Regulation of palmitic acid synthesis in cultured glial cells: effects of lipid on fatty acid synthetase, acetyl-CoA carboxylase, fatty acid and sterol synthesis.

Abstract— C6 glial cells in culture were utilized to study the regulation of the important lipogenic enzymes, fatty acid synthetase and acetyl-CoA carboxylase, and the synthesis of fatty acids and sterols. Regulation of these phenomena by lipid was demonstrated by the following observations. First, removal of serum from the culture medium was accompanied over the next five days by 2–3-fold increases in the lipogenic enzymatic activities and in 5–15-fold increases in rates of incorporation of acetate into fatty acids and sterols. Second, cells grown in delipidated serum exhibited approx 2-fold higher levels of activity of the lipogenic enzymes and 5–10-fold higher rates of synthesis of fatty acids and sterols than cells grown in normal calf serum. Third, cells grown in serum-free medium supplemented with concentrations of fatty acid comparable to those present in medium supplemented with serum exhibited activities of fatty acid synthetase comparable to those exhibited by cells grown in the serum-supplemented medium. The mechanism of these effects on fatty acid synthetase was shown by immunochemical techniques to involve alterations in content rather than in catalytic efficiency of the enzyme. The changes in content of the synthetase were caused by alterations in enzyme synthesis. In view of morphological and biochemical data suggesting that C6 cells are related to differentiating cells with properties of both astrocytes and oligodendroglia, the present data may indicate that regulation of palmitic acid synthesis by fatty acid or a product thereof occurs in brain during development.     

Effect of an unnatural phospholipid base analog,N-isopropylethanolamine,on 3-hydroxy-3-methylglutaryl coenzyme a reductase in cultured glial and neuronal cells

Cultured C-6 glial and neuroblastoma cells were utilized to study the effect of the unnatural amino alcohol, N-isopropylethanolamine, on the microsomal enzyme, 3-hydroxy-3-methylglutaryl coenzyme A reductase. Growth of both cell types in the presence of the compound was accompanied in 24 hr by a decrease in reductase activity to 25–35% of activity in control cells. The effect was accompanied by a comparable decrease in the rate of cholesterol synthesis. However, no comparable change occurred in cell growth, fatty acid synthetase activity, or in total protein synthesis from [³H]leucine. The data suggest that the polar head groups of microsomal membrane phospholipids play an important role in the regulation of reductase activity.     

Long-term regulation by theophylline of fatty acid synthetase, acetyl-CoA carboxylase and lipid synthesis in cultured glial cells.

The long-term regulation of fatty acid synthetase and acetyl-CoA carboxylase and of fatty acid and sterol synthesis was studied in C-6 glial cells in culture. When theophylline (10(-3) M) was added to the culture medium of these cells, rates of lipid synthesis from acetate and activities of synthetase and carboxylase became distinctly lower than in cells that were untreated. This effect appeared after approximately 12 h, and after 48 h enzymatic activities were reduced approx. 2-fold and rates of lipid synthesis from acetate 3- to 4-fold. The likelihood that the decrease in fatty acid synthesis from acetate was caused by the decrease in activities of fatty acid synthetase and acetyl-CoA carboxylase was established by several observations. These indicated that the locus of the effect probably did not reside at the level of acetate uptake into the cell, alterations in acetate pool sizes or conversion of acetate to acetyl-CoA. Moreover, de novo fatty acid synthesis was found to be the predominant pathway in these glial cells, whether treated with theophylline or not. The mechanism of the effect of theophylline on fatty acid synthetase was shown by immunochemical techniques to involve an alteration in content of enzyme rather than in catalytic efficiency. The change in content of fatty acid synthetase was shown by isotopic-immunochemical experiments to involve a decrease in synthesis of the enzyme. The mechanism whereby theophylline leads to a decrease in lipogenesis and in the synthesis of fatty acid synthetase may not be mediated entirely by inhibition of phosphodiesterase and an increase in cyclic AMP levels, because dibutyryl cyclic AMP (10(-3) M) only partially reproduced the effect.     

Regulation of palmitic acid synthesis in cultured glial cells: effects of glucocorticoid on fatty acid synthetase, acetyl-CoA carboxylase, fatty acid and sterol synthesis.

Abstract— C-6 glial cells in culture were utilized to define the role of glucocorticoid in the regulation of palmitic acid synthesis and the important lipogenic enzymes, fatty acid synthetase and acetyl-CoA carboxylase. Particular emphasis was given to fatty acid synthetase which exhibited more than a 50% reduction in specific activity when cells were exposed to hydrocortisone (10 μg/ml) for 1 week. Coordinate changes in acetyl-CoA carboxylase activity and in palmitic acid (and sterol) synthesis from acetate accompanied the alterations in fatty acid synthetase. Immunochemical techniques were utilized to show that the decrease in synthetase activity involved an alteration in enzyme content, not in catalytic efficiency. The changes in content of fatty acid synthetase were caused by alterations in enzyme synthesis. Glucocorticoids may regulate fatty acid synthetase in C-6 glial cells by a mechanism similar to that suggested for adipose tissue. The inhibition of palmitic acid synthesis may be relevant to other effects of glucocorticoids on developing brain.     

Estrogen regulation of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and acetyl-CoA carboxylase in xenopus laevis

Administration of estradiol-17 beta to male Xenopus laevis evokes the proliferation of the endoplasmic reticulum and the Golgi apparatus and the synthesis and secretion by the liver of massive amounts of the egg yolk precursor phospholipoglycoprotein, vitellogenin. We have investigated the effects of estrogen on three key regulatory enzymes in lipid biosynthesis, 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase, the major regulatory enzyme in cholesterol and isoprenoid synthesis, and acetyl-CoA carboxylase and fatty acid synthetase, which regulate fatty acid biosynthesis. HMG-CoA reductase activity and cholesterol synthesis increase in parallel following estrogen administration. Reductase activity in estrogen stimulated Xenopus liver cells peaks at 40-100 times the activity observed in control liver cells. The increased rate of reduction of HMG-CoA to mevalonic acid is not due to activation of pre-existing HMG-CoA reductase by dephosphorylation, as the fold induction is unchanged when reductase from control and estrogen-stimulated animals is fully activated prior to assay. The estrogen-induced increase of fatty acid synthesis is paralleled by a 16- to 20-fold increase of acetyl-CoA carboxylase activity, indicating that estrogen regulates fatty acid synthesis at the level of acetyl-CoA carboxylase. Fatty acid synthetase activity was unchanged during the induction of fatty acid biosynthesis by estrogen. The induction of HMG-CoA reductase and of acetyl-CoA carboxylase by estradiol-17 beta provides a useful model for regulation of these enzymes by steroid hormones.     

Comparative studies on lipogenesis and cholesterogenesis in lipemic BHE rats and normal Wistar rats.

The BHE strain of rat is characterized by early hyperinsulinemia and maturity onset hyperlipemia and hyperglycemia. Since we have previously shown that insulin is required for the coordinate regulation of a number of lipogenic enzymes in rat liver, a comparative study of the hepatic activities of the rate-limiting enzymes of lipid synthesis and the in vivo rates of fatty acid and cholesterol synthesis in the liver and the adipose tissue has been conducted in BHE and Wistar rats. In the liver, BHE rats had 25–28% higher acetyl-CoA carboxylase and fatty acid synthetase activities as measured in vitro but a 100% greater rate of fatty acid synthesis in vivo as compared to Wistar animals. These results strongly suggest that factors other than the amount of acetyl-CoA carboxylase, such as allosteric effectors, must be operating in vivo, thereby facilitating the carboxylase to function at its maximal capacity in BHE rats. Such a regulation of fatty acid biosynthesis by allosteric modifiers of acetyl-CoA carboxylase is already known, although the mechanism of this regulation is not fully understood. BHE rats also exhibited a twofold greater rate of fatty acid synthesis in the adipose tissue compared to the Wistar rats. Thus, increased lipogenic capacity and increased lipogenesis in BHE rats are consistent with early hyperinsulinemia in this strain. Furthermore, BHE rats had 71% more 3-hydroxy-3-methylglutaryl CoA reductase activity with a 97% greater rate of cholesterol synthesis as compared to Wistar rats. In contrast, cholesterol 7α-hydroxylase activity was only 20% greater in BHE rats compared to Wistar rats, suggesting that the BHE rat does not have the capacity to degrade cholesterol to bile acids at a rate commensurate with the increased rate of cholesterol synthesis. This difference in synthesis versus degradation might account for the hypercholesterolemia which occurs in BHE rats, but not in Wistar rats.     

A ROLE FOR MICROTUBULES IN THE REGULATION OF 3-HYDROXY-3-METHYLGLUTARYL COENZYME A REDUCTASE AND CHOLESTEROL BIOSYNTHESIS IN CULTURED GLIAL CELLS 1

—Cultured C-6 glial cells were utilized to evaluate the effect of the antimicrotubular drug, Colcemid, on 3-hydroxy-3-melhylglutaryl coenzyme A (HMG-CoA) reductase and cholesterol synthesis in cultured C-6 glial cells. The data indicate that Colcemid causes a marked inhibition of cholesterol synthesis (from [¹⁴C]acetate or ³H₂O) in these cells. A concentration of 0.5 μM led to a 50% lower rate of synthesis after 2 h and an 80–85% lower rate after 12 h or longer. That the effect of Colcemid is mediated at the level of HMG-CoA reductase was shown by defining closely coordinate temporal and quantitative changes in the activity of this enzyme under identical conditions. No comparable change in cell growth or in total protein synthesis accompanied the effect of Colcemid. The drug did lead to a decrease in the rate of DNA synthesis (from [³H]thymidine) but this effect was preceded by the decrease in the rate of cholesterol synthesis. Marked changes in glial cell shape were induced by exposure to Colcemid, and the temporal and quantitative aspects of these changes appeared to closely parallel the effects on reductase activily and cholesterol synthesis. The dala suggest that microtubules are involved in the regulation of HMG-CoA reductase and cholesterol synthesis in mammalian cells and that there are important interrelations between microtubules, glial differentiation and cholesterol synthesis.     

Coordinate regulation of acetyl coenzyme A carboxylase and fatty acid synthetase in liver cells of the developing chick in vivo and in culture.

The activities of hepatic acetyl-CoA carboxylase and fatty acid synthetase undergo two distinct types of development in the perinatal chick. The first increase begins prior to hatching, continues after hatching in the starved chick, and is independent of feeding. The second increase is caused by feeding and is reversed by starvation (A. G. Goodridge (1973) J. Biol. Chem.248, 1932–1938). We have purified these enzymes to homogeneity and raised antibodies to them in rabbits. Using immunochemical techniques we have established that the activity changes in both types of development were a function of changes in the concentrations of enzyme proteins. All activity changes were accompanied by similar changes in the relative rates of synthesis of the two enzymes. Regulation of the activities of acetyl-CoA carboxylase and fatty acid synthetase was further characterized in liver cells from 19-day-old embryos maintained in culture in a chemically defined medium. After 3 days in culture in the absence of hormones, the activities of the enzymes increased significantly with respect to the activities of the freshly prepared cells. Addition of either insulin or triiodothyronine alone caused additional small increases. Insulin plus triiodothyronine caused 8- and 15-fold increases in acetyl-CoA carboxylase and fatty acid synthetase, respectively, relative to cells incubated without hormones. In the presence of insulin alone glucagon had no effect on the activity of either enzyme. In the presence of insulin plus triiodothyronine, glucagon inhibited the increase in enzyme activities by about 75%. The results of quantitative immunoprecipitin tests indicated that activity changes caused by the various hormones were functions of changes in the concentrations of the enzyme proteins. The effects of the hormones on enzyme activities were accompanied by comparable or larger changes in the relative rates of synthesis of the enzymes. Under a wide variety of experimental conditions, both in vivo and in culture, the relative rates of synthesis of acetyl-CoA carboxylase and fatty acid synthetase are regulated coordinately. Under some of these conditions, synthesis of malic enzyme also is regulated coordinately with the syntheses of acetyl-CoA carboxylase and fatty acid synthetase. The common intracellular mechanisms underlying the coordinate control remain to be elucidated.     

Hepatosteatosis in peroxisome deficient liver despite increased β-oxidation capacity and impaired lipogenesis

Peroxisome deficiency in liver causes hepatosteatosis both in patients and in mice. Here, we studied the mechanisms that contribute to this lipid accumulation and to activation of peroxisome proliferator activated receptor α (PPARα) by using liver-specific Pex5^−/− mice (L-Pex5^−/− mice). Surprisingly, steatosis was accompanied both by increased mitochondrial β-oxidation capacity, confirming previous observations, and by impaired de novo lipid synthesis mediated by reduced expression of sterol regulatory element binding protein 1c and its targets. As a consequence, when challenged with a high fat diet, L-Pex5^−/− mice were protected from adiposity. Hepatic fatty acid uptake was strongly increased whereas the expression of apolipoproteins and the lipoprotein assembly factor microsomal triglyceride transfer protein were markedly reduced resulting in reduced secretion of very low density lipoproteins. Most of these changes seemed to be orchestrated by the endogenous activation of PPARα, challenging the assumption that PPARα activation in hepatocytes requires fatty acid synthase dependent de novo fatty acid synthesis. Expression of cholesterol synthesizing enzymes and cholesterol levels were not affected in peroxisome deficient liver. In conclusion, increased fatty acid uptake driven by endogenous PPARα activation and reduced fatty acid secretion cause hepatosteatosis in peroxisome deficient livers.     

Fatty Acid Synthesis in Hydrilla Chloroplasts

The rates of carboxylation, photophosphorylation and acetate incorporation have been compared in the intact and broken chloroplasts of Hydrilla verticillata Royle leaves in the presence and absence of certain inhibitors and metabolites. The intact chloroplasts showed low rates of photophosphorylation, high rates of carboxylation, and exhibited normal capacity for fatty acid biosynthesis. In broken chloroplasts a drastic decrease was observed in the rates of carboxylation and acetate incorporation. However, the rate of photophosphorylation was considerably increased. In the presence of light, inhibitors such as iodoacetamide, arsenite and sodium azide decreased the photophosphorylation rate. F-1,6-di-P and PGA stimulated CO₂ fixation rate. In the absence of artificial light, inhibitors such as sodium arsenite, gluconate-6-phosphate, sodium azide and iodoacetamide decreased the rate of CO₂ fixation. CoA, ATP, G-6-P, F-1,6-di-P Stimulated the synthesis of fatty acids. Exogenous supply of ADP. NADH, NADP and NADPH did not stimulate fatty acid biosynthesis probably because these compounds could not gain entry into the chloroplasts. Light was necessary for the in vitro fatty acid biosynthesis.     

Lipid composition of heteroploid cells in culture: effects of prednisolone

Prednisolone-induced alterations in the content and composition of the total lipids, cholesterol, phospholipids and fatty acids were studied in HeLa S3 cells. After relatively long exposure of the cell cultures to the hormone analog (24 to 72 hours), total cell lipid content was decreased. Partial inhibition of cholesterol and phospholipid metabolism resulted in a shift in the molar ratio of these two lipid constituents. Total fatty acid content was unaffected by prednisolone but there were minor changes in the relative distribution of the individual fatty acids. The observed decrease in cholesterol turnover after addition of prednisolone was primarily due to the reduced uptake of intact cholesterol from the culture medium. This was compensated, in part, by an increased de novo synthesis of cholesterol from acetate and glucose.     

Fatty acid transport protein 4 is the principal very long chain fatty acyl-CoA synthetase in skin fibroblasts

Fatty acid transport protein 4 (FATP4) is a fatty acyl-CoA synthetase that preferentially activates very long chain fatty acid substrates, such as C24:0, to their CoA derivatives. To gain better insight into the physiological functions of FATP4, we established dermal fibroblast cell lines from FATP4-deficient wrinkle-free mice and wild type (w.t.) mice. FATP4 -/- fibroblasts had no detectable FATP4 protein by Western blot. Compared with w.t. fibroblasts, cells lacking FATP4 had an 83% decrease in C24:0 activation. Peroxisomal degradation of C24:0 was reduced by 58%, and rates of C24:0 incorporation into major phospholipid species (54-64% decrease), triacylglycerol (64% decrease), and cholesterol esters (58% decrease) were significantly diminished. Because these lipid metabolic processes take place in different subcellular organelles, we used immunofluorescence and Western blotting of subcellular fractions to investigate the distribution of FATP4 protein and measured enzyme activity in fractions from w.t. and FATP4 -/- fibroblasts. FATP4 protein and acyl-CoA synthetase activity localized to multiple organelles, including mitochondria, peroxisomes, endoplasmic reticulum, and the mitochondria-associated membrane fraction. We conclude that in murine skin fibroblasts, FATP4 is the major enzyme producing very long chain fatty acid-CoA for lipid metabolic pathways. Although FATP4 deficiency primarily affected very long chain fatty acid metabolism, mutant fibroblasts also showed reduced uptake of a fluorescent long chain fatty acid and reduced levels of long chain polyunsaturated fatty acids. FATP4-deficient cells also contained abnormal neutral lipid droplets. These additional defects indicate that metabolic abnormalities in these cells are not limited to very long chain fatty acids.     

Isolation of Saccharomyces cerevisiae Mutants Defective in Acyl Coenzyme A Synthetase

Mutants of Saccharomyces cerevisiae defective in acyl-CoA synthetase (EC 6.2.1.3) were isolated. The mutants were concentrated by the radiation-suicide technique with the use of tritiated palmitic acid. Selection of the mutants was based on the premise that acyl-CoA synthetase activity would become indispensable when yeast cells in which fatty acid synthesis de novo is blocked are grown in a medium supplemented with fatty acid. The mutant strains isolated exhibited low acyl-CoA synthetase activity in vitro. Furthermore, they accumulated markedly more of the incorporated palmitic acid in the nonesterified form than did the wild- type strain. Some of the mutants showed thermosensitive acyl-CoA synthetase activity, indicating a mutation of the structural gene of the enzyme. Genetic studies on these mutants indicated that their phenotype resulted from a single, recessive mutation of a nuclear gene, designated faa 1 (fatty acid activation).     

Factors influencing the serum levels of cholesterol and beta-lipoproteins in starving rabbits]

The influence of starvation on the lipid metabolism was studied on male rabbits under usual conditions and in the presence of pyroxidine deficiency (4-deoxypyridoxine administration) and of thiamine (oxythiamine administration), and also in administration of neurotropic preparations (phenamine, seduxen). Starvation for 7 to 10 days led to increase of cholesterol and beta-lipoproteins level in the serum. Pyridoxine deficiency and phenamine administration caused a greater increase of cholesterol and especially or beta-lipoproteins. On the other hand, thiamine deficiency and seduxen administration limited an increase of cholesterol and beta-lipoproteins during hungry stress. Administration of aerovit for prophylactic purpose promoted a decrease of the metabolic shifts. The amount of cholesterol increased in the liver of hungry animals, especially after the phenamine administration and in the presence of pyridoxine deficiency; aerovit administration prevented increased cholesterol accumulation in the liver. The differences in the cholesterol level in the serum and and the liver can be explained by the changes of its biosynthesis during hungry stress.     

INHIBITION OF ASTAXANTHIN SYNTHESIS UNDER HIGH IRRADIANCE DOES NOT ABOLISH TRIACYLGLYCEROL ACCUMULATION IN THE GREEN ALGA HAEMATOCOCCUS PLUVIALIS (CHLOROPHYCEAE)1

Under stress conditions, Haematococcus pluvialis Flotow accumulates fatty acid–esterified astaxanthin, in extraplastidial lipid globules. The enhanced accumulation of fatty acids, mainly in triacylglycerols (TAG), among which oleic acid predominates, is linearly correlated with that of astaxanthin. We used inhibitors of either carotenoid or lipid biosynthesis to assess the interrelationship between carotenogenesis and TAG accumulation under high light irradiance as the stress factor. The two carotenogenesis inhibitors used—norflurazon, an inhibitor of phytoene desaturase, and diphenylamine (DPA), an inhibitor of β‐carotene C‐4 oxygenase—suppressed the accumulation of astaxanthin in a concentration‐dependent manner. Concurrently, the accumulation of neutral lipids was significantly less affected. The lipid biosynthesis inhibitor sethoxydim, which inhibits acetyl‐CoA carboxylase, significantly decreased de novo fatty acid synthesis and, in concert, drastically inhibited astaxanthin formation. In the presence of various concentrations of the three inhibitors, the inhibition of astaxanthin was not accompanied by a proportional decrease in oleic acid, which was used as a marker for TAG fatty acids. When astaxanthin synthesis was completely inhibited, the volumetric content of oleic acid was about 60% of the control value when the two carotenogenesis inhibitors (0.05 μM norflurazon or 20 μM DPA) were used and 27% of the control when the lipid‐synthesis inhibitor (50 μM) was used. We suggest therefore that TAG accumulation under high irradiance is not tightly coupled with astaxanthin accumulation, although the correlation between these two processes was demonstrated earlier. Furthermore, we propose that the accumulation of a certain amount of TAG is a prerequisite for the initiation of fatty acid–esterified astaxanthin accumulation in lipid globules.     

Control of membrane polar lipid composition in Acholeplasma laidlawii a by the extent of saturated fatty acid synthesis

The low level of endogenous fatty acid synthesis in Acholeplasma laidlawii A strain EF22 was found to be caused by a deficiency of pantetheine in the lipid-depleted growth medium. By supplementing the oleic acid-containing medium with increasing concentrations of pantetheine, saturated fatty acid synthesis was stimulated (having an apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ of 5 μM for pantetheine) and the incorporation of endogenously synthesized fatty acids in membrane lipids increased markedly. Furthermore, carotenoid biosynthesis was stimulated. Exogenous palmitic acid was found to inhibit partially the endogenous fatty acid synthesis. A gradual stimulation of fatty acid synthesis was accompanied by a linear increase in the molar proportion between the two dominating membrane glucolipids, monoglucosyldiacylglycerol and diglucosyldiacylglycerol. The total amount of charged membrane lipids decreased upon increasing the degree of fatty acid saturation. These regulations are discussed in terms of membrane stability, and influence of membrane molecular ordering and surface charge density on lipid polar head group synthesis.     

Lipogenic enzymes in rat maternal adipose tissue in the perinatal period.

1. The specific activities of fatty acid synthetase, acetyl-CoA carboxylase and pyruvate dehydrogenase were measured in rat adipose-tissue extracts in pregnancy and lactation. Fatty acid synthetase specific activity correlates very closely with the rate of fatty acid synthesis, the enzyme specific activity decreasing after mid-pregnancy in a manner very similar to the rate of fatty acid synthesis. Acetyl-CoA carboxylase specific activity also decreases dramatically after mid-pregnancy. Initial pyruvate dehydrogenase specific activity shows a decrease between 2 days pre partum and 2 days post partum, but total enzyme activity shows no significant change in the same period. 2. Immunotitrations of fatty acid synthetase and pyruvate dehydrogenase activities were carried out; the titrations showed that the change in the fatty acid synthetase activity is due to a change in the enzyme amount; the amount of pyruvate dyhydrogenase does not change. Therefore the decrease in fatty acid biosynthesis in subcutaneous and parametrial adipose tissue in late pregnancy and early lactation is associated with a decrease in the amount of at least one of the enzymes involved in fatty acid biosynthesis. The correlation of these events with known hormonal changes is discussed.     

The ergosterol biosynthesis inhibitor zaragozic acid promotes vacuolar degradation of the tryptophan permease Tat2p in yeast

Ergosterol is the yeast functional equivalent of cholesterol in mammalian cells. Deletion of the ERG6 gene, which encodes an enzyme catalyzing a late step of ergosterol biosynthesis, impedes targeting of the tryptophan permease Tat2p to the plasma membrane, but does not promote vacuolar degradation. It is unknown whether similar features appear when other steps of ergosterol biogenesis are inhibited. We show herein that the ergosterol biosynthesis inhibitor zaragozic acid (ZA) evoked massive vacuolar degradation of Tat2p, accompanied by a decrease in tryptophan uptake. ZA inhibits squalene synthetase (SQS, EC 2.5.1.21), which catalyzes the first committed step in the formation of cholesterol/ergosterol. The degradation of Tat2p was dependent on the Rsp5p-mediated ubiquitination of Tat2p and was not suppressed by deletions of VPS1, VPS27, VPS45 or PEP12. We will discuss ZA-mediated Tat2p degradation in the context of lipid rafts.     

Compensatory increase in lipogenic gene expression in adipose tissue of transgenic mice expressing constitutively active AMP-activated protein kinase-alpha1 in liver

We previously described a line of transgenic mice selectively expressing constitutively active AMPK-α1 under the control of liver-specific human apoE promoter with the hepatic control region sequence. In the short-term activation, the CA-AMPK-α1 transgenic mice at age 10–12 weeks exhibited normal hepatic triglyceride content as compared to wild-type mice due to compensatory increase in mRNA expression of genes in the cholesterol and fatty acid synthesis pathways. But it was not known whether the lipogenic gene expression in white adipose tissue also changed. Here we characterized mRNA expression profile of main lipogenic genes in the cholesterol and fatty acid biosynthesis pathway in white adipose tissue. The data show that short-term chronic activation of AMPK in liver caused marked compensatory increase in lipogenic gene expression both in liver due to induction of Srebp-2 and in white adipose tissue due to upregulation of Srebp-1c. These results support the notion that in addition to its well-recognized function for fat storage adipose tissue can play an adaptive role in fatty acid synthesis when fatty acid synthesis is severely reduced in liver, the main lipogenic organ in mammals.     

Relation of Cholesterol to Astrocytic Differentiation in C-6 Glial Cells

Abstract: The relation of cellular cholesterol content to a biochemical expression of astrocytic differentiation was investigated in cultured C-6 glial cells. The astrocytic marker, glutamine synthetase, was studied. Cellular sterol content was perturbed with compactin, a specific inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase and, thereby, cholesterol biosynthesis. Depletion of cellular sterol resulted in 72 h in a more than twofold increase in glutamine synthetase activity. Production of various degrees of sterol depletion with different concentrations of compactin demonstrated a striking inverse relationship between glutamine synthetase activity and the cellular sterol/phospholipid molar ratio. That the effect of compactin, in fact, is mediated by depletion of sterol was shown further by prevention of the compactin-induced increase in synthetase activity by simultaneous addition of exogenous cholesterol. Moreover, addition of cholesterol alone to the culture medium led to both a decrease in glutamine synthetase activity and an increase in the sterol/phospholipid molar ratio. The possibility that the compactin-induced increase in glutamine synthetase activity is caused by an increase in synthesis of the enzyme was suggested by prevention of the increase by cycloheximide. The data suggest that astrocytic differentiation is stimulated by a decrease in cellular sterol content. When considered with our previous observation that oli-godendroglial differentiation is inhibited by such a decrease, the findings suggest that cellular sterol content is a critical determinant of the direction of glial differentiation, i.e., whether along astrocytic or oligodendroglial lines.