Genetic Ablation of CCAAT/Enhancer Binding Protein α in Epidermis Reveals Its Role in Suppression of Epithelial Tumorigenesis

Abstract

CCAAT/enhancer binding protein A (C/EBPA) is a basic leucine zipper transcription factor that inhibits cell cycle progression and regulates differentiation in various cell types. C/EBPA is inactivated by mutation in acute myeloid leukemia (AML) and is considered a human tumor suppressor in AML. Although C/EBPA mutations have not been observed in malignancies other than AML, greatly diminished expression of C/EBPA occurs in numerous human epithelial cancers including lung, liver, endometrial, skin, and breast, suggesting a possible tumor suppressor function. However, direct evidence for C/EBPA as an epithelial tumor suppressor is lacking due to the absence of C/EBPA mutations in epithelial tumors and the lethal effect of C/EBPA deletion in mouse model systems. To examine the function of C/EBPA in epithelial tumor development, an epidermal-specific C/EBPA knockout mouse was generated. The epidermal-specific C/EBPA knockout mice survived and displayed no detectable abnormalities in epidermal keratinocyte proliferation, differentiation, or apoptosis, showing that C/EBPA is dispensable for normal epidermal homeostasis. In spite of this, the epidermal-specific C/EBPA knockout mice were highly susceptible to skin tumor development involving oncogenic Ras. These mice displayed decreased tumor latency and striking increases in tumor incidence, multiplicity, growth rate, and the rate of malignant progression. Mice hemizygous for C/ EBPA displayed an intermediate-enhanced tumor phenotype. Our results suggest that decreased expression of C/EBPA contributes to deregulation of tumor cell proliferation. C/ EBPA had been proposed to block cell cycle progression through inhibition of E2F activity. We observed that C/EBPA blocked Ras-induced and epidermal growth factor-induced E2F activity in keratinocytes and also blocked Ras-induced cell transformation and cell cycle progression. Our study shows that C/EBPA is dispensable for epidermal homeostasis and provides genetic evidence that C/EBPA is a suppressor of epithelial tumorigenesis. [Cancer Res 2007;67(14):6768-76]

Repression of Transcriptional Activity of C/EBPα by E2F-Dimerization Partner Complexes

“…Abrogation of C/EBP␣ functions has also been reported in AML-ETO and BCR-ABL leukemias (36,38), and ectopic expression of C/EBP␣ induces the differentiation of such leukemic cells (53,54). Moreover, the loss of C/EBP␣ expression in the epidermis increases susceptibility to Rasinduced skin tumorigenesis (28), similarly to the overexpression of E2F1 or DP1 transgenes in the mouse (40,55). To further delineate the contribution of failure to activate terminal differentiation genes in tumorigenesis, it would be interesting to identify a DP mutant that retains E2F coregulation but is defective for C/EBP␣ interaction.…”

“…Interestingly, C/EBP␣ and C/EBP␤ appear to be functionally distinct, as Sebastian and coworkers have shown that C/EBP␤ fails to repress E2F in an equivalent TKO cell line (46,48). It is known that despite their extended homology and partial redundancy (2,6,20), C/EBP␣ and C/EBP␤ may also have antagonistic functions, e.g., in the case of skin tumorigenesis, where the lack of C/EBP␣ promotes tumorigenesis (28), whereas the lack of C/EBP␤ protects against chemical-induced skin carcinogenesis (60). Curiously, pRB deficiency also protects from tumorigenesis in a murine skin carcinogenesis model (45).…”

Epidermal Barriers

“…Whereas C/EBP-b-deficient mice had mild epidermal hyperplasia, epidermis-specific C/EBP-a-deficient mice showed no skin phenotypic abnormalities in steady state (Zhu et al 1999;Loomis et al 2007). Keratinocyte-specific ablation of both C/EBP-a and C/ EBP-b revealed hyperproliferation of basal kerationocytes and halted terminal differentiation (Lopez et al 2009).…”

The emerging role of C/EBPs in glucocorticoid signaling: lessons from the lung

“…A number of studies have indicated a role for C/EBPa in regulating proliferation in several organs, including the lung (Flodby et al 1996, Diehl 1998, Loomis et al 2007. Deletion and ectopic expression of C/EBPa in the lung have revealed a vital role in regulating proliferation also in the alveolar epithelium (Flodby et al 1996, Sugahara et al 2001, Basseres et al 2006, Berg et al 2006, Didon et al 2010.…”