Comprehensive Characterization of Bovine Milk Lipids: Triglycerides

Overexpression of ATGL impairs lipid droplet accumulation by accelerating lipolysis in goat mammary epithelial cells

Adipose triglyceride lipase (ATGL) is the key enzyme for the degradation of triacylglycerols (TAGs). It functions in concert with other enzymes to mobilize TAG and supply fatty acids (FAs) for energy production. Dysregulated lipolysis leads to excess concentrations of circulating FAs, which may lead to destructive and lipotoxic effects to the organism. To understand the role of ATGL in mammary lipid metabolism, ATGL was overexpressed in goat mammary epithelial cells (GMECs) by using a recombinant adenovirus system. ATGL overexpression decreased lipid droplet (LD) accumulation and cellular TG content (p < 0.05) along with a decrease in the expression of the key enzyme that catalyzes the final step of TG synthesis (DGAT). Significant increases were observed in the expression of genes related to lipolysis (hormone-sensitive lipase [HSL]) and FA desaturation (SCD) by ATGL overexpression. Genes responsible for FA oxidation (PPARα), LD formation and secretion (ADRP and BTN1A1), and long-chain FA uptake (CD36) were all decreased by ATGL overexpression (p < 0.05). The primary products of TAG lipolysis, free FAs (FFAs), were notably increased in the ATGL-overexpressing cells. Taken together, our results demonstrated that ATGL activation impairs lipid formation partially through accelerating lipolysis in GMECs.

Lipidomics of coconut, almond and soybean milks - Comprehensive characterization of triacylglycerol class and comparison with bovine milk

Nowadays, plant-based milk consumption, as part of a healthy diet, is continuously increasing. In this paper, for the first time a lipidomic analysis on molecular species of triacylglycerol (TG) fraction of plant-based beverages (almond, soy, coconut) was performed by liquid chromatography quadrupole time-of-flight mass spectrometry. A total of 557 TG molecular species was measured, showing significantly different profiles between milk alternatives, compared with bovine milk. The most abundant TG molecular species were TG 18:1_18:1_18:1 and 18:1_18:1_18:2 for almond, TG 18:2_18:2_18:2 and 16:0_18:2_18:2 for soy, TG 12:0_10:0_12:0 and 12:0_12:0_14:0 for coconut. Unconventional fatty acids were detected in almond and soy. The main TG with ethereal linkage were TG-O 56:2, TG-O 56:4, and TG-O 56:5, while the main oxygenated TG was TG 54:5;1O. A total of 30 molecular species were identified as biomarkers for milk differentiation by principal component analysis, providing an interesting support for milk authentication and detection of adulteration on a larger sampling.

Optimization of chromatographic buffer conditions for the simultaneous analysis of phosphatidylinositol and phosphatidylinositol phosphate species in canola

The phosphatidylinositols and phosphatidylinositol phosphates are a set of closely related lipids known to influence various cellular functions. Irregular distributions of these molecules have been correlated with the development and progression of multiple diseases, including Alzheimer's, bipolar disorder, and various cancers. As a result, there is continued interest regarding the speciation of these compounds, with specific consideration on how their distribution may differ between healthy and diseased tissue. The comprehensive analysis of these compounds is challenging due to their varied and unique chemical characteristics, and current generalized lipidomics methods have proven unsuitable for phosphatidylinositol analysis and remain incapable of phosphatidylinositol phosphate analysis. Here we improved upon current methods by enabling the sensitive and simultaneous analysis of phosphatidylinositol and phosphatidylinositol phosphate species, whilst enhancing their characterization through chromatographic resolution between isomeric species. A 1 mM ammonium bicarbonate and ammonia buffer was determined optimal for this goal, enabling the identification of 148 phosphatidylinositide species, including 23 lyso-phosphatidylinositols, 51 phosphatidylinositols, 59 oxidized-phosphatidylinositols, and 15 phosphatidylinositol phosphates. As a result of this analysis, four distinct canola cultivars were differentiated based exclusively on their unique phosphatidylinositide-lipidome, indicating analyses of this type may be of use when considering the development and progression of the disease through lipidomic profiles.

Progress of Mass Spectrometry-Based Lipidomics in the Dairy Field

Lipids play important biological roles, such as providing essential fatty acids and signaling. The wide variety and structural diversity of lipids, and the limited technical means to study them, have seriously hampered the resolution of the mechanisms of action of lipids. With advances in mass spectrometry (MS) and bioinformatic technologies, large amounts of lipids have been detected and analyzed quickly using MS-based lipidomic techniques. Milk lipids, as complex structural metabolites, play a crucial role in human health. In this review, the lipidomic techniques and their applications to dairy products, including compositional analysis, quality identification, authenticity identification, and origin identification, are discussed, with the aim of providing technical support for the development of dairy products.

Linseed oil supplementation and DGAT1 K232A polymorphism affect the triacylglycerol composition and crystallization of milk fat

We studied the effect of dietary linseed oil (LSO) supplementation and DGAT1 K232A (DGAT1) polymorphism on the triacylglycerol composition and crystallization of bovine milk fat. LSO supplementation increased unsaturated triacylglycerols, notably in the C52-C54 carbon range, while reducing the saturated C29-C49 triacylglycerols. These changes were associated with an increase in the low-melting fraction and the crystal lamellar thickness, as well as a reduction in the medium and high-melting fractions and the formation of the most abundant crystal type at 20 °C (β'-2 polymorph). Furthermore, DGAT1 KK was associated with higher levels of odd-chain saturated triacylglycerols than DGAT1 AA, and it was also associated with an increase in the high-melting fraction and the endset melting temperature. An interaction between diet and DGAT1 for the unsaturated C54 triacylglycerols accentuated the effects of LSO supplementation with DGAT1 AA. These findings show that genetic polymorphism and cows' diet can have considerable effects on milk fat properties.

Analysis of milk with liquid chromatography–mass spectrometry: a review

As a widely consumed foodstuff, milk and dairy products are increasingly studied over the years. At the present time, milk profiling is used as a benchmark to assess the properties of milk. Modern biomolecular mass spectrometers have become invaluable to fully characterize the milk composition. This review reports the analysis of milk and its components using liquid chromatography coupled with mass spectrometry (LC–MS). LC–MS analysis as a whole will be discussed subdivided into the major constituents of milk, namely, lipids, proteins, sugars and the mineral fraction.

The FoxO1-ATGL axis alters milk lipolysis homeostasis through PI3K/AKT signaling pathway in dairy goat mammary epithelial cells

Goat milk is enriched in fatty acids which are beneficial to human health. Previous research has revealed that 98% of milk fat is composed of triglycerides. However, the mechanisms regulating milk fat composition remain unclear. Forkhead box protein O1 (FoxO1) is a crucial regulatory factor involved in lipid metabolism across various cell types. Chromatin immunoprecipitation sequencing (ChIP)-seq data) and RNA sequencing (RNA-seq) data revealed that have indicated a close association between FoxO1 was closely related to lipid metabolism during lactation in dairy goats. The objective of this study was to investigate the mechanisms by which FoxO1 regulates lipid metabolism in goat mammary epithelial cells (GMECs). FoxO1 knockdown significantly downregulated the expression of adipose triglyceride lipase (ATGL) and suppressed the activity of the ATGL promoter. Consistently, the number of lipid droplets decreased significantly in FoxO1-overexpressing cells and increased in ATGL-knockdown cells. To further verify the effect of FoxO1 on ATGL promoter activity, cells were transfected with four promoter fragments of different lengths. We found that the core region of the ATGL promoter was located between -882 bp and -524 bp, encompassing two FoxO1 binding sites (FKH1 and FKH2). Mutations in the FoxO1 binding sites significantly downregulated ATGL promoter activity in GMECs. Luciferase reporter assays demonstrated that FoxO1 overexpression markedly enhanced ATGL promoter activity. Furthermore, site-directed mutation confirmed that FKH1 and FKH2 sites were simultaneously mutated significantly attenuated the stimulatory effect of FoxO1 on ATGL promoter activities simultaneous mutation of FKH1 and FKH2 sites significantly attenuated the stimulatory effect of FoxO1 on ATGL promoter activity. ChIP assays showed that FoxO1 directly binds to the FKH2 element located in the ATGL promoter in vivo. Finally, immunofluorescence staining revealed that insulin promotes the translocation of FoxO1 from the nucleus to the cytoplasm, thereby attenuating the FoxO1-induced activation of the ATGL promoter. Collectively, these findings uncover a novel pathway where by FoxO1 may regulate lipid metabolism in GMECs specifically by modulating the transcriptional activity of ATGL.

Bioaccessibility of phospholipids in homogenized goat milk: Lipid digestion ecology through INFOGEST model

Phospholipids-rich goat milk provides health benefits to consumers. The effects of homogenization on the disruption and recombination of milk fat globule membrane and change the fatty acid positional distribution in glycerophospholipids profile by phosphatidylcholine metabolism pathways were investigated. Goat milk was homogenized at different intensity pressure. Homogenized samples were introduced into harmonized INFOGEST digestion model. Results showed that phosphatidylcholine increased significantly during storage in 30 MPa and were approximately twice that in raw milk (LOD 0.27-1.49 μg/L and LOQ 0.89-4.92 μg/L, respectively). Meanwhile, both linoleic acid (C18:2) and α-linolenic acid (C18:3ω-3), the foremost polyunsaturated acyl chains in homogenized milk extracts, showed upward trends. Notably, homogenization increased the number and altered the composition of Sn-1, 2 diacylglycerols via increasing trypsin and pancreatic lipase (PLRP2, MAUC15, CD36 and BSSL) expression and accelerated the phosphatidylcholine conversion. Ultimately, the relationship between homogenization and milk fat globule recombination and phospholipids bioaccessibility was preliminary established.

Optimized Identification of Triacylglycerols in Milk by HPLC-HRMS

This work has developed an optimized workflow for the targeted analysis of triacylglycerols (TAGs) in milk by liquid chromatography coupled with a Q-Exactive Orbitrap mass spectrometer. First, the effects of resolution (17,500; 35,000; 70,000; 140,000) and automatic gain control target (AGC, from 2×104, 2×105, 1×106, and 3×106) have been optimized with the goal to minimize the injection time, maximize the number of scans, and minimize the mass error. Then, the flow rate of the liquid chromatography system was also optimized by maximizing the number of theoretical plates. The resulting optimized parameters consisted of a flow rate of 200 μL/min, mass resolution of 35,000, and AGC target of 2×105. Such optimal conditions were applied for targeted TAG analysis of milk fat extracts. Up to 14 target triglycerides in milk fat were identified performing a data-dependent HPLC-HRMS-MS2 experiment (t-SIM-ddMS2). The findings reported here can be helpful for MS-based lipidomic workflows and targeted milk lipid analysis.

Seasonal variation in the positional distribution of fatty acids in bovine milk fat

The aim of this study was to determine the seasonal variation in the positional distribution of fatty acids (FA) in bovine milk fat. Bovine milk samples were collected from May 2017 to April 2018 in the Netherlands, and the FA composition in the sn-2 position was determined by using sn-1(3)-selective transesterification of Candida antarctica lipase B. The majority of the FA showed significant variation at sn-2 and sn-1(3) positions between different seasons. The seasonal variation in sn-2 position was higher than the sn-1(3) positions. Parallel to the changes in the diet of the cows throughout a year, we observed an increase in blood-derived FA (i.e. C18:0, C18:1 cis-9) concentrations and a decrease in de novo-synthesized FA during summer. In winter, more saturated FA were esterified in sn-2 position of milk fat. Highest concentrations of palmitic acid, C16:0, was observed in sn-2 position in winter, whereas the amount of unsaturated FA at this position was highest in summer. These results showed that the FA compositions in different regiospecific positions changed due to season; however, the proportions of a specific FA within the 3 positions of the triacylglycerols in milk fat did not change upon seasonal variation.

Multiple phase transitions and microstructural rearrangements shape milk fat crystal networks

HYPOTHESIS

The rheology of milk fat, which is strongly related to its functionality, reflects multiscale structural transitions in the colloidal network formed by crystallizing triacylglycerols.

EXPERIMENTS

To relate rheology to structure, early stages of milk fat crystallization at 15-22 °C were studied combining different techniques; XRD and microscopy to study structural changes, NMR to quantify the different structures, and rheology to evaluate their effect on macroscopic properties.

FINDINGS

Network strength increased with the synchronized formation of micro- and nanostructures. A rheological response was only obtained when these structures became visibly connected on a microscale, and internal transitional changes could be detected with rheology. On the nanoscale, transitions were linked to the formation of specific crystal polymorphs. We quantified the formation of polymorphs commonly found in milk fat (α-2 and β₁^'-2) and of two less commonly obtained polymorphs: β-2 and β₂^'-2. For the first time, the formation of these polymorphs was quantified and related to the composition of fat. Besides providing insights into the complex phase behavior of milk fat, this study shows that the structural transitions involved can be characterized and quantified by combining XRD with NMR and be detected at an early stage using rheology and microscopy.

Bovine Milk Triacylglycerol Regioisomer Ratio Shows Remarkable Inter-Breed and Inter-Cow Variation

Regioisomers (or positional isomers) of triacylglycerols (TAGs) of milk are known to show differential outcome in relation to human absorption. Quantitation of TAG regioisomers remains a big challenge due to the lack of facile chromatographic separation technique. The feasibility of using fragment ion intensity ratio to determine the ratio of co-eluting AAB/ABA-type regioisomer pairs was confirmed in this study. The ability of C30 stationary phase in resolving interfering TAG isomers was demonstrated for the first time. This allowed us to reveal the complexity of using fragment ion intensity to quantify 1,2-olein-3-palmitin (OOP), 1,3-olein-2-palmitin (OPO), 1,2-olein-3-stearin (OOS), and 1,3-olein-2-stearin (OSO) regioisomers in milk samples. A novel algorithm was proposed to consider the contribution of OPO/OOP and OSO/OOS double bond (DB)-isomers and to eliminate the interference of isobaric ions from other isomers, an aspect overlooked in previous studies. This liquid chromatography-mass spectrometry method that requires no pre-fractioning and a moderate chromatographic separation time of 36 min is simple and, thus, suitable for screening a large number of samples for genetic analysis of this trait. Preliminary results using a small cohort of animals showed that OPO/OOP ratio differs significantly between Jersey and Holstein cows, and a large variation was also observed across individual Holstein cows.

Effects of dairy processing on phospholipidome, in-vitro digestion and Caco-2 cellular uptake of bovine milk

Present work investigated the effects of processing (homogenization, sterilization) and cold storage on physicochemical properties, in vitro digestion and Caco-2 cellular uptake of bovine milk. Extreme heat sterilization and low temperature storage have significant impact on particle size and phospholipidome of bovine milk. In addition, cold storage of bovine milks led to formation of β' polymorphs crystals and endothermic peak with T_offset higher than body temperature. Processing and cold storage also increased the initial digestibility but reduced the overall digestibility of bovine milk. This might be related to the decreased particle size of the milk fat globules, changed in the phospholipidome of the MFGM and formation of β' polymorphs crystals in frozen milk. It is interesting to note that PE has relatively faster digestion meanwhile SM has relatively slower digestion. HTST milk which demonstrated lesser changed in terms of phospholipidome demonstrated highest cellular uptakes of most fatty acids.

TAILOR-MS, a Python Package that Deciphers Complex Triacylglycerol Fatty Acyl Structures: Applications for Bovine Milk and Infant Formulas

Liquid chromatography tandem mass spectrometry (LC/MS) and other mass spectrometric technologies have been widely applied for triacylglycerol profiling. One challenge for targeted identification of fatty acyl moieties that constitute triacylglycerol species in biological samples is the numerous combinations of 3 fatty acyl groups that can form a triacylglycerol molecule. Manual determination of triacylglycerol structures based on peak intensities and retention time can be highly inefficient and error-prone. To resolve this, we have developed TAILOR-MS, a Python (programming language) package that aims at assisting: (1) the generation of targeted LC/MS methods for triacylglycerol detection and (2) automating triacylglycerol structural determination and prediction. To assess the performance of TAILOR-MS, we conducted LC/MS triacylglycerol profiling of bovine milk and two infant formulas. Our results confirmed dissimilarities between bovine milk and infant formula triacylglycerol composition. Furthermore, we identified 247 triacylglycerol species and predicted the possible existence of another 317 in the bovine milk sample, representing one of the most comprehensive reports on the triacylglycerol composition of bovine milk thus far. Likewise, we presented here a complete infant formula triacylglycerol profile and reported >200 triacylglycerol species. TAILOR-MS dramatically shortened the time required for triacylglycerol structural identification from hours to seconds and performed decent structural predictions in the absence of some triacylglycerol constituent peaks. Taken together, TAILOR-MS is a valuable tool that can greatly save time and improve accuracy for targeted LC/MS triacylglycerol profiling.

Simple, Rapid Lipidomic Analysis of Triacylglycerols in Bovine Milk by Infusion-Electrospray Mass Spectrometry

Bovine milk is a complex mixture of lipids, proteins, carbohydrates, and other factors of which lipids comprise 3-5% of the total mass. Rapid analysis and characterization of the triacylglycerols (TAG) that comprise about 95% of the total lipid is daunting given the numerous TAG species. In the attached methods paper, we demonstrate an improved method for identifying and quantifying TAG species by infusion-based "shotgun" lipidomics. Because of the broad range of TAG species in milk, a single internal standard was insufficient for the analysis and required sectioning the spectrum into three portions based upon mass range to provide accurate quantitation of TAG species. Isobaric phospholipid interferences were removed using a simple dispersive solid-phase extraction step. Using this method, > 100 TAG species were quantitated by acyl carbon number and desaturation level in a sample of commercially purchased bovine milk.

A Liquid Chromatography-High Resolution Mass Spectrometry (LC-HRMS) Method for the Determination of Free Hydroxy Fatty Acids in Cow and Goat Milk

A liquid chromatography–high resolution mass spectrometry (LC-HRMS) method for the direct determination of various saturated hydroxy fatty acids (HFAs) in milk was developed for the first time. The method involves mild sample preparation conditions, avoids time-consuming derivatization procedures, and permits the simultaneous determination of 19 free HFAs in a single 10-min run. This method was validated and applied in 17 cow milk and 12 goat milk samples. This work revealed the existence of various previously unrecognized hydroxylated positional isomers of palmitic acid and stearic acid in both cow and goat milk, expanding our knowledge on the lipidome of milk. The most abundant free HFAs in cow milk were proven to be 7-hydroxystearic acid (7HSA) and 10-hydroxystearic acid (10HSA) (mean content values of 175.1 ± 3.4 µg/mL and 72.4 ± 6.1 µg/mL in fresh milk, respectively). The contents of 7HSA in cow milk seem to be substantially higher than those in goat milk.