Binding of terbium and cisplatin to C13* human ovarian cancer cells using time-resolved terbium luminescence

Terbium (Tb3+) has been shown to increase the cellular accumulation and cytotoxicity of cisplatin in cisplatin-resistant human breast and ovarian cancer cells. Time-resolved Tb3+ luminescence was used to describe the binding of cisplatin to cisplatin-resistant C13* cells. A high-affinity Tb3+ binding site was identified in the plasma membrane of the C13* cells (n = 105 +/- 2 fmol/cell and K-d = 36.3 +/- 5.2 mu M) The binding of Tb3+ is suggested to occur through a cation-pi interaction with tryptophan residues in the plasma membrane, resulting in an enhancement of the intensity and lifetime of Tb3+ Stern-Volmer quenching analysis revealed that the Tb3+ binding site is not readily accessible to the aqueous environment. The quenching of the Tb3+-C13* intensity by cisplatin occurred by static quenching processes, involving both a direct electron-exchange interaction as well as an indirect dipole-dipole resonant energy transfer mechanism. Formation of the Tb3+-C13* cisplatin complex does not interfere with the high-affinity binding of Tb3+; cisplatin and Tb3+ bind within 5 to 10 Angstrom of each other. A specific terbium/cisplatin binding protein is suggested to play a role in the cellular accumulation and cytotoxicity of cisplatin. Therefore, the transport of cisplatin across the plasma membrane must also involve a facilitated diffusion process. Our results indicate that the binding of Tb3+ to the plasma membrane may be potentially useful in the reversal of cisplatin resistance. (C) 1998 Elsevier Science B.V. All rights reserved.