Chemical stimuli

2,4-Dinitrofluorobenzene, which was revealed to induce AD-like pathology in mice [13], upregulates IL-24 expression in murine skin [14]. Sodium lauryl sulfate (a chemical irritant in human skin) stimulates IL-24 production in normal human keratinocytes [15]. IL-24 was overexpressed in an allergic contact dermatitis model induced by paraphenylenediamine exposure, in human and mouse skin [16]. IL-24 expression was also increased in imiquimod-stimulated HaCaT cells, an immortalized keratinocyte cell line [17]. Phorbol myristate acetate and ionomycin also induced IL-24 in T helper 2 (Th2) lymphocytes [10].

Some studies determined that IL-24 is the target cytokine of some oxidative environmental AHR agonists, such as 2, 3, 7, 8-tetrachloro-dibenzo-p-dioxin (TCDD), benzo(a)pyrene, particulate matter, and ultraviolet B irradiation, in primary human chorionic stromal cells, human lung adenocarcinoma cells, normal human bronchial epithelial cells, and normal keratinocytes, respectively [15, 18–21]. In line with these findings, we recently demonstrated that tapinarof, an antioxidative AHR modulator [22], stimulated normal human epidermal keratinocytes to generate IL-24 protein, whereas AHR depletion significantly attenuated the upregulation of IL-24 induced by tapinarof [7]. The AHR-binding sequence (GCGTG) is present in the promoter region of the IL-24 gene [18, 23]. These results indicated that AHR activation plays a vital role in the mechanism of AHR ligand-mediated IL-24 induction.

Microbial stimuli

Staphylococcal strains, both pathogenic (S. aureus) and commensal strains (S. epidermidis and S. saprophyticus), induce IL-24 mRNA and protein expression in human peripheral blood mononuclear cells [24]. The expression of IL-24 was also upregulated by S. aureus infection in human keratinocytes [25].

Lipopolysaccharide, a microbial product and Toll-like receptor activator, stimulates monocytes [26], macrophages [24, 27], and activated T cells [28] to produce IL-24. Pellino 1 ubiquitin E3 ligase is activated by innate pattern-recognition receptors such as Toll-like receptors. Overexpression of Pellino 1 induced a significant increase in the expression of IL-24 in HaCaT cells [29].

The Th2-type cytokines IL-4, IL-13, and IL-31

It is widely accepted that the Th2 cytokines IL-4 and IL-13 are driving factors in AD pathogenesis, as the blockade of IL-4/IL-13 signaling by dupilumab is effective in patients with severe AD [30]. The IL-24 gene has been identified as a dominant Th2 lineage-specific gene [31].

IL-4 increases IL-24 expression in normal human keratinocytes [9, 15], human monocytes [27], and T lymphocytes [28]. IL-24 expression is increased in the inflammatory skin lesions of IL-4-transgenic mice compared with that in wild-type mice [8]. IL-4 functions synergistically with IL-2 or lipopolysaccharide to induce IL-24 expression in natural killer (NK) cells and macrophages [32].

IL-13 upregulates periostin production via signal transducer and activator of transcription 6 (STAT6) activation, and periostin enhances IL-24 production in human keratinocytes [9]. STAT6 is critical for IL-4/ IL-13-induced IL-24 expression [9, 31]. In Th2 cells, the transcription factors STAT6, GATA3, and the activator protein-1 (AP-1) family member c-Jun were revealed to participate in the regulation of IL-24 production [31, 33, 34].

IL-31 stimulates keratinocytes to produce IL-24. The inhibition of Janus kinase (JAK), p38, and extracellular signal-regulated kinases (ERKs), but not c-Jun N-terminal kinases or phosphoinositide 3-kinases, interfere with the upregulation of IL-24 induced by IL-31 [35].

Cytokines IL-17A, IL-22, and IL-23

IL-17A upregulated IL-24 expression in normal human keratinocytes [15, 36]. Chong et al. [37] found that IL-17A induced IL-24 production in Th17 cells. Considering that IL-17A signals through the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway [38] and two potential NF-κB binding sites are present in the IL-24 gene, the researchers proposed that the binding of IL-17A to its receptor activates NF-κB signaling, leading to the transcription of IL-24 in Th17 cells [37]. IL-22 induced the production of IL-24 in normal human keratinocytes [15, 39, 40]. IL-17 synergistically induced the expression of IL-24 in keratinocytes with IL-22 or tumor necrosis factor alpha (TNFα) [36, 39, 40]. The injection of IL-23 into mouse skin elevated IL-24 expression [41].

Pro-inflammatory cytokines IL-1β, IL-6, and TNFα

IL-1β stimulation induced IL-24 protein secretion in human keratinocytes [14, 15, 42]. Inhibition of the p38 mitogen-activated protein kinase (MAPK) pathway was reported to significantly decrease IL-1β–induced IL-24 expression by reducing IL-24 mRNA stabilization, suggesting that p38 MAPK regulates IL-24 gene expression at the post-transcriptional level [43–45].

IL-6 induced the production of IL-24 in human keratinocytes [15, 46]. Suppressor of cytokine signaling-3 (SOCS3) inhibited JAK1, JAK2, and tyrosine protein kinase 2 but not JAK3, and it downregulated STAT3 activation [47]. The dysregulation of SOCS3 resulted in excess STAT3 activation in response to IL-6, leading to the upregulation of IL-24 expression by keratinocytes [48]. In an experimental model, physical stimulation (i.e. shaving) induced IL-24 expression at the site of stimulation in SOCS3 deficient mice, but not in control mice [48].

TNFα also upregulated IL-24 expression in keratinocytes [15, 49, 50–52]. Anti-TNFα antibody treatment almost completely inhibited IL-24 expression in psoriatic skin, indicating that IL-24 production is mainly driven by TNFα in psoriasis. Reactive oxygen species-activated ERK signaling was illustrated to mediate the upregulation of IL-24 in response to TNFα stimulation in keratinocytes [52].

IL-24 and keratinocyte differentiation/proliferation

Skin barrier dysfunction is a critical factor in the pathogenesis of AD. The disrupted barrier permits allergen penetration, accelerating immune response, pruritus, and trans-epidermal water loss [3]. In the epidermal barrier, FLG and LOR, markers of late differentiation of keratinocytes [3], are essential barrier-related proteins. They aggregate and align keratin bundles, contributing to skin barrier strength and integrity. In addition, FLG degradation products form a natural moisturizing factor, which has a role in skin hydration and barrier function [57]. In AD, overexpressed IL-4/IL-13 repressed FLG and LOR expression, leading to the impaired terminal differentiation and barrier function of the epidermis [2, 7]. IL-4/IL-13–mediated barrier dysfunction may be, at least in part, attributable to IL-24 because type 2 cytokine-induced IL-24 downregulates FLG and LOR expression via STAT3 [7, 9]. Depletion of IL-24 significantly restored the IL-13–induced downregulation of FLG [9]. Calmodulin-like 5 (CALML5) was revealed to be highly expressed in the differentiating epidermis. CALML5-knockout keratinocytes exhibit deficient FLG and LOR expression [58]. It was reported that IL-4 and IL-13 pathways converged on p63 to diminish CALML5 and FLG expression [59]. Interestingly, because IL-24 was also illustrated to inhibit the gene expression of CALML5 [39], the possible involvement of p63 in IL-24 signaling needs to be further studied. Collectively, these results indicate that IL-24 is a pivotal mediator of abnormal epidermal differentiation downstream of type 2 cytokine signals in AD.

Common inflammatory skin diseases, including AD, feature keratinocyte hyperproliferation [60, 61]. IL-24 was revealed to induce normal human epidermal keratinocyte proliferation in an epidermal growth factor receptor-independent manner, as blockade of this receptor in the monolayer culture system did not inhibit IL-24–mediated keratinocyte proliferation. However, keratinocyte proliferation might be further augmented through the upregulation of the EGF family ligands amphiregulin and heparin-binding epidermal growth factor induced by IL-24. IL-24 also increased the thickness of the reconstituted human epidermis [54]. Histological analysis revealed that IL-24 transgenic mice had a compact stratum corneum and markedly thickened epidermis compared with the findings in wild-type mice [42]. Additionally, IL-24 was reported to contribute to epidermal hyperplasia induced by IL-23, a major cytokine implicated in psoriasis, in mice [41]. Furthermore, expression of the epidermal proliferation-associated proteins (keratin 16 and S100As, particularly S100A7) was increased in IL-24–treated reconstituted human epidermis [54].

In summary, these data suggest that IL-24 induces abnormal epidermal differentiation and hyperplasia, contributing to skin barrier dysfunction in AD.

IL-24 and skin inflammatory responses

IL-24 might participate in a complex cascade of cytokines involved in skin inflammatory responses, as IL-24 can induce the expression of several cytokines and chemokines. When added to cultured human keratinocytes and/or reconstituted human epidermis, IL-24 upregulated the expression of many inflammatory mediators, including chemokine (C-X-C motif) ligand (CXCL)1 and IL-20 gene expression and CXCL8/IL-8, chemokine (C-C motif) ligand (CCL)20/macrophage inflammatory protein (MIP)-3α, matrix metalloproteinase (MMP)-1, and prostaglandin E2 (PGE2) protein secretion [15, 40, 52, 54]. Notably, IL-24 transgenic epidermis exhibited elevated induction of monocyte chemoattractant protein-1, a critical chemokine that mediates recruitment of monocytes/macrophages [42]. Furthermore, it was reported that IL-24-deficient mice were partially protected against allergic contact dermatitis and epidermal inflammatory infiltrate, particularly neutrophils, after exposure to paraphenylenediamine [16]. Altogether, these results indicate that IL-24 can promote skin inflammation, thereby contributing to the maintenance and exacerbation of AD.

IL-24 and itch

Intense itch induces scratching and skin lesion exacerbation that disturbs the quality of life of patients with AD [62]. The role of IL-24 in pruritus remains poorly explored. IL-31, a major pruritogenic cytokine in AD [62], induced IL-24 gene expression in keratinocytes [35]. IL-24 acts as an activator of STAT3, which is also activated by IL-31 to promote elongation of the nerve fibers, followed by enhanced itching [63]. Additionally, IL-24 upregulated the expression of CXCL1 [54], a chemokine that can evoke itch through multiple pathways [64, 65]. These data suggest the involvement of IL-24 in the mechanism of pruritus in AD.

IL-24 and skin infection

The skin of patients with AD is prone to microbial infections [66]. Atopic skin is preferentially colonized by S. aureus, and its colonization further contributes to disease exacerbation [67, 68]. Myles et al. [25] reported that S. aureus-induced IL-24 inhibits the induction of IL-1β in keratinocytes, followed by a decrease in IL-17A expression in γδ T cells, leading to reduced neutrophil recruitment and eventually resulting in more severe infection. They also suggested that IL-24 can promote cutaneous infection by S. aureus and that IL-20R2 blockade may have therapeutic potential for patients with S. aureus infection [25]. Conversely, in the human epidermis system, IL-24 was illustrated to induce several antimicrobial peptides that have a protective role against microbial infection, including S100A7, S100A12, S100A15, and β-defensins [52, 54]. These controversial findings imply that the anti-infection effects of IL-24 may vary depending on the infection conditions and timing.

IL-24 and therapeutics for AD

The aforementioned evidence suggests that IL-24 plays a significant role in the pathogenesis of AD and supports the potential efficacy of therapeutics that block the upstream and downstream pathways of IL-24 signaling. Dupilumab, an anti–IL-4Rα monoclonal antibody that inhibits IL-4 and IL-13 signaling, is efficacious in the treatment of moderate-to-severe AD. The drug significantly improves the clinical severity of AD. In parallel, the upregulation of IL-24 is suppressed in the lesional and non-lesional skin of patients with AD after 4 and 16 weeks of treatment with dupilumab [69]. It remains unclear whether other emerging drugs such as the IL-13 antibodies tralokinumab and lebrikizumab [70–72] and anti-IL-31Rα antibody nemolizumab [70, 73] affect IL-24 expression in AD.

JAK inhibitors, such as topical delgocitinib and oral baricitinib, have demonstrated efficacy against AD, mainly owing to their inhibitory action on JAK/STAT signaling, and they also possess the potential to inhibit the IL-24 signaling [74–76].

Tapinarof has been illustrated to attenuate the disease activity of AD in clinical studies [4–6]. Our recent study demonstrated that tapinarof upregulates FLG and LOR expression in an AHR-dependent manner in human keratinocytes, thereby improving barrier dysfunction in AD. Paradoxically, tapinarof induced the secretion of IL-24, and IL-24 sequentially activates JAK–STAT3 signaling, which negatively regulates FLG and LOR expression, partially via AHR activation. Therefore, inhibition of the IL-24–STAT3 axis during AHR activation using JAK inhibitors may further increase FLG and LOR expression induced by tapinarof treatment.

Therefore, co-treatment of tapinarof with JAK a inhibitor may be a promising strategy for managing AD [7].