Regulation of lipoprotein lipase translation by epinephrine in 3T3-L1 cells. Importance of the 3' untranslated region

Physiological regulation of lipoprotein lipase

The enzyme lipoprotein lipase (LPL), originally identified as the clearing factor lipase, hydrolyzes triglycerides present in the triglyceride-rich lipoproteins VLDL and chylomicrons. LPL is primarily expressed in tissues that oxidize or store fatty acids in large quantities such as the heart, skeletal muscle, brown adipose tissue and white adipose tissue. Upon production by the underlying parenchymal cells, LPL is transported and attached to the capillary endothelium by the protein GPIHBP1. Because LPL is rate limiting for plasma triglyceride clearance and tissue uptake of fatty acids, the activity of LPL is carefully controlled to adjust fatty acid uptake to the requirements of the underlying tissue via multiple mechanisms at the transcriptional and post-translational level. Although various stimuli influence LPL gene transcription, it is now evident that most of the physiological variation in LPL activity, such as during fasting and exercise, appears to be driven via post-translational mechanisms by extracellular proteins. These proteins can be divided into two main groups: the liver-derived apolipoproteins APOC1, APOC2, APOC3, APOA5, and APOE, and the angiopoietin-like proteins ANGPTL3, ANGPTL4 and ANGPTL8, which have a broader expression profile. This review will summarize the available literature on the regulation of LPL activity in various tissues, with an emphasis on the response to diverse physiological stimuli.

The lipoprotein lipase (LPL) S447X gain of function variant involves increased mRNA translation

OBJECTIVE

A common gain-of-function LPL variant, LPLS447X, has favorable clinical features and involves a C→G base change at nucleotide 1595 of the LPL cDNA, along with a haplotype, which includes other non-coding SNPs. The mechanism for the LPL gain-in-function is not clear. LPL translation is regulated by epinephrine by an RNA-protein complex, consisting of PKA subunits and an A kinase anchoring protein (AKAP), which targets the 3'UTR.

METHODS

To examine LPL translation of the LPLS447X variant, in vitro translation of LPL mRNA constructs was studied in the presence of cytoplasmic extracts from 3T3-F442A adipocytes treated with/without epinephrine.

RESULTS

When the C→G base change at nucleotide 1595 was introduced, LPL mRNA was less susceptible to inhibition by the adipocyte extract. Similarly, a lessened susceptibility to translation inhibition occurred when the complete haplotype was constructed in the full-length 3.6 kb LPL mRNA, when an irrelevant coding sequence was introduced into the LPL mRNA construct, and in response to the use of components of the RNA binding complex (PKA C and R subunits, and KH region of AKAP149).

CONCLUSION

These studies suggest that the LPLS447X gain of function may be due to the base change in the LPL mRNA resulting in a decreased susceptibility to translational inhibition.

Adiponectin translation is increased by the PPARgamma agonists pioglitazone and omega-3 fatty acids

Adiponectin, made exclusively by adipocytes, is a 30-kDa secretory protein assembled posttranslationally into low-molecular weight, middle-molecular weight, and high-molecular weight homo-oligomers. PPARgamma ligand thiozolidinediones, which are widely used in the treatment of type II diabetes, increase adiponectin levels. PPARgamma also has several putative ligands that include fatty acid derivatives. Overnight treatment of rat adipocytes with pioglitazone, docosahexaenoic acid (DHA), or eicosapentaenoic acid (EPA) triggered a twofold increase in the synthesis and secretion of HMW adiponectin, and this increase was blocked by the addition of PPARgamma inhibitor GW-9662. Inhibition of glycosylation using 2,2'-dipyridyl decreased the synthesis of high-molecular weight adiponectin by pioglitazone, EPA, and DHA, but there was increased secretion of trimeric adiponectin resulting from increased translation. Although pioglitazone, DHA, and EPA increased adiponectin synthesis by more than 60%, there was no increase in total protein synthesis and no corresponding change in adiponectin mRNA expression, indicating the upregulation of translation. We examined the possibility of transacting factors in the cytoplasmic extracts from adipocytes treated with pioglitazone or DHA. In vitro translation of adiponectin mRNA was inhibited by S-100 fraction of control adipocytes and increased by S-100 extracts from adipocytes treated with pioglitazone or DHA. Consistent with this observation, both pioglitazone and DHA treatments increased the association of adiponectin mRNA with the heavier polysome fractions. Together, these data suggest that pioglitazone and the fish oils DHA or EPA are PPARgamma agonists in adipocytes with regard to adiponectin expression, and the predominant mode of adiponectin stimulation is via an increase in translation.

Translational regulation of lipoprotein lipase in adipocytes: depletion of cellular protein kinase Calpha activates binding of the C subunit of protein kinase A to the 3'-untranslated region of the lipoprotein lipase mRNA

Adipose LPL (lipoprotein lipase) plays an important role in regulating plasma triacylglycerols and lipid metabolism. We have previously demonstrated that PKCalpha (protein kinase Calpha) depletion inhibits LPL translation in 3T3-F442A adipocytes. Using in vitro translation experiments, the minimum essential region on the 3'UTR (3'-untranslated region) of LPL mRNA required for the inhibition of translation was identified as the proximal 39 nt. These results were confirmed by RNase protection analysis using cytoplasmic proteins isolated from the adipocytes treated with PKCalpha antisense oligomers and the LPL 3'UTR transcript (LPL 3'UTR nt: 1512-1640). The protein components involved in this RNA-binding interaction from PKCalpha depletion were passed through an affinity column containing a sequence of the LPL 3'UTR and, after Western blotting, the RNA-binding proteins were identified as the catalytic and the regulatory subunits of PKA (protein kinase A), Calpha and RIIbeta, and AKAP (A-kinase-anchoring protein) 121. This RNA inhibitory complex consisted of the same RNA-binding proteins that have been identified previously as mediators of LPL translational inhibition by PKA activation, suggesting that PKCalpha depletion inhibits LPL translation through PKA activation. In additional experiments, PKC depletion by prolonged PMA treatment or PKCalpha antisense oligomers resulted in an increase in PKA activity in 3T3-F442A adipocytes, comparable with PKA activation with adrenaline (epinephrine) treatment. These results demonstrate that LPL translational inhibition occurs through an RNA-binding complex involving PKA subunits and AKAP121, and this complex can be activated either through traditional PKA activation methods or through the depletion of PKCalpha.

Functional significance of lipoprotein lipase HindIII polymorphism associated with the risk of coronary artery disease

Lipoprotein lipase (LPL) plays a pivotal role in lipid metabolism by hydrolyzing triglyceride (TG)-rich lipoprotein particles. Abnormalities in normal LPL function are associated with the risk of coronary artery disease (CAD). A number of genetic variants have been identified in the LPL gene that affects different functions of the LPL protein. A common HindIII polymorphism in intron 8 (T/G) of the LPL gene has been found to be associated with altered plasma TG and HDL-cholesterol, and CAD risk in several studies, but its functional significance is unknown. It has been shown that certain intronic sequence contain regulatory elements that are important for transcription and translational regulation of a gene. In this study we tested the hypothesis that this polymorphism affects the binding site of a transcription factor that regulates the transcription of LPL gene. Electrophoretic mobility shift assays (EMSAs) revealed that the HindIII site binds to a transcription factor and that the mutant allele has lower binding affinity than the wild type allele. Transcription assays containing the entire intron 8 sequence along with full-length human LPL promoter were carried out in COS-1 and human vascular smooth muscle cells. The mutant allele was associated with significantly decreased luciferase expression level compared to the wild type allele in both the muscle (3.394+/-0.022 vs. 4.184+/-0.028; P=4.7 x 10(-6)) and COS-1 (11.603+/-0.409 vs. 14.373+/-1.096; P<0.0001) cells. In conclusion, this study demonstrates for the first time that the polymorphic HindIII site in the LPL gene is functional because it affects the binding of a transcription factor and it also has an impact on LPL expression.

Effect of beta-adrenergic stimulation on whole-body and abdominal subcutaneous adipose tissue lipolysis in lean and obese men

AIMS/HYPOTHESIS

Obesity is characterised by increased triacylglycerol storage in adipose tissue. There is in vitro evidence for a blunted beta-adrenergically mediated lipolytic response in abdominal subcutaneous adipose tissue (SAT) of obese individuals and evidence for this at the whole-body level in vivo. We hypothesised that the beta-adrenergically mediated effect on lipolysis in abdominal SAT is also impaired in vivo in obese humans.

METHODS

We investigated whole-body and abdominal SAT glycerol metabolism in vivo during 3 h and 6 h [2H5]glycerol infusions. Arterio-venous concentration differences were measured in 13 lean and ten obese men after an overnight fast and during intravenous infusion of the non-selective beta-adrenergic agonist isoprenaline [20 ng (kg fat free mass)(-1) min(-1)].

RESULTS

Lean and obese participants showed comparable fasting glycerol uptake by SAT (9.7+/-3.4 vs 9.3+/-2.5% of total release, p=0.92). Furthermore, obese participants showed an increased whole-body beta-adrenergically mediated lipolytic response versus lean participants. However, their fasting lipolysis was blunted [glycerol rate of appearance: 7.3+/-0.6 vs 13.1+/-0.9 micromol (kg fat mass)(-1) min(-1), p<0.01], as was the beta-adrenergically mediated lipolytic response per unit SAT [Delta total glycerol release: 140+/-71 vs 394+/-112 nmol (100 g tissue)(-1) min(-1), p<0.05] compared with lean participants. Net triacylglycerol flux tended to increase in obese compared with lean participants during beta-adrenergic stimulation [Delta net triacylglycerol flux: 75+/-32 vs 16+/-11 nmol (100 g tissue)(-1) min(-1), p=0.06].

CONCLUSIONS/INTERPRETATION

We demonstrated in vivo that beta-adrenergically mediated lipolytic response is impaired systematically and in abdominal SAT of obese versus lean men. This may be important in the development or maintenance of increased triacylglycerol stores and obesity.

Adipogenic effect of alcohol on human bone marrow-derived mesenchymal stem cells

BACKGROUND

In addition to a decrease in bone mass in alcoholics their osteopenic skeletons show an increase in bone marrow adiposity. Human bone marrow mesenchymal stem cells (hMSC) in vivo differentiate into several phenotypes including osteogenic and adipogenic cells, both of which remain as resident populations of bone marrow. In vitro, the lineage commitment and differentiation of hMSC toward the adipogenic pathway can be promoted by alcohol.

METHODS

Human male and female mesenchymal stem cells from joint replacement surgery were cultured. Cells were grouped as: 1) Control (no additions to the culture medium), 2) EtOH (50 mm alcohol added to the culture medium), 3) OS (osteogenic inducers added to the culture medium), and 4) OS + EtOH (osteogenic inducers and 50 mm alcohol added to the culture medium). Cultures stained with Nile Red confirmed the development of differentiated adipocytes. Population analysis was performed using fluorescence-activated cell sorting. Gene expression of early, middle, late, and terminal differentiation stage markers (PPAR)gamma2, lipoprotein lipase, adipsin, leptin, and adipocyte P2 (aP2)] was studied by Northern hybridization, and protein synthesis of aP2 was determined by Western analysis.

RESULTS

Nile red staining confirmed increased adipocyte development 10 days after the onset of treatment with 50 mm alcohol and osteogenic induction. By day 21 the number of adipocytes increased to 13.6% of the total population. Alcohol up-regulated the gene expression of PPARgamma2 whereas no up-regulation was observed for the other genes. Protein production of aP2 was significantly increased in hMSC cells by culture in the presence of alcohol.

CONCLUSIONS

The data suggest that alcohol's adipogenic effect on cultured hMSC is through up-regulation of PPARgamma2 at the point of lineage commitment as well as through enhancement of lipid transport and storage through increased aP2 synthesis. The alcohol-induced expression and synthesis changes account for the increased Nile red staining of cultured hMSC.

Associations between acute lipid stress responses and fasting lipid levels 3 years later

The authors assessed the association between lipid responses to acute mental stress and fasting serum lipid levels 3 years later in 199 middle-aged men and women. Total cholesterol, low-density lipoprotein (LDL), and high-density lipoprotein (HDL) cholesterol increased following moderately stressful behavioral tasks. LDL cholesterol, HDL cholesterol, and total:HDL ratio measured 3 years later were predicted by acute stress responses independent of gender, age, socioeconomic position, change in body mass, smoking, alcohol consumption, or hormone replacement therapy baseline lipid levels. The odds of clinically elevated cholesterol were significantly greater in the highest compared with the lowest stress tertile, independent of baseline levels and covariates. Acute lipid stress responsivity may reflect processes that contribute to the development of elevated blood cholesterol concentration.

Transgenic mice expressing lipoprotein lipase in adipose tissue. Absence of the proximal 3'-untranslated region causes translational upregulation

Lipoprotein lipase (LPL) is a key enzyme in lipoprotein and adipocyte metabolism. Defects in LPL can lead to hypertriglyceridemia and the subsequent development of atherosclerosis. The mechanisms of regulation of this enzyme are complex and may occur at multiple levels of gene expression. Because the 3'-untranslated region (UTR) is involved in LPL translational regulation, transgenic mice were generated with adipose tissue expression of an LPL construct either with or without the proximal 3'-UTR and driven by the aP2 promoter. Both transgenic mouse colonies were viable and expressed the transgene, resulting in a 2-fold increase in LPL activity in white adipose tissue. Neither mouse colony exhibited any obvious phenotype in terms of body weight, plasma lipids, glucose, and non-esterified fatty acid levels. In the mice expressing hLPL with an intact 3'-UTR, hLPL mRNA expression approximately paralleled hLPL activity. However in the mice without the proximal 3'-UTR, hLPL mRNA was low in the setting of large amounts of hLPL protein and LPL activity. In previous studies, the 3'-UTR of LPL was critical for the inhibitory effects of constitutively expressed hormones, such as thyroid hormone and catecholamines. Therefore, these data suggest that the absence of the 3'-UTR results in a translationally unrepressed LPL, resulting in a moderate overexpression of adipose LPL activity.

Evaluation of thyroid hormone effects on liver P450 reductase translation

The expression of NADPH cytochrome P450 oxidoreductase (P450R) in rat liver is positively regulated by thyroid hormone (T3), at both the transcriptional and post-transcriptional levels. Here we investigate the effects of T3-induced hyperthyroidism on the regulation of P450R protein synthesis. T3 treatment of adult male rats led to a strong induction (up to approximately 10-fold) of liver P450R mRNA but little or no change in P450R protein and activity. Investigation of this discrepancy revealed that the association of hepatic P450R mRNA with polysomes was not altered by T3 treatment, suggesting that the discoordinate changes in P450R mRNA and protein levels do not reflect decreased recruitment of T3-induced P450R mRNA into polysomes. Moreover, polysome size distribution analysis of P450R mRNA did not show any T3-dependent changes. When assayed in an in vitro translation system, T3-induced and uninduced P450R mRNAs were translated with similar efficiencies. Moreover, liver cell extract from T3-treated rats did not selectively inhibit in vitro translation of T3-induced P450R mRNA. Thus, neither structural changes in P450R mRNA nor trans-acting binding proteins in liver cytosol were found to control translation of P450R mRNA in response to T3 treatment. Taken together, these data suggest that P450R may in part be regulated at the level of protein stability in hyperthyroid rat liver.

The translational regulation of lipoprotein lipase by epinephrine involves an RNA binding complex including the catalytic subunit of protein kinase A

The balance of lipid flux in adipocytes is controlled by the opposing actions of lipolysis and lipogenesis, which are controlled primarily by hormone-sensitive lipase and lipoprotein lipase (LPL), respectively. Catecholamines stimulate adipocyte lipolysis through reversible phosphorylation of hormone-sensitive lipase, and simultaneously inhibit LPL activity. However, LPL regulation is complex and previous studies have described translational regulation of LPL in response to catecholamines because of an RNA-binding protein that interacts with the 3'-untranslated region of LPL mRNA. In this study, we identified several protein components of an LPL RNA binding complex. Using an LPL RNA affinity column, we identified two of the RNA-binding proteins as the catalytic (C) subunit of cAMP-dependent protein kinase (PKA), and A kinase anchoring protein (AKAP) 121/149, one of the PKA anchoring proteins, which has known RNA binding activity. To determine whether the C subunit was involved in LPL translation inhibition, the C subunit was depleted from the cytoplasmic extract of epinephrine-stimulated adipocytes by immunoprecipitation. This resulted in the loss of LPL translation inhibition activity of the extract, along with decreased RNA binding activity in a gel shift assay. To demonstrate the importance of the AKAPs, inhibition of PKA-AKAP binding with a peptide competitor (HT31) prevented epinephrine-mediated inhibition of LPL translation. C subunit kinase activity was necessary for LPL RNA binding and translation inhibition, suggesting that the phosphorylation of AKAP121/149 or other proteins was an important part of RNA binding complex formation. The hormonal activation of PKA results in the reversible phosphorylation of hormone-sensitive lipase, which is the primary mediator of adipocyte lipolysis. These studies demonstrate a dual role for PKA to simultaneously inhibit LPL-mediated lipogenesis through inhibition of LPL translation.

Regulation of lipoprotein lipase by protein kinase C alpha in 3T3-F442A adipocytes

Lipoprotein lipase (LPL) is an important enzyme in adipocyte and lipid metabolism with complex cellular regulation. Previous studies demonstrated an inhibition of LPL activity and synthesis following depletion of protein kinase C (PKC) isoforms with long term treatment of 3T3-F442A adipocytes with 12-O-tetradecanoylphorbol-13-acetate. To identify the specific PKC isoforms involved, we treated cells with antisense oligonucleotides that block expression of specific PKC isoforms. An antisense oligonucleotide to PKC alpha inhibited LPL activity by 78 +/- 8%, whereas antisense oligonucleotides directed against PKC delta or PKC epsilon had no effect on LPL activity. The change in LPL activity was maximal at 72 h and was accompanied by a decrease in LPL protein and LPL synthetic rate but no change in LPL mRNA, suggesting regulation at the level of translation. However, PKC depletion resulted in no change in the polysome profile, indicating that translation initiation was not affected. However, the addition of cytoplasmic extracts from adipocytes treated with 12-O-tetradecanoylphorbol-13-acetate or PKC alpha antisense oligomers inhibited LPL translation in vitro. This inhibition of LPL translation in vitro was lost when the LPL mRNA transcript did not contain nucleotides 1599-3200, thus implicating the 3'-untranslated region of LPL in the regulation of translation by PKC depletion. Both LPL activity and Raf1 activity were decreased in parallel following depletion of either total PKC or specific inhibition of PKC alpha. An antisense oligonucleotide to RAF1, which inhibited RAF1 activity, also inhibited LPL activity by 48 +/- 10%, and this decrease in LPL activity was not accompanied by a change in LPL mRNA. Cells were treated with U0126, a specific inhibitor of the ERK-activating kinases MEK1 and MEK2. Although U0126 inhibited ERK1 and ERK2 phosphorylation, U0126 had no effect on LPL activity, indicating that MEK/ERK pathways were not involved in this mechanism of LPL regulation. Together, these data indicate that PKC alpha and RAF1 are important in the translational regulation of LPL in adipocytes and that the mechanism of regulation is probably through an ERK-independent pathway.

Abnormal expression of CTLA-4 by T cells from patients with myasthenia gravis: effect of an AT-rich gene sequence

Cytolytic T lymphocyte-associated antigen-4 (CTLA-4) plays a critical role in the down-regulation of antigen-activated immune responses. The aberrant CTLA-4 expression is characterized by low surface and intracellular levels of CTLA-4 protein, impaired up-regulation of CTLA-4 in T cells in response to ConA stimulation and high levels of soluble CTLA-4 (sCTLA-4) in serum. The serum levels of sCTLA-4 are positively correlated with the serum concentration of antibodies against the acetylcholine receptor. The (AT)(n) polymorphism in the 3'-untranslated region contributes to decreased mRNA stability and, hence, to reduced expression of CTLA-4.

The translational regulation of lipoprotein lipase in diabetic rats involves the 3'-untranslated region of the lipoprotein lipase mRNA

Adipose tissue lipoprotein lipase (LPL) activity is decreased in patients with poorly controlled diabetes, and this contributes to the dyslipidemia of diabetes. To study the mechanism of this decrease in LPL, we studied adipose tissue LPL expression in male rats with streptozotocin-induced diabetes. Heparin releasable and extractable LPL activity in the epididymal fat decreased by 75-80% in the diabetic group and treatment of the rats with insulin prior to sacrifice reversed this effect. Northern blot analysis indicated no corresponding change in LPL mRNA levels. However, LPL synthetic rate, measured using [(35)S]methionine pulse labeling, was decreased by 75% in the diabetic adipocytes, and insulin treatment reversed this effect. These results suggested regulation of LPL at the level of translation. Diabetic adipocytes demonstrated no change in the distribution of LPL mRNA associated with polysomes, suggesting no inhibition of translation initiation. Addition of cytoplasmic extracts from control and diabetic adipocytes to a reticulocyte lysate system demonstrated the inhibition of LPL translation in vitro. Using different LPL mRNA transcripts in this in vitro translation assay, we found that the 3'-untranslated region (UTR) of the LPL mRNA was important in controlling translation inhibition by the cytoplasmic extracts. To identify the specific region involved, gel shift analysis was performed. A specific shift in mobility was observed when diabetic cytoplasmic extract was added to a transcript containing nucleotides 1818-2000 of the LPL 3'-UTR. Thus, inhibition of translation is the predominant mechanism for the decreased adipose tissue LPL in this insulin-deficient model of diabetes. Translation inhibition involves the interaction of a cytoplasmic factor, probably an RNA-binding protein, with specific sequences of the LPL 3'-UTR.

Antagonistic effects of vitamin D and parathyroid hormone on lipoprotein lipase in cultured adipocytes

The effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) (calcitriol) and parathyroid hormone (PTH) on synthesis and secretion of lipoprotein lipase (LPL) were studied in 3T3-L1 adipocytes. Expression of the vitamin D receptor was demonstrated by saturation kinetics with radiolabeled calcitriol. Incubation with calcitriol (10(-8) M) for up to 4 d resulted in a time-dependent significant increase in heparin-releasable LPL activity (LPLa) accompanied by a significant increase in LPL mRNA. In contrast, incubation with intact (1-84) PTH (10(-6) to 10(-9) M) produced a time- and dose-dependent significant decrease in LPLa, but no change in LPL mRNA. The effect of PTH (24-h incubation, 10(-8) M) could be prevented by the calcium channel blocker verapamil. Coincubation with both calcitriol and PTH at equimolar concentration (10(-8) M) resulted in an increase in LPLa and LPL mRNA. These data indicate an antagonistic role for calcitriol and PTH in the regulation of LPL, possibly mediated by intracellular calcium, which may contribute to the alterations in lipoprotein metabolism occurring in uremia.

Influence of mental stress and circadian cycle on postprandial lipemia

BACKGROUND

Mental stress produces alterations in serum lipids and lipoproteins.

OBJECTIVE

The objective was to assess the effect of mental stress during the day and night on postprandial lipoproteins.

DESIGN

Fourteen healthy subjects aged 26.6 +/- 5.0 y were given randomly the same meal either at night (0100) or during the day (1300), with or without (control session) a mental stress challenge. The meal contained 40% of estimated daily energy needs. The mental task was performed on a computer and consisted of a task of choice reaction. Blood samples were drawn at baseline and hourly for 7 h after the meal.

RESULTS

Urinary epinephrine concentrations were higher (P < 0.012) during the mental task than during the control sessions. Repeated-measures analysis of variance showed that mean postprandial triacylglycerol concentrations were significantly higher (P < 0.02) and total cholesterol (P < 0.0001) and HDL-cholesterol concentrations were significantly lower (P < 0.0001) at night than during the day. The mean postprandial VLDL-triacylglycerol concentration was significantly higher (P < 0.04) during the mental task than during the control sessions. Similarly, the VLDL-cholesterol response, calculated as the area under the postprandial curve, was significantly greater (P < 0.02) during the mental task than during the control sessions. There was no interaction between mental stress and nyctohemeral cycle on postprandial lipoprotein responses, suggesting that both indexes act independently on postprandial lipid metabolism.

CONCLUSIONS

Mental stress is associated with increased concentrations of postprandial triacylglycerol-rich lipoprotein fractions. Therefore, postprandial hyperlipidemia is one possible mechanism contributing to the higher risk of ischemic heart disease in stressed people.

Role of protein kinase C in the translational regulation of lipoprotein lipase in adipocytes

The hypertriglyceridemia of diabetes is accompanied by decreased lipoprotein lipase (LPL) activity in adipocytes. Although the mechanism for decreased LPL is not known, elevated glucose is known to increase diacylglycerol, which activates protein kinase C (PKC). To determine whether PKC is involved in the regulation of LPL, we studied the effect of 12-O-tetradecanoyl phorbol 13-acetate (TPA) on adipocytes. LPL activity was inhibited when TPA was added to cultures of 3T3-F442A and rat primary adipocytes. The inhibitory effect of TPA on LPL activity was observed after 6 h of treatment, and was observed at a concentration of 6 nM. 100 nM TPA yielded maximal (80%) inhibition of LPL. No stimulation of LPL occurred after short term addition of TPA to cultures. To determine whether TPA treatment of adipocytes decreased LPL synthesis, cells were labeled with [35S]methionine and LPL protein was immunoprecipitated. LPL synthetic rate decreased after 6 h of TPA treatment. Western blot analysis of cell lysates indicated a decrease in LPL mass after TPA treatment. Despite this decrease in LPL synthesis, there was no change in LPL mRNA in the TPA-treated cells. Long term treatment of cells with TPA is known to down-regulate PKC. To assess the involvement of the different PKC isoforms, Western blotting was performed. TPA treatment of 3T3-F442A adipocytes decreased PKC alpha, beta, delta, and epsilon isoforms, whereas PKC lambda, theta, zeta, micro, iota, and gamma remained unchanged or decreased minimally. To directly assess the effect of PKC inhibition, PKC inhibitors (calphostin C and staurosporine) were added to cultures. The PKC inhibitors inhibited LPL activity rapidly (within 60 min). Thus, activation of PKC did not increase LPL, but inhibition of PKC resulted in decreased LPL synthesis by inhibition of translation, indicating a constitutive role of PKC in LPL gene expression.

Analysis of the 5' and 3'UTRs in the post-transcriptional regulation of the interleukin-5 gene

Post-transcriptional regulation is emerging as an important control point in cytokine gene expression. However, the role that it plays in IL-5 gene expression is unclear with some conflicting reports. Here we investigate the importance of post-transcriptional regulation and the role of the 5' and 3' untranslated regions (UTRs) in mIL-5 gene expression. To do this, IL-5 expression from a panel of cDNA constructs was compared. We found it essential to remove the 5' synthetic oligonucleotide tails, introduced during the cloning of the mIL-5 cDNA, when studying IL-5 expression. The presence of these oligo(G) tails acted as potent inhibitors of translation of both SV40 and SP6 transcripts. Furthermore, the length of the tails was found to be critical to the translational efficiency. Taking this into account, we found no evidence to suggest that IL-5 is regulated at the level of mRNA stability or translation efficiency by either the 5' or 3'UTR. These results suggest that post-transcriptional control is not a major factor regulating IL-5 expression.

Thiazolidinediones inhibit lipoprotein lipase activity in adipocytes

The thiazolidinediones troglitazone and BRL 49653 improve insulin sensitivity in humans and animals with insulin resistance. Adipose tissue lipoprotein lipase is an insulin-sensitive enzyme. We examined the effects of thiazolidinediones on lipoprotein lipase expression in adipocytes. When added to 3T3-F442A, 3T3-L1, and rat adipocytes in culture, troglitazone and BRL 49653 inhibited lipoprotein lipase activity. This inhibition was observed at concentrations as low as 0.1 microM and within 2 h after addition of the drug. Lipoprotein lipase activity was inhibited in differentiated adipocytes as well as the differentiating cells. Despite this decrease in enzyme activity, these drugs increased mRNA levels in undifferentiated 3T3-F442A and 3T3-L1 cells and had no effect on mRNA expression or synthesis of lipoprotein lipase in differentiated cells. Western blot analysis showed that these drugs did not affect the mass of the enzyme protein. Lipoprotein lipase activity in cultured Chinese hamster ovary cells was not inhibited by troglitazone. Glucose transport, biosynthesis of lipids from glucose or the biosynthesis of proteins were unaffected by thiazolidinediones in differentiated cells, whereas glucose transport and lipid biosynthesis were increased when these drugs were added during differentiation. These results show that troglitazone and BRL 49653 have a specific, post-translational inhibitory effect on lipoprotein lipase in adipocytes, yet they promote lipid accumulation and differentiation in preadipocytes.

Post-translational regulation of perilipin expression. Stabilization by stored intracellular neutral lipids

The perilipins are a family of polyphosphorylated proteins found exclusively surrounding neutral lipid storage droplets in adipocytes and steroidogenic cells. In steroidogenic cells, the cholesterol ester-rich lipid storage droplets are encoated with perilipins A and C. This study describes the dependence of perilipin levels on neutral lipid storage in cultured Y-1 adrenal cortical cells. The addition of fatty acids and cholesterol to the culture medium of Y-1 adrenal cortical cells greatly increased the storage of cholesterol esters and triacylglycerols concomitant with the formation of many new lipid storage droplets. The addition of fatty acids to the culture medium also produced a transient 6-fold increase in levels of perilipin A, but not C, mRNA, while much larger and stable increases in both perilipin A and C proteins were observed. The increases in perilipin protein levels were dependent upon the metabolism of fatty acids to triacylglycerol or cholesterol esters, since the incubation of cells with bromopalmitate, a poorly metabolized fatty acid, failed to yield large increases in lipid content or perilipin levels. Constitutive expression of epitope-tagged perilipins in transfected Y-1 adrenal cortical cells was regulated by lipid similarly to expression of the endogenous perilipins despite an absence of untranslated perilipin mRNA sequences in the expression constructs. Epitope-tagged perilipin A mRNAs were efficiently loaded with polyribosomes whether or not fatty acids were added to the culture medium; therefore, the increase in perilipin levels in the presence of fatty acids is likely due to factors other than increased translational efficiency. We suggest that the large increase in cellular perilipin levels upon lipid loading of cells is the result of post-translational stabilization of newly synthesized perilipins by stored neutral lipids.

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.

Translational regulation of lipoprotein lipase by epinephrine involves a trans-acting binding protein interacting with the 3' untranslated region

To better characterize the translational regulation of lipoprotein lipase (LPL) by epinephrine, cytoplasmic extracts were prepared from 3T3-L1 adipocytes, 3T3-F442A adipocytes, and other nonadipocyte cell lines (C2 cells, 3T3 fibroblasts, and Chinese hamster ovary cells). After treatment with epinephrine, cell extracts from the adipocytes inhibited LPL translation in an in vitro translation assay, whereas extracts from the C2 cells and 3T3 fibroblasts did not affect LPL translation. To identify the region on the LPL mRNA that controlled translation, in vitro translation was carried out using constructs containing different LPL sequences. Specific deletion of the first 50 (1601-1650) nucleotides of the 3' untranslated region (UTR) resulted in a loss of translation inhibition. The addition of LPL 3' UTR to a heterologous reporter gene construct resulted in an inhibition of translation. Inhibition of the reporter LPL 3' UTR translation was demonstrated by the addition of epinephrine-treated cell extracts to an in vitro translation assay, as well as by transfection of this construct into 3T3-F442A cells, followed by treatment of the cells with epinephrine. Competition for a trans-acting binding protein was demonstrated by the addition of sense mRNA strands corresponding to the proximal 135 nucleotides of the 3' UTR of LPL. To identify a RNA-binding protein, adipocyte extracts were incubated with 32P-labeled RNA sequences followed by RNase treatment. The epinephrine-treated cell extract protected a fragment of RNA when the RNA included sequences on the proximal 3' UTR of LPL. Cross-linking of this protected fragment and analysis by SDS-polyacrylamide gel electrophoresis revealed a protein that migrated at about 30 kDa. Thus, the addition of epinephrine to 3T3 adipocytes results in an inhibition of translation through the production of a RNA-binding protein that binds to a region on the proximal 3' UTR of the LPL mRNA.

Acute effects of adrenaline on hepatic lipase secretion by rat hepatocytes

Catecholamines are responsible for the daily changes in hepatic lipase (HL) expression associated with feeding and fasting. We have studied the mechanism by which adrenaline decreases HL secretion in suspensions of freshly isolated rat hepatocytes. Adrenaline acutely inhibited HL activity through activation of the alpha1-adrenergic pathway. The cells had significantly less HL activity in the presence of adrenaline versus cycloheximide, where protein de novo synthesis is completely blocked. The specific enzyme activity of secreted HL was not affected. Intracellular HL activity was decreased by adrenaline treatment. Pulse-labeling with [35S]methionine showed that de novo synthesis of the 53-kd endo-beta-N-acetylglucosaminidase H (Endo H)-sensitive HL protein was unaffected by adrenaline. During subsequent chase of the control cells, the 53-kd form was converted to a 58-kd Endo H-resistant HL protein, which was rapidly secreted into the medium. In the presence of adrenaline, formation of the 58-kd protein was markedly reduced, whereas the 53-kd protein disappeared at a rate similar to the rate in controls. This suggests that part of the HL protein was degraded. In contrast to adrenaline, inhibition of HL secretion by colchicine was accompanied by an intracellular accumulation of HL activity and of the 58-kd protein. We conclude that adrenaline inhibits HL secretion posttranslationally by retarding the maturation of the 53-kd HL precursor to an active 58-kd protein, possibly by stimulating degradation of newly synthesized HL protein.

Fat cells

The adipocyte is a metabolically active cell that functions to store energy for times of energy deprivation or enhanced need. Obesity is characterized by increased lipid accumulation and turnover compared with the nonobese state. Both triglyceride synthesis and lipolysis are regulated metabolic processes in the adipocyte. Current research on the metabolic activities of the human adipocyte focus on plasma triglyceride hydrolysis and uptake of fatty acids by LPL, esterification of these fatty acids, and the subsequent triglyceride breakdown by hormone-sensitive lipase in response to stimulation of adrenergic receptors. These topics are discussed in relationship to the development of obesity.