Heparanase and gallbladder cancer: New insights into understanding tumor growth and invasion

Gallbladder cancer (GBC) is a common gastrointestinal malignancy in the Asia–Pacific region.1 It has an overall dismal prognosis, as most cases have unresectable disease at presentation. Early detection is rare, the radical resection of GBC requires significant surgical expertise, and chemoradiotherapy is largely ineffective.1,2 Emerging novel targets, such as heparanase, may offer a fresh lease of life for affected patients. The role of heparanase has been studied in various tumors and has been found to be associated with tumor growth and invasion.3 Antiheparanase strategies are currently being evaluated in phase II trials in patients with hepatocellular carcinoma (HCC) and other malignancies.4 Studying the role of heparanase in GBC could pave the way for exploring therapeutic targets for this cancer. In this issue of the journal, Wu et al. have studied the association of heparanase with GBC and have shown that the presence of heparanase is associated with advanced tumor stage and poor outcome.5 An understanding of the role of heparanase in tumor biology is necessary before we proceed to analyze the present study. After any tumor becomes approximately 500 mm in diameter, its further growth is limited by tissue hypoxia and the surrounding extracellular matrix (ECM). Tumor adaptations are therefore essential to ensure survival. Such adaptations include the induction of angiogenesis by the production of factors that destroy the ECM and the elaboration of hypoxia-inducible factors (HIF).6 The accumulation of a series of mutations confers survival skills for the tumor under “hostile conditions”. One of the key steps in carcinogenesis is the mutation of p53. This protects the malignant cells from apoptosis and also results in the transcriptional activation of heparanase expression.7 Heparanase is a key factor that promotes tumor growth by promoting angiogenesis and destroying ECM, thereby providing more favorable adhesion sites for malignant cells.8 Heparan sulfate proteoglycan, an essential component of the ECM and the vessel wall, is enzymatically cleaved by heparanase. This results in the opening of channels between the vascular and the extravascular compartments, which in turn facilitates cell migration and cell–cell interactions.9,10 The heparanase-mediated destruction of ECM releases a host of bioactive growth factors anchored within the mesh of the ECM, and these promote cellular proliferation. Heparanase promotes angiogenesis in the tumor microenvironment by multiple mechanisms, which include enhancing Akt signaling, stimulating endothelial cell migration, increasing the synthesis and shedding of syndecan-1, and upregulating vascular epidermal growth factor activity.9–11 Heparanase also promotes cell adhesion, survival, and signaling events, independent of its enzymatic activity. Thus, transfection and overexpression of the heparanase gene converts normal T lymphocytes to those with malignant behavior, providing evidence for a direct role of heparanase in tumor metastasis.12 Interestingly, mammalian cells express a single dominant heparanase enzyme, which would make targeting this enzyme a more feasible proposition.13 In most normal tissues, heparanase activity is suppressed by the wild-type p53 gene, so that heparanase is active only in immune cells, platelets, and keratinocytes. However, heparanase is active during morphogenesis in the fetus and the placenta. By digesting the basement membrane and ECM, it promotes cell movement between the vascular and extravascular compartments. In this way, heparanase plays a crucial role in morphogenesis, inflammation, wound healing, autoimmune disorders, and malignancy. Estrogen, tumor necrosis factor-α, interferon-α, and p53 mutations upregulate heparanase gene expression and activity. Nuclear transcriptional factor-κB (NF-κB), interleukin-6, HIF-1α, and cyclooxygenase-2 are complexly interlinked in the regulation of heparanase activity.9 In the study reported by Wu et al. in this issue of Journal of Gastroenterology and Hepatology, the presence of heparanase was associated with larger tumor size, more frequent tumor invasion, and poorer survival for patients with GBC.5 Heparanase upregulation has been shown to be associated with increased tumor vascularity, tumor size, tumor invasion, and poor survival in patients with malignancies arising from various sites, such as ovary, breast, prostate, bladder, intestine, pancreas, liver, colon, stomach, and the buccal cavity.13 Also, HIF-1α and the NF-κB p65 protein (a component of the NF-κB pathway) have been frequently co-expressed with heparanase and associated with poor outcome. Accumulation of multiple mutations is required for tumor survival. The tumor clone that has most effectively adapted to the adverse microenvironment is the one likely to survive and further proliferate. At the time of clinical diagnosis, when the tumor mass is already sizeable, only such highly modified tumor clones are likely to be present. In the present study, the authors showed that the presence of heparanase and HIF-1α expression was associated with decreased survival by a univariate analysis. However, only lymph node metastasis and tumor size were independently related to survival based on a multivariate analysis. It is important to appreciate that heparanase and HIF-1α expression are the two identified changes, among the many unidentified changes which may have also occurred in the tumor cells. Thus, large tumor size and lymph node metastasis, which in themselves are a testimony to the survival capability of the tumor cells, have emerged as independent predictors of poor outcome for GBC. In this study, those cases with larger tumors and lymph node metastases were more often marker positive than small non-metastating cases. Heparanase has also become a prospective target in human cancer treatment because it is upregulated in most tumors. Various antiheparanase strategies have been found to be effective in controlling tumor growth under in vitro as well as in vivo conditions. Blocking heparanase has reduced the incidence of experimental metastases by 90% in one study.14 Sulfated oligosaccharide PI88 is an antiheparanase drug that has entered phase II trials for the treatment of HCC.15 Chang et al. have already shown that heparanase inhibitors inhibit the invasive potential of GBC cell lines in in vitro models.16 Heparin also has antiheparanase activity and has been found to be effective in tumor control, as shown in a number of animal models.17 While the use of heparin is limited by its anticoagulant properties, non-anticoagulant species of heparin have also been found to effectively inhibit tumor metastasis and autoimmunity and to stimulate neovascularization and wound healing.13,14,18 In summary, heparanase facilitates tumor cell invasion, vascularization, and survival, all critical events in cancer progression. The antitumor spreading effects of heparanase-inhibiting molecules and the identification of a single, predominant, functional heparanase in tumor cells support the concept that this enzyme could be a novel target for anticancer therapy. The present study provides the first evidence in humans that heparanase expression is associated with a more malignant phenotype of GBC, specifically with larger tumor size and lymph nodal metastases at the time of diagnosis with a resultant, poor overall outcome. It is now important to test whether the blockade of heparanase will impact on the otherwise dismal prognosis of patients with this distressing form of cancer.