Role of Specific CCAAT/Enhancer-binding Protein Isoforms in Intestinal Epithelial Cells

Abstract

Intestinal epithelial cells participate in the acute phase response in response to inflammation. We have shown that acute phase protein genes are induced during intestinal acute phase response, and that the CCAAT/enhancer binding protein family of transcription factors are involved. To address the role of specific C/EBP isoforms, we generated IEC-6 rat intestinal epithelial cell lines expressing different C/EBP isoforms, by retroviral infection. Overexpression of C/EBPα p30 and C/EBPδ led to increases in C/EBPβ LAP and C/EBPβ LIP endogenous protein levels, as determined by electrophoretic mobility shift assays and Western blot. Inhibition of C/EBP activity with dominant negative C/EBPs (C/EBPβ LIP, 3hF, 4hF) decreased glucocorticoid-, cAMP- and IL-1 responsiveness of the endogenous haptoglobin gene, while overexpression of each C/EBP isoform increased the responsiveness to these regulators. In contrast, dominant negative C/EBPs or C/EBP isoforms did not alter the expression of α-acid glycoprotein in response to dexamethasone and of C/EBPβ and C/EBPδ in response to various regulators as assessed by Northern blot. These data show that the three C/EBP isoforms are involved in the regulation of haptoglobin and that C/EBPβ, C/EBPδ, and α-acid glycoprotein expression are not induced by C/EBP isoforms in contrast to other cell types. C/EBPβ LAP-expressing cells showed an inhibition of cell growth characterized by a delay in p27 Kip1 decrease in response to serum and a decrease in cyclin D isoforms and cyclin E protein levels. Finally, C/EBP isoforms interact with the E2F4 transcription factor. Thus, specific C/EBP isoforms are involved in the differential expression of acute phase protein genes in response to hormones and cytokines. Furthermore, C/EBP isoforms may play a role in the control of cell cycle progression.