Psoralen plus UVA (PUVA) induced premature senescence as a model for stress-induced premature senescence

Inflammation and aging: signaling pathways and intervention therapies

Aging is characterized by systemic chronic inflammation, which is accompanied by cellular senescence, immunosenescence, organ dysfunction, and age-related diseases. Given the multidimensional complexity of aging, there is an urgent need for a systematic organization of inflammaging through dimensionality reduction. Factors secreted by senescent cells, known as the senescence-associated secretory phenotype (SASP), promote chronic inflammation and can induce senescence in normal cells. At the same time, chronic inflammation accelerates the senescence of immune cells, resulting in weakened immune function and an inability to clear senescent cells and inflammatory factors, which creates a vicious cycle of inflammation and senescence. Persistently elevated inflammation levels in organs such as the bone marrow, liver, and lungs cannot be eliminated in time, leading to organ damage and aging-related diseases. Therefore, inflammation has been recognized as an endogenous factor in aging, and the elimination of inflammation could be a potential strategy for anti-aging. Here we discuss inflammaging at the molecular, cellular, organ, and disease levels, and review current aging models, the implications of cutting-edge single cell technologies, as well as anti-aging strategies. Since preventing and alleviating aging-related diseases and improving the overall quality of life are the ultimate goals of aging research, our review highlights the critical features and potential mechanisms of inflammation and aging, along with the latest developments and future directions in aging research, providing a theoretical foundation for novel and practical anti-aging strategies.

Aging of the cells: Insight into cellular senescence and detection Methods

Cellular theory of aging states that human aging is the result of cellular aging, in which an increasing proportion of cells reach senescence. Senescence, from the Latin word senex, means "growing old," is an irreversible growth arrest which occurs in response to damaging stimuli, such as DNA damage, telomere shortening, telomere dysfunction and oncogenic stress leading to suppression of potentially dysfunctional, transformed, or aged cells. Cellular senescence is characterized by irreversible cell cycle arrest, flattened and enlarged morphology, resistance to apoptosis, alteration in gene expression and chromatin structure, expression of senescence associated- β-galactosidase (SA-β-gal) and acquisition of senescence associated secretory phenotype (SASP). In this review paper, different types of cellular senescence including replicative senescence (RS) which occurs due to telomere shortening and stress induced premature senescence (SIPS) which occurs in response to different types of stress in cells, are discussed. Biomarkers of cellular senescence and senescent assays including BrdU incorporation assay, senescence associated- β-galactosidase (SA-β-gal) and senescence-associated heterochromatin foci assays to detect senescent cells are also addressed.

MiR-23a-depressed autophagy is a participant in PUVA- and UVB-induced premature senescence

Autophagy is a cellular catabolic mechanism that is activated in response to stress conditions, including ultraviolet (UV) irradiation, starvation, and misfolded protein accumulation. Abnormalities in autophagy are associated with several pathologies, including aging and cancer. Furthermore, recent studies have demonstrated that microRNAs (miRNAs) are potent modulators of the autophagy pathway. As a result, the current study aims to elucidate the role of the autophagy-related miRNA miR-23ain the process of photoaging. Experiments demonstrated that the antagomir-mediated inactivation of miR-23a resulted in the stimulation of PUVA- and UVB-depressed autophagy flux and protected human fibroblasts from premature senescence. Furthermore, AMBRA1 was identified as a miR-23a target. AMBRA1 cellular levels increased following the introduction of miR-23a antagomirs. And a bioinformatics analysis revealed that the AMBRA1 3' UTR contains functional miR-23a responsive sequences. Finally, it was also demonstrated that both AMBRA1 overexpression and Rapamycin treatment were both able to rescue fibroblasts from PUVA and UVB irradiation-induced autophagy inhibition, but that these effects could also be mitigated by miR-23a overexpression. Therefore, this study concludes that miR-23a-regulated autophagy is a novel and important regulator of ultraviolet-induced premature senescence and AMBRA1 is a rate-limiting miRNA target in this pathway.

While reinforcing cell cycle arrest, rapamycin and Torins suppress senescence in UVA-irradiated fibroblasts

Sunlight predisposes to skin cancer and melanomas. Ultraviolet A (UVA), a long wave component of sunlight, can reach dermal fibroblasts. Here we studied UVA-induced senescence in human fibroblasts in vitro. It is known that senescence occurs, when cell cycle is arrested, but mTOR is still active, thus converting arrest to senescence (geroconversion). We showed that, while arresting cell cycle, UVA did not inhibit mTOR, enabling geroconversion. In UVA-treated cells, mTOR remained fully active. Rapamycin and Torins 1/ 2 prevented UVA-induced senescent phenotype, although they further re-enforced cell cycle arrest. Given that senescent stromal fibroblasts support tumorigenesis, we envision that mTOR inhibitors may potentially be used to prevent sunlight-caused tumors as well as skin photo-aging.

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.

SNEV(Prp19/PSO4) deficiency increases PUVA-induced senescence in mouse skin

Senescent cells accumulate during ageing in various tissues and contribute to organismal ageing. However, factors that are involved in the induction of senescence in vivo are still not well understood. SNEV(P) (rp19/) (PSO) (4) is a multifaceted protein, known to be involved in DNA damage repair and senescence, albeit only in vitro. In this study, we used heterozygous SNEV(+/-) mice (SNEV-knockout results in early embryonic lethality) and wild-type littermate controls as a model to elucidate the role of SNEV(P) (rp19/) (PSO) (4) in DNA damage repair and senescence in vivo. We performed PUVA treatment as model system for potently inducing cellular senescence, consisting of 8-methoxypsoralen in combination with UVA on mouse skin to induce DNA damage and premature skin ageing. We show that SNEV(P) (rp19/) (PSO) (4) expression decreases during organismal ageing, while p16, a marker of ageing in vivo, increases. In response to PUVA treatment, we observed in the skin of both SNEV(P) (rp19/) (PSO) (4) and wild-type mice an increase in γ-H2AX levels, a DNA damage marker. In old SNEV(P) (rp19/) (PSO) (4) mice, this increase is accompanied by reduced epidermis thickening and increase in p16 and collagenase levels. Thus, the DNA damage response occurring in the mouse skin upon PUVA treatment is dependent on SNEV(P) (rp19/) (PSO) (4) expression and lower levels of SNEV(P) (rp19/) (PSO) (4) , as in old SNEV(+/-) mice, result in increase in cellular senescence and acceleration of premature skin ageing.

UV, stress and aging

Skin is a model of choice in studies on aging. Indeed, skin aging can be modulated by internal and external factors, reflecting its complexity. Two types of skin aging have been identified: intrinsic, mainly genetically determined and extrinsic—also called "photo-aging"—resulting on the impact of environmental stress and more precisely of UV rays. Simplified in vitro models, based on cellular senescence, have been developed to study the relationship between UV and aging. These models vary on the cell type (fibroblasts or keratinocytes, normal or immortalized) and the type of UV used (UVA or UVB).

Cell senescence culturing methods

Development of therapeutic approaches that slow or ablate the adverse physiological and pathological changes associated with aging has been considered as an important goal for gerontological research. As cellular senescence is characterized as the basis for aging in organisms, culturing and subculturing of normal human diploid fibroblasts to mimic the in vivo aging processes have been developed as major methods to investigate molecular events involved in aging. It has been established that normal human diploid fibroblasts can proliferate in culture for only finite periods of time. There are many ways to study aging in vitro. In this chapter, we will discuss some of the basic laboratory procedures for cell senescence culturing methods.

Characterization of PLGA Microparticles as a Drug Carrier for 3-Ethoxycarbonyl-2H-Benzofuro[3,2-f]-1-Benzopyran-2-One. Ultrastructural Study of Cellular Uptake and Intracellular Distribution

Here we describe the application of microparticles (MPs) for the delivery and release of the drug a benzopsoralen. We also evaluated the intracellular distribution and cellular uptake of the drug by using an encapsulation technique for therapeutic optimization. MPs containing the compound 3-ethoxycarbonyl-2H-benzofuro[3,2-f]-1-benzopyran-2-one (psoralen A) were prepared by the solvent evaporation technique, and parameters such as particle size, drug encapsulation efficiency, effect of the encapsulation process on the drug's photochemistry, zeta potential, external morphology, and in vitro release behavior were evaluated. The intracellular distribution of MPs as well as their uptake by tissues were monitored. Size distribution studies using dynamic ligh scattering and scanning electron microscopy revealed that the MPs are spherical in shape with a diameter of 1.4 micro m. They present low tendency toward aggregation, as confirmed by their zeta potential (+10.6 mV). The loading efficiency obtained was 75%. As a consequence of the extremely low diffusivity of the drug in aqueous medium, the drug release profile of the MPs in saline phosphate buffer (pH 7.4) was much slower than that obtained in the biological environment. Among the population of peritoneal phagocytic cells, only macrophages were able to phagocytose poly-d,l-lactic-co-glycolic acid (PLGA) MP. The use of psoralen A in association with ultraviolet light (360 nm) revealed morphological characteristics of cell damage such as cytoplasmic vesiculation, mitochondria condensation, and swelling of both the granular endoplasmatic reticulum and the nuclear membrane. These results indicate that PLGA MP could be a promising delivery system for psoralen in connection with ultraviolet irradiation therapy (PUVA).

Contributions of DNA interstrand cross-links to aging of cells and organisms

Impaired DNA damage repair, especially deficient transcription-coupled nucleotide excision repair, leads to segmental progeroid syndromes in human patients as well as in rodent models. Furthermore, DNA double-strand break signalling has been pinpointed as a key inducer of cellular senescence. Several recent findings suggest that another DNA repair pathway, interstrand cross-link (ICL) repair, might also contribute to cell and organism aging. Therefore, we summarize and discuss here that (i) systemic administration of anti-cancer chemotherapeutics, in many cases DNA cross-linking drugs, induces premature progeroid frailty in long-term survivors; (ii) that ICL-inducing 8-methoxy-psoralen/UVA phototherapy leads to signs of premature skin aging as prominent long-term side effect and (iii) that mutated factors involved in ICL repair like ERCC1/XPF, the Fanconi anaemia proteins, WRN and SNEV lead to reduced replicative life span in vitro and segmental progeroid syndromes in vivo. However, since ICL-inducing drugs cause damage different from ICL and since all currently known ICL repair factors work in more than one pathway, further work will be needed to dissect the actual contribution of ICL damage to aging.

The gene expression profile of psoralen plus UVA-induced premature senescence in skin fibroblasts resembles a combined DNA-damage and stress-induced cellular senescence response phenotype

After a finite number of population doublings, normal human cells undergo replicative senescence accompanied by growth arrest. We previously described a model of stress-induced premature senescence by treatment of dermal fibroblasts with psoralen plus UVA, a common photodermatological therapy. Psoralen photoactivation has long been used as a therapy for hyperproliferative skin disorders. The repetitive therapeutical treatment is accompanied by premature aging of the skin. Treatment of fibroblasts in vitro with 8-methoxypsoralen (8-MOP) and subsequent ultraviolet A (UVA) irradiation results in growth arrest with morphological and functional changes reminiscent of replicative senescence. For gene expression profiling in two strains of human skin fibroblasts after PUVA treatment, we used a low-density DNA array representing 240 genes involved in senescence and stress response. Twenty-nine genes were differentially expressed after PUVA treatment in the two strains of human skin fibroblasts. These genes are involved in growth arrest, stress response, modification of the extracellular matrix and senescence. This study contributes further to the elucidation of the PUVA model and its validation as a useful stress-induced premature senescence model aiming to characterize the premature senescence of fibroblasts and to identify biomarkers that could be applied in vivo.

Evaluation of nanoparticles loaded with benzopsoralen in rat peritoneal exudate cells

Psoralens are widely used for the treatment of hyperproliferative skin disease. In this work, we prepared nanoparticles (NP) containing a benzopsoralen (3-ethoxy carbonyl-2H-benzofuro[3,2-f]-1-benzopiran-2-one) by the solvent evaporation technique. We evaluated important NP parameters such as particle size, drug encapsulation efficiency, effect of the encapsulation process over the drug's photochemistry, zeta potential, external morphology, and in vitro release behavior. We also investigated the nanoparticle as a drug delivery system (DDS), as well as its target delivery to the action site, which is a very important parameter to increase the therapeutic use of psoralens and to reduce their side effects. The uptake of benzopsoralen-loaded PLGA nanoparticles by different kinds of cells found in rat peritoneal exudates was also studied. The photodamage promoted by irradiation with UV light revealed morphological characteristics of cell damage such as cytoplasmic vesiculation, mitochondrial damage, and swelling of both the granular endoplasmatic reticulum and nuclear membrane. This encapsulation method maintained the drug's properties and improved drug delivery to the target cell.

Aging cell culture: methods and observations

Culturing and subcultivation of normal human diploid fibroblasts have advanced our understanding of the molecular events involved in aging. This progress is leading to the development of therapies that slow or ablate the adverse physiological and pathological changes associated with aging. It has been established that normal human diploid fibroblasts can proliferate in culture for only finite periods of time. Hayflick and Moorhead and others have described numerous types of cells, ranging from fetal to adult, that were incapable of indefinite proliferation. There are many ways to study aging in vitro, and this chapter summarizes some laboratory procedures.

Proteasome inhibitors shorten replicative life span and induce a senescent-like phenotype of human fibroblasts

The proteasome constitutes the main non-lysosomal cellular protease activity, and plays a crucial role not only in the disposal of unwanted material, but also in the regulation of numerous cellular processes. Previously, we have reported that during the replicative senescence of WI-38 fibroblasts there is a significant impairment in proteasome activity, which probably has important implications in the control of MAPK signaling and cellular proliferation. In this study, we report the potential role of the proteasome in the generation of the senescent phenotype in WI-38 fibroblasts. Our results indicate that inhibition of proteasome activity leads to an impairment in cell proliferation, and a shortening of the life span. The results also indicate that inhibition of the proteasome in young cells induces a premature senescent-like phenotype, as indicated by the increase in senescence-associated beta-galactosidase (SA beta-gal) activity and the abundance of both p21 and collagenase mRNAs, as well as a decreased level of EPC-1 mRNA known markers of cellular senescence, not previously shown to depend on proteasome activity. Together, our results suggest a molecular mechanism for the lack of responsiveness of human cells to growth factors, and point towards a role for the proteasome in the control of the life span of both cells and organisms.

Replicative senescence: a critical review

Human cells in culture have a limited proliferative capacity. After a period of vigorous proliferation, the rate of cell division declines and a number of changes occur in the cells including increases in size, in secondary lysosomes and residual bodies, nuclear changes and a number of changes in gene expression which provide biomarkers for senescence. Although human cells in culture have been used for over 40 years as models for understanding the cellular basis of aging, the relationship of replicative senescence to aging of the organism is still not clear. In this review, we discuss replicative senescence in the light of current information on signal transduction and mitogenesis, cell stress, apoptosis, telomere changes and finally we discuss replicative senescence as a model of aging in vivo.