Expression of breast cancer resistant protein and P-glycoprotein in cultured primary bovine mammary epithelial cells isolated from tissue and milk

A tight monolayer of mammary epithelial cells, in which the transporter expression is well characterised, could be a useful in vitro model for studying how chemicals cross the blood-milk-barrier (BMB) and accumulate in milk. The efflux transporter breast cancer resistance protein (BCRP) is the predominant efflux transporter in the BMB. In order for an in vitro cell culture model to mimic the in vivo BMB, the expression of BCRP should be higher than that of the efflux transporter p-glycoprotein (P-gp). The aim of the study reported in this communication was to determine the relationship between the transcription of ABCG2 and ABCB1 (encoding for BCRP and P-gp respectively) in primary bovine mammary epithelial cells in culture. Cells were sourced from both lactating udder tissue after routine slaughter of the donor, and then cryopreserved until use, as well as from fresh, raw milk of healthy cows in mid-lactation. In this study a total of 18 RNA samples from 18 individual trypsinisations, originating from 6 individual isolations (3 milk isolations and 3 udder tissue isolations) were analysed by rt-PCR. In all eighteen samples analysed, the mRNA detected for P-gp was higher per sample than the mRNA detected for BCRP. In six samples no mRNA for BCRP was detected at 40 amplification cycles. Our study showed that when pbMEC is cultured in the proliferation medium as described, P-gp mRNA expression is consistently higher than BCRP mRNA expression. This relationship is the inverse of the relationship between BCRP and P-gp in the BMB in vivo.