Isolation of adipophilin and butyrophilin from bovine milk and characterization of a cDNA encoding adipophilin

Molecular mechanisms of perilipin protein function in lipid droplet metabolism

Perilipins are abundant lipid droplet (LD) proteins present in all metazoans and also in Amoebozoa and fungi. Humans express five perilipins, which share a similar domain organization: an amino-terminal PAT domain and an 11-mer repeat region, which can fold into amphipathic helices that interact with LDs, followed by a structured carboxy-terminal domain. Variations of this organization that arose during vertebrate evolution allow for functional specialization between perilipins in relation to the metabolic needs of different tissues. We discuss how different features of perilipins influence their interaction with LDs and their cellular targeting. PLIN1 and PLIN5 play a direct role in lipolysis by regulating the recruitment of lipases to LDs and LD interaction with mitochondria. Other perilipins, particularly PLIN2, appear to protect LDs from lipolysis, but the molecular mechanism is not clear. PLIN4 stands out with its long repetitive region, whereas PLIN3 is most widely expressed and is used as a nascent LD marker. Finally, we discuss the genetic variability in perilipins in connection with metabolic disease, prominent for PLIN1 and PLIN4, underlying the importance of understanding the molecular function of perilipins.

Variation in milk fat globule size and composition: A source of bioactives for human health

Milk fat globules (MFGs) are secreted from the mammalian gland and are composed of a triacylglycerol core surrounded by a triple membrane structure, the milk fat globule membrane (MFGM). The MFGM contains complex lipids and proteins reported to have nutritional, immunological, neurological and digestive functions. Human and ruminant milk are shown to share a similar MFG structure but with different size, profile and abundance of protein and polar lipids. This review summarizes the reported data on human, bovine, caprine and ovine MFG composition and concentration of bioactive components in different MFG-size fractions. A comprehensive understanding of compositional variations between milk from different species and MFG size fractions may help promote various milk sources as targeted supplements to improve human development and health. MFG size and MFGM composition are species-specific and affected by lactation, diet and breed (or maternal origin). Purification and enrichment methods for some bioactive proteins and lipids present in the MFGM have yet to be established or are not scaled sufficiently to be used to supplement human diets. To overcome this problem, MFG size selection through fractionation or herd selection may provide a convenient way to pre-enrich the MFG fraction with specific protein and lipid components to fulfill human dietary and health requirements.

Insertion of perilipin 3 into a glycero(phospho)lipid monolayer depends on lipid headgroup and acyl chain species

Lipid droplets (LDs) are organelles that contribute to various cellular functions that are vital for life. Aside from acting as a neutral lipid storage depot, they are also involved in building new membranes, synthesis of steroid hormones, and cell signaling. Many aspects of LD structure and function are not yet well-understood. Here we investigate the interaction of perilipin 3, a member of the perilipin family of LD binding proteins, and three N-terminal truncation mutants with lipid monolayers. The interaction is studied as a function of surface pressure for a series of systematically chosen lipids. We find that the C terminus of perilipin 3 has different insertion behavior from that of the longer truncation mutants and the full-length protein. Inclusion of N-terminal sequences with the C terminus decreases the ability of the protein construct to insert in lipid monolayers. Coupling of anionic lipids to negative spontaneous curvature facilitates protein interaction and insertion. The C terminus shows strong preference for lipids with more saturated fatty acids. This work sheds light on the LD binding properties and function of the different domains of perilipin 3.

Development of high-sensitive ELISA method for detection of adipophilin levels in human colostrum and breast milk

AIM

To develop and validate high-sensitive (hs) ELISA method for detection of adipophilin (adipose differentiation-related protein, ADRP) in human breast milk (BM) and to analyze adipophilin levels in BM during 12 months of lactation.

METHODS

ADRP levels were determined using hsELISA method (Biovendor-Laboratory Medicine, Inc.) in colostrum (D0) and BM of 72 mothers was collected 1, 3, 6, and 12 months following delivery (M1, 3, 6, 12).

RESULTS

ADRP was detectable in BM up to 12 months of lactation. Mean levels at D0 were 1.98 ± 0.12; M1, 2.83 ± 0.21; M3, 2.39 ± 0.17; M6, 2.57 ± 0.16; and at M12 3.25 ± 0.21 μg/ml. Significantly higher levels of ADRP were found in M1 and M12 when compared to D0 and in M12 when compared to M3 (overall P = 0.0001). No significant correlation was seen between ADRP levels in BM and adiponectin, body weight of infants, their birth length, body weight gain during the first year of life, or BMI of mothers before pregnancy.

CONCLUSIONS

We developed and validated hsELISA for detection of ADRP in human BM. ADRP was detectable in human BM during the whole 12 months of lactation period and its levels were intraindividually well-conserved.

Multiple functions encoded by the N-terminal PAT domain of adipophilin

Adipophilin (ADPH), a member of the perilipin family of cytoplasmic lipid droplet (CLD)-binding proteins, is crucially dependent on triglyceride synthesis for stability. We have used cell lines expressing full-length or N-terminally modified forms of ADPH to investigate the role of the N-terminus in regulating ADPH stability and interactions with CLD. Full-length ADPH was unstable and could not be detected on CLDs unless cultures were incubated with oleic acid (OA) to stimulate triglyceride synthesis, or were treated with MG132 to block proteasomal degradation. By contrast, ADPH lacking amino acids 1-89 (Delta 2,3 ADPH), or N-terminally GFP-tagged full-length ADPH, was stable in the absence of OA or MG132, as was the closely related protein TIP47. However, none of these proteins localized to CLDs unless OA was added to the culture medium. Furthermore, immunofluorescence analysis showed that TIP47 localization to CLDs was prevented by full-length ADPH, but not by Delta 2,3 ADPH. These results suggest that the N-terminal region of ADPH mediates proteasomal degradation and access of TIP47 to the CLD surface and possibly contributes to CLD stability. Chimeras of ADPH and TIP47, generated by swapping their N- and C-terminal halves, showed that these properties are specific to ADPH.

Molecular determinants of milk lipid secretion

Mammary epithelial cells secrete lipids by an envelopment process that produces lipid droplets coated by membranes derived from the plasma membrane and possibly secretory vesicles. This secretion process, which resembles viral budding, is hypothesized to be mediated by specific interactions between molecules on the surface of intracellular lipids and membrane elements of the cell. Multiple lines of evidence indicate that milk lipid secretion occurs through a tripartite complex between the integral transmembrane protein, butyrophilin (BTN); the soluble metabolic enzyme, xanthine oxidoreductase (XOR); and the lipid droplet surface protein, adipophilin (ADPH). However, topological evidence from freeze-fracture replica immunolabelling (FRIL) challenge this model and suggests that milk lipid secretion is mediated by butyrophilin alone. Advances in our understanding of the molecular, structural, and functional properties of these proteins now make it possible to understand the physiological functions of each of these molecules in detail and to identify the specific molecular determinants that mediate milk lipid secretion.

Mammary transcriptome analysis of food-deprived lactating goats highlights genes involved in milk secretion and programmed cell death

Animal nutrition considerably affects milk composition that influences its nutritional quality. Milk component synthesis and secretion by the mammary gland involve expression of a large number of genes whose nutritional regulation remains poorly defined. In this study, we examined the effect of food deprivation (FD) on the expression of 8379 genes in caprine mammary gland using a bovine oligonucleotide microarray. Twelve lactating goats were assigned to 2 groups based on their feeding level (control diet ad libitum vs. 48-h FD). We identified 161 genes whose expression was altered by FD. Most of these genes (88%) were downregulated, suggesting a stress response by the mammary gland. In particular, the decrease in expression of genes involved in milk protein, lactose, and lipid metabolism could contribute together with the shortage of nutrients to the drop in milk protein, lactose, and fat secretion. In addition, this study highlights modification of the expression of at least 14 genes that could be responsible for a slowdown in mammary cell proliferation and differentiation and/or an increase in programmed cell death in response to 48-h FD in goats.

Secretion and fluid transport mechanisms in the mammary gland: comparisons with the exocrine pancreas and the salivary gland

Milk is a complex fluid composed of proteins, sugars, lipids and minerals, in addition to a wide variety of bioactive molecules including vitamins, trace elements and growth factors. The composition of these components reflects the integrated activities of distinct synthetic, secretion and transport processes found in mammary epithelial cells, and mirrors the differing nutritional and developmental requirements of mammalian neonates. Five general pathways have been described for secretion of milk components. With the exception of lipids, which are secreted a unique pathway, milk components are thought to be secreted by adaptations of pathways found in other secretory organs. However little is known about the molecular and cellular mechanisms that constitute these pathways or the physiological mechanisms by which they are regulated. Comparisons of current secretion and transport models in the mammary gland, exocrine pancreas and salivary gland indicate that significant differences exist between the mammary gland and other exocrine organs in how proteins and lipids are packaged and secreted, and how fluid is transported.

Butyrophilin controls milk fat globule secretion

The molecular mechanism underlying milk fat globule secretion in mammary epithelial cells ostensibly involves the formation of complexes between plasma membrane butyrophilin and cytosolic xanthine oxidoreductase. These complexes bind adipophilin in the phospholipid monolayer of milk secretory granules, the precursors of milk fat globules, enveloping the nascent fat globules in a layer of plasma membrane and pinching them off the cell. However, using freeze-fracture immunocytochemistry, we find these proteins in locations other than those previously inferred. Significantly, butyrophilin in the residual plasma membrane of the fat globule envelope is concentrated in a network of ridges that are tightly apposed to the monolayer derived from the secretory granule, and the ridges coincide with butyrophilin labeling in the globule monolayer. Therefore, we propose that milk fat globule secretion is controlled by interactions between plasma membrane butyrophilin and butyrophilin in the secretory granule phospholipid monolayer rather than binding of butyrophilin-xanthine oxidoreductase complexes to secretory granule adipophilin.

Regulation of ADRP expression by long-chain polyunsaturated fatty acids in BeWo cells, a human placental choriocarcinoma cell line

Transplacental transfer of maternal fatty acids is critical for fetal growth and development. In the placenta, a preferential uptake of fatty acids toward long-chain polyunsaturated fatty acids (LCPUFAs) has been demonstrated. Adipose differentiation-related protein (ADRP) is a lipid droplet-associated protein that has been ascribed a role in cellular fatty acid uptake and storage. However, its role in placenta is not known. We demonstrate that ADRP mRNA and protein are regulated by fatty acids in a human placental choriocarcinoma cell line (BeWo) and in primary human trophoblasts. LCPUFAs of the n-3 and n-6 series [arachidonic acid (20:4n-6), docosahexaenoic acid (22:6n-3), and eicosapentaenoic acid (20:5n-3)] were more efficient than shorter fatty acids at stimulating ADRP mRNA expression. The fatty acid-mediated increase in ADRP mRNA expression was not related to the differentiation state of the cells. Synthetic peroxisome proliferator-activated receptor and retinoic X receptor agonists increased ADRP mRNA level but had no effect on ADRP protein level in undifferentiated BeWo cells. Furthermore, we show that incubation of BeWo cells with LCPUFAs, but not synthetic agonists, increased the cellular content of radiolabeled oleic acid, coinciding with the increase in ADRP mRNA and protein level. These studies provide new information on the regulation of ADRP in placental trophoblasts and suggest that LCPUFA-dependent regulation of ADRP could be involved in the metabolism of lipids in the placenta.

Lipid droplet and milk lipid globule membrane associated placental protein 17b (PP17b) is involved in apoptotic and differentiation processes of human epithelial cervical carcinoma cells

The intracellular role of placental protein 17b (PP17b)/TIP47 has been controversial, because it is considered to be a protein required for mannose 6-phosphate receptor transport from endosome to trans-Golgi as well as a neutral lipid droplet-associated protein. The similarity between the amino acid sequences of PP17 variants, adipophilin and perilipins, and between their gene structures indicate that PP17b as well as other alternatively spliced PP17 variants belong to the lipid storage droplet protein family, containing also some differentiation factors. Using a specific antibody, PP17b was detected in lipid droplet fractions and co-localized with neutral lipid droplets stained by Nile red, and fluorescently labelled PP17 antibody in HeLa cells with confocal microscopy. PP17b was also detected in milk, associated to milk lipid globule membranes. Cytostatic agents induced apoptosis and PP17b synthesis in HeLa cells, which was significantly inhibited by protein kinase C (PKC) inhibitor, indicating the involvement of NF-kappa B and AP-1 transcription factors in this process, while protein kinase A (PKA) inhibitor had only a modest inhibitory effect. Cell differentiation induced by dibutyryl cyclic AMP or phorbol myristate acetate also increased PP17b synthesis, demonstrating its strong involvement in cell differentiation. PP17b synthesis was higher in M than in G0/G1 phases in control, apoptotic and differentiated cells. This data shows that PP17b is a neutral lipid droplet-associated protein, and its expression is regulated by PKC- and PKA-dependent pathways.

Adipose differentiation related protein: expression, purification of recombinant protein in Escherichia coli and characterization of its fatty acid binding properties

Adipose differentiation related protein (ADRP) is a 53 kDa protein encoded by a cDNA originally cloned by differential hybridization from murine adipocytes. ADRP is induced during the early onset of the adipose differentiation program and is expressed at high level in mature adipocytes. We have demonstrated that ADRP stimulated the uptake of fatty acids thereby providing evidence for a functional role of ADRP in lipid metabolism. In the present paper, the murine ADRP has been expressed as a recombinant histidine-tagged protein in Escherichia coli, and purified from expressing cultures in order to examine its biochemical properties. We report here that the purified recombinant ADRP binds fatty acids and exhibits stoichiometric saturable binding of NBD-stearic acid with a K(d)=0.145+/-0.003 microM and a B(max)=0.99+/-0.05. Analysis of fluorescence emission spectra indicates that the polarity of the ADRP binding site is near epsilon approximately 23, close to that observed for fatty acid binding sites in other lipid binding proteins such as the liver fatty acid binding protein. The data presented here provide evidence that isolated ADRP purified in the experimental conditions described here can be used for functional studies.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.