Transcriptional Regulation of Fatty Acid Transport by Sirtuin 2
The heart is a highly dynamic organ and under normal physiological conditions, it is heavily dependent on fatty acid oxidation to meet its energy requirements (Abumrad and Goldberg 2016). Fatty acid transport is a crucial parameter that governs intracellular lipid homeostasis (Glatz, Luiken et al. 2010). Dysregulation in fatty acid transport occurs in various pathological conditions including obesity, type-2 diabetes and cardiovascular disorders (Bayeva, Sawicki et al. 2013). In obesity and diabetes, myocardial fatty acid uptake is elevated due to increased expression of fatty acid transporters like CD36 and FATP-1 in the heart, thereby contributing to cardiac lipotoxicity (Wu, Ortegon et al. 2006; Glatz, Angin et al. 2013). Sirtuins play a key role in regulating fatty acid metabolism. They belong to the protein family of class-III histone deacetylases that are known to regulate diverse signalling pathways involved in lipid metabolism, glucose homeostasis, DNA repair and oxidative stress response (Simmons, Pruitt et al. 2015). Previous study in our laboratory has shown that SIRT2, which is predominantly cytosolic, plays a protecting role against development of pathological cardiac hypertrophy (Sarikhani, Maity et al. 2018). However, its role in regulating fatty acid transport has not been studied and this serves as the primary focus of the study. To test whether SIRT2 regulates fatty acid transport, expression of fatty acid transporters was checked in the heart of SIRT2-KO mice. Quantitative real-time PCR and western blotting analysis reveal that mRNA and protein levels of CD36 and FATP1 is elevated in the heart of SIRT2-KO mice compared to controls. In order to analyse the role of SIRT2 in regulating fatty acid transport in vitro, fatty acid uptake assay was performed in cardiomyocytes. Using Fluorescence-Activated Cell Sorting (FACS) analysis, it was found that SIRT2 overexpressing cardiomyocytes show lower fatty acid uptake. Additionally, confocal analysis shows lower accumulation of neutral lipids in SIRT2 overexpressing cardiomyocytes. Since muscles are a major site of fatty acid utilization (van Hall 2015), role of SIRT2 in regulating fatty acid transport in skeletal muscles was studied. Interestingly, CD36 mRNA expression was found to be upregulated in the muscle of SIRT2-KO mice. Hence, the mechanism by which SIRT2 regulates CD36 expression was sought to be studied. FoxO1 is known to regulate fatty acid uptake by causing membrane enrichment of CD36 (Bastie, Nahle et al. 2005; Wang and Tong 2009). Since SIRT2 deacetylates and activates FoxO1 (Wang and Tong 2009), role of FoxO1 in regulating CD36 expression via SIRT2 dependent mechanism was hypothesised. FoxO1 localization and its activity under SIRT2 overexpression were studied in C2C12 cells. The study indicates that although there is no change in FoxO1 localisation, its activity is enhanced upon SIRT2 overexpression in C2C12 cells. However, further study is necessary to understand the exact mechanism by which FoxO1 modulates CD36 gene expression, thereby regulating fatty acid uptake in cells.