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Very-low-density lipoprotein (VLDL) and chylomicrons (CMs) transport triacylglycerol (TAG) to peripheral tissues. Lipoprotein-TAG may gain access to target cells by lipoprotein lipase (LPL) hydrolysis or via receptor-mediated uptake; the principal routes of entry of VLDL and CM into heart are unknown, and different routes of entry may result in different metabolic fates. To examine this, isolated working rat hearts were perfused with rat VLDL and CMs, dual-labelled with [3H]TAG and [14C]cholesterol. Uptake and utilization of CM-TAG were significantly greater than VLDL-TAG, but both were decreased significantly (more than halved) by tetrahydrolipstatin (THL, an inhibitor of lipoprotein lipase). By contrast, uptake of VLDL-cholesterol was much higher than CM-cholesterol (P < 0.01), and suramin (a lipoprotein receptor antagonist) decreased cholesterol uptake of both forms. CM-TAG oxidation rate was more than 4-fold higher than VLDL-TAG oxidation. However, suramin decreased TAG oxidation from both VLDL and CM without affecting TAG uptake or total utilization, suggesting that the TAG gaining access through receptor-mediated pathways is preferentially ‘channelled’ towards oxidation. Most (79%) CM-TAG was oxidized whilst the proportion of VLDL-TAG oxidized was only about half (49%). In the presence of suramin, there was a significant increase in esterification (incorporation of assimilated [3H]TAG into myocardial tissue [3H]lipids, mainly TAG) of assimilated TAG from both VLDL and CMs, again suggesting that receptor-mediated TAG uptake is directed towards oxidation rather than esterification. The importance of this relatively small pool of TAG is indicated by the fact that cardiac mechanical function declined markedly when lipoprotein receptors were inhibited. These results suggest that CMs, most fatty acids of which gain access into cardiomyocytes through LPL-mediated hydrolysis, are the major supplier of TAG for hearts to oxidize; however, the metabolic fate of VLDL was split evenly between oxidation and deposition as myocardial tissue lipid. Most importantly, VLDL may play a regulatory role in heart lipid metabolism through a lipoprotein receptor-mediated mechanism.
Most (∼70%) of the cardiac energy requirement is supplied by fatty acid (FA) oxidation under normal physiological workload conditions (Lopaschuk et al. 1994; Calvani et al. 2000; van der Vusse et al. 2000). FAs are derived from two sources: (1) circulating non-esterified FAs (NEFAs) bound to plasma albumin, derived from adipose tissue lipolysis, which gain access to the cardiomyocytes both passively (Zakim, 1996) and via carrier-mediated pathways (Abumrad et al. 1998; Van Der Vusse et al. 2000) involving at least three proteins, namely fatty acid transport protein (FATP), fatty acid translocase (FAT) and fatty acid binding protein (FABPpm) (Schaap et al. 1998; Luiken et al. 1999); and (2) circulating esterified FA in the form of triacylglycerols (TAGs), which are transported in plasma within lipoproteins: very-low-density lipoprotein (VLDL) and chylomicrons (CMs). VLDLs are synthesized by the liver from endogenous lipids whereas CMs are synthesized by the intestine from exogenous dietary lipid. However, until recently the relative contributions of NEFAs and TAGs to the heart energy requirements were unknown. Although the metabolism of albumin-bound NEFAs by the heart has been studied extensively (Lopaschuk et al. 1994; Lopaschuk, 1997; Wang et al. 1998; Belke et al. 1999), and suggests that albumin–FAs are the primary energy source for the heart, the role of TAGs within VLDLs and CMs (VLDL-TAGs and CM-TAGs) in heart energy supply has been uncertain. Following development of a technique to prepare species-specific radiolabelled VLDL by liver perfusion (Bennett et al. 2000a), myocardial preference for NEFAs, VLDL-TAGs and CM-TAGs has been investigated in rat (Hauton et al. 2001). CMs (prepared by thoracic duct cannulation) are efficient substrates for heart, being utilized to a similar extent to NEFAs, but VLDLs were less so (Hauton et al. 2001). These studies indicated that whilst NEFAs suppress cardiac CM-TAG utilization, VLDL-TAG utilization was not altered by the presence of NEFAs. Furthermore, marked differences in the metabolic fates (oxidation versus tissue lipid deposition) of FA and TAG substrates were noted (Hauton et al. 2001; Mardy et al. 2001). Thus these studies provided preliminary evidence that intracellular channelling of FAs occurs, leading to differing metabolic fates (i.e. distribution between oxidation and incorporation into cellular lipid) according to the FA sources, and this may be due to different routes of FA and TAG uptake.
Hydrolysis of TAG by endothelium-bound lipoprotein lipase (EC 126.96.36.199; LPL) is widely regarded as the initial step for the ‘bulk’ uptake of lipoprotein-TAG by the heart (Braun & Severson, 1992; Goldberg, 1996; Merkel et al. 2002; Augustus et al. 2003). Following LPL-mediated hydrolysis of plasma TAG, the liberated FA product is assimilated by the cardiomyocyte as for NEFA (above). LPL is synthesized in the cardiomyocyte but translocated to its active site on the luminal surface of the capillary endothelium where it is bound to heparan sulphate proteoglycan. Endothelial LPL hydrolyses the triacylglycerol-rich lipoprotein (TGRLP) TAG core, releasing fatty acids which are then taken up into the cardiomyocyte via the endothelial cell by route(s) unknown.
However, besides LPL, plasma TAG may be taken up by the cardiomyocyte through lipoprotein receptor-mediated pathways. The recently described VLDL/apo-E receptor (VLDL-R) (Sakai et al. 1994; Tiebel et al. 1999; Kamataki et al. 2002) and the TGRLP/apo-B48 receptor (Gianturco et al. 1998; Brown et al. 2000) are highly expressed in the heart, skeletal muscle and adipose tissue (Tiebel et al. 1999). The tissue distribution of the VLDL receptor suggests a possible role in the delivery of lipoproteins, especially remnant lipoprotein particles, which are the products of LPL hydrolysis, to peripheral tissues, including heart for lipid metabolism. Furthermore, considering that the tissue distributions of VLDL-R and LPL are similar (Braun & Severson, 1992; Goldberg, 1996; Tiebel et al. 1999), an interaction between them could exist which can regulate TGRLP metabolism.
In the present study, we hypothesized that the pattern of intracellular metabolic fates (oxidation versus tissue esterification; pattern of incorporated tissue lipid distribution) differs between VLDLs and CMs in the heart and that this difference is due to the different routes of uptake of VLDLs and CMs by the heart. To test this hypothesis, we used dual-labelled ([3H]TAG; [14C]cholesterol) VLDLs and CMs to perfuse working rat hearts. Since cholesterol can only gain access to the cardiomyocyte through a receptor-mediated mechanism, this method allowed assessment of possible routes of uptake of TAG by hearts simultaneously. Further, we used tetrahydrolipstatin (THL), an active site inhibitor of LPL, and suramin, which inhibits the binding between lipoproteins and lipoprotein receptors, to assess the roles of LPL and lipoprotein receptors in TAG delivery to the heart.