Antioxidative effects of probiotics
Probiotics are deﬁned as viable microorganisms that exhibit beneﬁcial effects on the health of the host . Now, probiotics are known to possess physiological functions such as inhibition to pathogens, assisting digestion, immunoregulatory activity and antitumor activity . Here, we discuss the effects of probiotic on lipid metabolism from seven main aspects including history, antioxidant effect, impact on lipoprotein, microflora view, hormones, receptors and new mechanisms.
1.1. Past and present
As early as in 1974, Mann and Spoerry observed that inhabitants from African Maasai tribes maintained a lower level of blood lipids due to a high fermented milk intake . Further perspective suspected that live Lactobacilli included in fermented milk may contribute to reducing cholesterol . The cholesterol-reducing effect of probiotic has become more apparent with the discovery of bile salt deconjugating and cholesterol assimilating ability of
A new study by Lye et al showed that there existed ﬁve possible probiotic mechanisms including assimilation of cholesterol during growth, binding of cholesterol to cellular surface, disruption of cholesterol micelle, deconjugation of bile salt and bile salt hydrolase (BSH) activity . Now with the development of molecular biology, we can judge cholesterol-lowering effect firstly by detection of BSH gene and its expression in a probiotic genome. A recent study by Sridevi et al showed that
There are some reports that fermented soy milk by probiotics also showed favorable function of regulating lipids level . The advantages of fermented soy milk are that undesirable soybean oligosaccharides can be hydrolysed which provide nutritional components for probiotic and a large variety of peptides and amino acids are produced as well as active aglycon form of isoflavones . An improved cholesterol profile was observed with daily intake of a probiotic soy product . It seems possible that living probiotics and functional isoflavones cooperated in regulating lipid profile.
2. Antioxidant effect
Probiotic originated from longevity research by the well-known Eli Metchnikoff. As we all known, various published evidence suggested reduction of oxidative stress led to longevity-promoting consistent with Harman's Free Radical Theory of Aging . These two observations inspired the investigation of antioxidant ability of probiotics.
Oxidative stress induced by obesity tend to produce surplus reactive oxygen species (ROS) which may cause further damage by free radical chain reaction mechanism . ROS have some deleterious effects on polyunsaturated lipids in cell membrane leading to damage of cell structure and malondialdehyde (MDA), which was also toxic to DNA and protein and formed as a marker of lipid peroxidation at the same time  . As for the oxidative stress, human body has its own antioxidant defense system including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), glutathione (GSH) and so on . Many
|Probiotic yoghurt containing ||Type 2 diabetic patients||Serum MDA concentration significantly decreased|||
|Probiotic yoghurt containing Lactobacillus|
acidophilus LA-5 and Bifidobacterium BB-12
|Pregnant Women||Increased erythrocyte glutathione reductase levels, plasma glutathione and 8-oxo-7,8-dihydroguanine levels|||
|High-fat fed rat||A decrease of MDA and increase of SOD and GSH-Px in serum and liver|||
|pigs||Increased total antioxidant|
capacity, SOD and GSH-Px activity in serum as well as hepatic CAT and muscle SOD;
Decreased MDA level in serum and muscle
|Probiotic yoghurt containing Lactobacillus|
acidophilus LA-5 and Bifidobacterium BB-12
|human||An increase of SOD and catalase activity|||
|Rats with colon carcinoge-nesis||Lower plasma lipid peroxidation levels and higher plasma total antioxidant levels|||
|Probiotic dahi containing ||High fructose fed rats||Lower values of TBARS and higher values of glutathione in liver and pancreatic tissues|||
|human||Enhanced total antioxidative|
|High-Fat and cholestero-l fed rat||An increase in total radical trapping antioxidant potential (TRAP) and a decrease in conjugated dienes in plasma|||
|Iron overloade-d mice||A significant decrease of lipid peroxide in the colonic mucosa|||
|VSL#3||ob/ob mice||Lower fatty acid beta-oxidation|||
|rats||Higher GSH-Px activity in red blood cells|||
|rats||Inhibition of hemolysis of red blood cell under the condition of vitamin E dificient|||
3. Impact on lipoprotein
Lipoprotein transport play an important role in accumulation of host lipopolysaccharide level (LPS) . Studies by Cani et al showed that elevated LPS level was considered as a trigger factor involved in the pathogenesis of obesity and metabolic risk via innate immune mechanism . LPS-binding protein (LBP) and lipoproteins exert a synergistic effect on reducing the toxic LPS level.
Several fermented milk containing probiotics were demonstrated to reduce low-density lipoprotein cholesterol(LDL-c) level and very-low-density lipoprotein cholesterol (VLDL-c) in animal and human   .Recently,
4.The whole microflora view
Intestinal microbes not only
The human gut is consisting of a microbial community of 1014 bacteria with at least 1000 species and the whole microbiome is more than 100-fold the human genome .These researches highlight the significance of the whole gut microbiome contribute to energy harvest and the relationship between obesity and changes of gut microbiome . More detailed, obese is mainly characterized by elevated
Besides, such a huge microflora provide a large reservoir of LPS molecules to circulation through colonizing of Gram-negative bacteria in the gut . A recent study showed
The whole gut microflora is also known as a target for drug metabolism because of diverse microbial transformations . Manipulation of commensal microbial composition through antibiotics, probiotics or prebiotics was thought to enhance the metabolic activity and production of effective metabolites . Simvastatin, which is an inhibitor of HMG-CoA and widely used for regulating hepatic cholesterol production, was proposed to possess altered pharmacological properties by microflora degradation via changing its capacity to bind to the corresponding receptors . It is indicated that probiotics have potential to influence the metabolism of lipid-regulating drugs in gut.
5. Regulation of leptin, adiponectin and osteocalcin
Hormones such as leptin, adiponectin and osteocalcin play an important role in lipid metabolism. Obese population was characterized by significant lower levels of osteocalcin and adiponectin as well as high leptin level (leptin-resistant) which have been reported in literature. It is now increasingly accepted that leptin can regulate food intake and energy expenditure through hypothalamus and adiponectin can enhance tissue fat oxidation to downstream fatty acids levels and tissue triglyceride content associated with insulin sensitivity . As for osteocalcin, leptin assumed to modulate osteocalcin bioactivity and osteocalcin could stimulate the adiponectin synthesis  .
Leptin, an antiobesity hormone produced by adipose tissue, has been reported to regulate body weight by controlling food intake and energy expenditure . However, obesity tend to display markedly higher serum leptin level with a leptin-resistant symptom. Several studies reported a decrease of leptin by probiotic administration. In high-fat fed mice, Lee et al confirmed that
Leptin-lowering effect of probiotics was also observed in human. Similarly, Naruszewicz et al investigated whether oral administration of
As an adipocyte-derived serum protein, adiponectin play an important role in glucose and lipid metabolism since adiponectin deficiency are associated with insulin resistance, inflammation, dyslipidemia and risk of atherogenic vascular disease . In parallel, adiponectin has also been shown to suppress macrophage foam cell formation in atherosclerosis . Several studies showed that probiotic therapy improved adiponectin level or adiponectin gene expression. One comparative research performed in normal microflora (NMF) and germ-free (GF) mice revealed that adiponectin gene expression (Adipoq) was up-regulated in the groups of
Kadooka et al used a probiotic
In recent years, osteocalcin secreted by osteoblasts has aroused great interest linked to β cell function, adiponectin production, energy expenditure and adiposity . In humans, fat individuals kept a low level of serum osteocalcin . The only study by Naughton et al showed that osteocalcin levels was slightly increased in middle aged rats by consumption of inulin-rich milk fermented by
6. Interaction with receptors
Various Receptors are involved in regulating important genes in lipid transport and metabolism and selected as potential therapeutic targets for dyslipidemia and atherosclerosis. Recent studies have focused on nuclear receptors (NRs), G protein-coupled receptor (GPRs) and Toll-like receptors (TLRs) as factors regulated by probiotics administration. But the crosstalk among NRs,TLRs and GPRs have not been clearly elucidated. The only investigation about crosstalk of NRs,TLRs and microflora between specific pathogen-free (SPF) mice and germ-free (GF) mice have revealed that LXR alpha, ROR gamma and CAR expression were reduced while TLR-2 and TLR-5 increased in SPF compared with GF mice .
6.1. Nuclear receptors
According to the stated above, some probiotics were found to be effective in reducing blood cholesterol level and one possible mechanism is enhanced fecal bile acids level. As one of important lipid mediators, bile acids have been confirmed to influence a series of NRs including farnesoid X receptor (FXR), pregnane-X-receptor (PXR), constitutive androstane receptor (CAR), peroxisome proliferator-activated receptor (PPAR), liver X receptor (LXR), glucocorticoid receptor(GR) and vitamin D receptor(VDR) [74-76].
As we all known, PPARs play a key role in inflammation and blood glucose metabolism. Some studies have indicated that probiotic regulated the expression of PPARs in experimetal inflammatory model . In fact, PPARs is also a target gene of energy homeostasis and adipogenesis . Linked to ApoE gene transcription, PPAR-γ need LXR pathway for regulating adipocyte triglyceride balance . Avella et al reported that dietary probiotics could modify the expression of PPAR-α, PPAR-β, VDR-α, RAR-γand GR in a marine fish, suggesting extensive crosstalk among NRs activated by probiotic . Concerning about NRs and lipid metabolism linked with probiotic, Aronsson et al observed that
6.2. Toll-like receptors
As important pattern recognition receptors, TLRs participate in distinguishing and recognizing a range of microbial components such as peptidoglycan (TLR2) and LPS (TLR4) to activiate immune responses . Up to date, the relationship between TLRs and lipid metabolism is mainly from two aspects. On one hand,TLRs signaling can directly contact and interfere with cholesterol metabolism in macrophages . On the other hand, TLRs signaling (mainly TLR4) are involved in interaction LPS with fatty acid, lipoprotein and organ injury(especially liver and intestine). There is evidence that low dose of LPS can boost
TLR4 appears to be tightly linked to high-fat intake, LPS and inflammation. Probiotics are known to reduced LPS-containing gram-negative organisms (such as
Due to TLR4 deficiency with anti-obesigenic effects and susceptible to colitis, little information about influence of probiotic on lipid metabolism is obtained in TLR4 knockout model whereas protective effect of probiotic VSL#3 from inflammation was observed in TLR4 knockout mice  . With regard to the role of TLR4 in the development of metabolic disorders, Andreasen et al have considered that
Immunity homeostasis also have important effect on lipid metabolism. In general, it is well accepted that probiotic bacteria are able to maintain the Th1 and Th2 banlance of immunity through regulating pro-inflammatory and anti-inflammatory cytokines . In addition, Agrawal et al documented that TLR2-derived signaling mainly enhance Th2-cytokine release, while TLR4 triggered by LPS stimulates Th1-type responses . Interestingly, Voltan et al found that
6.3. G protein-coupled receptors
It has been well-established that probiotic bacteria exert beneﬁcial effects on the intestine especially the antimicrobial property by producing organic acids or regulating the organic acid-producing flora . It has been also reported that GPR41 and GPR43 can be activated by short-chain fatty acids(SCFAs). Thus, it is possible that probiotic may affect GPRs through production of SCFAs in gut. However, this relationship among these have not yet been well-established. Study performed in Gpr41-deficient mice under germ free or conventional environment revealed that present of microflora was associated with harvest of short-chain fatty acids from the diet which control the degree of adiposity .
By our knowledge, only one study has investigated the effect of prebiotic which can specifically increase intestinal probiotic bifidobacteria on GPR43 expression through modified lipid profile . Using a high-fat fed rodent model, the authors studied the effects of prebiotic on changes of microflora, adipose fatty acid profile and receptors expression. High fat diet is able to increase GPR43 and TLR4 expression as well as PPAR-γ expression due to oleic acid and α-linolenic acid production, while prebiotic decreases GPR43 and TLR4 overexpression.
7. New mechanisms exploration
In the past recent years, new mechanisms of probiotics on lipid metabolism were proposed. A research by Khedara et al showed lower nitric oxide level has been responsible for hyperlipidemia since endogenous nitric oxide can reduce fatty acid oxidation . Some probiotics had ability to induce nitric oxide synthesis through activation of inducible nitric oxide synthase  . Thus, modiﬁed NO availability by probiotics play an important role in lipid metabolism.
Moreover, Tanida et al demonstrated that
Omics technology provide a new insight into the mechanisms of lipid metabolism influenced by probiotics. Lee et al demostrated that gene ccpA (encodes catabolite control protein A) had function in cholesterol reduction in vivo by comparation of cholesterol-reducing strain
Microarray analysis of probiotic
Recently, small intestinal proteomes in weanling piglets that respond differently to probiotic (
In conclusion, probiotic is a better prevention and treatment strategy for regulating lipid homeostasis with the high prevalence of obesity, burden of amazing overweight and developing chronic diseases in the modern world. Despite the fact that people too pay attention to the thin result to neglect the drug side effect, probiotic can avoid this to achieve a healthy weight. Enhancing bile acids enflux and gut cholesterol assimilation was considered as the classic theory for cholesterol-reducing probiotics. Nevertheless, rencent studies focus on antioxidant activity and interaction with lipoprotein, hormones and the whole microbiota. Besides, crosstalk among NRs, GPRs and TLRs by probiotics is new frontiers for mechanical research. However, further investigations are needed to identify various responses related to lipid metabolism influenced by probiotics.
We thank professor Heping Zhang for revising this article. We also thank the members of the Laboratory in Department of Biological Science and Engineering directed by Yuzhen Wang at our university for useful advice on molecular biology.