Induction of apoptosis-like mitochondrial impairment triggers antioxidant and Bcl-2-dependent keratinocyte differentiation

Mitochondrial Activity Is Upregulated in Nonlesional Atopic Dermatitis and Amenable to Therapeutic Intervention

Previous work has shown increased expression of genes related to oxidative stress in nonlesional atopic dermatitis (ADNL) skin. Although mitochondria are key regulators of ROS production, their function in AD has never been investigated. Energy metabolism and the oxidative stress response were studied in keratinocytes (KCs) from patients with ADNL or healthy controls. Moreover, ADNL human epidermal equivalents were treated with tigecycline or MitoQ. We found that pyruvate and glucose were used as energy substrates by ADNL KCs. Increased mitochondrial oxidation of (very) long-chain fatty acids, associated with enhanced complexes I and II activities, was observed in ADNL KCs. Metabolomic analysis revealed increased tricarboxylic acid cycle turnover. Increased aerobic metabolism generated oxidative stress in ADNL KCs. ADNL human epidermal equivalents displayed increased mitochondrial function and an enhanced oxidative stress response compared with controls. Treatment of ADNL human epidermal equivalents with tigecycline or MitoQ largely corrected the AD profile, including high p-65 NF-κB, abnormal lamellar bodies, and cellular damage. Furthermore, we found that glycolysis supports but does not supersede mitochondrial metabolism in ADNL KCs. Thus, aerobic metabolism predominates in ADNL but leads to oxidative stress. Therefore, mitochondria could be a reservoir of potential therapeutic targets in atopic dermatitis.

Composition analysis and antioxidant activity evaluation of a high purity oligomeric procyanidin prepared from sea buckthorn by a green method

Procyanidin is an important polyphenol for its health-promoting properties, however, the study of procyanidin in sea buckthorn was limited. In this paper, sea buckthorn procyanidin (SBP) was obtained through a green isolation and enrichment technique with an extraction rate and purity of 9.1% and 91.5%. The structure of SBP was analyzed using Ultraviolet–visible spectroscopy (UV–vis), Fourier-transform infrared spectroscopy (FT-IR), and liquid chromatography-mass spectrometry (LC-MS/MS). The results show that SBP is an oligomeric procyanidin, mainly composed of (−)-epicatechin gallate, procyanidin B, (+)-gallocatechin-(+)-catechin, and (+)-gallocatechin dimer. SBP showed superior scavenging capacity on free radicals. Furthermore, the cleaning rate of the ABTS radical was 4.8 times higher than vitamin C at the same concentration. Moreover, SBP combined with vitamin C presented potent synergistic antioxidants with combined index values below 0.3 with concentration rates from 5:5 to 2:8. SBP also provided significant protection against oxidative stress caused by hydrogen peroxide (H₂O₂) on RAW264.7 cells. These findings prove the potential of SBP as a natural antioxidant in food additives and support the in-depth development of sea buckthorn resources.

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A green method for the extraction of procyanidin was proposed.

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An oligomeric procyanidin in sea buckthorn was identified for the first time.

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SBP combined with VC exerted strong synergistic antioxidant.

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SBP provided protection of macrophages against oxidative damage.

Nicotinamide Prevents UVB- and Oxidative Stress-Induced Photoaging in Human Primary Keratinocytes

Nicotinamide (NAM), a NAD⁺ precursor, is known for its benefits to skin health. Under standard culture conditions, NAM delays the differentiation and enhances the proliferation of human primary keratinocytes (HPKs), leading to the maintenance of stem cells. Here, we investigated the effects of NAM on photoaging in 2D HPK cultures and 3D organotypic epidermal models. In both models, we found that UVB irradiation and hydrogen peroxide induced HPK premature terminal differentiation and senescence. In 3D organotypics, the phenotype was characterized by a thickening of the granular layer expressing filaggrin and loricrin, but thinning of the epidermis overall. NAM limited premature differentiation and ameliorated senescence, as evidenced by the maintenance of lamin B1 levels in both models, with decreased lipofuscin staining and reduced IL-6/IL-8 secretion in 3D models, compared to UVB-only controls. In addition, DNA damage observed after irradiation was accompanied by a decline in energy metabolism, while both effects were partially prevented by NAM. Our data thus highlight the protective effects of NAM against photoaging and oxidative stress in the human epidermis, and pinpoint DNA repair and energy metabolism as crucial underlying mechanisms.

Epigenetic and metabolic regulation of epidermal homeostasis

Continuous exposure of the skin to environmental, mechanical and chemical stress necessitates constant self‐renewal of the epidermis to maintain its barrier function. This self‐renewal ability is attributed to epidermal stem cells (EPSCs), which are long‐lived, multipotent cells located in the basal layer of the epidermis. Epidermal homeostasis – coordinated proliferation and differentiation of EPSCs – relies on fine‐tuned adaptations in gene expression which in turn are tightly associated with specific epigenetic signatures and metabolic requirements. In this review, we will briefly summarize basic concepts of EPSC biology and epigenetic regulation with relevance to epidermal homeostasis. We will highlight the intricate interplay between mitochondrial energy metabolism and epigenetic events – including miRNA‐mediated mechanisms – and discuss how the loss of epigenetic regulation and epidermal homeostasis manifests in skin disease. Discussion of inherited epidermolysis bullosa (EB) and disorders of cornification will focus on evidence for epigenetic deregulation and failure in epidermal homeostasis, including stem cell exhaustion and signs of premature ageing. We reason that the epigenetic and metabolic component of epidermal homeostasis is significant and warrants close attention. Charting epigenetic and metabolic complexities also represents an important step in the development of future systemic interventions aimed at restoring epidermal homeostasis and ameliorating disease burden in severe skin conditions.

Keratin 14-dependent disulfides regulate epidermal homeostasis and barrier function via 14-3-3σ and YAP1

The intermediate filament protein keratin 14 (K14) provides vital structural support in basal keratinocytes of epidermis. Recent studies evidenced a role for K14-dependent disulfide bonding in the organization and dynamics of keratin IFs in skin keratinocytes. Here we report that knock-in mice harboring a cysteine-to-alanine substitution at Krt14's codon 373 (C373A) exhibit alterations in disulfide-bonded K14 species and a barrier defect secondary to enhanced proliferation, faster transit time and altered differentiation in epidermis. A proteomics screen identified 14-3-3 as K14 interacting proteins. Follow-up studies showed that YAP1, a transcriptional effector of Hippo signaling regulated by 14-3-3sigma in skin keratinocytes, shows aberrant subcellular partitioning and function in differentiating Krt14 C373A keratinocytes. Residue C373 in K14, which is conserved in a subset of keratins, is revealed as a novel regulator of keratin organization and YAP function in early differentiating keratinocytes, with an impact on cell mechanics, homeostasis and barrier function in epidermis.

Lysosomes Support the Degradation, Signaling, and Mitochondrial Metabolism Necessary for Human Epidermal Differentiation

Keratinocytes undergo significant structural remodeling during epidermal differentiation, including a broad transformation of the proteome coupled with a reduction in total cellular biomass. This suggests that intracellular digestion of proteins and organelles is necessary for keratinocyte differentiation. Here, we use both genetic and pharmacologic approaches to demonstrate that autophagy and lysosomal functions are required for keratinocyte differentiation in organotypic human skin. Lysosomal activity was required for mechanistic target of rapamycin signaling and mitochondrial oxidative metabolism. In turn, mitochondrial reactive oxygen species, produced as a natural byproduct of oxidative phosphorylation, were necessary for keratinocyte differentiation. Finally, treatment with exogenous reactive oxygen species rescued the differentiation defect in lysosome-inhibited keratinocytes. These findings highlight a reciprocal relationship between lysosomes and mitochondria, in which lysosomes support mitochondrial metabolism and the associated production of mitochondrial reactive oxygen species. The mitochondrial reactive oxygen species released to the cytoplasm in suprabasal keratinocytes triggers autophagy and lysosome-mediated degradation necessary for epidermal differentiation. As defective lysosome-dependent autophagy is associated with common skin diseases including psoriasis and atopic dermatitis, a better understanding of the role of lysosomes in epidermal homeostasis may guide future therapeutic strategies.

Optimization of Storage Temperature for Retention of Undifferentiated Cell Character of Cultured Human Epidermal Cell Sheets

Cultured epidermal cell sheets (CES) containing undifferentiated cells are useful for treating skin burns and have potential for regenerative treatment of other types of epithelial injuries. The undifferentiated phenotype is therefore important for success in both applications. This study aimed to optimize a method for one-week storage of CES for their widespread distribution and use in regenerative medicine. The effect of storage temperatures 4 °C, 8 °C, 12 °C, 16 °C, and 24 °C on CES was evaluated. Analyses included assessment of viability, mitochondrial reactive oxygen species (ROS), membrane damage, mitochondrial DNA (mtDNA) integrity, morphology, phenotype and cytokine secretion into storage buffer. Lowest cell viability was seen at 4 °C. Compared to non-stored cells, ABCG2 expression increased between temperatures 8–16 °C. At 24 °C, reduced ABCG2 expression coincided with increased mitochondrial ROS, as well as increased differentiation, cell death and mtDNA damage. P63, C/EBPδ, CK10 and involucrin fluorescence combined with morphology observations supported retention of undifferentiated cell phenotype at 12 °C, transition to differentiation at 16 °C, and increased differentiation at 24 °C. Several cytokines relevant to healing were upregulated during storage. Importantly, cells stored at 12 °C showed similar viability and undifferentiated phenotype as the non-stored control suggesting that this temperature may be ideal for storage of CES.

The proteasome maturation protein POMP increases proteasome assembly and activity in psoriatic lesional skin

BACKGROUND

The ubiquitin proteasome pathway is involved in the pathogenesis of psoriasis and proteasome subunits are increased in lesional psoriatic skin. Recent works have highlighted that proteasome levels can be regulated through modulation of proteasome assembly notably by the proteasome maturation protein POMP.

OBJECTIVES

To investigate whether proteasome assembly and POMP expression are modified in psoriatic skin.

METHODS

Proteasome assembly as well as expression of proteasome regulators were assessed in non-lesional and lesional psoriatic skin using native gel electrophoresis and western blots respectively. The protein and mRNA expression levels of POMP were compared by western blots, immunohistochemistry and quantitative polymerase chain reaction. The role of POMP in keratinocyte proliferation and differentiation was assessed by silencing POMP gene expression by RNA interference in human immortalized keratinocyte HaCaT cells.

RESULTS

Both 20S and 26S proteasomes (and their respective proteolytic activities) as well as the main proteasome regulators are increased in lesional psoriatic skin. POMP binds to 20S precursor complexes and is overexpressed in lesional epidermal psoriatic skin, supporting that POMP-mediated proteasome assembly is increased in psoriatic skin. POMP silencing inhibited HaCaT cell proliferation and induced apoptosis through the inhibition of the proteasome assembly. Moreover POMP partial depletion decreased the expression of the differentiation markers keratin 10 and involucrin during the [Ca²⁺]-induced HaCaT cells differentiation.

CONCLUSION

Altogether these results establish a potential role for POMP and proteasome assembly in psoriasis pathogenesis.

Possible Involvement of F1F0-ATP synthase and Intracellular ATP in Keratinocyte Differentiation in normal skin and skin lesions

The F1F0-ATP synthase, an enzyme complex, is mainly located on the mitochondrial inner membrane or sometimes cytomembrane to generate or hydrolyze ATP, play a role in cell proliferation. This study focused on the role of F1F0-ATP synthase in keratinocyte differentiation, and its relationship with intracellular and extracellular ATP (InATP and ExATP). The F1F0-ATP synthase β subunit (ATP5B) expression in various skin tissues and confluence-dependent HaCaT differentiation models was detected. ATP5B expression increased with keratinocyte and HaCaT cell differentiation in normal skin, some epidermis hyper-proliferative diseases, squamous cell carcinoma, and the HaCaT cell differentiation model. The impact of InATP and ExATP content on HaCaT differentiation was reflected by the expression of the differentiation marker involucrin. Inhibition of F1F0-ATP synthase blocked HaCaT cell differentiation, which was associated with a decrease of InATP content, but not with changes of ExATP. Our results revealed that F1F0-ATP synthase expression is associated with the process of keratinocyte differentiation which may possibly be related to InATP synthesis.

Periodic Exposure of Keratinocytes to Cold Physical Plasma: An In Vitro Model for Redox-Related Diseases of the Skin

Oxidative stress illustrates an imbalance between radical formation and removal. Frequent redox stress is critically involved in many human pathologies including cancer, psoriasis, and chronic wounds. However, reactive species pursue a dual role being involved in signaling on the one hand and oxidative damage on the other. Using a HaCaT keratinocyte cell culture model, we investigated redox regulation and inflammation to periodic, low-dose oxidative stress after two, six, eight, ten, and twelve weeks. Chronic redox stress was generated by recurrent incubation with cold physical plasma-treated cell culture medium. Using transcriptome microarray technology, we identified both acute ROS-stress responses as well as numerous adaptions after several weeks of redox challenge. We determined a differential expression (2-fold, FDR < 0.01, p < 0.05) of 260 genes that function in inflammation and redox homeostasis, such as cytokines (e.g., IL-6, IL-8, and IL-10), growth factors (e.g., CSF2, FGF, and IGF-2), and antioxidant enzymes (e.g., HMOX, NQO1, GPX, and PRDX). Apoptotic signaling was affected rather modestly, especially in p53 downstream targets (e.g., BCL2, BBC3, and GADD45). Strikingly, the cell-protective heat shock protein HSP27 was strongly upregulated (p < 0.001). These results suggested cellular adaptions to frequent redox stress and may help to better understand the inflammatory responses in redox-related diseases.

Effect of Storage Temperature on the Phenotype of Cultured Epidermal Cells Stored in Xenobiotic-Free Medium

PURPOSE

Cultured epidermal cell sheets (CECS) are used in the treatment of large area burns to the body and have potential to treat limbal stem cell deficiency (LSCD) as shown in animal studies. Despite widespread use, storage options for CECS are limited. Short-term storage allows flexibility in scheduling surgery, quality control and improved transportation to clinics worldwide. Recent evidence points to the phenotype of cultured epithelial cells as a critical predictor of post-operative success following transplantation of CECS in burns and in transplantation of cultured epithelial cells in patients with LSCD. This study, therefore assessed the effect of a range of temperatures, spanning 4-37 °C, on the phenotype of CECS stored over a 2-week period in a xenobiotic-free system.

MATERIALS AND METHODS

Progenitor cell (p63, ΔNp63α and ABCG2) and differentiation (C/EBPδ and CK10) associated marker expression was assessed using immunocytochemistry. Immunohistochemistry staining of normal skin for the markers p63, ABCG2 and C/EBPδ was also carried out. Assessment of progenitor cell side population (SP) was performed using JC1 dye by flow cytometry.

RESULTS

P63 expression remained relatively constant throughout the temperature range but was significantly lower compared to control between 20 and 28 °C (p < 0.05). High C/EBPδ together with low p63 suggested more differentiation beginning at 20 °C and above. Lower CK10 and C/EBPδ expression most similar to control was seen at 12 °C. The percentage of ABCG2 positive cells was most similar to control between 8 and 24 °C. Between 4 and 24 °C, the SP fluctuated, but was not significantly different compared to control. Results were supported by staining patterns indicating differentiation status associated with markers in normal skin sections.

CONCLUSIONS

Lower storage temperatures, and in particular 12 °C, merit further investigation as optimal storage temperature for maintenance of undifferentiated phenotype in CECS.

Pleiotropic age-dependent effects of mitochondrial dysfunction on epidermal stem cells

Tissue homeostasis declines with age partly because stem/progenitor cells fail to self-renew or differentiate. Because mitochondrial damage can accelerate aging, we tested the hypothesis that mitochondrial dysfunction impairs stem cell renewal or function. We developed a mouse model, Tg(KRT14-cre/Esr1) (20Efu/J) × Sod2 (tm1Smel) , that generates mitochondrial oxidative stress in keratin 14-expressing epidermal stem/progenitor cells in a temporally controlled manner owing to deletion of Sod2, a nuclear gene that encodes the mitochondrial antioxidant enzyme superoxide dismutase 2 (Sod2). Epidermal Sod2 loss induced cellular senescence, which irreversibly arrested proliferation in a fraction of keratinocytes. Surprisingly, in young mice, Sod2 deficiency accelerated wound closure, increasing epidermal differentiation and reepithelialization, despite the reduced proliferation. In contrast, at older ages, Sod2 deficiency delayed wound closure and reduced epidermal thickness, accompanied by epidermal stem cell exhaustion. In young mice, Sod2 deficiency accelerated epidermal thinning in response to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate, phenocopying the reduced regeneration of older Sod2-deficient skin. Our results show a surprising beneficial effect of mitochondrial dysfunction at young ages, provide a potential mechanism for the decline in epidermal regeneration at older ages, and identify a previously unidentified age-dependent role for mitochondria in skin quality and wound closure.

Epidermal TRPM8 channel isoform controls the balance between keratinocyte proliferation and differentiation in a cold-dependent manner

Deviation of the ambient temperature is one of the most ubiquitous stimuli that continuously affect mammals' skin. Although the role of the warmth receptors in epidermal homeostasis (EH) was elucidated in recent years, the mystery of the keratinocyte mild-cold sensor remains unsolved. Here we report the cloning and characterization of a new functional epidermal isoform of the transient receptor potential M8 (TRPM8) mild-cold receptor, dubbed epidermal TRPM8 (eTRPM8), which is localized in the keratinocyte endoplasmic reticulum membrane and controls mitochondrial Ca(2+) concentration ([Ca(2+)]m). In turn, [Ca(2+)]m modulates ATP and superoxide (O2(·-)) synthesis in a cold-dependent manner. We report that this fine tuning of ATP and O2(·-) levels by cooling controls the balance between keratinocyte proliferation and differentiation. Finally, to ascertain eTRPM8's role in EH in vivo we developed a new functional knockout mouse strain by deleting the pore domain of TRPM8 and demonstrated that eTRPM8 knockout impairs adaptation of the epidermis to low temperatures.

The protein deacetylase SIRT3 prevents oxidative stress-induced keratinocyte differentiation

Keratinocyte differentiation is a key process in the formation and maintenance of the protective skin barrier. Dysregulation in the balance of reactive oxygen species homeostasis may play a role in keratinocyte differentiation. We have identified the mitochondrial deacetylase SIRT3 as a key regulator of mitochondrial reactive oxygen species in keratinocytes. Our studies demonstrate that SIRT3 expression is down-regulated during keratinocyte differentiation, consistent with an increase in mitochondrial superoxide levels. Importantly, loss of SIRT3 expression in keratinocytes increased superoxide levels and promoted the expression of differentiation markers, whereas overexpression decreased superoxide levels and reduced the expression of differentiation markers. These findings identify a new role for SIRT3 in the suppression of epidermal differentiation via lowering oxidative stress.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated production of reactive oxygen species is an essential step in the mechanism of action to accelerate human keratinocyte differentiation

Chloracne is commonly observed in humans exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD); yet, the mechanism of toxicity is not well understood. Using normal human epidermal keratinocytes, we investigated the mechanism of TCDD-mediated enhancement of epidermal differentiation by integrating functional genomic, metabolomic, and biochemical analyses. TCDD increased the expression of 40% of the genes of the epidermal differentiation complex found on chromosome 1q21 and 75% of the genes required for de novo ceramide biosynthesis. Lipid analysis demonstrated that eight of the nine classes of ceramides were increased by TCDD, altering the ratio of ceramides to free fatty acids. TCDD decreased the expression of the glucose transporter, SLC2A1, and most of the glycolytic transcripts, followed by decreases in glycolytic intermediates, including pyruvate. NADH and Krebs cycle intermediates were decreased, whereas NAD(+) was increased. Mitochondrial glutathione (GSH) reductase activity and the GSH/glutathione disulfide ratio were decreased by TCDD, ultimately leading to mitochondrial dysfunction, characterized by decreased inner mitochondrial membrane potential and ATP production, and increased production of the reactive oxygen species (ROS), hydrogen peroxide. Aryl hydrocarbon receptor (AHR) antagonists blocked the response of many transcripts to TCDD, and the endpoints of decreased ATP production and differentiation, suggesting regulation by the AHR. Cotreatment of cells with chemical antioxidants or the enzyme catalase blocked the TCDD-mediated acceleration of keratinocyte cornified envelope formation, an endpoint of terminal differentiation. Thus, TCDD-mediated ROS production is a critical step in the mechanism of this chemical to accelerate keratinocyte differentiation.

The impact of UVB exposure and differentiation state of primary keratinocytes on their interaction with quantum dots

In this study we utilised an in vitro model system to gain insight into the potential cellular interactions that quantum dot (QD) nanoparticles may experience while transiting the viable skin epidermis, and we consider the effects of UVB exposure. UVB skin exposure is known to induce a skin barrier defect that facilitates QD stratum corneum penetration. Primary human keratinocytes were cultured in low and high calcium to induce basal and differentiated phenotypes, respectively. Results suggest that differentiation state plays a role in keratinocyte response to UVB exposure and exposure to negatively charged CdSe/ZnS core/shell QD. QD cell uptake increased with QD dose but association with differentiated cells was significantly lower than the basal keratinocyte phenotype. Differentiated keratinocytes were also less sensitive to the cytotoxic effects of UVB exposure. We did not observe an effect of UVB preexposure on QD cytotoxicity level despite the fact that fluorescent microscopy and flow cytometry data suggest that UVB may slightly increase QD uptake in the basal cell phenotype. The implications of these findings for assessing potential risk of human skin exposure are discussed.

Mitochondrial respiratory uncoupling promotes keratinocyte differentiation and blocks skin carcinogenesis

Decreased mitochondrial oxidative metabolism is a hallmark bioenergetic characteristic of malignancy that may have an adaptive role in carcinogenesis. By stimulating proton leak, mitochondrial uncoupling proteins (UCP1-3) increase mitochondrial respiration and may thereby oppose cancer development. To test this idea, we generated a mouse model that expresses an epidermal-targeted keratin-5-UCP3 (K5-UCP3) transgene and exhibits significantly increased cutaneous mitochondrial respiration compared with wild type (FVB/N). Remarkably, we observed that mitochondrial uncoupling drove keratinocyte/epidermal differentiation both in vitro and in vivo. This increase in epidermal differentiation corresponded to the loss of markers of the quiescent bulge stem cell population, and an increase in epidermal turnover measured using a bromodeoxyuridine (BrdU)-based transit assay. Interestingly, these changes in K5-UCP3 skin were associated with a nearly complete resistance to chemically-mediated multistage skin carcinogenesis. These data suggest that targeting mitochondrial respiration is a promising novel avenue for cancer prevention and treatment.

Mitochondrial oxidative stress caused by Sod2 deficiency promotes cellular senescence and aging phenotypes in the skin

Cellular senescence arrests the proliferation of mammalian cells at risk for neoplastic transformation, and is also associated with aging. However, the factors that cause cellular senescence during aging are unclear. Excessive reactive oxygen species (ROS) have been shown to cause cellular senescence in culture, and accumulated molecular damage due to mitochondrial ROS has long been thought to drive aging phenotypesin vivo. Here, we test the hypothesis that mitochondrial oxidative stress can promote cellular senescence in vivo and contribute to aging phenotypes in vivo, specifically in the skin. We show that the number of senescent cells, as well as impaired mitochondrial (complex II) activity increase in naturally aged mouse skin. Using a mouse model of genetic Sod2 deficiency, we show that failure to express this important mitochondrial anti-oxidant enzyme also impairs mitochondrial complex II activity, causes nuclear DNA damage, and induces cellular senescence but not apoptosis in the epidermis. Sod2 deficiency also reduced the number of cells and thickness of the epidermis, while increasing terminal differentiation. Our results support the idea that mitochondrial oxidative stress and cellular senescence contribute to aging skin phenotypes in vivo.

Differentiation-dependent expression of NADP(H):quinone oxidoreductase-1 via NF-E2 related factor-2 activation in human epidermal keratinocytes

BACKGROUND

NADP(H):quinone oxidoreductase-1 (NQO-1) is known for its protective role in skin carcinogenesis, but the expression of NQO-1 during keratinocyte (KC) differentiation has not been studied.

OBJECTIVE

The purpose of the current study was to evaluate modulation of NQO-1 and NF-E2-related factor-2 (Nrf2) during KC differentiation.

METHODS

Normal human epidermal keratinocytes (NHEKs) were induced to differentiation by prolonged culture after confluency (postconfluence).

RESULTS

NQO-1 was induced at the late stage of differentiation of NHEKs (7th day of postconfluence). The expression of postconfluence-induced NQO-1 was stimulated by 0.1 mM H(2)O(2), but attenuated by 5 mM N-acetylcysteine, implying that reactive oxygen species (ROS) are implicated in the expression of NQO-1 in differentiated KCs. Nrf2 was up-regulated at the earlier than NQO-1 induction (3rd day of postconfluence). The Nrf2-dependent expression of NQO-1 was further supported by Nrf2-siRNA experiments. A confocal study confirmed the differentiation-dependent induction and activation of NOQ-1 and Nrf-2 in NHEKs. Immunohistochemistry showed that NQO-1 was accentuated in the upper epidermal layers, supporting the notion that differentiation-dependent NQO-1 expression is functional in human skin in vivo.

CONCLUSION

These results demonstrate that NQO-1 is modulated during KC differentiation via Nrf2 pathway, suggesting the active role of NQO-1 in the differentiating epidermis.

Asymmetric distribution of epidermal growth factor receptor directs the fate of normal and cancer keratinocytes in vitro

Cancer cells are unequal in a tumor mass and in established cultures. This is attributable to cancer stem cells with the unique ability to self-renew and to generate differentiating progeny. This ability is controlled at the level of asymmetric division by mechanisms that are yet not well defined. We found that normal and cancer keratinocyte fate was linked to the asymmetric distribution of epidermal growth factor receptor (EGFR) during mitosis. Although essential for epithelial cell proliferation, differentiation, and survival, this receptor was not present on the surface of cells satisfying criteria for stem cells such as quiescence, competence to produce functionally distinct daughters, high proliferative and clonogenic potential, sphere formation ability, and expression of stem cell markers. In contrast, keratinocytes displaying EGFR acquired a more differentiated phenotype, suggesting that EGFR may be involved in a switch from stem to transient amplifying cell fate. This switch was associated with changes in the expression profile of cell cycle, survival, and mitochondria controlling proteins that varied between normal and cancer cells. In conclusion, it appears that an unequal distribution of EGFR at mitosis controls keratinocyte fate by balancing quiescence and cycling of EGFR(-) cells, clearly malfunctioning in cancer. We believe that our findings provide mechanistic insights into the development of resistance to anti-EGFR therapies.

Selective apoptosis induction by the cancer chemopreventive agent N-(4-hydroxyphenyl)retinamide is achieved by modulating mitochondrial bioenergetics in premalignant and malignant human prostate epithelial cells

Prostate tumorigenesis is coupled with an early metabolic switch in transformed prostate epithelial cells that effectively increases their mitochondrial bioenergetic capacity. The synthetic retinoid N-(4-hydroxyphenyl)retinamide (4HPR) inhibits prostate cancer development in vivo, and triggers reactive oxygen species (ROS)-dependent prostate cancer cell apoptosis in vitro. The possibility that 4HPR-induced ROS production is associated with mitochondrial bioenergetics and required for apoptosis induction in transformed prostate epithelial cells in vitro would advocate a prospective mechanistic basis for 4HPR-mediated prostate cancer chemoprevention in vivo. We investigated this tenet by comparing and contrasting 4HPR's effects on premalignant PWR-1E and malignant DU-145 human prostate epithelial cells. 4HPR promoted a dose- and/or time-dependent apoptosis induction in PWR-1E and DU-145 cells, which was preceded by and dependent on an increase in mitochondrial ROS production. In this regard, the PWR-1E cells were more sensitive than the DU-145 cells, and they consumed roughly twice as much oxygen as the DU-145 cells suggesting oxidative phosphorylation was higher in the premalignant cells. Interestingly, increasing the [Ca(2+)] in the culture medium of the PWR-1E cells attenuated their proliferation as well as their mitochondrial bioenergetic capacity and 4HPR's cytotoxic effects. Correspondingly, the respiration-deficient derivatives (i.e., rho(0) cells lacking mitochondrial DNA) of DU-145 cells were markedly resistant to 4HPR-induced ROS production and apoptosis. Together, these observations implied that the reduction of mitochondrial bioenergetics protected PWR-1E and DU-145 cells against the cytotoxic effects of 4HPR, and support the concept that oxidative phosphorylation is an essential determinant in 4HPR's apoptogenic signaling in transformed human prostate epithelial cells.

Matrix metalloproteinase-21 expression is associated with keratinocyte differentiation and upregulated by retinoic acid in HaCaT cells

In the skin, expression of several matrix metalloproteinases (MMPs) occurs in response to tissue injury, tumorigenesis, angiogenesis, apoptosis, and inflammation. The recently cloned MMP-21 has been implicated in skin development and various epithelial cancers. In this study, we found that it is also expressed by differentiated keratinocytes (KCs) in various benign skin disorders, in which it was not associated with KC apoptosis or proliferation, and in organotypic cultures. Furthermore, MMP-21 was induced in keratinocytes in association with increased calcium and presence of the differentiation marker filaggrin. In stably transfected A431 and HEK293 cell lines, MMP-21 increased invasion of cells but did not associate with increased apoptosis, proliferation, or epithelial-to-mesenchymal transition. Of various agents tested in HaCaT cell cultures, only retinoic acid (10(-6) M) and staurosporine (2.5 x 10(-8) M) upregulated MMP-21 mRNA and protein expression, whereas tumor promoters, hormones, or dexamethasone were without effect. Our results suggest that MMP-21 may be an important protease in the terminal differentiation of keratinocytes.

Variable Bax antigenicity is linked to keratinocyte position within epidermal strata and UV-induced apoptosis

Pro- and anti-apoptotic members of the Bcl-2 family are fundamental in the control of apoptosis. Among them, Bax plays a key role in apoptosis induction by mediating the release of apoptogenic factors from mitochondria to the cytosol. In this report, we investigated, by immunohistofluorescence, the in vivo distribution of Bax in normal human epidermis before and 24 h after exposure to solar-simulated radiation. Bax expression was evaluated with three different, Western blot pretested, anti-Bax antibodies (Ab) and correlated with markers of keratinocyte differentiation and apoptosis using anti-beta(1) integrin and anti-active caspase-3 Abs respectively. Using anti-Bax N20 and A-3533 polyclonal Ab, we found that, whereas undifferentiated keratinocytes of the basal proliferative compartment contained Bax in the cytosol, the differentiated suprabasal cells had Bax mainly in the nucleus. This immunoreactivity pattern was not modified by skin irradiation. Interestingly, the well known apoptosis-related Bax redistribution to mitochondria in response to a cell death signal, could be detected only with yet another, the 2D2 monoclonal Ab. This relocalization occurred specifically in apoptotic, active caspase-3 positive cells of irradiated epidermis. Our data highlight the differentiation- and apoptosis-associated changes in the pattern of Bax subcellular and cellular distribution as uncovered by different anti-Bax Abs and suggest that Bax undergoes successive activation that progresses in parallel with keratinocyte differentiation and apoptosis.

TRPC channels determine human keratinocyte differentiation: new insight into basal cell carcinoma

Aberrant keratinocyte differentiation is considered to be a key mechanism in the onset of hyperproliferative dermatological diseases, including basal cell carcinoma (BCC). It is, therefore, vital to understand what drives keratinocytes to develop such pathological phenotypes. The role of calcium in keratinocyte differentiation is uncontested but the mechanisms controlling calcium-induced differentiation have yet to be completely elucidated. This study was designed to investigate the role of calcium-permeable TRPC channels in human keratinocyte differentiation and BCC, using a combination of molecular and cell biology approaches, involving electrophysiology and Ca(2+)-imaging, on the HaCaT cell line, primary cultures of normal human keratinocytes, and BCC cells. We demonstrated that TRPC1/TRPC4 channel expression was important for keratinocyte differentiation, as knocking out these channels (by siRNA strategy) prevented the induction of Ca(2+)-induced differentiation. TRPC1/TRPC4-mediated calcium entry and endoplasmic reticulum Ca(2+) content increased significantly in differentiated keratinocytes. However, the failure of BCC cells to differentiate was related to a lack of TRPC channel expression and calcium entry. In summary, our data demonstrate that TRPC1 and TRPC4 channels are key elements in keratinocyte Ca(2+) homeostasis and differentiation and may therefore be responsible for skin pathologies.

TRPV6 is a Ca2+ entry channel essential for Ca2+-induced differentiation of human keratinocytes

Ca(2+) is an essential factor inducing keratinocyte differentiation due to the natural Ca(2+) gradient in the skin. However, the membrane mechanisms that mediate calcium entry and trigger keratinocyte differentiation had not previously been elucidated. In this study we demonstrate that Ca(2+)-induced differentiation up-regulates both mRNA and protein expression of a transient receptor potential highly Ca(2+)-selective channel, TRPV6. The latter mediates Ca(2+) uptake and accounts for the basal [Ca(2+)](i) in human keratinocytes. Our results show that TRPV6 is a prerequisite for keratinocyte entry into differentiation, because the silencing of TRPV6 in human primary keratinocytes led to the development of impaired differentiated phenotype triggered by Ca(2+). The expression of such differentiation markers as involucrin, transglutaminase-1, and cytokeratin-10 was significantly inhibited by small interfering RNA-TRPV6 as compared with differentiated control cells. TRPV6 silencing affected cell morphology and the development of intercellular contacts, as well as the ability of cells to stratify. 1,25-Dihydroxyvitamin D3, a cofactor of differentiation, dose-dependently increased TRPV6 mRNA and protein expression in human keratinocytes. This TRPV6 up-regulation led to a significant increase in Ca(2+) uptake in both undifferentiated and differentiated keratinocytes. We conclude that TRPV6 mediates, at least in part, the pro-differentiating effects of 1,25-dihydroxyvitamin D3 by increasing Ca(2+) entry, thereby promoting differentiation. Taken together, these data suggest that the TRPV6 channel is a key element in Ca(2+)/1,25-dihydroxyvitamin D3-induced differentiation of human keratinocytes.

Human epidermal keratinocytes accumulate superoxide due to low activity of Mn-SOD, leading to mitochondrial functional impairment

The energy metabolism of the epidermis has been the subject of controversy; thus we characterized the mitochondrial phenotype of human primary keratinocytes and fibroblasts, in cell culture and in human skin sections. We found that keratinocytes respire as much as fibroblasts, however, maximal activities of the respiratory chain (RC) complexes were 2- to 5-fold lower, whereas expression levels of RC proteins were similar. Maximal activities of aconitase and isocitrate dehydrogenase, two mitochondrial enzymes especially vulnerable to superoxide, were lower than in fibroblasts. Indeed, superoxide anion levels were much higher in keratinocytes, and keratinocytes displayed higher lipid peroxidation levels and a lower reduced glutathione/oxidized glutathione ratio, indicating enhanced oxidative stress. Although superoxide dismutase activity and especially expression of the mitochondrial superoxide dismutase, Mn-SOD, were drastically lower in keratinocytes, explaining the high superoxide levels, glutathione peroxidase activity and protein were almost undetectable in fibroblasts. Catalase activity and hydrogen peroxide levels were similar. In summary, we could show that keratinocytes actively use the mitochondrial RC not only for adenosine 5' triphosphate synthesis but also for the accumulation of superoxide anions, even at the expense of mitochondrial functional capacity, indicating that superoxide-driven mitochondrial impairment might be a prerequisite for keratinocyte differentiation.

An unexpected role for keratin 10 end domains in susceptibility to skin cancer

Keratin 10 (K10) is a type I keratin that is expressed in post-mitotic suprabasal keratinocytes of the skin. Based on cell culture experiments and transgenic mouse studies, it has been proposed that K10 suppresses cell proliferation and tumor formation in the skin. Furthermore, the ability of K10 to suppress cell proliferation was mapped to its unique N- and C-terminal protein domains. In the present study, we modified the endogenous keratin 14 (K14) gene of mice using a knock-in approach to encode a chimeric keratin that consists of the K14 rod domain fused to the K10 head and tail domains (K1014chim). This transgene was expressed in the basal layer of the epidermis and the outer root sheath of hair follicles. Unexpectedly, we found that the K10 end domains had no effect on basal keratinocyte proliferation in vivo. Moreover, when subjected to a chemical skin carcinogenesis protocol, papilloma formation in mutant mice was accelerated instead of being inhibited. Our data suggest that the increased tumor susceptibility of K1014chim mice is in part due to a suppression of apoptosis in mutant keratinocytes. Our results support the notion that intermediate filaments, in addition to their function as cytoskeletal components, affect tumor susceptibility of epithelial cells.