P16INK4a Positive Cells in Human Skin Are Indicative of Local Elastic Fiber Morphology, Facial Wrinkling, and Perceived Age

Regulation of ENPP5, a senescence-associated secretory phenotype factor, prevents skin aging

Aging negatively affects the appearance and texture of the skin owing to the accumulation of senescent fibroblasts within the dermis. Senescent cells undergo abnormal remodeling of collagen and the extracellular matrix through an inflammatory histolytic senescence-associated secretory phenotype (SASP). Therefore, suppression of SASP in senescent cells is essential for the development of effective skin anti-aging therapies. Ectonucleotide pyrophosphatase/phosphodiesterase family member 5 (ENPP5), an extracellular signaling molecule, has been implicated in vascular aging and apoptosis; however, its role in SASP remains unclear. Therefore, this study aimed to investigate the role of ENPP5 in SASP and skin aging using molecular techniques. We investigated the effects of siRNA-mediated ENPP5 knockdown, human recombinant ENPP5 (rENPP5) treatment, and lentiviral overexpression of ENPP5 on SASP and aging in human skin fibroblasts. Additionally, we investigated the effect of siRNA-mediated ENPP5 knockdown on the skin of C57BL/6 mice. We found that ENPP5 was significantly expressed in replication-aged and otherwise DNA-damaged human skin fibroblasts and that treatment with human rENPP5 and lentiviral overexpression of ENPP5 promoted SASP and senescence. By contrast, siRNA-mediated knockdown of ENPP5 suppressed SASP and the expression of skin aging-related factors. Additionally, ENPP5 knockdown in mouse skin ameliorated the age-related reduction of subcutaneous adipose tissue, the panniculus carnosus muscle layer, and thinning of collagen fibers. Conclusively, these findings suggest that age-related changes may be prevented through the regulation of ENPP5 expression to suppress SASP in aging cells, contributing to the development of anti-aging treatments for the skin.

Double-blind, vehicle-controlled clinical investigation of peptide OS-01 for skin rejuvenation

INTRODUCTION

Senescent cells contribute to age-related tissue deterioration, including the skin, which plays important roles in overall health and social interactions. This study aimed to assess the effects of the senotherapeutic peptide, OS-01 (a.k.a. Pep 14), on skin.

METHODS

A 12-week split-face, double-blinded, vehicle-controlled study involving 22 participants was conducted. The OS-01-containing formulation was applied to one side of the face, while the other side received an identical control formulation lacking the peptide. Skin characteristics were assessed using instrumental measurements, expert clinical grading, and subjective questionnaires.

RESULTS

Results showed that the OS-01 formulation significantly improved one aspect of skin barrier function, as evidenced by reduced trans-epidermal water loss compared to both baseline and vehicle control. Expert grading and Antera 3D image analysis revealed a reduction in wrinkle appearance and indentation in the periorbital area, and improved skin texture and radiance on both sides of the face, with the OS-01-containing formulation demonstrating superior results. Participants also perceived improvements in skin hydration, smoothness, radiance, and overall appearance.

CONCLUSION

The findings suggest that the OS-01 formulation promotes skin health by strengthening the skin barrier, protecting against dehydration, reducing the appearance of wrinkles, and improving skin texture and radiance. These effects are likely attributed to the senotherapeutic properties of OS-01 in reducing cellular senescence and its associated detrimental effects.

Positive and negative feedback regulation of the TGF-β1 explains two equilibrium states in skin aging

During aging, skin homeostasis is essential for maintaining appearance, as well as biological defense of the human body. In this study, we identified thrombospondin-1 (THBS1) and fibromodulin (FMOD) as positive and negative regulators, respectively, of the TGF-β1-SMAD4 axis in human skin aging, based on in vitro and in vivo omics analyses and mathematical modeling. Using transcriptomic and epigenetic analyses of senescent dermal fibroblasts, TGF-β1 was identified as the key upstream regulator. Bifurcation analysis revealed a binary high-/low-TGF-β1 switch, with THBS1 as the main controller. Computational simulation of the TGF-β1 signaling pathway indicated that THBS1 expression was sensitively regulated, whereas FMOD was regulated robustly. Results of sensitivity analysis and validation showed that inhibition of SMAD4 complex formation was a promising method to control THBS1 production and senescence. Therefore, this study demonstrated the potential of combining data-driven target discovery with mathematical approaches to determine the mechanisms underlying skin aging.

A Glance into the Destiny of Transcriptomic Activity, Embodied by the HOX Genes, in Neonatal and Aging Dermal Cells

Skin is an organ having a crucial role in the protection of muscle, bone, and internal organs and undergoing continuous self-renewal and aged. The growing interest in the prevention of skin aging and rejuvenation has sparked a surge of industrial and research studies focusing on the biological and transcriptional changes that occur during skin development and aging. In this study, the aim is to identify transcriptional differences between two main types of human skin cells: the human dermal fibroblasts (HDFs) and the human epidermis keratinocytes (HEKs) isolated from 30 neonatal and 30 adults (old) skin. Through differentially expressed gene (DEG) profiling using DEseq2, 604 up-, and 769 down-regulated genes are identified in the old group. A functional analysis using Metascape Gene Ontology and Reactome pathways revealed systematic transcriptomic shifts in key skin formation and maintenance markers, alongside a distinct difference in HOX gene families crucial for embryonic development and diverse biological processes. Among the 39 human HOX gene family, ten posterior HOX genes (HOXA10, 11, 13, HOXB13, HOXC11, and HOXD9-13) are significantly downregulated, and anterior 25 genes (HOXA2-7, HOXB1-9, HOXC4-6 and 8-9, and HOXD1,3,4 and 8) are upregulated, especially in the old HDFs. The study successfully demonstrates the correlation between HOX genes and the skin aging process, providing strong evidence that HOX genes are proposed as a new marker for skin aging assessment.

Aging in the dermis: Fibroblast senescence and its significance

Skin aging is characterized by changes in its structural, cellular, and molecular components in both the epidermis and dermis. Dermal aging is distinguished by reduced dermal thickness, increased wrinkles, and a sagging appearance. Due to intrinsic or extrinsic factors, accumulation of excessive reactive oxygen species (ROS) triggers a series of aging events, including imbalanced extracellular matrix (ECM) homeostasis, accumulation of senescent fibroblasts, loss of cell identity, and chronic inflammation mediated by senescence-associated secretory phenotype (SASP). These events are regulated by signaling pathways, such as nuclear factor erythroid 2-related factor 2 (Nrf2), mechanistic target of rapamycin (mTOR), transforming growth factor beta (TGF-β), and insulin-like growth factor 1 (IGF-1). Senescent fibroblasts can induce and accelerate age-related dysfunction of other skin cells and may even cause systemic inflammation. In this review, we summarize the role of dermal fibroblasts in cutaneous aging and inflammation. Moreover, the underlying mechanisms by which dermal fibroblasts influence cutaneous aging and inflammation are also discussed.

Hallmarks of Skin Aging: Update

Aging is defined as impaired physiological integrity, decreased function, increased susceptibility to external risk factors and various diseases. Skin, the largest organ in our body, may become more vulnerable to insult as time goes by and behave as aged skin. Here, we systemically reviewed three categories including seven hallmarks of skin aging. These hallmarks including genomic instability and telomere attrition, epigenetic alterations and loss of proteostasis, deregulated nutrient-sensing, mitochondrial damage and dysfunction, cellular senescence, stem cell exhaustion/dysregulation, and altered intercellular communication. These seven hallmarks can generally be divided into three categories including (i) causes of damages as primary hallmarks in skin aging; (ii) responses to damage as antagonistic hallmarks in skin aging; and (iii) culprits of the phenotype as integrative hallmarks in skin aging.

Cellular Senescence in Human Skin Aging: Leveraging Senotherapeutics

BACKGROUND

As the largest organ in the human body, the skin is continuously exposed to intrinsic and extrinsic stimuli that impact its functionality and morphology with aging. Skin aging entails dysregulation of skin cells and loss, fragmentation, or fragility of extracellular matrix fibers that are manifested macroscopically by wrinkling, laxity, and pigmentary abnormalities. Age-related skin changes are the focus of many surgical and nonsurgical treatments aimed at improving overall skin appearance and health.

SUMMARY

As a hallmark of aging, cellular senescence, an essentially irreversible cell cycle arrest with apoptosis resistance and a secretory phenotype, manifests across skin layers by affecting epidermal and dermal cells. Knowledge of skin-specific senescent cells, such as melanocytes (epidermal aging) and fibroblasts (dermal aging), will promote our understanding of age-related skin changes and how to optimize patient outcomes in esthetic procedures.

KEY MESSAGES

This review provides an overview of skin aging in the context of cellular senescence and discusses senolytic intervention strategies to selectively target skin senescent cells that contribute to premature skin aging.

Senescence-associated alterations in the extracellular matrix: deciphering their role in the regulation of cellular function

The extracellular matrix (ECM) is a dynamic structural network that provides a physical scaffolding, as well as biochemical factors that maintain normal tissue homeostasis and thus its disruption is implicated in many pathological conditions. On the other hand, senescent cells express a particular secretory phenotype, affecting the composition and organization of the surrounding ECM and modulating their microenvironment. As accumulation of senescent cells may be linked to the manifestation of several age-related conditions, senescence-associated ECM alterations may serve as targets for novel anti-aging treatment modalities. Here, we will review characteristic changes in the ECM elicited by cellular senescence and we will discuss the complex interplay between ECM and senescent cells, in relation to normal aging and selected age-associated pathologies.

Senotherapeutic peptide treatment reduces biological age and senescence burden in human skin models

Cellular senescence is known to play a role in age-related skin function deterioration which potentially influences longevity. Here, a two-step phenotypic screening was performed to identify senotherapeutic peptides, leading to the identification of Peptide (Pep) 14. Pep 14 effectively decreased human dermal fibroblast senescence burden induced by Hutchinson-Gilford Progeria Syndrome (HGPS), chronological aging, ultraviolet-B radiation (UVB), and etoposide treatment, without inducing significant toxicity. Pep 14 functions via modulation of PP2A, an understudied holoenzyme that promotes genomic stability and is involved in DNA repair and senescence pathways. At the single-cell level, Pep 14 modulates genes that prevent senescence progression by arresting the cell cycle and enhancing DNA repair, which consequently reduce the number of cells progressing to late senescence. When applied on aged ex vivo skin, Pep 14 promoted a healthy skin phenotype with structural and molecular resemblance to young ex vivo skin, decreased the expression of senescence markers, including SASP, and reduced the DNA methylation age. In summary, this work shows the safe reduction of the biological age of ex vivo human skins by a senomorphic peptide.

The expression of fibrosis-related genes is elevated in doxorubicin-induced senescent human dermal fibroblasts, but their secretome does not trigger a paracrine fibrotic response in non-senescent cells

Tissue fibrosis is associated with the aging process of most of our organs, and organ aging correlates with the chronic accumulation of senescent cells. Fibrosis occurs when fibroblasts proliferate and deposit pathological amounts of extracellular matrix (ECM), leading to progressive tissue scarring and organ dysfunction. Fibroblasts play a key role in fibrosis, especially in the skin where fibroblasts are the most abundant cell type in the dermis and are mainly responsible for the synthesis of ECM. This study aims to investigate how senescent fibroblasts and their secretome influence dermal fibrosis. Here we used human dermal fibroblasts (HDFs) treated with doxorubicin (doxo) to induce senescence. The senescent phenotype of these stress-induced premature senescent (SIPS) cells was confirmed with several markers. The expression of pro-fibrotic genes was quantified and finally, the impact of their secretome on the fibrotic response of non-senescent fibroblasts was assessed. Doxorubicin treatment, induced senescence in fibroblasts which has been confirmed with elevated senescence-associated β- galactosidase (SA-β-gal) activity, absence of BrdU incorporation, upregulation of p21, and loss of Lamin b1. Expression levels of the pro-fibrotic genes ACTA2 and FN1 increased in SIPS cells, but in contrast to studies using lung fibroblasts the secretome of these cells failed to induce a paracrine fibrotic response in non-senescent cells. In general, these results suggest that these senescent cells are potentially profibrotic, and their accumulation can trigger fibrosis in organs.

The Extracellular Matrix Vitalizer RATM Increased Skin Elasticity by Modulating Mitochondrial Function in Aged Animal Skin

Oxidative stress-induced cellular senescence and mitochondrial dysfunction result in skin aging by increasing ECM levels-degrading proteins such as MMPs, and decreasing collagen synthesis. MMPs also destroy the basement membrane, which is involved in skin elasticity. The extracellular matrix vitalizer RATM (RA) contains various antioxidants and sodium hyaluronate, which lead to skin rejuvenation. We evaluated whether RA decreases oxidative stress and mitochondrial dysfunction, eventually increasing skin elasticity in aged animals. Oxidative stress was assessed by assaying NADPH oxidase activity, which is involved in ROS generation, and the expression of SOD, which removes ROS. NADPH oxidase activity was increased in aged skin and decreased by RA injection. SOD expression was decreased in aged skin and increased by RA injection. Damage to mitochondrial DNA and mitochondrial fusion markers was increased in aged skin and decreased by RA. The levels of mitochondrial biogenesis markers and fission markers were decreased in aged skin and increased by RA. The levels of NF-κB/AP-1 and MMP1/2/3/9 were increased in aged skin and decreased by RA. The levels of TGF-β, CTGF, and collagen I/III were decreased in aged skin and increased by RA. The expression of laminin and nidogen and basement membrane density were decreased in aged skin and increased by RA. RA increased collagen fiber accumulation and elasticity in aged skin. In conclusion, RA improves skin rejuvenation by decreasing oxidative stress and mitochondrial dysfunction in aged skin.

Younger facial looks are associate with a lower likelihood of several age-related morbidities in the middle-aged to elderly

BACKGROUND

Looking older for one's chronological age is associated with a higher mortality rate. Yet it remains unclear how perceived facial age relates to morbidity and the degree to which facial ageing reflects systemic ageing of the human body.

OBJECTIVES

To investigate the association between ΔPA and age-related morbidities of different organ systems, where ΔPA represents the difference between perceived age (PA) and chronological age.

METHODS

We performed a cross-sectional analysis on data from the Rotterdam Study, a population-based cohort study in the Netherlands. High-resolution facial photographs of 2679 men and women aged 51.5-87.8 years of European descent were used to assess PA. PA was estimated and scored in 5-year categories using these photographs by a panel of men and women who were blinded for chronological age and medical history. A linear mixed model was used to generate the mean PAs. The difference between the mean PA and chronological age was calculated (ΔPA), where a higher (positive) ΔPA means that the person looks younger for their age and a lower (negative) ΔPA that the person looks older. ΔPA was tested as a continuous variable for association with ageing-related morbidities including cardiovascular, pulmonary, ophthalmological, neurocognitive, renal, skeletal and auditory morbidities in separate regression analyses, adjusted for age and sex (model 1) and additionally for body mass index, smoking and sun exposure (model 2).

RESULTS

We observed 5-year higher ΔPA (i.e. looking younger by 5 years for one's age) to be associated with less osteoporosis [odds ratio (OR) 0.76, 95% confidence interval (CI) 0.62-0.93], less chronic obstructive pulmonary disease (OR 0.85, 95% CI 0.77-0.95), less age-related hearing loss (model 2; B = -0.76, 95% CI -1.35 to -0.17) and fewer cataracts (OR 0.84, 95% CI 0.73-0.97), but with better global cognitive functioning (g-factor; model 2; B = 0.07, 95% CI 0.04-0.10).

CONCLUSIONS

PA is associated with multiple morbidities and better cognitive function, suggesting that systemic ageing and cognitive ageing are, to an extent, externally visible in the human face.

Current Understanding of the Role of Senescent Melanocytes in Skin Ageing

Melanocytes reside within the basal epidermis of human skin, and function to protect the skin from ultraviolet light through the production of melanin. Prolonged exposure of the skin to UV light can induce irreparable DNA damage and drive cells into senescence, a sustained cell cycle arrest that prevents the propagation of this damage. Senescent cells can also be detrimental and contribute to skin ageing phenotypes through their senescence-associated secretory phenotype. Senescent cells can act in both an autocrine and paracrine manner to produce widespread tissue inflammation and skin ageing. Recently, melanocytes have been identified as the main senescent cell population within the epidermis and have been linked to a variety of skin ageing phenotypes, such as epidermal thinning and the presence of wrinkles. However, the literature surrounding melanocyte senescence is limited and tends to focus on the role of senescence in the prevention of melanoma. Therefore, this review aims to explore the current understanding of the contribution of senescent melanocytes to human skin ageing.

Skin Aging in Long-Lived Naked Mole-Rats Is Accompanied by Increased Expression of Longevity-Associated and Tumor Suppressor Genes

Naked mole-rats (NMRs) (Heterocephalus glaber) are long-lived mammals that possess a natural resistance to cancer and other age-related pathologies, maintaining a healthy life span >30 years. In this study, using immunohistochemical and RNA-sequencing analyses, we compare skin morphology, cellular composition, and global transcriptome signatures between young and aged (aged 3‒4 vs. 19‒23 years, respectively) NMRs. We show that similar to aging in human skin, aging in NMRs is accompanied by a decrease in epidermal thickness; keratinocyte proliferation; and a decline in the number of Merkel cells, T cells, antigen-presenting cells, and melanocytes. Similar to that in human skin aging, expression levels of dermal collagens are decreased, whereas matrix metalloproteinase 9 and matrix metalloproteinase 11 levels increased in aged versus in young NMR skin. RNA-sequencing analyses reveal that in contrast to human or mouse skin aging, the transcript levels of several longevity-associated (Igfbp3, Igf2bp3, Ing2) and tumor-suppressor (Btg2, Cdkn1a, Cdkn2c, Dnmt3a, Hic1, Socs3, Sfrp1, Sfrp5, Thbs1, Tsc1, Zfp36) genes are increased in aged NMR skin. Overall, these data suggest that specific features in the NMR skin aging transcriptome might contribute to the resistance of NMRs to spontaneous skin carcinogenesis and provide a platform for further investigations of NMRs as a model organism for studying the biology and disease resistance of human skin.

Cellular Senescence in Aging, Tissue Repair, and Regeneration

SUMMARY

Society and our healthcare system are facing unprecedented challenges due to the expansion of the older population. As plastic surgeons, we can improve care of our older patients through understanding the mechanisms of aging that inevitably impact their outcomes and well-being. One of the major hallmarks of aging, cellular senescence, has recently become the focus of vigorous research in academia and industry. Senescent cells, which are metabolically active but in a state of stable cell cycle arrest, are implicated in causing aging and numerous age-related diseases. Further characterization of the biology of senescence revealed that it can be both detrimental and beneficial to organisms depending on tissue context and senescence chronicity. Here, we review the role of cellular senescence in aging, wound healing, tissue regeneration, and other domains relevant to plastic surgery. We also review the current state of research on therapeutics that modulate senescence to improve conditions of aging.

Targeting Cellular Senescence with Senotherapeutics: Development of New Approaches for Skin Care

SUMMARY

Aging of the skin is evidenced by increased wrinkles, age spots, dryness, and thinning with decreased elasticity. Extrinsic and intrinsic factors including UV, pollution, and inflammation lead to an increase in senescent cells (SnCs) in skin with age that contribute to these observed pathological changes. Cellular senescence is induced by multiple types of damage and stress and is characterized by the irreversible exit from the cell cycle with upregulation of cell cycle-dependent kinase inhibitors p16INK4a and p21CIP1. Most SnCs also developed an inflammatory senescence-associated secretory phenotype (SASP) that drives further pathology through paracrine effects on neighboring cells and endocrine effects on cells at a distance. Recently, compounds able to kill senescent cells specifically, termed senolytics, or suppress the SASP, termed senomorphics, have been developed that have the potential to improve skin aging as well as systemic aging in general. Here, we provide a summary of the evidence for a key role in cellular senescence in driving skin aging. In addition, the evidence for the potential application of senotherapeutics for skin treatments is presented. Overall, topical, and possibly oral senotherapeutic treatments have tremendous potential to eventually become a standard of care for skin aging and related skin disorders.

The Use of Cryotherapy in Cosmetology and the Influence of Cryogenic Temperatures on Selected Skin Parameters—A Review of the Literature

Cryotherapy is becoming an increasingly popular method used in medicine, physiotherapy, and cosmetology. It is used in the form of whole-body cryotherapy (WBC) and local cryotherapy. It is a tool for achieving analgesic and anti-inflammatory effects. Since the beginning of its use, its influence on the mental state has also been pointed out. The aim of this study was to analyze the available literature on the effect of cryogenic temperatures on the skin and the mechanisms induced by such a stimulus and its influence on well-being. A literature search of keywords or phrases was performed in PubMed®. Various effects of WBC on skin characteristics (hydration, pH, level of transepidermal water loss), mechanisms of anti-inflammatory effects, and effects on adipocytes were shown. Research on the impact of individual skin characteristics is not consistent. Positive effects on the reduction of inflammation and oxidative stress have been noted. Cryotherapy is also successfully used in dermatology to treat lentil spots, actinic keratosis, and ingrown toenails, remove viral warts, or relieve itching in atopic dermatitis. The results of the review also indicate the effectiveness of WBC as an adjunctive treatment for obesity. The number of papers available on the direct effects of WBC on the skin is still limited, despite the fact that it represents the first contact of the human body with cryogenic temperatures. Available data show that cold as a physical stimulus can be a safe and useful tool in cosmetology.

Ross J, Scott Herron G. Genetic and Epigenetic Influences on Cutaneous Cellular Senescence

Skin is the largest human organ system, and its protective function is critical to survival. The epithelial, dermal, and subcutaneous compartments are heterogeneous mixtures of cell types, yet they all display age-related skin dysfunction through the accumulation of an altered phenotypic cellular state called senescence. Cellular senescence is triggered by complex and dynamic genetic and epigenetic processes. A senescence steady state is achieved in different cell types under various and overlapping conditions of chronological age, toxic injury, oxidative stress, replicative exhaustion, DNA damage, metabolic dysfunction, and chromosomal structural changes. These inputs lead to outputs of cell-cycle withdrawal and the appearance of a senescence-associated secretory phenotype, both of which accumulate as tissue pathology observed clinically in aged skin. This review details the influence of genetic and epigenetic factors that converge on normal cutaneous cellular processes to create the senescent state, thereby dictating the response of the skin to the forces of both intrinsic and extrinsic aging. From this work, it is clear that no single biomarker or process leads to senescence, but that it is a convergence of factors resulting in an overt aging phenotype.

Downregulation of senescence-associated secretory phenotype by knockdown of secreted frizzled-related protein 4 contributes to the prevention of skin aging

There is growing evidence that the appearance and texture of the skin that is altered during the aging process are considerably enhanced by the accumulation of senescent dermal fibroblasts. These senescent cells magnify aging via an inflammatory, histolytic, and senescence-associated secretory phenotype (SASP). Secreted frizzled-related protein 4 (SFRP4) was previously determined to be expressed in dermal fibroblasts of aging skin, and its increased expression has been shown to promote cellular senescence. However, its role in the SASP remains unknown. We found that SFRP4 was significantly expressed in p16ink4a-positive human skin fibroblasts and that treatment with recombinant SFRP4 promoted SASP and senescence, whereas siRNA knockdown of SFRP4 suppressed SASP. Furthermore, we found that knockdown of SFRP4 in mouse skin ameliorates age-related reduction of subcutaneous adipose tissue, panniculus carnosus muscle layer, and thinning and dispersion of collagen fibers. These findings suggest a potential candidate for the development of new skin rejuvenation therapies that suppress SASP.

Effects of Air Pollution on Cellular Senescence and Skin Aging

The human skin is exposed daily to different environmental factors such as air pollutants and ultraviolet (UV) light. Air pollution is considered a harmful environmental risk to human skin and is known to promote aging and inflammation of this tissue, leading to the onset of skin disorders and to the appearance of wrinkles and pigmentation issues. Besides this, components of air pollution can interact synergistically with ultraviolet light and increase the impact of damage to the skin. However, little is known about the modulation of air pollution on cellular senescence in skin cells and how this can contribute to skin aging. In this review, we are summarizing the current state of knowledge about air pollution components, their involvement in the processes of cellular senescence and skin aging, as well as the current therapeutic and cosmetic interventions proposed to prevent or mitigate the effects of air pollution in the skin.

Focus on the Contribution of Oxidative Stress in Skin Aging

Skin aging is one of the most evident signs of human aging. Modification of the skin during the life span is characterized by fine lines and wrinkling, loss of elasticity and volume, laxity, rough-textured appearance, and pallor. In contrast, photoaged skin is associated with uneven pigmentation (age spot) and is markedly wrinkled. At the cellular and molecular level, it consists of multiple interconnected processes based on biochemical reactions, genetic programs, and occurrence of external stimulation. The principal cellular perturbation in the skin driving senescence is the alteration of oxidative balance. In chronological aging, reactive oxygen species (ROS) are produced mainly through cellular oxidative metabolism during adenosine triphosphate (ATP) generation from glucose and mitochondrial dysfunction, whereas in extrinsic aging, loss of redox equilibrium is caused by environmental factors, such as ultraviolet radiation, pollution, cigarette smoking, and inadequate nutrition. During the aging process, oxidative stress is attributed to both augmented ROS production and reduced levels of enzymatic and non-enzymatic protectors. Apart from the evident appearance of structural change, throughout aging, the skin gradually loses its natural functional characteristics and regenerative potential. With aging, the skin immune system also undergoes functional senescence manifested as a reduced ability to counteract infections and augmented frequency of autoimmune and neoplastic diseases. This review proposes an update on the role of oxidative stress in the appearance of the clinical manifestation of skin aging, as well as of the molecular mechanisms that underline this natural phenomenon sometimes accelerated by external factors.

Skin senescence: mechanisms and impact on whole-body aging

The skin is the largest organ and has a key protective role. Similar to any other tissue, the skin is influenced not only by intrinsic/chronological aging, but also by extrinsic aging, triggered by environmental factors that contribute to accelerating the skin aging process. Aged skin shows structural, cellular, and molecular changes and accumulation of senescent cells. These senescent cells can induce or accelerate the age-related dysfunction of other nearby cells from the skin, or from different origins. However, the extent and underlying mechanisms remain unknown. In this opinion, we discuss the possible relevant role of skin senescence in the induction of aging phenotypes to other organs/tissues, contributing to whole-body aging. Moreover, we suggest that topical administration of senolytics/senotherapeutics could counteract the overall whole-body aging phenotype.

Gremlin 2 suppresses differentiation of stem/progenitor cells in the human skin

INTRODUCTION

The skin is comprised of various kinds of cells and has three layers, the epidermis, dermis and subcutaneous adipose tissue. Stem cells in each tissue duplicate themselves and differentiate to supply new cells that function in the tissue, and thereby maintain the tissue homeostasis. In contrast, senescent cells accumulate with age and secrete senescence-associated secretory phenotype (SASP) factors that impair surrounding cells and tissues, which lowers the capacity to maintain homeostasis in each tissue. Previously, we found Gremlin 2 (GREM2) as a novel SASP factor in the skin and reported that GREM2 suppressed the differentiation of adipose-derived stromal/stem cells. In the present study, we investigated the effects of GREM2 on stem cells in the epidermis and dermis.

METHODS

To examine whether GREM2 expression and the differentiation levels in the epidermis and dermis are correlated, the expressions of GREM2, stem cell markers, an epidermal differentiation marker Keratin 10 (KRT10) and a dermal differentiation marker type 3 procollagen were examined in the skin samples (n = 14) randomly chosen from the elderly where GREM2 expression level is high and the individual differences of its expression are prominent. Next, to test whether GREM2 affects the differentiation of skin stem cells, cells from two established lines (an epidermal and a dermal stem/progenitor cell model) were cultured and induced to differentiate, and recombinant GREM2 protein was added.

RESULTS

In the human skin, the expression levels of GREM2 varied among individuals both in the epidermis and dermis. The expression level of GREM2 was not correlated with the number of stem cells, but negatively correlated with those of both an epidermal and a dermal differentiation markers. The expression levels of epidermal differentiation markers were significantly suppressed by the addition of GREM2 in the three-dimensional (3D) epidermis generated with an epidermal stem/progenitor cell model. In addition, by differentiation induction, the expressions of dermal differentiation markers were induced in cells from a dermal stem/progenitor cell model, and the addition of GREM2 significantly suppressed the expressions of the dermal differentiation markers.

CONCLUSIONS

GREM2 expression level did not affect the numbers of stem cells in the epidermis and dermis but affects the differentiation and maturation levels of the tissues, and GREM2 suppressed the differentiation of stem/progenitor cells in vitro. These findings suggest that GREM2 may contribute to the age-related reduction in the capacity to maintain skin homeostasis by suppressing the differentiation of epidermal and dermal stem/progenitor cells.

Skin Aging, Cellular Senescence and Natural Polyphenols

The skin, being the barrier organ of the body, is constitutively exposed to various stimuli impacting its morphology and function. Senescent cells have been found to accumulate with age and may contribute to age-related skin changes and pathologies. Natural polyphenols exert many health benefits, including ameliorative effects on skin aging. By affecting molecular pathways of senescence, polyphenols are able to prevent or delay the senescence formation and, consequently, avoid or ameliorate aging and age-associated pathologies of the skin. This review aims to provide an overview of the current state of knowledge in skin aging and cellular senescence, and to summarize the recent in vitro studies related to the anti-senescent mechanisms of natural polyphenols carried out on keratinocytes, melanocytes and fibroblasts. Aged skin in the context of the COVID-19 pandemic will be also discussed.

How good is the evidence that cellular senescence causes skin ageing?

Skin is the largest organ of the body with important protective functions, which become compromised with time due to both intrinsic and extrinsic ageing processes. Cellular senescence is the primary ageing process at cell level, associated with loss of proliferative capacity, mitochondrial dysfunction and significantly altered patterns of expression and secretion of bioactive molecules. Intervention experiments have proven cell senescence as a relevant cause of ageing in many organs. In case of skin, accumulation of senescence in all major compartments with ageing is well documented and might be responsible for most, if not all, the molecular changes observed during ageing. Incorporation of senescent cells into in-vitro skin models (specifically 3D full thickness models) recapitulates changes typically associated with skin ageing. However, crucial evidence is still missing. A beneficial effect of senescent cell ablation on skin ageing has so far only been shown following rather unspecific interventions or in transgenic mouse models. We conclude that evidence for cellular senescence as a relevant cause of intrinsic skin ageing is highly suggestive but not yet completely conclusive.

Promises and challenges of senolytics in skin regeneration, pathology and ageing

The research of the last two decades has defined a crucial role of cellular senescence in both the physiology and pathology of skin, and senescent cells have been detected in conditions including development, regeneration, aging, and disease. The pathophysiology of cellular senescence in skin is complex as the phenotype of senescence pertains to several different cell types including fibroblasts, keratinocytes and melanocytes, among others. Paradoxically, the transient presence of senescent cells is believed to be beneficial in the context of development and wound healing, while the chronic presence of senescent cells is detrimental in the context of aging, diseases, and chronic wounds, which afflict predominantly the elderly. Identifying strategies to prevent senescence induction or reduce senescent burden in the skin could broadly benefit the aging population. Senolytics, drugs known to specifically eliminate senescent cells while preserving non-senescent cells, are being intensively studied for use in the clinical setting. Here, we review recent research on skin senescence, on the methods for the detection of senescent cells and describe promises and challenges related to the application of senolytic drugs. This article is part of the Special Issue - Senolytics - Edited by Joao Passos and Diana Jurk.

Alterations of the pigmentation system in the aging process

Human skin aging is a natural phenomenon that results from continuous exposure to intrinsic (time, genetic factors, hormones) as well as extrinsic factors (UV exposure, pollution, tobacco). In areas that are frequently exposed to the sun, photoaging blends with the process of intrinsic aging, resulting in an increased senescent cells number and consequently accelerating the aging process. The severity of photodamage depends on constitutional factors, including skin phototype (skin color, tanning capacity), intensity, and duration of sunlight/UV exposure. Aging affects nearly every aspect of cutaneous biology, including pigmentation. Clinically, the phenotype of age pigmented skin has a mottled, uneven color, primarily due to age spots, with or without hypopigmentation. Uneven pigmentation might be attributed to the hyperactivation of melanocytes, altered distribution of pigment, and turnover. In addition to direct damage to pigment-producing cells, photodamage alters the physiological crosstalk between keratinocytes, fibroblasts, endothelial cells, and melanocytes responsible for natural pigmentation homeostasis. Interestingly, age-independent diffuse expression of senescence-associated markers in the dermal and epidermal compartment is also associated with vitiligo, suggesting that premature senescence plays an important role in the pathology.

Senescent cell removal via JAG1-NOTCH1 signalling in the epidermis

Emerging evidence has pointed to the noxious effects of senescent cells in various tissues, and senescent cells in the epidermis are known to accumulate with age. We hypothesized that there is a mechanism by which senescent cells in the epidermis are preferentially removed and that the function of such removal mechanism declines as age increases. In this study, we investigated whether Notch signalling is involved in such senescent cell removal. We found that Notch1 receptor was expressed more highly in p16INK4a-positive senescent cells than in surrounding cells in human epidermis both in young and old subjects. On the other hand, the expression of its ligand JAG1 was decreased in the epidermis of aged subjects. When normal epidermal cells and UVB-irradiated senescent cells were mixed and three-dimensional reconstructed epidermis was developed in vitro, the senescent cells were preferentially removed from the basal layer and located in the upper layer. We also found that the depletion of senescent cells from the basal layer was suppressed by JAG1 knockdown in normal cells or using a Notch signalling inhibitor. From these results, Notch signalling may be involved in senescent cell removal in the epidermis and the age-related decrease of JAG1 expression in the basal layer may lead to accumulation of senescent cells owing to reduced activation of Notch signalling.

Age-associated expression of p21and p53 during human wound healing

In mice, cellular senescence and senescence-associated secretory phenotype (SASP) positively contribute to cutaneous wound healing. In this proof-of-concept study, we investigated the expressions of p16, p21, and other senescence-associated biomarkers during human wound healing in 24 healthy subjects using a double-biopsy experimental design. The first punch biopsy created the wound and established the baseline. The second biopsy, concentric to the first and taken several days after wounding, was used to probe for expression of biomarkers by immunohistochemistry and RNA FISH. To assess the effects of age, we recruited 12 sex-matched younger (30.2 ± 1.3 years) and 12 sex-matched older (75.6 ± 1.8 years) subjects. We found that p21 and p53, but not p16, were induced during healing in younger, but not older subjects. A role for Notch signaling in p21 expression was inferred from the inducible activation of HES1. Further, other SASP biomarkers such as dipeptidyl peptidase-4 (DPP4) were significantly induced upon wounding in both younger and older groups, whereas matrix metallopeptidase 9 (MMP9) was induced only in the younger group. Senescence-associated β-galactosidase (SA-β-gal) was not detectable before or after wounding. This pilot study suggests the possibility that human cutaneous wound healing is characterized by differential expression of p21 and p53 between younger and older subjects.

Cellular Senescence and Inflammaging in the Skin Microenvironment

Cellular senescence and aging result in a reduced ability to manage persistent types of inflammation. Thus, the chronic low-level inflammation associated with aging phenotype is called “inflammaging”. Inflammaging is not only related with age-associated chronic systemic diseases such as cardiovascular disease and diabetes, but also skin aging. As the largest organ of the body, skin is continuously exposed to external stressors such as UV radiation, air particulate matter, and human microbiome. In this review article, we present mechanisms for accumulation of senescence cells in different compartments of the skin based on cell types, and their association with skin resident immune cells to describe changes in cutaneous immunity during the aging process.

Human skin aging is associated with increased expression of the histone variant H2A.J in the epidermis

Cellular senescence is an irreversible growth arrest that occurs as a result of damaging stimuli, including DNA damage and/or telomere shortening. Here, we investigate histone variant H2A.J as a new biomarker to detect senescent cells during human skin aging. Skin biopsies from healthy volunteers of different ages (18–90 years) were analyzed for H2A.J expression and other parameters involved in triggering and/or maintaining cellular senescence. In the epidermis, the proportions of H2A.J-expressing keratinocytes increased from ≈20% in young to ≈60% in aged skin. Inverse correlations between Ki67- and H2A.J staining in germinative layers may reflect that H2A.J-expressing cells having lost their capacity to divide. As cellular senescence is triggered by DNA-damage signals, persistent 53BP1-foci, telomere lengths, and telomere-associated damage foci were analyzed in epidermal keratinocytes. Only slight age-related telomere attrition and few persistent nuclear 53BP1-foci, occasionally colocalizing with telomeres, suggest that unprotected telomeres are not a significant cause of senescence during skin aging. Quantification of integrin-α6+ basal cells suggests that the number and function of stem/progenitor cells decreased during aging and their altered proliferation capacities resulted in diminished tissue renewal with epidermal thinning. Collectively, our findings suggest that H2A.J is a sensitive marker of epidermal aging in human skin.

Connective Tissue and Fibroblast Senescence in Skin Aging

There is increasing evidence that skin aging is significantly enforced by the accumulation of senescent dermal fibroblasts. Various stressors damaging macromolecules inside and outside fibroblasts are responsible. In addition, NK cells fail to adequately remove senescent (SEN) fibroblasts from tissues. SEN fibroblasts by the release of the proinflammatory, tissue degrading senescent-associated secretory phenotype factors and vesicles with distinct cargo impact on their endogenous niche and spread senescence and skin aging. In this review, we will further discuss less noticed facets, including the plasticity of distinct dermal fibroblast phenotypes, the underestimated impact of the extracellular matrix itself, and the depletion of fibroblast subsets on skin homeostasis and aging.

Cellular Senescence and the Senescence-Associated Secretory Phenotype as Drivers of Skin Photoaging

Chronic exposure to UVR is known to disrupt tissue homeostasis, accelerate the onset of age-related phenotypes, and increase the risk for skin cancer-a phenomenon defined as photoaging. In this paper, we review the current knowledge on how UV exposure causes cells to prematurely enter cellular senescence. We describe the mechanisms contributing to the accumulation of senescent cells in the skin and how the persistence of cellular senescence can promote impaired regenerative capacity, chronic inflammation, and tumorigenesis associated with photoaging. We conclude by highlighting the potential of senolytic drugs in delaying the onset and progression of age-associated phenotypes in the skin.

p16ink4a Positivity of Melanocytes in Non-Segmental Vitiligo

Cellular senescence is induced in response to cellular stressors such as increased levels of reactive oxygen species. The chronic accumulation of senescent cells is currently recognized as a contributor to the pathologic processes of diverse degenerative diseases. Vitiligo is characterized by the disappearance of melanocytes driven by cellular stress within melanocytes and autoimmune processes. In this study, we examined p16^INK4A positivity in the lesional and perilesional skin of 54 non-segmental vitiligo patients to explore cellular senescence in vitiligo. There were more p16^INK4A-positive melanocytes in the perilesional vitiligo skin samples than in control samples. It was also found that p16^INK4A immunoreactivity was not restricted to melanocytes but also existed in fibroblasts; the number of p16^INK4A-positive fibroblasts was significantly increased in lesional skin compared to perilesional skin and normal controls. However, in the subgroup analysis of sun-exposed and non-exposed samples, this outcome was only found at sun-exposed sites, suggesting that fibroblast senescence is an epiphenomenon related to the loss of pigment in skin with vitiligo. In summary, exploring p16^INK4A positivity in vitiligo revealed melanocyte senescence in perilesional skin, which may play a role in vitiligo pathogenesis.

A new model to investigate UVB-induced cellular senescence and pigmentation in melanocytes

Ultraviolet (UV) light is known to potentially damage human skin and accelerate the skin aging process. Upon UVB exposure, melanocytes execute skin protection by increasing melanin production. Senescent cells, including senescent melanocytes, are known to accumulate in aged skin and contribute to the age-associated decline of tissue function. However, melanocyte senescence is still insufficiently explored. Here we describe a new model to investigate mechanisms of UVB-induced senescence in melanocytes and its role in photoaging. Exposure to mild and repeated doses of UVB directly influenced melanocyte proliferation, morphology and ploidy. We confirmed UVB-induced senescence with increased senescence-associated β-galactosidase positivity and changed expression of several senescence markers, including p21, p53 and Lamin B1. UVB irradiation impaired proteasome and increased autophagic activity in melanocytes, while expanding intracellular melanin content. In addition, using a co-culture system, we could confirm that senescence-associated secretory phenotype components secreted by senescent fibroblasts modulated melanogenesis. In conclusion, our new model serves as an important tool to explore UVB-induced melanocyte senescence and its involvement in photoaging and skin pigmentation.

Cellular senescence and chronological age in various human tissues: A systematic review and meta-analysis

Senescent cells in tissues and organs are considered to be pivotal to not only the aging process but also the onset of chronic disease. Accumulating evidence from animal experiments indicates that the magnitude of senescence can vary within and between aged tissue samples from the same animal. However, whether this variation in senescence translates across to human tissue samples is unknown. To address this fundamental question, we have conducted a systematic review and meta‐analysis of all available literature investigating the magnitude of senescence and its association with chronological age in human tissue samples. While senescence is higher in aged tissue samples, the magnitude of senescence varies considerably depending upon tissue type, tissue section, and marker used to detect senescence. These findings echo animal experiments demonstrating that senescence levels may vary between organs within the same animal.

Topical rapamycin reduces markers of senescence and aging in human skin: an exploratory, prospective, randomized trial

Aging is a major risk factor for the majority of human diseases, and the development of interventions to reduce the intrinsic rate of aging is expected to reduce the risk for age-related diseases including cardiovascular disease, cancer, and dementia. In the skin, aging manifests itself in photodamage and dermal atrophy, with underlying tissue reduction and impaired barrier function. To determine whether rapamycin, an FDA-approved drug targeting the mechanistic target of rapamycin (mTOR) complex, can reduce senescence and markers of aging in human skin, an exploratory, placebo-controlled, interventional trial was conducted in a clinical dermatology setting. Participants were greater than 40 years of age with evidence of age-related photoaging and dermal volume loss and no major morbidities. Thirty-six participants were enrolled in the study, and nineteen discontinued or were lost to follow-up. A significant (P = 0.008) reduction in p16^INK4A protein levels and an increase in collagen VII protein levels (P = 0.0077) were observed among participants at the end of the study. Clinical improvement in skin appearance was noted in multiple participants, and immunohistochemical analysis revealed improvement in histological appearance of skin tissue. Topical rapamycin reduced the expression of the p16^INK4A protein consistent with a reduction in cellular senescence. This change was accompanied by relative improvement in clinical appearance of the skin and histological markers of aging and by an increase in collagen VII, which is critical to the integrity of the basement membrane. These results indicate that rapamycin treatment is a potential anti-aging therapy with efficacy in humans.Trial registration ClinicalTrials.gov Identifier: NCT03103893.

Senescent human melanocytes drive skin ageing via paracrine telomere dysfunction

Cellular senescence has been shown to contribute to skin ageing. However, the role of melanocytes in the process is understudied. Our data show that melanocytes are the only epidermal cell type to express the senescence marker p16^INK4A during human skin ageing. Aged melanocytes also display additional markers of senescence such as reduced HMGB1 and dysfunctional telomeres, without detectable telomere shortening. Additionally, senescent melanocyte SASP induces telomere dysfunction in paracrine manner and limits proliferation of surrounding cells via activation of CXCR3-dependent mitochondrial ROS. Finally, senescent melanocytes impair basal keratinocyte proliferation and contribute to epidermal atrophy in vitro using 3D human epidermal equivalents. Crucially, clearance of senescent melanocytes using the senolytic drug ABT737 or treatment with mitochondria-targeted antioxidant MitoQ suppressed this effect. In conclusion, our study provides proof-of-concept evidence that senescent melanocytes affect keratinocyte function and act as drivers of human skin ageing.

Are skin senescence and immunosenescence linked within individuals?

With advancing age, many organs exhibit functional deterioration. The age‐associated accumulation of senescent cells is believed to represent one factor contributing to this phenomenon. While senescent cells are found in several different organ systems, it is not known whether they arise independently in each organ system or whether their prevalence within an individual reflects that individual's intrinsic aging process. To address this question, we studied senescence in two different organ systems in humans, namely skin and T cells in 80 middle‐aged and older individuals from the Leiden Longevity Study. Epidermal p16INK4a positivity was associated with neither CD4⁺ nor CD8⁺ T‐cell immunosenescence phenotype composites (i.e., end‐stage differentiated/senescent T cells, including CD45RA⁺CCR7^‐CD28^‐CD27^‐CD57⁺KLRG1⁺ T cells). Dermal p16INK4a positivity was significantly associated with the CD4⁺, but not with the CD8⁺ immunosenescence composite. We therefore conclude that there is limited evidence for a link between skin senescence and immunosenescence within individuals.

Extracellular Vesicles in Human Skin: Cross-Talk from Senescent Fibroblasts to Keratinocytes by miRNAs

Extracellular vesicles (EVs) and their miRNA cargo are intercellular communicators transmitting their pleiotropic messages between different cell types, tissues, and body fluids. Recently, they have been reported to contribute to skin homeostasis and were identified as members of the senescence-associated secretory phenotype of human dermal fibroblasts. However, the role of EV-miRNAs in paracrine signaling during skin aging is yet unclear. Here we provide evidence for the existence of small EVs in the human skin and dermal interstitial fluid using dermal open flow microperfusion and show that EVs and miRNAs are transferred from dermal fibroblasts to epidermal keratinocytes in 2D cell culture and in human skin equivalents. We further show that the transient presence of senescent fibroblast derived small EVs accelerates scratch closure of epidermal keratinocytes, whereas long-term incubation impairs keratinocyte differentiation in vitro. Finally, we identify vesicular miR-23a-3p, highly secreted by senescent fibroblasts, as one contributor of the EV-mediated effect on keratinocytes in in vitro wound healing assays. To summarize, our findings support the current view that EVs and their miRNA cargo are members of the senescence-associated secretory phenotype and, thus, regulators of human skin homeostasis during aging.

Exceptional Human Longevity

Exceptional longevity represents an extreme phenotype. Current centenarians are survivors of a cohort who display delayed onset of age-related diseases and/or resistance to otherwise lethal illnesses occurring earlier in life. Characteristics of aging are heterogeneous, even among long-lived individuals. Associations between specific clinical or genetic biomarkers exist, but there is unlikely to be a single biomarker predictive of long life. Careful observations in the oldest old offer some empirical strategies that favor increased health span and life span, with implications for compression of disability, identification and implementation of lifestyle behaviors that promote independence, identification and measurement of more reliable markers associated with longevity, better guidance for appropriate health screenings, and promotion of anticipatory health discussions in the setting of more accurate prognostication. Comprehensive PubMed literature searches were performed, with an unbiased focus on mechanisms of longevity. Overall, the aggregate literature supports that the basis for exceptional longevity is multifactorial and involves disparate combinations of genes, environment, resiliency, and chance, all of which are influenced by culture and geography.

Selenium preserves keratinocyte stemness and delays senescence by maintaining epidermal adhesion

Skin is constantly exposed to environmental factors such as pollutants, chemicals and ultra violet radiation (UV), which can induce premature skin aging and increase the risk of skin cancer. One strategy to reduce the effect of oxidative stress produced by environmental exposure is the application of antioxidant molecules. Among the endogenous antioxidants, selenoproteins play a key role in antioxidant defense and in maintaining a reduced cellular environment. Selenium, essential for the activity of selenoproteins, is a trace element that is not synthesized by organisms and must be supplied by diet or supplementation. The aim of this study is to evaluate the effect of Selenium supplementation on skin aging, especially on keratinocytes, the main cells of the epidermis. Our results demonstrate for the first time to our knowledge, the major role of Selenium on the replicative life span of keratinocytes and on aging skin. Selenium protects keratinocyte stem cells (KSCs) against senescence via preservation of their stemness phenotype through adhesion to the basement membrane. Additionally, Selenium supplementation maintains the homeostasis of skin during chronological aging in our senescent skin equivalent model. Controlled supplementation with Selenium could be a new strategy to protect skin against aging.

Age-induced and photoinduced changes in gene expression profiles in facial skin of Caucasian females across 6 decades of age

BACKGROUND

Intrinsic and extrinsic factors, including ultraviolet irradiation, lead to visible signs of skin aging.

OBJECTIVE

We evaluated molecular changes occurring in photoexposed and photoprotected skin of white women 20 to 74 years of age, some of whom appeared substantially younger than their chronologic age.

METHODS

Histologic and transcriptomics profiling were conducted on skin biopsy samples of photoexposed (face and dorsal forearm) or photoprotected (buttocks) body sites from 158 women. 23andMe genotyping determined genetic ancestry.

RESULTS

Gene expression and ontologic analysis revealed progressive changes from the 20s to the 70s in pathways related to oxidative stress, energy metabolism, senescence, and epidermal barrier; these changes were accelerated in the 60s and 70s. The gene expression patterns from the subset of women who were younger-appearing were similar to those in women who were actually younger.

LIMITATIONS

Broader application of these findings (eg, across races and Fitzpatrick skin types) will require further studies.

CONCLUSIONS

This study demonstrates a wide range of molecular processes in skin affected by aging, providing relevant targets for improving the condition of aging skin at different life stages and defining a molecular pattern of epidermal gene expression in women who appear younger than their chronologic age.

UVB-Induced Senescence of Human Dermal Fibroblasts Involves Impairment of Proteasome and Enhanced Autophagic Activity

In the current study, we have extended previous findings aiming at a better understanding of molecular mechanisms underlying UVB-induced senescence of diploid human dermal fibroblasts (HDFs), an experimental model to study the process of photoaging in the skin. We provide evidence that the inhibition of proteasomal degradation of damaged proteins and the activation of autophagosome formation are early events in UVB-induced senescence of HDFs, dependent on UVB-induced accumulation of reactive oxygen species. Our data suggest that autophagy is required for the establishment of the senescent phenotype in UVB-treated HDFs and that inhibition of autophagy is sufficient to change the cell fate from senescence to cell death by apoptosis. Studies in reconstructed skin equivalents revealed that UVB irradiation triggers hallmarks of autophagy induction in the dermal layer. These findings have potential implications for fundamental as well as translational research into skin aging, in particular photoaging.

The impact of cellular senescence in skin ageing: A notion of mosaic and therapeutic strategies

Cellular senescence is now recognized as one of the nine hallmarks of ageing. Recent data show the involvement of senescent cells in tissue ageing and some age-related diseases. Skin represents an ideal model for the study of ageing. Indeed, skin ageing varies between individuals depending on their chronological age but also on their exposure to various exogenous factors (mainly ultraviolet rays). If senescence traits can be detected with ageing in the skin, the senescent phenotype varies among the various skin cell types. Moreover, the origin of cellular senescence in the skin is still unknown, and multiple origins are possible. This reflects the mosaic of skin ageing. Senescent cells can interfere with their microenvironment, either via the direct secretion of factors (the senescence-associated secretory phenotype) or via other methods of communication, such as extracellular vesicles. Knowledge regarding the impact of cellular senescence on skin ageing could be integrated into dermatology research, especially to limit the appearance of senescent cells after photo(chemo)therapy or in age-related skin diseases. Therapeutic approaches include the clearance of senescent cells via the use of senolytics or via the cooperation with the immune system.

Assessment of health status by molecular measures in adults ranging from middle-aged to old: Ready for clinical use?

In addition to measures already used in clinical practice, molecular measures have been proposed to assess health status, but these have not yet been introduced into clinical practice. We aimed to test the association of functional capacity measures used in current practice and molecular measures with age and health status. The cohort consisted of 178 middle-aged to old participants of the Leiden Longevity Study (range 42-82years). We tested associations between functional capacity measures (physical tests: grip strength, 4-meter walk, chair stand test; cognitive tests: Stroop test, digit symbol substitution test and 15-picture learning test) with age and with cardiovascular or metabolic disease as a measure of the health status. These associations with age and health status were also tested for molecular measures (C reactive protein (CRP), numbers of senescent p16INK4a positive cells in the epidermis and dermis and putative immunosenescence (presence of CD57+ T cells)). All functional capacity measures were associated with age. CRP and epidermal p16INK4a positivity were also associated with age, but with smaller estimates. Grip strength and the Stroop test were associated with cardiovascular or metabolic disease, as was epidermal p16INK4a positivity. All associations with cardiovascular or metabolic disease attenuated when adjusting for age. In conclusion, in middle-aged to old persons, the molecular measures tested here were more weakly associated with age and health status than functional capacity measures. Whether these molecular measures associate more closely with health status in the elderly or in specific groups of patients needs to be explored further.