A new enzyme: insights into mechanisms of cell trafficking

Abstract

Movement of leukocytes and tumour cells from blood vessels to tissue sites forms a crucial part of inflammatory responses and tumour metastasis. Many factors that influence this process are well understood, whereas others remain elusive.Proteoglycans are molecules that are widely distributed throughout the body and comprise functional glycosaminoglycan chains bound to a protein core. Proteoglycans that are rich in the glycosaminoglycan heparan sulfate are considered to possess a regulatory role in cell trafficking, being expressed on vascular endothelium and throughout basement membrane and extracellular matrices. Extravasation of leukocytes and metastatic tumour cells is thought to involve release of an endoglycosidase, heparanase, which cleaves these molecules. Furthermore, molecules that inhibit the activity of heparanase are known to limit tumour cell metastasis and inflammatory cell recruitment in animal models.McKenzie et al.1xCloning and expression profiling of hpa2, a novel mammalian heparanase family member. McKenzie, E. et al. Biochem. Biophys. Res. Com. 2000; 276: 1170–1177Crossref | PubMed | Scopus (105)See all References1 have cloned a novel heparanase enzyme, named heparanase 2, that exhibits 35% homology with the previously described heparanase 1 enzyme. The substrate specificity of this new enzyme is, to date, unknown. Expression of heparanase 1 and heparanase 2 were quantified, using real-time reverse-transcriptase polymerase chain reaction (RT-PCR), in RNA from a range of normal human tissue explants and cell lines and also in cDNA from human lung, breast, colon, prostate, ovarian and pancreatic tumour xenografts, propagated in athymic nude mice. Heparanase 1 mRNA was strongly expressed in placenta and lymph node with very low levels of expression in other normal tissues. Conversely, heparanase 2 mRNA has a much wider distribution in normal tissue, with the exception of lymph and placenta where little or no expression was detected. Furthermore, in all of the tumour cell lines and xenografts examined, heparanase 1 expression was strongly increased compared with levels found in normal tissue. However, heparanase 2 expression was observed only in breast and pancreatic tumour cell lines and xenografts. Interestingly, the significant expression of this enzyme found in breast tumour cells was not over and above that found in healthy breast tissue, whereas pancreatic tumour heparanase 2 expression was greatly increased with respect to tissue from normal pancreas. The results of the study indicate not only differential tissue distribution for these two related enzymes but also the possibility of disparity in their biological roles.Heparanase inhibition has been identified previously as a therapeutic target, particularly in relation to oncology, given that the metastatic potential of tumour cells correlates directly with increased heparanase expression. However, enzymatic disruption of endothelial and extracellular matrix glycosaminoglycans is a recognized feature in inflammatory disease, suggesting a further area in which specific heparanase inhibition could be beneficial. The results of the study indicate that heparanase 1 inhibition is likely to be of most benefit in both of these settings, being expressed by tumour and haematopoeitic cells. The biological function of the newly described heparanase 2, although requiring further investigation, appears to be involved largely with the function of normal tissue, with the notable exception of pancreatic tumour cells. Given the important role of heparan sulfate molecules in tissue structure and the wide distribution of the heparanase 2 enzyme, it is conceivable that heparanase 2 might be involved in ‘normal’ functions such as recycling of cellular proteoglycans, heparanase 1 being associated with pathophysiological conditions. The interesting observation that heparanase 2 is altered specifically in pancreatic tumours might eventually provide further insight into the aetiology of this particular condition. Certainly, further exploration of the role of this novel enzyme is warranted.