Ectoenzymes in epithelial ovarian carcinoma: potential diagnostic markers and therapeutic targets

Ectoenzymes are membrane-bounds proteins with the catalytic domain at the outer surface of the plasma membrane. These molecules can influence the extracellular environment through the products of their catalytic activities (for example, several among these products can function as second messenger or regulate the recruitment of cells); moreover, many ectoenzymes can function both as receptors and signaling molecules through mechanisms that are independent from their catalytic activity (1). Ectoenzymes are classified according to their enzymatic activity as peptidases, proteases, hydrolases, nucleotidases or oxidases. From the structural point of view, ectoenzymes are type II integral membrane proteins or glycosylphosphatidylinositol (GPI)-linked proteins, and many of them (including CD26, CD38, CD73, autotaxin and vascular adhesion protein 1) can be released in biological fluid as soluble proteins (1-4). Ectoenzymes have been extensively studied in the context of immune system, where they play multiple roles (both dependent and independent from their enzymatic activity) in the control of leukocyte trafficking (5). The role of ectoenzymes in cancer is poorly understood. However, it is emerging that many ectoenzymes play key roles during tumor development and progression and may have clinical utility. For example, CD38 is a negative prognostic marker in chronic lymphocytic leukemia (CLL) (6); low serum soluble vascular adhesion protein 1 (sVAP-1) is an indicator of poor prognosis and lymph node and liver metastasis in colorectal and gastric cancer (7); CD73 is associated with a prometastatic phenotype in melanoma and breast cancer (8). This evidence suggests that ectoenzymes could be explored as attractive candidates as targets for therapy. Ovarian cancer is the sixth most common cancer worldwide and the seventh leading cause of cancer-related deaths in women (9). Most (90%) ovarian cancers are epithelial in origin and, hence, are referred as epithelial ovarian cancers (EOC). Because of the lack of adequate screening tools, approximately 70% of patients are diagnosed at advanced stages, when the tumor has already metastasized to the peritoneum or distant sites (10). Despite advances in cytotoxic therapies, only 30% of patients with advanced-stage ovarian cancer survive 5 years after diagnosis. The current standard care consists of the combination of radical surgery and carboplatinum and paclitaxel combined chemotherapy. However, the usual failure of chemotherapy to eliminate all tumor cells promotes the development of drug-resistant tumors and systematic relapse. This scenario emphasizes the need for a greater awareness and understanding of ovarian cancer biology that can lead to the discovery of new tools for the screening and monitoring of the disease. Currently, few tumor markers are available to the clinicians caring for ovarian cancer patients. Great effort has been devoted to identify novel molecules that might facilitate screening, diagnosis, prognosis and monitoring response to treatment or relapse during follow-up, and that might provide specific targets for anti-tumor therapy with antibody-directed treatments, gene therapy or specific inhibitory molecules. This chapter presents an overview on ectoenzymes involved in ovarian cancer biology, development or progression (focusing on CD13, CD26, Autotaxin, CD157, CD73 and CD10) and highlights the potential role of these proteins as markers for ovarian cancer outcome and/or as therapeutic targets. CD10. Neutral endopeptidase 24.11 (NEP or CD10) is a cell surface aminopeptidase, which was originally characterized as T-cell differentiation antigen and was also detected in epithelial cells of several tissues (11). Recent reports showed that CD10 is involved in neoplastic transformation and tumor progression in selected human malignancies including lung, breast and prostate carcinomas, by degrading endothelin-1, which is an autocrin growth factor for these tumors (12, 13). CD10 expression was also detected in the stroma of borderline and malignant ovarian tumors, where it inversely correlated with histologic tumor grade (14). Kajiyama et al. described the expression of the molecule in ovarian cancer tissues and cell lines and showed that CD10 overexpression significantly decreases tumor cell proliferation and invasiveness, and enhances the susceptibility to paclitaxel resulting in increased apoptosis (15). CD13. Aminopeptidase N (APN or CD13) is a transmembrane ectopeptidase that cleaves N-terminal neutral amino acids of various peptides and proteins (16). Originally described as a marker for hematopoietic cells of myeloid lineage, CD13 expression has also been reported in non-hematopoietic cells and tissues, such as fibroblasts, epithelial cells of the liver, kidney, intestine, and pericytes in the brain. Furthermore, CD13 is expressed at high levels in various solid tumors and its expression level correlates with an increased tumor aggressiveness in prostate and colon cancers (17). In ovarian cancer, CD13 is expressed in epithelial tumor cells and endothelial cells, and its high expression affects cancer growth, vascular architecture, and response to chemotherapy. The role of CD13 in ovarian cancer cell migration is independent of aminopeptidase activity (18), while the increased resistance to chemotherapy is at least in part mediated by its enzymatic activity (19). CD26. Dipeptidyl peptidase IV (DPPIV or CD26) is a multifunctional cell surface aminopeptidase with ubiquitous expression and a variety of functional properties implicated in the development of human malignancies (20, 21). In ovarian carcinoma cells high CD26 expression induces a marked change in cellular morphology toward an epithelioid pattern and is accompanied by a significant decrease in the invasive potential both in vitro and in vivo (22, 23). It has been observed that CD26 expression is associated to sensitivity to paclitaxel in several EOC cell lines, suggesting that CD26 might be a useful marker to monitor ovarian cancer response to therapy. CD73. CD73 is a GPI-linked protein with ecto-5’-nucleotidase activity (24). Originally defined as a lymphocyte differentiation antigen, CD73 has a broad distribution being expressed in lymphocytes, endothelial cells and epithelial cells (25). CD73 expression has been reported in several types of cancer, including leukemia, glioblastoma, thyroid cancer, esophageal cancer and prostate cancer (26). CD73 expression is associated with a prometastatic phenotype in melanoma and breast cancer (8). Through its enzymatic activity, CD73 contributes to the generation of a suppressive microenvironment, thus promoting the growth of a number of tumors, including ovarian cancer (25). CD157. CD157 is the second member of a family of nicotinamide adenine dinucleotidases that also includes CD38. These ectoenzymes cleave extracellular nicotinamide adenine dinucleotide to produce ADP-ribose and cyclic ADP-ribose (27). CD157 is a GPI-anchored glycoprotein regulating leukocyte adhesion and diapedesis during inflammation (28). Recently, Ortolan et al. demonstrated that CD157 controls progression and peritoneal dissemination of ovarian tumors (29). CD157 is expressed in EOC primary cell cultures and cell lines, and it is involved in interactions among ovarian cancer cells, extracellular matrix proteins, and mesothelial cells, which ultimately control tumor cell migration and invasion. Moreover, CD157 is expressed in the majority of epithelial ovarian cancers, where its high expression is associated with rapid tumor relapse. Furthermore, in patients with serous ovarian cancer high CD157 expression is a powerful independent predictive variable for disease recurrence and survival (29). Autotaxin/CD203. Autotaxin (ATX), a member of the ectonucleotide pyrophosphatase and phosphodiesterase family of enzymes, is a transmembrane protein with a very short amino-terminal region. A soluble form derived by proteolytic cleavage has also been described (4). ATX was initially identified as a potent chemotactic factor able to increase invasiveness and metastatic potential in transformed cells (30). 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