Phosphorylation of Pkp1 by RIPK4 regulates epidermal differentiation and skin tumorigenesis

Abstract

Tissue homeostasis of skin is sustained by epidermal progenitor cells localized within the basal layer of the skin epithelium. Post-translational modification of the proteome, such as protein phosphorylation, plays a fundamental role in the regulation of stemness and differentiation of somatic stem cells. However, it remains unclear how phosphoproteomic changes occur and contribute to epidermal differentiation. In this study, we survey the epidermal cell differentiation in a systematic manner by combining quantitative phosphoproteomics with mammalian kinome cDNA library screen. This approach identified a key signaling event, phosphorylation of a desmosome component, PKP1 (plakophilin-1) by RIPK4 (receptor-interacting serine-threonine kinase 4) during epidermal differentiation. With genome-editing and mouse genetics approach, we show that loss of function of either or impairs skin differentiation and enhances epidermal carcinogenesis Phosphorylation of PKP1's N-terminal domain by RIPK4 is essential for their role in epidermal differentiation. Taken together, our study presents a global view of phosphoproteomic changes that occur during epidermal differentiation, and identifies RIPK-PKP1 signaling as novel axis involved in skin stratification and tumorigenesis.

Keratinocyte-Specific Ablation of RIPK4 Allows Epidermal Cornification but Impairs Skin Barrier Formation

“…This discrepancy could lie in the use of different K14-Cre transgenic lines, Tg(KRT14-cre)1 Cgn versus Tg(KRT14-cre)1 Efu , which may display different timings in full epidermal RIPK4 deletion. The fact that tamoxifen-induced RIPK4 deletion in adult mice resembles the RIPK4 EKO phenotype observed by Lee et al (2017) supports the idea that the timing of RIPK4 deletion affects the severity of the observed phenotype. The skin of the RIPK4 EKO mice generated by Lee et al is thickened with suprabasal expression of K14 and was explained by a lack of RIPK4-dependent PKP1 phosphorylation.…”

“…During the preparation of this manuscript, a RIPK4 EKO mouse has been described by others (Lee et al, 2017). Unlike our RIPK4 EKO mice, their RIPK4 EKO mice survived to adulthood, were runted, showed hair loss, and developed tumors at later age.…”

“…Recently, it was shown that plakophilin-1 (PKP1) is phosphorylated by RIPK4 during epidermal differentiation. Deletion of Pkp1 resulted in epidermal differentiation defects, disturbed tight junctions, and aberrant desmosomes (Lee et al, 2017). We assessed PKP1 localization in RIPK4 fl/fl and RIPK4 EKO skin sections, but found no marked difference between control and RIPK4-deficient skin (Supplementary Figure S3d).…”

RIPK4 suppresses the TGF‐β1 signaling pathway in HaCaT cells

“…Both TGF‐β1 and RIPK4 are important components of epidermal homeostasis (Sellheyer et al, ; Wang et al, ; Holland et al, ; Rountree et al, ; Buschke et al, ; De Groote et al, ). So far, RIPK4 has been shown to regulate NF‐κB, PKC, Wnt, and MAPK signaling pathways (Meylan et al, ; Huang et al, ; Kwa et al, ; Lee et al, ). In this study, we analyzed the effect of RIPK4 on TGF‐β1 signaling in keratinocyte HaCaT cells and identified RIPK4 as an inhibitor of the Smad‐mediated TGF‐β1 signaling.…”

Molecular and cytoskeletal regulations in epidermal development

“…In humans, germline mutations in RIPK4 have been linked to popliteal pterygium syndrome which is characterized by developmental defects in skin, craniofacial and genital systems [46, 47]. RIPK4 regulates epidermal differentiation through phosphorylation of the desmosome component, plakophilin-1 (PKP1) [48]. …”