Regulation of Photosensitivity by the Hippo Pathway in Lupus Skin

Objective: Photosensitivity is one of the most common manifestations of systemic lupus erythematosus (SLE), yet its pathogenesis is not well understood. The normal-appearing epidermis of patients with SLE exhibits increased ultraviolet B (UVB)-driven cell death that persists in cell culture. Here, we investigated the role of epigenetic modification and Hippo signaling in enhanced UVB-induced apoptosis seen in SLE keratinocytes.

Methods: We analyzed DNA methylation in cultured keratinocytes from SLE patients compared to keratinocytes from healthy controls (n = 6/group). Protein expression was validated in cultured keratinocytes using immunoblotting and immunofluorescence. An immortalized keratinocyte line overexpressing WWC1 was generated via lentiviral vector. WWC1-driven changes were inhibited using a large tumor suppressor kinase 1/2 (LATS1/2) inhibitor (TRULI) and small interfering RNA (siRNA). The interaction between the Yes-associated protein (YAP) and the transcriptional enhancer associate domain (TEAD) was inhibited by overexpression of an N/TERT cell line expressing a tetracycline-inducible green fluorescent protein-tagged protein that inhibits YAP-TEAD binding (TEADi). Apoptosis was assessed using cleaved caspase 3/7 and TUNEL staining.

Results: Hippo signaling was the top differentially methylated pathway in SLE versus control keratinocytes. SLE keratinocytes (n = 6) showed significant hypomethylation (Δβ = -0.153) and thus overexpression of the Hippo regulator WWC1 (P = 0.002). WWC1 overexpression increased LATS1/2 kinase activation, leading to YAP cytoplasmic retention and altered proapoptotic transcription in SLE keratinocytes. Accordingly, UVB-mediated apoptosis in keratinocytes could be enhanced by WWC1 overexpression or YAP-TEAD inhibition, mimicking SLE keratinocytes. Importantly, inhibition of LATS1/2 with either the chemical inhibitor TRULI or siRNA effectively eliminated enhanced UVB-apoptosis in SLE keratinocytes.

Conclusion: Our work unravels a novel driver of photosensitivity in SLE: overactive Hippo signaling in SLE keratinocytes restricts YAP transcriptional activity, leading to shifts that promote UVB apoptosis.