Fig 2: Schematic illustration of AD pathways induced by IL-4 and IL-13. IL-4 and IL-13 have partly shared receptor systems. Corneocytes express the IL-4Rα/IL-13Rα1 complex. IL-4 and IL-13 share IL-4Rα/IL-13Rα1 in NHEKs, activate the JAK1/JAK2/TYK2-STAT6 and -STAT3 pathways and inhibit the expression of EDC molecules such as FLG, LOR and IVL. NHEKs also express IL-13Rα2, a prey receptor for IL-13. IL-13Rα2 binds to IL-13 with high affinity but does not transmit a signal [13,14,15].

Fig 3: PM2.5 inhibits FLG through the MAPK/c-JNK pathway.Gene expression of FLG was examined using real-time RT-PCR in cultured HEKs stimulated with PM2.5 or combinations of each inhibitor and PM2.5. (A) NF-κB inhibitor. (B) ERK1/2 inhibitor. (C) P38 inhibitor. (D) SP600125 inhibitor. Data are representative of 3 independent experimental repetitions. The data are shown as the mean ± SEM. n = 3 per group. **P < 0.01 by 1-way ANOVA with Tukey-Kramer post hoc test.

Fig 4: PM2.5 inhibits FLG and causes skin barrier dysfunction in murine skin.Hairless mice were treated with a vehicle, PM2.5, R-7050, or a combination of PM2.5 and R-7050 on the back of each mouse twice daily for 10 days. FITC-dextran was applied to the left side of the back of each mouse for 60 minutes on day 10 and demonstrated enhanced barrier penetration of the PM2.5-treated skin. (A) Skin appearance and H&E staining (original magnification, ×100) of the skin biopsy samples in the study groups. Epidermal thickness (B) and TEWL (C) were evaluated and illustrated increased epidermal thickness and TEWL in PM2.5-exposed skin. (D) The penetration of FITC-dextran is enhanced in PM2.5-treated skin and is attenuated by TNF-α inhibitors. Protein expressions of FLG (E and F), TNF-α (G and H), and AHR (I and J) were evaluated using immunofluorescence staining. Arrows point to FLG staining (red). The dotted line represents a border between epidermis and dermis. Wheat germ agglutinin–conjugated FITC (green) was used to stain the cytoskeleton. Nuclei were visualized with DAPI (blue). Data are representative of 2 independent experiments. The data are shown as the mean ± SEM. Each point indicates individual mice, n = 7 mice per group. Scale bar: 25 μm. *P < 0.05, **P < 0.01, ***P < 0.001 by 1-way ANOVA with Tukey-Kramer post hoc test.

Fig 5: PM2.5-induced TNF-α inhibits FLG expression in human primary keratinocytes.Gene expression of FLG (A and B) was examined in cultured HEKs using real-time RT-PCR. Protein expression of FLG (C and D) was evaluated using Western blotting. Data are representative of 3 independent experimental repetitions. The data are shown as the mean ± SEM. n = 3 per group. *P < 0.05, **P < 0.01 by 1-way ANOVA with Tukey-Kramer post hoc test.