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.