Discussion

AD is a chronic inflammatory skin disease characterized by epidermal barrier dysfunction, which results from excessive inflammatory responses and leads to continuous exposure to environmental allergens.26,27 This creates a vicious cycle in AD pathogenesis that contributes to aggravating skin barrier deficiency.28 Therefore, rehabilitating skin barrier dysfunction induced by AD is increasingly recognized as a promising strategy for AD treatment.29 Former studies attributed AD-induced keratinocyte differentiation block mainly to a Th2 cytokine milieu in AD skin lesions.30 However, the AD microenvironment is a complicated ecosystem, and a variety of differences have been found to be involved in disease progression.30 Recently, high NaCl has been demonstrated to be a specific characteristic of skin lesions in AD.30 NaCl is a well-recognized immune modulating ion that can drive autoimmune disease by inducing pathogenic Th17 cells and dendritic cells via the activation of the p38/MAPK pathway and exert profound influence on the development and function of transforming growth factor-beta (TGF-β)-induced CD4+ Foxp3+ regulatory T cells.31, 32, 33 A recent study showed that high salt in the AD microenvironment exacerbated Th2 cell differentiation and Th2 cytokine production via an NFAT5-dependent mechanism.16,34 Another study reported that knockdown of the sodium channel Nax could alleviate skin inflammation and improve epidermal barrier function, suggesting that NaCl may influence keratinocyte differentiation.35 Restoring skin barrier function is essential for the treatment of AD and can be achieved through various approaches, such as the use of topical agents (e.g., emollients or lipid-based products) to repair the skin barrier and reduce water loss. There is also growing interest in the use of probiotics and prebiotics to modulate the skin microbiome and restore skin barrier function. Probiotics and prebiotics have been shown to improve skin hydration, reduce TEWL, and increase the expression of FLG and LOR.36 Another approach is to use immunomodulatory agents that target specific immune pathways that contribute to the development of AD. Several biological therapies targeting IgE, IL-4, or IL-13 have been approved for the treatment of moderate to severe AD. In addition, studies of the effects of high NaCl on skin barrier function and immune response are helping to advance our understanding of AD. Identifying specific immune pathways and environmental factors that contribute to the pathogenesis and exacerbation of AD will be critical to the development of effective therapies.

In this study, we present evidence that NaCl could not only affect keratinocytes but also disrupt skin barrier function and lead to the production of several inflammatory cytokines. Through further investigation, we have identified the salt-sensitive kinase SGK-1 as a key player in NaCl-induced progression of AD in keratinocytes (see Fig. 7H for a visual representation). NaCl modulates the expression of mTOR family proteins through SGK1 and activates downstream signaling pathways (Fig. S4). Our findings strongly suggest that either inhibiting NaCl uptake by cells or interfering with cellular sensing of NaCl could effectively reduce the detrimental effects of a high-salt microenvironment on skin barrier integrity and AD progression. This critical finding opens up new possibilities for therapeutic interventions that can target NaCl-related mechanisms underlying AD. Interestingly, while NaCl present in the AD microenvironment exacerbates disease progression, our experimental results with NaCl administered via drinking water showed an opposite effect. Indeed, we observed that NaCl intake via drinking water not only failed to promote AD progression but actually provided some relief from AD-like symptoms, as shown in Figure S5. This intriguing finding could be attributed to the regulation of the gut microbiome induced by NaCl intake, as previous research has shown that a high-salt diet can stimulate potent anti-tumor immunity by modulating the microbiota.37 Thus, our findings suggest that AD patients should not deliberately restrict their NaCl intake from dietary sources. Instead, an essential intake of NaCl may potentially offer benefits in alleviating their symptoms. This unexpected finding highlights the complex interplay between NaCl, the gut microbiome, and the pathophysiology of AD and calls for further investigation to fully understand these intricate interactions.

Keratinocytes, the major cell type in the epidermis, undergo a highly regulated process of differentiation to form a structured hierarchical epidermal layer.38 However, the specific mechanism by which AD promotes a block in keratinocyte differentiation is only partially understood. Previous studies have shown that the presence of Th2 cytokines in the AD microenvironment leads to the activation of JAK, which in turn phosphorylates STAT3. In addition, IL-33, a cytokine released during AD inflammation, has been shown to promote the phosphorylation of STAT3 by forming a complex with it. Once phosphorylated, STAT3 translocates to the nucleus of keratinocytes and inhibits their differentiation process.18,19,39 In addition, recent research has identified the transcription factor p63 as a critical factor that responds to another cytokine, IL-4/IL-13, present in the AD microenvironment. This interaction between IL-4/IL-13 and p63 leads to an inhibition of the differentiation of keratinocytes, further contributing to the skin barrier dysfunction observed in AD.40 In this study, we present novel evidence implicating the SGK-1–mTOR–STAT3 signaling pathway in the development of skin barrier dysfunction in AD. We show that SGK-1, a serine/threonine kinase, contributes significantly to the activation of STAT3 and the subsequent impairment of keratinocyte differentiation. Importantly, we identify an SGK-1 inhibitor as a potential therapeutic option for AD. Unlike JAK inhibitors that target only the JAK–STAT pathway, the SGK-1 inhibitor acts independently of JAK activation, suggesting that it could be used in combination with JAK inhibitors to achieve a more satisfactory clinical outcome. Our combination therapy using both the SGK-1 inhibitor and JAK inhibitors shows promising results in murine models of AD. It effectively alleviates AD-like symptoms in murine models and has great potential for future clinical use in AD patients. In conclusion, our study sheds light on the intricate mechanisms underlying the blockade of differentiation and inflammatory cytokine production of keratinocytes induced by a high-salt microenvironment in AD. The discovery of the SGK-1–mTOR–STAT3 pathway and the potential of an SGK-1 inhibitor as a therapeutic modality provide new avenues for the management and treatment of AD.

Taken together, in this study, we have discovered that within the context of AD, a high-NaCl microenvironment restrains the differentiation of keratinocytes, exacerbates the generation of inflammatory cytokines, and advances the progression of the disease through the activation of the SGK-1–mTOR pathway. Furthermore, we have successfully identified an inhibitor of SGK-1 that can effectively overturn the impediment to keratinocyte differentiation and cytokine production induced by NaCl, both in vitro and in vivo. Additionally, when combined with a JAK inhibitor, this inhibitor exhibits a synergistic effect in alleviating the symptoms of AD.