Regulation of 3-hydroxybutyrate formation and secretion of very-low-density-lipoprotein triacylglycerol by perfused livers from fed and starved rats.
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ABSTRACT:
SUBMITTER: Laker ME
PROVIDER: S-EPMC1158602 | biostudies-other | 1982 Aug
REPOSITORIES: biostudies-other
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Project description:The very-low-density-lipoprotein secretion rate of isolated hepatocytes obtained from rats fed a high-fat diet was half that of cells from control animals. In fat-fed rats, the initial cellular uptake of [l-14C]oleate in vitro was decreased by 25%, its esterification to triacylglycerols and phospholipids by 50% and its incorporation into very-low-density-lipoprotein triacylglycerols by 70%. Exogenous oleate was not the main precursor of very-low-density lipoproteins in these animals. Lipogenesis, a minor source of very-low-density lipoproteins with the control diet in our experimental conditions, was inhibited by 84% after fat-feeding. A short-term inhibition of lipogenesis in vitro did not result in a decrease in very-low-density-lipoprotein secretion rate. The results suggest that fat-feeding decreased availability of exogenous as well as endogenous fatty acids for synthesis of very-low-density lipoproteins.
| S-EPMC1163258 | biostudies-other
Project description:The effect of adrenaline on triacylglycerol synthesis and secretion was examined in isolated rat hepatocytes. Cells were incubated with 0.5 mM-[1-14C]oleate, and the accumulation of triacylglycerol and [14C]triacylglycerol was measured in the incubation medium. Triacylglycerol appearing in the medium was present in a form with properties similar to very-low-density lipoproteins. Triacylglycerol, [14C]triacylglycerol and [14C]phospholipid contents of hepatocytes were also determined. Addition of 10 microM-(-)adrenaline decreased accumulation of glycerolipid in the incubation medium and also decreased cellular [14C]phospholipid content. Prazosin abolished these effects, whereas propranolol did not. The hormone did not affect cellular triacylglycerol content or rates of incorporation of [1-14C]oleate into cell triacylglycerol. The effect of adrenaline on the removal of newly secreted triacylglycerol and the secretion of synthesized glycerolipid was also examined. The catecholamine did not affect rates of removal of newly secreted triacylglycerol. Adrenaline did inhibit the secretion of pre-synthesized lipid by the cells, as assessed by the appearance of radiolabelled triacylglycerol from hepatocytes that had been preincubated with [1,2,3-3H]-glycerol. Adrenaline did not affect rates of fatty acid uptake by hepatocytes, but did stimulate oxidation of [1-14C]oleate, principally to 14CO2.
| S-EPMC1148864 | biostudies-other
Project description:Hepatocytes from rats fed a chow (control) diet or from rats fed a chow diet supplemented with either orotic acid (OA; 1%, w/w) or fish oil (FO; 20%, v/w) were maintained in culture for periods up to 48 h. during the first 24 h period, the low rates of output of very-low-density lipoprotein (VLDL)-associated triacylglycerol (TAG) and apolipoprotein B (apoB) in hepatocytes from the FO- and OA-fed animals were associated with significantly lower rates of intracellular TAG lipolysis and re-esterification. Most of the VLDL TAG secreted was mobilized via lipolysis of the intracellular TAG pool, but the proportion of VLDL TAG secreted via this route in cells from the FO-fed and OA-fed animals was decreased compared with that in the control-fed animals' cells. In the presence of exogenous oleate the inhibitory effect of OA feeding on VLDL apoB and TAG secretion persisted in the derived hepatocytes for up to 48 h following isolation. However, when oleate was absent no inhibitory effect on the secretion of TAG and apoB was observed between 24 and 48 h. Under these conditions the rate of intracellular TAG turnover returned to normal. The initial inhibitory effect of FO feeding on VLDL TAG and apoB secretion did not persist in the derived hepatocytes between 24 h and 48 h of culture in the presence of exogenous oleate. Although intracellular TAG lipolysis and VLDL TAG and apoB secretion rates appear to be positively correlated, a causal relationship has not been conclusively established.
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Project description:In contrast to water-soluble fuels such as glucose or ketone bodies, the use of lipids as an energy source for tissues has required the development of complex structures for their transport through the aqueous plasma. In the case of endogenously synthesized triacylglycerol this is achieved by the assembly and secretion of hepatic VLDL which provides the necessary stability in an aqueous medium. An essential component of this assembly process is apo B. Dietary changes which require an increase in hepatic VLDL secretion appear to be accompanied by increases in the availability of functional apo B. Interesting questions relate to: (a) the intracellular site(s) of triacylglycerol association with apo B, and (b) the mechanism(s) by which the availability of functional apo B at this site responds to metabolic and hormonal signals which reflect dietary status and, thus, the need to secrete triacylglycerol. As regards the latter, although in some cases changes in apo B synthesis occur in response to VLDL secretion hepatic apo B mRNA levels appear to be quite stable in vitro. Intracellular switching of apo B between the secretory and degradative pathways may be important in controlling VLDL assembly and post-translational modifications of the apoprotein may also play a role by influencing its ability to bind to triacylglycerol. Transport is not the only problem associated with the utilization of a concentrated energy source such as triacylglycerol and the complex problems of waste product disposal and recycling have to be dealt with. In the case of triacylglycerol, potentially toxic waste products include atherogenic remnants and LDL. The overall problem, then, in the long-term, involves the development of a 'safe' means of utilizing triacylglycerol and this requirement accounts for much of the complexity of plasma lipoprotein metabolism. In this area, the rat could teach the human a few tricks. One of these appears to be the utilization of hepatic apo B48 rather than apo B100 for VLDL assembly in response to increases in the extrahepatic utilization of hepatically synthesized triacylglycerol. Under these conditions, the remnants of hepatic triacylglycerol utilization by peripheral tissues are cleared from the plasma much more readily via a process which seems to involve the cycling of more triacylglycerol back to the liver than that which occurs in humans. The means by which this is achieved, though, are obscure and may involve a chylomicron remnant receptor, the nature of which, itself, remains controversial.(ABSTRACT TRUNCATED AT 400 WORDS)
| S-EPMC1131384 | biostudies-other
Project description:Rats were fed ad libitum on either a standard, high-carbohydrate, chow diet or a similar diet supplemented with 15% unsaturated fat (corn oil). Hepatocytes were prepared either during the dark phase (D6-hepatocytes) or during the light phase (L2-hepatocytes) of the diurnal cycle. In hepatocytes from rats fed on the unsaturated-fat-containing diet, secretion of very-low-density lipoprotein (VLDL) triacylglycerol was inhibited to a greater extent in the D6- than in the L2-hepatocytes. Plasma non-esterified fatty acid concentrations were elevated to the same extent at both D6 and L2 in the unsaturated-fat-fed animals. The secretion of VLDL esterified and non-esterified cholesterol was relatively insensitive to changes in the unsaturated-fat content of the diet. This resulted in proportionate increases in the content of these lipid constituents compared with that of triacylglycerol in the nascent VLDL. There was also an increase in the ratio of esterified to non-esterified cholesterol in the nascent VLDL produced by hepatocytes of the unsaturated-fat-fed animals. In the D6-hepatocytes from the unsaturated-fat-fed animals, the decrease in the secretion of VLDL triacylglycerol could not be reversed by addition of exogenous oleate (0.7 mM) to the incubation medium. In contrast, addition of a mixture of lactate (10 mM) and pyruvate (1 mM) stimulated both fatty acid synthesis de novo and the rate of VLDL triacylglycerol secretion. Secretion of esterified and non-esterified cholesterol also increased under these conditions. Insulin suppressed the secretion of VLDL triacylglycerol and cholesteryl ester under a wide range of conditions in all types of hepatocyte preparations. Non-esterified cholesterol secretion was unaffected. In hepatocytes prepared from the fat-fed animals, these effects of insulin were more pronounced at D6 than at L2. Glucagon also inhibited VLDL lipid secretion in all types of hepatocyte preparations. The decrease in cholesterol secretion was due equally to decreases in the rates of secretion of both esterified and non-esterified cholesterol.
| S-EPMC1147881 | biostudies-other
Project description:Microsomal triglyceride transfer protein (MTP) is a heterodimeric protein consisting of a unique 97 kDa subunit and protein disulphide isomerase, that mediates the transfer of lipid between membranes and nascent lipoproteins. Mutations in the gene encoding the 97 kDa subunit of the protein cause the rare autosomal recessive disorder abetalipoproteinaemia. Recent findings in cultured cells indicate that the 5' promoter region contains an insulin-responsive element, suggesting that the gene is responsive to insulin regulation. In this study we examined two cases where very-low-density lipoprotein (VLDL) secretion is markedly reduced: the streptozotocin-diabetic rat and 10-day-old suckling rats. In both cases MTP activity was unaltered compared with that in control animals. Equal levels of MTP were also apparent in the livers of all groups of animals, as measured by immunoblotting with an anti-MTP antiserum. These findings indicate that factors other than MTP activity are responsible for the reduction in hepatic VLDL triglyceride secretion observed in the suckling and diabetic animals.
| S-EPMC1135847 | biostudies-other
Project description:Oxylipins are metabolites of polyunsaturated fatty acids that mediate cardiovascular health by attenuation of inflammation, vascular tone, hemostasis, and thrombosis. Very low-density lipoproteins (VLDL) contain oxylipins, but it is unknown whether the liver regulates their concentrations. In this study, we used a perfused liver model to observe the effect of inflammatory lipopolysaccharide (LPS) challenge and soluble epoxide hydrolase inhibition (sEHi) on VLDL oxylipins. A compartmental model of deuterium-labeled linoleic acid and palmitic acid incorporation into VLDL was also developed to assess the dependence of VLDL oxylipins on fatty acid incorporation rates. LPS decreased the total fatty acid VLDL content by 30% [6%,47%], and decreased final concentration of several oxylipins by a similar amount (13-HOTrE, 35% [4%,55%], -1.3 nM; 9(10)-EpODE, 29% [3%,49%], -2.0 nM; 15(16)-EpODE, 29% [2%,49%], -1.6 nM; AA-derived diols, 32% [5%,52%], -2.4 nM; 19(20)-DiHDPA, 31% [7%,50%], -1.0 nM). However, the EPA-derived epoxide, 17(18)-EpETE, was decreased by 75% [49%,88%], (-0.52 nM) with LPS, double the suppression of other oxylipins. sEHi increased final concentration of DHA epoxide, 16(17)-EpDPE, by 99% [35%,193%], (2.0 nM). Final VLDL-oxylipin concentrations with LPS treatment were not correlated with linoleic acid kinetics, suggesting they were independently regulated under inflammatory conditions. We conclude that the liver regulates oxylipin incorporation into VLDL, and the oxylipin content is altered by LPS challenge and by inhibition of the epoxide hydrolase pathway. This provides evidence for delivery of systemic oxylipin signals by VLDL transport.
| S-EPMC7903942 | biostudies-literature
Project description:A recirculating liver perfusion system was used to study the effects of dietary selenium (Se) on the hepatic secretion of very-low-density lipoprotein (VLDL). The perfusate from livers of rats fed on a Se-deficient diet incorporated about 50% more [1-14C]oleic acid into triacylglycerol (TG) and cholesteryl esters (ChoEs) than did the perfusate from livers of rats fed on a Se-supplemented diet. Similarly, livers from rats fed the Se-deficient diet secreted more VLDL and incorporated about 60% more [1-14C]oleic acid into VLDL TG and ChoEs than did livers from rats fed the Se-supplemented diet. The liver perfusate from rats in the Se-deficient group also showed significantly decreased fatty acid oxidation. We conclude that Se is a potent modulator of lipoprotein metabolism. A primary action of Se deficiency appears to be a decrease in fatty acid oxidation and a stimulation of fatty acid esterification, leading to increased VLDL TG and ChoEs formation and secretion.
| S-EPMC1151508 | biostudies-other
Project description:The metabolism of millimolar concentrations of R- or S-1,3-butanediol has been studied in perfused livers from fed and starved rats. Protocols were designed to measure in the same experiment (i) uptake of the diol, (ii) the contribution of the diol to ketogenesis, (iii) the contribution of the diol to total fatty acid plus sterol synthesis, and (iv) conversion of S-1,3-butanediol into S-3-hydroxybutyrate. Our data show that R- and S-1,3-butanediol are taken up by the liver at the same rate. Most of the metabolism of R-1,3-butanediol is accounted for by conversion to the physiological ketone bodies R-3-hydroxybutyrate and acetoacetate. Only 29-38% of S-1,3-butanediol uptake is accounted for by conversion into physiological ketone bodies. The balance of S-1,3-butanediol metabolism is conversion to S-3-hydroxybutyrate, lipids and CO2.
| S-EPMC1132838 | biostudies-other
Project description:The possibility that triacylglycerol (TAG) synthesis occurs on both aspects of the endoplasmic-reticular membrane during the process of incorporation of TAG into secreted very-low-density lipoprotein (VLDL) [Zammit (1996) Biochem. J. 314, 1-14] was investigated by measuring the latency of diacylglycerol acyltransferase (DGAT) in microsomal fractions obtained from rat liver homogenates. Permeabilization of microsomes with taurocholate resulted in the doubling of the activity, indicating that DGAT activities of approximately equal magnitude occur on either aspect of the microsomal membrane. The taurocholate concentrations required for exposure of the latent activity of DGAT were identical with those that resulted in the exposure of marker enzymes for the lumen of the endoplasmic reticulum. Fractionation of the microsomes into smooth and rough populations indicated that the distribution of overt and latent DGAT activities was the same throughout. The possibility that taurocholate effects may result from non-specific activation of the overt enzyme was excluded by employing the channel-forming peptide alamethicin to effect permeabilization, and by varying the mode of delivery of diacylglycerol substrate to the microsomal membranes. Permeabilization using alamethicin gave a slightly higher latent/overt ratio for DGAT. The possible roles of overt and latent DGAT activities in the synthesis and secretion of TAG by the liver are discussed.
| S-EPMC1218291 | biostudies-other