Attenuation of hepatic glucose output by D-chiro-inositol

Abstract

Over the past several decades, the mechanisms underlying insulin action have been extensively investigated. Insulin’s main role is to regulate blood glucose through activation of processes such as glycogen synthesis that encourage uptake of glucose and its conversion into glycogen. In the liver, insulin also regulates “hepatic glucose output” (HGO) by attenuating both gluconeogenesis and glycogen breakdown (glycogenolysis). There is a significant body of evidence based on observations with pyruvate dehydrogenase and cAMPdependent protein kinase suggesting that there are other mediators of insulin action distinct from the processes that occur through the insulin-signalling pathway. These mediators comprised of D-chiro-inositol-containing inositol phosphoglycan molecules (DCI-IPG). DCIIPGs have been shown to possess insulin-sensitising properties, however, their mechanism of action still remain largely unknown. The impetus for the present study was an isolated perfused liver study conducted previously which showed that DCI was able to attenuate HGO similar to that observed with insulin. It was therefore hypothesised that DCI possessed an insulin-mimic effect and the overall aim was to investigate whether this effect was mediated through the insulin-signalling pathway. A specific objective was to validate the use of liver lobes isolated throughout the perfusion time-course as an appropriate model to study insulin-signalling. Western blot analyses of known insulin-signalling targets as well as AMP-activated protein kinase (AMPK) and acetyl CoA carboxylase (ACC) were undertaken on isolated liver lobes samples from independent perfusions with either control (saline) or insulin, DCI, or myoinositol (MYO). Our results showed statistically significant stimulation of AKT by insulin in two out of the three perfusions. The magnitude of the responses were in the order of 10-20 fold and therefore consistent with a real effect. There was no evidence that DCI mediates its attenuating effects on HGO via AKT. No consistent effects were found with AMPK, ACC, glycogen synthase kinase (GSK) and glycogen synthase (GS) in any of the other treatment groups. However, there were some interesting effects within some of the individual perfusions. The results of this study revealed that the isolated liver is a practicable model for studying the insulin-signalling pathway and suggests that DCI’s observed effects on HGO are mediated by alternative mechanisms distinct to that of insulin.