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Breast Cancer |
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Estrogen plays a critical role in the development of breast cancer, because it can stimulate the growth of breast tissues. If the breast contains abnormal cells, they may also be stimulated by estrogen to proliferate and become a tumor. Estrogen and Its Receptors Two receptors for estrogen have been identified: ERa and ERb. We shall focus on ERa since its biological functions are relatively well understood. ERa is a transcription activator which can bind to the Estrogen Response Element of the target gene and then activate the gene transcription. In the absence of estrogen, ERa is inactivated by the binding of a heat shock protein (HSP90) within the nucleus. Upon estrogen binding, the receptor undergoes conformational change so that HSP90 is released, enabling ERa to activate gene transcription. ERa may also become a transcription repressor upon binding of certain antagonists such as tamoxifen - the most widely used drug for breast cancer. Details are given in this review article. More recently, it was found that ERa can also be activated in the absence of estrogen. The structural change required for its activation can also be induced by phosphorylation of some of its residues (reference). Since protein kinases (which catalyze phosphorylation) are activated during growth factor signaling, there is substantial "cross-talk" between growth factor and estrogen receptor signaling (review, reference 1, reference 2). The existence of estrogen-independent pathways suggests that drugs targeting estrogen alone (e.g., Aromatase inhibitors) may not be able to stop tumor growth completely. Its mechanism also provides explanation for tamoxifen resistance. As mentioned above, a major function of estrogen is to stimulate cell growth and proliferation. This can be achieved by expressing growth factors via the function of estrogen receptor (ER) as a transcription activator. The process of gene expression, however, is slow. Recently, it has been demonstrated that estrogen can also stimulate cell proliferation without expressing any molecules. This pathway (known as nongenomic pathway) does not involve ER, but requires a G-protein-coupled receptor called GPR30. Experiments suggest that estrogen may bind to GPR30, activating metalloproteinases to cleave heparan-bound EGF (epidermal growth factor). The released EGF can then stimulate cell proliferation by acting on its receptor EGFR (reference 1, reference 2). In this pathway, a functional growth factor is rapidly generated by enzymatic cleavage without invoking slow gene expression. The existence of ER-independent pathway suggests that drugs targeting ER only (e.g., ER downregulators) may not be sufficient to inhibit tumor growth. Prominent Genes in Breast Cancer Mutations of BRCA1 are found in approximately 8% of breast cancer cells. However, if a woman is inherited with BRCA1 mutations, the possibility to develop breast cancer by age 50 is as high as 60%. BRCA1 is a transcription regulator. It can repress the transcriptional activation function of ERa Mutated BRCA1 would allow ERa to activate transcription of target genes, thereby promoting proliferation of abnormal cells (reference). BRCA1 also participates in DNA repair (review). Related genes: BRCA2, BRCA3.
HER2 is also called ErbB2 or Neu, with structure and function similar to EGF receptor. This gene is overexpressed in 20-25% of breast cancer. This gene encodes the protein cyclin D1, involved in cell cycle progression. It is overexpressed in approximately 15% of breast cancer. Its frequency of mutation in breast cancer is 20-25%, well below its average of 50% in all types of cancer. Its frequency of mutation in breast cancer is less than 10%, also below its average of 25% in all types of cancer.
Mechanisms of Drug Actions Tamoxifen binds to ER and modulates its transcription function. Its usefulness is limited by the development of drug resistance: about half of patients with advanced breast cancer immediately fail to respond to tamoxifen, others eventually develop the drug resistance. Possible mechanisms:
These drugs destroy estrogen receptors (web link). Thus, they will not have drug resistance due to the mechanism 2. SERMs SERMs stands for "selective estrogen-receptor modulators". Examples: tamoxifen, raloxifene and arzoxifene (reference). Herceptin This drug is a monoclonal antibody that binds to HER2, and inactivates its function. The drug is used primarily for patients who overexpress HER2. Aromatase inhibitors These drugs bind to the body’s aromastase enzyme, which is responsible for producing estrogen. Example: Femara. Chemotherapy drugs These drugs attack dividing cells by altering DNA structure or interfering with microtubules. Examples: Adriamycin, Ellence, Cytoxan, and Taxol.
Review Articles: Estrogen receptors and human disease - J. Clin. Invest., 2006. Breast Cancer Metastasis to the Central Nervous System - Am. J. Pathology, 2005. Predictive Markers in Breast and Other Cancers: A Review - Clinical Chemistry, 2005. The Role of the BRCA1 Tumor Suppressor in DNA Double-Strand Break Repair - Mol. Cancer Research, 2005. Roles and Regulation of Stat Family Transcription Factors in Human Breast Cancer - Am. J. Pathol., 2004. The detection of circulating breast cancer cells in blood - J. Clin. Pathol., 2004. Molecular and Cellular Determinants of Estrogen Receptor a Expression - Mol. Cell Biology, 2004. Ovarian Tumorigenesis - Am. J. Pathology, 2004. HER-2-Targeted Therapy - Clinical Cancer Research, 2003. Mechanisms of Estrogen Action - Physiological Reviews, 2001. The initiation of breast and prostate cancer - Carcinogenesis, 2002. Cellular and Molecular Pharmacology of Antiestrogen Action and Resistance - Pharmacological Reviews, 2001. Understanding the genotoxicity of tamoxifen? - Carcinogenesis, 2001.
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