1 Department of Medicine, University of California, San Francisco 94143; Metabolism Section, Medical Service, Department of Veterans Affairs Medical Center, San Francisco, California 94121; and 2 Ross Products Division, Abbott Laboratories, Columbus, Ohio 43215
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ABSTRACT |
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Acyl-CoA synthetase (ACS) catalyzes the activation of fatty
acids (FA) to acyl-CoA esters, which are further metabolized in either
anabolic or catabolic pathways. Endotoxin [lipopolysaccharide (LPS)], tumor necrosis factor (TNF), and interleukin-1 (IL-1) enhance hepatic FA synthesis and reesterification and inhibit FA
oxidation. LPS also decreases triglyceride storage in adipose tissue
and inhibits the uptake of FA by heart and muscle. Therefore, in this
study we examined the effects of LPS and cytokines on ACS (now also
known as ACS1) mRNA expression and activity in multiple tissues in
Syrian hamsters. LPS markedly decreased ACS1 mRNA levels in liver,
adipose tissue, heart, and skeletal muscle. The inhibitory effects of
LPS on ACS1 mRNA levels in liver and adipose tissue were observed as
early as 2-4 h after administration, became maximal by 4-8 h,
and were sustained for 24 h. Very low doses of LPS (0.1-1
µg/100 g body wt) were needed to reduce ACS1 mRNA levels in liver and
adipose tissue. TNF and IL-1 mimicked the effect of LPS on ACS1 mRNA
levels in liver and adipose tissue. LPS decreased ACS activity in
adipose tissue, heart, and muscle. In liver, where ACS is localized in
several subcellular organelles, both LPS and cytokines decreased
mitochondrial ACS activity, whereas they increased microsomal ACS
activity. Taken together, these results indicate that LPS and cytokines
decrease ACS1 mRNA expression and ACS activity in tissues where FA
uptake and/or oxidation is decreased during sepsis. In liver,
where FA oxidation is decreased during sepsis but the reesterification
of FA is increased, LPS and cytokines decrease ACS1 mRNA and
mitochondrial ACS activity, which may inhibit FA oxidation, but
increase microsomal ACS activity, which may support the
reesterification of peripherally derived FA for triglyceride synthesis.
tumor necrosis factor; interleukin-1; sepsis; fatty acid oxidation; fatty acid transport
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INTRODUCTION |
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ACYL-COA SYNTHETASE (ACS) plays a central role in the
regulation of fatty acid (FA) metabolism by catalyzing the activation of long-chain FA (LCFA) to acyl-CoA esters (37). These acyl-CoA esters
are utilized either in the anabolic pathways that include synthesis of cellular lipids such as triglycerides, phospholipids, and cholesterol esters or in the catabolic pathway of
-oxidation. Because FA transport across the biological
membranes is a bidirectional process, the activation of FA to acyl-CoA
esters by ACS prevents the efflux of FA and renders FA transport
unidirectional (37). Acyl-CoA esters are also involved in several other
functions, such as posttranslational modification of proteins,
intracellular signaling, and regulation of gene expression (see review
in Ref. 6).
ACS is localized in microsomes, outer mitochondrial membranes, and peroxisomes in rat liver (31, 43). Purified ACS from these fractions of rat liver is identical with respect to several characteristics, including molecular weight, amino acid composition, substrate specificities, and kinetic properties (31, 43), suggesting that the same enzyme is localized in three different subcellular organelles. In contrast to liver, ACS activity is localized primarily to microsomes in adipose tissue and to outer mitochondrial membrane in heart and skeletal muscle. Several genes coding for ACS have recently been cloned that are expressed to various degrees in different tissues and may vary in their substrate specificity (12, 13, 19, 41, 44). ACS1 is the well-characterized gene that has been cloned from rat liver and murine adipocytes (41, 44). The mRNA for ACS1 is abundant in liver, adipose tissue, heart, and skeletal muscle (41, 44).
Despite a critical role of ACS in fuel homeostasis, very few studies have demonstrated ACS regulation in vivo. ACS has been considered a constitutive enzyme because early studies on the regulation of ACS under different nutritional conditions found no significant difference in enzyme activity (1, 24). However, recent studies have shown that the mRNA levels for ACS1 are markedly increased in rat liver by the feeding of a high carbohydrate or high fat diet (41). ACS1 gene expression is induced by insulin and triiodothyronine and inhibited by tumor necrosis factor (TNF) in 3T3-L1 adipocytes (20, 44).
The host response to infection and inflammation is accompanied by many changes in FA metabolism, such as increased adipose tissue lipolysis, enhanced hepatic lipogenesis and reesterification of FA in the liver, and inhibition of FA oxidation in multiple tissues including liver, heart, and skeletal muscle (3, 11, 22, 23, 39, 42). These metabolic changes can be induced by the administration of endotoxin or lipopolysaccharide (LPS), which mimics gram-negative infections, and by proinflammatory cytokines such as TNF and interleukin-1 (IL-1), which mediate many of the metabolic responses that occur during the host response to infectious and inflammatory stimuli (see review in Ref. 15). For example, LPS and cytokines increase serum triglyceride levels and very low-density lipoprotein (VLDL) production by stimulating hepatic lipogenesis, suppressing FA oxidation, and increasing reesterification of peripherally derived FA in the liver (10, 26, 28, 33).
We have recently shown that LPS and cytokines decrease mRNA levels of FA transport protein (FATP) and FA translocase (FAT) in adipose tissue, heart, and muscle in Syrian hamsters (27). LPS and cytokine also downregulate FATP mRNA expression in liver but upregulate hepatic FAT mRNA expression (27). The expression of FATP and FAT in cultured fibroblasts enhances the uptake of LCFA (2, 36). FA are esterified by ACS once they are inside the cells to prevent their efflux. Therefore, ACS is a logical site for metabolic regulation. Hence, in this study we have examined the effects of LPS and cytokines on ACS1 mRNA levels in liver, adipose tissue, heart, and muscle. Because ACS is present in different subcellular organelles in liver that channel FA to different metabolic pathways, we also examined the effect of LPS and cytokines on hepatic microsomal as well as mitochondrial ACS activity. Finally, we have examined the effect of LPS on ACS activity in adipose tissue, heart, and muscle.
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MATERIALS AND METHODS |
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Materials.
[-32P]dCTP (3,000 Ci/mmol) and
[U-14C]palmitic acid
(850 mCi/mmol) were purchased from New England Nuclear (Boston, MA).
Endotoxin (Escherichia coli 55:B5) was
purchased from Difco Laboratories (Detroit, MI) and was freshly diluted
to desired concentrations in pyrogen-free 0.9% saline (Kendall McGraw
Laboratories, Irvine, CA). Human TNF-
with a specific activity of 5 × 107 U/mg was kindly
provided by Genentech (South San Francisco, CA). Recombinant human
IL-1
with a specific activity of 1 × 109 U/mg was generously provided
by Immunex (Seattle, WA). IL-1
was used in this study because it is
the secreted form of IL-1, whereas IL-1
is primarily intracellular
or membrane associated (5). The cytokines were freshly diluted to
desired concentrations in pyrogen-free 0.9% saline containing 0.1%
human serum albumin. The multiprime DNA labeling system was purchased
from Amersham International (Amersham, UK); minispin G-50 columns were
from Worthington Biochemical (Freehold, NJ); Oligo(dT) cellulose, type 77F, was from Pharmacia LKB Biotechnology (Uppsala, Sweden);
nitrocellulose and Nytran were from Scleicher & Schuell (Keene, NH).
Kodak XAR5 film was used for autoradiography. cDNA for ACS1 was
prepared as described previously (44).
Animal procedures.
Male Syrian hamsters (~160-180 g) were purchased from Simonsen
Laboratories (Gilroy, CA). The animals were maintained on a normal
light cycle (6 AM to 6 PM light, 6 PM to 6 AM dark) and were provided
with rodent chow (Simonsen Laboratories) and water ad libitum. Animals
(n = 5 hamsters per group) were
injected intraperitoneally with either LPS, TNF, IL-1, or TNF plus IL-1
at the indicated doses or with 0.9% saline containing 0.1% human
serum albumin. Our previous studies have shown that LPS and cytokines
induce marked anorexia in Syrian hamsters (14). The induction of
anorexia by LPS is acute in onset, with a 90% or more decrease in food intake during the first 18 h after a 10-100 µg/100 g body wt LPS dose, and the anorexigenic effect of LPS persists for 72 h (14). Hence, in this study food was withdrawn from both control and treated
groups to avoid the effect of variations in food intake on ACS
regulation. Animals were studied between 1.5 and 24 h after LPS or 8 h
after cytokine administration. The doses of LPS used (0.1-100
µg/100 g body wt) are far below those required to cause death in
rodents in our laboratory (lethal dose ~5 mg/100 g body wt) but have
significant effects on triglyceride and cholesterol metabolism in
Syrian hamsters (7, 9). Similarly, the doses of TNF and IL-1 used (17 µg/100 g body wt and 1 µg/100 g body wt, respectively) in these
experiments were based on previous studies demonstrating that these
doses have marked effects on lipid metabolism and reproduce many of the
effects of LPS on lipid metabolism in Syrian hamsters (16, 17).
Isolation of RNA and Northern blotting. Total RNA was isolated by a variation of the guanidinium thiocyanate method (4), as described earlier (7). Total RNA from adipose tissue was used for Northern blotting, whereas poly(A)+ RNA from liver, heart, and muscle was isolated using oligo(dT) cellulose. Total or poly(A)+ RNA was quantified by measuring absorption at 260 nm. Equal amounts of total or poly(A)+ RNA were loaded on 1% agarose-formaldehyde gels and electrophoresed. The uniformity of sample applications was checked by ultraviolet visualization of the acridine orange-stained gels before transfer to Nytran membranes. We and others have found that LPS increases actin mRNA levels in liver by 2- to 5-fold in rodents (7, 32). TNF and IL-1 produce a 2-fold increase in actin mRNA levels. LPS also produced a 2-fold increase in hepatic mRNA levels for glyceraldehyde 3-phosphate dehydrogenase (G-3PD) and a 2.6-fold increase in cyclophilin mRNA (27). Therefore, the mRNA levels of actin, G-3PD, and cyclophilin, which are widely used for normalizing data, cannot be used to study LPS or cytokine-induced regulation of proteins in liver. However, the differing direction of the changes in mRNA levels for specific proteins after LPS or cytokine, the magnitude of the alterations, and the relatively small standard error of the mean make it unlikely that the changes observed are due to unequal loading of mRNA (7, 9, 16, 17, 27). cDNA probe hybridization was performed in 0.75 M sodium chloride, 0.075 M sodium citrate, 2% SDS, 10% dextran sulfate, 2 × Denhardt's solution, and 100 mg/ml sheared salmon sperm DNA at 65°C overnight. Blots were washed in 0.2× standard sodium citrate and 0.1% SDS at room temperature for 30 min and at 65°C for 1 h. The blots were exposed to X-ray films for various durations to ensure that measurements were done on the linear portion of the curve, and the bands were quantified by densitometry.
Acyl-CoA synthetase activity assay. Hepatic ACS activity was measured by a radioisotopic assay in liver homogenates, microsomes, or mitochondria by the method of Tanaka et al. (43). Postnuclear supernatants were used for measuring ACS activity in adipose tissue, heart, and skeletal muscle (38). Briefly, at indicated times after the administration of LPS or cytokines, the livers were removed and homogenized in a buffer [0.25 M sucrose, 1 mM dithiothreitol (DTT), and 1 mM EDTA], and mitochondria and microsomes were isolated by differential centrifugation. Similarly, adipose tissue, heart, and muscle were obtained and homogenized. The homogenates were centrifuged at 800 g for 5 min, and the resultant supernatants were used for measuring enzyme activity. As reported by others (43), our initial experiments also showed that the rate of reaction was linear up to 10 min and plateaued by 15 min. Hence, in all subsequent assays the reaction time was kept at 5 min. The assay mixture contained 0.1 M Tris · HCl buffer, pH 8.0, 5 mM DTT, 0.15 M KCl, 15 mM MgCl2, 1.6 mM Triton X-100, 10 mM ATP, 1 mM CoA, and 1 mM [U-14C]palmitic acid in a final reaction volume of 0.2 ml. The assay was initiated by addition of 5-10 µl of enzyme suspension (5 µg protein), and the reaction was carried out at 37°C for 5 min. The reaction was terminated by adding 2.5 ml of a mixture containing isopropanol-heptane-sulfuric acid 1 M (40:10:1, vol/vol/vol). The unreacted palmitic acid was extracted with heptane, and the radioactivity in the aqueous phase containing palmitoyl-CoA was measured (43). Control reactions containing no protein were run in parallel to correct for the background. ACS activity is expressed as nanomoles of palmitoyl-CoA formed per milligram protein per minute. Protein was assayed by the method of Bradford (Bio-Rad Laboratories).
Statistics. The results are presented as means ± SE. Statistical significance between two groups was determined by use of Student's t-test. Comparison among several groups was performed by analysis of variance, and statistical significance was calculated using Bonferroni's multiple comparison test.
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RESULTS |
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ACS1 mRNA levels in liver, adipose tissue, heart, and muscle in the fed and fasted states. It has been previously shown that short-term fasting has no significant effect on hepatic ACS1 mRNA levels in rats (41). In this study, we compared ACS1 mRNA levels in liver, adipose tissue, heart, and muscle from animals that were either fasted for 24 h or had free access to lab chow. Our results demonstrate that ACS1 mRNA levels are not significantly different between fed and 24-h-fasted animals in liver, heart, and muscle (Fig. 1). On the other hand, adipose tissue ACS1 mRNA levels were 70% lower in the fasted animals than in the fed group (Fig. 1).
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Effect of LPS on ACS1 mRNA levels in liver and adipose tissue. To investigate the effect of LPS on ACS1 mRNA levels in liver and adipose tissue, Syrian hamsters were injected with LPS (100 µg/100 g body wt) or saline (controls); both groups of animals were fasted to avoid the effect of variations in food intake on ACS1 mRNA level. Livers and adipose tissue were obtained at various time points, and total or poly(A)+ RNA was isolated from adipose tissue and liver, respectively, for Northern blot analysis. LPS markedly decreased ACS1 mRNA levels in both liver and adipose tissue (Fig. 2). ACS1 mRNA levels in liver were decreased by 50% at 90 min, 90% at 4 h, and 80% at 8 h after LPS administration (Fig. 2A). At 24 h after LPS, ACS1 mRNA levels were 40% of control values. In adipose tissue, ACS1 mRNA levels began to decrease by 4 h after LPS administration and were decreased by 87% at 8 h and 92% at 16 h after LPS treatment (Fig. 2B). At 24 h after LPS, ACS1 mRNA levels in adipose tissue were 23% of the control levels. These data demonstrate that LPS dramatically downregulates ACS1 mRNA levels in vivo in both liver and adipose tissue.
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Effect of cytokines on ACS1 mRNA levels in liver and adipose tissue. Because TNF and IL-1 mediate many of the metabolic effects of LPS, we next examined the effects of TNF (17 µg/100 g body wt) and IL-1 (1 µg/100 g body wt) on ACS1 mRNA levels in liver and adipose tissue (Fig. 4). These doses have previously been shown to mimic the metabolic effects of LPS on lipid and lipoprotein metabolism in Syrian hamsters, although the magnitude of the effect may vary (16, 17, 27). Eight hours after administration, TNF produced a 60% decrease in ACS1 mRNA levels in liver, whereas IL-1 either alone or in combination with TNF decreased hepatic ACS1 mRNA levels by 90% (Fig. 4A). Similarly, TNF produced a 60% decrease in ACS1 mRNA levels in adipose tissue, whereas IL-1 either alone or in combination with TNF produced an 80% decrease in ACS1 mRNA levels in adipose tissue (Fig. 4B). It is possible that a higher dose of TNF may produce an effect that is comparable in magnitude to that of IL-1.
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Effect of LPS on ACS1 mRNA levels in heart and muscle. In addition to liver and adipose tissue, ACS1 is also abundantly expressed in heart and skeletal muscle. Moreover, both of these organs metabolize FA as their primary oxidative fuel; hence, we next examined the effect of high-dose LPS (100 µg/100 g body wt) on ACS1 mRNA levels in heart and skeletal muscle. Sixteen hours after LPS treatment, ACS1 mRNA levels in heart and skeletal muscle were decreased by 40 and 80%, respectively (Fig. 5).
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Effect of LPS and cytokines on ACS activity.
Previous studies have shown that LPS and cytokines inhibit FA oxidation
and stimulate triglyceride synthesis in the liver (10, 26, 28, 33).
Because ACS is localized in several subcellular organelles in liver and
directs FA into anabolic or catabolic pathways, we investigated the
effect of LPS on ACS activity in liver homogenates as well as in
microsomal and mitochondrial fractions. Eight hours after LPS
treatment, ACS activity (nmol · mg
protein1 · min
1)
in liver homogenates was decreased by only 20% (control 680 ± 29, LPS 544 ± 32; P < 0.02) despite
a marked reduction in hepatic ACS1 mRNA after LPS treatment (Figs.
2A and 3). However, when the activity
of ACS was examined in mitochondrial and microsomal fractions, we
observed differential regulation of ACS activity. LPS decreased
mitochondrial ACS activity by 61%, whereas there was a 55% increase
in microsomal ACS activity (Fig.
6A).
Differential regulation of ACS activity was also observed after
cytokine treatment. TNF, IL-1, or the combination of both cytokines
produced a 37, 45, and 52% decrease, respectively, in mitochondrial
ACS activity (Fig. 6B). In contrast,
IL-1 produced a 60% increase in microsomal ACS activity, whereas TNF
had no effect on microsomal ACS activity and also did not synergize
with IL-1. It is possible that a higher dose of TNF may be needed to
produce an increase in microsomal ACS activity.
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DISCUSSION |
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During infection and inflammation, marked changes occur in FA, triglyceride, and cholesterol metabolism in several tissues (15, 39). Studies in experimental models of sepsis have shown that on one hand there is an increase in hepatic lipogenesis, reesterification, and VLDL production, whereas on the other hand there is decreased FA oxidation and ketone body production in liver (11, 22, 23, 42). LPS decreases the storage of triglycerides in adipose tissue and increases the mobilization of FA (3, 11, 23). Finally, the uptake of LCFA by heart and skeletal muscle is markedly reduced after LPS administration (35, 40). Cytokines, particularly TNF and IL-1, have been shown to mimic many of these effects, suggesting that they may be the in vivo mediators of metabolic changes produced during infection or inflammation (15).
The results of our present study demonstrate that LPS downregulates ACS1 mRNA levels and activity in adipose tissue. This downregulation of ACS1 mRNA is rapid, sustained, and elicited by very low doses of LPS. IL-1 and TNF also decreased ACS1 mRNA levels in adipose tissue; however, the effect of IL-1 was greater in magnitude than that of TNF. ACS is primarily associated with microsomes in adipose tissue to support the synthesis of triglycerides for storage of energy (18). We have previously shown that LPS and cytokines decrease the expression of mRNAs for FATP and FAT in adipose tissue (27). A coordinate decrease in FA transport proteins, ACS1 mRNA expression, and ACS activity in adipose tissue will prevent the storage of FA in adipose tissue during infection and inflammation and will promote the mobilization of FA. This conclusion is reinforced by previous studies that have shown that LPS and cytokines stimulate adipose tissue lipolysis and increase the mobilization of FA in rodents (10, 11, 26). A decrease in ACS1 mRNA under conditions of nutrient deprivation such as fasting, as reported here, will also prevent the activation and subsequent storage of FA in adipose tissue and will enhance FA mobilization to support the energy demands of other tissues.
Our results also demonstrate that LPS decreased ACS1 mRNA expression
and activity in heart and skeletal muscle. ACS is primarily associated
with outer mitochondrial membranes in heart and muscle for
-oxidation, as these tissues preferentially utilize FA as their
oxidative fuel (34). We have previously shown that LPS decreased the
mRNA expression for FATP and FAT in heart and muscle (27). Thus a
coordinate decrease in both FA transport proteins, coupled with a
decrease in ACS expression and activity, will decrease the uptake and
utilization of FA in heart and muscle. Although FA are the preferred
fuel substrate for heart and muscle under normal conditions, several
studies have shown that, during sepsis, their fuel preference is
altered. For example, Spitzer et al. (40) have shown that in vivo
myocardial uptake and oxidation of LCFA are decreased after LPS
treatment, whereas the uptake and utilization of lactate are elevated.
The uptake and oxidation of LCFA by skeletal muscle are also reduced
after LPS (35), whereas the uptake and utilization of glucose are
increased in several organs, including muscle after LPS treatment (29).
Lipoprotein lipase (LPL) is the key enzyme responsible for hydrolyzing triglyceride-rich lipoproteins. The FA released by the hydrolysis of triglycerides are either oxidized or reesterified. The activity of LPL is highest in tissues that oxidize FA for energy production (heart and skeletal muscle) or esterify FA for storage (adipose tissue). Hence, like ACS, LPL also plays a key role in the regulation of adipocyte lipid storage and provision of fuel substrate for heart and muscle (21). We and others have previously shown that LPS and several cytokines decrease the activity of LPL in adipose tissue, heart, and muscle in rodents (3, 8, 22). A coordinate decrease in LPL and ACS during infection and inflammation will also decrease the storage of FA in adipose tissue and decrease their utilization by heart and muscle.
In the liver, both LPS and cytokines markedly downregulated ACS1 mRNA expression. As seen in adipose tissue, the effect of IL-1 was greater than that of TNF on ACS1 mRNA in the liver and was comparable to that of LPS. It is of interest to note that, in the liver, where ACS is present in multiple subcellular organelles to facilitate the activation of FA for different metabolic pathways, LPS produced only a small but statistically significant decrease in ACS activity in liver homogenates, a marked decrease in mitochondrial ACS activity, and an increase in microsomal ACS activity. Previous studies have shown that LPS and cytokines suppress FA oxidation and ketone body production but stimulate FA reesterification, triglyceride synthesis, and VLDL production in the liver (15, 39). LPS and cytokines inhibit FA oxidation in part by raising the levels of malonyl-CoA (26), which is the first committed intermediate in FA synthesis and is an allosteric inhibitor of carnitine palmitoyltransferase I, the rate-limiting enzyme in FA oxidation (25). It is important to note that both FA oxidation and reesterification are highly compartmentalized, with FA oxidation taking place in mitochondria and reesterification occurring in the cytosol. We have recently shown that, in contrast to a coordinate downregulation of FATP and FAT in adipose tissue, heart, and muscle by LPS, the mRNAs for these proteins are differentially regulated in the liver (27). LPS and cytokines downregulate hepatic FATP mRNA levels, whereas they upregulate FAT mRNA expression. These results have suggested that FATP may transport FA toward mitochondria for oxidation, which is suppressed in sepsis, whereas FAT may transport FA to cytosol for reesterification, which is enhanced in sepsis. Our present results on the differential regulation of ACS activity in the liver support such a possibility. Thus, whereas a decrease in hepatic mitochondrial ACS activity will be an additional mechanism for inhibiting FA oxidation in liver, an increase in microsomal ACS activity is likely to lead to an activation of FA for hepatic triglyceride synthesis, which is enhanced during sepsis.
Although the decrease in mitochondrial ACS activity in liver corresponds to the decrease in hepatic ACS1 mRNA expression, the increase in microsomal ACS activity in the presence of a profound decrease in hepatic ACS1 mRNA levels is surprising. The mechanism for the discordance between decreased ACS1 mRNA expression and increased microsomal ACS activity is not clear. It is possible that microsomal ACS may be regulated by LPS and cytokines at the posttranslational level by the modification of protein through one of the several known mechanisms, such as phosphorylation, dephosphorylation, acylation, or isoprenylation; however, no such mechanism has been reported for the regulation of ACS. The decrease in mitochondrial ACS, coupled with increased microsomal activity, might represent a compartmental shift of the enzyme; however, we are not aware of any such example of enzyme translocation between mitochondria and endoplasmic reticulum. It is also possible that there is increased activation of the enzyme because of a conformational change induced by substrate binding. Thus an increase in FA delivery to the microsomal compartment induced by LPS and cytokines (by upregulating FAT and downregulating FATP) could preferentially enhance microsomal ACS activity.
An increase in hepatic microsomal ACS activity, despite a profound
decrease in ACS1 mRNA levels, also raises the possibility that
microsomal ACS may be coded by a different ACS gene product. This
hypothesis is supported by the fact that several ACS genes have been
recently cloned that are expressed to various degrees in different
tissues and may vary in their fatty acid specificity. ACS1, the mRNA
studied here, is the well-characterized gene that is most abundant in
liver, adipose tissue, heart, and muscle, with very low expression in
brain, lung, and small intestine, and ACS1 has a broad FA specificity
(41, 44). ACS2 is predominantly expressed in brain and to a much lesser
degree in heart, but it is not detectable in other tissues (13). ACS3
is highly expressed in brain (12). ACS4 is abundantly present in
steroidogenic tissues such as adrenal gland, ovary, and testis, but
substantial ACS4 mRNA expression is also seen in liver, brain, and lung
(19). Two distinct ACS genes have also been reported in yeast, with ACS1 being responsible for providing acyl-CoA exclusively for synthesis
of cellular lipids and ACS2 for producing acyl-CoA for degradation via
-oxidation (30). In light of this diversity of the ACS gene family
in mammals as well as in yeast, it is possible that there may be
another member of the ACS gene family present in liver that may code
for the microsomal isozyme. Further studies will be needed to address
these issues.
In summary, our results demonstrate that LPS and cytokines decrease ACS1 mRNA expression and activity in tissues where FA uptake and/or oxidation is decreased during sepsis. In the liver, where FA oxidation is suppressed but triglyceride synthesis is increased during sepsis, LPS and cytokines decrease ACS1 mRNA and mitochondrial ACS activity but increase microsomal ACS activity. This reciprocal regulation of ACS activity may inhibit FA oxidation and support the reesterification of peripherally derived FA for triglyceride synthesis.
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ACKNOWLEDGEMENTS |
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This work was supported by a grant from the Research Service of the Department of Veterans Affairs and National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-49448.
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FOOTNOTES |
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Address for reprint requests: R. A. Memon, Dept. of Veterans Affairs Medical Center, 4150 Clement St. (111F), San Francisco, CA 94121.
Received 8 December 1997; accepted in final form 9 March 1998.
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