Molecular mechanisms of hexavalent chromium-induced centrosome amplification.

Abstract

Lung cancer is the deadliest form of cancer and resulted in 1.8 million deaths worldwide in 2020. While cigarette smoking is the most familiar cause of lung cancer, up to 25% of cases occur in non-smokers, thus other environmental agents are also causative. Hexavalent chromium [Cr(VI)] is a known lung carcinogen and poses occupational and environmental exposure risks relevant to humans, wildlife, and ecosystems. This dissertation considers the carcinogenic mechanisms of a highly potent, particulate, hexavalent chromium compound, zinc chromate. The molecular mechanism of carcinogenesis induced by Cr(VI) is not fully understood, but it is known that chromosome instability is a key effect. Chromosome instability refers to structural instability characterized by breaks and translocations, and numerical instability characterized by changing numbers of chromosomes. This dissertation focuses on how hexavalent chromium causes numerical chromosome instability in human lung cells and uses the One Environmental Health approach to gain insights into the associated mechanism using whale cells as a comparative model. The hypothesis of this project is: Prolonged hexavalent chromium exposure targets securin in human lung cells, leading to centrosome amplification and numerical chromosome instability, while the ability of whale cells to retain normal securin levels confers resistance to these effects. A main driver of numerical chromosome instability is centrosome amplification, defined in this study as a single cell with more than two centrosomes. We previously found Cr(VI) induces centrosome amplification, which increased with duration and concentration of exposure and correlated with Cr(VI)-induced aneuploidy. In Aim 1 we focused on a novel potential target of Cr(VI), securin. Securin is an important centrosome regulator because it is the canonical inhibitor of separase. Separase is the enzyme that causes centriole disengagement and permits centrosome duplication. Prolonged Cr(VI) exposure decreased securin protein levels in a dose-dependent manner. Securin protein loss was not due to changes in protein degradation, but rather a loss of securin mRNA. Three measures of securin function were analyzed to determine if decreased securin levels were sufficient to control separase activity. Separase cleaves itself, kendrin, and cohesin. Prolonged Cr(VI) exposure caused increased separase autocleavage, increased kendrin cleavage, and increased separation at centromeres caused by cohesin cleavage. Securin knockdown increased levels of aneuploidy after acute Cr(VI) exposure, in contrast to untransfected cells which retained normal background levels. Together these data showed Cr(VI) disrupts securin, a key protein in the maintenance of numerical chromosome stability. Aim 2 sought to uncover the mechanism of Cr(VI)-induced