Cyclin E, complexed to its kinase partner, cyclin dependent kinase (CDK) 2, regulates G1
to S phase progression of the cell cycle.1,2
The cyclin E/CDK2 complex is normally negatively regulated by the cyclin dependent kinase inhibitors (CKIs) p21 and p27. The balance between G1
to S phase progression and the ability to halt the cell cycle is delicately controlled by cyclin/CDKs and CKIs in normal cells, but is often aberrant in cancer cells.3
In addition, normal cells, but not cancer cells, can exit the cell cycle before the restriction point if an environmental stressor is detected, such as a lack of nutrients. However, after the restriction point, cells are committed to replicate their DNA. Due to the crucial role that regulated expression and cyclin E activity plays in maintaining proliferative homeostasis, any defects in its expression could have a critical effect in tumorigenesis. Therefore, it is not surprising that cyclin E expression is aberrant in many types of cancers including colorectal, gastric, ovarian, melanoma as well as breast cancer.4–7
The form of cyclin E predominantly expressed in normal and tumor cells is the full-length 50 KDa isoform, considered wild-type and referred to as EL. However, tumor cells are uniquely capable of post-translationally cleaving the full-length cyclin E in to low molecular weight (LMW) isoforms. Elastase cleaves the full-length cyclin E in to 44 kDa and 33 kDa isoforms, which are then phosphorylated to generate doublets that have been designated as Trunk 1 (T1, 44–45 kDa) and Trunk 2 (T2, 33–35 kDa). Clinical studies have indicated that cyclin E overexpression occurs in 25% of breast cancer tumors and is associated with poor prognosis.8
We previously determined the clinical significance of LMW isoforms of cyclin E in 395 women with breast cancer by measuring cyclin E expression with western blot analysis. We showed that high levels of total or LMW cyclin E were the most powerful discriminants of disease-free and overall survival, outperforming currently used clinical criteria including nodal status, stage and estrogen-receptor status.8
For example, in multivariate analysis of factors predicted of disease-specific and overall survival, a high level of the lowmolecular-weight isoforms of cyclin E was strongly associated with poor outcome with a hazard ratio of 2.1.8
While, these findings suggest that there may be utility for determining the level of cyclin E expression in breast tumor specimens to better define prognosis in breast cancer patients, the specificity of the LMW isoforms of cyclin E to tumor cells has been questioned recently.9
Therefore, in the current study, we prospectively assessed the expression of LMW cyclin E versus the full-length form in breast tumor tissue and adjacent normal tissue from 340 breast cancer patients to validate our hypothesis that the LMW isoforms of cyclin E are in fact tumor specific.
LMW isoforms of cyclin E, compared to full-length cyclin E, de-regulate the cell cycle and have unique biochemical properties. For example, the overexpression of LMW cyclin E in MCF-7 breast cancer cells results in resistance to the growth inhibiting effects of anti-estrogens,10
due to their resistance to inhibition by p21 and p27.11
Compared with transgenic mice overexpressing full-length cyclin E, those expressing LMW cyclin E developed metastatic tumors and had significantly decreased survival.12
In this study, to directly link the LMW isoforms of cyclin E to the process of breast tumorigenesis, we assessed the phenotype of non-tumorigenic mammary epithelial cells, 76NE6, engineered to express the LMW cyclin E to determine whether the cells exhibited a phenotype characteristic of neoplastic transformation. This study shows that the LMW isoforms of cyclin E are indeed tumor-specific, and are both biologically and clinically relevant.