Cyclin E overexpression and associated events in human breast cancer

Sammanfattning: Unrestrained proliferation is a hallmark of cancer and genetic defects within G1/S-phase regulation and the pRb pathway occur frequently. Proliferation control can be circumvented either by excess cyclin D1 or cyclin E, alterations that can define two alternative tumour biologic pathways in breast cancer. By overexpressing cyclin E in a cell line model system we demonstrate that the capacity of cells to normalize the level of active cyclin E/cdk2 was dependent on the ability to upregulate and re-direct p21 and p27 to the active kinase complex, as could be observed in ER positive and cyclin D1 high, but not the ER negative and cyclin E high cell lines. The results further indicated that cyclin E and associated kinase activities might regulate proliferation independent of pRb. One alternative substrate for cyclin E kinase is the ID2 protein. Upon ID2 overexpression, the proliferative capacity increased but the invasive potential of breast cancer cell lines decreased. In primary breast cancer, high ID2 expression was associated with a favourable outcome and a less aggressive and more differentiated luminal breast cancer phenotype. Next we evaluated the incidence of aberrant cyclin E expression in the S/G2/M-phases, and investigated the potential tumour biologic characteristics associated with impaired cyclin E degradation. Cyclin E overexpressing breast cancer cells lines displayed varying ability to degrade excess cyclin E, and exhibited prolonged S-phase progression. Tumours with aberrant cell cycle specific expression pattern of cyclin E correlated inversely with survival status of patients and there was a favour for c-myc amplification within this group of tumours. We further characterised potential novel functions of cyclin E, and breast cancer cell lines overexpressing cyclin E induced significant changes in gene expression related to cell adhesion. Cells further showed an impaired capacity to migrate and invade through ECM. In primary breast cancer we observed a positive correlation between cyclin E expression and tumours with a pushing growth pattern and a medullary breast cancer type, clearly validating the cell line observations. Due to the link between cyclin E and growth pattern, as well as survival, cyclin E expression was consequently associated with impaired prognosis exclusively in patients with breast cancer with an infiltrative growth pattern. Thus, the effects of cyclin E expression and breast cancer progression are multiple, including altered proliferation as well as tumour growth pattern.