Glucose transport in lactation.

Lactose is the major carbohydrate of human milk and is also the major osmotic constituent of human milk. Therefore, synthesis of lactose is the major determinant of the volume of milk produced by the lactating human mammary gland. Lactose issynthesized from free glucose and UDP-galactose. Thus, glucose transport is required not only across the plasma membrane but also across an intracellular membrane to the compartment of lactose synthesis. The latter requirement is unique to mammary epithelial cells. Historically, based primarily on subcellular fractionation studies, lactose synthesis was thought to occur in Golgi. Yet, the only known glucose transporter isoform expressed in mammary gland is GLUT1, a plasma membrane glucose transporter. We therefore comprehensively studied the possible role of GLUT1 as a glucose transporter. We tested the hypothesis that changes in the amount, activity, and subcellular targeting of GLUT1 during lactation are consistent with an important role for GLUT1 in the regulation of lactation. The experiments described here summarize our recent work in the lactating mouse mammary gland and inmouse mammary epithelial cells in culture. Theresults demonstrate that GLUT1 is targeted to an intracellular compartment. However, studies in mammary epithelial cells in culture demonstrate that this is not a Golgi compartment, but a low-density, exquisitely Brefeldin A-sensitive compartment of Golgi-related vesicles. This raises the possibility that lactose synthesis does not take place in the Golgiproper. The results strongly suggest that GLUT1 appears to be important in delivery of substrate to the site of lactose synthesis.

Diminished milk synthesis in upstream stimulatory factor 2 null mice is associated with decreased circulating oxytocin and decreased mammary gland expression of eukaryotic initiation factors 4E and 4G.

Previous studies have suggested that upstream stimulatory factors (USFs) regulate genes involved with cell cycle progression. Because of the relationship of USFs to an important oncogene in breast cancer, c-myc, we chose to determine the importance of USF to normal mammary gland development in the mouse. Expression of USF in the mammary gland throughout development demonstrated only modest changes. Mutation of the Usf2 gene was associated with reduced fertility in females, but had no effect on prepartum mammary gland development. However, lactation performance in Usf2-/- females was only half of that observed in Usf2+/+ females, and both lactose and nitrogen were decreased in milk from Usf2-/- dams. This decrease was associated with diminished mammary tissue wet weight and luminal area by d 9 of lactation and with a decreased protein-DNA ratio. This decrease was associated with reduced abundance of the eukaryotic initiation factors eIF4E and eIF4G. Blood oxytocin concentrations on d 9 postpartum were also lower in Usf2-/- mice than Usf2+/+ mice. In contrast, the mutation had no effect on blood prolactin concentrations, mammary cell proliferation or apoptosis, mammary tissue oxytocin receptors, or milk protein gene expression. The mutation had only modest effects on maternal behavior. These data support the idea that USF is important to physiological processes necessary for the establishment and maintenance of normal lactation and suggest that USF-2 may impact lactation through both systemic and mammary cell-specific mechanisms.