Role of specific CCAAT/enhancer-binding protein isoforms in intestinal epithelial cells
Intestinal epithelial cells participate in the acute phase response (APR) during intestinal inflammation. We have shown that acute phase protein (APP) genes are induced during the intestinal APR, and that the CCAAT/enhancer binding protein family of transcription factors are involved in APP regulation. To determine the role of different C/EBP isoforms in vivo , we generated IEC-6 rat intestinal epithelial cell lines stably expressing different C/EBP isoforms and dominant-negative C/EBPs, by infection with the retroviral vector pBabepuro. Overexpression of C/EBP[alpha] p30 and C/EBP[delta] led to increases in C/EBP[bêta] LAP and C/EBP[bêta] LIP endogenous protein levels, as determined by electrophoretic mobility shift assays and Western blot. C/EBP[alpha] p30, C/EBP[bêta] LAP and C/EBP[delta] were directly involved in the glucocorticoid-, cAMP- and IL-1-dependent induction of APP gene haptoglobin. Inhibition of C/EBP activity with dominant-negative C/EBPs (C/EBP[bêta] LIP, 3hF, 4hF) decreased haptoglobin gene responsiveness to these regulators. These data show that the three C/EBPs isoforms are involved in the regulation of haptoglobin. [alpha]-acid glycoprotein induction by glucocorticoids was independent of dominant-negative C/EBPs or C/EBP isoform expression, In addition, the response of C/EBP[bêta] and C/EBP[delta] to various regulators was not affected by the overexpression of different C/EBP isoforms or dominant-negative C/EBPs, in contrast to other cell types, C/EBP[bêta] LAP expressing cells showed an increased doubling time characterized by a delay in pRb inactivation in response to serum, a decrease in cyclin D1/2 and cyclin E proteins levels and by an increase in p27[indice supérieur Kip1] protein levels. In addition, C/EBP[bêta] LAP seems to protect intestinal epithelial cells against anoikis. Thus, specific C/EBP isoforms are involved in the differential expression of APP genes in response to hormones and cytokines. Moreover, C/EBP isoforms may play an important role in the control of intestinal epithelial cell growth and apoptosis. Our results confirm the implication of C/EBP isoforms in intestinal inflammation.