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Introduction Myotonic dystrophy type 1 (DM1) is a complex disease affecting primarily skeletal muscle, causing myotonia, skeletal muscle weakness, and wasting (1). DM1 is caused by the expansion of polymorphic, [...], Myotonic dystrophy type 1 (DM1) is a complex neuromuscular disease characterized by skeletal muscle wasting, weakness, and myotonia. DM1 is caused by the accumulation of CUG repeats, which alter the biological activities of RNA-binding proteins, including CUG-binding protein 1 (CUGBP1). CUGBP1 is an important skeletal muscle translational regulator that is activated by cyclin D3-dependent kinase 4 (CDK4). Here we show that mutant CUG repeats suppress Cdk4 signaling by increasing the stability and activity of glycogen synthase kinase 3β (GSK3β). Using a mouse model of DM1 ([HSA.sup.LR]), we found that CUG repeats in the 3' untranslated region (UTR) of human skeletal actin increase active GSK3β in skeletal muscle of mice, prior to the development of skeletal muscle weakness. Inhibition of GSK3β in both DM1 cell culture and mouse models corrected cyclin D3 levels and reduced muscle weakness and myotonia in DM1 mice. Our data predict that compounds normalizing GSK3β activity might be beneficial for improvement of muscle function in patients with DM1.

Introduction Liver is a quiescent tissue that is able to regenerate itself in response to partial hepatectomy (PH) and after injury (1), (2). Under normal conditions, the quiescent stage of [...], Despite significant advancements in our understanding of cancer development, the molecular mechanisms that underlie the formation of liver cancer remain largely unknown. C/EBPα is a transcription factor that regulates liver quiescence. Phosphorylation of C/EBPα at serine 193 (S193-ph) is upregulated in older mice and is thought to contribute to age-associated liver dysfunction. Because development of liver tumors is associated with increasing age, we investigated the role of S193-ph in the development of liver cancer using knockin mice expressing a phospho-mimetic aspartic acid residue in place of serine at position 193 (S193D) of C/EBPα. The S193D isoform of C/EBPα was able to completely inhibit liver proliferation in vivo after partial hepatectomy. However, treatment of these mice with diethylnitrosamine/phenobarbital (DEN/PB), which induces formation of liver cancer, actually resulted in earlier development of liver tumors. DEN/PB treatment was associated with specific degradation of both the S193-ph and S193D isoforms of C/EBPα through activation of the ubiquitin-proteasome system (UPS). The mechanism of UPS-mediated elimination of C/EBPα during carcinogenesis involved elevated levels of gankyrin, a protein that was found to interact with the S193-ph isoform of C/EBPα and target it for UPS-mediated degradation. This study identifies a molecular mechanism that supports the development of liver cancer in older mice and potential therapeutic targets for the prevention of liver cancer.

Normal hepatocytes that escape negative control of proliferation lead to tumor cells. Activation of the PI3K/Akt pathway in live tumor cells block the growth inhibitory activity of C/EPP alpha.

Summary: Nonalcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide. NAFLD usually begins as low‐grade hepatic steatosis which further progresses in an age‐dependent manner to nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and hepatocellular carcinoma in some patients. Ghrelin is a hormone known to promote adiposity in rodents and humans, but its potential role in hepatic steatosis is unknown. We hypothesized that genetic ghrelin deletion will protect against the development of age‐related hepatic steatosis. To examine this hypothesis, we utilized ghrelin knockout (KO) mice. Although no different in young animals (3 months old), we found that at 20 months of age, ghrelin KO mice have significantly reduced hepatic steatosis compared to aged‐matched wild‐type (WT) mice. Examination of molecular pathways by which deletion of ghrelin reduces steatosis showed that the increase in expression of diacylglycerol O‐acyltransferase‐1 (DGAT1), one of the key enzymes of triglyceride (TG) synthesis, seen with age in WT mice, is not present in KO mice. This was due to the lack of activation of CCAAT/enhancer binding protein‐alpha (C/EBPα) protein and subsequent reduction of C/EBPα‐p300 complexes. These complexes were abundant in livers of old WT mice and were bound to and activated the DGAT1 promoter. However, the C/EBPα‐p300 complexes were not detected on the DGAT1 promoter in livers of old KO mice resulting in lower levels of the enzyme. In conclusion, these studies demonstrate the mechanism by which ghrelin deletion prevents age‐associated hepatic steatosis and suggest that targeting this pathway may offer therapeutic benefit for NAFLD. [ABSTRACT FROM AUTHOR]

CCAAT/Enhancer Binding Proteins family proteins are important regulators of liver functions. Here, we show the critical role of C/EBPα-mediated chromatin remodeling in the age-associated dysfunctions of the liver and in the maintenance of physiological homeostasis. Because ph-S193 isoform of C/EBPα is increased in livers of old mice, we have generated C/EBPα-S193D knockin mice, which mimic the ph-S193 isoform of C/EBPα. Analyses of these mice showed that the S193D mutation causes chromatin remodeling leading to histological appearance of ‘foci-like’ nodules, which are also observed in livers of old mice. These ‘foci-like’ structures contain K9 trimethylated histone H3, a marker of heterochromatin. The increase of heterochromatin regions in S193D mice correlates with the elevation of S193D-C/EBPα-HDAC1 complexes and with dys-regulation of gene expression including epigenetic silencing of cyclin D1 and D2 promoters and the inhibition of liver proliferation. The elimination of C/EBPα-HDAC1 complexes in S193D mice by inhibition of HDAC1 corrects chromatin structure and normalizes expression of cyclin D1 and D2. We found that epigenetic dys-regulation is also associated with the elevation of C/EBPβ and with the increase of C/EBPα/β heterodimers in S193D mice. The C/EBPα/β heterodimers activate transcription of Glut4 and increase the levels of Glut4. As the result, S193D livers have accelerated uptake of glucose and accumulation of glycogen in the liver. Thus, this study demonstrates that the phosphorylation of C/EBPα at S193 leads to the appearance of heterochromatin regions, which correlates with the development of age-related dysfunctions of the liver. [ABSTRACT FROM AUTHOR]

Liver tumor cells arise from normal hepatocytes that escape negative control of proliferation. The transcription factor C/EBPα maintains quiescence of hepatocytes through two pathways: inhibition of cdks and repression of E2F. Nevertheless, liver tumors and cultured hepatoma cell lines proliferate in the presence of C/EBPα. In this paper, we present evidence that the activation of the PI3K/Akt pathway in liver tumor cells blocks the growth inhibitory activity of C/EBPα through the PP2A-mediated dephosphorylation of C/EBPa on Ser 193, leading to a failure of C/EBPα to interact with and inhibit cdks and E2F. Mutation of Ser 193 to Ala also abolishes the ability of C/EBPα to cause growth arrest because of a lack of interactions with cdk2 and E2F-Rb complexes. These data provide a molecular basis for the development of liver tumors in which the activation of PI3K/Akt pathway neutralizes C/EBPα growth inhibitory activity. [ABSTRACT FROM AUTHOR]

CCAAT/enhancer binding protein alpha (C/EBPα) causes growth arrest via direct interaction with the cyclin-dependent kinases cdk2 and cdk4. In this paper, we present evidence showing that C/EBPα enhances a proteasome-dependent degradation of cdk4 during growth arrest in liver of newborn mice and in cultured cells. Overexpression of C/EBPα in several biological systems leads to a reduction of cdk4 protein levels, but not mRNA levels. Experiments with several tissue culture models reveal that C/EBPα enhances the formation of cdk4-ubiquitin conjugates and induces degradation of cdk4 through a proteasome- dependent pathway. As a result, the half-life of cdk4 is shorter and protein levels of cdk4 are reduced in cells expressing C/EBPα. Gel filtration analysis of cdk4 complexes shows that a chaperone complex cdk4-cdc37-Hsp90, which protects cdk4 from degradation, is abundant in proliferating livers that lack C/EBPα, but this complex is weak or undetectable in livers expressing C/EBPα. Our studies show that C/EBPα disrupts the cdk4-cdc37-Hsp90 complex via direct interaction with cdk4 and reduces protein levels of cdk4 by increasing proteasome-dependent degradation of cdk4. [ABSTRACT FROM AUTHOR]

Proteasome inhibitors have become the standard of care for multiple myeloma (MM). Blocking protein degradation particularly perturbs the homeostasis of short-lived polypeptides such as transcription factors and epigenetic regulators. To determine how proteasome inhibitors directly impact gene regulation, we performed an integrative genomics study in MM cells. We discovered that proteasome inhibitors reduce the turnover of DNA-associated proteins and repress genes necessary for proliferation through epigenetic silencing. Specifically, proteasome inhibition results in the localized accumulation of histone deacetylase 3 (HDAC3) at defined genomic sites, which reduces H3K27 acetylation and increases chromatin condensation. The loss of active chromatin at super-enhancers critical for MM, including the super-enhancer controlling the proto-oncogene c-MYC, reduces metabolic activity and cancer cell growth. Epigenetic silencing is attenuated by HDAC3 depletion, suggesting a tumor-suppressive element of this deacetylase in the context of proteasome inhibition. In the absence of treatment, HDAC3 is continuously removed from DNA by the ubiquitin ligase SIAH2. Overexpression of SIAH2 increases H3K27 acetylation at c-MYC-controlled genes, increases metabolic output, and accelerates cancer cell proliferation. Our studies indicate a novel therapeutic function of proteasome inhibitors in MM by reshaping the epigenetic landscape in an HDAC3-dependent manner. As a result, blocking the proteasome effectively antagonizes c-MYC and the genes controlled by this proto-oncogene., Competing Interests: Authors’ Disclosures: P.L. is an advisory board member of Karyopharm Therapeutics. All other authors declare they have no competing interests.