Depletion of RIPK4 parallels higher malignancy potential in cutaneous squamous cell carcinoma

The RIPK4 (receptor-interacting protein kinase 4), a member of the RIPK family, acts as an important regulator of epidermal differentiation, cutaneous inflammation, and cutaneous wound repair. However, Until now, the role of RIPK4 in tumorigenesis remains elusive. There have been no studies exploring the effects of RIPK4 on the signaling pathway in cutaneous squamous cell carcinoma (SCC). It remains unknown whether RIPK4 expression, which can affect the degree of epidermal differentiation can also influence the radiosensitivity of skin SCC. It is urgent to fully elucidate the biological mechanism by which RIPK4 promotes carcinogenesis in skin SCC and determine whether RIPK4 expression levels predicts the sensitivity to radiotherapy in skin SCC.

Human skin SCC cell line, A431, was transfected with either small interfering RNAs (siRNAs) targeting RIPK4 (siR-RIPK4) or negative control siRNA (siR-NC). Western blotting was used to detect the expression of RIPK4 and Raf/MEK/ERK pathway-related proteins. The cells were irradiated using an X-ray irradiator at 6 MV with different radiation doses (0, 2, 6, and 10 Gy). Cell proliferation analysis, colony formation assay, transwell cell migration and invasion assay, cell cycle and apoptosis analysis were conducted to investigate the effect of RIPK4 silencing on skin SCC malignancy and radiosensitivity.

RIPK4 protein expression was significantly decreased in the A431 cells transfected with siR-RIPK4, compared with the A431 cells transfected with siR-NC. RIPK4 silencing facilitated the proliferation, colony formation, migration, and invasion ability of A431 cell line, while cell cycle progression or cell apoptosis were not significantly influenced. In contrast with the previous literature, Raf/MEK/ERK pathway was not effected by RIPK4 knockdown in skin SCC. RIPK4 knockdown could not reverse the radiation resistance of A431 cells to irradiation in vitro.

In general, although depletion of RIPK4 cannot reverse the radiation resistance of A431 cells in vitro, it parallels higher malignancy potential in cutaneous SCC. To our knowledge, this is the first report of the effects of RIPK4 expression on the Raf/MEK/ERK signaling pathway and radiosensitivity in cutaneous SCC. The better understanding of the molecular mechanism of RIPK4 in cutaneous SCC may provide a promising biomarker for skin SCC prognosis and treatment.