INDIAN JOURNAL OF MEDICAL & PAEDIATRIC ONCOLOGY Vol. 27 Suppl. 1, 2007 19 Lysosomal Acidification Defect: A Plausible Mechanism of Cisplatin Resistance Shyam S. Chauhan Department of Biochemistry All India Institute of Medical Sciences, Ansari Nagar, New Delhi-110029 Cisplatin (CP) is a component of standard after uptake into lysosmes of KB-CP-r cells treatment regimens for ovarian, cervical, was TCA precipitable as compared to only testicular, bladder, head and neck and lung 10% released by sensitive cells. These results cancer. Adduct of DNA with CP induce indicate inefficient degradation of apoptosis leading to cell death. Intrinsic or internalised 125I-EGF in the lysosomes of KB- acquired resistance of tumourous cells to CP CP-r cells, consistent with slower under mines its clinical effectiveness. In processing of cathepsin L, a lysosomal order to understand the molecular basis CP cysteine protease. Treatment of KB cells by resistance we isolated human KB bafilomycin A 1, A known inhibitor of the adenocarcinoma cisplatin-resistant (CP-r) vacuolar proton pump, mimicked the cell lines with multidrug-resistance phenotype seen in KB-CP-r cells with phenotypes because of reduced reduced uptake of HRPO, 125 I-EGF, 14 C- accumulation of CP and other cytotoxic carboplatin and release of TCA precipitable compounds such as methotrexate and heavy 125I-EGF. KB-CP-r cells also had less acidic metals. The uptake of horseradish lysosomes. KB-CP-r cells were crossresistant peroxidase (HRPO) and Texas Red dextran to Pseudomonas exotoxin, and Pseudomonas was decreased several-fold in KB-CP-r cells, exotoxin-resistant KB cells were indicating a general defect in fluid phase crossresistant to CP. Since cells with endocytosis. In contrast, although EGF endosomal acidification defects are known to receptors were decreased in amount the be resistant to Pseudomonas exotoxin and kinetics of EGF uptake, a marker of blocking of endosomal acidification mimics receptormediated endocytosis, was similar the CP-r phenotype, we conclude that in sensitive and resistant cells. However, 40- defective endosomal acidification may 60% of the 125I-EGF released into the medium contribute to acquired CP resistance. !