CCAAT/enhancer binding protein-β is a mediator of keratinocyte survival and skin tumorigenesis involving oncogenic Ras signaling

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Abstract
The basic leucine zipper transcription factor CCAAT/enhancer binding protein-β (C/EBPβ) is expressed in many cell types, including keratinocytes. C/EBPβ activity can be increased by phosphorylation through pathways stimulated by oncogenic Ras, although the biological implications of Ras-C/EBPβ signaling are not currently understood. We report here that C/EBPβ-nullizygous mice are completely refractory to skin tumor development induced by a variety of carcinogens and carcinogenesis protocols, including 7,12-dimethylbenz[a]anthracene-initiation/12-O-tetradecanoylphorbol 13-acetate promotion, that produce tumors containing oncogenic Ras mutations. No significant differences in TPA-induced epidermal keratinocyte proliferation were observed in C/EBPβ-null versus wild-type mice. However, apoptosis was significantly elevated (17-fold) in the epidermal keratinocytes of 7,12-dimethylbenz[a]anthracene-treated C/EBPβ-null mice compared with wild-type mice. In v-Ha-ras transgenic mice, C/EBPβ deficiency also led to greatly reduced skin tumor multiplicity and size, providing additional evidence for a tumorigenesis pathway linking Ras and C/EBPβ. Oncogenic Ras potently stimulated C/EBPβ to activate a C/EBP-responsive promoter-reporter in keratinocytes and mutating an ERK1/2 phosphorylation site (T188) in C/EBPβ abolished this Ras effect. Finally, we observed that C/EBPβ participates in oncogenic Ras-induced transformation of NIH 3T3 cells. These findings indicate that C/EBPβ has a critical role in Ras-mediated tumorigenesis and cell survival and implicate C/EBPβ as a target for tumor inhibition.

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