iPS-cell-derived microglia promote brain organoid maturation via cholesterol transfer

Microglia are specialized brain-resident macrophages that arise from primitive macrophages colonizing the embryonic brain1. Microglia contribute to multiple aspects of brain development, but their precise roles in the early human brain remain poorly understood owing to limited access to relevant tissues2-6. The generation of brain organoids from human induced pluripotent stem cells recapitulates some key features of human embryonic brain development7-10. However, current approaches do not incorporate microglia or address their role in organoid maturation11-21. Here we generated microglia-sufficient brain organoids by coculturing brain organoids with primitive-like macrophages generated from the same human induced pluripotent stem cells (iMac)22. In organoid cocultures, iMac differentiated into cells with microglia-like phenotypes and functions (iMicro) and modulated neuronal progenitor cell (NPC) differentiation, limiting NPC proliferation and promoting axonogenesis. Mechanistically, iMicro contained high levels of PLIN2+ lipid droplets that exported cholesterol and its esters, which were taken up by NPCs in the organoids. We also detected PLIN2+ lipid droplet-loaded microglia in mouse and human embryonic brains. Overall, our approach substantially advances current human brain organoid approaches by incorporating microglial cells, as illustrated by the discovery of a key pathway of lipid-mediated crosstalk between microglia and NPCs that leads to improved neurogenesis.

A review and update on the current status of stem cell therapy and the retina

INTRODUCTION OR BACKGROUND

Many diseases of the retina result in irreversible visual loss. Stem cell (SC) therapy is a rapidly developing field and represents a novel approach to replace non-functioning neuro-retinal cells.

SOURCES OF DATA

A systematic computerized literature search was conducted on PubMed (http://www.ncbi.nlm.nih.gov/pubmed/).

AREAS OF AGREEMENT

The use of stem cells (SCs) in animal models of retinal diseases has resulted in improvement in visual function and performance. SC therapy represents an exciting prospect in restoring vision. Areas of controversy The use of human embryonic SCs raises ethical concerns.

GROWING POINTS

Human trials using SCs in retinal diseases have recently been approved.

AREAS TIMELY FOR DEVELOPING RESEARCH

The success of SCs in retinal therapy depends not only on implanted cell survival, but also on how well SCs migrate, integrate and form synapses. Further research will be needed to overcome these hurdles.

Digital mobile telephones and interference of ophthalmic equipment

AIM

To assess the effect of mobile telephone electromagnetic interference on electronic ophthalmic equipment.

METHODS

Prospective audit with mobile telephones placed at distances of 3 m, 1 m, and 30 cm from, and in contact with, electronic ophthalmic equipment. Any interruption or cessation of the function of the ophthalmic device was assessed with the mobile telephones in standby, and in dialling or receiving modes. Any alterations of displayed digital figures or numbers were also assessed.

RESULTS

A total of 23 electronic ophthalmic devices in two hospital ophthalmology outpatient departments were evaluated. All six mobile telephones used, and 22 (95.7%) of the 23 ophthalmic equipment evaluated had the Conformité Européene (CE) mark. No device showed any interruption or cessation of function. There were no alterations of displayed digital figures or numbers. The only effect of any kind was found with four instruments (1 non-CE marked), where there was temporary flickering on the screen, and only occurred when the mobile telephones were dialling or receiving at a distance of 30 cm or less from the instruments.

CONCLUSION

This study shows that among the electronic ophthalmic devices tested, none suffered failure or interruption of function, from mobile telephone interference. Although not comprehensive for all ophthalmic equipment, the results question the need for a complete ban of mobile telephones in ophthalmic departments. It highlights the need for a controlled, objectively measured study of the clinically relevant effects of mobile telephones in the ophthalmology outpatient setting.

Effects of cholesterol and apolipoprotein E on retinal abnormalities in ApoE-deficient mice

PURPOSE

To examine the pathologic changes in the retina of apolipoprotein E (apoE)-deficient mice fed a high-cholesterol diet.

METHODS

ApoE-deficient mice (ApoE) were maintained on either regular mouse chow (ApoE-R) or a high-cholesterol diet (ApoE-C) for 25 weeks. Age-matched control C57BL/6J mice (C57) were also maintained on either regular mouse chow (C57-R) or a cholesterol-containing diet (C57-C). Retinal function was assessed by dark-adapted electroretinography (ERG). The eyes were embedded, sectioned, and analyzed by histologic and immunohistochemical methods, as well as by light and transmission electron microscopy.

RESULTS

After the 25-week feeding period, ERG tracings of ApoE-C mice revealed significant increases of a- and b-wave implicit times when compared with the C57-R group of mice. In addition, there were reductions in oscillatory potential (OP) amplitudes in the ApoE-C group. However, a- and b-wave amplitudes appeared to be unchanged among the four groups of mice. Light microscopic examination of the retinas showed that compared with control C57-R mice, ApoE-C mice had significantly lower cell numbers in the inner and outer nuclear layers (85.1% +/- 4.6%, P < 0.05 and 81.4% +/- 3.7%, P < 0.01 of C57-R controls, respectively). Transmission electron microscopy of apoE-deficient mice revealed cells of the inner nuclear layer with condensation of nuclear chromatin and perinuclear vacuolization in focal areas. Bruch's membrane was also found to be thicker, and its elastic lamina appeared disorganized and discontinuous. Immunohistochemistry demonstrated diminished or no immunoreactivity for carbonic anhydrase II and calretinin in the retinal layers of apoE-deficient mice.

CONCLUSIONS

Overall, there were increasing abnormalities of retinal function and cellular morphology among the four groups of mice in the order of C57-R < C57-C < ApoE-R < ApoE-C. These findings suggest that apoE and/or cholesterol play an important role in retinal function.

Oxidized low-density lipoprotein regulates matrix metalloproteinase-9 and its tissue inhibitor in human monocyte-derived macrophages

BACKGROUND

Macrophages in human atherosclerotic plaques produce a family of matrix metalloproteinases (MMPs), which may influence vascular remodeling and plaque disruption. Because oxidized LDL (ox-LDL) is implicated in many proatherogenic events, we hypothesized that ox-LDL would regulate expression of MMP-9 and tissue inhibitor of metalloproteinase-1 (TIMP-1) in monocyte-derived macrophages. MWRHOSA AND RESULTS: Mononuclear cells were isolated from normal human subjects with Ficoll-Paque density gradient centrifugation, and adherent cells were allowed to differentiate into macrophages during 7 days of culture in plastic dishes. On day 7, by use of serum-free medium, the macrophages were incubated with various concentrations of native LDL (n-LDL) and copper-oxidized LDL. Exposure to ox-LDL (10 to 50 microg/mL) increased MMP-9 mRNA expression as analyzed by Northern blot, protein expression as measured by ELISA and Western blot, and gelatinolytic activity as determined by zymography. The increase in MMP-9 expression was associated with increased nuclear binding of transcription factor NF-kappaB and AP-1 complex on electromobility shift assay. In contrast, ox-LDL (10 to 50 microg/mL) decreased TIMP-1 expression. Ox-LDL-induced increase in MMP-9 expression was abrogated by HDL (100 microg/mL). n-LDL had no significant effect on MMP-9 or TIMP-1 expression.

CONCLUSIONS

These data demonstrate that unlike n-LDL, ox-LDL upregulates MMP-9 expression while reducing TIMP-1 expression in monocyte-derived macrophages. Furthermore, HDL abrogates ox-LDL-induced MMP-9 expression. Thus, ox-LDL may contribute to macrophage-mediated matrix breakdown in the atherosclerotic plaques, thereby predisposing them to plaque disruption and/or vascular remodeling.

Translational regulation of lipoprotein lipase by thyroid hormone is via a cytoplasmic repressor that interacts with the 3' untranslated region

To better characterize the increase in lipoprotein lipase (LPL) translation by hypothyroidism, adipocytes were prepared from control and hypothyroid rats. Whereas LPL synthesis was higher in hypothyroid adipocytes, with no change in mRNA levels, there was no increase in hormone-sensitive lipase (HSL) synthesis. To determine whether a transacting translation regulatory factor was present, a cytoplasmic fraction was prepared from control and hypothyroid adipocytes, and added to an in vitro translation system containing the hLPL mRNA. The hypothyroid cell fraction from adipose and heart yielded an increase in LPL translation, when compared to control extracts. Further experiments determined that the control adipocyte extract contained a translation-inhibitory factor that was 8-fold lower in activity in the hypothyroid extract. Using different LPL mRNA constructs in the in vitro translation reaction, the region that controlled translation was localized to nucleotides 1599 to 1638 (proximal 3' untranslated region (UTR)). To confirm the presence of a transacting factor, a sense RNA strand corresponding to this region was added to the in vitro translation reaction. This sense strand competed for the transacting factor in the control cell extract, yet had no effect on the hypothyroid cell extract. Thus, there is a translation repressor factor in the cytoplasm of rat adipocytes, and this factor is greatly reduced in activity in hypothyroid rat adipocytes. Because a similar mechanism of LPL regulation occurs in response to epinephrine, the absence of the translation repressor may be a mechanism for the loss of sensitivity of hypothyroid cells for catecholamines.

1,25-Dihydroxyvitamin D induces lipoprotein lipase expression in 3T3-L1 cells in association with adipocyte differentiation

1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] is known to modulate the development of bone and other mesenchymal cell types. Since osteoblasts and adipocytes are thought to arise in bone marrow from a common progenitor, this work examined the effects of 1,25-(OH)2D3 on adipocyte development, and in particular on the expression of lipoprotein lipase (LPL), which is an early marker for the differentiated adipocyte. 3T3-L1 preadipocytes were cultured in the presence of 1,25-(OH)2D3 (10(-9) to 10(-7) M) for up to 7 days. LPL activity was measured in the medium and cell extracts, and LPL messenger RNA levels were measured by Northern blotting. When compared to control cells, 10(-7) M 1,25-(OH)2D3 increased medium LPL activity by 2- to 3-fold and cellular LPL by 1.5-fold. Significant increases in medium and cellular LPL were observed at 10(-9) M and were maximal at 10(-7) M. Along with the increase in LPL activity, there was an increase in LPL messenger RNA by 2-fold at 5 days, and by 5-fold at 7 days. In addition to an increase in LPL, 1,25-(OH)2D3 increased expression of aP2, an adipocyte-specific marker associated with differentiation. After the addition of 1,25-(OH)2D3, there was a decrease in 3T3-L1 cell number, which is consistent with differentiation, and a decrease in vitamin D receptors. Finally, these cells developed a different morphology. 1,25-(OH)2D3-treated cells assumed a rounded appearance, although without detachment from the dish and without the degree of lipid accumulation usually associated with the addition of insulin, isbutylmethylxanthine, and dexamethasone. It is concluded that 1,25-(OH)2D3 induced LPL expression in 3T3-L1 cells through an induction of differentiation-dependent mechanism(s). These findings suggest an important role for 1,25-(OH)2D3 in normal adipocyte differentiation.

The expression of TNF alpha by human muscle. Relationship to insulin resistance

TNFalpha is orverexpressed in the adipose tissue of obese rodents and humans, and is associated with insulin resistance. To more closely link TNF expression with whole body insulin action, we examined the expression of TNF by muscle, which is responsible for the majority of glucose uptake in vivo. Using RT-PCR, TNF was detected in human heart, in skeletal muscle from humans and rats, and in cultured human myocytes. Using competitive RT-PCR, TNF was quantitated in the muscle biopsy specimens from 15 subjects whose insulin sensitivity had been characterized using the glucose clamp. technique. TNF expression in the insulin resistant subjects and the diabetic patients was fourfold higher than in the insulin sensitive subjects, and there was a significant inverse linear relationship between maximal glucose disposal rate and muscle TNF (r = -0.60, P < 0.02). In nine subjects, muscle cells from vastus lateralis muscle biopsies were placed into tissue culture for 4 wk, and induced to differentiate into myotubes. TNF was secreted into the medium from these cells, and cells from diabetic patients expressed threefold more TNF than cells from nondiabetic subjects. Thus, TNF is expressed in human muscle, and is expressed at a higher level in the muscle tissue and in the cultured muscle cells from insulin resistant and diabetic subjects. These data suggest another mechanism by which TNF may play an important role in human insulin resistance.

Effects of exercise training and feeding on lipoprotein lipase gene expression in adipose tissue, heart, and skeletal muscle of the rat

Lipoprotein lipase (LPL) is found in adipose tissue and muscle, and is important for the uptake of triglyceride-rich lipoproteins from plasma. This study examined the regulation of LPL in adipose tissue and muscle by exercise training in combination with the fed or fasted state. After training male rats on a treadmill for 6 weeks, LPL activity, mass, and mRNA levels were measured in adipose tissue, heart, soleus, and extensor digitorum longus (EDL) muscles and compared with levels in sedentary rats. Tissue LPL was measured as the heparin-released (HR) and cellular-extracted (EXT) fractions 16 hours following the last bout of exercise, during which time some animals were fasted and others were allowed free access to food. Training led to an increase in HR LPL activity and LPL protein mass in soleus and EDL, but had no effort on adipose tissue and heart LPL. The increase in soleus LPL with exercise was in the HR fraction only, whereas the increase in EDL LPL with training was in both the HR and EXT fractions. All these changes in LPL activity were accompanied by similar changes in LPL immunoreactive mass. However, there were no changes in LPL mRNA levels with training. Feeding induced a large increase in adipose tissue LPL activity and mass in both the HR and EXT fractions: however, there was no change in mRNA levels. In heart, feeding yielded a decrease in HR but no consistent change in EXT activity or mass, and a consistent decrease in mRNA levels. As compared with control rats, trained rats demonstrated different responses to feeding in all tissues, especially in soleus and EDL. Whereas feeding had no effect on LPL in soleus and EDL of control rats, feeding induced a decrease in HR and EXT LPL in the soleus of trained rats. In addition, feeding yielded a significant decrease in EXT LPL of the EDL of trained rats. Thus, these data demonstrate that adipose tissue and heart LPL are highly regulated by feeding and are not responsive to long-term exercise training. On the other hand, skeletal muscle LPL is increased in trained rats, but this increase is blunted considerably by feeding following the last bout of exercise. These changes in LPL activity and mass are mostly unaccompanied by changes in LPL mRNA levels, demonstrating that much physiologic regulation occurs posttranscriptionally.

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

Lipoprotein lipase (LPL) is a central enzyme in lipoprotein metabolism and is in part responsible for adipocyte lipid accumulation. Catecholamines are known to decrease the activity of LPL in adipocytes, and we have previously demonstrated that this inhibition occurs posttranscriptionally, with a prominent inhibition of LPL translation. To better characterize the inhibition of LPL translation, 3T3-L1 cells were differentiated into adipocytes, and exposed to epinephrine. Epinephrine induced a dose-dependent decrease in LPL synthesis using [35S]methionine incorporation, with no change in LPL mRNA levels, demonstrating translational regulation of LPL in this cell line. The poly A-enriched RNA from epinephrine-treated cells was translated well in vitro, and there was no difference in the polysome profiles from control and epinephrine-treated cells, suggesting that epinephrine did not affect mRNA editing, and did not induce an inhibition of translation initiation. To obtain evidence for the presence of an inhibitory factor, a cytoplasmic extract from control, and epinephrine-treated adipocytes was human. When compared to the control cell extract, the epinephrine-treated cell extract sharply inhibited LPL translation in vitro, yet had no effect on the translation of other mRNAs. Epinephrine-treated cells had fourfold more of this inhibitor activity than control cells, and this translation inhibition was partially reversed by heat treatment. To determine what region of the LPL mRNA was involved in the translation inhibition, different LPL constructs were synthesized. The inhibitory effect of the epinephrine-treated cell extract was dependent on the presence of the first 40 nucleotides of the 3' (untranslated region UTR) (nucleotides 1599-1638), whereas deletion of the 5' UTR and other areas of the 3' UTR had no effect on translation inhibition. When a sense RNA strand corresponding to this region was added to the in vitro translation reaction, it restored translation towards normal, suggesting that the sense strand was competing for a transacting binding protein. Thus, epinephrine-treated adipocytes produced a transacting factor, probably a protein, that interacted with a region on the LPL mRNA between nucleotides 1599 and 1638, resulting in an inhibition of translation. These studies add new insight into the hormonal regulation of LPL.

Effects of acromegaly treatment and growth hormone on adipose tissue lipoprotein lipase

Lipoprotein lipase (LPL) hydrolyzes lipoprotein triglyceride into nonesterified fatty acids, which are then reesterified and stored in adipose tissue. Previous studies have demonstrated increases in LPL in response to insulin-like growth factor I and GH when added in vitro. This study examined the effects of acromegaly treatment on adipose tissue LPL. Ten patients with clinically active acromegaly were recruited. A fasting adipose tissue biopsy was performed both before and 3 months after treatment with octreotide (8 patients) or surgery plus octreotide (2 patients). With treatment, mean baseline insulin-like growth factor I levels fell from 6.41 to 3.98 U/mL (normal, < 2.2 U/mL; P < 0.05), and serum glycohemoglobin fell from 8.6 to 7.2 (normal, < 6.8). Adipose LPL was measured in the heparin-released fraction as well as the cellular fraction extracted with nonionic detergent (EXT). After treatment of acromegaly, there was no change in heparin-released fraction LPL activity or immunoreactive mass. However, there was an increase in EXT activity from 0.73 +/- 0.33 to 1.83 +/- 0.58 nEq/min.10(6) cells (mean +/- SEM; P < 0.05) and an increase in EXT mass from 4.1 +/- 0.89 to 11.4 +/- 2.0 ng/10(6) cells (P < 0.05). There was no change in LPL messenger ribonucleic acid levels with treatment, determined using both quantitative polymerase chain reaction and Northern blotting. Thus, treatment of acromegaly resulted in an increase in the intracellular level of the LPL protein, with no change in messenger ribonucleic acid levels, suggesting posttranscriptional regulation of LPL. These changes in LPL may be due to improved insulin sensitivity, or to other changes associated with acromegaly treatment.

The expression of tumor necrosis factor in human adipose tissue. Regulation by obesity, weight loss, and relationship to lipoprotein lipase

A previous study reported the increased expression of the cytokine TNF in the adipose tissue of genetically obese rodents. To examine this paradigm in humans, we studied TNF expression in lean, obese, and reduced-obese human subjects. TNF mRNA was demonstrated in human adipocytes and adipose tissue by Northern blotting and PCR. TNF protein was quantitated by Western blotting and ELISA in both adipose tissue and the medium surrounding adipose tissue. Using quantitative reverse transcriptase PCR (RT-PCR), TNF mRNA levels were examined in the adipose tissue of 39 nondiabetic subjects, spanning a broad range of body mass index (BMI). There was a significant increase in adipose TNF mRNA levels with increasing adiposity. There was a significant correlation between TNF mRNA and percent body fat (r = 0.46, P < 0.05, n = 23). TNF mRNA tended to decrease in very obese subjects, but when subjects with a BMI > 45 kg/m2 were excluded, there was a significant correlation between TNF mRNA and BMI (r = 0.37, P < 0.05, n = 32). In addition, there was a significant decrease in adipose TNF with weight loss. In 11 obese subjects who lost between 14 and 66 kg (mean 34.7 kg, or 26.6% of initial weight), TNF mRNA levels decreased to 58% of initial levels after weight loss (P < 0.005), and TNF protein decreased to 46% of initial levels (P < 0.02). TNF is known to inhibit LPL activity. When fasting adipose LPL activity was measured in these subjects, there was a significant inverse relationship between TNF expression and LPL activity (r = -0.39, P < 0.02, n = 39). With weight loss, LPL activity increased to 411% of initial levels. However, the magnitude of the increase in LPL did not correlate with the decrease in TNF. Thus, TNF is expressed in human adipocytes. TNF is elevated in most obese subjects and is decreased by weight loss. In addition, there is an inverse relationship between TNF and LPL expression. These data suggest that endogenous TNF expression in adipose tissue may help limit obesity in some subjects, perhaps by increasing insulin resistance and decreasing LPL.

Tissue-specific expression of human lipoprotein lipase. Effect of the 3'-untranslated region on translation

Lipoprotein lipase (LPL) is a central enzyme in lipoprotein metabolism and is expressed predominantly in adipose tissue and muscle. In these tissues, the regulation of LPL is complex and often opposite in response to the same physiologic stimulus. In addition, much regulation of LPL occurs post-transcriptionally. The human LPL cDNA is characterized by a long 3'-untranslated region, which has two polyadenylation signals. In this report, human adipose tissue expressed two LPL mRNA species (3.2 and 3.6 kb) due to an apparent random choice of sites for mRNA polyadenylation, whereas human skeletal and heart muscle expressed predominantly the longer 3.6-kb mRNA form. To determine whether there was any functional significance to this tissue-specific mRNA expression, poly(A)-enriched RNA from adipose tissue and muscle were translated in vitro, and the poly(A)-enriched RNA from muscle was more efficiently translated into LPL protein. The increased translatability of the 3.6-kb form was also demonstrated by cloning the full-length 3.2- and 3.6-kb LPL cDNA forms, followed by in vitro translation of in vitro prepared transcripts. To confirm that this increased efficiency of translation occurred in vivo, Chinese hamster ovary cells were transfected with the 3.2- and 3.6-kb LPL cDNAs. Cells transfected with the 3.6-kb construct demonstrated increased LPL activity and synthesis, despite no increase in levels of LPL mRNA. Thus, human muscle expresses the 3.6-kb form of LPL due to a non-random choice of polyadenylation signals, and this form is more efficiently translated than the 3.2-kb form.

Expression of lipoprotein lipase in rat muscle: regulation by feeding and hypothyroidism

Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and is found predominantly in adipose tissue and muscle. We examined the mechanism of regulation of LPL in muscles composed of different fiber types (soleus, extensor digitorum longus, and heart) in fed, fasted, and hypothyroid rats. In all muscles, the detergent-extractable (EXT) fraction represented approximately 95% of total LPL activity and mass. LPL activity was similar in the heparin-releasable (HR) fractions of heart and soleus (predominantly type I fibers), while in the EXT fraction LPL activity in soleus was 418 +/- 48 nEq/min per g, and in heart was 272 +/- 30 nEq/min per g (P < 0.05). However, LPL activity in extensor digitorum longus (EDL, predominantly type II fibers) was considerably lower (7.9 +/- 0.8 nEq/min per g in EXT, P < 0.0001 versus heart and soleus). LPL immunoreactive mass followed a pattern similar to LPL activity. LPL mRNA levels were quantitated by both Northern blotting and reverse transcriptase-polymerase chain reaction (RT-PCR), and were approximately equal in heart and soleus, and 5-fold lower in EDL. In response to feeding, LPL activity, mass, and mRNA levels in heart were 30% to 50% lower than in fasted rat heart, although feeding had no effect on soleus or EDL. In hypothyroid animals, muscle LPL activity was increased by 3- to 4-fold in the HR (but not EXT) fractions of heart and soleus (P < 0.05), with no change in LPL mass or mRNA. Thus, muscles with oxidative, type I fibers expressed higher levels of LPL mRNA than muscles containing glycolytic, type II fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal regulation of hormone-sensitive lipase activity and mRNA levels in isolated rat adipocytes

Hormone-sensitive lipase (HSL) mediates the lipolysis of triacylglycerol from mammalian adipocytes, resulting in the release of non-esterified fatty acids and glycerol. Although numerous studies have examined the hormonal regulation of HSL, the measurement of HSL mRNA levels in response to hormonal regulators has not been studied. This study was designed to determine the effects of epinephrine, growth hormone, glucagon, and dexamethasone on HSL expression by measuring HSL mRNA levels and glycerol release in primary cultures of rat adipocytes. Exposure of adipocytes to epinephrine at 10(-7) M and 10(-5) M for 4 h resulted in an increase in medium glycerol (209 +/- 46%, and 284 +/- 58% of control, P < 0.001, respectively). However, no change in HSL mRNA levels occurred due to the epinephrine treatment. Similarly, the peptides glucagon (10(-7) M and 10(-5) M for 4 h) and growth hormone (100 ng/ml for 24 h) resulted in increased medium glycerol and had no effect on HSL mRNA levels in adipocytes. Dexamethasone was added to adipocyte cultures for 4 and 24 h, and resulted in a dose-dependent increase of medium glycerol (102 +/- 8%, 138 +/- 8% (P < 0.001), and 168 +/- 24% (P < 0.001) for 10(-8) M, 10(-7) M, and 10(-6) M, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of gemfibrozil on adipose tissue and muscle lipoprotein lipase

To better understand the mechanism of action of gemfibrozil on plasma triglycerides, lipoprotein lipase (LPL) concentration was measured in adipose tissue and muscle of 16 hypertriglyceridemic patients before and after treatment with gemfibrozil for 6 weeks. The patients were divided into three groups based on clinical criteria as follows: group 1, hypertriglyceridemia without secondary factors; group 2, hypertriglyceridemia with diabetes; and group 3, hypertriglyceridemia with renal insufficiency. LPL activity, immunoreactive mass, synthetic rate, and mRNA levels were measured in the adipose tissue samples, and LPL activity and mass in the muscle samples. Serum triglyceride levels were decreased by 46% by gemfibrozil, and patients demonstrated no change in diet, weight, or glycohemoglobin during the 6 weeks of treatment. Despite the decrease of blood triglyceride levels, there was no significant change in any measure of LPL either in adipose tissue or muscle. Although several patients demonstrated increases in muscle LPL activity, these changes were inconsistent and not statistically significant. Because there was no significant change in LPL, we conclude that gemfibrozil in these patients decreased circulating triglyceride levels predominantly by decreasing hepatic very-low-density lipoprotein (VLDL) secretion.

The regulation of adipose tissue and muscle lipoprotein lipase in runners by detraining

To study the mechanism of lipoprotein lipase (LPL) regulation by exercise, we recruited 16 healthy athletes to undergo a 2-wk period of detraining. Fasting fat and muscle biopsies were performed both before and after the detraining period. In muscle, detraining resulted in a decrease in LPL activity in both the heparin-releasable (HR) (-45%, P < 0.05) and cellular (extractable [EXT]) (-75%, P < 0.005) fractions, with no significant changes in LPL immunoreactive mass and mRNA levels. However, several subjects demonstrated parallel decreases in LPL mass and mRNA levels with detraining, suggesting that there is some degree of heterogeneity in response. In adipose tissue, detraining had the opposite effects on LPL activity. In the HR fraction, detraining resulted in an 86% increase (P < 0.005) in LPL activity, which was paralleled by a 100% (P = 0.02) increase in HR mass. However, there was no significant change in EXT LPL activity or EXT LPL mass. There were no changes in adipose LPL synthetic rate or LPL mRNA levels with detraining. The ratio of adipose tissue/muscle LPL, which may be an important indicator of the tendency for storage of circulating lipids in adipose tissue, increased significantly after detraining. The adipose/muscle LPL ratio was 0.51 +/- 0.17 in the exercising runners, and 4.45 +/- 2.46 in the same runners after detraining (P < 0.05). Thus, detraining of athletes resulted in a decrease in muscle LPL that occurred through post-translational mechanisms, whereas adipose tissue LPL increased, also due to posttranslational changes. This decrease in muscle LPL, coupled with an increase in adipose LPL, yielded a condition favoring adipose tissue storage.

Characterization of lipoprotein lipase activity, secretion, and degradation at different sites of post-translational processing in primary cultures of rat adipocytes

The regulation of adipose tissue lipoprotein lipase (LPL) by feeding and fasting occurs through post-translational changes in the LPL protein. In addition, LPL activity and secretion are decreased when N-linked glycosylation is inhibited. To better understand the role of oligosaccharide processing in the development of LPL activity and in LPL secretion, primary cultures of rat adipocytes were treated with inhibitors of oligosaccharide processing. LPL catalytic activity from the heparin-releasable fraction of adipocytes was inhibited by more than 70%, with similar decreases in LPL mass, when cells were cultured for 24 h in the presence of either tunicamycin or castanospermine. On the other hand, deoxymannojirimycin (DMJ) and swainsonine had no effect on LPL activity. LPL secretion was examined after pulse-labeling cells with [35S]methionine. The appearance of 35S-labeled LPL in the medium was blocked by treatment of cells with tunicamycin and castanospermine, whereas secretion was not affected by DMJ or swainsonine. To examine the effect of oligosaccharide processing on LPL intracellular degradation, adipocytes were treated with tunicamycin, castanospermine, and DMJ and then pulse-labeled with [35S]methionine, followed by a chase with unlabeled methionine for 120 min. The unglycosylated [35S]LPL that was synthesized in the presence of tunicamycin demonstrated essentially no intracellular degradation. In the presence of castanospermine and DMJ, the half-life of newly synthesized LPL was increased to 81 and 113 min, as compared to 65 min in control cells. Thus, castanospermine-treated adipocytes demonstrated a decrease in LPL activity and secretion, suggesting that the glucosidase-mediated cleavage of terminal glucose residues from oligosaccharides is a critical step in LPL maturation.(ABSTRACT TRUNCATED AT 250 WORDS)

The regulation of lipoprotein lipase gene expression by dexamethasone in isolated rat adipocytes

Lipoprotein lipase (LPL) is an enzyme found in adipose tissue that is important in the hydrolysis of triglyceride rich lipoproteins, and in the uptake of FFA lipid into the adipocyte. To examine the effects of glucocorticoids on adipose tissue LPL, male Sprague-Dawley rats were injected with dexamethasone (1 mg/kg) every other day for 10 days, followed by measurement of LPL in epididymal adipose tissue. Compared to sham-injected controls, heparin-releasable LPL activity and LPL mass in the dexamethasone-treated rats were 44% and 62% of those in control rats, respectively. Adipocytes were prepared from the fat pads and pulse labeled with [35S]methionine, demonstrating a decrease in the LPL synthetic rate in the treated rats to 57% of the rate in control rats. In addition, LPL mRNA was quantitated by Northern blotting, demonstrating a decrease in LPL mRNA in the dexamethasone-treated rats. A simultaneous decrease in the message for gamma-actin was also noted. To examine the effects of dexamethasone on LPL in vitro, adipocytes were prepared from normal rats and treated with dexamethasone for 24 h in vitro. Dexamethasone decreased heparin-releasable LPL activity in cultured adipocytes to 40 +/- 6% of the control value (P less than 0.01). This decrease in LPL activity was accompanied by a decrease in the LPL synthetic rate using [35S]methionine labeling, to 33% of the control value, and no specific change in LPL turnover or secretion. In addition, dexamethasone added to adipocytes decreased LPL mRNA levels. Because the combination of insulin plus dexamethasone has been shown to yield synergistic increases in LPL in adipose tissue pieces, insulin was added to isolated adipocytes in combination with dexamethasone. Whereas insulin and dexamethasone individually had opposite effects on LPL, the combination of insulin plus dexamethasone resulted in no change in any aspect of LPL gene expression. Thus, dexamethasone resulted in a decrease in adipocyte LPL mRNA levels both when added to cultured adipocytes in vitro as well as when injected into rats. This decreased LPL mRNA level yielded corresponding changes in the LPL synthetic rate and LPL activity.

Regulation of adipose tissue lipoprotein lipase gene expression by thyroid hormone in rats

Lipoprotein lipase (LPL) is an important enzyme in lipid metabolism, and adipose LPL activity is increased in rats that are deficient in thyroid hormone. To examine the mechanism of thyroid hormone's effect on LPL, LPL gene expression was assessed in the epididymal fat pads of hypothyroid rats. When compared to control rats, LPL activity, mass, and synthetic rate in hypothyroid rats were increased; heparin-releasable LPL activity and mass were increased to 448% and 300% of control, respectively, and [35S]methionine incorporation into LPL was increased to 250% of control. The increases in LPL activity and mass were reversed by treatment of hypothyroid rats with triiodothyronine (T3). However, there was no change in the level of LPL mRNA when compared to the level of gamma-actin mRNA and no effect on LPL transcription using run-off assays. Isolated adipocytes were prepared from normal rats and exposed to 2 nM T3 in vitro for 24 h. The addition of T3 to cultures of adipocytes resulted in a decrease in LPL activity, mass, and [35S]methionine incorporation, but still no change in LPL mRNA level. To determine whether thyroid hormone regulated catecholamine responsiveness, adipocytes were prepared from hypothyroid and control rats, and the responses to epinephrine were compared. Although epinephrine inhibited [35S]methionine incorporation into LPL in control rat adipocytes, there was essentially no effect in hypothyroid rat cells. In addition, T3 treatment of the hypothyroid rats restored the responsiveness to epinephrine. Thus, thyroid hormone regulates LPL in rat adipose tissue posttranscriptionally, resulting in parallel changes in LPL synthetic rate, immunoreactive mass, and activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Epinephrine inhibits lipoprotein lipase gene expression in rat adipocytes through multiple steps in posttranscriptional processing

Previous studies have demonstrated that in vitro treatment of adipocytes with catecholamines results in a decrease in the activity of the enzyme lipoprotein lipase (LPL). To examine the mechanism of this effect, primary cultures of rat adipocytes were cultured in the presence of various concentrations of epinephrine (10(-9)-10(-5) M). Epinephrine yielded a dose-dependent decrease in LPL activity; heparin-releasable LPL activity was reduced to 66% of control values after exposure to 10(-5) M epinephrine for 2 h. However, there was no effect of epinephrine on LPL immunoreactive mass, as measured by enzyme-linked immunosorbent assay. When cells were pulse labeled with [35S]methionine, there was a rapid and dose-dependent decrease in immunoprecipitable LPL. In spite of the decrease in LPL translation, neither epinephrine nor other catecholamines altered the level of LPL mRNA or the rate of LPL transcription. To further examine LPL posttranslational processing, cells were pulse labeled with [35S]methionine in the absence of epinephrine and then chased with unlabeled methionine in the presence of epinephrine. Cells exposed to epinephrine during the chase demonstrated a decrease in LPL secretion into the medium as well as a decrease in LPL degradation. The addition of epinephrine during LPL posttranslational processing did not alter the sensitivity of the newly synthesized LPL protein to endo-beta-N-acetylglucosaminidase-H. Thus, epinephrine had multiple effects on adipocyte LPL. Although there was a rapid decrease in LPL synthesis that was not due to changes in LPL mRNA, the level of LPL protein was unchanged under these conditions due to a decrease in LPL degradation and secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of improved diabetes control on the expression of lipoprotein lipase in human adipose tissue

Patients with diabetes commonly manifest hypertriglyceridemia along with decreased adipose tissue lipoprotein lipase (LPL) activity, and improved diabetes control tends to reverse these abnormalities. To better understand the mechanism of regulation of LPL in diabetes, 11 diabetic patients (3 type I, 8 type II) were brought under improved glycemic control, and adipose tissue LPL gene expression was assessed by performing paired fat biopsies. Six of the 11 patients attained improved control with insulin, with a decrease in glycohemoglobin (glyc Hgb) from 13.8 +/- 0.9 to 10.4 +/- 0.6%; 5 patients attained improved control with glyburide (glyc Hgb fell from 14.2 +/- 2.4 to 8.8 +/- 0.6%), and together they demonstrated a lowering of serum triglycerides and total cholesterol. No changes were observed in HDL cholesterol. Improved diabetes control resulted in a significant increase in LPL activity in both the heparin-releasable (HR) and extractable (EXT) fractions of adipose tissue, as well as in LPL immunoreactive mass. The change in LPL activity with improved control was variable, and showed a positive correlation with the HDL levels prior to treatment (r = 0.74, P less than 0.02). When adipose tissue was pulse-labeled with [35S]methionine, there was an increase in isotope incorporation into LPL after treatment, indicating an increase in LPL synthetic rate. However, improved diabetes control resulted in no significant change in LPL mRNA levels. Thus, improved glycemic control resulted in an increase in LPL activity which correlated with each patient's basal high density lipoprotein. This increase in LPL activity was accompanied by an increase in LPL immunoreactive mass, and an increase in LPL synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of cytokines on the production of lipoprotein lipase in cultured human macrophages

Macrophages are important cells in the pathogenesis of atherosclerosis because of their tendency to accumulate lipid and become transformed into foam cells. Cultured human monocyte-derived macrophages spontaneously secrete lipoprotein lipase (LPL), and LPL has been linked to increased lipid uptake by these cells. Because secretion of various macrophage products depends on activation by lymphokines, we studied the effects of immunoregulatory lymphokines on LPL secretion by cultured human macrophages. After culturing cells in RPMI 1640 medium with 20% fetal calf serum, recombinant human gamma-interferon (gamma-INF), interleukin-1 (IL-1), and interleukin-2 (IL-2) were added to the medium and LPL secretion was assessed by measuring LPL activity and/or LPL mass in the medium. Gamma-INF suppressed LPL production both when added to freshly plated cultures of human blood monocytes, as well as when added to monocyte/macrophages from mature cultures (day 6) that were producing large amounts of LPL. IL-1 inhibited medium LPL when added to freshly plated cultures, but not when added to mature cultures. On the other hand, IL-2 did not inhibit LPL in freshly plated cultures, but produced a dose-dependent suppression of LPL from mature cultures. None of the cytokines were cytotoxic to macrophages, and cells that were cultured in gamma-INF demonstrated partial recovery from LPL-suppressive doses of the cytokine. After exposure of cells to 50 U/ml of gamma-INF and 50 U/ml of IL-2 for 3 days, LPL mRNA levels, when expressed as LPL/gamma-actin ratios, were 42% and 53% of controls, respectively. Thus, activation of human macrophages in vitro by gamma-INF resulted in a suppression of LPL production.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of weight loss on the activity and expression of adipose-tissue lipoprotein lipase in very obese humans

Lipoprotein lipase is an enzyme in adipose tissue that hydrolyzes circulating triglycerides and thereby generates the fatty acids used in the synthesis of triglyceride in fat cells. To determine whether the activity and expression of lipoprotein lipase are affected by weight loss, we studied lipoprotein lipase in the adipose tissue of nine very obese subjects before and after a program of weight reduction. The subjects' mean (+/- SEM) initial weight was 136 +/- 7.3 kg, and the body-mass index (weight in kilograms divided by the square of the height in meters) ranged from 33.3 to 52.8 (mean, 43.0 +/- 2.5). Biopsies of adipose tissue were performed before weight loss and after it, when weight had been stable for three months. The weight reduction was achieved by a very-low-calorie diet (mean weight loss, 42.5 +/- 6.8 kg). After weight loss, the level of heparin-releasable lipoprotein lipase activity increased in all patients, from 3.8 +/- 1.1 to 7.1 +/- 1.6 neq of free fatty acid released per minute per 10(6) cells (P less than 0.05). In addition, the amount of lipoprotein lipase immunoreactive protein increased from 6.3 +/- 1.7 to 24.4 +/- 6.9 ng per 10(6) cells (P less than 0.05), and there was also an increase in the level of lipoprotein lipase messenger RNA as measured by Northern blotting. There was a strongly positive correlation between the initial body-mass index and the magnitude of the increase in lipoprotein lipase activity (r = 0.80, P less than 0.01) and immunoreactive protein (r = 0.92, P less than 0.01). We conclude that weight loss in very obese subjects leads to the increased activity and expression of lipoprotein lipase, thereby potentially enhancing lipid storage and making further weight loss more difficult.

Identification of lipoprotein lipase immunoreactive protein in pre- and postheparin plasma from normal subjects and patients with type I hyperlipoproteinemia

Postheparin plasma is a convenient source for the measurement of lipoprotein lipase (LPL) in humans. Previous studies have focused on the measurement of LPL catalytic activity, and have been unable to conveniently measure the LPL protein or identify possibly different plasma forms of the enzyme. Pre- and postheparin plasma was treated with a highly specific antibody raised against bovine milk LPL and the immunoprecipitate was analyzed by Western blotting. In normal subjects there were several species of LPL in plasma. A 56 kD protein increased after heparin injection, and likely represented active LPL. The anti-LPL antibody reacted specifically with this 56 kD protein, and also reacted specifically with proteins at 52 kD, 69 kD, as well as a 20 kD breakdown product. In addition, using peptide mapping, the 56 kD protein was structurally similar to the 52 and 69 kD LPL proteins. The antibodies were affinity purified, biotinylated, and used to quantitate LPL immunoreactive mass using an enzyme-linked immunosorbent assay (ELISA). LPL immunoreactive mass was present in all subjects in preheparin plasma. In postheparin plasma, five patients with type I hyperlipoproteinemia displayed decreased LPL immunoreactive mass when compared to normal subjects, although there was a wide range of specific activity of the small amount of enzyme present. When the LPL from the plasma of the patients was immunoprecipitated and Western blotted, there was considerable heterogeneity in the appearance of the LPL forms, and an overall decrease in LPL protein. Thus, several different immunoreactive LPL proteins were present in pre- and postheparin plasma. In preheparin plasma, as well as in patients with type I hyperlipoproteinemia, there was decreased immunoreactive LPL protein, and the LPL protein that was present was of low specific activity.

Effect of feeding and obesity on lipoprotein lipase activity, immunoreactive protein, and messenger RNA levels in human adipose tissue

Previous studies have demonstrated higher levels of adipose tissue lipoprotein lipase (LPL) catalytic activity in obese subjects, and in response to a meal. To examine the cellular mechanism of this increase in activity, LPL activity, immunoreactive mass, and mRNA level were measured in lean and obese subjects both before and 4 h after a carbohydrate-rich meal. Heparin-releasable (HR) LPL activity was approximately 2.5-fold higher in the 15 obese subjects, when compared with six lean subjects. However, there was no difference in LPL immunoreactive mass between the lean and obese subjects. In response to the meal, there was a 2.2-fold increase in total adipose tissue LPL activity in the lean subjects due to an increase in both the HR fraction, as well as the adipose fraction extracted with detergents. However, no increase in LPL immunoreactive mass was observed in any adipose tissue LPL fraction, resulting in an increase in LPL specific activity in response to the meal. In the obese subjects, there was no significant increase in LPL activity in response to feeding, and also no increase in immunoreactive mass or specific activity. After extraction of RNA, there was no difference in either the relative proportion of the 3.6- and 3.4-kb human LPL mRNA transcripts, nor in the quantity of LPL mRNA in response to feeding. Thus, these data suggest that the increase in LPL activity under these conditions occurs through a posttranslational activation of a previously inactive LPL precursor.

The role of glucose and glycosylation in the regulation of lipoprotein lipase synthesis and secretion in rat adipocytes

Several studies have suggested that insulin and glucose increase adipose tissue lipoprotein lipase (LPL). To study the mechanism of the glucose-induced stimulation of LPL, the effects of glucose and glycosylation were examined in primary rat adipocyte cultures. In cells cultured in the presence of 1 mg/ml glucose, a 55-kDa LPL protein was synthesized and secreted into the medium, whereas cells cultured in glucose-free medium synthesized a 49-kDa form of LPL which was not secreted. The treatment of the mature 55-kDa form of LPL with peptide:N-glycosidase-F resulted in the formation of a 49-kDa form of LPL. When cells were cultured in the presence of tunicamycin, a 49-kDa form of LPL was synthesized by the cells but was not secreted. In addition, LPL activity was reduced by 90% when glycosylation was blocked by either tunicamycin or glucose deprivation. LPL synthetic rate was examined in cells cultured in a spectrum of glucose concentrations. LPL synthetic rate increased directly with medium glucose concentration and was decreased 80% in the absence of glucose compared to the synthetic rate in the presence of 1 mg/ml glucose. In addition, LPL synthetic rate in the presence of insulin was approximately 200% of the synthetic rate in untreated control cells at all glucose concentrations and even in the absence of glucose. In spite of the effect of glucose on LPL synthetic rate, glucose had no effect on the level of LPL mRNA. In contrast, the mRNA for the 78-kDa glucose-regulated protein (GRP78) was increased in adipocytes cultured in glucose-free medium. In summary, glucose was essential for glycosylation of LPL, and glycosylation was essential for LPL catalytic activity and secretion. In addition, glucose stimulated LPL synthetic rate and potentiated the stimulatory effects of insulin, but had no specific effect on LPL mRNA. Whereas insulin stimulates LPL by increasing the level of LPL mRNA, glucose stimulates LPL translation and post-translational processing.

Insulin increases the synthetic rate and messenger RNA level of lipoprotein lipase in isolated rat adipocytes

Lipoprotein lipase (LPL) is the enzyme responsible for hydrolysis of circulating triglyceride-rich lipoproteins and is important for storage of adipocyte lipid. To study the regulation of LPL synthetic rate in adipose tissue, primary cultures of isolated rat adipocytes were pulse-labeled with [35S]methionine, and LPL was immunoprecipitated with an LPL-specific antibody. A pulse-chase experiment identified the cellular and secreted forms of LPL as a 55-57-kDa protein. In the presence of heparin, there was a large increase in secretion of newly synthesized LPL from the cells, although heparin did not stimulate cellular LPL synthetic rate. When cells were exposed to insulin for 2 h, pulse-labeling revealed that insulin stimulated a maximal dose-related increase in LPL synthetic rate of 300% of control. This increase in LPL synthetic rate was observed after an exposure to insulin for as little as 60 min and was accompanied by only a 10-25% increase in total protein synthesis. In addition, insulin had no effect on the turnover of intracellular LPL. Using a cDNA probe for LPL, insulin induced a 2-fold increase in the LPL mRNA. Thus, insulin stimulated an increase in specific LPL mRNA in isolated rat adipocytes. This increase in LPL mRNA then leads to an increase in the synthetic rate of the LPL protein.

Regulation of lipoprotein lipase immunoreactive mass in isolated human adipocytes

Previous studies of human adipose tissue lipoprotein lipase (LPL) have focused on enzyme catalytic activity, and have not measured the LPL protein directly. To study the regulation of the LPL protein, an antibody against purified bovine LPL was used. To demonstrate the specificity of the antiserum, adipose homogenates were Western blotted, and adipocytes were radiolabeled and the cell homogenates immunoprecipitated, yielding a single specific band at 53 kD. Breakdown products of LPL were demonstrated at 35 and 20 kD by Western blotting. An ELISA for human adipose LPL was established, in which LPL was sandwiched between affinity-purified antibody and biotinylated affinity-purified antibody. The standard curves for bovine LPL and human adipose LPL were parallel, and LPL activity correlated strongly with LPL immunoreactive mass. Thus, the bovine LPL standard curve was used to estimate LPL immunoreactive mass from human adipose tissue. The regulation of LPL activity and immunoreactive mass were compared in cultured adipocytes in the presence an absence of insulinlike growth factor-I/somatomedin C (IGF-I), insulin, and fetal bovine serum. IGF-I and a high insulin concentration (70 nM) stimulated only the heparin-releasable (HR) component of LPL activity and immunoreactive mass, and neither IGF-I nor insulin affected LPL specific activity. In contrast, 10% fetal bovine serum stimulated HR activity, HR mass, and cellular extractable (EXT) immunoreactive mass, with no effect on EXT activity. This resulted in a decrease in EXT specific activity in response to serum. The effects of the locally produced nucleosides adenosine and inosine were studied in a similar manner. As with serum, adenosine stimulated HR activity, HR mass, and EXT immunoreactive mass, resulting in a decrease in EXT specific activity. Inosine stimulated an increase in HR activity and HR mass, but had no effect on EXT, and thus did not change LPL specific activity. Thus, a sensitive ELISA for adipose tissue LPL has been developed using a specific, well-characterized antibody. Regulation of human LPL immunoreactive mass was demonstrated in vitro by IGF-I, serum, high concentrations of insulin, adenosine, and inosine. This method will permit further investigations into the regulation of the LPL protein.