Spontaneous transformation of cultured rat liver (TRL 1215) cells is associated with down-regulation of metallothionein: implications for sensitivity to cadmium cytotoxicity and genotoxicity.

摘要

Previously, we found cadmium (Cd) to be effective in suppressing liver and lung tumors in rodents. Thus, this study investigated the susceptibility of cultured cells to Cd during spontaneous transformation. The TRL 1215 cell line is an epithelial-like liver cell normally nontumorigenic. However, continuous passage can occasionally result in spontaneous transformation. In this study, we found that continuous passage (p) of TRL 1215 cells through p24-28 (high passage; HP) resulted in a spontaneous transformant. In contrast to low-passage (LP) cells (p15-19), the HP transformant had a fibroblast-like morphology and grew in soft agar. A passage-dependent decrease in Cd-induced DNA single-strand damage (SSD) was seen, indicating that HP cells were resistant to Cd genotoxicity. Both LP and HP cells exhibited similar sensitivity to gamma-irradiation-induced SSD, suggesting that resistance to Cd genotoxicity in HP cells was not indicative of generalized tolerance to DNA damage. In contrast to genotoxicity, HP cells were more sensitive to Cd-induced cytotoxicity than were LP cells. The LC50 for a 2-hour Cd exposure was approximately 1,060 microM for LP and 660 microM for HP cells. At genotoxic concentrations (500 microM) of Cd, LP cells accumulated approximately 1.8-fold more Cd than did HP cells, which may account for the reduced genotoxicity in HP cells but is not consistent with the enhanced cytotoxicity in the transformants. In contrast, LP cells had 7.4-fold greater basal metallothionein (MT) protein levels than did HP cells, which probably accounts for the increased cytotoxicity in HP cells. Basal levels of MT mRNA were similarly greater in LP cells. Thus, during spontaneous transformation of TRL 1215 cells, MT gene expression decreased, thereby increasing the susceptibility of these cells to Cd-induced cytotoxicity, which is consistent with an antitumor effect of Cd in some tumors that poorly express MT. However, MT expression, generally accepted to prevent almost all aspects of Cd toxicity, actually facilitated Cd genotoxicity, at least as assessed by SSD in LP cells.