51. Calmodulin Controls Liver Proliferation via Interactions with C/EBPβ-LAP and C/EBPβ-LIP
- Author
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Xiaoying Liu, Jingling Jin, Gou Li Wang, Polina Iakova, Daniel Orellana, and Nikolai A. Timchenko
- Subjects
Lipopolysaccharides ,Gene isoform ,animal structures ,Calmodulin ,Down-Regulation ,Cell Cycle Proteins ,Biology ,Retinoblastoma Protein ,Biochemistry ,CCAAT-Enhancer-Binding Protein-beta ,Cell Line ,Mice ,Downregulation and upregulation ,Animals ,Humans ,Acute-Phase Reaction ,Promoter Regions, Genetic ,Molecular Biology ,Cell Proliferation ,Cell Nucleus ,Ccaat-enhancer-binding proteins ,Cell growth ,Retinoblastoma protein ,Cell Biology ,Cell cycle ,Molecular biology ,E2F Transcription Factors ,Cell biology ,stomatognathic diseases ,Liver ,Trans-Activators ,biology.protein ,Calcium ,Protein Binding - Abstract
A truncated isoform of C/EBPbeta, C/EBPbeta-LIP, is required for liver proliferation. This isoform is expressed at high levels in proliferating liver and in liver tumors. However, high levels of C/EBPbeta-LIP are also observed in non-proliferating livers during acute phase response (APR). In this paper we present mechanisms by which liver regulates activities of C/EBPbeta-LIP. We found that calmodulin (CaM) inhibits the ability of C/EBPbeta-LIP to promote liver proliferation during APR through direct interactions. This activity of CaM is under negative control of Ca(2+), which is reduced in nuclei of livers with APR, whereas it is increased in nuclei of proliferating livers. A mutant CaM, which does not interact with C/EBPbeta-LIP, also fails to inhibit the growth promotion activity of C/EBPbeta-LIP. Down-regulation of CaM in livers of LPS-treated mice causes liver proliferation via activation of C/EBPbeta-LIP. Overexpression of C/EBPbeta-LIP above levels of CaM also initiates liver proliferation in LPS-treated mice. In addition, CaM regulates transcriptional activity of another isoform of C/EBPbeta, C/EBPbeta-LAP, and might control liver biology through the regulation of both isoforms of C/EBPbeta. In searching for molecular mechanisms by which C/EBPbeta-LIP promotes cell proliferation, we found that C/EBPbeta-LIP releases E2F.Rb-dependent repression of cell cycle genes by a disruption of E2F1.Rb complexes and by a direct interaction with E2F-dependent promoters. CaM inhibits these growth promotion activities of C/EBPbeta-LIP and, therefore, supports liver quiescence. Thus, our findings discover a new pathway of the regulation of liver proliferation that involves calcium-CaM signaling.
- Published
- 2010
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