Decreased survival of C/EBP beta-deficient keratinocytes is due to aberrant regulation of p53 levels and function

Recent studies have identified roles for C/EBP beta in cellular survival and tumorigenesis, however, the mechanisms through which C/EBPb regulates these processes are not fully understood. Previously, we demonstrated that C/EBP beta(-/-) mice are resistant to carcinogen-induced skin tumorigenesis and in response to topical carcinogen treatment display a 17-fold increase in keratinocyte apoptosis compared to wild-type mice. Here, we have investigated the mechanisms through which C/EBPb regulates apoptosis in response to carcinogenic stress. Analysis of carcinogen-treated C/EBP beta(-/-) mouse skin revealed a striking increase in the number of p53 immunopositive keratinocytes in the epidermis of C/EBP beta(-/-) mice compared to wild-type mice and this increase was temporally associated with a concomitant anomalous increase in apoptosis. The increased levels of p53 were functional as Mdm2, Bcl-2, C/EBP alpha and p21 were differentially regulated in the epidermis of carcinogenic treated C/EBP beta(-/-) mice. The increase in p53 protein was not associated with an increase in p53 mRNA levels. To determine whether p53 is required for the increased apoptosis in C/EBPb-/- mice, C/EBP beta/p53 compound knockout mice were generated. Carcinogen-treated C/EBP beta/p53 compound knockout mice did not display increased apoptosis demonstrating p53 is required for the proapoptotic phenotype in C/EBP beta(-/-) mice. Our results demonstrate that altered keratinocyte survival in C/EBP beta(-/-) mice results from aberrant regulation of p53 protein and function and indicate C/EBP beta has a role in the negative regulation of p53 protein levels in response to carcinogen-induced stress.