The p21 cyclin-dependent kinase inhibitor suppresses tumorigenicity in vivo

Lack of mutation in the WAF1/CIP1 gene during bovine leukemia virus-induced leukemogenesis

As a cyclin-dependent kinases (Cdks) inhibitor (CDI), the protein p21WAF1/CIP1 is able to interfere with cell cycle progression. Its expression is upregulated by wild-type p53, and the p21WAF1/CIP1 protein appears to be a potent effector of the p53-dependent cell cycle regulatory pathway. We have previously reported that p53 mutations frequently occur during bovine leukemia virus (BLV)-induced leukemogenesis in cattle but not in sheep. Therefore, we have investigated the involvement of p21WAF1/CIP1 mutations in the tumorigenic process associated with BLV. We first cloned the bovine and ovine WAF1 genes and determined the complete nucleotide sequences of their second coding exons. These sequences share respectively 79% and 80% homology with those of the human counterpart exon. In order to screen for mutations that could be associated with BLV-induced pathogenicity, we performed single strand conformation polymorphism (SSCP) assays on the WAF1 genes from BLV-induced tumors. No WAF1 mutations were detected in any of the ten BLV-induced bovine tumor samples. Among eleven sheep tumors and three ovine cell lines, only one sample revealed a single mutation in the WAF1 coding sequence, but this mutation was silent at the translational level. We concluded that mutations of the WAF1 gene are not involved in the development of the tumors during BLV-induced leukemogenesis.

ATM-dependent telomere loss in aging human diploid fibroblasts and DNA damage lead to the post-translational activation of p53 protein involving poly(ADP-ribose) polymerase

Telomere loss has been proposed as a mechanism for counting cell divisions during aging in normal somatic cells. How such a mitotic clock initiates the intracellular signalling events that culminate in G1 cell cycle arrest and senescence to restrict the lifespan of normal human cells is not known. We investigated the possibility that critically short telomere length activates a DNA damage response pathway involving p53 and p21(WAF1) in aging cells. We show that the DNA binding and transcriptional activity of p53 protein increases with cell age in the absence of any marked increase in the level of p53 protein, and that p21(WAF1) promoter activity in senescent cells is dependent on both p53 and the transcriptional co-activator p300. Moreover, we detected increased specific activity of p53 protein in AT fibroblasts, which exhibit accelerated telomere loss and undergo premature senescence, compared with normal fibroblasts. We investigated the possibility that poly(ADP-ribose) polymerase is involved in the post-translational activation of p53 protein in aging cells. We show that p53 protein can associate with PARP and inhibition of PARP activity leads to abrogation of p21 and mdm2 expression in response to DNA damage. Moreover, inhibition of PARP activity leads to extension of cellular lifespan. In contrast, hyperoxia, an activator of PARP, is associated with accelerated telomere loss, activation of p53 and premature senescence. We propose that p53 is post-translationally activated not only in response to DNA damage but also in response to the critical shortening of telomeres that occurs during cellular aging.

Multiple pathways control cell growth and transformation: overlapping and independent activities of p53 and p21Cip1/WAF1/Sdi1

Many tumour therapies act by inducing a cellular damage response pathway mediated by the tumour suppressor protein p53. Alternative outcomes of p53 induction include apoptosis or transient cell-cycle arrest, both thought to require the transcriptional activity of wild-type p53. Current research highlights the action of a p53-activated gene, p21Cip1/WAF1/Sdi1, which encodes a cyclin-kinase inhibitor important in mediating p53-dependent cell-cycle arrest, while programmed cell death in response to DNA damage requires transcriptionally active p53 but not activation of p21Cip1/WAF1/Sdi1. This review examines the roles of p53 and p21Cip1/WAF1/Sdi1 in controlling cell proliferation, in the light of a new study on expression of p53 and p21Cip1/WAF1/Sdi1 in squamous cell carcinoma of the larynx.

Expression of p21WAF1/CIP1 in fetal and adult tissues: simultaneous analysis with Ki67 and p53

AIMS

To determine the expression of p21WAF1/CIP1 in relation to the expression of Ki67 and p53 in various normal adult and fetal tissues, and to investigate its distribution throughout the cell cycle.

METHODS

The expression of p21WAF1/CIP1 in relation to Ki67 and p53 was analysed in adult and fetal tissues using immunohistochemical techniques. Heat induced epitope retrieval techniques were used to characterise the presence of p21WAF1/CIP1 in different tissues, as well as to detect its distribution throughout the cell cycle. In addition, flow cytometry and western blotting were used to test whether the level of p21WAF1/CIP1 expression varied at different phases of the cell cycle in phytohaemagglutinin (PHA) stimulated lymphocytes.

RESULTS

p21WAF1/CIP1 expression varied from one tissue to another, and it was restricted mainly to the squamous and glandular epithelium, where it appeared in association with p53. Human tissues in which p21WAF1/CIP1 was found showed a mutually exclusive topographical sequential expression between p21WAF1/CIP1 and Ki67. This was confirmed by double labelling studies, which showed that p21WAF1/CIP1 positive cells were in the G0 phase. Unlike these findings of a decline in p21WAF1/CIP1 expression after the G0 phase, PHA stimulated lymphocytes showed a level of p21WAF1/CIP1 expression that rose as the cell progressed through the cell cycle.

CONCLUSIONS

The analysis of p21WAF1/CIP1 expression in relation to the status of p53 should take into account the existence of variable p21WAF1/CIP1 expression in different tissues. This could provide an explanation for the varying frequency of p53 mutations in tumours of different cellular origin. In tissues characterised by regular p21WAF1/CIP1 expression, it appears in a pattern that is consistent with the proposed role of this inhibitor of cyclin dependent kinases in cell cycle arrest-that of inducing cell differentiation. The conflicting results of in vivo and in vitro studies could support the hypothesis that microenvironmental conditions may influence the location of p21WAF1/CIP1 in different phases of the cell cycle.

Delivery of tumor suppressor genes to reverse the malignant phenotype

Despite early enthusiasm and excitement, the treatment of cancer via gene therapy is a long way from reaching fruition. The objective of this review is to describe the rationale as to why the delivery of genes encoding functional proteins whose activity has been lost during the initiation and development of cancer may be a feasible therapeutic option. In addition we will evaluate the limitations of the current delivery systems and discuss how these limitations have impacted upon the progress of gene therapy. Finally, we will describe and discuss the most recent attempts to deliver tumor suppressor genes to rodent models of human cancer and perhaps more importantly to human patients. As will become apparent during this review the excitement and enthusiasm for gene therapy remains high, however, this should not diminish the challenges that must be overcome before gene therapy becomes routine.

Introduction of p21(Waf1/Cip1) gene into a carcinoma cell line of the uterine cervix with inactivated p53

In carcinomas of the uterine cervix of which p53 is frequently inactivated by papilloma viruses, gene transfer of p21, effector of p53, is an alternative tool to suppress cell growth. We introduced the p21 gene into HeLa cells. The transfectant with p21 showed a significant growth retardation by blockage of G1 to S transfer of the cell cycle. This cell line showed significantly reduced anchorage-independent growth as well as attenuated telomerase activity. These data suggest that gene transfer of p21 is an effective tool to lead carcinoma cells with inactivated p53 into less malignant phenotype.

p21WAF1 mutations and human malignancies

During the past few years, several categories of cyclin-dependent kinase inhibitors (CDKIs), which negatively regulate cyclin/cyclin-dependent kinase (CDK) activities, were cloned. The p21WAF1, also known as CIP1 or SDI1, was the first reported CDKI: it's expression is induced by wild-type p53. The p21WAF1 is a potent inhibitor of most cyclin/CDK complexes and also inhibits the ability of the proliferating cell nuclear antigen (PCNA) to activate DNA polymerase d. Alterations of the cell-cycle can cause cellular transformation. We analysed 471 primary samples from 15 types of human malignancies and 36 cell lines for structural alterations of the p21WAF1 gene. No changes were found in the coding region of p21WAF1 gene by polymerase-chain reaction-single-strand conformation polymorphism (PCR-SSCP) analysis. Many of these tumors had a normal p53 gene. Other investigators showed that p21WAF1 knockout mice did not have an increased incidence of cancer, while p53 knock-out mice did. Taken together, the absence of alterations of p21WAF1 in a series of malignancies suggests that p21WAF1 may not have a role in either onset or progression of most human cancers. Furthermore, p53 probably activates additional, critical tumor suppressor pathways.

Overexpression of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1) in human cutaneous malignant melanoma

p21(WAF1/CIP1) (p21) is an inhibitor of cyclin-dependent kinases recently identified as the downstream effector of wild-type p53-mediated cell cycle arrest. The gene coding for p21 may function as a negative regulator of melanoma growth, progression, and metastasis. Using immunohistochemistry and Western blotting, we investigated the expression of p21 in human melanocytic proliferations. Immunohistochemical staining was performed on 13 common acquired nevi, 12 dysplastic nevi, 23 primary malignant melanomas, and 12 metastatic melanomas. Common acquired nevi showed minimal p21 staining (1.8+/-0.3%, mean+/-SEM). The percentage of positive nuclei was slightly elevated in dysplastic nevi (8.9+/-1.7%). Both primary malignant melanoma (29+/-3%) and metastatic melanoma (33+/-5%) demonstrated a significantly increased number of p21-positive nuclei compared to benign lesions (p<0.001). p21 was strongly expressed even in actively proliferating lesions as confirmed by MIB-1 labelling, and although the majority of p21-positive cells likely represent a non-proliferating population, staining was occasionally observed in cells undergoing mitosis, suggesting abnormal function of this cell cycle inhibitor in malignant melanoma. Overexpression of p21 in metastatic melanoma compared to common acquired nevi was confirmed by Western blot analysis of human tumor samples. These findings suggest that increased p21 expression relative to benign nevi is not sufficient to control melanoma growth in vivo.

Functional expression of human p21(WAF1/CIP1) gene in rat glioma cells suppresses tumor growth in vivo and induces radiosensitivity

This study reports the successful growth suppression of a rat glioblastoma model (RT-2) both in vitro and in vivo by the insertion of p21(WAF1/CIP1), a negative cell cycle regulatory gene, into the tumor cells. Greater than 95% of the tumor cells expressed p21 protein after being infected with pCL based p21 retrovirus at 4x M.O.I. (multiplicity of infection). The p21-infected cells showed a 91% reduction in colony forming efficiency and a 66% reduction in growth rate. More prominent p21 staining was found in cells exhibiting histologic evidence of senescence. Intracranial implantation of the infected cells showed complete disappearance of the p21-infected cells at day 10 and long-term survival of the animals compared to controls. Injection of pCLp21 virus into tumor established in situ showed tumor necrosis and gene expression. In a clonogenic radiation survival assay, a 93% reduction of surviving colonies of p21-infected cells was seen in comparison to vector-infected control cells and to p53-infected cells after exposure to 8 Gy (800 rads).

Induction of differentiation by wild-type p53 gene in a human glioma cell line

Wild-type human p53 gene was transfected into the human glioma cell line T-98G. Transfectants were then isolated and characterized for growth potential and differentiation phenotype. Growth suppression, overexpression of GFAP, and accumulation in G1 phase were more commonly observed in transfectants than in T-98G cells. p21WAF1/CIP1 was overexpressed in transfectants, and the binding of PCNA and CDK 2 to p21WAF1/CIP1 were increased in transfectants. These results suggested the roles of p21WAF1/CIP1, PCNA, and CDK2 in regulation of differentiation in glioma cells and the gene transfer of wild-type p53 may be effective for the control of glial differentiation in glioma cells.

Adenovirally transferred p16INK4/CDKN2 and p53 genes cooperate to induce apoptotic tumor cell death

Repression of cell cycle progression by tumor suppressors might provide a means for tumor therapy. Here we demonstrate that ectopic overexpression of the p16INK4/CDKN2 tumor suppressor from an adenovirus vector in various cell lines results in block of cell division and, subsequently, in a gradual reduction of the levels of the product of retinoblastoma susceptibility gene, pRb. Overexpression of p53 and p16INK4/CDKN2, but not p53 on its own, induces apoptotic death only in tumor cells. Simultaneous adenoviral transfer of p16 and p53 genes leads to inhibition of tumor growth in nude mice. These results suggest that combined delivery of two cooperating genes like p16 and p53 could be the basis for the development of a new strategy for cancer gene therapy.

Isolation of a hamster cDNA homologous to the mouse and human cyclin kinase inhibitory protein p27Kip1

We report here the isolation, cloning and sequencing of a hamster cDNA homologous to the mouse cyclin kinase inhibitory protein p27Kip1. The full length hamster cDNA sequence (p30Kip1)5 revealed 91% similarity with the previously reported mouse and human cDNAs and coded for a protein of 198 amino acids, results which were very similar to that observed for the mouse and human protein. Western blotting analysis performed using a polyclonal antibody against a mouse cyclin kinase inhibitory protein, p27Kip1, revealed the presence of a strongly reactive protein band at 30 kDa (as opposed to 27 kDa in mouse and human cells) in cell lysate prepared from rat and hamster cells. Although the size of the cyclin kinase inhibitory protein cDNA transcript is similar in mouse and hamster, it is likely that the differential mobility of the hamster p30Kip1 protein compared to the mouse and human p27Kip1 protein could be due to post-translational modifications.

Inhibition of cyclin-dependent kinases by purine analogues: crystal structure of human cdk2 complexed with roscovitine

Cyclin-dependent kinases (cdk) control the cell division cycle (cdc). These kinases and their regulators are frequently deregulated in human tumours. A potent inhibitor of cdks, roscovitine [2-(1-ethyl-2-hydroxyethylamino)-6-benzylamino-9-isopropylpurin e], was identified by screening a series of C2,N6,N9-substituted adenines on purified cdc2/cyclin B. Roscovitine displays high efficiency and high selectivity (Meijer, L., Borgne, A., Mulner, O., Chong, J. P. J., Blow, J. J., Inagaki, N., Inagaki, M., Delcros, J.-G. & Moulinoux, J.-P. (1997) Eur. J. Biochem. 243, 527-536). It behaves as a competitive inhibitor for ATP binding to cdc2. We determined the crystal structure of a complex between cdk2 and roscovitine at 0.24-nm (2.4 A) resolution and refined to an Rfactor of 0.18. The purine portion of the inhibitor binds to the adenine binding pocket of cdk2. The position of the benzyl ring group of the inhibitor enables the inhibitor to make contacts with the enzyme not observed in the ATP-complex structure. Analysis of the position of this benzyl ring explains the specificity of roscovitine in inhibiting cdk2. The structure also reveals that the (R)-stereoisomer of roscovitine is bound to cdk2. The (R)-isomer is about twice as potent in inhibiting cdc2/cyclin B than the (S)-isomer. Results from structure/activity studies and from analysis of the cdk2/roscovitine complex crystal structure should allow the design of even more potent cdk inhibitors.

Biochemical and cellular effects of roscovitine, a potent and selective inhibitor of the cyclin-dependent kinases cdc2, cdk2 and cdk5

Cyclin-dependent kinases (cdk) play an essential role in the intracellular control of the cell division cycle (cdc). These kinases and their regulators are frequently deregulated in human tumours. Enzymatic screening has recently led to the discovery of specific inhibitors of cyclin-dependent kinases, such as butyrolactone I, flavopiridol and the purine olomoucine. Among a series of C2, N6, N9-substituted adenines tested on purified cdc2/cyclin B, 2-(1-ethyl-2-hydroxyethylamino)-6-benzylamino-9-isopropylpurine (roscovitine) displays high efficiency and high selectivity towards some cyclin-dependent kinases. The kinase specificity of roscovitine was investigated with 25 highly purified kinases (including protein kinase A, G and C isoforms, myosin light-chain kinase, casein kinase 2, insulin receptor tyrosine kinase, c-src, v-abl). Most kinases are not significantly inhibited by roscovitine. cdc2/cyclin B, cdk2/cyclin A, cdk2/cyclin E and cdk5/p35 only are substantially inhibited (IC50 values of 0.65, 0.7, 0.7 and 0.2 microM, respectively). cdk4/cyclin D1 and cdk6/cyclin D2 are very poorly inhibited by roscovitine (IC50 > 100 microM). Extracellular regulated kinases erk1 and erk2 are inhibited with an IC50 of 34 microM and 14 microM, respectively. Roscovitine reversibly arrests starfish oocytes and sea urchin embryos in late prophase. Roscovitine inhibits in vitro M-phase-promoting factor activity and in vitro DNA synthesis in Xenopus egg extracts. It blocks progesterone-induced oocyte maturation of Xenopus oocytes and in vivo phosphorylation of the elongation factor eEF-1. Roscovitine inhibits the proliferation of mammalian cell lines with an average IC50 of 16 microM. In the presence of roscovitine L1210 cells arrest in G1 and accumulate in G2. In vivo phosphorylation of vimentin on Ser55 by cdc2/cyclin B is inhibited by roscovitine. Through its unique selectivity for some cyclin-dependent kinases, roscovitine provides a useful antimitotic reagent for cell cycle studies and may prove interesting to control cells with deregulated cdc2, cdk2 or cdk5 kinase activities.

Decreased expression and rare somatic mutation of the CIP1/WAF1 gene in human hepatocellular carcinoma

CIP1/WAF1, a critical downstream effector of tumor suppressor p53, encodes a cyclin-dependent kinase inhibitor. By Northern blot analysis, the CIP1/WAF1 mRNA level in the tumor was significantly lower than that in the corresponding normal liver from 19 Japanese patients with hepatocellular carcinoma (P < 0.05). In the tumor from only one out of 19 patients (5%), somatic mutations of the CIP1/WAF1 as well as that of p53 gene were identified by RT-PCR/SSCP analysis. These results suggest that the decreased CIP1/WAF1 expression is involved in the carcinogenesis or the progression of hepatocellular carcinoma.

The absence of p21Cip1/WAF1 alters keratinocyte growth and differentiation and promotes ras-tumor progression

p21Cip1/WAF1 was the first cyclin-dependent kinase (CDK) inhibitor to be identified, as a mediator of p53 in DNA damage-induced growth arrest, cell senescence, and direct CDK regulation. p21 may also play an important role in differentiation-associated growth arrest, as its expression is augmented in many terminally differentiating cells. A general involvement of p21 in growth/differentiation control and tumor suppression has been questioned, as mice lacking p21 undergo a normal development, harbor no gross alterations in any of their organs, and exhibit no increase in spontaneous tumor development. However, a significant imbalance between growth and differentiation could be unmasked under conditions where normal homeostatic mechanisms are impaired. We report here that primary keratinocytes derived from p21 knockout mice, transformed with a ras oncogene, and injected subcutaneously into nude mice exhibit a very aggressive tumorigenic behavior, which is not observed with wild-type control keratinocytes nor with keratinocytes with a disruption of the closely related p27 gene. p21 knockout keratinocytes tested under well-defined in vitro conditions show a significantly increased proliferative potential, which is also observed but to a lesser extent with p27 knockout cells. More profound differences were found in the differentiation behavior of p21 versus p27 knockout keratinocytes, with p21 (but not p27) deficiency causing a drastic down-modulation of differentiation markers linked with the late stages of the keratinocyte terminal differentiation program. Thus, our results reveal a so far undetected role of p21 in tumor suppression, demonstrate that this function is specific as it cannot be attributed to the closely related p27 molecule, and point to an essential involvement of p21 in terminal differentiation control, which may account for its role in tumor suppression.

High-risk HPV-positive human cancer cell lines show different sensitivity to cisplatin-induced apoptosis correlated with the p21Waf1/Cip1 level

We investigated the sensitivity and cell-cycle inhibitory gene expression of human papillomavirus (HPV) 16- and 18-positive human cancer cell lines after DNA damage induced by treatment with the anti-cancer drug cisplatin. Four HPV-positive cell lines (Caski, SiHa, HeLa and KB) were treated with cisplatin at various concentrations. Apoptotic cell death was observed in a dose-dependent manner in all cell lines treated with cisplatin; however, colony assay for chemosensitivity revealed that HeLa and KB cells (HPV 18-positive cell lines) were more sensitive than SiHa and Caski cells (HPV 16-positive cell lines). Northern blot analyses showed that p53 and p21Waf1/Cip1 mRNA were detectable in all untreated cells, and increasing amounts of these transcripts were identified in all cell lines treated with cisplatin. However, signals were more prominent in HeLa and KB, HPV 18-positive-cells. Immunohistochemical detection of p21Waf1/Cip1 protein showed that the p21-positive cells with apoptotic features were more distinct in KB and HeLa cells (HPV 18-positive) than in SiHa and Caski cells (HPV 16-positive). Our results show that there were differences in sensitivity to cisplatin among four types of high risk HPV-positive cells, possibly due to different levels of p21Waf1/Cip1 up-regulation by functional p53.

The role of the cell cycle in genitourinary carcinoma

The understanding of the cell cycle and the control of cellular proliferation has increased dramatically over the past years. Cancer is most likely caused by multiple genetic aberrations, therefore it seems likely that one or more genes involved in tumorigenesis will regulate a portion of the cell cycle. Cell cycle related genes, such as p53 are becoming important in determining prognosis. In addition, gene replacement therapy for a deleted gene may hold promise in treating malignancies. Further understanding of cell proliferation should bring marked improvements in the diagnosis and treatment of genitourinary malignancies.

Role of the p21 cyclin-dependent kinase inhibitor in limiting intimal cell proliferation in response to arterial injury

Arterial injury induces a series of proliferative, vasoactive, and inflammatory responses that lead to vascular proliferative diseases, including atherosclerosis and restenosis. Although several factors have been defined which stimulate this process in vivo, the role of specific cellular gene products in limiting this response is not well understood. The p21 cyclin-dependent kinase inhibitor affects cell cycle progression, senescence, and differentiation in transformed cells, but its expression in injured blood vessels has not been investigated. In this study, we report that p21 protein is induced in porcine arteries following balloon catheter injury and suggest that p21 is likely to play a role in limiting arterial cell proliferation in vivo. Vascular endothelial and smooth muscle cell growth was arrested through the ability of p21 to inhibit progression through the G1 phase of the cell cycle. Following injury to porcine arteries, p21 gene product was detected in the neointima and correlated inversely with the location and kinetics of intimal cell proliferation. Direct gene transfer of p21 using an adenoviral vector into balloon injured porcine arteries inhibited the development of intimal hyperplasia. Taken together, these findings suggest that p21, and possibly related cyclin-dependent kinase inhibitors, may normally regulate cellular proliferation following arterial injury, and strategies to increase its expression may prove therapeutically beneficial in vascular diseases.