Metabolic roles of adenosine : studies using genetically modified mice and transfected cells

Abstract

The aim of this thesis was to further investigate the metabolic roles of adenosine. The endogenous nucleoside adenosine acts by binding to the four adenosine receptors, A1R, A2AR, A2BR and A3R. The A1R is the adenosine receptor that has been proposed to be the one most involved in metabolism. Hence, most of the studies in this thesis focus at the A1R. Genetically modified mice where the A1R has been deleted (A1R (-/-) mice), drugs acting at the adenosine receptors and adenosine deaminase, which removes endogenous adenosine, have been used in studies included in this thesis and metabolic actions of adenosine have mainly been examined in adipose tissue, skeletal muscle and pancreas. Adenosine has previously been reported to interact with the growth hormone secretagogue receptor 1a (GHSR1a), which is a receptor with a major role in metabolism. Interactions between adenosine and the GHSR1a and its endogenous ligand ghrelin have therefore also been investigated by using transfected cells and receptor binding drugs.

In adipose tissue, the results showed that adenosine is an important antilipolytic factor and the only adenosine receptor involved in regulating lipolysis is the A1R. The A1R couples to the Gi protein. However, deletion of the A1R in mice does not lead to compensatory increases in G protein-mediated antilipolytic actions of nicotinic acid or PGE2. Insulin inhibits lipolysis by another mechanism than adenosine that involves breakdown of cAMP. Our results show that adenosine and insulin inhibit lipolysis in an additive, but not synergistic manner. In lipogenesis, endogenous adenosine is not of major importance, but an adenosine analogue is able to stimulate lipogenesis via the A1R in the presence of insulin. Furthermore, there were no differences in mRNA expression of genes that have known functions in aspects of lipid synthesis in adipose tissue of A1R (-/-) versus wild type (A1R (+/+)) mice. In addition, the deletion of the A1R does not affect body weight in young mice, but older male A1R (-/-) mice have significantly higher body weight than the A1R (+/+) mice and they die earlier.

Following glucose injection, glucose tolerance was not appreciably altered in the A1R (-/-) mice. Glucose injection induced sustained increases in plasma insulin and glucagon levels in the A1R (-/-) mice, whereas A1R (+/+) mice showed the expected transient increase in insulin levels and decrease in glucagon levels. Hence, the A1R has a role in regulating the pancreatic hormones. Insulin- and contraction-mediated glucose uptake in skeletal muscle were not significantly different between the A1R (+/+) and A1R (-/-) mice and therefore, the A1R is not of major importance in regulation of glucose uptake in the skeletal muscle.

Adenosine was also shown to not be a full or partial agonist at the GHSR1a. Previous contrary results might be due to endogenous expression of adenosine receptors, presumably A2BRs, in the cell lines that have been used in these earlier studies. However, this study also showed that the cross-talk between the GHSR1a and A2BR is limited.

In conclusion, these studies emphasize the metabolic roles of adenosine and the findings may be of importance when developing drugs for treatment of metabolic diseases such as obesity and diabetes, if the adenosine receptors are used as drug targets. However, it will not be a useful strategy to use adenosine related substances when targeting the GHSR1a, e.g. for treatment of metabolic diseases. Adenosine receptor agonists may be used as drugs for lowering free fatty acid levels and adenosine receptor antagonists may be of importance for their role in regulating pancreatic release of insulin.