**2. Glucose uptake in mammary epithelial cells**

Glucose supply to the mammary gland is pivotal to maintain the high rate of proliferation of glandular epithelium in pregnancy and the continuous production of lactose, fat acids, and proteins during lactation [6]. Studies in cows demonstrate that between 60 and 85% of plasmatic glucose is distributed to the mammary gland during lactation and that duodenal glucose injection increases mammary gland glucose uptake and lactose synthesis glucose supply to the mammary gland during lactation, whereas the inhibition of this process or the renal reabsorption of glucose decreased them [7, 8]. On the other hand, in rodents, glucose uptake of the mammary gland duplicates 2 days before delivery, and it remains high during all lactation period, and in humans, 30% of glucose intake is used to lactose production in established lactation [9]. The whole organism adapts to the synthesis of milk; the initial negative energy balance is reversed by greater hepatic gluconeogenesis and decreased peripheral glucose use [10].

**11**

**Figure 2.**

*exported transporter.*

*Lactose Synthesis*

*DOI: http://dx.doi.org/10.5772/intechopen.91399*

The necessity of glucose supply for the proliferation of MEC is very important to lactation persistency, being, together with secretory activity, the two factors that define the lactation curve and peak [4, 11]. In rodents, milk production is mainly associated with a high proliferation of MEC, whereas in cows it is principally due to the increase in secretory activity per cell [12]. In general, MEC proliferation is low in virgin and early pregnancy (5%) in mice, where it has been associated with stem cell renewing, but it persists throughout lactation period, associated with cell replacement, with low net growth, associated with the expression of Ki67 [5, 11]. For example, in cows, the MEC replacement reaches 50%, whereas in rodents it is lower than 25% [11]. When lactation declines, proliferation decreases, and it is exceeded by apoptosis rate, but also the secretory activity by cell decreases [11]. In particular, in humans, mammary growth for lactation starts at week 20 of pregnancy, whereas in mice it starts at day 12 of pregnancy. Studies in mice have established that DNA content increases from the middle of pregnancy until day 5 of lactation, doubling every 6 days, maintaining a net proliferation rate of only 0.3%

during lactation, due to parallel apoptosis and loss of cells in milk [13].

The glucose transporters already identified in mammary epithelial cells of rodents, humans, and ruminants are facilitative glucose transporters GLUT1, GLUT8, and GLUT12, SGLT1, and the bidirectional sugar transporter SWEET1 (sugars will eventually be exported transporter) [7, 8, 14–16] (**Figure 2**). The GLUT transporters were initially identified due to the capacity of MEC to transport 3-O-methyl-D-Glucose and inhibition of this by cytochalasin B [17–19]. There are differences in the location and magnitude of peak expression of glucose transporters due to differences between species in the prevalence of cell proliferation or secretory activity [8, 20, 21]. In cows, the increase in the expression of GLUTs is in order of magnitude greater than in rodents, which reflects that its secretory activity

*Glucose uptake in mammary epithelial cell. Distribution of glucose transporters and glucose concentration in different compartments is detailed. MEC, mammary epithelial cells; myoMEC, mammary myoepithelial cells; GLUT, facilitative glucose transporter; SGLT, sodium-glucose cotransporter; SWEET, sugars will eventually be* 

#### *Lactose Synthesis DOI: http://dx.doi.org/10.5772/intechopen.91399*

The necessity of glucose supply for the proliferation of MEC is very important to lactation persistency, being, together with secretory activity, the two factors that define the lactation curve and peak [4, 11]. In rodents, milk production is mainly associated with a high proliferation of MEC, whereas in cows it is principally due to the increase in secretory activity per cell [12]. In general, MEC proliferation is low in virgin and early pregnancy (5%) in mice, where it has been associated with stem cell renewing, but it persists throughout lactation period, associated with cell replacement, with low net growth, associated with the expression of Ki67 [5, 11]. For example, in cows, the MEC replacement reaches 50%, whereas in rodents it is lower than 25% [11]. When lactation declines, proliferation decreases, and it is exceeded by apoptosis rate, but also the secretory activity by cell decreases [11]. In particular, in humans, mammary growth for lactation starts at week 20 of pregnancy, whereas in mice it starts at day 12 of pregnancy. Studies in mice have established that DNA content increases from the middle of pregnancy until day 5 of lactation, doubling every 6 days, maintaining a net proliferation rate of only 0.3% during lactation, due to parallel apoptosis and loss of cells in milk [13].

The glucose transporters already identified in mammary epithelial cells of rodents, humans, and ruminants are facilitative glucose transporters GLUT1, GLUT8, and GLUT12, SGLT1, and the bidirectional sugar transporter SWEET1 (sugars will eventually be exported transporter) [7, 8, 14–16] (**Figure 2**). The GLUT transporters were initially identified due to the capacity of MEC to transport 3-O-methyl-D-Glucose and inhibition of this by cytochalasin B [17–19]. There are differences in the location and magnitude of peak expression of glucose transporters due to differences between species in the prevalence of cell proliferation or secretory activity [8, 20, 21]. In cows, the increase in the expression of GLUTs is in order of magnitude greater than in rodents, which reflects that its secretory activity

#### **Figure 2.**

*Glucose uptake in mammary epithelial cell. Distribution of glucose transporters and glucose concentration in different compartments is detailed. MEC, mammary epithelial cells; myoMEC, mammary myoepithelial cells; GLUT, facilitative glucose transporter; SGLT, sodium-glucose cotransporter; SWEET, sugars will eventually be exported transporter.*

is highly dependent on the expression of glucose transporters [6, 14]. The maximum expression of GLUT1 observed was between late pregnancy and late lactation, reaching an increase of 5-fold at the protein level and 50-fold at mRNA level [5, 15, 17, 18, 22]. The increase in GLUT8 expression is lower, but it follows the same pattern, associated with cytokeratin 18 and Ki67 expression and MEC proliferation [5, 8]. However, some studies also found an increase of GLUT1 expression in MEC in early pregnancy, which could be related to the start of lipid synthesis in secretory activation phase, when sterol regulatory element-binding protein (SREBP), a transcription factor, appears, or to stem cell renewal [2, 5, 10, 23]. Although the majority of authors found over 60% of GLUT1 expression in plasma membrane in rat lactating gland, some studies found it almost exclusively at intracellular level [5, 19, 20, 24]. Interestingly, in early weaning of BalB/BalC mice, GLUT1 is also concentrated intracellularly, but, due to a decrease in lactation at this step, that could be associated with its accumulation in proteasomal compartment or to apoptotic bodies phagocyted by other epithelial cells acting as nonprofessional phagocytes [5, 25].

The intracellular concentration of glucose in the MEC is mainly determined by its incorporation by GLUT1 transporters in the basolateral membrane and by the activity of the cytosolic hexokinases, which transform glucose into glucose-6-phosphate [2, 23]. The induction of the expression of hexokinase II in the cytoplasm of MEC during the period of breastfeeding is essential in the determination of intracellular glucose levels, because this enzyme has low glucose affinity (Km 0.3 mM) [26]. On the other hand, glucose is also transported to the lumen of the alveolus, through GLUT12, reaching a concentration of 1.5 mM in milk, equivalent to the concentration found in the cytoplasm of the MEC [20, 26].
