Metallothionein-like proteins and cell resistance tocis-dichlorodiammineplatinum(II) in L1210 cells

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Abstract
Our studies on the mechanism of resistance of the murine leukemia L1210-PDD line tocis-dichlorodiammineplatinum(II) (cis-DDP) have not shown why it is 10-fold more resistant to the drug than the L1210 line. For this reason we investigated metallothionein-like proteins (‘MTs’) in these cells. Soluble protein extracts from cultures treated for 24 h withcis-DDP, zinc sulphate or saline were anaerobically eluted from columns of chemically reduced Sephadex G-75, and the profiles of zinc, copper and platinum were determined along with those for incorporated radioactive cyst(e)ien and tyrosine. Both salinetreated cell lines contained similar levels of ‘MTs’, which were induced by exposure to a minimally toxic level of zinc (100 μM). Zinc induction of ‘MTs’ was nearly 4-fold greater in L1210 than in L1210-PDD cells. The levels of mRNA for metallothionein I (MTI) and (MTII) in uninduced cells were measured by dot-blotting with a cDNA probe. The L1210-PDD cells contained 80% of the MTI and 41% of the MTII compared with L1210 cells, confirming the similar levels in uninduced cells. L1210-PDD cells were 2-fold more sensitive than L1210 cells to cadmium and equally sensitive to zinc. Thus, the resistance of L1210-PDD cells tocis-DDP was not associated with cross-resistance to group IIb metals, whereas their sensitivity to cadmium did reflect the relative inability of the cells to synthesize ‘MTs’. The L1210 cells produced ‘MTs’ when treated with 0.5 and 5.0 μM cis-DDP, but the L1210-DDP cells did not when treated with 5.0–40 μM cis-DDP. Small amounts of platinum (cis-DDP is neither an increased level of ‘MTs’ in the resistant cells nor an enhanced ability to increase the synthesis of ‘MTs’ after drug exposure.