Foxo1 represses expression of musclin, a skeletal muscle-derived secretory factor.
ABSTRACT Musclin is a novel skeletal muscle-derived secretory factor, whose mRNA level is markedly regulated by nutritional status. In the present study, we investigated the mechanism of musclin mRNA regulation by insulin. In C2C12 myocytes, insulin-induced upregulation of musclin mRNA was significantly decreased by treatment of phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, and was abolished in C2C12 myocytes stably expressing a constitutively active Foxo1 (Foxo1-3A), suggesting the involvement of Foxo1 in the regulation of musclin mRNA. Promoter deletion analysis of musclin promoter revealed that the region of -303/-123 is important for the repression of promoter activity by Foxo1. Chromatin immunoprecipitation assay showed that Foxo1 bound to musclin promoter. Musclin mRNA level was markedly downregulated in gastrocnemius muscle of Foxo1 transgenic mice. Our results demonstrated that Foxo1 downregulates musclin mRNA expression both in vitro and in vivo, which should explain insulin-mediated upregulation of this gene in muscle cells.
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ABSTRACT: The insulin-like growth factors (IGFs) are essential for development; bioavailable IGF is tightly regulated by six related IGF-binding proteins (IGFBPs). Igfbp5 is the most conserved and is developmentally up-regulated in key lineages and pathologies; in vitro studies suggest that IGFBP-5 functions independently of IGF interaction. Genetic ablation of individual Igfbps has yielded limited phenotypes because of substantial compensation by remaining family members. Therefore, to reveal Igfbp5 actions in vivo, we generated lines of transgenic mice that ubiquitously overexpressed Igfbp5 from early development. Significantly increased neonatal mortality, reduced female fertility, whole-body growth inhibition, and retarded muscle development were observed in Igfbp5-overexpressing mice. The magnitude of the response in individual transgenic lines was positively correlated with Igfbp5 expression. Circulating IGFBP-5 concentrations increased a maximum of only 4-fold, total and free IGF-I concentrations increased up to 2-fold, and IGFBP-5 was detected in high M(r) complexes; however, no detectable decrease in the proportion of free IGF-I was observed. Thus, despite only modest changes in IGF and IGFBP concentrations, the Igfbp5-overexpressing mice displayed a phenotype more extreme than that observed for other Igfbp genetic models. Although growth retardation was obvious prenatally, maximal inhibition occurred postnatally before the onset of growth hormone-dependent growth, regardless of Igfbp5 expression level, revealing a period of sensitivity to IGFBP-5 during this important stage of tissue programming.Proceedings of the National Academy of Sciences 04/2004; 101(12):4314-9. · 9.74 Impact Factor
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ABSTRACT: Emerging evidence documents a key function for the forkhead transcription factor FoxO1 in cellular metabolism. Here, we investigate the role of FoxO1 in the regulation of fatty acid (FA) metabolism in muscle cells. C2C12 cells expressing an inducible construct with either wild type FoxO1 or a mutant form (FoxO1/TSS) refractory to the protein kinase B inhibitory effects were generated. FoxO1 activation after myotube formation altered the expression of several genes of FA metabolism. Acyl-CoA oxidase and peroxisome proliferator-activated receptor delta mRNA levels increased 2.2-fold and 1.4-fold, respectively, whereas mRNA for acetyl-CoA carboxylase decreased by 50%. Membrane uptake of oleate increased 3-fold, and oleate oxidation increased 2-fold. Cellular triglyceride content was also increased. The enhanced FA utilization induced by FoxO1 was mediated by a severalfold increase in plasma membrane level of the fatty acid translocase FAT/CD36 and eliminated by cell treatment with the CD36 inhibitor sulfo-N-succinimidyl-oleate. We conclude that FoxO1 activation induces coordinate increases in FA uptake and oxidation and that these effects are mediated, at least in part, by membrane enrichment in CD36. The data suggest that FoxO1 contributes to preparing the muscle cell for the increased reliance on FA metabolism that is characteristic of fasting. Dysregulation of FoxO1 in muscle could contribute to intramuscular lipid accumulation and insulin resistance by maintaining activation of FA uptake.Journal of Biological Chemistry 05/2005; 280(14):14222-9. · 4.65 Impact Factor
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ABSTRACT: Phenotypic modulation of vascular smooth muscle cells (SMCs) in the blood vessel wall from a differentiated to a proliferative state during vascular injury and inflammation plays an important role in restenosis and atherosclerosis. Matrix metalloproteinase 9 (MMP9) is a member of the MMP family of proteases, which participate in extracellular matrix degradation and turnover. MMP9 is upregulated and required for SMC migration during the development of restenotic and atherosclerotic lesions. In this study, we show that FoxO4 activates transcription of the MMP9 gene in response to tumor necrosis factor alpha (TNF-alpha) signaling. Inhibition of FoxO4 expression by small interfering RNA or gene knockout reduces the abilities of SMCs to migrate in vitro and inhibit neointimal formation and MMP9 expression in vivo. We further show that both the N-terminal, Sp1-interactive domain and the C-terminal transactivation domain of FoxO4 are required for FoxO4-activated MMP9 transcription. TNF-alpha signaling upregulates nuclear FoxO4. Our studies place FoxO4 in the center of a transcriptional regulatory network that links gene transcription required for SMC remodeling to upstream cytokine signals and implicate FoxO4 as a potential therapeutic target for combating proliferative arterial diseases.Molecular and Cellular Biology 05/2007; 27(7):2676-86. · 5.37 Impact Factor