Loss of p21WAF1/CIP1 accelerates Ras oncogenesis in a transgenic/knockout mammary cancer model

p21WAF1/Cip1 is a negative transcriptional regulator of Wnt4 expression downstream of Notch1 activation

In keratinocytes, the cyclin/CDK inhibitor p21(WAF1/Cip1) is a direct transcriptional target of Notch1 activation; loss of either the p21 or Notch1 genes expands stem cell populations and facilitates tumor development. The Notch1 tumor-suppressor function was associated with down-regulation of Wnt signaling. Here, we show that suppression of Wnt signaling by Notch1 activation is mediated, at least in part, by down-modulation of Wnts gene expression. p21 is a negative regulator of Wnts transcription downstream of Notch1 activation, independently of effects on the cell cycle. More specifically, expression of the Wnt4 gene is under negative control of endogenous p21 both in vitro and in vivo. p21 associates with the E2F-1 transcription factor at the Wnt4 promoter and causes curtailed recruitment of c-Myc and p300, and histone hypoacetylation at this promoter. Thus, p21 acts as a selective negative regulator of transcription and links the Notch and Wnt signaling pathways in keratinocyte growth control.

Cell cycle in mouse development

Mice likely represent the most-studied mammalian organism, except for humans. Genetic engineering in embryonic stem cells has allowed derivation of mouse strains lacking particular cell cycle proteins. Analyses of these mutant mice, and cells derived from them, facilitated the studies of the functions of cell cycle apparatus at the organismal and cellular levels. In this review, we give some background about the cell cycle progression during mouse development. We next discuss some insights about in vivo functions of the cell cycle proteins, gleaned from mouse knockout experiments. Our text is meant to provide examples of the recent experiments, rather than to supply an extensive and complete list.

15-LOX-1 inhibits p21 (Cip/WAF 1) expression by enhancing MEK-ERK 1/2 signaling in colon carcinoma cells

Currently, some controversy exists regarding the precise role of 15-lipoxygenase-1 (15-LOX-1) in colorectal carcinogenesis and other aspects of cancer biology. The aim of this study was to evaluate the effect of 15-LOX-1 on p21 (Cip/WAF 1) expression and growth regulation in human colon carcinoma cells. The effect of 13-S-hydroxyoctadecadienoic acid (HODE), a product of 15-LOX-1, on p21 (Cip/WAF 1) expression was evaluated in Caco-2 cells treated with sodium butyrate (NaBT) and/or nordihydroguaiarectic acid (NDGA), a LOX inhibitor. The effect of transfecting HCT-116 cells with 15-LOX-1 was also examined. NaBT-induced p21 (Cip/WAF 1) expression was enhanced by treatment with NDGA and 13-S-HODE reversed NaBT-induced p21 (Cip/WAF 1) expression in Caco-2 cells. Overexpression of 15-LOX-1 induced extracellular signal-related kinase (ERK) 1/2 phosphorylation, decreased p21 (Cip/WAF 1) expression, and increased HCT-116 cell growth. Treatment with NDGA decreased ERK 1/2 phosphorylation, and increased p21 (Cip/WAF 1) expression in 15-LOX-1 overexpressing HCT-116 cells. Our experimental results support the hypothesis that 15-LOX-1 may have "pro-neoplastic" effects during the development of colorectal cancer.

Cooperative induction of rat mammary cancer by radiation and 1-methyl-1-nitrosourea via the oncogenic pathways involving c-Myc activation and H-ras mutation

Humans are continually exposed to various environmental carcinogens. Cancers may arise as a result of exposure to carcinogenic chemicals, ionizing radiation or a combination thereof. However, the mechanism of combined carcinogenesis has been only deduced from oncogenic actions of individual agents. Here, we analyzed experimental mammary carcinogenesis caused by a combination of radiation and a chemical carcinogen, 1-methyl-1-nitrosourea (MNU). Seven-week-old female Sprague-Dawley rats were divided into 4 groups: control, g gamma-irradiated (2 Gy), MNU-treated (40 mg/kg, i.p.) and combined treatment of radiation with subsequent MNU after 3 days. Rats with palpable tumors were sacrificed at 50 weeks of age to collect tumors for histologic typing and mutational analysis of the H-ras gene codon 12. The combined treatment induced adenocarcinomas, but not fibroadenomas, more efficiently than radiation or MNU alone. The H-ras mutation was not seen in radiation-induced carcinomas and was specific to MNU-induced carcinomas in individually treated groups. In the combined treatment group, H-ras-mutated, but not nonmutated, tumors were more frequent and developed significantly earlier than in the MNU-treated group. Significantly higher numbers of cells were stained for activated c-Myc protein in g gamma-ray- and combined treatment-induced cancers than in MNU-induced cancers. These results indicate that combined exposure to the 2 carcinogens elicits an unexpected cooperativity in which pre-irradiation enhances mammary carcinogenesis predominantly through the oncogenic pathway involving H-ras, possibly by synergism with c-Myc activation.

Different cooperating effect of p21 or p27 deficiency in combination with INK4a/ARF deletion in mice

The control exerted by the INK4a/ARF locus on cellular proliferation is crucial to restrict tumor development. In agreement with this, mice with defects in this locus are highly tumor prone. However, the potential contribution of other pathways in modulating tumorigenesis in the absence of INK4a/ARF is largely unexplored. In the present study, we investigated the consequences of the combined loss of either of two cyclin-dependent kinase inhibitors, p21 and p27, in cooperation with deletion of the INK4a/ARF locus. Our results show a clear differential effect in tumorigenesis depending on the CKI that is absent. The absence of p21 produced no overt alteration of the lifespan of the INK4a/ARF-null mice, although it modified their tumor spectrum, causing a significant increase in the incidence of fibrosarcomas and the appearance of a small number of rhabdomyosarcomas. In contrast, deficiency of p27 resulted in a significant increase in lethality due to accelerated tumor development, especially in the case of T-cell lymphomas. Finally, combined deficiency of INK4a/ARF and p27 resulted in a significant increase in the number of metastatic tumors. These results demonstrate genetically the oncogenic cooperation between defects on INK4a/ARF and p27, which are common alterations in human cancer.

RARgamma acts as a tumor suppressor in mouse keratinocytes

All-trans retinoic acid (RA), the principle biologically active form of vitamin A, is essential for many developmental process as well as homeostasis in the adult. Many lines of evidence also suggest that RA, acting through the RA receptors (RARs), can also suppress growth of tumors of diverse origin. To assess directly the role of the RARs in a model of epidermal tumorigenesis, we investigated the incidence of tumor formation using keratinocytes lacking specific RAR types. Our data suggest that loss of RARgamma, but not RARalpha, predisposed keratinocytes to v-Ha-Ras-induced squamous cell carcinoma. We also found that ablation of RARgamma, but not RARalpha, abolished RA-induced cell cycle arrest and apoptosis in these keratinocytes. Reconstitution of receptor expression into RAR-null cells restored sensitivity to RA, and reversed the tumorigenic potential of receptor-deficient keratinocytes. These data strongly support a tumor suppressor effect for the RARs, in particular endogenous RARgamma, in murine keratinocytes.

EphA4, RhoB and the molecular development of feather buds are maintained by the integrity of the actin cytoskeleton

The development of feather buds is a highly ordered process involving epithelial-mesenchymal signalling. Cellular morphology is determined by the actin cytoskeleton, which is controlled by networks of regulators such as the GTPases. EphA4 belongs to a receptor tyrosine kinase family that has been consistently shown to regulate the cytoskeleton via Rho family GTPases in neural development and is expressed in early stages of feather bud development though its role has not been defined. We therefore used an in vitro skin culture system to interfere with EphA4 levels in feather buds using anti-sense oligonucleotides, demonstrating a severe effect on both their number and morphological form. Analysis of the Rho family of GTPases revealed that this effect was mediated by the GTPase RhoB, the expression of which was altered in response to altered levels of EphA4. In addition, the inhibition of RhoB mimicked the effects of reduced EphA4 levels on feather development. Significantly, manipulation of cytoskeletal dynamics revealed that those cells undergoing morphogenetic change regulate the patterning signals responsible for initiating feather development. We propose that this molecular maintenance mechanism between EphA4-RhoB and the actin cytoskeleton converges or coordinates with other morphogenic signalling systems to control feather bud development.

Effect of HIV-1 Vpr on cell cycle regulators

Cell cycle is one of the most complex processes in the life of a dividing cell. It involves numerous regulatory proteins, which direct the cell through a specific sequence of events for the production of two daughter cells. Cyclin-dependent kinases (cdks), which complex with the cyclin proteins, are the main players in the cell cycle. They can regulate the progression of the cells through different stages regulated by several proteins including p53, p21(WAF1), p19, p16, and cdc25. Downstream targets of cyclin-cdk complexes include pRB and E2F. A cell cycle can be altered to the advantage of many viral agents, most notably polyomaviruses, papillomaviruses, adenoviruses, and retroviruses. In addition, viral protein R (Vpr) is a protein encoded by the human immunodeficiency virus type 1 (HIV-1). HIV-1, the causative agent of acquired immunodeficiency syndrome (AIDS), is a member of the lentivirus class of retroviruses. This accessory protein plays an important role in the regulation of the cell cycle by causing G(2) arrest and affecting cell cycle regulators. Vpr prevents infected cells from proliferating, and collaborates with the matrix protein (MA) to enable HIV-1 to enter the nucleus of nondividing cells. Studies from different labs including ours showed that Vpr affects the functions of cell cycle proteins, including p53 and p21(WAF1). Thus, the replication of HIV-1, and ultimately its pathogenesis, are intrinsically tied to cell-cycle control.

p27(Kip1) stabilization and G(1) arrest by 1,25-dihydroxyvitamin D(3) in ovarian cancer cells mediated through down-regulation of cyclin E/cyclin-dependent kinase 2 and Skp1-Cullin-F-box protein/Skp2 ubiquitin ligase

p27(Kip1) (p27) is a tumor suppressor whose stability is controlled by proteasome-mediated degradation, a process directed in part by cyclin-dependent kinase 2 (CDK2)-mediated phosphorylation of p27 at Thr(187) and its subsequent interaction with the Skp1-Cullin-F-box protein/Skp2 (Skp2) ubiquitin ligase. The present study shows that 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) arrests ovarian cancer cells in G(1) by stabilizing the p27 protein. 1,25(OH)(2)D(3) initiates a chain of events by decreasing the amounts of cyclin E and cyclin E-associated CDK2 activity. As a result, p27 phosphorylation at Thr(187) and consequently the interaction with Skp2 are decreased. 1,25(OH)(2)D(3) also increases p27 stability by decreasing the abundance of Skp2. It is the combined effect of 1,25(OH)(2)D(3) on both the CDK2-dependent phosphorylation of p27, and thus its affinity for Skp2, and Skp2 expression that dramatically increases the stability of the p27 protein. Similar to its effects in ovarian cancer cells, 1,25(OH)(2)D(3) induces p27 accumulation in wild type mouse embryo fibroblasts and arrests wild type but not p27-null mouse embryo fibroblasts in G(1). Stable expression of Skp2 in OVCAR3 cells diminishes the G(1) arrest and decreases the growth response to 1,25(OH)(2)D(3). Taken together, the results of this study identify p27 as the key mediator of 1,25(OH)(2)D(3)-induced growth suppression in G(1) and show that the hormone achieves this by decreasing the activity of CDK2 and reducing the abundance of Skp2, which act together to degrade p27.

Tbx2 directly represses the expression of the p21(WAF1) cyclin-dependent kinase inhibitor

T-box factors play a crucial role in the development of many tissues, and mutations in T-box factor genes have been implicated in multiple human disorders. Some T-box factors have been implicated in cancer; for example, Tbx2 and Tbx3 can suppress replicative senescence, whereas Tbx3 can cooperate with Myc and Ras in cellular transformation. The p21(WAF1) cyclin-dependent kinase inhibitor plays a key role in senescence and in cell cycle arrest after DNA damage. Here, using a combination of in vitro DNA-binding, transfection, and chromatin immunoprecipitation assays, we show that Tbx2 can bind and repress the p21 promoter in vitro and in vivo. Moreover, small interfering RNA-mediated down-regulation of Tbx2 expression results in a robust activation of p21 expression. Taken together, these results implicate Tbx2 as a novel direct regulator of p21 expression and have implications for our understanding of the role of T-box factors in the regulation of senescence and oncogenesis, as well as in development.

AIB1/SRC-3 deficiency affects insulin-like growth factor I signaling pathway and suppresses v-Ha-ras-induced breast cancer initiation and progression in mice

Although the amplified in breast cancer 1 (AIB1) coactivator is amplified and overexpressed in breast cancers, its role in mammary carcinogenesis remains unknown. We demonstrate that during mammary development and tumorigenesis, the elevation of AIB1 level and its nuclear localization correlate with normal and transformed mammary epithelial proliferation, whereas its lower expression and cytoplasmic localization correlate with mammary epithelial quiescence and differentiation. In this study, the role of AIB1 in breast tumor initiation, progression, and metastasis was studied by generating AIB1(+/+), AIB1(+/-), and AIB1(-/-) mice harboring the mouse mammary tumor virus/v-Ha-ras (ras) transgene that induces breast tumors. Breast tumor incidence was reduced dramatically in the intact AIB1(-/-)-ras virgin mice and inhibited completely in the ovariectomized AIB1(-/-)-ras mice. Breast tumor latency was delayed significantly in AIB1(-/-)-ras virgin mice with natural estrous cycles, multiparous mice with cyclically elevated reproductive hormones, and virgin mice bearing pituitary isografts with persistently elevated hormones. Although AIB1 deficiency significantly suppressed mammary tumorigenesis under all of the concentrations of ovarian hormones, it did not affect the promotional role of ovarian hormones on mammary tumorigenesis, suggesting that AIB1 and ovarian hormones contribute to mammary carcinogenesis through different pathways. AIB1 deficiency did not alter the expression of estrogen and progesterone-responsive genes in the mammary gland, but it caused partial resistance to the insulin-like growth factor I because of a significant reduction in the insulin receptor substrates. The impaired insulin-like growth factor I pathway in AIB1(-/-)-ras mammary epithelium and tumor cells was responsible in part for the suppression of mammary tumorigenesis and metastasis caused by inhibition of cell proliferation and migration. These results suggest that a more effective strategy to control breast cancer is to target AIB1-mediated and ovarian hormone-initiated pathways.

Rho GTPases as Key Transducers of Proliferative Signals in G1 Cell Cycle Regulation

Mitogenic growth factor- and integrin-dependent signaling pathways cooperate to control the proliferation of nontransformed cells. As integral mediators of these networks, the Rho family of GTPases play a pivotal role in G1 cell cycle progression, primarily through regulation of cyclin D1 expression, as well as the levels of the cyclin-dependent kinase inhibitors p21cip1 and p27kip1. Such dual control of both the critical positive and negative regulators of G1 progression make the Rho GTPases prime candidates to target the autonomous proliferation which typifies cancer cells. Cyclin D1 has been identified as an important oncogene and the cdk inhibitors as tumor suppressors in human breast carcinogenesis. Evidence pointing to the potential role of Rho-dependent pathways and their interaction with oncogenic Ras in contributing to such cell cycle abnormalities that characterize human breast cancer is also presented.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

Distinct roles for p53, p27Kip1, and p21Cip1 during tumor development

Mutations in p53 and reduced expression of the Cdk inhibitor p27Kip1 are frequently observed in a wide variety of human cancers, but it is not known whether alterations in these tumor suppressors interact to influence tumor progression. To address this, we measured tumor latency and spectrum in p53 and p27 single and compound mutant mice. p53-/- (null) mice developed T-cell lymphomas and soft-tissue sarcomas, while p27-/- mice developed adenomas of the pituitary and lung, but with much longer latency. The latency for tumor development in p53-/- p27-/- and p53-/- p27+/- compound mutant mice was significantly reduced, by 15-30%, compared to single mutant p53-/- mice. The tumor spectrum in the compound mutants was similar to that of p53-/- mice, and additional tumors of diverse histotypes. In tumors from p53-/- mice, p27 protein levels were reduced to a greater extent than were mRNA levels, indicating that p27 is downregulated in tumors at the transcriptional as well as post-transcriptional levels. In contrast, mice deficient in another Cdk inhibitor p21Cip1, which is also a transcriptional target and effector of p53, showed only a marginal increase in tumor predisposition in response to ENU treatment. Thus, downregulation of p27 is a common feature in p53-/- tumors. Germline deletion of one or both alleles of p27 accelerates tumor development and associated mortality in p53-/- mice, indicating potent synergy between loss of p27 and p53. Although p21 is functionally similar to both p53 and p27, it plays a lesser role in tumor suppression. These results further highlight the highly cooperative nature of p27 and its central role in tumor suppression.

Characterization of human palmitoyl-acyl transferase activity using peptides that mimic distinct palmitoylation motifs

The covalent attachment of palmitate to proteins commonly occurs on cysteine residues near either N-myristoylated glycine residues or C-terminal farnesylated cysteine residues. It therefore seems likely that multiple palmitoyl-acyl transferase (PAT) activities exist to recognize and modify these distinct palmitoylation motifs. To evaluate this possibility, two synthetic peptides representing these palmitoylation motifs, termed MyrGCK(NBD) and FarnCNRas(NBD), were used to characterize PAT activity under a variety of conditions. The human tumour cell lines MCF-7 and Hep-G2 each demonstrated high levels of PAT activity towards both peptides. In contrast, normal mouse fibroblasts (NIH/3T3 cells) demonstrated PAT activity towards the myristoylated substrate peptide but not the farnesylated peptide, while ras -transformed NIH/3T3 cells were able to palmitoylate the FarnCNRas(NBD) peptide. The kinetic parameters for PAT activity were determined using membranes from MCF-7 cells, and indicated that the K (m) values for palmitoyl-CoA were identical for PAT activity towards the two substrate peptides; however, the K (m) for MyrGCK(NBD) was 5-fold lower than the K (m) for FarnCNRas(NBD). PAT activity towards the two substrate peptides was dose-dependently inhibited by 2-bromopalmitate and 3-(1-oxo-hexadecyl)oxiranecarboxamide (16C; IC(50) values of approx. 4 and 1.3 microM, respectively); however, 2-bromopalmitate was found to be uncompetitive with respect to palmitoyl-CoA, whereas 16C was competitive. To seek additional evidence for multiple PATs, the effects of altering the assay conditions on the palmitoylation of MyrGCK(NBD) and FarnCNRas(NBD) were compared. PAT activity towards the two peptide substrates was modulated similarly by changing the ionic strength or incubation temperature, or by the addition of dithiothreitol. In contrast, the enzymic palmitoylation of the two peptides was differentially affected by N -ethylmaleimide and thermal denaturation. Overall, these data demonstrate that the enzymic palmitoylation of farnesyl- and myristoyl-containing peptide substrates can be differentiated, suggesting that multiple motif-specific PATs exist.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

PyV-mT-induced parotid gland hyperplasia as detected by altered lectin reactivity is not modulated by inducible nitric oxide deficiency

The parotid gland was one of the first organs recognised to be sensitive to the transforming effects of polyomavirus. This study examines parotid gland pathology in mice expressing the polyomavirus middle T (PyV-mT) under the control of the mouse mammary tumour virus long terminal repeat (MMTV-LTR) to (1) demonstrate the utility of this model for studying premalignant disease; (2) identify early lesions by lectin staining and (3) determine effects of the inducible nitric oxide synthase (iNOS) in modulating tumorigenesis. The middle T oncogene is expressed in the parotid glands in addition to the mammary glands and results in the formation of parotid hyperplasias in 100% of transgenic female mice. These hyperplasias have the features of intraepithelial neoplasia including hypertrophic cells with prominent nucleoli and abnormal mitoses. Focal areas of parotid hyperplasia can be identified using peanut agglutinin (PNA), a lectin that recognises the tumour associated T antigen. In contrast to normal parotid gland, areas of hyperplasia do not bind PNA. Mice deficient in iNOS, an enzyme implicated in the promotion of tumorigenesis, were bred into the PyV-mT model. The loss of iNOS did not impact on the number or size of parotid gland hyperplasias, suggesting that NO produced by this enzyme is not a key regulator of PyV-mT-induced parotid gland hyperplasia. The consistent development of parotid hyperplasias in PyV-mT mice and clear identification of these lesions by loss of PNA lectin reactivity provides a useful model for studying early molecular changes in parotid tumorigenesis.

P21Waf1 control of epithelial cell cycle and cell fate

As a broad-acting cyclin-dependent kinase inhibitor, p21(WAF1) occupies a central position in the cell cycle regulation of self-renewing tissues such as oral mucosa and skin. In addition to regulating normal cell cycle progression decisions, p21(WAF1) integrates genotoxic insults into growth arrest and apoptotic signaling pathways that ultimately determine cell fate. As a result of its complex interactions with cell cycle machinery and response to mutagenic agents, p21(WAF1) also has stage-specific roles in epithelial carcinogenesis. Finally, a view is emerging of p21(WAF1) as not merely a cyclin-dependent kinase inhibitor, but also as a direct participant in regulating genes involved in growth arrest, senescence, and aging, thus providing an additional layer of control over matters of the cell cycle. This review discusses these various roles played by p21(WAF1) in cell cycle control, and attempts to relate these to epithelial cell biology, with special emphasis on keratinocytes.

Loss of the cell cycle inhibitors p21(Cip1) and p27(Kip1) enhances tumorigenesis in knockout mouse models

Events that contribute to tumor formation include mutations in the ras gene and loss or inactivation of cell cycle inhibitors such as p21(Cip1) and p27(Kip1). In our previous publication, we showed that mice expressing the MMTV/v-Ha-ras transgene developed tumors earlier and at higher multiplicities in the absence than in the presence of p21(Cip1). To further evaluate the combinatorial role of genetic alterations and loss of cell cycle inhibitors in tumorigenesis, we performed two companion studies. In the first study, wild type and p21(Cip1)-null mice were exposed to the chemical carcinogen, urethane. Similar to its effects in v-Ha-ras mice, loss of p21(Cip1) accelerated tumor onset and increased tumor multiplicity in urethane-treated mice. Lung tumors were the predominant tumor type in urethane-treated mice regardless of p21(Cip1) status. In the second study, tumor formation was monitored in v-Ha-ras mice expressing or lacking p27(Kip1). Unlike p21(Cip1), the absence of p27(Kip1) had no effect on the timing or multiplicity of tumor formation, which was largely restricted to mammary and salivary glands. However, once tumors appeared, they grew faster in p27(Kip1)-null mice than in p27(Kip1)-wild type mice. Increases in growth rate were particularly striking for salivary tumors in ras/p27(-/-) mice. Loss of p21(Cip1), on the other hand, had no effect on tumor growth rate in v-Ha-ras mice. Collectively, our data suggest that p21(Cip1) suppresses tumor formation elicited by multiple agents and that p21(Cip1) and p27(Kip1) suppress tumor formation in different ways.

CDK4 coexpression with Ras generates malignant human epidermal tumorigenesis

Ras acts with other proteins to induce neoplasia. By itself, however, strong Ras signaling can suppress proliferation of normal cells. In primary epidermal cells, we found that oncogenic Ras transiently decreases cyclin-dependent kinase (CDK) 4 expression in association with cell cycle arrest in G1 phase. CDK4 co-expression circumvents Ras growth suppression and induces invasive human neoplasia resembling squamous cell carcinoma. Tumorigenesis is dependent on CDK4 kinase function, with cyclin D1 required but not sufficient for this process. In facilitating escape from G1 growth restraints, Ras and CDK4 alter the composition of cyclin D and cyclin E complexes and promote resistance to growth inhibition by INK4 cyclin-dependent kinase inhibitors. These data identify a new role for oncogenic Ras in CDK4 regulation and highlight the functional importance of CDK4 suppression in preventing uncontrolled growth.

The isolation of an immortalized and tumorigenic cell line from p21WAF1 null mouse bladders

Given a role for the deregulation of p21WAF1 in the progression of bladder tumors, we examined the growth of cultured urothelial cells from wild-type and p21WAF1 null bladders. Bladders were excised, minced from euthanized p21WAF1 and wild-type mice, treated overnight with dispase, and then placed into flasks coated with collagen type I in Dulbecco modified Eagle medium with 10% fetal calf serum. After an overnight incubation, the media was replaced with a serum-free media and a portion of explants were treated with 12-O-tetrade-canoylphorbol-13-acetate (TPA) on day 7 and continued for either 4 or 9 wk. The urothelial origin of any surviving epithelial cells was determined by reverse transcription-polymerase chain reaction (RT-PCR) using uroplakin II-specific primers, and the expression of the cell cycle-related proteins, p16INK4 and p19ARF, was examined by semiquantitative RT-PCR and Western blotting. Isolated wild-type and serially passaged p21WAF1 null epithelial-like cells were then injected subcutaneously into nude mice. We found that phorbol ester treatment at two different concentrations significantly enhanced uroepithelial colony formation from isolated wild-type mouse bladder tissue. On the other hand, significantly fewer urothelial colonies were derived from p21WAF1 null bladder cells treated with phorbol ester. Although there was apparent senescence and cell death of epithelial foci and stromal cells in phorbol ester-treated and -untreated p21WAF1 null cultures, after 3 mo there was an apparent subpopulation of epitheloid cells that overgrew each flask. There was a significant decrease in the number of these serially passaged cells in the G1 phase of the cell cycle when compared with initial explant wild-type or p21WAF1 null cells. This subpopulation of epitheloid cells expressed the mouse uroplakin II gene, indicating a urothelial phenotype, but did not express either the p16INKa or p19ARF proteins, whereas p21WAF1 null bladders express both proteins. There was also a high level of expression of the p53 protein and a significant decrease in the expression of the p19ARF transcript in both p21WAF1 null bladder and p21WAF1 null cells. These p21WAF1 null cells could be easily passaged and when injected subcutaneously into nude mice, large tumors developed. Therefore, it appears that a subpopulation of urothelial cells from the p21WAF1 null bladder can develop a tumorigenic phenotype in vitro.

Oncogenic functions of tumour suppressor p21(Waf1/Cip1/Sdi1): association with cell senescence and tumour-promoting activities of stromal fibroblasts

p21(Waf1/Cip1/Sdi1) is best known as a broad-specificity inhibitor of cyclin/cyclin-dependent kinase complexes, but p21 also interacts with many other regulators of transcription or signal transduction. p21 induction, which is mediated by p53 and by p53-independent mechanisms, is essential for the onset of cell cycle arrest in damage response and cell senescence. The effects of p21 knockout in mice and its expression patterns in human cancer are consistent with a role for p21 as both a tumour suppressor and an oncogene. Several functions of p21 are likely to promote carcinogenesis and tumour progression. These include endoreduplication and abnormal mitosis that develop in tumour cells after release from p21-induced growth arrest, the ability of p21 to inhibit apoptosis through several different mechanisms, and its ability to stimulate transcription of secreted factors with mitogenic and anti-apoptotic activities. The latter effects of p21 show close resemblance to paracrine activities of senescent cells and to tumour-promoting functions of stromal fibroblasts. Therapeutic strategies targeting the oncogenic consequences of p21 expression may provide a new approach to chemoprevention and treatment of cancer.

Negative regulation of cell growth and differentiation by TSG101 through association with p21(Cip1/WAF1)

TSG101 was discovered in a screen for tumor susceptibility genes and has since been shown to have a multiplicity of biological effects. However, the basis for TSG101's ability to regulate cell growth has not been elucidated. We report here that the TSG101 protein binds to the cyclin/cyclin-dependent kinase (CDK) inhibitor (CKI) p21(Cip1/WAF1) and increases stability of the p21 protein in HEK293F cells and differentiating primary keratinocytes, suppressing differentiation in a p21-dependent manner. In proliferating keratinocytes where the p21 protein is relatively stable, TSG101 does not affect the stability or expression of p21 but shows p21-dependent recruitment to cyclin/CDK complexes, inhibits cyclin/CDK activity, and causes strong growth suppression to a much greater extent in p21+/+ than in p21-/- cells. Conversely, suppression of endogenous TSG101 expression by an antisense TSG101 cDNA causes doubling of the fraction of keratinocytes in the S phase of the cell cycle as occurs during p21 deficiency. Our results indicate that TSG101 has a direct role in the control of growth and differentiation in primary epithelial cells, and that p21 is an important mediator of these TSG101 functions.

5' CpG island hypermethylation is associated with transcriptional silencing of the p21(CIP1/WAF1/SDI1) gene and confers poor prognosis in acute lymphoblastic leukemia

The p21 is a downstream effector of p53/p73 and belongs to the CIP/KIP family of cyclin-dependent kinase inhibitors (CDKIs). It is, therefore, a potential tumor suppressor gene and probably plays an important role in tumor development. Moreover, reduced expression of p21 has been reported to have prognostic value in several human malignancies. In contrast with other CDKIs, mutational inactivation of p21 is infrequent, but gene inactivation by an alternative mechanism seems to be the general pathway. In this study, we analyzed the methylation status of the p21 promoter region using semiquantitative polymerase chain reaction in 124 patients with acute lymphoblastic leukemia (ALL). We observed p21 hypermethylation in bone marrow cells from 41% (51 of 124) of ALL patients. Hypermethylation within promoter strongly correlated with decreased p21 messenger RNA expression in tumoral cells. Clinical, molecular, and laboratory features and complete remission rate did not differ significantly between hypermethylated and normally methylated patients. Estimated disease-free survival (DFS) and overall survival at 7 and 9 years, respectively, were 59% and 65% for healthy patients and 6% and 8% for hypermethylated patients (P =.00001 and P =.006). Multivariate analysis of potential prognostic factors demonstrated that p21 methylation status was an independent prognostic factor in predicting DFS (P =.0001). Our results indicate that the p21 gene is subject to methylation regulation at the transcription level in ALL and seems to be an important factor in predicting the clinical outcome of these patients.