Elongation and desaturation pathways in mammary gland epithelial cells are associated with modulation of fat and membrane composition

Molecular complexity of mammary glands development: a review of lactogenic differentiation in epithelial cells

The mammary gland is a dynamic organ with various physiological processes like cellular proliferation, differentiation, and apoptosis during the pregnancy-lactation-involution cycle. It is essential to understand the molecular changes during the lactogenic differentiation of mammary epithelial cells (MECs, the milk-synthesizing cells). The MECs are organized as luminal milk-secreting cells and basal myoepithelial cells (responsible for milk ejection by contraction) that form the alveoli. The branching morphogenesis and lactogenic differentiation of the MECs prepare the gland for lactation. This process is governed by many molecular mediators including hormones, growth factors, cytokines, miRNAs, regulatory proteins, etc. Interestingly, various signalling pathways guide lactation and understanding these molecular transitions from pregnancy to lactation will help researchers design further research. Manipulation of genes responsible for milk synthesis and secretion will promote augmentation of milk yield in dairy animals. Identifying protein signatures of lactation will help develop strategies for persistent lactation and shortening the dry period in farm animals. The present review article discusses in details the physiological and molecular changes occurring during lactogenic differentiation of MECs and the associated hormones, regulatory proteins, miRNAs, and signalling pathways. An in-depth knowledge of the molecular events will aid in developing engineered cellular models for studies related to mammary gland diseases of humans and animals.

Investigating lipid headgroup composition within epithelial membranes: a systematic review

Membrane lipid composition is often quoted within the literature, but with very little insight into how or why these compositions vary when compared to other biological membranes. One prominent area that lacks understanding in terms of rationale for lipid variability is the human gastro-intestinal tract (GIT). We have carried out a comprehensive systematic literature search to ascertain the key lipid components of epithelial membranes, with a particular focus on addressing the human GIT and to use compositional data to understand structural aspects of biological membranes. Both bacterial outer membranes and the human erythrocyte membrane were used as a comparison for the mammalian [epithelial] membranes and to understand variations in lipid presence. We show that phosphatidylcholine (PC) lipid types tend to dominate (33%) with phosphatidylethanolamines (PE) and cholesterol having very similar abundances (25 and 23% respectively). This systematic review presents a detailed insight into lipid headgroup composition and roles in various membrane types, with a summary of the distinction between the major lipid bilayer forming lipids and how peripheral lipids regulate charge and fluidity. The variety of lipids present in biological membranes is discussed and rationalised in terms function as well as cellular position.

Elongase of very long chain fatty acids 6 (ELOVL6) promotes lipid synthesis in buffalo mammary epithelial cells

Recent studies have shown elongase of very-long-chain fatty acids 6 (ELOVL6) is a vital protein for endogenous synthesis of saturated and monounsaturated long-chain fatty acids in some mammals. Nevertheless, its role in lipid synthesis in buffalo mammary gland is still unclear. In this work, the full-length coding sequence (CDS) of ELOVL6 was cloned and identified from buffalo mammary gland. As a result, the CDS of this gene is 795 bp, which encodes a polypeptide of 264 amino acid residues. The buffalo ELOVL6 contains an ELO domain which belongs to the ELO superfamily. Among the 10 tissues of buffalo in peak lactation detected by RT-qPCR, the expression level of ELOVL6 was the highest in the brain, followed by the spleen, and then decreased in the mammary gland, muscle, kidney, heart, liver, rumen, intestine and lung. However, only the expression in the brain and spleen was statistically different from that in other tissues (p < 0.05). Compared with that of the dry-off period, the mRNA abundance of ELOVL6 in the mammary gland was significantly increased in peak lactation. The experiments based on lentivirus transfection in buffalo mammary epithelial cells (BuMECs) displayed that the overexpression of ELOVL6 markedly promoted the expression of INSIG1, INSIG2, SREBP, PPARG, FASN, GPAM, DGAT2 and APGAT6 genes, and the knockdown of ELOVL6 significantly decreased the mRNA abundance of INSIG2, SREBP, FASN, SCD, GPAM, APGAT6 and TIP47 genes. In addition, the increase or decrease of ELOVL6 expression level also caused the corresponding change of total triglyceride content in the BuMECs. The results here suggest that the ELOVL6 can catalyse the synthesis of long-chain fatty acids in the BuMECs, and it can indirectly affect the expression of genes related to milk fat synthesis through its catalytic products to promote the lipid biosynthesis of BuMECs.

Lipidome analysis of milk composition in humans, monkeys, bovids, and pigs

Background

Lipids contained in milk are an essential source of energy and structural materials for a growing neonate. Furthermore, lipids’ long-chain unsaturated fatty acid residues can directly participate in neonatal tissue formation. Here, we used untargeted mass spectrometric measurements to assess milk lipid composition in seven mammalian species: humans, two macaque species, cows, goats, yaks, and pigs.

Results

Analysis of the main milk lipid class, triacylglycerides (TAGs), revealed species-specific quantitative differences in the composition of fatty acid residues for each of seven species. Overall, differences in milk lipid composition reflect evolutionary distances among species, with each species group demonstrating specific lipidome features. Among them, human milk contained more medium and long-chain unsaturated fatty acids compared to other species, while pig milk was the most distinct, featuring the highest proportion of long-chain polyunsaturated fatty acids.

Conclusions

We show that milk lipidome composition is dynamic across mammalian species, changed extensively in pigs, and contains features particular to humans.

Rapid communication: lipid metabolic gene expression and triacylglycerol accumulation in goat mammary epithelial cells are decreased by inhibition of SREBP-1

In mammals, sterol regulatory element binding protein-1 (SREBP-1) is the master regulator of fatty acid and triacylglycerol synthesis. Recent gene silencing studies in mammary cells indicate that SREBP-1 has a central role in milk fat synthesis. However, SREBP-1 knockdown studies in goat mammary cells have not been performed; hence, its direct role in controlling mRNA expression of lipid metabolism genes and triacylglycerol synthesis remains unknown. Inhibition of SREBP-1 in goat mammary epithelial cells (GMEC) by small interference RNA (siRNA) markedly reduced the content of cellular triacylglycerol (~50% decrease, P < 0.05) and was partly related to downregulation of AGPAT6, LPIN1, and DGAT2 (-23%, -28% and -19%, respectively. P < 0.05), which are key enzymes involved in triacylglycerol synthesis, cellular triacylglycerol content and lipid droplet accumulation all decreased by SREBP-1 inhibition. The expression of lipid droplet formation and secretion genes was not altered in response to treatment. Although the lack of effect on expression of ACACA and FASN (rate-limiting enzymes for de novo fatty acid synthesis) with SREBP-1 knockdown was unexpected (P > 0.05), the downregulation of genes related to synthesis of acetyl-CoA and acetate activation (ACLY, ACSS2, and IDH1, P < 0.05) suggests that lipogenesis was inhibited. SREBP-1 knockdown also resulted in decreased expression of genes associated with fatty acid desaturation and elongation (SCD1 and ELOVL6, P < 0.05), long-chain fatty acid (LCFA) activation and transport (ACSL1, FABP3, and SLC27A6, P < 0.05). The results underscored the essential role of SREBP-1 not only in fatty acid synthesis but also in desaturation, elongation, and esterification in GMEC. Clearly, the lack of effect on ACACA and FASN, both of which are considered the key lipogenic enzymes, implies that there may be different regulatory mechanisms in goat compared with bovine mammary cells.

Effect of different exogenous fatty acids on the cytosolic triacylglycerol content in bovine mammary cells

The objective of this study was to determine how cytosolic triacylglycerols (TAG) are stored in mammary cells and whether this depends on the individual chemical configuration of fatty acids (FA). This objective was accomplished by addition of different FA to a FA-free medium used to culture mammary alveolar cells-large T antigen cells (MAC-T). Treatments consisted of adding FA (palmitate, stearate, oleate, linoleate, rumenic acid [CLA], elaidate and vaccinate) solutions to the medium at 100, 200, 300 and 400 mmol/L concentrations for a 24-h incubation period. At the end of each incubation period, cytosolic TAG, DNA and protein contents were measured. Palmitate, vaccenate, linoleate and CLA increased (P < 0.05) cytosolic TAG (μg/mg protein). Palmitate and CLA increased (P < 0.05) cytosolic TAG adjusted for DNA content. Overall, effects on cytosolic TAG accumulation depended on individual FA structure (chain length, degree of saturation, and number and orientation of FA double bonds). In addition, the long-chain FA used in this study did not have a detrimental effect on MAC-T cells as indicated by cytosolic protein and DNA contents reflecting their biological role in lipid accumulation.

Fatty acid and phospholipid biosynthetic pathways are regulated throughout mammary epithelial cell differentiation and correlate to breast cancer survival

This work combined gene and protein expression, gas chromatography-flame ionization detector, and hydrophilic interaction liquid chromatography-tandem mass spectrometry to compare lipid metabolism changes in undifferentiated/proliferating vs. functionally differentiated mammary epithelial cells (MECs) and to study their correlation to breast cancer survival. Sixty-eight genes involved in lipid metabolism were changed in MEC differentiation. Differentiated cells showed induction of Elovl6 (2-fold), Scd1 (4-fold), and Fads2 (2-fold), which correlated with increased levels of C16:1 n-7 and C18:1 n-9 (1.5-fold), C20:3 n-6 (2.5-fold), and C20:4 n-6 (6-fold) fatty acids (FAs) and more phospholipids (PLs) containing these species. Further, increased expression (2- to 3-fold) of genes in phosphatidylethanolamine (PE) de novo biosynthesis resulted in a 20% PE increase. Proliferating/undifferentiated cells showed higher C16:0 (1.7-fold) and C18:2 n-6 (4.2-fold) levels and more PLs containing C16:0 FAs [PC(16:0/16:1), PG(16:0/18:2), PG(16:0/18:1), and SM(16:0/18:0)]. Kaplan-Meier analysis of data from 3455 patients with breast cancer disclosed a positive correlation for 59% of genes expressed in differentiated MECs with better survival. PE biosynthesis and FA oxidation correlated with better prognosis in patients with breast cancer, including the basal-like subtype. Therefore, genes involved in mammary gland FA and PL metabolism and their resulting molecular species reflect the cellular proliferative ability and differentiation state and deserve further studies as potential markers of breast cancer progression