Regulation of Filaggrin, Loricrin, and Involucrin by IL-4, IL-13, IL-17A, IL-22, AHR, and NRF2: Pathogenic Implications in Atopic Dermatitis

Atopic dermatitis (AD) is an eczematous, pruritic skin disorder with extensive barrier dysfunction and elevated interleukin (IL)-4 and IL-13 signatures. The barrier dysfunction correlates with the downregulation of barrier-related molecules such as filaggrin (FLG), loricrin (LOR), and involucrin (IVL). IL-4 and IL-13 potently inhibit the expression of these molecules by activating signal transducer and activator of transcription (STAT)6 and STAT3. In addition to IL-4 and IL-13, IL-22 and IL-17A are probably involved in the barrier dysfunction by inhibiting the expression of these barrier-related molecules. In contrast, natural or medicinal ligands for aryl hydrocarbon receptor (AHR) are potent upregulators of FLG, LOR, and IVL expression. As IL-4, IL-13, IL-22, and IL-17A are all capable of inducing oxidative stress, antioxidative AHR agonists such as coal tar, glyteer, and tapinarof exert particular therapeutic efficacy for AD. These antioxidative AHR ligands are known to activate an antioxidative transcription factor, nuclear factor E2-related factor 2 (NRF2). This article focuses on the mechanisms by which FLG, LOR, and IVL expression is regulated by IL-4, IL-13, IL-22, and IL-17A. The author also summarizes how AHR and NRF2 dual activators exert their beneficial effects in the treatment of AD.


Roles of IVL, LOR, FLG, and FLG2 in Epidermal Barrier Formation
IVL has high structural homology with LOR in the glutamine-and lysine-rich amino-and carboxy-terminal domains [5]. IVL is expressed in the upper spinous layer, but mainly in the granular layers, and is involved in the initial step of cornified envelope formation. Cornified envelope formation starts from desmosomes where IVL is crosslinked with envoplakin, periplakin, and keratin filaments by transglutaminase 1 [5,6]. This protein complex also becomes the scaffold for the corneocyte-bound lipid envelope [6,10].
LOR is the most abundant component of the cornified envelope [1,3,5]. It is very hydrophobic, insoluble, and is easily polymerized via disulfide crosslinking in ambient air, making it suitable as a protein that reinforces the cornified envelope [1,5]. LOR is expressed in the granular layer and is crosslinked to IVL, envoplakin, and periplakin scaffolds by transglutaminase 1 [1,5].
Profilaggrin consists of a conserved small N-terminal domain, 10-12 FLG repeats and a C-terminal domain [5]. Profilaggrin to FLG processing requires several proteases, such as profilaggrin endopeptidase 1, matriptase 1, and channel-activating protease 1. FLG is involved in aggregating the K1 and K10 filaments into higher-molecular-weight parallel structures that facilitate the incorporation of K1 and K10 into the cornified envelope and contribute to the thin granular keratinocyte shape [1,5,53]. FLG peptides are simultaneously degraded by caspase 14 and calpain 1 into free hydrophilic amino acids, which maintain the intracellular water content [1,5,6]. Ichthyosis vulgaris is caused by the loss-of-function mutation of FLG [54]. Loss-of-function mutations of FLG have been demonstrated in a subpopulation of patients with AD, at rates ranging from 10% to 50% depending on the ethnicity [55,56]. Therefore, AD is a significant comorbidity with ichthyosis vulgaris [57,58].
FLG2 contains two distinct repeat domains, A and B. The A domain presents high homology with hornerin repeats and the B domain is homologous to FLG [5,59]. FLG2 is also expressed in the keratohyalin granules in the granular layer [5,59]. The expression of FLG and FLG2 is downregulated in skin treated with 5% or 10% lactic acid, with such downregulation often used to define sensitive skin [60]. The expression of FLG and FLG2 has also been reported to be downregulated by tape stripping [61]. In a three-dimensional reconstituted human epidermis model, FLG2 downregulation was found to induce parakeratosis, compact stratum corneum, increased pH, and reduced amounts of free amino acids with reduced proteolytic processing of corneodesmosin, hornerin, and filaggrin in parallel with reduced amounts of caspase-14 [62]. In another report, the expression of FLG2 was described as being colocalized with corneodesmosin in the cornified cells [63]. The absence of FLG2 induces the marked reduction of corneodesmosin expression [63]. Thus, FLG2 exerts a specialized function different from FLG.
Under physiological conditions, IVL is detected from the uppermost spinous layer to granular layer keratinocytes (early-phase epidermal terminal differentiation), but the expression of LOR, FLG, and FLG2 is more confined to granular cell layer keratinocytes (late-phase epidermal terminal differentiation) [5,[64][65][66].

Upregulation of IVL, LOR, and FLG by AHR Activation
As a chemical sensor, AHR is one of the major transcription factors for EDC genes in keratinocytes. It was originally called dioxin receptor because environmental pollutants such as polycyclic aromatic hydrocarbons and dioxins bind to it with high affinity and generate ROS production [27][28][29][67][68][69]. In the absence of ligands, AHR resides in the cytoplasm, where it forms a protein complex with heat shock protein 90 (HSP90), hepatitis B virus X-associated protein 2 (XAP-2, also known as AIP or Ara9), p23, and c-Src protein kinase [70][71][72]. After ligand binding, AHR dissociates from the cytoplasmic complex and a nuclear translocation site of AHR is exposed. Then, AHR is translocated into the nucleus where it dimerizes with AHR-nuclear translocator (ARNT), binds DNA responsive elements called xenobiotic responsive elements (XREs), and upregulates the transcription of target genes, such as phase I metabolizing enzyme cytochrome P450 (CYP) members (i.e., CYP1A1, CYP1A2, and CYP1B1) [73,74] ( Figure 2). Under physiological conditions, IVL is detected from the uppermost spinous layer to granular layer keratinocytes (early-phase epidermal terminal differentiation), but the expression of LOR, FLG, and FLG2 is more confined to granular cell layer keratinocytes (late-phase epidermal terminal differentiation) [5,[64][65][66].

Upregulation of IVL, LOR, and FLG by AHR Activation
As a chemical sensor, AHR is one of the major transcription factors for EDC genes in keratinocytes. It was originally called dioxin receptor because environmental pollutants such as polycyclic aromatic hydrocarbons and dioxins bind to it with high affinity and generate ROS production [27][28][29][67][68][69]. In the absence of ligands, AHR resides in the cytoplasm, where it forms a protein complex with heat shock protein 90 (HSP90), hepatitis B virus X-associated protein 2 (XAP-2, also known as AIP or Ara9), p23, and c-Src protein kinase [70][71][72]. After ligand binding, AHR dissociates from the cytoplasmic complex and a nuclear translocation site of AHR is exposed. Then, AHR is translocated into the nucleus where it dimerizes with AHR-nuclear translocator (ARNT), binds DNA responsive elements called xenobiotic responsive elements (XREs), and upregulates the transcription of target genes, such as phase I metabolizing enzyme cytochrome P450 (CYP) members (i.e., CYP1A1, CYP1A2, and CYP1B1) [73,74] (Figure 2).

Figure 2.
There are many physiological or salubrious aryl hydrocarbon receptor (AHR) ligands such as tryptophan photoproduct 6-formylindolo [3,2-b] carbazole (FICZ), microbial products from Malassezia and Staphylococcus epidermidis, phytochemicals and drugs. AHR activated by ligands translocates into the nucleus and is heterodimerized with AHR-nuclear translocator (ARNT). The ligand-AHR-ARNT complex binds XRE regions with p300 cofactor and upregulates the transcription of target genes and associated protein expression, including for CYP1A1, OVOL1, filaggrin (FLG), loricrin (LOR), and involucrin (IVL). Cytoplasmic OVOL1 translocates into the nucleus and contributes to the upregulation of FLG and LOR, but not that of IVL. The effects of physiological or salubrious AHR ligands are transient because they are rapidly metabolized or degraded by CYP1A1. In contrast, hazardous AHR ligands such as polycyclic aromatic hydrocarbons, dioxins and benzo(a)pyrene are stable and long-lasting in the body because they are not easily metabolized by CYP1A1. The exaggerated AHR activation converts sebocyte differentiation from sebaceous cell differentiation to keratinocytic differentiation. This results in chloracne characterized by the loss of There are many physiological or salubrious aryl hydrocarbon receptor (AHR) ligands such as tryptophan photoproduct 6-formylindolo [3,2-b] carbazole (FICZ), microbial products from Malassezia and Staphylococcus epidermidis, phytochemicals and drugs. AHR activated by ligands translocates into the nucleus and is heterodimerized with AHR-nuclear translocator (ARNT). The ligand-AHR-ARNT complex binds XRE regions with p300 cofactor and upregulates the transcription of target genes and associated protein expression, including for CYP1A1, OVOL1, filaggrin (FLG), loricrin (LOR), and involucrin (IVL). Cytoplasmic OVOL1 translocates into the nucleus and contributes to the upregulation of FLG and LOR, but not that of IVL. The effects of physiological or salubrious AHR ligands are transient because they are rapidly metabolized or degraded by CYP1A1. In contrast, hazardous AHR ligands such as polycyclic aromatic hydrocarbons, dioxins and benzo(a)pyrene are stable and long-lasting in the body because they are not easily metabolized by CYP1A1.
Loertscher et al. were the first to demonstrate that exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces the premature or accelerated terminal differentiation of epidermal keratinocytes with upregulated expression of IVL, LOR, and FLG in three-dimensional skin equivalent models and in vivo models [65,75]. Their findings were later confirmed by Sutter's group [76]. In monolayer culture, the effects of AHR activation are apparent in human keratinocytes under high-calcium or high-cell-density conditions, in which keratinocytes undergo more differentiation than proliferation [77,78]. In addition, AHR signaling is preferentially activated in differentiated rather than in proliferating keratinocytes [79]. Reciprocally, growth-promoting conditions involving treatment with epidermal growth factor or transforming growth factor α attenuate the TCDD-AHR/ARNT-mediated CYP1A1 and FLG expression [16,78]. The recruitment of nuclear factor p300 plays an essential role in the AHR/ARNT-mediated transcription of target genes [78]. Epidermal growth factor receptor signaling also requires p300 for its proper activity. The competitive usage of p300 by epidermal growth factor receptor activation leads to repression of the recruitment of p300 to the AHR/ARNT transcriptional complex, and eventually suppresses the AHR/ARNT activity to induce transcription of the target gene CYP1A1 [78] ( Figure 2).
Activation of the AHR/ARNT system by TCDD is known to enhance the expression of IVL, LOR, FLG, and FLG2 genes, as well as many other EDC genes, such as hornerin, repetin, small proline-rich protein 2A, late cornified envelope protein 3A, and S100A7 [4,16,65]. The coordinated upregulation of EDC gene products by AHR/ARNT activation highlights the essential involvement of the AHR/ARNT system in epidermal terminal differentiation and skin barrier function. Moreover, AHR antagonists, GNF351 and CH223191, have been shown to inhibit FLG and IVL expression [79]. In addition, TCDD-induced AHR/ARNT activation increases the expression of 75% of the genes required for de novo ceramide biosynthesis, leading to the overproduction of ceramides 1-7 and 9 without affecting the levels of cholesterol and free fatty acids [4]. Moreover, the increased production of the cornified envelope by TCDD is blocked in the presence of antioxidative agents, indicating the important role of oxidative stress in the TCDD-AHR/ARNT-induced acceleration of epidermal terminal differentiation [4]. TCDD-induced ROS production is AHR-and CYP1A1-dependent because TCDD-induced ROS production was shown to be inhibited in AHR-silenced or CYP1A1-silenced cells [80].
Under physiological conditions, endogenous AHR ligands, such as tryptophan photoproducts [15,26,83] and microbial bioproducts [17,19,87], may upregulate the expression of EDC genes via AHR activation and maintain the healthy epidermal barrier. In line with this, the expression of FLG, LOR, and IVL is downregulated in keratinocytes with AHR knockdown or in the presence of AHR antagonists during the terminal differentiation of keratinocytes [79]. However, the effects of endogenous AHR ligands are transient because these ligands are rapidly degraded by AHR-induced CYP1A1 [83,96]. In contrast, TCDD and other dioxins are likely to induce long-lasting and exaggerated activation of AHR because these agents are chemically stable and resistant to degradation via CYP1A1 [27,28]. The high-level activation of AHR accelerates the keratinization process of sebocytes and keratinoctytes, leading to chloracne [27][28][29][30]97].
The expression of INV and LOR was also found to be decreased in the lesional and nonlesional skin of AD [31,110]. Abnormal skin barrier integrity also causes the increased colonization of microbes such as Staphylococcus aureus, which further exacerbates Th2-deviated skin inflammation [111,112].
Although the strongest genetic risk factors for AD are loss-of-function mutations in the FLG gene [95], FLG mutations were not found in all AD patients, were less common in Southern Europeans with AD [113] and were even absent in patients with AD from some African countries [114], suggesting that FLG mutations only partly explain FLG protein downregulation in AD. Moreover, FLG mutation was shown not to be related to the development of AD in patients from a subtropical island in Japan [115].
Notably, IL-4 and IL-13 are known to decrease the FLG [20,25,26,31,110,116], LOR [110], and IVL [31,110] expression in vitro. Therefore, a Th2-polarized inflammatory milieu in AD may be more influential in the downregulation of FLG expression than loss-of-function mutation of the FLG gene [91,106,117]. The pathogenic importance of IL-4 and IL-13 has recently been reinforced by the excellent treatment response of patients with AD to the anti-IL-4 receptor α (IL-4Rα, IL4R) antibody dupilumab, which inhibits both IL-4 and IL-13 signals [44,118]. More recently, large-scale transcriptomic analysis revealed the specific and dominant role of IL-13 in the lesional skin of AD because IL-4 expression was nearly undetectable [119]. Consistent with this notion, the anti-IL-13 antibody tralokinumab was shown to successfully improve AD [120].
In addition to FLG, the expression of LOR, IVL, and FLG2 is downregulated or occurs prematurely in the lesional and nonlesional skin of AD compared with their expression in the normal skin of healthy individuals [26,31,110,[121][122][123][124]. In line with these reports, topical steroids significantly improve clinical inflammatory signs and normalize transepidermal water loss in lesional AD skin with the upregulation of FLG and LOR expression [125]. These improvements are associated with downregulation of the Th2 (IL-13 and IL-31) signature [125]. Similar results have been obtained in dupilumab treatment. The expression of FLG and LOR is decreased in the lesional skin compared with that in the nonlesional skin in AD [44]. Dupilumab, but not placebo, restores the downregulation of FLG and LOR [44]. It is also known that the expression of FLG, FLG2, and LOR is downregulated in the patch test site of paraphenylenediamine in patients with allergic contact dermatitis [126].
It is still controversial whether IL-4/IL-13-mediated STAT6 activation is fully responsible for the downregulation of EDC molecules [127]. In Stat6 transgenic mice, the cutaneous expression of Lor and Ivl was shown to be significantly decreased compared with that in control wild-type mice [110]. In contrast, the permeability barrier function was found to be upregulated in Stat6-deficient mice with significantly increased Lor expression [139]. Moreover, the Flg and Ivl expression tended to be upregulated in Stat6-deficient mice, but this did not reach statistical significance [139].
Amano et al. highlighted a significant role of the IL-4/IL-13-mediated activation of STAT3, but not STAT6, in the downregulation of FLG and LOR using keratinocytes treated with specific small interfering RNA (siRNA) for STAT3 or STAT6 [127]. IL-4/IL-13-mediated downregulation of FLG and LOR was shown to be canceled in keratinocytes treated with STAT3 siRNA, but not in those with STAT6 siRNA [127]. Notably, IL-4/IL-13-mediated upregulation of CCL26 and CXCL6 expression was canceled in keratinocytes treated with STAT6 siRNA, but not in those with STAT3 siRNA [127]. These results indicate that the IL-4/IL-13-mediated activation of STAT3 transmits signals different from those by the IL-4/IL-13-mediated activation of STAT6, and that IL-4/IL-13-mediated STAT3 activation is probably responsible for the downregulation of EDC molecules in keratinocytes [127]. As both IL-4Rα/γc and IL-4Rα/IL-13Rα1 require the JAK family for their activation, different kinds of JAK inhibitors potently restore the IL-4/IL-13-mediated downregulation of EDC molecules (IVL, After ligation by IL-4, IL-4Rα/γc heterodimer activates Janus kinase 1 (JAK1) and JAK3 and induces the activation (phosphorylation) of signal transducer and activator of transcription (STAT)6 [129,134,135]. IL-4 and IL-13 bind IL-4Rα/IL-13Rα1, activate JAK1, JAK2, and tyrosine kinase 2 (TYK2), and induce the phosphorylation of STAT6 [129,130,136,137]. Although evidence for the STAT6 activation by IL-4 and IL-13 has been consistently found, several groups suggest the possibility that IL-4 and IL-13 activate other STAT family members including STAT1 [128], STAT3 [127,128], and STAT5 [138].
It is still controversial whether IL-4/IL-13-mediated STAT6 activation is fully responsible for the downregulation of EDC molecules [127]. In Stat6 transgenic mice, the cutaneous expression of Lor and Ivl was shown to be significantly decreased compared with that in control wild-type mice [110]. In contrast, the permeability barrier function was found to be upregulated in Stat6-deficient mice with significantly increased Lor expression [139]. Moreover, the Flg and Ivl expression tended to be upregulated in Stat6-deficient mice, but this did not reach statistical significance [139].
Amano et al. highlighted a significant role of the IL-4/IL-13-mediated activation of STAT3, but not STAT6, in the downregulation of FLG and LOR using keratinocytes treated with specific small interfering RNA (siRNA) for STAT3 or STAT6 [127]. IL-4/IL-13-mediated downregulation of FLG and LOR was shown to be canceled in keratinocytes treated with STAT3 siRNA, but not in those with STAT6 siRNA [127]. Notably, IL-4/IL-13-mediated upregulation of CCL26 and CXCL6 expression was canceled in keratinocytes treated with STAT6 siRNA, but not in those with STAT3 siRNA [127]. These results indicate that the IL-4/IL-13-mediated activation of STAT3 transmits signals different from those by the IL-4/IL-13-mediated activation of STAT6, and that IL-4/IL-13-mediated STAT3 activation is probably responsible for the downregulation of EDC molecules in keratinocytes [127]. As both IL-4Rα/γc and IL-4Rα/IL-13Rα1 require the JAK family for their activation, different kinds of JAK inhibitors potently restore the IL-4/IL-13-mediated downregulation of EDC molecules (IVL, LOR, FLG, and FLG2) and improve skin barrier function in vitro and in vivo [127,128].
OVOL1 is an important transcription factor for epidermal terminal differentiation. It resides in the cytoplasm in a steady-state condition, but activated OVOL1 translocates into the nucleus and regulates downstream gene expression [26,90,92]. OVOL1 suppresses c-Myc expression and inhibits keratinocyte proliferation [93,94], but it conversely promotes epidermal differentiation and upregulates the expression of FLG and LOR [22,26,90]. AHR activation upregulates the expression of OVOL1, induces its cytoplasmic-to-nuclear translocation, and increases the expression of FLG and LOR [22,26,90,92]. IL-4 does not affect or rather enhances OVOL1 expression, but it inhibits the cytoplasmic-to-nuclear translocation of OVOL1, which correlates with the downregulation of FLG expression [26,90]. AHR activation restores the IL-4-mediated inhibition of OVOL1 nuclear translocation and recovers the IL-4-induced FLG downregulation [26,90]. Interestingly, AHR activation also upregulates IVL expression, but its regulation is OVOL1-independent [22]. In addition, IL-4 and IL-13 themselves increase the mRNA and protein expression of AHR in B cells and keratinocytes [168,169]. This suggests a mutually compensatory (or seesaw) regulation between Th2 and AHR signaling. Keratinocytes are a rich source of pro-Th2 cytokines such as IL-33 [98]. AHR-mediated OVOL1 activation is also functional in inhibiting IL-33 production in keratinocytes [170].

Downregulation of IVL, LOR, and FLG by IL-22
Increased IL-17A and IL-22 signatures are also shown in AD [44,46,47,99,125,[171][172][173]. However, the pathogenic roles of Th17 and Th22 cell infiltration have not been fully elucidated in Th2-dominant AD. Guttman-Yassky et al. demonstrated that a blockade of Th2 signaling by dupilumab significantly decreased and normalized not only Th2 signatures, but also Th17 and Th22 signatures, in lesional skin of patients suffering from AD [44]. This suggests the possibility that the increased Th17 and Th22 signatures may be associated with Th2 dominance in AD. Initially, IL-22 was thought to be produced from Th17 cells, but recent human studies have revealed that Th22 cells, which do not produce IL-17A, are the main IL-22 producers [174,175]. Notably, IL-22 production from Th17/22, Th22, and innate lymphoid cells is dependent on AHR [176][177][178][179].
Recent clinical trials of the anti-IL-17A antibody secukinumab, a very potent therapeutic agent for treating psoriasis, did not report its satisfactory efficacy against AD (https://clinicaltrials.gov/ct2/show/ results/NCT02594098?term=atopic&cond=secukinumab&draw=2&rank=1). In contrast to IL-17A, IL-22 may exert additional pathogenic effects apart from those mediated by IL-4/IL-13 because the anti-IL-22 antibody fezakinumab shows weak therapeutic potential for treating patients with severe AD [46]. Moreover, fezakinumab is more efficacious for patients with high pretreatment expression of IL-22 than for those with low IL-22 expression [180].
Another important transcription factor for IL-17A signaling is the C/CAAT-enhancer-binding proteins (C/EBPs), particularly C/EBPB or C/EBPD [215]. In contrast, IL-17A is likely to inhibit the C/EBPA molecule [214]. The C/EBP family members are involved in epidermal keratinocyte differentiation [219] and are strongly upregulated in the lesional skin of psoriasis [215]. Together with the elevated CEBPB gene expression, the expression of keratinocyte terminal differentiation genes, such as IVL, FLG2, and TGM1, is upregulated in the lesional skin of psoriasis [215]. In the promoter region of the IVL gene, there is a binding site for C/EBP and the C/EBP transcription factor is necessary for the appropriate and continuous production of IVL protein [220]. In contrast to IVL, IL-17A is reported to downregulate the expression of K10 [51], LOR [51,215], and FLG [138,215,221]. In addition, the expression of S100A7 is upregulated by IL-17A [51,138,215]. Intriguingly, IL-4 and IL-13 are unlikely to affect the S100A7 expression or rather inhibit its expression in keratinocytes [138,222]. Notably, IL-17A is also a potent ROS producer in keratinocytes [192] and recent clinical trials have proven that topical treatment of the antioxidative AHR ligand tapinarof is efficacious for psoriasis [223,224].
Another important transcription factor for IL-17A signaling is the C/CAAT-enhancer-binding proteins (C/EBPs), particularly C/EBPB or C/EBPD [215]. In contrast, IL-17A is likely to inhibit the C/EBPA molecule [214]. The C/EBP family members are involved in epidermal keratinocyte differentiation [219] and are strongly upregulated in the lesional skin of psoriasis [215]. Together with the elevated CEBPB gene expression, the expression of keratinocyte terminal differentiation genes, such as IVL, FLG2, and TGM1, is upregulated in the lesional skin of psoriasis [215]. In the promoter region of the IVL gene, there is a binding site for C/EBP and the C/EBP transcription factor is necessary for the appropriate and continuous production of IVL protein [220]. In contrast to IVL, IL-17A is reported to downregulate the expression of K10 [51], LOR [51,215], and FLG [138,215,221]. In Figure 6. IL-17A has two receptor complexes, IL-17RA/IL-17RC and IL-17RA/IL-17RD, in keratinocytes. Signaling of both receptors occurs via downstream ACT1, TRAF6, and CARMA2 protein complexes, and activates nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and MAPKs. Activation of NF-κB and MAPKs induces cell proliferation and cyto/chemokine production in keratinocytes. IL-17A is not likely to directly activate JAK-STAT pathways. Another important transcription factor for IL-17A signaling is the C/CAAT-enhancer-binding protein β (C/EBPB) or C/EBPD. The IL-17A-C/EBPB pathway is likely to upregulate IVL expression, but to downregulate FLG and LOR expression.

Conclusions
The downregulation of EDC molecules, such as IVL, LOR, FLG, and FLG2, is the cardinal feature of the lesional skin of AD and is associated with skin barrier dysfunction. Although loss-of-function mutation of the FLG gene is the genetic abnormality most frequently associated with AD [95], an IL-4and IL-13-deviated milieu is probably far more important in the pathogenesis of AD when we take into account the excellent efficacy of the anti-IL-4 receptor α antibody dupilumab for AD. IL-4 and IL-13 do inhibit the expression of IVL, LOR, FLG, and FLG2 in keratinocytes in vitro, and the blockade of IL-4 and IL-13 by dupilumab restores the decreased expression of FLG and LOR in the lesional skin of AD.
In addition to IL-4 and IL-13 produced from Th2 cells, IL-22 and IL-17A, produced from Th22 and Th17 cells, are also known to participate in the pathogenesis of AD. IL-4 and IL-13 downregulate the expression of EDC molecules via STAT6 and STAT3 activation. IL-22 also activates STAT3 and inhibits the expression of EDC molecules. IL-22 is probably more potent than IL-17A in downregulating the EDC molecules.
In contrast to cytokine-mediated downregulation, the expression of EDC molecules is upregulated by AHR signaling. In addition to IL-24 [225], IL-4, IL-13, IL-22, and IL-17A are all potent inducers of oxidative stress; therefore, antioxidative AHR ligands with an NRF2-activating profile are expected to be useful for the treatment of AD. Medicinal coal tar and soybean tar glyteer are such AHR and NRF2 dual activators and have been shown to be efficacious in AD. However, these crude agents contain various compounds and have a bad smell. A single chemical compound, tapinarof, is another AHR and NRF2 dual activator, the efficacy for AD of which was recently proven in clinical trials. Of course, exaggerated NRF2 activation seen in the NRF2-transgenic mouse is known to be associated with dysregulated epidermal terminal differentiation [36]. The mechanisms by which EDC molecules are regulated by cytokines, AHR, and NRF2 are not fully understood and there is significant scope for additional investigation. Future studies should open up new strategies for the development of drugs for AD.