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 delta (C/EBPdelta) 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/EBPdelta in G(0) growth-arrested mouse MECs. The results of transcriptional inhibitor studies demonstrated that the C/EBPdelta mRNA exhibits a relatively short half-life in G(0) growth-arrested mouse MECs (t(1/2) similar to35 min). In contrast, C/EBPdelta mRNA has a longer half-life in G(0) growth-arrested mouse fibroblast cells (t(1/2) > 100 min). Oligo/RNase H cleavage analysis and rapid amplification of cDNA ends-poly(A) test both confirmed the short C/EBPdelta mRNA half-life observed in MECs and demonstrated that the C/EBPdelta mRNA poly(A) tail is relatively short (similar to100 nucleotides). In addition, the poly(A) tail length was not shortened during C/EBPdelta mRNA degradation, which suggested a deadenylation-independent pathway. The C/EBPdelta protein also exhibited a relatively short half-life in Go growth-arrested mouse MECs (t(1/2) similar to120 min). The C/EBPbeta 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/EBPdelta 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.