Heparin and Heparinoids—Far More Than Just Anticoagulants

In the era of rational drug design, heparin appears to be an atypical drug of an older generation, representing one of the few remaining drugs of animal origin in the market. Moreover, heparin is a highly sulfated glycosaminoglycan polysaccharide well known for its important structural variability and batch heterogeneity, qualities that have become tolerated. However, there are many strong arguments for the continued development and use of heparin in the modern drug market. In 2024, Proteoglycan Research published a special issue dedicated to heparin and heparinoids. Here, we will discuss the importance of these molecules to the extent that they received focus in a special issue. Heparin has been present as a potent anticoagulant in clinics for almost 90 years. Beyond being a life-saving pioneer in its ability to prevent blood coagulation, it has had an equally important role of inspiring research on HS and heparan sulfate proteoglycans (HSPGs) in physiological and pathophysiological contexts. Expressed by virtually all mammalian cells, HS share structural similarities with heparin. HS are, often in the form of HSPGs, of key importance for cells to communicate with the microenvironment. Furthermore, HS are considered a structural basis to assemble components of the extracellular matrix. Many of these interactions are based on proteins binding to negatively charged HS chains at the cell surface, including growth factors, chemokines, and other matrix components, which all have a strong impact on cell behavior. Notably, these binding events are sensitively balanced by the structure and charge density of HS, most likely induced by the sulfation pattern forming a complex picture of an “HS interactome,” which is challenging to interpret and understand [1]. However, heparin has been a helpful key when applied as an anticoagulant drug to incidentally bring many connections to light in those scenarios for a better understanding HS in pathophysiology. The application of heparin as an anticoagulant for the treatment and prophylaxis of thrombosis in oncology was an outstanding approach to decrypting HS-mediated pathologies. It revealed and targeted several processes in which HS and HSPG force malignancy and metastasis of tumors. Consequently, heparin was and is considered an antitumorigenic drug beyond its anticoagulant activity. Therefore, although orally available anticoagulants have been approved in the last two decades (e.g., FXa-inhibitors), being at least equivalent to low molecular weight heparins (LMWHs), LMWHs are still considered a first-line treatment of cancer-associated thrombosis according to guidelines analogous to other “modern” anticoagulants. Notably, after initial findings using heparin, heparin molecules served as a scaffold for preparing structurally modified HS-mimetics, referred to as “heparinoids” to further follow those findings on HS or HSPG mediated pathological processes and to understand structural predictions. To mention one prominent example, the role of heparanase, an endo-β-glucuronidase, is clinically highly accepted as a malignant factor of tumor progression. Targeting heparanase found its origin in the efforts to inhibit the enzyme by heparin and heparinoids [2, 3] (Figure 1). Derivative HS-mediated heparanase inhibitors underwent clinical trials for oncological treatment. Furthermore, heparin was the premier choice in blocking selectin adhesion receptor activity with strong impact on tumor cell metastatic spread [4] and immune cell transmigration. Thus, heparin was key to understand the kinetic binding peculiarities of this unique lectin receptor family [5]. Further attempts succeeded to separate anticoagulant and selectin blocking activities by using semi-synthetic heparin derivatives [6]. Furthermore, heparin was helpful in understanding signaling pathways of HSPGs, for example, the mechanistic role of syndecan-1 in tumor development [7]. The role of platelet activation by tumor cells, a thrombin-driven process for fostering metastatic spread of tumors, attracted much attention during the last years. Blocking platelet activation by heparin has further consequences to overcome immunosuppressive activities of platelet releasates, revealing a novel perspective for heparin-derived immune-oncological activities (Figure 1) [8]. A recent focus of heparin research has been the role of LMWH in SARS-CoV-2 viral pandemic. Like many pathogens, SARS-CoV-2 makes use of cell surface glycosaminoglycans as co-receptors for host cell invasion. Based on former knowledge of potential antiviral activities of heparin, it was discovered that heparin binds to SARS-CoV-2 spike S1-receptor-binding domain, thus attenuating the interaction with ACE2 and consequently the host cell invasion [9]. This offered a new view on HS in viral invasion [10] and supports the life-saving role of heparin and heparinoids by weakening the course of viral infections. As an indicator of the importance of glycosaminoglycan research, an annual meeting has been established in the 1990s started as an Italian-German scientific communication. These “Symposia on Glycosaminoglycans” in Villa Vigoni at Lake Como in Italy received a strong attention and reputation as a meeting of world-leading experts in glycosaminoglycan and proteoglycan research for scientific communication and discussion of research from a biochemical, pharmacological, medical, and pharmaceutical view, with emphasis on academic and industrial relevance and perspective. Heparinoids in the broadest sense of the term, that is, heparin analogues from plant and synthetic sources mimicking heparin biological functions, were covered in these discussions [11-13]. At the 30th anniversary of this symposium in 2023, editor in chief of Proteoglycan Research, Renato Iozzo, conceived the idea to celebrate the impact of heparin by awarding a Special Issue in the new journal. In these terms, we have collected a series of contributions to this Special Issue, meanwhile published by Proteoglycan Research representing the wide spectrum of research and activities in the field of glycosaminoglycan knowledge and investigation. A perfect overview of the field and perspective of heparin and heparinoids is given by Lena Kjellen and Ulf Lindahl, two outstanding, highly deserving experts in heparin research, providing the readers an “essence of sequence of HS.” A review by the Mourao group (Oliveira et al.) highlights the differences between bovine and porcine heparins, underlying the important advances in the production of a high-anticoagulant bovine heparin with properties comparable to porcine-derived heparin. Lanzi et al. give an excellent overview of the role of heparinoids, non-anticoagulant heparins in oncology, addressing the HS and heparanase activities in sarcoma. The group of Christian Gorzelanny (Grossdorf et al.) provides a fascinating piece of work using super-resolution microscopy in combination with single-molecule force spectroscopy (SMFS) to illustrate HS on cell surfaces at a molecular level, demonstrating the impressive progress in a visual understanding HS acitvities. The group of Rolland Reinbold (Angeli et al.) considered the complexity of glycosylation by a unique approach to study glycan processing enzymes by single cell sequencing of various cell types in tissues of human diseases using bioinformatics techniques. The role of glycosaminoglycans for targeting nanoparticles to tissues and organs is considered by Milesi et al. in all perspectives for drug delivery systems, a highly challenging technical approach with a pharmaceutical perspective. Edward Harris' group (Rohr et al.) addresses the catabolism of heparin, a pathway worthy of elucidation. To this purpose, the paper describes the cloning and characterization of recombinant rat stabilin-2 and its interaction with heparin and HA. Finally, an article by the Ronzoni group in collaboration with Keele and Liverpool universities (Uslenghi et al.) addresses the repurposing of heparin manufacturing by-products as potential anti-inflammatory therapeutics, through an in depth structural and functional investigation. As guest editors of this special issue, we hope to provide the readership of Proteoglycan Research an interesting and up-to-date insight into the wide spectrum of HS and HSPG research to prove that heparin and heparinoids are steadfastly modern drugs and unique tools to understand the fascinating world of HS in physiology and pathophysiology. The authors declare no conflicts of interest.