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CCAAT/enhancer binding protein alpha uses distinct domains to prolong pituitary cells in the growth 1 and DNA synthesis phases of the cell cycle.

Liu W, Enwright JF, Hyun W, Day RN, Schaufele F - BMC Cell Biol. (2002)

Bottom Line: C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.We found that C/EBPalpha utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells.Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

View Article: PubMed Central - HTML - PubMed

Affiliation: Metabolic Research Unit, Diabetes Research Center and Department of Medicine, University of California, San Francisco, CA 94143-0540, USA. liu_weiqun@hotmail.com

ABSTRACT

Background: A number of transcription factors coordinate differentiation by simultaneously regulating gene expression and cell proliferation. CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic/leucine zipper transcription factor that integrates transcription with proliferation to regulate the differentiation of tissues involved in energy balance. In the pituitary, C/EBPalpha regulates the transcription of a key metabolic regulator, growth hormone.

Results: We examined the consequences of C/EBPalpha 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/EBPalpha. Ectopic expression of C/EBPalpha 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/EBPalpha were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPalpha remained competent for G1 and S phase prolongation. C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.

Conclusion: We found that C/EBPalpha 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/EBPalpha remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPalpha transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

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A, Positions of the transcription activation (TA), DNA binding (basic) and dimerization (ZIP) domains along the linear sequence of C/EBPα. The numbers below the C/EBPα diagram indicate the amino acid positions at the boundaries of the domains. Also shown are C/EBPα sequences involved in known actions with the indicated cell-cycle proteins: p107 [34], p21 [25], CDK2 [25,36], CDK4 [36] and E2F [33]. B, Schematic of the GFP fusions with full-length and mutant C/EBPα used in the studies reported here.
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Figure 1: A, Positions of the transcription activation (TA), DNA binding (basic) and dimerization (ZIP) domains along the linear sequence of C/EBPα. The numbers below the C/EBPα diagram indicate the amino acid positions at the boundaries of the domains. Also shown are C/EBPα sequences involved in known actions with the indicated cell-cycle proteins: p107 [34], p21 [25], CDK2 [25,36], CDK4 [36] and E2F [33]. B, Schematic of the GFP fusions with full-length and mutant C/EBPα used in the studies reported here.

Mentions: Possible mechanisms of transcription-independent proliferation arrest by C/EBPα have been suggested by a number of studies. Decreased proliferation was associated with C/EBPα stabilization of the p21 protein [22,24]. p21 interacted directly with a large internal segment of C/EBPα that included transcription activation domain 3 [25] (see Fig. 1A). CDK2 and CDK4 also interacted with segments of C/EBPα close to, and within, transcription activation domain 3 [36]. CDK2 also interacted with the basic region of the C/EBPα [25]. p21 also has a second interaction site, within the leucine zipper of C/EBPα [25]. in vitro, C/EBPα enhanced p21 inhibition of CDK2 activity. C/EBPα inhibition of CDK2 activity correlated with p21 binding to C/EBPα transcription activation domain 3 [25]. However, proliferation arrest by C/EBPα still occurred in cell lines not containing p21 genes [40]. This indicated that proliferation arrest by C/EBPα did not rely solely upon C/EBPα enhancement of CDK inhibition by p21.


CCAAT/enhancer binding protein alpha uses distinct domains to prolong pituitary cells in the growth 1 and DNA synthesis phases of the cell cycle.

Liu W, Enwright JF, Hyun W, Day RN, Schaufele F - BMC Cell Biol. (2002)

A, Positions of the transcription activation (TA), DNA binding (basic) and dimerization (ZIP) domains along the linear sequence of C/EBPα. The numbers below the C/EBPα diagram indicate the amino acid positions at the boundaries of the domains. Also shown are C/EBPα sequences involved in known actions with the indicated cell-cycle proteins: p107 [34], p21 [25], CDK2 [25,36], CDK4 [36] and E2F [33]. B, Schematic of the GFP fusions with full-length and mutant C/EBPα used in the studies reported here.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC101385&req=5

Figure 1: A, Positions of the transcription activation (TA), DNA binding (basic) and dimerization (ZIP) domains along the linear sequence of C/EBPα. The numbers below the C/EBPα diagram indicate the amino acid positions at the boundaries of the domains. Also shown are C/EBPα sequences involved in known actions with the indicated cell-cycle proteins: p107 [34], p21 [25], CDK2 [25,36], CDK4 [36] and E2F [33]. B, Schematic of the GFP fusions with full-length and mutant C/EBPα used in the studies reported here.
Mentions: Possible mechanisms of transcription-independent proliferation arrest by C/EBPα have been suggested by a number of studies. Decreased proliferation was associated with C/EBPα stabilization of the p21 protein [22,24]. p21 interacted directly with a large internal segment of C/EBPα that included transcription activation domain 3 [25] (see Fig. 1A). CDK2 and CDK4 also interacted with segments of C/EBPα close to, and within, transcription activation domain 3 [36]. CDK2 also interacted with the basic region of the C/EBPα [25]. p21 also has a second interaction site, within the leucine zipper of C/EBPα [25]. in vitro, C/EBPα enhanced p21 inhibition of CDK2 activity. C/EBPα inhibition of CDK2 activity correlated with p21 binding to C/EBPα transcription activation domain 3 [25]. However, proliferation arrest by C/EBPα still occurred in cell lines not containing p21 genes [40]. This indicated that proliferation arrest by C/EBPα did not rely solely upon C/EBPα enhancement of CDK inhibition by p21.

Bottom Line: C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.We found that C/EBPalpha utilizes mechanistically distinct activities to prolong the cell cycle in G1 and S in pituitary progenitor cells.Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

View Article: PubMed Central - HTML - PubMed

Affiliation: Metabolic Research Unit, Diabetes Research Center and Department of Medicine, University of California, San Francisco, CA 94143-0540, USA. liu_weiqun@hotmail.com

ABSTRACT

Background: A number of transcription factors coordinate differentiation by simultaneously regulating gene expression and cell proliferation. CCAAT/enhancer binding protein alpha (C/EBPalpha) is a basic/leucine zipper transcription factor that integrates transcription with proliferation to regulate the differentiation of tissues involved in energy balance. In the pituitary, C/EBPalpha regulates the transcription of a key metabolic regulator, growth hormone.

Results: We examined the consequences of C/EBPalpha 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/EBPalpha. Ectopic expression of C/EBPalpha 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/EBPalpha were required for prolongation of G1, but not of S. Some transcriptionally inactive derivatives of C/EBPalpha remained competent for G1 and S phase prolongation. C/EBPalpha deleted of its leucine zipper dimerization functions was as effective as full-length C/EBPalpha in prolonging G1 and S.

Conclusion: We found that C/EBPalpha 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/EBPalpha remained transcriptionally active or retained the ability to dimerize via the leucine zipper. G1, but not S, arrest required a domain overlapping with C/EBPalpha transcription activation functions 1 and 2. Separation of mechanisms governing proliferation and transcription permits C/EBPalpha to regulate gene expression independently of its effects on proliferation.

Show MeSH
Related in: MedlinePlus