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Biomedical subjects

G F Gibbons

Publications and source records attributed to G F Gibbons.

At least 55 records · Page 3Linked to original sources
PubMed1995

Human triacylglycerol-rich lipoprotein subfractions as substrates for lipoprotein lipase.

In order to test the hypothesis that lipoprotein lipase (LPL) acts preferentially on larger lipoprotein particles, we determined the susceptibility of triacylglycerol-rich lipoprotein (TRL) subfractions to hydrolysis by LPL in vitro. Chylomicrons (Sf > 400), very low density lipoproteins (VLDL)1 (Sf 60-400) and VLDL2 (Sf 20-60) were isolated from six subjects with a range of plasma-triacylglycerol (TAG) concentrations following an overnight fast and for up to 6 h after the consumption of a mixed meal (41% fat). The percent of TRL-TAG hydrolysed by LPL in subfractions isolated following overnight fast was VLDL1 > VLDL2 (46.8 +/- 10.2 vs. 25.9 +/- 7.4%, P = 0.006) and 3 h after the meal it was chylomicrons > VLDL1 > VLDL2 (81.0 +/- 12.6 vs. 52.8 +/- 10.2 vs. 27.7 +/- 6.2%, chylomicrons vs. VLDL1 and VLDL1 vs. VLDL2, both P < or = 0.005). The percent of VLDL1-TAG hydrolysed increased both within and between subjects as VLDL1-TAG concentrations increased. This relationship could be explained by the positive correlation observed between VLDL1-TAG and VLDL1-TAG:apolipoprotein B. In conclusion, increasing the size and TAG content of a lipoprotein particle increases its susceptibility to hydrolysis by LPL.

Adult↗
PubMed1995

Decreased sensitivity to the inhibitory effect of insulin on the secretion of very-low-density lipoprotein in cultured hepatocytes from fructose-fed rats.

Hepatocytes were prepared from rats fed a chow diet (control-fed) and from rats fed a similar diet in which the drinking water contained 10% (wt/vol) fructose (fructose-fed). Both types of hepatocyte preparations were cultured for < or = 48 hours in supplemented Waymouth's medium containing increasing concentrations of bovine insulin (0 to 780 nmol/L). During the first 24 hours of culture, hepatocytes from fructose-fed rats secreted more very-low-density lipoprotein (VLDL) triacylglycerol (TAG) than hepatocytes from control-fed rats. This difference persisted at all concentrations of insulin. There was no difference in the rate of secretion of apolipoprotein B (apo B). In both control-fed and fructose-fed animals, the inhibitory effect of insulin on the secretion of VLDL was greater on the second versus the first day of culture. Under these conditions, hepatocytes from fructose-fed groups were less sensitive to insulin inhibition as compared with those from the control-fed group. This was evidenced by the following: (1) the decreased inhibitory effect of insulin on the secretion of both total and newly synthesized VLDL TAG, (2) the attenuated inhibitory effect of insulin on the secretion of VLDL apo B, (3) the decreased potency of insulin in suppressing the secretion of VLDL TAG in TAG-depleted hepatocytes from fructose-fed as compared with control-fed animals, and (4) the larger proportion of newly synthesized TAG secreted as VLDL in hepatocytes from fructose-fed rats as compared with controls. This difference was exacerbated at higher concentrations of insulin.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1995

Intracellular triacylglycerol lipase: its role in the assembly of hepatic very-low-density lipoprotein (VLDL).

Extracellular fatty acids entering the hepatocyte are either esterified to cytosolic TAG or oxidized to ketone bodies. Very little is esterified and secreted directly in association with VLDL. Thus, even when extracellular fatty acids are available, the major, direct source of VLDL TAG is the cytosolic pool. The recruitment of cytosolic TAG for VLDL assembly involves lipolysis followed by re-esterification. At least 70% of the secreted TAG is derived via this route. Fatty acids released at this lipolytic step are utilized exclusively for VLDL TAG synthesis and are not available for ketogenesis. Substantially more cytosolic TAG undergoes lipolysis than is required to meet the needs of VLDL assembly. The remaining fatty acids are re-esterified and re-cycled to the cell cytosol. From a physiological viewpoint, the presence of this indirect route for VLDL TAG recruitment would provide a means of regulation of VLDL secretion which is independent of the plasma fatty acid concentration. In this respect, several pathophysiological conditions are known in which there is a negative association between plasma fatty acid concentration and the rate of VLDL secretion. These are: (a) insulin-dependent diabetes, (b) starvation, (c) fat-feeding. Lipolysis of cytosolic TAG and transfer of fatty acids into the ER lumen may provide a regulatory focus for the control of hepatic VLDL output.

Adipose Tissue↗
PubMed1995

Impaired postprandial clearance of triacylglycerol-rich lipoproteins in adipose tissue in obese subjects.

Adipose tissue is an important site of clearance of circulating triacylglycerol (TAG), especially in the postprandial period. Postprandial lipemia is usually increased in obesity. We studied the extraction of TAG from plasma and TAG-rich lipoproteins (TRLs) in subcutaneous adipose tissue in 11 control and 8 obese subjects before and after a mixed meal. Clearance of plasma TAG and very low-density lipoprotein (VLDL)-TAG was decreased in the obese subjects after an overnight fast. After the meal, chylomicron-TAG extraction increased in the control group whereas VLDL-TAG clearance decreased; these changes were not seen in the obese group, in whom the VLDL particles appeared to be better able to compete with the chylomicrons for clearance by lipoprotein lipase. In the control subjects, removal of TAG from the TRL in the postprandial period was accompanied by a shift toward addition of cholesterol to the high-density lipoprotein (HDL) fraction; this was not observed in the obese subjects. We conclude that disturbed TRL-TAG clearance in adipose tissue is related both to the elevated plasma TAG concentrations and the depressed HDL-cholesterol concentrations typical of obesity.

Adipose Tissue↗
PubMed1995

VLDL output by hepatocytes from obese Zucker rats is resistant to the inhibitory effect of insulin.

The effects of insulin (0-780 nM) on the secretion of very-low-density lipoprotein (VLDL) apolipoprotein B (apoB) and triacylglycerol (TAG) in hepatocytes from obese Zucker rats and from their lean littermates were studied over a total period of 48 h in chemically defined culture medium. Cells from the obese Zucker rats initially secreted more TAG than those from the lean animals. Cells from the former were resistant to the inhibitory effect of insulin on the secretion of TAG. These changes were accompanied by an increased rate of TAG synthesis. Although the hepatocytes from the obese animals initially secreted less apoB than those from the lean, apoB output from the former could not be suppressed by insulin. Prolonging the length of the culture period resulted in the acquisition of sensitivity to the inhibitory effect of insulin in the hepatocytes from the obese rats. This occurred more rapidly for the secretion of TAG than of apoB. Under these conditions, the initial difference in the rate of TAG synthesis in the hepatocytes from the obese and from the lean animals was also abolished.

Animals↗
PubMed1994

Fasting plasma triacylglycerol concentrations predict adverse changes in lipoprotein metabolism after a normal meal.

The changes in lipoprotein metabolism which follow the ingestion of a large fat load have been well described. The hypothesis was tested that similar changes in lipoprotein metabolism would occur after a relatively normal meal. Plasma and lipoprotein triacylglycerol, cholesterol and apolipoprotein concentrations were determined in twenty subjects (ten female) given a mixed meal containing approximately one-third of the daily intake of major nutrients in the typical Western diet. Fasting plasma triacylglycerol concentrations (range 0.38-2.70 mm/l) and the postprandial rise in plasma triacylglycerol varied considerably between subjects and were significantly associated (P < 0.01). The rise in plasma triacylglycerol corresponded to marked increases in the triacylglycerol concentration of the triacylglycerol-rich lipoproteins (TRL; chylomicrons and very-low-density lipoproteins). TRL cholesterol also increased after the meal. An increase in high-density-lipoprotein (HDL)-triacylglycerol following the meal was accompanied by a decrease in HDL-cholesterol concentration, presumably due to the action of the cholesteryl-ester transfer protein. The increases in HDL-triacylglycerol and in TRL-cholesterol were correlated with the postprandial rise in triacylglycerol in the TRL (P < 0.01). We conclude that potentially adverse changes occur in both triacylglycerol-rich and high-density lipoproteins following a typical mixed meal, as they do after large fat loads. The changes are exaggerated in those subjects with greater fasting plasma triacylglycerol concentrations.

Adult↗
PubMed1994

A comparison of in-vitro models to study hepatic lipid and lipoprotein metabolism.

Removal of the liver from its physiological environment provides the opportunity to examine its response when challenged directly with substances which, in vivo, are associated with changes in lipid and lipoprotein metabolism. Many in-vitro models are currently available, ranging from intact, perfused livers to transformed cell lines. Selection of the most suitable model depends upon the precise experimental objective(s) and, in most cases, involves trading off one advantage in favour of another. This is particularly true in selecting a human model, and in this case, complementary studies involving more than one model system may provide the best results.

Animals↗
PubMed1994

Regulation of VLDL secretion in primary culture of rat hepatocytes: involvement of cAMP and cAMP-dependent protein kinases.

When hepatocytes were cultured for 24 h in the presence of forskolin (10(-4) mol l-1) or isobutylmethylxanthine (IBMX, 10(-3) mol l-1), the intracellular cAMP concentration peaked (320-380 pmol mg-1 protein) after 10-20 min of culture. This increase was accompanied by a decrease in the secretion of triacylglycerol, cholesterol and apoprotein B associated with VLDL. After 4 h cAMP levels had returned almost to basal values but the inhibition of VLDL secretion persisted. There was a small intracellular accumulation of triacylglycerol but not of apoprotein B. Addition of forskolin and IBMX together led to a further increase in intracellular cAMP and a further suppression of VLDL output. Similar effects on the secretion of VLDL were also observed after addition of Bt2cAMP. Exposure of cell cultures to glucagon (10(-7) mol l-1) for only 10 min raised cellular cAMP levels to > 200 pmol mg-1 protein, and suppressed VLDL secretion during the next 24 h to < 40% of control. All of the substances tested inhibited de novo synthesis of fatty acids but had little or no effect on cholesterol synthesis and did not inhibit oleate esterification to triacylglycerol. The cAMP-dependent protein kinase antagonist Rp-cAMPS prevented suppression of VLDL triacylglycerol secretion induced by glucagon (10(-7) mol l-1) and abolished glucagon-induced ketogenesis. Rp-cAMPS also inhibited Bt2cAMP (7.5 x 10(-6) mol l-1)-induced suppression of VLDL secretion and enhancement of ketogenesis. It is concluded that rat hepatic VLDL metabolism can be regulated by cAMP and cAMP-dependent protein kinases, and that the initial transient rise in cellular cAMP levels induced by glucagon is sufficient to maintain a long-term inhibitory effect on assembly and secretion of VLDL.

Animals↗
PubMed1994

Lipid balance in HepG2 cells: active synthesis and impaired mobilization.

The human hepatoma cell line HepG2 in culture medium synthesized fatty acids de novo (144 +/- 9 nmol fatty acid/mg protein per 24 h) at a rate similar to that observed in freshly prepared rat hepatocytes (192 +/- 8 nmol/mg per 24 h) and in primary cultures of rat hepatocytes (165.4 +/- 29.3 nmol/mg per 24 h). In HepG2 cells, fatty acid synthesis was inhibited by extracellular oleate (0.75 mM), and, to a lesser extent, by glucagon (10(-7) M). Insulin (7.8 x 10(-8) M) had a mild stimulatory effect. Fatty acid synthesis was not influenced by lipogenic precursors (lactate plus pyruvate), substances which, in rat hepatocytes, had pronounced stimulatory effects. Fatty acid synthesis rates were also unchanged in the presence of prostaglandin E2 (PGE2). In general, compared to rat hepatocytes, fatty acid synthesis in HepG2 cells was less sensitive to manipulation of the culture medium in vitro. HepG2 cells had a high capacity for triacylglycerol synthesis from extracellular oleate (469 +/- 43 nmol triacylglycerol/mg protein per 24 h) but phospholipid synthesis was relatively low (15.8 +/- 0.4% of total glycerolipids). Very little of the above newly synthesized triacylglycerol was secreted as lipoprotein (4.62 +/- 0.88 nmol triacylglycerol/mg protein per 24 h) resulting in a large intracellular accumulation of triacylglycerol. This was exacerbated by the absence of any detectable ketogenesis. The secretion of triacylglycerol produced from de novo synthesized fatty acids was also very low in HepG2 compared to that observed in primary cultures of rat hapatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Cells, Cultured↗
PubMed1993

Restoration in vitro of normal rates of very-low-density lipoprotein triacylglycerol and apoprotein B secretion in hepatocyte cultures from diabetic rats.

Hepatocytes derived from diabetic rats were cultured in serum-free Waymouth's medium containing various supplements, after an initial 4 h period during which the cells were allowed to attach to the culture dish in the presence of foetal-bovine serum (10%). After removal of serum, these cells secreted much less very-low-density lipoprotein (VLDL) apoprotein B (apoB) and triacylglycerol than those derived from normal rats when cultured for 24 h in the basal medium. Inclusion of oleate (0.75 mM) in the medium initially increased the output of apoB and triacylglycerol, but the rates remained lower than those observed in normal hepatocytes and declined to zero after 72 h. This time-dependent decline in VLDL output was prevented by addition of dexamethasone to the oleate-containing medium. Levels of apoB and triacylglycerol output characteristic of normal hepatocytes could only be completely restored, however, by further addition of a mixture of lipogenic substrates (lactate plus pyruvate) to the medium. Restoration of normal levels of VLDL secretion in diabetic hepatocytes in vitro by this means was accompanied by a normal inhibitory response of apoB and triacylglycerol output to short-term (24 h) treatment with insulin or glucagon. Exposure of the cells to insulin for 72 h enhanced the secretion of VLDL, whereas treatment with glucagon for the same period potentiated the original inhibitory effect. The defective secretion of VLDL apoB observed when diabetic hepatocytes were cultured in the basal medium for 24 h could also be rectified by inclusion of a mixture of oleate (0.75 mM), lactate (10 mM), pyruvate (1 mM), dexamethasone (1 microM) and insulin (78 nM) in the medium during the 4 h attachment period in the presence of serum. Under these conditions, the increase in the secretory response of triacylglycerol was not so pronounced.

Animals↗
PubMed1993

Post-prandial VLDL subfraction metabolism in normal and obese subjects.

The metabolic behaviour of very low density lipoprotein subfractions Sf 60-400 (VLDL1) and Sf 20-60 (VLDL2) was studied in five normal weight and five obese subjects following an overnight fast and after the consumption of a mixed meal. The VLDL1 showed increases in triacylglycerol (TG) and cholesterol concentrations after the meal, while the VLDL2 response was much smaller. The obese individuals had higher concentrations of TG, apolipoprotein B and cholesterol in both VLDL subfractions, but when the TG and cholesterol were expressed in terms of their content per VLDL particle, there was no difference between control and obese subjects. The abnormality in the obese group was identified as an elevated number of VLDL particles, of normal composition, which may predispose these individuals to coronary heart disease.

Adult↗
PubMed1992

Extracellular fatty acids are not utilized directly for the synthesis of very-low-density lipoprotein in primary cultures of rat hepatocytes.

In hepatocytes cultured in the presence of oleate (initial concn. 0.75 mM), the secretion of very-low-density lipoprotein (VLDL) triacylglycerol and, to a lesser extent, apoprotein B (apoB) increased with time, whereas there was a large decline in the extracellular concentration of fatty acid. There was thus no synchronous relationship between the extracellular fatty acid concentration and the secretion of VLDL. Rather, the appearance of VLDL in the medium was dependent on the intracellular triacylglycerol concentration. At a given concentration of extracellular fatty acid, cells depleted of triacylglycerol secreted less VLDL triacylglycerol and apoB than did control cells. A similar pattern was observed for triacylglycerol newly synthesized from extracellular [3H]oleate. By contrast, the synthesis and output of ketone bodies were directly dependent on the fatty acid concentration of the medium. These results suggest that, at least for oleic acid, extracellular fatty acids are not utilized directly for VLDL assembly, but first enter a temporary intracellular storage pool of triacylglycerol, which is the immediate precursor of secreted triacylglycerol. The size of this pool then determines the rate of secretion of VLDL triacylglycerol apoB. Ketogenesis, on the other hand, relies mainly on the direct utilization of extracellular fatty acids.

Animals↗
PubMed1992

The lipolysis/esterification cycle of hepatic triacylglycerol. Its role in the secretion of very-low-density lipoprotein and its response to hormones and sulphonylureas.

In hepatocyte cultures maintained in the absence of extracellular fatty acids, at least 70% of the secreted very-low-density lipoprotein (VLDL) triacylglycerol was derived via lipolysis of intracellular triacylglycerol. This proportion was unchanged when the cells were exposed for 24 h to insulin or glucagon, hormones which decreased the overall secretion of intracellular triacylglycerol, or to chloroquine or tolbutamide, agents which inhibit lysosomal lipolysis. The rate of intracellular lipolysis was 2-3-fold greater than that required to maintain the observed rate of triacylglycerol secretion. Most of the fatty acids released were returned to the intracellular pool. Neither insulin nor glucagon had any significant effect on the overall lipolysis and re-esterification of intracellular triacylglycerol. In these cases a greater proportion of the released fatty acids re-entered the cellular pool, rather than being recruited for VLDL assembly. Tolbutamide inhibited intracellular lipolysis, but suppressed VLDL secretion to a greater extent. 3,5-Dimethylpyrazole did not affect lipolysis or VLDL secretion. The increased secretion of VLDL triacylglycerol observed after exposure of cells to insulin for 3 days was not accompanied by an increased rate of intracellular lipolysis. However, a larger proportion of the triacylglycerol secreted under these conditions may not have undergone prior lipolysis.

Carbon Radioisotopes↗
PubMed1992

The role of pancreatic hormones in the regulation of lipid storage, oxidation and secretion in primary cultures of rat hepatocytes. Short- and long-term effects.

Exposure of cultured rat hepatocytes to a high concentration of insulin (78 nM) for 24 h in the presence of extracellular oleate (0.75 mM) resulted in a decrease in the secretion of apoprotein B (apoB) and triacylglycerol associated with very-low-density lipoprotein (VLDL). However, continuous exposure of the cells to insulin for longer periods (72 h) stimulated the secretion of apoB and triacylglycerol. Treatment of hepatocytes with glucagon (0.1 microM) for 24 h also suppressed the secretion of VLDL apoB, cholesterol and triacylglycerol. The cells remained responsive to the inhibitory effect of glucagon for at least 3 days. In contrast with insulin, however, exposure of the cells to glucagon for a continuous period of 72 h did not lead to a reversal of the initial inhibition. Glucagon also stimulated ketogenesis, and in this regard the cells were responsive for at least 3 days in culture. These changes were accompanied by a transient increase in intracellular cyclic AMP (cAMP) concentration, which reached a peak 10 min after addition of glucagon. Between 12 h and 24 h after glucagon addition, cAMP levels had returned almost to normal, but the secretion of VLDL remained suppressed during this period.

Animals↗
PubMed1992

Adipose tissue metabolism in obesity: lipase action in vivo before and after a mixed meal.

Physiological actions of insulin include suppression of fat mobilization from adipose tissue and activation of adipose tissue lipoprotein lipase. Here, we report measurements of adipose tissue hormone-sensitive lipase (HSL) and lipoprotein lipase (LPL) action in vivo in 10 normal and eight obese subjects, with the latter group having varying degrees of glucose intolerance. HSL and LPL actions (per gram of adipose tissue) were similar in the two groups, after an overnight fast. In the normal subjects, HSL action was suppressed after a meal (by 75% +/- 6% between 60 to 300 minutes, P less than .01), and the action of LPL was increased (clearance of circulating triacylglycerol [TAG] increased by 140% +/- 57% at 300 minutes, P less than .05). Despite hyperinsulinemia, these responses were blunted in the obese subjects (P less than .05 for each change being less than in normal group). The adipose tissue of the obese subjects showed continued nonesterified fatty acid (NEFA) release at a time when NEFA mobilization was completely suppressed in the normal group. Both impaired suppression of HSL and low fractional retention of fatty acids for reesterification within the adipose tissue contributed to this abnormal NEFA release. Impaired activation of LPL was associated with a greater absolute increase in plasma TAG concentration postprandially in the obese. In obese subjects, adipose tissue HSL and LPL fail to respond to immunoreactive insulin postprandially, which may be an important maladaptation in terms of lipoprotein metabolism and risk of coronary heart disease.

Adipose Tissue↗