Lecithin-cholesterol acyltransferase: role in lipoprotein metabolism, reverse cholesterol transport and atherosclerosis

In the past several years significant advances have been made in our understanding of lecithin-cholesterol acyltransferase (LCAT) function. LCAT beneficially alters the plasma concentrations of apolipoprotein B-containing lipoproteins, as well as HDL. In addition, its proposed role in facilitating reverse cholesterol transport and modulating atherosclerosis has been demonstrated in vivo. Analysis of LCAT transgenic animals has established the importance of evaluating HDL function, as well as HDL plasma levels, to predict atherogenic risk.

LCAT modulates atherogenic plasma lipoproteins and the extent of atherosclerosis only in the presence of normal LDL receptors in transgenic rabbits

Elevated low density lipoprotein cholesterol (LDL-C) and reduced high density lipoprotein cholesterol (HDL-C) concentrations are independent risk factors for coronary heart disease. We have previously demonstrated that overexpression of an enzyme with a well established role in HDL metabolism, lecithin:cholesterol acyltransferase (LCAT), in New Zealand White rabbits not only raises HDL-C concentrations but reduces those of LDL-C as well, ultimately preventing diet-induced atherosclerosis. In the present study, the human LCAT gene (hLCAT) was introduced into LDL receptor (LDLr)-deficient (Watanabe heritable hyperlipidemic) rabbits to (1) investigate the role of the LDLr pathway in the hLCAT-mediated reductions of LDL-C and (2) determine the influence of hLCAT overexpression on atherosclerosis susceptibility in an animal model of familial hypercholesterolemia. Heterozygosity or homozygosity for the LDLr defect was determined by polymerase chain reaction, and 3 groups of hLCAT-transgenic (hLCAT+) rabbits that differed in LDLr status were established: (1) LDLr wild-type (LDLr+/+), (2) LDLr heterozygotes (LDLr+/-), and (3) LDLr homozygotes (LDLr-/-). Data for hLCAT+ rabbits were compared with those of nontransgenic (hLCAT-) rabbits of the same LDLr status. Plasma HDL-C concentrations were significantly elevated in the hLCAT+ animals of each LDLr status. However, LDL-C levels were significantly reduced only in hLCAT+/LDLr+/+ and hLCAT+/LDLr+/- rabbits but not in hLCAT+/LDLr-/- rabbits (405+/-14 versus 392+/-31 mg/dL). Metabolic studies revealed that the fractional catabolic rate (FCR, d(-1)) of LDL apolipoprotein (apo) B-100 was increased in hLCAT+/LDLr+/+ (26+/-4 versus 5+/-0) and hLCAT+/LDLr+/- (4+/-1 versus 1+/-0) rabbits, whereas the FCR of LDL apoB-100 in both groups of LDLr-/- rabbits was nearly identical (0.16+/-0.02 versus 0.15+/-0.02). Consistently, neither aortic lipid concentrations nor the extent of aortic atherosclerosis was significantly different between hLCAT+/LDLr-/- and hLCAT-/LDLr-/- rabbits. Significant correlations were observed between the percent of aortic atherosclerosis and both LDL-C (r=0.985) and LDL apoB-100 FCR (-0.745), as well as between LDL-C and LDL apoB-100 FCR (-0.866). These data are the first to establish that LCAT modulates LDL metabolism via the LDLr pathway, ultimately influencing atherosclerosis susceptibility. Moreover, LCAT's antiatherogenic effect requires only a single functional LDLr allele, identifying LCAT as an attractive gene therapy candidate for the majority of dyslipoproteinemic patients.

Transgenic rabbits as models for atherosclerosis research

Several characteristics of the rabbit make it an excellent model for the study of lipoprotein metabolism and atherosclerosis. New Zealand White (NZW) rabbits have low plasma total cholesterol concentrations, high cholesteryl ester transfer protein activity, low hepatic lipase (HL) activity, and lack an analogue of human apolipoprotein (apo) A-II, providing a unique system in which to assess the effects of human transgenes on plasma lipoproteins and atherosclerosis susceptibility. Additionally, rabbit models of human lipoprotein disorders, such as the Watanabe Heritable Hyperlipidemic (WHHL) and St. Thomas' Hospital strains, models of familial hypercholesterolemia and familial combined hyperlipidemia, respectively, allow for the assessment of candidate genes for potential use in the treatment of dyslipoproteinemic patients. To date, transgenes for human apo(a), apoA-I, apoB, apoE2, apoE3, HL, and lecithin:cholesterol acyltransferase (LCAT), as well as for rabbit apolipoprotein B mRNA-editing enzyme catalytic poly-peptide 1 (APOBEC-1), have been expressed in NZW rabbits, whereas only those for human apoA-I and LCAT have been introduced into the WHHL background. All of these transgenes have been shown to have significant effects on plasma lipoprotein concentrations. In both NZW and WHHL rabbits, human apoA-I expression was associated with a significant reduction in the extent of aortic atherosclerosis, which was similarly the case for LCAT in rabbits having at least one functional LDL receptor allele. Conversely, expression of apoE2 in NZW rabbits caused increased susceptibility to atherosclerosis. These studies provide new insights into the mechanisms responsible for the development of atherosclerosis, emphasizing the strength of the rabbit model in cardiovascular disease research.

Superovulation of rabbits with FSH alters in vivo development of vitrified morulae

Morulae were flushed from the oviducts and uteri of New Zealand White (NZW) rabbits superovulated with either 6 (3 d) or 8 (4 d) injections of FSH and from non-superovulated controls. The percentages of embryos recovered from 4 d (100%, n = 8) donors was significantly higher (P < 0.01) than that of 3 d (76%, n = 16) and control (87%, n = 22) donors. Overall, fertilization rates were significantly lower for the 3 d embryos (P < 0.01). Most (86 to 90%) morulae were morphologically suitable for vitrification in an ethylene glycol-based solution. Following storage in liquid nitrogen, morulae were rapidly thawed and transferred to the uteri of pseudopregnant recipients. The total number of kits born for the 3 d, 4 d, and control groups was 40, 61 and 48, respectively. The percentage of live kits from morulae transferred was significantly lower for the 3 d (20%, n = 201) than either the 4 d (36%, n = 169; P < 0.01) or the control (31%, n = 157; P < 0.05) group. The mean number of kits born/recipient for the 3 d (2.4 +/- 2.9), 4 d (4.7 +/- 3.5), and control (3.0 +/- 2.2) protocols did not differ (P > 0.05). The estimated overall efficiency of producing kits based on normal morulae collected for control and 4 d groups, however, was nearly two-fold that for females given 6 FSH treatments. We conclude that the 4 d FSH superovulation regimen enhances the efficiency of rabbit reproductive biotechnology after embryo cryopreservation. These findings have important implications for rabbit colony management using embryo cryopreservation.

Lipoproteins and atherogenesis

Like many complex disease processes, atherogenesis represents the interaction of an array of genetic and environmental factors. From nonhuman animal models to the investigation of epidemiologic factors in man, no single, overriding cause for the development of this indolent vascular disease has been identified. However, the cholesterol-enriched lipoprotein particles are closely tied to the development of the disease. The genetic and environmental influences on the concentrations of specific lipoprotein subspecies provide a context for identifying patients at risk as well as for developing effective therapeutic strategies to influence and prevent the sequelae of atherogenesis.

Evaluation of the aortic root by MRI: insights from patients with homozygous familial hypercholesterolemia

BACKGROUND

In homozygous familial hypercholesterolemia (HFH), the aortic root is prone to develop atherosclerotic plaque at an early age. However, the aortic wall and plaque have not yet been assessed in this condition by MRI. We evaluated the aortic root by use of MRI in 17 HFH patients and 12 normal control subjects in a prospective, blinded, controlled study.

METHODS AND RESULTS

Morphological assessment of the aortic root was done with spin-echo and gradient-echo MRI scanning. Comparisons were made with a number of measures of disease severity, including cholesterol-year score, calcium score on electron-beam CT (EBCT), and size of Achilles tendon xanthomas. Atherosclerotic plaque, visible on fat-suppressed images but never on water-suppressed images, was present in 9 HFH patients (53%). Supravalvular aortic stenosis was present in 7 patients with HFH (41%). Maximum supravalvular aortic wall thickness was significantly greater and OD and lumen cross-sectional area (CSA) were smaller in patients than in control subjects (P=0.006, 0.0005, and 0.06, respectively). Maximum wall thickness was associated with a greater calcium score on electron-beam CT (P=0.02). Although the cumulative exposure of the aortic root to cholesterol (the cholesterol-year score) was significantly correlated with the Achilles tendon CSA and vascular calcification, this score did not correlate with the wall thickness or aortic CSA.

CONCLUSIONS

This study not only demonstrates the utility of MRI for detecting and characterizing aortic root atherosclerotic plaque and supravalvular aortic stenosis in HFH patients but also suggests that the LDL receptor plays a direct or indirect role in aortic mural development and vascular growth.

Correction of hypoalphalipoproteinemia in LDL receptor-deficient rabbits by lecithin:cholesterol acyltransferase

Familial hypercholesterolemia (FH), a disease caused by a variety of mutations in the low density lipoprotein receptor (LDLr) gene, leads not only to elevated LDL-cholesterol (C) concentrations but to reduced high density lipoprotein (HDL)-C and apolipoprotein (apo) A-I concentrations as well. The reductions in HDL-C and apoA-I are the consequence of the combined metabolic defects of increased apoA-I catabolism and decreased apoA-I synthesis. The present studies were designed to test the hypothesis that overexpression of human lecithin:cholesterol acyltransferase (hLCAT), a pivotal enzyme involved in HDL metabolism, in LDLr defective rabbits would increase HDL-C and apoA-I concentrations. Two groups of hLCAT transgenic rabbits were established: 1) hLCAT+/LDLr heterozygotes (LDLr+/-) and 2) hLCAT+/LDLr homozygotes (LDLr-/-). Data for hLCAT+ rabbits were compared to those of nontransgenic (hLCAT-) rabbits of the same LDLr status. In LDLr+/- rabbits, HDL-C and apoA-I concentrations (mg/dl), respectively, were significantly greater in hLCAT+ (62 +/- 8, 59 +/- 4) relative to hLCAT- rabbits (21 +/- 1, 26 +/- 2). This was, likewise, the case when hLCAT+/ LDLr-/- (27 +/- 2, 19 +/- 6) and hLCAT-/LDLr-/- (5 +/- 1, 6 +/- 2) rabbits were compared. Kinetic experiments demonstrated that the fractional catabolic rate (FCR, d(-1)) of apoA-I was substantially delayed in hLCAT+ (0.376 +/- 0.025) versus hLCAT- (0.588) LDLr+/- rabbits, as well as in hLCAT+ (0.666 +/- 0.033) versus hLCAT- (1.194 +/- 0.138) LDLr-/- rabbits. ApoA-I production rate (PR, mg x kg x d(-1)) was greater in both hLCAT+/LDLr+/- (10 +/- 2 vs. 6) and hLCAT+/LDLr-/- (9 +/- 1 vs. 4 +/- 1) rabbits. Significant correlations (P < 0.02) were observed between plasma LCAT activity and HDL-C (r = 0.857), apoA-I FCR (r = -0.774), and apoA-I PR (r = 0.771), while HDL-C correlated with both apoA-I FCR (-0.812) and PR (0.751). In summary, these data indicate that hLCAT overexpression in LDLr defective rabbits increases HDL-C and apoA-I concentrations by both decreasing apoA-I catabolism and increasing apoA-I synthesis, thus correcting the metabolic defects responsible for the hypoalphalipoproteinemia observed in LDLr deficiency.

Differential rate of cholesterol efflux from the apical and basolateral membranes of MDCK cells

Epithelial cells contain two distinct membrane surfaces, the apical and basolateral plasma membranes, which have different lipid and protein compositions. In order to assess the effect of the compositional differences of the apical and basolateral membranes on their ability to undergo cholesterol efflux, MDCK cells were radiolabeled with [3H]cholesterol and grown as a polarized monolayer on filter inserts, that separate the upper apical compartment from the lower basolateral compartment. The rate of cholesterol efflux from the basolateral membrane into media containing HDL in the basolateral compartment was 6.3%/h +/-0.7, whereas HDL-mediated efflux from the apical membrane was approximately 3-fold slower (1.9%/h +/-0.3). In contrast, Fu5AH cells, which do not form distinct polarized membrane domains, had a similar rate of HDL-mediated cholesterol efflux into the apical and basolateral compartments. Similar to HDL, other cholesterol acceptors, namely LDL, bovine serum albumin, and a lipid emulsion, also showed a decreased rate of cholesterol efflux from the apical membrane surface versus the basolateral membrane. Compared to the basolateral membrane, the apical membrane was also found to be more resistant to cholesterol oxidase treatment, to bind less HDL, and to take up less cholesterol from the medium. In conclusion, cholesterol efflux occurred less readily from the apical membrane than from the basolateral membrane for all types of acceptors tested. These results suggest that differences in the composition of the apical and basolateral membrane lead to a relative decrease in cholesterol desorption from the apical membrane and hence a reduced rate of cholesterol efflux.

Aortic hypoplasia in homozygous familial hypercholesterolemia

Diagnosis of hypoplastic aortic root with ultrafast computed tomography provides important clinical information in homozygous familial hypercholesterolemic patients with supravalvular aortic stenosis.

Overexpression of human lecithin:cholesterol acyltransferase in cholesterol-fed rabbits: LDL metabolism and HDL metabolism are affected in a gene dose-dependent manner

Lecithin:cholesterol acyltransferase (LCAT) is an enzyme well known for its involvement in the intravascular metabolism of high density lipoproteins; however, its role in the regulation of apolipoprotein (apo) B-containing lipoproteins remains elusive. The present study was designed to investigate the metabolic mechanisms responsible for the differential lipoprotein response observed between cholesterol-fed hLCAT transgenic and control rabbits. 131I-labeled HDL apoA-I and 125I-labeled LDL kinetics were assessed in age- and sex-matched groups of rabbits with high (HE), low (LE), or no hLCAT expression after 6 weeks on a 0.3% cholesterol diet. In HE, the mean total cholesterol concentration on this diet, mg/dl (230 +/- 50), was not significantly different from that of either LE (313 +/- 46) or controls (332 +/- 52) due to the elevated level of HDL-C observed in HE (127 +/- 19), as compared with both LE (100 +/- 33) and controls (31 +/- 4). In contrast, the mean nonHDL-C concentration for HE (103 +/- 33) was much lower than that for either LE (213 +/- 39) or controls (301 +/- 55). FPLC analysis of plasma confirmed that HDL was the predominant lipoprotein class in HE on the cholesterol diet, whereas cholesteryl ester-rich, apoB-containing lipoproteins characterized the plasma of LE and, most notably, of controls. In vivo kinetic experiments demonstrated that the differences in HDL levels noted between the three groups were attributable to distinctive rates of apoA-I catabolism, with the mean fractional catabolic rate (FCR, d-1) of apoA-I slowest in HE (0.282 +/- 0.03), followed by LE (0.340 +/- 0.01) and controls (0.496 +/- 0.04). A similar, but opposite, pattern was observed for nonHDL-C levels and LDL metabolism (h-1), such that HE had the lowest nonHDL-C levels with the fastest rate of clearance (0.131 +/- 0.027), followed by LE (0.057 +/- 0.009) and controls (0.031 +/- 0.001). Strong correlations were noted between LCAT activity and both apoA-I (r= -0.868, P < 0.01) and LDL (r = 0.670, P = 0.06) FCR, indicating that LCAT activity played a major role in the mediation of lipoprotein metabolism. In summary, these data are the first to show that LCAT overexpression can regulate both LDL and HDL metabolism in cholesterol-fed rabbits and provide a potential explanation for the prevention of diet-induced atherosclerosis observed in our previous study.

Altered compliance and residual strain precede angiographically detectable early atherosclerosis in low-density lipoprotein receptor deficiency

BACKGROUND

This study was performed to detect changes in vascular biomechanical properties early in atherogenesis.

METHODS AND RESULTS

Age- and weight-matched LDL-receptor deficient Watanabe hypercholesterolemic male rabbits (Group I: n = 11) and normal rabbits (Group II: n = 11) were studied. Fasting plasma lipoprotein concentrations, aortic angiography and intravascular ultrasound, in vivo aortic compliance evaluation, ex vivo aortic residual strain measurements, aortic lipid content and histopathology were determined. Plasma cholesterol was increased 9.8 fold and aortic cholesterol content was increased from 20 to 43 fold in Group I compared to Group II, respectively (P < .00005). Angiography revealed no stenoses in either group, whereas intravascular ultrasound and histological studies of Group I showed small circumferential plaques with < 10% cross-sectional area involvement. The residual strain in Group I was significantly increased in the ascending thoracic aorta (22.1 +/- 6.9% versus 10.4 +/- 3.2% in Group II, P < .0001), descending thoracic aorta (15.7 +/- 7.2% versus 4.8 +/- 1.3% in Group II, P < .0001), and abdominal aorta (18.0 +/- 4.8% versus 8.3 +/- 6.3% in Group II, P < .005). Changes in residual strain were inversely correlated with the aortic cholesterol content in the ascending thoracic aorta (r = -.72; P = -.001), descending thoracic aorta (r = -.95; P < .001), and abdominal aorta (r = -.51; P = .019).

CONCLUSIONS

Early atherosclerosis in LDL-receptor deficient rabbits, undetectable by angiography yet observed by intravascular ultrasound imaging and histology, is associated with marked changes in ex vivo residual strain. Alterations in vascular biomechanical properties, associated with changes in cholesterol content, may have physiologic consequences and may be useful in detecting and quantitating early atherosclerosis.

Active serum vitamin D levels are inversely correlated with coronary calcification

BACKGROUND

Arterial calcification is a common feature of atherosclerosis, occurring in >90% of angiographically significant lesions. Recent evidence from this and other studies suggests that development of atherosclerotic calcification is similar to osteogenesis; thus, we undertook the current investigation on the potential role of osteoregulatory factors in arterial calcification.

METHODS AND RESULTS

We studied two human populations (173 subjects) at high and moderate risk for coronary heart disease and assessed them for associations between vascular calcification and serum levels of the osteoregulatory molecules osteocalcin, parathyroid hormone, and 1alpha,25-dihydroxyvitamin D3 (1,25-vitamin D). Our results revealed that 1,25-vitamin D levels are inversely correlated with the extent of vascular calcification in both groups. No correlations were found between extent of calcification and levels of osteocalcin or parathyroid hormone.

CONCLUSIONS

These data suggest a possible role for vitamin D in the development of vascular calcification. Vitamin D is also known to be important in bone mineralization; thus, 1,25-vitamin D may be one factor to explain the long observed association between osteoporosis and vascular calcification.

Cumulative effects of high cholesterol levels, high blood pressure, and cigarette smoking on carotid stenosis

BACKGROUND

Single measurements of cardiovascular risk factors may not accurately reflect a person's past exposure to those risk factors. We therefore studied the long-term associations of cardiovascular risk factors such as high serum cholesterol levels, high blood pressure, and cigarette smoking with the prevalence of carotid stenosis.

METHODS

We studied cross-sectional and longitudinal information from a sample of 429 men and 661 women in the Framingham Heart Study who underwent B-mode ultrasound measurements of the carotid artery. Their mean age was 75 years, and each had attended most of the biennial clinic examinations over the 34 years before the carotid ultrasound study. We used time-integrated measurements to assess the associations between various cardiovascular risk factors and the degree of carotid stenosis.

RESULTS

Moderate carotid stenosis (> or =25 percent) was present in 189 men and 226 women. We assessed the odds ratios for this degree of stenosis as compared with minimal stenosis (<25 percent) according to increases in risk factors. In the men, the odds ratio for moderate carotid stenosis associated with an increase of 20 mm Hg in systolic blood pressure was 2.11 (95 percent confidence interval, 1.51 to 2.97). The odds ratio for an increase of 10 mg per deciliter (0.26 mmol per liter) in the cholesterol level was 1.10 (95 percent confidence interval, 1.03 to 1.16), and for an increase of five pack-years of smoking it was 1.08 (95 percent confidence interval, 1.03 to 1.13). The results were similar in the women. Time-integrated measurements of diastolic blood pressure showed significant associations with carotid stenosis in men and insignificant associations in women.

CONCLUSIONS

Over the long term, high systolic blood pressure, high cholesterol levels, and smoking were associated with an increased risk of carotid stenosis in this elderly population.

Lipoprotein lipase correlates positively and hepatic lipase inversely with calcific atherosclerosis in homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (FH) is a rare genetic disorder that leads to premature atherosclerosis due to a defective LDL receptor. There is, however, a large degree of phenotypic heterogeneity at the level of atherosclerosis even in patients with identical mutations of the LDL receptor protein. Lipoprotein lipase (LPL) and hepatic lipase (HL) are crucial enzymes in lipoprotein metabolism, and both have been proposed as having proatherogenic as well as antiatherogenic effects. To evaluate a potential role for these enzymes in the severity of atherosclerosis, we correlated postheparin LPL mass and activity as well as HL activity with the volume of total calcific atherosclerosis (heart and thoracic aorta), coronary artery calcific atherosclerosis, and Achilles tendon width as measured by computed tomography in 15 FH homozygotes. LPL dimer and total mass were positively correlated with all three parameters (r = .65 to .87, P < .01) as was LPL activity (r = .52 to .63, P < .05). HL activity was negatively correlated with total and coronary artery calcified lesion volume (r = -.55 to .57, P < .05). In a multiple regression model of the coronary artery lesion volume, LPL dimer mass and HL activity together accounted for 84% of the variability (r = .92, P < .0001). In a multiple regression model of the total calcified lesion volume, HL activity, total cholesterol, age, and LPL dimer mass together accounted for 85% of the variability (r = .92, P = .0005). These data demonstrate a significant correlation of LPL mass and activity with the extent of calcific atherosclerosis in homozygous FH. It is not clear whether LPL is the cause or consequence of the observed correlation, but if the association between LPL and coronary artery lesions is also present in patients with other genetic dyslipoproteinemias, LPL could constitute a new risk factor for cardiovascular disease.

Adenoviral delivery of low-density lipoprotein receptors to hyperlipidemic rabbits: receptor expression modulates high-density lipoproteins

Plasma concentrations of low-density lipoproteins (LDLs) and high-density lipoproteins (HDLs) are inversely related in several dyslipoproteinemias. To elucidate the interactions between these lipoproteins, we used a recombinant adenovirus (hLDLR-rAdV) to express human LDL receptors (hLDLRs) in LDL receptor-deficient rabbits. hLDLR-rAdV administration resulted in hepatocyte expression and a reduction of total, intermediate-density lipoprotein (IDL), and LDL cholesterol. In addition, we found that hLDLR-rAdV treatment induced (1) increased very-low-density lipoprotein (VLDL) cholesterol, (2) increased VLDL, IDL and LDL triglycerides, (3) decreased alpha- and pre-beta-migrating apolipoprotein E (apo E) and decreased pre-beta-migrating apo A-I at 2 to 4 days posttreatment, and (4) increased total plasma apo A-I and pre-beta-migrating apo A-I beginning 8 to 10 days posttreatment. Virtually all plasma apo A-I was present on alpha- and pre-beta-HDL. Pre-beta-HDL particles with size and electrophoretic properties consistent with nascent HDL demonstrated the greatest relative apo A-I enrichment following hLDLR-rAdV treatment. In summary, enhanced expression of hepatocyte LDLRs by hLDLR-rAdV treatment markedly altered apo A-I-containing lipoproteins and IDL and LDL. The use of recombinant viruses to express physiologically relevant genes in intact animals, analogous to transfection of cells in culture, provides a new strategy for the evaluation of effects of specific gene products on metabolic systems in vivo.

Overexpression of lecithin:cholesterol acyltransferase in transgenic rabbits prevents diet-induced atherosclerosis

Lecithin:cholesterol acyltransferase (LCAT) is a key plasma enzyme in cholesterol and high density lipoprotein (HDL) metabolism. Transgenic rabbits overexpressing human LCAT had 15-fold greater plasma LCAT activity that nontransgenic control rabbits. This degree of overexpression was associated with a 6.7-fold increase in the plasma HDL cholesterol concentration in LCAT transgenic rabbits. On a 0.3% cholesterol diet, the HDL cholesterol concentrations increased from 24 +/- 1 to 39 +/- 3 mg/dl in nontransgenic control rabbits (n = 10; P < 0.05) and increased from 161 +/- 5 to 200 +/- 21 mg/dl (P < 0.001) in the LCAT transgenic rabbits (n = 9). Although the baseline non-HDL concentrations of control (4 +/- 3 mg/dl) and transgenic rabbits (18 +/- 4 mg/dl) were similar, the cholesterol-rich diet raised the non-HDL cholesterol concentrations, reflecting the atherogenic very low density, intermediate density, and low density lipoprotein particles observed by gel filtration chromatography. The non-HDL cholesterol rose to 509 +/- 57 mg/dl in controls compared with only 196 +/- 14 mg/dl in the LCAT transgenic rabbits (P < 0.005). The differences in the plasma lipoprotein response to a cholesterol-rich diet observed in the transgenic rabbits paralleled the susceptibility to developing aortic atherosclerosis. Compared with nontransgenic controls, LCAT transgenic rabbits were protected from diet-induced atherosclerosis with significant reductions determined by both quantitative planimetry (-86%; P < 0.003) and quantitative immunohistochemistry (-93%; P < 0.009). Our results establish the importance of LCAT in the metabolism of both HDL and apolipoprotein B-containing lipoprotein particles with cholesterol feeding and the response to diet-induced atherosclerosis. In addition, these findings identify LCAT as a new target for therapy to prevent atherosclerosis.

Hyperalphalipoproteinemia in human lecithin cholesterol acyltransferase transgenic rabbits. In vivo apolipoprotein A-I catabolism is delayed in a gene dose-dependent manner

Lecithin cholesterol acyltransferase (LCAT) is an enzyme involved in the intravascular metabolism of high density lipoproteins (HDLs). Overexpression of human LCAT (hLCAT) in transgenic rabbits leads to gene dose-dependent increases of total and HDL cholesterol concentrations. To elucidate the mechanisms responsible for this effect, 131I-HDL apoA-I kinetics were assessed in age- and sex-matched groups of rabbits (n=3 each) with high, low, or no hLCAT expression. Mean total and HDL cholesterol concentrations (mg/dl), respectively, were 162+/-18 and 121+/-12 for high expressors (HE), 55+/-6 and 55+/-10 for low expressors (LE), and 29+/-2 and 28+/-4 for controls. Fast protein liquid chromatography analysis of plasma revealed that the HDL of both HE and LE were cholesteryl ester and phospholipid enriched, as compared with controls, with the greatest differences noted between HE and controls. These compositional changes resulted in an incremental shift in apparent HDL particle size which correlated directly with the level of hLCAT expression, such that HE had the largest HDL particles and controls the smallest. In vivo kinetic experiments demonstrated that the fractional catabolic rate(FCR, d(-1)) of apoA-I was slowest in HE (0.328+/-0.03) followed by LE (0.408+/-0.01) and, lastly, by controls (0.528+/-0.04). ApoA-I FCR was inversely associated with HDL cholesterol level (r=-0.851,P<0.01) and hLCAT activity (r=-0.816, P<0.01). These data indicate that fractional catabolic rate is the predominant mechanism by which hLCAT overexpression differentially modulates HDL concentrations in this animal model. We hypothesize that LCAT-induced changes in HDL composition and size ultimately reduce apoA-I catabolism by altering apoA-I conformation and/or HDL particle regeneration.

Relation of cholesterol-year score to severity of calcific atherosclerosis and tissue deposition in homozygous familial hypercholesterolemia

The high concentrations of low-density lipoprotein cholesterol in plasma lead to accelerated atherosclerosis in patients homozygous for familial hypercholesterolemia (FH). We addressed the hypothesis that lipid deposition in the arterial vasculature and in nonvascular tissues in these patients correlates with both the duration and severity of their hypercholesterolemia. The severity of calcific atherosclerosis was defined by calcification scores and a calcified volume determined by electron beam tomography. The extent of tendinous xanthomatosis was quantitated by computed tomography. A cholesterol-year score was calculated based on the age and the yearly mean serum cholesterol concentration of each patient. Seventeen patients homozygous for FH were followed up. The average total cholesterol concentration in the study group was 780 +/- 231 mg/dl (20.2 mmol/L), and the cholesterol-year scores ranged from 2,172 mg-year/dl (56 mmol-year/L) to 32,260 mg-year/dl (834 mmol-year/L). Achilles tendon width (r=0.86) and cross-sectional area (r=0.81; both p <0.001) were best correlated with the cholesterol-year score. In addition, the coronary (r=0.61; p<0.05), ostial (r=0.45; p<0.05), and total (r= 0.77; p<0.001) calcification atherosclerosis scores all were best correlated with the cholesterol-year score. Calcific atherosclerosis was not observed in these patients until the cholesterol-year score exceeded 10,000 mg-year/dl (260 mmol-year/L). These findings establish a direct association of cholesterol-year with extravascular lipid deposition in tissues of patients with FH. The cholesterol-year score may be useful in defining the risk of atherosclerosis in patients with more common forms of hypercholesterolemia.

Lecithin:cholesterol acyltransferase overexpression generates hyperalpha-lipoproteinemia and a nonatherogenic lipoprotein pattern in transgenic rabbits

Cholesterol esterification within plasma lipoprotein particles is catalyzed by lecithin:cholesterol acyltransferase (LCAT). The impact of the overexpression of this enzyme on plasma concentrations of the different plasma lipoproteins in an animal model expressing cholesteryl ester transfer protein was evaluated by generating rabbits expressing human LCAT. A 6.2-kilobase human genomic DNA construct was injected into the pronuclei of rabbit embryos. Of the 1002 embryos that were injected, 3 founder rabbits were characterized that expressed the human LCAT gene. As in mice and humans, the principal sites of mRNA expression in these rabbits is in the liver and brain, indicating that the regulatory elements required for tissue-specific expression among these species are similar. The alpha-LCAT activity correlated with the number of copies of LCAT that integrated into the rabbit DNA. Compared with controls, the high expressor LCAT-transgenic rabbits total and high density lipoprotein (HDL) cholesterol concentrations were increased 1.5-2.5-fold with a 3.1-fold increase in the plasma cholesterol esterification rate. Analysis of the plasma lipoproteins by fast protein liquid chromatography indicates that these changes reflected an increased concentration of apolipoprotein E-enriched, HDL1-sized particles, whereas atherogenic apolipoprotein B particles disappeared from the plasma. The concentrations of plasma HDL cholesterol were highly correlated with both human LCAT mass (r = 0.93; p = 0.001) and the log LCAT activity (r = 0.94; p < 0.001) in the transgenic rabbits. These results indicate that overexpression of LCAT in the presence of cholesteryl ester transfer protein leads to both hyperalpha-lipoproteinemia and reduced concentrations of atherogenic lipoproteins.

Dominant expression of type III hyperlipoproteinemia. Pathophysiological insights derived from the structural and kinetic characteristics of ApoE-1 (Lys146-->Glu)

Type III hyperlipoproteinemia is characterized by delayed chylomicron and VLDL remnant catabolism and is associated with homozygosity for the apoE-2 allele. We have identified a kindred in which heterozygosity for an apoE mutant, apoE-1 (Lys146-->Glu), is dominantly associated with the expression of type III hyperlipoproteinemia. DNA sequence analysis of the mutant apoE gene revealed a single-point mutation that resulted in the substitution of glutamic acid (GAG) for lysine (AAG) at residue 146 in the proposed receptor-binding domain of apoE. The pathophysiological effect of this mutation was investigated in vivo by kinetic studies in the patient and six normal subjects, and in vitro by binding studies of apoE-1 (Lys146-->Glu) to LDL receptors on human fibroblasts and to heparin. The kinetic studies revealed that apoE-1 (Lys146-->Glu) was catabolized significantly slower than apoE-3 in normals (P < 0.005). In the proband, the plasma residence times of both apoEs were substantially longer and the production rate of total apoE was about two times higher than in the control subjects. ApoE-1 (Lys146-->Glu) was defective in interacting with LDL receptors, and its ability to displace LDL in an in vitro assay was reduced to 7.7% compared with apoE-3. The affinity of apoE-1 (Lys146-->Glu) to heparin was also markedly reduced compared with both apoE-2 (Arg158-->Cys) and apoE-3. These abnormal in vitro binding characteristics and the altered in vivo metabolism of apoE-1 (Lys146-->Glu) are proposed to result in the functional dominance of this mutation in the affected kindred.

Carboxyl-terminal domain truncation alters apolipoprotein A-I in vivo catabolism

Apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins, facilitates reverse cholesterol transport from peripheral tissue to liver. To determine the structural motifs important for modulating the in vivo catabolism of human apoA-I (h-apoA-I), we generated carboxyl-terminal truncation mutants at residues 201 (apoA-I201), 217 (apoA-I217), and 226 (apoA-I226) by site-directed mutagenesis. ApoA-I was expressed in Escherichia coli as a fusion protein with the maltose binding protein, which was removed by factor Xa cleavage. The in vivo kinetic analysis of the radioiodinated apoA-I in normolipemic rabbits revealed a markedly increased rate of catabolism for the truncated forms of apoA-I. The fractional catabolic rates (FCR) of 9.10 +/- 1.28/day (+/- S.D.) for apoA-I201, 6.34 +/- 0.81/day for apoA-I217, and 4.42 +/- 0.51/day for apoA-I226 were much faster than the FCR of recombinant intact apoA-I (r-apoA-I, 0.93 +/- 0.07/day) and h-apoA-I (0.91 +/- 0.34/day). All the truncated forms of apoA-I were associated with very high density lipoproteins, whereas the intact recombinant apoA-I (r-apoA-I) and h-apoA-I associated with HDL2 and HDL3. Gel filtration chromatography revealed that in contrast to r-apoA-I, the mutant apoA-I201 associated with a phospholipid-rich rabbit apoA-I containing particle. Analysis by agarose gel electrophoresis demonstrated that the same mutant migrated in the pre-beta position, but not within the alpha position as did r-apoA-I. These results indicate that the carboxyl-terminal region (residue 227-243) of apoA-I is critical in modulating the association of apoA-I with lipoproteins and in vivo metabolism of apoA-I.

Polarized secretion of apoA-I and apoA-II by transfected MDCK cells

Apolipoproteins (apo) are secreted preferentially from the basolateral surface of hepatocytes and enterocytes. The polarized secretion of proteins is either mediated by a protein-dependent sorting signal or by a cell-dependent default pathway. In order to determine the mechanism for the polarized secretion of apolipoproteins, we examined the secretion of apoA-I and apoA-II in transfected Madin-Darby canine kidney (MDCK) cells. Transfected MDCK cells and Caco-2 cells were grown as a polarized monolayer on tissue culture inserts, which separate an upper apical compartment from the lower basolateral compartment, and the secretion of apoA-I and apoA-II into the apical and basolateral compartments was quantitated by immunoprecipitation. Caco-2 cells almost exclusively secreted apoA-I and apoA-II basolaterally, with an apical to basolateral ratio of 18:82 for apoA-I, and 11:89 for apoA-II. In contrast, transfected MDCK cells secreted significant amounts of apoA-I and apoA-II into both compartments, but with a bias toward apical secretion and an apical to basolateral ratio of 66:34 and 68:32, respectively. The polarized secretion of MDCK cells was not due to transcytosis, diffusion, or differential recovery. As assessed by density gradient ultracentrifugation, apoA-I and apoA-II secreted from either the apical or basolateral surface were relatively lipid-poor. Overall, these results suggest that the polarized secretion of apoA-I and apoA-II does not occur by a protein-dependent sorting signal, but by a cell-dependent default pathway that leads to preferential basolateral secretion by Caco-2 cells and both apical and basolateral secretion in MDCK cells, but with a bias toward apical secretion.

The low density lipoprotein receptor is not required for normal catabolism of Lp(a) in humans

Lipoprotein(a) [Lp(a)] is an atherogenic lipoprotein which is similar in structure to low density lipoproteins (LDL). The role of the LDL receptor in the catabolism of Lp(a) has been controversial. We therefore investigated the in vivo catabolism of Lp(a) and LDL in five unrelated patients with homozygous familial hypercholesterolemia (FH) who have little or no LDL receptor activity. Purified 125I-Lp(a) and 131I-LDL were simultaneously injected into the homozygous FH patients, their heterozygous FH parents when available, and control subjects. The disappearance of plasma radioactivity was followed over time. As expected, the fractional catabolic rates (FCR) of 131I-LDL were markedly decreased in the homozygous FH patients (mean LDL FCR 0.190 d-1) and somewhat decreased in the heterozygous FH parents (mean LDL FCR 0.294 d-1) compared with controls (mean LDL FCR 0.401 d-1). In contrast, the catabolism of 125I-Lp(a) was not significantly different in the homozygous FH patients (mean FCR 0.251 d-1), heterozygous FH parents (mean FCR 0.254 d-1), and control subjects (mean FCR 0.287 d-1). In summary, absence of a functional LDL receptor does not result in delayed catabolism of Lp(a), indicating that the LDL receptor is not a physiologically important route of Lp(a) catabolism in humans.

Identification of novel differentially expressed hepatic genes in cholesterol-fed rabbits by a non-targeted gene approach

Several key genes involved in cholesterol metabolism are known to be directly regulated by cholesterol. The possible indirect effect, however, of increased levels of cellular cholesterol on gene expression and its possible role in cholesterol metabolism and atherosclerosis has not been thoroughly explored. In order to determine the overall effect of cholesterol on gene expression, we isolated differentially expressed genes from a PCR-based subtraction library prepared from the liver of chow-fed and cholesterol-fed rabbits. A total of nine upregulated and four down-regulated cDNA fragments were isolated. As determined by Northern blot analysis, the expression of the isolated cDNAs began to change as early as the first week on the cholesterol-rich diet or as late as 4 weeks, which corresponded with hepatic cholesterol accumulation. Three of the cDNAs were identified by DNA sequence homology, whereas the remaining cDNAs had no significant homology match. CYP1A1, a cytochrome P450 isoenzyme, was found to be down-regulated in hepatocytes by cholesterol feeding. Osteopontin and Mac-2, which are produced by macrophages, were found to be up-regulated in Kupffer cells by cholesterol feeding. Overall these results demonstrate the usefulness of the subtraction library approach for identifying new candidate genes for exploring the pathogenesis of atherosclerosis.

Detection and quantitation of calcific atherosclerosis by ultrafast computed tomography in children and young adults with homozygous familial hypercholesterolemia

Ultrafast computed tomography (CT) is a new method for detecting calcific lesions in the coronary arteries. The ability of CT to detect and quantify coronary artery atherosclerosis in children and young adults at risk for malignant atherogenesis was evaluated. A total of 11 consecutive familial hypercholesterolemic (FH) homozygotes (3 to 37 years old) participated. Untreated total cholesterol concentrations were 488 to 1277 mg/dL (12.7 to 33.2 mmol/L). Angiography detected significant lesions in 7 of 11 patients. CT detected calcific atherosclerosis in all 9 of the patients older than 12 years of age, including all those with angina. CT was more sensitive in detecting aortic root and coronary ostial lesions, where atherosclerosis first appears in homozygous FH. The volume of calcification (in cubic millimeters) correlated with the severity and duration of the hypercholesterolemia (r = .62, P < .05) as well as with the presence of angina (P < .05). All patients with angina (7 of 7) had > 150 mm3 of calcified volume, whereas only 1 of 4 asymptomatic patients had a volume score > 150 mm3. We conclude that (1) coronary and aortic calcium phosphate deposits are common in young FH homozygotes; (2) these deposits are associated with the presence of angiographic stenoses, as has been seen in adults with coronary atherosclerosis; and (3) aortic calcific deposits are more common than calcific coronary lesions.

Quantitation of plasma apolipoproteins in the primary and secondary prevention of coronary artery disease

PURPOSE

To review current knowledge of apolipoprotein quantitation used in the clinical management of persons with or at risk for the development of premature coronary artery disease.

DATA SOURCES

The English-language literature was analyzed using MEDLINE (1975 to 1993) with key words "apolipoproteins," "quantitation," and "coronary artery disease." Article bibliographies were also reviewed to obtain additional references.

STUDY SELECTION

Published, peer-reviewed retrospective and prospective studies relevant to the association of plasma apolipoprotein levels with coronary artery disease in humans.

DATA SYNTHESIS

Most studies concerned apolipoprotein A-I (apo A-I), apolipoprotein B (apo B), and lipoprotein(a) [Lp(a)]. In retrospective cross-sectional studies, apo A-l levels were not substantially more predictive of coronary artery disease than were high-density lipoprotein (HDL) cholesterol levels. In contrast, levels of apo B and Lp(a) were often more strongly associated with coronary artery disease than were traditional lipid measurements. In studies of the relation between apolipoprotein levels in children and premature coronary artery disease in their parents, Lp(a) levels, but not apo A-l and apo B levels, were consistently predictive of familial coronary artery disease. Prospective studies have yielded variable results for all three apolipoproteins. Low apo A-l levels were consistently associated with coronary artery disease in six prospective studies but were not more predictive than HDL levels. Apolipoprotein B levels were strongly associated with coronary artery disease in four of five prospective studies but were more predictive of coronary artery disease than were total cholesterol levels in only two of the four studies. Lipoprotein(a) levels were strongly associated with coronary artery disease in five of seven prospective studies but were not associated in two of the four largest studies.

CONCLUSIONS

Too few large prospective studies of apolipoprotein quantitation using validated assay methods, both in general unselected populations and in subgroups of persons with premature coronary artery disease or family histories of premature coronary artery disease, are available to make definitive recommendations concerning clinical utility. The data do not support use of apolipoprotein quantitation as a screening tool to predict coronary artery disease risk in the general population. However, the data suggest that quantitation of apo B and Lp(a) may be indicated in subgroups of persons with premature coronary artery disease or with family histories of premature coronary artery disease. In these persons, an increased apo B or Lp(a) level or both could be a clinical indication for more aggressive treatment of low-density lipoprotein cholesterol.

Investigation of decreased availability of nitric oxide precursor as the mechanism responsible for impaired endothelium-dependent vasodilation in hypercholesterolemic patients

OBJECTIVES

The purpose of this study was to determine whether the impaired endothelium-dependent vasodilation of hypercholesterolemic patients is due to decreased availability of L-arginine, the substrate for nitric oxide.

BACKGROUND

Patients with hypercholesterolemia have impaired endothelium-dependent vasodilation that is related to a defect in the endothelium-derived nitric oxide system. However, the precise location of this abnormality has not been determined.

METHODS

The study included 12 hypercholesterolemic patients (6 men, 6 women; 52 +/- 9 years old; serum cholesterol > 240 mg/dl) and 15 normal volunteers (8 men, 7 women; 50 +/- 6 years old; serum cholesterol < 210 mg/dl). The forearm vascular responses to intraarterial infusion of acetylcholine, an endothelium-dependent vasodilator (7.5, 15, 30 micrograms/min), and sodium nitroprusside, a direct smooth muscle dilator (0.8, 1.6, 3.2 micrograms/min) were studied before and during infusion of L- or D-arginine (a stereoisomer of arginine that is not a nitric oxide precursor).

RESULTS

The response to acetylcholine was lower in hypercholesterolemic patients than in control subjects. However, no significant difference was observed with sodium nitroprusside infusion. L-Arginine augmented the response to acetylcholine in normal subjects (maximal blood flow increased from 14.4 +/- 7 to 18.9 +/- 10 ml/min per 100 ml, p < 0.002). In contrast, in the hypercholesterolemic patients, only a mild but not significant improvement in the response to acetylcholine was observed with the infusion of L-arginine (maximal blood flow increased from 6.8 +/- 4 to 8.4 +/- 5 ml/min per 100 ml; p = 0.16); however, a similar mild but not significant change was also observed with D-arginine (maximal blood flow increased from 6.8 +/- 4 to 8.3 +/- 4 ml/min per 100 ml, p = 0.07). L-Arginine did not modify the response to sodium nitroprusside in either group.

CONCLUSIONS

The augmentation of endothelium-dependent vasodilation by L-arginine, the nitric oxide precursor, is defective in hypercholesterolemic patients. This supports the concept of an abnormal endothelium-derived nitric oxide system in hypercholesterolemia and indicates that decreased availability of nitric oxide substrate is not responsible for the impaired endothelial function in this condition.

Aberrant hypothalamic-pituitary-ovarian axis in the Watanabe heritable hyperlipidemic rabbit

The WHHL rabbit has a defective low density lipoprotein receptor and is a model for familial hypercholesterolemia. WHHL rabbits are less fecund than NZW rabbits, the strain into which the defect has been inbred. This lower fecundity could be related to impaired ovarian steroidogenesis due to reduced intracellular availability of cholesterol. Here we compare the WHHL and NZW rabbits with regard to oocyte morphology and fertilization rates after stimulation with equine chorionic gonadotropin. We also compare hypothalamic-pituitary-ovarian axis function by measuring baseline and gonadotropin releasing hormone-stimulated plasma estradiol, progesterone, and gonadotropin levels, both before and after simvastatin inhibition of de novo cholesterol synthesis. WHHL rabbit oocytes remained encased in cumulus and had a lowered fertilization rate (9/50 vs. 83/87, P < 0.05). WHHL rabbits had lower baseline estradiol levels (7.1 +/- 0.72 vs. 10.2 +/- 0.94, P < 0.05) and had higher baseline follicle stimulating hormone (P < 0.05) and luteinizing hormone (P < 0.05) levels. Simvastatin lowered luteal progesterone concentrations only in WHHL rabbits (P < 0.05). We conclude that the hypothalamic-pituitary-ovarian axis in WHHL rabbits is abnormal. The reduced availability of intracellular cholesterol for progesterone synthesis by inhibition of de novo cholesterol biosynthesis leads to a significant reduction in plasma progesterone concentrations in the WHHL. These findings have implications for women with familial hypercholesterolemia, particularly regarding treatment with inhibitors of de novo cholesterol synthesis.

The role of nitric oxide in endothelium-dependent vasodilation of hypercholesterolemic patients

BACKGROUND

Patients with hypercholesterolemia have a reduced response to endothelium-dependent vasodilators. However, the regulatory function of the endothelium on vascular tone is mediated through the release of several vasoactive substances; therefore, a reduced response to endothelium-dependent agents does not identify which of the factors released by the endothelium is involved in this abnormality.

METHODS AND RESULTS

To investigate the role of nitric oxide in the endothelium-dependent vasodilation in hypercholesterolemia, we studied the effect of NG-monomethyl-L-arginine (L-NMMA), an inhibitor of endothelial nitric oxide synthesis, on basal vascular tone and on the responses to acetylcholine, an endothelium-dependent vasodilator, and to sodium nitroprusside, a direct smooth muscle dilator. The study included 33 hypercholesterolemic patients (17 men; 51 +/- 8 years; plasma cholesterol, > or = 240 mg/dL) and 23 normal controls (12 men; 48 +/- 7 years; plasma cholesterol, < 210 mg/dL). Drugs were infused into the brachial artery, and the response of the forearm vasculature was measured by strain-gauge plethysmography. Basal blood flow and vascular resistance were similar in hypercholesterolemic patients and normal controls (3.1 +/- 1 versus 2.6 +/- 0.8 mL/min per 100 mL and 32.1 +/- 13 versus 36.1 +/- 12 mm Hg/mL-1.min-1.100 mL-1, respectively). The reduction in basal blood flow and increase in vascular resistance produced by L-NMMA were not significantly different between the two groups. L-NMMA markedly blunted the response to acetylcholine in normals (maximum flow decreased from 16.4 +/- 8 to 7.0 +/- 3; P < .005); however, the arginine analogue did not significantly modify the response to acetylcholine in the hypercholesterolemic patients (maximum flow, 11.1 +/- 8 versus 10.0 +/- 8). L-NMMA did not modify the vasodilator response to sodium nitroprusside in either controls or patients.

CONCLUSIONS

These findings indicate that hypercholesterolemic patients have a defect in the bioactivity of nitric oxide that may explain their impaired endothelium-dependent vascular relaxation.

Homozygous familial hypercholesterolemia: a paradigm for phenotypic variation

Most, if not all, inborn errors of metabolism manifest phenotypic heterogeneity in their clinical presentation. The term "penetrance" has been used to describe the degree to which a given genotype expresses itself in the phenotype of the individual. Although many explanations for this phenomenon have been put forward, the molecular bases for this have been difficult to define. The investigation of the disease familial hypercholesterolemia (FH) has been used as a paradigm at many different levels. This condition, in which a wide variety of mutations in the low density lipoprotein (LDL) receptor gene leads to elevated concentrations of LDL particles has a wide array of clinical manifestations that are variably expressed in both patients who are heterozygous and homozygous for mutations at the LDL receptor allele. Progress in understanding lipoprotein metabolism, atherogenesis, and the development of molecular biology and transgenic expression techniques converge to utilize homozygous FH as a paradigm for understanding the molecular basis of penetrance. Elucidation of the key factors in altering the clinical features expressed by patients with FH have theoretical implications in understanding the polygenic nature of atherosclerosis as well as practical ramifications in the treatment of patients with FH.

O-linked glycosylation modifies the association of apolipoprotein A-II to high density lipoproteins

O-linked glycosylation is a common post-translational modification of apolipoproteins, but no structural or functional role for it has been identified. We examined the biosynthesis of apolipoprotein (apo) A-II in Hep G2 cells and in glycosylation-defective Chinese hamster ovary (CHO) cell mutants transfected with apoA-II cDNA. Three monomeric isoforms of apoA-II with an apparent molecular mass of 8.5, 9.8, and 11.4 kDa were synthesized by Hep G2 cells and transfected wild-type CHO cells. The 9.8- and 11.4-kDa isoforms were sialylated but not the 8.5-kDa isoform. Transfected 1dlD cells, which are defective in the biosynthesis of galactose and N-acetylgalactosamine, only produced the 8.5-kDa isoform; however, when grown in media supplemented with these sugars, ldlD cells produced all three isoforms of apoA-II. Pulse-chase analysis of ldlD cells showed that glycosylation was not necessary for secretion of apoA-II. Glycosylation did modify the association of apoA-II with nascent high density lipoprotein (HDL) secreted by Hep G2 cells. The sialylated isoforms were lipid-poor and were present in the lipoprotein-deficient density range, whereas the nonsialylated 8.5-kDa isoform was associated with LpA-I, A-II lipoprotein particles in the HDL density range. ApoA-II from transfected ldlD cells, regardless of glycosylation, were lipid-poor. When preincubated with HDL from serum, however, sialylated apoA-II from both ldlD cells and Hep G2 cells associated with lipoprotein particles within the HDL3 density, whereas nonsialylated apoA-II was found throughout the HDL density range. In summary, O-linked glycosylation is not necessary for the secretion of apoA-II but does modify the association of apoA-II to HDL and may, therefore, play an important role in the metabolism of HDL.

Regulation of LDL receptor, apoB, and apoE protein and mRNA in Hep G2 cells

The regulation of low-density lipoprotein (LDL) receptor activity, protein synthesis, and cellular mRNA content was evaluated in the human hepatoma cell line Hep G2. Incubation of the cells with LDL led to a complete downregulation of LDL receptor mRNA and LDL receptor protein synthesis. This LDL regulation of the LDL receptor and its mRNA was both time- and concentration-dependent. In contrast to protein synthesis and cellular mRNA concentrations of the LDL receptor, which were reduced to undetectable levels by prolonged incubation in the presence of LDL, LDL receptor activity was reduced to only 44% of preincubation levels. These findings support the presence of a second metabolic pathway for LDL uptake in human hepatocytic cells. The effect of LDL on cellular LDL receptor expression was specific for LDL because incubation in the presence of HDL did not affect any of these study end points. The potential coordinate regulation of the expression of the LDL receptor with its principal ligands, apolipoproteins (apo) B and E, was also investigated. In contrast to the LDL receptor mRNA downregulation with LDL incubation, cellular apoB and apoE mRNA concentrations were not affected by either LDL or HDL. Secretion of apoB, however, was significantly increased by incubating Hep G2 cells with LDL. These findings indicate that, in contrast to LDL receptor which is regulated at the mRNA level, the ligands for the LDL receptor are regulated either co- or post-translationally.

Ontogenetic regulation of apolipoprotein B mRNA editing during human and rat development in vivo

The solubilization and delivery of lipids in plasma rely on both forms of apolipoprotein B (apo B): apo B-100 and apo B-48. Apo B-48 is the translational product of apo B-100 mRNA that undergoes peritranscriptional conversion of C----U, replacing codon CAA (glutamine 2,153) with the inframe stop codon (UAA). We examined mRNA editing activity in the human and the rat by reverse transcription-polymerase chain reaction primer-extension analysis of intestine and liver total RNA. In rat intestine the percentage of apo B transcripts that undergo editing increases dramatically the day before birth (from approximately 1% to 80%), whereas the rat liver acquires an adult level of editing activity during the third postnatal week (rising from approximately 8% to 30%), when weaning is completed, bile acid composition matures, and plasma thyroid hormone levels peak. In contrast to the rat, the human intestine acquires adult levels of apo B mRNA editing relatively early in fetal development, rising from 10% at 10 weeks to approximately 80% by the end of the second trimester. Our results establish that apo B mRNA editing is 1) developmentally regulated in a tissue- and species-specific manner; 2) fully developed prenatally in both human and rat intestine, suggesting a crucial role of apo B-48 in mammalian fetal adaptation to extrauterine life; and 3) acquired early in human fetal intestine, implying a potential role for apo B-48 in prenatal lipid metabolism.

Apolipoprotein B upstream suppressor site: identification of an element which can decrease apolipoprotein B transcription

Elevated plasma levels of apolipoprotein B (apoB) may predispose to development of premature coronary atherosclerosis. We have identified the first well localized domain of the apoB gene which can effect negative regulation of its transcription. This region binds trans-activating factors present only in apoB producing cell lines. Mutagenesis of this region causes up-regulation of its transcriptional activity. We have termed this element apoB upstream suppressor site (aBUSS) and its trans-activators the apoB repressor proteins (ARP). aBUSS and ARP may play important roles in the transcriptional modulation of apoB.

Clinical and laboratory findings in the oculocerebrorenal syndrome of Lowe, with special reference to growth and renal function

BACKGROUND

The oculocerebrorenal syndrome of Lowe is an X-linked disorder whose clinical manifestations include congenital cataracts, mental retardation, and renal tubular dysfunction. We investigated growth, renal function, and serum chemistry values in patients with the oculocerebrorenal syndrome to determine the natural history of the disorder and its heterogeneity with respect to these characteristics.

METHODS

Twenty-three patients with the oculocerebrorenal syndrome, ranging in age from 4 months to 31 years, were examined. Height was compared with bone age. Renal function was assessed by measurements of proteinuria, urinary volume, and fractional excretions of potassium, phosphate, carnitine, and amino acids. Creatinine clearance was determined as a measure of glomerular function.

RESULTS

In the oculocerebrorenal syndrome, linear growth decreases after one year of age; bone age lies between chronologic age and height age. Renal dysfunction occurs in the first year of life, characterized by proteinuria (mean [+/- SD], 1.38 +/- 0.77 g of urinary protein per square meter of body-surface area per day; normal, less than or equal to 0.10), generalized aminoaciduria (mean, 686 +/- 505 mumol of urinary amino acid per kilogram of body weight per day; normal, 94 +/- 45), carnitine wasting (mean fractional excretion, 0.10 +/- 0.05; normal, 0.03 +/- 0.01), and phosphaturia progressing into the third decade. Urinary wasting of individual amino acids is milder than in cystinosis, and branched-chain amino acids are relatively spared. Reciprocal serum creatinine levels fall linearly with age, predicting renal failure in the fourth decade. Concentrations of the muscle enzymes creatine kinase, aspartate aminotransferase, and lactate dehydrogenase, as well as of total serum protein, serum alpha 2-globulin, and high-density lipoprotein cholesterol, are elevated.

CONCLUSIONS

Renal glomerular deterioration is slowly progressive in the oculocerebrorenal syndrome. Renal tubular dysfunction begins early and persists; most patients require alkalinization therapy, and many benefit from supplemental potassium, phosphate, calcium, or carnitine. Serum enzyme elevations suggest muscle involvement in the oculocerebrorenal syndrome.

Differential tissue-specific expression of human apoA-I and apoA-II

To evaluate the sources of high density lipoprotein (HDL) particles containing only apolipoprotein A-I (apoA-I), the synthesis of apoA-I and apolipoprotein A-II (apoA-II) was examined in human liver and small intestine as well as the human intestinally derived cell line, Caco-2. Human liver contained apoA-I, apoA-II as well as apolipoprotein B (apoB) mRNA. In contrast, human adult small intestine total and polyA+ RNA had little or no apoA-II despite the presence of apoA-I and apoB. Intestinal biopsies from normal individuals failed to show de novo apoA-II protein synthesis in the media of organ cultures during [35S]methionine pulse-chase labeling, whereas apoA-I could be readily detected. Caco-2 cells contained apoA-II mRNA and secreted apoA-II protein into the tissue culture media. These data indicate that the primary site of human apoA-II synthesis is in the liver and that the small intestine secretes apoA-I-containing high density lipoproteins.

Retroviral vector-mediated in vivo expression of low-density-lipoprotein receptors in the Watanabe heritable hyperlipidemic rabbit

We have achieved in vivo expression of recombinant low-density-lipoprotein (LDL) receptors in the Watanabe heritable hyperlipidemic (WHHL) rabbit, an animal model for the human disease familial hypercholesterolemia. A retroviral vector was constructed containing the human LDL receptor cDNA and was used to stably transduce primary skin fibroblasts from WHHL rabbits. The integrity and function of the introduced LDL receptor was established by immunoprecipitation, by a fluorescent LDL binding assay, and by the ability of the transduced cells to suppress 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase activity in response to exogenous cholesterol. Autologous transduced fibroblasts were reimplanted into donor rabbits; in vivo LDL receptor expression and the survival of the transduced cells were analyzed by immunohistochemistry and by LDL binding assays performed on cells recovered from the implants. LDL receptor-bearing cells could be identified on tissue sections and recovered from implants for up to four weeks. Total and LDL cholesterol levels decreased significantly after implantation of the transduced cells; however, control experiments indicated that the decreases were not mediated through the recombinant LDL receptor. While in vivo stable expression of recombinant LDL receptors in Watanabe rabbits is possible, consequent changes in lipid levels must be interpreted with caution. This system of site-specific in vivo expression of recombinant LDL receptors permits further evaluation of the role of LDL receptor-gene replacement in the therapy of hypercholesterolemia.

Pharmacologic and surgical treatment of dyslipidemic children and adolescents

A wide variety of treatment modalities have been used in children with dyslipidemias to reduce the concentrations of atherogenic lipoprotein particles. Most of the published experience has focused upon children with familial hypercholesterolemia (FH). A variety of pharmacologic regimens have been utilized with variable degrees of success. The bile acid sequestrants colestipol and cholestyramine, lovastatin, pantethine, paraminosalicylic acid, and fenofibrate have all been successful in reducing total blood cholesterol concentrations by 18-24% in hypercholesterolemic children. Of these medications, only the bile acid sequestrants are not absorbed into the circulation. This theoretic advantage is paralled by long-term safety studies which indicate the absence of serious adverse effects with bile acid sequestrant therapy. Therefore, the bile acid sequestrants represent the drugs of choice in treating severely dyslipidemic children. In selected cases of profoundly dyslipidemic children, other therapeutic strategies have been utilized. Most of these efforts have been directed in the treatment of the child homozygous for FH. Despite the lipid lowering effects of partial ileal bypass surgery in hypercholesterolemic adults, homozygous familial hypercholesterolemic children are not adequately treated by this approach. Portacaval shunt has reduced the total cholesterol concentrations by 20-35% in homozygous FH children without having a negative impact on growth and development. These children have, however, gone on to develop atherosclerotic cardiovascular disease despite therapy. Liver transplantation has led to virtual normalization of the plasma lipoprotein concentrations in 3 children homozygous for familial hypercholesterolemia, and there is evidence for regression of vascular lesions in the coronary arteries in one of these children. However, considering the expense, the difficulty in posttransplantation management, and the irreversible nature of the therapy, liver transplantation should be reserved as the therapy of last resort for homozygous FH. The best therapy for FH homozygotes is the frequent removal of the atherogenic lipoproteins by one of the several apheresis procedures currently available. Total plasma exchange, immunoadsorption, membrane filtration, dextran sulfate adsorption, and heparin extracorporeal precipitation have all been used successfully in significantly reducing the concentrations of total and low-density lipoprotein cholesterol. Studies currently under way will more extensively evaluate the long-term safety as well as the efficacy of apheresis procedures.

Both apolipoproteins B-48 and B-100 are synthesized and secreted by the human intestine

Apolipoprotein B (apoB), an apolipoprotein associated with very low density lipoproteins and the atherogenic low density lipoproteins (LDL), directs the metabolism of lipoprotein particles in plasma by interacting with the LDL receptor. Utilizing human intestinal biopsy organ cultures, we have studied the synthesis of intestinal apoB in man. Intestinal organ cultures from normal adults (n = 6) were incubated in the presence of protease inhibitors in media supplemented with [35S]methionine. Media from these cultures were evaluated by sequential NaDodSO4 polyacrylamide gel electrophoresis, radioautography, and Western blot analyses, and intestinal biopsies were studied using immunohistochemistry. The relative abundance of apoB-100 and apoB-48 mRNA was assessed using reverse transcriptase-polymerase chain reaction followed by primer extension. Although apoB-48 was the principal isoprotein that was newly synthesized by intestinal organ cultures, apoB-100 was also synthesized and secreted by human intestinal organ cultures with 16 +/- 3% of the intestinal apoB mRNA coding for apoB-100. These results establish that apoB-100 is produced by the human intestine. The synthesis of the atherogenic apoB-100 by the intestine has pathophysiologic implications for the development of diet-induced atherosclerosis.