Abstract
Heat shock protein 90 (Hsp90) is a molecular chaperone that facilitates the correct folding and functionality of its client protein. Numerous Hsp90-client proteins are involved in cancer development. Thus, Hsp90 inhibitors have potential applications as anti-cancer drugs. We previously discovered that Hsp90α expression increased in breast cancer stem cells (BCSCs), which can initiate tumorigenesis and metastasis and resist treatment. In the present study, we further demonstrated that 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG), an inhibitor of Hsp90, could suppress the self-renewal of BCSCs by downregulating B lymphoma Mo-MLV insertion region 1 homolog (BMI1), a polycomb family member with oncogenic activity in breast cancer. Through immunoprecipitation analysis, we found that BMI1 did not interact with Hsp90α and that the downregulation of BMI1 by 17-DMAG was mediated by the inhibition of c-Myc and enhancement of zeste homolog 2 (EZH2) expression. The transcriptional and BMI1 promoter-binding activities of c-Myc in BCSCs were inhibited by 17-DMAG treatment. The overexpression of EZH2 attenuated the inhibitory effect of 17-DMAG on BMI1 and c-Myc expression. Furthermore, Hsp90α could be co-immunoprecipitated with c-Myc and EZH2 and bind to the BMI1 promoter. Treatment with 17-DMAG decreased the nuclear expression of EZH2 and c-Myc but not that of Hsp90α. In conclusion, our data suggested that Hsp90α could positively regulate the self-renewal of BCSCs by facilitating the nuclear translocation of c-Myc and EZH2 to maintain BMI1 expression.
Highlights
Breast cancer is the most common cancer and the leading cause of cancer-related death among females worldwide [1]
Breast cancer stem/progenitor cells (BCSCs) are breast cancer cells that are characterized by the expression of specific markers, such as CD24-CD44+ [7], and the activity of intracellular aldehyde dehydrogenase (ALDH) [8]; these cells can be enriched through tumorsphere cultivation [9,10]
We previously discovered that Hsp90α expression is upregulated in ALDH+ BCSCs [22]
Summary
Breast cancer is the most common cancer and the leading cause of cancer-related death among females worldwide [1]. Hsp inhibitors, including geldanamycin and its analog 17-dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG), could decrease the population of ALDH+ BCSCs and the protein expression of Akt, a well-known client protein of Hsp that regulates cell survival and proliferation [22]. This finding suggested that Hsp inhibitors are potential and effective cancer chemotherapeutic drugs that target BCSCs. the mechanisms that underlie the anti-BCSC action of Hsp inhibitors should be further investigated. Our data suggested that Hsp90α could positively regulate the self-renewal of BCSCs by facilitating the nuclear translocation of c-Myc and EZH2 to maintain BMI1 expression
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