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Regulation and role of CCAAT/enhancer binding protein during differentiation of intestinal epithelial cells

Memişoğlu, Aslı Sade
Banerjee, Sreeparna
Background: The molecular mechanisms of differentiation of intestinal epithelial cells, is poorly understood and disruption of this balance may result in neoplastic transformation and malignant growth. The family of CCAAT/Enhancer Binding Protein (C/EBP) transcription factors is implicated in cellular growth, differentiation, inflammation and development and control differentiation in several cell types. The involvement of these transcription factors in intestinal differentiation is not known. The aim of this study is to elucidate the transcriptional, translational and post-translational regulation of C/EBP proteins and their role during the course of intestinal epithelial differentiation. Materials and Methods: The colorectal cancer cell line Caco-2, a well established differentiation model, was used. The expression of C/EBP genes was analyzed using publicly available microarray data. The protein levels and the phosphorylation status of translation initiation factors were checked by Western blot. The activity of C/EBP was determined by luciferase reporter and chromatin immunoprecipitation assays. Cytoplasmic calcium levels were determined fluorometrically by calcium binding with Fura-2AM. Results: Microarray data analyses of Caco-2 differentiation have shown that C/EBPa and b expression increases during differentiation. The major regulatory mechanism for C/EBP proteins is translational in which several isoforms are produced from alternative translation initiation sites. The ratio of the two isoforms C/EBPb1 to C/EBPb3 was found to be decreasing in differentiated Caco-2 cells. This change could be explained by the increase in the active translation initiation factor eIF2a. The activity of C/EBP was shown to increase together with an upregulation of its target genes. The post-translational regulation was investigated by determining SERCA-3 (less affinity to calcium) levels and was shown to decrease. This lead to an increase in the cytoplasmic calcium levels which in turn may cause activation of calcium dependent calmodulin kinase II, hence increase in C/EBP activity. Conclusions: In this study the involvement of C/EBP proteins in intestinal epithelial differentiation was investigated for the first time. We believe that the findings of this research would reveal important mechanisms underlying the intestinal differentiation-dedifferentiation processes and provide some candidate therapeutic targets for the prevention and treatment of colorectal cancer.