Regulation of Lin28a-miRNA let-7b-5p pathway in the skeletal muscle cells by PPARδ.

Lin28a/miRNA let-7b-5p pathway has emerged as a key regulators of energy homeostasis in the skeletal muscle. However, the mechanism through which this pathway is regulated in the skeletal muscle has remained unclear. We have found that eight weeks of aerobic training (Tr) markedly decreased let-7b-5p expression in murine skeletal muscle, whereas high-fat diet (Hfd) decreased it's expression. Conversely, Lin28a expression, a well-known inhibitor of let-7b-5p, was induced by Tr and decreased by Hfd. Similarly, in human muscle biopsies, Tr increased LIN28 expression and decreased let-7b-5p expression. Bioinformatics analysis of LIN28a DNA sequence revealed that it is enrichment in PPARδ binding sites, which is a well-known metabolic regulator of exercise. Treatment of primary mouse skeletal muscle cells or C2C12 cells with PPARδ activators GW501516 and AICAR increased Lin28a expression. Lin28a and let-7b-5p expression were also regulated by PPARδ co-regulators. While co-activator PGC1α increased Lin28a expression, co-repressor NCoR1 decreased it's expression. Furthermore, PGC1α markedly reduced the let-7b-5p expression. PGC1α-mediated induction of Lin28a expression was blocked by PPARδ inhibitor GSK0660. In agreement, Lin28a expression was downregulated in PPARδ knocked-down cells leading to increased let-7b-5p expression. Finally, we show that modulation of Lin28a-let-7b-5p pathway in muscle cells leads to changes in mitochondrial metabolism in PGC1α dependent fashion. In summary, we demonstrate that Lin28a-let-7b-5p is a direct target of PPARδ in the skeletal muscle, where its impacts mitochondrial respiration.