Re-evaluation of milk-fat globule EGF-factor VIII (MFG-E8) as an intrinsic component of the mouse milk-fat globule membrane

PTP1B mediates the inhibitory effect of MFGE8 on insulin signaling through the β5 integrin

Integrins are cell adhesion receptors that dimerize to mediate cell-cell interactions and regulate processes, including proliferation, inflammation, and tissue repair. The role of integrins in regulating insulin signaling is incompletely understood. We have previously shown that binding of the integrin ligand milk fat globule epidermal growth factor like 8 (MFGE8) to the αvβ5 integrin promotes termination of insulin receptor signaling in mice. Upon ligation of MFGE8, integrin β5 complexes with the insulin receptor beta (IRβ) in skeletal muscle, resulting in dephosphorylation of IRβ and reduction of insulin-stimulated glucose uptake. Here, we investigate the mechanism by which the interaction between β5 and IRβ impacts IRβ phosphorylation status. We show in in vitro and in vivo in skeletal muscle in mice that antibody-mediated blockade of the β5 integrin inhibits and recombinant MFGE8 promotes PTP1B binding to and dephosphorylation of IRβ resulting in increased or reduced insulin-stimulated glucose uptake, respectively. The β5-PTP1B complex is recruited by MFGE8 to IRβ leading to termination of canonical insulin signaling. β5 blockade enhances insulin-stimulated glucose uptake in wildtype but not Ptp1b KO mice indicating that PTP1B functions downstream of MFGE8 in modulating insulin receptor signaling. Furthermore, in a human cohort, we report serum MFGE8 levels correlate with indices of insulin resistance. These data provide mechanistic insights into the role of MFGE8 and β5 in regulating insulin signaling.

Knockout of butyrophilin subfamily 1 member A1 (BTN1A1) alters lipid droplet formation and phospholipid composition in bovine mammary epithelial cells

Background

Milk lipids originate from cytoplasmic lipid droplets (LD) that are synthesized and secreted from mammary epithelial cells by a unique membrane-envelopment process. Butyrophilin 1A1 (BTN1A1) is one of the membrane proteins that surrounds LD, but its role in bovine mammary lipid droplet synthesis and secretion is not well known.

Methods

The objective was to knockout BTN1A1 in bovine mammary epithelial cells (BMEC) via the CRISPR/Cas9 system and evaluate LD formation, abundance of lipogenic enzymes, and content of cell membrane phospholipid (PL) species. Average LD diameter was determined via Oil Red O staining, and profiling of cell membrane phospholipid species via liquid chromatography-tandem mass spectrometry (LC-MS/MS).

Results

Lentivirus-mediated infection of the Cas9/sgRNA expression vector into BMEC resulted in production of a homozygous clone BTN1A1^(−/−). The LD size and content decreased following BTN1A1 gene knockout. The mRNA abundance of fatty acid synthase (FASN) and peroxisome proliferator-activated receptor-gamma (PPARG) was downregulated in the BTN1A1^(−/−) clone. Subcellular analyses indicated that BTN1A1 and LD were co-localized in the cytoplasm. BTN1A1 gene knockout increased the percentage of phosphatidylethanolamine (PE) and decreased phosphatidylcholine (PC), which resulted in a lower PC/PE ratio.

Conclusions

Results suggest that BTN1A1 plays an important role in regulating LD synthesis via a mechanism involving membrane phospholipid composition.

Elevated serum milk fat globule-epidermal growth factor 8 levels in type 2 diabetic patients are suppressed by overweight or obese status

Inflammation is the most important link between obesity and type 2 diabetes (T2D). Although milk fat globule-epidermal growth factor 8 (MFG-E8) is a key mediator in anti-inflammatory responses, its role in obesity and diabetes is not yet completely understood. We aimed to measure MFG-E8 serum levels and to explore the role of MFG-E8 in obesity and T2D. Fasting serum MFG-E8 levels were quantified by enzyme-linked immunosorbent assay for 168 individuals, whose oral glucose tolerance test was conducted, and levels of inflammatory factors, including tumor necrosis factor-α (TNF-α) and C-reactive protein, were measured. The participants were subdivided into 66 newly diagnosed T2D individuals, 44 impaired glucose tolerance (IGT) subjects and 58 healthy controls. Their characteristics were further classified as lean or nonlean for investigation. MFG-E8 levels were significantly higher in T2D subjects than in healthy controls (P = 0.028). Decreased levels of MFG-E8 were found in overweight or obese individuals, compared to those in lean subjects, in both the T2D and IGT groups (P < 0.001). Interestingly, MFG-E8 levels showed a negative correlation with body mass index (BMI) and TNF-α levels in the total population and the T2D subgroup. Further, BMI and TNF-α concentrations were found to be independent predictors of MFG-E8 levels in all subjects. MFG-E8 levels are elevated in T2D but suppressed by increased adipose tissues, thereby allowing inflammatory factors to rise to high levels. MFG-E8 may serve as a potential biomarker for obesity and T2D in the clinical setting. © 2017 IUBMB Life, 69(2):63-71, 2017.

Milk fat globule epidermal growth factor 8 serves a novel biomarker of opisthorchiasis-associated cholangiocarcinoma

Milk fat globule epidermal growth factor 8 (MFG-E8) is a pleiotropic secreted glycoprotein to play roles in mediating immune tolerance and homeostasis maintenance and enhancing angiogenesis. To evaluate its value as a biomarker in opisthorchiasis-associated cholangiocarcinoma (CCA), the present study investigated MFG-E8 expression kinetics during the tumorigenesis in Opisthorchis viverrini infection-induced CCA, and demonstrated its expression in the tumor tissues of CCA patients and its serum level among them. During the tumorigenesis of CCA, MFG-E8 expression was increased in a time-dependent manner with the pathological processes. Absolutely higher expression of MFG-E8 messenger RNA was detected in the tumor tissues from CCA patients, compared with those in adjacent tissues. Immunobiochemical analysis showed that more than 90% CCA cases were positive and the positive reaction located in the membrane and cytoplasm of the tumor cells. Moreover, the average serum level in the CCA patients was significantly higher than that in healthy individuals and those with O. viverrini infection or other parasitosis. Correlation analysis of MFG-E8 expression with CCA clinicopathology revealed that a high expression of MFG-E8 protein was significantly bound with a poor differentiation, pathological advanced stage, and metastasis of CCA. The multivariation analysis indicated that MFG-E8 was an independent prognostic factor. In addition, short hairpin RNA-mediated MFG-E8 knockdown in CCA cell line obviously suppressed the cell proliferation. Our results strongly suggested that MFG-E8 is a promising biomarker for the diagnosis, prognosis, and therapy target of opisthorchiasis-associated CCA.