Chromium (VI)-induced cytotoxicity to osteoblast-derived cells

Chromium is well recognized as a carcinogen, and there are concerns about local and systemic cytotoxicity and carcinogenicity during the use of chromium-containing alloys for orthopaedic implants. We have investigated the cytotoxicity of Cr VI in immortalized rat osteoblast cells in vitro using alkaline phosphatase (ALP) activity as an index of toxicity. Cr VI caused a concentration-dependent decrease in ALP activity, thought to be mediated by intracellular reduction to Cr III. The role of several antioxidant vitamins, reduced glutathione (GSH) and DT-diaphorase in the reduction of Cr VI was investigated. The toxic response to Cr VI was partially prevented by treating the cells with ascorbic acid. In contrast, riboflavin and alpha-tocopherol did not alter the response. Ascorbic acid is thought to reduce Cr VI to Cr III extracellularly. This protects the cells because they are relatively impermeable to Cr III. Treatment of cells with dicoumarol, an inhibitor of DT-diaphorase activity, also decreased the toxicity of Cr VI, suggesting that this enzyme is involved in the intracellular reduction of the metal. GSH was not depleted during the metabolism of Cr VI and this was probably due to the activity of glutathione reductase which acts to recycle oxidized glutathione. However, depletion of intracellular GSH by buthionine sulfoximine increased the toxicity of Cr VI at early time points (after 1.5 and 3hr exposure). GSH and Cr VI therefore interact in the osteoblasts, and this may be through formation of a conjugate and/or by detoxification of reactive intermediates formed during redox cycling of the chromium.