The Low Molecular Weight Isoforms of Cyclin E Deregulate the Cell Cycle of Mammary Epithelial Cells

Abstract

Cyclin E is a positive regulator of the G1 to S phase transition of the cell cycle. In complex with CDK2 it is responsible for cells passing the restriction point, committing the cell to another round of cell division. Cyclin E is overexpressed and proteolytically cleaved into low molecular weight (LMW) isoforms in breast cancer cell lines and tumor tissues compared to normal cells and tissues. These alterations in cyclin E are linked to poor prognosis in breast cancer patients. Our laboratory has determined that the LMW forms of cyclin E are generated post-translationally, via elastase mediated cleavage at 2 specific sites in the amino-terminus of the full length cyclin E. In order to evaluate the biological effects of the LMW cyclin E, immortalized mammary epithelial cells, 76NE6, were stably transfected with each of the three cyclin E constructs. Our results reveal that the LMW forms of cyclin E (T1 and T2) are biologically functional, as their overexpression in the immortalized cells increases the ability of these cells to enter S and G2/M phase by 2 fold over full length or vector-alone transfected cells, concomitant with an increased rate of cell proliferation. In addition, these LMW isoforms are biochemically hyperactive, shown by their ability to phosphorylate substrates such as histone H1 4 fold more in cells transfected with T1 or T2 versus cells transfected with the EL form. These results suggest that overexpression of the LMW forms of cyclin E is mitogenic, stimulating the cells to progress through the cell cycle much more efficiently than the full length cyclin E.