Original articleSirt1 counteracts decrease in membrane phospholipid unsaturation and diastolic dysfunction during saturated fatty acid overload
Graphical abstract
Introduction
The prevalence of obesity is steadily increasing worldwide. Obesity-associated alterations of cardiac structure and function are a key factor for future occurrence of heart failure [[1], [2], [3]]. Epidemiological studies show that in humans saturated fatty acid (SFA) significantly worsen insulin-resistance and hypertension, while monounsaturated fatty acid (MUFA) and polyunsaturated fatty acid (PUFA) improve them through modifications in the fatty acid (FA) composition of membrane phospholipid which reflect at least in part dietary fat composition [4,5]. However, little is known whether dietary fat composition directly affects left ventricle (LV) structure and function independently of associated risk factors, such as insulin-resistance and hypertension.
Adult cardiomyocytes in actively contracting and load-bearing myocardium have high ATP demand accompanied by high fatty acid oxidation rates. Recent experimental studies demonstrated that a large fraction of the exogenous long chain FA is esterified into the triglyceride (TG) pool upon entry into the cardiomyocyte and then later de-esterified for release and entry into β-oxidation in the mitochondria [6]. By comparing the hearts of mice fed high fat diets (HFD) with different SFA and MUFA balances, we demonstrated that adult cardiomyocytes in situ channel a substantial amount of exogenous FA toward lipid membranes, like other cell types that have no incentive to burn fat. Although both SFA-rich HFD and MUFA-rich HFD cause the same degree of activation of myocardial FA uptake, triglyceride turnover, mitochondrial FA oxidation, and accumulation of toxic lipid intermediates, only SFA-rich HFD-fed heart showed a decrease in membrane phospholipid unsaturation (an increase in SFA/MUFA ratio of the membrane phospholipid), induction of unfolded protein response (UPR), and impaired diastolic function. These findings indicate that when FA overload in the heart overwhelms the cellular capacity to use it for energy, the surplus of FA is channeled into membrane phospholipid. In such situations, the FA composition of membrane phospholipid in adult cardiomyocytes is altered to reflect dietary fat composition [7].
SFA suppressed protein expression levels of Sirt1 and stearoyl-CoA desaturase-1 (Scd1), a rate-limiting enzyme in the cellular synthesis of MUFA from SFA which determines the SFA/MUFA balance in membrane phospholipid, while MUFA does not have this effect [7]. These findings suggested that the failure of homeostatic control of membrane FA composition is due to the reduced expression of Sirt1 and Scd1 which further worsens the decrease in membrane phospholipid unsaturation by SFA-rich HFD.
In this study, we explored a role of Sirt1 in homeostatic control of membrane FA composition. By using both cardiomyocyte-specific Sirt1 knockout mice and Sirt1 knockdown cultured cardiomyocytes, we found that cardiomyocyte Sirt1 counteracts the decreased membrane phospholipid unsaturation induced by SFA overload via regulating liver X receptor (LXR)-mediated transcription of the Scd1 gene. The Sirt1/LXR/Scd1 axis could be a novel target for prevention or treatment of diastolic dysfunction accompanied by inappropriately rich SFA overeating.
Section snippets
Materials and methods
Detailed Methods are provided in the online Data Supplement.
Cardiomyocyte-specific Sirt1 deletion shows vulnerability to diastolic dysfunction in response to an SFA-rich HFD
We investigated the role of Sirt1 in alterations in cardiac structure and function that occur in response to two HFD with different FA compositions. Cardiomyocyte-specific Sirt1 conditional knockout mice (Sirt1CKO) were generated by crossing Sirt1flox/flox mice with α-myosin heavy chain promoter-driven Cre mice (αMHC-Cre). αMHC-Cre− Sirt1flox/flox mice were used as a control. Four-week-old male Sirt1CKO and control mice were randomly divided into three groups and fed with one of two different
Discussion
In this study, we showed a role of Sirt1 in homeostatic control of membrane FA composition. By using both cardiomyocyte-specific Sirt1 knockout mice and Sirt1 knockdown cultured cardiomyocytes, we found that cardiomyocyte Sirt1 counteracts the decreased membrane phospholipid unsaturation induced by SFA overload via regulating LXR-mediated transcription of the Scd1 gene.
There has been heavy debate as to whether fats or carbohydrates are the culprit in the obesity epidemic. For years, because
Acknowledgments
The authors are grateful to Yoshiko Miyake (Keio University, Japan) for technical assistance and to Hiroki Matsui (Gunma University, Japan) for the plasmid used for the construction of the SCD1-targeting vector.
Sources of funding
This study was supported by JST PRESTO grants (2013–2015) and JSPS KAKENHI grants 15H04825 (2015–2017) and 15H01160 (2015–2016) (to Dr. Sano).
Disclosures
None.
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