The Mammalian Target of Rapamycin Complex 1 Regulates Leptin Biosynthesis in Adipocytes at the Level of Translation: The Role of the 5′-Untranslated Region in the Expression of Leptin Messenger Ribonucleic Acid

Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Molecular Endocrinology (Impact Factor: 4.02). 10/2008; 22(10):2260-7. DOI: 10.1210/me.2008-0148
Source: PubMed


Leptin production by adipose cells in vivo is increased after feeding and decreased by food deprivation. However, molecular mechanisms that control leptin expression in response to food intake remain unknown. Here, we test the hypothesis that leptin expression in adipose cells is regulated by nutrient- and insulin-sensitive mammalian target of rapamycin complex 1 (mTORC1)-mediated pathway. The activity of mTORC1 in 3T3-L1 adipocytes was up-regulated by stable expression of either constitutively active Rheb or dominant-negative AMP-activated protein kinase. In both cases, expression of endogenous leptin was significantly elevated at the level of translation. To investigate the role of leptin 5'-untranslated region (UTR) in the regulation of protein expression, we created bicistronic reporter constructs with and without the 5'-UTR. We found that the presence of leptin 5'-UTR renders mRNA resistant to regulation by mTORC1. It appears, therefore, that mTORC1 controls translation of leptin mRNA via a novel mechanism that does not require the presence of either the 5'-terminal oligopyrimidine tract or the 5'-UTR.

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    • "Leptin, an adipocytokine produced endogenously in the brain [12-15], has also been shown to reduce Aβ levels in vitro [16] as well as in vivo [17,18] and circulating leptin levels are reduced in AD [19]. Expression levels of leptin are regulated by the mammalian target of rapamycin complex 1 (mTORC1) [20-22]. Interestingly, IGF-1 and leptin are interconnected. "
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    ABSTRACT: Evidence shows that the insulin-like growth factor-1 (IGF-1) and leptin reduce β-amyloid (Aβ) production and tau phosphorylation, two major hallmarks of Alzheimer's disease (AD). IGF-1 expression involves the JAK/STAT pathway and the expression of leptin is regulated by the mammalian target of rapamycin complex 1 (mTORC1). We have previously shown that Aβ reduces leptin by inhibiting the mTORC1 pathway and Aβ was also suggested to inhibit the JAK/STAT pathway, potentially attenuating IGF-1 expression. As IGF-1 can activate mTORC1 and leptin can modulate JAK/STAT pathway, we determined the extent to which IGF-1 and leptin can upregulate the expression of one another and protect against Aβ-induced downregulation. We demonstrate that incubation of organotypic slices from adult rabbit hippocampus with Aβ42 downregulates IGF-1 expression by inhibiting JAK2/STAT5 pathway. Leptin treatment reverses these Aβ42 effects on IGF-1 and treatment with the STAT5 inhibitor completely abrogated the leptin-induced increase in IGF-1. Furthermore, EMSA and ChIP analyses revealed that leptin increases the STAT5 binding to the IGF-1 promoter. We also show that IGF-1 increases the expression of leptin and reverses the Aβ42-induced attenuation in leptin expression via the activation of mTORC1 signaling as the mTORC1 inhibitor rapamycin completely precluded the IGF-1-induced increase in leptin expression. Our results demonstrate for the first time that Aβ42 downregulates IGF-1 expression and that leptin and IGF-1 rescue one another from downregulation by Aβ42. Our study provides a valuable insight into the leptin/IGF-1/Aβ interplay that may be relevant to the pathophysiology of AD.
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    • "AMPK functions to restore cellular ATP levels by modifying diverse metabolic and cellular pathways [39]. Overexpressing dominant negative AMPK had been demonstrated to increase mTORC1 and leptin translation [40]. However, in this study, the level of phosphorylated activation of AMPK at T172 site was not affected by AA treatment (data not shown), suggesting that AMPK was not involved in regulation of leptin expression upon AA treatment in renal fibroblasts. "
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    ABSTRACT: Leptin is a peptide hormone playing pivotal role in regulating food intake and energy expenditure. Growing evidence has suggested the pro-inflammatory and fibrogenic properties of leptin. In addition, patients with renal fibrosis have higher level of plasma leptin, which was due to the increased leptin production. Aristolochic acid (AA) is a botanical toxin characterized to associate with the development of renal fibrosis including tubulointerstitial fibrosis. However, whether leptin is upregulated to participate in AA-induced kidney fibrosis remain completely unknown. In this study, leptin expression was increased by sublethal dose of AA in kidney fibroblast NRK49f determined by enzyme-linked immunosorbent assay and Western blot. Data from real-time reverse transcriptase-polymerase chain reaction revealed that leptin was upregulated by AA at transcriptional level. DNA binding activity of CCAAT enhancer binding protein α (C/EBP α), one of the transcription factors for leptin gene, was enhanced in DNA affinity precipitation assay and chromatin immunoprecipitation experiments. Knockdown of C/EBP α expression by small interfering RNA markedly reduced AA-induced leptin expression. Moreover, AA promoted Akt interaction with p-PDK1, and increased phosphorylated activation of Akt. Akt knockdown, and inhibition of Akt signaling by LY294002 and mTOR inhibitor rapamycin reduced leptin expression. Furthermore, treatment of LY294002 or rapamycin significantly suppressed AA-induced C/EBP α DNA-binding activity. These results suggest that Akt and C/EBP α activation were involved in AA-regulated leptin expression. Our findings demonstrate the first that AA could induce secretion and expression of fibrogenic leptin in kidney fibroblasts, which reveal potential involvement of leptin in the progression of kidney fibrosis in aristolochic acid nephropathy.
    Preview · Article · Feb 2011 · PLoS ONE
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    • "mTORC1 activity is enhanced by phosphorylation at Ser 2448 residue and can be measured by activation of the downstream proteins p70S6K1 and 4E-BP (Hara et al. 2002; Kim et al. 2002; Loewith et al. 2002). It has been established that mTORC1 regulates leptin biosynthesis at the level of translation (Roh et al. 2003; Cho et al. 2004; Chakrabarti et al. 2008) and Aβ has been demonstrated to inhibit mTORC1 (Chen et al. 2009). One can expect that inhibition of mTORC1 by Aβ may impair leptin translation and reduces its expression levels. "
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    ABSTRACT: J. Neurochem. (2010) 115, 373–384. High levels of the adipocytokine leptin are associated with reduced risk of Alzheimer’s disease. Leptin treatment also reduces β-amyloid (Aβ) levels in in vivo and in vitro models of Alzheimer’s disease. Aβ and leptin interact with the Akt/mammalian target of rapamycin complex 1 (mTORC1) signaling pathway. Akt/mTORC1 activation reduces tau phosphorylation through the inhibition of the downstream enzyme GSK-3β. mTORC1 also regulates translation of many proteins including leptin. While Aβ has been shown to inactivate Akt, inhibit mTORC1, and facilitate the phosphorylation of tau, leptin activates both Akt and mTORC1 and reduces tau phosphorylation. However, the extent to which Aβ may modulate leptin expression and increase tau phosphorylation involving Akt/mTORC1 has not been determined. In this study, we show that incubation of organotypic slices from rabbit hippocampus with Aβ down-regulates leptin expression, inhibits Akt, activates GSK-3β, increases tau phosphorylation, and inactivates mTORC1. Leptin treatment reverses Aβ effects by alleviating Akt inhibition, preventing GSK-3β activation, reducing tau phosphorylation, and activating mTORC1. On the other hand, Rapamycin, an allosteric inhibitor of mTORC1, down-regulates leptin expression, increases tau phosphorylation, and does not affect Akt and GSK-3β. Our results demonstrate for the first time that Aβ regulates leptin expression and tau phosphorylation through mTORC1.
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