Background and Aims: The molecular mechanisms driving non-alcoholic fatty liver disease (NAFLD) are poorly understood; however, microRNAs might play a key role in these processes. We hypothesize that let-7d- 5p could contribute to the pathophysiol-ogy of NAFLD and serve as a potential diagnostic biomarker.Methods: We evaluated let-7d- 5p levels and its targets in liver biopsies from a cross- sectional study including patients with NAFLD and healthy donors, and from a mouse model of NAFLD. Moreover, the induction of let-7d- 5p expression by fatty acids was evaluated in vitro. Further, we overexpressed let-7d- 5p in vitro to corroborate the results observed in vivo. Circulating let-7d- 5p and its potential as a NAFLD biomarker was determined in isolated extracellular vesicles from human plasma by RT-qPCR.Results: Our results demonstrate that hepatic let-7d- 5p was significantly up- regulated in patients with steatosis, and this increase correlated with obesity and a decreased expression of AKT serine/threonine kinase (AKT), insulin- like growth factor 1 (IGF1), IGF- I receptor (IGF1R) and insulin receptor (INSR). These alterations were corrobo-rated in a NAFLD mouse model. In vitro, fatty acids increased let-7d- 5p expression, and its overexpression decreased AKT, IGF-IR and IR protein expression. Furthermore, let- 7d- 5p hindered AKT phosphorylation in vitro after insulin stimulation. Finally, cir-culating let-7d- 5p significantly decreased in steatosis patients and receiver operating characteristic (ROC) analyses confirmed its utility as a diagnostic biomarker.Ministerio de Ciencia, Innovación y UniversidadesSantander-UCMUCMInstituto de Salud Carlos IIIFondo Europeo para el Desarrollo Regional (FEDER)American Heart AssociationDOD CDMRPNational Institutes of HealthDepto. de Bioquímica y Biología MolecularFac. de FarmaciaTRUEpu
