Insulin and insulin-like growth factor 1 antagonize the stimulation of ob gene expression by dexamethasone in cultured rat adipose tissue

The ob gene, specifically expressed in fat cells, encodes leptin, a hormone that induces satiety and increases energy expenditure. In this study, we investigated the interactions between glucocorticoids and insulin on ob gene expression in cultured explants of rat adipose tissue. Only low levels of ob mRNA were detected when adipose tissue from fasted rats was cultured for 12-24 h in minimal essential medium. However, the addition of dexamethasone to the medium increased ob gene expression in a concentration-dependent manner (EC50 10 nM). With 1 microM dexamethasone, ob mRNA levels were similar to those in fresh fat pads from fed rats, reaching a maximum after 12 h. The effect of dexamethasone was blocked by actinomycin D, which indicates an action on transcription. This effect was increased when a minimum amount of fuel (glucose or a mixture of lactate and pyruvate) was supplied in the medium. Unlike dexamethasone, insulin, even when combined with high glucose concentrations, did not induce ob expression, although it strongly increased the accumulation of mRNA species for fatty acid synthase (FAS), the insulin-sensitive glucose transporter GLUT4 and the gamma isoform of peroxisome proliferator-activated receptor (PPARgamma). Unexpectedly, insulin dose-dependently inhibited dexamethasone-induced ob mRNA accumulation. This effect was observed at low concentrations of insulin (IC50 1 nM) and was delayed in onset, beginning after 6-9 h of culture. It was mimicked by insulin-like growth factor 1 (IGF-1) (100 nM). The inhibition by insulin was only detectable when fuels were present and/or when a critical level of ob expression was reached. As this inhibitory effect was reversed by cycloheximide, this suggests that it required ongoing protein synthesis. In conclusion, unlike dexamethasone, insulin had no direct stimulatory effect on ob gene expression. On the other hand, insulin (and IGF-1) even inhibited the dexamethasone-induced accumulation of ob mRNA. The underlying mechanism involved ongoing synthesis of an inhibitory protein by insulin, which is in keeping with its delayed effect. Moreover, the expression of genes for FAS, GLUT4 and PPARgamma may be inversely related to that of ob.

Animal fatty acid synthase: functional mapping and cloning and expression of the domain I constituent activities

Animal fatty acid synthase (FAS; EC 2.3.1.85) is a homodimer of a multifunctional subunit protein and catalyzes the synthesis of palmitate from acetyl-CoA, malonyl-CoA, and NADPH. The subunit (Mr approximately 270,000) carries seven distinct component activities and a site for the prosthetic group 4'-phosphopantetheine (acyl carrier protein). Based on proteolytic mapping, the organization of the activity domains along the subunit polypeptide from the N terminus is as follows: beta-ketoacyl synthase, acetyl and malonyl transacylases, beta-hydroxyacyl dehydratase, enoyl reductase, beta-ketoacyl reductase, acyl carrier protein, and thioesterase. By comparing the amino acid sequences of the chicken, rat, and human synthases, we found that kallikrein cleavage sites occur in the least conserved regions of the FAS polypeptide subunit. Determining the amino acid sequences of the N-terminal end of the major kallikrein cleavage peptides helped delineate the most likely boundaries of the component activities in the cDNA-derived amino acid sequence. To confirm this organization, we cloned the chicken FAS cDNA coding for domain I and expressed it in Escherichia coli as a maltose-binding fusion protein. The isolated recombinant protein contained the activities of the acetyl and malonyl transacylases and the beta-hydroxyacyl dehydratase. Based on the boundaries of the acetyl and malonyl transacylases and the beta-hydroxyacyl dehydratase, we also cloned the appropriate cDNA fragments encoding the domains that contain the transacylases and the dehydratase in pET vectors and expressed them in E. coli as thioredoxin-6xHis fusion proteins. The purified recombinant proteins contained, respectively, the activities of the acetyl and malonyl transacylases and the dehydratase. These results not only confirmed the order of the component activities in domain I, but also paved the way for successful expression and characterization of the remaining activities.

Regulatory elements in the first intron of the rat fatty acid synthase gene

Sequence elements have been identified within the 1.2 kb-long first intron of the fatty acid synthase (FAS) gene that mediate both positive and negative effects on transcription. The negative regulatory element, when positioned downstream of either the FAS or simian virus 40 promoter, down-regulates the expression of a coupled reporter gene in an orientation-dependent manner. Sequences mediating this effect have been mapped, by deletion mutagenesis, to two regions approximately within nucleotides +405 to +768 and +924 to +1083. Both regions contain sequence elements that are strongly protected from DNase I digestion by nuclear extracts prepared from liver, but not by those prepared from spleen. The results of run-on assays performed with nuclei derived from tissues that express FAS at either high or low levels indicate that the different rates of transcription of the endogenous FAS gene result from differences in the extent of initiation, so it is unlikely that the negative effect is caused by transcriptional pausing in the first intron. The positive element maps to nt +292 to +297 and corresponds to an authentic binding site for upstream stimulatory factor (USF). This USF-binding element can up-regulate transcription from a heterologous promoter in a position- and orientation-independent manner. However, in the context of the entire FAS first intron, the effect of the USF-binding site is masked unless the effect of the negative elements is ablated by mutagenesis. These results suggest that the dominant negative element of the first intron may play a role in determining the tissue-specific expression of the FAS gene.

Androgens stimulate fatty acid synthase in the human prostate cancer cell line LNCaP

In addition to modulation of cell proliferation and stimulation of prostate-specific antigen secretion, one of the most striking effects of androgens on the human prostate cancer cell line LNCaP is the accumulation of neutral lipids. These lipids are synthesized de novo, suggesting that LNCaP cells express all enzymes required for endogenous lipogenesis and that the expression and/or activity of some of these enzymes is affected by androgens. One of the key enzymes involved in lipogenesis is fatty acid synthase (FAS), a potential prognostic enzyme and therapeutic target that is found to be frequently overexpressed in a variety of cancers including prostate cancer. Here, using Northern blot analysis, the gene encoding FAS is shown to be abundantly expressed in LNCaP cells and in two other prostate cancer cell lines tested (PC-3 and DU-145). In LNCaP cells, androgen treatment (10(-8) M R1881) causes a 3-4-fold increase in FAS mRNA levels. Concomitantly with the increase in FAS gene expression, androgens induce a 10-12-fold stimulation of FAS activity. Effects are dose- and time-dependent and follow courses similar to those of the androgen induction of lipid accumulation. In support of the involvement of the androgen receptor, steroid specificity of regulation of FAS activity is in agreement with the aberrant ligand specificity of the mutated androgen receptor in LNCaP cells. Stimulation of FAS activity is inhibited by the antiandrogen Casodex (bicalutamide) and is absent in the androgen receptor-negative cell lines PC-3 and DU-145. Taken together, these data demonstrate that androgens, mediated by the androgen receptor, stimulate the expression and activity of FAS and suggest that stimulation of FAS activity represents at least part of the mechanism by which androgens induce the accumulation of neutral lipids in LNCaP cells.

Elevated expression of fatty acid synthase and fatty acid synthetic activity in colorectal neoplasia

Expression of the primary enzyme catalyzing the synthesis of fatty acids, ie, fatty acid synthase (FAS), and ex vivo fatty acid synthetic activity were examined in colorectal epithelium and neoplasms, including the relationship to tumor progression and prognosis. Immunohistochemistry for FAS showed only faint staining of native colorectal mucosa, but increased expression was found in all sporadic adenomas (n = 18), adenomas associated with familial adenomatous polyposis (n = 7), hyperplastic polyps (n = 3), dysplasias arising in ulcerative colitis (n = 17), and colorectal carcinomas (n = 130) including 11 with contiguous adenomas. The intensity of staining was strong in 53% of carcinomas, intermediate in 38%, and weak in 9%. Activity of the fatty acid synthetic pathway measured by labeling of six surgical specimens with [U-14C]acetate was 2- to 7-fold higher in colorectal carcinomas than adjacent native mucosa (P = 0.006) and 6- to 16-fold higher than serosal fat (P = 0.01). Activity correlated with immunohistochemical expression (Spearman's rank correlation coefficient = 0.85; P < 0.001). There was no statistically significant association between patient survival and FAS staining intensity of carcinomas. Our study shows that FAS is expressed in all colorectal neoplasms and there is a concomitant increase in fatty acid synthesis. FAS may therefore represent a potential therapeutic target.

Expression of fatty acid synthase is closely linked to proliferation and stromal decidualization in cycling endometrium

Estrogen-driven proliferative phase growth is the most rapid physiological proliferative process that occurs in the adult. The tissue growth that occurs during this phase of the menstrual cycle requires incorporation of a substantial quantity of fatty acid into the structural lipids of cell membranes. Fatty acid synthase (FAS) is the major biosynthetic enzyme required for de novo synthesis of fatty acids. In this immunohistochemical study, we have observed that human endometrium displays distinct patterns of FAS expression in the proliferative and secretory phases of the normal menstrual cycle. Proliferative endometrial glands and stroma show high FAS expression that closely correlates with expression of Ki-67, estrogen and progesterone receptors, supporting the view that FAS expression plays a role in cellular proliferation in response to estrogen. FAS expression declines during early to midsecretory phase, then reappears in decidualized stromal cells in late secretory phase as well as in the decidua of pregnancy. The second wave of FAS expression correlates with progesterone-receptor localization in the decidual cells, a finding suggesting a second induction of FAS expression in the endometrium, associated with differentiation, that may be regulated by progesterone.

Insulin-like growth factor I and insulin induce adipogenic-related gene expression in fetal brown adipocyte primary cultures

Fetal rat brown adipocytes show high-affinity binding sites for both insulin-like growth factor I (IGF-I) and insulin. Cell culture for 24 h in the presence of IGF-I or insulin, independently, up-regulated the mRNA expression of adipogenic-related genes, such as fatty acid synthase (FAS), glycerol-3-phosphate de-hydrogenase and insulin-regulated glucose transporter Glut4, and down-regulated the expression of phosphoenolpyruvate carboxykinase mRNA in a dose-dependent manner. Moreover, both IGF-I and insulin increased the FAS gene transcription rate at 2 h, producing a time-dependent accumulation of FAS mRNA. Furthermore IGF-I or insulin increased glucose uptake and lipid content throughout the 24 h culture period. Our results suggest that both IGF-I and insulin are major signals involved in initiating and/or maintaining the expression of adipogenic-related genes in fetal rat brown adipocytes.

Regulation of gene expression for lipogenic enzymes in the liver and adipose tissue of hereditary hypertriglyceridemic, insulin-resistant rats: effect of dietary sucrose and marine fish oil

Hypertriglyceridemia is closely linked to insulin resistance. Increased dietary intake of n-3 polyunsaturated fatty acids reverses both hypertriglyceridemia and insulin resistance. To evaluate molecular mechanisms responsible for the hypotriglyceridemic effects of n-3 polyunsaturated fatty acids, the expression of genes for lipogenic enzymes in liver and white and brown adipose tissue was estimated in hereditary hypertriglyceridemic rats which underwent an euglycemic hyperinsulinemic clamp. Before the clamp, animals were fed a basal or a high (63%) sucrose diet with or without fish oil for two weeks. Results were compared to data obtained from control animals subjected to the identical protocol. In hereditary hypertriglyceridemic rats, gene expression for malic enzyme was increased in liver and in brown adipose tissue but not in white adipose tissue. The high sucrose diet raised malic enzyme mRNA levels in liver of both hereditary hypertriglyceridemic and control rats, and this effect was more pronounced in brown adipose tissue. Supplementing the high sucrose diet with fish oil led to a suppression of malic enzyme gene expression in liver and brown adipose tissue of control rats. However, this inhibitory effect was not as pronounced in the hereditary hypertriglyceridemic rats. Raised levels of fatty acid synthase mRNA in liver and brown adipose tissue of control rats fed high sucrose diet were suppressed by consumption of diet high in n-3 fatty acids. On the other hand, in hereditary hypertriglyceridemic rats fed high sucrose diet, fish oil supplementation failed to suppress increased levels of fatty acid synthase mRNA in liver and in brown adipose tissue. It appears that hereditary hypertriglyceridemic rats have elevated levels of mRNA for lipogenic enzymes in liver and brown adipose tissue and dietary control leading to an alteration of hypertriglyceridemia influences gene expression of lipogenic enzymes only under special dietary circumstances.

Nutritional regulation of lipogenic enzyme gene expression in rat epididymal adipose tissue

The time courses of gene expression, and the nutritional regulation of gene expression of lipogenic enzymes (acetyl-CoA carboxylase, fatty acid synthase, ATP citrate-lyase, malic enzyme, and glucose-6-phosphate dehydrogenase) in epididymal adipose tissue after refeeding food-deprived rats have been investigated and compared with those in liver (previously reported). The mRNA concentrations of lipogenic enzymes reached maximum levels at 24 h after the refeeding in adipose tissue and at 8-16 h in liver, while the enzyme induction reached maximum at 48-72 h in both tissues. Moreover, the mRNAs were more strongly induced in adipose tissue than in liver, whereas the enzyme induction (except malic enzyme) was lower. In adipose tissue of rats fed a carbohydrate diet without protein, the mRNA concentrations of acetyl-CoA carboxylase, ATP-citrate lyase, malic enzyme, and fatty acid synthase reached comparable levels to those of the carbohydrate/protein diet group. The protein feeding increased the enzyme induction in adipose tissue. As regards reduction of gene expression, lipogenic enzyme mRNA concentrations were not so markedly reduced by starvation or polyunsaturated fatty acids in adipose tissue as in liver. The differences in regulation of lipogenic enzyme gene expression and induction between adipose tissue and liver can be ascribed to tissue specificity.

Nutritional and hormonal regulation of fatty acid synthase

Fasting causes a decrease in the rate of synthesis of fatty acid synthase, the central enzyme in fatty acid synthesis, while refeeding carbohydrate increases synthesis. Insulin also increases the synthesis of fatty acid synthase, while glucagon causes a decline. The mechanism was shown to be transcriptional activation-mediated through a 2.1-kb stretch of the 5'-flanking sequence of the fatty acid synthase gene promoter that contains an insulin response element. These effects were confirmed by in vivo experiments with transgenic mice.

Fatty acid synthase (FAS): a target for cytotoxic antimetabolites in HL60 promyelocytic leukemia cells

Many human cancers express elevated levels of fatty acid synthase (FAS), with correspondingly increased fatty acid synthesis and abnormal fatty acid utilization. Recent studies have shown that the FAS inhibitor, cerulenin, is selectively cytotoxic to cell lines derived from human malignancies, suggesting that those carcinoma cells are dependent upon endogenous fatty acid synthesis for growth. These data further suggest that the fatty acid synthesis pathway is a potential target for chemotherapy development. The present studies demonstrate that cerulenin cytotoxicity is mediated by fatty acid pathway inhibition. Proliferating HL60 promyelocytic leukemia cells express high levels of FAS mRNA and protein and synthesize fatty acid predominantly for membrane phospholipid. Following exposure to 12-O-tetradecanoylphorbol-13-acetate, the FAS expression in HL60 cells is abolished, fatty acid synthesis diminishes, and the cells become insensitive to cerulenin while acquiring a differentiated, macrophage-like phenotype. HL60 cells adapted to growth in serum- and fatty acid-free medium show a dose-dependent sensitivity to cerulenin, which is reversed by palmitate, the major product of FAS, indicating that cerulenin cytotoxicity is mediated through fatty acid starvation. Cells grown in the presence of exogenous fatty acid partially downmodulate FAS expression and increase mean cell volume (phospholipid mass/cell) but retain their sensitivity to cerulenin, which is reversed by 3-fold excess oleate supplementation. These results demonstrate that malignant cells can retain dependence on endogenous fatty acid synthesis and sensitivity to FAS inhibitors in the presence of physiological fatty acid levels and thus support the notion that FAS inhibitors may be useful in treating cancer in vivo.

Regulation of fatty acid synthase expression by cholesterol in human cultured cells

The regulation of fatty acid synthase (FAS) expression by sterols in a cultured human hepatoblastoma cell line, Hep G2, was studied. When cells were treated with compactin in a medium containing lipoprotein deficient serum, FAS mRNA level increased 1.6-fold. A squalene synthase inhibitor, TAN1607A, decreased both free and esterified cholesterol contents in Hep G2 cells and increased mRNA levels for FAS, HMG-CoA reductase, squalene synthase and LDL receptor. However, for the increment of FAS mRNA, a 10-fold higher concentration of this inhibitor was needed. These results demonstrate that the concentration of cellular cholesterol which regulates FAS expression is necessarily lower than the levels which regulate other sterol sensitive genes. FAS mRNA was also increased by an inhibitor of SREBP degradation as well as chenodeoxycholic acid. These results indicate that FAS mRNA expression is regulated by cholesterol and is mediated through SREBPs. The implications of the different modes of sterol regulation of FAS and LDL receptor expression are discussed.

Construction of the complete rat fatty acid synthase cDNA and its expression in Saccharomyces cerevisiae

The 272 647-dalton polypeptide of fatty acid synthase (FAS) from Rattus norvegicus has been expressed in a proteinase-deficient strain of Saccharomyces cerevisiae. The seven overlapping cDNA clones for rat FAS spanning the entire coding region were the starting material for this undertaking. In a series of cloning steps an expression plasmid was constructed in which the cDNA was placed under the control of the yeast ADH1 promoter. Northern blotting of total RNA isolated from yeast transformed with this expression plasmid demonstrated a high rate of transcription of the 7.4-kb cDNA. However, a successful translation required further manipulation of the sequence immediately upstream of the rat FAS translational start codon. This was obtained when the 86 bp of the rat FAS cDNA immediately 5' to the start codon were replaced by a nonamer corresponding to the immediate 5'-vicinity of the translational start codon of the yeast ADH1 gene. Nevertheless, the translation product could be detected only by Western blotting. The FAS proteins of S. cerevisiae and rat are not functionally interchangeable. Using the purification protocol of rat FAS the heterologously expressed FAS could be enriched by at least one order of magnitude.

Hormonal and nutritional control of the fatty acid synthase promoter in transgenic mice

To study the molecular basis of tissue-specific and hormonally regulated expression of the fatty acid synthase (FAS) gene in vivo, we generated lines of transgenic mice carrying 2.1 kilobases of the 5'-flanking region (-2100 to +67) of the rat FAS gene fused to a chloramphenicol acetyltransferase (CAT) reporter gene. This reporter gene construct was strongly expressed in tissues that normally express high levels of FAS mRNA, which include liver and white adipose tissues. In contrast, CAT reporter activity was not detected in appreciable levels in lung, heart, kidney, and muscle tissues, which do not normally show significant levels of FAS activity. The relative levels of the CAT mRNA driven by the rat FAS promoter in various tissues of the transgenic animals approximated those of the endogenous mouse FAS mRNA. We also examined the hormonal and nutritional regulation of the FAS(2.1)-CAT reporter gene in transgenic mice. CAT activity was increased in both liver and white adipose tissue when fasted animals were refed a high carbohydrate, fat-free diet. These changes in CAT activity and CAT mRNA levels occurred in parallel to the changes in endogenous mouse FAS mRNA levels. On the other hand, fasting/refeeding did not change CAT activity appreciably in other tissues, such as muscle and brown adipose tissue. Administration of dibutyryl cAMP at the start of refeeding prevented an increase in CAT activity in liver. However, the cAMP effect was tissue-specific as cAMP treatment did not bring about change in CAT activity in adipose tissue. Next, to examine the effect of insulin, we made the transgenic mice insulin-deficient by streptozotocin treatment. Insulin treatment of the streptozotocin-diabetic mice increased both the CAT activity and CAT mRNA levels driven by the rat FAS promoter in liver and white adipose tissue. These changes in CAT expression by insulin paralleled those in endogenous FAS mRNA levels. Administration of glucocorticoids increased CAT activity in all tissues examined: liver, white and brown adipose tissues, lung, heart, and spleen. Overall, the first 2.1 kilobases of the 5'-flanking region of the rat FAS gene appear to contain sequence elements necessary to confer tissue-specific and hormonally regulated expression characteristic of the endogenous FAS gene.

Enoyl-acyl carrier protein reductase (fabI) plays a determinant role in completing cycles of fatty acid elongation in Escherichia coli

The role of enoyl-acyl carrier protein (ACP) reductase (E.C. 1.3.1.9), the product of the fabI gene, was investigated in the type II, dissociated, fatty acid synthase system of Escherichia coli. All of the proteins required to catalyze one cycle of fatty acid synthesis from acetyl-CoA plus malonyl-CoA to butyryl-ACP in vitro were purified. These proteins were malonyl-CoA:ACP transacylase (fabD), beta-ketoacyl-ACP synthase III (fabH), beta-ketoacyl-ACP reductase (fabG), beta-hydroxydecanoyl-ACP dehydrase (fabA), and enoyl-ACP reductase (fabI). Unlike the other enzymes in the cycle, FabA did not efficiently convert its substrate beta-hydroxybutyryl-ACP to crotonyl-ACP, but rather the equilibrium favored formation of beta-hydroxybutyryl-ACP over crotonyl-ACP by a ratio of 9:1. The amount of butyryl-ACP formed depended on the amount of FabI protein added to the assay. Extracts from fabI(Ts) mutants accumulated beta-hydroxybutyryl-ACP, and the addition of FabI protein to the fabI(Ts) extract restored both butyryl-ACP and long-chain acyl-ACP synthesis. FabI was verified to be the only enoyl-ACP reductase required for the synthesis of fatty acids by demonstrating that purified FabI was required for the elongation of both long-chain saturated and unsaturated fatty acids. These results were corroborated by analysis of the intracellular ACP pool composition in fabI(Ts) mutants that showed beta-hydroxybutyryl-ACP and crotonyl-ACP accumulated at the nonpermissive temperature in the same ratio found in the fabI(Ts) extracts and in the in vitro reconstruction experiments that lacked FabI. We conclude that FabI is the only enoyl-ACP reductase involved in fatty acid synthesis in E. coli and that the activity of this enzyme plays a determinant role in completing cycles of fatty acid biosynthesis.

Sterol regulation of fatty acid synthase promoter. Coordinate feedback regulation of two major lipid pathways

The gene encoding fatty acid synthase, the essential multi-functional enzyme of fatty acid biosynthesis, is shown to be regulated by cellular sterol levels similar to genes that encode important proteins of cholesterol metabolism. We show that expression of the endogenous FAS gene is repressed when regulatory sterols are included in the culture medium of HepG2 cells and that the FAS promoter is subject to similar regulation when fused to the luciferase reporter gene. Mutational studies demonstrate that sterol regulation is mediated by binding sites for the sterol regulatory element-binding protein (SREBP) and transcription factor Sp1, making it mechanistically similar to sterol regulation of the low density lipoprotein receptor gene. It is also demonstrated that SREBP and Sp1 synergistically activate the FAS promoter in Drosophila tissue culture cells, which lack endogenous Sp1. These experiments provide key molecular evidence that directly links the metabolism of fatty acids and cholesterol together.

Differentiation of rat brown adipocytes during late foetal development: role of insulin-like growth factor I

Rat brown adipocytes at day 22 of foetal development showed greater size, higher mitochondria content and larger amounts of lipids, as determined by flow cytometry, than 20-day foetal cells. Simultaneously, an inhibition on the percentage of brown adipocytes into S+G2/M phases of the cell cycle was observed between days 20 and 22 of foetal development. The expression of several adipogenesis-related genes, such as fatty acid synthase, malic enzyme, glucose-6-phosphate dehydrogenase and insulin-regulated glucose transporter, increased at the end of foetal life in brown adipose tissue. In addition, the lipogenic enzyme activities and the lipogenic flux increased during late foetal development, resulting in mature brown adipocytes showing a multilocular fat droplet phenotype. Concurrently, brown adipocytes induced the expression of the uncoupling protein (UP) mRNA and UP protein, as visualized by immunofluorescence. The three isoforms of CCAAT enhancer-binding proteins (C/EBPs) were expressed at the mRNA level in brown adipose tissue at day 20. C/EBP alpha decreased and C/EBP beta and delta increased their expression between days 20 and 22 of foetal development, respectively. Brown adipose tissue constitutively expressed insulin-like growth factor I (IGF-I) and IGF-I receptor (IGF-IR) mRNAs. Moreover, IGF-IR mRNA content increased between days 20 and 22 in parallel with the occurrence of tissue differentiation.

The expression of acetyl coenzyme A carboxylase is related to megakaryocyte maturation

We have previously demonstrated that de novo fatty acid synthesis predominantly occurs in later phases of megakaryocyte maturation. Therefore we have investigated the expression of fatty acid synthase (FAS) and acetyl coenzyme A carboxylase (ACC), key enzymes for fatty acid synthesis in megakaryocytes at different phases of maturation. Immature and mature megakaryocytes were isolated. Guinea pig-specific FAS and ACC cDNA probes were prepared by reverse transcriptase reaction-polymerase chain reaction (RT-PCR). The probes were used to assess the expression of mRNA for ACC and FAS by Northern blotting. The hybrids were quantitated by densitometry. Endogenous megakaryocyte ACC was quantitated by virtue of its biotin content by Western blotting with streptavidin and enhanced chemiluminescence (ECL). The ratio of ACC mRNA between mature and immature megakaryocytes was 2.43 +/- 0.86, and the ratio of FAS mRNA was 0.50 +/- 0.13 (mean +/- SD, n = 4). The ratio of endogenous ACC in mature and immature megakaryocytes was 1.96 +/- 0.62 (n = 6). The study showed that the FAS mRNA was expressed in all phases of megakaryocyte maturation. However, both mRNA for ACC and endogenous ACC were demonstrated primarily in mature megakaryocytes. Thus de novo fatty acid synthesis in megakaryocytes may depend on the expression of ACC in mature cells. The expression of ACC occurs during the terminal phases of megakaryocyte maturation and may be a marker of megakaryocyte maturity.

Transient induction of fatty acid synthase in rat liver after removal of a peroxisome proliferator

Removal of a peroxisome proliferator from the diet triggered the degradation of peroxisomes and induced the transient expression of a 220 kDa soluble protein in rat liver. The 220 kDa protein was purified by conventional methods and analyzed by amino acid sequencing. A total of 99 amino acid residues in 4 lysylendopeptidase-digested peptides completely matched those in rat fatty acid synthase. The transient induction of fatty acid synthase mRNA during peroxisome degradation was confirmed by Northern blotting.

Alteration of the specificity and regulation of fatty acid synthesis of Escherichia coli by expression of a plant medium-chain acyl-acyl carrier protein thioesterase

The expression of a plant (Umbellularia californica) medium-chain acyl-acyl carrier protein (ACP) thioesterase (BTE) cDNA in Escherichia coli results in a very high level of extractable medium-chain-specific hydrolytic activity but causes only a minor accumulation of medium-chain fatty acids. BTE's full impact on the bacterial fatty acid synthase is apparent only after expression in a strain deficient in fatty acid degradation, in which BTE increases the total fatty acid output of the bacterial cultures fourfold. Laurate (12:0), normally a minor fatty acid component of E. coli, becomes predominant, is secreted into the medium, and can accumulate to a level comparable to the total dry weight of the bacteria. Also, large quantities of 12:1, 14:0, and 14:1 are made. At the end of exponential growth, the pathway of saturated fatty acids is almost 100% diverted by BTE to the production of free medium-chain fatty acids, starving the cells for saturated acyl-ACP substrates for lipid biosynthesis. This results in drastic changes in membrane lipid composition from predominantly 16:0 to 18:1. The continued hydrolysis of medium-chain ACPs by the BTE causes the bacterial fatty acid synthase to produce fatty acids even when membrane production has ceased in stationary phase, which shows that the fatty acid synthesis rate can be uncoupled from phospholipid biosynthesis and suggests that acyl-ACP intermediates might normally act as feedback inhibitors for fatty acid synthase. As the fatty acid synthesis is increasingly diverted to medium chains with the onset of stationary phase, the rate of C12 production increases relative to C14 production. This observation is consistent with activity of the BTE on free acyl-ACP pools, as opposed to its interaction with fatty acid synthase-bound substrates.