Elucidating The Role Of Hpa2 In Breast Cancer

Heparanase is capable of cleaving heparan sulfate (HS) side chains of heparan sulfate proteoglycans (HSPG), an activity that is highly implicated in tumor metastasis, inflammation, and angiogenesis. Recent studies provide compelling evidence that tie heparanase with all aspects of tumor development including tumor initiation, growth, metastasis, and chemoresistance. Heparanase 2 (Hpa2) was cloned soon after cloning of the heparanase gene, based on sequence similarity. Unlike heparanase, Hpa2 lacks intrinsic HS-degrading activity typical of heparanase. Hpa2, nonetheless, retains the capacity to bind HS with potency even higher than that of heparanase. In head and neck cancer patients, Hpa2 expression was markedly elevated, correlating with prolonged time to disease recurrence and inversely correlating with tumor cell dissemination to regional lymph nodes, suggesting that Hpa2 functions as a tumor suppressor. The aim of the present study was to elucidate the role of Hpa2 in breast cancer.

Methods. Immunostaining was utilized to determine Hpa2 levels and cellular localization. Breast carcinoma cells were infected with Hpa2 cDNA or gene construct that directs its localization to the cell nucleus (Nuc-Hpa2). Orthotopic tumor model was established to examine the role of Hpa2 and Nuc-Hpa2 in tumor growth.

Results. In normal breast epithelium, Hpa2 was noted to be localized predominantly to the cell nucleus. In striking contrast, Hpa2 expression is markedly decreased in breast carcinoma and appears diffused in the cytoplasm of the tumor cells, suggesting that not only the expression level but also the cellular localization of Hpa2 play a role in tumorigenesis. In order to reveal the significance of nuclear Hpa2 in breast cancer growth, we have generated gene constructs that target Hpa2 to the cell nucleus via three NLS repeat sequences (Nuc-Hpa2). Notably, overexpression of Nuc-Hpa2 in ZR-75-1 breast cancer cells resulted in smaller tumors, critically implying that nuclear Hpa2 functions to restrain tumor growth. Interestingly, unlike Nuc-Hpa2, overexpression of Hpa2 in ZR-75-1 cells enhanced tumor growth. This may suggest that nuclear Hpa2 restrains, while the secreted Hpa2 promotes breast cancer growth. Indeed, overexpression of Hpa2 in ZR-75-1 and MCF10CA cells enhanced cell invasion and cell migration mediated by its interaction with cell surface HS. No such effect was noted in cells overexpressing Nuc-Hpa2.

Conclusions. The results add a new layer in our appreciation of Hpa2 function and suggest that nuclear localization may play a role in healthy breast epithelium, whereas the secreted form promotes breast cancer.