to the proteasome, as a sensitivity determinant for HDAC inhibitor-induced apoptosis. HR23B also governs tumor cell sensitivity to drugs that act directly on the proteasome. The level of HR23B influences the response of tumor cells to HDAC inhibitors, and HR23B is found at high levels in cutaneous T cell lymphoma in situ, a malignancy that responds favorably to HDAC inhibitor-based therapy. These results suggest that deregulated proteasome activity contributes to the anticancer activity of HDAC inhibitors.

A transcription cofactor required for the heat-shock response

The Stress-responsive activator of p300 (Strap) is a transcription cofactor that has an important role in the control of DNA damage response through its ability to regulate p53 activity. Here, we report that Strap is inducible by heat shock and stimulates the transcription of heat-shock genes. A chromatin-associated complex involving heat-shock factor 1 (HSF1), Strap and the p300 coactivator assembles on the heat-shock protein 70 (hsp70) promoter, and Strap augments HSF1 binding and chromatin acetylation in Hsp genes, most probably through the p300 histone acetyltransferase. Cells depleted of Strap do not survive under heat-shock conditions. These results indicate that Strap is an essential cofactor that acts at the level of chromatin control to regulate heat-shock-responsive transcription.

Mdm2 widens its repertoire

The p53 tumor suppressor protein is a DNA damage responsive transcription factor that affects diverse cellular processes which include transcription, DNA synthesis and repair, cell cycle arrest, senescence and apoptosis. The Mdm2 oncoprotein is a primary regulator of p53, mediating p53 control via ubiquitin-dependent proteasomal degradation. During DNA damage, the interaction between p53 and Mdm2 is reduced, which allows p53 levels to accumulate. p53 activity is tightly controlled and regulated at a multiplicity of levels, and the importance of co-factors that influence p53 activity is becoming increasingly evident. Recent studies have highlighted the role of Mdm2 in the control of p53 co-factors. Thus, Mdm2 targets JMY, a p53 co-factor, for ubiquitin-dependent Mdm2 targets JMY, a p53 co-factor, for ubiquitin-dependent proteasomal degradation and in doing so overcomes the ability of JMY to augment the p53 response. These results define a new functional relationship between control of p53 activity and Mdm2, and suggest that transcription co-factors which facilitate the p53 response are important targets through which Mdm2 mediates its oncogenic activity.

Mdm2 targets the p53 transcription cofactor JMY for degradation

We define here a new mechanism through which Mdm2 (mouse double minute 2) regulates p53 activity, by targeting the p53 transcription cofactor JMY. DNA damage causes an increase in JMY protein, and, in a similar manner, small molecule inhibitors of Mdm2 activity induce JMY in unperturbed cells. At a mechanistic level, Mdm2 regulation of JMY requires the Mdm2 RING (really interesting new gene) finger, which promotes the ubiquitin-dependent degradation of JMY. However, regulation of JMY occurs independently of the p53-binding domain in Mdm2 and p53 activity. These results define a new functional relationship between the p53 cofactor JMY and Mdm2, and indicate that transcription cofactors that facilitate p53 activity are important targets for Mdm2 in suppressing the p53 response.

Histone deacetylase inhibitors: gathering pace

Reversible histone acetylation is one of the key mechanisms involved in the epigenetic control of gene expression. A variety of recent studies has revealed a role for acetylation in a much broader repertoire of physiological processes, including proliferation control and protein folding, and has highlighted how a variety of non-histone regulatory proteins are influenced by acetylation. Inhibition of histone deacetylase (HDAC) prompts tumour cells to enter apoptosis and, as a consequence, several HDAC inhibitors have entered clinical trials. It is likely that HDAC inhibitor drugs will provide an important class of new mechanism-based therapeutics for cancer.

Chromatin control and cancer-drug discovery: realizing the promise

Recent years have seen major advances in elucidating the complexity of chromatin and its role as an epigenetic regulator of gene expression in eukaryotes. We now have a basic understanding of chromatin control and the enzymatic modifications that impart diverse regulatory cues to the functional activity of the genome. Most importantly, although research into chromatin has uncovered fascinating insights into the control of gene expression, it has also generated a large body of information that is being harnessed to develop new therapeutic modalities for treating cancer. Here, we discuss recent advances that support the contention that future generations of chromatin-modulating drugs will provide a significant group of new, mechanism-based therapeutics for cancer.

DNA-damage-responsive acetylation of pRb regulates binding to E2F-1

The pRb (retinoblastoma protein) tumour suppressor protein has a crucial role in regulating the G1- to S-phase transition, and its phosphorylation by cyclin-dependent kinases is an established and important mechanism in controlling pRb activity. In addition, the targeted acetylation of lysine (K) residues 873/874 in the carboxy-terminal region of pRb located within a cyclin-dependent kinase-docking site hinders pRb phosphorylation and thereby retains pRb in an active state of growth suppression. Here, we report that the acetylation of pRb K873/874 occurs in response to DNA damage and that acetylation regulates the interaction between the C-terminal E2F-1-specific domain of pRb and E2F-1. These results define a new role for pRb acetylation in the DNA damage signalling pathway, and suggest that the interaction between pRb and E2F-1 is controlled by DNA-damage-dependent acetylation of pRb.

14-3-3 proteins integrate E2F activity with the DNA damage response

The E2F family is composed of at least eight E2F and two DP subunits, which in cells exist as E2F/DP heterodimers that bind to and regulate E2F target genes. While DP-1 is an essential and widespread component of E2F, much less is known about the DP-3 subunit, which exists as a number of distinct protein isoforms that differ in several respects including the presence of a nuclear localisation signal (NLS). We show here that the NLS region of DP-3 harbours a binding site for 14-3-3epsilon, and that binding of 14-3-3epsilon alters the cell cycle and apoptotic properties of E2F. DP-3 responds to DNA damage, and the interaction between DP-3 and 14-3-3epsilon is under DNA damage-responsive control. Further, 14-3-3epsilon is present in the promoter region of certain E2F target genes, and reducing 14-3-3epsilon levels induces apoptosis. These results identify a new level of control on E2F activity and, at a more general level, suggest that 14-3-3 proteins integrate E2F activity with the DNA damage response.

E2F-8: an E2F family member with a similar organization of DNA-binding domains to E2F-7

E2F is a family of transcription factors implicated in cell cycle control. To understand the role of E2F in controlling cell cycle progression, it is necessary to clarify the breadth of the E2F family. To date, seven E2F subunits have been identified. We report here the characterization of a new E2F subunit, E2F-8, which resembles the organization of E2F-7 in the presence of two separate DNA-binding domains, the integrity of which is required for E2F-8 to bind to DNA. Furthermore, like E2F-7, we find that E2F-8 can repress transcription and delay cell cycle progression. The similarities between E2F-7 and E2F-8 define a new subgroup of the E2F family, and further imply that E2F-7 and E2F-8 may act through overlapping mechanisms in mediating cell cycle control.

The emerging role of E2F-1 in the DNA damage response and checkpoint control

Genotoxic stress triggers a myriad of cellular responses including cell cycle arrest, stimulation of DNA repair and apoptosis. A central role for the E2F-1 transcription factor in the DNA damage response pathway is gaining support. E2F-1 is phosphorylated by DNA damage responsive protein kinases, which leads to E2F-1 accumulation and the induction of apoptosis. In addition, emerging information suggests that E2F-1 may play a role in the detection and subsequent repair of damaged DNA.

Signalling DNA damage by regulating p53 co-factor activity

In response to DNA damage the related phosphatidylinositol-3-OH-kinase-like-kinases ATM and ATR phosphorylate downstream protein targets which facilitate the DNA damage response. A new pathway in which ATM phosphorylates the transcriptional co-factor Strap has been elucidated. Phosphorylation causes the stabilization of nuclear Strap and favours the formation of a stress-responsive co-activator complex. Strap activity enhances p53 acetylation, and augments the response to DNA damage. Most interestingly, in AT cells Strap remains cytoplasmic, and a mutant derivative that cannot be phosphorylated by ATM is similarly localised to the cytoplasm. These results argue that Strap is an important downstream effector in the DNA damage response.

Histone deacetylase inhibitors open new doors in cancer therapy

Cancer drug development has moved from conventional cytotoxic chemotherapeutics to a more mechanism-based targeted approach towards the common goal of tumour growth arrest. The rapid progress in chromatin research has supplied a plethora of potential targets for intervention in cancer. Here, we focus on the histone deacetylase (HDAC) inhibitors, together with their current status of clinical development and potential utility in cancer therapy. HDACs have been widely implicated in growth and transcriptional control, and inhibition of HDAC activity using small molecules causes apoptosis in tumour cells. We discuss the rationale for the development of HDAC inhibitors as novel anti-cancer agents, the potential clinical application and explore ideas on how we may move towards patient stratification with the possibility of increasing efficacy in the clinic.