Cellular Senescence: What, Why, and How

All age-related chronic diseases may be caused in part by convergence of the basic aging mechanisms that underlie age-related tissue dysfunction, including chronic sterile (not pathogen-associated) inflammation, macromolecular damage, progenitor cell dysfunction, and cellular senescence.¹ In the past decade, cellular senescence has emerged as a possible cause of general tissue dysfunction and aging phenotypes.^2,3 Cellular senescence is an essentially irreversible growth arrest that occurs in response to various cellular stressors, such as telomere erosion, DNA damage, oxidative stress, and oncogenic activation, and it is thought to be an antitumor mechanism.⁴ 

Cellular senescence is a stress response that links the degenerative and hyperplastic pathologies of aging. This degeneration or gradual loss of function occurs at the molecular, cellular, tissue, and organismal levels. Age-related loss of function is a feature of virtually all organisms that age, ranging from single-celled creatures to large, complex animals. In mammals, age-related degeneration gives rise to well-recognized pathologies such as sarcopenia, atherosclerosis, heart failure, osteoporosis, macular degeneration, pulmonary insufficiency, renal failure, neural degeneration and prominent neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases, and many more age-related pathologies. Although species vary in their susceptibilities to specific age-related pathologies, those diseases generally rise with exponential kinetics, beginning at about the midpoint of the species-specific lifespan (eg, aged 50–60 years for humans).^5,6

Cellular senescence refers to the essentially irreversible arrest of cell proliferation (growth) that occurs when cells experience potentially oncogenic stress.⁴ The permanence of the senescence growth arrest enforces the idea that senescence response evolved at least in part to suppress the development of cancer.⁷ A senescence arrest is considered irreversible because no known physiologic stimuli can stimulate senescence cells to re-enter the cell cycle. The senescence arrest is stringent; it is established and maintained by at least 2 major tumor suppressor pathways: the proteins p53/p21 and the p16^Ink4a/retinoblastoma protein (pRb). These pathways are now recognized as a formidable barrier to malignant tumorigenesis.  

In addition to arrested growth, senescence cells show widespread changes in chromatin organization and gene expression. These changes include the secretion of numerous proinflammatory cytokines, chemokines, growth factors, and proteases, which is a featured function of senescence-associated secretory phenotypes (SASPs) that will be discussed in detail throughout this article. The SASP has powerful paracrine activities, the nature of which suggests that senescence response is not solely a mechanism for preventing cancer. Rather, cellular senescence and the SASP likely evolved to suppress the development of cancer and to promote tissue repair or regeneration in the face of injury.

Through the SASP, a low absolute number of senescent cells in a tissue (typically < 20%) may be able to exert systemic effects.⁴ For example, obesity-associated senescent cells may promote chronic, low-grade, sterile inflammation. In this way, senescent cells might be a link between obesity and inflammation that contributes to the development and progression of type II diabetes.^8,9 Although cellular senescence is normally a defense mechanism against tumor development, the presence or persistence of a high number of senescent cells can promote tumor progression because of inflammation, tissue disruption, and growth signals due to the SASP.¹⁰ Senescent cells can also initiate a deleterious positive feedback mechanism by promoting the spread of senescence to nearby cells.^11-13

Senescent cell burden is low in young individuals but increases with aging in several tissues, including adipose, skeletal muscle, kidney, and skin.^14-16 In particular, components of the metabolic syndrome, including abdominal obesity, diabetes, hypertension, and atherosclerosis, are among the many pathologies that are associated with increased senescent cell burden.^17-19 Senescent cell accumulation can occur due to a variety of factors such as various age-related chronic diseases, oxidative stress, hormonal milieu, developmental factors, chronic infection (eg, human immunodeficiency virus [HIV]), certain medications (chemotherapy or certain HIV protease inhibitors), and radiation exposure.^2,3,20,21

There are different types of cellular senescence that have been identified, including oncogene-induced senescence, stress-induced premature senescence as seen in patients with diabetes, and the classical replicative senescence.³ Therefore, senescent cells can contribute to aging and all age-related pathologies by accelerating loss of tissue regeneration through the depletion of stem cells and progenitor cells. Cellular senescence is indicated in every pathological condition associated with aging.²²

Cellular senescence was formally described by Hayflick^23,24  in the 1960s. He showed that after undergoing a certain number of divisions, normal human diploid fibroblasts enter an irreversible nondividing state or replicative senescence. Research has demonstrated that normal human diploid fibroblasts can divide 50 to 60 times, but afterwards they stop dividing irreversibly.^25,26 Thus, the number of divisions cells complete before reaching the end of their replicative lifespan has been termed as the Hayflick Limit.

Senescence has been reported to occur in a number of other cell types such as keratinocytes, melanocytes, endothelial cells, epithelial cells, glial cells, adrenocortical cells, T lymphocytes, and even tissue stem cells.^27-34 

Even though senescence is induced by multiple factors such as repeated cell culture, telomere attrition, irradiation, oncogene activation, and oxidative damage, it can also be caused by the perturbation of mitochondrial homeostasis, which may accelerate age-related phenotypes.^35-37 Because mitochondria can generate reactive oxygen species (ROS), it is proposed that excessive mitochondrial ROS are important to establishing cellular senescence. Perturbations of mitochondrial homeostasis will include excessive ROS production, impaired mitochondrial dynamics, electron transport chain defects, bioenergetic imbalances or increased adenosine monophosphate-activated protein kinase activity, decreased mitochondrial nicotinamide adenine dinucleotide or altered metabolism, and mitochondrial calcium accumulation.³⁸ 

As previously mentioned, there are several causes that can induce cellular senescence, which can also include telomere shortening, genomic damage, strong mitogen-associated signals, epigenomic damage, and activation of tumor suppressors. Replicative senescence is indeed not dependent on chronological time and culture, but rather depends on the number of divisions that cells undergo in culture.^39-42 It is thought that telomere shortening, which occurs at each cell division because of incomplete replication, is the counting mechanism for the induction of replicative senescence.⁴³ Functional telomeres prevent DNA repair machineries from recognizing chromosome ends as DNA double-stranded breaks, which the cells rapidly respond to and attempt to repair. In the case of telomeres, repair followed by cell division will cause rampant genomic instability through cycles of chromosome fusion and breakage, which are major risk factors for developing cancer. Therefore, repeated and extensive divisions in the absence of telomerase eventually cause 1 or more telomeres to become critically short and dysfunctional. Dysfunctional telomeres elicit a DNA damage response (DDR) but suppress attempted DNA repair. In turn, this DDR arrests cell division primarily through activities of the p53 tumor suppressor, thereby preventing genomic instability. Telomeres become critically short after extensive division, and telomere ends are recognized as DNA double-strand breaks. The telomere ends aggravate a DDR in cell divisions and are then arrested by the activated DDR, mainly through p53 tumor suppressor activity.^43-46

The mechanism behind the finite replicative lifespan of normal cells is now understood quite well. Because polymerase that copy DNA templates are unidirectional and require a labile primer, the ends of linear DNA molecules cannot be completely replicated. Thus, telomeres, the DNA protein structures that cap the ends of linear chromosomes, shorten with each cell division. Telomere shortening does not occur in cells that express telomerase, the reverse transcriptase that can replenish the repetitive de novo telomeric DNA.^47,48

The numbers and types of telomerase-expressing cells vary widely among species. In humans, however, such cells are rare.^49-51 Telomerase-positive human cells include most cancer cells, embryonic stem cells, certain adult stem cells, and a few somatic cells (eg, activated T cells).^52,53

Dysfunctional telomeres appear to be irreparable; consequently, cells with such telomeres experience persistent DDR signaling and p53 activation, which enforce the senescence growth arrest.^44,45 The signaling for DDR also establishes and maintains the SASP.  The remaining causes of cellular senescence are beyond the scope of this article. Therefore, when there are pathologies that cause DNA damage enough to stimulate and prevent repair of DNA, this will stimulate cellular senescence significantly.^54,55

An important feature of many senescent cells is the SASP. The SASP is arguably the most striking feature of senescent cells, because it has the potential to explain the role of cellular senescence in organismal aging and age-related pathologies.^56,57 Consistent with its complexity, the SASP biological activities are myriad. The SASP can stimulate cell proliferation, owing to proteins such as growth-related oncogenes and amphiregulin, as well as stimulate new blood vessel formation due to proteins such as vascular endothelial growth factor.²² However, the SASP can also include proteins that have complex effects on cells. For example, the biphasic WNT modulator secreted frizzled-related protein 1 and interleukin (IL)-6 and IL-8, which can stimulate or inhibit WNT signaling cell proliferation, depend on the physiological context.

Chronic WNT signaling can drive both differentiated cells and stem cells into senescence. Also, some SASP factors induce an epithelial-to-mesenchymal transition in susceptible cells. Thus, these aforementioned SASP factors can alter stem cell proliferation and differentiation or modify stem cell niches.^58-68 In addition, many SASP components directly or indirectly promote inflammation, which is of particular importance to the role of cellular senescence in aging and age-related disease. These factors include IL-6 and IL-8, a variety of monocyte chemoattractant proteins and macrophage inflammatory proteins, and proteins that regulate multiple aspects of inflammation such as granulocyte-macrophage colony-stimulating factor. The secretion of these and similar proteins by senescent cells is predicted to cause chronic inflammation, at least locally and possibly systemically.

Chronic inflammation, of course, is a cause of or an important contributor to virtually every major age-related disease, both degenerative and hyperplastic.^{14,56,57,69-75} The SASP is also a plastic phenotype; this means proteins that are included in the SASP vary among cell types and, to some extent, with the stimulus that induced the senescence response. Nevertheless, there is substantial overlap among SASPs; proinflammatory cytokines are the most highly conserved feature, cutting across many different cell types and senescence-inducing stimuli.^63,76-78

Senescent cells have been implicated in many age-associated degenerative phenotypes, both normal and pathological. In most cases, senescent cells have been shown to drive degenerative changes, largely through their secreted proteins from their SASP. Senescent cells can disrupt normal tissue structures, which are essential for normal tissue function. Senescent cells and SASPs can also fuel overt age-related diseases. For example, indirect evidence shows that senescence and associated SASPs of astrocytes can promote the age-related neurodegeneration that gives rise to cognitive impairment as well as to Alzheimer’s and Parkinson’s diseases.^79,80 In addition, the presence of SASP in senescent chondrocytes, which are prominent in age-related osteophytic joints and degenerated intervertebral discs, are thought to play a major role in etiology and promotion of these pathologies.^81,82 

Moreover, senescent epithelial, endothelial, and smooth muscle cells have been implicated in the genesis and promotion of age-related cardiovascular disease.⁸³ The list of age-related pathologies in which senescent cells have been observed and proposed to cause or contribute is long and includes macular degeneration, chronic obstructive pulmonary disease, emphysema, and insulin insensitivity, among others. Therefore, senescent cells are a smoking gun present at the right time and place to drive these age-related pathologies.

The beneficial effects of senescent cells on tissue repair pose a paradox, as wound healing and tissue repair decline with age. Given that senescent cells increase with age and age-related pathology, why does tissue repair not improve with age? 

Physicians should understand the molecular, cellular, and genomic derangements that are going on within the chronic wound bed as they try to stimulate repair and regenerative mechanisms in an area of tissue that is devoid of functional cells, extracellular matrices, and proteins. With the knowledge that cells within a chronic wound are nonfunctional, nonmigratory, and nonproliferative, why do clinicians utilize acellular treatments to reestablish proliferative pathways when these cells are quite dysfunctional as the evidence provided has shown? Therefore, once clinicians understand the cellular senescence pathway underlies most age-related pathologies and not just wound healing, they then can start to search for those treatments that resurrect these deficient cellular mechanisms in order to propagate the restoration of cellular function to contribute to proper healing.

From the Wound Institute of Ocean County, Toms River, NJ (Medical Director); and Ocean County Foot and Ankle Surgical Associates, Toms River, NJ (Partner) 

Address correspondence to:
Matthew J. Regulski, DPM, ABMSP, FASPM, FAPWH(c)
1104 Seashell Avenue 
Manahawkin, NJ 08050
mregulski@comcast.net

Disclosure: The author discloses no financial or other conflicts of interest.