Yosef Buganim

p53‐repressed miRNAs are involved with E2F in a feed‐forward loop promoting proliferation

Normal cell growth is governed by a complicated biological system, featuring multiple levels of control, often deregulated in cancers. The role of microRNAs (miRNAs) in the control of gene expression is now increasingly appreciated, yet their involvement in controlling cell proliferation is still not well understood. Here we investigated the mammalian cell proliferation control network consisting of transcriptional regulators, E2F and p53, their targets and a family of 15 miRNAs. Indicative of their significance, expression of these miRNAs is downregulated in senescent cells and in breast cancers harboring wild‐type p53. These miRNAs are repressed by p53 in an E2F1‐mediated manner. Furthermore, we show that these miRNAs silence antiproliferative genes, which themselves are E2F1 targets. Thus, miRNAs and transcriptional regulators appear to cooperate in the framework of a multi‐gene transcriptional and post‐transcriptional feed‐forward loop. Finally, we show that, similarly to p53 inactivation, overexpression of representative miRNAs promotes proliferation and delays senescence, manifesting the detrimental phenotypic consequence of perturbations in this circuit. Taken together, these findings position miRNAs as novel key players in the mammalian cellular proliferation network.

Transcriptional Programs following Genetic Alterations in p53, INK4A, and H-Ras Genes along Defined Stages of Malignant Transformation

The difficulty to dissect a complex phenotype of established malignant cells to several critical transcriptional programs greatly impedes our understanding of the malignant transformation. The genetic elements required to transform some primary human cells to a tumorigenic state were described in several recent studies. We took the advantage of the global genomic profiling approach and tried to go one step further in the dissection of the transformation network. We sought to identify the genetic signatures and key target genes, which underlie the genetic alterations in p53, Ras, INK4A locus, and telomerase, introduced in a stepwise manner into primary human fibroblasts. Here, we show that these are the minimally required genetic alterations for sarcomagenesis in vivo. A genome-wide expression profiling identified distinct genetic signatures corresponding to the genetic alterations listed above. Most importantly, unique transformation hallmarks, such as differentiation block, aberrant mitotic progression, increased angiogenesis, and invasiveness, were identified and coupled with genetic signatures assigned for the genetic alterations in the p53, INK4A locus, and H-Ras, respectively. Furthermore, a transcriptional program that defines the cellular response to p53 inactivation was an excellent predictor of metastasis development and bad prognosis in breast cancer patients. Deciphering these transformation fingerprints, which are affected by the most common oncogenic mutations, provides considerable insight into regulatory circuits controlling malignant transformation and will hopefully open new avenues for rational therapeutic decisions.

Prostate stromal cells produce CXCL-1, CXCL-2, CXCL-3 and IL-8 in response to epithelia-secreted IL-1

It is well accepted that tumor microenvironment is essential for tumor cells survival, cancer progression and metastasis. However, the mechanisms by which tumor cells interact with their surrounding at early stages of cancer development are largely unidentified. The aim of this study was to identify specific molecules involved in stromal-epithelial interactions that might contribute to early stages of prostate tumor formation. Here, we show that conditioned medium (CM) from immortalized non-transformed prostate epithelial cells stimulated immortalized prostate stromal cells to express cancer-related molecules. CM obtained from epithelial cells triggered stromal cells to express and secrete CXCL-1, CXCL-2, CXCL-3 and interleukin (IL)-8 chemokines. This effect was predominantly mediated by the cytokines of the IL-1 family secreted by the epithelial cells. Thus, prostate epithelial cells induced the secretion of proinflammatory and cancer-promoting chemokines by prostate stromal cells. Such interactions might contribute to prostatic inflammation and progression at early stages of prostate cancer formation.

Modulated expression of WFDC1 during carcinogenesis and cellular senescence

Fibroblasts located adjacent to the tumor [cancer-associated fibroblasts (CAFs)] that constitute a large proportion of the cancer-associated stroma facilitate the transformation process. In this study, we compared the biological behavior of CAFs that were isolated from a prostate tumor to their normal-associated fibroblast (NAF) counterparts. CAFs formed more colonies when seeded at low cell density, exhibited a higher proliferation rate and were less prone to contact inhibition. In contrast to the general notion that high levels of α-smooth muscle actin serve as a marker for CAFs, we found that prostate CAFs express it at a lower level compared with prostate NAFs. Microarray analysis revealed a set of 161 genes that were altered in CAFs compared with NAFs. We focused on whey acidic protein four-disulfide core domain 1 (WFDC1), a known secreted protease inhibitor, and found it to be downregulated in the CAFs. WFDC1 expression was also dramatically downregulated in highly prolific mesenchymal cells and in various cancers including fibrosarcomas and in tumors of the lung, bladder and brain. Overexpression of WFDC1 inhibited the growth rate of the fibrosarcoma HT1080 cell line. Furthermore, WFDC1 level was upregulated in senescent fibroblasts. Taken together, our data suggest an important role for WFDC1 in inhibiting proliferation of both tumors and senescent cells. Finally, we suggest that the downregulation of WFDC1 might serve as a biomarker for cellular transformation.

Mutant p53 Attenuates the SMAD-Dependent Transforming Growth Factor β1 (TGF-β1) Signaling Pathway by Repressing the Expression of TGF-β Receptor Type II

ABSTRACT Both transforming growth factor beta (TGF-β) and p53 have been shown to control normal cell growth. Acquired mutations either in the TGF-β signaling pathway or in the p53 protein were shown to induce malignant transformation. Recently, cross talk between wild-type p53 and the TGF-β pathway was observed. The notion that mutant p53 interferes with the wild-type p53-induced pathway and acts by a “gain-of-function” mechanism prompted us to investigate the effect of mutant p53 on the TGF-β-induced pathway. In this study, we show that cells expressing mutant p53 lost their sensitivity to TGF-β1, as observed by less cell migration and a reduction in wound healing. We found that mutant p53 attenuates TGF-β1 signaling. This was exhibited by a reduction in SMAD2/3 phosphorylation and an inhibition of both the formation of SMAD2/SMAD4 complexes and the translocation of SMAD4 to the cell nucleus. Furthermore, we found that mutant p53 attenuates the TGF-β1-induced transcription activity of SMAD2/3 proteins. In searching for the mechanism that underlies this attenuation, we found that mutant p53 reduces the expression of TGF-β receptor type II. These data provide important insights into the molecular mechanisms that underlie mutant p53 “gain of function” pertaining to the TGF-β signaling pathway.

p53 Regulates the Ras Circuit to Inhibit the Expression of a Cancer-Related Gene Signature by Various Molecular Pathways

In this study, we focus on the analysis of a previously identified cancer-related gene signature (CGS) that underlies the cross talk between the p53 tumor suppressor and Ras oncogene. CGS consists of a large number of known Ras downstream target genes that were synergistically upregulated by wild-type p53 loss and oncogenic H-Ras(G12V) expression. Here we show that CGS expression strongly correlates with malignancy. In an attempt to elucidate the molecular mechanisms underling the cooperation between p53 loss and oncogenic H-Ras(G12V), we identified distinguished pathways that may account for the regulation of the expression of the CGS. By knocking-down p53 or by expressing mutant p53, we revealed that p53 exerts its negative effect by at least two mechanisms mediated by its targets B-cell translocation gene 2 (BTG2) and activating transcription factor 3 (ATF3). Whereas BTG2 binds H-Ras(G12V) and represses its activity by reducing its GTP loading state, which in turn causes a reduction in CGS expression, ATF3 binds directly to the CGS promoters following p53 stabilization and represses their expression. This study further elucidates the molecular loop between p53 and Ras in the transformation process.

Amplification of the 20q Chromosomal Arm Occurs Early in Tumorigenic Transformation and May Initiate Cancer

Duplication of chromosomal arm 20q occurs in prostate, cervical, colon, gastric, bladder, melanoma, pancreas and breast cancer, suggesting that 20q amplification may play a causal role in tumorigenesis. According to an alternative view, chromosomal imbalance is mainly a common side effect of cancer progression. To test whether a specific genomic aberration might serve as a cancer initiating event, we established an in vitro system that models the evolutionary process of early stages of prostate tumor formation; normal prostate cells were immortalized by the over-expression of human telomerase catalytic subunit hTERT, and cultured for 650 days till several transformation hallmarks were observed. Gene expression patterns were measured and chromosomal aberrations were monitored by spectral karyotype analysis at different times. Several chromosomal aberrations, in particular duplication of chromosomal arm 20q, occurred early in the process and were fixed in the cell populations, while other aberrations became extinct shortly after their appearance. A wide range of bioinformatic tools, applied to our data and to data from several cancer databases, revealed that spontaneous 20q amplification can promote cancer initiation. Our computational model suggests that 20q amplification induced deregulation of several specific cancer-related pathways including the MAPK pathway, the p53 pathway and Polycomb group factors. In addition, activation of Myc, AML, B-Catenin and the ETS family transcription factors was identified as an important step in cancer development driven by 20q amplification. Finally we identified 13 cancer initiating genes, located on 20q13, which were significantly over-expressed in many tumors, with expression levels correlated with tumor grade and outcome suggesting that these genes induce the malignant process upon 20q amplification.

Various p53 mutant proteins differently regulate the Ras circuit to induce a cancer-related gene signature

Summary Concomitant expression of mutant p53 and oncogenic Ras, leading to cellular transformation, is well documented. However, the mechanisms by which the various mutant p53 categories cooperate with Ras remain largely obscure. From this study we suggest that different mutant p53 categories cooperate with H-Ras in different ways to induce a unique expression pattern of a cancer-related gene signature (CGS). The DNA-contact p53 mutants (p53R248Q and p53R273H) exhibited the highest level of CGS expression by cooperating with NF&kgr;B. Furthermore, the Zn+2 region conformational p53 mutants (p53R175H and p53H179R) induced the CGS by elevating H-Ras activity. This elevation in H-Ras activity stemmed from a perturbed function of the p53 transcription target gene, BTG2. By contrast, the L3 loop region conformational mutant (p53G245S) did not affect CGS expression. Our findings were further corroborated in human tumor-derived cell lines expressing Ras and the aforementioned mutated p53 proteins. These data might assist in future tailor-made therapy targeting the mutant p53–Ras axis in cancer.

Mutant p53 protects cells from 12-O-tetradecanoylphorbol-13-acetate-induced death by attenuating activating transcription factor 3 induction.

Mutations in p53 are ubiquitous in human tumors. Some p53 mutations not only result in loss of wild-type (WT) activity but also grant additional functions, termed gain of function. In this study, we explore how the status of p53 affects the immediate response gene activating transcription factor 3 (ATF3) in the 12-O-tetradecanoylphorbol-13-acetate (TPA)-protein kinase C (PKC) pathway. We show that high doses of TPA induce ATF3 in a WT p53-independent manner correlating with PKCs depletion and cell death. We show that cells harboring mutant p53 have attenuated ATF3 induction and are less sensitive to TPA-induced death compared with their p53-null counterparts. Mutagenesis analysis of the ATF3 promoter identified the regulatory motifs cyclic AMP-responsive element binding protein/ATF and MEF2 as being responsible for the TPA-induced activation of ATF3. Moreover, we show that mutant p53 attenuates ATF3 expression by two complementary mechanisms. It interacts with the ATF3 promoter and influences its activity via the MEF2 site, and additionally, it attenuates transcriptional expression of the ATF3 activator MEF2D. These data provide important insights into the molecular mechanisms that underlie mutant p53 gain of function.

A Novel Translocation Breakpoint within the BPTF Gene Is Associated with a Pre-Malignant Phenotype

Partial gain of chromosome arm 17q is an abundant aberrancy in various cancer types such as lung and prostate cancer with a prominent occurrence and prognostic significance in neuroblastoma – one of the most common embryonic tumors. The specific genetic element/s in 17q responsible for the cancer-promoting effect of these aberrancies is yet to be defined although many genes located in 17q have been proposed to play a role in malignancy. We report here the characterization of a naturally-occurring, non-reciprocal translocation der(X)t(X;17) in human lung embryonal-derived cells following continuous culturing. This aberrancy was strongly correlated with an increased proliferative capacity and with an acquired ability to form colonies in vitro. The breakpoint region was mapped by fluorescence in situ hybridization (FISH) to the 17q24.3 locus. Further characterization by a custom-made comparative genome hybridization array (CGH) localized the breakpoint within the Bromodomain PHD finger Transcription Factor gene (BPTF), a gene involved in transcriptional regulation and chromatin remodeling. Interestingly, this translocation led to elevation in the mRNA levels of the endogenous BPTF. Knock-down of BPTF restricted proliferation suggesting a role for BPTF in promoting cellular growth. Furthermore, the BPTF chromosomal region was found to be amplified in various human tumors, especially in neuroblastomas and lung cancers in which 55% and 27% of the samples showed gain of 17q24.3, respectively. Additionally, 42% percent of the cancer cell lines comprising the NCI-60 had an abnormal BPTF locus copy number. We suggest that deregulation of BPTF resulting from the translocation may confer the cells with the observed cancer-promoting phenotype and that our cellular model can serve to establish causality between 17q aberrations and carcinogenesis.