**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 [73].

#### **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].

Recently, *Lactobacillus acidophilus* ATCC 4356 could act as a liver X receptor (LXR) receptor agonist and inhibited the cellular uptake of micellar cholesterol in Caco-2 cells [77]. A similar study conducted with Yoon et al using a combination of *L. rhamnosus* BFE5264 and *L. plantarum* NR74 also showed a up-regulating the expression of LXR and promotion of cholesterol efflux in Caco-2 cells [78]. This is identical to effect of bile acid sequestrants drug which can also induce an increase of LXR activity in liver[79].

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 [80]. In fact, PPARs is also a target gene of energy homeostasis and adipogenesis [81]. Linked to ApoE gene transcription, PPAR-γ need LXR pathway for regulating adipocyte triglyceride balance [82]. 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 [83]. Concerning about NRs and lipid metabolism linked with probiotic, Aronsson et al observed that *L. paracasei* F19 could reduce the fat storage associated with the drastic changes of PPARs [84]. One most recent study by Zhao et al have also demonstrated probiotic *Pediococcus pentosaceus* LP28 could also acted as a PPAR-γ agonist concomitantly with the great reduction of triglyceride and cholesterol in obese mice [85].

#### **6.2. Toll-like receptors**

448 Lipid Metabolism

**5.3. Osteocalcin** 

**6. Interaction with receptors** 

compared with GF mice [73].

**6.1. Nuclear receptors** 

In recent years, osteocalcin secreted by osteoblasts has aroused great interest linked to β cell function, adiponectin production, energy expenditure and adiposity [69]. In humans, fat individuals kept a low level of serum osteocalcin [70]. 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 *Lactobacillus* GG and *Bifidobacterium lactis* [71]. It is interesting that osteocalcin is an vitamin K-dependent protein and two main types including vitamin K1 and vitamin K2 are respectively produced from dietary vegetable and microflora [72]. As an effective way to alter microflora, probiotics have potential to enhance vitamin K2

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

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),

Recently, *Lactobacillus acidophilus* ATCC 4356 could act as a liver X receptor (LXR) receptor agonist and inhibited the cellular uptake of micellar cholesterol in Caco-2 cells [77]. A similar study conducted with Yoon et al using a combination of *L. rhamnosus* BFE5264 and *L. plantarum* NR74 also showed a up-regulating the expression of LXR and promotion of cholesterol efflux in Caco-2 cells [78]. This is identical to effect of bile acid sequestrants drug

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 [80]. In fact, PPARs is also a target gene of energy homeostasis and adipogenesis [81]. Linked to ApoE gene transcription, PPAR-γ need LXR pathway for regulating adipocyte triglyceride balance [82]. Avella et al reported that dietary

glucocorticoid receptor(GR) and vitamin D receptor(VDR) [74-76].

which can also induce an increase of LXR activity in liver[79].

production and related osteocalcin level through changing the microflora.

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 [86]. 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 [87]. 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 *de novo* fatty acid synthesis and lipolysis and lipoprotein production in liver which leading to hepatic hypertriglyceridemia [88]. In mice, moderately higher LPS level could be increased by a fat-enriched diet and contributed to low grade inflammation [34]. In rabbits, high cholesterol intake plus with low dose LPS accelerated the development of atherosclerosis [89]. These two studies are considered as the result of crosstalk between LPS and TLRs leads to intestinal mucosal injury associated with inflammatory response. Besides, foam cell formation in atherosclerosis has been shown to be mediated by TLR2 and 4 and other TLRs such as TLR3, 7, and 9 may also participate in atherosclerosis [90] [91].

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 *E. coli*) in the gut and influx of LPS into circulation [92] [93]. A great number of probiotics are also able to specically modulate the NF-κB pathway (one of most important inflammatory pathways)in intestinal epithelial cells and macrophages [94].

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 [95] [96]. With regard to the role of TLR4 in the development of metabolic disorders, Andreasen et al have considered that *L. acidophilus* NCFM may reduce overow of LPS from the gut to the circulation and downregulate the TLR4 signalling and pro-inammatory cytokines in human subjects [97].

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 [98]. In addition, Agrawal et al documented that TLR2-derived signaling mainly enhance Th2-cytokine release, while TLR4 triggered by LPS stimulates Th1-type responses [99]. Interestingly,

#### 450 Lipid Metabolism

Voltan et al found that *L. crispatus* M247 could increase TLR2 mRNA level and reduced TLR4 mRNA and protein levels in the colonic mucosa, suggesting that *L. crispatus* M247 maintain the Th1 / Th2 homeostasis through TLR2 / TLR4 banlance [100].

The Effect of Probiotics on Lipid Metabolism 451

[111]. In addition, six main different expressed proteins involved in these two different strains *in vitro* were identified by proteomic analysis including transcription regulator, FMN-binding protein, major facilitator superfamily permease, glycogen phosphorylase,

Microarray analysis of probiotic *L. casei* Zhang effect on liver of high fat diet-fed rats revealed that *L. casei* Zhang administration promote the β-oxidation of fatty acid metabolism through up-regulating five genes expression (Acsl1, Hadh, Acaa2, Acads, and gcdH). Moreover, *L. casei* Zhang could strongly activate expression of glucocorticoid receptor (NR3C1 gene) which might be related to protect against high-fat induced low grade

Recently, small intestinal proteomes in weanling piglets that respond differently to probiotic (*Lactobacillus fermentum* I5007) and antibiotic (Aureomycin) supplementation in terms of lipid metabolism have shown that probiotic enhanced mucosal SAR1B abundance could prevent weanling piglets from fat malabsorption. More importantly, high mucosal abundance of EIF4A and KRT10 in probiotic-treated piglets may contribute to improve

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

overall gut integrity, suggesting a potential reduction of LPS influx [113].

various responses related to lipid metabolism influenced by probiotics.

Wang at our university for useful advice on molecular biology.

*Key Laboratory of Dairy Biotechnology and Engineering, Education Ministry of P. R. China, Department of Food Science and Engineering, Inner Monglia Agricultural University, Hohhot,* 

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

YknV protein, and fructose/tagatose bisphosphate aldolase.

inflammation [112].

**8. Conclusion** 

**Author details** 

**Acknowledgement** 

*China* 

Yong Zhang and Heping Zhang
