FOS knockdown or pharmacological inhibition in human SCC cell lines induces p53-dependent differentiation and growth inhibition. (A) Immunohistochemistry of FOS in human skin SCCs. Original magnification, ×10 (upper panels); ×20 (lower panels). Arrowheads indicate positive FOS staining. Scale bar: 100 μm. See Supplemental Table 1 for complete analyses. (B) Immunohistochemistry of TACE in human skin SCCs. Membranous TACE expression is restricted to differentiated cells only. Original magnification, ×10 (upper panels); ×20 (lower panels). Arrowheads indicate positive TACE staining. Scale bar: 100 μm. See Supplemental Table 1 for complete analyses. (C) qRT-PCR analyses upon Fos shRNA–mediated knockdown in SCCO12 (SCC12) show increased expression of p53 and TACE, Notch1, and Keratin 1. (D) qRT-PCR analyses upon Fos shRNA–mediated knockdown in SCC9. No induction of p21 and TACE, Notch1, and Keratin 1 upon Fos knockdown. (E) EdU proliferation assay performed after FOS inhibition (48 hours treatment) in HEK, SCCO12, and SCC9. Experiment was performed 2 times. (F) EdU proliferation assay performed after FOS inhibition (48 hour treatment) in SCCO12 where p53 was knocked down. Experiment was performed 2 times. (G) qRT-PCR analyses upon WT p53 overexpression in SCC9 cells show increased expression of TACE, Notch1, and Keratin 1. Experiment was performed 2 times. (H) EdU proliferation assay after p53 overexpression in SCC9 cells. Experiment was performed 2 times. (I) qRT-PCR upon TACE overexpression through transient transfection in SCC9 cells. *P < 0.05. Experiment was performed 2 times. (J) Model illustrating how the FOS/p53/TACE pathway controls keratinocyte differentiation and skin tumor suppression. p53 directly induces TACE and NOTCH1 expression, thereby promoting keratinocyte differentiation, leading to skin tumor suppression. Red lines indicate the pathways described in this manuscript. Solid lines indicate direct interaction. Dotted line indicates indirect interaction.