Semester

Summer

Date of Graduation

2002

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Microbiology, Immunology, and Cell Biology

Committee Chair

Xianglin Shi.

Abstract

Vanadate is potent toxic and carcinogenic agent. The mechanisms of its toxic and carcinogenic actions are still under investigation. Cell cycle arrest is an important mechanism in maintaining genomic stability and integrity in response to environmental stress. The present study investigated the possible mechanisms involved in vanadate-induced cell cycle arrest and its signal transduction pathway. The results show that vanadate caused G2/M phase arrest A549 cells. p21 and cdc25C affected on cyclin B/cdc2 complex, resulting in the cell cycle arrest at G2/M phase. p38 and extracellular signal-regulated protein kinase (ERK) were involved in the regulation of these cell cycle regulatory proteins. Furthermore, reactive oxygen species (ROS), especially, hydrogen peroxide (H2O2), mediated vanadate-induced G2/M phase arrest. The effect of p53, a tumor suppressor gene, also was examined in the present study. It has been shown that vanadate-induced S phase arrest was dependent on p53 through p21 pathway. Both p38 and ERK acted as upstream regulators of p53. However, the role of p53 is both phase- and cell line-specific. Vanadate-induced G2/M phase arrest is p53-independent in mouse embryo fibroblasts. It has been reported that phosphatidylinositol-3 kinase (PI3K) and its down stream kinases, protein kinase B (PKB)/Akt and p70 S6 kinase (p70S6K) play an essential role in control of transcription and protein synthesis, which impact on cell growth, survival and metabolism. The present study investigated the effects of these kinases on vanadate-induced cell cycle arrest and its regulation. It has been found that vanadate activated both PI3K and Akt, which in turn caused hyperphosphorylation of retinoblastoma protein (pRb) and increased E2F1 expression. Induction of E2F1 activated cyclin E which triggered G1/S transition. Vanadate was also able to cause phosphorylation of p70S6K through PI3K-dependent pathway. Activation of p70S6K affected on p21, and cdc25A, leading to inactivation of cdk2. Therefore, inactivation of cdk2 caused the cells being unable to progress from S to G2 phase. The present study provides a molecular basis for understanding the mechanisms of vanadate-induced cellular responses, and furthers understanding of the carcinogenic process induced by this metal.

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