Skeletal muscle TET3 promotes insulin resistance through destabilisation of PGC-1

Aim/hypothesis The peroxisome proliferator-activated receptor-gamma coactivator alpha (PGC-1 alpha) plays a critical role in the maintenance of glucose, lipid and energy homeostasis by orchestrating metabolic programs in multiple tissues in response to environmental cues. In skeletal muscles, PGC-1 alpha dysregulation has been associated with insulin resistance and type 2 diabetes but the underlying mechanisms have remained elusive. This research aims to understand the role of TET3, a member of the ten-eleven translocation (TET) family dioxygenases, in PGC-1 alpha dysregulation in skeletal muscles in obesity and diabetes. Methods TET expression levels in skeletal muscles were analysed in humans with or without type 2 diabetes, as well as in mouse models of high-fat diet (HFD)-induced or genetically induced (ob/ob) obesity/diabetes. Muscle-specific Tet3 knockout (mKD) mice were generated to study TET3's role in muscle insulin sensitivity. Genome-wide expression profiling (RNA-seq) of muscle tissues from wild-type (WT) and mKD mice was performed to mine deeper insights into TET3-mediated regulation of muscle insulin sensitivity. The correlation between PGC-1 alpha and TET3 expression levels was investigated using muscle tissues and in vitro-derived myotubes. PGC-1 alpha phosphorylation and degradation were analysed using in vitro assays. Results TET3 expression was elevated in skeletal muscles of humans with type 2 diabetes and in HFD-fed and ob/ob mice compared with healthy controls. mKD mice exhibited enhanced glucose tolerance, insulin sensitivity and resilience to HFD-induced insulin resistance. Pathway analysis of RNA-seq identified 'Mitochondrial Function' and 'PPAR alpha Pathway' to be among the top biological processes regulated by TET3. We observed higher PGC-1 alpha levels (similar to 25%) in muscles of mKD mice vs WT mice, and lower PGC-1 alpha protein levels (similar to 25-60%) in HFD-fed or ob/ob mice compared with their control counterparts. In human and murine myotubes, increased PGC-1 alpha levels following TET3 knockdown contributed to improved mitochondrial respiration and insulin sensitivity. TET3 formed a complex with PGC-1 alpha and interfered with its phosphorylation, leading to its destabilisation. Conclusions/interpretation Our results demonstrate an essential role for TET3 in the regulation of skeletal muscle insulin sensitivity and suggest that TET3 may be used as a potential therapeutic target for the metabolic syndrome. Data availability equences are available from the Gene Expression Omnibus (https://www.ncbi.nlm.nih.gov/geo/) with accession number of GSE224042.