Thesis (Ph.D.) - Cairo University - National Cancer Institute - Department of Cancer Biology

Background:Tamoxifen is the gold-standard endocrine therapy for estrogen receptor positive (ER+) breast cancer. Nonetheless, tamoxifen resistance is a challenging clinical problem. Transcriptional activation of ER-Ü can be stimulated by cyclin-dependent kinase 7 (CDK7), which plays an important role in the regulation of cell cycle and the expression of genes involved in tumor progression and resistance to tamoxifen. Aim of the work: To study the potential modulatory role of CDK7 inhibitor in tamoxifen response and to explore its mechanistic pathways in the tamoxifen sensitive, MCF-7 and tamoxifen-resistant LCC2, breast cancer cell lines; in two orthotopic xenograft models, as well as in clinical samples. Methods: The cytotoxic effect of CDK7 inhibition, alone and in combination with tamoxifen was studied in vitro, in both tamoxifen-sensitive and resistant cell lines, as well as in orthotopic breast cancer mouse models. Drug interaction was evaluated using the isobologram equation. Apoptosis was evaluated by flow cytometry and TUNEL assay. The expression of angiogenesis marker (CD31), cell cycle regulatory proteins as well as downstream oncogenic markers were determined by flow cytometry and western blotting. Moreover, analysis of TCGA breast cancer data and the microarray data of patients receiving tamoxifen were investigated to evaluate the role of CDK7 in tamoxifen resistance. Results: The current in vitro study showed that CDK7 inhibitors either by siRNA or pharmacologically by THZ1 decreased the expression of activated estrogen receptor-Ü (p-ER-Ü S118) and downregulated the of essential oncogenes such as MYC, STAT3 and Ý{u2013}catenin expression which plays a crucial role in tamoxifen resistance

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