Glucose and palmitate uncouple AMPK from autophagy in human aortic endothelial cells

Abstract

Weikel KA, Cacicedo JM, Ruderman NB, Ido Y. Glucose and palmitate uncouple AMPK from autophagy in human aortic endothelial cells. Am J Physiol Cell Physiol 308: C249 -C263, 2015. First published October 29, 2014 doi:10.1152/ajpcell.00265.2014.-Dysregulated autophagy and decreased AMP-activated protein kinase (AMPK) activity are each associated with atherogenesis. Atherogenesis is preceded by high circulating concentrations of glucose and fatty acids, yet the mechanism by which these nutrients regulate autophagy in human aortic endothelial cells (HAECs) is not known. Furthermore, whereas AMPK is recognized as an activator of autophagy in cells with few nutrients, its effects on autophagy in nutrient-rich HAECs has not been investigated. We maintained and passaged primary HAECs in media containing 25 mM glucose and incubated them subsequently with 0.4 mM palmitate. These conditions impaired basal autophagy and rendered HAECs more susceptible to apoptosis and adhesion of monocytes, outcomes attenuated by the autophagy activator rapamycin. Glucose and palmitate diminished AMPK activity and phosphorylation of the uncoordinated-51-like kinase 1 (ULK1) at Ser555, an autophagy-activating site targeted by AMPK. 5-Aminoimidazole-4-carboxamide-1-␤-D-ribofuranoside (AICAR)-mediated activation of AMPK phosphorylated acetyl-CoA carboxylase, but treatment with AICAR or other AMPK activators (A769662, phenformin) did not restore ULK1 phosphorylation or autophagosome formation. To determine whether palmitate-induced ceramide accumulation contributed to this finding, we overexpressed a ceramidemetabolizing enzyme, acid ceramidase. The increase in acid ceramidase expression ameliorated the effects of excess nutrients on ULK1 phosphorylation, without altering the effects of the AMPK activators. Thus, unlike low nutrient conditions, AMPK becomes uncoupled from autophagy in HAECs in a nutrient-rich environment, such as that found in patients with increased cardiovascular risk. These findings suggest that combinations of AMPK-independent and AMPK-dependent therapies may be more effective alternatives than either therapy alone for treating nutrient-induced cellular dysfunction. endothelium; autophagy; palmitate; glucose; AMPK PATIENTS WITH THE METABOLIC SYNDROME or type 2 diabetes are more likely to develop cardiovascular complications such as atherosclerosis (26,44,54,62,77). Many cellular processes have been implicated in driving this pathology, but the predominant mechanisms remain unclear. Intriguing reports from humans and murine models of vascular disease indicate that dysregulation of macroautophagy, a pathway by which cellular components are recycled for energy utilization, may play a prominent role in this disease progression. Thus it has been shown that impairment of macroautophagy contributes to arterial aging (56), cardiomyocyte apoptosis (34), and aortic lesion formation (74).Macroautophagy (hereinafter referred to as "autophagy") is a process by which cellular components are engulfed in a double-membraned vesicle, the au...

Unraveling the actions of AMP-activated protein kinase in metabolic diseases: Systemic to molecular insights

“…Although AMPK stimulates autophagy under low-nutrient conditions, activation of AMPK alone does not restore this process when cells are passaged in high concentrations of glucose and then acutely exposed to high concentrations of free fatty acids as might occur in the setting of diabetes. [7] Intriguingly, these data suggest that epigenetic changes are another potential mechanism that could lead to AMPK inhibition, impairment of basal autophagy and distortion of AMPK-mediated signaling in metabolic disease.…”

“…For example, in HAECs, nutrient excess conditions decrease AMPK activity but do not increase mTORC1 activity. [7] Impairment of lysosomal cathepsin activity in HAECs by excess nutrients,[7] coupled with evidence of mTORC1 and AMPK regulation on the lysosomal membrane in other contexts[54] suggests that the lysosome may be a key factor regulating mTORC1 and AMPK in HAECs.…”

“…[7, 58] Studies in HeLa cells, HEK293 cells and rat hepatocytes have also found that lysosomal pH is associated with mTORC1 activity, likely via TFEB. [45, 59, 60] In addition to genes directly involved in lysosome function, genomic analysis of HeLa cells indicated that TFEB regulates the expression of PRKAG2, the gene that encodes for AMPKγ2.…”

“…They include high concentrations of glucose, fatty acids such as palmitate, and glucocorticoids and a decreased concentration of adiponectin; the same factors that predispose individuals to metabolic diseases. [3, 7] However, the mechanisms by which AMPK is dysregulated are not clear and may vary among these factors and between different types of cells.…”

Uncoupling AMPK from autophagy: a foe that hinders the beneficial effects of metformin treatment on metabolic syndrome-associated atherosclerosis? Focus on “Glucose and palmitate uncouple AMPK from autophagy in human aortic endothelial cells”

“…In this issue of American Journal of Physiology-Cell Physiology, Weikel et al (9) established a unique cell culture model and dissected the role of AMPK-mTOR-ULK1 in autophagy in human aortic endothelial cells (HAECs) (9). In this model, HAECs were maintained and passaged in media containing 25 mM glucose (to mimic chronic hyperglycemia conditions) and were further acutely challenged with 0.4 mM of the saturated fatty acid palmitate (a physiologically relevant concentration that is often achieved in diabetes patients during fasting or exercise).…”