Date of Award

2016

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Pharmaceutical Sciences

First Advisor

Zahir A. Shaikh

Abstract

Cadmium (Cd) is carcinogenic and a well-established human health risk. Epidemiological and experimental studies have demonstrated that Cd is associated with breast cancer. We reported previously that Cd promoted breast cancer cell growth. Whether Cd treatment also causes epithelial-mesenchymal transition (EMT) and progression to metastasis in breast epithelial cells is unknown. The dissertation described here investigated the effect of Cd in breast cancer progression.

Noncancerous breast epithelial MCF10A cells, non-metastatic HCC 1937 and HCC 38 cells, and metastatic MDA-MB-231 cells were treated with 1 or 3 µM Cd for 4 weeks. The increase in the expressions of N-cadherin and vimentin but decrease in the expressions of E-cadherin and claudin-1 indicated that these epithelial cells were transformed to mesenchymal-like upon prolonged Cd treatment. Treatment of MCF10A-Zcad and MDA-MB-231-Zcad cells with 1 or 3 µM Cd showed the suppression of E-cadherin transcription and an increase in the translation of ZEB-1, suggesting that transcription factors were involved in the EMT process. The accumulation of Snail in the nucleus of cells treated with Cd for 4 weeks confirmed that Cd affected the expression of the EMT regulator. Inhibition of transcription by actinomycin D eliminated the Cd-induced Snail accumulation. However, inhibition of nuclear export by leptomycin B had an additive effect on Cd-induced Snail expression. Based on these results, it appears that Cd induces EMT in breast epithelial cells by modulating the transcription of Snail.

The effects of Cd on metastasis-associated phenotypes were investigated in MDA-MB-231 cells. Treatment of the metastatic cells with 1 or 3 µM Cd for 1-6 h resulted in cell spreading. The activation of integrin β1, FAK, Src, and Rac1 indicated the involvement of integrin signaling in Cd-induced cell spreading. Treatment with 1 or 3 μM Cd also inhibited GSK3β and induced T-cell factor/lymphoid enhancer factor (TCF/LEF) transcription, suggesting the involvement of Wnt/β-catenin pathway. Furthermore, treatment with 3 μM Cd for 4 weeks increased β-catenin expression, resulting in enhanced TCF/LEF transcription. Prolonged Cd treatment elevated the expression of integrin α5 and β1, paxillin, and vimentin. This indicated cytoskeleton remodeling leading to malignancy, as evidenced by enhanced matrix metalloprotease 2/9 (MMP2/9) secretion and cell invasion. Furthermore, prolonged Cd treatment increased cell growth rate. Together, these results suggest that Cd alters integrin and β-catenin signaling involved in regulation of cytoskeleton and enhances cancer cell migration, adhesion, invasion, and proliferation.

Available for download on Friday, November 30, 2018

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