CCAAT/Enhancer Binding Protein alpha uses distinct domains to prolong pituitary cells in the Growth 1 and DNA Synthesis phases of the cell cycle

We examined the consequences of C/EBPα expression on proliferation of the transformed, mouse GHFT1-5 pituitary progenitor cell line. In contrast to mature pituitary cells, GHFT1-5 cells do not contain C/EBPα. Ectopic expression of C/EBPα in the progenitor cells resulted in prolongation of both growth 1 (G1) and the DNA synthesis (S) phases of the cell cycle. Transcription activation domain 1 and 2 of C/EBPα were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPα remained competent for G1 and S phase prolongation. C/EBPα deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPα in prolonging G1 and S.We found that C/EBPα utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells. G1 and S phase prolongation did not require that C/EBPα remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPα transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPα to regulate gene expression independently of its effects on proliferation.Differentiation is commonly associated with intermingled changes in gene expression and cellular proliferation. In some differentiating cell types, changes in both gene transcription and proliferation are regulated by the same transcription factor [1-9]. CCAAT/Enhancer Binding Protein alpha (C/EBPα) is a transcription factor that is required for the differentiation of a number of tissues [10-18]. Mice homozygous for C/EBPα null alleles have severe defects in tissues involved in metabolic homeostasis [19-21]. Cellular proliferation is elevated in the liver of these knockout mice [22] suggesting that C/EBPα blocks proliferation in vivo. In cultured cells, C/EBPα expression leads to decreased colony formation upon antibiotic selection [23-25], decreased