Calcium receptor binding of cisplatin and terbium in human breast tumor cells after hyperthermia

Terbium phosphorescence was used to investigate a thermosensitive calcium binding protein located in the plasma membrane of human MCF-7 breast tumor cells. The cells were pretreated at 25, 37, or 43 degrees C for 1 h, before binding experiments were performed at 25 degrees C. The delta Imax for the Tb(3+)-MCF/7 complex increased significantly after cellular exposure to 43 degrees C. The Kd for the Tb(3+)-MCF/7 complex did not change with the temperature. These results suggested that hyperthermia can unmask preexisting calcium binding sites on the surface of the plasma membrane, without affecting their affinity. After treatment at 43 degrees C, the quenching of the Tb(3+)-MCF/7 intensity by cisplatin was significantly greater than that at 37 degrees C. Hyperthermia was determined to increase the affinity of cisplatin for the calcium binding protein. The effect of hyperthermia on the calcium receptor binding of cisplatin is hypothesized to be the basis for the synergism between their cytotoxicities.