LOW EXTERNAL PH AND OSMOTIC SHOCK INCREASE THE EXPRESSION OF HUMAN MDR PROTEIN

We have studied the effects of extracellular pH (pH(o)) and osmotic st rength on the expression of the human multidrug resistance (MDR) prote in. Both lowered pH(o) and hypertonic shock increase the level of hu M DR protein 5-10-fold in membranes isolated from the human colon carcin oma cell lines SW620 and HCT15 and the human kidney carcinoma line SKR C-39. Increased protein expression is dependent on the duration of aci d or osmotic shock and is reversed within several days when normal gro wth conditions are restored. Quantitative northern blot analysis with a hu MDR 1 specific probe reveals increased MDR mRNA in the acid and h ypertonically shocked cells. Interestingly, we find a greater increase in mRNA levels for hypotonically shocked colon cells, without an appa rent increase in protein levels. Overexpressing cells are found to ret ain less [H-3]vinblastine relative to cells cultured under normal cond itions, and they are resistant to the cytotoxic effects of doxorubicin , vinblastine, and colchicine, but not methotrexate. This resistance a ppears to be reversed by treatment with verapamil. In contrast, SW620 cells previously induced to overexpress MDR protein via the administra tion of differentiation agents [Mickley et al. (1989) J. Biol. Chem. 2 64, 18031-18040] did not exhibit decreased retention of [H-3]vinblasti ne; thus low-pH(o)-induced overexpression of MDR protein in these cell s may provide additional factors that promote the full expression of t he MDR phenotype. These data may help to explain why many solid tumors (e.g., of colon and kidney origin) develop MDR prior to chemotherapy, since they usually grow under similar acidic conditions. These data a lso support the contention that MDR protein may play a role in intrace llular pH and volume homeostasis.