Monks Jenifer, Dzieciatkowska Monika, Bales Elise S, Orlicky David J, Wright Richard M, McManaman James L
Division of Reproductive Sciences, Department of Obstetrics and Gynecology, School of Medicine, University of Colorado Anschutz Medical Center, Aurora, CO, 80045, USA.
Biochemistry Department, School of Medicine, University of Colorado Anschutz Medical Center, Aurora, CO, 80045, USA.
J Physiol. 2016 Oct 15;594(20):5899-5921. doi: 10.1113/JP272390. Epub 2016 Aug 3.
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.
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.
黄嘌呤氧化还原酶（XOR）调节乳汁脂质分泌和泌乳起始。在乳汁脂质分泌过程中，XOR是膜内嗜乳脂蛋白1a1聚集所必需的。XOR介导乳汁脂质分泌过程中的顶端膜重组。XOR缺失会延迟乳脂肪球的分泌。XOR缺失会改变乳脂肪球的蛋白质组。
外分泌腺的上皮细胞利用顶浆分泌。这些细胞将膜结合颗粒芽生到腺腔内，在分泌产物中损失一部分细胞质。泌乳乳腺通过这种机制分泌乳汁脂质，长期以来人们一直认为黄嘌呤氧化还原酶（XOR）在功能上很重要。我们构建了乳腺特异性XOR基因敲除（MGKO）小鼠，预期泌乳会失败。基因敲除腺体的组织学检查显示，乳腺腺泡细胞内包裹着非常大的脂滴，但乳汁分析表明这些大脂滴被分泌了。嗜乳脂蛋白是一种已知与XOR结合的膜蛋白，在野生型腺体中，它聚集在细胞质脂滴与顶端质膜的接触点处，而在基因敲除腺体中则没有，这表明XOR介导了与该膜的“对接”。从野生型和XOR MGKO母鼠的乳汁中分离出分泌的乳脂肪球，并进行液相色谱-串联质谱分析以分析蛋白质成分。蛋白质组学结果表明，XOR缺失导致与分泌的乳脂肪球相关的细胞质、细胞骨架、高尔基体和脂质代谢蛋白增加。XOR与脂滴的结合导致膜对接，分泌细胞能更有效地保留细胞质成分。XOR缺失随后导致恢复到一种更原始、效率更低的顶浆分泌机制，但并不阻止乳脂肪球的分泌。
