Inhibition of Serum- and Glucocorticoid-Regulated Protein Kinase-1 Aggravates Imiquimod-Induced Psoriatic Dermatitis and Enhances Proinflammatory Cytokine Expression through the NF-kB Pathway

Although the anti-inflammatory effect of serum- and glucocorticoid-regulated protein kinase 1 (SGK1) has been established in other diseases, the possible regulatory role of SGK1 in psoriasis and the underlying molecular mechanisms remain largely unknown. In this study, we found that SGK1 expression was decreased in macrophages from patients with psoriasis. Moreover, a specific pharmacological SGK1 inhibitor, EMD638683, significantly enhanced imiquimod-mediated toll-like receptor 7/8 activity and proinflammatory cytokine production in RAW264.7 cells, and this result was confirmed by Sgk1 small interfering RNA. Further mechanistic data showed that SGK1 inhibition increased the phosphorylation of Bruton's agammaglobulinemia tyrosine kinase; moreover, Bruton's agammaglobulinemia tyrosine kinase inhibition abrogated the proinflammatory effects of the SGK1 inhibitor on toll-like receptor 7/8 activation, thereby validating that SGK1 inhibition enhances the toll-like receptor 7/8 pathway by increasing Bruton's agammaglobulinemia tyrosine kinase phosphorylation. In addition, our in vivo results showed that SGK1 inhibition significantly increased the secretion of proinflammatory cytokines, including IL-1β, IL-6, and TNF-α, and the infiltration of T helper 17 cells in an imiquimod-induced psoriasis mouse model. Altogether, these results show that SGK1 plays a critical role in the pathogenesis of psoriasis by modulating inflammatory responses in skin lesions, indicating that SGK1‒Bruton's agammaglobulinemia tyrosine kinase signaling could be a novel therapeutic target for the control of psoriasis.