Growth suppression by p16ink4 requires functional retinoblastoma protein

ASCL2 is a key regulator of the proliferation-differentiation equilibrium in the esophageal epithelium

The esophagus is protected from the hostile environment by a stratified epithelium, which renews rapidly. Homeostasis of this epithelium is ensured by a rare population of stem cells in the basal layer: Keratin 15+ (Krt15+) cells. However, little is known about the molecular mechanisms regulating their distinct features, namely self-renewal, potency and epithelial regeneration. Achaete-scute family BHLH transcription factor 2 (ASCL2) is strongly upregulated in Krt15+ stem cells and is known to contribute to stem cell maintenance in other tissues. Herein, we investigated the role of ASCL2 in maintaining homeostasis under normal and stress conditions in the esophageal epithelium. ASCL2 overexpression severely dysregulated cell differentiation and cell fate. Proliferation was also reduced due potentially to a blockage in the G1 phase of the cell cycle or an induction of quiescence. Mass spectrometry analysis confirmed alterations in several proteins associated with differentiation and the cell cycle. In addition, overexpression of ASCL2 enhanced resistance to radiation and chemotherapeutic drugs. Overall, these results denote the role of ASCL2 as a key regulator of the proliferation-differentiation equilibrium in the esophageal epithelium.

SNIP1 and PRC2 coordinate cell fates of neural progenitors during brain development

Stem cell survival versus death is a developmentally programmed process essential for morphogenesis, sizing, and quality control of genome integrity and cell fates. Cell death is pervasive during development, but its programming is little known. Here, we report that Smad nuclear interacting protein 1 (SNIP1) promotes neural progenitor cell survival and neurogenesis and is, therefore, integral to brain development. The SNIP1-depleted brain exhibits dysplasia with robust induction of caspase 9-dependent apoptosis. Mechanistically, SNIP1 regulates target genes that promote cell survival and neurogenesis, and its activities are influenced by TGFβ and NFκB signaling pathways. Further, SNIP1 facilitates the genomic occupancy of Polycomb complex PRC2 and instructs H3K27me3 turnover at target genes. Depletion of PRC2 is sufficient to reduce apoptosis and brain dysplasia and to partially restore genetic programs in the SNIP1-depleted brain in vivo. These findings suggest a loci-specific regulation of PRC2 and H3K27 marks to toggle cell survival and death in the developing brain.

Cellular senescence and neurodegeneration

Advancing age is a major risk factor of Alzheimer's disease (AD). The worldwide prevalence of AD is approximately 50 million people, and this number is projected to increase substantially. The molecular mechanisms underlying the aging-associated susceptibility to cognitive impairment in AD are largely unknown. As a hallmark of aging, cellular senescence is a significant contributor to aging and age-related diseases including AD. Senescent neurons and glial cells have been detected to accumulate in the brains of AD patients and mouse models. Importantly, selective elimination of senescent cells ameliorates amyloid beta and tau pathologies and improves cognition in AD mouse models, indicating a critical role of cellular senescence in AD pathogenesis. Nonetheless, the mechanisms underlying when and how cellular senescence contributes to AD pathogenesis remain unclear. This review provides an overview of cellular senescence and discusses recent advances in the understanding of the impact of cellular senescence on AD pathogenesis, with brief discussions of the possible role of cellular senescence in other neurodegenerative diseases including Down syndrome, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis.

CDK4/6 Inhibitors in Pancreatobiliary Cancers: Opportunities and Challenges

Existing treatment strategies for pancreatobiliary malignancies are limited. Nowadays, surgery is the only path to cure these types of cancer, but only a small number of patients present with resectable tumors at the time of diagnosis. The notoriously poor prognosis, lack of diverse treatment options associated with pancreaticobiliary cancers, and their resistance to current therapies reflect the urge for the development of novel therapeutic targets. Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors have emerged as an attractive therapeutic strategy in a number of cancers since their approval for treatment in patients with ER+/HER- breast cancer in combination with antiestrogens. In this article, we discuss the therapeutic potential of CDK4/6 inhibitors in pancreatobiliary cancers, notably cholangiocarcinoma and pancreatic ductal adenocarcinoma.

Cell cycle related long non-coding RNAs as the critical regulators of breast cancer progression and metastasis

Cell cycle is one of the main cellular mechanisms involved in tumor progression. Almost all of the active molecular pathways in tumor cells directly or indirectly target the cell cycle progression. Therefore, it is necessary to assess the molecular mechanisms involved in cell cycle regulation in tumor cells. Since, early diagnosis has pivotal role in better cancer management and treatment, it is required to introduce the non-invasive diagnostic markers. Long non-coding RNAs (LncRNAs) have higher stability in body fluids in comparison with mRNAs. Therefore, they can be used as efficient non-invasive markers for the early detection of breast cancer (BCa). In the present review we have summarized all of the reported lncRNAs involved in cell cycle regulation in BCa. It has been reported that lncRNAs mainly affect the cell cycle in G1/S transition through the CCND1/CDK4-6 complex. Present review paves the way of introducing the cell cycle related lncRNAs as efficient markers for the early detection of BCa.

Cancer cell cycle dystopia: heterogeneity, plasticity, and therapy

The mammalian cell cycle has been extensively studied regarding cancer etiology, progression, and therapeutic intervention. The canonical cell cycle framework is supported by a plethora of data pointing to a relatively simple linear pathway in which mitogenic signals are integrated in a stepwise fashion to allow progression through G1/S with coordinate actions of cyclin-dependent kinases (CDK)4/6 and CDK2 on the RB tumor suppressor. Recent work on adaptive mechanisms and intrinsic heterogeneous dependencies indicates that G1/S control of the cell cycle is a variable signaling pathway rather than an invariant engine that drives cell division. These alterations can limit the effectiveness of pharmaceutical agents but provide new avenues for therapeutic interventions. These findings support a dystopian view of the cell cycle in cancer where the canonical utopian cell cycle is often not observed. However, recognizing the extent of cell cycle heterogeneity likely creates new opportunities for precision therapeutic approaches specifically targeting these states.

CDK4 and CDK6 kinases: From basic science to cancer therapy

Cyclin-dependent kinases 4 and 6 (CDK4 and CDK6) and their activating partners, D-type cyclins, link the extracellular environment with the core cell cycle machinery. Constitutive activation of cyclin D–CDK4/6 represents the driving force of tumorigenesis in several cancer types. Small-molecule inhibitors of CDK4/6 have been used with great success in the treatment of hormone receptor–positive breast cancers and are in clinical trials for many other tumor types. Unexpectedly, recent work indicates that inhibition of CDK4/6 affects a wide range of cellular functions such as tumor cell metabolism and antitumor immunity. We discuss how recent advances in understanding CDK4/6 biology are opening new avenues for the future use of cyclin D–CDK4/6 inhibitors in cancer treatment.

Biology of HPV Mediated Carcinogenesis and Tumor Progression

Human papillomavirus (HPV) is a ubiquitous DNA virus that infects squamous epithelia. Though HPV only encodes 8 genes, it is capable of causing cellular transformation and ultimately cancer in host cells. In this article we review the classification of HPV viruses, their genetic structure and life cycle, viral gene biology, and provide an overview of the role of HPV in cancer. We explain how the viral life cycle can lead to integration of viral DNA into the host genome leading to increased cell cycle progression, decreased apoptosis, altered DNA repair, and chromosomal instability. We describe the multifaceted roles of the canonical oncogenes E6 and E7 in promoting tumorigenesis and the important role of other viral genes in regulating cancer development. We also review how the virus actively suppresses innate and adaptive immunity to evade immune detection and promote a pro-tumorigenic microenvironment. The biology presented here will serve as a foundation to the other chapters in this edition and we hope it will incite enthusiasm for continued research on this fascinating virus that causes significant morbidity and mortality worldwide.

PARP-1 via regulation of p53 and p16, is involved in the hydroquinone-induced malignant transformation of TK6 cells by decelerating the cell cycle

Poly(ADP-ribose)polymerase-1 (PARP-1) plays a crucial role in DNA damage repair and could be viewed as both a tumor promoter and tumor-suppressor gene. However, the effects of PARP-1 in hydroquinone-induced malignant transformation of TK6 cells remain to be further elucidated. The present research evaluated the potential mechanism of PARP-1 in hydroquinone-induced malignant transformation of TK6 cells. The results indicated that high PARP-1 inhibited TK6 cells malignant transformation after chronic exposure to HQ. We further confirmed that PARP-1 overexpression blocked cell proliferation, and decelerated cell cycle progression in vitro and in vivo. The immunoblotting analysis indicated that PARP-1 regulated cell cycle progression via p16/Rb and p53. Therefore, we conclude that PARP-1 is involved in HQ-induced malignant transformation associated with increasing p16/Rb and p53 which resulting in decelerating the cell cycle progression.

Breast Cancer Resistance to Cyclin-Dependent Kinases 4/6 Inhibitors: Intricacy of the Molecular Mechanisms

Breast cancer is a common malignant tumor in women, with a highest incidence and mortality among all of the female malignant tumors. Notably, targeted therapy has achieved impressive success in the treatment of breast cancer. As one class of the anti-tumor targeted therapeutics, Cyclin-Dependent Kinases 4/6CDK4/6inhibitors have shown good clinical activity in treating breast cancer. Nevertheless, despite the promising clinical outcomes, intrinsic or acquired resistance to CDK4/6 inhibitors has limited the benefits of this novel target therapy. In the present review, we provide an overview of the currently known molecular mechanisms of resistance to CDK4/6 inhibitors, and discuss the potential strategies to overcoming drug resistance improving the outcomes for breast cancer patients treated with CDK4/6 inhibitors.

Metabolic Control by DNA Tumor Virus-Encoded Proteins

Viruses co-opt a multitude of host cell metabolic processes in order to meet the energy and substrate requirements for successful viral replication. However, due to their limited coding capacity, viruses must enact most, if not all, of these metabolic changes by influencing the function of available host cell regulatory proteins. Typically, certain viral proteins, some of which can function as viral oncoproteins, interact with these cellular regulatory proteins directly in order to effect changes in downstream metabolic pathways. This review highlights recent research into how four different DNA tumor viruses, namely human adenovirus, human papillomavirus, Epstein-Barr virus and Kaposi's associated-sarcoma herpesvirus, can influence host cell metabolism through their interactions with either MYC, p53 or the pRb/E2F complex. Interestingly, some of these host cell regulators can be activated or inhibited by the same virus, depending on which viral oncoprotein is interacting with the regulatory protein. This review highlights how MYC, p53 and pRb/E2F regulate host cell metabolism, followed by an outline of how each of these DNA tumor viruses control their activities. Understanding how DNA tumor viruses regulate metabolism through viral oncoproteins could assist in the discovery or repurposing of metabolic inhibitors for antiviral therapy or treatment of virus-dependent cancers.

Mechanisms of CDK4/6 Inhibitor Resistance in Luminal Breast Cancer

As a new-generation CDK inhibitor, a CDK4/6 inhibitor combined with endocrine therapy has been successful in the treatment of advanced estrogen receptor–positive (ER+) breast cancer. Although there has been overall progress in the treatment of cancer, drug resistance is an emerging cause for breast cancer–related death. Overcoming CDK4/6 resistance is an urgent problem. Overactivation of the cyclin-CDK-Rb axis related to uncontrolled cell proliferation is the main cause of CDK4/6 inhibitor resistance; however, the underlying mechanisms need to be clarified further. We review various resistance mechanisms of CDK4/6 inhibitors in luminal breast cancer. The cell signaling pathways involved in therapy resistance are divided into two groups: upstream response mechanisms and downstream bypass mechanisms. Finally, we discuss possible strategies to overcome CDK4/6 inhibitor resistance and identify novel resistance targets for future clinical application.

Knockout of the caspase 8-associated protein 2 gene improves recombinant protein expression in HEK293 cells through up-regulation of the cyclin-dependent kinase inhibitor 2A gene

Cell lines used in bioproduction are routinely engineered to improve their production efficiency. Numerous strategies, such as random mutagenesis, RNA interference screens, and transcriptome analyses have been employed to identify effective engineering targets. A genome-wide small interfering RNA screen previously identified the CASP8AP2 gene as a potential engineering target for improved expression of recombinant protein in the HEK293 cell line. Here, we validate the CASP8AP2 gene as an engineering target in HEK293 cells by knocking it out using CRISPR/Cas9 genome editing and assessing the effect of its knockout on recombinant protein expression, cell growth, cell viability, and overall gene expression. HEK293 cells lacking CASP8AP2 showed a seven-fold increase in specific expression of recombinant luciferase and a 2.5-fold increase in specific expression of recombinant SEAP, without significantly affecting cell growth and viability. Transcriptome analysis revealed that the deregulation of the cell cycle, specifically the upregulation of the cyclin-dependent kinase inhibitor 2A (CDKN2A) gene, contributed to the improvement in recombinant protein expression in CASP8AP2 deficient cells. The results validate the CASP8AP2 gene is a viable engineering target for improved recombinant protein expression in the HEK293 cell line.

Mechanisms of resistance to estrogen receptor modulators in ER+/HER2- advanced breast cancer

Endocrine therapy represents a mainstay adjuvant treatment of estrogen receptor-positive (ER+) breast cancer in clinical practice with an overall survival (OS) benefit. However, the emergence of resistance is inevitable over time and is present in one-third of the ER+ breast tumors. Several mechanisms of endocrine resistance in ER+/HER2- advanced breast cancers, through ERα itself, receptor tyrosine signaling, or cell cycle pathway, have been identified to be pivotal in endocrine therapy. The epigenetic alterations also contribute to ensuring tumor cells' escape from endocrine therapies. The strategy of combined hormone therapy with targeted pharmaceutical compounds has shown an improvement of progression-free survival or OS in clinical practice, including three different classes of drugs: CDK4/6 inhibitors, selective inhibitor of PI3Kα and mTOR inhibitors. Many therapeutic targets of cell cycle pathway and cell signaling and their combination strategies have recently entered clinical trials. This review focuses on Cyclin D-CDK4/6-RB axis, PI3K pathway and HDACs. Additionally, genomic evolution is complex in tumors exposed to hormonal therapy. We highlight the genomic alterations present in ESR1 and PIK3CA genes to elucidate adaptive mechanisms of endocrine resistance, and discuss how these mutations may inform novel combinations to improve clinical outcomes in the future.

Cysteine oxidation triggers amyloid fibril formation of the tumor suppressor p16INK4A

The tumor suppressor p16INK4A induces cell cycle arrest and senescence in response to oncogenic transformation and is therefore frequently lost in cancer. p16INK4A is also known to accumulate under conditions of oxidative stress. Thus, we hypothesized it could potentially be regulated by reversible oxidation of cysteines (redox signaling). Here we report that oxidation of the single cysteine in p16INK4A in human cells occurs under relatively mild oxidizing conditions and leads to disulfide-dependent dimerization. p16INK4A is an all α-helical protein, but we find that upon cysteine-dependent dimerization, p16INK4A undergoes a dramatic structural rearrangement and forms aggregates that have the typical features of amyloid fibrils, including binding of diagnostic dyes, presence of cross-β sheet structure, and typical dimensions found in electron microscopy. p16INK4A amyloid formation abolishes its function as a Cyclin Dependent Kinase 4/6 inhibitor. Collectively, these observations mechanistically link the cellular redox state to the inactivation of p16INK4A through the formation of amyloid fibrils.

Palbociclib in Patients With Pancreatic and Biliary Cancer With CDKN2A Alterations: Results From the Targeted Agent and Profiling Utilization Registry Study

PURPOSE

The Targeted Agent and Profiling Utilization Registry (TAPUR) Study identifies signals of antitumor activity of commercially available targeted agents in patients with advanced cancers that harbor genomic alterations known as drug targets. In this article, data from two cohorts of patients with pancreatic and biliary cancers with CDKN2A loss or mutation treated with palbociclib are reported.

METHODS

Eligible patients age 12 years and older with advanced measurable or evaluable solid tumors are provided treatment according to protocol-specified genomic matching rules. The primary study end point is objective response or stable disease of at least 16 weeks duration. For each cohort, a Simon two-stage design was used with a futility evaluation after 10 patients. Secondary end points include safety, progression-free survival (PFS), and overall survival (OS).

RESULTS

Between July 2016 and November 2017, 12 and 10 patients with pancreatic and biliary cancer, respectively, with CDKN2A loss or mutation were treated with palbociclib. Twenty evaluable patients (10 per cohort) were included in the analysis. No patients had objective response or stable disease at 16 weeks, and both cohorts were closed. Two patients, neither with response, were determined to be ineligible. All patients were evaluated for safety, PFS, and OS. A median PFS of 7.2 weeks (90% CI, 4.0 to 8.0 weeks) and median OS of 12.4 weeks (90% CI, 4.7 to 23.1 weeks) were observed in the pancreatic cohort. A median PFS of 7.3 weeks (90% CI, 3.9 to 7.9 weeks) and median OS of 11.1 weeks (90% CI, 5.1 to 14.0 weeks) were observed in the biliary cohort. No unexpected toxicities were observed.

CONCLUSION

Palbociclib monotherapy does not have clinical activity in patients with advanced pancreatic or biliary cancers with CDKN2A loss or mutation. Toxicity is similar to reported experience with palbociclib in other tumor types.

Molecular mechanisms of resistance to CDK4/6 inhibitors in breast cancer: A review

Deregulation of the cyclin D-CDK4/6-INK4-RB pathway leading to uncontrolled cell proliferation, is frequently observed in breast cancer. Currently, three selective CDK4/6 inhibitors have been FDA approved: palbociclib, ribociclib and abemaciclib. Despite promising clinical outcomes, intrinsic or acquired resistance to CDK4/6 inhibitors has limited the success of these treatments; therefore, the development of various strategies to overcome this resistance is of great importance. We highlight the various mechanisms that are directly or indirectly responsible for resistance to CDK4/6 inhibitors, categorizing them into two broad groups; cell cycle-specific mechanisms and cell cycle-nonspecific mechanisms. Elucidation of the diverse mechanisms through which resistance to CDK4/6 inhibitors occurs, may aid in the design of novel therapeutic strategies to improve patient outcomes. This review summarizes the currently available knowledge regarding mechanisms of resistance to CDK4/6 inhibitors, and possible therapeutic strategies that may overcome this resistance as well.

Prognostic impact of high p16/cyclin D1 index in breast cancer

Proteins p16 and cyclin D1 (CCND1) are known to tightly regulate the G1/S transition during the cell cycle, but their role in breast cancer development and progression is not clear. We investigated 224 cases of breast cancer from the Kangbuk Samsung Medical Center between 2000-2005. Expression levels of p16 and CCND1 were assessed by tissue microarray-based immunohistochemistry. A p16/CCND1 index was divided into low- and high-expression groups using receiver operating characteristic curves. The p16/CCND1 index was significantly different across molecular subtypes and a high p16/CCND1 index was statistically correlated with survival rates. This p16/CCND1 index may be an indicator of poor patient outcome and thus, represents a potential therapeutic target.

Histone Deacetylation in the Promoter of p16 Is Involved in Fluoride-Induced Human Osteoblast Activation via the Inhibition of Sp1 Binding

Chronic fluorosis is a systemic condition which principally manifests as defects in the skeleton and teeth. Skeletal fluorosis is characterized by aberrant proliferation and activation of osteoblasts, however, the underlying mechanisms of osteoblast activation induced by fluoride are not fully understood. Therefore, we investigated the pathogenic mechanism of human primary osteoblast proliferation and activation in relation to histone acetylation of the promoter p16, a well-known cell cycle regulation-related gene. The results showed that sodium fluoride (NaF) induced deacetylation and decreased expression of the p16 gene via inhibition of specificity protein 1 (Sp1) binding to its response element, which accounts for NaF increasing cell viability and promoting proliferation in human primary osteoblasts. These results reveal the regulatory mechanism of histone acetylation of the p16 gene on osteoblast activation in skeletal fluorosis.

Mechanisms of therapeutic CDK4/6 inhibition in breast cancer

Cyclin dependent kinase (CDK) 4/6 inhibitors have advanced the treatment of metastatic breast cancer by targeting the cell cycle machinery, interrupting intracellular and mitogenic hormone signals that stimulate proliferation of malignant cells. Preclinical evidence demonstrated that derangements of cyclin D1, CDK4/6, and retinoblastoma expression are common in breast cancer, and suggested a therapeutic benefit from interrupting this axis required for cell cycle progression. Studies of cell lines and animal models of breast cancer have demonstrated the complex interplay between the cell cycle and estrogen receptor and human epidermal growth receptor 2 signaling, which informs our understanding of synergistic use of CDK4/6 inhibitors with endocrine therapy, as well as mechanisms of resistance to endocrine therapy. Interestingly, estrogen receptor activity leads to upregulation of cyclin D1 expression, but the estrogen receptor is also in turn activated by cyclin D1, independent of estrogen binding. Early CDK inhibitors were nonspecific and limited by systemic toxicities, while the current generation of CDK4/6 inhibitors have shown promise in the treatment of hormone receptor-positive breast cancer. Preclinical investigations of the three CDK4/6 inhibitors approved by the US Food and Drug Administration (palbociclib, ribociclib, and abemaciclib) lend further insight into their mechanism of action, which will hopefully inform the future use and refinement of these therapies. Finally, we summarize evidence for additional novel CDK4/6 inhibitors currently in development.

Deletion of the p16INK4a tumor suppressor and expression of the androgen receptor induce sarcomatoid carcinomas with signet ring cells in the mouse prostate

The tumor suppressor p16^Ink4a, encoded by the INK4a gene, is an inhibitor of cyclin D-dependent kinases 4 and 6, CDK4 and CDK6. This inhibition prevents the phosphorylation of the retinoblastoma protein (pRb), resulting in cellular senescence through inhibition of E2F-mediated transcription of S phase genes required for cell proliferation. The p16^Ink4a plays an important role in tumor suppression, whereby its deletion, mutation, or epigenetic silencing is a frequently observed genetic alteration in prostate cancer. To assess its roles and related molecular mechanisms in prostate cancer initiation and progression, we generated a mouse model with conditional deletion of p16^Ink4a in prostatic luminal epithelium. The mice underwent oncogenic transformation and developed prostatic intraepithelial neoplasia (PIN) from eight months of age, but failed to develop prostatic tumors. Given the prevalence of aberrant androgen signaling pathways in prostate cancer initiation and progression, we then generated R26hAR^L/wt:p16^L/L: PB-Cre4 compound mice, in which conditional expression of the human AR transgene and deletion of p16^Ink4a co-occur in prostatic luminal epithelial cells. While R26hAR^L/wt:PB-Cre4 mice showed no visible pathological changes, R26hAR^L/wt:p16^L/L: PB-Cre4 compound mice displayed an early onset of high-grade PIN (HGPIN), prostatic carcinoma, and metastatic lesions. Strikingly, we observed tumors resembling human sarcomatoid carcinoma with intermixed focal regions of signet ring cell carcinoma (SRCC) in the prostates of the compound mice. Further characterization of these tumors showed they were of luminal epithelial cell origin, and featured characteristics of epithelial to mesenchymal transition (EMT) with enhanced proliferative and invasive capabilities. Our results not only implicate a biological role for AR expression and p16^Ink4a deletion in the pathogenesis of prostatic SRCC, but also provide a new and unique genetically engineered mouse (GEM) model for investigating the molecular mechanisms for SRCC development.

Targeting the Cardiomyocyte Cell Cycle for Heart Regeneration

Adult mammalian cardiomyocytes (CMs) exhibit limited proliferative capacity, as cell cycle activity leads to an increase in DNA content, but mitosis and cytokinesis are infrequent. This makes the heart highly inefficient in replacing with neoformed cardiomyocytes lost contractile cells as occurs in diseases such as myocardial infarction and dilated cardiomyopathy. Regenerative therapies based on the implant of stem cells of diverse origin do not warrant engraftment and electromechanical connection of the new cells with the resident ones, a fundamental condition to restore the physiology of the cardiac syncytium. Consequently, there is a growing interest in identifying factors playing relevant roles in the regulation of the CM cell cycle to be targeted in order to induce the resident cardiomyocytes to divide into daughter cells and thus achieve myocardial regeneration with preservation of physiologic syncytial performance.

Hyper-phosphorylation of Rb S249 together with CDK5R2/p39 overexpression are associated with impaired cell adhesion and epithelial-to-mesenchymal transition: Implications as a potential lung cancer grading and staging biomarker

Prediction of lung cancer metastasis relies on post-resection assessment of tumor histology, which is a severe limitation since only a minority of lung cancer patients are diagnosed with resectable disease. Therefore, characterization of metastasis-predicting biomarkers in pre-resection small biopsy specimens is urgently needed. Here we report a biomarker consisting of the phosphorylation of the retinoblastoma protein (Rb) on serine 249 combined with elevated p39 expression. This biomarker correlates with epithelial-to-mesenchymal transition traits in non-small cell lung carcinoma (NSCLC) cells. Immunohistochemistry staining of NSCLC tumor microarrays showed that strong phospho-Rb S249 staining positively correlated with tumor grade specifically in the squamous cell carcinoma (SCC) subtype. Strong immunoreactivity for p39 positively correlated with tumor stage, lymph node invasion, and distant metastases, also in SCC. Linear regression analyses showed that the combined scoring for phospho-Rb S249, p39 and E-cadherin in SCC is even more accurate at predicting tumor staging, relative to each score individually. We propose that combined immunohistochemistry staining of NSCLC samples for Rb phosphorylation on S249, p39, and E-cadherin protein expression could aid in the assessment of tumor staging and metastatic potential when tested in small primary tumor biopsies. The intense staining for phospho-Rb S249 that we observed in high grade SCC could also aid in the precise sub-classification of poorly differentiated SCCs.

Up-regulated expression of E2F2 is necessary for p16INK4a-induced cartilage injury

Background

Cartilage degradation would result in osteoarthritis (OA). p16INK4awas found in some age-related diseases. In this study, we aimed to determine the role of p16INK4a during OA and to investigate the underlying mechanisms.

Methods

Enzyme-linked immunosorbent assay (ELISA) was performed to test the activity of senescence-associated secretory phenotype (SASP). Real-time PCR (RT-PCR) and Western blot were used to determine the expressions of target genes.

Results

The increased expressions of p16INK4a and E2F2 were accompanied with cartilage degradation induced by IL-1β. Over-expression of p16INK4a enhanced the secretion of SASP markers (TGFβ, IL-6, IL-8, IL-1α, MMP3 and MMP13), reduced the expression of type II procollagen (COL2A1).Thus, the over-expression of p16INK4a lead to cartilage injury. Moreover, we found that the expression of E2F2 was enhanced in p16INK4a over-expression group, and that cartilage injury caused by p16INK4a was alleviated by depleting E2F2.

Conclusions

p16INK4a was up-regulated during the cartilage injury in OA. p16INK4a promoted cartilage injury by increasing the expression of E2F2. Thus, this study extends the molecular regulation network for understanding pathological progression of OA, and provides potential therapeutic target for OA.

Proteins of the retinoblastoma pathway, FEN1 and MGMT are novel potential prognostic biomarkers in pancreatic adenocarcinoma

BACKGROUND

We studied the expression of some major proteins involved in cell-cycle regulation and DNA repair, the roles of which are not well known in pancreatic ductal adenocarcinoma (PDAC), but which have a significant impact on carcinogenesis of many other cancers.

METHODS

We immunohistochemically assessed expression levels of the cell-cycle regulators Rb1, p16 and cyclin-dependent kinase 4 (CDK4), and the DNA repair enzymes O6-methylguanine-DNA-alkyltransferase (MGMT) and flap endonuclease-1 (FEN1) separately in malignant tissue and benign tissue from resection margins in 102 cases of PDAC. Nearly all (95.1%) patients had undergone pancreaticoduodenectomy.

RESULTS

The studied proteins showed wide but somewhat variable expression in both benign and malignant pancreatic tissues. Strong CDK4 expression in islets of Langerhans predicted poor relapse-free survival (RFS) (HR 2.874; 95% CI 1.261-6.550; p = .012) and within T3-4 tumors CDK4 expression in adenocarcinoma cells also predicted poor disease-free survival (DFS) (RR 2.148; 95% CI 1.081-4.272; p = .029). Strong MGMT expression was associated in N1 patients with weak local relapse-free survival (RFS), DFS and overall survival; all significantly in Cox regression analysis. FEN1 was also an independent predictor of decreased DFS (in the whole study population) and worse RFS (in the patients with T3-4 tumors).

CONCLUSIONS

Major cell-cycle regulator also have predictive significance, but further studies are required to evaluate this.

Abnormalities in Cell Cycle Control in Cancer and Their Clinical Implications

Recent studies indicate that the functions of several genes that control the cell cycle are altered during the carcinogenic process and that these changes perturb both cell proliferation and genomic stability, thus promoting cell transformation and enhancing the process of tumor progression. The purpose of this paper is to review current information on the role of cyclins and related genes in the control of the mammalian cell cycle, the types of abnormalities in these genes found in human tumors and the possible clinical implications of these findings.

Long non-coding PANDAR as a novel biomarker in human cancer: A systematic review

OBJECTIVES

Long non-coding RNAs (lncRNAs) are characterized as a group of RNAs that more than 200 nucleotides in length and have no protein-coding function. More and more evidences provided that lncRNAs serve as key molecules in the development of cancer. Deregulation of lncRNAs functions as either oncogenes or tumour suppressor genes in various diseases. Recently, increasing studies about PANDAR in cancer progression were reported. In our review, we will focus on the current research on the character of PANDAR include the clinical management, tumour progression and molecular mechanisms in human cancers.

MATERIALS AND METHODS

We summarize and analyze current studies concerning the biological functions and mechanisms of lncRNA PANDA in tumour development. The related studies were obtained through a systematic search of Pubmed.

RESULTS

PANDAR was a well-characterized oncogenic lncRNA and widely overexpressed in many tumours. PANDAR is upregulated in many types of cancer, including colorectal cancer, lung cancer, renal cell carcinoma, cholangiocarcinoma, osteosarcoma, thyroid cancer and other cancers. Upregulation of PANDAR was significantly associated with advanced tumour weights, TNM stage and overall survival. Furthermore, repressed of PANDAR would restrain proliferation, migration and invasion.

CONCLUSION

PANDAR may act as a powerful tumour biomarker for cancer diagnosis and treatment.

Genes coding for cyclin-dependent kinase inhibitors are fragile in Xenopus

Cell proliferation is strictly regulated by the dosage balance among cell-cycle regulators such as CDK/cyclin complexes and CDK-Inhibitors. Even in the allotetraploid genome of Xenopus laevis, the dosage balance must be maintained for animals to stay alive, and the duplicated homeologous genes seem to have gradually changed, through evolution, resulting in the best genes for them to thrive. In the Xenopus laevis genome, while homeologous gene pairs of CDKs are fundamentally maintained and a few cyclin genes are amplified, homeologous gene pairs of the important CDK-Inhibitors, CDKn1c and CDKn2a, are deleted from chromosomes L and S. Although losses of CDKn1c and CDKn2a can lead to diseases in humans, their loss in X. laevis does not affect the animals' health. Also, another gene coding CDKn1b is lost besides CDKn1c and CDKn2a in the genome of Xenopus tropicalis. These findings suggest a high resistance of Xenopus to diseases. We also found that CDKn2c.S expression is higher than that of CDKn2c.L, and a conserved noncoding sequence (CNS) of CDKn2c genomic loci on X. laevis chromosome S and X. tropicalis has an enhancement activity in regulating the different expression. These findings together indicate a surprising fragility of CDK inhibitor gene loci in the Xenopus genome in spite of their importance, and may suggest that factors other than CDK-inhibitors decelerate cell-cycling in Xenopus.

Diagnostic accuracy of p16INK4a immunohistochemistry in oropharyngeal squamous cell carcinomas: A systematic review and meta-analysis

The accurate diagnosis of human papillomavirus (HPV) causality in oropharyngeal squamous cell carcinomas (OPSCC) is likely to influence therapeutic decisions in affected patients in the near future. We conducted a systematic review and meta-analysis to determine the diagnostic accuracy of p16^INK4a immunohistochemistry (IHC) to identify HPV-induced OPSCC. We identified all studies that performed p16^INK4a IHC (index test) and HPV E6/E7 mRNA detection using an amplification-based method (gold standard to indicate a transforming relevance of HPV) in OPSCC. Testing with one or more comparator tests (HPV DNA PCR, HPV DNA in situ hybridization (ISH) and p16^INK4a IHC/HPV DNA PCR combined testing) was an optional criterion for inclusion. Among 1,636 retrieved studies 24 fulfilled the inclusion criteria. The pooled sensitivity of p16^INK4a IHC, HPV DNA PCR, HPV DNA ISH and p16^INK4a IHC/HPV DNA PCR combined testing was 94% (95%-confidence interval (CI) 91-97%), 98% (CI 94-100%), 85% (CI 76-92%) and 93% (CI 87-97%), respectively. The pooled specificity was 83% (CI 78-88%), 84% (CI 74-92%), 88% (CI 78-96%) and 96% (CI 89-100%), respectively. p16^INK4a IHC/HPV DNA PCR combined testing was as sensitive as either p16^INK4a IHC or HPV DNA PCR alone but significantly more specific than either separate test. In conclusion, p16^INK4a IHC is highly sensitive but moderately specific to diagnose HPV-transformed OPSCC when used as a single test. Combined p16^INK4a IHC and HPV DNA PCR testing significantly enhances specificity while maintaining high sensitivity. This diagnostic test combination thus represents an attractive testing strategy for the reliable diagnosis of HPV-induced OPSCC in the clinical setting and may constitute an inclusion criterion for future therapeutic trials.

Targeting CDK4 and CDK6: From Discovery to Therapy

Biochemical and genetic characterization of D-type cyclins, their cyclin D-dependent kinases (CDK4 and CDK6), and the polypeptide CDK4/6 inhibitor p16(INK4)over two decades ago revealed how mammalian cells regulate entry into the DNA synthetic (S) phase of the cell-division cycle in a retinoblastoma protein-dependent manner. These investigations provided proof-of-principle that CDK4/6 inhibitors, particularly when combined with coinhibition of allied mitogen-dependent signal transduction pathways, might prove valuable in cancer therapy. FDA approval of the CDK4/6 inhibitor palbociclib used with the aromatase inhibitor letrozole for breast cancer treatment highlights long-sought success. The newest findings herald clinical trials targeting other cancers.

SIGNIFICANCE

Rapidly emerging data with selective inhibitors of CDK4/6 have validated these cell-cycle kinases as anticancer drug targets, corroborating longstanding preclinical predictions. This review addresses the discovery of these CDKs and their regulators, as well as translation of CDK4/6 biology to positive clinical outcomes and development of rational combinatorial therapies.

Treating cancer with selective CDK4/6 inhibitors

Uncontrolled cellular proliferation, mediated by dysregulation of the cell-cycle machinery and activation of cyclin-dependent kinases (CDKs) to promote cell-cycle progression, lies at the heart of cancer as a pathological process. Clinical implementation of first-generation, nonselective CDK inhibitors, designed to inhibit this proliferation, was originally hampered by the high risk of toxicity and lack of efficacy noted with these agents. The emergence of a new generation of selective CDK4/6 inhibitors, including ribociclib, abemaciclib and palbociclib, has enabled tumour types in which CDK4/6 has a pivotal role in the G1-to-S-phase cell-cycle transition to be targeted with improved effectiveness, and fewer adverse effects. Results of pivotal phase III trials investigating palbociclib in patients with advanced-stage oestrogen receptor (ER)-positive breast cancer have demonstrated a substantial improvement in progression-free survival, with a well-tolerated toxicity profile. Mechanisms of acquired resistance to CDK4/6 inhibitors are beginning to emerge that, although unwelcome, might enable rational post-CDK4/6 inhibitor therapeutic strategies to be identified. Extending the use of CDK4/6 inhibitors beyond ER-positive breast cancer is challenging, and will likely require biomarkers that are predictive of a response, and the use of combination therapies in order to optimize CDK4/6 targeting.

LncRNA PANDAR regulates the G1/S transition of breast cancer cells by suppressing p16(INK4A) expression

It has been reported that lncRNA PANDAR (promoter of CDKN1A antisense DNA damage-activated RNA) is induced as a result of DNA damage, and it regulates the reparation of DNA damage. In this study, we investigated the role of lncRNA PANDAR in the progression of breast cancer and found that PANDAR was up-regulated in breast cancer tissues and cell lines. The knockdown of PANDAR suppresses G1/S transition of breast cancer cells. We demonstrated mechanistically that the regulation of G1/S transition by PANDAR was partly due to the transcriptional modulation of p16^INK4A. Moreover, we showed that PANDAR impacted p16^INK4A expression by regulating the recruitment Bmi1 to p16^INK4A promoter. To our knowledge, this is the first study which showed the functional roles and mechanisms of PANDAR in regulating the progression of breast cancer. The PANDAR/Bmi1/p16^INK4A axis could serve as novel targets for breast cancer therapy.

Evidence for a CDK4-dependent checkpoint in a conditional model of cellular senescence

Cellular senescence, the stable cell cycle arrest elicited by various forms of stress, is an important facet of tumor suppression. Although much is known about the key players in the implementation of senescence, including the pRb and p53 axes and the cyclin dependent kinase inhibitors p16^INK4a and p21^CIP1, many details remain unresolved. In studying conditional senescence in human fibroblasts that express a temperature sensitive SV40 large T-antigen (T-Ag), we uncovered an unexpected role for CDK4. At the permissive temperature, where pRb and p53 are functionally compromised by T-Ag, cyclin D-CDK4 complexes are disrupted by the high p16^INK4a levels and reduced expression of p21^CIP1. In cells arrested at the non-permissive temperature, p21^CIP1 promotes reassembly of cyclin D-CDK4 yet pRb is in a hypo-phosphorylated state, consistent with cell cycle arrest. In exploring whether the reassembled cyclin D-CDK4-p21 complexes are functional, we found that shRNA-mediated knockdown or chemical inhibition of CDK4 prevented the increase in cell size associated with the senescent phenotype by allowing the cells to arrest in G1 rather than G2/M. The data point to a role for CDK4 kinase activity in a G2 checkpoint that contributes to senescence.

Transcriptional regulation of Sox2 by the retinoblastoma family of pocket proteins

Cellular reprogramming to iPSCs has uncovered unsuspected links between tumor suppressors and pluripotency factors. Using this system, it was possible to identify tumor suppressor p27 as a repressor of Sox2 during differentiation. This led to the demonstration that defects in the repression of Sox2 can contribute to tumor development. The members of the retinoblastoma family of pocket proteins, pRb, p107 and p130, are negative regulators of the cell cycle with tumor suppressor activity and with roles in differentiation. In this work we studied the relative contribution of the retinoblastoma family members to the regulation of Sox2 expression. We found that deletion of Rb or p130 leads to impaired repression of Sox2, a deffect amplified by inactivation of p53. We also identified binding of pRb and p130 to an enhancer with crucial regulatory activity on Sox2 expression. Using cellular reprogramming we tested the impact of the defective repression of Sox2 and confirmed that Rb deficiency allows the generation of iPSCs in the absence of exogenous Sox2. Finally, partial depletion of Sox2 positive cells reduced the pituitary tumor development initiated by Rb loss in vivo. In summary, our results show that Sox2 repression by pRb is a relevant mechanism of tumor suppression.

Molecular Alterations of Colorectal Cancer with Inflammatory Bowel Disease

Inflammatory bowel disease (IBD) is an important etiologic factor in the development of colorectal cancer (CRC). The risk of CRC begins to increase 8 or 10 years after the diagnosis of IBD. This type of cancer is called colitis-associated CRC (CA-CRC). The molecular pathogenesis of inflammatory epithelium might play a critical role in the development of CA-CRC. Genetic alterations detected in CA-CRC such as genetic mutations, microsatellite instability, and DNA hypermethylation are also recognized in sporadic CRC; however, there are differences in the timing and frequency of molecular events between CA-CRC and sporadic CRC. Interaction between gene-environmental factors, including inflammation, lifestyle, psychological stress, and prior appendectomy, might be associated with the etiopathology of IBD. The mucosal inflammatory mediators, such as oxidant stress, free radicals, and chemokines, may cause the genetic alterations. Understanding the molecular mechanisms of CA-CRC might be important to develop clinical efficacies for patients with IBD. This review discusses the molecular characteristics of CA-CRC, especially ulcerative colitis-associated CRC, including clinical features, signaling pathways, and interactions between genetic alterations and environment involved in inflammatory carcinogenesis.

Intracellular CD24 disrupts the ARF-NPM interaction and enables mutational and viral oncogene-mediated p53 inactivation

CD24 is overexpressed in nearly 70% human cancers, whereas TP53 is the most frequently mutated tumour-suppressor gene that functions in a context-dependent manner. Here we show that both targeted mutation and short hairpin RNA (shRNA) silencing of CD24 retard the growth, progression and metastasis of prostate cancer. CD24 competitively inhibits ARF binding to NPM, resulting in decreased ARF, increase MDM2 and decrease levels of p53 and the p53 target p21/CDKN1A. CD24 silencing prevents functional inactivation of p53 by both somatic mutation and viral oncogenes, including the SV40 large T antigen and human papilloma virus 16 E6-antigen. In support of the functional interaction between CD24 and p53, in silico analyses reveal that TP53 mutates at a higher rate among glioma and prostate cancer samples with higher CD24 mRNA levels. These data provide a general mechanism for functional inactivation of ARF and reveal an important cellular context for genetic and viral inactivation of TP53.

P53 is a tumour suppressor that is frequently mutated or downregulated in cancer. Here, Wang et al. show that CD24, a molecule frequently overexpressed in cancer, promotes p53 degradation by disrupting a regulatory ARF–MDM2 interaction, and silencing CD24 prevents the downregulation of p53.

HAUSP, a novel deubiquitinase for Rb - MDM2 the critical regulator

Tumor suppressor retinoblastoma-associated protein (Rb) is an important cell cycle regulator, arresting cells in early G1. It is commonly inactivated in cancers and its level is maintained during the cell cycle. Rb is regulated by various post-translational modifications such as phosphorylation, acetylation, ubiquitination and so on. Several E3 ligases including murine double minute 2 (MDM2) promote the degradation of Rb. This study focuses on the role of HAUSP (herpes virus associated ubiquitin specific protease) on Rb. Here, we show that HAUSP colocalizes and interacts with Rb to stabilize it from proteasomal degradation by removing wild-type and K48-linked ubiquitin chains in human embryonic kidney 293 (HEK293) cells. HAUSP deubiquitinates Rb in vivo and in vitro, leading to an increased cell population in the G1 phase. Hence, HAUSP is a novel deubiquitinase for Rb. Immunohistochemistry, western blotting and cell-based assays show that HAUSP is overexpressed in glioma and contributes towards glioma progression. However, HAUSP activity on Rb is abrogated in glioma (cancer), where these two proteins show an inverse relationship. MDM2 (a known substrate of HAUSP) serves as a better target for HAUSP-mediated deubiquitination in cancer cells, facilitating degradation of Rb and oncogenic progression. This novel regulatory axis is proteasome mediated, p53 independent, and the level of MDM2 is critical. The shift in equilibrium by differential deubiquitination in regulation of Rb explains a subtle difference existing between normal and cancer cells. This leads to speculation about a new possibility for distinguishing cancer cells from normal cells at the molecular level, which may be investigated for therapeutic intervention in the future.

p16(INK4a) protects against dysfunctional telomere-induced ATR-dependent DNA damage responses

Dysfunctional telomeres limit cellular proliferative capacity by activating the p53-p21- and p16(INK4a)-Rb-dependent DNA damage responses (DDRs). The p16(INK4a) tumor suppressor accumulates in aging tissues, is a biomarker for cellular senescence, and limits stem cell function in vivo. While the activation of a p53-dependent DDR by dysfunctional telomeres has been well documented in human cells and mouse models, the role for p16(INK4a) in response to telomere dysfunction remains unclear. Here, we generated protection of telomeres 1b p16-/- mice (Pot1bΔ/Δ;p16-/-) to address the function of p16(INK4a) in the setting of telomere dysfunction in vivo. We found that deletion of p16(INK4a) accelerated organ impairment and observed functional defects in highly proliferative organs, including the hematopoietic system, small intestine, and testes. Pot1bΔ/Δ;p16-/- hematopoietic cells exhibited increased telomere loss, increased chromosomal fusions, and telomere replication defects. p16(INK4a) deletion enhanced the activation of the ATR-dependent DDR in Pot1bΔ/Δ hematopoietic cells, leading to p53 stabilization, increased p21-dependent cell cycle arrest, and elevated p53-dependent apoptosis. In contrast to p16(INK4a), deletion of p21 did not activate ATR, rescued proliferative defects in Pot1bΔ/Δ hematopoietic cells, and significantly increased organismal lifespan. Our results provide experimental evidence that p16(INK4a) exerts protective functions in proliferative cells bearing dysfunctional telomeres.

Common drugs and treatments for cancer and age-related diseases: revitalizing answers to NCI's provocative questions

In 2011, The National Cancer Institute (NCI) has announced 24 provocative questions on cancer. Some of these questions have been already answered in “NCI's provocative questions on cancer: some answers to ignite discussion” (published in Oncotarget, 2011, 2: 1352.) The questions included “Why do many cancer cells die when suddenly deprived of a protein encoded by an oncogene?” “Can we extend patient survival by using approaches that keep tumors static?” “Why are some disseminated cancers cured by chemotherapy alone?” “Can we develop methods to rapidly test interventions for cancer treatment or prevention?” “Can we use our knowledge of aging to enhance prevention or treatment of cancer?” “What is the mechanism by which some drugs commonly and chronically used for other indications protect against cancer?” “How does obesity contribute to cancer risk?” I devoted a single subchapter to each the answer. As expected, the provocative questions were very diverse and numerous. Now I choose and combine, as a single problem, only three last questions, all related to common mechanisms and treatment of age-related diseases including obesity and cancer. Can we use common existing drugs for cancer prevention and treatment? Can we use some targeted “cancer-selective” agents for other diseases and … aging itself.

Up-regulation of survivin during immortalization of human myofibroblasts is linked to repression of tumor suppressor p16(INK4a) protein and confers resistance to oxidative stress

Survivin is an essential component of the chromosomal passenger complex and a member of the inhibitor of apoptosis family. It is expressed at high levels in a large variety of malignancies, where it has been implicated in drug resistance. It was also shown previously that survivin is up-regulated during telomerase-mediated immortalization, which occurs at a relatively early stage during carcinogenesis. This study shows that up-regulation of survivin during immortalization of human myofibroblasts is an indirect consequence of the repression of p16(INK4a). Survivin and p16(INK4a) were functionally linked by assays that showed that either the up-regulation of survivin or repression of p16(INK4a) rendered telomerase-transduced MRC-5 myofibroblasts resistant to oxidative stress. Conversely, siRNA-mediated down-regulation of survivin activated caspases and enhanced the sensitivity of immortal MRC-5 cells to oxidative stress. The E2F1 transcription factor, which is negatively regulated by the pRB/p16(INK4a) tumor suppressor pathway, was implicated in the up-regulation of survivin. Using the ChIP assay, it was shown that E2F1 directly interacted with the survivin gene (BIRC5) promoter in cells that spontaneously silenced p16(INK4a) during telomerase-mediated immortalization. E2F1 binding to the BIRC5 was also enhanced in telomerase-transduced cells subjected to shRNA-mediated repression of p16(INK4a). Together, these data show that repression of p16(INK4a) contributes to the up-regulation of survivin and thereby provides a survival advantage to cells exposed to oxidative stress during immortalization. The up-regulation of survivin during immortalization likely contributes to the vulnerability of immortal cells to transformation by oncogenes that alter intracellular redox state.

The switch from pRb/p105 to Rb2/p130 in DNA damage and cellular senescence

Cellular senescence is a response to genotoxic stress that results in an irreversible cell cycle arrest. Activation of this pathway relies on the activity of the retinoblastoma proteins and proteins of the DNA damage response cascade. Here, we discuss the functional relevance of the switch from pRb/p105 to Rb2/p130 that becomes apparent when cells enter senescent arrest.

Molecular damage in cancer: an argument for mTOR-driven aging

Despite common belief, accumulation of molecular damage does not play a key role in aging. Still, cancer (an age-related disease) is initiated by molecular damage. Cancer and aging share a lot in common including the activation of the TOR pathway. But the role of molecular damage distinguishes cancer and aging. Furthermore, an analysis of the role of both damage and aging in cancer argues against "a decline, caused by accumulation of molecular damage" as a cause of aging. I also discuss how random molecular damage, via rounds of multiplication and selection, brings about non-random hallmarks of cancer.

Prognostic significance of p16INK4a/p53 in Tunisian patients with breast carcinoma

Infiltrating ductal carcinoma (IDC) of the breast is a result of genetic alterations that affect the regulation of the cell cycle check-point and apoptosis. The aim of the present study was analysis using immunohistochemical localization of mouse double minute-2 (mdm2), p16INK4a, p53, bax and bcl-2 markers in Tunisian patients with breast IDC and to determine if there was correlation with the major clinico-pathological parameters and with survival of patients. We showed that the expression of p53, p16INK4a, mdm2, bcl-2, and bax was observed in 46.3%, 20.7%, 38%, 50% and 11.9% of cases, respectively. Statistical analysis revealed that positive expression of mdm2 was associated with larger tumors (P=0.013), whereas bax positivity was more prevalent in younger patients and in tumors of smaller size (P=0.008 and P=0.012 respectively). Furthermore, the expression of p16INK4a correlated with advanced grade (P<0.0001), triple negative tumors (ER-/PR-/HER2-, P=0.001) and mdm2 expression (P=0.017). The absence of nuclear p53 accumulation was predictive of good prognosis as well as when it was associated with negative expression of p16INK4a. Our findings suggest that among the biomarkers tested, p16INK4a might have a useful clinical and prognostic significance in infiltrating ductal carcinoma of the breast.

The meaning of p16(ink4a) expression in tumors: functional significance, clinical associations and future developments

The CDKN2A gene is a tumor suppressor that encodes the CDK4/6 inhibitor p16(ink4a). Loss of this tumor suppressor contributes to the bypass of critical senescent signals and is associated with progression to malignant disease. However, the high-level expression of p16(ink4a) in tumors is associated with aggressive subtypes of disease, and in certain clinical settings elevated p16(ink4a) expression is an important determinant for disease prognosis and therapeutic response. These seemingly contradictory facets of p16(ink4a) expression have lead to confusion related to the meaning of this tumor suppression in tumor pathobiology. As reviewed here, the alternative expression of p16(ink4a) represents an ideal marker for considering RB-pathway function, tumor heterogeneity, and novel means for directing therapy.

DOT1L, the H3K79 methyltransferase, is required for MLL-AF9-mediated leukemogenesis

Chromosomal translocations of the mixed lineage leukemia (MLL) gene are a common cause of acute leukemias. The oncogenic function of MLL fusion proteins is, in part, mediated through aberrant activation of Hoxa genes and Meis1, among others. Here we demonstrate using a tamoxifen-inducible Cre-mediated loss of function mouse model that DOT1L, an H3K79 methyltransferase, is required for both initiation and maintenance of MLL-AF9-induced leukemogenesis in vitro and in vivo. Through gene expression and chromatin immunoprecipitation analysis we demonstrate that mistargeting of DOT1L, subsequent H3K79 methylation, and up-regulation of Hoxa and Meis1 genes underlie the molecular mechanism of how DOT1L contributes to MLL-AF9-mediated leukemogenesis. Our study not only provides the first in vivo evidence for the function of DOT1L in leukemia, but also reveals the molecular mechanism for DOT1L in MLL-AF9 mediated leukemia. Thus, DOT1L may serve as a potential therapeutic target for the treatment of leukemia caused by MLL translocations.