Diminished expression of C/EBPα in skin carcinomas is linked to oncogenic ras and reexpression of C/EBPα in carcinoma cells inhibits proliferation

Abstract

The basic leucine zipper transcription factor, CCAAT/enhancer binding protein α (C/EBPα) is involved in mitotic growth arrest and has been implicated as a human tumor suppressor in acute myeloid leukemia. We have previously shown that C/EBPα is abundantly expressed in mouse epidermal keratinocytes. In the current study, the expression of C/EBPα was evaluated in seven mouse skin squamous cell carcinoma (SCC) cell lines that contain oncogenic Ha-Ras. C/EBPα mRNA and protein levels were greatly diminished in all seven SCC cell lines compared with normal primary keratinocytes, whereas C/EBPβ levels were not dramatically changed. Reexpression of C/EBPα in these SCC cell lines resulted in the inhibition in SCC cell proliferation. To determine whether the decrease in C/EBPα expression observed in the SCC cell lines also occurred in the carcinoma itself, immunohistochemical staining for C/EBPα in mouse skin SCCs was conducted. All 14 SCCs evaluated displayed negligible C/EBPα protein expression and normal C/EBPβ levels compared with the epidermis and all 14 carcinomas contained mutant Ras. To determine whether oncogenic Ras is involved in the down-regulation of C/EBPα, BALB/MK2 keratinocytes were infected with a retrovirus containing Ras12V, and C/EBPα protein, mRNA and DNA binding levels were determined. Keratinocytes infected with the retrovirus containing oncogenic Ras12V displayed greatly diminished C/EBPα protein, mRNA and DNA binding levels. In addition, BALB/MK2 cells containing endogenous mutant Ras displayed diminished C/EBPα expression and the ectopic expression of a dominant-negative RasN17 partially restored C/EBPα levels in these cells. These results indicate that oncogenic Ras negatively regulates C/EBPα expression and the loss of C/EBPα expression may contribute to the development of skin SCCs.