Conjugated linoleic acid isomers and cancer

We reviewed the literature regarding the effects of conjugated linoleic acid (CLA) preparations enriched in specific isomers, cis9, trans11-CLA (c9, t11-CLA) or trans10, cis12-CLA (t10, c12-CLA), on tumorigenesis in vivo and growth of tumor cell lines in vitro. We also examined the potential mechanisms by which CLA isomers may alter the incidence of cancer. We found no published reports that examined the effects of purified CLA isomers on human cancer in vivo. Incidence of rat mammary tumors induced by methylnitrosourea was decreased by c9, t11-CLA in all studies and by t10, c12-CLA in just a few that included it. Those 2 isomers decreased the incidence of forestomach tumors induced by benzo (a) pyrene in mice. Both isomers reduced breast and forestomach tumorigenesis. The c9, t11-CLA isomer did not affect the development of spontaneous tumors of the intestine or mammary gland, whereas t10, c12-CLA increased development of genetically induced mammary and intestinal tumors. In vitro, t10, c12-CLA inhibited the growth of mammary, colon, colorectal, gastric, prostate, and hepatoma cell lines. These 2 CLA isomers may regulate tumor growth through different mechanisms, because they have markedly different effects on lipid metabolism and regulation of oncogenes. In addition, c9, t11-CLA inhibited the cyclooxygenase-2 pathway and t10, c12-CLA inhibited the lipooxygenase pathway. The t10, c12-CLA isomer induced the expression of apoptotic genes, whereas c9, t11-CLA did not increase apoptosis in most of the studies that assessed it. Several minor isomers including t9, t11-CLA; c11, t13-CLA; c9, c11-CLA; and t7, c11-CLA were more effective than c9, t11-CLA or t10, c12-CLA in inhibiting cell growth in vitro. Additional studies with purified isomers are needed to establish the health benefit and risk ratios of each isomer in humans.     

Conjugated linoleic acid

Conjugated linoleic acid (CLA) is a collective term for metabolic by‐products resulting from the conversion of linoleic acid to oleic acid by rumen bacteria. Consequently CLA is found in foods and fats of animal origin. There is a growing body of information regarding effects of dietary CLA in health and disease, but not yet any definitive mechanisms relating to its mode(s) of action. The review paper by Professor Kritchevsky and accompanying commentary by Professor Williams summarise the recent evidence from animal studies for anti‐tumourigenic, anti‐lipogenic and anti‐atherogenic effects of CLA. These findings may open new avenues of research in several normal and disease states. Furthermore, if the health benefits suggested by animal studies can be shown to apply in human populations, the consequence of advice to reduce fat intakes, may prove to have important public health implications.     

Conjugated linoleic acid content of human plasma

Conjugated linoleic acid (CLA), a naturally occurring anticarcinogen found in dairy products, is an intermediary product of ruminal biohydrogenation of polyunsaturated fatty acids. Few data exist on the CLA content of the human blood plasma. The determination of a "normal" content could help in estimating if a person consumes satisfactory amounts of CLA with the diet and thus takes advantage of its potential beneficial effects on health. The purpose of this study was to compare the plasma CLA content of individuals not consuming dairy products (group 1, n = 12), individuals consuming normal amounts of dairy products (group 2, n = 77) and individuals consuming CLA supplement (group 3, n = 12). The only CLA isomer that presented higher percentage than the detection limit (0.03% of total fatty acids) was rumenic acid (cis9, trans11-octadecadienoic acid). An interesting finding is that compared to the other two groups, group 3 members show the highest average plasma content in rumenic acid, i.e. 0.20% of total fatty acids. The present study could be characterized as the first step in the direction of establishing a normal CLA content of human plasma. Based on these results, it could be suggested that the lower limit of the plasma CLA content is approximately 0.1% of total fatty acids.     

Conjugated linoleic acid and human health-related outcomes

Summary There has been increasing interest in health benefits of conjugated linoleic acid (CLA) based on findings with laboratory animals. Some human studies have also suggested health benefits of CLA, but because of the mixes used these could not be readily associated with a particular isomer of CLA. A recent study examined the separate effects of near‐pure cis‐9,trans‐11 CLA (c9,t11 CLA) or trans‐10,cis‐12 CLA (t10,c12 CLA) on health‐related outcomes in healthy young males. The CLA isomers were provided in capsules and at three doses (up to about 2.5 g/day) each for 8 weeks. Both c9,t11 and t10,c12 CLA were incorporated in a dose–response fashion into blood lipids and cells. At the doses and durations used, neither isomer of CLA affected bodyweight, body mass index or body composition, insulin sensitivity, immune function or markers of inflammation. However, at the doses and durations used, c9,t11 and t10,c12 CLA had opposing effects on blood lipid concentrations. Altered dairy cow‐feeding practices were used to produce c9,t11 CLA‐rich milk and, from this ultra heat‐treated milk, cheese and butter were produced. The milk and the dairy products made from it had ninefold higher contents of c9,t11 CLA, higher contents of n‐3 fatty acids and lower contents of total fat and of saturated fatty acids. They also contained much higher contents of trans‐vaccenic acid (tVA). The modified dairy products were used in a 6‐week controlled dietary intervention study in healthy middle‐aged males. c9,t11 CLA and tVA were incorporated from dairy products into blood lipids and cells. Consumption of the CLA‐rich (and tVA‐rich) dairy products did not affect bodyweight or body mass index, insulin sensitivity or inflammatory markers. However, there were some detrimental effects on blood lipids. These effects may be due to tVA rather than to c9,t11 CLA, as they are consistent with the effects of trans fatty acids and not consistent with the effects of c9,t11 CLA identified in the earlier study with c9,t11 CLA in capsules.     

Conjugated linoleic acid inhibits proliferation and modulates protein kinase C isoforms in human prostate cancer cells

Prostate cancer is the second most common cancer in men. The disease etiology is poorly understood, but diet and lifestyle are contributory factors. Conjugated linoleic acids (CLAs), naturally occurring fatty acids in ruminant food products, have antitumor properties in animal models of cancer and antiproliferative effects on cancer cells in vitro. The cellular mechanisms by which CLAs elicit these effects are unclear, particularly for prostate cancer cells. We have previously identified protein kinase C (PKC) isoforms, alpha, delta, iota, mu, and zeta in LNCaP prostate cancer cells. The objective of this study was to determine the effects of CLAs (individual cis-9, trans-11 and trans-10, cis-12 isoforms and a 50:50 mixture) on PKC isoform abundance in LNCaP cells. Confluent cells were treated with 6, 25, and 50 microM CLA for 0.5, 6, and 24 h. Cytosol and membrane protein fractions were assayed for PKC isoforms (mainly alpha and delta but also iota, mu, and zeta) by Western blot analysis using specific antibodies. CLAs clearly modulated the abundance of these PKC isoforms, both positively and negatively, depending on the isoform, concentration of CLAs, and period of treatment. Increased PKC-delta and decreased PKC-iota membrane abundance was consistent with CLAs eliciting increased apoptosis and, in part, with their antitumor effects.     

Conjugated linoleic acid alters caveolae phospholipid fatty acid composition and decreases caveolin-1 expression in MCF-7 breast cancer cells

Conjugated linoleic acid (CLA) refers to a group of biologically active fatty acids that exhibit anticarcinogenic properties; however, the mechanism of action remains poorly understood. Caveolae are specialized plasma membrane structures that affect many facets of cancer cell function, including growth, cell signaling, and apoptosis. Therefore, one potential mechanism could be alteration of caveolae lipid composition and function. We hypothesized that CLA can alter the lipid microenvironment of caveolae and alter expression of the major caveolae-resident protein, caveolin-1. MCF-7 human breast cancer cells were treated with a vehicle control, linoleic acid (LA), or CLA for 3 days after which total cell lysate, plasma membrane, and caveolae membrane fractions were isolated. Our findings indicate that CLA readily incorporates into caveolae (Δ9cis,11trans-18:2 being the major isomer) and maybe preferentially enriched in specific phospholipid species. Furthermore, caveolin-1 localization to caveolae after treatment with CLA was decreased relative to either control- or LA-treated cells, without changes in total cellular levels of protein relative to vehicle-control treated cells. Taken together, our results suggest that further investigation of a potential therapeutic role for CLA in modulating caveolae function in breast cancer is merited.     

Conjugated linoleic acid isomers and mammary cancer prevention

There is increasing evidence that individual isomers of conjugated linoleic acid (CLA) may have unique biological or biochemical effects. A primary objective of this study was to determine whether there might be differences in the anticancer activity of 9,11-CLA and 10,12-CLA. This was achieved by evaluating the reduction in premalignant lesions and carcinomas in the mammary gland of rats that had been treated with a single dose of methylnitrosourea and given 0.5% of either highly purified CLA isomer in the diet. Our results showed that the anticancer efficacies of the two isomers were very similar. At 6 wk after carcinogen administration, the total number of premalignant lesions was reduced by 33-36%. At 24 wk, the total number of mammary carcinomas was reduced by 35-40%. The concentration of each CLA isomer and its respective metabolites was analyzed in the mammary fat pad. Tissue level of 10,12-CLA was much lower than that of 9,11-CLA. The pool of metabolites from each isomer was very similar between the two groups and represented only a small fraction of total conjugated diene fatty acids. Feeding of 9,11-CLA resulted in minimal changes in other unsaturated fatty acids. In contrast, feeding of 10,12-CLA produced a wider spectrum of perturbations. Small but significant increases in 16:1 and 16:2 were detected; these were accompanied by decreases in 20:2, 20:3, 20:4, 22:4, and 22:6. The above observation suggests that 10,12-CLA might be more potent than 9,11-CLA in interfering with elongation and desaturation of linoleic and linolenic acids. In summary, our study showed that, at the 0.5% dose level, the anticancer activity of 9,11-CLA and 10,12-CLA was very similar, even though accumulation of 10,12-CLA in the mammary tissue was considerably less than that of 9,11-CLA. These confounding changes of the other unsaturated fatty acids in contributing to the effect of 10,12-CLA need to be clarified.     

Conjugated linoleic acid increased C-reactive protein in human subjects

We previously showed that conjugated linoleic acid (CLA) increases 15-keto-dihydro-prostaglandin F2alpha, a marker for cyclooxygenase-mediated lipid peroxidation and thus an indicator of cyclooxygenase-mediated inflammation. The aim of the present study was to investigate the effects of CLA on other indicators of inflammation in human subjects, including C-reactive protein, TNF-alpha, TNF-alpha receptors 1 and 2, and vascular cell adhesion molecule-1. In a double-blind, placebo-controlled study, fifty-three human subjects were supplemented with a mixture (4.2 g/d) of the isomers cis-9,trans-11 CLA and trans-10,cis-12 CLA or control oil for 3 months. CLA supplementation increased levels of C-reactive protein (P=0.003) compared with the control group. However, no changes in TNF-alpha, TNF-alpha receptors 1 and 2, and vascular cell adhesion molecule-1 were detected.     

Conjugated linoleic acid content in breast adipose tissue of breast cancer patients and the risk of metastasis

The association between the level of conjugated linoleic acid (CLA) in breast adipose tissue at the time of diagnosis and the subsequent development of metastasis was examined in a cohort of 209 patients presenting with an initially localized breast cancer. CLA level in breast adipose tissue was used as a qualitative biomarker of its past dietary intake. Biopsies of adipose tissue were obtained at the time of initial surgery. A CLA-enriched fraction was prepared by high performance liquid chromatography and CLA measured as a percentage of total fatty acids, using capillary gas chromatography. Mean CLA level was low (0.44% of total fatty acids) and the range between patients was narrow (0.19-0.85). With a median follow-up time of 7.5 yr, 45 patients developed metastases. A Cox proportional hazard regression model was used to identify prognostic factors. We did not find any significant association between CLA level in adipose fat and either the prognostic factor (tumor size, nodal status, histoprognostic grade, mitotic index, and estrogen or progesterone receptors) or the risk of metastasis or death. We concluded that CLA are unlikely to be involved in survivorship. However, the hypothesis that a higher intake of CLA might have a protective effect on the risk of metastasis cannot be ruled out from these data, since the level of CLA in breast cancer patients' adipose tissue is likely to be too low and the range of CLA distribution too narrow for any protection to be detectable.     

共轭亚油酸对人乳腺癌细胞生长的抑制作用

目的 研究共轭亚油酸（c9,t11-CLA)对人乳腺癌细胞（MCF－7）生长的影响。方法 采用细胞核分裂指数、细胞生长曲线、细胞集落形成试验、^3H-TdR掺入试验和软琼脂培养方法,所设剂量（μmol/L）为25,50,100,200,以96％乙醇为溶剂对照。结果 在细胞核分裂指数、细胞生长曲线和细胞集落形成试验中,可见c9,t11-CLA对MCF－7细胞生长有明显的抑制作用,其作用于MCF－7细胞8d后的抑制率（％）分别为27．18、35．43、91．05和92．86;在^3H-TdR掺入试验中,可见随着c9,t11-CLA剂量的增加,^3H-TdR掺入到MCF－7细胞中明显的减少,与阴性对照组相比差异有显性;由软琼脂培养试验结果可见,随着c9,t11-CLA剂量的增加,MCF－7细胞的集落形成逐渐降低,除了25 μmol/L剂量组外与阴性对照组相比差异有显性。结论 c9,t11-CLA对MCF－7细胞的生长有明显的抑制作用。     

Conjugated linoleic acid and bone biology

Osteoporosis, osteoarthritis and inflammatory joint disease afflict millions of people worldwide. Inflammatory cytokines inhibit chondrocyte proliferation and induce cartilage degradation for which part of the response is mediated by PGE2. Excess production of PGE2 is linked to osteoporosis and arthritis and is associated with bone and proteoglycan loss. PGE2 also influences the IGF-I/IGFBP axis to facilitate bone and cartilage formation. Recent investigations with growing rats given butter fat and supplements of CLA demonstrated an increased rate of bone formation and reduced ex vivo bone PGE2 production, respectively. Furthermore, the supplements of CLA isomers resulted in their enrichment in lipids of various bone compartments of animals. The effects of CLA on bone biology in rats (IGF action and cytokines) appear to be dependent on the level of n-6 and n-3 fatty acids in the diet; however, these studies generally showed that CLA decreased ex vivo bone PGE2 production and in osteoblast-like cultures. Anti-inflammatory diets, including nutraceutical applications of CLA, may be beneficial in moderating cyclooygenase 2 (COX-2) activity or expression (influencing PGE2 biosynthesis) and might help to reduce rheumatoid arthritis (secondary osteoporosis). This review summarizes findings of CLA on bone modeling in rats and effects on cellular functions of osteoblasts and chondrocytes. These experiments indicate that CLA isomers possess anti-inflammatory activity in bone by moderating prostanoid formation.     

Conjugated linoleic acid upregulates LDL receptor gene expression in HepG2 cells

Conjugated linoleic acid (CLA) exerts anticarcinogenic and antiatherosclerotic effects in animals. The present study was conducted to examine the effects of CLA on LDL receptor (LDLr) expression in HepG2 cells, and to evaluate whether the sterol response element binding protein 1 (SREBP-1) and acyl CoA:cholesterol acyltransferase (ACAT) were involved in the regulation of LDLr expression by CLA. When HepG2 cells were cultured with serum-free DMEM for 48 h, there was a three- to fivefold (P<0.05) increase in LDLr protein and mRNA levels. Incubation of HepG2 cells in serum-free medium supplemented with 25-hydroxycholesterol (25OH, 5 mg/L) for 24 h decreased LDLr protein and mRNA by 50-70% (P<0.05) and mature SREBP-1 by 20-40% (P<0.05). CLA, but not linoleic acid, antagonized the depressive effects of 25OH and increased both LDLr protein and mRNA abundance twofold (P<0.05). LDLr protein and mRNA abundance were not different when HepG2 cells were cultured with CLA (0.4 mmol/L) plus 25OH in the presence or absence of an ACAT inhibitor (58-035, 1 mg/L). Furthermore, CLA had no effect on SREBP-1 abundance. These results suggest that CLA upregulates LDLr expression via a mechanism that is independent of ACAT and SREBP-1.     

Conjugated linoleic acid reduces early aortic atherosclerosis greater than linoleic acid in hypercholesterolemic hamsters

Thirty-six male F₁B hamsters, 10 weeks of age, were divided into 3 groups of 12 based on similar body weights. The experimental diets comprised of a chow-based hypercholesterolemic diet supplemented with 20% coconut oil, 2% safflower oil, and 0.12% cholesterol (HCD); the HCD plus either 1% CLA as the free fatty acid (CLA), or 1% LA as the free fatty acid (LA) and were fed for 12 weeks. Plasma total cholesterol (TC) and nonHDL-C (very low- and low-density lipoprotein cholesterol) were significantly lower in the CLA and LA relative to the HCD (P < 0.05). The CLA had significantly less maximum number of dienes formed relative to the LA and HCD (P < 0.05). The CLA developed significantly less early aortic atherosclerosis relative to both the HCD and LA (P < 0.05). Thus it appears CLA reduces the development of early aortic atherosclerosis to a greater degree than LA possibly through changes in LDL oxidative susceptibility in hypercholesterolemic hamsters.     

Conjugated linoleic acid isomers may diminish human macrophages adhesion to endothelial surface

Dysfunction of endothelial cells and activation of monocytes in the vascular wall are important pathogenetic factors of atherosclerosis. Conjugated linoleic acids (CLAs) can modulate the function of immune system in humans: reduce the concentration of atherogenic lipoproteins, and the intensity of inflammatory processes in the plasma. In this paper, we focus on macrophage's surface integrins (β1 integrin CD49d/CD29-(VLA4); Mac-1 as well as endothelial human vein endothelial cell (HUVEC) surface adhesins: vascular cell adhesion molecule-1 (VCAM-1) and intracellular cell adhesion molecule-1 (ICAM-1)) expression in relation to CLA isomer used during cell culture. Both CLA isomers decreased expression of VLA-4 and Mac-1 on macrophages compared with control cells (cultured with bovine serum albumine (BSA) or oxidized form of low-density lipoproteins). cis-9, trans-11 CLA isomer reduced ICAM-1 and VCAM-1 expression on the endothelium surface. Strong tendency to reduce of adhesion of macrophages to HUVEC in the cells cultured with CLA isomers was observed. The potential role of cis-9, trans-11 CLA in the reduction of adhesion of macrophages to the HUVEC – one of the important steps in the inflammatory process, can be considerate. These mechanisms may contribute to the potent anti-atherosclerotic effects of CLA in vivo.     

Conjugated linoleic acid isomers may diminish human macrophages adhesion to endothelial surface

Dysfunction of endothelial cells and activation of monocytes in the vascular wall are important pathogenetic factors of atherosclerosis. Conjugated linoleic acids (CLAs) can modulate the function of immune system in humans: reduce the concentration of atherogenic lipoproteins, and the intensity of inflammatory processes in the plasma. In this paper, we focus on macrophage's surface integrins (β1 integrin CD49d/CD29-(VLA4); Mac-1 as well as endothelial human vein endothelial cell (HUVEC) surface adhesins: vascular cell adhesion molecule-1 (VCAM-1) and intracellular cell adhesion molecule-1 (ICAM-1)) expression in relation to CLA isomer used during cell culture. Both CLA isomers decreased expression of VLA-4 and Mac-1 on macrophages compared with control cells (cultured with bovine serum albumine (BSA) or oxidized form of low-density lipoproteins). cis-9, trans-11 CLA isomer reduced ICAM-1 and VCAM-1 expression on the endothelium surface. Strong tendency to reduce of adhesion of macrophages to HUVEC in the cells cultured with CLA isomers was observed. The potential role of cis-9, trans-11 CLA in the reduction of adhesion of macrophages to the HUVEC--one of the important steps in the inflammatory process, can be considerate. These mechanisms may contribute to the potent anti-atherosclerotic effects of CLA in vivo.     

Conjugated linoleic acid alters matrix metalloproteinases of metastatic mouse mammary tumor cells

Conjugated linoleic acid (CLA) is a group of linoleic acid derivatives that has been implicated in animal studies to reduce a number of components of mammary tumorigenesis. Previously, we showed that CLA could alter the latency and metastasis of the highly metastatic transplantable line 4526 mouse mammary tumor. Several possible mechanisms have been proposed for the actions of CLA, but here we assessed how CLA may act to alter the expression and activity of matrix-modifying proteins within tumors from line 4526. In vitro, highly metastatic mouse mammary tumor cells had significantly decreased invasiveness after treatment with CLA, an indication that matrix-modifying proteins may have been altered. Using these same highly metastatic cells, primary tumors were grown in mice of separate groups fed 0, 0.1, 0.5, and 1% CLA (wt:wt) and evaluated for their levels and activities of matrix-modifying enzymes, enzyme inhibitors, and enzyme activators. The addition of CLA to the diet increased steady-state levels of messenger RNA (mRNA) of the matrix metalloproteinases (MMP) -2 and -9 in primary tumors removed from mice. However, western analysis revealed that although relative levels of the proform of MMP-9 were consistent with the mRNA observations, MMP-2 proform levels were actually decreased by dietary CLA. The activity of MMP-2 was barely detectable, but gelatin zymography and an in vitro activity assay showed that MMP-9 activity was significantly decreased by CLA. The steady-state mRNA and protein levels of tissue inhibitors of metalloproteinase-1 (TIMP-1) and TIMP-2, natural inhibitors of MMP, were increased at higher dietary CLA levels relative to low or no CLA. Suppression of MMP activity, therefore, may be 1 pathway through which CLA reduces tumor invasion and spread.     

Conjugated linoleic acid and human health: a critical evaluation of the evidence

PURPOSE OF REVIEW: This review critically evaluates studies investigating the effects of conjugated linoleic acid on human health, including effects on body composition, blood lipids, liver metabolism, insulin sensitivity and immune function. It focuses mainly on human intervention studies, but includes some reference to animal and cellular studies which provide insight into potential molecular mechanisms of action of conjugated linoleic acid. RECENT FINDINGS: Human studies continue to report inconsistent effects of conjugated linoleic acid on human health. Some of these reports are based on overinterpretation of marginal effects of supplementation. Recent data suggest that the effects of the substance may be isomer dependent and that cis-9,trans-11 and trans-10,cis-12 conjugated linoleic acids have opposing effects on blood lipids and on metabolism in adipocytes and hepatic cells. SUMMARY: Claims that conjugated linoleic acid is beneficial for health remain as yet unconvincing. Human studies investigating the effects of conjugated linoleic acid supplements have tended to use mixtures of isomers and have been inconsistent. More recent studies have attempted to use relatively pure preparations of single isomers and these studies suggest that the effects of conjugated linoleic acid may be isomer-specific. These recent data suggest a relative detrimental effect of trans-10,cis-12 conjugated linoleic acid on blood lipids. There appears to be little effect of conjugated linoleic acid on immune function and the effects on insulin sensitivity remain unclear.