Endocytosis of therapeutic macromolecules in tumor cells - Mechanistic aspects of the proteoglycan receptor function
Abstract: Novel therapeutics to combat cancer are urgently needed. Most current pharmacological therapies have severe side effects and are seldom curative. Macromolecular drugs, and in particular nucleic acid based drugs, offer a potential remedy for this situation. Currently, the absence of efficacious and safe methods to deliver nucleic acids to intracellular sites of action is the main impediment to the introduction of nucleic acid based therapies in the clinic. Viral delivery methods have been demonstrated to efficiently deliver nucleic acids, but also to be associated with severe, occasionally life threatening, immune reactions. Non-viral delivery methods are, so far, not sufficiently efficient for use in the clinic. Many viral and virtually all non-viral macromolecular delivery methods depend on cell surface heparan sulfate proteoglycans (HS PGs) for efficient uptake, however the details of this mechanism and the exact role of the PG has been unclear. The aim of this thesis was to clarify the role of the cell surface PG in macromolecular uptake processes. It is demonstrated that mammalian cells can internalize extracellular DNA by a pathway strictly dependent on cell surface PGs and this pathway is characterized. Secreted, positively charged, proteins and peptides including the antimicrobial peptide LL-37, are shown to facilitate the uptake process. It is also demonstrated that specific HS epitopes, present on cell surface HS PGs, are pivotal for the uptake of diverse HS binding ligands including polyamines and macromolecular antibody complexes. Finally, using a newly developed method for the isolation of endocytic vesicles, it is demonstrated that both classes of cell surface HS PGs, syndecans and glypicans, are true internalizing receptors capable of intracellular macromolecular delivery. This thesis advances our understanding of PGs as potential targets for macromolecular delivery vehicles. This understanding will be of aid for the development of future macromolecular drugs to the benefit of the patient.
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