Dietary Fats High in Linoleic Acids Impair Antitumor T-cell Responses by Inducing E-FABP-Mediated Mitochondrial Dysfunction.

The most recent American Dietary Guidelines (2020-2025) recommend shifting dietary fats from solid saturated fats to unsaturated oils. Dietary oils contain different compositions of unsaturated fatty acids (UFA). Oleic acid (OA) and linoleic acid (LA) are the most common UFA in dietary oils. How individual UFA in oils regulate immune cell function and cancer risk remains unclear. Here we demonstrated that high-fat diets (HFD) rich either in OA or LA induced a similar degree of murine obesity, but the LA-rich HFD specifically promoted mammary tumor growth. LA impaired antitumor T-cell responses by promoting naïve T-cell apoptosis and inhibiting TNFα production. While exogenous OA and LA were taken up by T cells with similar efficacy, only LA induced significant mitochondrial reactive oxygen species production and lipid peroxidation. Importantly, naïve T cells predominantly expressed epidermal fatty acid binding protein (E-FABP), which is central in facilitating LA mitochondrial transport and cardiolipin incorporation. Genetic depletion of E-FABP rescued LA-impaired T-cell responses and suppressed LA-rich HFD-associated mammary tumor growth. Collectively, these data suggest that dietary oils high in LA promote mammary tumors by inducing E-FABP-mediated T-cell dysfunction. SIGNIFICANCE: These findings suggest that modulation of dietary oil composition and inhibition of E-FABP activity may represent novel strategies to enhance T-cell function in the prevention and treatment of obesity-associated cancers.

Species differences in toxicokinetic parameters of glycidol after a single dose of glycidol or glycidol linoleate in rats and monkeys.

Glycidol fatty acid esters (GEs) have been identified as contaminants in refined edible oils. Although the possible release of glycidol (G) from GEs is a concern, little is known about the conversion of GEs to G in the human body. This study addressed the toxicokinetics of glycidol linoleate (GL) and G in male Crl:CD(SD) rats and cynomolgus monkeys. Equimolar amounts of GL (341 mg/kg) or G (75 mg/kg) were administered by gavage to each animal. G was found in both species after the G and GL administration, while plasma GL concentrations were below the lower limit of quantification (5 ng/ml) in both species. In rats, the administration of GL or G produced similar concentration-time profiles for G. In monkeys, the C(max) and AUC values after GL administration were significantly lower than those after G administration. The oral bioavailability of G in monkeys (34.3%) was remarkably lower than that in rats (68.8%) at 75 mg/kg G administration. In addition, plasma G concentrations after oral administration at three lower doses of GL or G were measured in both species. In monkeys, G was detected only at the highest dose of G. In contrast, the rats exhibited similar plasma G concentration-time profiles after GL or G administration with significantly higher G levels than those in monkeys. In conclusion, these results indicate that there are remarkable species differences in the toxicokinetics of GEs and G between rodents and primates, findings that should be considered when assessing the human risk of GEs.

Amended safety assessment of tall oil acid, sodium tallate, potassium tallate, and ammonium tallate.

Tall oil acid is a mixture of oleic and linoleic acids (fatty acids) and rosin acids derived from tall oil, a by-product of pulp from resinous woods, used in cosmetic products as a surfactant at concentrations up to 8%. Ammonium, potassium, and sodium salts also are listed as cosmetic ingredients. In addition to the studies summarized in this report, extensive toxicity, genotoxicity, and carcinogenicity studies in animals are available for oleic, lauric, palmitic, myristic, and stearic fatty acids as published earlier by the Cosmetic Ingredient Review (CIR). These data may be extrapolated to tall oil acid and its salts. There are no reports of current uses or use concentration data for ammonium tallate, nor are use concentration data available for the other salts. The CIR Expert Panel found tall oil acid, ammonium tallate, potassium tallate, and sodium tallate to be safe cosmetic ingredients in the given practices of use and concentration.

Steroidal constituents of rice (Rryza sativa) hulls with algicidal and herbicidal activity against blue-green algae and duckweed.

Two new compounds, 14-methyl stigmast-9(11)-en-3alpha-ol-3beta-D-glucopyranoside (1) and cholest-11-en-3beta, 6beta, 7alpha, 22beta-tetraol-24-one-3beta-palmitoleate (2), along with the known compound beta-sitosteryl-3beta-D-glucopyranosyl-6'-linoleiate (3), were isolated from the methanolic extract of rice (Oryza sativa) hulls. The structures of the two new compounds were elucidated using one- and two-dimensional NMR in combination with IR, EI/MS, FAB/MS, HR-EI/MS and HR-FAB/MS. In bioassays with blue-green algae, Microcystis aeruginosa UTEX 2388 and duckweed, Lemna paucicostata Hegelm 381, the efficacy of bioactivity of the two new compounds linearly increased as the concentration increased from 0.3 to 300 IgM. Compared with momilactone A, compounds 1 and 2 showed similar and higher inhibitory activities against the growth of M. aeruginosa at a concentration of 300 microM. However, compound 2 was similar to momilactone A in inhibiting L. paucicostata growth at a concentration of 300 microM. As a result, compound 2 appears to have a strong potential for the environmentally friendly control of weed and algae that are harmful to water-logged rice.

Suppression of tumour-promoting factors in fat-induced colon carcinogenesis by the antioxidants caroverine and ubiquinone.

Fatty acid hydroperoxides are produced from unsaturated fatty acids in the presence of oxygen at elevated temperatures during food processing. Their effects on gene expression in colorectal tumour cells were studied using linoleic acid hydroperoxide (LOOH) as a model compound. Addition of LOOH to the medium of LT97 adenoma and SW480 carcinoma cells enhanced the production of hydrogen peroxide. Both cell lines were observed to increase VEGF factors based on mRNA. High consumption of dietary fat promotes colon carcinogenesis in the long-term. While this effect is well known, the underlying mechanisms are not understood. An approach was made starting from the assumption that LOOH is present in dietary fats as a result of heating. LOOH undergoes homolytic cleavage in the presence of iron. Various radicals are formed on mixing LT97 or SW480 cells with LOOH. The expression of tumour-promoting factors was inhibited by caroverine and ubiquinone, which may be justified as active chemopreventive agents.

Effects of conjugated linoleic acid on prostaglandins produced by cells isolated from maternal intercotyledonary endometrium, fetal allantochorion and amnion in late pregnant ewes.

The anticarcinogenic properties of conjugated linoleic acid (CLA) are, at least partially, attributed to its ability to interrupt the n-6 polyunsaturated fatty acid (PUFA) metabolic pathway for the biosynthesis of eicosanoids, including prostaglandins (PG). Both PGE(2) and PGF(2alpha) play key roles in parturition. In the present study, we compared the effects of CLA (a mixture of cis- and trans-9, 11- and -10, 12-octadecadienoic acid) and linoleic acid (LA) on PG production by cells isolated from maternal intercotyledonary endometrium, fetal allantochorion and amnion from late pregnant ewes. The results demonstrated that supplementation of LA and CLA significantly affected both the proportions and the amounts of PGs produced by all three tissue types. The ability of the uterus and placenta to respond to oxytocin (OT, endometrium only) and lipopolysaccharide (LPS) was also affected. LA inhibited PGE(2) and PGF(2alpha) production in the absence or presence of either oxytocin or LPS. In endometrial cells with or without oxytocin or LPS, CLA dose-dependently suppressed PGF(2alpha) generation, whereas low doses of CLA (20 microM) increased PGE(2) generation. Supplementation with CLA therefore increased the PGE(2)/PGF(2alpha) ratio in the endometrial cells. These results suggest that dietary supplementation of LA or CLA may affect both the initiation and progression of parturition.

Conjugated linoleic acid supplementation reduces adipose tissue by apoptosis and develops lipodystrophy in mice.

Conjugated linoleic acid (CLA) is a naturally occurring group of dienoic derivatives of linoleic acid found in beef and dairy products. CLA has been reported to reduce body fat. To examine the mechanism(s) of CLA reduction of fat mass, female C57BL/6J mice were fed standard semipurified diets (10% fat of total energy) with or without CLA (1% wt/wt). Terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick endlabeling (TUNEL) and DNA fragmentation analysis revealed that fat-mass decrease by CLA was mainly due to apoptosis. Tumor necrosis factor (TNF)-alpha and uncoupling protein (UCP)-2 mRNA levels increased 12- and 6-fold, respectively, in isolated adipocytes from CLA-fed mice compared with control mice. Because it is known that TNF-alpha induces apoptosis of adipocytes and upregulates UCP2 mRNA, a marked increase of TNF-alpha mRNA with an increase of UCP2 in adipocytes caused CLA-induced apoptosis. However, with a decrease of fat mass, CLA supplementation resulted in a state resembling lipoatrophic diabetes: ablation of brown adipose tissue, a marked reduction of white adipose tissue, marked hepatomegaly, and marked insulin resistance. CLA supplementation decreased blood leptin levels, but continuous leptin infusion reversed hyperinsulinemia, indicating that leptin depletion contributes to the development of insulin resistance. These results demonstrate that intake of CLA reduces adipose tissue by apoptosis and results in lipodystrophy, but hyperinsulinemia by CLA can be normalized by leptin administration.

Gastrointestinal glutathione peroxidase prevents transport of lipid hydroperoxides in CaCo-2 cells.

BACKGROUND & AIMS: Gastrointestinal glutathione peroxidase (GI-GPx), 1 of the 4 types of selenium-dependent glutathione peroxidases, is expressed exclusively in the gastrointestinal system and has therefore been suggested to function as a barrier against the absorption of dietary hydroperoxides. METHODS: The selenium-dependent expression of GI-GPx and cytosolic GPx (cGPx) was analyzed by Western blotting. Transport of 13-hydroperoxy octadecadienoic acid (13-HPODE) was investigated in a CaCo-2 cell monolayer modulated in GI-GPx and cGPx by selenium restriction or repletion. Localization of GI-GPx in rat intestine was visualized by immunohistochemistry. RESULTS: Low but significant GI-GPx levels were detected in selenium-deficient CaCo-2 cells and in the gastrointestinal tract of selenium-deficient rats, whereas cGPx was completely absent. Selenium supplementation of CaCo-2 cells resulted in a 5-fold increase of GI-GPx protein, whereas total GPx activity increased by a factor of 13, with most of the GPx activity under selenium-adequate conditions being cGPx. Irrespective of the selenium status, 13-HPODE did not reach the basolateral side of an intact CaCo-2 cell monolayer. Depending on the selenium status, hydroperoxides damaged the monolayer as evidenced by loss of transepithelial resistance and paracellular diffusion of lucifer yellow. Only under these conditions was unmetabolized 13-HPODE detectable at the basolateral side. CONCLUSIONS: Low GI-GPx levels, as present in selenium deficiency, suffice to prevent transport of 13-HPODE. GI-GPx may thus function as a barrier against hydroperoxide absorption. cGPx contributes to balance major oxidative challenge.

Newly recognized cytotoxic effect of conjugated trienoic fatty acids on cultured human tumor cells.

We investigated the cytotoxic effect of conjugated trienoic fatty acids on various human tumor cell lines: DLD- 1, colorectal; HepG2, hepatoma; A549, lung; MCF-7, breast; and MKN-7, stomach. Conjugated linoleic acid (CLA) and conjugated linolenic acid were prepared from linoleic acid (18:2, n-6) and alpha-linolenic acid (18:3, n-3), respectively, by treatment with 6.6% or 21% potassium hydroxide. Spectrophotometric readings at 235 nm for the conjugated diene formation, and at 268 nm for the conjugated triene, were confirmed for the respective conjugated fatty acids. In addition, tung oil (Aleurites fordii) fatty acids consisting principally of a conjugated triene (eleostearic acid, approximately 80% of total fatty acids) were prepared using an alkaline saponification procedure. All tumor cells were incubated for 24 h with 5-100 microM of the conjugated fatty acids, and MTT dye reduction was measured to verify the cell viability. Among the conjugated fatty acids examined, conjugated linolenic acid and tung oil fatty acids exhibited the most intense cytotoxic effects on DLD-1, HepG2, A549, MCF-7 and MKN-7 cells, while CLA was not cytotoxic to the tumor cells. These results demonstrate that conjugated trienoic fatty acids are more cytotoxic to human tumor cells than the conjugated dienoic fatty acid, CLA.

The acute pathology of fatty acid anilides and linoleic diester of 3-phenylamino-1,2-propanediol in mice: possible implication as aetiologic agents for the toxic oil syndrome.

Two groups of compounds, the fatty acid anilides and the mono- and diester of 3-phenylamino-1,2-propanediol (PAP) are suspected as aetiologic agents for the toxic oil syndrome (TOS). Intraperitoneal administration of oleoyl and linoleoyl anilides in mice caused severe weight loss followed by death in 50% of the animals and histopathological changes mainly to the lungs. Linoleic diester of PAP led to weight loss, haemorrhage, congestion and emphysema in the lungs and an increase in blood eosinophilia. Although not producing the full spectrum of symptoms the effects of the substances resemble the acute human disease. Possibly, the two groups of substances led together to the full spectrum of disease manifestations seen in TOS.

Expression of human phospholipid hydroperoxide glutathione peroxidase gene for protection of host cells from lipid hydroperoxide-mediated injury.

A cDNA encoding human phospholipid hydroperoxide glutathione peroxidase (PHGPx) was obtained by PCR amplification from human testis cDNA and was inserted into the plasmid pRc/CMV to construct an expression vector for human PHGPx. Guinea pig cell line 104C1 cells were transfected with the expression vector. One of the transfectants, designated 104Cl/O4C, expressed high glutathione peroxidase activity toward dilinoleoyl phosphatidylcholine hydroperoxide and linoleic acid hydroperoxide. Western blot analysis revealed a large amount of protein immunoreactive against anti-PHGPx antibody in the transfectant. When the cells were incubated with these hydroperoxides, the parental cells suffered from serious cell injury, whereas the transfectant was extremely resistant against lipid hydroperoxide-mediated injury.

Interactions of cis-fatty acids and their anilides with formyl-methionyl-leucyl-phenylalanine, phorbol myristate acetate and dioctanoyl-s,n-glycerol in human leukocytes.

Aniline-denaturated rape-seed food oils that contained anilides of linoleic and oleic acids caused a poisoning epidemic, known as Toxic Oil Syndrome, in Spain in 1981. Toxic Oil Syndrome affected mainly the lungs and the immune system of exposed individuals. Linoleic and oleic acids, and linoleic and oleic anilides increased the production of reactive oxygen metabolites in human polymorphonuclear leukocytes. Both cis-fatty acids inhibited a chemotactic peptide-, fMLP-induced production of reactive oxygen metabolites without affecting fMLP-induced elevation of intracellular calcium levels. Linoleic acid anilide slightly amplified fMLP-induced respiratory burst, whereas oleic acid anilide was without an effect. However, both fatty acid anilides decreased fMLP-induced elevation of levels of free intracellular calcium. Moreover, both cis-fatty acids and their anilides inhibited phorbol myristate acetate (PMA)- and dioctanoyl-s,n-glycerol (DiC8)-induced production of reactive oxygen metabolites. Thus, both cis-fatty acids and their anilides inhibited agonist-stimulated production of reactive oxygen metabolites; this is most likely due to interactions with cell signalling events. These results suggest that both linoleic and oleic acids and their anilides may inhibit immunological responses of leukocytes.

Immunotoxicity of polyunsaturated fatty acids in serum-free medium.

To test the effect of purified polyunsaturated fatty acids on immune cells in vitro, human peripheral blood mononuclear cells and murine spleen cells were incubated in Opti-MEM medium without serum or even albumin and with 2-mercapto-ethanol, insulin, transferrin and selenium as supplements. The human cells were stimulated with phytohemagglutinin and the murine cells were stimulated with Concanavalin A or lipopolysaccharide. Both human and murine cells were stimulated with recombinant human interleukin-2 to generate lymphokine activated killer cells. Linoleic and linolenic acids inhibited all of the immune responses tested, whereas docosahexaenoic and eicosapentaenoic acids did not. Similar effects were observed with cultured B16 F10 murine melanoma cells. Mixtures of linoleic and docosahexaenoic or eicosapentaenoic acids also inhibited the mitogenic response to phytohemagglutinin. Inhibition of lipid mediator production by indomethacin, quercetin, rutin, or nordihydroguariaretic acid, and addition of vitamins C and E with anti-oxidant activity failed to reverse the effects of linoleic acid. Thus, linoleic and linolenic acids appear to directly inhibit immune and tumor cells, at least under these conditions.

Modulation of growth and cell turnover of preneoplastic lesions and of prostaglandin levels in rat pancreas by dietary fish oil.

In the present study the modulating effects of dietary fish oil (MaxEPA) on unsaturated fat-promoted pancreatic carcinogenesis in azaserine-treated rats were investigated. Three groups of 20 rats (each group comprised five saline-treated and 15 azaserine-treated animals) were fed an AIN76-based purified diet containing (i) 5 wt% fat, (ii) 25 wt% fat including 5 wt% linoleic acid or (iii) 25 wt% fat including 5 wt% linoleic acid and 9.4 wt% (20 cal%) MaxEPA for 6 months. The number and size of pancreatic atypical acinar cell foci was significantly higher (P < 0.01) in azaserine-treated animals maintained on a high fat diet than in those fed a low fat diet. MaxEPA did not influence the promoting effect of the high fat diet. The labeling index of atypical acinar cell foci in animals maintained on both a low fat or a high fat/MaxEPA diet was significantly (P < 0.01) lower than that in rats fed a high fat diet without MaxEPA. The linoleic acid concentration was higher, whereas the arachidonic acid concentration was lower, in blood plasma and to a lesser extent also in the pancreas of animals given MaxEPA in comparison with the other groups. Furthermore, animals fed MaxEPA showed lower 6-keto-prostaglandin F1 alpha, prostaglandin F2 alpha and thromboxane B2 levels, but not prostaglandin E2 levels in pancreatic tissue in comparison with the other groups. It is concluded that a high fat diet containing 5 wt% linoleic acid has a strong promoting effect on pancreatic carcinogenesis in azaserine-treated rats. Dietary MaxEPA did not influence the promoting effect of unsaturated fat on pancreatic carcinogenesis, although it caused a decrease in both cell proliferation in atypical acinar cell foci and prostaglandin levels in the pancreas.

Ozone-induced lung toxicity: mediated by ozonides?

In an in vitro study a comparison was made between the cytotoxicities of a model ozonide, methyl linoleate-9,10-ozonide (MLO), and a model peroxidative agent, cumene hydroperoxide (CumOOH), by measuring the effects of both compounds on the phagocytosing capacity of rat alveolar macrophages. Toxicity as well as detoxication characteristics of the ozonide were found to be similar to those of ozone: (1) vitamin E protected the alveolar macrophages in vitro against the ozonide, (2) glutathione (GSH) depletion enhanced the sensitivity of the cells towards the ozonide and (3) the ozonide did not enhance lipid peroxidation. This also suggests that GSH depletion, followed by lipid peroxidation, does not underlie the MLO toxicity. This was supported by the differences in protection provided by vitamin C, vitamin E and GSH. Supplementation of the macrophages with vitamin C resulted in a decrease in their sensitivity towards MLO and an increase in their sensitivity towards CumOOH. Following GSH depletion the sensitivity of the cells towards CumOOH had increased more than that towards MLO. Exposure to CumOOH led to a more extensive vitamin E depletion. The results of an in vivo study on the toxicity of MLO (in the rat) largely confirmed the findings of the in vitro study: partial contributions of vitamin E and the glutathione system to the protection against MLO.

Effects of dietary linoleic acid on pancreatic carcinogenesis in rats and hamsters.

It has been suggested that linoleic acid (LA) is responsible for the promoting effect of dietary polyunsaturated fat on pancreatic carcinogenesis via an accelerated prostaglandin synthesis, caused by metabolism of LA-derived arachidonic acid in (pre)neoplastic tissue. The purpose of the present study was to investigate whether dietary LA is the cause of pancreatic tumor promotion by a high fat diet. Five groups of 30 azaserine-treated rats and 5 groups of 30 N-nitrosobis(2-oxopropyl)amine-treated hamsters were maintained for 6 months (rats) and 12 months (hamsters) on high fat (25 weight %) AIN diets containing 2, 4, 6, 10, or 15 weight % LA. The results indicated that the strongest enhancing effect on the growth of pancreatic (pre)neoplastic lesions in rats and hamsters was obtained with 4 and 2 weight % of dietary LA, respectively. At higher LA levels the tumor response seemed to decrease rather than increase. In both rats and hamsters the fatty acid profiles of blood plasma and pancreas showed an accurate reflection of the dietary fatty acid profiles: a proportional increase in LA levels was observed in plasma and pancreas with increasing dietary LA. In both species plasma and pancreatic AA levels remained constant, except for arachidonic acid levels in rat plasma, which significantly increased with increasing dietary LA levels. Fatty acid profiles in hamster pancreatic tumors did not differ from fatty acid profiles in nontumorous pancreatic tissue from hamsters fed the same diet. Prostaglandin (PG) E2, 6-keto-PGF1 alpha, PGF2 alpha, and thromboxane B2-concentrations in nontumorous pancreatic tissue were similar among the diet groups. Ductular adenocarcinomas from hamster pancreas showed significantly higher levels of 6-keto-PGF1 alpha, PGF2 alpha, and thromboxane B2, but not of PGE2 in comparison with nontumorous pancreas. It is concluded that the strongest pancreatic tumor promotion by dietary LA is 4 weight % in rats and 2 weight % or less in hamsters, and that PGs may be involved in the development of ductular adenocarcinomas induced in hamster pancreas by N-nitrosobis(2-oxopropyl)amine.

Toxic effects of fatty acid anilides on the oxygen defense systems of guinea pig lungs and erythrocytes.

Toxic oil syndrome (TOS) is caused by ingestion of denatured edible oils. Even though the etiology and pathogenesis of this disease are not fully known, it is quite clear that generation of free radicals caused by ingestion of fatty acid anilides is responsible for the pathogenetic mechanism in many TOS patients. Fatty acid anilides may also alter the free radical status of lungs and erythrocytes; this possibility may shed some light on understanding toxic oil syndrome. The present study describes the effects of oral administration of fatty acid anilides on the activities of major enzymes involved in the oxygen defense systems of lungs and erythrocytes. Feeding fatty acid anilides caused an increase in the superoxide dismutase (SOD) activity in erythrocytes, whereas it caused a decrease in the SOD activity in lungs. GSH-Px activity was not significantly changed in erythrocytes but was decreased in lungs. Although the activity of catalase was increased only by a higher dose in the erythrocytes, it was not affected in the lung at any dosage. Even though the ingestion of fatty acid anilides caused an increase in the SOD activity in the erythrocytes and a decrease in the SOD activity in the lungs, there was an increase in the lipid peroxidation in both cases. The increase in lipid peroxidation in erythrocytes is probably caused by the accumulation of H2O2, and that in the lungs is due to the accumulation of superoxide anion.

Effects of fatty acids on invasion through reconstituted basement membrane ('Matrigel') by a human breast cancer cell line.

An in vitro invasion assay system was used to examine the effects of linoleic acid, an omega-6 fatty acid, and two omega-3 fatty acids, eicosapentaenoic acid and docosahexaenoic acid, on the invasive capacity of MDA-MB-435 human breast cancer cells. Linoleic acid stimulated, and the omega-3 fatty acids inhibited, tumor cell invasion at concentrations of 0.25 and 0.5 microgram/ml. Indomethacin, 20 micrograms/ml, completely suppressed the stimulatory activity of linoleic acid, suggesting that these fatty acid effects are mediated via eicosanoid biosynthesis.

Cyclooxygenase and lipoxygenase arachidonate metabolites synthesized by mouse peritoneal macrophages: in vitro effect of N-phenyllinoleamide from toxic oil samples.

N-phenyllinoleamide (NPLA) has been detected as extraneous compound in adulterated cooking oils associated with a unique epidemic disease known as the Toxic Oil Syndrome (TOS). In this communication we report on the action of NPLA on the endogenous cyclooxygenase and lipoxygenase arachidonate metabolism. Results show that mouse peritoneal macrophages (MPM) exposed to 1 mM NPLA for 2 h undergo significant increases of 6-keto prostaglandin F1a, prostaglandin E2, leukotriene B4, 12- and 15-hydroxyeicosatetraenoic acids. MPM prelabelled with 3H-AA showed an enhanced release when exposed to NPLA. Thus, it is concluded that NPLA potentiates AA release from cell membrane phospholipids and the subsequent cyclooxygenase and lipoxygenase oxidative metabolism of this precursor to various eicosanoids. This is in agreement with the implication of peroxidative process mediated by fatty acids anilides in TOS.

Mechanism of lipid mobilization associated with cancer cachexia: interaction between the polyunsaturated fatty acid, eicosapentaenoic acid, and inhibitory guanine nucleotide-regulatory protein.

During a study of the mechanism of cancer cachexia, a debilitating condition in which catabolism of host muscle and adipose tissue occurs, it has been observed that the process can be effectively reversed in vivo by the polyunsaturated fatty acid, eicosapentaenoic acid (EPA), but not by other PUFA of either the n-3 or n-6 series. In vitro studies showed that EPA blocked the action of a tumour-produced catabolic factor at the level of the adipocyte, and that the effect of EPA also extended to beta-adrenergic stimuli and polypeptide hormones. Again the effect was specific to EPA and appeared to arise from an inhibition of the elevation of cyclic AMP levels in adipocytes in response to varied stimuli. Using isoprenaline stimulated lipolysis as a model system we have shown that EPA has a direct inhibitory effect on isoprenaline-stimulated adenylate cyclase in isolated plasma membrane fractions with half maximal inhibition at a concentration of 165 microM. The inhibitory effect was specific for EPA and was not shown by docosahexaenoic or arachidonic acids. The inhibitory effect of EPA on adenylate cyclase showed properties similar to hormonal inhibition of the enzyme in that it was (i) GTP-dependent, (ii) non-competitive with isoprenaline, (iii) eliminated following treatment of either adipocytes or plasma membrane fractions with pertussis toxin, which is known to ADP-ribosylate the alpha-subunit of an inhibitory guanine nucleotide-regulatory protein (Gi), thus leading to its inactivation. This suggests that inhibition of cyclic AMP formation by EPA was due, at least in part, to a Gi-mediated inhibition of adenylate cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)