The effects of chronic and acute toxicological exposures on the expression of the tumor suppressor p16INK4a

In mammals, expression of p16INK4a is highly regulated. Excess expression can lead to cellular senescence and aging, while impaired activation is associated with cancer. The precise mechanism of p16INK4a regulation in vivo is poorly understood. In vitro systems have limited utility since proliferation in culture induces p16INK4a . Both extrinsic (chemotherapy and ionizing radiation) and intrinsic (telomere shortening and improper DNA damage repair) stimuli can induce p16INK4a , but the kinetics of and cellular responses to these genomic insults have not been examined in vivo. To address this question, we developed a murine strain with firefly luciferase `knocked-in' to the endogenous p16INK4a locus and under control of the p16INK4a promoter (p16LUC). To determine the expression of p16INK4a after chronic exposure, we exposed p16LUC mice to 50 ppm arsenic (As), 42% fat diet (HFD), 350 J/m2 Ultraviolet B light (UVB), or cigarette smoke (CS) for a minimum of 6 months. Every other month, p16LUC mice were imaged to measure luciferase induction. At 30 weeks of exposure, mice exposed to CS displayed 2 times higher levels of whole body luciferase activity than ambient air (AA) controls. Additionally, mice exposed to UVB exhibited 1.5 times higher levels of luciferase activity by 6 weeks of exposure that reached 8 times over control mice by 24 weeks. In As exposed mice, there was a slight, but statistically significant induction of p16INK4a by 24 weeks. We observed no differences in p16INK4a expression in mice on HFD compared to normal diet (ND, 4%) after 78 weeks. We can deduce the direct DNA damaging agents, CS and UVB, are leading to an induction of p16INK4a while in direct DNA-damaging agents (e.g. As) cause a slight induction of p16INK4a . However, non-DNA damaging agents, such as HFD, show no changes in expression when compared to ND controls. To determine the regulation of transient p16INK4a expression, mice were exposed to a high dose of UVB light (2000 J/m2). The mice were then imaged for luciferase induction at various time points over one month, allowing the burn to heal. To manipulate the induction of p16INK4a the mice were treated with pharmacological inhibitors, including dexamethasone (dex), clodrosome (clod), and compound A (cA). Dex, a known immunosuppression, showed a decrease in p16LUC expression, during the healing process along with clod, known to be toxic to macrophages, and cA, a p65 inhibitor. This data suggests that inflammation at the site of the burn can lead to the induction of p16INK4a through the recruitment of macrophages along the Nfκb pathway. The data generated from these experiments will demonstrate how environmental exposures are associated with expression of p16INK4a , a mediator of tumor suppression and aging.Doctor of Philosoph

Cells exhibiting strong p16INK4a promoter activation in vivo display features of senescence

The activation of cellular senescence throughout the lifespan promotes tumor suppression, whereas the persistence of senescent cells contributes to aspects of aging. This theory has been limited, however, by an inability to identify and isolate individual senescent cells within an intact organism. Toward that end, we generated a murine reporter strain by “knocking-in” a fluorochrome, tandem-dimer Tomato (tdTom), into exon 1α of the p16 INK4a locus. We used this allele (p16 tdTom ) for the enumeration, isolation, and characterization of individual p16 INK4a -expressing cells (tdTom + ). The half-life of the knocked-in transcript was shorter than that of the endogenous p16 INK4a mRNA, and therefore reporter expression better correlated with p16 INK4a promoter activation than p16 INK4a transcript abundance. The frequency of tdTom + cells increased with serial passage in cultured murine embryo fibroblasts from p16 tdTom/+ mice. In adult mice, tdTom + cells could be readily detected at low frequency in many tissues, and the frequency of these cells increased with aging. Using an in vivo model of peritoneal inflammation, we compared the phenotype of cells with or without activation of p16 INK4a and found that tdTom + macrophages exhibited some features of senescence, including reduced proliferation, senescence-associated β-galactosidase (SA-β-gal) activation, and increased mRNA expression of a subset of transcripts encoding factors involved in SA-secretory phenotype (SASP). These results indicate that cells harboring activation of the p16 INK4a promoter accumulate with aging and inflammation in vivo, and display characteristics of senescence

Defining the toxicology of aging

Mammalian aging is complex and incompletely understood. While significant effort has been spent addressing the genetics or, more recently, the pharmacology of aging, the toxicology of aging has been relatively understudied. Just as an understanding of `carcinogens' has proven critical to modern cancer biology, an understanding of environmental toxicants that accelerate aging (`gerontogens') will inform gerontology. In this review, we discuss the evidence for the existence of mammalian gerontogens, as well as describe biomarkers needed to measure the age-promoting activity of a given toxicant. We focus on the effects of putative gerontogens on the in vivo accumulation of senescent cells, a characteristic feature of aging that plays a causal role in some age-associated phenotypes

Preclinical development of G1T38: A novel, potent and selective inhibitor of cyclin dependent kinases 4/6 for use as an oral antineoplastic in patients with CDK4/6 sensitive tumors

Inhibition of the p16INK4a/cyclin D/CDK4/6/RB pathway is an effective therapeutic strategy for the treatment of estrogen receptor positive (ER+) breast cancer. Although efficacious, current treatment regimens require a dosing holiday due to severe neutropenia potentially leading to an increased risk of infections, as well as tumor regrowth and emergence of drug resistance. Therefore, a next generation CDK4/6 inhibitor that can inhibit proliferation of CDK4/6-dependent tumors while minimizing neutropenia could reduce both the need for treatment holidays and the risk of inducing drug resistance

Scar-Free Laparoscopy in BRCA-Mutated Women

Background and Objectives: BRCA 1 and 2 mutations have a cumulative risk of developing ovarian cancer at 70 years of 41% and 15%, respectively, while a cumulative risk of breast cancer by 80 years of age was 72% for BRCA1 mutation carriers and 69% for BRCA2 mutation carriers. The NCCN recommends risk-reducing salpingo-oophorectomy (RRSO), typically between 35 and 40 years, and upon completion of childbearing in BRCA1 mutation, while it is reasonable to delay RRSO for management of ovarian cancer risk until age 40–45 years in patients with BRCA2. In recent years there have been two main lines of evolution in laparoscopy. The former concerning the development of a single-site laparoscopic and the latter concerning the miniaturisation of laparoscopic instruments (mini/micro-laparoscopy). Materials and Methods: In this case report, we show our experience in prophylactic adnexectomy, on a mutated-BRCA patient, using the MiniLap® percutaneous surgical system. Results: This type of technique is safe and effective and does not require a particular learning curve compared to single-port laparoscopy. Conclusions: The considerable aesthetic advantage of the scars, we believe, albeit to a lesser extent, is useful to find in these patients burdened by an important stress loa

p16INK4a reporter mice reveal age-promoting effects of environmental toxicants

While murine-based systems to identify cancer-promoting agents (carcinogens) are established, models to identify compounds that promote aging (gerontogens) have not been described. For this purpose, we exploited the transcription of p16INK4a, which rises dynamically with aging and correlates with age-associated disease. Activation of p16INK4a was visualized in vivo using a murine strain that harbors a knockin of the luciferase gene into the Cdkn2a locus (p16LUC mice). We exposed p16LUC mice to candidate gerontogens, including arsenic, high-fat diet, UV light, and cigarette smoke and serially imaged animals to monitor senescence induction. We show that exposure to a high-fat diet did not accelerate p16INK4a expression, whereas arsenic modestly augmented, and cigarette smoke and UV light potently augmented, activation of p16INK4a-mediated senescence. This work provides a toxicological platform to study mammalian aging and suggests agents that directly damage DNA promote molecular aging

Epigenetic Alterations in Liver of C57BL/6J Mice after Short-Term Inhalational Exposure to 1,3-Butadiene

Background1,3-Butadiene (BD) is a high-volume industrial chemical and a known human carcinogen. The main mode of BD carcinogenicity is thought to involve formation of genotoxic epoxides.ObjectivesIn this study we tested the hypothesis that BD may be epigenotoxic (i.e., cause changes in DNA and histone methylation) and explored the possible molecular mechanisms for the epigenetic changes.Methods and ResultsWe administered BD (6.25 and 625 ppm) to C57BL/6J male mice by inhalation for 2 weeks (6 hr/day, 5 days a week) and then examined liver tissue from these mice for signs of toxicity using histopathology and gene expression analyses. We observed no changes in mice exposed to 6.25 ppm BD, but glycogen depletion and dysregulation of hepatotoxicity biomarker genes were observed in mice exposed to 625 ppm BD. We detected N-7-(2,3,4-trihydroxybut-1-yl)guanine (THB-Gua) adducts in liver DNA of exposed mice in a dose-responsive manner, and also observed extensive alterations in the cellular epigenome in the liver, including demethylation of global DNA and repetitive elements and a decrease in histone H3 and H4 lysine methylation. In addition, we observed down-regulation of DNA methyltransferase 1 (Dnmt1) and suppressor of variegation 3–9 homolog 1, a histone lysine methyltransferase (Suv39h1), and up-regulation of the histone demethylase Jumonji domain 2 (Jmjd2a), proteins responsible for the accurate maintenance of the epigenetic marks. Although the epigenetic effects were most pronounced in the 625-ppm exposure group, some effects were evident in mice exposed to 6.25 ppm BD.ConclusionsThis study demonstrates that exposure to BD leads to epigenetic alterations in the liver, which may be important contributors to the mode of BD carcinogenicity

Transient CDK4/6 inhibition protects hematopoietic stem cells from chemotherapy-induced exhaustion

Conventional cytotoxic chemotherapy is highly effective in certain cancers, but causes dose-limiting damage to normal proliferating cells, especially hematopoietic stem and progenitor cells (HSPCs). Serial exposure to cytotoxics causes a long-term hematopoietic compromise (‘exhaustion’), which limits the use of chemotherapy and success of cancer therapy. Here, we show that the co-administration of G1T28 (trilaciclib), a small-molecule inhibitor of cyclin-dependent kinases 4 and 6 (CDK4/6), contemporaneously with cytotoxic chemotherapy protects murine hematopoietic stem cells (HSCs) from chemotherapy-induced exhaustion in a serial 5-fluorouracil (5FU) treatment model. Consistent with a cell intrinsic effect, we show directly preserved HSC function resulting in a more rapid recovery of peripheral blood counts, enhanced serial transplantation capacity and reduced myeloid skewing. When administered to healthy human volunteers, G1T28 demonstrated excellent in vivo pharmacology and transiently inhibited bone marrow (BM) HSPC proliferation. These findings suggest that the combination of CDK4/6 inhibitors (CDK4/6i) with cytotoxic chemotherapy should provide a means to attenuate therapy-induced BM exhaustion in patients with cancer