Posttranscriptional and Posttranslational Regulation of C/EBPδ in G0 Growth-arrested Mammary Epithelial Cells

Previous work from our laboratory demonstrated that CCAAT/enhancer-binding protein δ (C/EBPδ) functions in the initiation and maintenance of G 0 growth arrest in mouse mammary epithelial cells (MECs). In this report, we investigated the posttranscriptional and posttranslational regulation of C/EBPδ in G 0 growth-arrested mouse MECs. The results of transcriptional inhibitor studies demonstrated that the C/EBPδ mRNA exhibits a relatively short half-life in G 0 growth-arrested mouse MECs ( t ∼35 min). In contrast, C/EBPδ mRNA has a longer half-life in G 0 growth-arrested mouse fibroblast cells ( t >100 min). Oligo/RNase H cleavage analysis and rapid amplification of cDNA ends-poly(A) test both confirmed the short C/EBPδ mRNA half-life observed in MECs and demonstrated that the C/EBPδ mRNA poly(A) tail is relatively short (∼100 nucleotides). In addition, the poly(A) tail length was not shortened during C/EBPδ mRNA degradation, which suggested a deadenylation-independent pathway. The C/EBPδ protein also exhibited a relatively short half-life in G 0 growth-arrested mouse MECs ( t ∼120 min). The C/EBPδ protein was degraded in a ubiquitin-dependent manner, primarily in the nucleus, during G 0 growth arrest. In conclusion, these studies indicated that the C/EBPδ mRNA and protein content are under tight regulation in G 0 growth-arrested mouse MECs, despite the general concept that G 0 growth arrest is associated with a decrease in cellular activity.