1.1 Polyunsaturated fatty acids
Polyunsaturated fatty acids (PUFAs) are fatty acids that contain more than one double bond in their backbone, and can be classified into various groups by their chemical structure:
1. Methylene-Interrupted Polyenes: These fatty acids have 2 or more cis double bonds that are separated from each other by a single methylene group. (This form is also sometimes called a divinylmethane pattern.). The essential fatty acids are all omega-3 and -6 methylene-interrupted fatty acids, such as Arachidonic acid (AA;
20:4 (n-6)), Docosadienoic acid (22:2 (n-6)), Docosapentaenoic acid (22:5 (n-6)), Docosahexaenoic acid (DHA; 22:6 (n-3)), Eicosapentaenoic acid (EPA, 20:5 (n-3)).
And Omega-9 fatty acids are mono- and polyunsaturated, including Oleic acid (18:1 (n-9)), Mead acid (20:3 (n-9)).
2. Conjugated fatty acids: Conjugated fatty acids have two or more conjugated double bonds, such as Rumenic acid (18:2 (n-7)), Punicic acid (18:3 (n-5)).
3. Other Polyunsaturates: Pinolenic acid (18:3 (n-6)), Podocarpic acid (20:3 (n-6)).
1.2 Obese epidemic
Overweight is a major concern of health issues by the public now. According to some reports health, there are around 1.46 billion adults worldwide have BMI of 25 kg/m2 or greater in 2008, of which 6 million adults (2.05 million men and 2.97 million women) were obese 8. In both developed and developing countries, obesity becomes epidemic issue across in all age groups with the increasing prevalence of obesity, a large burden on health care use and costs may impact the finance of the countries.
Weight loss can improve health and related diseases and economic benefits. Dietary
therapy, physical activity and lifestyle modification are good effective for weight loss strategies. Obese or overweight patients with concomitant obesity-related risk factors or diseases thus may need drug therapy9.
1.3 The effect and benefit of polyunsaturated fatty acids in obesity and lipid metabolism
Mice and humans exhibited lower body weight and reduced fat deposition when fed with with high PUFA diet10. PUFAs exert protective effects through direct or indirect pathways in lipid metabolism. Fish oil enriched with PUFA treated in the rats including metabolic syndrome, the blood pressure, serum insulin, cholesterol, TAG and NEFA were decreased, although there was no change in plasma TNFα concentration or fat accumulation11. Patients with metabolic syndrome have a high risk to develop cardiovascular disease and PUFA administration for at least 3 months significantly decreased serum triglycerides by 7% to 25% and is helpful to prevent cardiovascular diseases12.
1.4 Regulation of inflammation by long-chain n-3 PUFAs
Long-chain n-3 PUFA can exert anti-inflammatory effects by reducing pro-inflammatory cytokine expressions. Therefore, long-chain n-3 PUFA may provide an anti-inflammatory strategy to improve obesity-related diseases13.
Long-chain n-3 PUFA regulate gene expressions not only through transcription factors such as PPAR and NF-kappaB but also via eicosanoid production, reducing pro-inflammatory cytokine production from different types of cells including macrophages. Long-chain n-3 PUFA may therefore offer a useful anti-inflammatory strategy7.
Obesity nowadays is regarded as a to low-grade chronic inflammatory condition and characterized as inflammation with increased macrophage infiltration in the adipose tissue14. Furthermore, neutrophils and T-cells, may enter the adipose tissue first and contribute to the recruitment of microphages into adipose tissue. Obese individuals express more amount of pro-inflammatory cytokines as compared to lean ones, those pro-inflammatory molecules include interleukin 6 (IL-6), tumor necrosis factor alpha (TNFα), C-reactive protein, soluble intercellular adhesion molecules, inducible NO synthase, transforming growth factor b1, monocyte chemotactic protein (MCP)-1, tissue factor and factor VII, plasminogen activator inhibitor type 12,15,16. Recent reports suggest that stromal vascular cells, which contain endothelial cells, immune cells and pre-adipocytes also produce pro-inflammatory molecules into the richest adipose tissue. The evidences support that increased infiltration of macrophages into the adipose tissue elute adipose tissue into low-grade inflammation status. Adipose tissue with enhanced adiposity are constantly stimulated by pro-inflammatory molecules that are only from adipocytes, but derived from microphages 17. Those pro-inflammation cytokine in adipose tissue, for example, IL-1b, TNFα and IL-6 have been considered in promoting the development insulin resistance and obesity17.
Many reports have shown move macrophages infiltration into adipose tissue by HFD treatment in the mouse model for type 2 diabetes and in genetically diabetic db/db mice18. Dietary induction of LC n-3 PUFA reduces
macrophage infiltration into adipose tissue and suppresses inflammatory gene expression and c-Jun N-terminal kinase phosphorylation in HFD induced dialectics mice without changes of body weight18.
1.5 Regulation of transcription factors and cytokines by long-chain n-3 PUFAs PUFA also expresses expression of same lipid accumulation-related genes including cytokines and transcription factors in the adipose tissue. Past studies suggested that serum IL-6 can significantly reduced and TNFα has trend-wise reduction by 4 g/day n3-PUFA12. PUFA also regulates extensively the lipid metabolism by cytokines through transcription factors in adipose tissue19. The major fatty acids in PUFA are DHA and EPA, DHA is implicated in modulating homeostasis and lipid metabolism through transcription factors and protein kinase20, such as PPAR, liver X receptors and sterol regulatory element binding protein-l(SREBP1) adipocyte determination and differentiation dependent factor 1. These transcription factors regulate fatty acid metabolism and expression of various cytokines. including leptin, adiponectin, resistin21, TNFα, IL-6 and etc. 7,22.
PUFAs inhibit expression of SREBP1 in porcine adipocytes. SREBP1 is a transcription factor that stimulates expression of genes involved in lipogenesis. Our group reported that porcine primary adipocytes treated with DHA, expression of steady-state transcription factor SREBP1 mRNA and lipogenesis related genes and cytokines were decreased through a mechanism related to fatty acid peroxidation in adipocytes and liver 23-25.
The metabolic products from PUFAs could be PPAR activators, docosahexaenoic acid and particularly eicosapentaenoic acid can activate PPARγ in porcine. and then induce gene expressions responsible for fatty acid oxidation such as hormone sensitive lipase, acyl-CoA oxidase, hydoxymethylglutaryl-CoA synthase and fatty
acyl-CoA synthetase26,27. Also, when PPARγ+/+ transgenic mice were treated with diet enriched with fish oil, expressions of adipogenic and glucose uptake genes, such as Glut4 are increased in the muscle. These results indicate that the glucose uptake in vivo may be regulated by PUFAs as a natural regulator through PPARγ activation
16.
Various adipocyte-derived hormones (adipocytokines) have been described capable to affect insulin sensitivity, inflammation, and stress, and thereby are potentially linked to obesity, diabetes and cardiovascular diseases 28,29. The relationship between PUFA and cytokines related to obesity and inflammation are discussed below.
1.6 Regulation of adipocytokine, SAA by long-chain n-3 PUFAs
Serum amyloid A (SAA) is a family of apolipoproteins associated with high-density lipoprotein and primarily produced from the liver. SAA is also an acute-phase protein induced by inflammation and obesity. SAA1 responds to tissue damage and inflammation. In the acute-phase stage, SAA is induced primarily by IL-1, TNF α , and IL-6 through the down-regulation of NF-κB, CCAAT/enhancer-binding protein (C/EBP) family30.
Polyunsaturated fatty acids (PUFA) have been showed to decrease lipid concentration in blood, promote insulin sensitivity and immune responses. They also reduce expression of adipogenesis-related genes and lipid accumulation.
Docosahexaenoic acid (DHA), one of n-3 PUFA, downregulates hepatic lipogenic genes in the liver of pigs. Using the suppression subtractive hybridization technique31. We demonstrated that hepatic SAA mRNA concentration was promoted by dietary DHA in pigs. Thus, DHA may regulate lipid metabolism through increased SAA
expression. When porcine primary hepatic cell was treated with DHA, the expression of SAA mRNA was induced. SAA treatment also reduced the express of genes related to lipid metabolism, including PPARγ and lipoprotein lipase (LPL). SAA mRNA aboudance also significantly correlated with inflammatory and adiposity markers, including interleukin 6 (IL6) and tumor necrosis factor α (TNFα) 32. Since SAA1 is correlated significantly with the degree of obesity and the risk of cardiovascular disease, therefore SAA1 is considered as a marker for obesity and cardiovascular disease . Both of in the procine and human adipocytes studies, we discovered the effect of SAA1 in lipid metabolism and suggest that SAA1 is a mediator to reduce fat deposition1,33.
1.7 Introduction of TN
TN also named Clec3B, is a homotrimeric C-type superfamily lectin, also called C-type lectin domain family 3, member B (CLEC3B). Tetranectin binds to plasminogen at the presence of calcium to form tetranectin-plasminogen complex with tissue-type plasminogen activator (tPA) 34. The tetranectin-plasminogen complex, as an inducer, drives fibrinolysis. The tPA-catalyzed plasminogen is activated in fibrinolysis and participates in thrombus dissolution 35,36. Tetranectin is reduced in serum after clotting of plasma, and tetranectin is presence in clot lysates 35. These results suggested that tetranectin is associated with clotlysisclinical research, when stroke patients were treated with recombinant tPA (rtPA) and a low-dose of argatroban (a direct thrombin inhibitor), 43% patients with clot in brain artery were completely broken 37. Acute myocardial infarction (AIM) patients have significant lower plasma TN as compared to healthy subjects. When AIM patients underwent thrombolytic treatment with rtPA, the plasma tetranectin increased at the end of rtPA
infusion and some patients established early blood vessel recanalization. Tetranectin returned to the baseline levels 12 h after rtPA treatment 38. Because tetranectin is involved in the dissolution of fibrin clot and vascular homeostasis, it may exert a protective role against cardiovascular disease.
If human TN gene is consisted by three exons which encodes specific functional domains. The heparin binding site is encoded in first exon 1 of TN. The second exon encodes an a helix domain, which conducts the trimerisation of TN monomers by assembling them into a triple helical coiled structural element. The 3th exon encodes a C-type lectin-like domain, which is homologous to the carbohydrate recognition domains of calcium dependent animal lectins. Moreover, TN has O linked glycosylation at threonine39,40.
In addition, many reports suggested indicated that serum TN concentrations in patients with malignant tumors correlate with disease stage and survival and TN expression correlate well with tumor histological grading41 . In human breast cancer, TN play a possible role in the invasiveness and metastatic spread as a cancer marker42.