Retinoic Acid Amplifies <scp>IL</scp> ‐4/ <scp>IL</scp> ‐13–Induced Eotaxin Expression via <scp>CRABP1</scp> in Eosinophilic Chronic Rhinosinusitis

Eosinophilic chronic rhinosinusitis (ECRS) is a type 2 inflammatory disease characterized by persistent eosinophilic infiltration and recurrence [1]. Eotaxins mediate eosinophil recruitment and are correlated with disease severity [2]. While IL-4 and IL-13 are known to induce eotaxin expression, inter-patient variability in eotaxin levels and heterogeneous responses to Th2 cytokines in primary cells from ECRS patients suggest the presence of additional regulatory mechanisms, potentially related to intrinsic cellular metabolism [3]. Retinoic acid (RA), a vitamin A metabolite, regulates differentiation, tissue homeostasis, and inflammation. Intracellular RA availability is controlled by cellular retinoic acid–binding proteins (CRABPs), which regulate RA degradation and receptor-mediated transcription. RA has been shown to decrease eotaxin expression and is implicated in the regulation of eosinophilic inflammation [4]. Based on this, we hypothesized that upregulation of CRABP1 in ECRS fibroblasts shifts intracellular RA signaling toward a catabolic pathway, promoting RA degradation and thereby impairing RA-mediated suppression of CCL11 and CCL26 expression. Single-cell RNA sequencing of sinonasal tissues from healthy controls and patients with ECRS (HRA000772) identified fibroblasts as a major source of C-C motif chemokine ligands 11 (CCL11) and 26 (CCL26) (Figure 1A–C). Differential expression analysis revealed upregulation of CCL11, CCL26, and CRABP1 in ECRS fibroblasts (Figure 1D), which was associated with enhanced IL-4/IL-13 signaling (Figure 1E). ECRS fibroblasts exhibited a distinct expression pattern of RA metabolic regulators, characterized by significant upregulation of CRABP1 compared with fibroblasts from healthy controls. These changes suggest a shift in RA metabolic balance in ECRS fibroblasts (Figure 1F). To determine whether a Th2 inflammatory milieu alters RA responsiveness and metabolism in fibroblasts, primary nasal fibroblasts derived from healthy donors were stimulated with IL-4 or IL-13 prior to RA exposure. RA alone moderately increased CRABP1 and CRABP2 expression (2–3-fold). In contrast, RA treatment following Th2 stimulation markedly enhanced CRABP1 expression (9–15-fold), without affecting CRABP2 levels (Figure 2A), which was confirmed by confocal microscopy (Figure 2B). CRABP1 facilitates intracellular RA catabolism, whereas CRABP2 transports RA to the nucleus to promote transcriptional regulation [5]. Therefore, the selective upregulation of CRABP1 in Th2 cytokine–stimulated fibroblasts upon RA exposure suggests a shift toward a CRABP1-dominant, catabolic RA signaling pathway. Consistent with this metabolic shift, intracellular RA levels were reduced in fibroblasts pre-stimulated with IL-4 or IL-13 and subsequently treated with RA compared with RA alone. CRABP1 inhibition using siRNA prevented the reduction in intracellular RA levels under the same treatment conditions (Figure 2C,D). RA treatment following Th2 stimulation markedly increased CCL11 and CCL26 expression beyond levels induced by Th2 cytokines alone (Figure 2E), whereas CRABP1 knockdown significantly inhibited eotaxin expression (Figure 2F). These findings indicate that aberrant CRABP1 upregulation (Figure 2A) drives RA catabolism (Figure 2D), preventing RA from exerting its suppressive effects on CCL11 and CCL26, and CRABP1 inhibition preserves intracellular RA (Figure 2D) and restores its chemokine-suppressive function (Figure 2F). Although the amplified induction of CCL11 and CCL26 under RA treatment following Th2 stimulation cannot be fully explained by RA degradation alone, recent studies suggest that CRABP1 mediates non-genomic RA pathways linked to inflammation [6]. Increased eotaxin expression was further confirmed by confocal microscopy (Figure 2G), and CRABP1-positive fibroblasts were identified in association with CCL11 and CCL26 expression in sinonasal tissues from ECRS patients (Figure 2H). In conclusion, CRABP1-mediated RA dysregulation enhances CCL11 and CCL26 production in ECRS, contributing to eosinophilic inflammation. These findings highlight CRABP1 as a potential diagnostic or therapeutic target in ECRS. Future studies will aim to determine whether CRABP1-driven RA dysregulation induces pro-inflammatory signaling under Th2 conditions, and to elucidate the underlying mechanisms involving nuclear receptors such as RAR and RXR, as well as RA-catabolizing enzymes, including the CYP26 family. Hyun-Woo Yang conceived and designed the study, performed the experiments, analyzed the data, wrote and revised the manuscript. Yeong-In Jo and Hwa-Eun Yang performed the experiments. Joo-Hoo Park, Hyeong-Guk Son, Jee Won Moon, and Il-Ho Park reviewed the data and contributed to the discussion of the results. Jee Won Moon and Il-Ho Park supervised the study and revised the manuscript. All authors contributed to the manuscript and approved the final submitted version. This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (Grants RS-2025-00557445 and RS-2025-25436857). The authors declare no conflicts of interest. The data that support the findings of this study are available from the corresponding author upon reasonable request.