Psoralen photoactivation promotes morphological and functional changes in fibroblasts in vitro reminiscent of cellular senescence

Fluorofenidone inhibits UV-A induced senescence in human dermal fibroblasts via the mammalian target of rapamycin-dependent SIRT1 pathway

The aim of this study was to investigate the protective effect of fluorofenidone (5‐methyl‐1‐[3‐fluorophenyl]‐2‐[1H]‐pyridone, AKF‐PD) on ultraviolet (UV)‐A‐induced senescence in human dermal fibroblasts (HDF) and examine the mechanisms involved. HDF were treated with AKF‐PD. Senescence‐associated (SA)‐β‐galactosidase level, cell viability and expression of p16 were evaluated. In addition, UV‐A‐irradiated HDF were treated with AKF‐PD, rapamycin and MHY1485; SA‐β‐galactosidase staining, 3‐(4 5‐dimethylthiazol‐2‐yl)‐2 5‐diphenyltetrazolium bromide assay and western blot for SIRT1 were performed; and phosphorylated mammalian target of rapamycin (p‐mTOR) expression and reactive oxygen species (ROS) levels were measured. Intracellular ROS was detected by the 2′,7′‐dichlorofluroescein diacetate probe. Our results showed that AKF‐PD substantially attenuated the changes of p16 expression, SA‐β‐galactosidase staining and cellular proliferation induced by UV‐A irradiation in HDF. AKF‐PD rescued the increased mTOR phosphorylation and reduced SIRT1 expression induced by UV‐A irradiation in HDF. AKF‐PD and rapamycin together had a synergistic effect on p‐mTOR reduction and SIRT1 increase. mTOR activator MHY1485 partly blocked the above effects. Moreover, intracellular ROS level induced by UV‐A irradiation could partly decrease by AKF‐PD, and MHY1485 could reduce this effect. Our results indicated that AKF‐PD could alleviate HDF senescence induced by UV‐A‐irradiation by inhibiting the p‐mTOR and increasing SIRT1. Moreover, AKF‐PD may be a potential treatment material for skin.

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.

Disseminated Scleroderma of a Japanese Patient Successfully Treated with Bath PUVA Photochemotherapy

Background:

Localized scleroderma is known to be resistant to therapies. Recently, it has been reported that bath PUVA photochemotherapy is effective for treating this dermatosis.

Objectives:

Although according in earlier reports mainly white populations have been treated successfully with bath PUVA therapy, there is little knowledge of whether it is effective in treating colored populations. We treated a 64-year-old Japanese woman suffering from disseminated scleroderma with bath PUVA photochemotherapy to see its effects.

Conclusion:

Although rather high cumulative UVA doses were required for this patient compared with those needed in earlier reports, no adverse effects were observed. The lesions were markedly improved, suggesting that this therapeutic modality is well-tolerated and useful for colored patients such as the Japanese. Furthermore, it turns out that the thermographical assessment is useful to estimate clinical improvement of this sclerosing skin disorder.

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.

Modulation of PPARγ provides new insights in a stress induced premature senescence model

Peroxisome proliferator-activated receptor gamma (PPARγ) may be involved in a key mechanism of the skin aging process, influencing several aspects related to the age-related degeneration of skin cells, including antioxidant unbalance. Therefore, we investigated whether the up-modulation of this nuclear receptor exerts a protective effect in a stress-induced premature senescence (SIPS) model based on a single exposure of human dermal fibroblasts to 8-methoxypsoralen plus + ultraviolet-A-irradiation (PUVA). Among possible PPARγ modulators, we selected 2,4,6-octatrienoic acid (Octa), a member of the parrodiene family, previously reported to promote melanogenesis and antioxidant defense in normal human melanocytes through a mechanism involving PPARγ activation. Exposure to PUVA induced an early and significant decrease in PPARγ expression and activity. PPARγ up-modulation counteracted the antioxidant imbalance induced by PUVA and reduced the expression of stress response genes with a synergistic increase of different components of the cell antioxidant network, such as catalase and reduced glutathione. PUVA-treated fibroblasts grown in the presence of Octa are partially but significantly rescued from the features of the cellular senescence-like phenotype, such as cytoplasmic enlargement, the expression of senescence-associated-β-galactosidase, matrix-metalloproteinase-1, and cell cycle proteins. Moreover, the alterations in the cell membrane lipids, such as the decrease in the polyunsaturated fatty acid content of phospholipids and the increase in cholesterol levels, which are typical features of cell aging, were prevented. Our data suggest that PPARγ is one of the targets of PUVA-SIPS and that its pharmacological up-modulation may represent a novel therapeutic approach for the photooxidative skin damage.

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).

UVA irradiation following treatment with topical 8-methoxypsoralen improves bleomycin-induced scleroderma in a mouse model, by reducing the collagen content and collagen gene expression levels in the skin

BACKGROUND

Recent studies have demonstrated that systemic or topical PUVA therapy, i.e., ultraviolet A (UVA) irradiation following treatment with 8-methoxypsoralen (8-MOP), is effective against the sclerotic skin lesions in systemic sclerosis. However, the mechanisms still remain unknown.

OBJECTIVE

To clarify the mechanisms of this therapy, we created a mouse model of bleomycin (BLM) injection-induced scleroderma and evaluated the effects of PUVA on the fibrotic lesions of scleroderma in this mouse model.

METHODS

BLM was injected subcutaneously once a day into the mice for 24 days. During the injection period, one group of mice was irradiated with UVA following local application of 8-MOP. Control groups were also set up, which were injected with phosphate-buffered saline, instead of BLM. Skin tissue samples examined histopathologically changes, measured of the content of hydroxyproline, and checked for the expression of genes encoding type I collagen, type III collagen, and transforming growth factor-β1 (TGF-β1).

RESULTS

The mouse models of scleroderma was found to show an increase in the density of the collagen fibers and thickening of the dermis and increased expressions of type I collagen, type III collagen, and TGF-β1. However, the combination of BLM treatment and topical PUVA treatment mice appeared reduced the dermal thickness and hydroxyproline content, down-regulation of expressions of the type I and type III collagen genes was observed while the expression of the TGF-β1 gene remained unchanged.

CONCLUSION

These results suggest that the effectiveness of topical PUVA therapy is attributable to the down-regulation of the expressions of the collagen genes by this treatment. The results additionally suggest that is not mediated by down-regulated expression of the TGF-β1.

Role of fibroblast-derived growth factors in regulating hyperpigmentation of solar lentigo

BACKGROUND

Cutaneous pigmentation is regulated by a complex melanogenic network in which both keratinocytes and fibroblasts synthesize growth factors and cytokines. Solar lentigo (SL) is characterized by hyperpigmented lesions occurring on photodamaged skin areas. Despite the association of SL to ultraviolet (UV) exposure, the mechanisms underlying the development of these spots are not completely defined.

OBJECTIVES

To analyse the involvement of the fibroblast-derived growth factors, hepatocyte growth factor (HGF), keratinocyte growth factor (KGF) and stem cell factor (SCF) in SL hyperpigmentation; to evaluate whether the photoageing process occurring in fibroblasts could be responsible for the altered expression of these cytokines; and to investigate a new possible role of KGF in regulating pigmentation through the specific induction of melanogenic cytokines by keratinocytes.

METHODS

We performed immunohistochemical analysis of HGF, KGF and SCF on SL biopsies. We analysed the mRNA expression of these cytokines using an in vitro model of photoageing induced on fibroblasts. Finally, we evaluated the effects of KGF on the expression of melanogenic cytokines at the mRNA and protein levels on keratinocytes.

RESULTS

We found positive staining for HGF, KGF and SCF in the upper dermis of SL lesions and a significant induction of the three cytokines in photoaged fibroblasts. We also demonstrated the contribution of KGF to pigmentation, showing its ability specifically to modulate the expression of SCF in keratinocytes.

CONCLUSIONS

Fibroblasts may be persistently activated by UV exposure to release melanogenic growth factors; this inducible cytokine network acts both directly and indirectly through keratinocytes and may contribute to the hyperpigmentation of SL.

Proteomic identification of cathepsin B and nucleophosmin as novel UVA-targets in human skin fibroblasts

Solar UVA exposure plays a causative role in skin photoaging and photocarcinogenesis. Here, we describe the proteomic identification of novel UVA-targets in human dermal fibroblasts following a two-dimensional-difference-gel-electrophoresis (2D-DIGE) approach. Fibroblasts were exposed to noncytotoxic doses of UVA or left untreated, and total protein extracts underwent CyDye-labeling followed by 2D-DIGE/mass-spectrometric identification of differentially expressed proteins, confirmed independently by immunodetection. The protein displaying the most pronounced UVA-induced upregulation was identified as the nucleolar protein nucleophosmin. The protein undergoing the most pronounced UVA-induced downregulation was identified as cathepsin B, a lysosomal cysteine-protease displaying loss of enzymatic activity and altered maturation after cellular UVA exposure. Extensive lysosomal accumulation of lipofuscin-like autofluorescence and osmiophilic material occurred in UVA-exposed fibroblasts as detected by confocal fluorescence microscopy and transmission electron microscopy, respectively. Array analysis indicated UVA-induced upregulation of oxidative stress response gene expression, and UVA-induced loss of cathepsin B enzymatic activity in fibroblasts was suppressed by antioxidant intervention. Pharmacological cathepsin B inhibition using CA074Me mimicked UVA-induced accumulation of lysosomal autofluorescence and deficient cathepsin B maturation. Taken together, these data support the hypothesis that cathepsin B is a crucial target of UVA-induced photo-oxidative stress causatively involved in dermal photodamage through the impairment of lysosomal removal of lipofuscin.

Kynurenine-induced photo oxidative damage to lens in vitro: protective effect of caffeine

Photochemical generation of reactive species of oxygen in the lens and aqueous and consequent physiological damage to the tissue has been implicated in the genesis of human cataracts. The present studies were undertaken to examine the feasibility of possible prevention of such damage to the lens initiated by UV activation of kynurenine, a well-known photosensitizer in the human lens. The studies were done by organ culturing intact mouse lenses in medium containing kynurenine and exposed to UVA. Tissue damage was assessed by the inhibition of its ability to carry active transport of rubidium ions and the associated decrements in the levels of GSH and ATP. These deleterious effects were significantly prevented by caffeine, an alkaloid present in many common beverages and known to chemically deactivate the said oxygen derivatives. Further studies on the pharmacological significance of the findings are hence in progress.

Donor age-dependent acceleration of cellular aging by repeated ultraviolet A irradiation of human dermal fibroblasts derived from a single donor

The relationship between cellular aging and aging of entire organisms has been studied extensively.The findings are confusing, however, and no clear relationships have been demonstrated.The conflicting data may be due to individual differences among the donors of the studied cells.It is crucial to identify the changes in cellular properties that are the result of the aging process.Here, we used human dermal fibroblast cell lines established from a single donor at different ages to assess the influence of ultraviolet A (UVA) on cellular aging. These cell lines have the same genetic background and were obtained from a restricted body region. The results indicated that cellular aging was accelerated by UVA irradiation in a donor age-dependent manner. The ratio of lifespan shortening increased with donor age. Increased donor age not only decreased cell division, but also increased the growth arrest response to UVA irradiation. The characteristics of the cultured cells reflected the age-related changes in dermal fibroblasts.

Increased levels of a particular phosphatidylcholine species in senescent human dermal fibroblasts in vitro

Plasma membranes are essential components of living cells, and phospholipids are major components of cellular membranes. Here, we used liquid chromatography/mass spectrometry to investigate changes in the membrane phospholipid content that occur in association with aging. Our results indicate that the levels of a particular species of phosphatidylcholine comprised of stearic acid and arachidonic acid increased with age. To determine the reason for the increased levels of this particular phosphatidylcholine, we examined the effect of highly unsaturated fatty acids, such as arachidonic acid and eicosapentaenoic acid, on cellular aging. Applied arachidonic acid was incorporated into phosphatidylcholine molecules, but neither arachidonic acid nor other related unsaturated fatty acids had any effect. We conclude that increased levels of this distinctive phosphatidylcholine are a result of in vitro senescence.

Small molecular antioxidants effectively protect from PUVA-induced oxidative stress responses underlying fibroblast senescence and photoaging

Exposure of human fibroblasts to 8-methoxypsoralen plus ultraviolet-A irradiation (PUVA) results in stress-induced cellular senescence in fibroblasts. We here studied the role of the antioxidant defense system in the accumulation of reactive oxygen species (ROS) and the effect of the antioxidants alpha-tocopherol, N-acetylcysteine, and alpha-lipoic acid on PUVA-induced cellular senescence. PUVA treatment induced an immediate and increasing generation of intracellular ROS. Supplementation of PUVA-treated fibroblasts with alpha-tocopherol (alpha-Toc), N-acetylcysteine (NAC), or alpha-lipoic acid (alpha-LA) abrogated the increased ROS generation and rescued fibroblasts from the ROS-dependent changes into the cellular senescence phenotype, such as cytoplasmic enlargement, enhanced expression of senescence-associated-beta-galactosidase and matrix-metalloproteinase-1, hallmarks of photoaging and intrinsic aging. PUVA treatment disrupted the integrity of cellular membranes and impaired homeostasis and function of the cellular antioxidant system with a significant decrease in glutathione and hydrogen peroxide-detoxifying enzymes activities. Supplementation with NAC, alpha-LA, and alpha-Toc counteracted these changes. Our data provide causal evidence that (i) oxidative stress due to an imbalance in the overall cellular antioxidant capacity contributes to the induction and maintenance of the PUVA-induced fibroblast senescence and that (ii) low molecular antioxidants protect effectively against these deleterious alterations.

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.

Lessons learned from DNA repair defective syndromes

Genomic instability is the driving force behind cancer development. Human syndromes with DNA repair deficiencies comprise unique opportunities to study the clinical consequences of faulty genome maintenance leading to premature aging and premature cancer development. These syndromes include chromosomal breakage syndromes with defects in DNA damage signal transduction and double-strand break repair, mismatch repair defective syndromes as well as nucleotide excision repair defective syndromes. The same genes that are severely affected in these model diseases may harbour more subtle variations in the 'healthy' normal population leading to genomic instability, cancer development, and accelerated aging at later stages of life. Thus, studying those syndromes and the molecular mechanisms behind can significantly contribute to our understanding of (skin) cancerogenesis as well as to the development of novel individualized preventive and therapeutic anticancer strategies. The establishment of centers of excellence for studying rare genetic model diseases may be helpful in this direction.

Senescence of human fibroblasts after psoralen photoactivation is mediated by ATR kinase and persistent DNA damage foci at telomeres

Cellular senescence is a phenotype that is likely linked with aging. Recent concepts view different forms of senescence as permanently maintained DNA damage responses partially characterized by the presence of senescence-associated DNA damage foci at dysfunctional telomeres. Irradiation of primary human dermal fibroblasts with the photosensitizer 8-methoxypsoralen and ultraviolet A radiation (PUVA) induces senescence. In the present study, we demonstrate that senescence after PUVA depends on DNA interstrand cross-link (ICL) formation that activates ATR kinase. ATR is necessary for the manifestation and maintenance of the senescent phenotype, because depletion of ATR expression before PUVA prevents induction of senescence, and reduction of ATR expression in PUVA-senesced fibroblasts releases cells from growth arrest. We find an ATR-dependent phosphorylation of the histone H2AX (gamma-H2AX). After PUVA, ATR and gamma-H2AX colocalize in multiple nuclear foci. After several days, only few predominantly telomere-localized foci persist and telomeric DNA can be coimmunoprecipitated with ATR from PUVA-senesced fibroblasts. We thus identify ATR as a novel mediator of telomere-dependent senescence in response to ICL induced by photoactivated psoralens.

Impaired apoptosis and immune senescence - cause or effect?

Aged animals and humans exhibit a decreased T-cell activation response although they also exhibit increased susceptibility to responses to self-antigens and a loss of self-tolerance. The age-related alteration in T-cell reactivity, polyclonal expansion of T cells, and enhanced production of autoantibodies may reflect the numerous age-associated alterations in the T-cell arm of the immune system that have been revealed in numerous studies. These studies suggest that subpopulations of T cells are not deleted appropriately in older animals. They further suggest that an age-related impairment of Fas/Fas ligand (FasL)-mediated apoptosis - which plays a major role in activation-induced cell death (AICD) of T cells - may contribute to compromised regulation of the immune system. The likely mechanisms that may lead to impaired induction of FasL in AICD senescent T cells include an age-related shift from the apoptosis-sensitive T-helper 1 cell (Th1) response to the AICD-resistant Th2 response, aberrant T-cell receptor/CD3 downstream-signaling pathways, and altered CD28/B7-mediated T-cell costimulatory signals. Pathologically, accumulation of AICD-senescent T cells is associated with a defective cytotoxic T lymphocyte response and generation of autoreactive T cells. Based on the accumulating evidence, we propose that the emergence of the FasL(lo) AICD-senescent T cells is not only an effect of immune aging but also an important cause of T-cell proliferative senescence in both humans and mice.

Overexpression of Phospholipid-hydroperoxide Glutathione Peroxidase in Human Dermal Fibroblasts Abrogates UVA Irradiation-induced Expression of Interstitial Collagenase/Matrix Metalloproteinase-1 by Suppression of Phosphatidylcholine Hydroperoxide-mediated NFκB Activation and Interleukin-6 Release

Phospholipid-hydroperoxide glutathione peroxidase (PHGPx) exhibits high specific activity in reducing phosphatidylcholine hydroperoxides (PCOOHs) and thus may play a central role in protecting the skin against UV irradiation-triggered detrimental long term effects like cancer formation and premature skin aging. Here we addressed the role of PHGPx in the protection against UV irradiation-induced expression of matrix metalloproteinase-1 (MMP-1). For this purpose, we created human dermal fibroblast cell lines overexpressing human PHGPx. Overexpression led to a significant increase in PHGPx activity. In contrast to a maximal 4.5-fold induction of specific MMP-1 mRNA levels in vector-transfected cells at 24 h after UVA irradiation, no MMP-1 induction occurred at any studied time point after UVA treatment of PHGPx-overexpressing fibroblasts. As interleukin-6 (IL-6) was earlier shown to mediate the UVA induction of MMP-1, we studied whether PHGPx overexpression might interfere with the NFkappaB-mediated IL-6 induction and downstream signaling. Using transient transfections of IL-6 promoter constructs containing NFkappaB binding sites, we observed a high induction of the reporter gene luciferase in vector-transfected control cells and a significantly lower induction in PHGPx-overexpressing fibroblasts following UVA irradiation. Consistently both UVA irradiation and treatment of fibroblasts with PCOOHs led to phosphorylation and nuclear translocation of the p65 subunit, whereas cells overexpressing PHGPx exhibited impaired NFkappaB activation, p65 phosphorylation, and nuclear translocation. In line with this, the PHGPx-overexpressing fibroblasts showed a reduced constitutive and UVA irradiation-induced IL-6 release. After incubating PHGPx-overexpressing cells with PCOOHs a reduced induction of IL-6 was observed. This together with the suppression of UVA irradiation-induced IL-6 release in the presence of Trolox, a chain breaker of PCOOH-initiated lipid peroxidation, indicates that UVA irradiation-induced PCOOHs and subsequent lipid peroxides initiate the NFkappaB-mediated induction of IL-6, which mediates the induction of MMP-1. Our finding is particularly relevant in light of the already available small molecule mimetics of PHGPx.

Photoaging as a consequence of natural and therapeutic ultraviolet irradiation—studies on PUVA-induced senescence-like growth arrest of human dermal fibroblasts

Premature aging of the skin is a prominent side effect of psoralen photoactivation, a therapy widely and successfully used for different skin disorders. Recently, we demonstrated that treatment of fibroblasts with 8-methoxypsoralen and ultraviolet A irradiation resulted in growth arrest with morphological and functional changes reminiscent of replicative senescence. In this minireview we will focus on the similarities between intrinsic and extrinsic aging and PUVA-induced senescence-like growth arrest both resulting in the loss of the structural integrity of the dermal connective tissue as a hallmark of intrinsic aging and photoaging (extrinsic aging) of the skin, and we will discuss the important role of oxidative stress related telomere attrition in the PUVA-induced phenotype of dermal fibroblasts. With the PUVA-induced growth arrest of fibroblasts a new model has been added to the growing number of in vitro models with longterm growth arrest upon exposure to sublethal stressors (i.e. hyperoxia, hydrogen peroxide, ethanol), which are characterized by morphological and functional changes common for cellular senescence. This model may be particularly suited for further studies addressing mechanisms of stress-induced senescence-like growth arrest in vitro and in vivo, since many dermatological patients are treated with PUVA allowing the analysis of putative stress-induced premature senescence in vivo.

Long-term growth arrest of PUVA-treated fibroblasts in G2/M in the absence of p16INK4a, p21CIP1or p53

Premature aging of the skin is a prominent side-effect of psoralen photoactivation, a therapy used for different skin disorders. Recently, we demonstrated that treatment of fibroblasts with 8-methoxypsoralen and ultraviolet A irradiation resulted in growth arrest with morphological and functional changes reminiscent of replicative senescence. To further elucidate the underlying molecular mechanisms, we analysed the cell-cycle phases of the growth-arrested fibroblasts. After PUVA treatment, fibroblasts arrested in G2/M, in contrast to spontaneously senesced fibroblasts arresting in a cell-cycle phase with many features similar to G1. To address the role of the cell-cycle controlling genes p16(INK4a), p21(CIP1) and p53, we analysed the expression of these genes. p16(INK4a), p21(CIP1) and p53 protein levels increased substantially with different time kinetics in growth-arrested fibroblasts. Because p16(INK4a), p21(CIP1) and p53 are involved in replicative senescence, we applied the PUVA regimen to fibroblasts deficient in either of these genes. p16(INK4a), p21(CIP1) and p53 null mutant fibroblast strains underwent growth arrest with a senescent phenotype similar to wild-type human fibroblasts. Based on these results, we propose that redundant or alternate pathways are involved in the response of dermal fibroblasts to PUVA treatment resulting in a phenocopy of replicative senescence in vitro.

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

Following psoralen photoactivation (PUVA treatment) human dermal fibroblasts undergo long-term growth arrest as well as morphological and functional changes reminiscent of replicative senescence. Although the molecular description of cellular senescence is still incomplete, replicative senescence of cultured human cells has been suggested to reflect cellular aging in vitro. Recently, the term stress-induced premature senescence (SIPS) was introduced to define in vitro models with longterm growth arrest upon exposure to sublethal stressors (i.e. hyperoxia, hydrogen peroxide, ethanol), which are characterized by morphological and functional changes common for replicative senescence. This mini review focuses on the morphological and functional changes in the fibroblast phenotype following exposure to psoralen plus UVA (PUVA) leading to SIPS and the role of reactive oxygen species in the switch from the proliferative to the post mitotic cell. Additionally, we will discuss the possible in vivo relevance of PUVA-SIPS fibroblasts in PUVA-treated patients.

Free radicals in the physiological control of cell function

At high concentrations, free radicals and radical-derived, nonradical reactive species are hazardous for living organisms and damage all major cellular constituents. At moderate concentrations, however, nitric oxide (NO), superoxide anion, and related reactive oxygen species (ROS) play an important role as regulatory mediators in signaling processes. Many of the ROS-mediated responses actually protect the cells against oxidative stress and reestablish "redox homeostasis." Higher organisms, however, have evolved the use of NO and ROS also as signaling molecules for other physiological functions. These include regulation of vascular tone, monitoring of oxygen tension in the control of ventilation and erythropoietin production, and signal transduction from membrane receptors in various physiological processes. NO and ROS are typically generated in these cases by tightly regulated enzymes such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. In a given signaling protein, oxidative attack induces either a loss of function, a gain of function, or a switch to a different function. Excessive amounts of ROS may arise either from excessive stimulation of NAD(P)H oxidases or from less well-regulated sources such as the mitochondrial electron-transport chain. In mitochondria, ROS are generated as undesirable side products of the oxidative energy metabolism. An excessive and/or sustained increase in ROS production has been implicated in the pathogenesis of cancer, diabetes mellitus, atherosclerosis, neurodegenerative diseases, rheumatoid arthritis, ischemia/reperfusion injury, obstructive sleep apnea, and other diseases. In addition, free radicals have been implicated in the mechanism of senescence. That the process of aging may result, at least in part, from radical-mediated oxidative damage was proposed more than 40 years ago by Harman (J Gerontol 11: 298-300, 1956). There is growing evidence that aging involves, in addition, progressive changes in free radical-mediated regulatory processes that result in altered gene expression.

PUVA downregulates whn expression in primary mouse keratinocytes

Psoralen photochemotherapy (PUVA) is one of the most efficient treatment regimens for psoriasis and other skin diseases. In order to evaluate keratinocyte-specific PUVA effects, we investigated the impact of clinically relevant PUVA doses on whn, the 'nude' gene. This transcription factor plays an important role in epidermal homeostasis, and epidermal whn over-expression results in a psoriasis-like phenotype. We demonstrated a persistent down-regulation of whn mRNA 48-72 h after PUVA treatment but not after UVA alone. Using transgenic animals, we also demonstrated dose-dependent down-regulation of whn promoter activity. Finally, whn-null ('nude') keratinocytes were more resistant to PUVA-induced suppression of DNA synthesis than wild-type cells. Our results suggest that whn suppression may be involved in mediating the anti-proliferative effect of PUVA on keratinocytes.

Solar UV irradiation and dermal photoaging

The skin is increasingly exposed to ambient UV-irradiation thus increasing risks for photooxidative damage with long-term detrimental effects like photoaging, characterized by wrinkles, loss of skin tone and resilience. Photoaged skin displays alterations in the cellular component and extracellular matrix with accumulation of disorganized elastin and its microfibrillar component fibrillin in the deep dermis and a severe loss of interstitial collagens, the major structural proteins of the dermal connective tissue. The unifying pathogenic agents for these changes are UV-generated reactive oxygen species (ROS) which deplete and damage non-enzymatic and enzymatic antioxidant defense systems of the skin. As well as causing permanent genetic changes, ROS activate cytoplasmic signal transduction pathways in resident fibroblasts that are related to growth, differentiation, senescence and connective tissue degradation. This review focuses on the role of UV-induced ROS in the photodamage of the skin resulting in clinical and biochemical characteristics of photoaging. In addition, the relationship of photoaging to intrinsic aging of the skin will be briefly discussed. A decrease in the overall ROS load by efficient sunscreens or other protective agents may represent promising strategies to prevent or at least minimize ROS-induced photoaging.

Chronological ageing and photoageing of the fibroblasts and the dermal connective tissue

In recent years, the exposure of human skin to environmental and artificial UV irradiation has increased dramatically. This is due not only to increased solar UV irradiation as a consequence of stratospheric ozone depletion, but also to inappropriate social behaviour with the use of tanning salons still being very popular in the public view. Besides this, leisure activities and a lifestyle that often includes travel to equatorial regions add to the individual annual UV load. In addition to the common long-term detrimental effects such as immunosuppression and skin cancer, the photo-oxidative damage due to energy absorption of UV photons in an oxygenized environment leads to quantitative and qualitative alterations of cells and structural macromolecules of the dermal connective tissue responsible for tensile strength, resilience and stability of the skin. The clinical manifestations of UV/reactive oxygen species (ROS)-induced disturbances result in photoaged skin with wrinkle formation, laxity, leathery appearance as well as fragility, impaired wound healing capacities and higher vulnerability. Strategies to prevent or at least minimize ROS-induced photo-ageing and intrinsic ageing of the skin necessarily include protection against UV irradiation and antioxidant homeostasis.

Different pattern of collagen cross-links in two sclerotic skin diseases: lipodermatosclerosis and circumscribed scleroderma

Changes in the process of cross-linking of collagen molecules are associated with defects in the biomechanical stability of the extracellular matrix. Fibrosis of skin is characterized by an increase in pyridinolines, which are hydroxylysine aldehyde derived cross-links usually absent in healthy skin. In this study, we analyzed cross-links in lipodermatosclerosis and localized scleroderma to address the question whether all the mature cross-links currently characterized are increased in fibrosis in addition to the increase in pyridinolines. As psoralen plus ultraviolet A treatment leads to clinical improvement of fibrotic plaques in localized scleroderma we analyzed the cross-link content in lesional skin after bath psoralen plus ultraviolet A therapy. In skin from patients with localized scleroderma an increase in the total number of mature cross-links was found to be due to an increase in both pyridinolines and dehydro-histidinohydroxymerodesmosine. The concentration of histidinohydroxylysinonorleucine was unchanged. By contrast, the total number of mature cross-links was decreased in lipodermatosclerosis. This decrease was caused by a decrease of lysine aldehyde derived cross-links (dehydro-histidinohydroxymerodesmosine and histidinohydroxylysinonorleucine), whereas the concentration of pyridinolines increased. A decrease in the content of pyridinolines after bath psoralen plus ultraviolet A treatment was found in six out of nine patients with localized scleroderma, which might reflect a remodeling of the extracellular matrix. Our data provide evidence that sclerosis of skin is associated with either an increase in the number of cross-links per molecule of collagen or a change in the molecular nature of the cross-links formed.

Disseminated scleroderma of a Japanese patient successfully treated with bath PUVA photochemotherapy

BACKGROUND

Localized scleroderma is known to be resistant to therapies. Recently, it has been reported that bath PUVA photochemotherapy is effective for treating this dermatosis.

OBJECTIVES

Although according in earlier reports mainly white populations have been treated successfully with bath PUVA therapy, there is little knowledge of whether it is effective in treating colored populations. We treated a 64-year-old Japanese woman suffering from disseminated scleroderma with bath PUVA photochemotherapy to see its effects.

CONCLUSION

Although rather high cumulative UVA doses were required for this patient compared with those needed in earlier reports, no adverse effects were observed. The lesions were markedly improved, suggesting that this therapeutic modality is well-tolerated and useful for colored patients such as the Japanese. Furthermore, it turns out that the thermographical assessment is useful to estimate clinical improvement of this sclerosing skin disorder.

Isolation and identification of psoralen plus ultraviolet A (PUVA)-induced genes in human dermal fibroblasts by polymerase chain reaction-based subtractive hybridization

Premature aging of the skin is a prominent side-effect of psoralen photoactivation, a therapy used for a variety of skin disorders. Recently, we demonstrated that treatment of human dermal fibroblasts with 8-methoxypsoralen and ultraviolet A irradiation resulted in a permanent growth arrest with a switch of mitotic to postmitotic fibroblasts. Furthermore, an upregulation of matrix-degrading metalloproteinases and a high level of de novo expression of the senescence-associated beta-galactosidase was detected in the PUVA-treated postmitotic fibroblasts. The molecular basis for this PUVA-induced change in the functional and morphologic phenotype of fibroblasts resembling or mimicking replicative senescence is, however, unknown. Herein after, we have used a polymerase chain reaction-based subtractive hybridization protocol to identify human genes that are induced by PUVA treatment. Application of polymerase chain reaction-Select resulted in the cloning of four PUVA genes. Sequence analysis and homology searches identified three cDNA clones of known genes related to cell cycle regulation (p21waf1/cip1), stress response (ferritin H) and connective tissue metabolism (tissue inhibitor of metalloproteinases-3), whereas one cDNA clone represented a novel gene (no. 478). Northern blot analyses were performed to confirm a PUVA-dependent increase in specific mRNA levels in human dermal fibroblasts in vitro. This report on the identification of growth arrest related genes in PUVA-treated fibroblasts may stimulate further research addressing the causal role of these known and novel genes in extrinsic and intrinsic aging processes on a molecular and cellular level.

Atmospheric oxygen accelerates the induction of a post-mitotic phenotype in human dermal fibroblasts: the key protective role of glutathione

It has been proposed that ageing of human dermal fibroblasts occurs as a multi-stage process during which cells progress from a mitotic to a post-mitotic state. We describe the development of a simple and novel cell-cloning model for identifying and quantifying the different fibroblast morphotypes associated with the induction of post mitotic behaviour. We have found that under atmospheric (20%) oxygen tension a significant proportion of human dermal fibroblasts are rapidly induced to switch from a mitotic to a post-mitotic phenotype. In contrast, under more physiological (4%) oxygen conditions, the induction of a post-mitotic phenotype is largely prevented. Increasing oxidative stress by addition of hydrogen peroxide or depletion of glutathione also induced a switch from a mitotic to a post-mitotic phenotype in these cells, whereas addition of the anti-oxidant N-acetylcysteine under atmospheric (20%) oxygen tension potently inhibited this process. In addition, a statistically significant correlation was observed between the magnitude of intracellular glutathione depletion and the reduction in the population of mitotic cells in this model. We propose that the switch from a mitotic to a post-mitotic phenotype represents a process of cellular ageing and that standard atmospheric oxygen tension imposes a substantial oxidative stress on dermal fibroblasts which accelerates this process in culture. The data also suggest that intracellular glutathione levels strongly influence the induction of a post-mitotic phenotype and that, by implication, depletion of glutathione may play a significant role in the progression of cellular ageing in human skin.

Research on ageing in Germany

The present review on ageing research in Germany is biased towards experimental biogerontology, because this is the field which will define the future of ageing research as a whole. In absolute numbers of publications between 1995 and 1999, Germany is comparable to other large European countries. However, Germany ranks definitively last among 10 major developed countries if the numbers of scientific papers per year are seen in relation to the economic capability. This is true for the whole of biomedical research, but it is even more exaggerated for ageing research. There are potent groups in German ageing research capable of producing a good fraction of high-impact papers, however. There are many more researchers in areas highly relevant to gerontology which recently became attracted by gerontological problems. However, the importance of modern biogerontology has not made clear to decision-makers in Germany, so that structural and financial limitations will probably prevent any significant rise in the near future, which would be necessary to keep Germany along with other developed countries.

Photoaging of the skin from phenotype to mechanisms

The skin is increasingly exposed to ambient UV-irradiation thus increasing its risk for photooxidative damage with longterm detrimental effects like photoaging, which is characterized by wrinkles, loss of skin tone, and resilience. Photoaged skin displays prominent alterations in the cellular component and the extracellular matrix of the connective tissue with an accumulation of disorganized elastin and its microfibrillar component fibrillin in the deep dermis and a severe loss of interstitial collagens, the major structural proteins of the dermal connective tissue. The unifying pathogenic agents for these changes are UV-generated reactive oxygen species (ROS) that deplete and damage non-enzymatic and enzymatic antioxidant defense systems of the skin. As well as causing permanent genetic changes, ROS activate cytoplasmic signal transduction pathways in resident fibroblasts that are related to growth, differentiation, senescence, and connective tissue degradation. This review focuses on the role of UV-induced ROS in the photodamage of the skin resulting in biochemical and clinical characteristics of photoaging. In addition, the relationship of photoaging to intrinsic aging of the skin will be discussed. A decrease in the overall ROS load by efficient sunscreens or other protective agents may represent promising strategies to prevent or at least minimize ROS induced photoaging.

Ultraviolet-B induction of interstitial collagenase and stromelyin-1 occurs in human dermal fibroblasts via an autocrine interleukin-6-dependent loop

Ultraviolet-B irradiation of human dermal fibroblasts has earlier been shown to induce matrix-degrading metalloproteinases, thus driving connective tissue degradation in photoaging and photocarcinogenesis. Herein, we report that Ultraviolet-B irradiation led to a dramatic increase in specific mRNA and protein levels of interstitial collagenase, stromelysin and interleukin-6. By contrast, the major tissue inhibitor of matrix-degrading metalloproteinases, TIMP-1, was unaffected. Monospecific neutralizing antibodies directed against human interleukin-6 significantly reduced the interstitial collagenase and stromelysin-1 protein levels. Taken together, our data provide the first evidence that Ultraviolet-B induction of interstitial collagenase and stromelysin-1 occurs via the synthesis and release of interleukin-6. Hence, this newly identified autocrine mechanism may contribute to dermal photodamage.