| Summary: | Cancer of the mammary gland is the most common type of cancer in women worldwide, and the vast majority of breast cancers originate from a cluster of malignant cells in the epithelial tissue of the breast, which initially confines the ductal carcinoma in situ. Research has shown that the signalling pathways that increase differentiation and maintain proliferation in normal epithelial cells are of utmost importance for sustaining this barrier against malignant cells. As a model for normal mammary epithelial cells, the MCF-12A cell line was used to determine factors that are required for cell cycle progression of these cells. A discontinuous treatment assay was developed in which the MCF-12A cells were treated with epidermal growth factor (EGF) and insulin at two distinct times to induce cell cycle re-entry. The use of these chemically defined growth factors enabled us to determine that continuous stimulation with mitogenic factors is not required for these cells to re-enter the cell cycle. An initial activation of the MAP kinase pathway and an up-regulation of the transcription factor c-Myc, followed by activation of the PI3K pathway, resulted in full competence to progress into S phase. The order in which the growth factors were applied, and thus the sequence in which the subsequent proteins were triggered, was of great importance for successful S phase entry. We found that estradiol (E2) was unable to induce the factors necessary for cell cycle progression. Furthermore, we report for the first time that E2 did not affect estrogen-regulated genes which normally are under the control of a ligand-bound estrogen receptor (ER). We suggest that the mechanism by which the ligand-activated ER usually interferes with the estrogen responsive element in the promoter region of the target genes is defective in the MCF-12A cell line. The results presented here may contribute to new approaches in chemotherapy, taking advantage of the diverse molecular mechanism in place for cell cycle progression and proliferation in malignant cells compared to normal mammary epithelial cells.
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