Immunohistochemical analysis of p16INK4a,p14ARF,p18INK4c,p21CIP1, p27KIP1 and p73 expression in 271 meningiomas correlation with tumor grade and clinical outcome

Routine pathological examination cannot distinctively predict the clinical course of meningiomas because even histologically benign tumors may recur after gross total resection. Numerous efforts have been made for the evaluation of different immunohistochemical assays in meningioma prognosis. We investigated the prognostic significance of p16INK4a, p14ARF, p18INK4c, p21CIP1, p27KIP1 and p73 expression by immunohistochemical analysis of 271 meningiomas. All tumors were additionally stained for the proliferation markers Ki-67 and DNA topoisomerase II alpha (TopoIIalpha). Significant differences between the number of p16INK4a-, p18INK4c- and p21CIP1-positive cases were noted among the 3 grades of meningiomas. p16INK4a- and p21CIP-positive tumors were found to prevail among benign meningiomas, whereas p18INK4c immunostaining was closely associated to anaplastic meningiomas. The number of p16INK4a- and p21CIP-positive cases was significantly lower in the cohort of recurrent meningiomas. In contrast, p18INK4c-positive cases were clustered among recurrent meningiomas regardless of tumor grade. Immunoreactivity of p14ARF, p27KIP1 and p73 did not show any differences between meningiomas of various histology and clinical outcomes. Multivariate analysis revealed that only tumor grade and TopoIIalpha index are independent criteria for predicting meningioma recurrence. Thus, the immunohistochemical assessment of p16INK4a, p14ARF, p18INK4c, p21CIP1, p27KIP1 and p73 expression in meningiomas does not appear to provide prognostically useful information. Further studies are needed to identify more reliable prognostic markers and to address in more detail the role of cell cycle aberrations in these tumors.     

Human p14(ARF)-mediated cell cycle arrest strictly depends on intact p53 signaling pathways

The tumor suppressor ARF is transcribed from the INK4a/ARF locus in partly overlapping reading frames with the CDK inhibitor p16(Ink4a). ARF is able to antagonize the MDM2-mediated ubiquitination and degradation of p53, leading to either cell cycle arrest or apoptosis, depending on the cellular context. However, recent data point to additional p53-independent functions of mouse p19(ARF). Little is known about the dependency of human p14(ARF) function on p53 and its downstream genes. Therefore, we analysed the mechanism of p14(ARF)-induced cell cycle arrest in several human cell types. Wild-type HCT116 colon carcinoma cells (p53(+/+)p21(CIP1+/+) 14-3-3sigma(+/+)), but not p53(-/-) counterparts, underwent G(1) and G(2) cell cycle arrest following infection with a p14(ARF)-adenovirus. In p21(CIP1-/-) cells, p14(ARF) did not induce G(1) or G(2) arrest, while 14-3-3sigma(-/-) counterparts were mainly arrested in G(1), pointing to essential roles of p21(CIP1) in G(1) and G(2) arrest and cooperative roles of p21 and 14-3-3sigma in ARF-mediated G(2) arrest. Our data demonstrate a strict p53 and p21(CIP1) dependency of p14(ARF)-induced cell cycle arrest in human cells.     

Transfer of p14ARF gene in drug-resistant human breast cancer MCF-7/Adr cells inhibits proliferation and reduces doxorubicin resistance

The INK4a/ARF locus on human chromosome 9p21 encodes two tumor suppressors, p16INK4a and p14ARF, that restrain cell growth by affecting the functions of the retinoblastoma protein and p53, respectively. Overexpression of ARF results in cell cycle arrest in both G1 and G2. To elucidate the effect of p14ARF gene on multidrug-resistant tumor cells, we transferred a p14ARF cDNA into p53-mutated MCF-7/Adr human breast cancer cells. In this report we demonstrated for the first time that p14ARF expression was able to greatly inhibit the MCF-7/Adr cell proliferation. Furthermore, p14ARF expression resulted in decrease of MDR-1 mRNA and P-glycoprotein production, which linked to the reducing resistance of MCF-7/Adr cells to doxorubicin. These results imply that drug resistance might be effectively reversed by the wild-type p14ARF expression in human breast cancer cells.     

Mechanisms of inactivation of p14ARF, p15INK4b, and p16INK4a genes in human esophageal squamous cell carcinoma.

The 9p21 gene cluster, harboring growth suppressive genes p14ARF, p15INK4b, and p16INK4a, is one of the major aberration hotspots in human cancers. It was shown that p14ARF and p16INK4a play active roles in the p53 and Rb tumor suppressive pathways, respectively, and p15INK4b is a mediator of the extracellular growth inhibition signals. To elucidate specific targets and aberrations affecting this subchromosomal region, we constructed a detailed alteration map of the 9p21 gene cluster by analyzing homozygous deletion, hypermethylation, and mutation of the p14ARF, p15INK4b, and p16INK4a genes individually in 40 esophageal squamous cell carcinomas (ESCCs) and compared the genetic alterations with mRNA expression in 18 of these samples. We detected aberrant promoter methylation of the p16INK4a gene in 16 (40%), of p14ARF in 6 (15%), and of p15INK4b in 5 (12.5%) tumor samples. Most p16INK4a methylations were exclusive, whereas all but one of the p14ARF/p15INK4b methylations were accompanied by concomitant p16INK4a methylation. We detected homozygous deletion of p16INK4a in 7 (17.5%), of p14ARF-E1beta in 13 (33%), and of p15INK4b in 16 (40%) tumor samples. Most deletions occurred exclusively on the E1beta-p15INK4b loci. Two samples contained p14ARF deletion but with p16INK4a and p15INK4b intact. No mutation was detected in the p14ARF and p16INK4a genes. Comparative RT-PCR showed good concordance between suppressed mRNA expression and genetic alteration for p15INK4b and p16INK4a genes in the 18 frozen samples, whereas 5 of the 13 cases with suppressed p14ARF mRNA expression contained no detectable E1beta alteration but aberrations in the p16INK4a locus. Our results show that in human ESCCs, p14ARF is a primary target of homozygous deletion along with p15INK4b, whereas p16INK4a is the hotspot of hypermethylation of the 9p21 gene cluster. The frequent inactivation of the p14ARF and p16INK4a genes may be an important mechanism for the dysfunction of both the Rb and p53 growth regulation pathways during ESCC development.     

Cooperative effect of p21Cip1/WAF-1 and 14-3-3sigma on cell cycle arrest and apoptosis induction by p14ARF

P14(ARF) (p19(ARF) in the mouse) plays a central role in the regulation of cellular proliferation. Although the capacity of p14(ARF) to induce a cell cycle arrest in G1 phase depends on a functional p53/p21-signaling axis, the G2 arrest triggered by p14(ARF) is p53/p21-independent. Using isogeneic HCT116 cells either wild-type or homozygously deleted for p21, 14-3-3sigma or both, we further investigated the cooperative effect of p21 and 14-3-3sigma on cell cycle regulation and apoptosis induction by p14(ARF). In contrast to DNA damage, which induces mitotic catastrophe in 14-3-3sigma-deficient cells, we show here that the expression of p14(ARF) triggers apoptotic cell death, as evidenced by nuclear DNA fragmentation and induction of pan-caspase activities, irrespective of the presence or absence of 14-3-3sigma. The activation of the intrinsic mitochondrial apoptosis pathway by p14(ARF) was confirmed by cytochrome c release from mitochondria and induction of caspase-9- (LEHDase) and caspase-3/7-like (DEVDase) activities. Moreover, 14-3-3sigma/p21 double-deficient cells were exceedingly sensitive to apoptosis induction by p14(ARF) as compared to wild-type cells or cells lacking either gene alone. Notably, p14(ARF)-induced apoptosis was preceded by an arrest in the G2 phase of cell cycle, which coincided with downregulation of cdc2 (cdk1) protein expression and lack of its nuclear localization. This indicates that p14(ARF) impairs mitotic entry by targeting the distal DNA damage-signaling pathway and induces apoptotic cell death, rather than mitotic catastrophe, out of a transient G2 arrest. Furthermore, our data delineate that the disruption of G2/M cell cycle checkpoint control critically determines the sensitivity of the cell toward p14(ARF)-induced mitochondrial apoptosis.     

Loss of p21 disrupts p14 ARF-induced G1 cell cycle arrest but augments p14 ARF-induced apoptosis in human carcinoma cells

The human INK4a locus encodes two structurally unrelated tumor suppressor proteins, p16 INK4a and p14 ARF (p19 ARF in the mouse), which are frequently inactivated in human cancer. Both the proapoptotic and cell cycle-regulatory functions of p14 ARF were initially proposed to be strictly dependent on a functional p53/mdm-2 tumor suppressor pathway. However, a number of recent reports have implicated p53-independent mechanisms in the regulation of cell cycle arrest and apoptosis induction by p14 ARF. Here, we show that the G1 cell cycle arrest induced by p14 ARF entirely depends on both p53 and p21 in human HCT116 and DU145 carcinoma cells. In contrast, neither loss of p53 nor p21 impaired apoptosis induction by p14 ARF as evidenced by nuclear DNA fragmentation, phosphatidyl serine exposure, and caspase activation, which included caspase-3/7- and caspase-9-like activities. However, lack of functional p21 resulted in the accumulation of cells in G2/M phase of the cell cycle and markedly enhanced p14 ARF-induced apoptosis that was, nevertheless, efficiently inhibited by the cell permeable broad-spectrum caspase inhibitor zVAD-fmk (valyl-alanyl-aspartyl-(O)-methyl)-fluoromethylketone). Thus, loss of cell cycle restriction point control in the absence of p21 may interfere with p14 ARF-induced apoptosis. Finally, these data indicate that the signaling events required for G1 cell cycle arrest and apoptosis induction by p14 ARF dissociate upstream of p53.     

The alternative reading frame tumor suppressor inhibits growth through p21-dependent and p21-independent pathways

The alternative reading frame (ARF) tumor suppressor mediates growth arrest or apoptosis through activation of the p53 tumor suppressor. A prevailing concept is that ARF uses p21Cip1/Waf1, a p53-responsive gene and cyclin-dependent kinase (Cdk) inhibitor, to block cell cycle progression. Using p21 nullizygous cells, we demonstrate that p21 is nonessential for the antiproliferative activity of ARF and p53, although it likely governs the arrest through Cdk inactivation when present. ARF overexpression in p21-positive and p21-negative mouse embryo fibroblasts (MEFs), but not in primary cells lacking p53, induced a biphasic (G1 and G2) cell cycle arrest. The ARF-induced growth arrest, regardless of p21 status, coincided with activation of p53 and accumulation of hypophosphorylated retinoblastoma protein (retinoblastoma protein). In ARF-arrested p21-positive cells, the presence of growth-inhibitory retinoblastoma protein correlated with an absence of Cdk2-dependent kinase activity, an increase in p21 association with inactive Cdks, and a lack of cyclin A expression. In contrast, p21-/- mouse embryo fibroblasts were arrested by ARF despite containing elevated levels of cyclin A protein and highly active Cdk2-dependent kinases. These findings provide evidence that ARF can block growth through a p21-independent pathway(s) that overrides Cdk2 activation.     

Chromosome 9p21 gene copy number and prognostic significance of p16 in ESFT

Chromosome 9p21 gene copy number in Ewing's sarcoma family of tumour (ESFT) cell lines and primary ESFT has been evaluated using Multiplex Ligation-dependent probe amplification, and the clinical significance of CDKN2A loss and p16/p14(ARF) expression investigated. Homozygous deletion of CDKN2A was identified in 4/9 (44%) of ESFT cell lines and 4/42 (10%) primary ESFT; loss of one copy of CDKN2A was identified in a further 2/9 (22%) cell lines and 2/42 (5%) tumours. CDKN2B was co-deleted in three (33%) cell lines and two (5%) tumours. Co-deletion of the MTAP gene was observed in 1/9 (11%) cell lines and 3/42 (7%) tumours. No correlation was observed between CDKN2A deletion and clinical parameters. However, co-expression of high levels of p16/p14(ARF) mRNA predicted a poor event-free survival (P=0.046, log-rank test). High levels of p16/p14(ARF) mRNA did not correlate with high expression of p16 protein. Furthermore, p16 protein expression did not predict event-free or overall survival. Methylation is not a common mechanism of p16 gene silencing in ESFT. These studies demonstrate that loss (homozygous deletion or single copy) of CDKN2A was not prognostically significant in primary ESFT. However, high levels of p16/p14(ARF) mRNA expression were predictive of a poor event-free survival and should be investigated further.     

Dysfunction of p53 pathway in human colorectal cancer: analysis of p53 gene mutation and the expression of the p53-associated factors p14ARF, p33ING1, p21WAF1 and MDM2

Mutations of p53 tumor suppressor gene increase with tumor progression in colorectal cancers. In this study, we examined the expressions of p33ING1, p14ARF, MDM2 and p21WAF1 mRNA in 25 advanced colorectal cancers by quantitative RT-PCR method, and compared the expression levels of p33ING1, p14ARF, p21WAF1 and MDM2 in relation to p53 status in the tumors. Fifteen of 25 colorectal cancers (60%) showed abnormal accumulation of p53 protein in the nucleus, and the remaining 10 colorectal cancers (40%) were negative for p53 immunostaining. We found a G --> T transition (nonsense mutation) at the first nucleotide of codon 298 (exon 8) in one p53-negative case, and a frame shift mutation on exon 7 in another p53-negative case. In remaining eight p53-negative cases, there was no mutation in the entire open reading frame of p53 cDNA. Interestingly, in eight cases with p53 wild-type gene, 6 cases (75%) showed a marked down-regulation of p14ARF mRNA, and three cases (37.5%) over-expressed MDM2 mRNA. Only one case with wild-type p53 gene showed normal level expression of p53 regulatory-factors (p33ING1, p14ARF, and MDM2). Thus, p53 tumor suppressor pathway was disrupted in 24 of 25 colorectal cancers (96%).     

Frequent alterations of the p14(ARF) and p16(INK4a) genes in primary central nervous system lymphomas

To elucidate the role of p53/p16(INK4a)/RB1 pathways in the tumorigenesis of primary central nervous system lymphomas (PCNSLs), we have analyzed p14(ARF), p16(INK4a), RB1, p21(Waf1), and p27(Kip1) status in a series of their 18 sporadic cases of diffuse large B-cell lymphoma, using methylation-specific PCR, differential PCR, and immunohistochemistry. Homozygous deletion or methylation of p14(ARF) was detected in 10 (56%) PCNSLs, and they were almost entirely deletions (except 1 case). A total of 11 (61%) PCNSLs demonstrated homozygous deletion (6 cases) or methylation (5 cases) of p16(INK4a). Six tumors showed both p14(ARF) and p16(INK4a) homozygous deletions. Hypermethylation of the RB1 and the p27(Kip1) promoter region was detected in 2 (11%) cases, whereas p21(Waf1) methylation was not detected in any. Immunohistochemistry revealed loss of p14(ARF) and p16(INK4a) expression in 10 (56%) samples, correlating with the gene status. Four cases showed independent negative immunoreactivity for pRB and p27(Kip1), and nearly one-half of cases (8 of 18; 44%) were characterized by lack of p21(Waf1) expression. These results indicate that inactivation of p14(ARF) and p16(INK4a) by either homozygous deletion or promoter hypermethylation represents an important molecular pathogenesis in PCNSLs. Hypermethylation of RB1, p21(Waf1), and p27(Kip1) appears to be of minor significance, these genes being independently methylated in PCNSLs.     

Expression of the cell cycle regulators p14(ARF) and p16(INK4a) in chronic myeloid leukemia

Expression of p14(ARF) and p16(INK4a) tumor suppressor genes was investigated in 109 patients with chronic myeloid leukemia (CML). The p14(ARF) and p16(INK4a) mRNA levels were significantly low in patients in chronic phase (CP) at presentation and high in patients treated with interferon-alpha (IFN-alpha), especially in non-responders. A moderate overexpression of p14(ARF) with a normal expression of p16(INK4a) was observed in imatinib-resistant patients. Although protein expression did not consistently match mRNA levels, a role for the two cell cycle regulators in the IFN-alpha signaling pathway is suggested as well as a relation with the resistance to IFN-alpha or imatinib therapy.     

Expression of DNA mismatch repair proteins in transformed non-Hodgkin's lymphoma: relationship to smoking

It has been hypothesized that defects in DNA-mismatch repair are associated with smoking in certain types of transformed non-Hodgkin lymphoma (NHL). We have analyzed biopsy samples from two indolent B-cell lymphomas, follicular lymphoma (FL) and chronic lymphocytic leukemia/small lymphocytic leukemia (CLL/SLL), that have transformed to diffuse-large B-cell lymphoma (DLBCL). We correlated the presence or absence of DNA-mismatch repair enzymes by immunostaining as well as the p53 status to smoking history. Of all patients (n = 30), 37% showed negative immunostaining of MLH1, 16% showed negative immunostaining of MSH2 and 63% had p53 mutations and/or protein expression. Eighteen out of 20 transformed follicular lymphomas and seven out of 10 CLL/SLL that have transformed to DLBCL (Richter's syndrome) were informative for smoking histories. We found that the relative risk of negative immunostaining for either MLH1 or MSH2 was 2.2 times higher in smokers than non-smokers (relative risk = 2.2041, 95% confidence interval: 0.89714, 5.41491). No direct correlation was found between smoking and the mutations in the p53 gene. These results suggest that cigarette smoking may play a role in the development of transformed lymphomas through defective mismatch repair.     

Correlation between p14(ARF)/p16(INK4A) expression and HPV infection in uterine cervical cancer

The CDKN2A locus on human chromosome 9p21 encodes two tumor suppressors, p14(ARF) and p16(INK4A), which enhance the growth-suppressive functions of the retinoblastoma (Rb) and the p53 proteins, respectively. Conversely, the E6 and E7 oncoproteins of the high-risk human papillomaviruses (HPVs) causally associated with carcinogenesis of the uterine cervix contributes to tumor development by inactivating p53 and Rb. Nevertheless, a correlation between expression of p14(ARF)/p16(INK4A) and HPV infection in uterine cervix is less clear. To clarify this, we examined 25 cervical cancers and 11 normal uterine cervixes. HPV was detected in 21 of 25 cervical cancers (84%) and their subtype was determined by PCR-RFLP. Quantitative real-time RT-PCR assays showed overexpression of p14(ARF) mRNA in all 21 HPV-positive cases (100%). p16(INK4A) mRNA was overexpressed in 17 cases of the HPV-positive cases (81%). In four HPV-negative cancers, reduced expression of p14(ARF) mRNA was detected in two cases (50%) and reduced p16(INK4A) mRNA in three cases (75%). Our data indicate that the overexpression of p14(ARF) and p16(INK4A) strongly associates with HPV-positive cervical cancers and that reduced expression of p14(ARF) and p16(INK4A) correlates with HPV-negative cervical cancers. These findings may indicate that impaired p14(ARF) and p16(INK4A) mRNA expression contribute to tumor development in HPV-negative cervical cancers by failure to support p53 and Rb instead of their inactivation by HPV E6 and E7.     

Growth suppression by a p14(ARF) exon 1beta adenovirus in human tumor cell lines of varying p53 and Rb status

We have analyzed the ability of an adenoviral vector encoding the exon 1beta region of the p14(ARF) tumor suppressor (ARF) to suppress the growth and viability of an array of tumor cell lines of various origins and varying p53 and Rb status, in order to establish the clinical potential of ARF. An important activity of ARF is regulation of p53 stability and function through binding to the mdm2 protein. By sequestering mdm2, ARF may promote growth suppression through the Rb pathway as well because mdm2 can bind to Rb and attenuate its function. Whereas the high frequency of ARF gene deletion in human cancers, accounting for some 40% of cancers overall, suggests that ARF would be a strong candidate for therapeutic application, the possible dependence of ARF activity on p53 and Rb function presents a potential limitation to its application, as these functions are often impaired in cancer. We show here that a replication-defective adenovirus, Ad1beta, encoding the exon 1beta region of ARF is most effective in tumor cells expressing endogenous wild-type p53. Nevertheless, Ad1beta suppresses tumor cell growth and viability in vitro and in vivo, inducing G1 or G2 cell cycle arrest and cell death even in tumor cells lacking both functional Rb and p53 pathways, and independently of induction of the p53 downstream targets, p21, bax, and mdm2. These results point to an activity of ARF in human tumor cells that is independent of Rb or p53, and suggest that therapeutic applications based on ARF would have a broad clinical application in cancer.     

p19ARF prevents G1 cyclin-dependent kinase activation by interacting with MDM2 and activating p53 in mouse fibroblasts.

p19ARF encoded by the INK4a tumor suppressor gene locus functions upstream of p53 to induce cell cycle arrest. p19ARF can interact with MDM2 and p53 in cells ectopically overexpressing these three components, but the biochemical cascades from p19ARF to cell cycle arrest has not been fully elucidated. In this study, we generated stably transfected NIH3T3 cells that express exogenous p19ARF under the control of a heavy metal-inducible metalothionine promoter. Cells arrested in G1 by ectopically expressed p19ARF contained considerably reduced G1 cyclin dependent kinase (cdk2 and cdk4) activities. The expression of cyclin A (a regulatory subunit of cdk2) markedly decreased, while cyclin D1, the major cdk4 partner in fibroblasts, expressed at a slightly higher level and formed complexes with cdk2 and cdk6 in addition to cdk4. Induction of p19ARF activated p53 by increasing its stability, and allowed the expression of p21Cip1, which bound to all of the cyclin D1-cdk complexes (cyclin D1-cdk2, -cdk4, and -cdk6) thereby inhibiting their kinase activities. p19ARF formed complexes with several cellular proteins including mouse MDM2. The majority of MDM2 was found in the complex with p19ARF, while no p53 was detected in association with p19ARF. Thus, we propose that p19ARF neutralizes MDM2 by sequestration from p53, which results in activation of p53, inhibition of G1 cyclin-cdk activities, and G1 arrest.