Induction of senescence in human malignant glioma cells by p16INK4A

Adenovirus-mediated wt-p16 reintroduction induces cell cycle arrest or apoptosis in pancreatic cancer

Pancreatic cancer has long carried poor prognosis. The development of new therapeutic approaches is particularly urgent. Inactivation of the tumor-suppressor gene p16(INK4a/CDKN2), a specific inhibitor of the cyclin-dependent kinases CDK4 and CDK6, is the most common genetic alteration in human pancreatic cancer, making it an ideal target for gene replacement. Here we transfected tumor cells using a recombinant adenovirus containing the wt-p16 cDNA (Ad5RSV-p16). The overexpression of p16 decreased cell proliferation in all four human pancreatic tumor cell lines (NP-9, NP-18, NP-29, and NP-31). However, G1 arrest and senescence were observed in only three. In contrast, the fourth (NP-18) showed a significant increase in apoptosis. This differential behavior may be related to the differences found in the expression level of E2F-1. Experiments on subcutaneous pancreatic xenografts demonstrated the effectiveness of p16 in the inhibition of pancreatic tumor growth in vivo. Taken together, our results indicate that approaches involving p16 replacement are promising in pancreatic cancer treatment.

p53 and its homologues, p63 and p73, induce a replicative senescence through inactivation of NF-Y transcription factor

Recent studies have identified two p53 homologues, p63 and p73. They activate p53-responsive promoters and induce apoptosis when overexpressed in certain human tumors. Here, we report that p63, like p53 and p73, induces replicative senescence when expressed in a tetracycline-regulated manner in EJ cells lacking a functional p53. In addition to transcription activation of p53-responsive genes, we found that p63 and p73 repress transcription of the cdk1 and cyclin B genes, both of which are irreversibly repressed in senescent human fibroblast. In transient transfection assay, p63 and p73 repress the cdk1 promoter regardless of the presence of a dominant negative mutant form of p53. Furthermore, we found that DNA binding activity of NF-Y transcription factor, which is essential for transcription of the cdk1 and cyclin B genes and inactivated in senescent fibroblast, is significantly decreased by expression of either of p53, p63, or p73. Since NF-Y binds to many promoters besides the cdk1 and cyclin B promoters, inactivation of NF-Y by p53 family genes may be a general mechanism for transcription repression in replicative senescence.

Induction of a senescence-like phenotype in bovine aortic endothelial cells by ionizing radiation

Treatment of confluent monolayers of bovine aortic endothelial cells (BAEC) with gamma rays resulted in the delayed appearance of cells with an enlarged surface area that were morphologically similar to senescent cells. The majority of these cells stained positively for senescence-associated beta-galactosidase (SA-beta-gal), indicating that these cells are biochemically similar to senescent cells. The incidence of the senescence-like phenotype increased with dose (5-15 Gy) and time after irradiation. Cells with a senescence-like phenotype began to appear in the monolayer several days after irradiation. The onset of the appearance of this phenotype was accelerated by subculturing 24 h after irradiation. This acceleration was not entirely due to stimulation of progression through the cell cycle, since a high percentage of the senescent-like cells that appeared after subculture were not labeled with BrdUrd during the period after subculture. Prolonged up-regulation of expression of CDKN1A (also known as p21(CIP1/WAF1)) after irradiation was noted by Western blot analysis, again suggesting a similarity to natural senescence. Phenotypically altered endothelial cells were present in the irradiated monolayers as long as 20 weeks after irradiation, suggesting that a subpopulation of altered endothelial cells that might be functionally deficient could persist in the vasculature of irradiated tissue for a prolonged period after irradiation.

Dual growth arrest pathways in astrocytes and astrocytic tumors in response to Raf-1 activation

Normal human fibroblasts have been shown to undergo a p16(Ink4a)-associated senescence-like growth arrest in response to sustained activation of the Ras/Raf/MEK/ERK pathway. We noted a similar p16(Ink4a)-associated, senescence-like arrest in normal human astrocytes in response to expression of a conditional form of Raf-1. While HPV16 E7-mediated functional inactivation of the p16(Ink4a)/pRb pathway in astrocytes blocked the p16(Ink4a)-associated growth arrest in response to activation of Raf-1, it also revealed a second p21(Cip1)-associated, senescence-associated, beta-galactosidase-independent growth arrest pathway. Importantly, the p21(Cip1)-associated pathway was present not only in normal astrocytes but also in p53-, p14(ARF)-, and p16(Ink4a)/pRb-deficient high grade glioma cells that lacked the p16(Ink4a)-dependent arrest mechanism. These results suggest that normal human cells have redundant arrest pathways, which can be activated by Raf-1, and that even tumors that have dismantled p16(Ink4a)-dependent growth arrest pathways are potentially regulated by a second p21(Cip1)-dependent growth arrest pathway.

Alterations of the p53 and pRB pathways in human astrocytoma

Human astrocytomas are characterized by a number of molecular changes affecting two critical tumor suppressor pathways: the pRB and the p53 pathways. Genetic alterations functionally eliminate pRB and p53 themselves or upstream and/or downstream molecules such as products of the Ink4a/ARF locus, p16Ink4a and p14ARF. As a result, malignant cells are defective in critical cell cycle and apoptosis regulatory elements contributing to unrelenting tumour growth and invasion. Current research aims to discover effective means of reconstituting p53 and pRB pathway components in an effort to attenuate the aggressive phenotype of astrocytoma.

p16(MTS-1/CDKN2/INK4a) in cancer progression

Since its discovery as an inhibitor of cyclin-dependent kinases 4 and 6, the tumor suppressor p16 has continued to gain widespread importance in cancer. The high frequency of deletions of p16 in tumor cell lines first suggested an important role for p16 in carcinogenesis. This initial genetic evidence was subsequently strengthened by numerous studies documenting p16 inactivation in kindreds with familial melanoma. Moreover, a high frequency of p16 gene alterations was found in primary tumors, while recent studies have identified p16 promoter methylation as a major mechanism of tumor-suppressor-gene silencing. Additional insight into p16's role in cancer has come from the genetic analysis of precancerous lesions and various tissue culture models. It is now believed that loss of p16 is an early and often critical event in tumor progression. Consequently, p16 is a major tumor-suppressor gene whose frequent loss occurs early in many human cancers.

p53-independent upregulation of p21WAF1 in NIH 3T3 cells malignantly transformed by mot-2

Mot-2 protein is shown to interact with p53 and inhibit its transcriptional activation function. Mot-2 overexpressing stable clones of NIH 3T3 cells were malignantly transformed, however, they had a high level of expression of a p53 downstream gene, p21WAF1. The present study was undertaken to elucidate possible molecular mechanism(s) of such upregulation. An increased level of p21WAF1 expression was detected in stable transfectants although an exogenous reporter gene driven by p21WAF1 promoter exhibited lower activity in these cells suggesting that some post-transcriptional mechanism contributes to upregulation. Western analyses of transient and stable clones revealed that upregulation of p21WAF1 in stable NIH 3T3/mot-2 cells may be mediated by cyclin D1 and cdk-2.

The INK4A/ARF locus: role in cell cycle control and apoptosis and implications for glioma growth

The unique INK4A/ARF locus at chromosome 9p21 encodes two distinct proteins that intimately link the pRB and p53 tumour suppressor pathways. p16INK4A has been identified as an inhibitor of the cell cycle, capable of inducing arrest in G1 phase. p14/p19ARF on the other hand can induce both G1 and G2 arrest due to its stabilizing effects on the p53 transcription factor. In addition to their roles in growth arrest, both proteins are involved in cellular senescence and apoptosis. The frequent mutation or deletion of INK4A/ARF in human tumours as well as the occurence of tumours in the murine knockout models have identified both p16 and ARF as bona fide tumour suppressors.

Adenoviral transfer of cyclin-dependent kinase inhibitor genes suppresses collagen-induced arthritis in mice

In rheumatoid synovial tissues, synovial fibroblasts are activated by proinflammatory cytokines and proliferate to develop hyperplastic pannus tissues, which irreversibly damage the affected joints. We recently reported that the cyclin-dependent kinase inhibitors p16(INK4a) and p21(Cip1) are not expressed in vivo in rheumatoid synovial fibroblasts, but are readily inducible in vitro. This observation was followed by the successful treatment of rat adjuvant arthritis by local p16(INK4a) gene transfer, showing that the inhibition of the cell cycle of the synovial cells ameliorates the arthritis. In this study, we show that another animal model of rheumatoid arthritis, murine collagen-induced arthritis, can be effectively treated by local gene transfer of p21(Cip1) as well as that of p16(INK4a). The anti-arthritic effects were observed even when the treatment was conducted after the arthritis had developed. Furthermore, the effects included suppression of the expression of proinflammatory cytokines such as IL-1ss, IL-6, and TNF-alpha. Our results demonstrate that the ectopic expression of cyclin-dependent kinase inhibitors not only prevents synovial overgrowth but also ameliorates the proinflammatory milieu in the affected joints. The induction of p21(Cip1) in rheumatoid synovial tissues by pharmacological agents may also be an effective strategy to treat rheumatoid arthritis.

5-Aza-2'-deoxycytidine leads to down-regulation of aberrant p16INK4A RNA transcripts and restores the functional retinoblastoma protein pathway in hepatocellular carcinoma cell lines

The inactivation of the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor p16INK4A may be caused by gene deletion, mutation or promoter hypermethylation. We have previously reported that p16INK4A in hepatocellular carcinoma (HCC) tissues and cell lines is inactivated predominantly by promoter hypermethylation rather than genomic aberrations. In the present experiments, we have studied the effects of the demethylating agent, 5-aza-2'-deoxycytidine (5-AZA/decitabine), on the expression of aberrant p16INK4A RNA transcripts and the CDK-retinoblastoma gene pathway in HCC cell lines with p16INK4A promoter hypermethylation. The expression of aberrant p16INK4A RNA transcripts was down-regulated and p16INK4A protein was strongly re-expressed in the HCC cell lines, SNU 354, 398, 423 and 475 after 5-AZA/decitabine treatment for 5 days. The re-expressed p16INK4A was functional, because it bound to and inhibited CDK4 kinase activity, and increased the concentrations of the hypophosphorylated form of retinoblastoma protein (pRB) in cells with a wild type RB gene. Moreover, treatment with the demethylating agent led not only to G1 cell cycle arrest, but also to the increased expression of the senescence-associated marker beta-galactosidase. This up-regulation of p16INK4A mRNA and protein correlated with demethylation of the p16INK4A promoter, and with the down-regulation or disappearance of aberrant p16INK4A transcripts. These results suggest that the aberrant p16INK4A RNA transcript can be transcribed from the methylated p16INK4A gene, and endogenous reactivation of functional p16INK4A mRNA by a demethylating agent can restore the pRB pathway in HCC, and foster the terminal differentiation of the malignant cells. Therefore, demethylating agents, such as 5-AZA/decitabine, may have potential in the treatment of HCC.

PML is induced by oncogenic ras and promotes premature senescence

Oncogenic ras provokes a senescent-like arrest in human diploid fibroblasts involving the Rb and p53 tumor suppressor pathways. To further characterize this response, we compared gene expression patterns between ras-arrested and quiescent IMR90 fibroblasts. One of the genes up-regulated during ras-induced arrest was promyelocytic leukemia (PML) protein, a potential tumor suppressor that encodes a component of nuclear structures known as promyelocytic oncogenic domains (PODs). PML levels increased during both ras-induced arrest and replicative senescence, leading to a dramatic increase in the size and number of PODs. Forced PML expression was sufficient to promote premature senescence. Like oncogenic ras, PML increased the levels of p16, hypophosphorylated Rb, phosphoserine-15 p53, and expression of p53 transcriptional targets. The fraction of Rb and p53 that colocalized with PML markedly increased during ras-induced arrest, and expression of PML alone forced p53 to the PODs. E1A abolished PML-induced arrest and prevented PML induction and p53 phosphorylation in response to oncogenic ras. These results imply that PML acts with Rb and p53 to promote ras-induced senescence and provide new insights into PML regulation and activity.

Expression of p57(KIP2) potently blocks the growth of human astrocytomas and induces cell senescence

Astrocytic tumors frequently exhibit defects in the expression or activity of proteins that control cell-cycle progression. Inhibition of kinase activity associated with cyclin/cyclin-dependent kinase co-complexes by cyclin-dependent kinase inhibitors is an important mechanism by which the effects of growth signals are down-regulated. We undertook the present study to determine the role of p57(KIP2) (p57) in human astrocytomas. We demonstrate here that whereas p57 is expressed in fetal brain tissue, specimens of astrocytomas of varying grade and permanent astrocytoma cell lines do not express p57, and do not contain mutations of the p57 gene by multiplex-heteroduplex analysis. However, the inducible expression of p57 in three well-characterized human astrocytoma cell lines (U343 MG-A, U87 MG, and U373 MG) using the tetracycline repressor system leads to a potent proliferative block in G(1) as determined by growth curve and flow cytometric analyses. After the induction of p57, retinoblastoma protein, p107, and E2F-1 levels diminish, and retinoblastoma protein is shifted to a hypophosphorylated form. Morphologically, p57-induced astrocytoma cells became large and flat with an expanded cytoplasm. The inducible expression of p57 leads to the accumulation of senescence-associated beta-galactosidase marker within all astrocytoma cell lines such that approximately 75% of cells were positive at 1 week after induction. Induction of p57 in U373 astrocytoma cells generated a small population of cells ( approximately 15%) that were nonviable, contained discrete nuclear fragments on Hoechst 33258 staining, and demonstrated ultrastructural features characteristic of apoptosis. Examination of bax and poly-(ADP ribose) polymerase levels showed no change in bax, but decreased expression of poly-(ADP ribose) polymerase after p57 induction in all astrocytoma cell lines. These data demonstrate that the proliferative block imposed by p57 on human astrocytoma cells results in changes in the expression of a number of cell cycle regulatory factors, cell morphology, and a strong stimulus to cell senescence.

Phorbol 12-myristate 13-acetate induces protein kinase ceta-specific proliferative response in astrocytic tumor cells

Protein kinase C (PKC) activation has been implicated in cellular proliferation in neoplastic astrocytes. The roles for specific PKC isozymes in regulating this glial response, however, are not well understood. The aim of this study was to characterize the expression of PKC isozymes and the role of PKC-eta expression in regulating cellular proliferation in two well characterized astrocytic tumor cell lines (U-1242 MG and U-251 MG) with different properties of growth in cell culture. Both cell lines expressed an array of conventional (alpha, betaI, betaII, and gamma) and novel (theta and epsilon) PKC isozymes that can be activated by phorbol myristate acetate (PMA). Another novel PKC isozyme, PKC-eta, was only expressed by U-251 MG cells. In contrast, PKC-delta was readily detected in U-1242 MG cells but was present only at low levels in U-251 MG cells. PMA (100 nm) treatment for 24 h increased cell proliferation by over 2-fold in the U-251 MG cells, whereas it decreased the mitogenic response in the U-1242 MG cells by over 90%. When PKC-eta was stably transfected into U-1242 MG cells, PMA increased cell proliferation by 2.2-fold, similar to the response of U-251 MG cells. The cell proliferation induced by PMA in both the U-251 MG and U-1242-PKC-eta cells was blocked by the PKC inhibitor bisindolylmaleimide (0.5 micrometer) and the MEK inhibitor, PD 98059 (50 micrometer). Transient transfection of wild type U-251 with PKC-eta antisense oligonucleotide (1 micrometer) also blocked the PMA-induced increase in [(3)H]thymidine incorporation. The data demonstrate that two glioblastoma lines, with functionally distinct proliferative responses to PMA, express different novel PKC isozymes and that the differential expression of PKC-eta plays a determining role in the different proliferative capacity.

Gene therapy for arthritis

Abstract An accumulating body of evidence shows that gene therapy can be successfully used to treat animal models of arthritis. Based on this success, a clinical trial of gene therapy for rheumatoid arthritis has been initiated. We review the methods and genes used for the previous gene transfer experiments, including our own. Retroviral ex vivo gene transfer and adenoviral in vivo gene transfer were utilized most frequently. Most of the gene transfer strategies aimed at suppression of synovial inflammation, while our study attempted to convert a phenotype of synovial cells. Gene transfer could be used for part of the future therapy for RA. In basic research studies, gene transfer is of great help in defining new target molecules to treat arthritis.

Long-term cultured IL-2-dependent T cell lines demonstrate p16(INK4a) overexpression, normal pRb/p53, and upregulation of cyclins E or D2

Acquisition of an immortal phenotype by circumvention of the normal senescence program can be an important step in tumor development and progression. The regulation of life-span checkpoints is complex and abrogation of these processes can occur at different levels. To better understand these mechanisms in long-term cultured lymphocytes we have characterized two human long-term cultured IL-2-dependent T cell lines regarding telomere length, telomerase activity, and the expression of selected cell cycle regulators (pRb, p53, cyclin E, cyclin D1, cyclin D2, cyclin D3, cdk4, p16(INK4a), p21(WAF1), p27(KIP1), c-myc, bcl-2, and NPAT). We compared these cell lines with a primary T lymphoblast population with a limited life span from the same donor. Both T cell lines with extraordinary growth capacity showed telomere length stabilization, high telomerase activity and demonstrated wild-type pattern of pRb and p53 but strong p16(INK4a) protein expression. The growth inhibitory activity of p16(INK4a) seemed to be abrogated by enhanced expression of cyclin D2, cdk4, and c-myc in one T cell line and overexpression of cyclin E in the second T cell line.

Expression of p16/INK4a in posttransplantation lymphoproliferative disorders

It was recently demonstrated that classification of posttransplantation lymphoproliferative disorders (PT-LPDs) into morphological and molecular categories is clinically relevant. It was also reported that PT-LPD not associated with Epstein-Barr virus (EBV) had a more aggressive course than most lesions associated with EBV. Because the cyclin-dependent kinase inhibitor p16/INK4a has been reported to be frequently inactivated in high-grade lymphomas, we evaluated 17 PT-LPD to determine whether p16/INK4a expression could be correlated to morphology, EBV detection, and a Ki-67 labeling index. We demonstrated that tumors with no p16/INK4a expression (n = 8) had a predominantly monomorphic appearance, and most were EBV negative (respectively, 7/8 and 5/8), whereas lesions with p16/INK4a expression (n = 9) were mostly polymorphic PT-LPD (6/9) (P = 0.049) and associated with EBV (9/9) (P = 0.015). In particular, strong p16/INK4a expression was observed in atypical immunoblasts and Reed-Sternberg-like cells. Furthermore, the proliferation index was significantly higher in tumors lacking p16/INK4a expression than in other lesions (P = 0.0008). In conclusion, down-regulation of p16/INK4a was mostly observed in PT-LPD lesions known to follow more aggressive courses: monomorphic tumors and EBV-negative PT-neoplasms. Conversely, overexpression of p16/INK4a was associated with EBV-positive PT-LPD. While p16/INK4a might play a role in the proliferative rate of LP-LPD, further investigations are needed to assess the clinical relevance of p16/INK4a expression in predicting the evolution of tumors and to explain how EBV could favor p16/INK4a protein accumulation in lesions.

LMP1 of Epstein-Barr virus suppresses cellular senescence associated with the inhibition of p16INK4a expression

Epstein-Barr virus is associated with a number of human proliferative and malignant diseases. It is capable of immortalizing human primary B-lymphocytes in vitro. Studies indicate that latent membrane protein LMP1 is one of the viral proteins essential for this process. In this report, LMP1 was shown to prevent primary mouse embryonic fibroblasts from entering into replicative senescence in vitro. It further suppresses the senescence-associated induction of p16INK4a, commonly believed to be a key regulator of replicative senescence. In addition, LMP1 was shown to prevent premature senescence provoked by oncogenic ras in mouse embryonic fibroblasts, and to inhibit the oncogene ras-mediated induction of p16INK4a and p21WAF1. In parallel, LMP1 also prevents ras-induced premature senescence in rat embryonic fibroblasts REF52 and human diploid fibroblasts IMR90. Moreover, LMP1 is capable of suppressing the p16INK4a promoter in REF52 and Saos-2 cells in a promoter reporter assay. Our findings suggest that with the expression of p16INK4a and replicative senescence being suppressed, LMP1 may play a key role in Epstein-Barr virus-associated proliferative diseases, and it may further contribute to cancer development by preventing premature senescence induced by mitogenic oncogenes.

p16 INK4a can initiate an autonomous senescence program

The tumor suppressor p16INK4a is a potent mediator of cell cycle arrest in transient expression studies, is induced in senescing cells, and can impose morphological features of senescence. Nonetheless, it is unclear whether p16INK4a can block cell proliferation irreversibly. We explored this issue using osteogenic sarcoma cell clones with inducible p16INK4a expression. Induction of p16INK4a for 1 day arrested most cells in G1 phase. If the induction was then interrupted, p16INK4a levels returned to baseline and robust growth resumed within 3-5 days. When p16INK4a was induced for 6 days DNA synthesis remained strongly inhibited and the cells acquired morphological features of senescence. Moreover, if p16INK4a induction was interrupted at this point and the cells were followed for 12 more days, most cells retained these morphologic features and either failed to divide or died. This occurred despite the prompt return of p16INK4a expression and retinoblastoma protein phosphorylation toward baseline levels. In fact, some senescing cells appeared to enter S phase. These results demonstrate that a sustained period of p16INK4a expression is sufficient in this setting to impose a durable block to cell proliferation and that this state becomes independent of p16INK4a expression, hypophosphorylation of pRB, or a strict G1 arrest.

p16/MTS1/INK4A suppresses prostate cancer by both pRb dependent and independent pathways

Tumor suppressor gene p16 is a cyclin-dependent kinase inhibitor and an important negative cell cycle regulator. The inactivation of p16 appears to be a common event in prostate cancer. Replacement of p16 inhibits prostate tumor cell growth, but the mechanism is not known. Human prostate cancer cell lines PPC-1, which has an inactivated p16, and DU145, which has a nonfunctional retinoblastoma Rb protein (pRb), were used to determine the possible mechanism of p16 mediated growth inhibition. PPC-1 cells treated with 5-aza-2'-deoxycytidine (5-aza-dC), a demethylating agent, induced p16 expression, inhibited cell growth, and induced senescence. Similarly, PPC-1 cells transduced by an adenoviral vector containing the p16 gene (AdRSVp16) produced a p16 protein that suppressed cellular proliferation and induced senescence. Co-staining of AdRSVp16-transduced PPC-1 cells by p16 immunohistochemistry and by beta-galactosidase substrate X-gal showed that the morphologically enlarged cells expressed both p16 and senescence-associated beta-galactosidase. In contrast, AdRSVp16 did not induce senescence in DU145 cells, but did inhibit its growth. However, when wild-type pRb was introduced in DU145 cells, AdRSVp16 was able to induce senescence. Thus, the mechanism by which p16 suppressed prostate cancer was dependent on the pRb functional status of cells whereby p16 caused pRb+ cells to undergo inhibition by senescence, whereas pRb- cells were also inhibited, but not by senescence.

Telomere shortening is an in vivo marker of myocyte replication and aging

To determine whether adult cardiac myocytes are capable of multiple divisions and whether this form of growth is restricted to a subpopulation of cells that retain this capacity with age, telomere lengths were measured in myocyte nuclei isolated from the left ventricle of fetal and neonatal Fischer 344 rats and rats at 4, 12, and 27 months after birth. Two independent methodologies were used for this analysis: laser scanning cytometer and confocal microscopy. In each case, fluorescence intensity of a peptide nucleic acid probe specific for telomeric sequence was evaluated. The two techniques yielded comparable results. Telomeric shortening increased with age in a subgroup of myocytes that constituted 16% of the entire cell population. In the remaining nondividing cells, progressive accumulation of a senescent associated nuclear protein, p16(INK4), was evidenced. In conclusion, a significant fraction of myocytes divides repeatedly from birth to senescence, counteracting the continuous death of cells in the aging mammalian rat heart.

Retinoic acid extends the in vitro life span of normal human oral keratinocytes by decreasing p16(INK4A) expression and maintaining telomerase activity

Retinoic acid (RA) plays an important role in the regulation of cell growth and differentiation. To investigate whether RA extends in vitro the life span of human epithelial cells, we examined the effect of all-trans RA on both the cumulative population-doubling level (PDL) and the replicative senescence of cultured oral keratinocytes. When proliferating oral keratinocytes were cultured in medium containing 1 nM of all-trans RA, the in vitro life span of the cells was increased 1.5- to 1.8-fold compared to the vehicle control and the replicative senescence of the cells was significantly inhibited. Since the replicative senescence of human epithelial cells is associated with a steady increase of p16(INK4A) and a loss of telomerase activity, we expected that RA could delay the replicative senescence of oral keratinocytes by decreasing p16(INK4A) expression and/or inhibiting the loss of telomerase activity. To test this possibility, we examined the expression of replicative senescence-associated genes and the telomerase activities of different PDL numbers of oral keratinocytes exposed to 1 nM of all-trans RA. The protein level of cellular p16(INK4A) in the RA-treated oral keratinocytes was gradually but significantly enhanced by an increased PDL number; however, the level was significantly lower than that of the vehicle control at all of the same PDL numbers. In contrast, the telomerase activity was maintained in oral keratinocytes with increasing PDL numbers induced by RA treatment. Summarizing, these results indicate that RA induces the in vitro life-span extension of oral keratinocytes, which is linked to a decreased cellular level of p16(INK4A) and the maintenance of telomerase activity.

Melanoma

Cutaneous melanoma is becoming more common. Although still rarer than some cancers it is important, particularly as it afflicts young adults relatively frequently. Furthermore early detection with resultant better prognosis should be achievable if the public was to become more aware, and if health-care professionals were more familiar with melanoma and the behaviour of normal moles.

Replicative senescence in human uroepithelial cells

PURPOSE

Normal human uroepithelial cells (HUCs) proliferate rapidly in culture during early passage and then spontaneously undergo replicative senescence. We previously reported that the cyclin D1-CDK4/6 inhibitor, p16INK4a, is elevated at senescence in HUCs. Hence, we proposed that p16INK4a may play a critical role in mediating senescence in this cell type. In the current study, we further characterized the senescent state in HUCs. We also tested the possible roles of changes in other cell cycle proteins, including p53, p21WAF1, pRb, and cyclin D1 in HUC senescence.

METHODS

Normal HUCs cultured from explants of ureteral mucosa were used for these studies. Senescence associated-beta-galactosidase activity (SA-beta-gal) was used to identify cells in senescence. Flow cytometric analysis was used to determine changes in cell cycle distribution at senescence. Response of cells to serum stimulation was determined by Northern analysis of c-fos. Western analysis was used to assess changes in p53, p21WAF, p16INK4a, cyclin D1 and plasminogen activator inhibitor-1 (PAI-1) levels at senescence.

RESULTS

beta-gal-positive HUCs were blocked at G1/S in senescence and failed to show c-fos induction in response to serum stimulation. As previously reported, senescent HUCs also showed elevated p16INK4a. However, unlike human fibroblasts, neither p53 nor p21WAF1 elevation accompanied HUCs senescence. PAI-1 levels were also not elevated in HUC senescence.

CONCLUSION

These findings support a model in which elevation of p16INK4a, but not p53 or p21WAF1 plays a critical role in HUC replicative senescence. These findings elucidate the tumor suppressor mechanism of p16INK4a and the frequent loss of either p16INK4a or pRb in invasive human bladder tumors.

Functional evaluation of tumour-specific variants of p16INK4a/CDKN2A: correlation with protein structure information

Inherited mutations in the CDKN2A/INK4a/MTS1 tumour suppressor gene on chromosome 9p21 are associated with familial predisposition to melanoma and other tumour types. Nonsense and missense mutations are also found in a variety of sporadic cancers, and over 140 sequence variants have already been recorded in the literature. In assessing the relevance of these variants and for counselling members of affected families, it is important to distinguish inactivating mutations from harmless polymorphisms. Existing functional assays have frequently reached conflicting conclusions and no single test appears adequate. Here we evaluate a number of alternatives including a novel assay based on retroviral delivery of p16INK4a cDNAs into human diploid fibroblasts. Among the 17 sequence variants analysed, three distinct categories can be distinguished: those that abrogate the binding of p16INK4a to CDK4 and CDK6, those that alter the properties of the protein without preventing it from interacting with CDKs, and those that have no discernible effect on protein function. These distinctions can be rationalized by considering the impact of the amino acid changes on the three-dimensional structure of the protein.

Role of p53 and p21waf1/cip1 in senescence-like terminal proliferation arrest induced in human tumor cells by chemotherapeutic drugs

Exposure of human tumor cell lines to moderate doses of anticancer agents induces terminal proliferation arrest accompanied by morphologic and enzymatic changes that resemble senescence of normal cells. We have investigated the role of p53 and p21waf1/cip1 in the induction of this response in drug-treated tumor cells. Doxorubicin treatment induced the senescence-like phenotype (SLP) and its associated terminal growth arrest in wild-type HCT116 colon carcinoma cells; this response was strongly decreased but not abolished in HCT116 lines with homozygous knockout of p53 or p21. Transduction of HT1080 fibrosarcoma cells with a genetic inhibitor of p53 also decreased the induction of SLP and increased drug-induced mitotic cell death. To determine if drug-stimulated p21 expression was responsible for senescence-like growth arrest, we have expressed different levels of p21 from an inducible promoter. While high-level overexpression of p21 was sufficient to induce SLP in HT1080 cells, the levels of p21 expressed in doxorubicin-treated cells could account for only a fraction of doxorubicin-induced SLP. Our results indicate that p53 and p21 act as positive regulators of senescence-like terminal proliferation arrest, but their function is neither sufficient nor absolutely required for this treatment response in tumor cells.

Telomerase-independent senescence of human immortal cells induced by microcell-mediated chromosome transfer

Maintenance of telomeres, commonly through expression of telomerase activity, is necessary but may not be sufficient for human cells to escape from the cellular senescence program and become immortal. We report here that human tumor cells could undergo cellular senescence in the presence of telomerase activity when a specific normal human chromosome was introduced via microcell-mediated chromosome transfer. The cell models studied include SiHa (uterine cervical carcinoma cells expressing E6 and E7 oncoproteins of human papillomavirus type 16) with a transferred chromosome 2, CC1 (choriocarcinoma cells expressing an amino-terminally truncated p53 protein) with a transferred chromosome 7, and JTC-32 (bladder carcinoma cells) with a transferred chromosome 11. The microcell hybrids with the indicated chromosomes ceased to divide after five to 10 population doublings and showed senescence-associated beta-galactosidase activity but still expressed the genes encoding three components of human telomerase, consistent with the retention of telomerase activity. These results are evidence for barriers to human cell immortalization, which involve activation of unidentified senescence-inducing genes that function independently of inactivation of telomerase.

Induction of the p16INK4a senescence gene as a new therapeutic strategy for the treatment of rheumatoid arthritis

Synovial tissue affected by rheumatoid arthritis is characterized by proliferation, which leads to irreversible cartilage and bone destruction. Current and experimental treatments have been aimed mainly at correcting the underlying immune abnormalities, but these treatments often prove ineffective in preventing the invasive destruction. We studied the expression of cyclin-dependent kinase inhibitors in rheumatoid synovial cells as a means of suppressing synovial cell proliferation. Synovial cells derived from hypertrophic synovial tissue readily expressed p16INK4a when they were growth-inhibited. This was not seen in other fibroblasts, including those derived from normal and osteoarthritis-affected synovial tissues. In vivo adenoviral gene therapy with the p16INK4a gene efficiently inhibited the pathology in an animal model of rheumatoid arthritis. Thus, the induction of p16INK4a may provide a new approach to the effective treatment of rheumatoid arthritis.

p21Waf1/Cip1/Sdi1 induces permanent growth arrest with markers of replicative senescence in human tumor cells lacking functional p53

We have shown previously that wild type p53 can rapidly induce replicative senescence in EJ human bladder carcinoma cells lacking functional p53. A major effector of p53 functions is p21Waf1/Cip1/Sdi1, a potent cyclin-dependent kinase inhibitor. p21Waf1/Cip1/Sdi1 has been shown to be involved in both p53 dependent and independent control of cell proliferation, differentiation and death. To directly investigate the effects of p21Waf1/Cip1/Sdi1 in the p53 response observed in EJ tumor cells, we established p21Waf1/Cip1/Sdi1 inducible lines using the tetracycline-regulatable vector system. p21Waf1/Cip1/Sdi1 induction caused irreversible cell cycle arrest in both G1 and G2/M, and diminished Cdk2 kinase activity. In addition, p21Waf1/Cip1/Sdi1 induction led to morphological alterations characteristic of cells undergoing replicative senescence with morphological, biochemical and ultrastructural markers of the senescent phenotype. Furthermore, sustained p21Waf1/Cip1/Sdi1 induction sensitized EJ cells to apoptotic cell death induced by mitomycin C, a cross-linking DNA damaging agent. These findings support the function of p21Waf1/Cip1/Sdi1 as an inducer of replicative senescence and a major mediator of this phenomenon in response to p53. Moreover, our results imply that therapeutic intervention in human cancers might be aimed at sustained elevation of p21Waf1/Cip1/Sdi1 expression.

Induction of p21(WAF1/CIP1) and inhibition of Cdk2 mediated by the tumor suppressor p16(INK4a)

The tumor suppressor p16(INK4a) inhibits cyclin-dependent kinases 4 and 6. This activates the retinoblastoma protein (pRB) and, through incompletely understood events, arrests the cell division cycle. To permit biochemical analysis of the arrest, we generated U2-OS osteogenic sarcoma cell clones in which p16 transcription could be induced. In these clones, binding of p16 to cdk4 and cdk6 abrogated binding of cyclin D1, p27(KIP1), and p21(WAF1/CIP1). Concomitantly, the total cellular level of p21 increased severalfold via a posttranscriptional mechanism. Most cyclin E-cdk2 complexes associated with p21 and became inactive, expression of cyclin A was curtailed, and DNA synthesis was strongly inhibited. Induction of p21 alone, in a sibling clone, to the level observed during p16 induction substantially reproduced these effects. Overexpression of either cyclin E or A prevented p16 from mediating arrest. We then extended these studies to HCT 116 colorectal carcinoma cells and a p21-null clone derived by homologous recombination. In the parental cells, p16 expression also augmented total cellular and cdk2-bound p21. Moreover, p16 strongly inhibited DNA synthesis in the parental cells but not in the p21-null derivative. These findings indicate that p21-mediated inhibition of cdk2 contributes to the cell cycle arrest imposed by p16 and is a potential point of cooperation between the p16/pRB and p14(ARF)/p53 tumor suppressor pathways.

Induced p21waf expression in H1299 cell line promotes cell senescence and protects against cytotoxic effect of radiation and doxorubicin

The CDK inhibitor p21waf is a principal mediator of p53 function but can also be transactivated by many p53-independent stimuli leading to cell growth arrest or differentiation. In order to study the function of p21waf in a p53-deficient environment, we established an inducible expression of p21waf in the p53-null lung cancer cell line H1299, based on the muristerone-regulated system. Overexpression of p21waf led cells to growth arrest which after several days became irreversible and the arrested cells acquired a senescent phenotype as judged by cell shape, the senescence-associated beta-gal marker and inhibition of colony formation. The effect of p21waf overexpression, in the absence of p53, on the cytotoxicity caused by irradiation, doxorubicin and taxol was studied. Expression of p21waf provided protection against the cytotoxic effect of radiation and doxorubicin but not of taxol. These results are relevant to treatment of cancer when p53 is inactive.

Induced expression of p16(INK4a) inhibits both CDK4- and CDK2-associated kinase activity by reassortment of cyclin-CDK-inhibitor complexes

To investigate the mode of action of the p16(INK4a) tumor suppressor protein, we have established U2-OS cells in which the expression of p16(INK4a) can be regulated by addition or removal of isopropyl-beta-D-thiogalactopyranoside. As expected, induction of p16(INK4a) results in a G1 cell cycle arrest by inhibiting phosphorylation of the retinoblastoma protein (pRb) by the cyclin-dependent kinases CDK4 and CDK6. However, induction of p16(INK4a) also causes marked inhibition of CDK2 activity. In the case of cyclin E-CDK2, this is brought about by reassortment of cyclin, CDK, and CDK-inhibitor complexes, particularly those involving p27(KIP1). Size fractionation of the cellular lysates reveals that a substantial proportion of CDK4 participates in active kinase complexes of around 200 kDa. Upon induction of p16(INK4a), this complex is partly dissociated, and the majority of CDK4 is found in lower-molecular-weight fractions consistent with the formation of a binary complex with p16(INK4a). Sequestration of CDK4 by p16(INK4a) allows cyclin D1 to associate increasingly with CDK2, without affecting its interactions with the CIP/KIP inhibitors. Thus, upon the induction of p16(INK4a), p27(KIP1) appears to switch its allegiance from CDK4 to CDK2, and the accompanying reassortment of components leads to the inhibition of cyclin E-CDK2 by p27(KIP1) and p21(CIP1). Significantly, p16(INK4a) itself does not appear to form higher-order complexes, and the overwhelming majority remains either free or forms binary associations with CDK4 and CDK6.

Exogenous expression of p16INK4a is associated with decrease in telomerase activity

In this study, gene transfection was used to determine whether the exogenous expression of p16INK4a modulated the biological characteristics of glioblastoma cells. The human glioblastoma cell line U87MG was doubly transfected with the plasmids pVgRXR and pIND harboring the wild-type p16 gene. The expression of p16INK4a in the resulting transfectants was regulated by the addition of the ecdysone homologue, muristerone A. When the cells expressed p16INK4a, their growth capacity was reduced and morphological changes such as an increase in cell size and cellular flattening were observed. The analysis of cell cycle regulation provided evidence that cells expressing p16INK4a were inhibited from entry into the cell cycle, as assessed by Ki-67 antigen expression. In addition, it was observed that the exogenous expression of p16INK4a was associated with decrease in telomerase activity.

Transforming growth factor beta triggers two independent-senescence programs in cancer cells

Transforming growth factor-beta (TGF-beta)TG has been shown to play a multifunctional role in tumorigenesis. Here we demonstrate that TGF-beta induces a morphological change and expression of senescence-associated beta-galactosidase activity in the human lung adenocarcinoma cell line A549 cells within a week after the addition. These TGF-beta induced phenotypic changes are thought to characterize the rapid onset of senescence. When A549 cells were treated with TGF-beta, cell growth was not completely arrested, but the activity of telomerase was down regulated via transcriptional repression of telomerase reverse transcriptase, which led to a shortening of the telomere during long-term culture and finally resulted in replicative senescence. These results indicate that TGF-beta is able to induce a rapid senescence in A549 cells without significantly inhibiting cell growth and can further direct A549 cells to a replicative senescence state via the suppression of telomerase which culminates in telomere shortening. All these experimental results suggest that TGF-beta transmits several separate and independent signals to shift A549 cells back to a normal senescent cell.

Premature senescence involving p53 and p16 is activated in response to constitutive MEK/MAPK mitogenic signaling

Oncogenic Ras transforms immortal rodent cells to a tumorigenic state, in part, by constitutively transmitting mitogenic signals through the mitogen-activated protein kinase (MAPK) cascade. In primary cells, Ras is initially mitogenic but eventually induces premature senescence involving the p53 and p16(INK4a) tumor suppressors. Constitutive activation of MEK (a component of the MAPK cascade) induces both p53 and p16, and is required for Ras-induced senescence of normal human fibroblasts. Furthermore, activated MEK permanently arrests primary murine fibroblasts but forces uncontrolled mitogenesis and transformation in cells lacking either p53 or INK4a. The precisely opposite response of normal and immortalized cells to constitutive activation of the MAPK cascade implies that premature senescence acts as a fail-safe mechanism to limit the transforming potential of excessive Ras mitogenic signaling. Consequently, constitutive MAPK signaling activates p53 and p16 as tumor suppressors.

The p16(INK4A) protein and flavopiridol restore yeast cell growth inhibited by Cdk4

Cyclin-dependent kinase 4 (Cdk4) activity is misregulated in most cancers. Loss of Cdk4 regulation can occur through overexpression of Cdk4 catalytic subunit or its regulatory partner cyclin D1, or if the Cdk4-specific inhibitory protein p16(INK4A) is inactive. We have attempted to express the two human subunits, Cdk4 and cyclin D1, in the yeast Saccharomyces cerevisiae. Surprisingly, expression of Cdk4 alone, under control of the strong GAL promoter, inhibits cell growth. Coexpression of both subunits allows formation of an active Cdk4-cyclin D1 complex which accentuates growth arrest. In cells expressing Cdk4 only, growth is restored by overexpressing human Cdc37, a Cdk4-binding molecular chaperone. Interestingly, the effect of Cdk4 on yeast is also overcome by both p16- and p21-families of Cdk-inhibitory proteins. Moreover, flavopiridol, a compound which inhibits Cdk4 enzyme activity, restores cell division. The fact that p16(INK4A) and flavopiridol negate Cdk4-mediated suppression of yeast cell growth implies that this simple system can be used as a screen for identifying Cdk4-specific antagonists which may mimic p16(INK4A) in the cancer cell cycle.

Features of replicative senescence induced by direct addition of antennapedia-p16INK4A fusion protein to human diploid fibroblasts

The p16INK4A cyclin-dependent kinase (Cdk) inhibitor is now recognized as a major tumor suppressor that is inactivated by a variety of mechanisms in a wide range of human cancers. It is also implicated in the mechanisms underlying replicative senescence since p16INK4A RNA and protein accumulate as cells approach their proscribed limit of population doublings in tissue culture. To obtain further evidence of its role in senescence, we have sought ways of overexpressing p16INK4A in primary human diploid fibroblasts (HDF). To circumvent the low transfection efficiency of primary cells we have exploited a recombinant form of the full-length p16INK4A protein fused to a 16 amino acid peptide from the Drosophila antennapedia protein. This peptide has the capacity to cross both cytoplasmic and nuclear membranes allowing the direct introduction of the active protein to primary cells. Here, we show that antennapedia-tagged wild-type p16INK4A protein, but not a functionally compromised tumor-specific variant, causes G1 arrest in early passage HDFs by inhibiting the phosphorylation of the retinoblastoma protein. Significantly, the arrested cells display several phenotypic features that are considered characteristic of senescent cells. These data support a role for p16INK4A in replicative senescence and raise the possibility of using the antennapedia-tagged protein therapeutically.

Inhibitors of cyclin-dependent kinases induce features of replicative senescence in early passage human diploid fibroblasts

After a limited number of population doublings (PDs), cultures of normal mammalian diploid cells undergo an irreversible growth arrest known as replicative senescence [1]. As well as contributing to cellular ageing, senescence is viewed as an important mechanism of tumour suppression by preventing the emergence of immortal cell clones [2-4]. Senescent cells have a number of characteristics that distinguish them from cycling or quiescent cells including elevated levels of two cyclin-dependent kinase (Cdk) inhibitors, p16INK4a and p21CIP1 [5-11]. Here, we demonstrate that both of these Cdk inhibitors, as well as other members of their protein families (the INK4 and CIP/KIP families, respectively [12]), induce several facets of the senescent phenotype when ectopically expressed in young human diploid fibroblasts. These include a reduced proliferative capacity, an altered size and shape, the presence of underphosphorylated retinoblastoma protein (pRb), increased expression of plasminogen activator inhibitor (PAI-1) and the appearance of senescence-associated beta-galactosidase (SA-beta-gal) activity [2,3,13-15]. A 20 amino acid peptide from p16INK4a that inhibits Cdks active in the G1 phase of the cell cycle [16] produces similar effects in a dose-dependent manner suggesting that, in primary fibroblasts, inhibition of G1-specific Cdk activity is sufficient to induce phenotypic changes that normally occur at the end of their finite lifespan.