Functional regulation of xanthine oxidoreductase expression and localization in the mouse mammary gland: evidence of a role in lipid secretion

Comparative proteomic analysis of the changes in mare milk associated with different lactation stages and management systems

Mare milk has traditionally been attributed a number of health promoting properties. However, knowledge on its composition and functionality remains scarce, with particularly limited studies on mare milk proteomics. This study deeply characterized mare milk proteome accounting for both caseins and proteins in the whey fraction, also addressing the impact of lactation stage and different management systems. Milk samples from Basque Mountain Horse breed mares belonging to three different farms and three lactation stages were analysed after in-gel and in-solution digestion using nLC-MS/MS. Among the 469 proteins identified, the content of alpha-1 antitrypsin was significantly higher in pasture-based compared to other systems. Moreover, lactation stage significantly affected the content of beta-lactoglobulin II, immunoglobulin-like domain-containing protein, interferon alpha-inducible protein 27, lactotransferrin, polypeptide N-acetylgalactosaminyltransferase, and transforming acidic coiled-coil containing protein 2. This study contributes to the deep characterization of mare milk proteome and provides new insights into the effect of different production factors.

Comparative proteomic analysis of human milk fat globules and paired membranes and mouse milk fat globules identifies core cellular systems contributing to mammary lipid trafficking and secretion

Introduction: Human milk delivers critical nutritional and immunological support to human infants. Milk fat globules (MFGs) and their associated membranes (MFGMs) contain the majority of milk lipids and many bioactive components that contribute to neonatal development and health, yet their compositions have not been fully defined, and the mechanisms responsible for formation of these structures remain incompletely understood. Methods: In this study, we used untargeted mass spectrometry to quantitatively profile the protein compositions of freshly obtained MFGs and their paired, physically separated MFGM fractions from 13 human milk samples. We also quantitatively profiled the MFG protein compositions of 9 pooled milk samples from 18 lactating mouse dams. Results: We identified 2,453 proteins and 2,795 proteins in the majority of human MFG and MFGM samples, respectively, and 1,577 proteins in mouse MFGs. Using paired analyses of protein abundance in MFGMs compared to MFGs (MFGM-MFG; 1% FDR), we identified 699 proteins that were more highly abundant in MFGMs (MFGM-enriched), and 201 proteins that were less abundant in MFGMs (cytoplasmic). MFGM-enriched proteins comprised membrane systems (apical plasma membrane and multiple vesicular membranes) hypothesized to be responsible for lipid and protein secretion and components of membrane transport and signaling systems. Cytoplasmic proteins included ribosomal and proteasomal systems. Comparing abundance between human and mouse MFGs, we found a positive correlation (R 2 = 0.44, p < 0.0001) in the relative abundances of 1,279 proteins that were found in common across species. Discussion: Comparative pathway enrichment analyses between human and mouse samples reveal similarities in membrane trafficking and signaling pathways involved in milk fat secretion and identify potentially novel immunological components of MFGs. Our results advance knowledge of the composition and relative quantities of proteins in human and mouse MFGs in greater detail, provide a quantitative profile of specifically enriched human MFGM proteins, and identify core cellular systems involved in milk lipid secretion.

Mammalian lipid droplets: structural, pathological, immunological and anti-toxicological roles

Mammalian lipid droplets (LDs) are specialized cytosolic organelles consisting of a neutral lipid core surrounded by a membrane made up of a phospholipid monolayer and a specific population of proteins that varies according to the location and function of each LD. Over the past decade, there have been significant advances in the understanding of LD biogenesis and functions. LDs are now recognized as dynamic organelles that participate in many aspects of cellular homeostasis plus other vital functions. LD biogenesis is a complex, highly-regulated process with assembly occurring on the endoplasmic reticulum although aspects of the underpinning molecular mechanisms remain elusive. For example, it is unclear how many enzymes participate in the biosynthesis of the neutral lipid components of LDs and how this process is coordinated in response to different metabolic cues to promote or suppress LD formation and turnover. In addition to enzymes involved in the biosynthesis of neutral lipids, various scaffolding proteins play roles in coordinating LD formation. Despite their lack of ultrastructural diversity, LDs in different mammalian cell types are involved in a wide range of biological functions. These include roles in membrane homeostasis, regulation of hypoxia, neoplastic inflammatory responses, cellular oxidative status, lipid peroxidation, and protection against potentially toxic intracellular fatty acids and lipophilic xenobiotics. Herein, the roles of mammalian LDs and their associated proteins are reviewed with a particular focus on their roles in pathological, immunological and anti-toxicological processes.

Genome-wide identification and skin expression of immunoglobulin superfamily in discus fish (Symphysodon aequifasciatus) reveal common genes associated with vertebrate lactation

Discus Symphysodon spp. employs an unusual parental care behavior where fry feed on parental skin mucus after hatching. Studies on discus immunoglobulin superfamily (IgSF) especially during parental care are scarce. Here, a total of 518 IgSF members were identified based on discus genome and clustered into 12 groups, unevenly distributing on 30 linkage groups. A total of 92 pairs of tandem duplication and 40 pairs of segmental duplication that underwent purifying selection were identified. IgSF genes expressed differentially in discus skin during different care stages and between male and female parents. Specifically, the transcription of btn1a1, similar with mammalian lactation, increased after spawning, reached a peak when fry started biting on parents' skin mucus, and then decreased. The expression of btn2a1 and other immune members, e.g., nect4, fcl5 and cd22, were up-regulated when fry stopped biting on mucus. These results suggest the expression differentiation of IgSF genes in skin of discus fish during parental care.

Proteomic analysis of milk fat globule membranes from small-sized milk fat globules and their function in promoting lipid droplet fusion in bovine mammary epithelial cells

In mammary epithelial cells, milk fat is synthesized as lipid droplets and secreted in the form of globules. Milk fat globules (MFGs) are covered by a lipid-protein membrane known as the milk fat globule membrane (MFGM). We randomly divided 12 Holstein cows into control and conjugated linoleic acid (CLA) groups. The control group was fed a basal diet, while the CLA group was fed the basal diet + CLA (15 g per kg DM) for 10 days. Cow performance, milk composition, and MFG size were measured daily. On day 10, we extracted MFGM proteins (n = 3) and identified them via quantitative proteomic analysis. We investigated the effects of the MFGM proteins from control and CLA-treated milk on the lipid droplet formation in MAC-T cells. Compared with the control group, the CLA group had reduced milk fat content (3.39 g/100 mL vs. 2.45 g/100 mL) and MFG size parameters (D_[4,3] of 3.85 μm vs. 3.37 μm; D_[3,2] of 3.24 μm vs. 2.83 μm). The specific surface area (SSA) increased in the CLA group. A total of 361 differentially expressed proteins were identified in the CLA group by iTRAQ quantitative proteomic analysis. Among these proteins, 100 were upregulated and 251 were downregulated (p < 0.05). In MAC-T cells, CLA-MFGM proteins increased the diameter of the lipid droplets to 1.32 μm. CLA-MFGM proteins decreased the proportion of the small lipid droplets (15.33% vs. 47.78%) and increased the proportion of the large lipid droplets (25.04% vs. 11.65%). CLA-MFGM proteins promoted lipid droplet fusion. Therefore, MFGM proteins play an important role in the regulation of the lipid droplet size.

Environmental chemical TCPOBOP disrupts milk lipid homeostasis during pregnancy and lactation

Humans are exposed to different kinds of environmental contaminants or drugs throughout their lifetimes. The widespread presence of these compounds has raised concerns about the consequent adverse effects on lactating women. The constitutive androstane receptor (CAR, Nr1i3) is known as a xenobiotic sensor for environmental pollution or drugs. In this study, the model environmental chemical 1, 4-bis [2-(3, 5-dichloropyridyloxy)] benzene, TCPOBOP (TC), which is a highly specific agonist of CAR, was used to investigate the effects of exogenous exposure on lactation function and offspring health in mice. The results revealed that TC exposure decreased the proliferation of mammary epithelial cells during pregnancy. This deficiency further compromised lobular-alveolar structures, resulting in alveolar cell apoptosis, as well as premature stoppage of the lactation cycle and aberrant lactation. Furthermore, TC exposure significantly altered the size and number of milk lipid droplets, suggesting that TC exposure inhibits milk lipid synthesis. Additionally, TC exposure interfered with the milk lipid metabolism network, resulting in the inability of TC-exposed mice to efficiently secrete nutrients and feed their offspring. These findings demonstrated that restricted synthesis and secretion of milk lipids would indirectly block mammary gland form and function, which explained the possible reasons for lactation failure and retarded offspring growth.

Milk fat globule membrane proteins are involved in controlling the size of milk fat globules during conjugated linoleic acid-induced milk fat depression

Milk fat globule membrane (MFGM) proteins surround the triacylglycerol core comprising milk fat globules (MFG). We previously detected a decrease in the size of fat globules during conjugated linoleic acid (CLA)-induced milk fat depression (MFD), and other studies have reported that some MFGM proteins play a central role in regulating mammary cellular lipid droplet size. However, little is known about the relationship between MFD, MFG size, and MFGM proteins in bovine milk. The aim of this study was to investigate the profile of MFGM proteins during MFD induced by CLA. Sixteen mid-lactating Holstein cows (145 ± 24 d in milk) with similar body condition and parity were divided into control and CLA groups over a 10-d period. Cows were fed a basal diet (control, n = 8) or control plus 15 g/kg of dry matter (DM) CLA (n = 8) to induce MFD. Cow performance, milk composition, and MFG size were measured daily. On d 10, MFGM proteins were extracted and identified by quantitative proteomic analysis, and western blotting was used to verify a subset of the identified MFGM proteins. Compared with controls, supplemental CLA did not affect milk production, DM intake, or milk protein and lactose contents. However, CLA reduced milk fat content (3.73 g/100 mL vs. 2.47 g/100 mL) and the size parameters volume-related diameter D_[4,3] (3.72 μm vs. 3.35 μm) and surface area-related diameter D_[3,2] (3.13 μm vs. 2.80 μm), but increased specific surface area of MFG (1,905 m²/kg vs. 2,188 m²/kg). In total, 177 differentially expressed proteins were detected in milk from cows with CLA-induced MFD, 60 of which were upregulated and 117 downregulated. Correlation analysis showed that MFG size was negatively correlated with various proteins, including XDH and FABP3, and positively correlated with MFG-E8, RAB19, and APOA1. The results provide evidence for an important role of MFGM proteins in regulating MFG diameter, and they facilitate a mechanistic understanding of diet-induced MFD.

Perilipin-2 promotes lipid droplet-plasma membrane interactions that facilitate apocrine lipid secretion in secretory epithelial cells of the mouse mammary gland

Secretory epithelial cells (sMEC) in mammary glands of lactating animals secrete lipids by a novel apocrine mechanism in which cytoplasmic lipid droplets (LD) contact and are enveloped by elements of the apical plasma membrane (APM) before being released into the lumen of the gland as membrane bound structures. The molecular properties of LD-APM contacts and the mechanisms regulating LD membrane envelopment and secretion are not fully understood. Perilipin-2 (Plin2) is a constitutive LD protein that has been proposed to tether LD to the APM through formation of a complex with the transmembrane protein, butyrophilin1a1 (BTN) and the redox enzyme, xanthine oxidoreductase (XOR). Using mice lacking Plin2 and physiological inhibition of apocrine lipid secretion, we demonstrate that LD-APM contact and envelopment are mechanistically distinct steps that they are differentially regulated by Plin2 and independent of LD secretion. We find that Plin2 is not required for formation of LD-APM contacts. However, it increases the percentage of LD that contact the APM and mediates enlargement of the LD-APM contact zone as LD undergo membrane envelopment. The effects of Plin2 LD-APM interactions are associated with increased abundances of BTN, XOR and Cidea, which are implicated as mediators of LD-APM contact formation, on membranes surrounding secreted LD, and with promotion of glycocalyx remodeling at LD-APM contact sites. We propose that Plin2 does not directly mediate contact between LD and the APM but acts by enhancing molecular interactions that stabilize LD-APM contacts and govern membrane envelopment of LD during apocrine lipid secretion. Plin2 does not appear to significantly affect the lipid content of milk in fully lactating animals, but it does increase lipid secretion at the onset of lactation in primaparous dams, which suggest a role in facilitating apocrine lipid secretion in sMEC during their initial transition to a secretory phenotype.

The double faced role of xanthine oxidoreductase in cancer

Xanthine oxidoreductase (XOR) is a critical, rate-limiting enzyme that controls the last two steps of purine catabolism by converting hypoxanthine to xanthine and xanthine to uric acid. It also produces reactive oxygen species (ROS) during the catalytic process. The enzyme is generally recognized as a drug target for the therapy of gout and hyperuricemia. The catalytic products uric acid and ROS act as antioxidants or oxidants, respectively, and are involved in pro/anti-inflammatory actions, which are associated with various disease manifestations, including metabolic syndrome, ischemia reperfusion injury, cardiovascular disorders, and cancer. Recently, extensive efforts have been devoted to understanding the paradoxical roles of XOR in tumor promotion. Here, we summarize the expression of XOR in different types of cancer and decipher the dual roles of XOR in cancer by its enzymatic or nonenzymatic activity to provide an updated understanding of the mechanistic function of XOR in cancer. We also discuss the potential to modulate XOR in cancer therapy.

Hello from the other side: Membrane contact of lipid droplets with other organelles and subsequent functional implications

Lipid droplets (LDs) are ubiquitous organelles that play crucial roles in response to physiological and environmental cues. The identification of several neutral lipid synthesizing and regulatory protein complexes have propelled significant advance on the mechanisms of LD biogenesis in the endoplasmic reticulum (ER). Increasing evidence suggests that distinct proteins and regulatory factors, which localize to membrane contact sites (MCS), are involved not only in interorganellar lipid exchange and transport, but also function in other important cellular processes, including autophagy, mitochondrial dynamics and inheritance, ion signaling and inter-regulation of these MCS. More and more tethers and molecular determinants are associated to MCS and to a diversity of cellular and pathophysiological processes, demonstrating the dynamics and importance of these junctions in health and disease. The conjugation of lipids with proteins in supramolecular complexes is known to be paramount for many biological processes, namely membrane biosynthesis, cell homeostasis, regulation of organelle division and biogenesis, and cell growth. Ultimately, this physical organization allows the contact sites to function as crucial metabolic hubs that control the occurrence of chemical reactions. This leads to biochemical and metabolite compartmentalization for the purposes of energetic efficiency and cellular homeostasis. In this review, we will focus on the structural and functional aspects of LD-organelle interactions and how they ensure signaling exchange and metabolites transfer between organelles.

Quantitative analysis of differentially expressed milk fat globule membrane proteins between donkey and bovine colostrum based on high-performance liquid chromatography with tandem mass spectrometry proteomics

This study was designed to provide novel insights into milk fat globule membrane (MFGM) proteins in donkey colostrum (DC) and bovine colostrum (BC) using quantitative proteomics. In total, 179 (DC) and 195 (BC) MFGM proteins were characterized, including 71 shared, 108 DC-specific, and 124 BC-specific proteins. Fifty-one shared proteins were selected as differentially expressed MFGM proteins, including 21 upregulated and 30 downregulated proteins in DC. Gene ontology analysis showed that these proteins were mainly enriched in cellular components, including the extracellular exosome, extracellular space, and plasma membrane. Additionally, they were further involved in metabolic pathways, including cholesterol metabolism, the peroxisome proliferator-activated receptor signaling pathway, and purine metabolism. Furthermore, several key protein factors with high connectivity were identified via protein-protein interaction analysis. These results provide more comprehensive knowledge of differences in the biological properties of MFGM proteins in DC and BC as well as pave the way for future studies of the nutritional and functional requirements of these important ingredients toward the development of dairy products based on multiple milk sources.

Improving Human Health with Milk Fat Globule Membrane, Lactic Acid Bacteria, and Bifidobacteria

The milk fat globule membrane (MFGM), the component that surrounds fat globules in milk, and its constituents have gained significant attention for their gut function, immune-boosting properties, and cognitive-development roles. The MFGM can directly interact with probiotic bacteria, such as bifidobacteria and lactic acid bacteria (LAB), through interactions with bacterial surface proteins. With these interactions in mind, increasing evidence supports a synergistic effect between MFGM and probiotics to benefit human health at all ages. This important synergy affects the survival and adhesion of probiotic bacteria through gastrointestinal transit, mucosal immunity, and neurocognitive behavior in developing infants. In this review, we highlight the current understanding of the co-supplementation of MFGM and probiotics with a specific emphasis on their interactions and colocalization in dairy foods, supporting in vivo and clinical evidence, and current and future potential applications.

Milk somatic cell derived transcriptome analysis identifies regulatory genes and pathways during lactation in Indian Sahiwal cattle (Bos indicus)

BACKGROUND

The present study is an effort to understand the genomic drivers of lactation in Sahiwal (Bos indicus), the best milch cattle breed of the tropics.

METHODS

RNA sequencing of four animals from early, mid and late lactation stages was performed using milk somatic cells as source of RNA.

RESULTS

The genes encoding the milk casein and whey proteins showed highest expression in early and mid lactation, with a declining trend towards the late stage. The enhanced expression of PLIN2, FABP5 and FABP3 genes in mid lactation suggests enrichment of the PPARα pathway which is linked to fatty acid metabolism. A gradual decline in the percentage of genes involved in metabolism of proteins, mRNA and insulin synthesis from early to late lactation reflected transition from lactogenesis to involution. Major biological pathways maintained throughout lactation were adaptive immune system, FGF signaling, EGFR signaling, activated TLR4 signaling, NFkB and MAP kinases activation mediated by TLR4 signaling repertoire. Differential expression analysis revealed 547, 1010 and 1313 differentially expressed genes (p < 0.05) between early-late, early-mid and mid-late stages, respectively. The topmost regulatory genes identified by network analysis from the differentially expressed genes, were involved in Chemokine receptor, GPCR and EGFR1 pathways.

CONCLUSION

The genes and pathways delineated in this study have regulatory implications in cell morphogenesis, lipid droplet formation and protein synthesis in the course of lactation. The study provides an insight into the expression profile of genes influencing milk properties and lactation in Sahiwal cattle.

Xanthine oxidase-lactoperoxidase system and innate immunity: Biochemical actions and physiological roles

The innate immune system in mammals is the first-line defense that plays an important protective role against a wide spectrum of pathogens, especially during early life before the adaptive immune system develops. The enzymes xanthine oxidase (XO) and lactoperoxidase (LPO) are widely distributed in mammalian tissues and secretions, and have a variety of biological functions including in innate immunity, provoking much interest for both in vitro and in vivo applications. The enzymes are characterized by their generation of reactive oxygen and nitrogen species, including hydrogen peroxide, hypothiocyanite, nitric oxide, and peroxynitrite. XO is a major generator of hydrogen peroxide and superoxide that subsequently trigger a cascade of oxidative radical pathways, including those produced by LPO, which have bactericidal and bacteriostatic effects against pathogens including opportunistic bacteria. In addition to their role in host microbial defense, reactive oxygen and nitrogen species play important physiological roles as second messenger cell signaling molecules, including cellular proliferation, differentiation and gene expression. There are several indications that the reactive species generated by peroxide have positive effects on human health, particularly in neonates; however, some important in vivo aspects of this system remain obscure. The primary dependence of the system on hydrogen peroxide has led us to propose it is particularly relevant to neonate mammals during milk feeding.

Organellar Contacts of Milk Lipid Droplets

Milk-secreting epithelial cells of the mammary gland are functionally specialized for the synthesis and secretion of large quantities of neutral lipids, a major macronutrient in milk from most mammals. Milk lipid synthesis and secretion are hormonally regulated and secretion occurs by a unique apocrine mechanism. Neutral lipids are synthesized and packaged into perilipin-2 (PLIN2) coated cytoplasmic lipid droplets within specialized cisternal domains of rough endoplasmic reticulum (ER). Continued lipid synthesis by ER membrane enzymes and lipid droplet fusion contribute to the large size of these cytoplasmic lipid droplets (5–15 μm in diameter). Lipid droplets are directionally trafficked within the epithelial cell to the apical plasma membrane. Upon contact, a molecular docking complex assembles to tether the droplet to the plasma membrane and facilitate its membrane envelopment. This docking complex consists of the transmembrane protein, butyrophilin, the cytoplasmic housekeeping protein, xanthine dehydrogenase/oxidoreductase, the lipid droplet coat proteins, PLIN2, and cell death-inducing DFFA-like effector A. Interactions of mitochondria, Golgi, and secretory vesicles with docked lipid droplets have also been reported and may supply membrane phospholipids, energy, or scaffold cytoskeleton for apocrine secretion of the lipid droplet. Final secretion of lipid droplets into the milk occurs in response to oxytocin-stimulated contraction of myoepithelial cells that surround milk-secreting epithelial cells. The mechanistic details of lipid droplet release are unknown at this time. The final secreted milk fat globule consists of a triglyceride core coated with a phospholipid monolayer and various coat proteins, fully encased in a membrane bilayer.

TDP-43 facilitates milk lipid secretion by post-transcriptional regulation of Btn1a1 and Xdh

Milk lipid secretion is a critical process for the delivery of nutrition and energy from parent to offspring. However, the underlying molecular mechanism is less clear. Here we report that TDP-43, a RNA-binding protein, underwent positive selection in the mammalian lineage. Furthermore, TDP-43 gene (Tardbp) loss induces accumulation of large lipid droplets and severe lipid secretion deficiency in mammary epithelial cells to outside alveolar lumens, eventually resulting in lactation failure and pup starvation within three weeks postpartum. In human milk samples from lactating women, the expression levels of TDP-43 is positively correlated with higher milk output. Mechanistically, TDP-43 exerts post-transcriptional regulation of Btn1a1 and Xdh mRNA stability, which are required for the secretion of lipid droplets from epithelial cells to the lumen. Taken together, our results highlights the critical role of TDP-43 in milk lipid secretion, providing a potential strategy for the screening and intervention of clinical lactation insufficiency.

Milk lipid secretion is a critical process for the delivery of nutrition and energy from parent to offspring. Here the authors found that TDP-43, a RNA-binding protein, is required for milk lipid secretion by post-transcriptional regulation of Btn1a1 and Xdh mRNA stability.

Formation and degradation of lipid droplets in human adipocytes and the expression of aldehyde oxidase (AOX)

Lipid droplet (LD) binding proteins in mammary glands and in adipocytes were previously compared and striking similar sets of these specific proteins demonstrated. Xanthine oxidoreductase (XOR) together with perilipins and the lactating mammary gland protein butyrophilin play an important role in the secretion process of LDs into milk ducts. In contrast, in adipose tissue and in adipocytes, mainly perilipins have been described. Moreover, XOR was reported in mouse adipose tissue and adipocyte culture cells as “novel regulator of adipogenesis”. This obvious coincidence of protein sets prompted us to revisit the formation of LDs in human-cultured adipocytes in more detail with special emphasis on the possibility of a LD association of XOR. We demonstrate by electron and immunoelectron microscopy new structural details on LD formation in adipocytes. Surprisingly, by immunological and proteomic analysis, we identify in contrast to previous data showing the enzyme XOR, predominantly the expression of aldehyde oxidase (AOX). AOX could be detected tightly linked to LDs when adipocytes were treated with starvation medium. In addition, the majority of cells show an enormous interconnected, tubulated mitochondria network. Here, we discuss that (1) XOR is involved—together with perilipins—in the secretion of LDs in alveolar epithelial cells of the lactating mammary gland and is important in the transcytosis pathway of capillary endothelial cells. (2) In cells, where LDs are not secreted, XOR cannot be detected at the protein level, whereas in contrast in these cases, AOX is often present. We detect AOX in adipocytes together with perilipins and find evidence that these proteins might direct LDs to mitochondria. Finally, we here report for the first time the exclusive and complementary localization of XOR and AOX in diverse cell types.

Symposium review: Intravital imaging of the lactating mammary gland in live mice reveals novel aspects of milk-lipid secretion

Milk fat comprises membrane-coated droplets of neutral lipid, which constitute the predominant source of lipids for survival of the suckling neonate. From the perspective of the dairy industry, they are the basis for the manufacture of butter and essential ingredients in the production of cheese, yogurt, and specialty dairy produce. To provide mechanistic insight into the assembly and secretion of lipid droplets during lactation, we developed novel intravital imaging techniques using transgenic mice, which express fluorescently tagged marker proteins. The number 4 mammary glands were surgically prepared under a deep plane of anesthesia and the exposed glands positioned as a skin flap with intact vascular supply on the stage of a laser-scanning confocal microscope. Lipid droplets were stained by prior exposure of the glands to hydrophobic fluorescent BODIPY (boron-dipyrromethene) dyes and their formation and secretion monitored by time-lapse subcellular microscopy over periods of 1 to 2 h. Droplets were transported to the cell apex by directed (superdiffusive) motion at relatively slow and intermittent rates (0-2 µm/min). Regardless of size, droplets grew by numerous fusion events during transport and as they were budding from the cell enveloped by apical membranes. Surprisingly, droplet secretion was not constitutive but required an injection of oxytocin to induce contraction of the myoepithelium with subsequent release of droplets into luminal spaces. These novel results are discussed in the context of the current paradigm for milk fat synthesis and secretion and as a template for future innovations in the dairy industry.

Milk lipid regulation at the maternal-offspring interface

Milk lipids provide a large proportion of energy, nutrients, essential fatty acids, and signaling molecules for the newborns, the synthesis of which is a tightly controlled process. Dysregulated milk lipid production and composition may be detrimental to the growth, development, health and survival of the newborns. Many genetically modified animal models have contributed to our understanding of milk lipid regulation in the lactating mammary gland. In this review, we discuss recent advances in our knowledge of the mechanisms that control milk lipid biosynthesis and secretion during lactation, and how maternal genetic and dietary defects impact milk lipid composition and consequently offspring traits.

Th-POK regulates mammary gland lactation through mTOR-SREBP pathway

The Th-inducing POK (Th-POK, also known as ZBTB7B or cKrox) transcription factor is a key regulator of lineage commitment of immature T cell precursors. It is yet unclear the physiological functions of Th-POK besides helper T cell differentiation. Here we show that Th-POK is restrictedly expressed in the luminal epithelial cells in the mammary glands that is upregulated at late pregnancy and lactation. Lineage restrictedly expressed Th-POK exerts distinct biological functions in the mammary epithelial cells and T cells in a tissue-specific manner. Th-POK is not required for mammary epithelial cell fate determination. Mammary gland morphogenesis in puberty and alveologenesis in pregnancy are phenotypically normal in the Th-POK-deficient mice. However, Th-POK-deficient mice are defective in triggering the onset of lactation upon parturition with large cellular lipid droplets retained within alveolar epithelial cells. As a result, Th-POK knockout mice are unable to efficiently secret milk lipid and to nurse the offspring. Such defect is mainly attributed to the malfunctioned mammary epithelial cells, but not the tissue microenvironment in the Th-POK deficient mice. Th-POK directly regulates expression of insulin receptor substrate-1 (IRS-1) and insulin-induced Akt-mTOR-SREBP signaling. Th-POK deficiency compromises IRS-1 expression and Akt-mTOR-SREBP signaling in the lactating mammary glands. Conversely, insulin induces Th-POK expression. Thus, Th-POK functions as an important feed-forward regulator of insulin signaling in mammary gland lactation.

Disruption of xanthine oxidoreductase gene attenuates renal ischemia reperfusion injury in mice

AIMS

We examined the roles of xanthine oxidoreductase (XOR) in renal ischemia reperfusion (IR) injury.

MAIN METHODS

XOR+/+ and XOR+/- mice were subjected to 24-h reperfusion after a 45-min bilateral renal artery occlusion or sham operation. We evaluated the renal damage based on the concentrations of blood urea nitrogen (BUN) and serum creatinine (Cr), and histological changes were detected by PAS staining. Xanthine dehydrogenase, oxidase (XO) and XOR activities, amounts of blood and urine 8-OHdG, and expressions of TNF-α and MCP-1 mRNA were examined. F4/80 and nitrotyrosine-positive cells were assessed by immunohistochemical staining.

KEY FINDINGS

The BUN and Cr concentrations in the XOR+/+IR mice were increased significantly compared to those in XOR+/-IR and allopurinol-treated XOR+/+IR mice. XO and XOR activity, which were increased in IR mice, were reduced in the allopurinol-treated XOR+/+IR and XOR+/-IR mice compared to the XOR+/+IR mice. The concentrations of blood and urine 8-OHdG, and the expressions of MCP-1 and TNF-α mRNA were increased significantly in the XOR+/+IR mice compared to those in the XOR+/-IR mice. The histological analysis revealed that the XOR+/-IR and allopurinol-treated XOR+/+IR mice showed less tubular injury than the XOR+/+IR mice in the cortex regions, with the reduction of inflammation and oxidative stress assessed by the immunohistological staining for F4/80 and nitrotyrosine.

SIGNIFICANCE

Both the disruption of XOR gene in XOR+/- mice and the reduction of XOR activity in allopurinol-treated XOR+/+IR mice attenuated renal tissue injury in this IR model. Reduced XOR activity during renal IR could be a beneficial treatment target.

Ultrastructural and immunocytochemical evidence for the reorganisation of the milk fat globule membrane after secretion

This paper reports a detailed ultrastructural and immunocytochemical investigation of the structure of the milk fat globule membrane (MFGM) in a variety of species. The process follows the same pattern in all mammals so far investigated. The initial (or primary) MFGM immediately on release from the mammary cell is a continuous unit membrane with a thin underlying layer of cytoplasmic origin and a monolayer of phospholipid separating it from the core lipid. This structure changes rapidly as the milk fat globule (MFG) moves into the alveolar lumen. The unit membrane plus the underlying layer of cytoplasm modifies drastically into discontinuous patches and networks. These are superimposed upon a continuous apparently structureless sheet of electron dense material stabilising the MFG and similar to that which bounded the lipid in the cell. The underlying layer of the patches increases in electron density and immunocytochemistry demonstrates localisation of MFGM proteins in this layer. In four species, the dense material shows ordered paracrystalline molecular arrays in section and en face views. All the arrays show the same basic pattern and unit size as determined by optical diffraction. Similar patches, networks and arrays are present on the surface of expressed MFG. Negative staining of lipid-extracted expressed MFGs shows similar patches and networks of membrane. These also occasionally show the crystalline arrays and label with MFGM protein antibodies. Similar networks and strands of plasma membrane on the MFG surface are shown by our CLSM examination of unfixed expressed MFG from mice genetically modified to express a fluorescent molecule as a normal plasma membrane constituent.

Xanthine oxidoreductase mediates membrane docking of milk-fat droplets but is not essential for apocrine lipid secretion

KEY POINTS

Xanthine oxidoreductase (XOR) modulates milk lipid secretion and lactation initiation. XOR is required for butyrophilin1a1 clustering in the membrane during milk lipid secretion. XOR mediates apical membrane reorganization during milk lipid secretion. Loss of XOR delays milk fat globule secretion. XOR loss alters the proteome of milk fat globules.

ABSTRACT

Apocrine secretion is utilized by epithelial cells of exocrine glands. These cells bud off membrane-bound particles into the lumen of the gland, losing a portion of the cytoplasm in the secretion product. The lactating mammary gland secretes milk lipid by this mechanism, and xanthine oxidoreductase (XOR) has long been thought to be functionally important. We generated mammary-specific XOR knockout (MGKO) mice, expecting lactation to fail. Histology of the knockout glands showed very large lipid droplets enclosed in the mammary alveolar cells, but milk analysis showed that these large globules were secreted. Butyrophilin, a membrane protein known to bind to XOR, was clustered at the point of contact of the cytoplasmic lipid droplet with the apical plasma membrane, in the wild-type gland but not in the knockout, suggesting that XOR mediates 'docking' to this membrane. Secreted milk fat globules were isolated from mouse milk of wild-type and XOR MGKO dams, and subjected to LC-MS/MS for analysis of protein component. Proteomic results showed that loss of XOR leads to an increase in cytoplasmic, cytoskeletal, Golgi apparatus and lipid metabolism proteins associated with the secreted milk fat globule. Association of XOR with the lipid droplet results in membrane docking and more efficient retention of cytoplasmic components by the secretory cell. Loss of XOR then results in a reversion to a more rudimentary, less efficient, apocrine secretion mechanism, but does not prevent milk fat globule secretion.

Xanthine oxidoreductase in cancer: more than a differentiation marker

Human xanthine oxidoreductase (XOR) catalyzes the last two steps of purine catabolism and is present in two interconvertible forms, which may utilize O2 or NAD(+) as electron acceptors. In addition to uric acid, XOR products may comprise reactive oxygen and nitrogen species that have many biologic effects, including inflammation, endothelial dysfunction, and cytotoxicity, as well as mutagenesis and induction of proliferation. XOR is strictly modulated at the transcriptional and post-translational levels, and its expression and activity are highly variable in cancer. Xanthine oxidoreductase (XOR) expression has been negatively associated with a high malignity grade and a worse prognosis in neoplasms of the breast, liver, gastrointestinal tract, and kidney, which normally express a high level of XOR protein. However, the level of XOR expression may be associated with a worse outcome in cancer of low XOR-expressing cells, in relation to the inflammatory response elicited through the tissue damage induced by tumor growth. Xanthine oxidoreductase (XOR) has been implicated in the process of oncogenesis either directly because it is able to catalyze the metabolic activation of carcinogenic substances or indirectly through the action of XOR-derived reactive oxygen and nitrogen species. The role of uric acid is characterized by both oxidant and antioxidant action; thus, it is still debatable whether control of uricemia may be helpful to improve the outcomes of tumor illness.

XBP1 Regulates the Biosynthetic Capacity of the Mammary Gland During Lactation by Controlling Epithelial Expansion and Endoplasmic Reticulum Formation

Cells composing the mammary secretory compartment have evolved a high capacity to secrete not only proteins but also triglycerides and carbohydrates. This feature is illustrated by the mouse, which can secrete nearly twice its own weight in milk proteins, triglycerides and lactose over a short 20-day lactation. The coordination of synthesis and export of products in other secretory cells is orchestrated in part by the transcription factor X-box binding protein 1 (XBP1). To assess the role of XBP1 in mammary epithelial cells (MEC), we studied floxed XBP1 female mice lacking (wild type; WT) or expressing the Cre recombinase under the control of the ovine β-lactoglobulin promoter (ΔXBP1(MEC)). Pregnant ΔXBP1(MEC) females had morphologically normal mammary development and gave birth to the same number of pups as WT mice. Their litters, however, suffered a weight gain deficit by lactation day 3 (L3)3 that grew to 80% by L14. ΔXBP1(MEC) dams had only modest changes in milk composition (-21% protein, +24% triglyceride) and in the expression of associated genes in isolated MEC. By L5, WT glands were fully occupied by dilated alveoli, whereas ΔXBP1(MEC) glands contained fewer, mostly unfilled alveoli and retained a prominent adipocyte population. The smaller epithelial compartment in ΔXBP1(MEC) glands was explained by lower MEC proliferation and increased apoptosis. Finally, endoplasmic reticulum ribbons were less abundant in ΔXBP1(MEC) at pregnancy day 18 and failed to increase in abundance by L5. Collectively, these results show that XBP1 is required for MEC population expansion during lactation and its ability to develop an elaborate endoplasmic reticulum compartment.

Redox modulation of adipocyte differentiation: hypothesis of "Redox Chain" and novel insights into intervention of adipogenesis and obesity

In view of the global prevalence of obesity and obesity-associated disorders, it is important to clearly understand how adipose tissue forms. Accumulating data from various laboratories implicate that redox status is closely associated with energy metabolism. Thus, biochemical regulation of the redox system may be an attractive alternative for the treatment of obesity-related disorders. In this work, we will review the current data detailing the role of the redox system in adipocyte differentiation, as well as identifying areas for further research. The redox system affects adipogenic differentiation in an extensive way. We propose that there is a complex and interactive "redox chain," consisting of a "ROS-generating enzyme chain," "combined antioxidant chain," and "transcription factor chain," which contributes to fine-tune the regulation of ROS level and subsequent biological consequences. The roles of the redox system in adipocyte differentiation are paradoxical. The redox system exerts a "tridimensional" mechanism in the regulation of adipocyte differentiation, including transcriptional, epigenetic, and posttranslational modulations. We suggest that redoxomic techniques should be extensively applied to understand the biological effects of redox alterations in a more integrated way. A stable and standardized "redox index" is urgently needed for the evaluation of the general redox status. Therefore, more effort should be made to establish and maintain a general redox balance rather than to conduct simple prooxidant or antioxidant interventions, which have comprehensive implications.

Immunocytochemical Evidence for Golgi Vesicle Involvement in Milk Fat Globule Secretion

The exact mechanism of secretion of the milk fat globule (MFG) from the mammary secretory cell is still controversial. We have previously suggested close involvement of Golgi vesicles in this process. This paper provides direct immunocytochemical evidence that butyrophilin is present in the Golgi stack and vesicles in ovine and caprine mammary glands. We suggest that it is the butyrophilin in the Golgi vesicle membrane that forms the specific association with the adipophilin on the lipid surface in the cytoplasm. Exocytosis of the associated Golgi vesicle will then initiate the process of MFG secretion. Further exocytosis of associated Golgi vesicles will continue and complete the process. Areas of the plasmalemma that have butyrophilin delivered by previous non-lipid associated Golgi exocytoses may also contribute to the process of forming the milk fat globule membrane (MFGM).

Milk lipid composition is modified by perinatal exposure to bisphenol A

To evaluate whether bisphenol A (BPA) modifies the synthesis, composition and/or profile of fatty acids (FAs) in the mammary glands of perinatally exposed animals, pregnant rats were orally exposed to 0, 0.6 or 52 µg BPA/kg/day from gestation day (GD) 9 until weaning. F1 females were bred, and on GD21, lactation day 2 (LD2) and LD10, mammary glands were obtained. On LD10, milk samples were collected, and FA profiles and lipid compositions were established. On GD21 and LD2, BPA exposure delayed mammary alveolar maturation and modified the synthesis of milk fat globules. On LD10, mammary gland histo-architecture was restored; however, the milk of BPA-exposed F1 dams had a FA profile and lipid concentration different from those of the control milk. Furthermore, the body weight gain of BPA52 F2 pups was increased compared with control animals. Thus, perinatal exposure to BPA modifies milk quality, compromising the normal growth of offspring.

Proteomics-driven analysis of ovine whey colostrum

The aim of this study was to shed light in to the complexity of the ovine colostrum proteome, with a specific focus on the low abundance proteins. The ovine colostrum is characterized by a few dominating proteins, as the immunoglobulins, but it also contains less represented protein species, equally important for the correct development of neonates. Ovine colostrum, collected immediately after lambing, was separated by 1D SDS-PAGE. Proteins bands were digested with trypsin and the resulting peptides were analyzed by LC-MS/MS. On the basis of the Swiss-Prot database, a total of 343 unique proteins were identified. To our knowledge, this study represents the most comprehensive analysis of ovine colostrum proteome.

Cholesterol transport and regulation in the mammary gland

The milk-producing alveolar epithelial cells secrete milk that remains after birth the principal source of nutrients for neonates. Milk secretion and composition are highly regulated processes via integrated actions of hormones and local factors which involve specific receptors and downstream signal transduction pathways. Overall milk composition is similar among mammalian species, although the content of individual constituents such as lipids may significantly differ from one species to another. The milk lipid fraction is essentially composed of triglycerides, which represent more than 95 % of the total lipids in human and commercialized bovine milk. Though sterols, including cholesterol, which is the major milk sterol, represent less than 0.5 % of the total milk lipid fraction, they are of key importance for several biological processes. Cholesterol is required for the formation of biological membranes especially in rapidly growing organisms, and for the synthesis of sterol-based compounds. Cholesterol found in milk originates predominantly from blood uptake and, to a certain extent, from local synthesis in the mammary tissue. The present review summarizes current knowledge on cellular mechanisms and regulatory processes determining intra- and transcellular cholesterol transport in the mammary gland. Cholesterol exchanges between the blood, the mammary alveolar cells and the milk, and the likely role of active cholesterol transporters in these processes are discussed. In this context, the hormonal regulation and signal transduction pathways promoting active cholesterol transport as well as potential regulatory crosstalks are highlighted.

Structural and functional insights into the catalytic inactivity of the major fraction of buffalo milk xanthine oxidoreductase

Background

Xanthine oxidoreductase (XOR) existing in two interconvertible forms, xanthine dehydrogenase (XDH) and xanthine oxidase (XO), catabolises xanthine to uric acid that is further broken down to antioxidative agent allantoin. XOR also produces free radicals serving as second messenger and microbicidal agent. Large variation in the XO activity has been observed among various species. Both hypo and hyper activity of XOR leads to pathophysiological conditions. Given the important nutritional role of buffalo milk in human health especially in south Asia, it is crucial to understand the functional properties of buffalo XOR and the underlying structural basis of variations in comparison to other species.

Methods and Findings

Buffalo XO activity of 0.75 U/mg was almost half of cattle XO activity. Enzymatic efficiency (k_cat/K_m) of 0.11 sec⁻¹ µM⁻¹ of buffalo XO was 8–10 times smaller than that of cattle XO. Buffalo XOR also showed lower antibacterial activity than cattle XOR. A CD value (Δε_{430 nm}) of 46,000 M⁻¹ cm⁻¹ suggested occupancy of 77.4% at Fe/S I centre. Buffalo XOR contained 0.31 molybdenum atom/subunit of which 48% existed in active sulfo form. The active form of XO in buffalo was only 16% in comparison to ∼30% in cattle. Sequencing revealed 97.4% similarity between buffalo and cattle XOR. FAD domain was least conserved, while metal binding domains (Fe/S and Molybdenum) were highly conserved. Homology modelling of buffalo XOR showed several variations occurring in clusters, especially close to FAD binding pocket which could affect NAD⁺ entry in the FAD centre. The difference in XO activity seems to be originating from cofactor deficiency, especially molybdenum.

Conclusion

A major fraction of buffalo milk XOR exists in a catalytically inactive form due to high content of demolybdo and desulfo forms. Lower Fe/S content and structural factors might be contributing to lower enzymatic efficiency of buffalo XOR in a minor way.

Mice heterozygous for the xanthine oxidoreductase gene facilitate lipid accumulation in adipocytes

OBJECTIVE

Xanthine oxidoreductase (XOR) catalyzes the production of uric acid with concomitant generation of reactive oxygen species. XOR has been shown to regulate adipogenesis through the control of peroxisome proliferator-activated receptor γ, but its role in adipose tissue remains unclear. The aim of this study was to examine the role of XOR in adipose tissue using XOR genetically modified mice.

APPROACH AND RESULTS

Experiments were performed using 2-, 4-, and 18-month-old XOR heterozygous mice (XOR(+/-)) and their wild-type littermates to evaluate the physiological role of XOR as the mice aged. Stromal vascular fraction cells were prepared from epididymal white adipose tissue in 2-month-old XOR mice to assess adipogenesis. At 18 months, XOR(+/)- mice had significantly higher body weight, higher systolic blood pressure, and higher incidence of insulin resistance compared with wild-type mice. At 4 months, blood glucose and the expressions of CCAAT enhancer-binding protein β, peroxisome proliferator-activated receptor γ, monocyte chemoattractant protein-1, and tumor necrosis factor α mRNA in epididymal white adipose tissue were significantly higher in XOR(+/-) than in wild-type mice. Furthermore, histological analysis of epididymal white adipose tissue in XOR(+/-) mice revealed that adipocyte size and the F4/80-positive macrophage count were increased. Experiments with a high-fat diet exhibited that body weight gain was also significantly higher in XOR(+/-) than in wild-type mice. In stromal vascular fraction cells derived from XOR(+/-) mice, the levels of peroxisome proliferator-activated receptor γ, fatty acid-binding protein 4, and CCAAT enhancer-binding protein α mRNA were upregulated, and oxidative stress levels were elevated during differentiation into adipocytes.

CONCLUSIONS

These results suggest that the reduction in XOR gene expression in mice augments lipid accumulation in adipocytes, accompanied by an increase in oxidative stress, and induces obesity with insulin resistance in older age.

A test of current models for the mechanism of milk-lipid droplet secretion

Milk lipid is secreted by a unique process, during which triacylglycerol droplets bud from mammary cells coated with an outer bilayer of apical membrane. In all current schemes, the integral protein butyrophilin 1A1 (BTN) is postulated to serve as a transmembrane scaffold, which interacts either with itself or with the peripheral proteins, xanthine oxidoreductase (XOR) and possibly perilipin-2 (PLIN2), to form an immobile bridging complex between the droplet and apical surface. In one such scheme, BTN on the surface of cytoplasmic lipid droplets interacts directly with BTN in the apical membrane without binding to either XOR or PLIN2. We tested these models using both biochemical and morphological approaches. BTN was concentrated in the apical membrane in all species examined and contained mature N-linked glycans. We found no evidence for the association of unprocessed BTN with intracellular lipid droplets. BTN-enhanced green fluorescent protein was highly mobile in areas of mouse milk-lipid droplets that had not undergone post-secretion changes, and endogenous mouse BTN comprised only 0.5-0.7% (w/w) of the total protein, i.e. over 50-fold less than in the milk-lipid droplets of cow and other species. These data are incompatible with models of milk-lipid secretion in which BTN is the major component of an immobile global adhesive complex and suggest that interactions between BTN and other proteins at the time of secretion are more transient than previously predicted. The high mobility of BTN in lipid droplets marks it as a potential mobile signaling molecule in milk.

Lipid droplet accumulation is associated with an increase in hyperglycemia-induced renal damage: prevention by liver X receptors

Dyslipidemia is a frequent component of the metabolic disorder of diabetic patients contributing to organ damage. Herein, in low-density lipoprotein receptor-deficient hyperlipidemic and streptozotozin-induced diabetic mice, hyperglycemia and hyperlipidemia acted reciprocally, accentuating renal injury and altering renal function. In hyperglycemic-hyperlipidemic kidneys, the accumulation of Tip47-positive lipid droplets in glomeruli, tubular epithelia, and macrophages was accompanied by the concomitant presence of the oxidative stress markers xanthine oxidoreductase and nitrotyrosine, findings that could also be evidenced in renal biopsy samples of diabetic patients. As liver X receptors (LXRα,β) regulate genes linked to lipid and carbohydrate homeostasis and inhibit inflammatory gene expression in macrophages, the effects of systemic and macrophage-specific LXR activation were analyzed on renal damage in hyperlipidemic-hyperglycemic mice. LXR stimulation by GW3965 up-regulated genes involved in cholesterol efflux and down-regulated proinflammatory/profibrotic cytokines, inhibiting the pathomorphology of diabetic nephropathy, renal lipid accumulation, and improving renal function. Xanthine oxidoreductase and nitrotyrosine levels were reduced. In macrophages, GW3965 or LXRα overexpression significantly suppressed glycated or acetylated low-density lipoprotein-induced cytokines and reactive oxygen species. Specifically, in mice, transgenic expression of LXRα in macrophages significantly ameliorated hyperlipidemic-hyperglycemic nephropathy. The results demonstrate the presence of lipid droplet-induced oxidative mechanisms and the pathophysiologic role of macrophages in diabetic kidneys and indicate the potent regulatory role of LXRs in preventing renal damage in diabetes.

Milk lipid secretion: recent biomolecular aspects

Neonates of most species depend on milk lipids for calories, fat-soluble vitamins, and bioactive lipid components for growth and development during the postnatal period. To meet neonatal nutrition and development needs, the mammary gland has evolved efficient mechanisms for synthesizing and secreting large quantities of lipid during lactation. Although the biochemical steps involved in milk lipid synthesis are understood, the identities of the genes mediating these steps and the molecular physiology of milk lipid production and secretion have only recently begun to be understood in detail through advances in mouse genetics, gene expression analysis, protein structural properties, and the cell biology of lipid metabolism. This review discusses emerging data about the molecular, cellular, and structural determinants of milk lipid synthesis and secretion within the context of physiological functions.

Mutations associated with functional disorder of xanthine oxidoreductase and hereditary xanthinuria in humans

Xanthine oxidoreductase (XOR) catalyzes the conversion of hypoxanthine to xanthine and xanthine to uric acid with concomitant reduction of either NAD⁺ or O₂. The enzyme is a target of drugs to treat hyperuricemia, gout and reactive oxygen-related diseases. Human diseases associated with genetically determined dysfunction of XOR are termed xanthinuria, because of the excretion of xanthine in urine. Xanthinuria is classified into two subtypes, type I and type II. Type I xanthinuria involves XOR deficiency due to genetic defect of XOR, whereas type II xanthinuria involves dual deficiency of XOR and aldehyde oxidase (AO, a molybdoflavo enzyme similar to XOR) due to genetic defect in the molybdenum cofactor sulfurase. Molybdenum cofactor deficiency is associated with triple deficiency of XOR, AO and sulfite oxidase, due to defective synthesis of molybdopterin, which is a precursor of molybdenum cofactor for all three enzymes. The present review focuses on mutation or chemical modification studies of mammalian XOR, as well as on XOR mutations identified in humans, aimed at understanding the reaction mechanism of XOR and the relevance of mutated XORs as models to estimate the possible side effects of clinical application of XOR inhibitors.

The evolution of milk secretion and its ancient origins

Lactation represents an important element of the life history strategies of all mammals, whether monotreme, marsupial, or eutherian. Milk originated as a glandular skin secretion in synapsids (the lineage ancestral to mammals), perhaps as early as the Pennsylvanian period, that is, approximately 310 million years ago (mya). Early synapsids laid eggs with parchment-like shells intolerant of desiccation and apparently dependent on glandular skin secretions for moisture. Mammary glands probably evolved from apocrine-like glands that combined multiple modes of secretion and developed in association with hair follicles. Comparative analyses of the evolutionary origin of milk constituents support a scenario in which these secretions evolved into a nutrient-rich milk long before mammals arose. A variety of antimicrobial and secretory constituents were co-opted into novel roles related to nutrition of the young. Secretory calcium-binding phosphoproteins may originally have had a role in calcium delivery to eggs; however, by evolving into large, complex casein micelles, they took on an important role in transport of amino acids, calcium and phosphorus. Several proteins involved in immunity, including an ancestral butyrophilin and xanthine oxidoreductase, were incorporated into a novel membrane-bound lipid droplet (the milk fat globule) that became a primary mode of energy transfer. An ancestral c-lysozyme lost its lytic functions in favor of a role as α-lactalbumin, which modifies a galactosyltransferase to recognize glucose as an acceptor, leading to the synthesis of novel milk sugars, of which free oligosaccharides may have predated free lactose. An ancestral lipocalin and an ancestral whey acidic protein four-disulphide core protein apparently lost their original transport and antimicrobial functions when they became the whey proteins β-lactoglobulin and whey acidic protein, which with α-lactalbumin provide limiting sulfur amino acids to the young. By the late Triassic period (ca 210 mya), mammaliaforms (mammalian ancestors) were endothermic (requiring fluid to replace incubatory water losses of eggs), very small in size (making large eggs impossible), and had rapid growth and limited tooth replacement (indicating delayed onset of feeding and reliance on milk). Thus, milk had already supplanted egg yolk as the primary nutrient source, and by the Jurassic period (ca 170 mya) vitellogenin genes were being lost. All primary milk constituents evolved before the appearance of mammals, and some constituents may have origins that predate the split of the synapsids from sauropsids (the lineage leading to 'reptiles' and birds). Thus, the modern dairy industry is built upon a very old foundation, the cornerstones of which were laid even before dinosaurs ruled the earth in the Jurassic and Cretaceous periods.

Curcumin decreases renal triglyceride accumulation through AMPK-SREBP signaling pathway in streptozotocin-induced type 1 diabetic rats

Diabetic kidney disease has been associated with the presence of lipid deposits. We assumed that curcumin, a polyphenol, would attenuate the tissue dyslipidemic condition through activation of 5' adenosine monophosphate (AMP)-activated protein kinase (AMPK) phosphorylation and suppression of sterol regulatory element-binding protein (SREBP)-1c in the kidney and would prevent renal progression in experimental type 1 diabetic rats. Diabetes was induced with streptozotocin (STZ) (55 mg/kg) by intraperitoneal injection in male Sprague-Dawley rats. Three weeks after STZ injection, rats were divided into three groups, namely, control, diabetic and diabetic treated with curcumin (100 mg/kg/day) by gavage for 8 weeks. We found that curcumin decreased plasma triglyceride and the amount of renal triglyceride significantly. Furthermore, treatment of diabetic rats with curcumin increased the phosphorylation of AMPK and prevented the increased renal expression of SREBP-1c and, as a result, decreased the expression of acetyl CoA carboxylase and fatty acid synthase as well as adipose differentiation-related protein, a marker of cytoplasmic droplets. We also demonstrate that curcumin significantly suppressed the increased expression of transforming growth factor β, vascular endothelial growth factor and extracellular matrix proteins such as type IV collagen and fibronectin. In addition, curcumin treatment increased nephrin expression to near-normal levels in diabetic rats. These results demonstrated that curcumin protects against the development of diabetic nephropathy through the AMPK-SREBP pathway and the reduction of renal triglyceride accumulation which could be a possible mechanism by which curcumin preserves renal function in diabetes.

Cidea is an essential transcriptional coactivator regulating mammary gland secretion of milk lipids

Adequate lipid secretion by mammary glands during lactation is essential for the survival of mammalian offspring. However, the mechanism governing this process is poorly understood. Here we show that Cidea is expressed at high levels in lactating mammary glands and its deficiency leads to premature pup death as a result of severely reduced milk lipids. Furthermore, the expression of xanthine oxidoreductase (XOR), an essential factor for milk lipid secretion, is markedly lower in Cidea-deficient mammary glands. Conversely, ectopic Cidea expression induces the expression of XOR and enhances lipid secretion in vivo. Unexpectedly, as Cidea has heretofore been thought of as a cytoplasmic protein, we detected it in the nucleus and found it to physically interact with transcription factor CCAAT/enhancer-binding protein β (C/EBPβ) in mammary epithelial cells. We also observed that Cidea induces XOR expression by promoting the association of C/EBPβ onto, and the dissociation of HDAC1 from, the promoter of the Xdh gene encoding XOR. Finally, we found that Fsp27, another CIDE family protein, is detected in the nucleus and interacts with C/EBPβ to regulate expression of a subset of C/EBPβ downstream genes in adipocytes. Thus, Cidea acts as a previously unknown transcriptional coactivator of C/EBPβ in mammary glands to control lipid secretion and pup survival.

Decreased xanthine oxidoreductase (XOR) is associated with a worse prognosis in patients with serous ovarian carcinoma

OBJECTIVE

Xanthine oxidoreductase (XOR) is a key enzyme in the degradation of DNA, RNA and high-energy phosphates. In the human cancers previously studied, down-regulated XOR identifies patients with unfavorable prognosis. We assessed the clinical relevance of XOR expression in serous ovarian cancer.

METHODS

XOR protein was determined in tissue microarrays from 474 patients with serous ovarian cancer and analyzed with respect to clinical parameters and survival.

RESULTS

XOR was down regulated in 64% of the tumors as compared to the corresponding normal tissue. Decreased XOR was associated with a poorly differentiated tumor and an abnormal p53 expression, but not with age at diagnosis, FIGO stage, Ki-67 or tumor size. XOR expression was associated with outcome, and the five year ovarian cancer specific survival in patients with strong XOR expression was 59% compared to 44% in those with moderate (hazard ratio, HR; 1.44; P=0.0083) and 26% in patients with lack of XOR (HR, 2.07; P=0.0003). This was also true in patients whose tumors were highly differentiated (HR, 3.67; P=0.008) and in patients with a small (<1cm) residual tumor (HR, 2.62; P=0.017), and in patients whose tumors show a low Ki-67 protein expression (HR, 3.79; P<0.0001). In multivariate survival analysis, absence of XOR emerged as an independent prognostic factor (HR, 1.82; P=0.015).

CONCLUSIONS

Decreased XOR is associated with poorer prognosis in patients with serous ovarian cancer especially in those with an otherwise more favorable prognostic profile.

Functional genomic screen and network analysis reveal novel modifiers of tauopathy dissociated from tau phosphorylation

A functional genetic screen using loss-of-function and gain-of-function alleles was performed to identify modifiers of tau-induced neurotoxicity using the 2N/4R (full-length) isoform of wild-type human tau expressed in the fly retina. We previously reported eye pigment mutations, which create dysfunctional lysosomes, as potent modifiers; here, we report 37 additional genes identified from ∼1900 genes screened, including the kinases shaggy/GSK-3beta, par-1/MARK, CamKI and Mekk1. Tau acts synergistically with Mekk1 and p38 to down-regulate extracellular regulated kinase activity, with a corresponding decrease in AT8 immunoreactivity (pS202/T205), suggesting that tau can participate in signaling pathways to regulate its own kinases. Modifiers showed poor correlation with tau phosphorylation (using the AT8, 12E8 and AT270 epitopes); moreover, tested suppressors of wild-type tau were equally effective in suppressing toxicity of a phosphorylation-resistant S11A tau construct, demonstrating that changes in tau phosphorylation state are not required to suppress or enhance its toxicity. Genes related to autophagy, the cell cycle, RNA-associated proteins and chromatin-binding proteins constitute a large percentage of identified modifiers. Other functional categories identified include mitochondrial proteins, lipid trafficking, Golgi proteins, kinesins and dynein and the Hsp70/Hsp90-organizing protein (Hop). Network analysis uncovered several other genes highly associated with the functional modifiers, including genes related to the PI3K, Notch, BMP/TGF-β and Hedgehog pathways, and nuclear trafficking. Activity of GSK-3β is strongly upregulated due to TDP-43 expression, and reduced GSK-3β dosage is also a common suppressor of Aβ42 and TDP-43 toxicity. These findings suggest therapeutic targets other than mitigation of tau phosphorylation.

Filter-aided sample preparation with dimethyl labeling to identify and quantify milk fat globule membrane proteins

Bovine milk is a major nutrient source in many countries and it is produced at an industrial scale. Milk is a complex mixture of proteins, fats, carbohydrates, vitamins and minerals. The composition of the bovine milk samples can vary depending on the genetic makeup of the bovine species as well as environmental factors. It is therefore important to study the qualitative and quantitative differences of bovine milk samples. Proteins in milk can be present in casein micelles, in the serum (the water soluble fraction) or in fat globules. These fat globules have a double membrane layer with proteins being bound to or being incapsulated in the membrane layer. The identification and molecular composition of the milk proteins have gained increased interest in recent years. Proteomic techniques make it now possible to identify up to many thousands of proteins in one sample, however quantification of proteins is as yet not straightforward. We analyzed the proteins of the milk fat globule membrane using dimethyl labeling methods combined with a filter-aided sample preparation protocol. Using these methods, it is now possible to quantitatively study the detailed protein composition of many milk samples in a short period of time.

Effect of the level and type of starchy concentrate on tissue lipid metabolism, gene expression and milk fatty acid secretion in Alpine goats receiving a diet rich in sunflower-seed oil

The potential benefits on human health have prompted an interest in developing nutritional strategies for reducing saturated and increasing specific unsaturated fatty acids (FA) in ruminant milk. The impact of the level and type of starchy concentrate added to diets supplemented with sunflower-seed oil on caprine milk FA composition and on mammary, omental and perirenal adipose, and liver lipid metabolism was examined in fourteen Alpine goats in a replicated 3 × 3 Latin square with 21 d experimental periods. Treatments were a grass hay-based diet with a high level of forage (F) or a high level of concentrate with either maize grain (CM) or flattened wheat (CW) as source of starch and supplemented with 130 g/d sunflower-seed oil. Milk yield was enhanced (P < 0·01) and milk fat content was decreased on the CM and CW diets compared with the F diet, resulting in similar milk fat secretion. Both high-concentrate diets increased (P < 0·05) milk yield of 10 : 0-16 : 0 and decreased trans-9,11-18 : 1 and cis-9, trans-11-18 : 2. The CW diet decreased (P < 0·05) the output of Σ C18 and Σ cis-18 : 1 and increased (P < 0·05) the output of trans-10-18 : 1 in milk. The expression and/or activity of fourteen proteins involved in the major lipogenic pathways in mammary tissues and of lipogenic genes in adipose and liver tissues were similar among treatments. In conclusion, high starch concentrates alter milk FA yield via mechanisms independent of changes in mammary, liver or adipose tissue lipogenic gene expression. Furthermore, data provided indications that mammary lipogenic responses to starch-rich diets differ between caprine and bovine ruminants.

Adipophilin regulates maturation of cytoplasmic lipid droplets and alveolae in differentiating mammary glands

Milk lipids originate by secretion of triglyceride-rich cytoplasmic lipid droplets (CLDs) from mammary epithelial cells. Adipophilin (ADPH)/Plin2, a member of the perilipin family of CLD binding proteins, is hypothesized to regulate CLD production in these cells during differentiation of the mammary gland into a secretory organ. We tested this hypothesis by comparing CLD accumulation in differentiating mammary glands of wild-type and ADPH-deficient mice. ADPH deficiency did not prevent CLD formation; however, it disrupted the increase in CLD size that normally occurs in differentiating mammary epithelial cells. Failure to form large CLDs in ADPH-deficient mice correlated with localization of adipose triglyceride lipase (ATGL) to the CLD surface, suggesting that ADPH promotes CLD growth by inhibiting lipolytic activity. Significantly, mammary alveoli also failed to mature in ADPH-deficient mice, and pups born to these mice failed to survive. The possibility that CLD accumulation and alveolar maturation defects in ADPH-deficient mice are functionally related was tested by in vivo rescue experiments. Transduction of mammary glands of pregnant ADPH-deficient mice with adenovirus encoding ADPH as an N-terminal GFP fusion protein prevented ATGL from localizing to CLDs and rescued CLD size and alveolar maturation defects. Collectively, these data provide direct in vivo evidence that ADPH inhibition of ATGL-dependent lipolysis is required for normal CLD accumulation and alveolar maturation during mammary gland differentiation. We speculate that impairing CLD accumulation interferes with alveolar maturation and lactation by disrupting triglyceride homeostasis in mammary epithelial cells.

Contribution of xanthine oxidoreductase to mammary epithelial and breast cancer cell differentiation in part modulates inhibitor of differentiation-1

Loss of xanthine oxidoreductase (XOR) has been linked to aggressive breast cancer in vivo and to breast cancer cell aggressiveness in vitro. In the present study, we hypothesized that the contribution of XOR to the development of the normal mammary gland may underlie its capacity to modulate breast cancer. We contrasted in vitro and in vivo developmental systems by differentiation marker and microarray analyses. Human breast cancer microarray was used for clinical outcome studies. The role of XOR in differentiation and proliferation was examined in human breast cancer cells and in a mouse xenograft model. Our data show that XOR was required for functional differentiation of mammary epithelial cells both in vitro and in vivo. Poor XOR expression was observed in a mouse ErbB2 breast cancer model, and pharmacologic inhibition of XOR increased breast cancer tumor burden in mouse xenograft. mRNA microarray analysis of human breast cancer revealed that low XOR expression was significantly associated with time to tumor relapse. The opposing expression of XOR and inhibitor of differentiation-1 (Id1) during HC11 differentiation and mammary gland development suggested a potential functional relationship. While overexpression of Id1 inhibited HC11 differentiation and XOR expression, XOR itself modulated expression of Id1 in differentiating HC11 cells. Overexpression of XOR both inhibited Id1-induced proliferation and -stimulated differentiation of Heregulin-β1-treated human breast cancer cells. These results show that XOR is an important functional component of differentiation whose diminished expression contributes to breast cancer aggressiveness, and they support XOR as both a breast cancer biomarker and a target for pharmacologic activation in therapeutic management of aggressive breast cancer.

Determinants of adipophilin function in milk lipid formation and secretion

In many species the lactating mammary gland is one of the most lipogenic organs of the body. The majority of the lipid produced during lactation is secreted into milk by a novel process of membrane envelopment of cytoplasmic lipid droplets (CLDs). Adipophilin (ADRP/ADPH/PLIN2), a member of the perilipin (PAT) family of lipid droplet proteins, is hypothesized to play a pivotal role in both formation and secretion of milk lipids. Production of milk lipids is the only known example of CLD secretion, and the only process in which PAT family members undergo secretion. This review discusses emerging data on the structural and functional properties of adipophilin that determine its physiological actions and mediate its effects on milk lipid formation and secretion.

Comparative Genomics and Evolution of Molybdenum Utilization

The trace element molybdenum (Mo) is the catalytic component of important enzymes involved in global nitrogen, sulfur, and carbon metabolism in both prokaryotes and eukaryotes. With the exception of nitrogenase, Mo is complexed by a pterin compound thus forming the biologically active molybdenum cofactor (Moco) at the catalytic sites of molybdoenzymes. The physiological roles and biochemical functions of many molybdoenzymes have been characterized. However, our understanding of the occurrence and evolution of Mo utilization is limited. This article focuses on recent advances in comparative genomics of Mo utilization in the three domains of life. We begin with a brief introduction of Mo transport systems, the Moco biosynthesis pathway, the role of posttranslational modifications, and enzymes that utilize Mo. Then, we proceed to recent computational and comparative genomics studies of Mo utilization, including a discussion on novel Moco-binding proteins that contain the C-terminal domain of the Moco sulfurase and that are suggested to represent a new family of molybdoenzymes. As most molybdoenzymes need additional cofactors for their catalytic activity, we also discuss interactions between Mo metabolism and other trace elements and finish with an analysis of factors that may influence evolution of Mo utilization.