The effects of chronic uremia and dexamethasone on triglyceride kinetics in the rat

Dyslipidemia of chronic renal failure: the nature, mechanisms, and potential consequences

Chronic renal failure (CRF) results in profound lipid disorders, which stem largely from dysregulation of high-density lipoprotein (HDL) and triglyceride-rich lipoprotein metabolism. Specifically, maturation of HDL is impaired and its composition is altered in CRF. In addition, clearance of triglyceride-rich lipoproteins and their atherogenic remnants is impaired, their composition is altered, and their plasma concentrations are elevated in CRF. Impaired maturation of HDL in CRF is primarily due to downregulation of lecithin-cholesterol acyltransferase (LCAT) and, to a lesser extent, increased plasma cholesteryl ester transfer protein (CETP). Triglyceride enrichment of HDL in CRF is primarily due to hepatic lipase deficiency and elevated CETP activity. The CRF-induced hypertriglyceridemia, abnormal composition, and impaired clearance of triglyceride-rich lipoproteins and their remnants are primarily due to downregulation of lipoprotein lipase, hepatic lipase, and the very-low-density lipoprotein receptor, as well as, upregulation of hepatic acyl-CoA cholesterol acyltransferase (ACAT). In addition, impaired HDL metabolism contributes to the disturbances of triglyceride-rich lipoprotein metabolism. These abnormalities are compounded by downregulation of apolipoproteins apoA-I, apoA-II, and apoC-II in CRF. Together, these abnormalities may contribute to the risk of arteriosclerotic cardiovascular disease and may adversely affect progression of renal disease and energy metabolism in CRF.

The role of lipogenesis in the development of uremic hyperlipidemia

BACKGROUND

It is well documented that hypertriglyceridemia in renal failure mostly is a result of impaired plasma triglyceride (TG) removal. However, the role of TG production in its development is obscure. Therefore, our attention was given to the gene expression of lipogenic enzymes participating in TG biosynthesis.

METHODS

We measured some lipogenic enzyme activities, protein abundance (Western blot analysis), and messenger RNA level (Northern blot analysis) in liver and epididymal white adipose tissue (WAT) of rats with surgically induced renal failure (two-stage subtotal nephrectomy). Simultaneously, plasma TG and very low-density lipoprotein (VLDL) concentrations in uremic animals were determined.

RESULTS

An increase in plasma TG and VLDL concentrations in rats with renal failure was observed. It was associated with an increase in fatty acid synthase and adenosine triphosphate-citrate lyase (ACL) gene expression in liver and WAT. Moreover, increased activities of malic enzyme, glucose-6-phosphate dehydrogenase, and 6-phosphogluconate dehydrogenase were found.

CONCLUSION

Results of the present study provide some evidence that the accumulation of TG-rich lipoproteins in renal insufficiency could be related in part to increased lipogenic enzyme gene expression and, consequently, TG overproduction.

Lipogenesis in experimental chronic renal failure in rats

Hyperlipidemia is a common occurance in patients with chronic renal failure (CRF) and has been the subject of many clinical and experimental studies. Despite this, the role of lipogenesis in the development of hyperlipidemia is still obscure. The present study is based on a rat model of CRF involving a two-stage subtotal nephrectomy. In this study, we measured the activity of fatty acid synthase (FAS). This is the rate-limiting enzyme of lipogenesis and is present in liver and white adipose tissue (WAT). Using isotopic methods, we also determined the rate of lipogenesis in vivo in liver and WAT. In both liver and WAT, the results of the analyses were similar. In the uremic rats, there was a tendency for the FAS activity to rise. However, the difference was not statistically significant. Furthermore, there was no increase in the rate of lipogenesis in vivo in either tissue. In summary, the results of our study confirm the thesis that lipogenesis does not play a role in the development of hypertriglyceridemia seen in an experimental CRF in rats.

Effect of oyster mushroom (Pleurotus Ostreatus) and its ethanolic extract in diet on absorption and turnover of cholesterol in hypercholesterolemic rat

The effect of the diet containing 5% of powdered oyster mushroom (Pleurotus ostreatus) or an equivalent amount of mushroom ethanolic extract on cholesterol content in serum and liver, on its distribution in lipoproteins, absorption and turnover was studied in male Wistar rats (initial body weight about 70 g) fed a diet with 0.3% cholesterol. 12 weeks of feeding with whole oyster mushroom or mushroom extract reduced cholesterol level in serum by 52 and 33%, respectively. However, cholesterol content in liver was reduced only by whole oyster mushroom (by 20%). Diminished serum cholesterol level was mediated in 60% by reduction of cholesterol in very-low-density lipoproteins. Both whole oyster mushroom and mushroom extract increased the concentration of cholesterol in high-density lipoproteins. Consuming whole oyster mushroom decreased cholesterol absorption (estimated by dual-isotope plasma ratio method) by nearly 16% while no significant effect of mushroom extract could be demonstrated. Feeding the diet containing whole oyster mushroom or its extract reduced the half-times of decay curve of cholesterol-4-14C by 29 and 35%, respectively and reciprocally increased the fractional catabolic rate of plasma cholesterol.

Mechanism of hypocholesterolemic effect of oyster mushroom (Pleurotus ostreatus) in rats: reduction of cholesterol absorption and increase of plasma cholesterol removal

The content of cholesterol in the serum and liver of male Wistar rats fed, for the period of 8 weeks shortly after weaning, a diet containing 0.3% of cholesterol was reduced by 33 and 27% by the addition of 5% of dried oyster mushroom powder. Although the level of serum triacylglycerols was not affected by oyster mushroom, their content in liver of rats on mushroom diet was reduced by 41%. Very-low-density lipoproteins and low-density lipoproteins participated by 55 and 38%, respectively, in the total reduction of serum cholesterol. Cholesterol content in high-density lipoproteins was not significantly affected by oyster mushroom. Cholesterol absorption as determined by dual-isotope plasma ratio method was significantly reduced by 14% with oyster mushroom diet. Similarly, this diet increased by 42% the fractional catabolic rate of cholesterol determined by the analysis of decay curve of [4-14C]cholesterol.

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure

Catabolism of low-density lipoprotein is altered in experimental chronic renal failure. In patients with chronic renal failure, cardiovascular disease accounts for a significant proportion of all deaths. Several factors contribute to the "accelerated" atherosclerosis in this population, including hyperlipidemia, the pathogenesis of which is multifactorial. We investigated low-density lipoprotein (LDL) metabolism in a remnant model of chronic renal failure in the guinea pig. After one and two-thirds nephrectomy, creatinine clearance decreased to one-sixth normal. Plasma cholesterol and triglyceride (TG) levels increased with induction of renal failure. Analysis of lipoprotein composition disclosed significant TG enrichment of both uremic very-low-density lipoprotein (VLDL) and uremic LDL compared with control lipoproteins. Plasma clearance of homologous LDL was evaluated in turnover studies in control and uremic guinea pigs. To discriminate between differences in catabolism related to the uremic lipoprotein particle and to the uremic host milieu, a crossover protocol was used comparing the fractional catabolic rate (FCR) after simultaneous injection into control and uremic animals of 125I-control LDL and 131I-uremic LDL. The FCR of native LDL was slower in uremic animals than in controls. In addition, FCR of uremic LDL was significantly less than that of control LDL in both groups. Degradation studies in cultured fibroblasts indicated substantially reduced degradation of uremic LDL compared with control LDL. These results suggest dual abnormalities of LDL catabolism in renal failure that are not only related to alterations in clearance mechanisms in the uremic environment, but also suggest important functional differences in the LDL particle itself isolated from uremic animals.

The role of lipids in the pathogenesis of glomerulosclerosis in the rat following subtotal nephrectomy

Similarities between atherosclerosis and glomerulosclerosis suggest that hyperlipidaemia may contribute to glomerular injury. Dietary supplementation with 4% cholesterol + 1% cholic acid was administered to rats 4 weeks after 1 1/3 nephrectomy and continued for 7 weeks. There was a significant increase in serum cholesterol (peak = 11.52 +/- 1.09 mmol l-1 vs. 4.73 +/- 0.31 on control diet, P less than 0.001) and triglyceride concentrations (peak = 2.31 +/- 0.27 mmol l-1 vs. 1.41 +/- 0.29, P less than 0.05) and a marked increase in beta-migrating lipoproteins. The severity of hypercholesterolaemia was significantly correlated with proteinuria (control diet: r = 0.600, cholesterol diet: r = 0.672, P less than 0.0001) as was hypertriglyceridaemia (control diet: r = 0.544, cholesterol diet: r = 0.678, P less than 0.0001). The percentage of glomeruli containing lipid deposits was increased from 21% to 60% (P less than 0.05). The kidney total cholesterol content was increased from 29.2 +/- 0.8 to 47.7 +/- 3.3 mumols g-1 dry weight (P less than 0.0001), with esterified cholesterol increasing from 7.5 +/- 0.4% to 14.5 +/- 2.1% of total (P less than 0.01). Serum cholesterol concentration was significantly correlated with both glomerular lipid deposition (rs = 0.7195, P less than 0.0001) and tissue total cholesterol content (rs = 0.6053, P less than 0.001). Lipid vacuolation was prominent in the paramesangium and within mesangial cells. Despite these changes hypertension, uraemia, proteinuria and glomerulosclerosis were not significantly increased on the cholesterol diet. Cholesterol deposition in the glomeruli occurs secondary to hyperlipidaemia in rats following subtotal nephrectomy but over 7 weeks no exacerbation of glomerulosclerosis is detectable.

Evaluation of hepatobiliary disorders in the horse

This article addresses clinical problems that present in equine liver disease. It also discusses the variety of laboratory tests available to the clinician that can differentiate the type and degree of liver dysfunction. This is followed by a more specific discussion regarding unique features of equine bilirubin and bile acid metabolism.

An increased apo A-IV serum concentration of patients with chronic renal failure on hemodialysis

Serum apolipoprotein (apo) A-IV concentration was determined in 20 patients with chronic renal failure on hemodialysis. The mean value of apo A-IV was 33.5 +/- 6.3 mg/dl, that was increased by approximately 3 times compared with that in healthy controls (11.1 +/- 2.7 mg/dl, n = 23). No significant correlation of apo A-IV was observed with the other apolipoproteins (apo A-I, A-II, B, C-II, C-III, and E) levels, serum lipids (TC, TG, and HDL-C) levels, and lipoprotein concentrations in both the patients and the controls. However, apo A-IV was significantly correlated with serum creatinine, BUN, and beta 2-microglobulin levels (p less than 0.05) in the patients. On the other hand, in patients with nephrotic syndrome and in that of a patient with Fanconi's syndrome, apo A-IV was detected in their urine. These results, in turn, suggest from their well-known pathogenesis that apo A-IV can readily transverse the glomerular filter due to its small molecular mass size and that it is also probably taken up and catabolized by renal tubular cells in the same fashion as other low molecular mass proteins. These observations suggest that a decreased glomerular filtration and/or reduced catabolism of apo A-IV by tubular cells may be one of the important causes of the increase in the serum apo A-IV level in patients with chronic renal failure besides a decreased catabolism of chylomicron remnants containing apo A-IV.

Factors of increase in serum triglyceride-rich lipoproteins in uremic rats

The possible mechanisms of the increase in serum triglycerides (TG) and TG-rich lipoproteins were studied in chronically uremic (U) rats by comparison with either ad-lib fed control (C) rats or diet-restricted (DR), sham-operated pair-fed control rats. A first series of animals was studied in the fed state and a second series after a 16-hr fast. In U animals the concentration of serum TG and TG-rich particles was lower than that of C rats in the fed state but significantly higher than that of C and DR rats after a 16-hr fast. Serum glucose and lactate concentrations in the fed or fasted state were unchanged by uremia. Serum insulin concentration was significantly decreased in U rats as compared to C and DR rats in both series. The fast did not increase the concentration of serum nonesterified fatty acids (NEFA) in U or DR animals to the same extent as in C rats, whereas the serum concentration of beta-hydroxybutyrate (BOB), which was higher than that of C rats in the fed state, was significantly lower after a 16-hr fast. In U animals, as compared to control rats of either series, a significant decrease of epididymal lipoprotein lipase (LPL) activity was observed during both nutritional states when expressing the enzymic activity per number of cells. In conclusion, our data provide evidence against hepatic over-production of TG-rich lipoproteins in rats with chronic renal failure and strongly point to an LPL-mediated defect of their peripheral catabolism, probably related to the insulin deficiency state.

Insulin resistance in uremia. Characterization of lipid metabolism in freshly isolated and primary cultures of hepatocytes from chronic uremic rats

We have studied the mechanism(s) of hyperlipidemia and liver insulin sensitivity in a rat model of severe chronic uremia (U). Basal lipid synthesis was decreased in freshly isolated hepatocytes from U when compared with sham-operated ad lib.-fed controls (alfC). Basal lipid synthesis in pair-fed controls (pfC) was in between U and alfC. Similarly, the activity of liver acetyl CoA carboxylase, fatty acid synthetase, citrate cleavage enzyme, malate dehydrogenase, and glucose-6-phosphate dehydrogenase was diminished in U. Muscle and adipose tissue lipoprotein lipase was also decreased. Insulin stimulated lipid synthesis in freshly isolated hepatocytes from alfC. Hepatocytes from U and pfC were resistant to this effect of insulin. To ascertain if the insulin resistance in U was due to starvation (chow intake 50% of alfC) or to uremia itself, the U and pfC were intragastrically fed an isocaloric diet via a Holter pump the last week of the experimental period. Hepatocytes from orally fed U and pfC were also cultured for 24 h in serum-free medium. While freshly isolated and cultured U hepatocytes remained insulin resistant, those from pfC normalized, in vivo and in vitro, when they were provided with enough nutrients.

CONCLUSIONS

(a) Hyperlipidemia in uremia is not due to increased synthesis, but to defect(s) in clearance. (b) Insulin does not stimulate lipid synthesis in uremia. This finding, along with our recent demonstration that insulin binding and internalization are not decreased in the uremic liver, suggests that a post-binding defect(s) in the liver plays an important role in the mechanism(s) of insulin resistance in uremia. (c) Cultured hepatocytes from uremic rats remain insulin resistant. This quality renders these cells useful in studying the postinsulin binding events responsible for the insulin-resistant state in the absence of complicating hormonal and substrate changes that occur in vivo.

[Alterations of fat metabolism in renal disease - pathogenetic mechanisms (author's transl)]

Hypertriglyceridaemia is often observed in patients (1) with chronic renal insufficiency, (2) on haemodialysis and (3) after successful renal transplantation. HDL cholesterol is reduced in all three groups of patients and plasma cholesterol is elevated after renal transplantation. In these three patient groups type IV hyperlipoproteinaemia is found most frequently and after renal transplantation there is a relative increase in the incidence of type II hyperlipoproteinaemia. The role of glucagon resistance and carnitine deficiency in the alteration of fat metabolism seen in patients with chronic renal failure and patients on haemodialysis is discussed. Other factors which may influence fat metabolism in uraemia include calcium and vitamin D status as well as beta adrenergic receptor blocking agents and diuretics. Steroid therapy may be one cause of the hypercholesterolaemia and hypertriglyceridaemia seen after renal transplantation. PHLP lipase activity is reduced in all three groups of patients. In nephrotic syndrome, if hypercholesterolaemia occurs, the HDL cholesterol fraction is increased and thus the cardiovascular risk may be lower than in the three patient group mentioned above.

Role of glucagon as a contributor to glucose intolerance in acute and chronic uremia

It has been suggested that increased sensitivity to glucagon may contribute to glucose intolerance in uremia. In order to evaluate this possibility systematically, we have assessed the effect of glucagon on hepatic glucose outflow, formation of cAMP, and activation of adenylate cyclase by livers obtained from acutely and chronically uremic rats and their respective sham operated controls. Glucagon infused at rates of 6 ng/min/kg rat resulted in minimal and equivalent increases in hepatic glucose outflow and cAMP accumulation when livers from acutely uremic and control rats were perfused for 30 min. However, at glucagon infusion rates of 18 ng/min/kg, glucose efflux from perfused livers of acutely uremic rats was significantly reduced (p less than 0.001) compared to perfused livers of control rats (4.64 +/- .9 vs 12.7 +/- 2.4 mumol/g liver) and cAMP accumulation was also significantly lower (p less than 0.01) (1352 +/- 222 vs 3100 +/- 348 pmol/g liver). Basal adenylate cyclase activity of hepatic membranes obtained from uremic and control rats was similar, and was stimulated by glucagon concentrations ranging from 10(-8) to 10(-6) at equivalent rates in both groups. In livers from chronically uremic rats, glucagon infused at rates of 6 ng/kg/min significantly increased hepatic glucose outflow (32.5 +/- 6.9 mumol/g liver). However this was not greater than that of control animals (37.6 +/- 9.2). Furthermore, cAMP accumulation was significantly lower (p less than .02) in chronically uremic rats than in controls, and activation of adenylate cyclase by glucagon was similar in both groups. These findings indicate that glucagon does not increase glucose efflux, cAMP accumulation or enhance activation of adenylate cyclase by isolated perfused livers from either acutely or chronically uremic rats. Thus, glucose intolerance in uremic rats does not appear to be due to increase hepatic glucose output resulting from increased sensitivity to glucagon.

Plasma lipid alterations in patients with chronic renal disease

Hyperlipidemia is common in patients with renal disease. This fact may be of great clinical relevance in view of the overwhelming evidence associating disturbed lipid metabolism and atherogenesis. Thus, hyperlipidemia may predispose to vascular disease in patients with chronic renal disorders and premature atherosclerosis could be an important risk in renal disease and a major factor limiting survival of patients on long-term maintenance hemodialysis. The aim of the present review is to present a brief but clinically relevant description of lipoprotein physiology and then to survey the now considerable literature concerned with lipoprotein and thus lipid abnormalities in patients with renal disease. A particular emphasis is placed on the role of the plasma lipoproteins in forming an integrated and controlled pathway for lipid metabolism, and how altered regulatory control within the pathway may be associated with pathogenic mechanisms. Finally, the evidence for accelerated development of vascular disease associated with these lipid abnormalities is briefly considered.

Impaired plasma triglyceride clearance as a feature of both uremic and posttransplant triglyceridemia

After accurate plasma volume calculation, endogenous plasma very-low-density-lipoprotein (VLDL)-triglyceride turnover rates were measured in 20 undialyzed patients with chronic renal failure (CRF) and in 16 renal transplant recipients with stable graft function. When kinetic criteria were based on a group of healthy subjects (Vmax = 36.7 mumoles/hr/kg), it was clear that, on the whole, CRF patients had a reduced capacity for VLDL-triglyceride removal (Vmax = 14.0 mumoles/hr/kg), as did graft recipients (Vmax = 19.5 mumoles/hr/kg). In transplant recipients with impaired graft function, however, extremes of both under removal and over production of VLDL-triglycerides were observed. In CRF, defective clearance was accompanied by a reduction in postheparin lipoprotein and hepatic lipase activities, although there was no statistical relationship. Enzyme activities were not reduced, however, after transplantation, and the metabolic factors responsible for defective clearance were not clearly identified.

Hyperlipoproteinemia in experimental chronic renal insufficiency in the rat

Lipid metabolism was studied in experimental uremia. Uremic (U) rats were compared with sham-operated, pair-fed (PF) controls and with ad-lib-fed (AL) controls. In U animals, fasting glucose concentrations were normal, immunoreactive serum insulin (IRI) levels were decreased, and immunoreactive glucagon levels were increased. A significant increase in the serum concentration of all lipid classes was observed: triglycerides were elevated 10-fold above the values in PF and AL controls; phospholipids, twofold; total cholesterol, threefold; and free cholesterol, sixfold. Cholesterol concentration was increased in beta- and pre-beta-lipoproteins and even more so in alpha- and pre-alpha-lipoproteins. There was an increase in the ratio of free cholesterol/total cholesterol. The fatty acid composition of serum lipoproteins was unchanged. Concomitantly, in liver tissue, there was no change in lipid content (triglyceride, cholesterol) and fatty acid composition. These findings argue against glucose- or insulin-mediated changes in hepatic de novo fatty acid synthesis, chain elongation, or poly-desaturation. In U animals, the HMG-CoA-reductase activity of liver microsomes was slightly, but not significantly, reduced as was tritiated water incorporation into cholesterol in isolated perfused liver preparations. In adipose tissue, there was a decrease in triglyceride content. The results provide evidence against insulin-mediated hepatic overproduction as a major cause of hyperlipoproteinemia in this model of experimental renal insufficiency and point to peripheral under-utilization of lipoproteins.