The Heparanase-CD24-L1CAM Axis in Glioma Tumorigenesis

Introduction. Heparanase is an endo-β-D-glucuronidase that cleaves heparan sulfate (HS) side chains of heparan sulfate proteoglycans (HSPG). This activity is responsible for remodeling of the extracellular matrix (ECM), thereby promoting cell dissemination associated with tumor metastasis, angiogenesis and inflammation. Heparanase expression is low in normal epithelia but its expression is up-regulated in many carcinomas as well as sarcomas and hematological malignancies. Notably, cancer patients exhibiting high levels of heparanase had a significant shorter postoperative survival time than patients whose tumors exhibit low levels of heparanase, thus supporting its pro-metastatic function. More recent studies provided compelling evidence that tie heparanase levels with all steps of tumor formation including tumor initiation, growth, metastasis, and chemo-resistance. These and other results indicate that heparanase is causally involved in cancer progression and hence is a valid target for anti-cancer drug development. This notion is reinforced by preclinical studies revealing a marked inhibition of tumor growth in mice treated with heparanase inhibitors, now in phase I/II clinical trials in cancer patients.

Methods. In order to reveal the molecular mechanism(s) underlying the pro-tumorigenic properties of heparanase, we established an inducible (Tet-on) system of heparanase in U87 human glioma cells and applied gene array methodology in order to discover novel alterations in gene expression associated with heparanase induction.

Results. We found that CD24, a mucin-like cell adhesion protein, is consistently upregulated by heparanase as revealed by qPCR, FACS and immunostaining, whereas heparanase gene silencing was associated with decreased CD24 expression in human glioma cell lines. This finding was further substantiated by a similar pattern of heparanase- and CD24- immunostaining in core patient samples derived from high-grade glioma, low-grade glioma and nonmalignant control brain tissue. Noteworthy, over-expression of CD24 stimulated glioma cell migration, invasion, colony formation in soft agar, and tumor growth in mice, suggesting that CD24 functions to promote tumor growth. Likewise, anti-CD24 neutralizing monoclonal antibody (mAb) attenuated glioma tumor growth, and a similar inhibition was observed in mice treated with a neutralizing mAb directed against L1CAM, a ligand for CD24. Importantly, a significant difference in patient survival was found between heparanase-high/CD24-high tumors and heparanase-high/CD24-low tumors for both high-grade and low-grade glioma.

Conclusions. Our results strongly support the notion that the heparanase-CD24-L1CAM axis plays a significant role in glioma tumorigenesis. Therefore, targeting this axis might be a useful strategy to hinder glioma tumor progression.