Mutant p53 drives invasion by promoting integrin recycling

Correlation of IDH1 mutation with clinicopathologic factors and prognosis in primary glioblastoma: a report of 118 patients from China

It has been reported that IDH1 (IDH1R132) mutation was a frequent genomic alteration in grade II and grade III glial tumors but rare in primary glioblastoma (pGBM). To elucidate the frequency of IDH1 mutation and its clinical significance in Chinese patients with pGBM, one hundred eighteen pGBMs were assessed by pyro-sequencing for IDH1 mutation status, and the results were correlated with clinical characteristics and molecular pathological factors. IDH1 mutations were detected in 19/118 pGBM cases (16.1%). Younger age, methylated MGMT promoter, high expression of mutant P53 protein, low expression of Ki-67 or EGFR protein were significantly correlated with IDH1 mutation status. Most notably, we identified pGBM cases with IDH1 mutation were mainly involved in the frontal lobe when compared with those with wild-type IDH1. In addition, Kaplan-Meier survival analysis revealed a highly significant association between IDH1 mutation and a better clinical outcome (p = 0.026 for progression-free survival; p = 0.029 for overall survival). However, in our further multivariable regression analysis, the independent prognostic effect of IDH1 mutation is limited when considering age, preoperative KPS score, extent of resection, TMZ chemotherapy, and Ki-67 protein expression levels, which might narrow its prognostic power in Chinese population in the future.

Rab25 and CLIC3 collaborate to promote integrin recycling from late endosomes/lysosomes and drive cancer progression

Here we show that Rab25 permits the sorting of ligand-occupied, active-conformation α5β1 integrin to late endosomes/lysosomes. Photoactivation and biochemical approaches show that lysosomally targeted integrins are not degraded but are retrogradely transported and recycled to the plasma membrane at the back of invading cells. This requires CLIC3, a protein upregulated in Rab25-expressing cells and tumors, which colocalizes with active α5β1 in late endosomes/lysosomes. CLIC3 is necessary for release of the cell rear during migration on 3D matrices and is required for invasion and maintenance of active Src signaling in organotypic microenvironments. CLIC3 expression predicts lymph node metastasis and poor prognosis in operable cases of pancreatic ductal adenocarcinoma (PDAC). The identification of CLIC3 as a regulator of a recycling pathway and as an independent prognostic indicator in PDAC highlights the importance of active integrin trafficking as a potential drive to cancer progression in vivo.

Redox signaling at invasive microdomains in cancer cells

Redox signaling contributes to the regulation of cancer cell proliferation, survival, and invasion and participates in the adaptation of cancer cells to their microenvironment. NADPH oxidases are important mediators of redox signaling in normal and cancer cells. Redox signal specificity in normal cells is in part achieved by targeting enzymes that generate reactive oxygen species to specific subcellular microdomains such as focal adhesions, dorsal ruffles, lipid rafts, or caveolae. In a similar fashion, redox signal specificity during cancer cell invasion can be regulated by targeting reactive oxygen generation to invasive microdomains such as invadopodia. Here we summarize recent advances in the understanding of the redox signaling processes that control the cancer cell proinvasive program by modulating cell adhesion, migration, and proteolysis as well as the interaction of cancer cells with the tumor microenvironment. We focus on redox signaling events mediated by invadopodia NADPH oxidase complexes and their contribution to cancer cell invasion.

Activation of p53-p21 is closely associated with the acquisition of resistance to apoptosis caused by β1-integrin silencing in head and neck cancer cells

The issue of whether aberrant expression of β1-integrin is associated with cancer progression and development of resistance to cytotoxic therapy is of considerable interest. Studies to date have shown that the anchorage-independent survival of cancer is attributed, in part, to epithelial-to-mesenchymal transition (EMT). Here, we have reported a novel alternative mechanism of anchorage-independent survival of cancer cells. Cell lines derived from head and neck cancer patients (AMC-HN-3 and AMC-HN-9) and the well-known EMT cancer cell line, MDA-MB231, were examined. The EMT features of AMC-HN-9 cells were comparable to those of MDA-MB231, whereas AMC-HN-3 cells showed no EMT characteristics. Although the pattern and degree of β1-integrin expression were similar in all three cell lines, sensitivities of the cells to β1-integrin knockdown with small interfering RNA (siRNA) were different. Cancer cells with no EMT features underwent cell death to a more significant extent following β1-integrin silencing than those with EMT. Intriguingly, we observed reactive activation of the p53-p21 pathway after β1-integrin silencing in AMC-HN-9 cells lacking an apparent cell death response. Simultaneous knockdown of wild-type p53 and β1-integrin in this cell line promoted cell death. Our data collectively indicate that β1-integrin-related cell death is closely associated with EMT phenotypes and activation of the p53-p21 pathway is partly involved in the acquisition of resistance to apoptosis induced by β1-integrin silencing. Further clarification of the mechanisms underlying p53 integration with β1-integrin signaling may facilitate the development of novel anti-cancer strategies.

Microarray analysis reveals increased expression of ΔNp63α in seborrhoeic keratosis

BACKGROUND

Seborrhoeic keratoses (SKs) are very common benign epidermal lesions without malignant potential. Ultraviolet radiation, old age and viruses are well-known risk factors for disease development. However, the pathomechanisms of SK are not fully understood.

OBJECTIVES

To detect and characterize the genes that are involved in the pathogenesis of SK.

METHODS

We performed a gene expression study using paired lesional and nonlesional skin samples from patients with SK.

RESULTS

We identified and validated 19 differentially expressed genes in SK. Of these 19 genes, we focused on p63 transcription factor, which plays a pivotal role in epidermal development by regulating its transcriptional programme. We found by immunofluorescence that the expression of ΔNp63α, the most abundantly expressed p63 isoform, was significantly increased in SK as compared with normal skin. Moreover, siRNA-mediated knockdown of ΔNp63 led to the downregulation of 11 genes, including a member of the tensin family TNS4. Chromatin immunoprecipitation assay revealed that TNS4 was a target gene of p63.

CONCLUSIONS

We identified upregulated genes in SK using genome-wide cDNA microarray and elucidated the functional contribution of p63 to the disease transcriptome by gene-silencing assay. Taken together, these data may provide a novel insight into the molecular basis of these benign skin lesions.

ERK2 drives tumour cell migration in 3D microenvironments by suppressing expression of Rab17 and Liprin-β2

Upregulation of the extracellular signal-regulated kinase (ERK) pathway has been shown to contribute to tumour invasion and progression. Since the two predominant ERK isoforms (ERK1 and ERK2) are highly homologous and have indistinguishable kinase activities in vitro, both enzymes were believed to be redundant and interchangeable. To challenge this view, here we show that ERK2 silencing inhibits invasive migration of MDA-MB-231 cells, and re-expression of ERK2 but not ERK1 restores the normal invasive phenotype. A detailed quantitative analysis of cell movement on 3D matrices indicates that ERK2 knockdown impairs cellular motility by decreasing the migration velocity as well as increasing the time that cells spend not moving. We used gene expression arrays to identify rab17 and liprin-β2 as genes whose expression was increased by knockdown of ERK2 and restored to normal levels following re-expression of ERK2, but not ERK1. Both Rab17 and Liprin-β2 play inhibitory roles in the invasive behaviour of three independent cancer cell lines. Importantly, knockdown of either Rab17 or Liprin-β2 restores invasiveness of ERK2-depleted cells, indicating that ERK2 drives invasion of MDA-MB-231 cells by suppressing expression of these genes.

Mutant p53 gain of function is interwoven into the hallmarks of cancer

Cancer is viewed as being governed by several aberrant biological events defined by Weinberg and Hanahan as 'hallmarks'. In most human cancers the tumour suppressor p53 is mutated, leading to its malfunction and to the acquirement of oncogenic activities, termed 'gain of function'. This commentary links mutant p53 activities to the hallmarks of cancer, describing its involvement in resistance to apoptosis, genomic instability, aberrant cell cycle, invasion and metastasis, tumour microenvironment, and inflammation. Recent work published in The Journal of Pathology by Acin and colleagues, summarized here, reveals an interesting mechanism by which mutant p53 accelerates mitosis entry. Collectively, the growing body of evidence relating mutant p53 and the hallmarks of cancer reinforces the notion that targeting mutant p53 pathways might be beneficial for anti-cancer therapy.

Cytokinesis failure due to derailed integrin traffic induces aneuploidy and oncogenic transformation in vitro and in vivo

Aneuploidy is frequently detected in solid tumors but the mechanisms regulating the generation of aneuploidy and their relevance in cancer initiation remain under debate and are incompletely characterized. Spatial and temporal regulation of integrin traffic is critical for cell migration and cytokinesis. Impaired integrin endocytosis, because of the loss of Rab21 small GTPase or mutations in the integrin β-subunit cytoplasmic tail, induces failure of cytokinesis in vitro. Here, we describe that repeatedly failed cytokinesis, because of impaired traffic, is sufficient to trigger the generation of aneuploid cells, which display characteristics of oncogenic transformation in vitro and are tumorigenic in vivo. Furthermore, in an in vivo mouse xenograft model, non-transformed cells with impaired integrin traffic formed tumors with a long latency. More detailed investigation of these tumors revealed that the tumor cells were aneuploid. Therefore, abnormal integrin traffic was linked with generation of aneuploidy and cell transformation also in vivo. In human prostate and ovarian cancer samples, downregulation of Rab21 correlates with increased malignancy. Loss-of-function experiments demonstrate that long-term depletion of Rab21 is sufficient to induce chromosome number aberrations in normal human epithelial cells. These data are the first to demonstrate that impaired integrin traffic is sufficient to induce conversion of non-transformed cells to tumorigenic cells in vitro and in vivo.

Gain-of-function mutant p53 upregulates CXC chemokines and enhances cell migration

The role of dominant transforming p53 in carcinogenesis is poorly understood. Our previous data suggested that aberrant p53 proteins can enhance tumorigenesis and metastasis. Here, we examined potential mechanisms through which gain-of-function (GOF) p53 proteins can induce motility. Cells expressing GOF p53 -R175H, -R273H and -D281G showed enhanced migration, which was reversed by RNA interference (RNAi) or transactivation-deficient mutants. In cells with engineered or endogenous p53 mutants, enhanced migration was reduced by downregulation of nuclear factor-kappaB2, a GOF p53 target. We found that GOF p53 proteins upregulate CXC-chemokine expression, the inflammatory mediators that contribute to multiple aspects of tumorigenesis. Elevated expression of CXCL5, CXCL8 and CXCL12 was found in cells expressing oncogenic p53. Transcription was elevated as CXCL5 and CXCL8 promoter activity was higher in cells expressing GOF p53, whereas wild-type p53 repressed promoter activity. Chromatin immunoprecipitation assays revealed enhanced presence of acetylated histone H3 on the CXCL5 promoter in H1299/R273H cells, in agreement with increased transcriptional activity of the promoter, whereas RNAi-mediated repression of CXCL5 inhibited cell migration. Consistent with this, knockdown of the endogenous mutant p53 in lung cancer or melanoma cells reduced CXCL5 expression and cell migration. Furthermore, short hairpin RNA knockdown of mutant p53 in MDA-MB-231 cells reduced expression of a number of key targets, including several chemokines and other inflammatory mediators. Finally, CXCL5 expression was also elevated in lung tumor samples containing GOF p53, indicating relevance to human cancer. The data suggest a mechanistic link between GOF p53 proteins and chemokines in enhanced cell motility.

The Arp2/3 activator WASH regulates α5β1-integrin-mediated invasive migration

The actin cytoskeleton provides scaffolding and physical force to effect fundamental processes such as motility, cytokinesis and vesicle trafficking. The Arp2/3 complex nucleates actin structures and contributes to endocytic vesicle invagination and trafficking away from the plasma membrane. Internalisation and directed recycling of integrins are major driving forces for invasive cell motility and potentially for cancer metastasis. Here, we describe a direct requirement for WASH and Arp2/3-mediated actin polymerisation on the endosomal membrane system for α5β1 integrin recycling. WASH regulates the trafficking of endosomal α5β1 integrin to the plasma membrane and is fundamental for integrin-driven cell morphology changes and integrin-mediated cancer cell invasion. Thus, we implicate WASH and Arp2/3-driven actin nucleation in receptor recycling leading to invasive motility.

Mutations in the p53 Tumor Suppressor Gene: Important Milestones at the Various Steps of Tumorigenesis

Inactivation of the p53 tumor suppressor is a frequent event in tumorigenesis. In most cases, the p53 gene is mutated, giving rise to a stable mutant protein whose accumulation is regarded as a hallmark of cancer cells. Mutant p53 proteins not only lose their tumor suppressive activities but often gain additional oncogenic functions that endow cells with growth and survival advantages. Interestingly, mutations in the p53 gene were shown to occur at different phases of the multistep process of malignant transformation, thus contributing differentially to tumor initiation, promotion, aggressiveness, and metastasis. Here, the authors review the different studies on the involvement of p53 inactivation at various stages of tumorigenesis and highlight the specific contribution of p53 mutations at each phase of cancer progression.

Integrin trafficking and tumor progression

Integrins are major mediators of cancer cell adhesion to extracellular matrix. Through this interaction, integrins play critical roles in cell migration, invasion, metastasis, and resistance to apoptosis during tumor progression. Recent studies highlight the importance of integrin trafficking, endocytosis and recycling, for the functions of integrins in cancer cells. Understanding the molecular mechanisms of integrin trafficking is pivotal for understanding tumor progression and for the development of anticancer drugs.

Mutant p53 disrupts role of ShcA protein in balancing Smad protein-dependent and -independent signaling activity of transforming growth factor-β (TGF-β)

Biomarkers are lacking for identifying the switch of transforming growth factor-β (TGF-β) from tumor-suppressing to tumor-promoting. Mutated p53 (mp53) has been suggested to switch TGF-β to a tumor promoter. However, we found that mp53 does not always promote the oncogenic role of TGF-β. Here, we show that endogenous mp53 knockdown enhanced cell migration and phosphorylation of ERK in DU145 prostate cancer cells. Furthermore, ectopic expression of mp53 in p53-null PC-3 prostate cancer cells enhanced Smad-dependent signaling but inhibited TGF-β-induced cell migration by down-regulating activated ERK. Reactivation of ERK by the expression of its activator, MEK-1, restored TGF-β-induced cell migration. Because TGF-β is known to activate the MAPK/ERK pathway through direct phosphorylation of the adaptor protein ShcA and MAPK/ERK signaling is pivotal to tumor progression, we investigated whether ShcA contributed to mp53-induced ERK inhibition and the conversion of the role of TGF-β during carcinogenesis. We found that mp53 expression led to a decrease of phosphorylated p52ShcA/ERK levels and an increase of phosphorylated Smad levels in a panel of mp53-expressing cancer cell lines and in mammary glands and tumors from mp53 knock-in mice. By manipulating ShcA levels to regulate ERK and Smad signaling in human untransformed and cancer cell lines, we showed that the role of TGF-β in regulating anchorage-dependent and -independent growth and migration can be shifted between growth suppression and migration promotion. Thus, our results for the first time suggest that mp53 disrupts the role of ShcA in balancing the Smad-dependent and -independent signaling activity of TGF-β and that ShcA/ERK signaling is a major pathway regulating the tumor-promoting activity of TGF-β.

A p53/miRNA-34 axis regulates Snail1-dependent cancer cell epithelial-mesenchymal transition

Expression of the essential EMT inducer Snail1 is inhibited by miR-34 through a p53-dependent regulatory pathway.

Snail1 is a zinc finger transcriptional repressor whose pathological expression has been linked to cancer cell epithelial–mesenchymal transition (EMT) programs and the induction of tissue-invasive activity, but pro-oncogenic events capable of regulating Snail1 activity remain largely uncharacterized. Herein, we demonstrate that p53 loss-of-function or mutation promotes cancer cell EMT by de-repressing Snail1 protein expression and activity. In the absence of wild-type p53 function, Snail1-dependent EMT is activated in colon, breast, and lung carcinoma cells as a consequence of a decrease in miRNA-34 levels, which suppress Snail1 activity by binding to highly conserved 3′ untranslated regions in Snail1 itself as well as those of key Snail1 regulatory molecules, including β-catenin, LEF1, and Axin2. Although p53 activity can impact cell cycle regulation, apoptosis, and DNA repair pathways, the EMT and invasion programs initiated by p53 loss of function or mutation are completely dependent on Snail1 expression. These results identify a new link between p53, miR-34, and Snail1 in the regulation of cancer cell EMT programs.

Genomic profiling of glioblastoma: convergence of fundamental biologic tenets and novel insights

With advances in genomic profiling and sequencing technology, we are beginning to understand the landscape of the genetic events that accumulated during the neoplastic process. The insights gleamed from these genomic profiling studies with regards to glioblastoma etiology has been particularly satisfying because it cemented the clinical pertinence of major concepts in cancer biology-concepts developed over the past three decades. This article will review how the glioblastoma genomic data set serves as an illustrative platform for the concepts put forward by Hanahan and Weinberg on the cancer phenotype. The picture emerging suggests that most glioblastomas evolve along a multitude of pathways rather than a single defined pathway. In this context, the article will further provide a discussion of the subtypes of glioblastoma as they relate to key principles of developmental neurobiology.

Mutant p53 drives multinucleation and invasion through a process that is suppressed by ANKRD11

Mutations of p53 in cancer can result in a gain of function associated with tumour progression and metastasis. We show that inducible expression of several p53 'hotspot' mutants promote a range of centrosome abnormalities, including centrosome amplification, increased centrosome size and loss of cohesion, which lead to mitotic defects and multinucleation. These mutant p53-expressing cells also show a change in morphology and enhanced invasive capabilities. Consequently, we sought for a means to specifically target the function of mutant p53 in cancer cells. This study has identified ANKRD11 as a key regulator of the oncogenic potential of mutant p53. Loss of ANKRD11 expression with p53 mutation defines breast cancer patients with poor prognosis. ANKRD11 alleviates the mitotic defects driven by mutant p53 and suppresses mutant p53-mediated mesenchymal-like transformation and invasion. Mechanistically, we show that ANKRD11 restores a native conformation to the mutant p53 protein and causes dissociation of the mutant p53-p63 complex. This represents the first evidence of an endogenous protein with the capacity to suppress the oncogenic properties of mutant p53.

Myc, Aurora Kinase A, and mutant p53(R172H) co-operate in a mouse model of metastatic skin carcinoma

Clinical observations, as well as data obtained from the analysis of genetically engineered mouse models, firmly established the gain-of-function (GOF) properties of certain p53 mutations. However, little is known about the underlying mechanisms. We have used two independent microarray platforms to perform a comprehensive and global analysis of tumors arising in a model of metastatic skin cancer progression, which compares the consequences of a GOF p53(R172H) mutant vs p53 deficiency. DNA profiling revealed a higher level of genomic instability in GOF vs loss-of-function (LOF) p53 squamous cell carcinomas (SCCs). Moreover, GOF p53 SCCs showed preferential amplification of Myc with a corresponding increase in its expression and deregulation of Aurora Kinase A. Fluorescent in situ hybridization confirmed amplification of Myc in primary GOF p53 SCCs and its retention in metastatic tumors. We also identified by RNA profiling distinct gene expression profiles in GOF p53 tumors, which included enriched integrin and Rho signaling, independent of tumor stage. Thus, the progression of GOF p53 papillomas to carcinoma was marked by the acquisition of epithelial-to-mesenchymal transition and metastatic signatures. In contrast, LOF p53 tumors showed enrichment of genes associated with cancer proliferation and chromosomal instability. Collectively, these observations suggest that genomic instability has a prominent role in the early stages of GOF p53 tumor progression (that is, papillomas), whereas it is implicated at a later stage in LOF p53 tumors (that is, SCCs). This model will allow us to identify specific targets in mutant p53 SCCs, which may lead to the development of new therapeutic agents for the treatment of metastatic SCCs.

Gain-of-function mutant p53 but not p53 deletion promotes head and neck cancer progression in response to oncogenic K-ras

Mutations in p53 occur in over 50% of the human head and neck squamous cell carcinomas (SCCHN). The majority of these mutations result in the expression of mutant forms of p53, rather than deletions in the p53 gene. Some p53 mutants are associated with poor prognosis in SCCHN patients. However, the molecular mechanisms that determine the poor outcome of cancers carrying p53 mutations are unknown. Here, we generated a mouse model for SCCHN and found that activation of the endogenous p53 gain-of-function mutation p53$^{\rm{R172H}}$, but not deletion of p53, cooperates with oncogenic K-ras during SCCHN initiation, accelerates oral tumour growth, and promotes progression to carcinoma. Mechanistically, expression profiling of the tumours that developed in these mice and studies using cell lines derived from these tumours determined that mutant p53 induces the expression of genes involved in mitosis, including cyclin B1 and cyclin A, and accelerates entry in mitosis. Additionally, we discovered that this oncogenic function of mutant p53 was dependent on K-ras because the expression of cyclin B1 and cyclin A decreased, and entry in mitosis was delayed, after suppressing K-ras expression in oral tumour cells that express p53$^{\rm{R172H}}$. The presence of double-strand breaks in the tumours suggests that oncogene-dependent DNA damage resulting from K-ras activation promotes the oncogenic function of mutant p53. Accordingly, DNA damage induced by doxorubicin also induced increased expression of cyclin B1 and cyclin A in cells that express p53$^{\rm{R172H}}$. These findings represent strong in vivo evidence for an oncogenic function of endogenous p53 gain-of-function mutations in SCCHN and provide a mechanistic explanation for the genetic interaction between oncogenic K-ras and mutant p53.

β3 integrin-EGF receptor cross-talk activates p190RhoGAP in mouse mammary gland epithelial cells

Active RhoA localizes to plasma membrane, where it stimulates formation of focal adhesions and stress fibers. RhoA activity is inhibited by p190RhoGAP following integrin-mediated cell attachment to allow sampling of new adhesive environments. p190RhoGAP is itself activated by Src-dependent tyrosine phosphorylation, which facilitates complex formation with p120RasGAP. This complex then translocates to the cell surface, where p190RhoGAP down-regulates RhoA. Here we demonstrate that the epidermal growth factor receptor (EGFR) cooperates with β3 integrin to regulate p190RhoGAP activity in mouse mammary gland epithelial cells. Adhesion to fibronectin stimulates tyrosine phosphorylation of the EGFR in the absence of receptor ligands. Use of a dominant inhibitory EGFR mutant demonstrates that fibronectin-activated EGFR recruits p120RasGAP to the cell periphery. Expression of an inactive β3 integrin subunit abolishes p190RhoGAP tyrosine phosphorylation, demonstrating a mechanistic link between β3 integrin-activated Src and EGFR regulation of the RhoA inhibitor. The β3 integrin/EGFR pathway also has a positive role in formation of filopodia. Together our data suggest that EGFR constitutes an important intrinsic migratory cue since fibronectin is a key component of the microenvironment in normal mammary gland development and breast cancer. Our data also suggest that EGFR expressed at high levels has a role in eliciting cell shape changes associated with epithelial-to-mesenchymal transition.

MDM2 promotes SUMO-2/3 modification of p53 to modulate transcriptional activity

The tumor suppressor p53 is extensively regulated by post-translational modification, including modification by the small ubiquitin-related modifier SUMO. We show here that MDM2, previously shown to promote ubiquitin, Nedd8 and SUMO-1 modification of p53, can also enhance conjugation of endogenous SUMO-2/3 to p53. Sumoylation activity requires p53-MDM2 binding but does not depend on an intact RING finger. Both ARF and L11 can promote SUMO-2/3 conjugation of p53. However, unlike the previously described SUMO-1 conjugation of p53 by an MDM2-ARF complex, this activity does not depend on the ability of MDM2 to relocalize to the nucleolus. Interestingly, the SUMO consensus is not conserved in mouse p53, which is therefore not modified by SUMO-2/3. Finally, we show that conjugation of SUMO-2/3 to p53 correlates with a reduction of both activation and repression of a subset of p53-target genes.

Mechanisms by which the extracellular matrix and integrin signaling act to regulate the switch between tumor suppression and tumor promotion

Cell adhesion to the extracellular matrix (ECM) is necessary for development of the mammary gland, and to maintain the normal architecture and function of the gland. Cells adhere to the ECM via the integrin family of trans-membrane receptors, which signal to control mammary-specific gene expression and regulate cell proliferation and survival. During tumor formation, the ECM is extensively remodeled and signaling through integrins is altered such that cells become proliferative and invasive. A key regulator of whether integrin-mediated adhesion will promote tumor suppression or tumor formation is the stiffness of the stromal ECM. The normal mammary gland is typically surrounded by a loose collagenous stroma. An increase in the deposition of collagen and other stromal components is associated with mammographic density, which is one of the greatest risk factors for developing breast carcinoma. Several groups have demonstrated that increased stromal ECM density results in a matrix that is stiffer. Cells sense the stiffness of their surrounding ECM by Rho-mediated contraction of the actin-myosin cytoskeleton. If the surrounding ECM is stiffer than the cell's ability to contract it, then the tensile forces that result are able to drive the clustering of integrins and assemble adhesion signaling complexes. The result is subsequent activation of signaling pathways including FAK, ERK, and PI3K that drive cell proliferation and survival. In contrast, focal complexes are not formed in a compliant matrix, and activation of FAK and pERK is diminished, resulting in control of proliferation. Signaling from FAK moreover regulates p53 and miR-200 members, which control apoptosis and epithelial phenotype, such that a compliant matrix is predicted to promote normal mammary gland architecture and suppress tumor formation.

Human DNAJ in cancer and stem cells

The heat shock protein 40kDa (HSP40/DNAJ) co-chaperones constitute the largest and most diverse sub-group of the heat shock protein (HSP) family. DNAJ are widely accepted as regulators of HSP70 function, but also have roles as co-chaperones for the HSP90 chaperone machine, and a growing number of biological functions that may be independent of either of these chaperones. The DNAJ proteins are differentially expressed in human tissues and demonstrate the capacity to function to both promote and suppress cancer development by acting as chaperones for tumour suppressors or oncoproteins. We review the current literature on the function and expression of DNAJ in cancer, stem cells and cancer stem cells. Combining data from gene expression, proteomics and studies in other systems, we propose that DNAJ will be key regulators of cancer, stem cell and possibly cancer stem cell function. The diversity of DNAJ and their assorted roles in a range of biological functions means that selected DNAJ, provided there is limited redundancy and that a specific link to malignancy can be established, may yet provide an attractive target for specific and selective drug design for the development of anti-cancer treatments.

The new faces of endocytosis in signaling

It is becoming clear that intracellular signaling events are intimately linked with the membrane transport processes. In addition to the long known role of endocytosis in downregulating plasma membrane receptors, more recent data uncover several sophisticated modes by which endocytosis affects the type and duration of signals. Particularly striking are various roles of endocytic compartments as membrane platforms for compartmentalized assembly or sequestration of specific signaling complexes. Here we review some recent examples illustrating how endosomes may mediate ligand-stimulated apoptotic signaling and how multivesicular bodies affect Wnt signaling by regulated sequestration of signaling molecules or their secretion in exosomes. We also discuss evidence documenting the involvement of endocytic proteins in the regulation of p53 activity and stability, which suggests a possible cross-talk between endocytic processes and transcriptional responses.

Wild-type p53 controls cell motility and invasion by dual regulation of MET expression

Recent observations suggest that p53 mutations are responsible not only for growth of primary tumors but also for their dissemination. However, mechanisms involved in p53-mediated control of cell motility and invasion remain poorly understood. By using the primary ovarian surface epithelium cell culture, we show that conditional inactivation of p53 or expression of its mutant forms results in overexpression of MET receptor tyrosine kinase, a crucial regulator of invasive growth. At the same time, cells acquire increased MET-dependent motility and invasion. Wild-type p53 negatively regulates MET expression by two mechanisms: (i) transactivation of MET-targeting miR-34, and (ii) inhibition of SP1 binding to MET promoter. Both mechanisms are not functional in p53 absence, but mutant p53 proteins retain partial MET promoter suppression. Accordingly, MET overexpression, cell motility, and invasion are particularly high in p53-null cells. These results identify MET as a critical effector of p53 and suggest that inhibition of MET may be an effective antimetastatic approach to treat cancers with p53 mutations. These results also show that the extent of advanced cancer traits, such as invasion, may be determined by alterations in individual components of p53/MET regulatory network.

Inhibition of p53 expression by peptide-conjugated phosphorodiamidate morpholino oligomers sensitizes human cancer cells to chemotherapeutic drugs

The p53 tumor suppressor gene encodes a transcription factor that is commonly mutated in cancer. Tumors arise when premalignant cells are unable to undergo p53-dependent apoptosis, cell cycle arrest or DNA repair. The p53-signaling pathway affects not only tumor development, but also the response of tumors to chemotherapeutic drugs. In this study, we use cell penetrating peptide conjugates of phosphorodiamidate morpholino oligomers (PPMOs) to inhibit p53 expression. We examine the functional properties of endogenous p53 isoforms that are produced upon PPMO-mediated inhibition of p53 translation and splicing, and report that loss of N-terminal or C-terminal sequences interferes with the transcriptional activity of p53. Importantly, we report that PPMO-mediated inhibition of p53 expression sensitizes human cancer cells with wild-type p53 to chemotherapeutic drugs.

Competitive binding of Rab21 and p120RasGAP to integrins regulates receptor traffic and migration

P120RasGAP competes with Rab21 for binding to the cytoplasmic domain of integrin α-subunits, thereby promoting receptor escape from early endosomes and recycling to the plasma membrane.

Integrin trafficking from and to the plasma membrane controls many aspects of cell behavior including cell motility, invasion, and cytokinesis. Recruitment of integrin cargo to the endocytic machinery is regulated by the small GTPase Rab21, but the detailed molecular mechanisms underlying integrin cargo recruitment are yet unknown. Here we identify an important role for p120RasGAP (RASA1) in the recycling of endocytosed α/β1-integrin heterodimers to the plasma membrane. Silencing of p120RasGAP attenuated integrin recycling and augmented cell motility. Mechanistically, p120RasGAP interacted with the cytoplasmic domain of integrin α-subunits via its GAP domain and competed with Rab21 for binding to endocytosed integrins. This in turn facilitated exit of the integrin from Rab21- and EEA1-positive endosomes to drive recycling. Our results assign an unexpected role for p120RasGAP in the regulation of integrin traffic in cancer cells and reveal a new concept of competitive binding of Rab GTPases and GAP proteins to receptors as a regulatory mechanism in trafficking.

p63 is a suppressor of tumorigenesis and metastasis interacting with mutant p53

p53 mutations, occurring in two-thirds of all human cancers, confer a gain of function phenotype, including the ability to form metastasis, the determining feature in the prognosis of most human cancer. This effect seems mediated at least partially by its ability to physically interact with p63, thus affecting a cell invasion pathway, and accordingly, p63 is deregulated in human cancers. In addition, p63, as an 'epithelial organizer', directly impinges on epidermal mesenchimal transition, stemness, senescence, cell death and cell cycle arrest, all determinant in cancer, and thus p63 affects chemosensitivity and chemoresistance. This demonstrates an important role for p63 in cancer development and its progression, and the aim of this review is to set this new evidence that links p63 to metastasis within the context of the long conserved other functions of p63.

A Pin1/mutant p53 axis promotes aggressiveness in breast cancer

TP53 missense mutations dramatically influence tumor progression, however, their mechanism of action is still poorly understood. Here we demonstrate the fundamental role of the prolyl isomerase Pin1 in mutant p53 oncogenic functions. Pin1 enhances tumorigenesis in a Li-Fraumeni mouse model and cooperates with mutant p53 in Ras-dependent transformation. In breast cancer cells, Pin1 promotes mutant p53 dependent inhibition of the antimetastatic factor p63 and induction of a mutant p53 transcriptional program to increase aggressiveness. Furthermore, we identified a transcriptional signature associated with poor prognosis in breast cancer and, in a cohort of patients, Pin1 overexpression influenced the prognostic value of p53 mutation. These results define a Pin1/mutant p53 axis that conveys oncogenic signals to promote aggressiveness in human cancers.

FLIM-FRET imaging in vivo reveals 3D-environment spatially regulates RhoGTPase activity during cancer cell invasion

Many conceptual advances in biology have been achieved by experimental studies using planar two-dimensional cell culture systems. Recent adaptations of molecular techniques to three-dimensional model systems are bridging the gap in our understanding of biological events in vitro and in vivo in the study of disease progression. Recently, in vitro studies using Förster resonance energy transfer (FRET) have shown that the prototypical RhoGTPases Cdc42, Rac and RhoA are temporally and spatially synchronized during cell migration, with initial RhoA activity inducing protrusion prior to activation of Rac. This simultaneous FRET approach illustrates the tight control and dynamic regulation of RhoGTPase activity necessary for coordinated cell migration in vitro. Here, we discuss our recent work using FLIM-FRET analysis in a three-dimensional setting to reveal another layer of regulation in which RhoA activity is governed by the extracellular microenvironment. We demonstrate that RhoA is spatially regulated into discrete fractions of activity at the leading edge and rear of cells during invasion in vivo or within three-dimensional matrices. Significantly, this spatial regulation of RhoA was absent in two-dimensional in vitro settings. This distinct sub-cellular regulation of RhoA at the poles of invading cells in three-dimensions sets a precedent that other RhoGTPases or signaling proteins may also be differentially regulated in a con-text-dependent manner during key biological processes such as invasion.

Cooperation between integrins and growth factor receptors in signaling and endocytosis

All multicellular animals express receptors for growth factors (GFs) and extracellular matrix (ECM) molecules. Integrin-type ECM receptors anchor cells to their surroundings and concomitantly activate intracellular signal transduction pathways. The same signaling mechanisms are regulated by GF receptors (GFRs). Recently, intensive research efforts have revealed novel mechanisms describing how the two receptor systems collaborate at many different levels. Integrins can directly bind to GFs and promote their activation. Adhesion receptors also organize signaling platforms and assist GFRs or even activate them via ligand-independent mechanisms. Furthermore, integrins can orchestrate endocytosis and recycling of GFRs. Here, we review the present knowledge about the interplay between integrins and GFRs and discuss recent ideas of how this collaboration may explain some previous controversies in integrin research.

Novel p63 target genes involved in paracrine signaling and keratinocyte differentiation

The transcription factor p63 is required for proper epidermal barrier formation and maintenance. Herein, we used chromatin immunoprecipitation coupled with DNA sequencing to identify novel p63 target genes involved in normal human epidermal keratinocyte (NHEKs) growth and differentiation. We identified over 2000 genomic sites bound by p63, of which 82 were also transcriptionally regulated by p63 in NHEKs. Through the discovery of interleukin-1-α as a p63 target gene, we identified that p63 is a regulator of epithelial–mesenchymal crosstalk. Further, three-dimensional organotypic co-cultures revealed TCF7L1, another novel p63 target gene, as a regulator of epidermal proliferation and differentiation, providing a mechanism by which p63 maintains the proliferative potential of basal epidermal cells. The discovery of new target genes links p63 to diverse signaling pathways required for epidermal development, including regulation of paracrine signaling to proliferative potential. Further mechanistic insight into p63 regulation of epidermal cell growth and differentiation is provided by the identification of a number of novel p63 target genes in this study.

A direct role for Met endocytosis in tumorigenesis

Compartmentalization of signals generated by receptor tyrosine kinase (RTK) endocytosis has emerged as a major determinant of various cell functions. Here, using tumour-associated Met-activating mutations, we demonstrate a direct link between endocytosis and tumorigenicity. Met mutants exhibit increased endocytosis/recycling activity and decreased levels of degradation, leading to accumulation on endosomes, activation of the GTPase Rac1, loss of actin stress fibres and increased levels of cell migration. Blocking endocytosis inhibited mutants' anchorage-independent growth, in vivo tumorigenesis and metastasis while maintaining their activation. One mutant resistant to inhibition by a Met-specific tyrosine kinase inhibitor was sensitive to endocytosis inhibition. Thus, oncogenicity of Met mutants results not only from activation but also from their altered endocytic trafficking, indicating that endosomal signalling may be a crucial mechanism regulating RTK-dependent tumorigenesis.

SAHA shows preferential cytotoxicity in mutant p53 cancer cells by destabilizing mutant p53 through inhibition of the HDAC6-Hsp90 chaperone axis

Mutant p53 (mutp53) cancers are surprisingly dependent on their hyperstable mutp53 protein for survival, identifying mutp53 as a potentially significant clinical target. However, exploration of effective small molecule therapies targeting mutp53 has barely begun. Mutp53 hyperstabilization, a hallmark of p53 mutation, is cancer cell-specific and due to massive upregulation of the HSP90 chaperone machinery during malignant transformation. We recently showed that stable complex formation between HSP90 and its mutp53 client inhibits E3 ligases MDM2 and CHIP, causing mutp53 stabilization. Histone deacetylase (HDAC) inhibitors (HDACi) are a new class of promising anti-cancer drugs, hyperacetylating histone and non-histone targets. Currently, suberoylanilide hydroxamic acid (SAHA) is the only FDA-approved HDACi. We show that SAHA exhibits preferential cytotoxicity for mutant, rather than wild-type and null p53 human cancer cells. Loss/gain-of-function experiments revealed that although able to exert multiple cellular effects, SAHA's cytotoxicity is caused to a significant degree by its ability to strongly destabilize mutp53 at the level of protein degradation. The underlying mechanism is SAHA's inhibition of HDAC6, an essential positive regulator of HSP90. This releases mutp53 and enables its MDM2- and CHIP-mediated degradation. SAHA also strongly chemosensitizes mutp53 cancer cells for chemotherapy due to its ability to degrade mutp53. This identifies a novel action of SAHA with the prospect of SAHA becoming a centerpiece in mutp53-specific anticancer strategies.

Mutant p53 disrupts MCF-10A cell polarity in three-dimensional culture via epithelial-to-mesenchymal transitions

Mutant p53 is not only deficient in tumor suppression but also acquires additional activity, called gain of function. Mutant p53 gain of function is recapitulated in knock-in mice that carry one null allele and one mutant allele of the p53 gene. These knock-in mice develop aggressive tumors compared with p53-null mice. Recently, we and others showed that tumor cells carrying a mutant p53 are addicted to the mutant for cell survival and resistance to DNA damage. To further define mutant p53 gain of function, we used the MCF-10A three-dimensional model of mammary morphogenesis. MCF-10A cells in three-dimensional culture undergo a series of morphological changes and form polarized and growth-arrested spheroids with hollow lumen, which resembles normal glandular architectures in vivo. Here, we found that endogenous wild-type p53 in MCF-10A cells was not required for acinus formation, but knockdown of endogenous wild-type p53 (p53-KD) led to partial clearance of cells in the lumen due to decreased apoptosis. Consistent with this, p53-KD altered expression patterns of the cell adhesion molecule E-cadherin, the cytoskeletal marker β-catenin, and the extracellular matrix protein laminin V. We also found that ectopic expression of the mutant G245S led to a phenotype similar to p53-KD, whereas a combination of ectopic expression of siRNA-resistant G245S with p53-KD led to a less cleared lumen. In contrast, ectopic expression of mutant R248W, R175H, and R273H disrupted normal acinus architectures with filled lumen and led to formation of irregular and multiacinus structures regardless of p53-KD. In addition, these mutants altered normal expression patterns and/or levels of E-cadherin, β-catenin, laminin V, and tight junction marker ZO-1. Furthermore, epithelial-to-mesenchymal transitions (EMT) markers, Snail, Slug, and Twist, were highly induced by mutant p53 and/or p53-KD. Together, we postulate that EMT represents a mutant p53 gain of function and mutant p53 alters cell polarity via EMT.

Escape from p21-mediated oncogene-induced senescence leads to cell dedifferentiation and dependence on anti-apoptotic Bcl-xL and MCL1 proteins

Oncogene-induced senescence (OIS) is a tumor suppressor response that induces permanent cell cycle arrest in response to oncogenic signaling. Through the combined activation of the p53-p21 and p16-Rb suppressor pathways, OIS leads to the transcriptional repression of proliferative genes. Although this protective mechanism has been essentially described in primary cells, we surprisingly observed in this study that the OIS program is conserved in established colorectal cell lines. In response to the RAS oncogene and despite the inactivation of p53 and p16(INK4), HT29 cells enter senescence, up-regulate p21(WAF1), and induce senescence-associated heterochromatin foci formation. The same effect was observed in response to B-RAF(v600E) in LS174T cells. We also observed that p21(WAF1) prevents the expression of the CDC25A and PLK1 genes to induce cell cycle arrest. Using ChIP and luciferase experiments, we have observed that p21(WAF1) binds to the PLK1 promoter to induce its down-regulation during OIS induction. Following 4-5 weeks, several clones were able to resume proliferation and escape this tumor suppressor pathway. Tumor progression was associated with p21(WAF1) down-regulation and CDC25A and PLK1 reexpression. In addition, OIS and p21(WAF1) escape was associated with an increase in DNA damage, an induction of the epithelial-mesenchymal transition program, and an increase in the proportion of cells expressing the CD24(low)/CD44(high) phenotype. Results also indicate that malignant cells having escaped OIS rely on survival pathways induced by Bcl-xL/MCL1 signaling. In light of these observations, it appears that the transcriptional functions of p21(WAF1) are active during OIS and that the inactivation of this protein is associated with cell dedifferentiation and enhanced survival.

p63, a story of mice and men

The transcription factor p63 is essential for the formation of the epidermis and other stratifying epithelia. This is clearly demonstrated by the severe abnormality of p63-deficient mice and by the development of certain types of ectodermal dysplasias in humans as a result of p63 mutations. Investigation of the in vivo functions of p63 is complicated by the occurrence of 10 different splicing isoforms and by its interaction with the other family members, p53 and p73. In vitro and in vivo models have been used to unravel the functions of p63 and its different isoforms, but the results or their interpretation are often contradictory. This review focuses on what mammalian in vivo models and patient studies have taught us in the last 10 years.

Cancer early dissemination: cancerous epithelial-mesenchymal transdifferentiation and transforming growth factor β signalling

Contrary to the long believed hypothesis, it is now evident that breast cancer cells can disseminate from the early phases of the oncogenesis; and that such early disseminated cells sometimes survive at the sites of dissemination and may outgrow after a long latency of years and decades. For cancer cells to leave their origin, they must at least transiently loosen their adhesion with adjacent epithelial cells and stroma, and become motile while avoiding anoikis. Such processes resemble epithelial-mesenchymal transdifferentiation (EMT), which normally takes place in situations such as embryogenesis and wound healing. Interestingly, the occurrence of an EMT-like process in breast cancer cells has been implicated in the generation of cancer stem-like cells, in which TGFβ1 signalling often plays core roles. Here, I discuss the current knowledge regarding cancerous EMT and its signalling pathways with the aim to consider the possible mechanisms of early dissemination, and also the generation of cancer stem-like cells in mammary tumour.

p53-mediated transcriptional regulation and activation of the actin cytoskeleton regulatory RhoC to LIMK2 signaling pathway promotes cell survival

The central arbiter of cell fate in response to DNA damage is p53, which regulates the expression of genes involved in cell cycle arrest, survival and apoptosis. Although many responses initiated by DNA damage have been characterized, the role of actin cytoskeleton regulators is largely unknown. We now show that RhoC and LIM kinase 2 (LIMK2) are direct p53 target genes induced by genotoxic agents. Although RhoC and LIMK2 have well-established roles in actin cytoskeleton regulation, our results indicate that activation of LIMK2 also has a pro-survival function following DNA damage. LIMK inhibition by siRNA-mediated knockdown or selective pharmacological blockade sensitized cells to radio- or chemotherapy, such that treatments that were sub-lethal when administered singly resulted in cell death when combined with LIMK inhibition. Our findings suggest that combining LIMK inhibitors with genotoxic therapies could be more efficacious than single-agent administration, and highlight a novel connection between actin cytoskeleton regulators and DNA damage-induced cell survival mechanisms.

Mutant p53 protein is targeted by arsenic for degradation and plays a role in arsenic-mediated growth suppression

p53 is frequently mutated in tumor cells, and mutant p53 is often highly expressed due to its increased half-life. Thus, targeting mutant p53 for degradation might be explored as a therapeutic strategy to manage tumors that are addicted to mutant p53 for survival. Arsenic trioxide, a drug for patients with acute promyelocytic leukemia, is found to target and degrade a class of proteins with high levels of cysteine residues and vicinal thiol groups, such as promyelocytic leukemia protein (PML) and PML-retinoic acid receptor α fusion protein. Interestingly, wild type p53 is accumulated in cells treated with arsenic compounds, presumably due to arsenic-induced oxidative stresses. In this study, we found that wild type p53 is induced by arsenic trioxide in tumor cells, consistent with published studies. In contrast, we found that arsenic compounds degrade both endogenous and ectopically expressed mutant p53 in time- and dose-dependent manners. We also found that arsenic trioxide decreases the stability of mutant p53 protein through a proteasomal pathway, and blockage of mutant p53 nuclear export can alleviate the arsenic-induced mutant p53 degradation. Furthermore, we found that knockdown of endogenous mutant p53 sensitizes, whereas ectopic expression of mutant p53 desensitizes, tumor cells to arsenic treatment. Taken together, we found that mutant p53 is a target of arsenic compounds, which provides an insight into exploring arsenic compound-based therapy for tumors harboring a mutant p53.

BRAF(V600E) and microenvironment in thyroid cancer: a functional link to drive cancer progression

Papillary thyroid cancer (PTC) rates continue to increase in the United States and Europe, and, although most patients do well, some recur and die of their disease. Patients with PTC harboring the BRAF(V600E) mutation seem to display a more aggressive clinical behavior, but little is known about the role of this mutation in crucial processes in the tumor microenvironment, such as tumor adhesion, migration, invasion, and metastasis. The extracellular matrix (ECM) microenvironment is not merely a structural scaffold for the cellular elements of the epithelial and stromal microenvironment, but it also elicits a profound influence on cell behavior affecting viability, proliferation, adhesion, and motility. The effects of BRAF(V600E) on cell surface receptors (i.e., integrins) and ECM noncellular components [i.e., thrombospondin-1 (TSP-1) and fibronectin (FN)] seem to trigger different pathologic biological processes in a cell context-dependent manner. This review focuses on the recent progress in understanding the role of BRAF(V600E) in the regulation of some ECM noncellular components and trans-membrane receptors of the microenvironment in PTC in order to design novel targeted therapies directed at the BRAF(V600E) multifaceted signaling cascades. Some of these targeted therapeutics, such as ATP-competitive BRAF(V600E) inhibitors (i.e., orally bioavailable PLX4720 and PLX4032 compounds), are already under investigation.

Syndecan-1 and -4 differentially regulate oncogenic K-ras dependent cell invasion into collagen through α2β1 integrin and MT1-MMP

Syndecans function as co-receptors for integrins on different matrixes. Recently, syndecan-1 has been shown to be important for α2β1 integrin-mediated adhesion to collagen in tumor cells by regulating cell adhesion and migration on two-dimensional collagen. However, the function of syndecans in supporting α2β1 integrin interactions with three-dimensional (3D) collagen is less well studied. Using loss-of-function and overexpression experiments we show that in 3D collagen syndecan-4 supports α2β1-mediated collagen matrix contraction. Cell invasion through type I collagen containing 3D extracellular matrix (ECM) is driven by α2β1 integrin and membrane type-1 matrix metalloproteinase (MT1-MMP). Here we show that mutational activation of K-ras correlates with increased expression of α2β1 integrin, MT1-MMP, syndecan-1, and syndecan-4. While K-ras-induced α2β1 integrin and MT1-MMP are positive regulators of invasion, silencing and overexpression of syndecans demonstrate that these proteins inhibit cell invasion into collagen. Taken together, these data demonstrate the existence of a complex interplay between integrin α2β1, MT1-MMP, and syndecans in the invasion of K-ras mutant cells in 3D collagen that may represent a mechanism by which tumor cells become more invasive and metastatic.

p53 and its mutants in tumor cell migration and invasion

In about half of all human cancers, the tumor suppressor p53 protein is either lost or mutated, frequently resulting in the expression of a transcriptionally inactive mutant p53 protein. Loss of p53 function is well known to influence cell cycle checkpoint controls and apoptosis. But it is now clear that p53 regulates other key stages of metastatic progression, such as cell migration and invasion. Moreover, recent data suggests that expression of mutant p53 is not the equivalent of p53 loss, and that mutant p53s can acquire new functions to drive cell migration, invasion, and metastasis, in part by interfering with p63 function.

Ras mutation cooperates with β-catenin activation to drive bladder tumourigenesis

Mutations in the Ras family of proteins (predominantly in H-Ras) occur in approximately 40% of urothelial cell carcinoma (UCC). However, relatively little is known about subsequent mutations/pathway alterations that allow tumour progression. Indeed, expressing mutant H-Ras within the mouse bladder does not lead to tumour formation, unless this is expressed at high levels. The Wnt signalling pathway is deregulated in approximately 25% of UCC, so we examined if this correlated with the activation of MAPK signalling in human UCC and found a significant correlation. To test the functional significance of this association we examined the impact of combining Ras mutation (H-Ras(Q61L) or K-Ras(G12D)) with an activating β-catenin mutation within the mouse bladder using Cre-LoxP technology. Although alone, neither Ras mutation nor β-catenin activation led to UCC (within 12 months), mice carrying both mutations rapidly developed UCC. Mechanistically this was associated with reduced levels of p21 with dependence on the MAPK signalling pathway. Moreover, tumours from these mice were sensitive to MEK inhibition. Importantly, in human UCC there was a negative correlation between levels of p-ERK and p21 suggesting that p21 accumulation may block tumour progression following Ras mutation. Taken together these data definitively show Ras pathway activation strongly cooperates with Wnt signalling to drive UCC in vivo.

CKIα ablation highlights a critical role for p53 in invasiveness control

The mature gut renews continuously and rapidly throughout adult life, often in a damage-inflicting micro-environment. The major driving force for self-renewal of the intestinal epithelium is the Wnt-mediated signalling pathway, and Wnt signalling is frequently hyperactivated in colorectal cancer. Here we show that casein kinase Iα (CKIα), a component of the β-catenin-destruction complex, is a critical regulator of the Wnt signalling pathway. Inducing the ablation of Csnk1a1 (the gene encoding CKIα) in the gut triggers massive Wnt activation, surprisingly without causing tumorigenesis. CKIα-deficient epithelium shows many of the features of human colorectal tumours in addition to Wnt activation, in particular the induction of the DNA damage response and cellular senescence, both of which are thought to provide a barrier against malignant transformation. The epithelial DNA damage response in mice is accompanied by substantial activation of p53, suggesting that the p53 pathway may counteract the pro-tumorigenic effects of Wnt hyperactivation. Notably, the transition from benign adenomas to invasive colorectal cancer in humans is typically linked to p53 inactivation, underscoring the importance of p53 as a safeguard against malignant progression; however, the mechanism of p53-mediated tumour suppression is unknown. We show that the maintenance of intestinal homeostasis in CKIα-deficient gut requires p53-mediated growth control, because the combined ablation of Csnk1a1 and either p53 or its target gene p21 (also known as Waf1, Cip1, Sdi1 and Cdkn1a) triggered high-grade dysplasia with extensive proliferation. Unexpectedly, these ablations also induced non-proliferating cells to invade the villous lamina propria rapidly, producing invasive carcinomas throughout the small bowel. Furthermore, in p53-deficient gut, loss of heterozygosity of the gene encoding CKIα caused a highly invasive carcinoma, indicating that CKIα functions as a tumour suppressor when p53 is inactivated. We identified a set of genes (the p53-suppressed invasiveness signature, PSIS) that is activated by the loss of both p53 and CKIα and which probably accounts for the brisk induction of invasiveness. PSIS transcription and tumour invasion were suppressed by p21, independently of cell cycle control. Restraining tissue invasion through suppressing PSIS expression is thus a novel tumour-suppressor function of wild-type p53.

p53 regulation of podosome formation and cellular invasion in vascular smooth muscle cells

The p53 transcription factor, discovered in 1979 ( 1;2) , is well known as a potent suppressor of tumor development by inhibiting cell cycle progression, and promoting senescence or apoptosis, when the genome is compromised or under oncogenic stress ( 3) . Accumulating evidence has pointed to an alternative role of p53 in the curtailment of tumor progression and colonization of secondary sites by negatively regulating tumor cell metastasis ( 4;5) . Recently, we have found that p53 suppresses Src-induced formation of podosomes and associated invasive phenotypes in fibroblasts and vascular smooth muscle cells (VSMC) ( 6;7) . In this review, I will focus on some recent studies that have identified p53 as a suppressor of cell migration and invasion in general, and VSMC podosome formation and ECM degradation in particular.