Amanda S. Coutts

p53-cofactor JMY is a multifunctional actin nucleation factor

Many cellular structures are assembled from networks of actin filaments, and the architecture of these networks depends on the mechanism by which the filaments are formed. Several classes of proteins are known to assemble new filaments, including the Arp2/3 complex, which creates branched filament networks, and Spire, which creates unbranched filaments. We find that JMY, a vertebrate protein first identified as a transcriptional co-activator of p53, combines these two nucleating activities by both activating Arp2/3 and assembling filaments directly using a Spire-like mechanism. Increased levels of JMY expression enhance motility, whereas loss of JMY slows cell migration. When slowly migrating HL-60 cells are differentiated into highly motile neutrophil-like cells, JMY moves from the nucleus to the cytoplasm and is concentrated at the leading edge. Thus, JMY represents a new class of multifunctional actin assembly factor whose activity is regulated, at least in part, by sequestration in the nucleus.

Arginine methylation controls growth regulation by E2F‐1

E2F transcription factors are implicated in diverse cellular functions. The founding member, E2F‐1, is endowed with contradictory activities, being able to promote cell‐cycle progression and induce apoptosis. However, the mechanisms that underlie the opposing outcomes of E2F‐1 activation remain largely unknown. We show here that E2F‐1 is directly methylated by PRMT5 (protein arginine methyltransferase 5), and that arginine methylation is responsible for regulating its biochemical and functional properties, which impacts on E2F‐1‐dependent growth control. Thus, depleting PRMT5 causes increased E2F‐1 protein levels, which coincides with decreased growth rate and associated apoptosis. Arginine methylation influences E2F‐1 protein stability, and the enhanced transcription of a variety of downstream target genes reflects increased E2F‐1 DNA‐binding activity. Importantly, E2F‐1 is methylated in tumour cells, and a reduced level of methylation is evident under DNA damage conditions that allow E2F‐1 stabilization and give rise to apoptosis. Significantly, in a subgroup of colorectal cancer, high levels of PRMT5 frequently coincide with low levels of E2F‐1 and reflect a poor clinical outcome. Our results establish that arginine methylation regulates the biological activity of E2F‐1 activity, and raise the possibility that arginine methylation contributes to tumourigenesis by influencing the E2F pathway.

TES is a novel focal adhesion protein with a role in cell spreading

Previously, we identified TES as a novel candidate tumour suppressor gene that mapped to human chromosome 7q31.1. In this report we demonstrate that the TES protein is localised at focal adhesions, actin stress fibres and areas of cell-cell contact. TES has three C-terminal LIM domains that appear to be important for focal adhesion targeting. Additionally, the N-terminal region is important for targeting TES to actin stress fibres. Yeast two-hybrid and biochemical analyses yielded interactions with several focal adhesion and/or cytoskeletal proteins including mena, zyxin and talin. The fact that TES localises to regions of cell adhesion suggests that it functions in events related to cell motility and adhesion. In support of this, we demonstrate that fibroblasts stably overexpressing TES have an increased ability to spread on fibronectin.

A transcription co-factor integrates cell adhesion and motility with the p53 response.

Despite its obvious importance in tumorigenesis, little information is available on the mechanisms that integrate cell motility and adhesion with nuclear events. JMY is a transcription co-factor that regulates the p53 response. In addition, JMY contains a series of WH2 domains that facilitate in vitro actin nucleation. We show here that the ability of JMY to influence cell motility is dependent, in part, on its control of cadherin expression as well as the WH2 domains. In DNA damage conditions JMY undergoes nuclear accumulation, which drives the p53 transcription response but reduces its influence on cell motility. Consequently, the role of JMY in actin nucleation is less in damaged cells, although the WH2 domains remain functional in the nucleus where they impact on p53 activity. Together, these findings demonstrate a pathway that links the cytoskeleton with the p53 response, and further suggest that the ability of JMY to regulate actin and cadherin is instrumental in coordinating cell motility with 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.

Actin nucleators in the nucleus: an emerging theme

Summary 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.

Actin nucleation by WH2 domains at the autophagosome

Autophagy is a catabolic process whereby cytosolic components and organelles are degraded to recycle key cellular materials. It is a constitutive process required for proper tissue homoeostasis but can be rapidly regulated by a variety of stimuli (for example, nutrient starvation and chemotherapeutic agents). JMY is a DNA damage-responsive p53 cofactor and actin nucleator important for cell survival and motility. Here we show that JMY regulates autophagy through its actin nucleation activity. JMY contains an LC3-interacting region, which is necessary to target JMY to the autophagosome where it enhances the autophagy maturation process. In autophagosomes, the integrity of the WH2 domains allows JMY to promote actin nucleation, which is required for efficient autophagosome formation. Thus our results establish a direct role for actin nucleation mediated by WH2 domain proteins that reside at the autophagosome.

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.

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.

Mdm2 widens its repertoire.

The p53 tumour 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 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.