Loss of protein phosphatase 6 in mouse keratinocytes enhances K‐ras G12D ‐driven tumor promotion

Here, we address the function of protein phosphatase 6 (PP6) loss on K‐ras‐initiated tumorigenesis in keratinocytes. To do so, we developed tamoxifen‐inducible double mutant ( K‐ras G12D ‐expressing and Ppp6c ‐deficient) mice in which K‐ras G12D expression is driven by the cytokeratin 14 (K14) promoter. Doubly‐mutant mice showed early onset tumor formation in lips, nipples, external genitalia, anus and palms, and had to be killed by 3 weeks after induction by tamoxifen, while comparably‐treated K‐ras G12D ‐expressing mice did not. H&E‐staining of lip tumors before euthanasia revealed that all were papillomas, some containing focal squamous cell carcinomas. Immunohistochemical analysis of lips of doubly‐mutant vs K‐ras G12D mice revealed that cell proliferation and cell size increased approximately 2‐fold relative to K‐ras G12D ‐expressing mutants, and epidermal thickness of lip tissue greatly increased relative to that seen in K‐ras G12D ‐only mice. Moreover, AKT phosphorylation increased in K‐ras G12D ‐expressing/ Ppp6c ‐deficient cells, as did phosphorylation of the downstream effectors 4EBP1, S6 and GSK3, suggesting that protein synthesis and survival signals are enhanced in lip tissues of doubly‐mutant mice. Finally, increased numbers of K14‐positive cells were present in the suprabasal layer of doubly‐mutant mice, indicating abnormal keratinocyte differentiation, and γH2AX‐positive cells accumulated, indicating perturbed DNA repair. Taken together, Ppp6c deficiency enhances K‐ras G12D ‐dependent tumor promotion.