Summary Restoration of anti-tumor immunity by blocking PD-L1 signaling using antibodies has proven to be beneficial in cancer therapy. Here we show that BET bromodomain inhibition suppresses PD-L1 expression and limits tumor progression in ovarian cancer. CD274 (encoding PD-L1) is a direct target of BRD4-mediated gene transcription. In mouse models, treatment with the BET inhibitor JQ1 significantly reduced PD-L1 expression on tumor cells and tumor-associated dendritic cells and macrophages, which correlated with an increase in the activity of anti-tumor cytotoxic T cells. The BET inhibitor limited tumor progression in a cytotoxic T cell dependent manner. Together, these data demonstrate a small molecule approach to block PD-L1 signaling. Given the fact that BET inhibitors have been proven safe with manageable reversible toxicity in clinical trials, our findings indicate that pharmacological BET inhibitors represent a treatment strategy for targeting PD-L1 expression.

The emergence of tumor cells with certain stem-like characteristics such as high aldehyde dehydrogenase (ALDH) activity due to ALDH1A1 expression contributes to chemotherapy resistance and tumor relapse. However, clinically applicable inhibitors of ALDH activity have not been reported. There is evidence to suggest that epigenetic regulation of stem-related genes contributes to chemotherapy efficacy. Here we show that bromodomain and extra-terminal (BET) inhibitors suppress ALDH activity by abrogating BRD4-mediated ALDH1A1 expression through a super-enhancer element and its associated enhancer RNA. The clinically applicable small molecule BET inhibitor JQ1 suppressed the outgrowth of cisplatin-treated ovarian cancer cells both in vitro and in vivo. Combination of JQ1 and cisplatin improved the survival of ovarian cancer bearing mice in an orthotopic model. These phenotypes correlate with inhibition of ALDH1A1 expression through a super-enhancer element and other stem-related genes in promoter regions bound by BRD4. Thus, targeting the BET protein BRD4 using clinically applicable small molecule inhibitors such as JQ1 is a promising strategy for targeting ALDH activity in epithelial ovarian cancer.

Summary PARP inhibition is known to be an effective clinical strategy in BRCA-mutant cancers, but PARP inhibition has not been applied to BRCA-proficient tumors. Here we show synergy of BET bromodomain inhibition with PARP inhibition in BRCA-proficient ovarian cancers due to mitotic catastrophe. Treatment of BRCA-proficient ovarian cancer cells with the BET inhibitor JQ1 downregulated the G2-M cell cycle checkpoint regulator WEE1 and the DNA damage response factor TOPBP1. When combined with the PARP inhibitor Olaparib, we observed a synergistic increase in DNA damage and checkpoint defects, which allowed cells to enter mitosis despite the accumulation of DNA damage, ultimately causing mitotic catastrophe. Moreover, JQ1 and Olaparib showed synergistic suppression of growth of BRCA-proficient cancer in vivo in a xenograft ovarian cancer mouse model. Our findings indicate that a combination of BET inhibitor and PARP inhibitor represents a potential therapeutic strategy for BRCA-proficient cancers.

Cancer cells exhibit a variety of features indicative of atypical nuclei. However, the molecular mechanisms underlying these phenomena remain to be elucidated. The linker of nucleoskeleton and cytoskeleton (LINC) complex, a nuclear envelope protein complex consisting mainly of the SUN and nesprin proteins, connects nuclear lamina and cytoskeletal filaments and helps to regulate the size and shape of the nucleus. Using immunohistology, we found that a nuclear lamina component, lamin A/C and all of the investigated LINC complex components, SUN1, SUN2, and nesprin-2, were downregulated in human breast cancer tissues. In the majority of cases, we observed lower expression levels of these analytes in samples' cancerous regions as compared to their cancer-associated noncancerous regions (in cancerous regions, percentage of tissue samples exhibiting low protein expression: lamin A/C, 85% [n = 73]; SUN1, 88% [n = 43]; SUN2, 74% [n = 43]; and nesprin-2, 79% [n = 53]). Statistical analysis showed that the frequencies of recurrence and HER2 expression were negatively correlated with lamin A/C expression (P < 0.05), and intrinsic subtype and ki-67 level were associated with nesprin-2 expression (P < 0.05). In addition, combinatorial analysis using the above four parameters showed that all patients exhibited reduced expression of at least one of four components despite the tumor's pathological classification. Furthermore, several cultured breast cancer cell lines expressed less SUN1, SUN2, nesprin-2 mRNA, and lamin A/C compared to noncancerous mammary gland cells. Together, these results suggest that the strongly reduced expression of LINC complex and nuclear lamina components may play fundamental pathological functions in breast cancer progression.

Global histone modification patterns correlate with tumor phenotypes and prognostic factors in multiple tumor types. Recent studies suggest that aberrant histone modifications play an important role in cancer. However, the effects of global epigenetic rearrangements on cell functions remain poorly understood. In this study, we show that the histone H3 lysine 9 (H3K9) methyltransferase SUV39H1 is clearly involved in regulating cell migration in vitro. Overexpression of wild-type SUV39H1, but not enzymatically inactive SUV39H1, activated migration in breast and colorectal cancer cells. Inversely, migration was reduced by knockdown of SUV39H1 or chemical inhibition by chaetocin. In addition, H3K9 trimethylation (H3K9me3) was specifically increased in invasive regions of colorectal cancer tissues. Moreover, the presence of H3K9me3 positively correlated with lymph node metastasis in colorectal cancer patients. Furthermore, overexpression of SUV39H1 drove tumorigenesis in mouse, resulting in a considerable decrease in survival rate. These data indicate that H3K9 trimethylation plays an important role in human colorectal cancer progression, possibly by promoting collective cell invasion. (Cancer Sci 2013; 104: 889-895) G lobal changes in the epigenetic landscape are a hallmark of cancer.(1,2) For example, genome-wide loss of lysine 16 acetylation and lysine 20 trimethylation of histone H4 (H4K16ac and H4K20me3, respectively) is observed in multiple types of human cancers. (3,4) Moreover, global histone modification patterns predict clinical outcome, and correlate with tumor phenotypes and prognostic factors, in tumors including breast, prostate, lung, and gastric cancer.(5-10) Although somatic mutation is widely accepted as the origin of cancer, recent studies suggest that epigenetic alterations may be key initiating events, and that genetic and epigenetic alterations interact at all stages of cancer development and cancer progression. (11,12) Importantly, epigenetic aberrations, unlike genetic mutations, are potentially reversible and can potentially be restored to their normal state by epigenetic therapy. (13,14)