Resumen de The role of FGF21 in the metabolic response to amino acid restriction
[eng] Obesity and associated metabolic diseases have reached epidemic proportions, affecting not only high-income countries but also low- and middle-income ones. In this context, the search for therapeutic approaches to treat obesity is becoming a priority worldwide. In this regard, the metabolic hormone fibroblast growth factor 21 (FGF21) has been identified as a potential candidate for the treatment of obesity and metabolic syndrome. Previous work by our group described that FGF21 is highly induced in liver in response to leucine deprivation and that the transcription factor ATF4 mediates this induction. The present work is the follow-up of this initial observation. To delve deeper into the molecular mechanisms that regulate FGF21 expression during leucine deprivation, we focused on the transcriptional repressor Rev-erb?, which functions both as a core repressive component of the cell autonomous clock and as a regulator of metabolic genes. Our results reveal a consistent negative correlation between Fgf21 and the Pgc-1?/heme/Rev-erb? axis across various nutritional states and that a decrease in Rev-erb? activity enhances the ATF4-mediated upregulation of the human FGF21 promoter. Consequently, we propose a model whereby the induction of Fgf21 upon leucine deprivation is the consequence of the sum of two factors: binding of the activator ATF4 to the promoter and the absence of the repressor Rev-erb?. Given the coincidence between the effects of leucine deprivation and those observed during FGF21 treatment, we analysed the role of FGF21 during leucine deprivation. In the current study, we demonstrate that weight loss, downregulation of key lipogenic genes in liver and WAT, and BAT activation in response to leucine deprivation are partly FGF21-dependent. Given the unfeasibility to translate single amino acid deprivation to humans, we focussed on low-protein diets (LPDs) as a more realistic approach. The LPD increased circulating FGF21 levels with an associated upregulated expression in liver. Analysis of serum human samples from the PREDIMED study extended the correlation between LPD and FGF21 to humans. The ATF4-mediated upregulation of Fgf21 in liver was partially responsible for the weight loss observed in mice fed a LPD, since the liver specific Fgf21 knockout mice (LFgf21KO) mice were partially protected from this loss. Focusing on the effects of FGF21 on scWAT and given the capacity of FGF21 to produce the browning of white fat depots, we examined the activation of the thermogenic programme in this tissue. Accordingly, scWAT browning caused by the LPD did not occur in mice lacking hepatic Fgf21. As UCP1 activity is related to EE, the blunted induction of Ucp1 in the LPD-fed LFgf21KO mice, may contribute to the reduction in weight loss observed in this mouse model under these circumstances. The administration of the b-blocker propranolol to protein-restricted mice allowed us to distinguish between the roles of FGF21 and noradrenaline. While Ucp1 expression was upregulated independently of adrenergic signalling, Dio2 and Ppar? expression was blunted by propranolol treatment. These results point to the induction of Ucp1 as a direct effect of liver-delivered FGF21 on scWAT and discard a CNS-mediated effect. In addition, the LPD improved glucose tolerance, and this improvement was not observed in LFgf21KO mice, indicating a role of FGF21 in glucose metabolism during protein restriction. Our findings show that the effects of a LPD depend, at least in part, on the circulating levels of FGF21 and consequently on the liver production of this growth factor. Given the parallelism between the results of our study in humans and those in mice, we postulate that modulation of dietary protein content can bring about changes in the circulating levels of FGF21 in mice and humans. [cat] L’obesitat i les malalties metabòliques que en deriven són un problema de salut mundial. En aquest context, la recerca d’estratègies terapèutiques pel tractament de l’obesitat ha esdevingut una prioritat. En aquest sentit, el factor metabòlic fibroblast growth factor 21 (FGF21), ha estat identificat com a un prometedor candidat pel tractament de l’obesitat i la síndrome metabòlica. El nostre laboratori va descriure que en resposta a la privació de leucina els nivells de FGF21 augmenten dràsticament i que el factor de transcripció activating transcription factor (ATF4) n’és el responsable. Els resultats d’aquesta tesi són la continuació i desenvolupament d’aquesta observació inicial. Aprofundint en els mecanismes de regulació que controlen l’expressió de FGF21 en resposta a la privació de leucina, els nostres resultats indiquen que el repressor transcripcional Rev-erb? participa significativament en aquesta regulació i que la disminució dels nivells de Rev-erb? correlacionen amb una activació del promotor de FGF21. A més a més, en aquest estudi demostrem que la pèrdua de pes, la disminució de l’expressió de gens lipogènics en fetge i teixit adipós blanc, així com l’activació del teixit adipós marró en resposta a la privació de leucina són, al menys parcialment, dependents de FGF21. Finalment, amb la finalitat de fer els nostres resultats traslladables a humans, demostrem que una dieta amb baix contingut proteic augmenta les nivells circulants de FGF21 en ratolins i humans, i que això succeeix a través de l’increment d’ATF4. L’increment dels nivells de FGF21 degut a la restricció proteica provoquen l’increment de l’expressió dels gens termogènics en el teixit adipós blanc subcutani, la pèrdua de pes i la millora la tolerància a la glucosa. El conjunt d’aquest resultats destaquen el paper clau del factor FGF21 com a mediador dels efectes metabòlics que es produeixen durant la restricció d’aminoàcids i suggereixen la disminució del contingut de proteïna de la dieta com a estratègia per incrementar-ne els nivells.
