The role of AMPK in bone and cholesterol metabolism

© 2012 Dr. Shanna Shiu-Nam TamThe central focus of this thesis explores the biology of AMP-activated protein kinase (AMPK), an important whole-body energy regulator, firstly in bone metabolism, and secondly in the regulation of cholesterol metabolism. AMPK is a ubiquitously-expressed αβγ heterotrimeric enzyme. Since AMPK plays important roles in modulating metabolism in response to diet and exercise, both of which influence bone mass, the influence of AMPK on bone mass was investigated in mice. In the first part, I will present data that the non-specific AMPK activator, AICAR, enhanced the formation of osteoclasts (bone-resorbing cells) in vitro and caused bone loss in vivo. However, the in vitro effects were shown to be AMPK-independent. Next, I will present genetic evidence that AMPK is important for the maintenance of normal bone mass, as germline deletion of either AMPK β1 or β2 subunit isoforms resulted in reduced trabecular bone density and mass, but without affecting osteoblast or osteoclast numbers. In Chapter 6, I focussed on one of the substrates of AMPK named 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR), which catalyses the rate limiting step in the mevalonate pathway. Mevalonate is essential for the synthesis of important bioactive sterols (e.g. cholesterol) and non-sterol isoprenoids, which are important for processes such as protein prenylation. Phosphorylation of residue S871 of HMGR by AMPK inhibits its activity, presumably to conserve cellular energy by shutting down cholesterol synthesis. To study the physiological importance of this phosphorylation control of HMGR by AMPK, our laboratory generated a knock-in mutation in mice by homologous recombination and Cre-mediated excision to introduce a modified exon 20 to encode Alanine at residue 871. The data suggests that lack of regulation of HMGR by AMPK in mice did not lead to any overt physical phenotypic differences or compensatory up- or down-regulation in the expression of other mevalonate pathway genes. However, these mice displayed slightly altered lipid profiles in the brain, liver and plasma. The lack of a more prominent phenotype may reflect the importance of the mevalonate pathway and its multivalent backup regulation