Stat3 is activated in skin lesions by the local application of imiquimod, a ligand of TLR7, and inhibited by the recombinant peptide aptamer rS3-PA.

Abstract Signal transducer and activator of transcription 3 (Stat3) assumes central functions in the regulation of apoptosis, proliferation, angiogenesis, and immune responses in normal cells. It also plays crucial roles in inflammatory and malignant diseases and in the cellular communication in the tissue microenvironment. Signaling interactions among normal endothelial cells, immune cells, and tumor cells, mediated by the release of cytokines, chemokines, and growth factors, often result in the activation of Stat3 and promotion of cancer cell proliferation, invasion, angiogenesis, and immune evasion. Stat3 also causes the differentiation and activation of T helper 17 (Th17) cells, which is involved, e.g., in psoriasis, an inflammatory autoimmune disease of the skin. Here, we describe molecular characteristics of a mouse model triggered by the treatment of mouse skin with the immune modulator imiquimod. The application of this compound causes the local release of proinflammatory cytokines and symptoms that resemble human psoriasis. We show that this process is accompanied by strong Stat3 activation. We also investigated the effects of a membrane-permeable, peptide-based Stat3 inhibitor, recombinant Stat3-specific peptide aptamer (rS3-PA). This molecule specifically interacts with Stat3 and prevents its transactivation potential in cultured cells. rS3-PA is able to penetrate the skin, enter cells, and reduce the level of activated Stat3. The topical applications of rS3-PA to the skin could thus possibly become useful in the treatment of inflammatory skin diseases and skin cancer.