Enhanced C/EBPß function promotes hypertrophic versus hyperplastic fat tissue growth and prevents steatosis in response to high-fat diet feeding.

Chronic obesity is correlated with severe metabolic and cardiovascular diseases as well as with an increased risk for developing cancers. Obesity is usually characterized by fat accumulation in enlarged - hypertrophic - adipocytes that are a source of inflammatory mediators, which promote the development and progression of metabolic disorders. Yet, in certain healthy obese individuals, fat is stored in metabolically more favorable hyperplastic fat tissue that contains an increased number of smaller adipocytes that are less inflamed. In a previous study, we demonstrated that loss of the inhibitory protein-isoform C/EBPß-LIP and the resulting augmented function of the transactivating isoform C/EBPß-LAP promotes fat metabolism under normal feeding conditions and expands health- and lifespan in mice. Here, we show that in mice on a high-fat diet, LIP-deficiency results in adipocyte hyperplasia associated with reduced inflammation and metabolic improvements. Furthermore, fat storage in subcutaneous depots is significantly enhanced specifically in LIP-deficient male mice. Our data identify C/EBPß as a regulator of adipocyte fate in response to increased fat intake, which has major implications for metabolic health and aging.

Tuberous sclerosis complex is required for tumor maintenance in MYC-driven Burkitt's lymphoma.

The tuberous sclerosis complex (TSC) 1/2 is a negative regulator of the nutrient-sensing kinase mechanistic target of rapamycin complex (mTORC1), and its function is generally associated with tumor suppression. Nevertheless, biallelic loss of function of TSC1 or TSC2 is rarely found in malignant tumors. Here, we show that TSC1/2 is highly expressed in Burkitt's lymphoma cell lines and patient samples of human Burkitt's lymphoma, a prototypical MYC-driven cancer. Mechanistically, we show that MYC induces TSC1 expression by transcriptional activation of the TSC1 promoter and repression of miR-15a. TSC1 knockdown results in elevated mTORC1-dependent mitochondrial respiration enhanced ROS production and apoptosis. Moreover, TSC1 deficiency attenuates tumor growth in a xenograft mouse model. Our study reveals a novel role for TSC1 in securing homeostasis between MYC and mTORC1 that is required for cell survival and tumor maintenance in Burkitt's lymphoma. The study identifies TSC1/2 inhibition and/or mTORC1 hyperactivation as a novel therapeutic strategy for MYC-driven cancers.

Reduced expression of C/EBPß-LIP extends health and lifespan in mice.

Ageing is associated with physical decline and the development of age-related diseases such as metabolic disorders and cancer. Few conditions are known that attenuate the adverse effects of ageing, including calorie restriction (CR) and reduced signalling through the mechanistic target of rapamycin complex 1 (mTORC1) pathway. Synthesis of the metabolic transcription factor C/EBPß-LIP is stimulated by mTORC1, which critically depends on a short upstream open reading frame (uORF) in the Cebpb-mRNA. Here, we describe that reduced C/EBPß-LIP expression due to genetic ablation of the uORF delays the development of age-associated phenotypes in mice. Moreover, female C/EBPßΔuORF mice display an extended lifespan. Since LIP levels increase upon aging in wild type mice, our data reveal an important role for C/EBPß in the aging process and suggest that restriction of LIP expression sustains health and fitness. Thus, therapeutic strategies targeting C/EBPß-LIP may offer new possibilities to treat age-related diseases and to prolong healthspan.

Wnt/ß-catenin signaling via Axin2 is required for myogenesis and, together with YAP/Taz and Tead1, active in IIa/IIx muscle fibers.

Canonical Wnt/ß-catenin signaling plays an important role in myogenic differentiation, but its physiological role in muscle fibers remains elusive. Here, we studied activation of Wnt/ß-catenin signaling in adult muscle fibers and muscle stem cells in an Axin2 reporter mouse. Axin2 is a negative regulator and a target of Wnt/ß-catenin signaling. In adult muscle fibers, Wnt/ß-catenin signaling is only detectable in a subset of fast fibers that have a significantly smaller diameter than other fast fibers. In the same fibers, immunofluorescence staining for YAP/Taz and Tead1 was detected. Wnt/ß-catenin signaling was absent in quiescent and activated satellite cells. Upon injury, Wnt/ß-catenin signaling was detected in muscle fibers with centrally located nuclei. During differentiation of myoblasts expression of Axin2, but not of Axin1, increased together with Tead1 target gene expression. Furthermore, absence of Axin1 and Axin2 interfered with myoblast proliferation and myotube formation, respectively. Treatment with the canonical Wnt3a ligand also inhibited myotube formation. Wnt3a activated TOPflash and Tead1 reporter activity, whereas neither reporter was activated in the presence of Dkk1, an inhibitor of canonical Wnt signaling. We propose that Axin2-dependent Wnt/ß-catenin signaling is involved in myotube formation and, together with YAP/Taz/Tead1, associated with reduced muscle fiber diameter of a subset of fast fibers.

Deficiency in mTORC1-controlled C/EBPß-mRNA translation improves metabolic health in mice.

The mammalian target of rapamycin complex 1 (mTORC1) is a central regulator of physiological adaptations in response to changes in nutrient supply. Major downstream targets of mTORC1 signalling are the mRNA translation regulators p70 ribosomal protein S6 kinase 1 (S6K1p70) and the 4E-binding proteins (4E-BPs). However, little is known about vertebrate mRNAs that are specifically controlled by mTORC1 signalling and are engaged in regulating mTORC1-associated physiology. Here, we show that translation of the CCAAT/enhancer binding protein beta (C/EBPß) mRNA into the C/EBPß-LIP isoform is suppressed in response to mTORC1 inhibition either through pharmacological treatment or through calorie restriction. Our data indicate that the function of 4E-BPs is required for suppression of LIP. Intriguingly, mice lacking the cis-regulatory upstream open reading frame (uORF) in the C/EBPß-mRNA, which is required for mTORC1-stimulated translation into C/EBPß-LIP, display an improved metabolic phenotype with features also found under calorie restriction. Thus, our data suggest that translational adjustment of C/EBPß-isoform expression is one of the key processes that direct metabolic adaptation in response to changes in mTORC1 activity.

Bioinformatic and functional analysis of TGFBR1 polymorphisms.

OBJECTIVES: The transforming growth factor-beta (TGFB) pathway has substantial impact on cellular functions, cell proliferation, and apoptosis. We used bioinformatics, gene expression, and cell biological assays to evaluate the functionality of frequent inherited germline polymorphisms in the TGFB receptor 1 (TGFBR1). METHODS: In an exploratory (n=55) and confirmatory (n=106) study, we analyzed the TGFB1 pathway after incubation with TGFbeta1 ligand and after exposure to X-rays in peripheral blood human mononuclear cells. Expression of TGFB pathway genes was assessed by real-time PCR, and cellular viability was analyzed by flow cytometry. A total of six polymorphisms including the deletion variant (*6A) were identified to tag currently known common genetic variations in TGFBR1 and were analyzed in relation to the phenotypes. RESULTS: In accordance with a negative feedback mechanism, incubations with the ligand TGFbeta1 was followed by up-regulation of the intracellular SMAD7 and down-regulation of the SMAD3 mRNA molecules. The TGFBR1*6A deletion variant attenuated the suppression of SMAD3 in response to TGFbeta1 (P=0.02, in both studies). Moreover, cells harboring *6A were more sensitive toward cytotoxic effects of irradiation (P=0.001 after adjustment for age and sex). Cells were particularly prone toward radiation toxicity when carrying, in addition to *6A, the variant allele of rs11568785, which exhibits a strong genetic selection signature. CONCLUSION: The *6A deletion and the linked rs11568785 polymorphisms seem to attenuate TGFB signaling. This should be considered not only for clinical-epidemiological studies on cancer susceptibility but may also be relevant for side effects from drugs or radiotherapy.