A Critical Role of Endothelial Skp2/PTEN Axis in Angiogenesis and Psoriasis.

BACKGROUND: Psoriasis is a common, chronic skin disorder pathologically featured with abnormal epidermal proliferation, infiltrating inflammatory cells and increased angiogenesis in the dermis. Aberrant expressions of E3 ubiquitin ligase and dysregulated protein ubiquitination system are implicated in the pathogenesis.

OBJECTIVE: We aimed to examine the potential role of S-phase kinase-associated protein 2 (Skp2), an E3 ligase and oncogene, in psoriasis.

METHODS: Gene expression and protein levels were evaluated by using qRT-PCR, western blotting, immunohistochemistry and immunofluorescence staining in the skin samples from the patients with psoriasis vulgaris and the imiquimod (IMQ)-induced mouse model as well as in the cultured endothelial cells (ECs). Protein interaction, substrate ubiquitination and degradation were examined using co-immunoprecipitation, western blotting and cycloheximide (CHX) chase assay in human umbilical vein ECs (HUVECs). Angiogenesis was measured in vitro using human dermal microvascular ECs (HDMECs) for BrdU incorporation, migration and tube formation. Angiogenesis in vivo assays included chick embryonic chorioallantoic membrane (CAM), the Matrigel plug assay and quantification of vasculature in the mouse lesions. Skp2 gene global knockout mice and endothelial-specific conditional knockout mice were used.

RESULTS: Skp2 was increased in the skin samples from the psoriatic patients and IMQ-induced mouse lesions. Immunofluorescent double staining indicated a close association of Skp2 expression with excessive vascularity in the lesional dermal papillae. In HDMECs, Skp2 overexpression enhanced whereas Skp2 knockdown inhibited EC proliferation, migration, and tube-like structure formation. Mechanistically, phosphatase and tensin homolog (PTEN), the suppressor of PI3K/Akt pathway, was identified to be a novel substrate for Skp2-mediated ubiquitination. A selective inhibitor of Skp2 (C1) or Skp2 siRNA significantly reduced the VEGF-triggered PTEN ubiquitination and degradation. In addition, the Skp2-mediated ubiquitination depended on the phosphorylation of PTEN by glycogen synthase kinase 3β (GSK3β). In the mouse model, Skp2 gene deficiency alleviated the IMQ-induced psoriasis. Importantly, tamoxifen-induced endothelial-specific Skp2 knockout mice developed significantly ameliorated psoriasis with diminished papillae angiogenesis. Furthermore, topical use of the Skp2 inhibitor C1 effectively prevented the experimental psoriasis.

CONCLUSION: The Skp2/PTEN axis may play an important role in psoriasis-associated angiogenesis. Thus, targeting Skp2-driven angiogenesis may be a potential approach to treat psoriasis.