Heparin can interact and regulate the activities of a wide range of proteins that are essentials to important biological processes such as
- blood clotting
- pathogen infection
- cell differentiation
- cell growth and migration
While these interactions are primarily electrostatic, there are evidences that the oligosacharidic sequence is also important.
The presence in some heparin chains of pentasaccharidic sequence that constitutes the antithrombin-binding site makes heparin a very potent anticoagulant. Heparin is capable of interacting with coagulation cascade proteins such as antithrombinaccelerating the inhibition of procoagulant factors such as the Xa and IIa factors.
Viruses and othe pathogens use the heparan sulphate chains on the wall of the target cells as receptors for a coat protein. This interaction is usually one of the first and critical steps leading to infection. Since exogenous heparin or its derivatives can interfere the binding by competing with the heparan sulphate the infection can be prevented.
One of the most well-known examples of this is malaria infection. Some strains of the parasite which causes malaria are capable of binding to cell-surface heparan sulphate, probably through the TRAP protein. The aggregation of red cells known as erythrocyte rosetting, (E-rosetting) can be inhibited by heparin or modified heparins (Rosetting refers to the immunological reaction between an antigenic determinant on the cell surface and a antibody on a red blood cell) In fact, curdlan sulphate, a semisynthetic sulphated polysaccharide with an anticoagulant activity 10 times lower than that of heparin, has been investigated as treatment for severe malaria. The 3-O-sulphated sequences found in Heparin Sulphate inhibit the entry of Herpex Simplex Virus (HSV-1) into corneal fibroblast, which points out to the possibility of using heparin-based drugs as antivirals.
Cell Differentiation, growth and migration.
Heparan sulphate molecules are expressed endogenously as proteoglycans that are components of the cellular surface or the extracellular matrix. Most biological functions of heparan sulphate are mediated by interactions with proteins that can be promoted or inhibited by exogenous heparin.
One of the major functions of heparan sulphate proteoglycans is the ability to regulate the activity of growth factors by different mechanism. They can act as co-receptor of the growth factor cell surface receptor; control the growth factor diffusion through the extracellular matrix and extend their half-life.
HS is therefore important in both normal tissue development and in tumour growth and metastasis.
Heparin is an inhibitor of the heparanase. However a major problem in developing useful therapeutic agents from heparin for use as, for example, anti-inflammatory or anti-cancer agents is the difficulty of isolating heparin structures specific to the desired activity. Several clinical trials have demonstrated that heparin is an efficient inhibitor of metastasis - it inhibits heparanase, whose expression is associated with tumour growth and metastasis. Furthermore, non- or low-anticoagulant heparins still have an anti-metastatic activity; therefore, anticoagulation is not a necessary component to attenuate metastasis.
Thrombosis is often associated with aggressive types of cancers. In fact, there are numerous clinical trials investigating the relation between the use of heparin and low molecular weight heparin and the morbidity and mortality rate of cancer patients. Researchers have proved that heparin therapies do improve survival rate for cancer patients. Moreover, there are also evidences that heparin might also possess antineoplastic properties. It has been proved in animal models that non-anticoagulant heparin significantly inhibits the migration of breast cancer cells and that anticoagulant heparin can reduce migration and metastasis of cancer cells. It has been suggested that the anti-metastatic effect might be caused by its ability to inhibit the binding of growth factor on the cell surface, which in turn inhibits tumor angiogenesis. Heparin can also inhibit the adhesion of growth factors by acting on the cell adhesion molecules L-selectin and P-selectin.
The potential of heparin as an anti-inflammatory drug has been shown by modest clinical trials with patients suffering bronchial asthma, ulcerative colitis and burns. This effect seems to be related with the ability of heparin to modulate the interactions between the leukocytes and the vascular endothelium. It is now well established that heparin inhibit the function of adhesion molecules involved in different stages of leukocyte extravasation; an essential event of the inflammatory response. In addition, heparin can inhibit complement activation, modulate the synthesis and activity of chemokines and reduce the activity of growth and angiogenic factors.
There are under study at least four potential mechanisms explaining the anti-inflammatory effects of heparin. They are:
- Binding proteins. There are more than 100 heparin-binding proteins. Heparin modulates the function and activity of a number of proteins, including acute phase proteins (i.e. proteins whose plasma concentrations increase or decrease in response to inflammation) and complement proteins (proteins forming the complement system). This property of heparin may explain the anti inflammatory activity of heparin. Researchers think that the binding of a cytokine to heparin-like molecules can protect the cytokine protein from proteolytic degradation. The glycosaninoglycan-cytokine interaction has potential therapeutic uses: heparin can attenuate ongoing tissue damage because it can bind and neutralize a wide range of mediators released from inflammatory cells. Heparin binds to many chemokines and cytokines therefore preventing these pro-inflammatory elements from interacting with their respective receptors.
- Selectin-mediated cell adhesion. Heparin interferes with the adhesion of leukocytes to the endothelium (leukocyte adhesion and activation is key in the response to inflammation). Excessive leukocyte activation releases toxic oxygen radicals and proteolytic enzymes which in turn contribute to vascular and tissue damage. These findings further support the hypothesis that the ability to bind to P-selectin of heparin and heparin-like products is the reason why it can inhibit neutrophil adherence to activated endothelial cells, thus blocking severe inflammation.
- Nuclear factor kappa B (NF-κB). Heparin blocks the NF-κB transcription factor. The blocking of this transcription factor can potentially reduce activation of inflammatory genes and regulate the gene expression and production of pro inflammatory cytokines, chemokines and adhesion molecules.
- Apoptotic cell death. Heparin and related compounds have been proved to modulate the activity of Tumor Necrosis Factor and NF-κB, which are involved in apoptotic cell death. Therefore, future research should investigate therapeutic potential of heparin in this area. The use of heparin as an anti-inflammatory agent has been limited for the high bleeding risk at the doses where the anti-inflammatory activity is expressed. Consequently, during the last years research work has been focused on the discovery of non-anticoagulant heparin derivatives or mimics. Ongoing investigations must focus on elucidating the mechanisms of action of these non anticoagulant heparins in cellular and animal models of acute and chronic inflammation.