p73 cooperates with Ras in the activation of MAP kinase signaling cascade

A novel TAp73-inhibitory compound counteracts stemness features of glioblastoma stem cells

Glioblastoma (GB) is the most common and fatal type of primary malignant brain tumor for which effective therapeutics are still lacking. GB stem cells, with tumor-initiating and self-renewal capacity, are mostly responsible for GB malignancy, representing a crucial target for therapies. The TP73 gene, which is highly expressed in GB, gives rise to the TAp73 isoform, a pleiotropic protein that regulates neural stem cell biology; however, its role in cancer has been highly controversial. We inactivated TP73 in human GB stem cells and revealed that TAp73 is required for their stemness potential, acting as a regulator of the transcriptional stemness signatures, highlighting TAp73 as a possible therapeutic target. As proof of concept, we identified a novel natural compound with TAp73-inhibitory capacity, which was highly effective against GB stem cells. The treatment reduced GB stem cell-invasion capacity and stem features, at least in part by TAp73 repression. Our data are consistent with a novel paradigm in which hijacking of p73-regulated neurodevelopmental programs, including neural stemness, might sustain tumor progression, pointing out TAp73 as a therapeutic strategy for GB.

The Metabolic Landscape of RAS-Driven Cancers from biology to therapy

Our understanding of how the RAS protein family, and in particular mutant KRAS promote metabolic dysregulation in cancer cells has advanced significantly over the last decade. In this Review, we discuss the metabolic reprogramming mediated by oncogenic RAS in cancer, and elucidating the underlying mechanisms could translate to novel therapeutic opportunities to target metabolic vulnerabilities in RAS-driven cancers.

Serum-dependent and -independent regulation of PARP2

PARP2 belongs to a family of proteins involved in cell differentiation, DNA damage repair, cellular energy expenditure, and chromatin modeling. In addition to these overlapping functions with PARP1, PARP2 participates in spermatogenesis, T-cell maturation, extra-embryonic endoderm formation, adipogenesis, lipid metabolism, and cholesterol homeostasis. Knowledge of the functions of PARP2 is far from complete, and the mechanism(s) by which the gene and protein are regulated are unknown. In this study, we found that two different mechanisms are used in vitro to regulate PARP2 levels. In the presence of serum, PARP2 is degraded through the ubiquitin-proteasome pathway; however, when serum is removed or dialyzed with a 3.5 kDa molecular cut membrane, PARP2 rapidly becomes sodium dodecyl sulphate- and urea-insoluble. Despite the presence of a putative serum response element in the PARP2 gene, transcription is not affected by serum deprivation, and PARP2 levels are restored when serum is replaced. The loss of PARP2 affects cell differentiation and gene expression linked to cholesterol and lipid metabolism. These observations highlight the critical roles that PARP2 plays under different physiological conditions, and reveal that PARP2 is tightly regulated by distinct pathways.

p73 induction by Abrus agglutinin facilitates Snail ubiquitination to inhibit epithelial to mesenchymal transition in oral cancer

BACKGROUND

Epithelial-to-mesenchymal transition (EMT), a key step in oral cancer progression, is associated with invasion, metastasis, and therapy resistance, thus targeting the EMT represents a critical therapeutic strategy for the treatment of oral cancer metastasis. Our previous study showed that Abrus agglutinin (AGG), a plant lectin, induces both intrinsic and extrinsic apoptosis to activate the tumor inhibitory mechanism.

OBJECTIVE

This study aimed to investigate the role of AGG in modulating invasiveness and stemness through EMT inhibition for the development of antineoplastic agents against oral cancer.

METHODS

The EMT- and stemness-related proteins were studied in oral cancer cells using Western blot analysis and fluorescence microscopy. The potential mechanisms of Snail downregulation through p73 activation in FaDu cells were evaluated using Western blot analysis, immunoprecipitation, confocal microscopy, and molecular docking analysis. Immunohistochemical staining of the tumor samples of AGG-treated FaDu-xenografted nude mice was performed.

RESULTS

At the molecular level, AGG-induced p73 suppressed Snail expression, leading to EMT inhibition in FaDu cells. Notably, AGG promoted the translocation of Snail from the nucleus to the cytoplasm in FaDu cells and triggered its degradation through ubiquitination. In this setting, AGG inhibited the interaction between Snail and p73 in FaDu cells, resulting in p73 activation and EMT inhibition. Moreover, in epidermal growth factor (EGF)-stimulated FaDu cells, AGG abolished the upregulation of extracellular signal-regulated kinase (ERK)1/2 that plays a pivotal role in the upregulation of Snail to regulate the EMT phenotypes. In immunohistochemistry analysis, FaDu xenografts from AGG-treated mice showed decreased expression of Snail, SOX2, and vimentin and increased expression of p73 and E-cadherin compared with the control group, confirming EMT inhibition as part of its anticancer efficacy against oral cancer.

CONCLUSION

In summary, AGG stimulates p73 in restricting EGF-induced EMT, invasiveness, and stemness by inhibiting the ERK/Snail pathway to facilitate the development of alternative therapeutics for oral cancer.

p73 regulates ependymal planar cell polarity by modulating actin and microtubule cytoskeleton

Planar cell polarity (PCP) and intercellular junctional complexes establish tissue structure and coordinated behaviors across epithelial sheets. In multiciliated ependymal cells, rotational and translational PCP coordinate cilia beating and direct cerebrospinal fluid circulation. Thus, PCP disruption results in ciliopathies and hydrocephalus. PCP establishment depends on the polarization of cytoskeleton and requires the asymmetric localization of core and global regulatory modules, including membrane proteins like Vangl1/2 or Frizzled. We analyzed the subcellular localization of select proteins that make up these modules in ependymal cells and the effect of Trp73 loss on their localization. We identify a novel function of the Trp73 tumor suppressor gene, the TAp73 isoform in particular, as an essential regulator of PCP through the modulation of actin and microtubule cytoskeleton dynamics, demonstrating that Trp73 is a key player in the organization of ependymal ciliated epithelia. Mechanistically, we show that p73 regulates translational PCP and actin dynamics through TAp73-dependent modulation of non-musclemyosin-II activity. In addition, TAp73 is required for the asymmetric localization of PCP-core and global signaling modules and regulates polarized microtubule dynamics, which in turn set up the rotational PCP. Therefore, TAp73 modulates, directly and/or indirectly, transcriptional programs regulating actin and microtubules dynamics and Golgi organization signaling pathways. These results shed light into the mechanism of ependymal cell planar polarization and reveal p73 as an epithelial architect during development regulating the cellular cytoskeleton.

p73 is required for appropriate BMP-induced mesenchymal-to-epithelial transition during somatic cell reprogramming

The generation of induced pluripotent stem cells (iPSCs) by somatic cell reprogramming holds great potential for modeling human diseases. However, the reprogramming process remains very inefficient and a better understanding of its basic biology is required. The mesenchymal-to-epithelial transition (MET) has been recognized as a crucial step for the successful reprogramming of fibroblasts into iPSCs. It has been reported that the p53 tumor suppressor gene acts as a barrier of this process, while its homolog p63 acts as an enabling factor. In this regard, the information concerning the role of the third homolog, p73, during cell reprogramming is limited. Here, we derive total Trp73 knockout mouse embryonic fibroblasts, with or without Trp53, and examine their reprogramming capacity. We show that p73 is required for effective reprogramming by the Yamanaka factors, even in the absence of p53. Lack of p73 affects the early stages of reprogramming, impairing the MET and resulting in altered maturation and stabilization phases. Accordingly, the obtained p73-deficient iPSCs have a defective epithelial phenotype and alterations in the expression of pluripotency markers. We demonstrate that p73 deficiency impairs the MET, at least in part, by hindering BMP pathway activation. We report that p73 is a positive modulator of the BMP circuit, enhancing its activation by DNp73 repression of the Smad6 promoter. Collectively, these findings provide mechanistic insight into the MET process, proposing p73 as an enhancer of MET during cellular reprogramming.

Retinoic Acid Reduces Stem Cell-Like Features in Pancreatic Cancer Cells

OBJECTIVES

Retinoic acid (RA) has important functions during embryonic development being involved in cell growth and differentiation. Although approved for the treatment of acute promyelocytic leukemia, it is still under investigation for different solid tumors including pancreatic cancer. The objective of this study was to analyze how RA affects pancreatic cancer stem cells and how its combination with chemotherapy could impact cell growth.

METHODS

Using different pancreatic cancer cell lines, we evaluated the effect of RA alone or in combination with chemotherapy regulating cancer stem cells properties and pathways.

RESULTS

Retinoic acid treatment reduces the expression of pancreatic stem cell markers CD24, CD44, CD133, and aldehyde dehydrogenase 1 but not c-Met. Although gemcitabine treatment increases the expression of some of these markers especially CD44 when it is combined with RA, a notable reduction in all of them is observed. Retinoic acid induces a G0/G1 arrest and combined with gemcitabine increases the apoptotic effect produced by chemotherapy probably as a consequence of a regulation of specific stem cell transcription factors.

CONCLUSIONS

Retinoic acid regulates self-renewal capacity of cells in pancreatic tumors and should be further investigated in combination with chemotherapy as therapeutic strategy in pancreatic cancer.

Zoledronic acid inhibits the pentose phosphate pathway through attenuating the Ras-TAp73-G6PD axis in bladder cancer cells

Zoledronic acid (ZA) is the current standard of care for the therapy of patients with bone metastasis or osteoporosis. ZA inhibits the prenylation of small guanosine‑5'-triphosphate (GTP)‑binding proteins, such as Ras, and thus inhibit Ras signaling. The present study demonstrated that ZA inhibited cell proliferation and the pentose phosphate pathway (PPP) in bladder cancer cells. In addition, the expression of glucose‑6‑phosphate dehydrogenase (G6PD, the rate‑limiting enzyme of the PPP) was found to be inhibited by ZA. Furthermore, the stability of TAp73, which activates the expression G6PD was decreased in zoledronic acid treated cells. Decreased levels of Ras‑GTP and phosphorylated‑extracellular signal-regulated kinase 1/2 were also observed following treatment with ZA. This may be due to the fact that activated Ras was reported to stabilize TAp73 inducing its accumulation. The inhibition of Ras activity by PT inhibitor II also significantly reduced the levels of TAp73 and G6PD and the PPP flux. Moreover, knockdown of TAp73, attenuated the PPP flux and eliminated the affection of ZA on the PPP flux. In conclusion, it was proposed that ZA can inhibit stability of TAp73 and attenuate the PPP via blocking Ras signaling in bladder cancer cells.

∆Np73 is capable of inducing apoptosis by co-ordinately activating several BH3-only proteins

Inactivation of p53 is one of the most relevant events in human cancer, since it allows transformed cells to escape their own proliferation control and leave them irresponsive to drugs that aim to damage their DNA. When p53 falls, other members of its family may become targets to attack tumoural cells. p73 has shown capacity to mediate these attacks. However, its N-terminal truncated isoforms have been associated with oncogenesis due to their capacity to act as dominant negatives of p53 and the transactivation (TA) isoforms of p73. We previously found a relationship between the overexpression of N-terminus-truncated p73 isoform (∆Np73) and that of the proapoptotic gene Bcl-2-interacting killer (BIK). In the present report we demonstrate that ∆Np73-α has the capacity to induce apoptosis through the co-ordinated activation of a group of genes harbouring GC-rich elements in their regulatory regions. ∆Np73-α synergizes with specificity protein (Sp1) on these elements but the overall response of these genes probably depends on the additional presence of consensus p53 elements. We explore the domains of ∆Np73-α involved in this transactivation capacity and found divergences with the previously described functions for them. Moreover, we found that the transforming mutation V12 of HRas impairs this transactivation capacity of ∆Np73-α, further supporting the anti-tumoural function of this later. Our data add complexity to the action of p73 on the induction of apoptosis and tumourogenesis, opening new interpretations to the expression profile of p73 isoforms in different human neoplasias.

p73 is required for endothelial cell differentiation, migration and the formation of vascular networks regulating VEGF and TGFβ signaling

Vasculogenesis, the establishment of the vascular plexus and angiogenesis, branching of new vessels from the preexisting vasculature, involves coordinated endothelial differentiation, proliferation and migration. Disturbances in these coordinated processes may accompany diseases such as cancer. We hypothesized that the p53 family member p73, which regulates cell differentiation in several contexts, may be important in vascular development. We demonstrate that p73 deficiency perturbed vascular development in the mouse retina, decreasing vascular branching, density and stability. Furthermore, p73 deficiency could affect non endothelial cells (ECs) resulting in reduced in vivo proangiogenic milieu. Moreover, p73 functional inhibition, as well as p73 deficiency, hindered vessel sprouting, tubulogenesis and the assembly of vascular structures in mouse embryonic stem cell and induced pluripotent stem cell cultures. Therefore, p73 is necessary for EC biology and vasculogenesis and, in particular, that DNp73 regulates EC migration and tube formation capacity by regulation of expression of pro-angiogenic factors such as transforming growth factor-β and vascular endothelial growth factors. DNp73 expression is upregulated in the tumor environment, resulting in enhanced angiogenic potential of B16-F10 melanoma cells. Our results demonstrate, by the first time, that differential p73-isoform regulation is necessary for physiological vasculogenesis and angiogenesis and DNp73 overexpression becomes a positive advantage for tumor progression due to its pro-angiogenic capacity.

Transcription addiction: can we garner the Yin and Yang functions of E2F1 for cancer therapy?

Classically, as a transcription factor family, the E2Fs are known to regulate the expression of various genes whose products are involved in a multitude of biological functions, many of which are deregulated in diseases including cancers. E2F is deregulated and hyperactive in most human cancers with context dependent, dichotomous and contradictory roles in almost all cancers. Cancer cells have an insatiable demand for transcription to ensure that gene products are available to sustain various biological processes that support their rapid growth and survival. In this context, cutting-off hyperactivity of transcription factors that support transcription dependence could be a valuable therapeutic strategy. However, one of the greatest challenges of targeting a transcription factor is the global effects on non-cancerous cells given that they control cellular functions in general. Recently, there is growing realization regarding the possibility to target the oncogenic activation of transcription factors to modulate transcription addiction without affecting the normal activity required for cell functions. In this review, we used E2F1 as a prototype transcription factor to address transcription factor activity in cancer cell functions. We focused on melanoma considering that E2F1 executes critical functions in response to UV, an etiological factor of cutaneous melanoma and lies immediately downstream of the CDKN2A/pRb axis, which is frequently deregulated in melanoma. Further, activation of E2F1 in melanomas can also occur independent of loss of CDKN2A. Given its activated status and the ability to transcriptionally control a plethora of genes involved in regulating melanoma development and progression, we review the current literature on its differential role in controlling signaling pathways involved in melanoma as well as therapeutic resistance, and discuss the practical value of weaning melanoma cells from E2F1-mediated transcription dependence for melanoma management.

SOX2 promotes dedifferentiation and imparts stem cell-like features to pancreatic cancer cells

SOX2 (Sex-determining region Y (SRY)-Box2) has important functions during embryonic development and is involved in cancer stem cell (CSC) maintenance, in which it impairs cell growth and tumorigenicity. However, the function of SOX2 in pancreatic cancer cells is unclear. The objective of this study was to analyze SOX2 expression in human pancreatic tumors and determine the role of SOX2 in pancreatic cancer cells regulating CSC properties. In this report, we show that SOX2 is not expressed in normal pancreatic acinar or ductal cells. However, ectopic expression of SOX2 is observed in 19.3% of human pancreatic tumors. SOX2 knockdown in pancreatic cancer cells results in cell growth inhibition via cell cycle arrest associated with p21(Cip1) and p27(Kip1) induction, whereas SOX2 overexpression promotes S-phase entry and cell proliferation associated with cyclin D3 induction. SOX2 expression is associated with increased levels of the pancreatic CSC markers ALDH1, ESA and CD44. Importantly, we show that SOX2 is enriched in the ESA(+)/CD44(+) CSC population from two different patient samples. Moreover, we show that SOX2 directly binds to the Snail, Slug and Twist promoters, leading to a loss of E-Cadherin and ZO-1 expression. Taken together, our findings show that SOX2 is aberrantly expressed in pancreatic cancer and contributes to cell proliferation and stemness/dedifferentiation through the regulation of a set of genes controlling G1/S transition and epithelial-to-mesenchymal transition (EMT) phenotype, suggesting that targeting SOX2-positive cancer cells could be a promising therapeutic strategy.

Regulatory feedback loop between TP73 and TRIM32

The p73 transcription factor is one of the members of the p53 family of tumor suppressors with unique biological functions in processes like neurogenesis, embryonic development and differentiation. For this reason, p73 activity is tightly regulated by multiple mechanisms, including transcription and post-translational modifications. Here, we identified a novel regulatory loop between TAp73 and the E3 ubiquitin ligase tripartite motif protein 32 (TRIM32). TRIM32, a new direct p73 transcriptional target in the context of neural progenitor cells, is differentially regulated by p73. Although TAp73 binds to the TRIM32 promoter and activates its expression, TAp73-induced TRIM32 expression is efficiently repressed by DNp73. TRIM32 in turn physically interacts with TAp73 and promotes its ubiquitination and degradation, impairing p73-dependent transcriptional activity. This mutual regulation between p73 and TRIM32 constitutes a novel feedback loop, which might have important implications in central nervous system development as well as relevance in oncogenesis, and thus emerges as a possible therapeutic target.

Effect of connective tissue growth factor on protein kinase expression and activity in human corneal fibroblasts

PURPOSE

To investigate signal transduction pathways for connective tissue growth factor (CTGF) in human corneal fibroblasts (HCF).

METHODS

Expression of 75 kinases in cultures of serum-starved (HCF) were investigated using protein kinase screens, and changes in levels of phosphorylation of 31 different phosphoproteins were determined at 0, 5, and 15 minutes after treatment with CTGF. Levels of phosphorylation of three signal transducing phosphoproteins (extracellular regulated kinase 1 [ERK1], extracellular regulated kinase 2 [ERK2] [MAPKs], and signal transducer and activator of transcription 3 [STAT3]) were measured at nine time points after exposure to CTGF using Western immunoblots. Inhibition of Ras, MEK1/2 (MAPKK), and ERK1/2, on CTGF-stimulated fibroblast proliferation and collagen gel contraction was assessed using selective inhibitors farnesylthiosalicylic acid, PD-98059, and SB203580, respectively.

RESULTS

Thirty two of the 75 kinases (43%) evaluated by the kinase screen were detected in extracts of quiescent HCF, suggesting these kinases are available to respond acutely to CTGF exposure. Addition of CTGF increased levels of phosphorylation of five phosphoproteins (ERK1 and 2, MEK1/2 [MAPKK], STAT3, and SAPK/JNK), and decreased levels of phosphorylation of 14 phosphoproteins (including protein kinases B and C) after 5 and 15 minutes. Further analysis of ERK1 and 2 and STAT3 phosphorylation showed rapid increases within 1 minute of CTGF exposure that peaked between 5 and 10 minutes then returned to pretreatment levels by 30 minutes. Treatment of HCF with selective inhibitors of Ras, MEK 1/2, and ERK1/2 individually blocked both CTGF induced cell proliferation, and collagen gel contraction.

CONCLUSIONS

Results from protein kinase screens and selective kinase inhibitors demonstrate Ras/MEK/ERK/STAT3 pathway is required for CTGF signaling in HCF.

Transcription factors Sp1 and p73 control the expression of the proapoptotic protein NOXA in the response of testicular embryonal carcinoma cells to cisplatin

Testicular germ cell tumors (TGCTs) are highly responsive to and curable by cisplatin-based chemotherapy even in advanced stages. We have studied the molecular mechanisms involved in the induction of apoptosis in response to cisplatin, and found that proapoptotic Noxa is transcriptionally up-regulated following cisplatin exposure, even in the absence of p53, in NTERA2 cisplatin-sensitive cells but not in 1411HP-resistant cells. Blockade of Noxa reduced the apoptotic response of embryonal carcinoma (EC) NTERA2 cells to cisplatin. A detailed analysis of the Noxa promoter revealed that p73 and Sp1-like factors, Sp1 and KLF6, played key roles in the transcriptional control of this gene. Overexpression of TAp73 induced Noxa whereas the dominant negative isoform ΔNp73, reduced the levels of Noxa after cisplatin exposure in NTERA2 and 2102EP. Interestingly, down-regulation of Sp1 increased Noxa expression in response to cisplatin. However, blockade of KLF6 decreased cisplatin-induced up-regulation of Noxa in EC cell lines. In addition, tissue microarray analyses of TGCTs revealed that expression of Noxa correlates with good clinical prognosis in patients with embryonal carcinoma. Thus, our data show the transcriptional network that regulates Noxa in EC cells, which is key for their apoptotic response to cisplatin-based chemotherapy, and propose Noxa as a predictive factor of therapeutic response.

The prognostic value of p73 overexpression in colorectal carcinoma: a clinicopathologic, immunohistochemical, and statistical study of 204 patients

INTRODUCTION

The protein p73 is the first identified homolog of the tumor suppressor gene p53, but its function in tumor development has not been established. Indeed, the results regarding the p73 implication in colorectal cancers is still controversial.

AIM

We investigated whether the p73 is implicated in colorectal cancer, whether the p73 expression is related to prognosis and whether the p73 expression is correlated with p21-ras or p53.

MATERIALS AND METHODS

We performed a comparative immunohistochemical analysis of p73, p53, and p21ras proteins in primary colorectal tumor with matched normal mucosa and metastasis from 204 patients with colorectal cancer. We correlated these expressions with clinicopathologic variables and we compared the different profiles between nonmucinous carcinoma and mucinous carcinoma.

RESULTS

In this study, we did not find any correlation between p73 expression, sex, age, site, differentiation and stage. Overexpression of p73 was significantly correlated with infiltrating growth pattern (P<0.0001) and nonmucinous carcinoma (P<0.0001). Furthermore, frequency and intensity of p73 expression were marquedly increased from normal mucosa (26%), to primary tumors (75%) and to metastasis (97%). Furthermore, expression of p73 was also correlated with shorter survival period. The prognostic significance of p73 expression remained, even after adjustment for the clinical and pathologic variables. The p73 expression was positively correlated only with p21ras expression (P<0.0001).

CONCLUSIONS

All these findings prove that p73 expression should be considered as a valuable poor prognostic marker. Our data also suggest that TP73 gene may play a role in colorectal carcinoma development.

TAp73 is one of the genes responsible for the lack of response to chemotherapy depending on B-Raf mutational status

BACKGROUND

Although there have been many studies on the p73 gene, some of its functions still remain unclear. There is little research on the relationship between p73 gene transcription and its protein expression and the response to certain drugs such as oxaliplatin and cetuximab, which are drugs currently used in colorectal cancer.The purpose of this study was to evaluate the impact of TAp73 expression on oxaliplatin and cetuximab-based chemotherapy in colorectal cancer cell lines with different K-Ras and B-Raf mutational status.

METHODS

TAp73 was analyzed in three colorectal tumor cell lines HT-29, SW-480 and Caco-2. mRNA TAp73 was determined using Real time PCR; TAp73 protein by immunoblotting and cell viability was analyzed by the MTT method.

RESULTS

We found that mRNA and TAp73 protein were decreased in cells treated with oxaliplatin (in monotherapy or combined with cetuximab) when B-Raf is mutated. This was statistically significant and was also associated with higher cell viability after the treatment.

CONCLUSIONS

Here, for the first time we report, that there is a signaling loop between B-Raf activation and p73 function.Low expression of TAp73 in colorectal cancer cell lines with mutated B-Raf may be involved in the lack of response to oxaliplatin in monotherapy or combined with cetuximab.

p73 plays a role in erythroid differentiation through GATA1 induction

The TP73 gene gives rise to transactivation domain-p73 isoforms (TAp73) as well as DeltaNp73 variants with a truncated N terminus. Although TAp73alpha and -beta proteins are capable of inducing cell cycle arrest, apoptosis, and differentiation, DeltaNp73 acts in many cell types as a dominant-negative repressor of p53 and TAp73. It has been proposed that p73 is involved in myeloid differentiation, and its altered expression is involved in leukemic degeneration. However, there is little evidence as to which p73 variants (TA or DeltaN) are expressed during differentiation and whether specific p73 isoforms have the capacity to induce, or hinder, this differentiation in leukemia cells. In this study we identify GATA1 as a direct transcriptional target of TAp73alpha. Furthermore, TAp73alpha induces GATA1 activity, and it is required for erythroid differentiation. Additionally, we describe a functional cooperation between TAp73 and DeltaNp73 in the context of erythroid differentiation in human myeloid cells, K562 and UT-7. Moreover, the impaired expression of GATA1 and other erythroid genes in the liver of p73KO embryos, together with the moderated anemia observed in p73KO young mice, suggests a physiological role for TP73 in erythropoiesis.

ERK activation is regulated by E2F1 and is essential for E2F1-induced S phase entry

The E2F family of transcription factors regulates a diverse array of cellular functions including cell cycle progression, cell differentiation and apoptosis. Recent studies indicate that E2F1 influences the activity of signal transduction pathways. We identify here a novel link between E2F1 and the Ras/Raf/MEK/ERK signaling pathway, namely that E2F1 levels affect growth factor-induced ERK phosphorylation. Specifically, downregulating E2F1 inhibits PDGF-induced ERK phosphorylation and ectopic expression of E2F1 sensitizes cells to PDGF. We demonstrate that E2F1 induces ERK activation via a transcriptional mechanism and upregulates the expression of two guanine nucleotide exchange factors, RASGRP1 and RASGEF1B, which promote Ras activation. Furthermore, we show that E2F1-induced ERK activity is essential for E2F1-induced S phase entry. Current literature dictates that the cyclin D/pRB/E2F pathway lies downstream of the mitogenically activated Ras/Raf/MEK/ERK cascade. Our results indicate that the relationship between these signaling modules is not a simple unidirectional linear one and suggests there exists a positive feedback loop that may enhance both ERK signaling and E2F1 activity.

p73 poses a barrier to malignant transformation by limiting anchorage-independent growth

p53 is known to prevent tumour formation by restricting the proliferation of damaged or oncogene-expressing cells. In contrast, how the p53 family member p73 suppresses tumour formation remains elusive. Using a step-wise transformation protocol for human cells, we show that, in premalignant stages, expression of the transactivation-competent p73 isoform TAp73 is triggered in response to pRB pathway alterations. TAp73 expression at this stage of transformation results in increased sensitivity to chemotherapeutic drugs and oxidative stress and inhibits proliferation and survival at high cell density. Importantly, TAp73 triggers a transcriptional programme to prevent anchorage-independent growth, which is considered a crucial hallmark of fully transformed cells. An essential suppressor of anchorage-independent growth is KCNK1, which is directly transactivated by TAp73 and commonly downregulated in glioma, melanoma and ovarian cancer. Oncogenic Ras switches p73 expression from TAp73 to the oncogenic deltaNp73 isoform in a phosphatidyl-inositol 3-kinase-dependent manner. Our results implicate TAp73 as a barrier to anchorage-independent growth and indicate that downregulation of TAp73 is a key transforming activity of oncogenic Ras mutants.