Current Drug Targets - Volume 17, Issue 4, 2016

Cellular senescence is the state of permanent proliferation cessation. There are two types of cell senescence. One is replicative senescence, which relies on telomere length-dependent limit of cell divisions. The second is stress-induced premature senescence (SIPS) which is telomere- independent. Cell senescence is a barrier to cancer. Paradoxically senescent cells, which are metabolically active secrete factors which can be procancerogenic. The main culprit of cell senescence is DNA damage and DNA damage response. Although cancer cells frequently possess mutations in two main signalling pathways involved in cell senescence, namely p53/p21 and p16/Rb, they still preserve the ability to undergo DNA damage-induced senescence. Cancer cell senescence is a new promising target for anticancer therapy. It was shown that many types of cancer cells can undergo SIPS. Senescent cancer cells have generally the same features as normal cells, such as enlarged size, accumulation of DNA damage foci and increased activity of Senescence-Associated β- galactosidase. Moreover senescent cancer cells are frequently polyploid and it was shown that polyploidy might be connected with abnormal cell division, which leads to the appearance of small descendants. In this review we will focus on morphological hallmarks of senescent cancer cells as well as their functional capabilities, such as secretion, polyploidization, and stemness. We will also discuss links with autophagy, mitotic catastrophe and the propensity of senescent cells to regain proliferative activities. We would like to show the complexity of cancer cell phenotype arising after anticancer treatment and difficulties in interpretation of the experimental data.

Aging is accompanied by a progressive decline of endothelial function and a progressive drift toward a systemic pro-inflammatory status that has been designated “inflammaging”. Both phenomena are accelerated and exacerbated in patients with the most common age-related diseases (ARDs), including cancer. The finding that chronic cell stress activates a pro-inflammatory program leading to acquisition of the senescence-associated secretory phenotype (SASP) and to the propagation of senescence to surrounding cells through the secretome, suggests that cell senescence may have a role in both processes. Here we: i) describe the role of cell senescence in endothelial dysfunction, ii) emphasize the contribution of the endothelial cell SASP to inflammaging, and iii) suggest that selective removal of senescent endothelial cells may not only hinder such harmful processes, but also reduce the risk of developing ARDs and their complications. Although in vivo detection and targeting of senescent endothelial cells are still being investigated, it is likely that therapeutic strategies based on antioxidant and anti-inflammatory compounds would involve generalized anti-aging effects also benefiting endothelial cells. MicroRNA (miRNAs) - single-stranded, non-coding RNAs expressed by all living cells and involved in the epigenetic modulation of all transcriptional programs - may constitute an innovative, valuable tool to detect and target senescent endothelial cells and to devise treatments that can slow down the pro-inflammatory program activated in senescent endothelial cells.

When DNA damage occurs, cells stop the cell cycle and DNA repair can take place. However, if DNA damage exceeds DNA repair capacities, cells undergo either apoptosis or senescence. These mechanisms preclude the proliferation of cells with heavily damaged DNA, thus protecting the organism against tumour development. When individuals are exposed to stress, the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic- adrenal-medullary (SAM) system can be activated leading to secretion of corticosteroids and catecholamines, respectively. The influences of these stress-related hormones have been proposed to promote cellular senescence. But paradoxically, chronic stimulation of the HPA axis is associated with higher risk of developing cancer. Focusing on the DNA damage response pathway, this review discusses whether stress hormones induce senescence or tumour progression or both and presents historical and recent data that might help resolve some of these controversies.

Cancer and aging are two similar processes representing the final outcome of timedependent accumulation of various irreversible dysfunctions, mainly caused by stress-induced DNA and cellular damages. Apoptosis and senescence are two types of cellular response to damages that are altered in both cancer and aging, albeit through different mechanisms. Carcinogenesis is associated with a progressive reduction in the ability of the cells to trigger apoptosis and senescence. In contrast, in aging tissues, there is an increased accumulation of senescent cells, and the nature of apoptosis deregulation varies depending on the tissue. Thus, the prevailing model suggests that apoptosis and cellular senescence function as two essential tumor-suppressor mechanisms, ensuring the health of the individual during early and reproductive stages of life, but become detrimental and promote aging later in life. The recent discovery that various anticancer agents, including canonical inducers of apoptosis, act also as inducers of cellular senescence indicates that pro-senescence strategies may have applications in cancer prevention therapy. Therefore, dissection of the mechanisms mediating the delicate balance between apoptosis and cellular senescence will be beneficial in the therapeutic exploitation of both processes in the development of future anticancer and anti-aging strategies, including minimizing the side effects of such strategies. Here, we provide an overview of the roles of apoptosis and cellular senescence in cancer and aging.

Endogenous retroelements (ERs) represent nearly half of the human genome. Considered up to recent years as “functionless” DNA sequences, they are now known to be involved in important cellular functions such as stress response and generation of non coding regulatory RNAs. Moreover, an increasing amount of data supports the idea of ERs as key players in cellular senescence and in different senescence-related pathogenic cellular processes, including those leading to inflammation, cancer and major age-related multifactorial diseases. The involvement of ERs in these biological mechanisms can suggest new therapeutic strategies in neoplasms, inflammatory/autoimmune diseases and in different age-related pathologies, such as macular degeneration, diabetes, cardiovascular diseases and major age-related neurodegenerative disorders. The therapeutic approaches which can be suggested range from a set of well-known, common drugs that have been shown to modulate ERs activity, to immune therapy against ER-derived tumor antigens, to more challenging strategies such as those based on anti-ERs RNA interference.

Before the last decade, attempts to identify the genetic factors involved in the susceptibility to age-related complex diseases such as cardiovascular disease, diabetes and cancer had very limited success. Recently, two important advancements have provided new opportunities to improve our knowledge in this field. Firstly, it has emerged the concept of studying the molecular mechanisms underlying the age related decline of the organism (such as cellular senescence), rather than the genetics of single disorders. In addition, advances in DNA technology have uncovered an incredible number of common susceptibility variants for several complex traits. Despite these progresses, the translation of these discoveries into clinical practice has been very difficult. To date, several attempts in translating genomics to medicine are being carried out to look for the best way by which genomic discoveries may improve our understanding of fundamental issues in the prediction and prevention of some complex diseases. The successful strategy seems to be testing simultaneously multiple susceptibility variants in combination with traditional risk factors. In fact, such approach showed that genetic factors substantially improve the prediction of complex diseases especially for coronary heart disease and prostate cancer, making possible appropriate behavioural and medical interventions. In the future, the identification of new genetic variants and their inclusion into current risk profile models will probably improve the discrimination power of these models for other complex diseases such as type 2 diabetes mellitus and breast cancer. On the other hand, for traits with low heritability, this improvement will probably be negligible, and this will urge further researches on the role played by traditional and newly discovered non-genetic risk factors.

In the 70-80th of last century, it has been shown that the antidiabetic biguanide drugs phenformin (PF) and buformin (BF) can exert an inhibitory action on carcinogenesis in animal models and increase from 5 to 10-years survival of cancer patients. Since 2005, after first evidence publication of the capacity of metformin (MF), another biguanide, to prevent development of malignant tumors in individuals with type 2 diabetes (T2D) and suppress tumorigenesis in mice, the burst of studies started in this field focused on breast, pancreas, prostatic, endometrial and some other cancers. Colorectal cancer (CRC) is the most prevalent among digestive system cancers. In this mini-review are presented the available data on the capacity of antidiabetic biguanide drugs to prevent colorectal carcinogenesis and growth in vitro and in vivo in experimental models and clinical observations.

The possibility to target cellular senescence with natural bioactive substances open interesting therapeutic perspective in cancer and aging. Engaging senescence response is suggested as a key component for therapeutic intervention in the eradication of cancer. At the same time, delaying senescence or even promote death of accumulating apoptosis-resistant senescent cells is proposed as a strategy to prevent age related diseases. Although these two desired outcome present an intrinsic dichotomy, there are examples of promising natural compounds that appear to satisfy all the requirements to develop senescence- targeted health promoting nutraceuticals. Tocotrienols (T3s) and quercetin (QUE), albeit belonging to different phytochemical classes, display similar and promising effects “in vitro” when tested in normal and cancer cells. Both compounds have been shown to induce senescence and promote apoptosis in a multitude of cancer lines. Conversely, they display senescence delaying activity in primary cells and rejuvenating effects in senescent cells. More recently, QUE has been shown to display senolytic effects in some primary senescent cells, likely as a consequence of its inhibitory effects on specific anti-apoptotic genes (i.e. PI3K and other kinases). Senolytic activity has not been tested for T3s but part of metabolic and apoptotic pathways affected by these compounds in cancer cells overlap with those of QUE. This suggests that the rejuvenating effects of T3s and QUE on pre-senescent and senescent primary cells might be the net results of a senolytic activity on senescent cells and a selective survival of a sub-population of non-senescent cells in the culture. The meaning of this hypothesis in the context of adjuvant therapy of cancer and preventive anti-aging strategies with QUE or T3s is discussed.

Senescence was originally identified by the finite lifespan of normal cells that is a consequence of telomere shortening with each cycle of DNA replication. Cells undergoing replicative senescence display pronounced morphological and biochemical changes such as flattening and/or enlargement, increases in p21WAF1 and/or p16INK4A, a senescence-associated secretory phenotype, and often senescence-associated heterochromatic foci. Senescence also occurs in tumor cells in response to various forms of chemotherapy or radiation (therapy-induced senescence), which could be the basis for prolonged or (ideally) permanent growth arrest. Alternatively, therapy-induced senescence could represent a means whereby tumor cells evade the potential toxicity of chemotherapy and radiation, allowing for the eventual re-emergence or escape from senescence that could lead to disease recurrence. This review discusses the experimental data in the literature that support the premise that senescence is potentially reversible through the inactivation of p53, p16INK4A and/or Rb, over-expression of Cdc2/cdk1 and survivin, the development of polyploidy, the survival of cancer stem cells and/or restoration of the nuclear landscape. If senescence is truly reversible, then the re-emergence of tumor cells from senescent arrest induced by chemotherapy or radiation could represent a barrier to the development of effective and curative cancer therapies.

Cardiovascular and cerebrovascular diseases (CCVD) are a major and increasing health burden worldwide. Although treatments have been made a certain progress, the development of novel therapies for patients remains a major research object. There are a lot of drugs against CCVD, but most of them lack tissue specificity with a short half-life, which seriously limits their extensive clinical application. Utilization of some pathophysiological changes caused by CCVD, targeted drug distribution in CCVD patients could be achieved. The current review aims to capture various drug delivery systems transporting drug specifically to vascular disease, including passive, active and physicochemical targeted delivery carriers. Their design strategies and mechanisms are described in detail. The present limitations and future perspective are also discussed.

Articular cartilage is a physiologically non-self-renewing avascular tissue with a unique cell type, the chondrocyte, which functions as producing and maintaining the extracellular matrix of cartilage. Cartilage differentiation and maintenance of homeostasis are finely tuned by a complex network of signaling molecules. The network sheds light on these mechanisms that appear to be highly relevant to both the identification of pathogenic key factors, and the development of biological approaches for cartilage regeneration. Wnt/β-catenin signaling has been recognized as a key regulator of development and homeostasis in bone, cartilage, and joint. It plays important roles in many biological processes, including the condensation and differentiation of mesenchymal cells, the maintenance of mature articular cartilage phenotype, the hypertrophic maturation in the process of endochondral ossification, and tissue degeneration and regeneration. With regard to the importance of Wnt signaling pathways in regulating chondrocytes physiological and pathological activities, this article reviews the role of Wnt/β-catenin signaling in chondrogenesis, chondrocytes development, degeneration, and the inhibitors of Wnt/β-catenin signaling.