The microRNA-29 family: role in metabolism and metabolic disease.

The microRNA-29 family members miR-29a-3p, miR-29b-3p, and miR-29c-3p are ubiquitously expressed and consistently increased in various tissues and cell types in conditions of metabolic disease, obesity, insulin resistance, and type 2 diabetes. In pancreatic β cells, miR-29a is required for normal exocytosis, but increased levels are associated with impaired β-cell function. Similarly, in liver, miR-29 species are higher in models of insulin resistance and type 2 diabetes, and either knock-out or depletion using a microRNA inhibitor improves hepatic insulin resistance. In skeletal muscle, miR-29 family upregulation is associated with insulin resistance and altered substrate oxidation, and similarly, in adipocytes, overexpression of miR-29a leads to insulin resistance. Blocking miR-29a using nucleic acid antisense therapeutics show promising results in preclinical animal models of obesity and type 2 diabetes, although the widespread expression pattern of miR-29 family members complicates the exploration of single target tissues. However, in fibrotic diseases, such as in late complications of diabetes and metabolic disease (diabetic kidney disease, nonalcoholic steatohepatitis), miR-29 species expression is suppressed by TGF-β allowing increased extracellular matrix collagen to form. In the clinical setting, circulating levels of miR-29a and miR-29b are consistently increased in type 2 diabetes and in gestational diabetes and are also possible prognostic markers for deterioration of glucose tolerance. In conclusion, miR-29 family miRNAs play an essential role in various organs relevant to intermediary metabolism and its upregulation contributes to impaired glucose metabolism, whereas it suppresses fibrosis development. Thus, a correct balance of levels of miR-29 family miRNA seems important for cellular and organ homeostasis in metabolism.