Leptin induces interleukin‐6 production in keratinocytes via decreased expression of caveolin‐1: a possible link between obesity and psoriatic inflammation

Dear Editor, It has been reported that obesity may be an aggravating factor for psoriasis; however, the pathological mechanism remains elusive. The adipocyte‐derived hormone leptin primarily controls energy homeostasis by regulating appetite and modulates immunity. Strikingly, serum leptin levels are elevated in obese patients and positively correlate with body mass index.1 Additionally, leptin levels also correlate with severity of psoriasis,2 and knocking out leptin suppresses imiquimod‐induced psoriatic inflammation in mice.3 These data indicate that leptin may induce or aggravate psoriatic inflammation, although obese people with high leptin levels may not necessarily have psoriasis. Caveolin (Cav)‐1 is a membrane protein essential for the formation of caveolae, which serve as platforms for various cellular processes, such as lipid metabolism and signal transduction. Cav‐1 negatively regulates signalling molecules and receptors that interact with Cav‐1 scaffolding domain (CSD) (aa 82–101). We have previously reported that Cav‐1 is significantly downregulated in psoriatic epidermis.4 Cav‐1 downregulation activates the Janus kinase/signal transducers and activators of transcription pathway during psoriatic inflammation, thereby furthering keratinocyte proliferation and cytokine/chemokine production. CSD peptide interferes with psoriatic inflammation in mice.4 Furthermore, we have also shown that Cav‐1 downregulation in the monocytes of patients with psoriasis induces cytokine production and promotes monocyte infiltration into the skin.5 This study aimed to examine the aggravating role of leptin on keratinocytes through reduction of Cav‐1 in psoriasis. Firstly, the leptin receptor (Ob‐R) expression in human primary keratinocytes was examined. Immunofluorescent staining and immunoprecipitation analyses revealed that Ob‐R and Cav‐1 colocalized and interacted with each other (Figure 1a,b). Accordingly, Ob‐R and Cav‐1 form a protein complex that presumably modulates the leptin signalling. As patients with psoriasis have reduced Cav‐1 levels in keratinocytes, exacerbating psoriatic inflammation, we hypothesized that leptin might aggravate inflammation through reducing Cav‐1 levels in keratinocytes. Therefore, primary human keratinocytes were treated with a small interfering RNA targeting Cav‐1 and simultaneously stimulated with leptin and interleukin (IL)‐17A, a key propsoriasis cytokine used for mimicking psoriatic inflammation. Levels of psoriasis‐related cytokines/chemokines and antimicrobial peptides were evaluated by semiquantitative polymerase chain reaction (PCR) (Figure 1c). Although leptin stimulation alone did not induce any significant effect, Cav‐1 RNAi and leptin cotreatment somewhat upregulated CXCL10 and chemokine ligand 20 and significantly increased IL‐6 mRNA levels. The IL‐6 upregulation was confirmed by quantitative PCR and enzyme‐linked immunosorbent assay (Figure 1d,e). Furthermore, restoring Cav‐1 function by using the CSD peptide significantly inhibited the leptin‐induced IL‐6 aggravation in Cav‐1‐silenced keratinocytes (Figure 1f). (a) Colocalization of Caveolin (Cav)‐1 and the leptin receptor (Ob‐R) was determined by immunocytochemical staining. Human keratinocytes with leptin stimulation were stained for Ob‐R (green) and Cav‐1 (red). Scale bar = 20 μm. (b) Keratinocyte lysates were immunoprecipitated (IP) with an anti‐Ob‐R or control IgG. Coprecipitation of anti‐Cav‐1 antibody was examined by immunoblotting (IB). Experiments were repeated thrice. (c) Representative semiquantitative polymerase chain reaction (PCR) images of psoriasis‐related cytokines/chemokines and antimicrobial peptides are shown. Cav‐1‐silenced (siCav‐1) and control (siNeg) keratinocytes were stimulated with or without leptin in the presence of interleukin (IL)‐17 for 16 h, then gene expression of CXCL8, CXCL10, chemokine ligand (CCL) 20, beta defensin (BD)1, BD2, BD3 and IL‐6 were analysed. The relative gene and protein expression levels of IL‐6 were further determined by quantitative‐PCR (d) and enzyme‐linked immunosorbent assay (e). Unpaired Student's t‐test. *P < 0·05, **P < 0·01. (f) Suppression of leptin‐induced IL‐6 production in siCav‐1 keratinocytes by restoring Cav‐1 function using Cav‐1 scaffolding domain (CSD) peptide. CSD or scrambled peptides were added to siCav‐1 keratinocytes 1 h prior to leptin stimulation, then IL‐6 expression was examined by quantitative‐PCR. One‐way analysis of variance (anova) with post hoc Tukey's test. **P < 0·01, ***P < 0·001. DAPI, 4',6‐diamidino‐2‐phenylindole. We hypothesized that leptin played an important role in the pathogenesis of psoriasis through downregulating Cav‐1 in keratinocytes, thereby linking the aggravating roles of leptin in psoriasis and obesity. It has been reported that leptin stimulates the production of proinflammatory proteins in keratinocytes. However, we observed no significant increase in cytokine expression in keratinocytes stimulated by leptin alone. Although the cause of this discrepancy is unclear, differences in cellular states, such as inflammatory conditions, may have affected the results. We observed that Cav‐1 reduction upregulated several cytokines/chemokines under psoriatic inflammation as previously described,5 and leptin dramatically enhanced IL‐6. These data suggest that leptin may enhance inflammation, especially in patients with psoriasis who show reduced keratinocyte Cav‐1 levels. Several studies on obesity and Cav‐1 have been reported. Cav‐1 is upregulated in the adipose tissue of obese patients6 but downregulated in their monocytes. We showed that the Cav‐1 level in the psoriatic epidermis was decreased, and such downregulation might also exacerbate obesity‐associated psoriasis. Although we did not examine whether the Cav‐1 level was modulated by obesity itself, impaired binding of insulin receptors to Cav‐1 in type II diabetes is known to destabilize their levels in the adipose tissue, thereby resulting in insulin resistance.7 Taken together, Cav‐1 expression and function may be intricately regulated by differences in cell types and inflammatory states; however, further research is needed for clarification. IL‐6 has many biological effects, including keratinocyte activation and differentiation of T helper 17 cells. Lindroos et al. have shown that knocking out IL‐6 decreases skin inflammation in a mouse model of psoriasis induced by intradermal injection of IL‐23.8 However, anti‐IL‐6 or IL‐6 receptor antibodies are ineffective against psoriasis. Thus, IL‐6 may not be a major player in psoriasis to serve as a therapeutic target, but it may play a role in exacerbation and initiation. Obesity occasionally aggravates pre‐existing psoriasis. This study demonstrated that the obesity‐related hormone leptin might worsen psoriasis via Cav‐1 downregulation in keratinocytes, thus potentially linking obesity and psoriasis. Further research is required to confirm this observation, and the therapeutic potential of restoring Cav‐1 levels by CSD peptide administration should be explored in future studies. Appendix S1 Details on data sources.