The plasma membrane glycoprotein 170 (P-gp), responsible for multidrug resistance, MDR, may also function as an efflux pump for chemical carcinogens and is regulated by dietary nutrients. P-gp is found in normal tissues, predominately in the cells lining the luminal space of a variety of tissues, including the placenta and the endometrium of the gravid uterus. Recently, our laboratory demonstrated that P-gp mediates the efflux of chemical carcinogens, benzo(a)pyrene and dimethylbenzanthracene, and we proposed that P-gp in normal tissues may serve as a first line of defense against carcinogens. In order to examine the expression of P-gp in normal cells, we used normal rat placental cells immortalized with SV40 temperature-sensitive A (tsA) mutant. We found at 33 degrees C (transformed phenotype) P-gp was not detectable but at 39.5 degrees C (normal differentiated phenotype) cells expressed significant amounts of P- gp protein as measured by Western immunoprecipitation with the monoclonal antibody, C219. Progesterone effectively blocks adriamycin efflux in the differentiated placental cells, as well as vinblastine accumulation, indicating that P-gp in placental cells may be regulated by progesterone. However, progesterone is not a substrate for P-gp since progesterone accumulation and efflux at 33 degrees C is similar to that at 39 degrees C. We further developed doxorubicin-resistant placental cells from these SV40- tsA rat placental cells and found a 2-4 fold increase of P-gp in doxorubicin-resistant placental cells at the differentiated phenotype. This is the first demonstration of normal differentiated placental cells with high expression of P-gp. The functional role of P-gp in normal tissues has not been determined. However, our recent findings indicate that P-gp mRNA is developmentally regulated in human placental tissues. Currently, a developmental study in rats of P-gp expression in normal tissues during gestation is in progress. Rat placentas, ovaries, uteri, adrenals, kidneys, and colon muscosal cells at 0, 6, 9, 12, 15, and 18 days of gestation are being processed for examination of P-gp expression at the mRNA and protein levels. In addition, we are also studying the regulation of P-gp in a human endometrial adenocarcinoma cell line and a human cervical carcinoma cell line. Dietary effectors such as retinoic acid (which is known to enhance P-gp) and the flavonoid, kaempferol (which decreases P-gp) are being investigated in the human endometrial and cervical cell lines to determine their effects. The role of mdr1 and mdr3 in the placental-related cell lines is under investigation to determine their expression under normal physiological conditions.
