Physiologic and genetic evidence links hemopexin to triglycerides in mice and humans

BACKGROUND/OBJECTIVES

Elevated triglycerides predict insulin resistance and vascular disease in obesity, but how the inert triglyceride molecule is related to development of metabolic disease is unknown. To pursue novel potential mediators of triglyceride-associated metabolic disease, we used a forward genetics approach involving inbred mice and translated our findings to human subjects.

SUBJECTS/METHODS

Hemopexin (HPX) was identified as a differentially expressed gene within a quantitative trait locus associated with serum triglycerides in an F₁₆ advanced intercross between the LG/J and SM/J strains of mice. Hpx expression was evaluated in both the reproductive fat pads and livers of mice representing three strains, LG/J (n=25), SM/J (n=27) and C57Bl/6J (n=19), on high- and low-fat diets. The effect of altered Hpx expression on adipogenesis was studied in 3T3-L1 cells. Circulating HPX protein along with HPX expression were characterized in subcutaneous white adipose tissue samples obtained from a cohort of metabolically abnormal (n=18) and of metabolically normal (n=24) obese human subjects. We further examined the relationship between HPX and triglycerides in human atherosclerotic plaques (n=18).

RESULTS

HPX expression in mouse adipose tissue, but not in liver, was regulated by dietary fat regardless of genetic background. HPX increased in concert with adipogenesis in 3T3-L1 cells, and disruption of its expression impaired adipocyte differentiation. RNAseq data from the adipose tissue of obese humans showed differential expression of HPX based on metabolic disease status (P<0.05), and circulating HPX levels were correlated with serum triglycerides in these subjects (r=0.33; P=0.03). HPX was also found in an unbiased proteomic screen of human atherosclerotic plaques and shown to display differential abundance based on the extent of disease and triglyceride content (P<0.05).

CONCLUSIONS

Our findings suggest that HPX is associated with triglycerides and provide a framework for understanding mechanisms underlying lipid metabolism and metabolic disease.

Chronic activation of AMP kinase results in NRF-1 activation and mitochondrial biogenesis

The underlying mechanism by which skeletal muscle adapts to exercise training or chronic energy deprivation is largely unknown. To examine this question, rats were fed for 9 wk either with or without beta-guanadinopropionic acid (beta-GPA; 1% enriched diet), a creatine analog that is known to induce muscle adaptations similar to those induced by exercise training. Muscle phosphocreatine, ATP, and ATP/AMP ratios were all markedly decreased and led to the activation of AMP-activated protein kinase (AMPK) in the beta-GPA-fed rats compared with control rats. Under these conditions, nuclear respiratory factor-1 (NRF-1) binding activity, measured using a cDNA probe containing a sequence encoding for the promoter of delta-aminolevulinate (ALA) synthase, was increased by about eightfold in the muscle of beta-GPA-fed rats compared with the control group. Concomitantly, muscle ALA synthase mRNA and cytochrome c content were also increased. Mitochondrial density in both extensor digitorum longus and epitrochlearis from beta-GPA-fed rats was also increased by more than twofold compared with the control group. In conclusion, chronic phosphocreatine depletion during beta-GPA supplementation led to the activation of muscle AMPK that was associated with increased NRF-1 binding activity, increased cytochrome c content, and increased muscle mitochondrial density. Our data suggest that AMPK may play an important role in muscle adaptations to chronic energy stress and that it promotes mitochondrial biogenesis and expression of respiratory proteins through activation of NRF-1.

Leucine and insulin activate p70 S6 kinase through different pathways in human skeletal muscle

Amino acids and insulin have anabolic effects in skeletal muscle, but the mechanisms are poorly understood. To test the hypothesis that leucine and insulin stimulate translation initiation in human skeletal muscle by phosphorylating 70-kDa ribosomal protein S6 kinase (p70(S6k)), we infused healthy adults with leucine alone (n = 6), insulin alone (n = 6), or both leucine and insulin (n = 6) for 2 h. p70(S6k) and protein kinase B (PKB) serine(473) phosphorylation were measured in vastus lateralis muscles. Plasma leucine increased from approximately 116 to 343 micromol/l during the leucine-alone and leucine + insulin infusions. Plasma insulin increased to approximately 400 pmol/l during the insulin-alone and leucine + insulin infusions and was unchanged during the leucine-alone infusion. Phosphorylation of p70(S6k) increased 4-fold in response to leucine alone, 8-fold in response to insulin alone, and 18-fold after the leucine + insulin infusion. Insulin-alone and leucine + insulin infusions increased PKB phosphorylation, but leucine alone had no effect. These results show that physiological concentrations of leucine and insulin activate a key mediator of protein synthesis in human skeletal muscle. They suggest that leucine stimulates protein synthesis through a nutrient signaling mechanism independent of insulin, raising the possibility that administration of branched-chain amino acids may improve protein synthesis in insulin-resistant states.

PPARalpha deficiency reduces insulin resistance and atherosclerosis in apoE-null mice

PPARalpha is a ligand-dependent transcription factor expressed at high levels in the liver. Its activation by the drug gemfibrozil reduces clinical events in humans with established atherosclerosis, but the underlying mechanisms are incompletely defined. To clarify the role of PPARalpha in vascular disease, we crossed PPARalpha-null mice with apoE-null mice to determine if the genetic absence of PPARalpha affects vascular disease in a robust atherosclerosis model. On a high-fat diet, concentrations of atherogenic lipoproteins were higher in PPARalpha(-/-)apoE(-/-) than in PPARalpha(+/+)apoE(-/-) mice, due to increased VLDL production. However, en face atherosclerotic lesion areas at the aortic arch, thoracic aorta, and abdominal aorta were less in PPARalpha-null animals of both sexes after 6 and 10 weeks of high-fat feeding. Despite gaining as much or more weight than their PPARalpha(+/+)apoE(-/-) littermates, PPARalpha(-/-)apoE(-/-) mice had lower fasting levels of glucose and insulin. PPARalpha-null animals had greater suppression of endogenous glucose production in hyperinsulinemic clamp experiments, reflecting less insulin resistance in the absence of PPARalpha. PPARalpha(-/-)apoE(-/-) mice also had lower blood pressures than their PPARalpha(+/+)apoE(-/-) littermates after high-fat feeding. These results suggest that PPARalpha may participate in the pathogenesis of diet-induced insulin resistance and atherosclerosis.

Glucose and insulin stimulate heparin-releasable lipoprotein lipase activity in mouse islets and INS-1 cells. A potential link between insulin resistance and beta-cell dysfunction

Lipoprotein lipase (LpL) provides tissues with triglyceride-derived fatty acids. Fatty acids affect beta-cell function, and LpL overexpression decreases insulin secretion in cell lines, but whether LpL is regulated in beta-cells is unknown. To test the hypothesis that glucose and insulin regulate LpL activity in beta-cells, we studied pancreatic islets and INS-1 cells. Acute exposure of beta-cells to physiological concentrations of glucose stimulated both total cellular LpL activity and heparin-releasable LpL activity. Glucose had no effect on total LpL protein mass but instead promoted the appearance of LpL protein in a heparin-releasable fraction, suggesting that glucose stimulates the translocation of LpL from intracellular to extracellular sites in beta-cells. The induction of heparin-releasable LpL activity was unaffected by treatment with diazoxide, an inhibitor of insulin exocytosis that does not alter glucose metabolism but was blocked by conditions that inhibit glucose metabolism. In vitro hyperinsulinemia had no effect on LpL activity in the presence of low concentrations of glucose but increased LpL activity in the presence of 20 mm glucose. Using dual-laser confocal microscopy, we detected intracellular LpL in vesicles distinct from those containing insulin. LpL was also detected at the cell surface and was displaced from this site by heparin in dispersed islets and INS-1 cells. These results show that glucose metabolism controls the trafficking of LpL activity in beta-cells independent of insulin secretion. They suggest that hyperglycemia and hyperinsulinemia associated with insulin resistance may contribute to progressive beta-cell dysfunction by increasing LpL-mediated delivery of lipid to islets.

Resistance exercise decreases skeletal muscle tumor necrosis factor alpha in frail elderly humans

Skeletal muscle protein and function decline with advancing age but the underlying pathophysiology is poorly understood. To test the hypothesis that the catabolic cytokine tumor necrosis factor alpha (TNF-alpha) contributes to this process, we studied the effects of aging and resistance exercise on TNF-alpha expression in human muscle. Using in situ hybridization, TNF-alpha message was localized to myocytes in sections of skeletal muscle from elderly humans. Both TNF-alpha mRNA and protein levels were elevated in skeletal muscle from frail elderly (81+/-1 year) as compared to healthy young (23+/-1 year) men and women. To determine whether resistance exercise affects TNF-alpha expression, frail elderly men and women were randomly assigned to a training group or to a nonexercising control group. Muscle biopsies were performed before and after 3 months. Muscle TNF-alpha mRNA and protein levels decreased in the exercise group but did not change in the control group. Muscle protein synthesis rate in the exercise group was inversely related to levels of TNF-alpha protein. These data suggest that TNF-alpha contributes to age-associated muscle wasting and that resistance exercise may attenuate this process by suppressing skeletal muscle TNF-alpha expression.

Skeletal muscle respiratory uncoupling prevents diet-induced obesity and insulin resistance in mice

To determine whether uncoupling respiration from oxidative phosphorylation in skeletal muscle is a suitable treatment for obesity and type 2 diabetes, we generated transgenic mice expressing the mitochondrial uncoupling protein (Ucp) in skeletal muscle. Skeletal muscle oxygen consumption was 98% higher in Ucp-L mice (with low expression) and 246% higher in Ucp-H mice (with high expression) than in wild-type mice. Ucp mice fed a chow diet had the same food intake as wild-type mice, but weighed less and had lower levels of glucose and triglycerides and better glucose tolerance than did control mice. Ucp-L mice were resistant to obesity induced by two different high-fat diets. Ucp-L mice fed a high-fat diet had less adiposity, lower levels of glucose, insulin and cholesterol, and an increased metabolic rate at rest and with exercise. They were also more responsive to insulin, and had enhanced glucose transport in skeletal muscle in the setting of increased muscle triglyceride content. These data suggest that manipulating respiratory uncoupling in muscle is a viable treatment for obesity and its metabolic sequelae.

Macrophage lipoprotein lipase promotes foam cell formation and atherosclerosis in low density lipoprotein receptor-deficient mice

The role of macrophage lipoprotein lipase (LPL) expression in atherosclerotic lesion formation was examined in low density lipoprotein receptor (LDLR(-/-)) mice using dietary conditions designed to induce either fatty streak lesions or complex atherosclerotic lesions. First, LDLR(-/-) mice chimeric for macrophage LPL expression were created by transplantation of lethally irradiated female LDLR(-/-) mice with LPL(-/-) (n = 12) or LPL(+/+) (n = 14) fetal liver cells as a source of hematopoietic cells. To induce fatty streak lesions, these mice were fed a Western diet for 8 weeks, resulting in severe hypercholesterolemia. There were no differences in plasma post-heparin LPL activity, serum lipid levels, or lipoprotein distribution between these two groups. The mean lesion area in the proximal aorta in LPL(-/-) --> LDLR(-/-) mice was significantly reduced by 33% compared with LPL(+/+) --> LDLR(-/-) mice, and a similar reduction (38%) in lesion area was found by en face analysis of the aortae. To induce complex atherosclerotic lesions, female LDLR(-/-) mice were lethally irradiated, transplanted with LPL(-/-) (n = 14), LPL(+/-) (n = 13), or LPL(+/+) (n = 14) fetal liver cells, and fed the Western diet for 19 weeks. Serum cholesterol and triglyceride levels did not differ between the three groups. After 19 weeks of diet, the lesions in the proximal aorta were complex with relatively few macrophages expressing LPL protein and mRNA in LPL(+/+) --> LDLR(-/-) mice. Analysis of cross-sections of the proximal aorta demonstrated no differences in the extent of lesion area between the groups, whereas en face analysis of the aortae revealed a dose-dependent effect of macrophage LPL on mean aortic lesion area in LPL(-/-) --> LDLR(-/-), LPL(-/+) --> LDLR(-/-), and LPL(+/+) --> LDLR(-/-) mice (1.8 +/- 0. 2%, 3.5 +/- 0.5% and 5.9 +/- 0.8%, respectively). Taken together, these data indicate that macrophage LPL expression in the artery wall promotes atherogenesis during foam cell lesion formation, but this impact may be limited to macrophage-rich lesions.

Exercise induces lipoprotein lipase and GLUT-4 protein in muscle independent of adrenergic-receptor signaling

Exercise increases the expression of lipoprotein lipase (LPL) and GLUT-4 in skeletal muscle. Intense exercise increases catecholamines, and catecholamines without exercise can affect the expression of both LPL and GLUT-4. To test the hypothesis that adrenergic-receptor signaling is central to the induction of LPL and GLUT-4 by exercise, six untrained individuals [age 28 +/- 4 (SD) yr, peak oxygen uptake 3.6 +/- 0.3 l/min] performed two exercise bouts within 12 days. Exercise consisted of cycling at approximately 65% peak oxygen uptake for 60 min with (block trial) and without (control trial) adrenergic-receptor blockade. Exercise intensity was the same during the block and control trials. Plasma catecholamine concentrations were significantly higher and heart rates were significantly lower during the block trial compared with the control trial, consistent with known effects of adrenergic-receptor blockade. However, blockade did not prevent the induction of either LPL or GLUT-4 proteins assayed in biopsies of skeletal muscle. LPL was significantly increased by 170-240% and GLUT-4 was significantly increased by 32-51% at 22 h after exercise compared with before exercise during both the control and block trials. These findings provide evidence that exercise increases muscle LPL and GLUT-4 protein content via signals generated by alterations in cellular homeostasis and not by adrenergic-receptor stimulation.

The role of osteoprogenitors in vascular calcification

Calcification is a component of vascular disease that usually occurs in concert with atheroma formation but through distinct pathophysiological processes. Vessel wall osteoprogenitor cells known as calcifying vascular cells can form bone matrix proteins and calcified nodules, analogous to osteoblastic differentiation in bone. These cells have been isolated from the tunica media of bovine and human arteries, and both in-vitro tissue culture models and mouse models of vascular calcification have been established. Studies of the effects of diabetes mellitus, hyperlipidemia, estrogens and glucocorticoids on calcifying vascular cell function provide insight into the relationship between common human disease states and vascular calcification.

Relative hypoglycemia and hyperinsulinemia in mice with heterozygous lipoprotein lipase (LPL) deficiency. Islet LPL regulates insulin secretion

Lipoprotein lipase (LPL) provides tissues with fatty acids, which have complex effects on glucose utilization and insulin secretion. To determine if LPL has direct effects on glucose metabolism, we studied mice with heterozygous LPL deficiency (LPL+/-). LPL+/- mice had mean fasting glucose values that were up to 39 mg/dl lower than LPL+/+ littermates. Despite having lower glucose levels, LPL+/- mice had fasting insulin levels that were twice those of +/+ mice. Hyperinsulinemic clamp experiments showed no effect of genotype on basal or insulin-stimulated glucose utilization. LPL message was detected in mouse islets, INS-1 cells (a rat insulinoma cell line), and human islets. LPL enzyme activity was detected in the media from both mouse and human islets incubated in vitro. In mice, +/- islets expressed half the enzyme activity of +/+ islets. Islets isolated from +/+ mice secreted less insulin in vitro than +/- and -/- islets, suggesting that LPL suppresses insulin secretion. To test this notion directly, LPL enzyme activity was manipulated in INS-1 cells. INS-1 cells treated with an adeno-associated virus expressing human LPL had more LPL enzyme activity and secreted less insulin than adeno-associated virus-beta-galactosidase-treated cells. INS-1 cells transfected with an antisense LPL oligonucleotide had less LPL enzyme activity and secreted more insulin than cells transfected with a control oligonucleotide. These data suggest that islet LPL is a novel regulator of insulin secretion. They further suggest that genetically determined levels of LPL play a role in establishing glucose levels in mice.

Respiratory uncoupling induces delta-aminolevulinate synthase expression through a nuclear respiratory factor-1-dependent mechanism in HeLa cells

Nuclear respiratory factor (NRF)-1 appears to be important for the expression of several respiratory genes, but there is no direct evidence that NRF-1 transduces a physiological signal into the production of an enzyme critical for mitochondrial biogenesis. We generated HeLa cells containing plasmids allowing doxycycline-inducible expression of uncoupling protein (UCP)-1. In the absence of doxycycline, UCP-1 mRNA and protein were undetectable. In the presence of doxycycline, UCP-1 was expressed and oxygen consumption doubled. This rise in oxygen consumption was associated with an increase in NRF-1 mRNA. It was also associated with an increase in NRF-1 protein binding activity as determined by electrophoretic mobility shift assay using a functional NRF-1 binding site from the delta-aminolevulinate (ALA) synthase promoter. Respiratory uncoupling also caused a time-dependent increase in protein levels of ALA synthase, an early marker for mitochondrial biogenesis. ALA synthase induction by respiratory uncoupling was prevented by transfecting cells with an oligonucleotide antisense to the region of the NRF-1 initiation codon; a scrambled oligonucleotide with the same base composition had no effect. Respiratory uncoupling increases oxygen consumption and lowers energy reserves. In HeLa cells, uncoupling also increases ALA synthase, an enzyme critical for mitochondrial respiration, but only if translatable mRNA for NRF-1 is available. These data suggest that the transcription factor NRF-1 plays a key role in cellular adaptation to energy demands by translating physiological signals into an increased capacity for generating energy.

Macrophage lipoprotein lipase promotes foam cell formation and atherosclerosis in vivo

Expression of lipoprotein lipase (LPL) by the macrophage has been proposed to promote foam cell formation and atherosclerosis, primarily on the basis of in vitro studies. LPL-deficient mice might provide a model for testing the role of LPL secretion by the macrophage in an in vivo system. Unfortunately, homozygous deficiency of LPL in the mouse is lethal shortly after birth. Because the fetal liver is the major site of hematopoiesis in the developing fetus, transplantation of C57BL/6 mice with LPL-/- fetal liver cells (FLCs) was used to investigate the physiologic role of macrophage LPL expression in vivo. Thirty-four female C57BL/6 mice were lethally irradiated and reconstituted with FLCs from day 14 LPL+/+, LPL+/-, and LPL-/- donors. No significant differences were detected in plasma levels of post-heparin LPL activity or in serum cholesterol or triglyceride levels between the 3 groups on either a chow diet or an atherogenic diet. After 19 weeks on the atherogenic diet, aortae were collected for quantitative analysis of the extent of aortic atherosclerosis. LPL expression was detected by immunocytochemistry and in situ hybridization in macrophages of aortic atherosclerotic lesions of LPL+/+-->C57BL/6 and LPL+/--->C57BL/6 mice, but not in LPL-/--->C57BL/6 mice, whereas myocardial cells expressed LPL in all groups. The mean aortic lesion area was reduced by 55% in LPL-/--->C57BL/6 mice compared with LPL+/+-->C57BL/6 mice and by 45% compared with LPL+/--->C57BL/6 mice, respectively. These data demonstrate in vivo that LPL expression by macrophages in the artery wall promotes foam cell formation and atherosclerosis. off

Diet-induced diabetes activates an osteogenic gene regulatory program in the aortas of low density lipoprotein receptor-deficient mice

Vascular calcification is common in people with diabetes and its presence predicts premature mortality. To clarify the underlying mechanisms, we used low density lipoprotein receptor-deficient (LDLR -/-) mice to study vascular calcification in the ascending aorta. LDLR -/- mice on a chow diet did not develop obesity, diabetes, atheroma, or vascular calcification. In contrast, LDLR -/- mice on high fat diets containing cholesterol developed obesity, severe hyperlipidemia, hyperinsulinemic diabetes, and aortic atheroma. A high fat diet without cholesterol also induced obesity and diabetes, but caused only moderate hyperlipidemia and did not result in significant aortic atheroma formation. Regardless of cholesterol content, high fat diets induced mineralization of the proximal aorta (assessed by von Kossa staining) and promoted aortic expression of Msx2 and Msx1, genes encoding homeodomain transcription factors that regulate mineralization and osseous differentiation programs in the developing skull. Osteopontin (Opn), an osteoblast matrix protein gene also expressed by activated macrophages, was up-regulated in the aorta by these high fat diets. In situ hybridization showed that peri-aortic adventitial cells in high fat-fed mice express Msx2. Opn was also detected in this adventitial cell population, but in addition was expressed by aortic vascular smooth muscle cells and macrophages of the intimal atheroma. High fat diets associated with hyperinsulinemic diabetes activate an aortic osteoblast transcriptional regulatory program that is independent of intimal atheroma formation. The spatial pattern of Msx2 and Opn gene expression strongly suggests that vascular calcification, thought to be limited to the media, is an active process that can originate from an osteoprogenitor cell population in the adventitia.

Skeletal muscle lipoprotein lipase: molecular regulation and physiological effects in relation to exercise

LPL directs the body wide distribution of fatty acids derived from circulating triglycerides. This is accomplished by tissue-specific regulation. In adipose tissue, LPLA per gram is higher than in muscle tissue. Eating increases adipose tissue LPLA and may increase blood flow. Exercise greatly increases SM blood flow and LPLA over a longer time frame as compared to the effect of eating on adipose tissue LPLA. The regulation of LPLA occurs at several levels and is better understood in adipose tissue models. In muscle, the study of regulation has been neglected. LPL expression in muscle may be more complex than in adipose tissue owing to the changes in blood flow and metabolism associated with contractile activity, as well as to other factors intrinsic to contraction, such as electrical events and cellular deformation. Sixty to 90 minutes of continuous leg exercise at 60% of VO2 max induces muscle LPL expression, increases LPL mRNA in humans with 4 hours of exercise, and raises immunoreactive mass by 8 hours post-exercise. Within 24 hours, both LPL and mRNA and mass have returned to normal levels. Increased muscle LPL mass following exercise may serve to replenish intramyofibral stores of triglyceride, which are depleted with endurance exercise and are greater in aerobically-trained individuals as compared to untrained individuals. The post-exercise increase in muscle LPL mass coincides with the post-exercise acute fall in circulating triglycerides typically observed in subjects capable of exercising for 60-90 minutes at 60% of VO2 max. The low fasting triglyceride levels often seen in highly trained individuals are due in part to their high levels of muscle LPLA. Both the physiological mediator and the molecular mediator of the exercised-induced induction of muscle LPL expression are known. Hopefully, the next decade will see careful studies aimed at better defining the molecular physiology of LPL expression in muscle.

Effects of heterozygous lipoprotein lipase deficiency on diet-induced atherosclerosis in mice

Heterozygous lipoprotein lipase deficiency (LPL+/-) is common and has been implicated in premature atherosclerosis in epidemiologic studies. However, in vitro data suggest that LPL deficiency in the vascular wall may be antiatherogenic. To address the role of LPL in atherosclerosis, LPL+/- mice in the C57BL/6J background were fed an atherogenic diet for 8 months. LPL+/- mice were more dyslipidemic than +/+ animals due to increased concentrations of non-HDL lipoproteins. There was no difference in aortic origin atherosclerosis between LPL+/- (n=56) and +/+ (n=55) mice. LPL+/- mice in the low density lipoprotein receptor knockout (LDLR-/-) background were fed the same atherogenic diet for 3 months. LPL+/-LDLR-/- mice were more dyslipidemic than LPL+/+LDLR-/- animals. There was no difference in atherosclerosis assayed for the entire aorta and no difference in aortic sterol content between LPL+/-LDLR-/- (n=28) and LPL+/+LDLR-/- (n=15) mice. LPL protein was detected in murine lesions in a consistent layered pattern. More luminal, lipid-laden macrophages generally did not stain for LPL, but deeper, lipid-poor macrophages as well as necrotic core regions contained immunoreactive LPL. LPL protein was more abundant in lesions from LPL+/+ LDLR-/- than LPL+/-LDLR-/- mice. After eating an atherogenic diet, LPL+/- as compared to LPL+/+ mice have more dyslipidemia, but no more atherosclerosis, and less LPL protein in atherosclerotic lesions. These data suggest that lipoprotein lipase deficiency in the vascular wall could prevent the retention of atherogenic lipoproteins.

Properties and purification of a glucose-inducible human fatty acid synthase mRNA-binding protein

Glucose stabilizes the mRNA for human fatty acid synthase (FAS), an enzyme relevant to diverse human disorders, including hyperlipidemia, obesity, and malignancy. To determine the underlying mechanisms, RNA gel mobility shift assays were used to demonstrate that human Hep G2 cells contain a cytoplasmic factor that binds specifically to the 3'-terminus of the human FAS mRNA. D-Glucose increased RNA-binding activity by 2.02-fold (P = 0.0033), with activity peaking 3 h after glucose feeding. Boiling or treatment of extracts with proteinase K abolished binding. Ultraviolet cross-linking of the FAS mRNA-binding factor followed by SDS-PAGE resolved a proteinase K-sensitive band with an apparent molecular mass of 178 +/- 7 kDa. The protein was purified to homogeneity using nondenaturing polyacrylamide gels as an affinity matrix. Acid phosphatase treatment of the protein prevented binding to the FAS mRNA, but binding activity was unaffected by modification of sulfhydryl groups and was not Mg2+ or Ca2+ dependent. Deletion and RNase T1 mapping localized the binding site of the protein to 37 nucleotides characterized by the repetitive motif ACCCC and found within the first 65 bases of the 3'-UTR. Hybridization of the FAS transcript with an oligonucleotide antisense to this sequence abolished binding. These findings indicate that a 178-kDa glucose-inducible phosphoprotein binds to an (ACCCC)n-containing sequence in the 3'-UTR of the FAS mRNA within the same time frame that glucose stabilizes the FAS message. This protein may participate in the posttranscriptional control of FAS gene expression.

Correction of hypertriglyceridemia and impaired fat tolerance in lipoprotein lipase-deficient mice by adenovirus-mediated expression of human lipoprotein lipase

Humans homozygous or heterozygous for mutations in the lipoprotein lipase (LPL) gene demonstrate significant disturbances in plasma lipoproteins, including raised triglyceride (TG) and reduced HDL cholesterol levels. In this study we explored the feasibility of adenovirus-mediated gene replacement therapy for LPL deficiency. A total of 5 x 10(9) plaque-forming units (pfu) of an E1/E3-deleted adenovirus expressing either human LPL (Ad-LPL) or the bacterial beta-galactosidase gene (Ad-LacZ) as a control were administered to mice heterozygous for targeted disruption in the LPL gene (n = 57). Peak expression of total postheparin plasma LPL activity was observed at day 7 in Ad-LPL mice versus Ad-LacZ controls (834 +/- 133 vs 313 +/- 89 mU/mL, P < .01), and correlated with human-specific LPL activity (522 +/- 219 mU/mL) and mass (9214 +/- 782 ng/mL), a change that was significant to 14 and 42 days, respectively. At day 7, plasma TGs were significantly reduced relative to Ad-LacZ mice (0.17 +/- 0.07 vs 1.90 +/- 0.89 mmol/L, P < .01) but returned to endogenous levels by day 42. Ectopic liver expression of human LPL was confirmed by in situ hybridization analysis and from raised LPL activity and mass in liver homogenates. Analysis of plasma lipoprotein composition revealed a marked decrease in VLDL-derived TGs. Severely impaired oral and intravenous fat-load tolerance in LPL-deficient mice was subsequently corrected after Ad-LPL administration and closely paralleled that observed in wild-type mice. These findings suggest that liver-targeted adenovirus-mediated LPL gene transfer offers an effective means for transient correction of altered lipoprotein metabolism and impaired fat tolerance due to LPL deficiency.

The mystery of diabetes and atherosclerosis: time for a new plot

Most patients with diabetes die from macrovascular complications. Little is known about the pathogenesis of diabetic vascular disease, but recent advances in molecular genetics and oxidation chemistry provide clues to the mystery of diabetes and atherosclerosis. Genetic variants of well-known proteins such as lipoprotein lipase and apolipoprotein E are common. These proteins are suitable candidates for mediating diabetic vascular risk because their variants can produce hypertriglyceridemia, a risk factor for atherosclerosis in diabetes. However, mutations could have different effects on lipoprotein flux across arteries depending on whether expression is dominant in the vascular space or the vascular wall. Lipoproteins retained in the arterial wall are subject to oxidative modification, which could be dependent on glycoxidation, the enzyme myeloperoxidase, or reactive nitrogen species derived from nitric oxide. Accelerated vascular disease in diabetes is likely the result of complex interactions between metabolic derangements such as hyperglycemia, mutations in genes controlling lipid metabolism, and antioxidant defense mechanisms.

Induction of human skeletal muscle lipoprotein lipase gene expression by short-term exercise is transient

Exercise increases skeletal muscle lipoprotein lipase (LPL) expression, but the time course of this response is not known. In the present study, we examined the time course of the LPL response to both short-term and acute exercise and measured circulating levels of putative regulators of muscle LPL. Nine adults underwent short-term exercise training (60-90 min of stationary cycling at 55-70% of leg ergometer peak oxygen uptake on 5 consecutive days). Five vastus lateralis biopsies were performed: before training, 20 h after the fourth bout (immediately before the 5th bout), and 0.2, 4, and 8 h after the fifth bout. After four bouts of exercise in 4 days, there was no increase in LPL mass or LPL mRNA exactly 20 h after the fourth bout. However, when tissues were sampled closer to the exercise bout on the 5th day, transient increases were seen. On day 5, LPL mRNA increased by 127% (P < 0.05) at 4 h postexercise and was followed by an increase in LPL mass of 93% (P < 0.05) at 8 h postexercise. Serum triglycerides decreased by 23% (P < 0.05) during the protocol. Two nonexercising subjects showed no consistent change in LPL mRNA or mass. Acute exercise transiently increased levels of norepinephrine (9-fold) and epinephrine (5-fold) and reduced insulin levels. Acute exercise preceded by four daily bouts of exercise induces a transient rise in LPL mRNA followed by rise in LPL mass, suggesting that these responses are temporally related. This induction of LPL gene expression may result from dynamic changes in serum catecholamines, plasma insulin, or events intrinsic to muscle contraction itself.

Essential amino acids regulate fatty acid synthase expression through an uncharged transfer RNA-dependent mechanism

To better understand the regulation of gene expression by amino acids, we studied the effects of these macronutrients on fatty acid synthase (FAS), an enzyme crucial for energy storage. When HepG2 cells were fed serum-free media selectively deficient in each amino acid, the omission of any single classic essential amino acid as well as Arg or His (essential in some rapidly growing cells) resulted in FAS mRNA levels that were about half of those in complete medium. Control message levels were unaffected and omission of nonessential amino acids did not alter FAS expression. FAS mRNA levels peaked 12-16 h after feeding complete and Ser (nonessential)-deficient media but did not increase in cells fed Lys (essential)-deficient medium. With Lys, FAS mRNA increased over the physiologic concentration range of 15-150 microM, and low concentrations of lysine decreased FAS but not apoB protein mass. Transcription inhibitors mimicked treatment with Lys-deficient media, and nuclear run-off assays showed that Lys-deficient media abolished FAS but not apoB transcription. After treatment with Lys-deficient media, the intracellular Lys pool was rapidly depleted in association with an increase of uncharged (deacylated) tRNA Lys from < 1 to 64% of available tRNA Lys. Even in the presence of the essential amino acid His, increasing the level of uncharged tRNA His with histidinol, a competitive inhibitor of the histidinyl-tRNA synthetase, blocked FAS expression. Tyrosinol treatment did not alter FAS mRNA levels. These results suggest that essential amino acids regulate FAS expression by altering uncharged tRNA levels, a novel mechanism for nutrient control of gene expression in mammalian cells.

Human fatty acid synthase mRNA: tissue distribution, genetic mapping, and kinetics of decay after glucose deprivation

To better understand the accelerated decay of fatty acid synthase (FAS) message that occurs after glucose deprivation (J. Biol. Chem. 1993. 268: 6961-6970), we characterized the 3' terminus of the human message and the kinetics of FAS mRNA decay in HepG2 cells. The FAS gene was localized to human chromosome 17q24-25 and to syntenic distal mouse chromosome 11. Expression of the FAS message in human tissues was ubiquitous with high levels in liver, lung, and intra-abdominal adipose tissue. The 806 nucleotide 3' untranslated region of the human mRNA contained two regions with the instability pentamer AUUUA. Unlike short-lived messages containing AUUUA motifs, FAS mRNA decay after glucose deprivation was not first order, and there were no detectable changes in the poly(A) tail. Glucose deprivation transiently caused FAS message to sediment more rapidly than control message in density gradients. In vivo treatment with different translational inhibitors showed that translation per se was not necessary for FAS mRNA decay; association of polysomes with FAS message protected it from decay. In cell-free decay experiments, FAS mRNA decay was more rapid using components from glucose-deprived than glucose-treated cells. These data suggest that glucose regulates cytoplasmic HepG2 FAS mRNA stability by partitioning the message between a translated pool not subject to degradation and a decay compartment, features reminiscent of regulated stability for other diet-responsive messages.

COOH-terminal disruption of lipoprotein lipase in mice is lethal in homozygotes, but heterozygotes have elevated triglycerides and impaired enzyme activity

The role of the enzyme lipoprotein lipase (LPL) in atherosclerosis is uncertain. To generate an animal model of LPL deficiency, we targeted the LPL gene in embryonic stem cells with a vector designed to disrupt the COOH terminus of the protein and used these cells to generate LPL-deficient mice. Germ line transmission of the disrupted LPL allele was achieved with two chimeric males, and offspring from each of these animals were phenotypically identical. Pups homozygous (-/-) for LPL deficiency died within 48 h of birth with extreme elevations of serum triglycerides (13,327 mg/dl) associated with essentially absent LPL enzyme activity in heart and carcass. Newborn heterozygous (+/-) LPL-deficient pups had lower LPL enzyme activity and higher triglycerides (370 versus 121 mg/dl) than wild type (+/+) littermates. Adult heterozygotes had higher triglycerides than wild type mice with ad libitum feeding (236 mg/dl for +/- versus 88 mg/dl for +/+) and after fasting for 4 h (98 mg/dl for +/- versus 51 for +/+) or 12 h (109 mg/dl for +/- versus 56 mg/dl for +/+). Triglycerides were present as very low density lipoprotein particles and chylomicrons, but high density lipoprotein cholesterol levels were not decreased in +/- animals. Plasma heparin-releasable LPL activity was 43% lower in +/- versus +/+ adult animals. LPL activity, mRNA, and protein were lower in the tissues of +/- versus +/+ mice. Homozygous LPL deficiency caused by disruption of the COOH terminus of the enzyme is lethal in mice. Heterozygous LPL deficiency caused by this mutation is associated with mild to moderate hypertriglyceridemia without affecting static HDL cholesterol levels. Heterozygous LPL-deficient mice could be useful for determining if hypertriglyceridemia, independently or in combination with other discrete defects, influences atherosclerosis.

Exercise induces human lipoprotein lipase gene expression in skeletal muscle but not adipose tissue

Lipoprotein lipase (LPL) is regulated by exercise in humans, but the effects of exercise on LPL expression in different tissues and the molecular mechanisms involved are unclear. We assessed the effects of 5-13 consecutive days of supervised exercise on tissue LPL expression as well as fasting plasma lipids and lipoproteins in 32 sedentary, weight-stable adult men. In skeletal muscle, exercise training increased the mean LPL mRNA level by 117% (P = 0.037), LPL protein mass by 53% (P = 0.038), and total LPL enzyme activity by 35% (P = 0.025). In adipose tissue, mean LPL mRNA, protein mass, and activity did not change. Exercise decreased triglycerides [from 172 +/- 4.3 to 127 +/- 3.2 (SE) mg/dl, P = 0.002], total cholesterol (from 188 +/- 1.2 to 181 +/- 1.0 mg/dl, P = 0.011), and very low-density lipoprotein-cholesterol (from 30.1 +/- 0.9 to 22.0 +/- 0.8, P = 0.004) and increased high-density lipoprotein cholesterol (HDL-C; from 43.4 +/- 0.35 to 45.0 +/- 0.37, P = 0.030) and HDL2-C (from 6.6 +/- 0.21 to 7.7 +/- 0.19, P = 0.021). Changes in muscle but not adipose tissue heparin-releasable LPL activity were inversely correlated (r = -0.435, P < 0.034) with changes in triglycerides. These data suggest the existence of an exercise stimulus intrinsic to skeletal muscle, which raises LPL activity in part by pretranslational mechanisms, a process that contributes to the improvement in circulating lipids seen with physical activity.

Increased low density lipoprotein receptor expression mediated through the insulin-like growth factor-I receptor in cultured fibroblasts

Plasma insulin-like growth factor-I (IGF-I) levels are inversely correlated with apolipoprotein B and low density lipoprotein (LDL) cholesterol in humans. To identify a molecular basis for this observation, the effects of IGF-I on LDL receptor expression in fibroblasts were studied. IGF-I increased LDL receptors in cultured human skin fibroblasts at concentrations greater than 25 ng/ml. However, IGF-I effects were not easily quantitated due to secretion of inhibitory IGF-binding proteins by the cells. To circumvent this difficulty, QAYL, an IGF-I analog that binds to the IGF-I receptor but not to IGF-binding proteins, was used. QAYL increased LDL receptor number 56-72% with half-maximum effect at 0.6 ng/ml. alpha-IR3, a monoclonal antibody directed toward the IGF-I receptor, blocked this effect. QAYL treatment increased synthesis of LDL receptor protein without increasing LDL receptor mRNA levels or altering protein stability. Both QAYL and IGF-I increased LDL receptors prominently in cells that had been treated with physiological amounts of LDL cholesterol. IGF-I, acting through the IGF-I receptor and modulated by IGF-binding proteins, may contribute to the regulation of LDL metabolism by increasing translation of LDL receptor message.

Exercise induces rapid increases in GLUT4 expression, glucose transport capacity, and insulin-stimulated glycogen storage in muscle

GLUT4 glucose transporter content and glucose transport capacity are closely correlated in skeletal muscle. In this study, we tested the hypothesis that a rapid increase in GLUT4 expression occurs as part of the early adaptive response of muscle to exercise and serves to enhance glycogen storage. Rats exercised by swimming had a approximately 2-fold increase in GLUT4 mRNA and a 50% increase in GLUT4 protein expression in epitrochlearis muscle 16 h after one prolonged exercise session. After a 2nd day of exercise, muscle GLUT4 protein was increased further to approximately 2-fold while there was no additional increase in GLUT4 mRNA. Muscle hexokinase activity also doubled in response to 2 days of exercise. Glucose transport activity maximally stimulated with insulin, contractions, or hypoxia was increased roughly in proportion to the adaptive increase in GLUT4 protein in epitrochlearis muscles. Treatment with insulin prior to subcellular fractionation of muscle resulted in a approximately 2-fold greater increase in GLUT4 content of a plasma membrane fraction in the 2-day swimmers than in controls. When epitrochlearis muscles were incubated with glucose and insulin, glycogen accumulation over 3 h was twice as great in muscles from 2-day swimmers as in control muscles. Our results show that a rapid increase in GLUT4 expression is an early adaptive response of muscle to exercise. This adaptation appears to be mediated by pretranslational mechanisms. We hypothesize that the physiological role of this adaptation is to enhance replenishment of muscle glycogen stores.

Exercise training decreases plasma cholesteryl ester transfer protein

To assess the effect of exercise on the plasma concentration of cholesterol ester transfer protein (CETP) and its possible influence in mediating the exercise-associated redistribution of cholesterol among plasma lipoproteins, we measured plasma CETP in 57 healthy normolipidemic men and women before and after 9 to 12 months of exercise training. The training protocol resulted in significant changes in VO2max (mean +/- SD, +5.3 +/- 3.5 mL.kg-1 x min-1), body weight (-2.5 +/- 3.5 kg), plasma triglycerides (-25.7 +/- 36.3 mg/dL), high-density lipoprotein cholesterol (HDL-C) (+2.6 +/- 6.2 mg/dL), and ratios of total cholesterol to HDL-C (-0.30 +/- 0.52) and low-density lipoprotein cholesterol (LDL-C) to HDL-C (-0.18 +/- 0.45) (all P < or = .05) but no change in lipoprotein(a). CETP concentration (in milligrams per liter) fell significantly in response to training in both men (n = 28, 2.47 +/- 0.66 to 2.12 +/- 0.43; % delta = 14.2%; P < .005) and women (n = 29, 2.72 +/- 1.01 to 2.36 +/- 0.76; % delta = 13.2%; P < .047). The CETP change was observed both in subjects who lost weight (n = 28, delta mean weight = -5.0 kg; delta CETP = -0.42 +/- 0.79; % delta = 15.4%; P < .009) and in those who were weight stable (n = 29, delta mean weight = -0.12 kg; delta CETP = -0.29 +/- 0.78; % delta = 10.4%; P < .055).(ABSTRACT TRUNCATED AT 250 WORDS)

Physiologic concentrations of glucose regulate fatty acid synthase activity in HepG2 cells by mediating fatty acid synthase mRNA stability

Carbohydrate feeding of animals results in striking increases in hepatic fatty acid synthesis but much of this induction is presumed to be hormone mediated. To clarify the mechanisms responsible for the specific effect of carbohydrate on fatty acid synthesis, the effects of D-glucose on the expression of human fatty acid synthase (FAS) in HepG2 cells cultured in serum-free medium were studied. Northern blots of total RNA from these cells showed a single FAS mRNA band of 9.3 kilobases that was increased 2.7-5.4-fold in the presence of D-glucose. Lactate and citrate but not L-glucose mimicked this effect. Glucose induction of FAS mRNA was time- and concentration-dependent and most of the increase in FAS message was detected within the range of physiologic glucose concentrations. Glucose induction of FAS mRNA, protein synthetic rate, and enzyme activity were similar suggesting that glucose regulates FAS expression at a pretranslational level in this system. Transcription run-off experiments showed that glucose feeding was associated with no change in the FAS transcription initiation rate despite a 5-fold increase in FAS mRNA. FAS mRNA stability as determined by actinomycin D chase was 7-fold greater in the presence of glucose. Differences between FAS mRNA levels in cells grown in the presence versus the absence of glucose were inhibited by cycloheximide but not puromycin, suggesting that glucose regulation of FAS mRNA stability is not dependent on translation. Glucose, at physiologic concentrations and in the absence of hormones, appears to regulate FAS gene expression in HepG2 cells predominantly by mediating FAS mRNA stability.

Adaptation of muscle to creatine depletion: effect on GLUT-4 glucose transporter expression

Feeding rats beta-guanidinopropionic acid (beta-GPA), a creatine analogue, results in depletion of creatine and phosphocreatine and induces increases in mitochondrial oxidative enzymes and hexokinase in skeletal muscle. Comparisons of different muscle types and studies of the adaptation to exercise suggest that 1) the levels of the insulin-responsive glucose transporter (GLUT-4), mitochondrial oxidative enzymes, and hexokinase may be coregulated and 2) GLUT-4 content can determine maximal glucose transport activity in muscle. To further evaluate these possibilities, we examined the effects of feeding rats 1% beta-GPA in their diet for 6 wk on muscle GLUT-4 expression and glucose transport activity. beta-GPA feeding induced 40-50% increases in cytochrome c concentration, citrate synthase activity, and hexokinase activity in plantaris muscle. GLUT-4 protein concentration was increased approximately 50% in plantaris and epitrochlearis muscles, while GLUT-4 mRNA was increased approximately 40% in plantaris muscles of beta-GPA-fed rats. Glucose transport activity maximally stimulated by insulin was increased in parallel with GLUT-4 protein concentration in the epitrochlearis. These results provide evidence that chronic creatine depletion increases GLUT-4 expression by pretranslational mechanisms. They support the hypothesis that the levels of mitochondrial enzymes, hexokinase, and GLUT-4 protein are coregulated in striated muscles. They also support the concept that the GLUT-4 content of a muscle determines its maximal glucose transport activity when the signaling pathways for glucose transport activation are intact.

Short-term interruption of training affects both fasting and post-prandial lipoproteins

Exercise training alters plasma lipoprotein profiles in a manner compatible with decreased coronary artery disease risk. The aim of this study was to ascertain whether interruption of training (detraining) was associated with potentially undesirable changes in the metabolism of post-prandial lipoproteins and plasma levels of Lp(a). Eight normolipidemic, male runners who ran 30-40 miles/week were studied in the trained state and after 14-22 days of detraining. Two of the subjects were studied in the reverse order to control for any confounding effects of exercise sequence. Detraining resulted in (1) a 12% (P = 0.002) reduction in the subjects' aerobic capacity, (2) a 7.7% (P = 0.007) reduction in fasting concentrations of high density lipoprotein cholesterol (HDL-C), (3) a 21% (P = 0.01) reduction in post-heparin lipoprotein lipase activity. Lp(a) concentrations did not change significantly (mean increase 15%, P = 0.076). Fasting plasma concentrations of total cholesterol (TC), triglycerides (TG) and low density lipoprotein-cholesterol (LDL-C) did not change in the detrained state. There was little fluctuation over 24 h in plasma concentrations of TC, LDL-C and HDL-C in either the trained or detrained states. TG concentrations fluctuated over the 24 h in accord with food intake, but there were no exercise-related changes. Exercise had a dramatic effect on chylomicron and chylomicron remnant metabolism as measured by retinyl palmitate measurements. The mean areas under the concentration vs. time curves (AUC) for chylomicron-retinyl esters increased by 41% (P = 0.013) and for chylomicron remnant-retinyl ester by 37% (P = 0.058) following detraining.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesteryl ester storage disease: complex molecular effects of chronic lovastatin therapy

To better characterize the in vivo effects of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibition on human lipid metabolism, an adolescent male with cholesteryl ester storage disease (CESD) was treated chronically with lovastatin. Therapy was associated with decreased liver-spleen size, improved but not normal serum lipids, a 26% decrease in hepatic cholesteryl ester, a 12% decrease in unesterified hepatic cholesterol, and a fourfold increase in hepatic low density lipoprotein (LDL) receptor protein. Hepatic mRNA levels for the LDL receptor and apolipoprotein (apo) B standardized to levels of hepatic gamma actin mRNA were unchanged with therapy. Kinetic studies revealed no change in the LDL fractional catabolic rate and a decrease in the LDL production rate. Size exclusion chromatography showed striking reductions in plasma very low density lipoprotein (VLDL) cholesterol and intermediate density lipoprotein (LDL) cholesterol but not LDL cholesterol with therapy. Mean LDL particle size and the LDL particle size range were increased by treatment. However, there was no difference in the ability of pretreatment or treatment LDL to bind to the LDL receptor on cultured cells consistent with previous studies in animals, indicating that lovastatin may alter LDL particles to impair interaction with the LDL receptor in vivo but not in vitro. Lovastatin therapy in CESD appears to be clinically beneficial and has complex effects on lipid metabolism that may include a dominant inhibitory effect on hepatic lipoprotein production, posttranscriptionally mediated induction of the LDL receptor, and alterations of LDL particles that interfere with their clearance by the LDL receptor in vivo.

A missense (Asp250----Asn) mutation in the lipoprotein lipase gene in two unrelated families with familial lipoprotein lipase deficiency

We have identified the molecular basis for familial lipoprotein lipase (LPL) deficiency in two unrelated families with the syndrome of familial hyperchylomicronemia. All 10 exons of the LPL gene were amplified from the two probands' genomic DNA by polymerase chain reaction. In family 1 of French descent, direct sequencing of the amplification products revealed that the patient was heterozygous for two missense mutations, Gly188----Glu (in exon 5) and Asp250----Asn (in exon 6). In family 2 of Italian descent, sequencing of multiple amplification products cloned in plasmids indicated that the patient was a compound heterozygote harboring two mutations, Arg243----His and Asp250----Asn, both in exon 6. Studies using polymerase chain reaction, restriction enzyme digestion (the Gly188----Glu mutation disrupts an Ava II site, the Arg243----His mutation, a Hha I site, and the Asp250----Asn mutation, a Taq I site), and allele-specific oligonucleotide hybridization confirmed that the patients were indeed compound heterozygous for the respective mutations. LPL constructs carrying the three mutations were expressed individually in Cos cells. All three mutant LPLs were synthesized and secreted efficiently; one (Asp250----Asn) had minimal (approximately 5%) catalytic activity and the other two were totally inactive. The three mutations occurred in highly conserved regions of the LPL gene. The fact that the newly identified Asp250----Asn mutation produced an almost totally inactive LPL and the location of this residue with respect to the three-dimensional structure of the highly homologous human pancreatic lipase suggest that Asp250 may be involved in a charge interaction with an alpha-helix in the amino terminal region of LPL. The occurrence of this mutation in two unrelated families of different ancestries (French and Italian) indicates either two independent mutational events affecting unrelated individuals or a common shared ancestral allele. Screening for the Asp250----Asn mutation should be included in future genetic epidemiology studies on LPL deficiency and familial combined hyperlipidemia.

Structural and functional roles of highly conserved serines in human lipoprotein lipase. Evidence that serine 132 is essential for enzyme catalysis

The structure of human lipoprotein lipase was recently deduced from its cDNA sequence. It contains 8 serine residues (residues 45, 132, 143, 172, 193, 244, 251, and 363) that are absolutely conserved in both lipoprotein lipase and hepatic lipase across all species studied. The high homology between lipoprotein lipase, hepatic lipase, and pancreatic lipase suggests that the catalytic functions of these enzymes share a common mechanism and that one of the 8 conserved serines in human lipoprotein lipase must play a catalytic role as does serine 152 in the case of pancreatic lipase (Winkler, F. K., D'Arcy, A., and Hunziker, W. Nature 343, 771-774). We expressed wild-type and site-specific mutants of human lipoprotein lipase in COS cells in vitro. We produced two to four substitution mutants involving each of the 8 serines and assayed a total of 22 mutants for both enzyme activity and the amount of immunoreactive enzyme mass produced. Immunoreactive lipase was detected in all cases. With the exception of Ser132, for each of the 8 serine mutants we studied, at least one of several mutants at each position showed detectable enzyme activity. All three substitution mutants at Ser132, Ser----Thr, Ser----Ala, and Ser----Asp, were totally inactive. Ser132 occurs in the consensus sequence Gly-Xaa-Ser-Xaa-Gly present in all serine proteinases and in human pancreatic lipase. The x-ray crystallography structure of human pancreatic lipase suggests that the analogous serine residue in human pancreatic lipase, Ser152, is the nucleophilic residue essential for catalysis. Our biochemical data strongly support the conclusion that Ser132 in human lipoprotein lipase is the crucial residue required for enzyme catalysis. The observed specific activities of the variants involving the other seven highly conserved serines in human lipoprotein lipase are consistent with the interpretation that this enzyme has a three-dimensional structure very similar to that of human pancreatic lipase.

A protein partially expressed on the surface of HepG2 cells that binds lipoproteins specifically is nucleolin

Nucleolin, a major nucleolar protein of rapidly growing eukaryotic cells, has been thought to be predominantly if not exclusively located in the nucleolus. Recent data however [Borer, R.A., Lehner, C.F., Eppenberger, H.M., & Nigg, N.A. (1989) Cell 56, 379-390] suggest that the protein shuttles constantly between the nucleus and cytoplasm. Ligand blotting studies of whole cell extracts of HepG2 cells identified, in addition to the LDL receptor, another LDL binding protein of Mr 109,000. The 109-kDa protein was partially purified by HPLC and, like the LDL receptor, bound apoB- and apoE-containing lipoproteins but not HDL. However, unlike the LDL receptor, the 109-kDa protein bound lipoproteins in the presence of EDTA and reducing agents, had a lower affinity for lipoproteins than the LDL receptor, and did not react with two antibodies raised against the LDL receptor. The protein sequences of three separate peptides derived from the partially purified 109-kDa species were determined and were identical except for one residue to three separate regions of the published sequence of nucleolin. On immunoblot analysis the 109-kDa protein reacted with a nucleolin-specific antibody, and purified nucleolin reacted both with anti-109-kDa antibody and with LDL. When intact HepG2 cells were treated with Pronase before harvest, there was a 46% decrease in 109-kDa protein while recovery of actin, an intracellular protein, was unaffected. When intact HepG2 cells were surface iodinated and the proteins subjected to HPLC fractionation, the 109-kDa protein was found to be iodinated.(ABSTRACT TRUNCATED AT 250 WORDS)

Apolipoprotein B mRNA editing. Validation of a sensitive assay and developmental biology of RNA editing in the rat

Apolipoprotein B (apoB) 48 mRNA is the product of a C----U conversion of the first base of the codon CAA for Gln-2153 in apoB-100 mRNA, changing it to an in-frame stop codon (UAA). Methods for measuring the ratio of apoB-48 to apoB-100 mRNA that have been authenticated with standard mixtures of the two apoB mRNA species have not been described. Using RNA mixtures consisting of known proportions of in vitro transcripts of apoB-100 and apoB-48, we directly compared four different assays. We found that a procedure based on the polymerase chain reaction, cDNA cloning, and oligonucleotide colony hybridization was the most sensitive and accurate assay. Total RNAs isolated from adult rat small intestine, adult liver, Day 15 and term placentas, and term fetal membranes were found to contain apoB mRNA in the following relative amounts: 100, 59.8, 0.9, 6.96, and 1087, respectively. They all contained both apoB-48 and apoB-100 mRNAs, with the former constituting 95.8, 59.2, 4.6, 1.3, and 0.8%, respectively, of the apoB mRNA. We examined the ontogeny of apoB-48 mRNA biogenesis in the liver and intestine in the rat prenatally on Days 17, 19, and 20 of gestation, and postnatally on Days 1, 3, 7, 13, 20, 24, and 37 after birth. Slot-blot hybridization demonstrated that apoB mRNA showed a peak at birth (Days 1-3 in the liver and Days 1-7 in the small intestine) and then decreased on Days 7 (in the liver) and 13 (in the intestine) before it increased again on Day 20 toward the adult level. Quantitation of the ratio of apoB-48 to apoB-100 mRNA at the different time points showed that RNA editing became highly competent prenatally on Day 19 of gestation in the small intestine, but postnatally on Day 24 after birth in the liver. The asynchronous nature for this developmentally regulated process in the liver and small intestine of the rat has implications for the mechanism of RNA editing and lipid homeostasis in this animal.

In vitro expression and site-specific mutagenesis of the cloned human lipoprotein lipase gene. Potential N-linked glycosylation site asparagine 43 is important for both enzyme activity and secretion

Detailed structure-function information about human lipoprotein lipase (LPL) is unavailable because it is difficult to purify large amounts of the enzyme for study. To circumvent this problem, we constructed an in vitro LPL expression vector. Human LPL cDNA was cloned and inserted into the expression vector p91023(B). After transfection of COS M-6 cells with the human LPL cDNA construct, LPL enzyme activity was detected in cell extracts and culture medium. Purified human apolipoprotein C-II caused a 5-fold stimulation of the recombinant human LPL expressed in vitro. Using site-specific mutagenesis, Ala residues were substituted for Asn residues at two potential N-linked glycosylation sites (positions 43 and 359) and at a third unrelated Asn (position 257) in the LPL cDNA. RNA blot analysis demonstrated the presence of a single mRNA species in COS cells transfected with wild-type and mutant LPL expression vectors. Intracellular and secreted LPL activity was absent in the construct containing an Ala for Asn mutation at position 43, whereas the same substitutions at positions 257 and 359 did not appreciably affect activity. LPL activity was also absent in another construct containing a Gln for Asn mutation at position 43. Quantitation of LPL protein mass concomitant with measurement of enzyme activity showed that substitution of Ala or Gln for Asn at position 43 resulted in the production of an enzymatically inactive protein which accumulated intracellularly but was not secreted into the culture medium. Our report represents an initial documentation of the expression of cloned human LPL in vitro and of the importance of Asn-43 for both enzyme activity and secretion.

Insulin regulation of lipoprotein lipase activity in 3T3-L1 adipocytes is mediated at posttranscriptional and posttranslational levels

Insulin is a major regulator of lipoprotein lipase (LPL) activity. The molecular events associated with LPL regulation by insulin in 3T3-L1 adipocytes were studied by determining LPL enzyme activity, mRNA levels, protein synthetic rate, and transcription run-off activity. Adipocytes treated with insulin (10(-6) M for 48 h) had substantially higher LPL activity (mean difference compared to carrier-treated cells 146%) with little difference in LPL mRNA levels (mean level 109% of control). Insulin regulation of LPL activity was dose-dependent but changes in LPL mRNA were not. Within 2 h of hormone addition, LPL activity was higher in insulin-treated versus carrier-treated adipocytes although their LPL mRNA levels were similar. In [35S]methionine pulse-labeled adipocytes, insulin decreased LPL protein synthetic rate measured by immunoprecipitation 42-48%, although increases (75-340%) in heparin-releasable LPL activity were detected in the same cells. In contrast, during differentiation of 3T3-L1 fibroblasts to the adipocyte state, 5-80-fold increases of heparin-releasable LPL activity were closely associated with similar (8-60-fold) increases in LPL mRNA levels. LPL synthetic rate was 16-fold greater, and LPL gene transcription initiation measured by transcriptional run-off was 10-fold higher in adipocytes than in undifferentiated cells. Differentiation of 3T3-L1 fibroblasts increases transcription of the LPL gene leading to increased LPL mRNA, protein synthetic rate, and enzyme activity. Insulin regulation of LPL activity in 3T3-L1 adipocytes, however, is mediated entirely at posttranscriptional and posttranslational levels.

Quantitative relationship between plasma lipids and glycohemoglobin in type I patients. Longitudinal study of 212 patients

Plasma cholesterol, triglycerides, glycohemoglobin, and other covariates were measured in 212 type I (insulin-dependent) diabetic subjects on entry into a longitudinal study of diabetes and again after an average interval of 3.7 yr. Changes in individual cholesterol and triglyceride values over time were significantly correlated with changes in glycohemoglobin. After adjustment for potentially confounding covariates, plasma cholesterol declined by 2.2% (0.1 mM) for each percentage-point reduction in glycohemoglobin and plasma triglycerides declined by 8% (0.08 mM) per percentage point glycohemoglobin. Increased insulin dose was independently associated with increased plasma triglycerides, after adjusting for glycohemoglobin level and other covariates. However, insulin dose diabetic metabolic control, measured as declining glycohemoglobin, is the variable most closely associated with reduced plasma lipids in a population of typical type I diabetic patients.

Lipoprotein lipase and hepatic lipase mRNA tissue specific expression, developmental regulation, and evolution

Lipoprotein lipase (LPL) and hepatic lipase (HL) enzyme activities were previously reported to be regulated during development, but the underlying molecular events are unknown. In addition, little is known about LPL evolution. We cloned and sequenced a complete mouse LPL cDNA. Comparison of sequences from mouse, human, bovine, and guinea pig cDNAs indicated that the rates of evolution of mouse, human, and bovine LPL are quite low, but guinea pig LPL has evolved several times faster than the others. 32P-Labeled mouse LPL and rat HL cDNAs were used to study lipase mRNA tissue distribution and developmental regulation in the rat. Northern gel analysis revealed the presence of a single 1.87 kb HL mRNA species in liver, but not in other tissues including adrenal and ovary. A single 4.0 kb LPL mRNA species was detected in epididymal fat, heart, psoas muscle, lactating mammary gland, adrenal, lung, and ovary, but not in adult kidney, liver, intestine, or brain. Quantitative slot-blot hybridization analysis demonstrated the following relative amounts of LPL mRNA in rat tissues: adipose, 100%; heart, 94%; adrenal, 6.6%; muscle, 3.8%; lung, 3.0%; kidney, 0%; adult liver, 0%. The same quantitative analysis was used to study lipase mRNA levels during development. There was little postnatal variation in LPL mRNA in adipose tissue; maximal levels were detected at the earliest time points studied for both inguinal and epididymal fat. In heart, however, LPL mRNA was detected at low levels 6 days before birth and increased 278-fold as the animals grew to adulthood.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma lipids in patients with type I diabetes mellitus. Influence of race, gender, and plasma glucose control: lipids do not correlate with glucose control in black women

Plasma lipids and hemoglobin A1 were measured in 544 type I diabetic patients. Hemoglobin A1 was positively correlated with the levels of total plasma cholesterol, total triglycerides, and low-density lipoprotein cholesterol and negatively correlated with the level of high-density lipoprotein cholesterol in the entire biracial group. These relationships between plasma lipids and hemoglobin A1 were not present in black women. In the white diabetic population a reduction in hemoglobin A1 of one percentage point was statistically associated with a decrease of 0.16 to 0.17 mmol/L in total plasma cholesterol, a decrease of 0.10 to 0.13 mmol/L in low-density lipoprotein cholesterol, and a reduction of 0.12 to 0.14 mmol/L in triglycerides. These findings suggest that race and gender are important determinants of the response of plasma lipids to glucose control in type I diabetes mellitus.