Effect of cisplatin exposure on the degree of N-myc amplification in small cell lung carcinoma cell lines with N-myc amplification.

To know whether the degree of N-myc gene amplification would change or not by the treatment with chemotherapeutic agents, we studied the effect of cisplatin (CDDP) exposure on the degree of amplification of the N-myc gene in two small cell lung carcinoma (SCLC) cell lines with N-myc amplification (H-69, SBC-4). The N-myc gene was amplified around 60-fold in H-69 and 40-fold in SBC-4 cells compared with placenta tissues, and these two cell lines were more resistant to CDDP than the nine other SCLC cell lines without N-myc amplification. H-69/CP cells became 4.1 times and SBC-4/CP cells 2.1 times more resistant to CDDP compared to their parent cells by continuous in vitro exposure to CDDP. However, there was no significant difference in the degree of amplification or overexpression of the N-myc gene between parent and CDDP-resistant cells in both cell lines. Namely, the degree of N-myc amplification did not show a significant change by making these cells more resistant to CDDP. These results suggest that (1) the amplification of the N-myc gene may be unchangeable by chemotherapeutic agents, and (2) the degree of CDDP resistance may not correlate with the degree of amplification or over-expression of the N-myc gene, if activation of the N-myc gene would be related to the drug resistance.