Hypersensitivity of Human Testicular Tumors to Etoposide-induced Apoptosis Is Associated with Functional p53 and a High Bax:Bcl-2 Ratio

Abstract

Metastatic testicular cancers are curable, whereas bladder cancers and most other solid tumors are not. Cell lines derived from human testicular (GH, GCT27, and 833K) and bladder (RT4, RT112, and HT1376) tumors retain this differential chemosensitivity in vitro. We have investigated the hypothesis that differential sensitivity to chemotherapy is related to differences in the threshold of susceptibility to undergoing apoptosis. Sensitivity to etoposide was not directly related to the frequency of DNA strand breaks. DNA damage was on average 2-fold greater in the testicular than the bladder tumor cell lines; in contrast, the testicular tumor lines were 15-fold more sensitive to etoposide cytotoxicity than the bladder tumor lines (IC₉₀ values of 19 ± 6 versus 293 ± 180 µm, respectively). Using equidamaging (550 rad equivalents) etoposide treatments, the percentage of cells that underwent drug-induced apoptosis was on average higher in the testicular tumor cell lines than the bladder tumor cell lines. The testicular tumor lines have two characteristics that could confer sensitivity to drug-induced apoptosis. First, they have functional p53: the product of the p53-dependent gene waf-1 was increased after etoposide treatment. Second, the testicular tumor lines expressed relatively high levels of the apoptosis-promoting protein Bax, but there was no expression of the suppressor of apoptosis Bcl-2. In contrast, only one of the three bladder cell lines (RT4) had functional p53, and all of the bladder lines had readily detectable levels of Bcl-2 and low levels of Bax. In the testicular cell lines, increases in p53 and p53-transactivated genes were associated with apoptosis but not arrest in G₁. In contrast, in the bladder cell line (RT4), increases in p53 and Waf-1 were associated with both arrest in G₁ and apoptosis. The differences in the ratio of Bax:Bcl-2 could contribute to the differential sensitivity of the two tumor types. However, in contrast to earlier reports, the ratio of Bax and Bcl-2 was not perturbed by DNA damage.