Effects of the (1199G > A) Polymorphism on Steroid Sex Hormone-Induced P-Glycoprotein Expression, ATPase Activity, and Hormone Efflux.

This study examined how the 1199G > A polymorphism in the gene encoding P-glycoprotein (P-gp) affects the protein's expression, ATPase activity, and ability to pump female steroid sex hormones out of LLC-PK1 cells. The (1199G) or (1199A) allele was transfected into cells, which were incubated for 48 h with various hormone concentrations, then analyzed by Western blotting to examine expression of P-gp protein and by reverse transcription-polymerase chain reaction (RT-PCR) to examine expression of mRNA. Cells were also compared in terms of their transepithelial permeability to steroid sex hormones in the presence and absence of the specific P-gp inhibitor GF120918. P-gp ATPase activity induced by steroid sex hormones was also assayed. Estriol and ethynyl estradiol up-regulated levels of mRNA in a concentration-dependent manner, with (1199A) mRNA showing greater up-regulation than (1199G) mRNA. Estrone, estriol, and ethynyl estradiol were substrates of both types of P-gp in transepithelial permeability assays, and the (1199A) protein showed a significantly higher net efflux ratio for estrone (13.4 7.4, < 0.005), estriol (5.6 3.3, < 0.05), and ethynyl estradiol (12.7 5.3, < 0.005). Induction of P-gp ATPase activity by ethynyl estradiol and progesterone increased with increasing hormone concentration, and the magnitude of stimulation was greater for (1199A) P-gp than for (1199G) P-gp. These results indicate that the (1199G > A) polymorphism influences steroid sex hormone-induced expression and function of P-gp, which may help to explain inter-patient differences in P-gp-mediated chemotherapy resistance .