The Effects of Zinc on Homeostasis, Growth and Oxidative Stress Responses in Cultured Cells

Zinc is involved in many physiological processes and is required as a catalytic cofactor for more than 300 metalloenzymes. Zinc levels are tightly regulated for maintaining cellular homeostasis. Previous studies from our laboratory with radioactive 65Zn demonstrated that cancer cell lines decrease zinc efflux when zinc availability is deprived with the extracellular metal chelator DTPA (diethylenetriaminepentaacetic acid). Primary cell lines, however, give out more zinc under the same conditions. The focus of this dissertation is to further examine these differential homeostatic responses and to investigate the underlying mechanisms. ^ This dissertation is comprised of three majorstudies investigating (1) the differential zinc homeostasis in rat lymphoma and hepatoma cell lines, (2) the effects of immortalizing rat hepatocytes and knock down of the plasma membrane localized zinc efflux transporter ZnT-1 on cellular responses to DTPA, and (3) the effects of altered zinc availability on viability and oxidative stress in a hepatoma cancer cell line. Findings suggest that (1) depletion of extracellular zinc induces a decrease in retention of zinc, a depletion of glutathione levels and an increase in oxidative stress in lymphoma cells, (2) transformation of hepatocytes result in greater retention of zinc in the face of limited availability, (3) ZnT-1 is crucial for zinc retention during zinc deficiency in cancer cells, and (4) both zinc and zinc deficiency induce oxidative stress, activating p53 and leading to two different mechanisms of apoptotic cell death. Taken together, these data suggest that transformed cells increase zinc retention to meet their growth demands under conditions of zinc deprivation, perhaps by altering the activity of ZnT-1. Although, cancer cells alter homeostatic response to survive with limited zinc availability, exposure to diminished or increased zinc concentrations leads to apoptosis and/or necrosis. Overall, these studies provide greater insights for understanding cellular zinc homeostasis and the complex roles that zinc plays in physiological processes.