Tumor-specific proteolytic processing of cyclin E generates hyperactive lower-molecular-weight forms

Cyclin E is a G(1) cyclin essential for S-phase entry and has a profound ro le in oncogenesis. Previously this laboratory found that cyclin E is overex pressed and present in lower-molecular-weight (LMW) isoforms in breast canc er cells and tumor tissues compared to normal cells and tissues. Such alter ation of cyclin E is linked to poor patient outcome. Here we report that th e LMW forms of cyclin E are hyperactive biochemically and they can more rea dily induce G(1)-to-S progression in transfected normal cells than the full -length form of the protein can. Through biochemical and mutational analyse s we have identified two proteolytically sensitive sites in the amino termi nus of human cyclin E that are cleaved to generate the LMW isoforms found i n tumor cells. Not only are the LMW forms of cyclin E functional, as they p hosphorylate substrates such as histone H1 and GST-Rb, but also their activ ities are higher than the full-length cyclin E. These nuclear localized LMW forms of cyclin E are also biologically functional, as their overexpressio n in normal cells increases the ability of these cells to enter S and G(2)/ M. Lastly, we show that cyclin E is selectively cleaved in vitro by the ela stase class of serine proteases to generate LMW forms similar to those obse rved in tumor cells. These studies suggest that the defective entry into an d exit from S phase by tumor cells is in part due to the proteolytic proces sing of cyclin E, which generates hyperactive LMW isoforms whose activities have been modified from that of the full-length protein.