Contemplating chemosensitivity of basal-like breast cancer based on BRCA1 dysfunction

A DNA repair BRCA1 estrogen receptor and targeted therapy in breast cancer

BRCA1 is a key mediator of DNA repair pathways and participates in the maintenance of the genomic integrity of cells. The control of DNA damage repair mechanisms by BRCA1 is of great interest since molecular defects in this pathway may reflect a predictive value in terms of a cell’s sensitivity to DNA damaging agents or anticancer drugs. BRCA1 has been found to exhibit a hormone-dependent pattern of expression in breast cells. Wild-type BRCA1 is required for the inhibition of the growth of breast tumor cells in response to the pure steroidal ERα antagonist fulvestrant. Also a loss of BRCA1-mediated transcriptional activation of ERα expression results in increased resistance to ERα antagonists. Platinum-based drugs, poly(ADP-ribose) polymerase (PARP) inhibitors, and their combination are currently included in chemotherapy regimens for breast cancer. Preclinical and clinical studies in a BRCA1-defective setting have recently indicated a rationale for the use of these compounds against hereditary breast cancers. Initial findings indicate that neoadjuvant use of cisplatin results in high rates of complete pathological response in patients with breast cancer who have BRCA1 mutations. Cisplatin produces a better response in triple-negative breast cancer (TNBC) than in non-TNBC diseases in both the neoadjuvant and adjuvant settings. This implies that TNBC cells may harbor a dysfunctional BRCA1 repair pathway.

ER-/ER+ breast cancer cell lines exhibited different resistance to paclitaxel through pulse selection

In this study, we established two PTX-resistant breast cancer cell lines, 231 TIM10 and MCF-7 TIM10, from ER-negative MDA-MB-231 cells and ER-positive MCF-7 cells by pulse selection, respectively. We found that 231 TIM10 variants acquired higher drug resistance than MCF-7 TIM10 variants by the pulse selection, although ER-positive MCF-7 cells were not as sensitive as ER-negative MDA-MB-231 to the initial pulses with PTX. 231 TIM10 had 11.9-fold greater resistance (RI = 11.9) than the parental MDA-MB-231 cells, while MCF-7 TIM10 got 5.5-fold resistance (RI = 5.5) when compared with the parental MCF-7 cells. In the presence of 5nM PTX, 231 TIM10 cells formed colonies, but no colony formed when MCF-7 TIM10 cells were cultured in the same condition. These data have two implications. First, the ER expression state might be an important determinant for the response of breast cancer cells to paclitaxel treatment. Second, ER-negative and ER-positive breast cancer cells develop drug-resistance phenotype with distinctive mechanisms. Our work not only established useful models for studying the paclitaxel resistance but also provides interesting clues to understand the mechanisms underlying the drug resistance of ER-negative and ER-positive breast cancer cells.

Genome-wide approaches to systematically identify substrates of the ubiquitin-proteasome pathway

The ubiquitin-proteasome system handles the majority of controlled proteolysis in eukaryotes. Defects in the ubiquitin-proteasome system have been implicated in diseases ranging from cancers to neurodegenerative disorders. However, the precise role of ubiquitin-proteasome-mediated degradation in health and disease is far from clear. A major challenge is to link specific substrates directly to a particular degradation pathway. Here, we review genome-wide approaches that have been developed in recent years to comprehensively identify ubiquitylated substrates of a particular pathway. Components of the ubiquitin-proteasome system are attractive drug targets, as illustrated by the efficacy of some proteasome inhibitors in the treatment of multiple myeloma. Information that has emerged from these studies could reveal novel drug targets and strategies for treating human diseases.