Transport proteins in mammary epithelial cells

Abstract: Transporters localized in membranes of secreting mammary epithelial cells are involved in delivery of essential nutrients into milk. However, drugs and other xenobiotics may be substrates of these transporters and thus be actively secreted into milk, which may pose a health threat to breastfed infants and dairy consumers. Aims of the thesis were to determine expressions of drug transporters in mammary gland tissue and to assess mammary cell models for studies of these proteins. Gene expressions of members of the ABC (BCRP/ABCG2, MDR1/ABCB1, MRP1/ABCC1) and SLC (OATP1A2/SLCO1A2, OCTN1/SLC22A4, OCT1/SLC22A1) families were measured in murine and bovine mammary tissue and in murine (HC11) and bovine (BME-UV) mammary epithelial cell lines. BCRP function was assessed by transport experiments with mitoxantrone (MX) in HC11 cells. Effects of the imidazole fungicide prochloraz on transporter expression and function in HC11 and BME-UV cells were examined. Expressions of BCRP and OCT1 in murine mammary glands were increased during gestation and lactation, whereas MDR1, MRP1, OATP1A2 and OCTN1 were decreased, compared to expression in virgins. All transporters measured in mammary glands of mice were detected in bovine mammary tissue. All transporters investigated in vivo were also detected in HC11 cells, while only MDR1 and MRP1 were detected in BME-UV cells. Differentiation of HC11 cells resulted in increased BCRP protein expression, while MDR1 expression was reduced. The BCRP inhibitor elacridar reduced secretion and increased accumulation of MX in both undifferentiated and differentiated HC11 cells. An increased accumulation of MX was observed in BCRP gene silenced HC11 cells. Prochloraz treatment induced MDR1 gene expression and protein function in both differentiated HC11 and BME-UV cells, resulting in decreased accumulation of the MDR1 substrate digoxin. In conclusion, we demonstrated that the HC11 and BME-UV mammary cell models are valuable tools for identifying substrates, inhibitors and inducers of transport proteins expressed in the mammary epithelium during lactation. The models can be used both to examine if chemical compounds are actively transported into milk and if they disrupt the normal function of transporters which may result in a disturbed delivery of essential nutrients into milk.