Interleukin-38 interacts with destrin/actin-depolymerizing factor in human keratinocytes

Interleukin-38 alleviates hepatic steatosis through AMPK/autophagy-mediated suppression of endoplasmic reticulum stress in obesity models

Interleukin-38 (IL-38), recently recognized as a cytokine with anti-inflammatory properties that mitigate type 2 diabetes, has been associated with indicators of insulin resistance and nonalcoholic fatty liver disease (NAFLD). This study investigated the impact of IL-38 on hepatic lipid metabolism and endoplasmic reticulum (ER) stress. We assessed protein expression levels using Western blot analysis, while monodansylcadaverine staining was employed to detect autophagosomes in hepatocytes. Oil red O staining was utilized to examine lipid deposition. The study revealed elevated serum IL-38 levels in high-fat diet (HFD)-fed mice and IL-38 secretion from mouse keratinocytes. IL-38 treatment attenuated lipogenic lipid accumulation and ER stress markers in hepatocytes exposed to palmitate. Furthermore, IL-38 treatment increased AMP-activated protein kinase (AMPK) phosphorylation and autophagy. The effects of IL-38 on lipogenic lipid deposition and ER stress were nullified in cultured hepatocytes by suppressing AMPK through small interfering (si) RNA or 3-methyladenine (3MA). In animal studies, IL-38 administration mitigated hepatic steatosis by suppressing the expression of lipogenic proteins and ER stress markers while reversing AMPK phosphorylation and autophagy markers in the livers of HFD-fed mice. Additionally, AMPK siRNA, but not 3MA, mitigated IL-38-enhanced fatty acid oxidation in hepatocytes. In summary, IL-38 alleviates hepatic steatosis through AMPK/autophagy signaling-dependent attenuation of ER stress and enhancement of fatty acid oxidation via the AMPK pathway, suggesting a therapeutic strategy for treating NAFLD.

Emerging functions and therapeutic targets of IL-38 in central nervous system diseases

Interleukin (IL)-38 is a newly discovered cytokine of the IL-1 family, which binds various receptors (i.e., IL-36R, IL-1 receptor accessory protein-like 1, and IL-1R1) in the central nervous system (CNS). The hallmark physiological function of IL-38 is competitive binding to IL-36R, as does the IL-36R antagonist. Emerging research has shown that IL-38 is abnormally expressed in the serum and brain tissue of patients with ischemic stroke (IS) and autism spectrum disorder (ASD), suggesting that IL-38 may play an important role in neurological diseases. Important advances include that IL-38 alleviates neuromyelitis optica disorder (NMOD) by inhibiting Th17 expression, improves IS by protecting against atherosclerosis via regulating immune cells and inflammation, and reduces IL-1β and CXCL8 release through inhibiting human microglial activity post-ASD. In contrast, IL-38 mRNA is markedly increased and is mainly expressed in phagocytes in spinal cord injury (SCI). IL-38 ablation attenuated SCI by reducing immune cell infiltration. However, the effect and underlying mechanism of IL-38 in CNS diseases remain inadequately characterized. In this review, we summarize the biological characteristics, pathophysiological role, and potential mechanisms of IL-38 in CNS diseases (e.g., NMOD, Alzheimer's disease, ASD, IS, TBI, and SCI), aiming to explore the therapeutic potential of IL-38 in the prevention and treatment of CNS diseases.

Cell Intrinsic IL-38 Affects B Cell Differentiation and Antibody Production

IL-38 is an IL-1 family receptor antagonist with an emerging role in chronic inflammatory diseases. IL-38 expression has been mainly observed not only in epithelia, but also in cells of the immune system, including macrophages and B cells. Given the association of both IL-38 and B cells with chronic inflammation, we explored if IL-38 affects B cell biology. IL-38-deficient mice showed higher amounts of plasma cells (PC) in lymphoid organs but, conversely, lower levels of plasmatic antibody titers. Exploring underlying mechanisms in human B cells revealed that exogenously added IL-38 did not significantly affect early B cell activation or differentiation into plasma cells, even though IL-38 suppressed upregulation of CD38. Instead, IL-38 mRNA expression was transiently upregulated during the differentiation of human B cells to plasma cells in vitro, and knocking down IL-38 during early B cell differentiation increased plasma cell generation, while reducing antibody production, thus reproducing the murine phenotype. Although this endogenous role of IL-38 in B cell differentiation and antibody production did not align with an immunosuppressive function, autoantibody production induced in mice by repeated IL-18 injections was enhanced in an IL-38-deficient background. Taken together, our data suggest that cell-intrinsic IL-38 promotes antibody production at baseline but suppresses the production of autoantibodies in an inflammatory context, which may partially explain its protective role during chronic inflammation.

IL-38 orchestrates proliferation and differentiation in human keratinocytes

Interleukin (IL)-38 is a member of the IL-1 cytokine family with reported anti-inflammatory activity. The highest constitutive IL-38 expression is detected in the skin, where it is mainly produced by differentiating keratinocytes. However, little data are available regarding its biological functions. In this study, we investigated the role of IL-38 in skin physiology. We demonstrate here that dermal fibroblasts and epithelial cells of skin appendages, such as eccrine sweat glands and sebaceous glands, also express IL-38. Next, using two- and three-dimensional cell cultures, we show that endogenous expression of IL-38 correlates with keratinocyte differentiation and its ectopic overexpression inhibits keratinocyte proliferation and enhances differentiation. Accordingly, immunohistochemical analysis revealed downregulation of IL-38 in skin pathologies characterized by keratinocyte hyperproliferation, such as psoriasis and basal or squamous cell carcinoma. Finally, intracellular IL-38 can shuttle between the nucleus and the cytoplasm and its overexpression modulates the activity of the transcription regulators YAP and ID1. Our results indicate that IL-38 can act independently from immune system activation and suggest that it may affect the epidermis directly by decreasing proliferation and promoting differentiation of keratinocytes. These data suggest an important role of keratinocyte-derived IL-38 in skin homeostasis and pathologies characterized by epidermal alterations.

Epigenetic remodeling of downstream enhancer regions is linked to selective expression of the IL1F10 gene in differentiated human keratinocytes

Interleukin (IL)-38, encoded by the IL1F10 gene, is a member of the IL-1 family of cytokines. IL-38 is constitutively expressed in epithelia in healthy humans, and in particular in epidermal keratinocytes in the skin. IL-38 expression is closely correlated with keratinocyte differentiation. The aim of this study was to further characterize the regulation of IL1F10 expression and the mechanisms involved in its selective induction in differentiated human keratinocytes. We observed coordinated expression of two IL1F10 transcripts, transcribed from two different promoters, upon differentiation of primary human keratinocytes. Using ENCODE datasets and ChIP-qPCR on ex vivo isolated normal human epidermis, we identified regulatory regions located downstream of the IL1F10 gene, which displayed features of differentiated keratinocyte-specific enhancers. Expression of the IL1F10 gene was linked to changes in the epigenetic landscape at these downstream enhancer regions in human epidermis. Overexpression of the transcription factors KLF4 and TAp63β in an immortalized normal human keratinocyte (iNHK) cell line promoted the expression of mRNA encoding the differentiation markers keratin 10 and involucrin, and of IL1F10. ChIP-qPCR experiments indicated that KLF4 and TAp63β overexpression also modified the chromatin state of the proximal downstream enhancer region, suggesting a role for KLF4 and TAp63β in directly or indirectly regulating IL1F10 transcription. In conclusion, expression of the IL1F10 gene in differentiated keratinocytes in normal human epidermis involves coordinated transcription from two promoters and is linked to epigenetic remodeling of enhancer regions located downstream of the gene.

Immunobiological Properties and Clinical Applications of Interleukin-38 for Immune-Mediated Disorders: A Systematic Review Study

Exponential growth in the usage of "cytokines" (as seroimmunobiomarkers) has facilitated more accurate prognosis, early diagnosis, novel, and efficient immunotherapeutics. Numerous studies have reported immunopathophysiological and immunopathological processes of interleukin-38 (IL-38). Therefore, in this systematic review article, the authors aimed to present an updated comprehensive overview on the immunobiological mechanisms, diagnostic, and immune gene-based therapeutic potentials of IL-38. According to our inclusion and exclusion criteria, a total of 216 articles were collected from several search engines and databases from the January 2012 to July 2021 time interval by using six main keywords. Physiologic or pathologic microenvironments, optimal dosage, and involved receptors affect the functionalities of IL-38. Alterations in serum levels of IL-38 play a major role in the immunopathogenesis of a wide array of immune-mediated disorders. IL-38 shows anti-inflammatory activities by reduction or inhibition of pro-inflammatory cytokines, supporting the therapeutic aspects of IL-38 in inflammatory autoimmune diseases. According to the importance of pre-clinical studies, it seems that manipulation of the immune system by immunomodulatory properties of IL-38 can increase the accuracy of diagnosis, and decipher optimal clinical outcomes. To promote our knowledge, more collaboration is highly recommended among laboratory scientists, internal/infectious diseases specialists, oncologists, immunologists, diseases-specific biomarkers scientists, and basic medical researchers.

IL-1 Family Antagonists in Mouse and Human Skin Inflammation

Interleukin (IL)-1 family cytokines initiate inflammatory responses, and shape innate and adaptive immunity. They play important roles in host defense, but excessive immune activation can also lead to the development of chronic inflammatory diseases. Dysregulated IL-1 family signaling is observed in a variety of skin disorders. In particular, IL-1 family cytokines have been linked to the pathogenesis of psoriasis and atopic dermatitis. The biological activity of pro-inflammatory IL-1 family agonists is controlled by the natural receptor antagonists IL-1Ra and IL-36Ra, as well as by the regulatory cytokines IL-37 and IL-38. These four anti-inflammatory IL-1 family members are constitutively and highly expressed at steady state in the epidermis, where keratinocytes are a major producing cell type. In this review, we provide an overview of the current knowledge concerning their regulatory roles in skin biology and inflammation and their therapeutic potential in human inflammatory skin diseases. We further highlight some common misunderstandings and less well-known observations, which persist in the field despite recent extensive interest for these cytokines.

Biology of interleukin-38 and its role in chronic inflammatory diseases

Interleukin (IL)-38 is the tenth member of the IL-1 cytokine family. IL-38 shares high similarity with IL-36Ra and IL-1Ra and can bind to their receptors, thus exerting an anti-inflammatory effect. Despite the lack of a signal peptide, IL-38 can be released from several cell types, but its maturation process remains obscure. The role of IL-38 in numerous inflammatory diseases, especially in autoimmune diseases, has been extensively studied. In this review, we discuss the characteristics, biological functions and pathways of IL-38, as well as its role in several chronic inflammatory diseases. Better understanding the role of IL-38 will pave the way for clinical treatments in the near future.

IL-38 Ablation Reduces Local Inflammation and Disease Severity in Experimental Autoimmune Encephalomyelitis

IL-38 is an IL-1 family receptor antagonist that restricts IL-17-driven inflammation by limiting cytokine production from macrophages and T cells. In the current study, we aimed to explore its role in experimental autoimmune encephalomyelitis in mice, which is, among others, driven by IL-17. Unexpectedly, IL-38-deficient mice showed strongly reduced clinical scores and histological markers of experimental autoimmune encephalomyelitis. This was accompanied by reduced inflammatory cell infiltrates, including macrophages and T cells, as well as reduced expression of inflammatory markers in the spinal cord. IL-38 was highly expressed by infiltrating macrophages in the spinal cord, and in vitro activated IL-38-deficient bone marrow-derived macrophages showed reduced expression of inflammatory markers, accompanied by altered cellular metabolism. These data suggest an alternative cell-intrinsic role of IL-38 to promote inflammation in the CNS.

IL-36 family cytokines in protective versus destructive inflammation

The IL-1 family of cytokines and receptors are critical regulators of inflammation. Within the IL-1 family and in contrast to its IL-1 and IL-18 subfamilies, the IL-36 subfamily is still poorly characterized. Three pro-inflammatory agonists IL-36α, IL-36β, IL-36γ, one IL-36 receptor (IL-1R6) antagonist, IL-36RA, and one putative IL-1R6 antagonist, IL-38, have been grouped into the IL-36 cytokine subfamily. IL-36 agonists signal through a common receptor complex to serve as early triggers of inflammatory responses by activating and cross-regulating a number of inflammatory pathways including NF-κB, MAPK and IFN signaling. IL-36RA binds to IL-1R6 to limit inflammatory signaling, while IL-38 may be an antagonist of more than one IL-1 family receptor. Expression patterns of IL-36 family cytokines, being most prominently expressed in epithelial barrier tissues such as the skin and intestines as well as in immune cells, suggest a role in protecting these barriers from infection. Dysregulation of IL-36 family cytokine signaling at physiological barriers, most prominently the skin, induces autoimmune inflammation. However, transferring the potential of IL-36 to induce tissue damage to tumors might benefit cancer patients. Here we summarize signaling pathways regulated by IL-36 family cytokines, including IL-38, and the consequences for physiological protective and pathophysiological destructive inflammation. Moreover, we discuss the limits of current knowledge on IL-36 family function to open potential avenues for research in the future.

The Role of New IL-1 Family Members (IL-36 and IL-38) in Atopic Dermatitis, Allergic Asthma, and Allergic Rhinitis

PURPOSE OF REVIEW

Since the discovery of its very first member in 1974, the IL-1 family has expanded into a group of 11 potent molecules which are essential in both innate and acquired immunity. Pro-inflammatory cytokines IL-36α, IL-36β, and IL-36γ and their receptor antagonists IL-36Ra and IL-38, which belong to the IL-36 subfamily, are some of the most recently identified members. Recent studies show that these members possess pro-inflammatory and anti-inflammatory activities and may take part in the pathogenesis of allergy. In this review, the involvement and importance of these newly described IL-1 family members in the most common allergic diseases, i.e., atopic dermatitis (AD), allergic asthma, and allergic rhinitis, will be discussed.

RECENT FINDINGS

Dysregulation of IL-36 and IL-38 was observed in the skin and respiratory tract of AD, allergic rhinitis, and allergic asthma individuals. Although the upregulation in IL-36α and IL-36γ observed in the lesional skin of AD patients was unexpectedly small, IL-36 may play an important role in AD pathogenesis especially upon Staphylococcus aureus colonization. While IL-36γ regulates eosinophils to induce an inflammatory response in allergic rhinitis, IL-36α was found to regulate Th17 immunity. IL-36 receptor antagonists, IL-36Ra and IL-38, however, both show promising anti-inflammatory activities against allergic asthma. Of note, IL-38 in allergic asthmatic children is significantly lower than their healthy counterparts, while the anti-inflammatory effects of IL-38 in allergic asthma exacerbation upon viral-like infection were demonstrated in in vitro, HDM-induced, and humanized mice models. Dysregulated expression of IL-36 and IL-38 observed in allergic patients and mice models revealed that they may have essential roles in the pathogenesis in AD, allergic rhinitis, and allergic asthma, especially during the host defense against pathogens at inflammatory sites. Their receptor antagonists, IL-36Ra and IL-38, could also be promising biologics in the control of allergy. Since allergic diseases are phenotypically complex, contradictory data obtained in different studies may be explained if further stratification of disease endotypes is explored. Genetically modified mice model and investigation in anti-IL-36 treatment may be useful to characterize the therapeutic potential of these cytokines in the regulation of allergy in the future.

LDB1 and the SWI/SNF complex participate in both transcriptional activation and repression by Caenorhabditis elegans BLIMP1/PRDM1

Transcription factors of the BLIMP1/PRDM1 family are important regulators of development. BLIMP1/PRDM1 can both activate and repress gene expression, however, the mechanism of activation is not well understood. Therefore, we looked for factors involved in gene activation by C. elegans BLMP-1, the ortholog of BLIMP1/PRDM1. BLMP-1 activates the expression of bed-3, a gene involved in vulval development. By screening nuclear proteins that function in vulval development, we identified two proteins (LDB-1 and HAM-3) required for BLMP-1 dependent bed-3 expression. LDB-1 is the sole C. elegans member of the LIM Binding Protein (LDB) family, whereas HAM-3 is an accessory subunit of the SWI/SNF complex (ortholog of human SMARCD3/BAF60C). A core SWI/SNF subunit SWSN-1 (ortholog of human SMARCC1/BAF155) is also involved. We found that LDB-1 and HAM-3 bind to BLMP-1, suggesting that BLMP-1 recruits LDB-1 and the SWI/SNF complex to activate bed-3 expression. Interestingly, LDB-1 and HAM-3 are involved in both transcriptional activation and repression. In particular, BLMP-1, LDB-1 and HAM-3 co-regulate a set of hypodermal genes including bed-3 (activated), col-124 (activated) and lin-29 (repressed). On the other hand, LDB-1 and HAM-3 are not required for activation or repression of some genes regulated by BLMP-1 (e.g. T09D3.8, nas-10). We also found that human LDB1, SMARCD3/BAF60C and SMARCC1/BAF155 all physically interact with human BLIMP1/PRDM1 in vitro and are closely associated with BLIMP1/PRDM1 in vivo. Taken together, these results identify LDB1 and SWI/SNF as likely conserved cofactors of BLIMP1/PRDM1, which participate in activation and repression of a subset of BLIMP1/PRDM1-regulated genes.