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    • The focus of this review is on

    2020-04-14

    The focus of this review is on the diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) enzymes, DGAT1 and DGAT2, that catalyze the esterification of a fatty acyl moiety to a DAG molecule to produce TAG. These enzymes commit DAG to being stored as TAG rather than being diverted for the synthesis of phospholipids [16]. As catalysts of TAG synthesis, DGATs are involved in intestinal fat WAY-100635 maleate salt [17], flux of lipoproteins between adipose tissue and the liver, regulation of lipid concentrations in plasma [18], and energy metabolism in muscle [19]. Early studies characterizing DGAT activity in eukaryotic cells showed a noticeable increase in DGAT activity of embryonic mouse fibroblast NIH 3T3-L1 cells as they differentiated into adipocytes [20]. It has since been shown that the DGATs are essential for TAG synthesis in adipocytes and play a critical role in the formation of LDs [14]. Moreover, Dgat2 gene expression also affects the skin permeability barrier function in mice [21]. In humans, a decreased DGAT2 mRNA in the sebaceous glands of patients with psoriasis highlights the vital role of this gene and its product [22]. While the finer details of the functions of each enzyme are still being worked out, the discovery and clinical development of several DGAT1 small molecule inhibitors for the potential treatment of an array of metabolic disorders has provided biological insights that have once again highlighted the central role lipids and the DGAT enzymes play in cellular homeostasis [23]. Besides drawing the interest of pharmaceutical companies, there are commercial incentives to modulating the role of the DGATs to improve milk lipid quality [24], and the DGAT enzymes are central to the quest to increase oil outputs from single cell oil sources [25] and oleaginous plants [26]. This review summarises the present knowledge on the functional convergence, polymorphism and effects of knocking out the genes that code for the DGAT enzymes. Also included are the membrane topology of the enzymes, DGAT-protein interactions, preferences for fatty acids from different sources, and the differential roles of the DGAT isozymes in TAG synthesis and secretion. This information will provide rationales and perspectives on how DGAT activities of the two enzymes are important for lipid homeostasis. Such a review has not been presented since the DGATs were last reviewed in 2012 [27] and 2013 [28].
    Genes encoding the DGAT enzymes
    Membrane topology and structure-function relationships The evolutionary divergence in the gene sequences of the DGAT enzymes is reflected in their predicted protein structures [73]. Through multiple sequence alignment of DGATs, including 59 DGAT1s from 48 organisms and 58 DGAT2s from 44 organisms, Cao et al. [74] revealed that a DGAT1 isoform has an average number of 515 ± 44 amino acid residues with 41 amino acid residues completely conserved. A DGAT2 isoform has an average number of 344 ± 29 amino acid residues with 16 completely conserved. Both DGAT1 and DGAT2 contain about 40% hydrophobic amino acid residues [74]. Due to their hydrophobic nature, structural investigation to date of DGATs has been limited to topological analysis. Initial computational models predicted that DGAT1 could have between 8 and 10 hydrophobic regions that are believed to form the transmembrane domains (TMDs) [27]. An analysis of the protein sequence available for the human DGAT1 in the UniProt database (UniProt_O75907) suggests that the protein contains 36% hydrophobic amino acid residues (Fig. 2a and b) and that it is predicted by the transmembrane hidden Markov model to form as many as 9 TMDs (Fig. 2e) (TMHMM server v.2.0) [75]. The UniProt database describes human DGAT1 (UniProt_O75907) as a multi-pass membrane protein based on sequence similarity between DGATs of different species [76]. Experimental data for murine DGAT1 reported by McFie et al. [77] suggest that the murine isoform may have only three TMDs, with the N-terminus oriented toward the cytosol and the C-terminal region accounting for approximately 50% of the sequence present in the ER lumen (Fig. 2g) and the catalytic activity exclusively on the luminal side of the ER membrane. The second and third TMDs of the three may form a hemi-loop such that they are embedded within the lipid bilayer without traversing it. The proposed membrane topology serves as a limited guide to the actual membrane topology of DGAT isoforms in other species. Wurie et al. [78] have suggested that DGAT1 has dual topology with approximately equal activity on both the cytosolic and luminal sides of the ER membrane in HepG2 cells. Further experimental verification would be required to explain the DGAT1 activity reported by Wurie et al. [78].