Abstract
Type 2 diabetes mellitus is a chronic disease characterised by progressive insulin resistance and loss of β-cell function. An incomplete understanding of its pathogenesis is hindering the effective treatment of this disease. Roux-en Y gastric bypass surgery (RYGB); however, causes rapid remission of liver insulin resistance and type 2 diabetes, and therefore affords us an opportunity to examine some fundamental characteristics of these conditions. Gathering evidence suggests that liver insulin resistance may be a crucial contributor to development of diabetes. In this thesis, we used liver biopsies taken before, and in some individuals after, RYGB surgery to explore or identify several molecular processes involved in the pathogenesis of type 2 diabetes. The study cohort included individuals with normal glucose tolerance and others with type 2 diabetes.
Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) may cause insulin resistance through its inhibitory action on insulin signalling. This thesis provides evidence to the contrary, suggesting that liver ENPP1 is not a contributor to liver insulin resistance. We found liver ENPP1 protein abundance was lower in individuals with type 2 diabetes than in those with normal glucose tolerance, and increased after RYGB surgery in those individuals who had remission of diabetes. ENPP1 positively correlated with insulin sensitivity at the liver which is contrary to what others have reported in muscle and adipose tissue. We reasoned that our findings are likely due to the hypothesized role of ENPP1 as a natural modulator of insulin signalling and the unique role the liver has in insulin processing.
Changes in the expression ratio of insulin receptor (IR) isoform A (IR-A) and B (IRB) have previously been implicated in the pathogenesis of type 2 diabetes. The metabolically active IR-B isoform has been shown to predominate in the liver, with the liver IR-B:A ratio being reported to be 9.8. By assaying levels of IR-A and IR-B mRNA expression we found that the ratio of liver IR-B:A was abnormally low in individuals with type 2 diabetes (5.2) and increased with remission of diabetes (5.4 to 8.6). The change in ratio was due to a diminished IR-A expression following remission of diabetes. Further work with an in vitro cell model showed that insulin’s ability to inhibit gluconeogenesis in Hep G2 cells overexpressing IR-A was reduced, suggesting that the altered liver IR-B:A ratio observed in diabetes may have a detrimental effect on glucose homeostasis.
Access to liver tissue before and after RYGB surgery afforded a rare opportunity to observe changes in liver gene expression before and after remission of diabetes. Using gene microarray analysis, we showed that the majority of genes that were differentially regulated after RYGB surgery are involved in lipotoxicity, inflammation, and ER stress, particularly in those individuals who had remission of type 2 diabetes. Although there were many significant observations, the apparent inter-organ communication between the liver and the pancreas presents a hitherto unconfirmed relationship whereby the liver can not only mediate pancreatic β cell size, but can also regulate insulin secretion. This work has identified a mechanism which may be exploited to develop novel treatments for type 2 diabetes.
In conclusion this thesis describes several novel findings with respect to the pathogenesis of type 2 diabetes. It provides novel data on the role of ENPP1 and IR isoforms in insulin signalling which have furthered our understanding of the insulin signalling pathway. In addition, microarray gene analysis in liver tissue from before and after improvements in insulin resistance and remission of type 2 diabetes has allowed us to identify several candidate genes that are worthy of further investigation.