Altered Subcellular Localization of Tumor-Specific Cyclin E Isoforms Affects Cyclin-Dependent Kinase 2 Complex Formation and Proteasomal Regulation

Departments of ¹Experimental Radiation Oncology and ²Surgical Oncology, University of Texas at M. D. Anderson Cancer Center, Houston Texas

^* To whom correspondence should be addressed. E-mail: kkeyomar{at}mdanderson.org.

	Abstract

In tumors, alternative translation and posttranslational proteolytic cleavage of full-length cyclin E (EL) produces tumorigenic low molecular weight cyclin E (LMW-E) isoforms that lack a portion of the EL amino-terminus containing a nuclear localization sequence. Therefore, we hypothesized that LMW-E isoforms have altered subcellular localization. To explore our hypothesis, we compared EL versus LMW-E localization in cell lysates and in vivo using fractionation and protein complementation assays. Our results reveal that LMW-E isoforms preferentially accumulate in the cytoplasm where they bind the cyclin E kinase partner, cyclin-dependent kinase 2 (Cdk2), and have associated kinase activity. The nuclear ubiquitin ligase Fbw7 targets Cdk2-bound cyclin E for degradation; thus, we examined if altered subcellular localization affected LMW-E degradation. We found that cytoplasmic LMW-E/Cdk2 was less susceptible to Fbw7-mediated degradation. One implication of our findings is that altered LMW-E and LMW-E/Cdk2 subcellular localization may lead to aberrant LMW-E protein interactions, regulation, and activity, ultimately contributing to LMW-E tumorigenicity. [Cancer Res 2009;69(7):2817–25]

Key Words: cyclin E, low molecular weight cyclin E, subcellular localization, Fbw7