Proteomic Map of ER+ Breast Cancer Cell Cycle

Cancer is characterized by a deregulation of the cell cycle resulting in abnormal proliferation of cells that can bypass tightly regulated molecular checkpoints. Breast cancer is the most common cancer diagnosed in women, ~70% of cases displaying an estrogen receptor positive (ER+) phenotype. The aim of the present work was to generate a comprehensive overview of the biological mechanisms, molecular pathways and specific proteins involved in cell cycle progression in ER+ breast cancer cells. We focused on the G1-to-S phase transition of the cell cycle because major differences in cell proliferation mechanisms between normal and cancerous cells are observed at this point. We developed a large-scale proteomics strategy to enable the comparison of MCF-7 ER+ (cancer) and MCF-10A (non-tumorigenic) epithelial breast cells. Samples were analyzed by liquid chromatography tandem mass spectrometry (LC-MS/MS) followed by a label-free quantitation approach, i.e., spectral counting, for differential protein expression analysis. The study was divided into three distinct parts: 1) qualitative profiling of MCF-7 cells arrested in the G1-phase and released into the S-phase of the cell cycle, 2) differential expression profiling of MCF-7 cells in G1 and S, and 3) differential expression profiling of the G1-phases of MCF-7 and MCF-10A cells. The qualitative evaluation of MCF-7 proteomic data resulted in the identification of >2700 proteins (p-score<0.001). A large number of these proteins were involved in cell cycle relevant processes, being representative of all hallmarks of cancer. Differential expression analysis of the MCF-7 G1 and S-phases resulted in the identification of >250 proteins with roles in DNA repair, transcription, translation, chromatin maintenance and signaling. The MCF-7/MCF-10 comparison revealed that major cellular processes that require DNA access, such as the ones identified in the MCF-7 analysis, are up-regulated in the nucleus of MCF-7 cells during starvation, possibly allowing these cancerous cells to bypass the restriction point. Several proliferative and anti-proliferative markers were identified in both MCF-7 and MCF-10A cells.