Milk lipid and protein traffic in mammary epithelial cells: joint and independent pathways

Abstract

-In mammary epithelial cells, milk lipids and proteins are synthesised in the same compartment, the endoplasmic reticulum. Lipids, carried through the cytoplasm, associate with the apical membrane which then pinches off and releases the lipid globule. Proteins, carried through membrane compartments are released in the lumen after fusion of secretory vesicles with the apical membrane. These processes assure a relatively constant composition of milk but it is not known whether lipid and protein secretion are linked. The protein composition of the milk fat globule membrane and the stimulatory effects of prolactin and oxytocin on lipid and protein secretion suggest that these processes are coupled and co-regulated. However, it is possible to observe a dissociation between the formation and the secretion of the two constituents, during differentiation and in various experimental conditions, and this suggests that coupling is not strictly required. milk lipids / mammary epithelial cells / secretionRésumé -Transport des lipides et des protéines du lait dans les cellules épithéliales mammaires. Dans la cellule épithéliale mammaire, les lipides et les protéines du lait sont synthéti-sés dans le même compartiment, le reticulum endoplasmique. Les lipides transportés à travers le cytoplasme s'accolent à la membrane plasmique apicale qui les enveloppe avant leur sécrétion. Les protéines, transportées dans des compartiments membranaires jusqu'aux vésicules de sécrétion, sont libérées dans la lumière après fusion des vésicules avec la membrane apicale. Ces processus assurent une composition relativement constante du lait. Cependant, on ne sait pas si ces deux types de sécré-tion sont liés. La composition protéique de la membrane du globule gras et l'effet stimulant de la prolactine et de l'ocytocine sur la sécrétion des deux constituants suggèrent que synthèse et sécrétion sont liées et co-régulées. Cependant, la possibilité d'observer une dissociation entre la formation et la sécrétion des deux constituants suggère que ce couplage n'est pas obligatoire.lipides du lait / cellule épithéliale mammaire / sécrétion

Relationships between milks differentiated on native milk fat globule characteristics and fat, protein and calcium compositions

“…The synthesis and secretion of the caseins (via the endoplasmic reticulum -golgi -secretory Differences between groups at P < 0.05. ***P < 0.001. vesicle axis) and lipids (via the endoplasmic reticulum -CLD -MFG axis) involve joint and independent secretory pathways in lactocytes (Ollivier-Bousquet, 2002).…”

“…Most of the studies dealing with the effect of the composition of the milk proteins and MFG diameter looked at very specific proteins such as butyrophiline, xanthine oxidoreductase and adipophiline (also known as perilipin-2 or adipose differentiation-related protein) (McManaman et al, 2007;Jeong et al, 2013) or proteins involved in lipid synthesis (Cohen et al, 2015). These three proteins, by forming a bridge between the CLD membrane and the apical membrane, are involved in the membrane enwrapping process during MFG secretion by budding (Ollivier-Bousquet, 2002;Jeong et al, 2013) as well as the phosphatidylethanolamine content in the membrane, as mentioned above (Keenan and Huang, 1972). Our results cannot support these explanations and further work is still needed.…”

“…Triglycerides are synthesized in the reticulum endoplasmic membrane and then released into the cytoplasm in the form of a triglyceride core coated by a monolayer of phospholipids derived from the reticulum membrane, the so-called cytoplasmic lipid droplets (CLD). The droplets grow during their migration to the apical pole of the cell due to the continuation of the synthesis of lipids (Ollivier-Bousquet, 2002;Heid and Keenan, 2005;McManaman et al, 2007). The secretion would correspond to the membrane enwrapping the CLD and its secretion in association with the exocytosis of the golgiderived secretory vesicles containing caseins micelles (Wu et al, 2000;Keenan and Mather, 2006;Robenek et al, 2006;McManaman et al, 2007;Truchet et al, 2014).…”

Recent advances in the regulation of milk fat synthesis

“…Adipose tissue mobilization accounts for less than 10% of preformed FA in milk fat, except during periods of negative energy balance when their proportion increases substantially ). Lastly, due to the hydrophobic nature of esterified FA, milk fat is secreted from the mammary epithelial cell as a lipid droplet surrounded by a protein-rich polar lipid coat called the milk fat globule membrane (MFGM; see reviews by Mather and Keenan (1998), Keenan (2001) and Olivier-Bousquet (2002)). The origin of the MFGM and the mechanism of cellular milk fat secretion continue to be areas of intense investigation.…”

Genetic Factors that Regulate Milk Protein and Lipid Composition in Goats

“…There is a certain controversy about the influence of CSN1S1 polymorphism on the mRNA expression of lipid metabolism genes, with studies that support a regulatory effect (Badaoui 2008) and others that do not (Leroux et al 2003). Since lipids and proteins are synthesized in the endoplasmic reticulum and their transport is, to a certain extent, coupled and co-regulated, it has been hypothesized that the perturbation of casein transport induced by the CSN1S1 genotype might also alter lipid trafficking resulting in a reduced fat secretion (Ollivier-Bousquet et al 2002).…”