ARF promotes MDM2 degradation and stabilizes p53: ARF-INK4a locus deletion impairs both the Rb and p53 tumor suppression pathways

[Cyclin dependent kinase inhibitors and replicative senescence]

Replicative senescence is defined for human diploid fibroblasts in culture as a cell growth arrest appearing beyond 50 +/- 10 population doublings and associated with telomeres' shortening. This phenomenon shows an increased expression of growth cell inhibitors: p21Waf1 described as an universal CDK inhibitor and p16INK4a as a specific inhibitor for both G1 phase kinases CDK4 and CDK6. The cell proliferation inhibitor p14ARF, product of INK4a/ARF locus is involved in replicative senescence too. Overexpression or homozygotic deletion of these inhibitors demonstrated their role in senescence induction. These proteins are involved in two different metabolic pathways, the first including p53, represented by E2F, ARF, MDM2, p53, p21Waf1, and the second concerning pRb and p16INK4a. These two pathways present numerous interactions and the polymerase (PARP) in relation with p53 and activated by telomere shortening might represent via p21Waf1 a link between this shortening and cell cycle control. An another metabolic pathway involving PTEN and p27KIP1 is discussed in senescent-like phenotype induction, but its activity in replicative senescent is uncertain.

Concurrent inactivation of RB1 and TP53 pathways in anaplastic oligodendrogliomas

Oligodendrogliomas are characterized by frequent loss of heterozygosity (LOH) on chromosomes 1p and 19q, but additional genetic alterations are likely to be involved. In this study, we screened 28 oligodendrogliomas (WHO grade II) and 20 anaplastic oligodendrogliomas (WHO grade III) for alterations in the RB1/CDK4/p16INK4a/p15INK4b and TP53/p14ARF/MDM2 pathways. In oligodendrogliomas, hypermethylation of RB1 (1 case) and p14ARF (6 cases) were the only detectable genetic changes (7/28, 25%). In anaplastic oligodendrogliomas, the RB1/CDK4/p16INK4a/p15INK4b signaling pathway regulating the G1 -->3 S transition of the cell cycle was altered in 13/20 (65%) cases, by either RBI alteration, CDK4 amplification, or p16IN4a/p15INK4b homozygous deletion or promoter hypermethylation. Further, 50% (10/20) of anaplastic oligodendrogliomas showed alterations in the TP53 pathway through promoter hypermethylation or homozygous deletion of the p14ARF gene and, less frequently, through TP53 mutation or MDM2 amplification. Of 13 anaplastic astrocytomas with an altered RB1 pathway, 9 (69%) also showed a dysregulated TP53 pathway. Thus, simultaneous disruption of the RB1/CDK4/p16INK4a/p15INK4b and the TP53/p14ARF/MDM2 pathways occurs in 45% (9/20) of anaplastic oligodendrogliomas, suggesting that these phenomena contribute to their malignant phenotype.

Alterations of INK4a(p16-p14ARF)/INK4b(p15) expression and telomerase activation in meningioma progression

Dysregulation of cell cycle progression and telomerase activation have been implicated in malignant tumor progression as well as in the evasion of senescence and immortalization. We have investigated expression of the cell cycle control and tumor suppressor genes INK4a(p16-p14ARF), INK4b(p15-p10) and RB, and their relation to telomerase activation during malignant meningioma progression. 7/26 (27%) benign, 3/12 (25%) atypical but 4/7 (57%) anaplastic tumors lacked both, p16 and p15 protein expression. 14/39 (36%) benign and atypical but 5/7 (71%) anaplastic meningiomas contained no p14ARF mRNA. 2/46 (4%) tumors failed to express pRB. We observed frequent differential loss of expression of the alternatively spliced INK4a tumor suppressors p16 and p14ARF. Exclusive expression of the alternative INK4b transcript p10 possibly at the expense of p15 and therefore resulting in loss of p15 tumor suppressor activity was noted in two meningiomas. We have previously described telomerase activity or expression of the telomerase catalytic subunit hTERT in this meningioma series. Telomerase activation was detected in 10/27 (37%) benign, but 18/19 (95%) non-benign meningiomas. We observed no significant overall correlation between loss of INK4a/INK4b expression and telomerase activation. In conclusion, our results suggest a greater role for losses of INK4a/INK4b gene products in meningioma formation and malignant progression than previously thought. Inactivation of p16/p15- and pl4ARF-dependent pathways possibly in conjunction with telomerase activation might be critical steps for a meningioma cell towards escape from senescence, that is, immortalization.

Aberrations of the p14(ARF) and p16(INK4a) genes in renal cell carcinomas

The INK4a / ARF locus on chromosome 9p21, which encodes two distinct genes, p14(ARF) and p16(INK4a), is frequently altered in human neoplasms. To investigate the potential roles of p14(ARF) and p16(INK4a) genes in human renal cell carcinomas (RCCs), we analyzed 6 human RCC cell lines and 91 primary RCCs for homozygous deletion, promoter hypermethylation and expression of the p14(ARF) and p16(INK4a) gene products using differential PCR, methylation-specific PCR, and immunohistochemistry, respectively. Five cell lines showed homozygous co-deletion of both genes and one demonstrated promoter hypermethylation of the p16(INK4a) gene only. Eight of 91 RCCs showed aberrations of p14(ARF) or p16(INK4a) status and six of these featured gross extension into the renal vein. The results suggest that p14(ARF) and p16(INK4a) aberrations may play roles in the relatively late stage of renal tumorigenesis associated with tumor progression.

Biological activities and molecular targets of the human papillomavirus E7 oncoprotein

The human papillomavirus (HPV) E7 protein is one of only two viral proteins that remain expressed in HPV-associated human cancers. HPV E7 proteins share structural and functional similarities with oncoproteins encoded by other small DNA tumor viruses such as adenovirus E1A and SV40 large tumor antigen. The HPV E7 protein plays an important role in the viral life cycle by subverting the tight link between cellular differentiation and proliferation in normal epithelium, thus allowing the virus to replicate in differentiating epithelial cells that would have normally withdrawn from the cell division cycle. The transforming activities of E7 largely reflect this important function.

Defining the molecular basis of Arf and Hdm2 interactions

Understanding the interaction of Arf and Hdm2 has recently become a central issue in cancer biology. In response to hyperproliferative signals, p14(Arf) stabilizes p53 by binding to Hdm2 and inhibits the ubiquitination and subsequent proteosome-dependent degradation of p53. The medical importance of the Arf-Hdm2-p53 regulatory system is highlighted by the finding that either p53 or p14(Arf) are lost or modified in virtually all human cancers. Isolated Arf and Hdm2 domains are dynamically disordered in solution, yet they retain the ability to interact in vitro and in cellular assays. Upon binding, domains of both Arf and Hdm2 undergo a dramatic transition from disordered conformations to extended structures comprised of beta-strands. The presence of domains from both proteins are necessary and sufficient for the formation of the highly stable extended beta structures. We have mapped sites within Arf and Hdm2 that interact at a resolution of five amino acid residues using surface plasmon resonance. Surface plasmon resonance and circular dichroism spectropolarimetry confirm the presence of multiple interaction domains within each protein. Both p14(Arf) (human) and p19(Arf) (mouse) interact with Hdm2 through two short motifs present in their N termini. The Arf interacting region of Hdm2 is also composed of two short sequences located in the central acidic domain, between residues 235-264 and 270-289. The binding-induced structural transition is also induced by short peptides, 15 amino acids in length, that contain the binding motifs. Micro-injection and live cell imaging of proteins tagged with fluorescent labels was used to confirm the in vivo function of the interaction domains. Arf and Hdm2 thus appear to interact through a novel mechanism that exerts control over the cell division cycle. The novel molecular mechanism of interaction and the limited size of the protein domains involved provide opportunities for the development of anticancer therapeutics.

Mutations in the INK4a/ARF melanoma susceptibility locus functionally impair p14ARF

The INK4a/ARF locus encodes two cell cycle regulatory proteins, the cyclin-dependent kinase inhibitor, p16(INK4a), and the p53 activator, p14(ARF). Germline mutations in this locus are associated with melanoma susceptibility in 20-40% of multiple case melanoma families. Many of these mutations specifically impair p16(INK4a), whereas mutations uniquely targeting p14(ARF) are rare. Nevertheless, the importance of p14(ARF) has not been excluded because more than 40% of INK4a/ARF alterations affect p16(INK4a) and p14(ARF). We now report that p14(ARF) is functionally impaired in melanoma kindreds carrying INK4a/ARF mutations. Of the seven INK4a/ARF mutations tested, three altered the subcellular distribution of p14(ARF) and diminished the ability of p14(ARF) to activate the p53 pathway. This work establishes the importance of p14(ARF) in melanoma predisposition.

HER-2/neu induces p53 ubiquitination via Akt-mediated MDM2 phosphorylation

HER-2/neu amplification or overexpression can make cancer cells resistant to apoptosis and promotes their growth. p53 is crucial in regulating cell growth and apoptosis, and is often mutated or deleted in many types of tumour. Moreover, many tumours with a wild-type gene for p53 do not have normal p53 function, suggesting that some oncogenic signals suppress the function of p53. In this study, we show that HER-2/neu-mediated resistance to DNA-damaging agents requires the activation of Akt, which enhances MDM2-mediated ubiquitination and degradation of p53. Akt physically associates with MDM2 and phosphorylates it at Ser166 and Ser186. Phosphorylation of MDM2 enhances its nuclear localization and its interaction with p300, and inhibits its interaction with p19ARF, thus increasing p53 degradation. Our study indicates that blocking the Akt pathway mediated by HER-2/neu would increase the cytotoxic effect of DNA-damaging drugs in tumour cells with wild-type p53.

Immunohistochemical [corrected] detection of the alternate INK4a-encoded tumor suppressor protein p14(ARF) in archival human cancers and cell lines using commercial antibodies: correlation with p16(INK4a) expression

The INK4a locus encodes two structurally unrelated tumor suppressor proteins, p16(INK4a) and p14(ARF). Although the former is one of the most common targets for inactivation in human neoplasia, the frequency of p14(ARF) abrogation is not established. We have developed an immunohistochemical assay that allows the evaluation of p14(ARF) expression in formalin-fixed, paraffin-embedded tissues, using commercially available antibodies. p14(ARF) positive cells showed nuclear/nucleolar staining, which was absent in all cell lines and tumors with homozygous deletions of the INK4a gene. The assay was applied to 34 paraffin-embedded cell buttons, 30 non-small cell lung cancers and 28 pancreatic carcinomas, and the staining results were correlated with p16(INK4a) expression. Loss of p14(ARF) expression was common but less frequent than down-regulation of p16(INK4a) (53% versus 76% of all specimens). The p14(ARF) and p16(INK4a) expression pattern was concordant in 65 of 92 cases (71%). Significantly, 24 cases were p16(INK4a)-/p14(ARF)+, while the opposite staining pattern was observed in three cases, consistent with the notion that the two proteins have nonredundant functions. The immunohistochemical assay described here may facilitate studies on the prevalence and significance of aberrant p14(ARF) expression in human tumors.

PML interaction with p53 and its role in apoptosis and replicative senescence

A network of control pathways has been characterized that arrest growth or induce apoptosis in response to potentially tumorogenic events such as genotoxic stress or oncogene expression. Ablation, or functional disruption, of these pathways is frequently observed during multistep carcinogenesis. Analysis of those genes most commonly compromized in tumours has led to the identification of the transcription factor p53 and the E2F binding protein Retinoblastoma (Rb), as key regulators of these processes. This review discusses recent data, demonstrating that the Promyelocytic Leukemia (PML) protein can physically and functionally interact with both p53 and Rb, suggesting that PML may be a novel regulator of these pathways.

Cocompartmentalization of p53 and Mdm2 is a major determinant for Mdm2-mediated degradation of p53

The product of the Mdm2 oncogene directly interacts with p53 and promotes its ubiquitination and proteasomal degradation. Initial biological studies identified nuclear export sequences (NES), similar to that of the Rev protein from the human immunodeficiency virus, both in Mdm2 and p53. The reported phenotypes resulting from mutation of these NESs, together with results obtained using the nuclear export inhibitor leptomycin B (LMB), have led to a model according to which nuclear export of p53 (via either the NES of Mdm2 or its own NES) is required for efficient p53 degradation. In this study we demonstrate that Mdm2 can promote degradation of p53 in the nucleus or in the cytoplasm, provided both proteins are colocalized. We also investigated if nuclear export is an obligate step on the p53 degradation pathway. We find that (1) when proteasome activity is inhibited, ubiquitinated p53 accumulates in the nucleus and not in the cytoplasm; (2) Mdm2 with a mutated NES can efficiently mediate degradation of wild type p53 or p53 with a mutated NES; (3) the nuclear export inhibitor LMB can increase the steady-state level of p53 by inhibiting Mdm2-mediated ubiquitination of p53; and (4) LMB fails to inhibit Mdm2-mediated degradation of the p53NES mutant, demonstrating that Mdm2-dependent proteolysis of p53 is feasible in the nucleus in the absence of any nuclear export. Therefore, given cocompartmentalization, Mdm2 can promote ubiquitination and proteasomal degradation of p53 with no absolute requirement for nuclear to cytoplasmic transport.

Apoptosis regulators and their role in tumorigenesis

It has become clear that, together with deregulated growth, inhibition of programmed cell death (PCD) plays a pivotal role in tumorigenesis. In this review, we present an overview of the genes and mechanisms involved in PCD. We then summarize the evidence that impaired PCD is a prerequisite for tumorigenesis, as indicated by the fact that more and more neoplastic mutations appear to act by interfering with PCD. This has made the idea of restoration of corrupted 'death programs' an intriguing new area for potential cancer therapy.

Selection for loss of p53 function in T-cell lymphomagenesis is alleviated by Moloney murine leukemia virus infection in myc transgenic mice

Thymic lymphomas induced by Moloney murine leukemia virus (MMLV) have provided many examples of oncogene activation, but the role of tumor suppressor pathways in these tumors is less clear. These tumors display little evidence of loss of heterozygosity, and MMLV is only weakly synergistic with the Trp53 null genotype, suggesting that viral lymphomagenesis involves mechanisms which do not require mutational loss of Trp53 function. To explore this relationship in greater depth, we infected CD2-myc transgenic mice with MMLV and examined the role of Trp53 in the genesis of these tumors. Most (19 of 27) of the tumors from MMLV-infected, CD2-myc Trp53(+/-) mice retained the wild-type Trp53 allele in vivo while tumors of uninfected CD2-myc Trp53(+/-) mice invariably showed allele loss from a significant fraction of primary tumor cells. The functional integrity of the Trp53 gene in these tumors was indicated by ongoing allele loss or selection for mutational stabilization during in vitro propagation and by the radiosensitivity of selected Trp53(+/-) tumor cell lines. An inverse correlation was noted between retention of the wild-type Trp53 allele and expression of p19(ARF), providing further evidence of negative-feedback control of the latter by p53. However, expression of p19(ARF) does not appear to be counterselected in the absence of p53, and its integrity in Trp53(+/-) tumors was indicated by its transcriptional upregulation on Trp53 wild-type allele loss in vitro in selected tumor cell lines. The role of MMLV was investigated further by analysis of proviral insertion sites in tumors of CD2-myc transgenic mice sorted for Trp53 genotype. A proportion of tumors showed insertions at Runx2, an oncogene which has been shown to collaborate independently with CD2-myc and with the Trp53 null genotype, and at a novel common integration site (ptl-1) on chromosome 8. Genotypic analysis of the panel of tumors suggested that neither of these integrations is functionally redundant with loss of p53, but it appears that the combination of the MMLV oncogenic program with the CD2-myc oncogene relegates p53 loss to a late step in tumor progression or in vitro culture. While the means by which these tumors preempt the p53 tumor suppressor response remains to be established, this study provides further evidence that irreversible inactivation of this pathway is not a prerequisite for tumor development in vivo.

Role of p14(ARF) in replicative and induced senescence of human fibroblasts

Following a proliferative phase of variable duration, most normal somatic cells enter a growth arrest state known as replicative senescence. In addition to telomere shortening, a variety of environmental insults and signaling imbalances can elicit phenotypes closely resembling senescence. We used p53(-/-) and p21(-/-) human fibroblast cell strains constructed by gene targeting to investigate the involvement of the Arf-Mdm2-p53-p21 pathway in natural as well as premature senescence states. We propose that in cell types that upregulate p21 during replicative exhaustion, such as normal human fibroblasts, p53, p21, and Rb act sequentially and constitute the major pathway for establishing growth arrest and that the telomere-initiated signal enters this pathway at the level of p53. Our results also revealed a number of significant differences between human and rodent fibroblasts in the regulation of senescence pathways.

Analysis of TP53 mutations in relapsed childhood acute lymphoblastic leukemia

TP53 is the most commonly mutated gene in human cancer, but TP53 mutations are present in less than 5% of children with acute lymphoblastic leukemia (ALL) at initial presentation. Mutations are detected more frequently in children with relapsed T-cell ALL, but the potential role of TP53 mutations in relapsed B-lineage childhood ALL is not understood as well. The authors determined the nucleotide sequence of amplified DNA from exons 5 to 8 of the TP53 gene in leukemic cells obtained from 17 children with ALL at the time of first bone marrow relapse. All 17 contained only germline TP53 sequences. Review of the published literature disclosed that TP53 mutations have been found in 22% of cases of relapsed ALL. To understand the role of p53 abnormalities in this clinical setting, it will be important for future studies to analyze cases of relapsed ALL with assays capable of interrogating the functional integrity of the p53 pathway.

Inactivation of p21 by E1A leads to the induction of apoptosis in DNA-damaged cells

A major impediment to successful chemotherapy is the propensity for some tumor cells to undergo cell cycle arrest rather than apoptosis. It is well established, however, that the adenovirus E1A protein can sensitize these cells to the induction of apoptosis by anticancer agents. To further understand how E1A enhances chemosensitivity, we have made use of a human colon carcinoma cell line (HCT116) which typically undergoes cell cycle arrest in response to chemotherapeutic drugs. As seen by the analysis of E1A mutants, we show here that E1A can induce apoptosis in these cells by neutralizing the activities of the cyclin-dependent kinase inhibitor p21. E1A's ability to interact with p21 and thereby restore Cdk2 activity in DNA-damaged cells correlates with the reversal of G(1) arrest, which in turn leads to apoptosis. Analysis of E1A mutants failing to bind p300 (also called CBP) or Rb shows that they are almost identical to wild-type E1A in their ability to initially overcome a G(1) arrest in cells after DNA damage, while an E1A mutant failing to bind p21 is not. However, over time, this mutant, which can still target Rb, is far more efficient in accumulating cells with a DNA content greater than 4N but is similar to wild-type E1A and the other E1A mutants in releasing cells from a p53-mediated G(2) block following chemotherapeutic treatment. Thus, we suggest that although E1A requires the binding of p21 to create an optimum environment for apoptosis to occur in DNA-damaged cells, E1A's involvement in other pathways may be contributing to this process as well. A model is proposed to explain the implications of these findings.

UV-induced DNA damage, repair, mutations and oncogenic pathways in skin cancer

Repair of UV induced DNA damage is of key importance to UV-induced skin carcinogenesis. Specific signal transduction pathways that regulate cell cycling, differentiation and apoptosis are found to be corrupted in skin cancers, e.g., the epidermal growth-stimulating Hedgehog pathway in basal cell carcinomas (BCCs). Mutations in genes coding for proteins in these pathways lead to persistent disturbances that are passed along to daughter cells, e.g., mutations in the gene for the Patched (PTCH) protein in the Hedgehog pathway. Thus far only the point mutations in the P53 gene from squamous cell carcinomas and BCCs, and in PTCH gene from BCC of xeroderma pigmentosum (XP) patients appear to be unambiguously attributable to solar UV radiation. Solar UVB radiation is most effective in causing these point mutations. Other forms of UV-induced genetic changes (e.g., deletions) may, however, contribute to skin carcinogenesis with different wavelength dependencies.

Restoration of endogenous wild-type p53 activity in a glioblastoma cell line with intrinsic temperature-sensitive p53 induces growth arrest but not apoptosis

p53 protein is a transcription factor involved in multiple tumor-suppressor activities including cell cycle control and apoptosis. TP53 gene is frequently mutated in glioblastoma, suggesting the importance of inactivation of this gene product in gliomagenesis. Restoration of p53 function in glioblastoma cell lines deficient for p53 has shown that p53 induces growth arrest or apoptosis depending on the cell line and vector used to transduce wild-type TP53 alleles. Considering that astrocytes grow and express p53, it is not clear whether these results reflect physiologic responses or the result of p53 overexpression in combination with cellular responses to viral vector infection. Here, we reassessed this issue using a glioblastoma cell line (LN382) that expresses an endogenous temperature-sensitive mutant p53. This cell line expresses TP53 alleles (100% as determined by a p53 transcriptional assay in yeast) mutated at codon 197 GTG (Val) > CTG (Leu). We found that the p53 protein in these cells acted as an inactive mutant at 37 degrees C and as a functional wild-type p53 below 34 degrees C as demonstrated by several lines of evidence, including (i) restoration of transactivating ability in yeast, (ii) induction of p53-modulated genes such as CDKN1(p21) and transforming growth factor-alpha, (iii) disappearance of accumulated p53 protein in the nucleus and (iv) decrease in steady state p53 protein levels. This temperature switch allowed p53 levels, which were close to physiological levels to dramatically reduce LN382 cell proliferation by inducing a G(1)/S cell cycle block, but not to induce apoptosis. The lack of apoptosis was considered to be a result of the low level p53 expression, because increasing wild-type p53 levels by adenoviral-mediated gene transfer caused apoptosis in these cells. The LN382 cell line will be extremely useful for investigations into the roles of p53 in cellular responses to a variety of stimuli or damages.

The developmental biology of brain tumors

Tumors of the central nervous system (CNS) can be devastating because they often affect children, are difficult to treat, and frequently cause mental impairment or death. New insights into the causes and potential treatment of CNS tumors have come from discovering connections with genes that control cell growth, differentiation, and death during normal development. Links between tumorigenesis and normal development are illustrated by three common CNS tumors: retinoblastoma, glioblastoma, and medulloblastoma. For example, the retinoblastoma (Rb) tumor suppressor protein is crucial for control of normal neuronal differentiation and apoptosis. Excessive activity of the epidermal growth factor receptor and loss of the phosphatase PTEN are associated with glioblastoma, and both genes are required for normal growth and development. The membrane protein Patched1 (Ptc1), which controls cell fate in many tissues, regulates cell growth in the cerebellum, and reduced Ptc1 function contributes to medulloblastoma. Just as elucidating the mechanisms that control normal development can lead to the identification of new cancer-related genes and signaling pathways, studies of tumor biology can increase our understanding of normal development. Learning that Ptc1 is a medulloblastoma tumor suppressor led directly to the identification of the Ptc1 ligand, Sonic hedgehog, as a powerful mitogen for cerebellar granule cell precursors. Much remains to be learned about the genetic events that lead to brain tumors and how each event regulates cell cycle progression, apoptosis, and differentiation. The prospects for beneficial work at the boundary between oncology and developmental biology are great.

A melanoma-associated germline mutation in exon 1beta inactivates p14ARF

The INK4a/ARF locus encodes the cyclin dependent kinase inhibitor, p16(INK4a) and the p53 activator, p14ARF. These two proteins have an independent first exon (exon 1alpha and exon 1beta, respectively) but share exons 2 and 3 and are translated in different reading frames. Germline mutations in this locus are associated with melanoma susceptibility in 20-40% of multiple case melanoma families. Although most of these mutations specifically inactivate p16(INK4a), more than 40% of the INK4a/ARF alterations located in exon 2, affect both p16(INK4a) and p14ARF. We now report a 16 base pair exon 1beta germline insertion specifically altering p14ARF, but not p16(INK4a), in an individual with multiple primary melanomas. This mutant p14ARF, 60ins16, was restricted to the cytoplasm, did not stabilize p53 and was unable to arrest the growth of a p53 expressing melanoma cell line. This is the first example of an exon 1beta mutation that inactivates p14ARF, and thus implicates a role for this tumour suppressor in melanoma predisposition.

Molecular analysis of the INK4A and INK4B gene loci in human breast cancer cell lines and primary carcinomas

The INK4A and INK4B loci are located at 9p21 and have been implicated in the tumorigenesis of various human malignancies. The INK4A gene encodes two cell cycle regulators, p16(INK4A) and ARF, while INK4B encodes p15(INK4B). Previously, we have shown that the p16(INK4) tumor suppressor was not mutated or deleted in primary breast carcinomas. However, primary and metastatic breast carcinomas exhibited a relative hypomethylation of p16(INK4A), which is associated with expression, compared to normal breast tissue. The present study was conducted to determine if inactivation of p15(INK4B) and INK4A exon 1beta (ARF) are common events in breast carcinoma. Mutational analysis was performed by PCR-SSCP, and mRNA expression was evaluated by RT-PCR. Methylation-specific PCR was used to determine the methylation status of the p15(INK4B) promoter. Our results demonstrate that the p15(INK4B) gene was altered in 3 (21%) of the 14 breast cell lines; one had a silent mutation and two had homozygous deletion of the gene. None of the cell lines showed methylation of p15(INK4B). Two (14%) cell lines had homozygous deletion of INK4A exon 1beta. All normal and malignant breast tissue samples were wild-type and non-methylated for p15(INK4B) and wild-type for exon 1beta. Our results show that these structurally and functionally related genes are not invariably affected together, and the most frequently observed alteration at the INK4A and INK4B loci in breast carcinoma appears to be p16(INK4A) hypomethylation.

Changes in p14(ARF) do not play a primary role in human chondrosarcoma tissues

The locus encoding the tumor suppressor p16 has been found to code for a second, different protein. This protein, p14(ARF), has been shown to protect p53 from degradation. Like p16, its gene is often altered in different cancers. In this study, the first unique exon, exon 1 beta, of p14(ARF), has been studied in 22 chondrosarcoma tissues using polymerase chain reaction, DNA sequencing and methylation-specific polymerase chain reaction. One chondrosarcoma was found to have exon 1 beta homozygously deleted, but neither mutations nor methylations were found in any of the chondrosarcomas. This indicates that genetic changes of p14(ARF) are a rare event in chondrosarcoma.

Different p16INK4a and p14ARF expression patterns in acute myeloid leukaemia and normal blood leukocytes

The p16INK4a gene is often disrupted or transcriptionally silenced by CpG island methylation in human cancers. However, in acute myeloid leukaemia (AML) alterations of the INK4a-ARF tumour suppressor locus are rarely found despite the noted variable p16INK4a mRNA and protein levels. The p14ARF, an alternative reading frame protein encoded from the same INK4a-ARF locus, is a potent tumour suppressor functionally linked to p53. There is little known regarding the role of p14ARF in primary human tumours. Therefore, we analysed the expression patterns of these two tumour suppressors in 37 cases of AML. The relative expression of p16INK4a and p14ARF mRNA in AML blasts, measured by a specific p16INK4a/p14ARF multiplex RT-PCR, was significantly shifted towards p14ARF whereas relatively lower levels of p16INK4a were detected. Quantitative RT-PCR revealed significantly higher expression of both transcripts in AML blasts when compared to normal differentiated myeloid cells or CD34+ progenitor cells. Furthermore, a good correlation between p16INK4a protein and mRNA was observed, whereas no correlation was found with p14ARF. Our results suggest: a) increased levels of both p16INK4a and p14ARF may participate in the pathogenesis of AML, b) that high p14ARF mRNA expression might influence p16INK4a transcription and c) that post-transcriptional regulatory mechanisms are important for p14ARF expression.

Correlation of immunohistochemical molecular staging of bladder biopsies and radical cystectomy specimens

PURPOSE

To determine the relationship of p53, retinoblastoma (RB), and p16 expression between precystectomy transurethral resection bladder (TURB) biopsy and matched cystectomy specimens; and to determine the value of p53 immunoreactivity for predicting progression and survival in patients undergoing radical cystectomy.

METHODS AND MATERIALS

We performed p53 immunohistochemical staining on matched archival TURB and cystectomy specimens taken from 40 patients. Twenty-seven and 26 of these patients were also evaluated for RB and p16 expression, respectively.

RESULTS

Twenty-eight (70%) of the TURB and 22 (55%) of the cystectomy specimens stained positive for p53. RB and p16 protein expression were altered in 19 (70%) and 19 (73%) of the TURB specimens, respectively, and 19 (70%) and 19 (73%) of the cystectomy specimens, respectively. There was a strong correlation between p53, RB, and p16 expression and TURB and cystectomy specimens (all p < 0.001). In preoperative and postoperative multivariate analyses, biopsy p53 and cystectomy p53 were independently associated with disease progression (p = 0.049 and p = 0.034, respectively) and bladder cancer-related death (p = 0.044 and p = 0.037, respectively).

CONCLUSION

p53, RB, and p16 expression patterns on TURB specimens correlate with cystectomy specimens. p53 immunoreactivity is an independent predictor of disease progression and bladder cancer survival. These data support the potential of prognostic staging using immunohistochemical analysis on bladder biopsy specimens prior to neoadjuvant or definitive therapy.

Combined analysis of p53 and RB pathways in epithelial ovarian cancer

Disruptions of the p16-CDK4/cyclin D1-pRb pathway (RB pathway) and the p14ARF-MDM2-p53 pathway (p53 pathway) are important mechanisms in the development of human malignancies. In this study, we investigated RB and p53 pathways in 46 epithelial ovarian cancers (EOCs). In the RB pathway, 16 (34.8%) of 46 cases had p16 gene alterations or loss of expression. The deletion of the p16 gene was a rare event. In 7 cases, we observed methylation in the 5'CpG island in the promoter region of the p16 gene. Abnormal expressions of pRb and CDK4/cyclin D1 were 10.9% and 30.4%, respectively. In the p53 pathway, 10 (21.7%) of 46 cases had p14ARF gene alterations or abnormal expression. In 4 cases, methylation in the 5'CpG island in the promoter region of the p14ARF gene was present. MDM2 overexpression was a rare event. Thirty-six (78.3%) of 46 patients had p53 gene alterations or expression. In our studied cases, p14ARF abnormalities were independent of p16 abnormalities. Abnormal RB and p53 pathways were present in 60.9% and 80.4% of cases, respectively. In conclusion, disruptions of p53 and RB pathways are frequent events and the inverse correlations were present between the abnormality of p16 and p14ARF in EOCs.

Glioma models

Gliomas are primary central nervous system tumors that arise from astrocytes, oligodendrocytes or their precursors. Gliomas can be classified into several groups according to their histologic characteristics, the most malignant of the gliomas is glioblastoma multiforme. In contrast to the long-standing and well-defined histopathology, the underlying molecular and genetic bases for gliomas are only just emerging. Many genetic alterations have been identified in human gliomas, however, establishing unequivocal correlation between these genetic alterations and gliomagenesis requires accurate animal models for this disease. Here we are reviewing the existing animal models for gliomas with different strategies and our current knowledge on the important issues about this disease, such as activation of signal transduction pathways, disruption of cell cycle arrest pathways, cell-of-origin of gliomas, and therapeutic strategies.

Different effects of p14ARF on the levels of ubiquitinated p53 and Mdm2 in vivo

Mdm2 has been shown to promote its own ubiquitination and the ubiquitination of the p53 tumour suppressor by virtue of its E3 ubiquitin ligase activity. This modification targets Mdm2 and p53 for degradation by the proteasome. The p14ARF tumour suppressor has been shown to inhibit degradation of p53 mediated by Mdm2. Several models have been proposed to explain this effect of p14ARF. Here we have compared the effects of p14ARF overexpression on the in vivo ubiquitination of p53 and Mdm2. We report that the inhibition of the Mdm2-mediated degradation of p53 by p14ARF is associated with a decrease in the proportion of ubiquitinated p53. The levels of polyubiquitinated p53 decreased preferentially compared to monoubiquitinated species. p14ARF overexpression increased the levels of Mdm2 but it did not reduce the overall levels of ubiquitinated Mdm2 in vivo. This is unexpected because p14ARF has been reported to inhibit the ubiquitination of Mdm2 in vitro. In addition we show that like p14ARF, the proteasome inhibitor MG132 can promote the accumulation of Mdm2 in the nucleolus and that this can occur in the absence of p14ARF expression. We also show that the mutation of the nucleolar localization signal of Mdm2 does not impair the overall ubiquitination of Mdm2 but is necessary for the effective polyubiquitination of p53. These studies reveal important differences in the regulation of the stability of p53 and of Mdm2.

Differential effects of p19(Arf) and p16(Ink4a) loss on senescence of murine bone marrow-derived preB cells and macrophages

Establishment of cell lines from primary mouse embryo fibroblasts depends on loss of either the Arf tumor suppressor or its downstream target, the p53 transcription factor. Mouse p19(Arf) is encoded by the Ink4a-Arf locus, which also specifies a second tumor suppressor protein, the cyclin D-dependent kinase inhibitor p16(Ink4a). We surveyed bone marrow-derived cells from wild-type, Ink4a-Arf-null, or Arf-null mice for their ability to bypass senescence during continuous passage in culture. Unlike preB cells from wild-type mice, those from mice lacking Arf alone could be propagated indefinitely when placed onto stromal feeder layers engineered to produce IL-7. The preB cell lines remained diploid and IL-7-dependent and continued to express elevated levels of p16(Ink4a). By contrast, Arf-null bone marrow-derived macrophages that depend on colony-stimulating factor-1 for proliferation and survival in culture initially grew at a slow rate but gave rise to rapidly and continuously growing, but still growth factor-dependent, variants that ceased to express p16(Ink4a). Wild-type bone marrow-derived macrophages initially expressed both p16(Ink4a) and p19(Arf) but exhibited an extended life span when p16(Ink4a) expression was extinguished. In all cases, gene silencing was accompanied by methylation of the Ink4a promoter. Therefore, whereas Arf loss alone appears to be the major determinant of establishment of murine fibroblast and preB cell lines in culture, p16(Ink4a) provides an effective barrier to immortalization of bone marrow-derived macrophages.

Mdm2 mutant defective in binding p300 promotes ubiquitination but not degradation of p53: evidence for the role of p300 in integrating ubiquitination and proteolysis

Turnover of the p53 tumor suppressor protein is mediated by Mdm2 through the ubiquitin proteolysis pathway. p300, a co-activator for p53, also participates in this process by complexing with Mdm2. We now report that the mutant Mdm2, defective in p53 binding, does not promote p53 ubiquitination and degradation in vivo or inhibit p53 transcriptional activation. By contrast, the mutant Mdm2, defective in p300 binding, still retains its activity to promote p53 ubiquitination and to inhibit p53 transcriptional activation but fails in promoting p53 degradation. We also show that both wild-type Mdm2 and the mutant Mdm2, defective in p300 binding, can promote the ubiquitination of cancer-derived p53 mutants, but only wild-type Mdm2 can cause their degradation. Furthermore, adenoviral oncoprotein, 12S.E.1A, but not its deletion mutant that lacks p300 binding, was shown to decrease in vivo ubiquitination of mutant p53. Taken together, these results provide genetic evidence that p300 plays a pivotal role in the regulation of Mdm2-mediated p53 turnover by integrating the cellular ubiquitination and proteolytic processes.

Concurrent hypermethylation of multiple genes is associated with grade of oligodendroglial tumors

Current evidence suggests that epigenetic changes play an important role in the evolution of human cancers. In this study, we evaluated whether hypermethylation of CpG islands at the gene promotor regions of several tumor-related genes is involved in the carcinogenesis of oligodendroglial tumors. We examined the methylation status of 11 genes in a series of 43 oligodendroglial tumors (19 oligodendrogliomas, 13 anaplastic oligodendrogliomas, 9 oligoastrocytomas, and 2 anaplastic oligoastrocytomas) by methylation-specific polymerase chain reaction. Our results showed that hypermethylation of CpG islands was detectable in 8 of 11 genes studied and 74% of tumors were hypermethylated in at least 1 gene. Promotor hypermethylations were detected in O6-methylguanine-DNA methyltransferase (MGMT), RB1, estrogen receptor, p73, p16INK4a, death-associated protein kinase, p15INK4b, and p14ARF at 60%, 34%, 30%, 16%, 12%, 10%, 7%, and 2%, respectively. No hypermethylation was detected in the promotors of glutathione-S-transferase P1, von Hippel-Lindau or the DNA mismatch repair (hMLH1) genes. Statistical analysis revealed that concordant hypermethylation of at least 2 genes, p16INK4a and p15INK4b were significantly associated with anaplastic oligodendroglial tumors, and hypermethylation of MGMT was significantly associated with loss of chromosome 19q and with combined loss of chromosomes 1p and 19q. More importantly, several candidate tumor suppressor genes such as p16INK4a, p15INK4b, and p73 that were previously reported as unmutated in oligodendroglial tumors were found to be hypermethylated in their CpG islands. Taken together, we conclude that hypermethylation of CpG islands is a common epigenetic event that is associated with the development of oligodendroglial tumors.

What we could do now: molecular pathology of bladder cancer

There is much information on the genetic alterations that contribute to the development of bladder cancer. Because it is hypothesised that the genotype of the cancer cell plays a major role in determining phenotype, this genetic information should impact on clinical practice. To date however, this has not happened. Some of the alterations identified in bladder cancer have clear associations with outcome-for example, mutational inactivation of the cell cycle regulator proteins p53 and the retinoblastoma protein (Rb). However, as single markers, these events have insufficient predictive power to be applied in the management of individual patients. The use of panels of markers is a potential solution to this problem. Examples of suitable panels include those genes/proteins with known impact on specific cell cycle checkpoints or with impact on cellular phenotypes, such as immortalisation, invasion, or metastasis. To evaluate such marker panels, large tumour series will be needed-for example, archival samples from completed clinical trials. The use of these valuable resources will require coordination of sample provision. This might involve central collection and distribution of tissue blocks, sections, or tissue arrays and the provision of patient follow up information to laboratories participating in a study. With the availability of microarray technologies, including cDNA and comparative genomic hybridisation arrays, the transcriptome and genome of transitional cell carcinomas of different phenotypes can be compared and will undoubtedly provide a wealth of information with potential diagnostic and prognostic uses. Although these studies can be initiated using small local tissue collections, high quality collection of fresh tissues from new clinical trials will be crucial for proper evaluation of associations with clinical outcome. Funding for molecular pathological studies to date has been poor. To begin to translate molecular information from the laboratory to the clinic and to make maximum use of valuable urological patient resources in the UK, adequate funding and scientific energy are required. Whereas the latter is not in doubt, present funding for this type of translational research is inadequate.

DNA damage and cell cycle checkpoints in hyperoxic lung injury: braking to facilitate repair

The beneficial use of supplemental oxygen therapies to increase arterial blood oxygen levels and reduce tissue hypoxia is offset by the knowledge that it injures and kills cells, resulting in increased morbidity and mortality. Although many studies have focused on understanding how hyperoxia kills cells, recent findings reveal that it also inhibits proliferation through activation of cell cycle checkpoints rather than through overt cytotoxicity. Cell cycle checkpoints are thought to be protective because they allow additional time for injured cells to repair damaged DNA and other essential molecules. During recovery in room air, the lung undergoes a burst of proliferation to replace injured and dead cells. Failure to terminate this proliferation has been associated with fibrosis. These observations suggest that growth-suppressive signals, which inhibit proliferation of injured cells and terminate proliferation when tissue repair has been completed, may play an important role in the pulmonary response to hyperoxia. Because DNA replication is coupled with DNA repair, activation of cell cycle checkpoints during hyperoxia may be a mechanism by which cells protect themselves from oxidant genotoxic stress. This review examines the effect of hyperoxia on DNA integrity, pulmonary cell proliferation, and cell cycle checkpoints activated by DNA damage.

A common founder for the V126D CDKN2A mutation in seven North American melanoma-prone families

One of the most common melanoma-related CDKN2A mutations reported in North America is the V126D mutation. We examined nine markers surrounding CDKN2A in three American and four Canadian families carrying the V126D mutation. All seven families had a haplotype consistent with a common ancestor/founder for this mutation. In addition, the mutation appears to have originated 34-52 generations ago (1-LOD-unit support interval 13-98 generations).

Apoptosis in atherosclerosis: pathological and pharmacological implications

Normal embryonic development, tissue differentiation and repair in the eukaryote requires a tightly regulated apoptosis, or programmed cell death. Apoptosis also plays an essential role in different pathological processes including atherosclerosis, in which it affects all cell types in the atherosclerotic lesion, including endothelial cells, vascular smooth muscle cells, and macrophages. During atherosclerosis progression, pro- and anti-apoptotic signals abound in the evolving lesion. Apoptosis limits the number of a particular cell type that accumulates in the lesion and slows down the overall progression of the lesion. On the other hand, it contributes to the production of unstable plaques. Many pharmacological agents used to treat cardiovascular and lipid disorders have pro- or/and anti-apoptotic effects. Pharmaceuticals that modulate apoptosis in specific types of cell can potentially serve as anti-atherogenic agents. However, to develop agents for clinical use requires a thorough knowledge of the pathophysiology of apoptosis in atheromatous lesions, a highly cell-specific process. Here we review our current understanding of the process to provide a background for future pharmacological research in the area.

Cyclin A-CDK Phosphorylation Regulates MDM2 Protein Interactions

The product of the MDM2 gene interacts with and regulates a number of proteins, in particular the tumor suppressor p53. The MDM2 protein is likely to be extensively modified in vivo, and such modification may regulate its functions in cells. We identified a potential cyclin-dependent kinase (CDK) site in murine MDM2, and found the protein to be efficiently phosphorylated in vitro by cyclin A-containing complexes (cyclin A-CDK2 and cyclin A-CDK1), but MDM2 was either weakly or not phosphorylated by other cyclin-containing complexes. Moreover, a peptide containing a putative MDM2 cyclin recognition motif specifically inhibited phosphorylation by cyclin A-CDK2. The site of cyclin A-CDK2 phosphorylation was identified as Thr-216 by two-dimensional phosphopeptide mapping and mutational analysis. Phosphorylation of MDM2 at Thr-216 both weakens its interaction with p53 and modestly augments its binding to p19(ARF). Interestingly, an MDM2-specific monoclonal antibody, SMP14, cannot recognize MDM2 phosphorylated at Thr-216. Changes in SMP14 reactivity of MDM2 in staged cell extracts indicate that phosphorylation of MDM2 at Thr-216 in vivo is most prevalent at the onset of S phase when cyclin A first becomes detectable.

Analysis of genetic and epigenetic processes that influence p14ARF expression in breast cancer

The INK4a/ARF locus encodes two unrelated cell cycle-regulatory proteins that both function in tumor suppression, p16INK4a and p14ARF. In human tumors including breast cancer, alterations affecting selectively p14ARF have been poorly analysed. We have performed a comprehensive analysis of the inactivation mechanisms (mutation, homozygous and hemizygous deletion, and promoter hypermethylation) in a large series of 100 primary breast carcinomas. RT-PCR showed expression variable of the p14ARF transcript, with 17% demonstrating overexpression and 26% demonstrating decreased expression. No detectable alterations were observed in the majority of cases with overexpressed p14ARF mRNA, but 77% of tumors with decreased expression presented at least one of these genetic/epigenetic alterations. Nevertheless, a statistically significant correlation was observed between decreased p14ARF expression and several poor prognostic parameters.

An alternatively spliced HDM2 product increases p53 activity by inhibiting HDM2

The human counterpart hdm2 of the murine double-minute 2 (mdm2) gene encodes a 90-kD protein (HDM2) that inhibits the function of the p53 tumor suppressor. Hdm2 is amplified in approximately 30% of sarcomas, leading to overproduction of HDM2 and inactivation of p53. Using immunohistochemistry to screen a panel of human tumors for HDM2 overproduction, we detected high levels of HDM2 in the cytoplasm in 25% of lung tumors as opposed to its normal localization in the nucleus. These samples contained full-length hdm2 and several alternate-splice forms of hdm2 mRNA. Sequence analysis revealed deletions in the alternate-splice forms of the p53 binding domain and absence of a nuclear localization signal. In transient transfection assays, one of the alternate-splice forms, HDM2(ALT1), bound and sequestered full-length HDM2 in the cytoplasm. In addition, the binding of HDM2(ALT1) to HDM2 inhibited the interaction of HDM2 with p53, thus enhancing p53 transcriptional activity. These data suggest the existence of another level of regulation of HDM2 which increases the activity of p53.

Myc requires distinct E2F activities to induce S phase and apoptosis

Previous work has shown that the Myc transcription factor induces transcription of the E2F1, E2F2, and E2F3 genes. Using primary mouse embryo fibroblasts deleted for individual E2F genes, we now show that Myc-induced S phase and apoptosis requires distinct E2F activities. The ability of Myc to induce S phase is impaired in the absence of either E2F2 or E2F3 but not E2F1 or E2F4. In contrast, the ability of Myc to induce apoptosis is markedly reduced in cells deleted for E2F1 but not E2F2 or E2F3. From this data, we propose that the induction of specific E2F activities is an essential component in the Myc pathways that control cell proliferation and cell fate decisions.

Detailed gene expression analysis but not microsatellite marker analysis of 9p21 reveals differential defects in the INK4a gene locus in the majority of head and neck cancers

The INK4a gene locus on chromosome 9p21 encodes two proteins, p16(INK4a) and p14(ARF), which influence cell cycle control regulated by pRb and p53. The objective of this study was to use different methods for the analysis of the incidence of changes at the INK4a locus in head and neck cancer (HNSCC). Primary tumours were analysed for allelic imbalances (AI) with microsatellite markers for chromosome 9, by immunohistochemistry (IHC) and IHC with enhanced sensitivity by tyramide signal amplification (TSA-IHC), and by RT-PCR. No homozygous deletions at 9p21 were detected. AI at 9p21, which was found in approximately 60% of the tumours, completely failed to indicate the functional inactivation of the two INK4a gene products. Immunostaining of normal squamous epithelia revealed very low levels of p16(INK4a), whereas p14(ARF) was readily detectable. In 160 tumours, IHC suggested a loss of p16(INK4a) expression in 90%. However, by TSA-IHC, only 53.7% showed loss of p16(INK4a) expression, and this was consistent with the RT-PCR analyses. In 100 tumours analysed for both proteins, selective loss of p16(INK4a) occurred in 37%; loss of p14(ARF) was found in only 15%, and selective loss in only 4%; 11% of the tumours had lost both proteins. We conclude that only IHC with high sensitivity and the combined expression analysis of mRNAs and proteins is suitable for studying the role of INK4a in HNSCC. The INK4a gene expression defects are frequent but not universal and primarily affect p16(INK4a). Their clinical impact is still not clear.

The bmi-1 oncoprotein is differentially expressed in non-small cell lung cancer and correlates with INK4A-ARF locus expression

Genes of the polycomb group function by silencing homeotic selector genes that regulate embryogenesis. In mice, downregulation of one of the polycomb genes, bmi-1, leads to neurological alterations and severe proliferative defects in lymphoid cells, whilst bmi-1 overexpression, together with upregulation of myc-1, induces lymphoma. An oncogenic function has been further supported in primary fibroblast studies where bmi-1 overexpression induces immortalization due to repression of p16/p19ARF, and where together with H-ras, it readily transforms MEFs. It was the aim of this study to assess the expression of bmi-1 in resectable non-small cell lung cancer (NSCLC) in association with p16 and p14ARF (=human p19ARF). Tumours (48 resectable NSCLC (32 squamous, 9 adeno-, 2 large cell, 4 undifferentiated carcinomas and 1 carcinoid); stage I, 29, II, 7, III, 12; T1, 18, T2, 30; differentiation: G1 12, G2 19, G3 17) were studied by immunohistochemistry for protein expression and by comparative multiplex PCR for gene amplification analysis. In tumour-free, normal lung tissue from patients, weak - moderate bmi-1 staining was seen in some epithelial cells, lymphocytes, glandular cells and in fibroblasts, whereas blood, endothelial, chondrocytes, muscle cells and adipocytes did not exhibit any bmi-1 expression. In tumours, malignant cells were negative/weakly, moderately and strongly positive in 20, 22 and 6 cases, respectively. As assessed by multiplex PCR, bmi-1 gene amplification was not the reason for high-level bmi-1 expression. Tumours with moderate or strong bmi-1 expression were more likely to have low levels of p16 and p14ARF (P = 0.02). Similarly, tumours negative for both, p16 and p14ARF, exhibit moderate-strong bmi-1 staining. 58% of resectable NSCLC exhibit moderate-high levels of bmi-1 protein. The inverse correlation of bmi-1 and the INK4 locus proteins expression (p16/p14ARF) supports a possible role for bmi-1 misregulation in lung carcinogenesis.

A p53 amino-terminal nuclear export signal inhibited by DNA damage-induced phosphorylation

The p53 protein is present in low amounts in normally growing cells and is activated in response to physiological insults. MDM2 regulates p53 either through inhibiting p53's transactivating function in the nucleus or by targeting p53 degradation in the cytoplasm. We identified a previously unknown nuclear export signal (NES) in the amino terminus of p53, spanning residues 11 to 27 and containing two serine residues phosphorylated after DNA damage, which was required for p53 nuclear export in colloboration with the carboxyl-terminal NES. Serine-15-phosphorylated p53 induced by ultraviolet irradiation was not exported. Thus, DNA damage-induced phosphorylation may achieve optimal p53 activation by inhibiting both MDM2 binding to, and the nuclear export of, p53.

[Molecular mechanisms controlling the cell cycle: fundamental aspects and implications for oncology]

INTRODUCTION

Comprehension of cell cycle regulation mechanisms has progressed very quickly these past few years and regulators of the cell cycle have gained widespread importance in cancer. This review first summarizes major advances in the understanding of the control of cell cycle mechanisms. Examples of how this control is altered in tumoral cells are then described.

CURRENT KNOWLEDGE AND KEY POINTS

The typical mammalian cell cycle consists of four distinct phases occurring in a well-defined order, each of which should be completed successfully before the next begins. Progression of eukaryotic cells through major cell cycle transitions is mediated by sequential assembly and activation of a family of serine-threonine protein kinases, the cyclin dependent kinases (CDK). The timing of their activation is determined by their post-translational modifications (phosphorylations/dephosphorylations), and by the association of a protein called cyclin, which is the regulatory subunit of the kinase complex. The cyclin family is divided into two main classes. The 'G1 cyclins' include cyclins C, D1-3, and E, and their accumulation is rate-limiting for progression from the G1 to S phase. The 'mitotic or G2 cyclins', which include cyclin A and cyclin B, are involved in the control of G2/M transition and mitosis. The cyclins bind to and activate the CDK, which leads to phosphorylation (and then inhibition) of the tumor suppressor protein, pRb. pRb controls commitment to progress from the G1 to S phase, at least in part by repressing the activity of the E2F transcription factors known to promote cell proliferation. Both the D-type cyclins and their partner kinases CDK4/6 have proto-oncogenic properties, and their activity is carefully regulated at multiple levels including negative control by two families of CDK inhibitors. While members of the INK4 family (p16INK4A, p15INK4B, p18INK4C, p19INK4D) interact specifically with CDK4 and CDK6, the CIP/KIP inhibitors p21CIP1/WAF1, p27KIP1 and p57KIP2 inhibit a broader spectrum of CDK. The interplay between p16INK4A, cyclin D/CDK, and pRb/E2F together constitute a functional unit collectively known as the 'pRb pathway'. Each of the major components of this mechanism may become deregulated in cancer, and accumulating evidence points to the 'pRb pathway' as a candidate obligatory target in multistep oncogenesis of possibly all human tumor types.

FUTURE PROSPECTS AND PROJECTS

Major advances in the understanding of cell cycle regulation mechanisms provided a better knowledge of the molecular interactions involved in human cancer. This progress has led to the promotion of new therapeutic agents presently in clinical trials or under development. Moreover, the components of the cell cycle are probably involved in other non-cancerous diseases and their role must be defined.

p14ARF homozygous deletion or MDM2 overexpression in Burkitt lymphoma lines carrying wild type p53

The hallmark of Burkitt lymphoma (BL) is a constitutively activated c-myc gene that drives tumor cell growth. A majority of BL-derived cell lines also carry mutant p53. In addition, the p16INK4a promoter is hypermethylated in most BL biopsies and BL cell lines, leading to silencing of this gene. Activation of c-myc and/or cell cycle dysregulation can induce ARF expression and p53-dependent apoptosis. We therefore investigated the p14ARF-MDM2-p53 pathway in BL cell lines. p14ARF was expressed and localized to nucleoli in all BL carrying mutant p53. Three out of seven BL carrying wt p53 had a homozygous deletion of the CDKN2A locus that encodes both p14ARF and p16INK4a. Three BL carrying wild type p53 retained the CDKN2A locus and overexpressed MDM2. DNA sequencing revealed a point mutation in CDKN2A exon 2 in one of these BL, Seraphine. However, this point mutation did not affect p14ARF's nucleolar localization or ability to induce p53. The Bmi-1 protein that negatively regulates the p14ARF promoter and co-operates with c-myc in tumorigenesis was expressed at low to moderate levels in all BL analysed. Our results indicate that inactivation of the ARF-MDM2-p53 pathway is an essential step during the development of Burkitt lymphoma, presumably as a mechanism to escape c-myc induced apoptosis.

Different expression of P14ARF defines two groups of breast carcinomas in terms of TP73 expression and TP53 mutational status

In 95 breast carcinomas, we investigated P14ARF and TP73 mRNA expression and their relationship to TP53 mutations, determined by an immunohistochemical method, studying several clinicopathologic features of the tumors. P14ARF and TP73 mRNA levels were determined by semiquantitative reverse transcription polymerase chain reaction (RT-PCR), using beta-actin as a control. P14ARF was overexpressed in 19% of the cases and underexpressed in 24%. TP73 was overexpressed in 22% of the tumors, and normal levels were found in the remaining 78%. The analysis of TP53 showed positive immunostaining in 38% of cases. The association of P14ARF and TP73 overexpression was statistically significant, as was the association between positive TP53 staining and TP73 overexpression. P14ARF was related to TP53 only in those cases in which there was low expression of P14ARF. Concomitant overexpression of P14ARF and TP73 was statistically related to positive TP53 immunostaining. The analysis of concomitant P14ARF and TP73 overexpression and clinicopathologic parameters of the tumors showed a statistically significant difference with respect to peritumoral vessel invasion (P = 0.01), lymph node metastasis (P = 0.03), negative ERBB2 expression (P = 0.005), and more advanced pathologic stages (P = 0.03). These results suggest that overexpression of P14ARF and TP73 could be implicated in breast carcinoma tumorigenesis and, ultimately, in the phenotypic features of these lesions.

Increased expression of the INK4a/ARF locus in polycythemia vera

The retinoblastoma (Rb), cyclin-dependent kinase (CDK), and CDK inhibitor genes regulate cell generation, and deregulation can produce increased cell growth and tumorigenesis. Polycythemia vera (PV) is a clonal myeloproliferative disease where the mechanism producing increased hematopoiesis is still unknown. To investigate possible defects in cell-cycle regulation in PV, the expression of Rb and CDK inhibitor gene messenger RNAs (mRNAs) in highly purified human erythroid colony-forming cells (ECFCs) was screened using an RNase protection assay (RPA) and 11 gene probes. It was found that RNA representing exon 2 of p16INK4a and p14ARF was enhanced by 2.8- to 15.9-fold in 11 patients with PV. No increase of exon 2 mRNA was evident in the T cells of patients with PV, or in the ECFCs and T cells from patients with secondary polycythemia. p27 also had elevated mRNA expression in PV ECFCs, but to a lesser degree. Because the INK4a/ARF locus encodes 2 tumor suppressors, p16INK4a and p14ARF with the same exon 2 sequence, the increased mRNA fragment could represent either one. To clarify this, mRNA representing the unique first exons of INK4a and ARF were analyzed by semiquantitative reverse transcription–polymerase chain reaction. This demonstrated that mRNAs from the first exons of both genes were increased in erythroid and granulocyte-macrophage cells and Western blot analysis showed that the INK4a protein (p16INK4a) was increased in PV ECFCs. Sequencing revealed no mutations of INK4a or ARF in 10 patients with PV. p16INK4a is an important negative cell-cycle regulator, but in contrast with a wide range of malignancies where inactivation of theINK4a gene is one of the most common carcinogenetic events, in PV p16 INK4a expression was dramatically increased without a significant change in ECFC cell cycle compared with normal ECFCs. It is quite likely that p16INK4a and p14ARF are not the pathogenetic cause of PV, but instead represent a cellular response to an abnormality of a downstream regulator of proliferation such as cyclin D, CDK4/CDK6, Rb, or E2F. Further work to delineate the function of these genes in PV is in progress.

Selective deletion of p14(ARF) exon 1beta of the INK4a locus in oral squamous cell carcinomas of Indians

The tumor suppressor gene - p16 INK4/CDKN2/MTS1 and its alternate splice product p14 (ARF), constitute the INK4a locus. We have examined the integrity of exon 1beta of p14(ARF) gene of oral squamous cell carcinomas (n=58) in untreated Indian patients. No mutations were detected in this region by PCR-SSCP analysis of the tumor DNA's. Further, PCR-based analysis revealed homozygous deletions of exon 1beta in 14 of the 58 tumors; these results were confirmed by hybridization of tumor DNAs with exon 1beta specific probe. The deletions were limited to the exon 1beta while the exons coding p16/INK4 were not affected. Except in two cases these deletions were mutually exclusive to the p53 inactivating mutations. These observations suggest an alternate mechanism of loss of p14(ARF) in the genesis of oral squamous cell carcinomas.

Pex19p dampens the p19ARF-p53-p21WAF1 tumor suppressor pathway

We isolated a 33-kDa protein, Pex19p/HK33/HsPXF, as a p19ARF-binding protein in a yeast two-hybrid screen. We demonstrate here that Pex19p interacts with p19ARF in the cell cytoplasm and excludes p19ARF from the nucleus, leading to a concurrent inactivation of p53 function. Down-regulation of Pex19p by its antisense expression resulted in increased levels of p19ARF, increased p53 function, and a p53/p21WAF1-mediated senescence-like cell cycle arrest. The data demonstrated a novel mechanism of down-regulation of the p19ARF-p53 pathway.

Genetic dissection of melanoma pathways in the mouse

The frequent loss of the INK4a/ARF locus, encoding for both p16(INK4a)and p19(ARF)in human melanoma, raises the question as to which INK4a/ARF gene product functions to suppress melanoma-genesis in vivo. Studies in the mouse have shown that activated RAS mutation can cooperate with INK4a(Delta 2/3)deficiency (null for both p16(INK4a)and p19(ARF)) to promote development of melanoma, and these melanomas retain wild-type p53. Given the functional link between p19(ARF)and p53, we have now shown that activated RAS can also cooperate with p53 deficiency to produce melanoma in the mouse. Moreover, genome-wide analysis of RAS-induced p53 mutant melanomas reveals alterations of key components governing RB-regulated G1/S transition, such as c-Myc. These experimental findings suggest that both RB and p53 pathways function to suppress melanocyte transformation in vivo in the mouse.