Endothelial LKB1/AMPK signaling pathway in regulating energy and vascular homeostasis

Abstract

Liver kinase B1 (LKB1), a serine/threonine kinase, is responsible for the activation of AMP-activated protein kinase (AMPK), the master regulator of energy metabolism. LKB1/AMPK signaling pathway possesses a wide range of biological functions in regulating cell cycle progression, cell polarity, senescence and inflammation. In cultured endothelial cells, the pro-senescence function of LKB1/AMPK signaling pathway has been observed. However, the mechanisms by which LKB1 is regulated in endothelial cells remain largely uncharacterized. Furthermore, little is known about the effects of activated endothelial LKB1/AMPK signaling pathway on vascular and energy homeostasis. The present study aimed to investigate the upstream molecular mechanisms regulating LKB1 protein stability during endothelial senescence and the downstream pathophysiological effects of hyperactivated AMPK signaling in endothelial cells. In cultured model of cellular senescence, the lysine (K) 64 residue of LKB1 was found to be crucial for mediating its pro-senescence activities. The protein stability and intracellular localization of LKB1 mutant with K64 replaced by arginine (R) was different from the wild type protein. K64R exhibited enhanced effects on promoting endothelial senescence. Moreover, mutation of this residue attenuated the binding to HERC2, a newly identified E3 ubiquitin ligase for LKB1, in turn preventing its ubiquitination and degradation. Using a transgenic mouse model that selectively over-expresses a constitutively active AMPK α1 subunit (EC-AMPK) in endothelial cells, the influence of hyperactivated AMPK signaling on metabolic and vascular functions was investigated. Under standard chow condition, the metabolic phenotypes were similar between wild type and EC-AMPK mice; under high fat diet condition, EC-AMPK mice showed more severe obesity-induced fatty liver injury. Selective activation of AMPK in endothelial cells caused vascular and hepatic inflammation. Cyclooxygenase-2 (COX-2) was found to be the mediator for the pro-inflammatory functions of AMPK in vascular endothelial cells and facilitated to the development of obesity-induced fatty liver injury in EC-AMPK mice. Evaluation using isolated arteries revealed an increased systolic blood pressure and abnormal endothelial function in EC-AMPK miceunder high fat diet. AMPK activation in endothelium of the blood vessel could not block vascular remodeling associated with dietary obesity. Taken in conjunction, the above findings suggest that continuous activation of LKB1/AMPK signaling elicits adverse effects on both energy and vascular homeostasis.