Oxidative Stress in Cr(VI)-Induced Carcinogenesis

Hexavalent chromium [Cr(VI)] is a known environmental carcinogen. Environmental exposure to Cr(VI)-containing compounds causes cancers of various organs. Its mechanism of carcinogenic actions remains to be investigated. Recent studies have demonstrated that reduction of Cr(VI) to its lower oxidation states is important for Cr(VI)-induced carcinogenesis. It has been shown that cellular reduction of Cr(VI) generates reactive oxygen species (ROS) which cause various cellular injuries. We hypothesize that Cr(VI) induces generation of ROS, which activate nuclear transcription factors, leading to cell transformation and tumorigenesis. In Specific Aim 1, we will identify ROS generated in Cr(VI) treated human lung bronchial epithelial (BEAS-2B) cells and the mechanism involved. We will emphysize the role of CDC42 signaling and the involvement of p47phox and NADPH oxidases in the molecular mechanism of Cr(VI)-induced ROS production. In Specific Aim 2, we will detect reactive Cr(V) intermediates produced in Cr(VI) reduction by living animals and identify the chemical structures of these intermediates. We will also detect and identify superoxide (020 -) and hydroxyl (oOH) radicals produced by intact animals exposed to Cr(VI). Major methods to be used in this aim are low frequency electron spin resonance (ESR) spectroscopy and ESR spin trapping. In Specific Aim 3, we will investigate Cr(VI)-induced activation of NF-kappaB and AP-1 in BEAS-2B cells and in transgenic mice. We will use a dominant negative mutant MEK1/2, a domiant negative mutant ERK2, or a dominant negative mutant c-jun (TAM67), respectively, to investigate the role of the MEK1/2-ERKs-AP-1 pathway in Cr(VI)-induced cell transformation. We will also use a dominant negative mutant IkappaBa or a dominant negative mutant IKKI3, respectively, to determine the role of the IKKI3-lkappaBa-NF-kappaB pathway in Cr(VI)-induced cell transformation. The involvement of ROS in these pathways will be investigated using expressions of specific antioxidant enzymes. Tumorigenesis will be investigated by injecting the transformed cells into athymic nude mice. We anticipate that Cr(VI) causes activation of transcription factors through ROS reactions, leading to cell transformation and tumorigenesis. We attempt to link the cell transformation, and tumorigenesis with specific transcription factors and specific reactive oxygen species.