title: Binding of terbium and cisplatin to C13* human ovarian cancer cells using time-resolved terbium luminescence.
abstract: 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 Kd=36. 3+/-5.2 microM). 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 A 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.
authors.affiliation: Laboratory of Biophysical Cytochemistry, Department of Physiology and Biophysics, Howard University College of Medicine, Washington, DC 20059, USA.
authors.forename: R G
authors.initials: RG
authors.lastname: Canada
authors.forename: D N
authors.initials: DN
authors.lastname: Paltoo
mesh_terms: D000970:Antineoplastic Agents / Q000378:metabolism*; D001665:Binding Sites; D001692:Biological Transport; D002945:Cisplatin / Q000378:metabolism*; D004347:Drug Interactions; D004351:Drug Resistance; D005260:Female; D006801:Humans; D008163:Luminescent Measurements; D008962:Models, Theoretical; D010051:Ovarian Neoplasms / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D013725:Terbium / Q000737:chemistry / Q000378:metabolism*
chemical_list: D000970:Antineoplastic Agents; D013725:Terbium; D002945:Cisplatin