Cadmium-induced malignant transformation in rat liver cells: role of aberrant oncogene expression and minimal role of oxidative stress.
Our study examined the role of oxidative stress and aberrant gene expression in malignant transformation induced by chronic, low-level cadmium exposure in non-tumorigenic rat liver epithelial cell line, TRL 1215. Cells were cultured in 1.0 microM cadmium (as CdCl(2)) for up to 28 weeks and compared to passage-matched control cells. The level of cadmium used for transformation produced no evidence of increased superoxide (O(2) (-*.)) or hydrogen peroxide (H(2)O(2)) levels in the early stages of exposure (</=24 hr). The chronic cadmium exposed liver epithelial cells (CCE-LE) were hyperproliferative with a growth rate about 3-fold higher than control cells. CCE-LE cells produced highly aggressive tumors upon inoculation into mice confirming malignant transformation. Analysis of cellular reactive oxygen species (ROS) showed that CCE-LE cells possessed markedly lower basal levels of intracellular O(2) (-*.)and H(2)O(2) and were very tolerant to high-dose (50 microM) cadmium-induced ROS. Time course studies showed the production of ROS by high-dose cadmium was abolished well in advance of malignant transformation. In contrast, marked overexpression of the oncogenes c-myc and c-jun occurred in transformed CCE-LE cells as evidenced by up to 10-fold increases in both transcript and protein. A significant increase in DNA-binding activity of the transcription factors AP-1 and NF-kappaB occurred in CCE-LE cells. Increases in oncogene expression and transcription factor activity occurred concurrently with malignant transformation. Thus, cadmium-induced ROS occurs as an early, high-dose event but is abolished well in advance of malignant transformation. Low-level chronic cadmium triggers oncogene overexpression possibly by altering critical transcription factor activity. Such changes in cellular gene expression likely culminate in the loss of growth control and cadmium-induced neoplastic transformation in CCE-LE cells, whereas generation of ROS by cadmium seemed to play a minimal role in this transformation.
Qu
Wei
W
Inorganic Carcinogenesis Section, Laboratory of Comparative Carcinogenesis, National Cancer Institute at the National Institute of Environmental Health Sciences, Research Triangle Park, NC 27709, USA.
Diwan
Bhalchandra A
BA
Reece
Jeffrey M
JM
Bortner
Carl D
CD
Pi
Jingbo
J
Liu
Jie
J
Waalkes
Michael P
MP
D000818:Animals; D002104:Cadmium / Q000633:toxicity*; D002471:Cell Transformation, Neoplastic / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D004847:Epithelial Cells; D020869:Gene Expression Profiling; D005786:Gene Expression Regulation / Q000187:drug effects; D008099:Liver / Q000166:cytology / Q000473:pathology; D008113:Liver Neoplasms / Q000503:physiopathology* / Q000662:veterinary; D009857:Oncogenes / Q000235:genetics*; D018384:Oxidative Stress; D051381:Rats; D011916:Rats, Inbred F344; D017382:Reactive Oxygen Species; D014157:Transcription Factors / Q000096:biosynthesis / Q000494:pharmacology*
D017382:Reactive Oxygen Species; D014157:Transcription Factors; D002104:Cadmium