Jessica L. Bowser

Another Surprise from Metformin: Novel Mechanism of Action via K-Ras Influences Endometrial Cancer Response to Therapy

Metformin is an oral biguanide commonly used for the treatment of type II diabetes and has recently been demonstrated to possess antiproliferative properties that can be exploited for the prevention and treatment of a variety of cancers. The mechanisms underlying this effect have not been fully elucidated. Using both in vitro and in vivo models, we examined the effects of metformin on endometrial tumors with defined aberrations in the PI3K/PTEN/mTOR and MAPK signaling pathways to understand metformin mechanism of action and identify clinically useful predictors of response to this agent. In vitro assays of proliferation, cytotoxicity, and apoptosis were used to quantify the effects of metformin on endometrial cancer cell lines with mutations in the PI3K/PTEN/mTOR and MAPK signaling pathways. The in vivo effects of oral metformin on tumor progression were further examined using xenograft mouse models of endometrial cancer. K-Ras localization was analyzed by confocal microscopy using GFP-labeled oncogenic K-Ras and by immunoblot following subcellular fractionation. Metformin inhibited cell proliferation, induced apoptosis, and decreased tumor growth in preclinical endometrial cancer models, with the greatest response observed in cells harboring activating mutations in K-Ras. Furthermore, metformin displaces constitutively active K-Ras from the cell membrane, causing uncoupling of the MAPK signaling pathway. These studies provide a rationale for clinical trials using metformin in combination with PI3K-targeted agents for tumors harboring activating K-Ras mutations, and reveal a novel mechanism of action for metformin. Mol Cancer Ther; 12(12); 2847–56. ©2013 AACR.

Mechanistic Contribution of Ubiquitous 15-Lipoxygenase-1 Expression Loss in Cancer Cells to Terminal Cell Differentiation Evasion

Loss of terminal cell differentiation promotes tumorigenesis. 15-Lipoxygenase-1 (15-LOX-1) contributes to terminal cell differentiation in normal cells. The mechanistic significance of 15-LOX-1 expression loss in human cancers to terminal cell differentiation suppression is unknown. In a screen of 128 cancer cell lines representing more than 20 types of human cancer, we found that 15-LOX-1 mRNA expression levels were markedly lower than levels in terminally differentiated cells. Relative expression levels of 15-LOX-1 (relative to the level in terminally differentiated primary normal human–derived bronchial epithelial cells) were lower in 79% of the screened cancer cell lines than relative expression levels of p16 (INK4A), which promotes terminal cell differentiation and is considered one of the most commonly lost tumor suppressor genes in cancer cells. 15-LOX-1 was expressed during terminal differentiation in three-dimensional air–liquid interface cultures, and 15-LOX-1 expression and terminal differentiation occurred in immortalized nontransformed bronchial epithelial but not in lung cancer cell lines. 15-LOX-1 expression levels were lower in human tumors than in paired normal lung epithelia. Short hairpin RNA–mediated downregulation of 15-LOX-1 in Caco-2 cells blocked enterocyte-like differentiation, disrupted tight junction formation, and blocked E-cadherin and ZO-1 localization to the cell wall membrane. 15-LOX-1 episomal expression in Caco-2 and HT-29 colon cancer cells induced differentiation. Our findings indicate that 15-LOX-1 downregulation in cancer cells is an important mechanism for terminal cell differentiation dysregulation and support the potential therapeutic utility of 15-LOX-1 reexpression to inhibit tumorigenesis. Cancer Prev Res; 4(12); 1961–72. ©2011 AACR.

MUC1 stimulates EGFR expression and function in endometrial cancer

The current standard of care for endometrial cancer patients involves hysterectomy with adjuvant radiation and chemotherapy, with no effective treatment for advanced and metastatic disease. MUC1 is a large, heavily glycosylated transmembrane protein that lubricates and protects cell surfaces and increases cellular signaling through the epidermal growth factor receptor (EGFR). We show for the first time that MUC1 stimulates EGFR expression and function in endometrial cancer. siRNA knockdown and CRISPR/Cas knockout of MUC1 reduced EGFR gene expression, mRNA, protein levels and signaling. MUC1 bound strongly to two regions of the EGFR promoter: −627/−511 and −172/−64. MUC1 knockout also reduced EGFR-dependent proliferation in two dimensional culture, as well as growth and survival in three dimensional spheroid cultures. MUC1 knockout cells were more sensitive to the EGFR inhibitor, lapatinib. Finally, MUC1 and EGFR co-expression was associated with increased cellular proliferation in human endometrial tumors. These data demonstrate the importance of MUC1-driven EGFR expression and signaling and suggest dual-targeted therapies may provide improved response for endometrial tumors.

Loss of polarity alters proliferation and differentiation in low-grade endometrial cancers by disrupting Notch signaling

Cell adhesion and apicobasal polarity together maintain epithelial tissue organization and homeostasis. Loss of adhesion has been described as a prerequisite for the epithelial to mesenchymal transition. However, what role misregulation of apicobasal polarity promotes tumor initiation and/or early progression remains unclear. We find that human low-grade endometrial cancers are associated with disrupted localization of the apical polarity protein Par3 and Ezrin while, the adhesion molecule E-cadherin remains unchanged, accompanied by decreased Notch signaling, and altered Notch receptor localization. Depletion of Par3 or Ezrin, in a cell-based model, results in loss of epithelial architecture, differentiation, increased proliferation, migration and decreased Notch signaling. Re-expression of Par3 in endometrial cancer cell lines with disrupted Par3 protein levels blocks proliferation and reduces migration in a Notch dependent manner. These data uncover a function for apicobasal polarity independent of cell adhesion in regulating Notch-mediated differentiation signals in endometrial epithelial cells.

CD73s protection of epithelial integrity: Thinking beyond the barrier

ABSTRACT The prevailing view of CD73 in cancer is that it is overexpressed in tumors and promotes cancer progression by dampening local T cell-mediated immune responses. We recently found that CD73 is down-regulated in poorly-differentiated and advanced stage endometrial carcinoma compared to normal endometrium and well-differentiated, early stage tumors. We revealed that CD73-generated adenosine induces a physiological response to protect epithelial integrity in well-differentiated, early stage endometrial carcinoma. The ability of CD73-generated adenosine to protect the barrier is not so different from its ability to induce immunosuppression and other physiological responses in cancerous tissues. In this commentary we examine the complexity of CD73 in cancer and suggest that a “one size fits all” approach to the role of CD73/adenosine in cancer is no longer warranted. Given that tumors often hijack normal cellular responses, we also provide consideration on how CD73s known role to protect barrier function may have implications in promoting tumor progression.

Coordination of ENT2-dependent adenosine transport and signaling dampens mucosal inflammation

Intestinal epithelial barrier repair is vital for remission in inflammatory bowel disease (IBD). Extracellular adenosine signaling has been implicated in promoting restoration of epithelial barrier function. Currently, no clinically approved agents target this pathway. Adenosine signaling is terminated by uptake from the extracellular space via equilibrative nucleoside transporters (ENTs). We hypothesized that ENT inhibition could dampen intestinal inflammation. Initial studies demonstrated transcriptional repression of ENT1 and ENT2 in IBD biopsies or in murine IBD models. Subsequent studies in mice with global Ent1 or Ent2 deletion revealed selective protection of Ent2-/- mice. Elevated intestinal adenosine levels in conjunction with abolished protection following pharmacologic blockade of A2B adenosine receptors implicate adenosine signaling as the mechanism of gut protection in Ent2-/- mice. Additional studies in mice with tissue-specific deletion of Ent2 uncovered epithelial Ent2 as the target. Moreover, intestinal protection provided by a selective Ent2 inhibitor was abolished in mice with epithelium-specific deletion of Ent2 or the A2B adenosine receptor. Taken together, these findings indicate that increased mucosal A2B signaling following repression or deletion of epithelial Ent2 coordinates the resolution of intestinal inflammation. This study suggests the presence of a targetable purinergic network within the intestinal epithelium designed to limit tissue inflammation.

Abstract 1538: Inhibition of adenosine-mediated stabilization of intercellular adhesions promotes hypoxia-stressed endometrial carcinoma cell metastasis

Hypoxic microenvironments are commonly encountered in multiple epithelial malignancies. Epithelial cells classically up-regulate cellular adhesive mechanisms to counter the noxious effects of hypoxia. Inhibiting these tissue protective mechanisms is necessary for carcinoma cell invasion and metastasis. Our previous studies utilizing a transgenic model deficient for 5’nucleotidase (CD73), an enzyme responsible for adenosine biosynthesis, demonstrated that adenosine is required to circumvent hypoxia-mediated disruption of cellular adhesiveness in the normal endometrium. In endometrial carcinoma (EC), CD73 is down-regulated in poorly-differentiated, invasive, and metastatic ECs, but is expressed and functional in well-differentiated and non-metastatic disease. Based on these findings, we hypothesized that the malignant progression of EC is dependant on the loss of adenosine-mediated regulation of intercellular adhesions. Modified and unmodified Boyden chambers were used to assess migration and invasion of endometrial adenocarcinoma cells HEC-1A and HEC-1B. Hypoxia (5% or 1.5% O 2 ) significantly reduced HEC-1A and HEC-1B migration and invasion in vitro; this effect was abrogated by CD73 RNA interference and the CD73 enzymatic inhibitor, α, β- methylenediphosphate (AoPCP). The inhibitory effect of CD73 on EC cell migration and invasion was restored by the stable, receptor-targeting adenosine analog, 5’-N-ethylcarboxamido adenosine (NECA). Quantitative RT-PCR identified two adenosine receptors, A 1 R and A 2B R, to be expressed in HEC-1A and HEC-1B. Inhibition of the A 2B R by MRS1754 significantly increased HEC-1B migration and invasion, while A 1 R antagonism (DPCPX) had no effect. A 2B R inhibition in HEC-1A monolayers also resulted in increased paracellular permeability to FITC-labeled dextrans. Moreover, A 2B R antagonism lowered cellular levels of junctional adhesion proteins claudin-1 and claudin-2, and resulted in increased accumulation of β-catenin within the cytoplasm. Intracellular pools of β-catenin re-organized to the cell membrane with NECA application. In summary, we have shown that CD73-generated adenosine is necessary to maintain normal protective epithelial barrier function in the uterus. Our results demonstrate for the first time that inhibition of adenosine-mediated stabilization of intercellular adhesions promotes cancer cell invasion and metastasis in a hypoxic microenvironment. (NIH TL1RR024147 and NIH 1P50CA098258-01) Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1538. doi:10.1158/1538-7445.AM2011-1538

Abstract 3384: Down-regulation of 5’-nucleotidase (CD73)-generated adenosine: A novel mechanism for regulating endometrial cancer metastasis

Metastasis remains a significant concern for women with advanced or recurrent endometrial cancer (EC), as adjuvant chemotherapy and hormonal therapy have little effect. Therefore, more attention recently has been focused on the development of targeted therapies. The purinergic pathway, a metabolic cascade responsible for adenosine biosynthesis and regulation, has been clinically targeted in chronic lung and neurodegenerative diseases. Of interest is the association of adenosine dysregulation with epithelial-to-mesenchymal transition (EMT) in chronic lung diseases, as EMT is an essential hallmark of tumor metastasis. Adenosine and the purinergic pathway9s involvement in cancer EMT are unknown. In this study, we evaluated the expression of purinergic pathway components (adenosine receptor subtypes, A 1 R, A 2A R, A 2B R, and A 3 R; adenosine biosynthesis enzyme, 5′-nucleotidase (CD73); nucleoside transporter, equilibrative nucleoside transporter 1 (ENT1); and adenosine metabolizing enzyme, adenosine deaminase (ADA)), by quantitative real-time RT-PCR in 12 normal endometrial tissues and 46 endometrial carcinomas of varying histotypes. CD73 was the single component significantly altered in EC. Specifically, CD73 was down-regulated in poorly-differentiated, invasive and metastatic ECs. Similar correlations were observed by immunoassays. CD73 was also evaluated in EC cell lines representative of various EMT stages (HEC-1A, well- and KLE, poorly-differentiated, primary tumor; AN3CA, poorly-differentiated, metastatic lesion). CD73 transcript levels were highest in HEC-1A, low in KLE, and undetectable in AN3CA. Immunoanalyses confirmed these findings and revealed CD73 to be primarily localized to extracellular membranes in direct contact with neighboring cells. Using reverse phase HPLC, we demonstrated CD739s potential to function catalytically, and in addition, confirmed a direct correlation between transcript expression and production of adenosine in endometrial tissues and cell lines (p 2 =0.9388; p 2 =0.9816, respectfully). Moreover, as seen by enzyme histochemistry and electron microscopy, CD73 activity occurred exclusively at established intercellular adhesions. Our results suggest a novel function for CD73-generated adenosine in maintaining endometrial epithelial adhesions, with its down-regulation being an essential event of EC EMT. For a variety of different cancer types, clinical correlations have been previously established between tumor cells retaining intercellular adhesions and therapeutic responsiveness, but factors regulating these adhesions have not yet been successfully directly targeted. The drugable nature of adenosine and the purinergic pathway and its likely role in mediating endometrial epithelial intercellular adhesions represents a potential therapeutic strategy for metastatic EC. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3384.