Cr(VI) is a common and well-recognized environmental carcinogen that causes lung and other cancers in human. Despite considerable research effort to understand the molecular and cellular mechanisms of Cr(VI) carcinogenesis, crucial facets of these mechanisms remain largely unknown; identification of the key molecular regulators and processes, the mechanisms of action, and strategies for prevention are urgently needed. Remarkable progress has been made that highlights the functional importance of mRNA translation and protein synthesis in cancer development and progression. However, translational control of Cr(VI) carcinogenesis has not yet been investigated. We recently demonstrated that chronic low dose Cr(VI) exposure in human lung bronchial epithelial cells can induce cancer stem cell (CSC)-like properties, cell transformation and tumor formation through upregulation of histone-lysine methyltransferases including G9a, SUV39H1 and EZH2. Notably, we found that chronic Cr(VI) exposure can activate mTOR kinase leading to phosphorylation (inactivation) of the translational repressor 4E-BP1, and thereby promote the recruitment of capping mRNAs and ribosomal subunits to the eIF4F translation initiation complex (eIF4E+eIF4G+eIF4A) for active translation in polysomes. Furthermore, we found that on chronic Cr(VI) exposure, the EZH2 mRNA is actively translated in polysomes with no changes in its transcriptional level. Importantly, targeted inhibition of mTOR kinase can inhibit translation of EZH2 mRNA in polysomes and result in growth inhibition of Cr(VI)-transformed cells. Based on these findings, the central hypothesis of the proposed study is that the activation of cap-dependent mRNA translation machinery by mTOR kinase is required to selectively upregulate key oncoproteins that confer Cr(VI)- induced tumorigenesis. To test this hypothesis, two specific aims are proposed.
Aim 1 will define the role of mTOR-activated translation machinery in Cr(VI)-induced tumorigenesis using a combination of molecular, cellular, biochemical, and pharmacologic approaches, as well as genetically engineered mouse models with clinical association schemes.
Aim 2 will characterize the molecular mechanism of mTOR-activated cap- dependent translation in Cr(VI) carcinogenicity using a polysome profiling approach to systematically identify a set of key mRNAs that are selectively recruited to polysomes and translated in Cr(VI)-transformed human bronchial epithelial cells, followed by characterization of their functional importance in Cr(VI)-induced tumorigenicity. Our proposed study will investigate the innovative concept that Cr(VI) induces carcinogenesis through dysregulation of the mTOR/4E-BP1-mediated translation initiation process. Elucidating this process will provide novel insights into the biology and clinical relevance of translational regulation in Cr(VI) carcinogenesis. This research will have the added benefit of identifying Cr(VI)-induced translationally-regulated targets for potential preventive or interventional therapy.
Hexavalent chromium [Cr(VI)] is one of the most common environmental and occupational pollutants, and exposure to Cr(VI) is a world-wide public health concern, including for the region of Appalachian Kentucky. This proposal investigates the dysregulation of the mTOR/4E-BP1-mediated mRNA translation initiation process in Cr(VI) carcinogenesis, a study that has never before been attempted. This study is expected to identify new molecular targets and biomarkers for improving prevention and treatment of Cr(VI)-induced cancers.