The transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) is a strong inhibitor of cell proliferation. We found that C/EBPalpha directly interacts with cdk2 and cdk4 and arrests cell proliferation by inhibiting these kinases. We mapped a short growth inhibitory region of C/EBPalpha between amino acids 175 and 187. This portion of C/EBPalpha is responsible for direct inhibition of cyclin-dependent kinases and causes growth arrest in cultured cells. C/EBPalpha inhibits cdk2 activity by blocking the association of cdk2 with cyclins. Importantly, the activities of cdk4 and cdk2 are increased in C/EBPalpha knockout livers, leading to increased proliferation. Our data demonstrate that the liver-specific transcription factor C/EBPalpha brings about growth arrest through direct inhibition of cdk2 and cdk4.

The liver is capable of completely regenerating itself in response to injury and after partial hepatectomy. In liver of old animals, the proliferative response is dramatically reduced, the mechanism for which is unknown. The liver specific protein, C/EBPalpha, normally arrests proliferation of hepatocytes through inhibiting cyclin dependent kinases (cdks). We present evidence that aging switches the liver-specific pathway of C/EBPalpha growth arrest to repression of E2F transcription. We identified an age-specific C/EBPalpha-Rb-E2F4 complex that binds to E2F-dependent promoters and represses these genes. The C/EBPalpha-Rb-E2F4 complex occupies the c-myc promoter and blocks induction of c-myc in livers of old animals after partial hepatectomy. Our results show that the age-dependent switch from cdk inhibition to repression of E2F transcription causes a loss of proliferative response in the liver because of an inability to induce E2F target genes after partial hepatectomy providing a possible mechanism for the age-dependent loss of liver regenerative capacity.

Accumulation of RNA CUG repeats in myotonic dystrophy type 1 (DM1) patients leads to the induction of a CUG-binding protein, CUGBP1, which increases translation of several proteins that are required for myogenesis. In this paper, we examine the role of overexpression of CUGBP1 in DM1 muscle pathology using transgenic mice that overexpress CUGBP1 in skeletal muscle. Our data demonstrate that the elevation of CUGBP1 in skeletal muscle causes overexpression of MEF2A and p21 to levels that are significantly higher than those in skeletal muscle of wild type animals. A similar induction of these proteins is observed in skeletal muscle of DM1 patients with increased levels of CUGBP1. Immunohistological analysis showed that the skeletal muscle from mice overexpressing CUGBP1 is characterized by a developmental delay, muscular dystrophy, and myofiber-type switch: increase of slow/oxidative fibers and the reduction of fast fibers. Examination of molecular mechanisms by which CUGBP1 up-regulates MEF2A shows that CUGBP1 increases translation of MEF2A via direct interaction with GCN repeats located within MEF2A mRNA. Our data suggest that CUGBP1-mediated overexpression of MEF2A and p21 inhibits myogenesis and contributes to the development of muscle deficiency in DM1 patients. DM11 is a multisystem disease mainly characterized by defects in skeletal muscle with the involvement of many tissues and systems such as cardiac muscle, brain, eye, and endocrine system (1). DM1 is caused by an expansion of CTG trinucleotide repeats within the 3Ј-untranslated region of the myotonin protein kinase gene (2). In DM1 patients, the size of DNA CTG expansion correlates with the severity of the disease. Patients with CTG expansion containing 50 -80 CTG repeats are almost asymptomatic. Individuals bearing the myotonin protein kinase gene with 100 -500 CTG repeats develop a disease in adult life (classical adult form of DM1) that is characterized by a progressive muscle wasting with myotonia. The most severe form of DM1, congenital disease, affects patients before or after birth and is associated with long CTG expansions (up to 2,000 repeats). This form of disease is characterized by a delay or arrest of skeletal muscle development (3). Although there is an overlap in range of repeats between different forms of DM1, there is a clear correlation of repeat number with severity of phenotype and reduction of age of onset.Investigations of molecular alterations in DM1 suggest that the expansion of CTG repeats causes the DM1 pathology through different mechanisms, mediated at both DNA and RNA levels. It has been shown that CTG repeats reduce expression of myotonin protein kinase in cis (4), causing abnormalities in cardiac muscle (5). CTG repeats also affect transcription of genes adjacent to myotonin protein kinase (6), leading to the development of cataracts (7,8). A number of recent studies indicate that other symptoms in DM1 such as myotonia (9, 10), delay of skeletal muscle differentiation (11,12), and a resistance to insulin (13) are mediated ...

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Differentiation of skeletal muscle is affected in myotonic dystrophy (DM) patients. Analysis of cultured myoblasts from DM patients shows that DM myoblasts lose the capability to withdraw from the cell cycle during differentiation. Our data demonstrate that the expression and activity of the proteins responsible for cell cycle withdrawal are altered in DM muscle cells. Skeletal muscle cells from DM patients fail to induce cytoplasmic levels of a CUG RNA binding protein, CUGBP1, while normal differentiated cells accumulate CUGBP1 in the cytoplasm. In cells from normal patients, CUGBP1 up-regulates p21 protein during differentiation. Several lines of evidence show that CUGBP1 induces the translation of p21 via binding to a GC-rich sequence located within the 5' region of p21 mRNA. Failure of DM cells to accumulate CUGBP1 in the cytoplasm leads to a significant reduction of p21 and to alterations of other proteins responsible for the cell cycle withdrawal. The activity of cdk4 declines during differentiation of cells from control patients, while in DM cells cdk4 is highly active during all stages of differentiation. In addition, DM cells do not form Rb/E2F repressor complexes that are abundant in differentiated cells from normal patients. Our data provide evidence for an impaired cell cycle withdrawal in DM muscle cells and suggest that alterations in the activity of CUGBP1 causes disruption of p21-dependent control of cell cycle arrest.