Cancer stem cells (CSC) play a central role in cancer metastasis and development of drug resistance. miRNA are important in regulating CSC properties and are considered potential therapeutic targets. Here we report that miR-328–3p (miR-328) is significantly upregulated in ovarian CSC. High expression of miR-328 maintained CSC properties by directly targeting DNA damage binding protein 2, which has been shown previously to inhibit ovarian CSC. Reduced activity of ERK signaling in ovarian CSC, mainly due to a low level of reactive oxygen species, contributed to the enhanced expression of miR-328 and maintenance of CSC. Inhibition of miR-328 in mouse orthotopic ovarian xenografts impeded tumor growth and prevented tumor metastasis. In summary, our findings provide a novel mechanism underlying maintenance of the CSC population in ovarian cancer and suggest that targeted inhibition of miR-328 could be exploited for the eradication of CSC and aversion of tumor metastasis in ovarian cancer. Significance: These findings present inhibition of miR-328 as a novel strategy for efficient elimination of CSC to prevent tumor metastasis and recurrence in patients with epithelial ovarian cancer.

◥Poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) are approved to treat recurrent ovarian cancer with BRCA1 or BRCA2 mutations, and as maintenance therapy for recurrent platinumsensitive ovarian cancer (BRCA wild-type or mutated) after treatment with platinum. However, the acquired resistance against PARPi remains a clinical hurdle. Here, we demonstrated that PARP inhibitor (olaparib)-resistant epithelial ovarian cancer (EOC) cells exhibited an elevated aldehyde dehydrogenase (ALDH) activity, mainly contributed by increased expression of ALDH1A1 due to olaparib-induced expression of BRD4, a member of bromodomain and extraterminal (BET) family protein. We also revealed that ALDH1A1 enhanced microhomology-mediated end joining (MMEJ) activity in EOC cells with inactivated BRCA2, a key protein that promotes homologous recombination (HR) by using an intrachromosomal MMEJ reporter. Moreover, NCT-501, an ALDH1A1-selective inhibitor, can synergize with olaparib in killing EOC cells carrying BRCA2 mutation in both in vitro cell culture and the in vivo xenograft animal model. Given that MMEJ activity has been reported to be responsible for PARPi resistance in HR-deficient cells, we conclude that ALDH1A1 contributes to the resistance to PARP inhibitors via enhancing MMEJ in BRCA2 À/À ovarian cancer cells. Our findings provide a novel mechanism underlying PARPi resistance in BRCA2-mutated EOC cells and suggest that inhibition of ALDH1A1 could be exploited for preventing and overcoming PARPi resistance in EOC patients carrying BRCA2 mutation.

Radiotherapy resistance is one of the major factors limiting the efficacy of radiotherapy in lung cancer patients. The extensive investigations indicate the diversity in the mechanisms underlying radioresistance. Here, we revealed that DNA damage binding protein 2 (DDB2) is a potential regulator in the radiosensitivity of non-small cell lung cancer (NSCLC) cells. DDB2, originally identified as a DNA damage recognition factor in the nucleotide excision repair, promotes the survival and inhibits the apoptosis of NSCLC cell lines upon ionizing radiation (IR). Mechanistic investigations demonstrated that DDB2 is able to facilitate IR-induced phosphorylation of Chk1, which plays a critical role in the cell cycle arrest and DNA repair in response to IR-induced DNA double-strand breaks (DSBs). Indeed, knockdown of DDB2 compromised the G2 arrest in the p53-proficient A549 cell line and reduced the efficiency of homologous recombination (HR) repair. Taken together, our data indicate that the expression of DDB2 in NSCLC could be used as a biomarker to predict radiosensitivity of the patients. Targeting Chk1 can be used to increase the efficacy of radiotherapy in patients of NSCLC possessing high levels of DDB2.

Epithelial ovarian cancer is the most lethal malignancy of the female reproductive tract. A healthy ovary expresses both Estrogen Receptor a (ERa) and b (ERb). Given that ERa is generally considered to promote cell survival and proliferation, thereby, enhancing tumor growth, while ERb shows a protective effect against the development and progression of tumors, the activation of ERb by its agonists could be therapeutically beneficial for ovarian cancer. Here, we demonstrate that the activation of ERb using a newly developed ERb agonist, OSU-ERb-12, can impede ovarian cancer cell expansion and tumor growth in an ERα-independent manner. More interestingly, we found that OSU-ERb-12 also reduces the cancer stem cell (CSC) population in ovarian cancer by compromising non-CSC-to-CSC conversion. Mechanistically, we revealed that OSU-ERb-12 decreased the expression of Snail, a master regulator of the epithelial-to-mesenchymal transition (EMT), which is associated with de novo CSC generation. Given that ERα can mediate EMT and facilitate maintenance of the CSC subpopulation and that OSU-ERb-12 can block the transactivity of ERα, we conclude that OSU-ERb-12 reduces the CSC subpopulation by inhibiting EMT in an ERα-dependent manner. Taken together, our data indicate that the ERb agonist OSU-ERb-12 could be used to hinder tumor progression and limit the CSC subpopulation with the potential to prevent tumor relapse and metastasis in patients with ovarian cancer.

Ovarian cancer (OC) is one of the most lethal female malignancy which accounts for just 2.5% of female cancer cases but 5% of deaths because of low survival. 90% of OC are epithelial ovarian cancer (EOC), with an overall 5-year relative survival rate of 47% and only 29% for patients diagnosed with distant-stage. 20% of OC cases are estimated to be due to inherited mutations that confer elevated risk, especially cancer susceptibility genes BRCA1 and BRCA2. PARP inhibitors (PARPi) are novel and promising cancer-targeted drugs. PAPRi are approved by FDA for clinical treatment of advanced EOC patients with BRCA1/2 gene mutation. However, patients gradually gained resistance to PARPi with continuously increased recurrence rate (>90%). Thus, understanding the mechanism underlying PARPi resistance is an urgent need for improving the PARPi efficacy. Aldehyde dehydrogenase (ALDH) activity is considered as a cancer stem cell (CSC) marker and also relative to drug resistance. However, the relationship between ALDH activity and PAPPi resistance remains unclear. In this study, we generated two olaparib-resistant EOC cell lines by continuously treating BRCA2-mutated PEO1 and Kuramochi cell lines for 6 months, and found that these resistant cell lines exhibited higher ALDH activity compared to their corresponding parent cell lines. In addition, short-term treatment of PEO1 and Kuramochi cells with olaparib (7 days) also increased the ALDH+ cell population in these cells, and olaparib-induced ALDH--to-ALDH+ conversion contributed to the expansion of the ALDH+ cell population after olaparib treatment. qRT-PCR analysis demonstrated that ALDH1A1 is the major gene in the ALDH gene family that was induced by olaparib. Overexpression of ALDH1A1 increased olaparib resistance in a panel of EOC cells lines including both BRCA2-muated and BRCA2-wild type cell lines. In summary, our data indicate that olaparib is able to induce ALDH1A1 gene expression, which results in the enhanced ALDH activity. The enhanced ALDH activity can contribute to olaparib resistance in BRCA2-mutated EOC cells. Citation Format: Lu Liu, Shurui Cai, Chunhua Han, Ananya Benerjee, Dayong Wu, Tiantian Cui, Guozhen Xie, Yanfang Zheng, and Qi-En Wang. PARPI-INDUCED ALDH1A1 EXPRESSION CONTRIBUTES TO PARPI RESISTANCE IN OVARIAN CANCER CELLS [abstract]. In: Proceedings of the 12th Biennial Ovarian Cancer Research Symposium; Sep 13-15, 2018; Seattle, WA. Philadelphia (PA): AACR; Clin Cancer Res 2019;25(22 Suppl):Abstract nr NT-100.