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  • The preferences for source of FAs for DGAT

    2020-05-22

    The preferences for source of FAs for DGAT1 and DGAT2 are related to their membrane topologies, subcellular locations, interactions with other proteins or organelles (discussed in the former section), and their differential expression, substrate specificities, and enzyme kinetics. According to UniProt protein database (https://www.uniprot.org), human DGAT1 is expressed at a high level in adrenal gland and small intestine, at a medium level in liver and pancreas, and at low level in heart muscle, kidney, and skin, whereas human DGAT2 is expressed predominantly in liver and white adipose tissue, at a lower level in mammary gland and peripheral blood leukocytes, and is also expressed in sebaceous glands of normal skin but decreased ipsoriatic skin. In mouse, DGAT1 is expressed at a medium level in liver, kidney, and heart and at a low level in skeletal muscle tissue, whereas DGAT2 is expressed at a high level in liver, testis, brain, and lung. DGAT1 has broader substrate specificity synthesizing retinal esters, ether lipids, and waxes in addition to TAG [111], and with a higher K than DGAT2 [119], indicating DGAT1 requires a higher substrate concentration for apparent DGAT activity. It is reported that DGAT1 has an apparent Km of 13.9 μM for palmitoyl coenzyme A [120], whereas the Km for DGAT2 for oleoyl-CoA is 8.3 μM [54]. DGAT1 is subject to allosteric regulation with positive cooperativity in terms of acyl-CoA levels [110,111]. It has been shown that DGAT1 is more active at higher (>200 μM) oleoyl-CoA concentrations associated with an influx of exogenous FA in an in vivo system, whereas DGAT2 is more active at lower oleoyl-CoA concentrations (50 μM or less) [42]. This suggests that DGAT1 activity is proportionately increased over DGAT2 at high substrate concentrations, such as with exogenous addition of FA or with the high levels of lipolysis seen as a complication in metabolic syndrome and insulin resistance [42,45,108].
    Role of DGAT comt inhibitor in TAG metabolism DGAT activity of hepatic microsome preparations has been divided into overt (cytosolic) activity of intact microsomes and latent (luminal) activity that appears following permeablization of microsomes [121]. The latent activity in rat liver microsomes is almost twice the overt activity, and suggests a sidedness of DGAT activity [113]. Owen et al. [122] reported that DGAT activity is about equal magnitude on both sides of the microsomal membrane and by extension the ER membrane. This sidedness implies that while both or either DGAT enzyme may contribute to the overt activity responsible for the synthesis of TAG for cytosolic LDs, DGAT1 is capable of synthesising the TAG packaged in luminal LDs in the ER [123]. It has also been argued that the DGAT1 active site may be present on either side of the ER membrane due to dual topology of the protein [78]. Overexpression of Dgat1 in mouse liver has been correlated with an increased latent activity and VLDL secretion [124]. Overexpression of DGAT1 in rat hepatoma McArdle-RH7777 cells results in accumulation of small LDs around the cell periphery and increased secretion of LDs [126]. Inhibition of DGAT2 in mouse yields submaximal suppression of VLDL secretion [125]. DGAT1 has been shown to synthesize cytosolic TAG in hepatocytes that is preferentially channelled to FA oxidation [116]. Given that the large C-terminal part that contains the active site of DGAT2 lies on the cytosolic face of the ER membrane and a decline in overt activity is seen when DGAT2 is inhibited by niacin, DGAT2 is believed to be entirely overt [54]. Overexpression of DGAT2 in rat hepatoma McArdle-RH7777 cells has shown large cytosolic LDs [126]. Overexpression of Dgat2 in mouse liver leads to increases in liver TAG content but not VLDL secretion [124]. Liver specific Dgat1 knockout mice reduced steatosis and had lower serum TAG levels during fasting [112], indicating DGAT2 can support production of TAG for hepatic VLDL secretion using exogenous FAs from white adipose tissue during fasting, independent of DGAT1. In addition, DGAT2 forms protein complexes with MAGT2 [105] and FATP1 [107] in both the ER membrane and the phospholipid monolayer of cytosolic LDs. DGAT2 also associates with SCD1 in close proximity in the ER membrane [106]. Such interactions promote TAG synthesis because DGAT2 uses the DAG produced by MGAT2, and fatty acyl-CoAs generated by SCD1 exogenously and FATP1 from pre-formed FAs, to synthesize TAG within close proximity avoiding diffusion of the substrates to the active site. Together, these results suggest that DGAT1 and DGAT2 have differentiated roles in TAG synthesis and secretion, which are summarised in Fig. 1.