Ligand-activated PPARδ inhibits UVB-induced senescence of human keratinocytes via PTEN-mediated inhibition of superoxide production

Peroxisome Proliferator-Activated Receptors and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) function as nuclear transcription factors upon the binding of physiological or pharmacological ligands and heterodimerization with retinoic X receptors. Physiological ligands include fatty acids and fatty-acid-derived compounds with low specificity for the different PPAR subtypes (alpha, beta/delta, and gamma). For each of the PPAR subtypes, specific pharmacological agonists and antagonists, as well as pan-agonists, are available. In agreement with their natural ligands, PPARs are mainly focused on as targets for the treatment of metabolic syndrome and its associated complications. Nevertheless, many publications are available that implicate PPARs in malignancies. In several instances, they are controversial for very similar models. Thus, to better predict the potential use of PPAR modulators for personalized medicine in therapies against malignancies, it seems necessary and timely to review the three PPARs in relation to the didactic concept of cancer hallmark capabilities. We previously described the functions of PPAR beta/delta with respect to the cancer hallmarks and reviewed the implications of all PPARs in angiogenesis. Thus, the current review updates our knowledge on PPAR beta and the hallmarks of cancer and extends the concept to PPAR alpha and PPAR gamma.

PPARδ Inhibits Hyperglycemia-Triggered Senescence of Retinal Pigment Epithelial Cells by Upregulating SIRT1

Emerging evidence shows that peroxisome proliferator-activated receptor delta (PPARδ) plays a pivotal role in cellular aging. However, its function in retinal disease processes such as hyperglycemia-associated diabetic retinopathy is unclear. Here, we demonstrate that PPARδ inhibits premature senescence of retinal pigment epithelial (RPE) cells induced by high glucose (HG) through SIRT1 upregulation. A specific ligand GW501516-activation of PPARδ suppressed premature senescence and production of reactive oxygen species induced by HG in ARPE-19 cells, a spontaneously arising human RPE cell line. These effects were accompanied by the regulation of the premature senescence-associated genes p53, p21, and SMP-30. Furthermore, GW501516-activated PPARδ almost completely abolished the effects of HG treatment on the formation of phosphorylated H2A histone family member X (γ-H2A.X) foci, a molecular marker of aging. These inhibitory effects of GW501516 were significantly reversed in ARPE-19 cells stably expressing small hairpin RNA targeting PPARδ. Notably, GW501516 significantly increased the mRNA and protein levels of SIRT1, indicating that GW501516-activated PPARδ exerted its beneficial effects through SIRT1. In addition, GW501516 restored HG-suppressed SIRT1 expression, corroborating the role of SIRT1 in the anti-senescence function of PPARδ. The effects of PPARδ on HG-induced premature senescence and the expression of the senescence-associated genes p53, p21, and SMP-30 were mimicked by the SIRT1 activator resveratrol, but blocked by the SIRT1 inhibitor sirtinol. Collectively, these results indicate that GW501516-activated PPARδ inhibits HG-triggered premature senescence of RPE cells by modulating SIRT1 signaling.

Fisetin-induced PTEN expression reverses cellular senescence by inhibiting the mTORC2-Akt Ser473 phosphorylation pathway in vascular smooth muscle cells

Cellular senescence is caused by a wide range of intracellular and extracellular stimuli and influences physiological functions, leading to the progression of age-related diseases. Many studies have shown that cellular senescence is related to phosphatase and tension homolog deleted on chromosome ten (PTEN) loss and mammalian target of rapamycin (mTOR) activation. Although it has been reported that mTOR complex 1 (mTORC1) is major anti-aging target in several cell types, the functions and mechanisms of mTOR complex 2 (mTORC2) during aging have not been elucidated in vascular smooth muscle cells (VSMCs). Therefore, the aim of this study was to reveal the relationship between PTEN and mTORC2 during VSMC senescence. We found adriamycin-induced VSMC senescence was accompanied by reduced PTEN protein expression and upregulation of the mTORC2-Akt (Ser 473) pathway and that fisetin treatment reduced VSMC senescence by increasing PTEN and decreasing mTORC2 protein levels. Furthermore, PTEN played a primary role in the anti-aging effect of fisetin, and fisetin-activated PTEN directly regulated the mTORC2-Akt (Ser 473) signaling pathway, and attenuated senescence phenotypes such as senescence-associated β-galactosidase (SA-β-gal) and the p53-p21 signaling pathway in VSMCs. In mouse aortas, fisetin delayed aging by regulating the PTEN-mTORC2-Akt (Ser473) signaling pathway. These results suggest PTEN and mTORC2 are associated with cellular senescence in VSMCs and that the mTORC2-Akt (Ser 473) signaling pathway be considered a new target for preventing senescence-related diseases.

Peroxisome proliferator-activated receptor-δ-mediated upregulation of catalase helps to reduce ultraviolet B-induced cellular injury in dermal fibroblasts

BACKGROUND

Previous studies suggested that the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-δ plays an essential role in cellular responses against oxidative stress.

OBJECTIVE

To investigate how PPAR-δ elicits cellular responses against oxidative stress in primary human dermal fibroblasts (HDFs) exposed to ultraviolet B (UVB).

METHODS

The present study was undertaken in HDFs by performing real-time polymerase chain reaction, gene silencing, cytotoxicity and reporter gene assay, analyses for catalase and reactive oxygen species, and immunoblot analyses.

RESULTS

The PPAR-δ activator GW501516 upregulated expression of catalase and this upregulation was attenuated by PPAR-δ-targeting siRNA. GW501516-activated PPAR-δ induced catalase promoter activity through a direct repeat 1 response element. Mutation of this response element completely abrogated transcriptional activation, indicating that this site is a novel type of PPAR-δ response element. In addition, GW501516-activated PPAR-δ counteracted the reductions in activity and expression of catalase induced by UVB irradiation. These recovery effects were significantly attenuated in the presence of PPAR-δ-targeting siRNA or the specific PPAR-δ antagonist GSK0660. GW501516-activated PPAR-δ also protected HDFs from cellular damage triggered by UVB irradiation, and this PPAR-δ-mediated reduction of cellular damage was reversed by the catalase inhibitor or catalase-targeting siRNA. These effects of catalase blockade were positively correlated with accumulation of reactive oxygen species in HDFs exposed to UVB. Furthermore, GW501516-activated PPAR-δ targeted peroxisomal hydrogen peroxide through catalase in UVB-irradiated HDFs.

CONCLUSION

The gene encoding catalase is a target of PPAR-δ, and this novel catalase-mediated pathway plays a critical role in the cellular response elicited by PPAR-δ against oxidative stress.

PPAR Beta/Delta and the Hallmarks of Cancer

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear hormone receptor family. Three different isoforms, PPAR alpha, PPAR beta/delta and PPAR gamma have been identified. They all form heterodimers with retinoic X receptors to activate or repress downstream target genes dependent on the presence/absence of ligands and coactivators or corepressors. PPARs differ in their tissue expression profile, ligands and specific agonists and antagonists. PPARs attract attention as potential therapeutic targets for a variety of diseases. PPAR alpha and gamma agonists are in clinical use for the treatment of dyslipidemias and diabetes. For both receptors, several clinical trials as potential therapeutic targets for cancer are ongoing. In contrast, PPAR beta/delta has been suggested as a therapeutic target for metabolic syndrome. However, potential risks in the settings of cancer are less clear. A variety of studies have investigated PPAR beta/delta expression or activation/inhibition in different cancer cell models in vitro, but the relevance for cancer growth in vivo is less well documented and controversial. In this review, we summarize critically the knowledge of PPAR beta/delta functions for the different hallmarks of cancer biological capabilities, which interplay to determine cancer growth.

Regulatory mechanisms mediated by peroxisome proliferator-activated receptor-β/δ in skin cancer

Considerable progress has been made during the past 20 years towards elucidating the role of peroxisome proliferator-activated receptor-β/δ (PPARβ/δ) in skin cancer. In 1999, the original notion that PPARβ/δ was involved with epithelial cell function was postulated based on a correlation between PPARβ/δ expression and the induction of messenger RNAs encoding proteins that mediate terminal differentiation in keratinocytes. Subsequent studies definitively revealed that PPARβ/δ could induce terminal differentiation and inhibit proliferation of keratinocytes. Molecular mechanisms have since been discovered to explain how this nuclear receptor can be targeted for preventing and treating skin cancer. This includes the regulation of terminal differentiation, mitotic signaling, endoplasmic reticulum stress, and cellular senescence. Interestingly, the effects of activating PPARβ/δ can preferentially target keratinocytes with genetic mutations associated with skin cancer. This review provides the history and current understanding of how PPARβ/δ can be targeted for both nonmelanoma skin cancer and melanoma and postulates how future approaches that modulate PPARβ/δ signaling may be developed for the prevention and treatment of these diseases.

Self-regulation of the inflammatory response by peroxisome proliferator-activated receptors

The peroxisome proliferator-activated receptor (PPAR) family includes three transcription factors: PPARα, PPARβ/δ, and PPARγ. PPAR are nuclear receptors activated by oxidised and nitrated fatty acid derivatives as well as by cyclopentenone prostaglandins (PGA₂ and 15d-PGJ₂) during the inflammatory response. This results in the modulation of the pro-inflammatory response, preventing it from being excessively activated. Other activators of these receptors are nonsteroidal anti-inflammatory drug (NSAID) and fatty acids, especially polyunsaturated fatty acid (PUFA) (arachidonic acid, ALA, EPA, and DHA). The main function of PPAR during the inflammatory reaction is to promote the inactivation of NF-κB. Possible mechanisms of inactivation include direct binding and thus inactivation of p65 NF-κB or ubiquitination leading to proteolytic degradation of p65 NF-κB. PPAR also exert indirect effects on NF-κB. They promote the expression of antioxidant enzymes, such as catalase, superoxide dismutase, or heme oxygenase-1, resulting in a reduction in the concentration of reactive oxygen species (ROS), i.e., secondary transmitters in inflammatory reactions. PPAR also cause an increase in the expression of IκBα, SIRT1, and PTEN, which interferes with the activation and function of NF-κB in inflammatory reactions.

Ligand-activated PPARδ inhibits angiotensin II-stimulated hypertrophy of vascular smooth muscle cells by targeting ROS

We investigated the effect of peroxisome proliferator-activated receptor δ (PPARδ) on angiotensin II (Ang II)-triggered hypertrophy of vascular smooth muscle cells (VSMCs). Activation of PPARδ by GW501516, a specific ligand of PPARδ, significantly inhibited Ang II-stimulated protein synthesis in a concentration-dependent manner, as determined by [3H]-leucine incorporation. GW501516-activated PPARδ also suppressed Ang II-induced generation of reactive oxygen species (ROS) in VSMCs. Transfection of small interfering RNA (siRNA) against PPARδ significantly reversed the effects of GW501516 on [3H]-leucine incorporation and ROS generation, indicating that PPARδ is involved in these effects. By contrast, these GW501516-mediated actions were potentiated in VSMCs transfected with siRNA against NADPH oxidase (NOX) 1 or 4, suggesting that ligand-activated PPARδ elicits these effects by modulating NOX-mediated ROS generation. The phosphatidylinositol 3-kinase inhibitor LY294002 also inhibited Ang II-stimulated [3H]-leucine incorporation and ROS generation by preventing membrane translocation of Rac1. These observations suggest that PPARδ is an endogenous modulator of Ang II-triggered hypertrophy of VSMCs, and is thus a potential target to treat vascular diseases associated with hypertrophic changes of VSMCs.

Bioactive Dietary VDR Ligands Regulate Genes Encoding Biomarkers of Skin Repair That Are Associated with Risk for Psoriasis

Treatment with 1,25-dihydroxyvitamin D₃ (1,25D) improves psoriasis symptoms, possibly by inducing the expression of late cornified envelope (LCE)3 genes involved in skin repair. In psoriasis patients, the majority of whom harbor genomic deletion of LCE3B and LCE3C (LCE3C_LCE3B-del), we propose that certain dietary analogues of 1,25D activate the expression of residual LCE3A/LCE3D/LCE3E genes to compensate for the loss of LCE3B/LCE3C in the deletant genotype. Herein, human keratinocytes (HEKn) homozygous for LCE3C_LCE3B-del were treated with docosahexaenoic acid (DHA) and curcumin, two low-affinity, nutrient ligands for the vitamin D receptor (VDR). DHA and curcumin induce the expression of LCE3A/LCE3D/LCE3E mRNAs at concentrations corresponding to their affinity for VDR. Moreover, immunohistochemical quantitation revealed that the treatment of keratinocytes with DHA or curcumin stimulates LCE3 protein expression, while simultaneously opposing the tumor necrosis factor-alpha (TNFα)-signaled phosphorylation of mitogen activated protein (MAP) kinases, p38 and Jun amino-terminal kinase (JNK), thereby overcoming inflammation biomarkers elicited by TNFα challenge. Finally, DHA and curcumin modulate two transcription factors relevant to psoriatic inflammation, the activator protein-1 factor Jun B and the nuclear receptor NR4A2/NURR1, that is implicated as a mediator of VDR ligand-triggered gene control. These findings provide insights into the mechanism(s) whereby dietary VDR ligands alter inflammatory and barrier functions relevant to skin repair, and may provide a molecular basis for improved treatments for mild/moderate psoriasis.

Activation of PPARδ attenuates neurotoxicity by inhibiting lipopolysaccharide-triggered glutamate release in BV-2 microglial cells

Neuroinflammation-associated release of glutamate from activated microglia has been implicated in the progression of neurodegenerative diseases. However, the regulatory mechanisms underlying this glutamate release are poorly understood. Here, we show that peroxisome proliferator-activated receptor delta (PPARδ) modulates neurotoxicity by inhibiting glutamate release in lipopolysaccharide (LPS)-activated BV-2 microglial cells. Activation of PPARδ by GW501516, a specific PPARδ agonist, inhibited glutamate release in BV-2 cells. This effect of GW501516 was significantly blocked by shRNA-mediated knockdown of PPARδ and by treatment with GSK0660, a specific PPARδ antagonist, indicating that PPARδ is associated with blockade of glutamate release. Additionally, GW501516-activated PPARδ suppressed generation of reactive oxygen species and expression of gp91phox, a functional subunit of NADPH oxidase 2, in BV-2 cells stimulated with LPS. The inhibitory effect of GW501516 on gp91phox expression and glutamate release was further potentiated in the presence of AG490, a specific inhibitor of janus kinase 2 (JAK2), leading to the inhibition of signal transducer and activator of transcription 1 (STAT1). By contrast, GW501516 upregulated the expression of suppressor of cytokine signaling 1 (SOCS1), an endogenous inhibitor of JAK2. Furthermore, neurotoxicity induced by conditioned media from LPS-stimulated BV-2 cells was significantly reduced when conditioned media from BV-2 cells treated with both LPS and GW501516 were used. These results indicate that PPARδ attenuates LPS-triggered neuroinflammation by enhancing SOCS1-mediated inhibition of JAK2/STAT1 signaling, thereby inhibiting neurotoxicity associated with glutamate release.

Apoptotic effect of the selective PPARβ/δ agonist GW501516 in invasive bladder cancer cells

GW501516 is a selective and high-affinity synthetic agonist of peroxisome proliferator-activated receptor β/δ (PPARβ/δ). This molecule promoted the inhibition of proliferation and apoptosis in few cancer cell lines, but its anticancer action has never been investigated in bladder tumor cells. Thus, this study was undertaken to determine whether GW501516 had antiproliferative and/or apoptotic effects on RT4 and T24 urothelial cancer cells and to explore the molecular mechanisms involved. Our results indicated that, in RT4 cells (derived from a low-grade papillary tumor), GW501516 did not induce cell death. On the other hand, in T24 cells (derived from an undifferentiated high-grade carcinoma), this PPARβ/δ agonist induced cytotoxic effects including cell morphological changes, a decrease of cell viability, a G2/M cell cycle arrest, and the cell death as evidenced by the increase of the sub-G1 cell population. Furthermore, GW501516 triggered T24 cell apoptosis in a caspase-dependent manner including both extrinsic and intrinsic apoptotic pathways through Bid cleavage. In addition, the drug led to an increase of the Bax/Bcl-2 ratio, a mitochondrial dysfunction associated with the dissipation of ΔΨm, and the release of cytochrome c from the mitochondria to the cytosol. GW501516 induced also ROS generation which was not responsible for T24 cell death since NAC did not rescue cells upon PPARβ/δ agonist exposure. For the first time, our data highlight the capacity of GW501516 to induce apoptosis in invasive bladder cancer cells. This molecule could be relevant as a therapeutic drug for high-grade urothelial cancers.

Anti-photoaging properties of the phosphodiesterase 3 inhibitor cilostazol in ultraviolet B-irradiated hairless mice

We investigated whether cilostazol, an activator of cyclic adenosine monophosphate (cAMP)-dependent intracellular signaling, could inhibit ultraviolet B (UVB) irradiation-induced photoaging in HR-1 hairless mice. Cilostazol decreased wrinkle formation and skin thickness in UVB-irradiated mice, as well as increased staining of collagen fibers and inhibition of reactive oxygen species (ROS) formation in the skin. Moreover, the proteolytic activities of gelatinase matrix metalloproteinase (MMP)-9 and collagenase MMP-3 were significantly decreased in UVB-irradiated mice treated with cilostazol. Western blotting showed that UVB-induced activation of p38 mitogen-activated protein kinases (MAPK) and nuclear factor (NF)-κB was significantly inhibited by cilostazol, whereas the activation of Akt was significantly enhanced by cilostazol. Confirmation of localized protein expression in the skin revealed marked p38 MAPK and NF-κB activation that was mainly detected in the dermis. Marked Akt activation was mainly detected in the epidermis. Our results suggest that cilostazol may have anti-photoaging effects on UVB-induced wrinkle formation by maintaining the extracellular matrix density in the dermis, which occurs via regulation of ROS and related p38 MAPK and NF-κB signaling, and subsequent down-regulation of MMPs. Therefore, cilostazol may protect against photoaging-induced wrinkle formation.

Identification of a novel PPARβ/δ/miR-21-3p axis in UV-induced skin inflammation

Although excessive exposure to UV is widely recognized as a major factor leading to skin perturbations and cancer, the complex mechanisms underlying inflammatory skin disorders resulting from UV exposure remain incompletely characterized. The nuclear hormone receptor PPARβ/δ is known to control mouse cutaneous repair and UV-induced skin cancer development. Here, we describe a novel PPARβ/δ-dependent molecular cascade involving TGFβ1 and miR-21-3p, which is activated in the epidermis in response to UV exposure. We establish that the passenger miRNA miR-21-3p, that we identify as a novel UV-induced miRNA in the epidermis, plays a pro-inflammatory function in keratinocytes and that its high level of expression in human skin is associated with psoriasis and squamous cell carcinomas. Finally, we provide evidence that inhibition of miR-21-3p reduces UV-induced cutaneous inflammation in ex vivo human skin biopsies, thereby underlining the clinical relevance of miRNA-based topical therapies for cutaneous disorders.

Oxygen regulates molecular mechanisms of cancer progression and metastasis

Oxygen is the basic molecule which supports life and it truly is "god's gift to life." Despite its immense importance, research on "oxygen biology" has never received the light of the day and has been limited to physiological and biochemical studies. It seems that in modern day biology, oxygen research is summarized in one word "hypoxia." Scientists have focused on hypoxia-induced transcriptomics and molecular-cellular alterations exclusively in disease models. Interestingly, the potential of oxygen to control the basic principles of biology like homeostatic maintenance, transcription, replication, and protein folding among many others, at the molecular level, has been completely ignored. Here, we present a perspective on the crucial role played by oxygen in regulation of basic biological phenomena. Our conclusion highlights the importance of establishing novel research areas like oxygen biology, as there is great potential in this field for basic science discoveries and clinical benefits to the society.

Ethanol extract of Dalbergia odorifera protects skin keratinocytes against ultraviolet B-induced photoaging by suppressing production of reactive oxygen species

Dalbergia odorifera T. Chen (Leguminosae), an indigenous medicinal herb, has been widely used in northern and eastern Asia to treat diverse diseases. Here, we investigated the anti-senescent effects of ethanolic extracts of Dalbergia odorifera (EEDO) in ultraviolet (UV) B-irradiated skin cells. EEDO significantly inhibited UVB-induced senescence of human keratinocytes in a concentration-dependent manner, concomitant with inhibition of reactive oxygen species (ROS) generation. UVB-induced increases in the levels of p53 and p21, biomarkers of cellular senescence, were almost completely abolished in the presence of EEDO. Sativanone, a major constituent of EEDO, also attenuated UVB-induced senescence and ROS generation in keratinocytes, indicating that sativanone is an indexing (marker) molecule for the anti-senescence properties of EEDO. Finally, treatment of EEDO to mice exposed to UVB significantly reduced ROS levels and the number of senescent cells in the skin. Thus, EEDO confers resistance to UVB-induced cellular senescence by inhibiting ROS generation in skin cells.

Senescence suppressors: their practical importance in replicative lifespan extension in stem cells

Recent animal and clinical studies report promising results for the therapeutic utilization of stem cells in regenerative medicine. Mesenchymal stem cells (MSCs), with their pluripotent nature, have advantages over embryonic stem cells in terms of their availability and feasibility. However, their proliferative activity is destined to slow by replicative senescence, and the limited proliferative potential of MSCs not only hinders the preparation of sufficient cells for in vivo application, but also draws a limitation on their potential for differentiation. This calls for the development of safe and efficient means to increase the proliferative as well as differentiation potential of MSCs. Recent advances have led to a better understanding of the underlying mechanisms and significance of cellular senescence, facilitating ways to manipulate the replicative lifespan of a variety of primary cells, including MSCs. This paper introduces a class of proteins that function as senescence suppressors. Like tumor suppressors, these proteins are lost in senescence, while their forced expression delays the onset of senescence. Moreover, treatments that increase the expression or the activity of senescence suppressors, therefore, cause expansion of the replicative and differentiation potential of MSCs. The nature of the activities and putative underlying mechanisms of the senescence suppressors will be discussed to facilitate their evaluation.

Peroxisome proliferator-activated receptor δ modulates MMP-2 secretion and elastin expression in human dermal fibroblasts exposed to ultraviolet B radiation

BACKGROUND

Changes in skin connective tissues mediated by ultraviolet (UV) radiation have been suggested to cause the skin wrinkling normally associated with premature aging of the skin. Recent investigations have shown that peroxisome proliferator-activated receptor (PPAR) δ plays multiple biological roles in skin homeostasis.

OBJECTIVE

We attempted to investigate whether PPARδ modulates elastin protein levels and secretion of matrix metalloproteinase (MMP)-2 in UVB-irradiated human dermal fibroblasts (HDFs) and mouse skin.

METHODS

These studies were undertaken in primary HDFs or HR-1 hairless mice using Western blot analyses, small interfering (si)RNA-mediated gene silencing, and Fluorescence microscopy.

RESULTS

In HDFs, UVB irradiation induced increased secretion of MMP-2 and reduced levels of elastin. Activation of PPARδ by GW501516, a ligand specific for PPARδ, markedly attenuated UVB-induced MMP-2 secretion with a concomitant increase in the level of elastin. These effects were reduced by the presence of siRNAs against PPARδ or treatment with GSK0660, a specific inhibitor of PPARδ. Furthermore, GW501516 elicited a dose- and time-dependent increase in the expression of elastin. Modulation of MMP-2 secretion and elastin levels by GW501516 was associated with a reduction in reactive oxygen species (ROS) production in HDFs exposed to UVB. Finally, in HR-1 hairless mice, administration of GW501516 significantly reduced UVB-induced MMP-2 expression with a concomitant increase in elastin levels, and these effects were significantly reduced by the presence of GSK0660.

CONCLUSION

Our results suggest that PPARδ-mediated modulation of MMP-2 secretion and elastin expression may contribute to the maintenance of skin integrity by inhibiting ROS generation.

The role of AKT/mTOR pathway in stress response to UV-irradiation: implication in skin carcinogenesis by regulation of apoptosis, autophagy and senescence

Induction of DNA damage by UVB and UVA radiation may generate mutations and genomic instability leading to carcinogenesis. Therefore, skin cells being repeatedly exposed to ultraviolet (UV) light have acquired multilayered protective mechanisms to avoid malignant transformation. Besides extensive DNA repair mechanisms, the damaged skin cells can be eliminated by induction of apoptosis, which is mediated through the action of tumor suppressor p53. In order to prevent the excessive loss of skin cells and to maintain the skin barrier function, apoptotic pathways are counteracted by anti-apoptotic signaling including the AKT/mTOR pathway. However, AKT/mTOR not only prevents cell death, but is also active in cell cycle transition and hyper-proliferation, thereby also counteracting p53. In turn, AKT/mTOR is tuned down by the negative regulators being controlled by the p53. This inhibition of AKT/mTOR, in combination with transactivation of damage-regulated autophagy modulators, guides the p53-mediated elimination of damaged cellular components by autophagic clearance. Alternatively, p53 irreversibly blocks cell cycle progression to prevent AKT/mTOR-driven proliferation, thereby inducing premature senescence. Conclusively, AKT/mTOR via an extensive cross talk with p53 influences the UV response in the skin with no black and white scenario deciding over death or survival.

Neonauclea reticulata (Havil.) Merr Stimulates Skin Regeneration after UVB Exposure via ROS Scavenging and Modulation of the MAPK/MMPs/Collagen Pathway

In this study, we investigated whether the protective effects of Neonauclea reticulata water extract against ultraviolet B (UVB) irradiation in human skin fibroblast cell cultures (Hs68) are governed by its ability to protect against oxidative stress and expression of matrix metalloproteinases (MMPs). We found that Neonauclea reticulata extract exhibited DPPH scavenging activity and inhibited AAPH-induced haemolysis of erythrocytes in a dose- and time-dependent manner. We also found that pretreatment of fibroblasts with Neonauclea reticulata water extract resulted in markedly lower levels of MMP-1, -3, and -9 expressions. Furthermore, our results indicate that Neonauclea reticulata extract inhibits the expression of MMPs by inhibiting ERK, JNK, and p38 phosphorylation. Our results also demonstrate that treatment with Neonauclea reticulata extract protects against UVB-induced depletion of collagen. In addition, Neonauclea reticulata extract did not have a cytotoxic effect. These findings indicate that the antioxidant activity of Neonauclea reticulata extract resulted in inhibition of MMP-1, -3, and -9 expressions and in increased levels of collagen activity. Our results suggest that Neonauclea reticulata extract can protect against photoaging.

PPARδ inhibits UVB-induced secretion of MMP-1 through MKP-7-mediated suppression of JNK signaling

In the present study, we investigated the role of peroxisome proliferator-activated receptor (PPAR) δ in modulating matrix-degrading metalloproteinases and other mechanisms underlying photoaging processes in the skin. In human dermal fibroblasts (HDFs), activation of PPARδ by its specific ligand GW501516 markedly attenuated UVB-induced secretion of matrix metalloproteinase (MMP)-1, concomitant with decreased generation of reactive oxygen species. These effects were significantly reduced in the presence of PPARδ small interfering RNA and GSK0660. Furthermore, c-Jun N-terminal kinase (JNK), but not p38 or extracellular signal-regulated kinase, mediated PPARδ-dependent inhibition of MMP-1 secretion in HDFs exposed to UVB. PPARδ-mediated messenger RNA stabilization of mitogen-activated protein kinase phosphatase (MKP)-7 was responsible for the GW501516-mediated inhibition of JNK signaling. Inhibition of UVB-induced secretion of MMP-1 by PPARδ was associated with the restoration of types I and III collagen to levels approaching those in cells not exposed to UVB. Finally, in HR-1 hairless mice exposed to UVB, administration of GW501516 significantly reduced wrinkle formation and skin thickness, downregulated MMP-1 and JNK phosphorylation, and restored the levels of MKP-7, types I and III collagen. These results suggest that PPARδ-mediated inhibition of MMP-1 secretion prevents some effects of photoaging and maintains the integrity of skin by inhibiting the degradation of the collagenous extracellular matrix.

Nuclear hormone receptor functions in keratinocyte and melanocyte homeostasis, epidermal carcinogenesis and melanomagenesis

Skin homeostasis is maintained, in part, through regulation of gene expression orchestrated by type II nuclear hormone receptors in a cell and context specific manner. This group of transcriptional regulators is implicated in various cellular processes including epidermal proliferation, differentiation, permeability barrier formation, follicular cycling and inflammatory responses. Endogenous ligands for the receptors regulate actions during skin development and maintenance of tissue homeostasis. Type II nuclear receptor signaling is also important for cellular crosstalk between multiple cell types in the skin. Overall, these nuclear receptors are critical players in keratinocyte and melanocyte biology and present targets for cutaneous disease management.