Cooperation of p53 loss of function and v-Ha-ras in transformation of mouse keratinocyte cell lines

Wip1 phosphatase: between p53 and MAPK kinases pathways

Cells undergoing oncogenic transformation frequently inactivate tumor suppressor pathways that could prevent their uncontrolled growth. Among those pathways p53 and p38MAPK pathways play a critical role in regulation of cell cycle, senescence and cell death in response to activation of oncogenes, stress and DNA damage. Consequently, these two pathways are important in determining the sensitivity of tumor cells to anti-cancer treatment. Wild type p53-induced phosphatase, Wip1, is involved in governance of both pathways. Recently, strategies directed to manipulation with Wip1 activity proposed to advance current day anticancer treatment and novel chemical compounds synthesized to improve specificity of manipulation with Wip1 activity. Here we reviewed the history of Wip1 studies in vitro and in vivo, in genetically modified animal models that support Wip1 role in tumorigenesis through regulation of p53 and p38MAPK pathways. Based on our knowledge we propose several recommendations for future more accurate studies of Wip1 interactions with other pathways involved in tumorigenesis using recently developed tools and for adoption of Wip1 manipulation strategies in anti-cancer therapy.

Frequent p53 and H-ras Mutations in Benzene- and Ethylene Oxide-Induced Mammary Gland Carcinomas from B6C3F1 Mice

Benzene and ethylene oxide are multisite carcinogens in rodents and classified as human carcinogens by the National Toxicology Program. In 2-year mouse studies, both chemicals induced mammary carcinomas. We examined spontaneous, benzene-, and ethylene oxide-induced mouse mammary carcinomas for p53 protein expression, using immunohistochemistry, and p53 (exons 5–8) and H -ras (codon 61) mutations using cycle sequencing techniques. p53 protein expression was detected in 42% (8/19) of spontaneous, 43% (6/14) of benzene-, and 67% (8/12) of ethylene oxide-induced carcinomas. However, semiquantitative evaluation of p53 protein expression revealed that benzene- and ethylene oxide-induced carcinomas exhibited expression levels five- to six-fold higher than spontaneous carcinomas. p53 mutations were found in 58% (7/12) of spontaneous, 57% (8/14) of benzene-, and 67% (8/12) of ethylene oxide-induced carcinomas. H -ras mutations were identified in 26% (5/19) of spontaneous, 50% (7/14) of benzene-, and 33% (4/12) of ethylene oxide-induced carcinomas. When H- ras mutations were present, concurrent p53 mutations were identified in 40% (2/5) of spontaneous, 71% (5/7) of benzene-, and 75% (3/4) of ethylene oxide-induced carcinomas. Our results demonstrate that p53 and H -ras mutations are relatively common in control and chemically induced mouse mammary carcinomas although both chemicals can alter the mutational spectra and more commonly induce concurrent mutations.

Regulation of fibulin-2 gene expression by integrin α3β1 contributes to the invasive phenotype of transformed keratinocytes

The laminin-binding integrin α3β1 is highly expressed in epidermal keratinocytes, where it regulates both cell-autonomous and paracrine functions that promote wound healing and skin tumorigenesis. However, the roles for α3β1 in regulating gene expression programs that control the behaviors of immortalized or transformed keratinocytes remain underexplored. In the current study, we used a microarray approach to identify genes that are regulated by α3β1 in immortalized keratinocytes. α3β1-Responsive genes included several genes that are involved in extracellular matrix proteolysis or remodeling, including fibulin-2 and secreted protein acidic and rich in cysteine. However, α3β1-dependent induction of specific target genes was influenced by the genetic lesion that triggered immortalization, as α3β1-dependent fibulin-2 expression occurred in cells immortalized by either SV40 large T antigen or p53-null mutation, whereas α3β1-dependent expression of secreted protein acidic and rich in cysteine occurred only in the former cells. Interestingly, quantitative PCR arrays did not reveal strong patterns of α3β1-dependent gene expression in freshly isolated primary keratinocytes, suggesting that this regulation is acquired during immortalization. p53-null keratinocytes transformed with oncogenic RasV12 retained α3β1-dependent fibulin-2 expression, and RNAi-mediated knockdown of fibulin-2 in these cells reduced invasion, although not their tumorigenic potential. These findings demonstrate a prominent role for α3β1 in immortalized/transformed keratinocytes in regulating fibulin-2 and other genes that promote matrix remodeling and invasion.

The atypical E2F family member E2F7 couples the p53 and RB pathways during cellular senescence

Oncogene-induced senescence is an anti-proliferative stress response program that acts as a fail-safe mechanism to limit oncogenic transformation and is regulated by the retinoblastoma protein (RB) and p53 tumor suppressor pathways. We identify the atypical E2F family member E2F7 as the only E2F transcription factor potently up-regulated during oncogene-induced senescence, a setting where it acts in response to p53 as a direct transcriptional target. Once induced, E2F7 binds and represses a series of E2F target genes and cooperates with RB to efficiently promote cell cycle arrest and limit oncogenic transformation. Disruption of RB triggers a further increase in E2F7, which induces a second cell cycle checkpoint that prevents unconstrained cell division despite aberrant DNA replication. Mechanistically, E2F7 compensates for the loss of RB in repressing mitotic E2F target genes. Together, our results identify a causal role for E2F7 in cellular senescence and uncover a novel link between the RB and p53 pathways.

The transcriptional regulatory function of p53 is essential for suppression of mouse skin carcinogenesis and can be dissociated from effects on TGF-beta-mediated growth regulation

Transcriptional regulation by p53 is critical for p53-mediated tumour suppression; however, p53-mediated transactivation has been dissociated from p53-mediated biological processes including apoptosis, DNA repair, and differentiation. We compared the effects of a mutant allele, p53(QS - val135), containing a double mutation in the amino-terminus abrogating transactivation activity and a modification at amino acid 135 partially affecting DNA binding, to complete loss of p53. We applied in vitro endpoints correlated with epithelial tumourigenesis and an in vivo assay of tumour phenotype to assess whether loss of p53-mediated transcriptional regulation underlies the malignant phenotype of p53(-/-)/v-ras(Ha)-overexpressing keratinocytes. Transactivation deficiency of p53QS-val135 was confirmed by reporter gene assays in fibroblasts and differentiating keratinocytes. Ras oncogene-induced senescence was lost in both p53(QS - val135/QS - val135) and p53(-/-) keratinocytes. Similarly, p53(QS - val135/QS - val135), like p53(-/-), cooperated with v-ras(Ha) to enhance malignant conversion. The tumours arising in p53(QS - val135/QS - val135) keratinocytes displayed strong nuclear p53 expression; thus, the p53(QS - val135) allele was maintained and the deficient transactivation function of the expressed p53QS mutant protein was supported by absence of p21(waf1) in these tumours. The p53(QS - val135) allele did not confer a dominant-negative phenotype, as p53(+/QS - val135) keratinocytes senesced normally in response to v-ras(Ha) expression and formed benign tumours. While p53(-/-) keratinocytes displayed diminished response to TGF-beta, p53(QS - val135/QS - val135) and p53(+/+) keratinocytes responded equivalently, indicating that the requirement for p53 in maximizing TGF-beta-mediated growth regulation is independent of its transactivation domain and that the ability of keratinocytes to respond to TGF-beta is insufficient to suppress the malignant phenotype in this model. Furthermore, TGF-beta enhances p53QS-induced activation of a dual p53-TGF-beta responsive reporter in a keratinocyte cell line. These findings support an essential role for p53-mediated transcriptional regulation in suppressing malignancies arising from ras-induced skin tumours, consistent with previous findings in spontaneous carcinogenesis in other organs, and highlight the potential importance of senescence for tumour suppression in vivo.

Cyfip1 is a putative invasion suppressor in epithelial cancers

Identification of bona fide tumor suppressors is often challenging because of the large number of genetic alterations present in most human cancers. To evaluate candidate genes present within chromosomal regions recurrently deleted in human cancers, we coupled high-resolution genomic analysis with a two-stage genetic study using RNA interference (RNAi). We found that Cyfip1, a subunit of the WAVE complex, which regulates cytoskeletal dynamics, is commonly deleted in human epithelial cancers. Reduced expression of CYFIP1 is commonly observed during invasion of epithelial tumors and is associated with poor prognosis in this setting. Silencing of Cyfip1 disturbed normal epithelial morphogenesis in vitro and cooperated with oncogenic Ras to produce invasive carcinomas in vivo. Mechanistically, we have linked alterations in WAVE-regulated actin dynamics with impaired cell-cell adhesion and cell-ECM interactions. Thus, we propose Cyfip1 as an invasion suppressor gene.

An immortalization-dependent switch in integrin function up-regulates MMP-9 to enhance tumor cell invasion

Integrins, the major receptors for cell adhesion to the extracellular matrix, play important roles during tumor progression. However, it is still unclear whether genetic lesions that occur during carcinoma development can lead to altered integrin function, and how changes in integrin function contribute to subsequent carcinoma progression. Loss-of-function mutations in p53 and activating mutations in H-Ras, which immortalize and transform epithelial cells, respectively, are common causal events in squamous cell carcinoma (SCC). Phenotypes resulting from these two genetic lesions promote SCC progression and are, therefore, potential targets for anticancer therapies. We developed a model system of keratinocyte transformation that has allowed us to investigate the individual roles of p53 mutation and oncogenic Ras mutation in the acquisition of integrin alpha3beta1-regulated phenotypes that promote SCC progression. Using this model, we show that keratinocyte immortalization by p53-null mutation causes a switch in alpha3beta1 function that induces matrix metalloproteinase (MMP)-9 gene expression in tumorigenic cells. This acquired alpha3beta1-dependent regulation of MMP-9 was maintained during subsequent transformation by oncogenic Ras, and it promoted invasion of tumorigenic keratinocytes. Our results show that loss of p53 function leads to changes in integrin-mediated gene regulation that occur during SCC progression and play a critical role in tumor cell invasion.

Persistent nicotine treatment potentiates amplification of the dihydrofolate reductase gene in rat lung epithelial cells as a consequence of Ras activation

Although nicotine has been suggested to promote lung carcinogenesis, the mechanism of its action in this process remains unknown. The present investigation demonstrates that the treatment of rat lung epithelial cells with nicotine for various periods differentially mobilizes multiple intracellular pathways. Protein kinase C and phosphoinositide 3-OH-kinase are transiently activated after the treatment. Also, Ras and its downstream effector ERK1/2 are activated after long term exposure to nicotine. The activation of Ras by nicotine treatment is responsible for the subsequent perturbation of the methotrexate (MTX)-mediated G1 cell cycle restriction as well as an increase in production of reactive oxygen species. When p53 expression is suppressed by introducing E6, persistent exposure to nicotine enables dihydrofolate reductase gene amplification in the presence of methotrexate (MTX) and the formation of the MTX-resistant colonies. Altering the activity of phosphoinositide 3-OH-kinase has no effect on dihydrofolate reductase amplification. However, the suppression of protein kinase C dramatically affects the colony formation in soft agar. Thus, our data suggest that persistent exposure to nicotine perturbs the G1 checkpoint and causes DNA damage through the increase of the production of reactive oxygen species. However, a third element rendered by loss of p53 is required for the initiation of the process of gene amplification. Under p53-deficient conditions, the establishment of a full oncogenic transformation, in response to long term nicotine exposure, is achieved through the cooperation of multiple signaling pathways.

Conditional transformation of mouse pancreatic epithelial cells: an in vitro model for analysis of genetic events in pancreatocarcinogenesis

Pancreatic ductal adenocarcinomas arise through the accumulation of certain genetic alterations including ras, p16, p53, and DPC4. We found that activation of ras and inactivation of p53 could cooperatively induce in vitro tumorigenicity in conditionally immortalized pancreatic epithelial (IMPE) cells. IMPE cells were established from transgenic mice bearing a temperature-sensitive mutant SV40 Large T (LT) antigen. IMPE cells grew continuously under permissive conditions (33 degrees C with interferon-gamma), but rapidly suffered growth arrest under non-permissive conditions (39 degrees C without interferon-gamma). The cells showed strong expression of E-cadherin and beta-catenin as epithelial markers, and cytokeratin 19, a specific ductal cell marker. Cell proliferation under permissive conditions was associated with down-regulation of p21 expression through inactivation of p53 after overexpression of LT antigen. Intriguingly, the shift from the permissive to non-permissive culture conditions caused G2/M arrest of IMPE cells. Although the cells did not form colonies when cultured in soft agar without activation of ras, cells with ras activation via an adenovirus vector formed colonies under permissive conditions. These findings suggest that activation of ras and inactivation of p53 can cooperatively induce anchorage-independent growth of IMPE cells. This cell line might be useful for studying the processes involved in pancreatocarcinogenesis.

mtCLIC/CLIC4, an organellular chloride channel protein, is increased by DNA damage and participates in the apoptotic response to p53

mtCLIC/CLIC4 (referred to here as mtCLIC) is a p53- and tumor necrosis factor alpha-regulated cytoplasmic and mitochondrial protein that belongs to the CLIC family of intracellular chloride channels. mtCLIC associates with the inner mitochondrial membrane. Dual regulation of mtCLIC by two stress response pathways suggested that this chloride channel protein might contribute to the cellular response to cytotoxic stimuli. DNA damage or overexpression of p53 upregulates mtCLIC and induces apoptosis. Overexpression of mtCLIC by transient transfection reduces mitochondrial membrane potential, releases cytochrome c into the cytoplasm, activates caspases, and induces apoptosis. mtCLIC is additive with Bax in inducing apoptosis without a physical association of the two proteins. Antisense mtCLIC prevents the increase in mtCLIC levels and reduces apoptosis induced by p53 but not apoptosis induced by Bax, suggesting that the two proapoptotic proteins function through independent pathways. Our studies indicate that mtCLIC, like Bax, Noxa, p53AIP1, and PUMA, participates in a stress-induced death pathway converging on mitochondria and should be considered a target for cancer therapy through genetic or pharmacologic approaches.

P53 is necessary for the apoptotic response mediated by a transient increase of Ras activity

The tumor suppressor p53 eliminates cancer-prone cells via multiple mechanisms, including apoptosis. Ras elicits apoptosis in cells after protein kinase C (PKC) downregulation. However, the role of p53 in Ras-mediated apoptosis has not been fully investigated. Here, we demonstrate that mouse fibroblasts that express wild-type p53 are more susceptible to apoptosis elicited by PKC inhibition if Ras is transiently expressed or upregulated as opposed to stably expressed. In the latter case, p53 is frequently mutated. Transiently increased Ras activity induces Bax, and PKC inhibition augments this induction. Overexpression of E6 inactivates p53 and thereby suppresses both Bax induction and apoptosis. In contrast, Bax is not induced in stable ras transfectants, regardless of PKC inhibition. The data suggest that short- and long-term activation of Ras use a different mechanism(s) to initiate apoptosis. The status of p53 may contribute to such differences.

A p21(waf1)-independent pathway for inhibitory phosphorylation of cyclin-dependent kinase p34(cdc2) and concomitant G(2)/M arrest by the chemopreventive flavonoid apigenin

Apigenin, a nonmutagenic flavonoid, has been shown to inhibit ultraviolet light-induced skin tumorigenesis when topically applied to mouse skin. Our previous studies have shown that apigenin treatment of cultured mouse keratinocytes induces G(2)/M arrest accompanied by an increase in p53 protein stability and expression of p21(waf1). In this study, we determined whether the G(2)/M arrest induced by apigenin was dependent upon the presence of the cyclin dependent kinase inhibitor p21(waf1). We exposed WWT.8 (p21(waf1) wild-type) and WKO.16 (p21(waf1) null) mouse keratinocytes to various doses of apigenin for 24 h and observed G(2)/M arrest in both cell lines, thereby establishing that the apigenin-induced G(2)/M arrest was p21(waf1) independent. A 4-h treatment with apigenin induced increases in p53 protein level by sixfold and tenfold in the WWT.8 p21(waf1) wild-type cells and WKO.16 p21(waf1) null cells, respectively. After 24 h in WWT.8 cells, p21(waf1) protein also was induced in a dose-dependent manner, but it was not expressed in WKO.16 keratinocytes. We then measured the effect of apigenin treatment on the mammalian homologue of the yeast cdc2 gene (p34(cdc2)) cyclin-dependent kinase and cyclin B1 (cycB1), because these proteins complex to regulate G(2)/M progression. Apigenin treatment decreased the protein level of p34(cdc2), and p34(cdc2) kinase activity was inhibited in both p21(waf1)(+/+) and p21(waf1)(-/-) cell lines by approximately 40%. The inhibition of p34(cdc2) kinase activity by apigenin treatment correlated with increasing levels of p34(cdc2) phosphorylation at Tyr15, a site in the p34(cdc2) kinase that undergoes inhibitory phosphorylation by Wee1 kinase. Apigenin treatment also had no effect on the protein level or activity of the competing phosphatase, cdc25c, which dephosphorylates p34(cdc2) kinase at Tyr15. Apigenin had little effect on the accumulation of cycB1 protein. These results supported the conclusion that G(2)/M arrest induced by apigenin was accompanied by inhibition of the p34(cdc2) cyclin-dependent kinase protein level and activity in a p21(waf1)-independent manner.

The p53 codon 249 mutant--derived from human functional adrenal tumors--can modify the cell shape of normal adrenocortical transfected cells

Our previous study has indicated that p53 gene mutation occurred in 73% of human functional adrenal tumors, and the mutation hot spots were focused on codons 100, 102 (exon 4), and 249 (exon 7). Furthermore, a transcriptional activity study revealed that the mutant p53 protein derived from human functional adrenal tumors lost 90% transcriptional activity and the ability to bind with the p53 sequence. In order to investigate the influence of the mutant protein extracted from adrenocortical tumors on normal adrenal cells, we first cloned p53 cDNA from the human primary aldosteronism and constituted it with isopropyl thiogalactoside (IPTG) inducible expression vector as recombinant plasmid. The recombinant plasmid was then transfected to normal bovine adrenocortical cells through electroporation. The results showed that the p53 protein mutations at codons 100 and 102 could neither affect the cell morphology nor enable cell growth on the soft agar. In addition, no significant difference was found in cortisol level between the p53 transfected and the control cells. On the other hand, cell morphological changes and cell proliferation rate increase were observed when we used IPTG to induce the expression of the p53 protein, which mutated at codon 249, in adrenocortical cells. The cell morphology changes included less flattened and decreased elongation when compared to non-transfected cells. However, the cortisol level in transfected cells was not affected by the p53 mutants. Taken together, we concluded that the mutant p53 protein indeed participates in adrenal carcinogenesis; however, it has no influence on hormone production and secretion.

Increased sensitivity of transforming growth factor (TGF) beta 1 null cells to alkylating agents reveals a novel link between TGFbeta signaling and O(6)-methylguanine methyltransferase promoter hypermethylation

Inactivation of the transforming growth factor beta (TGFbeta)-signaling pathway and gene silencing through hypermethylation of promoter CpG islands are two frequent alterations in human and experimental cancers. Here we report that nonneoplastic TGFbeta1-/- keratinocyte cell lines exhibit increased sensitivity to cell killing by alkylating agents, and this is due to lack of expression of the DNA repair enzyme O(6)-methylguanine DNA methyltransferase (MGMT). In TGFbeta1-/- but not TGFbeta1+/- cell lines, the CpG dinucleotides in the MGMT promoter are hypermethylated, as measured by restriction enzyme analysis and methylation specific polymerase chain reaction. In one unstable TGFbeta1+/- cell line, loss of the wild type TGFbeta1 allele correlates with the appearance of methylation in the MGMT promoter. Bisulfite sequencing shows that in the KO3 TGFbeta1-/- cell line nearly all of the 28 CpG sites in the MGMT promoter 475 base pairs upstream of the start site of transcription are methylated, whereas most are unmethylated in the H1 TGFbeta1+/- line. Treatment of the TGFbeta1-/- cell lines with 5-azacytidine causes reexpression of MGMT mRNA and demethylation of CpG islands in the promoter. Analysis of the time course of methylation using methylation-specific polymerase chain reaction shows a lack of methylation in primary TGFbeta1-/- keratinocytes and increasing methylation with passage number of immortalized clones. Subcloning of early passage clones reveals a remarkable heterogeneity and instability of the methylation state in the TGFbeta1-/- keratinocytes. Thus, the TGFbeta1-/- genotype does not directly regulate MGMT methylation but predisposes cells to immortalization-associated MGMT hypermethylation.

p53 is phosphorylated at the carboxyl terminus and promotes the differentiation of human HaCaT keratinocytes

The p53 phosphoprotein acts as a tumor-suppressor gene product through the inhibition of cell growth and induction of apoptosis in a transcription-dependent manner. These functions require p53 activation through different biochemical postranslational modifications. Given the relevance of this protein in ultraviolet light-induced carcinogenesis, whose targets are primarily skin keratinocytes, we studied the functions of p53 in epidermal cell differentiation. We selected HaCaT cells, a human keratinocyte cell line bearing point-mutated, transcriptionally inactive, but highly stable p53, which facilitates immunochemical and biochemical analysis. In addition, a reliable in vitro differentiation system has been developed with these cells (Paramio et al. Oncogene 17:949, 1998). We report that during HaCaT differentiation there is a loss of immunoreactivity of p53 against antibodies that specifically recognize epitopes located at the carboxyl terminus of the protein. Because treatment with phosphatase restores this immunoreactivity, we conclude that p53 is phosphorylated at the carboxyl terminus during keratinocyte differentiation. This biochemical modification has been associated with the transcriptional activation of the molecule, and because p53 is involved in differentiation processes in other cell types, we investigated the potential functions of p53 during epidermal differentiation. To this end, we generated HaCaT clones expressing a murine temperature-sensitive p53 (Mp53ts) by transfection because the endogenous p53 is not functional even with phosphorylation. We characterized the expression and effects of the transfected protein in different selected clones. The ultraviolet-light response of these clones was restored, demonstrating the functionality of Mp53ts in these cells. We also observed that, with induction of differentiation, Mp53ts transfected cells differentiate faster than the parental or vector-transfected control cells, demonstrating that p53 promotes epidermal differentiation. The sustained expression of p53 in differentiating cells leads to massive cell death and detachment, a phenomenon that may be similar to epidermal terminal differentiation. In addition, we observed that the expression of p53-dependent genes such as p21waf/cip1 and mdm2 (which are known to participate in epidermal differentiation) increases during HaCaT differentiation, i.e., in a p53-independent manner.

p53 and tumor necrosis factor alpha regulate the expression of a mitochondrial chloride channel protein

A novel chloride intracellular channel (CLIC) gene, clone mc3s5/mtCLIC, has been identified from differential display analysis of differentiating mouse keratinocytes from p53+/+ and p53-/- mice. The 4.2-kilobase pair cDNA contains an open reading frame of 762 base pairs encoding a 253-amino acid protein with two putative transmembrane domains. mc3s5/mtCLIC protein shares extensive homology with a family of intracellular organelle chloride channels but is the first shown to be differentially regulated. mc3s5/mtCLIC mRNA is expressed to the greatest extent in vivo in heart, lung, liver, kidney, and skin, with reduced levels in some organs from p53-/- mice. mc3s5/mtCLIC mRNA and protein are higher in p53+/+ compared with p53-/- basal keratinocytes in culture, and both increase in differentiating keratinocytes independent of genotype. Overexpression of p53 in keratinocytes induces mc3s5/mtCLIC mRNA and protein. Exogenous human recombinant tumor necrosis factor alpha also up-regulates mc3s5/mtCLIC mRNA and protein in keratinocytes. Subcellular fractionation of keratinocytes indicates that both the green fluorescent protein-mc3s5 fusion protein and the endogenous mc3s5/mtCLIC are localized to the cytoplasm and mitochondria. Similarly, mc3s5/mtCLIC was localized to mitochondria and cytoplasmic fractions of rat liver homogenates. Furthermore, mc3s5/mtCLIC colocalized with cytochrome oxidase in keratinocyte mitochondria by immunofluorescence and was also detected in the cytoplasmic compartment. Sucrose gradient-purified mitochondria from rat liver confirmed this mitochondrial localization. This represents the first report of localization of a CLIC type chloride channel in mitochondria and the first indication that expression of an organellular chloride channel can be regulated by p53 and tumor necrosis factor alpha.