Liver-specific knockdown of class IIa HDACs has limited efficacy on glucose metabolism but entails severe organ side effects in mice

Histone deacetylases (HDACs) are important regulators of epigenetic gene modification that are involved in the transcriptional control of metabolism. In particular class IIa HDACs have been shown to affect hepatic gluconeogenesis and previous approaches revealed that their inhibition reduces blood glucose in type 2 diabetic mice. In the present study, we aimed to evaluate the potential of class IIa HDAC inhibition as a therapeutic opportunity for the treatment of metabolic diseases. For that, siRNAs selectively targeting HDAC4, 5 and 7 were selected and used to achieve a combinatorial knockdown of these three class IIa HDAC isoforms. Subsequently, the hepatocellular effects as well as the impact on glucose and lipid metabolism were analyzed in vitro and in vivo. The triple knockdown resulted in a statistically significant decrease of gluconeogenic gene expression in a murine hepatic cell line as well as in human primary hepatocytes. Despite a similar HDAC-induced downregulation of hepatic genes involved in gluconeogenesis in mice using a liver-specific lipid nanoparticle siRNA formulation, the in vivo effects on whole body glucose metabolism were only limited and did not outweigh the safety concerns observed by histopathological analysis in spleen and kidney. Mechanistically, Affymetrix gene chip analysis and gene expression studies provide evidence that class IIa HDACs directly target and thus regulate the expression of HNF4α and FOXP1 in the liver, thereby modifying gene regulatory mechanisms mediating glucose and lipid metabolism and transport. In conclusion, the combinatorial knockdown of HDAC4, 5 and 7 by therapeutic siRNAs affected multiple pathways in vitro and in vivo leading to the downregulation of genes involved in gluconeogenesis. However, the effects on the gene expression level were not paralleled by a significant reduction of gluconeogenesis in mice, as shown in pyruvate tolerance tests. However, the liver-specific inhibition of these HDAC isoforms was associated with severe adverse effects in vivo, making this approach not a viable treatment option for chronic metabolic disorders like type 2 diabetes.

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