Loss of protein phosphatase 6 in mouse keratinocytes enhances K‐rasG12D‐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‐rasG12D‐expressing and Ppp6c‐deficient) mice in which K‐rasG12D 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‐rasG12D‐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‐rasG12D mice revealed that cell proliferation and cell size increased approximately 2‐fold relative to K‐rasG12D‐expressing mutants, and epidermal thickness of lip tissue greatly increased relative to that seen in K‐rasG12D‐only mice. Moreover, AKT phosphorylation increased in K‐rasG12D‐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‐rasG12D‐dependent tumor promotion. We have defined synergy between Ppp6c deficiency and expression of mutant K‐ras in skin tumorigenesis. Highly activated AKT seen in double mutant cells likely plays an important role in skin carcinogenesis (namely, normal skin, to hyperplastic epidermis, to papilloma, and then to SCC).