Differential Processing of Cyclin E Variants in Normal vs Tumor Cells and Their Role in Breast Cancer Oncogenesis

摘要

Cyclin E is a positive regulator, which controls the transition of the G1 to S phase of the cell cycle. When associated with CDK2, it is responsible for cells passing through the restriction point, which is the barrier between G1 and S. This commits the cells to complete one round of cell division. Previous findings by this laboratory have found that overexpression of cyclin E and the presence of lower molecular weigh isoforms (LMW) are found more often in breast tumors and cancer cell lines when compared to normal tissues and cells. Also, tumor cells, but not normal cells have the mechanisms to proteolytically cleave the full length cyclin E into these LMW forms. An altered cyclin E may contribute to the deregulation of the G1 to S checkpoint and lead to tumorigenesis. Our laboratory has also identified through mutational and biochemical analysis, the region of cyclin E that is proteolytically cleaved to generate the LMW forms. Critical phosphorylation sites of cyclin E are responsible for the appearance of the LMW forms of cyclin E. To investigate the possible role of phosphorylation in the processing of cyclin E into these lower forms, two approaches have been employed. First incubation of breast cancer cell line extracts expressing the LMW forms with phosphatases was examined via western blot analysis. Visualization of cyclin E showed downward shifts in both the full length and lower forms in the presence of active dephosphorylation.