NRF2 Augments Epidermal Antioxidant Defenses and Promotes Atopy

Ozonated Sunflower Oil (OSO) Alleviates Inflammatory Responses in Oxazolone-Induced Atopic Dermatitis (AD)-Like Mice and LPS-Treated RAW 264.7 Cells

Ozone, a highly reactive oxidant molecule, is widely used as a complementary therapy for various skin diseases, including wound healing, pressure ulcers, diabetic foot, and infections. However, there is limited research on the effectiveness of ozone for atopic dermatitis (AD). Ozonated sunflower oil (OSO) is an active ingredient obtained from partially ozonated sunflower oil (SO). OSO markedly reduced the LPS-induced increase in IL-1β and nitric oxide (NO) levels in RAW 264.7 mouse macrophage cells. Oxazolone (OXZ) was applied to hairless mice to induce AD-like skin symptoms and immune response. OSO significantly alleviated the OXZ-induced increases in the number of infiltrating mast cells, epidermal thickness, AD symptoms, thymic stromal lymphopoietin (TSLP), and filaggrin, as well as the serum levels of NO, IgE, IL-1β, and TNF-α. Furthermore, OSO inhibited the IL-4/STAT3/MAPK pathway and the expression of NF-κB. Our results suggest that OSO treatment could relieve AD-mediated skin damage through its anti-inflammatory and antioxidant activities. Therefore, it can be used as a therapeutic agent against AD-related skin diseases.

Excessive Production of Hydrogen Peroxide in Mitochondria Contributes to Atopic Dermatitis

Atopic dermatitis (AD) is a complex disease characterized by chronic recurring eczema and pruritus. In addition, patients with AD display increased cutaneous and systemic levels of oxidative damage markers, whose source remains elusive. In this study, we investigated oxidative and mitochondrial stress in AD epidermis. The levels of superoxide dismutase 2 and hydrogen peroxide are augmented in the mitochondria of flaky tail (ft/ft) mouse keratinocytes, which is associated with the inhibition of the glutathione system and catalase. Furthermore, reduced levels of glutathione peroxidase 4 are associated with accumulation of malondialdehyde, 4-hydroxy-2-nonenal, and oxidized phosphatidylcholines in ft/ft epidermis. Cytochrome c is markedly increased in ft/ft epidermis, hence showing mitochondrial stress. Topical application of MitoQ, which is a mitochondrial-targeting antioxidant, to ft/ft mouse skin reduced damage to macromolecules and inflammation and restored epidermal homeostasis. Absence of alteration in the expression of superoxide dismutase 2, catalase, and glutathione peroxidase 4 and limited lipid peroxidation as well as oxidized phosphatidylcholines in the epidermis of Flg-/- mice suggest that FLG deficiency marginally contributes to oxidative stress in ft/ft epidermis. Increased superoxide dismutase 2, lipid peroxidation, and cytochrome c in the epidermis of patients with AD, associated with reduced antioxidant response in primary AD keratinocytes, corroborate mitochondrial dysfunction and lack of cellular adjustment to oxidative stress in AD epidermis.

Ursolic acid ameliorates DNCB-induced atopic dermatitis-like symptoms in mice by regulating TLR4/NF-κB and Nrf2/HO-1 signaling pathways

BACKGROUND

Ursolic acid (UA) is a triterpenoid compound found in natural plants. It has been reported to have anti-inflammatory, antioxidant, and immunomodulatory properties. However, its role in atopic dermatitis (AD) is unknown. This study aimed to evaluate the therapeutic effect of UA in AD mice and explore the underlying mechanisms.

METHODS

Balb/c mice were treated with 2, 4-dinitrochlorobenzene (DNCB) to induce AD-like lesions. During modeling and medication administration, dermatitis scores and ear thickness were measured. Subsequently, histopathological changes, levels of T helper cytokines, and oxidative stress markers levels were evaluated. Immunohistochemistry staining was used to assess changes in the expression of the nuclear factor of kappa B (NF-κB) and NF erythroid 2-related factor 2 (Nrf2). Furthermore, CCK8 assay, reactive oxygen species (ROS) assay, real-time PCR, and western blotting were employed to evaluate the effects of UA on ROS levels, inflammatory mediator production, and the NF-κB and Nrf2 pathways in TNF-α/IFN-γ-stimulated HaCaT cells.

RESULTS

The results showed that UA significantly reduced dermatitis score and ear thickness, effectively inhibited skin proliferation and mast cell infiltration in AD mice, and decreased the expression level of T helper cytokines. Meanwhile, UA improved oxidative stress in AD mice by regulating lipid peroxidation and increasing the activity of antioxidant enzymes. In addition, UA inhibited ROS accumulation and chemokine secretion in TNF-α/IFN-γ-stimulated HaCaT cells. It might exert anti-dermatitis effects by inhibiting the TLR4/NF-κB pathway and activating the Nrf2/HO-1 pathway.

CONCLUSION

Taken together, our results suggest that UA may have potential therapeutic effects on AD and could be further studied as a promising drug for AD treatment.

NRF2 in the Epidermal Pigmentary System

Melanogenesis is a major part of the environmental responses and tissue development of the integumentary system. The balance between reduction and oxidation (redox) governs pigmentary responses, for which coordination among epidermal resident cells is indispensable. Here, we review the current understanding of melanocyte biology with a particular focus on the "master regulator" of oxidative stress responses (i.e., the Kelch-like erythroid cell-derived protein with cap'n'collar homology-associated protein 1-nuclear factor erythroid-2-related factor 2 system) and the autoimmune pigment disorder vitiligo. Our investigation revealed that the former is essential in pigmentogenesis, whereas the latter results from unbalanced redox homeostasis and/or defective intercellular communication in the interfollicular epidermis (IFE). Finally, we propose a model in which keratinocytes provide a "niche" for differentiated melanocytes and may "imprint" IFE pigmentation.

The Role of KEAP1-NRF2 System in Atopic Dermatitis and Psoriasis

The Kelch-like erythroid cell-derived protein with cap‘n’collar homology-associated protein 1 (KEAP1)-nuclear factor erythroid-2-related factor 2 (NRF2) system, a thiol-based sensor-effector apparatus, exerts antioxidative and anti-inflammatory effects and maintains skin homeostasis. Thus, NRF2 activation appears to be a promising treatment option for various skin diseases. However, NRF2-mediated defense responses may deteriorate skin inflammation in a context-dependent manner. Atopic dermatitis (AD) and psoriasis are two common chronic inflammatory skin diseases caused by a defective skin barrier, dysregulated immune responses, genetic predispositions, and environmental factors. This review focuses on the role of the KEAP1-NRF2 system in the pathophysiology of AD and psoriasis and the therapeutic approaches that utilize this system.

Activation of the transcription factor NRF2 mediates the anti-inflammatory properties of a subset of over-the-counter and prescription NSAIDs

Nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit cyclooxygenase (COX) enzymes and are ubiquitously used for their anti-inflammatory properties. However, COX inhibition alone fails to explain numerous clinical outcomes of NSAID usage. Screening commonly used NSAIDs in primary human and murine myeloid cells demonstrated that NSAIDs could be differentiated by their ability to induce growth/differentiation factor 15 (GDF15), independent of COX specificity. Using genetic and pharmacologic approaches, NSAID-mediated GDF15 induction was dependent on the activation of nuclear factor erythroid 2-related factor 2 (NRF2) in myeloid cells. Sensing by Cysteine 151 of the NRF2 chaperone, Kelch-like ECH-associated protein 1 (KEAP1) was required for NSAID activation of NRF2 and subsequent anti-inflammatory effects both in vitro and in vivo. Myeloid-specific deletion of NRF2 abolished NSAID-mediated tissue protection in murine models of gout and endotoxemia. This highlights a noncanonical NRF2-dependent mechanism of action for the anti-inflammatory activity of a subset of commonly used NSAIDs.

Loricrin at the Boundary between Inside and Outside

Cornification is a specialized mode of the cell-death program exclusively allowed for terrestrial amniotes. Recent investigations suggest that loricrin (LOR) is an important cornification effector. As the connotation of its name (“lorica” meaning an armor in Latin) suggests, the keratin-associated protein LOR promotes the maturation of the epidermal structure through organizing covalent cross-linkages, endowing the epidermis with the protection against oxidative injuries. By reviewing cornification mechanisms, we seek to classify ichthyosiform dermatoses based on their function, rather than clinical manifestations. We also reviewed recent mechanistic insights into the Kelch-like erythroid cell-derived protein with the cap “n” collar homology-associated protein 1/nuclear factor erythroid 2-related factor 2 (NRF2) signaling pathway in skin health and diseases, as LOR and NRF2 coordinate the epidermis-intrinsic xenobiotic metabolism. Finally, we refine the theoretical framework of cross-talking between keratinocytes and epidermal resident leukocytes, dissecting an LOR immunomodulatory function.

The Complicated Role of Nuclear Factor Erythroid-Derived 2-Like 2 in Allergy and Asthma

Nuclear factor erythroid-derived 2-like 2 (Nrf2) is a stress-activated transcription factor that is highly responsive to oxidative stress and electrophilic stimuli. Upon activation, Nrf2 upregulates a battery of cytoprotective genes meant to prevent cell death or damage. In many models of inflammation, Nrf2 protects against the immune response and decreases injury, including in the context of asthma and allergy. However, in some models of asthma and allergy, Nrf2 either does not play a role or can even exacerbate inflammation. In general, the reasons behind these discrepancies are not clear and the mechanisms by which Nrf2 modulates immune response are largely uncharacterized. The aim of this review is to highlight current literature assessing the role of Nrf2 in allergy and asthma to understand Nrf2 as a potential therapeutic target. SIGNIFICANCE STATEMENT: Nuclear factor erythroid-derived 2-like 2 (Nrf2) is an important immune mediator that modulates numerous immune cell types in various inflammatory diseases, including allergy and asthma. There is considerable interest in Nrf2 as a drug target in inflammation, which is complicated by the complex nature of Nrf2 in the immune system. This review focuses on the role of Nrf2 in asthma and allergy, including in regulating immune cell function and in detoxifying xenobiotics that exacerbate these diseases.

Aryl Hydrocarbon Receptor/nuclear factor E2-related factor 2 (AHR/NRF2) Signaling: A Novel Therapeutic Target for Atopic Dermatitis

Aryl hydrocarbon receptor (AHR)/nuclear factor-erythroid 2-related factor 2 (NRF2) modulation are emerging as novel targets in the treatment of atopic dermatitis and other inflammatory skin disorders. Agonist activation of this pathway has downstream effects on epidermal barrier function, immunomodulation, oxidative stress reduction, and cutaneous microbiome modulation. Tapinarof, a dual agonist of the AHR/NRF2 signaling pathway, has shown promise in phase 2 trials for atopic dermatitis. In this review, we summarize current knowledge of the AHR/NRF2 pathway and implications in skin disease process. We also review the therapeutic potential of current AHR agonists and propose future directions to address knowledge gaps.

Loricrin and NRF2 Coordinate Cornification

Cornification involves cytoskeletal cross-linkages in corneocytes (the brick) and the secretion of lipids/adhesion structures to the interstitial space (the mortar). Because the assembly of lipid envelopes precedes corneocyte maturation, loricrin is supposed to be dispensable for the protection against desiccation. Although the phenotypes of Lor knockout (LKO) mice are obscure, the antioxidative response on the KEAP1/NRF2 signaling pathway compensates for the structural defect in utero. In this study, we asked how the compensatory response is evoked after the defects are repaired. To this end, the postnatal phenotypes of LKO mice were analyzed with particular attention to the permeability barrier function primarily maintained by the mortar. Ultrastructural analysis revealed substantially thinner cornified cell envelopes and increased numbers of lamellar granules in LKO mice. Superficial epidermal damages triggered the adaptive repairing responses that evoke the NRF2-dependent upregulation of genes associated with lamellar granule secretion in LKO mice. We also found that corneodesmosomes are less degraded in LKO mice. The observation suggests that loricrin and NRF2 are important effectors of cornification, in which proteins need to be secreted, cross-linked, and degraded in a coordinated manner.

The Epidermis: Redox Governor of Health and Diseases

A functional epithelial barrier necessitates protection against dehydration, and ichthyoses are caused by defects in maintaining the permeability barrier in the stratum corneum (SC), the uppermost protective layer composed of dead cells and secretory materials from the living layer stratum granulosum (SG). We have found that loricrin (LOR) is an essential effector of cornification that occurs in the uppermost layer of SG (SG1). LOR promotes the maturation of corneocytes and extracellular adhesion structure through organizing disulfide cross-linkages, albeit being dispensable for the SC permeability barrier. This review takes psoriasis and AD as the prototype of impaired cornification. Despite exhibiting immunological traits that oppose each other, both conditions share the epidermal differentiation complex as a susceptible locus. We also review recent mechanistic insights on skin diseases, focusing on the Kelch-like erythroid cell-derived protein with the cap "n" collar homology-associated protein 1/NFE2-related factor 2 signaling pathway, as they coordinate the epidermis-intrinsic xenobiotic metabolism. Finally, we refine the theoretical framework of thiol-mediated crosstalk between keratinocytes and leukocytes in the epidermis that was put forward earlier.

Cutaneous Squamous Cell Carcinoma in the Age of Immunotherapy

Cutaneous squamous cell carcinoma (cSCC) is the second most prevalent skin cancer globally. Because most cSCC cases are manageable by local excision/radiotherapy and hardly become life-threatening, they are often excluded from cancer registries in most countries. Compared with cutaneous melanoma that originates from the melanin-producing, neural crest-derived epidermal resident, keratinocyte (KC)-derived cancers are influenced by the immune system with regards to their pathogenetic behaviour. Congenital or acquired immunosurveillance impairments compromise tumoricidal activity and raises cSCC incidence rates. Intriguingly, expanded applications of programmed death-1 (PD-1) blockade therapies have revealed cSCC to be one of the most amenable targets, particularly when compared with the mucosal counterparts arisen in the esophagus or the cervix. The clinical observation reminds us that cutaneous tissue has a peculiarly high immunogenicity that can evoke tumoricidal recall responses topically. Here we attempt to redefine cSCC biology and review current knowledge about cSCC from multiple viewpoints that involve epidemiology, clinicopathology, molecular genetics, molecular immunology, and developmental biology. This synthesis not only underscores the primal importance of the immune system, rather than just a mere accumulation of ultraviolet-induced mutations but also reinforces the following hypothesis: PD-1 blockade effectively restores the immunity specially allowed to exist within the fully cornified squamous epithelium, that is, the epidermis.

"Structural imprinting" of the cutaneous immune effector function

Keratinization provides tolerance to desiccation and mechanical durability. Loricrin, which is an epidermal thiol-rich protein, efficiently stabilizes terminally differentiated keratinocytes and maintains redox homeostasis. The discovery of the largely asymptomatic loricrin knockout (LKO) phenotype decades ago was rather unpredicted. Nevertheless, when including redox-driven, NF-E2-related factor 2-mediated backup responses, LKO mice provide opportunities for the observation of altered or "quasi-normal" homeostasis. Specifically, given that the tissue structure, as well as the local metabolism, transmits immunological signals, we sought to dissect the consequence of truncated epidermal differentiation program from immunological perspectives. Through a review of the aggregated evidence, we have attempted to generate an integrated view of the regulation of the peripheral immune system, which possibly occurs within the squamous epithelial tissue with truncated differentiation. This synthesis might not only provide insights into keratinization but also lead to the identification of factors intrinsic to the epidermis that imprint the immune effector function.

Honey and Chamomile Activate Keratinocyte Antioxidative Responses via the KEAP1/NRF2 System

Introduction

The stratum corneum protects against the entry of pathogens, allergens, and irritants while preventing dehydration. The Kelch-like erythroid cell-derived protein with cap-n-collar homology-associated protein 1 (KEAP1)/NF-E2-related factor 2 (NRF2) system maintains skin barrier homeostasis. Aggregated evidence suggests that NRF2-mediated antioxidative response is hardwired into the stratified squamous epithelia. Honey and chamomile have long been regarded as natural antioxidants. Nonetheless, it is still unclear whether they activate the KEAP1/NRF2 system in the epidermis and could promote epidermal barrier recovery.

Methods

To address the abovementioned issue, we explored the antioxidative property of honey/chamomile extract by using non-cell-based KEAP1-inhibition assay and cultured human epidermal keratinocytes.

Results

Herein we report that the extract inhibited KEAP1-NRF2 interaction and induced keratinocyte production of antioxidant small proline-rich protein.

Conclusion

Our results may offer an opportunity to develop cosmetic products that boost NRF2-mediated antioxidative/antiaging, epidermis-intrinsic bio-responses.

The KEAP1/NRF2 Signaling Pathway in Keratinization

Keratinization is a tissue adaptation, but aberrant keratinization is associated with skin disorders such as ichthyoses, atopic dermatitis, psoriasis, and acne. The disease phenotype stems from the interaction between genes and the environment; therefore, an understanding of the adaptation machinery may lead to a new appreciation of pathomechanisms. The KEAP1/NRF2 signaling pathway mediates the environmental responses of squamous epithelial tissue. The unpredicted outcome of the Keap1-null mutation in mice allowed us to revisit the basic principle of the biological process of keratinization: sulfur metabolism establishes unparalleled cytoprotection in the body wall of terrestrial mammals. We summarize the recent understanding of the KEAP1/NRF2 signaling pathway, which is a thiol-based sensor-effector apparatus, with particular focuses on epidermal differentiation in the context of the gene-environment interaction, the structure/function principles involved in KEAP1/NRF2 signaling, lessons from mouse models, and their pathological implications. This synthesis may provide insights into keratinization, which provides physical insulation and constitutes an essential innate integumentary defense system.