Dietary fish oil effects on biliary lipid secretion and cholesterol gallstone formation in the African green monkey

Identification of blood metabolites linked to the risk of cholelithiasis: a comprehensive Mendelian randomization study

BACKGROUND AND AIMS

Observational and Mendelian randomization (MR) studies have identified several modifiable risk factors of cholelithiasis. However, there is limited evidence about the causal effect of blood metabolites on the cholelithiasis risk.

METHODS

To have a comprehensive understanding to causal relations between blood metabolites and cholelithiasis, for the primary discovery, we applied two MR methods to explore the associations between 249 circulating metabolites and cholelithiasis. For secondary validations, we replicated the examinations using another metabolic dataset with 123 metabolites. The summary statistics of cholelithiasis were retrieved from FinnGen Consortium Release 5 and UK Biobank. Inverse-variance weighted, weight median and MR-egger methods were used for calculating causal estimates. Furthermore, Bayesian model averaging MR (MR-BMA) method was employed to detect the dominant causal metabolic traits with adjustment for pleiotropy effects.

RESULTS

In the primary analysis, sphingomyelin showed consistent protective causal associations with cholelithiasis; while plasma cholesterol-associated traits showed generally inverse correlation with cholelithiasis risk. Notably, large numbers of traits within the (un)saturated fatty acid category demonstrated significant causal effects. Secondary analyses demonstrated similar results, with traits related to the levels of bisallylic groups in fatty acids showing protective effects. Lastly, MR-BMA analyses discovered that the degree of unsaturation plays a predominant role in reducing the risk of cholelithiasis.

CONCLUSION

Our MR study provides a complete atlas of associations between plasma metabolites on cholelithiasis risk. It highlighted that genetically predicted sphingomyelin and degree of unsaturation of fatty acid were causally associated with the reduced risk of cholelithiasis.

Association between omega-3/6 fatty acids and cholelithiasis: A mendelian randomization study

Background

Omega-3 and omega-6 may be protective factors for cholelithiasis. However, this relationship has not yet been demonstrated clearly. Therefore, we attempted to identify these causal relationships.

Materials and methods

The omega-3/6 fatty acid discovery dataset was obtained from UK Biobank and contained 114,999 individuals. The validation set was derived from an independent genome-wide association study (GWAS) and contained 13,544 individuals. The cholelithiasis dataset was derived from FinnGen and contained 19,023 cases and 195,144 controls. The inverse variance weighting (IVW) method was used as the main method of analysis in this study. Multiple methods of analysis were also used in the repeated methods, including the MR-Egger, weighted median, MR-pleiotropic residual sum (MR-PRESSO), outliers, and maximum likelihood methods. In addition, we used multiple sensitivity analyses to identify the potential pleiotropy.

Result

In the discovery stage, the results of the random effect IVW analysis showed that higher omega-3 levels were correlated inversely with the risk of cholelithiasis (β = -0.22, 95% CI [-0.32 to -0.12], P = 1.49 × 10^-5). When the replication analysis was performed using another set of instrumental variables (IVs), the causal relationship between omega-3 fatty acids and cholelithiasis remained stable (β = -0.42, 95% CI [-0.66 to -0.18], P = 5.49 × 10^-4), except for the results obtained using the MR-Egger method, which were not significant. The results of the IVW approach showed that each SD increase in omega-6 levels was associated negatively with the risk of cholelithiasis, both in the discovery (β = -0.21, 95% CI [-0.35 to -0.06], P = 4.37 × 10^-3) and the validation phases (β = -0.21, 95% CI [-0.40 to -0.02], P = 3.44 × 10^-2).

Conclusion

The results of our MR study suggest that omega-3/6 is associated with cholelithiasis risk. Attention to the risk of cholelithiasis in individuals with low serum omega-3/6 levels is necessary.

Combination treatment with n-3 polyunsaturated fatty acids and ursodeoxycholic acid dissolves cholesterol gallstones in mice

The increasing prevalence of cholesterol gallstone disease places an economic burden on the healthcare system. To identify novel therapeutics, we assessed the effects of n-3 polyunsaturated fatty acids (PUFA) in combination with UDCA in a mouse model of cholesterol gallstones. Gallstone dissolution, gallbladder wall thickness, mucin gene expression in the gallbladder, and levels of phospholipids, cholesterol, and bile acids in bile and serum were analysed. RNA was extracted from the liver for mRNA sequencing and gene expression profiling. Combination treatment resulted in greater gallstone dissolution compared with the control group, and PUFA and combination treatments reduced the thickness of the gallbladder wall. Expression levels of mucin genes were significantly lower in the UDCA, PUFA, and combination groups. Transcriptome analyses revealed that combination treatment modulated hepatic lipid metabolism. The PUFA and combination groups showed elevated bile phospholipid and bile acid levels and a lower cholesterol saturation index. Combination treatment with PUFA and UDCA dissolves cholesterol gallstones in mice by decreasing mucin production, increasing levels of phospholipids and bile acids in bile, and decreasing cholesterol saturation. Further studies of the therapeutic effects of combination PUFA and UDCA treatment in patients with cholesterol gallstones are warranted.

The Role of Diet in the Pathogenesis of Cholesterol Gallstones

Cholesterol gallstone disease is a major health problem in Westernized countries and depends on a complex interplay between genetic factors, lifestyle and diet, acting on specific pathogenic mechanisms. Overweigh, obesity, dyslipidemia, insulin resistance and altered cholesterol homeostasis have been linked to increased gallstone occurrence, and several studies point to a number of specific nutrients as risk- or protective factors with respect to gallstone formation in humans. There is a rising interest in the identification of common and modifiable dietetic factors that put the patients at risk of gallstones or that are able to prevent gallstone formation and growth. In particular, dietary models characterized by increased energy intake with highly refined sugars and sweet foods, high fructose intake, low fiber contents, high fat, consumption of fast food and low vitamin C intake increase the risk of gallstone formation. On the other hand, high intake of monounsaturated fats and fiber, olive oil and fish (ω-3 fatty acids) consumption, vegetable protein intake, fruit, coffee, moderate alcohol consumption and vitamin C supplementation exert a protective role. The effect of some confounding factors (e.g., physical activity) cannot be ruled out, but general recommendations about the multiple beneficial effects of diet on cholesterol gallstones must be kept in mind, in particular in groups at high risk of gallstone formation.

Potentiation of anti-cholelithogenic influence of dietary tender cluster beans (Cyamopsis tetragonoloba) by garlic (Allium sativum) in experimental mice

BACKGROUND & OBJECTIVES

Dietary fibre-rich tender cluster beans (Cyamopsis tetragonoloba; CB) are known to exert beneficial cholesterol lowering influence. We examined the influence of a combination of dietary tender CB and garlic (Allium sativum) in reducing the cholesterol gallstone formation in mice.

METHODS

Cholesterol gallstones were induced in Swiss mice by feeding a high-cholesterol diet (HCD) for 10 wk. Dietary interventions were made with 10 per cent CB and 1 per cent garlic included individually or together along with HCD. A total of 100 mice were divided into five groups of 20 mice each.

RESULTS

Dietary CB, garlic and CB+garlic reduced the formation of cholesterol gallstones by 44, 25 and 56 per cent, respectively, lowered cholesterol by 23-48, 16-24, and 24-58 in bile, serum, and liver, respectively. Cholesterol saturation index in bile and cholesterol: phospholipid ratio in circulation and hepatic tissue were significantly lowered by these dietary interventions, with highest beneficial effect from CB+garlic. Activities of hepatic cholesterol metabolizing enzymes were modulated by CB, garlic and CB+garlic. Elevation in lipid peroxides caused by HCD was also countered by these dietary interventions, the combination producing the highest effect.

INTERPRETATION & CONCLUSIONS

The results showed that the prevention of experimentally induced formation of cholesterol gallstones by dietary CB and garlic was due to decreased biliary cholesterol secretion and increased cholesterol saturation index. In addition of anti-lithogenic effect, dietary CB and garlic in combination had a beneficial antioxidant effect.

The effect of Macrotyloma uniflorum seed on bile lithogenicity against diet induced cholelithiasis on mice

Background:

The seeds of Macrotyloma uniflorum Lam. (Family Fabaceae) contain extractable total and tannins with reported hepatoprotective, hypocholesterolemic and antioxidant activity. In this study, dietary M. uniflorum seed, methanolic and acetone extracts (ME and AE) were examined for their bile-antilithogenic potential.

Materials and Methods:

Mice fed with 1% cholesterol and 0.5% cholic acid lithogenic (LG) diet for 8 weeks resulted in cholesterol super saturation in gallbladder bile, which promotes the formation of cholesterol gallstones (CGSs).

Results:

AE reduced the CGS incidence by 60.21%, and serum total cholesterol, triglyceride (TG), very low density lipoprotein (LDL) and LDL compared to control animals. Seed extracts at 300 mg/kg dose markedly reduced biliary cholesterol (BC) and decreased bile salt content. The ratio of BC to phospholipid which was 2.64 in the LG diet group was reduced to 1.57–1.35 in the M. uniflorum seed extracts treated groups. Liver cholesterol and TG were decreased significantly by feeding of ME and AE at 300 mg/kg dose. AE significantly reversed the changes in apolipoproteins A-I and C-II level disturbed by LG diet.

Conclusions:

M. uniflorum seed exerted antilithogenic influence by decreasing the cholesterol hyper-secretion into bile and increasing the bile acid output, thus decreasing the formation of LG bile in mice. The effect was maximum in the AE as it also reduced papillary proliferation of gallbladder and fatty degeneration of the liver. The potential antilithogenic effect of the AE of M. uniflorum may be due to antioxidant property of its rich total polyphenol and tannins content.

The incorporation of fatty acids of different chain length into liver and biliary lipids in the perfused rat liver

In an attempt to correlate the incorporation of fatty acids (FA) of different chain length into liver and biliary lipids, isolated rat livers were perfused for 2 h with Krebs-Ringer bicarbonate containing 1% albumin and 10 mumol of [1-14C]-labeled FA: C2, C8, C10, C12, C16, and C18:1. One to 1.36 mumol of medium-chain fatty acids (MCFA, C8, C10, and C12) and 6.6 mumol of long-chain FA (LCFA) were incorporated into liver lipids, 40% of the latter into phosphatidylcholine (PC). 14C-acetate (13 nmol) was incorporated into biliary cholesterol; 14C-MCFA contributed only 3.2-5 nmol; LCFA did not lead to newly synthesized cholesterol. Newly synthesized liver PC (2.75 to 3.25%) and newly synthesized liver cholesterol (6.5 to 10%) were secreted into bile. The specific radioactivity of biliary PC after infusion of all-saturated FA was 3.8-6.8 times higher than that of liver PC; for C18:1 it was only 1.7-fold. The specific radioactivity of biliary cholesterol, as compared to liver cholesterol, was 12 times higher for C2 and five times higher for MCFA. This indicates that a considerable proportion of the newly synthesized lipids was secreted into bile prior to significant mixing with preexisting liver PC and cholesterol pools. Liver PC contained 8% of unchanged 14C-C12; while 14C-C10 was not detected. Biliary PC, in contrast, contained 18% of unchanged 14C-C12 and 3% 14C-C10. These results suggest that after prolonged infusion of medium-chain triacylglycerols/long-chain triacylglycerols to patients, biliary PC may become enriched with MCTA. In addition, the oxidation of these FA may provide C-2 units which increase cholesterol synthesis.

Thyroid hormone is required for dietary fish oil to induce hypersecretion of biliary cholesterol in the rat

In the rat, both fish oil diet and thyroid hormone replacement are reported to augment bile cholesterol secretion out of proportion to bile flow or secretion of other bile lipids. We sought common mechanisms for these effects and evaluated the role of phospholipid fatty acid composition in the process. Methimazole-treated hypothyroid rats were fed low-fat chow or chow supplemented with 10% corn oil or fish oil, and were studied before and after thyroid hormone treatment. Serum, hepatic, and bile lipids were measured, phospholipid fatty acid composition determined, and hepatic 3-hydroxy-3-methylglutaryl CoA reductase activity assayed. Fish oil diet stimulated cholesterol secretion into bile only after thyroid hormone was given, and this action was synergistic with that of thyroid hormone. Reduced serum cholesterol in fish oil-treated rats was associated with increased biliary cholesterol secretion and diminished hepatic cholesterol content. This suggests that augmented biliary cholesterol secretion may contribute to the fish oil-induced reduction of serum cholesterol. No definite relationship between hepatic or biliary phospholipid fatty acid composition and biliary secretion was apparent, although high bile cholesterol secretion was associated with a low percentage of hepatic and bile phospholipid linoleic acid.