Abstract
BackgroundStudying the biological pathways involved in mammalian milk production during lactation could have many clinical implications. The mammary gland is unique in its requirement for transport of free glucose into the cell for the synthesis of lactose, the primary carbohydrate in milk.ObjectiveTo study GLUT1 trafficking and subcellular targeting in living mammary epithelial cells (MEC) in culture.MethodsImmunocytochemistry was used to study GLUT1 hormonally regulated subcellular targeting in human MEC (HMEC). To study GLUT1 targeting and recycling in living mouse MEC (MMEC) in culture, we constructed fusion proteins of GLUT1 and green fluorescent protein (GFP) and expressed them in CIT3 MMEC. Cells were maintained in growth medium (GM), or exposed to secretion medium (SM), containing prolactin.ResultsGLUT1 in HMEC localized primarily to the plasma membrane in GM. After exposure to prolactin for 4 days, GLUT1 was targeted intracellularly and demonstrated a perinuclear distribution, co-localizing with lactose synthetase. The dynamic trafficking of GFP-GLUT1 fusion proteins in CIT3 MMEC suggested a basal constitutive GLUT1 recycling pathway between an intracellular pool and the cell surface that targets most GLUT1 to the plasma membrane in GM. Upon exposure to prolactin in SM, GLUT1 was specifically targeted intracellularly within 90–110 minutes.ConclusionsOur studies suggest intracellular targeting of GLUT1 to the central vesicular transport system upon exposure to prolactin. The existence of a dynamic prolactin-induced sorting machinery for GLUT1 could be important for transport of free glucose into the Golgi for lactose synthesis during lactation.
Highlights
Our studies suggest intracellular targeting of GLUT1 to the central vesicular transport system upon exposure to prolactin
The existence of a dynamic prolactin-induced sorting machinery for GLUT1 could be important for transport of free glucose into the Golgi for lactose synthesis during lactation
In secretion medium GLUT1 green signal colocalized with the blue signal of plasmid vector ECFP (pECFP)-Golgi (Figure 2A, B, C)
Summary
Biology of Milk Production and Transport Pathways of Milk ConstituentsFemales of all mammalians bear mammary glands, and milk secretion and lactation is a characteristic feature of all mammalian species, which are the only organisms that produce copious glandular skin secretions to feed their young.[1,2] Lactation is a highly complex and evolutionarily ancient strategy of all mammals, providing their offspring with a highly digestible, concentrated, nutritionally balanced diet, while allowing adult mammals to evolve a wide range of developmental and reproductive strategies and specialize on diets that could either be too difficult to capture or digest or would be insufficient to cover the high nutritional needs of their small rapidly growing offspring.[1,2,3] Lactation helps mammalian mothers cope with unreliable food supplies, because lactating females can draw on their nutrient reserves for milk production, suggesting an evolutionary advantage for their dependent offspring, since milk intake promotes growth, fitness, and survival of the young.[2,4,5] Beyond nourishment of the neonate, milk helps establish immunological and endocrine competence in the offspring. Biology of Milk Production and Transport Pathways of Milk Constituents. Milk’s nutrient composition varies extensively across mammalian species, as a function of evolutionary history, maternal nutrient intake, duration of milk production, and stage of lactation.[2,5] Milk is a complex mixture whose composition reflects different transport and secretion mechanisms within the mammary gland that aim to answer the different nutritional needs of mammalian neonates.[6]. The mammary gland is unique in its requirement for transport of free glucose into the cell for the synthesis of lactose, the primary carbohydrate in milk
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