ToxSci Advance Access originally published online on September 8, 2005
Toxicological Sciences 2005 88(2):602-613; doi:10.1093/toxsci/kfi318
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Basal and Zinc-Induced Metallothionein in Resistance to Cadmium, Cisplatin, Zinc, and tertButyl Hydroperoxide: Studies Using MT Knockout and Antisense-Downregulated MT in Mammalian Cells
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* Cancer Research Laboratory Program, London Regional Cancer Program, London Health Sciences Centre, London, Ontario, Canada N6A 4L6; Division of Basic Medical Sciences, Mercer University School of Medicine, Macon, Georgia; Departments of Oncology, Physiology and Pharmacology, Pathology, and Microbiology and Immunology, University of Western Ontario; and Lawson Health Research Institute, London, Ontario, Canada N6A 4L6
Received July 28, 2005; accepted September 6, 2005
Metallothioneins (MTs) mediate resistance to metal and non-metal toxicants. To differentiate the role of MTs from other protective factors, resistance to zinc (Zn), cadmium (Cd), tertbutyl hydroperoxide (tBH), and cisplatin (CDDP) was compared in renal cell lines from wild type (MT-WT) and MT-1/MT-2 knockout (MT-KO) mice. MT-WT cells were more resistant to tBH than MT-KO cells but, unexpectedly, were more sensitive to Zn, Cd, and CDDP. Thus, basal expression of MT conferred resistance to tBH, but not to Cd or CDDP. Pretreatment with Zn increased MT expression and enhanced resistance to Cd and CDDP only in MT-WT cells, indicating a critical role for MT in this form of resistance. By contrast, Zn-pretreatment increased resistance to subsequent Zn exposure, but did not alter resistance to tBH, regardless of MT-status. Therefore, Zn-induced resistance to subsequent exposure to Zn (but not to Cd or CDDP) was mediated by non-MT factors, and neither Zn-induced MT nor other factors affected tBH sensitivity. Furthermore, antisense down-regulation of MT in human HeLa cells reduced basal MT levels and resistance to TBH, but not to Cd or CDDP. Therefore, basal MT alone can mediate resistance to TBH (but not to Cd or CDDP) in mouse and human cells. These data suggest that MT can mediate resistance to toxicants by different mechanisms, some of which correlate with the cellular content of MT protein. Moreover, resistance to some agents (Cd and CDDP) can be enhanced by inducing MT. Resistance to other agents (tBH) requires only basal (non-induced) MT levels.
Key Words: metallothionein; knockout; antisense; drug sensitivity; drug resistance; cisplatin; cadmium; reactive oxygen species; tertbutyl hydroperoxide; zinc.