{beta}3-integrin expression on tumor cells inhibits tumor progression, reduces metastasis, and is associated with a favorable prognosis in patients with ovarian cancer

Therapeutic targeting of tumor-stroma interactions

INTRODUCTION

Cancers exist within a complex microenvironment populated by diverse cell types within a protein-rich extracellular matrix. It is becoming increasingly apparent that molecular interactions between epithelial cells and cells in the surrounding stroma promote growth, invasion and spread of the tumor itself and thus represents a crucial underlying driving force in tumorigenesis.

AREAS COVERED

This article reviews how key interactions between tumor epithelial cells and surrounding mesenchymal and immune cells can promote tumor progression and highlights molecular elements that might represent novel therapeutic targets.

EXPERT OPINION

The tumor microenvironment is increasingly being viewed as a potential therapeutic target with a number of strategies being developed to disrupt tumor-stroma interactions, in order to delay or circumvent tumor progression. Targeting elements of the tumor microenvironment, or signaling pathways in tumor cells activated as a consequence of stromal interactions, may prove a useful therapeutic strategy to prevent tumor development and progression. However, given the tumor cells' ability to circumvent various therapeutic agents when given as monotherapy, the success of these agents is likely to be seen when used in combination with existing treatments.

Targeting the urokinase plasminogen activator receptor inhibits ovarian cancer metastasis

PURPOSE

To understand the functional and preclinical efficacy of targeting the urokinase plasminogen activator receptor (u-PAR) in ovarian cancer.

EXPERIMENTAL DESIGN

Expression of u-PAR was studied in 162 epithelial ovarian cancers, including 77 pairs of corresponding primary and metastatic tumors. The effect of an antibody against u-PAR (ATN-658) on proliferation, adhesion, invasion, apoptosis, and migration was assessed in 3 (SKOV3ip1, HeyA8, and CaOV3) ovarian cancer cell lines. The impact of the u-PAR antibody on tumor weight, number, and survival was examined in corresponding ovarian cancer xenograft models and the mechanism by which ATN-658 blocks metastasis was explored.

RESULTS

Only 8% of all ovarian tumors were negative for u-PAR expression. Treatment of SKOV3ip1, HeyA8, and CaOV3 ovarian cancer cell lines with the u-PAR antibody inhibited cell invasion, migration, and adhesion. In vivo, anti-u-PAR treatment reduced the number of tumors and tumor weight in CaOV3 and SKOV3ip1 xenografts and reduced tumor weight and increased survival in HeyA8 xenografts. Immunostaining of CaOV3 xenograft tumors and ovarian cancer cell lines showed an increase in active-caspase 3 and TUNEL staining. Treatment with u-PAR antibody inhibited α(5)-integrin and u-PAR colocalization on primary human omental extracellular matrix. Anti-u-PAR treatment also decreased the expression of urokinase, u-PAR, β(3)-integrin, and fibroblast growth factor receptor-1 both in vitro and in vivo.

CONCLUSIONS

This study shows that an antibody against u-PAR reduces metastasis, induces apoptosis, and reduces the interaction between u-PAR and α(5)-integrin. This provides a rationale for targeting the u-PAR pathway in patients with ovarian cancer and for further testing of ATN-658 in this indication.

Ovarian cancer development and metastasis

The biology of ovarian carcinoma differs from that of hematogenously metastasizing tumors because ovarian cancer cells primarily disseminate within the peritoneal cavity and are only superficially invasive. However, since the rapidly proliferating tumors compress visceral organs and are only temporarily chemosensitive, ovarian carcinoma is a deadly disease, with a cure rate of only 30%. There are a number of genetic and epigenetic changes that lead to ovarian carcinoma cell transformation. Ovarian carcinoma could originate from any of three potential sites: the surfaces of the ovary, the fallopian tube, or the mesothelium-lined peritoneal cavity. Ovarian cacinoma tumorigenesis then either progresses along a stepwise mutation process from a slow growing borderline tumor to a well-differentiated carcinoma (type I) or involves a genetically unstable high-grade serous carcinoma that metastasizes rapidly (type II). During initial tumorigenesis, ovarian carcinoma cells undergo an epithelial-to-mesenchymal transition, which involves a change in cadherin and integrin expression and up-regulation of proteolytic pathways. Carried by the peritoneal fluid, cancer cell spheroids overcome anoikis and attach preferentially on the abdominal peritoneum or omentum, where the cancer cells revert to their epithelial phenotype. The initial steps of metastasis are regulated by a controlled interaction of adhesion receptors and proteases, and late metastasis is characterized by the oncogene-driven fast growth of tumor nodules on mesothelium covered surfaces, causing ascites, bowel obstruction, and tumor cachexia.

CD95 promotes tumour growth

CD95 (also called Fas and APO-1) is a prototypical death receptor that regulates tissue homeostasis mainly in the immune system through induction of apoptosis 1-3. During cancer progression CD95 is frequently downregulated or cells are rendered apoptosis resistant 4,5 raising the possibility that loss of CD95 is part of a mechanism for tumour evasion. However, complete loss of CD95 is rarely seen in human cancers 4 and many cancer cells express large quantities of CD95 and are highly sensitive to CD95 mediated apoptosis in vitro. Furthermore, cancer patients frequently have elevated levels of the physiological ligand for CD95, CD95L 6. These data raise the intriguing possibility that CD95 could actually promote the growth of tumours through its nonapoptotic activities 7. Here we show that cancer cells in general, regardless of their CD95 apoptosis sensitivity, depend on constitutive activity of CD95, stimulated by cancer-produced CD95L, for optimal growth. Consistently, loss of CD95 in mouse models of ovarian cancer and liver cancer reduces cancer incidence as well as the size of the tumours. The tumorigenic activity of CD95 is mediated by a pathway involving JNK and c-Jun. These results demonstrate that CD95 plays a growth promoting role during tumorigenesis and suggest that efforts to inhibit its activity rather than to enhance its activation should be considered during cancer therapy.

Constitutive activation of BMP signalling abrogates experimental metastasis of OVCA429 cells via reduced cell adhesion

Background

Activation of bone morphogenetic protein (BMP)4 signalling in human ovarian cancer cells induces a number of phenotypic changes in vitro, including altered cell morphology, adhesion, motility and invasion, relative to normal human ovarian surface epithelial cells. From these in vitro analyses, we had hypothesized that active BMP signalling promotes the metastatic potential of ovarian cancer.

Methods

To test this directly, we engineered OVCA429 human ovarian cancer cells possessing doxycycline-inducible expression of a constitutively-active mutant BMP receptor, ALK3^QD, and administered these cells to immunocompromised mice. Further characterization was performed in vitro to address the role of activated BMP signalling on the EOC phenotype, with particular emphasis on epithelial-mesenchymal transition (EMT) and cell adhesion.

Results

Unexpectedly, doxycycline-induced ALK3^QD expression in OVCA429 cells reduced tumour implantation on peritoneal surfaces and ascites formation when xenografted into immunocompromised mice by intraperitoneal injection. To determine the potential mechanisms controlling this in vivo observation, we followed with several cell culture experiments. Doxycycline-induced ALK3^QD expression enhanced the refractile, spindle-shaped morphology of cultured OVCA429 cells eliciting an EMT-like response. Using in vitro wound healing assays, we observed that ALK3^QD-expressing cells migrated with long, cytoplasmic projections extending into the wound space. The phenotypic alterations of ALK3^QD-expressing cells correlated with changes in specific gene expression patterns of EMT, including increased Snail and Slug and reduced E-cadherin mRNA expression. In addition, ALK3^QD signalling reduced β1- and β3-integrin expression, critical molecules involved in ovarian cancer cell adhesion. The combination of reduced E-cadherin and β-integrin expression correlates directly with the reduced EOC cell cohesion in spheroids and reduced cell adhesion to the extracellular matrix substrates fibronectin and vitronectin that was observed.

Conclusions

We propose that the key steps of ovarian cancer metastasis, specifically cell cohesion of multicellular aggregates in ascites and cell adhesion for reattachment to secondary sites, may be inhibited by overactive BMP signalling, thereby decreasing the ultimate malignant potential of ovarian cancer in this model system.

Integrins and mutant p53 on the road to metastasis

Understanding how tumor cells invade tissues is key to developing drugs to block metastasis. In this issue, Muller et al. (2009) report that a mutant form of the tumor suppressor p53 in cancer cells boosts the endocytic recycling of the adhesion molecule integrin alpha5beta1 and of epidermal growth factor receptor, promoting invasion and metastasis.