Clofibrate treatment increases stearoyl-CoA desaturase mRNA level and enzyme activity in mouse liver

The PPAR pan-agonist tetradecylthioacetic acid promotes redistribution of plasma cholesterol towards large HDL

Tetradecylthioacetic acid (TTA) is a synthetic fatty acid with a sulfur substitution in the β-position. This modification renders TTA unable to undergo complete β-oxidation and increases its biological activity, including activation of peroxisome proliferator activated receptors (PPARs) with preference for PPARα. This study investigated the effects of TTA on lipid and lipoprotein metabolism in the intestine and liver of mice fed a high fat diet (HFD). Mice receiving HFD supplemented with 0.75% (w/w) TTA had significantly lower body weights compared to mice fed the diet without TTA. Plasma triacylglycerol (TAG) was reduced 3-fold with TTA treatment, concurrent with increase in liver TAG. Total cholesterol was unchanged in plasma and liver. However, TTA promoted a shift in the plasma lipoprotein fractions with an increase in larger HDL particles. Histological analysis of the small intestine revealed a reduced size of lipid droplets in enterocytes of TTA treated mice, accompanied by increased mRNA expression of fatty acid transporter genes. Expression of the cholesterol efflux pump Abca1 was induced in the small intestine, but not in the liver. Scd1 displayed markedly increased mRNA and protein expression in the intestine of the TTA group. It is concluded that TTA treatment of HFD fed mice leads to increased expression of genes involved in uptake and transport of fatty acids and HDL cholesterol in the small intestine with concomitant changes in the plasma profile of smaller lipoproteins.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Peroxisome proliferator-activated receptor α is not the exclusive mediator of the effects of dietary cyclic FA in mice

Cyclic FA monomers (CFAM) formed during heating of alpha-linolenic acid have been reported to interfere in hepatic metabolism in a putatively peroxisome proliferator-activated receptor alpha (PPARalpha)-dependent manner. In the present work, CFAM (0.5% of the diet) were administered for 3 wk to wild-type and PPARalpha-null mice of both genders to elucidate the role of PPARalpha in mediating the effects of CFAM on the activity of acyl-CoA oxidase (ACO) and omega-laurate hydroxylase (CYP4A), the regulation of which is known to be dependent on the PPARalpha. Dietary CFAM enhanced CYP4A activity threefold in male and female wild-type mice. This effect was abolished in PPARalpha-null mice. A twofold induction of ACO activity was found in wild-type female mice fed CFAM; however, no effect was seen in males. In wild-type animals, (omega-1)-laurate hydroxylase (CYP2E1) activity, the expression of which has not been shown to be PPARalpha dependent, was not affected by the CFAM diet. In contrast, stearoyl-CoA desaturase activity was reduced in wild-type mice. CFAM feeding reduced the activities of ACO, CYP2E1, and stearoyl-CoA desaturase and caused accumulation of lipids in the livers of female PPARalpha-null mice. These data show that CFAM apparently activate gene expression via the PPARalpha and have profound effects on lipid homeostasis, exacerbating the disturbances preexisting in mice lacking functional PPARalpha. Although the data emphasize the importance of PPARalpha in the metabolism of the CFAM, these results show that PPARalpha is not the exclusive mediator of the effects of CFAM in lipid metabolism in mice.

Regulation by carbohydrate and clofibric acid of palmitoyl-CoA chain elongation in the liver of rats

Regulation of palmitoyl-CoA chain elongation (PCE) and its contribution to oleic acid formation were investigated in rat liver in comparison with stearoyl-CoA desaturase (SCD). Hepatic PCE activity was induced by the administration of 20% wt/vol glucose or fructose in the drinking water of normal rats. In streptozotocin-induced diabetic rats, the activities of both PCE and SCD were suppressed, and fructose, but not glucose, feeding caused an increase in the activities of both enzymes. Treatment of normal rats with clofibric acid in combination with carbohydrate further increased PCE, but not SCD, activity. FA analysis of hepatic lipids revealed that the proportion of oleic acid (18:1 n-9) increased upon administration of carbohydrate or clofibric acid. The treatment of rats with clofibric acid in combination with carbohydrate greatly increased the proportion of 18:1 n-9. A significant correlation was observed between PCE activity and the hepatic proportion of 18:1 n-9 (r2 = 0.874, P < 0.01), whereas the relationship between SCD activity and the proportion of 18:1 n-9 was not significant (r2 = 0.552, P > 0.05). Taken together, these results suggest that carbohydrate induces PCE as well as SCD activity to increase the hepatic 18:1 content in rat liver, and the increased PCE activity seems to be responsible for the further increase in 18:1 n-9 when carbohydrate is administered in combination with clofibric acid.

Role of stearoyl-coenzyme A desaturase in lipid metabolism

Stearoyl-CoA desaturase (SCD) (EC 1.14.99.5) is an endoplasmic reticulum-bound enzyme that catalyzes the delta9-cis desaturation of saturated fatty acyl-CoAs, the preferred substrates being palmitoyl- and stearoyl-CoA, which are converted to palmitoleoyl- and oleoyl-CoA, respectively. These monounsaturated fatty acids are used as substrates for the synthesis of triglycerides, wax esters, cholesteryl esters and membrane phospholipids. The saturated to monounsaturated fatty acid ratio affects membrane phospholipid composition and alteration in this ratio has been implicated in a variety of disease states including cardiovascular disease, obesity, diabetes, neurological disease, skin disorders and cancer. Thus, the expression of SCD is of physiological importance in normal and disease states. Several mammalian SCD genes have been cloned. A single human, three mouse and two rat are the best characterized SCD genes. The physiological role of each SCD isoform and the reason for having three or more SCD gene isoforms in the rodent genome are currently unknown. A clue as to the physiological role of the SCD, at least SCD1 gene and its endogenous products came from recent studies of asebia mouse strains that have a natural mutation in the SCD1 gene and a mouse model with a targeted disruption of the SCD1 gene. In this review we discuss our current understanding of the physiological role of SCD in lipid synthesis and metabolism.

Effects of sterculic acid on stearoyl-CoA desaturase in differentiating 3T3-L1 adipocytes

The effects of sterculic acid on cell size, adiposity, and fatty acid composition of differentiating 3T3-L1 adipocytes are correlated with stearoyl-CoA desaturase (SCD) expression (mRNA and protein levels) and enzyme activity. Fluorescence-activated cell scanning (FACS) analysis showed that adipocytes differentiated with methylisobutylxanthine, dexamethasone, and insulin (MDI) plus 100 microM sterculic acid comprised a population of predominantly large cells with reduced adiposity compared to MDI-treated cells. Although both groups had similar amounts of total fat, their fatty acid profiles were strikingly different: MDI-treated cells had high levels of the unsaturated palmitoleic (Delta(9)-16:1) and oleic (Delta(9)-18:1) acids, whereas the cells cultured with MDI plus sterculic acid accumulated palmitic (16:0) and stearic (18:0) acids together with a marked reduction in Delta(9)-16:1. Although the cells treated with MDI plus sterculic acid had similar levels of scd1 and scd2 mRNAs and antibody-detectable SCD protein as the MDI-treated cells, the SCD enzyme activity was inhibited more than 90%. The accumulation of 16:0 and 18:0, together with normal levels of fatty acid synthase (FAS) and aP2 mRNAs, shows that de novo synthesis and elongation of fatty acids, as well as cell differentiation, were not affected by sterculic acid. Because of the increase in cell size in the sterculic acid-treated cells, the insulin-stimulated 2-deoxyglucose (2-DOG) uptake was determined. Compared to MDI-treated cells, the 2-DOG uptake in the cells treated with sterculic acid was not affected. These results indicate that sterculic acid directly inhibits SCD activity, possibly by a turnover-dependent reaction, without affecting the processes required for adipocyte differentiation, scd gene expression or SCD protein translation.

The biochemistry of hypo- and hyperlipidemic fatty acid derivatives: metabolism and metabolic effects

A selection of amphipatic hyper- and hypolipidemic fatty acid derivatives (fibrates, thia- and branched chain fatty acids) are reviewed. They are probably all ligands for the peroxisome proliferation activation receptor (PPARalpha) which has a low selectivity for its ligands. These compounds give hyper- or hypolipidemic responses depending on their ability to inhibit or stimulate mitochondrial fatty acid oxidation in the liver. The hypolipidemic response is explained by the following metabolic effects: Lipoprotein lipase is induced in liver where it is normally not expressed. Apolipoprotein CIII is downregulated. These two effects in liver lead to a facilitated (re)uptake of chylomicrons and VLDL, thus creating a direct transport of fatty acids from the gut to the liver. Fatty acid metabolizing enzymes in the liver (CPT-I and II, peroxisomal and mitochondrial beta-oxidation enzymes, enzymes of ketogenesis, and omega-oxidation enzymes) are induced and create an increased capacity for fatty acid oxidation. The increased oxidation of fatty acids "drains" fatty acids from the body, reduces VLDL formation, and ultimately explains the antiadiposity and improved insulin sensitivity observed after administration of peroxisome proliferators.

Differential induction of stearoyl-CoA desaturase and acyl-CoA oxidase genes by fibrates in HepG2 cells

We studied whether two typical effects of fibrates, induction of stearoyl-CoA desaturase (EC 1.14.99.5) and peroxisome proliferation, are related. The effect of bezafibrate on the activity and mRNA of stearoyl-CoA desaturase and acyl-CoA oxidase in the liver and epididymal white adipose tissue of male Sprague-Dawley rats was determined. The same parameters were measured in HepG2 cells, a cell line resistant to peroxisome proliferation, following incubation with ciprofibrate. Bezafibrate increased the hepatic mRNA levels (14.5-fold on day 7) and activity (9.3-fold on day 15) of acyl-CoA oxidase. Stearoyl-CoA desaturase mRNA levels were transiently increased (2.7-fold on day 7), while its activity remained increased at the end of the treatment (2.4-fold). In white adipose tissue, bezafibrate increased the mRNA (5-fold) and activity (1.9-fold) of acyl-CoA oxidase, while stearoyl-CoA desaturase was not modified. Ciprofibrate addition to HepG2 cells cultured in 7% fetal bovine serum (FBS) only increased the stearoyl-CoA desaturase mRNA (1.9-fold). When cells were cultured in 0.5% FBS, ciprofibrate increased acyl-CoA oxidase mRNA (2.2-fold), while the increase in stearoyl-CoA desaturase mRNA was identical (1.9-fold). Further, its activity was also increased (1.5-fold). Incubation of HepG2 cells in the presence of cycloheximide did not alter the capacity of ciprofibrate to induce stearoyl-CoA desaturase mRNA, whereas the presence of actinomycin abolished the induction. In addition, preincubation of HepG2 cells with ciprofibrate increased the rate of stearoyl-CoA desaturase mRNA degradation. The results presented in this study suggest that fibrates induce stearoyl-CoA desaturase activity and mRNA levels independently of peroxisome proliferation.

Effects of polyunsaturated fatty acids and clofibrate on chicken stearoyl-coA desaturase 1 gene expression

In chicken, adiposity is influenced by hepatic stearoyl-CoA desaturase (SCD) 1. This gene is up-regulated by low-fat high-carbohydrate diet and down-regulated by addition of polyunsaturated fatty acids (PUFA). In this study, we present evidence for an inhibition of chicken SCD1 expression by PUFA using reporter gene constructs in transient transfection assays. This inhibition does not involve the peroxisome proliferator-activated receptor pathway, in contrast with what has been observed in rodents. We were able to localise a PUFA as well as an insulin response element within the -372/+125 bp region of the promoter. Sequence analyses of this region allowed identification of several cis-regulatory elements: A sterol regulatory element (SRE) and a juxtaposed NF-Y element which have been shown to be involved in the regulation of mouse SCD genes by PUFA. In addition, we identified an overlapping Sp1/USF motif, which was described to play a role in insulin/glucose and PUFA regulation of fatty synthase, ATP-citrate-lyase, and leptin genes. These data provide the first characterisation of the chicken SCD1 promoter and putative cis-sequences involved in the regulation of this gene by PUFA and insulin.

Trans-vaccenic acid is desaturated to conjugated linoleic acid in mice

Mice were fed pure trans11 octadecenoic acid (trans-vaccenic acid; TVA) to determine whether it is desaturated to cis9, trans11 octadecadienoic acid, a predominant isomer of conjugated linoleic acid (CLA). In a preliminary trial, 12% of the TVA consumed during a 2-wk feeding period was recovered in the carcass as CLA. As a proportion of TVA in the tissues available for bioconversion, 48.8% was desaturated. We tested whether desaturation could be modified by supplementing no modifier, 0.5% clofibric acid to stimulate desaturation, or increasing the polyunsaturated fatty acids (PUFA) (10% corn oil vs. 4% corn oil) to inhibit desaturation in diets with or without 1% TVA. These diets were fed to six groups of mice in a 3x2 factorial arrangement of treatments. Feeding 1% TVA with 10% corn oil decreased feed intake (2.70 vs. 3.73 g/d, SEM 0.23; P<0.05). Bioconversion of dietary TVA was 12.0, 7.5 and 5.1% for mice fed no modifier of desaturation, clofibrate and increased PUFA, respectively. Conversion based on TVA available for desaturation was 52.6, 55.5 and 37.0%, respectively. Thus, clofibrate did not increase bioconversion, but increasing PUFA decreased conversion by 30%. To test whether TVA decreases food intake directly or after conversion to CLA, four groups of mice were fed diets containing 1% stearic, TVA, elaidic or conjugated linoleic acid. Dietary CLA decreased food intake and body fat, but did not change body protein. CLA was found in the carcass only when TVA or CLA was fed. CLA was found in both triacylglycerol and phospholipids when CLA was fed, but only in triacylglycerol when TVA was fed, suggesting that bioconversion occurred in the adipose tissue. In three trials, conversion of dietary TVA to CLA was 11.4+/-1.25%; conversion of stored TVA was 50.8+/-1.91%. Similar bioconversion of TVA in humans would increase current estimates of CLA available for the general population by 6- to 10-fold.

Hormonal regulation of stearoyl coenzyme-A desaturase 1 activity and gene expression in primary cultures of chicken hepatocytes

Previous studies have provided evidence for the important role of liver stearoyl-CoA desaturase (SCD) in excessive adiposity in the chicken and suggest that the difference in SCD activity between fat and lean chickens could be explained by a difference in SCD1 gene expression. In the present study, the regulation of SCD1 gene expression was analyzed as the result of insulin and glucagon action, using primary cultures of 6-week-old chicken hepatocytes. Insulin increased SCD1 activity and mRNA levels, whereas glucagon decreased dramatically both the enzyme activity and the mRNA levels. Nuclear run-on transcription assays and mRNA stability investigations demonstrated that insulin and glucagon effects on SCD1 gene expression was primarily transcriptional. Furthermore, the results indicated that the glucagon-mediated inhibition of SCD1 gene transcription was more potent than just counteracting the insulin-mediated effect. These data represent the first demonstration that the glucagon effect on the SCD1 gene expression is primarily transcriptional. Moreover, among hepatic genes involved in lipid metabolism in chicken, SCD1 is the first gene shown to be regulated at the transcriptional level by insulin, in the absence of triiodothyronine. These data point out the potency of the growing chicken hepatocyte culture model in contrast with the embryonic cell culture model as regards the investigations of the insulin effect on gene expression.

Early modulation of genes encoding peroxisomal and mitochondrial beta-oxidation enzymes by 3-thia fatty acids

The aim of the present study was to elucidate the effects of a single dose of 3-thia fatty acids (tetradecylthioacetic acid and 3-thiadicarboxylic acid) over a 24-hr study period on the expression of genes related to peroxisomal and mitochondrial beta-oxidation in liver of rats. The plasma triglyceride level decreased at 2-4 hr, 4-8 hr, and 8-24 hr, respectively, after a single dose of 150, 300, or 500 mg of 3-thia fatty acids/kg body weight. Four to eight hours after administration of 3-thia fatty acids, a several-fold-induced gene expression of peroxisomal multifunctional protein, fatty acyl-CoA oxidase (EC 1.3.3.6), fatty acid binding protein, and 2,4-dienoyl-CoA reductase (EC 1.3.1.43) resulted, concomitant with increased activity of 2,4-dienoyl-CoA reductase and fatty acyl-CoA oxidase. The expression of carnitine palmitoyltransferase-I and carnitine palmitoyltransferase-II increased at 2 and 4 hr, respectively, although at a smaller scale. In cultured hepatocytes, 3-thia fatty acids stimulated fatty acid oxidation after 4 hr, and this was both L-carnitine- and L-aminocarnitine-sensitive. The hepatic content of eicosapentaenoic acid and docosahexaenoic acid decreased throughout the study period. In contrast, the hepatic content of oleic acid tended to increase after 24 hr and was significantly increased after repeated administration of 3-thia fatty acids. Similarly, the expression of delta9-desaturase was unchanged during the 24-hr study, but increased after feeding for 5 days. To conclude, carnitine palmitoyltransferase-I expression seemed to be induced earlier than 2,4-dienoyl-CoA reductase and fatty acid binding protein, and not later than the peroxisomal fatty acyl-CoA oxidase. The expression of delta9-desaturase showed a more delayed response.

Thia fatty acids as substrates and inhibitors of stearoyl-CoA desaturase

Thia fatty acids are fatty acid analogues, where sulfur atoms substitute methylene groups in the carbon chain. In 7800 C1 Morris hepatoma cells and in hepatocytes 9-thia and 10-thia stearic acid are strong inhibitors of stearoyl-Co desaturase, while 3,9-dithia stearic acid and 3,10-dithia stearic acid are much weaker inhibitors. No effect on the stearoyl-CoA desaturase can be observed with 3-thia stearic acid. In microsomes, an equimolar concentration of 9-thia stearoyl-CoA inhibits the delta9 desaturation of [1-14C]stearoyl-CoA approximately 75%, while 3,9-dithia stearoyl-CoA and 3,10-dithia stearoyl-CoA again are weak inhibitors. 3-Thia stearoyl-CoA has no effect on the desaturation of [1-14C]stearoyl-CoA. [2-14C]3-Thia stearoyl-CoA is delta9 desaturated to [2-14C]thia oleic acid. This desaturation is inhibited by unlabelled stearoyl-CoA, which therefore is the preferred substrate. These results show that a sulfur atom in the 3 position reduces the affinity of the CoA ester for the enzyme, but permits desaturation. A sulfur in the 9 or 10 position does not affect binding to the enzyme. The 9-thia and 10-thia stearoyl-CoA, which cannot be desaturated, therefore are strong inhibitors.

Lipid level and type alter stearoyl CoA desaturase mRNA abundance differently in mice with distinct susceptibilities to diet-influenced diseases

Chronic diseases develop in susceptible individuals following exposure to environmental conditions including high fat diets. Inbred strains of mice differing in susceptibility to atherosclerosis, diabetes, obesity and certain cancers are models for understanding the genetic basis and molecular mechanisms whereby diet influences these polygenic and multifactorial disorders. Expression sequence tags (EST) and disease quantitative trait loci (QTL) are also being identified with these strains. Reported here are comparisons of food intake, growth, nonfasting serum lipids and expression of mRNA for hepatic apolipoprotein E (ApoE), hepatic stearoyl CoA desaturase (Scd1) and heart lipoprotein lipase (Lpl) in a 2 x 2 x 2 design with C57BL/6J and BALB/cByJ mice fed semipurified diets with 4 or 20% saturated (coconut) or unsaturated (corn) oils for 4 mo. Histological studies of aortas and coronary arteries are also reported for these animals. After 4 mo, BALB/cByJ mice were significantly heavier and had significantly higher total serum cholesterol, HDL cholesterol and triglyceride concentrations in the fed state than C57BL/6J mice. Efficiency of utilizing dietary energy did not differ consistently between strains. Oil level affected serum total cholesterol, triglycerides and HDL cholesterol, which were significantly greater in mice fed high fat diets. Lpl and ApoE mRNA expression levels were not significantly affected by mouse strain, oil source or oil level. Scd1 mRNA expression, however, was significantly higher in C57BL/6J than in BALB/cByJ mice and was lower in all mice fed 20% compared with those fed 4% fat diets. Genes regulated differently by diet among strains with distinct susceptibility to diet-influenced disease may be associated with molecular pathways contributing to incidence or severity.

Thia fatty acids, metabolism and metabolic effects

(1) The chemical properties of thia fatty acids are similar to normal fatty acids, but their metabolism (see below: points 2-6) and metabolic effects (see below: points 7-15) differ greatly from these and are dependent upon the position of the sulfur atom. (2) Long-chain thia fatty acids and alkylthioacrylic acids are activated to their CoA esters in endoplasmatic reticulum. (3) 3-Thia fatty acids cannot be beta-oxidized. They are metabolized by extramitochondrial omega-oxidation and sulfur oxidation in the endoplasmatic reticulum followed by peroxisomal beta-oxidation to short sulfoxy dicarboxylic acids. (4) 4-Thia fatty acids are beta-oxidized mainly in mitochondria to alkylthioacryloyl-CoA esters which accumulate and are slowly converted to 2-hydroxy-4-thia acyl-CoA which splits spontaneously to an alkylthiol and malonic acid semialdehyde-CoA ester. The latter presumably is hydrolyzed and metabolized to acetyl-CoA and CO2. (5) Both 3- and 4-thiastearic acid are desaturated to the corresponding thia oleic acids. (6) Long-chain 3- and 4-thia fatty acids are incorporated into phospholipids in vivo, particularly in heart, and in hepatocytes and other cells in culture. (7) Long-chain 3-thia fatty acids change the fatty acid composition of the phospholipids: in heart, the content of n-3 fatty acids increases and n-6 fatty acids decreases. (8) 3-Thia fatty acids increase fatty acid oxidation in liver through inhibition of malonyl-CoA synthesis, activation of CPT I, and induction of CPT-II and enzymes of peroxisomal beta-oxidation. Activation of fatty acid oxidation is the key to the hypolipidemic effect of 3-thia fatty acids. Also other lipid metabolizing enzymes are induced. (9) Fatty acid- and cholesterol synthesis is inhibited in hepatocytes. (10) The nuclear receptors PPAR alpha and RXR alpha are induced by 3-thia fatty acids. (11) The induction of enzymes and of PPAR alpha and RXR alpha are increased by dexamethasone and counteracted by insulin. (12) 4-Thia fatty acids inhibit fatty acid oxidation and induce fatty liver in vivo. The inhibition presumably is explained by accumulation of alkylthioacryloyl-CoA in the mitochondria. This metabolite is a strong inhibitor of CPT-II. (13) Alkylthioacrylic acids inhibits both fatty acid oxidation and esterification. Inhibition of esterification presumably follows accumulation of extramitochondrial alkylthioacryloyl-CoA, an inhibitor of microsomal glycerophosphate acyltransferase. (14) 9-Thia stearate is a strong inhibitor of the delta 9-desaturase in liver and 10-thia stearate of dihydrosterculic acid synthesis in trypanosomes. (15) Some attempts to develop thia fatty acids as drugs are also reviewed.

Effects of chain length and sulphur position of thia fatty acids on their incorporation into phospholipids in 7800 C1 hepatoma cells and isolated rat hepatocytes, and their effects on fatty acid composition of phospholipids

Incorporation of thia fatty acids and their effects on the fatty acid composition in phospholipids has been investigated in 7800 C1 hepatoma cells and cultured hepatocytes. 3-Thia fatty acids of chain lengths from dodecyl-to hexadecyl-thioacetic acid were incorporated into phospholipids during a 3-day incubation. Longer and shorter 3-thia fatty acids were barely detectable. Tetradecylthioacetic acid, 3-thia stearate, and their delta9- desaturated derivatives were maximally incorporated into whole-cell phospholipids. The amount of tetradecylthioacetic acid incorporated into phospholipids of hepatoma cells remained almost identical in cells cultured for 3 days or adapted over a period of 1 year. Delta9-desaturated metabolites of long chain thia fatty acids (C13-to C16-S-acetic acid) were identified by GC-MS in phospholipids. 3-Thia stearate appeared to be the best substrated for delta9 desaturase. Incubation of hepatoma cells with thia fatty acids led to alterations in the amount of normal fatty acids in total phospholipids. The amounts of 16:0 and 18:1 decreased and 18:2 (n-6) and 20:5 (n-3) increased. Changes in the normal fatty acid composition of phospholipids were seen both with thia acids incorporated into phospholipids and those not incorporated. These effects, therefore, may be only partially dependent on displacement of normal fatty acids by thia fatty acids. Morris 7800 C1 hepatoma cell acyl-CoA synthetase (ACS) and peroxisomal acyl-CpA oxidase (ACO) were induced by thia fatty acids of all chain lengths, and with the sulphur atom(s) in different positions. Control experiments with hepatocytes revealed a similar incorporation of thia fatty acids in these physiologically more normal cells.