Esko A. Nikkilä

Evidence for the role of hepatic endothelial lipase in the metabolism of plasma high density lipoprotein2 in man.

Previous animal studies have shown that the heparin-releasable hepatic lipase (HL) is located on the luminal surface of the liver endothelial cells and may have a function in the removal of high density lipoprotein lipids from plasma. We therefore examined the relationship between plasma HDL levels and the HL activity of postheparin plasma in a group of young, very fit men who were living under strictly controlled comparable conditions (military academy studients). HDL2 cholesterol, HDL2 phospholipid and HDL2 protein concentrations each showed a highly significant negative correlation with postheparin HL activity. A similar but slightly lower inverse relationship was also present between total HDL lipids and HL activity, whereas no correlation could be observed between any of the HDL3 lipids and HL activity. The cholesterol/protein ratio of HDL2 correlated negatively with the HL activity. These results support the hypothesis that the hepatic endothelial lipase has a physiological role in the degradation and removal of circulating HDL2.

Lipoprotein lipase activity in adipose tissue and skeletal muscle of runners: Relation to serum lipoproteins

Physically well-trained people generally have lower VLDL-triglyceride and higher HDL-cholesterol levels than sedentary subjects. To examine the underlying mechanisms of this lipoprotein pattern, we measured the lipoprotein lipase (LPL) activity in needle biopsy specimens of adipose tissue and skeletal muscle of competitive runners and of body weight-matched, physically less-active controls. The active sportsmen were either sprinters, whose training program consisted mainly of athletics of short duration or long distance runners undergoing a strenuous endurance exercise program. In sprinters (all males) the serum lipid and lipoprotein concentrations did not differ significantly from those of controls and the mean LPL activities in muscle and adipose tissue were also similar in these two groups. The long distance runners (both sexes), on the other hand, had higher means levels of HDL-cholesterol than the respective controls. The LPL-activity of both adipose tissue (p less than 0.05) and skeletal muscle (p less than 0.01) was significantly higher in male long distance runners than in control males. Female runners had higher muscle LPL activity than controls (p less than 0.01) but in adipose tissue the difference in LPL activity was not significant. Rough estimates calculated for LPL activity present in whole body adipose tissue and skeletal muscle indicated that total LPL activity was 2.3 times higher in male long distance runners and 1.5 times higher in female long distance runners than in the respective controls. In combined groups of male runners and controls, there was a highly significant positive correlation between the serum HDL-cholesterol level and the LPL activity of adipose tissue expressed per tissue weight (r = +0.72, p less than 0.001) or per whole body fat (r = +0.62, p less than 0.001). The group means of HDL-cholesterol and adipose tissue LPL activity in the five cohorts studied (male sprinters, distance runners and controls and female distance runners and controls) were also positively correlated (r = +0.94). It is concluded that endurance training is associated with an adaptive increase of LPL activity not only in skeletal muscle but also in adipose tissue. These changes are not observed in sprinters who are trained by exercises of shorter duration. The high HDL-cholesterol levels of physically well-trained people are probably accounted for, at least partly, by the increased LPL activity and the concomitant rapid turnover or triglyceride-rich lipoproteins.

An immunochemical method for the selective measurement of two triglyceride lipases in human postheparin plasma

A new method for the selective measurement of postheparin plasma lipoprotein lipase and hepatic lipase is described and validated. The activity of lipoprotein lipase is determined at 0.1 M NaCl after removal of hepatic lipase by specific antiserum, and the hepatic lipase is assayed in a medium containing 1.0 M NaCl but no additional serum. The optimal conditions for the determination of the two postheparin plasma triglyceride hydrolases were shown to be similar to those described for the purified enzymes. The new assay methods are simple, accurate and highly specific for the two lipase activities. VLDL and LDL do not interfere with the measurement, making the methods suitable for studies of patients with various hyperlipidemias. More than 90% of the total triglyceride hydrolase activity in postheparin plasma is precipitated with antisera raised against purified human postheparin plasma hepatic lipase and bovine milk lipoprotein lipase. The time and dose dependence of the two postheparin plasma lipase responses differ. For optimal activity of both enzymes, plasma taken 15 minutes after intravenous administration of 100 I.U./kg of heparin, should be used. The activity of postheparin plasma lipoprotein lipase and hepatic lipase in 12 young, healthy males is reported.

Changes in serum lipoprotein pattern induced by acute infections

To study the effects of acute infections on serum lipids and lipoproteins we measured the concentration and composition of different lipoproteins, apoproteins A-I, A-II, and B, and the activities of plasma postheparin lipolytic enzymes, lipoprotein lipase (LPL) and hepatic lipase (HL) during acute and convalescence phase and after complete recovery in 72 infectious patients (33 with viral infection and 39 with bacterial infection). The mass concentrations of both low density lipoprotein (LDL) (P less than .001) and high density lipoprotein (HDL)2 (P less than .002) were reduced during acute infections due to the lowering of their cholesterol, phospholipid, and protein contents. The reduction of LDL cholesterol was maximal at the acute stage of infection (change -15%, P less than .001) while the reduction of HDL2 cholesterol was maximal during the convalescence (change -35%, P less than .001). During acute infections LDL became triglyceride-enriched (11.8 v 8.6%, P less than .0001) but cholesterol-poor (36.6 v 39.3%, P less than .0001). The ratio of HDL cholesterol/LDL cholesterol was significantly reduced during the convalescence (0.42 +/- 0.15 v 0.53 +/- 0.19, P less than .0001). The concentrations of apo A-I and apo A-II were decreased during acute infections (changes -22%, P less than .001, and -16%, P less than .001, respectively). The very low density lipoprotein (VLDL) was 18% higher during the convalescence period than after the recovery due to the elevations of VLDL triglycerides, cholesterol, and phospholipids. The activity of LPL was reduced both in the acute and convalescence phase, whereas that of HL was reduced only in the acute phase of infections.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic endothelial lipase antiserum influences rat plasma low and high density lipoproteins in vivo

Postheparin plasma contains two separate lipolytic enzymes, lipoprotein lipase and hepatic lipase, which are released from vascular endothelial cells. The former is located in extrahepatic capillary beds and has a well-defined role in the catabolism of plasma triglycerides. The hepatic lipase has been recently shown to be located on the surface of hepatic endothelial cells [l] but the physiological function of this enzyme is still far from clear. Hepatic endothelial lipase can hydrolyze chylomicron and VLDL triglycerides in vitro [2] but its activity in postheparin plasma has no correlation to plasma triglyceride levels [3]. It has been suggested that hepatic lipase could be involved in the uptake of chylomicron remnants [4], intermediate density lipoprotein [5] or LDL [6] by the liver but there is little experimental evidence for any of these possibilities. We have recently purified the heparin-releasable lipase from rat liver perfusates [7] and used the enzyme preparation for production of antiserum. The availability of this anti-hepatic lipase serum offered a good opportunity to study the function of the enzyme by searching whether plasma lipoproteins are influenced by specific inhibition of the hepatic endothehal lipase in vivo.

Plasma triglyceride transport kinetics in diabetes mellitus

Plasma endogenous triglyceride transport has been measured in 74 adult patients with different types of diabetes and compared to similar data obtained in 35 healthy normoglyceridemic control subjects. The diabetic material was divided into subgroups by a number of criteria including ketoacidosis, insulin-dependence, relative body weight, control of blood glucose, and severity of hypertriglyceridemia. In ketoacidosis, a marked elevation of plasma triglyceride production rate was the rule, and this probably accounted for the moderate hypertriglyceridemia even though the fractional transport was simultaneously lowered. In uncontrolled but nonketotic juvenile-type diabetes the mean plasma triglyceride turnover rate and concentration were both significantly increased while the fractional turnover was in most cases within normal range. When the disease was brought into good control by insulin the production rate decreased but not completely in line with blood glucose. Patients with adult-onset-type diabetes showed also an increase of triglyceride turnover rate, and the magnitude of hypertriglyceridemia was consistent with this increment so that most patients were “on line” of the enzyme kinetic Michaelis curves extrapolated from the normal material. On the other hand, of nonketotic diabetics with severe hyperglyceridemia (more than 4.0 mM) some had an increased turnover rate, while others were characterized by a normal production rate. Therefore, it is probable that one minor group of diabetics has a true removal defect, while in other forms of diabetes, the hypertriglyceridemia, if present, is primarily due to enhanced hepatic secretion of triglycerides into plasma. The exogenous fat tolerance was tested in 15 uncontrolled diabetics and 10 control subjects with intravenous intralipid emulsion. The fractional disappearance rate was either normal or decreased in diabetes but, when compared at similar basal plasma triglyceride concentration, there was no difference in the elimination of exogenous particulate fat between diabetics and nondiabetics. The different mechanisms of hypertriglyceridemia associated with diabetes are discussed on the basis of the results obtained in this study. In this connection a new concept on the regulation of removal kinetics by plasma triglyceride production rate is suggested.

Serum lipids and lipoproteins in insulin-treated diabetes. Demonstration of increased high density lipoprotein concentrations.

In view of the high incidence of coronary heart disease in insulin-dependent diabetes, we determined cholesterol and triglyceride concentrations in serum and in three major lipoprotein fractions of 170 nonuremic middle-aged diabetic patients who had been treated with insulin for a minimum of 10 years. In addition, postheparin plasma lipoprotein lipase activity was measured in a subsample of the diabetic subjects. The diabetics had serum cholesterol and triglyceride concentrations similar to those of nondiabetic control subjects of the same age and sex. However, the concentration of high density lipoprotein (HDL)-cholesterol was significantly higher in both male and female diabetic patients than in respective controls (P < 0.001). Male patients with poor diabetic control or with overweight had elevated triglyceride and very low density lipoprotein (VLDL)-triglyceride concentrations, whereas the patients with good diabetic control had significantly lower triglyceride and VLDL-triglyceride concentrations than did nondiabetic subjects. Overweight diabetic patients also had elevated serum cholesterol and LDL-cholesterol concentrations. The HDL-cholesterol of diabetic subjects, on the other hand, was not dependent on the degree of diabetic control or on the relative body weight. Diabetic patients had higher lipoprotein lipase activity in postheparin plasma than did normal subjects of corresponding age and sex. The HDL-cholesterol concentration showed a significant positive correlation with postheparin plasma lipoprotein lipase activity. We conclude that the average serum lipid and lipoprotein pattern of insulin-treated chronic diabetic patients is not more. atherogenic than that of nondiabetic subjects of similar age and sex. On the contrary, the increase of HDL-cholesterol observed in the diabetic subjects should make them less liable to develop coronary heart disease. Thus, the excess cardiovascular disease that is associated with insulin-dependent diabetes must be accounted for by other factors.

Plasma triglyceride metabolism in thyroid disease

Plasma endogenous triglyceride transport kinetics were determined in 16 hyperthyroid and in 12 hypothyroid patients and the results compared with those of euthyroid control subjects. In addition, the removal of exogenous particulate fat (Intralipid; Vitrum, Sweden) from the circulation and the postheparin plasma lipolytic activity (PHLA) were studied in these patients for further characterization of the alterations of plasma triglyceride metabolism in thyroid disease. In thyrotoxicosis the average plasma triglyceride level was slightly but significantly increased above that of control subjects. This change was associated with augmented production of triglycerides whereas the mean fractional removal rate was not different from normal. There was a significant linear correlation between the concentration and turnover rate of plasma triglycerides in both hyperthyroid and euthyroid subjects but the concentration/turnover rate ratio was less in the former group suggesting that the efficiency of removal of triglycerides from the circulation was improved in thyroid hyperfunction. The elimination of intravenously administered particulate fat occurred more rapidly in untreated hyperthyroid patients than in euthyroid control subjects. The mean PHLA was also above normal in thyrotoxicosis. Upon adequate treatment of the hyperthyroid state the fasting plasma triglyceride concentration was further increased. Hypothyroid patients showed another pattern of alteration of triglyceride kinetics. The synthesis of plasma triglycerides was normal but the fractional removal of both endogenous and exogenous triglycerides was markedly reduced and this change seems to account for the hypertriglyceridemia associated with thyroid hypofunction. The plasma PHLA was also clearly decreased in the hypothyroid state. Plasma FFA and glycerol levels were increased in hyperthyroidism and plasma FFA was slightly decreased in hypothyroid patients, but these variables were not significantly correlated with any parameter of triglyceride metabolism. Endogenous triglyceride turnover rate was significantly correlated with serum protein-bound iodine (PBI) and T3 uptake in thyrotoxicosis but not in hypothyroidism. Removal of exogenous fat was not related to postheparin plasma lipolytic activity but the fractional endogenous triglyceride transport showed a highly significant relationship to this lipase activity in a mixed group of hyper- and hypothyroid patients. The results suggest that thyroid hormones control both production and removal of plasma triglycerides. Different mechanisms for these interactions are considered.

Relation of plasma high-density lipoprotein cholesterol to lipoprotein-lipase activity in adipose tissue and skeletal muscle of man.

Lipoprotein-lipase activity (LPL) was measured in biopsies of adipose tissue and skeletal muscle of normal human subjects, and the results were related to concentrations of cholesterol and triglycerides in plasma lipoproteins. Adipose-tissue LPL activity was significantly higher in females than in males, whereas no sex difference was observed in skeletal-muscle LPL activity. A highly significant positive correlation was present between the plasma high density lipoprotein (HDL) cholesterol level and LPL activity in adipose tissue (r = +0.66, P less than 0.001) but not between HDL and skeletal-muscle LPL. The results suggest that the activity of LPL in adipose tissue and the rate of catabolism of triglyceride-rich lipoproteins might be one of the factors that determine the concentration of HDL in plasma and at least partly account for the known sex difference in plasma HDL level.

Regulation of hepatic lipase and serum lipoproteins by sex steroids

The influence of sex steroids on the serum lipoprotein pattern was recognized more than 30 years ago, and it still remains among the areas of major interest. This is because of the compatible sex difference in plasma lipoprotein pattern and in coronary heart disease risk. Recent discoveries of the role of hepatic lipase in lipoprotein metabolism have elucidated mechanisms behind sex steroid-induced changes in lipoproteins. These steroids regulate the activity of hepatic lipase, an enzyme bound to the endothelial cells of liver sinusoids. Hepatic lipase has a central role in the removal of phospholipids and triglycerides from subfractions of high-density lipoprotein (HDL2) particles, but it may also function in the lipolysis of triglyceride-rich particles. Some older and more recent developments in this area will be reviewed.