p16INK4a-induced senescence is disabled by melanoma-associated mutations

The p16(INK4a)-Rb tumour suppressor pathway is required for the initiation and maintenance of cellular senescence, a state of permanent growth arrest that acts as a natural barrier against cancer progression. Senescence can be overcome if the pathway is not fully engaged, and this may occur when p16(INK4a) is inactivated. p16(INK4a) is frequently altered in human cancer and germline mutations affecting p16(INK4a) have been linked to melanoma susceptibility. To characterize the functions of melanoma-associated p16(INK4a) mutations, in terms of promoting proliferative arrest and initiating senescence, we utilized an inducible expression system in a melanoma cell model. We show that wild-type p16(INK4a) promotes rapid cell cycle arrest that leads to a senescence programme characterized by the appearance of chromatin foci, activation of acidic beta-galactosidase activity, p53 independence and Rb dependence. Accumulation of wild-type p16(INK4a) also promoted cell enlargement and extensive vacuolization independent of Rb status. In contrast, the highly penetrant p16(INK4a) variants, R24P and A36P failed to arrest cell proliferation and did not initiate senescence. We also show that overexpression of CDK4, or its homologue CDK6, but not the downstream kinase, CDK2, inhibited the ability of wild-type p16(INK4a) to promote cell cycle arrest and senescence. Our data provide the first evidence that p16(INK4a) can initiate a CDK4/6-dependent autonomous senescence programme that is disabled by inherited melanoma-associated mutations.     

Tumor suppressor INK4: refinement of p16INK4A structure and determination of p15INK4B structure by comparative modeling and NMR data

Within the tumor suppressor protein INK4 (inhibitor of cyclin-dependent kinase 4) family, p15INK4B is the smallest and the only one whose structure has not been determined previously, probably due to the protein's conformational flexibility and instability. In this work, multidimensional NMR studies were performed on this protein. The first tertiary structure was built by comparative modeling with p16INK4A as the template, followed by restrained energy minimization with NMR constraints (NOE and H-bonds). For this purpose, the solution structure of pl6INK4A, whose quality was also limited by similar problems, was refined with additional NMR experiments conducted on an 800 MHz spectrometer and by structure-based iterative NOE assignments. The nonhelical regions showed major improvement with root-mean-square deviation (RMSD) improved from 1.23 to 0.68 A for backbone heavy atoms. The completion of p15INK4B coupled with refinement of p16INK4A made it possible to compare the structures of the four INK4 members in depth, and to compare the structures of p16INK4A in the free form and in the p16INK4A-CDK6 complex. This is an important step toward a comprehensive understanding of the precise functional roles of each INK4 member.     

Expression and characterization of Syrian golden hamster p16, a homologue of human tumor suppressor p16 INK4A

The p16(INK4A)/CDKN2A tumor suppressor gene is known to be inactivated in up to 98% of human pancreatic cancer specimens and represents a potential target for novel therapeutic intervention. Chemically induced pancreatic tumors in Syrian golden hamsters have been demonstrated to share many morphologic and biological similarities with human pancreatic tumors and this model may be appropriate for studying therapies targeting p16(INK4A)/CDKN2A. The purpose of this study was to investigate the fundamental biochemistry of hamster P16 protein. Using both in vivo and in vitro approaches, the CDK4 binding affinity, kinase inhibitory activity, and thermodynamic stability of hamster and human P16 proteins were evaluated. Furthermore, a structural model of hamster P16 protein was generated. These studies demonstrate that hamster P16 protein is biochemically indistinguishable from human P16 protein. From a biochemical perspective, these data strongly support the study of p16-related pancreatic oncogenesis and cancer therapies in the hamster model.     

A novel CDKN2A variant (p16L117P) in a patient with familial and multiple primary melanomas

Germline mutations in CDKN2A (p16) are commonly found in patients with family history of melanoma or personal history of multiple primary melanomas. The p16 tumor suppressor gene regulates cell cycle progression and senescence through binding of cyclin‐dependent kinases (CDK) and also regulates cellular oxidative stress independently of cell cycle control. We identified a germline missense (c.350T>C, p.Leu117Pro) CDKN2A mutation in a patient who had history of four primary melanomas, numerous nevi, and self‐reported family history of melanoma. This particular CDKN2A mutation has not been previously reported in prior large studies of melanoma kindreds or patients with multiple primary melanomas. Compared with wild‐type p16, the p16^L117P mutant largely retained binding capacity for CDK4 and CDK6 but exhibited impaired capacity for repressing cell cycle progression and inducing senescence, while retaining its ability to reduce mitochondrial reactive oxygen species. Structural modeling predicted that the Leu117Pro mutation disrupts a putative adenosine monophosphate (AMP) binding pocket involving residue 117 in the fourth ankyrin domain. Identification of this new likely pathogenic variant extends our understanding of CDKN2A in melanoma susceptibility and implicates AMP as a potential regulator of p16.     

小鼠p16~(INK4a)基因位点的结构和功能研究

p1 6INK4a基因的失活与多种肿瘤的发生和发展有联系。通过筛选小鼠基因组文库 ,获得长度为 1 4.5kb的p1 6INK4a基因组DNA片段。对上述 1 4.5kbDNA测序后进行生物信息学分析表明 :该片段包含 3个外显子 ,编码 1个由 1 68个氨基酸残基组成的多肽 ,其相对分子质量的理论计算值为 1 7941 ,有 7个可能的磷酸化位点 ,说明p1 6INK4a蛋白的功能可能受到磷酸化的调控。该DNA片段的非编码区分布着大量短散布元件、长散布元件和简单重复序列 ,这样的结构为转座和同源重组提供了结构基础 ,提示了部分肿瘤细胞中p1 6INK4a基因缺失的可能原因。对第一外显子序列与已发表的相应序列比较发现其DNA序列和所编码的多肽存在多态性     

p16INK4a deficiency promotes DNA hyper‐replication and genetic instability in melanocytes

Activated oncogenes restrict cell proliferation and transformation by triggering a DNA damage‐dependent senescence checkpoint in response to DNA hyper‐replication. Here, we show that loss of the p16^INK4a cyclin‐dependent kinase inhibitor and melanoma tumour suppressor facilitates a DNA damage response after a hyper‐replicative phase in human melanocytes. Unlike cells expressing activated oncogenes, however, melanocytes depleted for p16^INK4a display enhanced proliferation and an extended replicative lifespan in the presence of replication‐associated DNA damage. Analysis of human benign naevi confirmed that DNA damage and loss of p16^INK4a expression co‐segregate closely. Thus, we propose that loss of p16^INK4a facilitates tumourigenesis by promoting the proliferation of genetically unstable cells.     

Retinoic acid inhibits the growth of bone marrow mesenchymal stem cells and induces p27Kip1 and p16INK4A up-regulation

The importance of bone marrow mesenchymal stem cells in hemopoiesis has been definitely demonstrated. Thus, their impairment might cause profound alteration on production and maturation of blood cells. In the present paper, we investigated, for the first time, the effect of retinoic acid, an important antileukemic molecule, on the proliferation of primary cultures of human bone marrow mesenchymal stem cells. We demonstrated that retinoic acid, at a pharmacological concentration, hampers strongly the growth of the cells, without inducing osteoblastic differentiation. The analysis of cell division cycle machinery showed that the antiproliferative effect is associated with (i) the up-regulation of two cyclin-dependent kinase inhibitors, namely p27^Kip1 and p16^INK4A, and (ii) the down-regulation of cyclin-dependent kinase 2 activity and pRB phosphorylation. The reported findings represent novel insights into the antileukemic effects of the drug and contribute in clarifying the molecular mechanism of its pharmacological activity.     

Loss of CDKN2A expression is a frequent event in primary invasive melanoma and correlates with sensitivity to the CDK4/6 inhibitor PD0332991 in melanoma cell lines

We have investigated the potential for the p16‐cyclin D‐CDK4/6‐retinoblastoma protein pathway to be exploited as a therapeutic target in melanoma. In a cohort of 143 patients with primary invasive melanoma, we used fluorescence in situ hybridization to detect gene copy number variations (CNVs) in CDK4, CCND1, and CDKN2A and immunohistochemistry to determine protein expression. CNVs were common in melanoma, with gain of CDK4 or CCND1 in 37 and 18% of cases, respectively, and hemizygous or homozygous loss of CDKN2A in 56%. Three‐quarters of all patients demonstrated a CNV in at least one of the three genes. The combination of CCND1 gain with either a gain of CDK4 and/or loss of CDKN2A was associated with poorer melanoma‐specific survival. In 47 melanoma cell lines homozygous loss, methylation or mutation of CDKN2A gene or loss of protein (p16^INK4A) predicted sensitivity to the CDK4/6 inhibitor PD0332991, while RB1 loss predicted resistance.     

Identification and characterization of a novel protein ISOC2 that interacts with p16INK4a

p16(INK4a) is a multiple tumor suppressor, playing an important role in proliferation and tumorigenesis. To screen the p16(INK4a)-associated proteins, we performed a yeast two-hybrid assay and identified a novel protein isochorismatase domain containing 2 (ISOC2). ISOC2 conserves in different species, and encodes 205 and 210 amino acids in human and mouse, respectively. The expression of ISOC2 in mouse is universal but predominantly in uterus, stomach, and urinary tract system. Interaction between ISOC2 and p16(INK4a) was verified using in vitro pull-down assays and in vivo co-immunoprecipitation. Confocal microscopy studies using green and cyan fluorescent fusion proteins determined that ISOC2 co-localizes with p16(INK4a). Over-expressed ISOC2 is able to inhibit p16(INK4a) in dose-dependent manner. Our data indicated that ISOC2 is a novel functional protein, which is able to bind and co-localize with a tumor suppressor gene p16(INK4a). Over-expressed ISOC2 inhibits the expression of p16(INK4a), suggesting that this novel gene may play a role during the tumor development by interacting with p16(INK4a).     

p16INK4a and p14ARF methylation as a potential biomarker for human bladder cancer

Promoter hypermethylation is one of the putative mechanisms underlying the inactivation of negative cell-cycle regulators. We examined whether the methylation status of p16(INK4a) and p14(ARF), genes located upstream of the RB and p53 pathway, is a useful biomarker for the staging, clinical outcome, and prognosis of human bladder cancer. Using methylation-specific PCR (MSP), we examined the methylation status of p16(INK4a) and p14(ARF) in 64 samples from 45 bladder cancer patients (34 males, 11 females). In 19 patients with recurrent bladder cancer, we examined paired tissue samples from their primary and recurrent tumors. The methylation status of representative samples was confirmed by bisulfite DNA sequencing analysis. The median follow-up duration was 34.3 months (range 27.0-100.1 months). The methylation rate for p16(INK4a) and p14(ARF) was 17.8% and 31.1%, respectively, in the 45 patients. The incidence of p16(INKa) and p14(ARF) methylation was significantly higher in patients with invasive (>or=pT2) than superficial bladder cancer (pT1) (p=0.006 and p=0.001, respectively). No MSP bands for p16(INK4a) and p14(ARF) were detected in the 8 patients with superficial, non-recurrent tumors. In 19 patients with tumor recurrence, the p16(INK4a) and p14(ARF) methylation status of the primary and recurrent tumors was similar. Of the 22 patients who had undergone cystectomy, 8 (36.4%) manifested p16(INKa) methylation; p16(INK4a) was not methylated in 23 patients without cystectomy (p=0.002). Kaplan-Meier analysis revealed that patients with p14(ARF) methylation had a significantly poorer prognosis than those without (p=0.029). This is the first study indicating that MSP analysis of p16(INK4a) and p14(ARF) genes is a useful biomarker for the pathological stage, clinical outcome, and prognosis of patients with bladder cancer.     

Expression of p16(INK4a) as a biomarker of T-cell aging in HIV-infected patients prior to and during antiretroviral therapy

The p16(INK4a) tumor suppressor gene is a mediator of cellular senescence and has been suggested to be a biomarker of 'molecular' age in several tissues including T cells. To determine the association of both active and suppressed HIV infection with T-cell aging, T-cell p16(INK4a) expression was compared between 60 HIV+ suppressed subjects, 23 HIV+ untreated subjects, and 18 contemporaneously collected HIV-negative controls, as well as 148 HIV-negative historical samples. Expression did not correlate with chronologic age in untreated HIV+ patients, consistent with an effect of active HIV replication on p16(INK4a) expression. In patients on cART with suppressed viral loads, however, p16(INK4a) levels were similar to uninfected controls and correlated with chronologic age, with a trend toward an inverse correlation with CD4 count. These data show that p16(INK4a) is a reliable biomarker of T-cell aging in HIV+ patients with suppressed viral loads and suggest that poor CD4 cell recovery on cART may be associated with increased T-cell expression of p16(INK4a) , a marker of cellular senescence.     

p16INK4A is a robust in vivo biomarker of cellular aging in human skin

The cell-cycle regulating gene, p16INK4A, encoding an inhibitor of cyclin-dependent kinases 4 and 6, is considered to play an important role in cellular aging and in premature senescence. Although there is an age-dependent increase of p16INK4A expression in human fibroblast senescence in vitro, no data are available regarding the age dependency of p16INK4A in vivo. To determine whether p16INK4A expression in human skin correlates with donor age, p16INK4A expression was analyzed by immunohistochemistry as well as the expression of the p16INK4A repressor BMI1. Samples from the age groups 0-20, 21-70, and 71-95 years were selected from a bank of healthy human skin. We show that the number of p16INK4A positive cells is significantly higher in elderly individuals compared to the younger age groups. The number of p16INK4A positive cells was found to be increased in both epidermis and dermis, compartments with strictly different proliferative activities. BMI1 gene expression was significantly down-regulated with increasing donor age, whereas no striking age differences were observed for Ki67. In immunofluorescence co-expression studies, Ki67-positive cells were negative for p16INK4A and BMI1-expressing cells also stained negatively for Ki67. In conclusion, we provide for the first time evidence that p16INK4A expression directly correlates with chronological aging of human skin in vivo. p16INK4A therefore is a biomarker for human aging in vivo. The data reported here suggest a model for changes in regulatory gene expression that drive aging in human skin.     

5-Fluorouracil or gemcitabine combined with adenoviral-mediated reintroduction of p16INK4A greatly enhanced cytotoxicity in Panc-1 pancreatic adenocarcinoma cells

BACKGROUND: Pancreatic cancer is one of the most lethal of all the common gastrointestinal malignancies. Although surgery offers the best chance for survival, it is not appropriate for all cases. The only adjuvant treatment to show promise is chemotherapy. Hence new treatments are urgently sought. We previously reported that adenoviral (Ad)-mediated delivery of p53 (Adp53) and p16(INK4A) (Adp16) significantly inhibited the growth of pancreatic cancer cell lines and established subcutaneous pancreatic tumours in nude mice (Ghaneh P, et al. Adenovirus mediated transfer of p53 and p16INK4A results in pancreatic cancer regression in vitro and in vivo. Gene Ther 2001; 8: 199-208). In this study we examine whether combining Ad-mediated delivery of p53 or p16(INK4A) with clinically relevant chemotherapeutic drugs has therapeutic potential for pancreatic cancer. METHODS AND RESULTS: Four pancreatic adenocarcinoma cell lines were evaluated for their sensitivity to 5-fluorouracil (5-FU) and gemcitabine and two of these, Suit-2 and Panc-1, were chosen for combination experiments because they showed moderate and poor sensitivity, respectively, to 5-FU and gemcitabine. We found no evidence for enhanced cytotoxicity when either cell line was transduced with Adp53 before or after incubation with chemotherapeutic drugs. In contrast, incubation of Panc-1 cells with either 5-FU or gemcitabine followed by Ad-mediated overexpression of p16(INK4A) resulted in a substantial reduction in cell viability under conditions where the drugs alone had minimal cytotoxicity. Incubation of Suit-2 cells with 5-FU followed by Ad-mediated overexpression of p16(INK4A) also resulted in a significant reduction in cell viability. This, however, was observed only with higher concentrations of 5-FU and viral vector. Cell cycle analysis of Panc-1 cells showed that the combination of cytotoxic drugs and Adp16 resulted in an increase in the sub-G1 population suggesting an increase in apoptosis. Dual labelling of these cells with annexin V and propidium iodide (PI) confirmed that the combination of 5-FU and Adp16 resulted in a significant increase in early apoptotic cells (annexin V positive and PI negative) compared with controls. Moreover, overexpression of p16(INK4A) was associated with a reduction in pRb levels in these cells-high levels of pRb have been proposed to contribute to chemoresistance in pancreatic cancer cells. CONCLUSIONS: We have shown that the currently used chemotherapeutic drugs for pancreatic adenocarcinoma combined with restoration of p16(INK4A) expression hold promise for the adjuvant treatment of this disease. Importantly, the combination facilitated the use of chemotherapeutic drugs at lower concentrations than would otherwise be effective.     

p16INK4a在早孕小鼠子宫内膜的表达及其对胚泡着床的影响

本研究旨在检测肿瘤抑制基因p16INK4a(inhibitor of cyclin-dependent kinase 4a)在早孕小鼠子宫内膜中的表达规律,探讨p16INK4a在小鼠胚胎着床过程中的作用.采用荧光定量PCR(FQ-PCR)和免疫组织化学方法分别检测未孕小鼠及孕小鼠第2、3、4、5、7天子宫内膜p16INK4a mRNA和蛋白的表达;子宫角注射p16INK4a抗体观察胚泡着床数.FQ-PCR结果显示孕小鼠子宫内膜组织p16INK4amRNA的表达高于未孕小鼠,且随着妊娠天数的增加呈现表达逐渐增强的趋势,到妊娠第5天达到最高,后渐降.免疫组织化学分析显示p16INK4a蛋白在子宫内膜的表达规律与mRNA结果一致.子宫角注射p16INK4a抗体后胚泡着床数明显减少.以上结果提示,P161INK4a在妊娠早期子宫内膜持续表达,可能参与胚泡着床.     

P16^INK4、Rb基因表达在肺癌发生中协调性研究

采用mRNA原位双杂交和免疫组织化学方法对31例非小细胞肺癌组织进行P16^INK、Rb基因、Rb基因表达水平及其相关性的研究。结果显示,以Dig-碱性磷酸酶－NBT／BCIP系统检测P16^INK4基因转录,阳性结果呈蓝色,阴性杂交率为22．6％（7／31）;以Bio－辣根过氧化物酶－AEC系统检测Rb基因转录,阳性结果为红色,阴性率为16．1％（5／31）。免疫组织化学检测显示P16^INK4蛋白质阴性率为42％（13／31）;Rb蛋白表达阴性率为19．4％（6／31）。Rb、P16^INK4基因在非小细胞肺癌发生中起协同调控作用,以P16^INK4基因表达异常为主。