The evolution of chemoresistance is a fundamental characteristic of cancer that ultimately defeats its clinical management. However, it may be possible improve patient outcomes significantly by defeating nodal resistance mechanisms which cancers rely upon during the evolution to an untreatable state.Here we report an essential role for upregulation of the stem cell reprogramming factor PBX1 in mediating chemoresistance in recurrent ovarian carcinomas. In clinical specimens, high levels of PBX1 expression correlated with shorter survival in post-chemotherapy ovarian cancer patients. In tumor cells with low endogenous expression of PBX1, its enforced expression promoted cancer stem cell-like phenotypes, including most notably an increase in resistance to platinum-based therapy used most commonly for management of this disease. Conversely, silencing PBX1 in platinum-resistant cells that overexpressed PBX1 sensitized them to platinum treatment and reduced their stem-likeproperties. An analysis of published genome-wide chromatin immunoprecipitation data indicated that PBX1 binds directly to promoters of genes involved in stem cell maintenance and the response to tissue injury. We confirmed direct regulation of one of these genes, STAT3, demonstrating that the PBX1 binding motif at its promoter acted to positively regulate STAT3 transcription. Pursing this connection, we determined that a STAT3/JAK2 inhibitor could potently sensitize platinum-resistant cells to carboplatin and suppress their growth in vivo. Our findings offer a mechanistic rationale to target the PBX1/STAT3 axis to defeat a key mechanism of chemoresistance in ovarian cancers and possibly other human cancers.3

Background: Cancer recurrence and chemoresistance contribute greatly to the high mortality of ovarian cancers; however, molecular players involved in these processes remain under-determined. Several theories have been proposed to account for the development of resistance to chemotherapeutic agents. One of the emerging concepts is tumor heterogeneity, which argues for the presence of different cell subpopulations in bulk tumor with varying degrees of tumor initiating potential and drug sensitivity. The fractions of cells with enhanced tumor initiating potential are referred to as cancer stem-like cells (CSC). These cells have defining “stemness” phenotypes, including unlimited cell division, repopulation by a minimum cell number and resistance to cytotoxic agents and irradiation. New Findings: To identify embryonic stem cell factors that are potentially involved in tumor recurrence and chemoresistance, we have screened expression levels of four of these factors by immunohistochemistry in pairs of matched recurrent and primary tumor tissues from 41 patients. Our data demonstrated that PBX1 is significantly up-regulated in recurrent/chemoresistant ovarian tumors. Moreover, PBX1 overexpression in ascites tumors correlates with shorter overall survival in post-chemotherapy ovarian cancer patients. When PBX1 is ectopically expressed, it promotes cancer stem cell-like phenotypes, including increased side population and ALDH1 activity, enhanced tumorigenicity at low cell density, and increased resistance to platinum-based therapy. In platinum-resistant cell lines that overexpress PBX1, silencing PBX1 expression using RNA interference sensitizes cells to platinum treatment and reduces their stem cell-like phenotypes. Gene expression and chromatin immunoprecipitation analyses identified PBX1 direct target genes involved in multi-drug resistance, stem cell maintenance, immunomodulation, and DNA damage response. In the TCGA ovarian cancer recurrent series, expression of several PBX1 direct target genes, including ABCA1, Nanog, BMP3, and ATM, is tightly associated with PBX1 expression, further supporting the tissue-level transcriptional regulation of these genes by PBX1. Conclusion: The above findings establish PBX1 as an upstream regulator of key functional networks that mediate cancer stem-like and drug resistant phenotypes. Studies on the hematopoietic system have indicated that PBX1 maintains a viable pool of quiescent stem cells. Recently, in a renal cell carcinoma system, chemotherapy has been shown to induce damage signaling and to stimulate cell division and repopulation of quiescent CSCs. We speculate that chemotherapy in ovarian cancer may induce comparable injury responses and trigger PBX1 signaling, which in turn may activate downstream networks that support CSC survival and create a microenvironment niche that is essential for CSC repopulation at the tissue level. Collectively, the association between PBX1 and a cascade of stemness pathways points to a potential Achilles's heel critical to responding to chemotherapy and developing chemoresistance and argues for the development of antagonists of PBX1 signaling as anticancer agents. Citation Format: Jin-Gyoung Jung, Tae-Hoen Kim, Emily Gerry, Jen-Chun Kuan, Ayse Ayhan, Ben Davidson, Ie-Ming Shih, Tian-Li Wang. PBX1, a transcriptional regulator, promotes stemness and chemoresistance in ovarian cancer. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr A32.

<div>Abstract<p><b>Purpose:</b> Statins are among the most frequently prescribed drugs because of their efficacy and low toxicity in treating hypercholesterolemia. Recently, statins have been reported to inhibit the proliferative activity of cancer cells, especially those with <i>TP53</i> mutations. Because <i>TP53</i> mutations occur in almost all ovarian high-grade serous carcinoma (HGSC), we determined whether statins suppressed tumor growth in animal models of ovarian cancer.</p><p><b>Experimental Design:</b> Two ovarian cancer mouse models were used. The first one was a genetically engineered model, mogp-TAg, in which the promoter of oviduct glycoprotein-1 was used to drive the expression of SV40 T-antigen in gynecologic tissues. These mice spontaneously developed serous tubal intraepithelial carcinomas (STICs), which are known as ovarian cancer precursor lesions. The second model was a xenograft tumor model in which human ovarian cancer cells were inoculated into immunocompromised mice. Mice in both models were treated with lovastatin, and effects on tumor growth were monitored. The molecular mechanisms underlying the antitumor effects of lovastatin were also investigated.</p><p><b>Results:</b> Lovastatin significantly reduced the development of STICs in mogp-TAg mice and inhibited ovarian tumor growth in the mouse xenograft model. Knockdown of prenylation enzymes in the mevalonate pathway recapitulated the lovastatin-induced antiproliferative phenotype. Transcriptome analysis indicated that lovastatin affected the expression of genes associated with DNA replication, Rho/PLC signaling, glycolysis, and cholesterol biosynthesis pathways, suggesting that statins have pleiotropic effects on tumor cells.</p><p><b>Conclusions:</b> The above results suggest that repurposing statin drugs for ovarian cancer may provide a promising strategy to prevent and manage this devastating disease. <i>Clin Cancer Res; 21(20); 4652–62. ©2015 AACR</i>.</p></div>

<div>Abstract<p><b>Purpose:</b> Statins are among the most frequently prescribed drugs because of their efficacy and low toxicity in treating hypercholesterolemia. Recently, statins have been reported to inhibit the proliferative activity of cancer cells, especially those with <i>TP53</i> mutations. Because <i>TP53</i> mutations occur in almost all ovarian high-grade serous carcinoma (HGSC), we determined whether statins suppressed tumor growth in animal models of ovarian cancer.</p><p><b>Experimental Design:</b> Two ovarian cancer mouse models were used. The first one was a genetically engineered model, mogp-TAg, in which the promoter of oviduct glycoprotein-1 was used to drive the expression of SV40 T-antigen in gynecologic tissues. These mice spontaneously developed serous tubal intraepithelial carcinomas (STICs), which are known as ovarian cancer precursor lesions. The second model was a xenograft tumor model in which human ovarian cancer cells were inoculated into immunocompromised mice. Mice in both models were treated with lovastatin, and effects on tumor growth were monitored. The molecular mechanisms underlying the antitumor effects of lovastatin were also investigated.</p><p><b>Results:</b> Lovastatin significantly reduced the development of STICs in mogp-TAg mice and inhibited ovarian tumor growth in the mouse xenograft model. Knockdown of prenylation enzymes in the mevalonate pathway recapitulated the lovastatin-induced antiproliferative phenotype. Transcriptome analysis indicated that lovastatin affected the expression of genes associated with DNA replication, Rho/PLC signaling, glycolysis, and cholesterol biosynthesis pathways, suggesting that statins have pleiotropic effects on tumor cells.</p><p><b>Conclusions:</b> The above results suggest that repurposing statin drugs for ovarian cancer may provide a promising strategy to prevent and manage this devastating disease. <i>Clin Cancer Res; 21(20); 4652–62. ©2015 AACR</i>.</p></div>

<p>Including Supplementary Figures and Methods</p>