Cofactor Strap regulates oxidative phosphorylation and mitochondrial p53 activity through ATP synthase.

Metabolic reprogramming is a hallmark of cancer cells. Strap (stress-responsive activator of p300) is a novel TPR motif OB-fold protein that contributes to p53 transcriptional activation. We show here that, in addition to its established transcriptional role, Strap is localised at mitochondria where one of its key interaction partners is ATP synthase. Significantly, the interaction between Strap and ATP synthase downregulates mitochondrial ATP production. Under glucose-limiting conditions, cancer cells are sensitised by mitochondrial Strap to apoptosis, which is rescued by supplementing cells with an extracellular source of ATP. Furthermore, Strap augments the apoptotic effects of mitochondrial p53. These findings define Strap as a dual regulator of cellular reprogramming: first as a nuclear transcription cofactor and second in the direct regulation of mitochondrial respiration.

Pleiotropic effect of somatic mutations in the E2F subunit DP-1 gene in human cancer.

Transcription factor E2F-1 and its interaction with pRb provide a key point of control in cell proliferation. E2F-1 participates in both cell cycle progression and apoptosis, and in cells exists with a DP dimerization partner protein, the most prominent being DP-1. By mining the tumor tissue and cancer cell line encyclopedia genomic databases, we identified the first somatic mutations in the DP-1 gene and describe 53 distinct mutation events here. The mutations are mostly missense mutations, but also include nonsense and frame-shift mutations that result in truncated DP-1 derivatives. Mutation occurs throughout the DP-1 gene but generally leaves protein dimerization activity intact. This allows the mutant derivatives to affect the properties of the E2F-1/DP-1 heterodimer through a transdominant mechanism, which changes the DNA binding, transcriptional activation and pRb-binding properties of the heterodimer. In particular, many DP-1 mutants were found to impair E2F-1-dependent apoptosis. Our results establish that somatic mutations in DP-1 uncouple normal control of the E2F pathway, and thus define a new mechanism that could contribute to aberrant proliferation in tumor cells.

A regulatory circuit that involves HR23B and HDAC6 governs the biological response to HDAC inhibitors.

Histone deacetylase (HDAC) is an emergent anticancer target, and HR23B is a biomarker for response to HDAC inhibitors. We show here that HR23B has impacts on two documented effects of HDAC inhibitors; HDAC inhibitors cause apoptosis in cells expressing high levels of HR23B, whereas in cells with low level expression, HDAC inhibitor treatment is frequently associated with autophagy. The mechanism responsible involves the interaction of HDAC6 with HR23B, which downregulates HR23B and thereby reduces the level of ubiquitinated substrates targeted to the proteasome, ultimately desensitising cells to apoptosis. Significantly, the ability of HDAC6 to downregulate HR23B occurs independently of its deacetylase activity. An analysis of the HDAC6 interactome identified HSP90 as a key effector of HDAC6 on HR23B levels. Our results define a regulatory mechanism that involves the interplay between HR23B and HDAC6 that influences the biological outcome of HDAC inhibitor treatment.

Diversity within the pRb pathway: is there a code of conduct?

The failure of cell proliferation to be properly regulated is a hallmark of tumourigenesis. The retinoblastoma protein (pRb) pathway represents a key component in the regulation of the cell cycle and tumour suppression. Recent findings have revealed new levels of complexity reflecting a repertoire of post-translational modifications that occur on pRb together with its key effector E2F-1. Here we provide an overview of the modifications and consider the possibility of a 'code' that endows pRb with the ability to function in diverse physiological settings.

Hypoxia-driven cell motility reflects the interplay between JMY and HIF-1α.

Junction-mediating and regulatory protein (JMY) is a novel p53 cofactor that regulates p53 activity during stress. JMY interacts with p300/CBP, which are ubiquitous transcriptional co-activators that interact with a variety of sequence-specific transcription factors, including hypoxia-inducible factor-1α (HIF-1α). In addition, JMY is an actin-nucleating protein, which, through its WH2 domains, stimulates cell motility. In this study, we show that JMY is upregulated during hypoxia in a HIF-1α-dependent manner. The JMY gene contains HIF-responsive elements in its promoter region and HIF-1α is recruited to its promoter during hypoxia. HIF-1α drives transcription of JMY, which accounts for its induction under hypoxia. Moreover, the enhanced cell motility and invasion that occurs during hypoxia requires JMY, as depleting JMY under hypoxic conditions causes decreased cell motility. Our results establish the interplay between JMY and HIF-1α as a new mechanism that controls cell motility under hypoxic stress.

E2F-1 regulation by an unusual DNA damage-responsive DP partner subunit.

E2F activity is negatively regulated by retinoblastoma protein (pRb) through binding to the E2F-1 subunit. Within the E2F heterodimer, DP proteins are E2F partner subunits that allow proper cell cycle progression. In contrast to the other DP proteins, the newest member of the family, DP-4, downregulates E2F activity. In this study we report an unexpected role for DP-4 in regulating E2F-1 activity during the DNA damage response. Specifically, DP-4 is induced in DNA-damaged cells, upon which it binds to E2F-1 as a non-DNA-binding E2F-1/DP-4 complex. Consequently, depleting DP-4 in cells re-instates E2F-1 activity that coincides with increased levels of chromatin-bound E2F-1, E2F-1 target gene expression and associated apoptosis. Mutational analysis of DP-4 highlighted a C-terminal region, outside the DNA-binding domain, required for the negative control of E2F-1 activity. Our results define a new pathway, which acts independently of pRb and through a biochemically distinct mechanism, involved in negative regulation of E2F-1 activity.

Lysine methylation regulates the pRb tumour suppressor protein.

The pRb tumour suppressor protein has a central role in coordinating early cell cycle progression. An important level of control imposed on pRb occurs through post-translational modification, for example, phosphorylation. We describe here a new level of regulation on pRb, mediated through the targeted methylation of lysine residues, by the methyltransferase Set7/9. Set7/9 methylates the C-terminal region of pRb, both in vitro and in cells, and methylated pRb interacts with heterochromatin protein HP1. pRb methylation is required for pRb-dependent cell cycle arrest and transcriptional repression, as well as pRb-dependent differentiation. Our results indicate that methylation can influence the properties of pRb, and raise the interesting possibility that methylation modulates pRb tumour suppressor activity.

HDAC inhibitor-based therapies and haematological malignancy.

Reversible acetylation mediated by histone deacetylase (HDAC) influences a broad repertoire of physiological processes, many of which are aberrantly controlled in tumour cells. Since HDAC inhibition prompts tumour cells to enter apoptosis, small-molecule HDAC inhibitors have been developed as a new class of mechanism-based anticancer agent, many of which have entered clinical trials. While the clinical picture is evolving and the precise utility of HDAC inhibitors remains to be determined, it is noteworthy that certain tumour types undergo a favourable response, in particular haematological malignancies. Vorinostat (suberoylanilide hydroxamic acid) has 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 anticancer effects of HDAC inhibitors and relate this information to the emerging clinical picture for the application of HDAC inhibitors in haematological malignancies.

A functionally distinct member of the DP family of E2F subunits.

E2F transcription factors regulate genes involved in cell-cycle progression. In mammalian cells, physiological E2F exists as an E2F/DP heterodimer. Currently, eight E2F and two DP subunits have been characterized. We report here the characterization of a new member of the DP family, DP-4. While DP-4 exhibits certain similarities with members of the DP family, it also possesses a number of significant differences. Thus, DP-4 forms a heterodimer with E2F subunits, binds to the E2F site and associates with pocket proteins including pRb. In contrast to DP-1, however, DP-4/E2F-1 complexes exhibit reduced DNA binding activity. Furthermore, DP-4 interferes with E2F-1-dependent transcription and delays cell-cycle progression. These results highlight an emerging complexity in the DP family of E2F subunits, and suggest that DP-4 may endow E2F heterodimers with distinct transcription properties.

Histone deacetylase inhibitors and 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 and understanding epigenetic control has supplied a plethora of potential targets for intervention in cancer. Histone deacetylases (HDACs) have been widely implicated in growth and transcriptional control, and inhibition of HDAC activity using small molecules causes apoptosis in tumour cells. Here, we review HDAC inhibitors, together with their current status of clinical development and potential utility in cancer therapy.

Role of LXCXE motif-dependent interactions in the activity of the retinoblastoma protein.

Cell cycle control by pRb requires the integrity of the pocket domain, which is a region necessary for interactions with a variety of proteins, including E2F and LXCXE-motif containing proteins. Through knowledge of the crystal structure of pRb we have prepared a panel of pRb mutant derivatives in which a cluster of lysine residues that demark the LXCXE peptide binding domain were systematically mutated. One of the mutant derivatives, Rb6A, exhibits significantly reduced LXCXE-dependent interactions with HPV E7, cyclinD1 and HDAC2, but retained LXCXE-independent binding to E2F. Consistent with these results, Rb6A could down-regulate E2F-1-dependent activation of different E2F responsive promoters, but was compromised in Rb-dependent repression. Most importantly, Rb6A retained wild-type growth arrest activity, and colony forming activity similar to wild-type pRb. It is compatible with these results that directly targeting HDAC2 to E2F responsive promoters as an E2F/HDAC hybrid protein failed to effect cell cycle arrest. These results suggest that LXCXE-dependent interactions are not essential for pRb to exert growth arrest.

p300/CBP proteins: HATs for transcriptional bridges and scaffolds.

p300/CBP transcriptional co-activator proteins play a central role in co-ordinating and integrating multiple signal-dependent events with the transcription apparatus, allowing the appropriate level of gene activity to occur in response to diverse physiological cues that influence, for example, proliferation, differentiation and apoptosis. p300/CBP activity can be under aberrant control in human disease, particularly in cancer, which may inactivate a p300/CBP tumour-suppressor-like activity. The transcription regulating-properties of p300 and CBP appear to be exerted through multiple mechanisms. They act as protein bridges, thereby connecting different sequence-specific transcription factors to the transcription apparatus. Providing a protein scaffold upon which to build a multicomponent transcriptional regulatory complex is likely to be an important feature of p300/CBP control. Another key property is the presence of histone acetyltransferase (HAT) activity, which endows p300/CBP with the capacity to influence chromatin activity by modulating nucleosomal histones. Other proteins, including the p53 tumour suppressor, are targets for acetylation by p300/CBP. With the current intense level of research activity, p300/CBP will continue to be in the limelight and, we can be confident, yield new and important information on fundamental processes involved in transcriptional control.

A TPR motif cofactor contributes to p300 activity in the p53 response.

The transcription of p53 target genes involves p300/CBP coactivators, which are multiprotein complexes that interact with the p53 activation domain. We report a cofactor in the p300 coactivator complex, Strap, which has an unusual structure, being composed almost entirely of a tandem series of six tetratricopeptide repeat (TPR) motifs. The TPR motif functions as a protein interaction domain, and it is consistent with this property that Strap harbors distinct and dedicated domains that allow it to bind and augment the interaction between different components of the p300 complex. Strap facilitates p53 activity in response to stress, in part through the stress-responsive accumulation of Strap protein and interfering with the MDM2-dependent downregulation of p53.

Acetylation control of the retinoblastoma tumour-suppressor protein.

The retinoblastoma tumour-suppressor protein (pRb) and p300/CBP co-activator proteins are important for control of proliferation and in tumour cells these are sequestered by viral oncoproteins such as E1A. pRb is involved in negatively regulating growth, and p300/CBP proteins have histone acetyltransferase (HAT) activity, which influences gene expression. Although it is known that phosphorylation by G1 cyclin-dependent kinases (CDKs) regulates pRb activity, the nature and role of other post-translational modifications is not understood. Here we identify acetylation as a new type of modification and level of control in pRb function. Adenovirus E1A, which binds p300/CBP through an amino-terminal transformation-sensitive domain, stimulates the acetylation of pRb by recruiting p300 and pRb into a multimeric-protein complex. Furthermore, pRb acetylation is under cell-cycle control, and acetylation hinders the phosphorylation of pRb by cyclin-dependent kinases. pRb binds more strongly when acetylated to the MDM2 oncoprotein, which indicates that acetylation may regulate protein-protein interactions in the pRb pathway. The acetylation of pRb defines a new level of cell-cycle control mediated by HAT. Furthermore, our results establish a relationship between p300, pRb and acetylation in which E1A acts to recruit and target a cellular HAT activity to pRb.

Control of gene expression and the cell cycle.

The pRb tumour suppressor protein is an essential component of the cell-cycle clock, integrating both positive and negative signals for cellular growth and proliferation with the transcription machinery. pRb exerts its tumour suppression function by both antagonizing and synergizing with downstream effectors, such as E2F. pRb has two modes of action, it can inactivate E2F transcription activity or it can assemble an active repression complex with E2F. Apart from E2F, pRb interacts with various factors to promote cellular differentiation. The differentiation properties of pRb are likely to contribute partly to its tumour suppressor function. It is also clear that pRb is a master regulator for transcription. It can both activate and repress transcription in a context-dependent manner. pRb interacts directly with histone acetyltransferase, histone deacetylases and SWI/SNF proteins, all of which are classes of proteins involved in chromatin remodelling. Last, but not least, pRb regulates transcription driven by all three polymerases, thereby integrating the cell-cycle clock with the biosynthetic capacity of the cell in controlling cellular proliferation and growth.

Functional interaction between nucleosome assembly proteins and p300/CREB-binding protein family coactivators.

The p300/CREB-binding protein (CBP) family of proteins consists of coactivators that influence the activity of a wide variety of transcription factors. Although the mechanisms that allow p300/CBP proteins to achieve transcriptional control are not clear, it is believed that the regulation of chromatin is an important aspect of the process. Here, we describe a new level of p300-dependent control mediated through the functional interaction between p300/CBP and members of the family of nucleosome assembly proteins (NAP), which includes NAP1, NAP2, and TAF1. We find that NAP proteins, which have previously been implicated in the regulation of transcription factor binding to chromatin, augment the activity of different p300 targets, including p53 and E2F, through a process that is likely to involve the physical interaction between p300 and NAP. NAP proteins can form oligomers, and the results show that NAP proteins can bind to both core histones and p300 coactivator proteins, perhaps in a multicomponent ternary complex. We also provide data in support of the idea that histones can influence the interaction between p300 and NAP protein. These results argue that NAP is a functionally important component of the p300 coactivator complex and suggest that NAP may serve as a point of integration between transcriptional coactivators and chromatin.

Regulation of E2F transcription by cyclin E-Cdk2 kinase mediated through p300/CBP co-activators.

The E2F proteins form a family of transcription factors that regulate the transition from the G1 to the S phase in the cell cycle. E2F activity is regulated by members of the retinoblastoma protein (pRb) family, ensuring the tight control of E2F-responsive genes. During the G1 phase, phosphorylation of pRb by cyclin-dependent kinases (CDKs), most notably cyclin D-CDK complexes, releases pRb from E2F, facilitating cell-cycle progression by the timely induction of E2F-targeted genes such as cyclin E. However, it is not known whether E2F proteins are directly targeted by CDKs. Here we show that E2F-5 is phosphorylated by the cyclin E-Cdk2 complex, which functions in the late G1 phase, but not by the early-G1-phase-acting cyclin D-CDK complex. A phosphorylation site in the trans-activation domain of E2F-5 stimulates transcription and cell-cycle progression by the recruitment of the p300/CBP family of co-activators, whose binding to E2F-5 is stabilized upon phosphorylation by cyclin E-Cdk2. These results indicate that E2F activity may be directly regulated by cyclin E-Cdk2, and imply an autoregulatory mechanism for cell-cycle-dependent transcription through the CDK-stimulated interaction of E2F with p300/CBP co-activators.

Apoptotic and growth-promoting activity of E2F modulated by MDM2.

E2F integrates and coordinates cell cycle progression with the transcription apparatus through its cyclical interactions with important regulators of cellular proliferation, such as pRb, cyclins, and cdk's. Physiological E2F is a heterodimeric transcription factor composed of an E2F and a DP family member, and while E2F proteins can stimulate proliferation, certain members of the family are known to be endowed with growth-inhibitory and tumor suppressor-like activity. We have investigated the product of the human mdm2 oncogene, hDM2, and report on its ability to regulate E2F-dependent apoptosis in a fashion that is independent of p53. hDM2 can prevent p53(-/-) cells from entering E2F-dependent apoptosis, an outcome that is dependent upon the presence of the DP subunit. Cells rescued from apoptosis possess lower levels of E2F subunits, although the rescued cells show an increase in DNA synthesis and possess enhanced viability that reflects cooperation between E2F-DP and hMD2. Furthermore, the regulation of E2F activity correlates with an hDM2-dependent effect on the intracellular distribution of DP-1, since hDM2 causes the nuclear accumulation of DP-1. The control of E2F by hDM2 therefore has certain parallels with the targeted degradation by MDM2 of p53. However, the domains in hDM2 required for the regulation of E2F activity can be distinguished from those necessary for p53 degradation, suggesting that control of E2F and p53 by hDM2 may be mechanistically distinct. These experiments define a new level of interplay between E2F and hDM2 whereby hDM2 has a profound impact on the physiological consequences of E2F activation. They suggest that the oncogenic properties of hDM2 may in part be mediated by an antiapoptotic activity that converts E2F from a negative to a positive regulator of cell cycle progression and thereby retains E2F at a level that contributes to a continual state of growth stimulation.

A novel cofactor for p300 that regulates the p53 response.

The ability of p53 to function as a transcription factor is instrumental in facilitating the response to cellular stress, and p300/CBP proteins, which act as coactivators for diverse transcription factors, participate in regulating p53 activity. We report a novel cofactor for p300 that facilitates the p53 response by augmenting p53-dependent transcription and apoptosis. JMY and p300 associate in physiological conditions, and, during the cellular stress response, the p300/JMY complex is recruited to activated p53. The bax gene is efficiently activated by JMY, and protein isoforms that arise through alternative splicing alter the functional outcome of the p53 response. The results provide compelling evidence that the p300/JMY coactivator complex plays a central role in facilitating the p53 response.