Differential Processing of Cyclin E Variants in Normal vs Tumor cells and their Role in Breast Cancer Oncogenesis
Abstract
Cyclin E is a posftive regulator, which controls the transition of the Ol 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 Gi and S. This commits the cell 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 weight 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 fall length cyclin E into these LMW forms. An altered cyclin E may contribute to the deregulation of the Gi to S checkpoint and lead to tumorigenesis. Our laboratory has also identified through mutational and biochemical analysis, the region of cyclin E that is proteolyticaly cleaved to generate the LMW forms. Critical phosphorylation sites of cyclin B are responsible for the appearance of the LMW forms of cyclin B. To invesrigate the possible role of phosphorylation in the processing of cyclin B 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. (isualization of cyclin B showed downward shifts in both the fall length and lower forms in the presence of active dephosphorylation. Second, fall length and truncated cyclin E cDNAs were mutated at critical phosphorylation residues via site-directed mutagenesis. Transfection of these mutants into tumor cells capable of LMW form processing followed by western analysis indicates that at least one of the mutations results in the loss of2 lower forms of cyclin B. However, assays for kinase activity in the transfected tumor cells demonstrate no change activity of the mutants deficient in these phosphorylation sites.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 01, 2003
- Accession Number
- ADA421770
Entities
People
- Khandan Keyomarsi
- Mollianne J. Mcgahren
Organizations
- The University of Texas MD Anderson Cancer Center