Small-Molecule-Mediated Stabilization of PP2A Modulates the Homologous Recombination Pathway and Potentiates DNA Damage-Induced Cell Death.

High-grade serous carcinoma (HGSC) is the most common and lethal ovarian cancer subtype. PARP inhibitors (PARPi) have become the mainstay of HGSC-targeted therapy, given that these tumors are driven by a high degree of genomic instability (GI) and homologous recombination (HR) defects. Nonetheless, approximately 30% of patients initially respond to treatment, ultimately relapsing with resistant disease. Thus, despite recent advances in drug development and an increased understanding of genetic alterations driving HGSC progression, mortality has not declined, highlighting the need for novel therapies. Using a small-molecule activator of protein phosphatase 2A (PP2A; SMAP-061), we investigated the mechanism by which PP2A stabilization induces apoptosis in patient-derived HGSC cells and xenograft (PDX) models alone or in combination with PARPi. We uncovered that PP2A genes essential for cellular transformation (B56α, B56γ, and PR72) and basal phosphatase activity (PP2A-A and -C) are heterozygously lost in the majority of HGSC. Moreover, loss of these PP2A genes correlates with worse overall patient survival. We show that SMAP-061-induced stabilization of PP2A inhibits the HR output by targeting RAD51, leading to chronic accumulation of DNA damage and ultimately apoptosis. Furthermore, combination of SMAP-061 and PARPi leads to enhanced apoptosis in both HR-proficient and HR-deficient HGSC cells and PDX models. Our studies identify PP2A as a novel regulator of HR and indicate PP2A modulators as a therapeutic therapy for HGSC. In summary, our findings further emphasize the potential of PP2A modulators to overcome PARPi insensitivity, given that targeting RAD51 presents benefits in overcoming PARPi resistance driven by BRCA1/2 mutation reversions.

Targeting DNA Damage Response and Replication Stress in Pancreatic Cancer.

BACKGROUND & AIMS: Continuing recalcitrance to therapy cements pancreatic cancer (PC) as the most lethal malignancy, which is set to become the second leading cause of cancer death in our society. The study aim was to investigate the association between DNA damage response (DDR), replication stress, and novel therapeutic response in PC to develop a biomarker-driven therapeutic strategy targeting DDR and replication stress in PC. METHODS: We interrogated the transcriptome, genome, proteome, and functional characteristics of 61 novel PC patient-derived cell lines to define novel therapeutic strategies targeting DDR and replication stress. Validation was done in patient-derived xenografts and human PC organoids. RESULTS: Patient-derived cell lines faithfully recapitulate the epithelial component of pancreatic tumors, including previously described molecular subtypes. Biomarkers of DDR deficiency, including a novel signature of homologous recombination deficiency, cosegregates with response to platinum (P < .001) and PARP inhibitor therapy (P < .001) in vitro and in vivo. We generated a novel signature of replication stress that predicts response to ATR (P < .018) and WEE1 inhibitor (P < .029) treatment in both cell lines and human PC organoids. Replication stress was enriched in the squamous subtype of PC (P < .001) but was not associated with DDR deficiency. CONCLUSIONS: Replication stress and DDR deficiency are independent of each other, creating opportunities for therapy in DDR-proficient PC and after platinum therapy.

miR-181a initiates and perpetuates oncogenic transformation through the regulation of innate immune signaling.

Genomic instability (GI) predisposes cells to malignant transformation, however the molecular mechanisms that allow for the propagation of cells with a high degree of genomic instability remain unclear. Here we report that miR-181a is able to transform fallopian tube secretory epithelial cells through the inhibition of RB1 and stimulator-of-interferon-genes (STING) to propagate cells with a high degree of GI. MiR-181a targeting of RB1 leads to profound nuclear defects and GI generating aberrant cytoplasmic DNA, however simultaneous miR-181a mediated inhibition of STING allows cells to bypass interferon mediated cell death. We also found that high miR-181a is associated with decreased IFNγ response and lymphocyte infiltration in patient tumors. DNA oncoviruses are the only known inhibitors of STING that allow for cellular transformation, thus, our findings are the first to identify a miRNA that can downregulate STING expression to suppress activation of intrinsic interferon signaling. This study introduces miR-181a as a putative biomarker and identifies the miR-181a-STING axis as a promising target for therapeutic exploitation.

HNF4A and GATA6 Loss Reveals Therapeutically Actionable Subtypes in Pancreatic Cancer.

Pancreatic ductal adenocarcinoma (PDAC) can be divided into transcriptomic subtypes with two broad lineages referred to as classical (pancreatic) and squamous. We find that these two subtypes are driven by distinct metabolic phenotypes. Loss of genes that drive endodermal lineage specification, HNF4A and GATA6, switch metabolic profiles from classical (pancreatic) to predominantly squamous, with glycogen synthase kinase 3 beta (GSK3ß) a key regulator of glycolysis. Pharmacological inhibition of GSK3ß results in selective sensitivity in the squamous subtype; however, a subset of these squamous patient-derived cell lines (PDCLs) acquires rapid drug tolerance. Using chromatin accessibility maps, we demonstrate that the squamous subtype can be further classified using chromatin accessibility to predict responsiveness and tolerance to GSK3ß inhibitors. Our findings demonstrate that distinct patterns of chromatin accessibility can be used to identify patient subgroups that are indistinguishable by gene expression profiles, highlighting the utility of chromatin-based biomarkers for patient selection in the treatment of PDAC.

ATDC mediates a TP63-regulated basal cancer invasive program.

Basal subtype cancers are deadly malignancies but the molecular events driving tumor lethality are not completely understood. Ataxia-telangiectasia group D complementing gene (ATDC, also known as TRIM29), is highly expressed and drives tumor formation and invasion in human bladder cancers but the factor(s) regulating its expression in bladder cancer are unknown. Molecular subtyping of bladder cancer has identified an aggressive basal subtype, which shares molecular features of basal/squamous tumors arising in other organs and is defined by activation of a TP63-driven gene program. Here, we demonstrate that ATDC is linked with expression of TP63 and highly expressed in basal bladder cancers. We find that TP63 binds to transcriptional regulatory regions of ATDC and KRT14 directly, increasing their expression, and that ATDC and KRT14 execute a TP63-driven invasive program. In vivo, ATDC is required for TP63-induced bladder tumor invasion and metastasis. These results link TP63 and the basal gene expression program to ATDC and to aggressive tumor behavior. Defining ATDC as a molecular determinant of aggressive, basal cancers may lead to improved biomarkers and therapeutic approaches.

HNF1A is a novel oncogene that regulates human pancreatic cancer stem cell properties.

The biological properties of pancreatic cancer stem cells (PCSCs) remain incompletely defined and the central regulators are unknown. By bioinformatic analysis of a human PCSC-enriched gene signature, we identified the transcription factor HNF1A as a putative central regulator of PCSC function. Levels of HNF1A and its target genes were found to be elevated in PCSCs and tumorspheres, and depletion of HNF1A resulted in growth inhibition, apoptosis, impaired tumorsphere formation, decreased PCSC marker expression, and downregulation of POU5F1/OCT4 expression. Conversely, HNF1A overexpression increased PCSC marker expression and tumorsphere formation in pancreatic cancer cells and drove pancreatic ductal adenocarcinoma (PDA) cell growth. Importantly, depletion of HNF1A in xenografts impaired tumor growth and depleted PCSC marker-positive cells in vivo. Finally, we established an HNF1A-dependent gene signature in PDA cells that significantly correlated with reduced survivability in patients. These findings identify HNF1A as a central transcriptional regulator of PCSC properties and novel oncogene in PDA.

UM-164: A Potent c-Src/p38 Kinase Inhibitor with In Vivo Activity against Triple-Negative Breast Cancer.

PURPOSE: c-Src has been shown to play a pivotal role in breast cancer progression, metastasis, and angiogenesis. In the clinic, however, the limited efficacy and high toxicity of existing c-Src inhibitors have tempered the enthusiasm for targeting c-Src. We developed a novel c-Src inhibitor (UM-164) that specifically binds the DFG-out inactive conformation of its target kinases. We hypothesized that binding the inactive kinase conformation would lead to improved pharmacologic outcomes by altering the noncatalytic functions of the targeted kinases. EXPERIMENTAL DESIGN: We have analyzed the anti-triple-negative breast cancer (TNBC) activity of UM-164 in a comprehensive manner that includes in vitro cell proliferation, migration, and invasion assays (including a novel patient-derived xenograft cell line, VARI-068), along with in vivo TNBC xenografts. RESULTS: We demonstrate that UM-164 binds the inactive kinase conformation of c-Src. Kinome-wide profiling of UM-164 identified that Src and p38 kinase families were potently inhibited by UM-164. We further demonstrate that dual c-Src/p38 inhibition is superior to mono-inhibition of c-Src or p38 alone. We demonstrate that UM-164 alters the cell localization of c-Src in TNBC cells. In xenograft models of TNBC, UM-164 resulted in a significant decrease of tumor growth compared with controls, with limited in vivo toxicity. CONCLUSIONS: In contrast with c-Src kinase inhibitors used in the clinic (1, 2), we demonstrate in vivo efficacy in xenograft models of TNBC. Our results suggest that the dual activity drug UM-164 is a promising lead compound for developing the first targeted therapeutic strategy against TNBC. Clin Cancer Res; 22(20); 5087-96. ©2016 AACR.

The Notch pathway is important in maintaining the cancer stem cell population in pancreatic cancer.

BACKGROUND: Pancreatic cancer stem cells (CSCs) represent a small subpopulation of pancreatic cancer cells that have the capacity to initiate and propagate tumor formation. However, the mechanisms by which pancreatic CSCs are maintained are not well understood or characterized. METHODS: Expression of Notch receptors, ligands, and Notch signaling target genes was quantitated in the CSC and non-CSC populations from 8 primary human pancreatic xenografts. A gamma secretase inhibitor (GSI) that inhibits the Notch pathway and a shRNA targeting the Notch target gene Hes1 were used to assess the role of the Notch pathway in CSC population maintenance and pancreatic tumor growth. RESULTS: Notch pathway components were found to be upregulated in pancreatic CSCs. Inhibition of the Notch pathway using either a gamma secretase inhibitor or Hes1 shRNA in pancreatic cancer cells reduced the percentage of CSCs and tumorsphere formation. Conversely, activation of the Notch pathway with an exogenous Notch peptide ligand increased the percentage of CSCs as well as tumorsphere formation. In vivo treatment of orthotopic pancreatic tumors in NOD/SCID mice with GSI blocked tumor growth and reduced the CSC population. CONCLUSION: The Notch signaling pathway is important in maintaining the pancreatic CSC population and is a potential therapeutic target in pancreatic cancer.

Altered EGFR localization and degradation in human breast cancer cells with an amphiregulin/EGFR autocrine loop.

The epidermal growth factor receptor (EGFR) and its ligand amphiregulin (AR) have been shown to be co-over expressed in breast cancer. We have previously shown that an AR/EGFR autocrine loop is required for SUM149 human breast cancer cell proliferation, motility and invasion. We also demonstrated that AR can induce these altered phenotypes when expressed in the normal mammary epithelial cell line MCF10A, or by exposure of these cells to AR in the medium. In the present studies, we demonstrate that SUM149 cells and immortalized human mammary epithelial MCF10A cells that over express AR (MCF10A AR) or are cultured in the presence of exogenous AR, express higher levels of EGFR protein than MCF10A cells cultured in EGF. Pulse-chase analysis showed that EGFR protein remained stable in the presence of AR, yet was degraded in the presence of EGF. Consistent with this observation, tyrosine 1045 on the EGFR, the c-cbl binding site, exhibited less phosphorylation following stimulation with AR than following stimulation with EGF. Ubiquitination of the receptor was also dramatically less following stimulation with AR than following stimulation with EGF. Flow cytometry analysis showed that EGFR remained on the cell surface following stimulation with AR but was rapidly internalized following stimulation with EGF. Immunofluorescence and confocal microscopy confirmed the flow cytometry results. EGFR in MCF10A cells cultured in the presence of EGF exhibited a predominantly intracellular, punctate localization. In stark contrast, SUM149 cells and MCF10A cells growing in the presence of AR expressed EGFR predominantly on the membrane and at cell-cell junctions. We propose that AR alters EGFR internalization and degradation in a way that favors accumulation of EGFR at the cell surface and ultimately leads to changes in EGFR signaling.

Cooperative interactions of HER-2 and HPV-16 oncoproteins in the malignant transformation of human mammary epithelial cells.

To better understand the mechanisms of transformation by the oncogene HER-2, we transduced the human mammary epithelial (HME) cell line MCF-10A with HER-2 and developed a cell line that appeared to moderately overexpress HER-2. These MCF-10HER-2 cells were unable to grow in the absence of epidermal growth factor (EGF). However, coexpression of HER-2 with the HPV-16 oncoproteins E6 and E7 resulted in EGF-independent cells that expressed very high levels of constitutively activated HER-2. Interestingly, coexpression of E7 with HER-2 resulted in cells that were EGF-independent for growth but did not express HER-2 to high levels, and coexpression of E6 with HER-2 resulted in cells expressing higher levels of HER-2, which were still dependent on EGF for growth and survival. The MCF-10HER-2E7 and HER-2/E6E7 cells exhibited constitutive activation of a form of epidermal growth factor receptor (EGFR) that had a faster electrophoretic mobility than EGFR activated by exogenous growth factors. Exposure of cells with EGFR activation to ZD1839 (Iressa), at concentrations specific for EGFR, had little or no influence on proliferation of cells with amplified HER-2 but little or no EGFR. These results indicate that HER-2, E6, and E7 cooperate with endogenous EGFR to yield fully transformed cells.

Y-box-binding protein 1 confers EGF independence to human mammary epithelial cells.

The epidermal growth factor receptor (EGFR) is linked to poor outcome in breast cancer, and resistance to hormonal therapy is often accompanied by activation of growth factor receptors. To investigate the mechanism(s) by which EGFR becomes activated in breast cancer, we screened a cDNA expression library for genes that mediate EGF-independent proliferation of human mammary epithelial cells (HMECs). We isolated the NSEP1 cDNA encoding Y-box-binding protein 1 (YB-1), a multifunctional transcriptional and translational regulator. This cDNA conferred growth factor independence to HMECs. YB-1-transduced cells overexpressed EGFR, but ErbB-2 (Her-2/neu) levels were unchanged. Moreover, EGFR was constitutively phosphorylated in the absence of exogenous ligand. In these cells, an EGFR-blocking antibody failed to inhibit proliferation, conditioned medium activity could not be detected, and the synthesis of EGFR ligands was reduced compared to parental cells. This suggests that EGFR is activated in a ligand-independent fashion. However, cell growth could be blocked with an ErbB kinase inhibitor, indicating that EGFR signaling plays a major role in YB-1-induced growth factor independence. Taken together, our results demonstrate that YB-1 overexpression can induce EGF independence in HMECs via activation of the EGFR pathway. This could represent one of the mechanisms by which YB-1 contributes to breast tumor aggressiveness.

Stat3 modulates heat shock 27kDa protein expression in breast epithelial cells.

The constitutive activation of signal transducer and activator of transcription 3 (Stat3) is frequently detected in breast carcinoma cell lines but not in normal breast epithelial cells. Stat3 has been classified as an oncogene because activated Stat3 can mediate oncogenic transformation in cultured cells and tumor formation in nude mice. In this study, we investigated potential Stat3 regulated genes in breast cells. Upon expression of Stat3-C, a constitutively active Stat3 form, in nonmalignant telomerase immortalized breast cells (TERT), cell lysate was subjected to 2-dimensional (2-D) protein gel analysis. Our results showed that heat shock 27kDa protein (HSP27) was markedly induced by Stat3-C expression. Further analysis demonstrated that phosphorylation of HSP27 at serine residue 78 was induced by Stat3-C in TERT breast cells as well as in MCF-10A and MDA-MB-453 breast cells. RT-PCR result confirmed that HSP27 mRNA was induced by Stat3-C in TERT cells. As the result of Stat3 knock-down by Stat3 short interfering RNA oligonucleotides in MDA-MB-468 human breast carcinoma cells, HSP27 was markedly reduced consistent with Stat3 reduction. Furthermore, we observed that Stat3 was physically associated with HSP27 and HSP90 in MDA-MB-468 breast carcinoma cells. Taken together, our findings demonstrate that constitutively activated Stat3 up-regulates HSP27 and may facilitate phosphorylation of HSP27 at serine residue 78. The up-regulation of HSP27 may be one of the underlying mechanisms with which aberrant Stat3 signaling induces cell malignancies.