The Molecular Sieve and Tumor Cells
The matrix of our connective tissue is an amorphous gelatinous substance made of biological macromolecules; it is colorless and transparent and has a certain viscosity. Its interstices are filled with tissue fluid, which lies between the connective tissue cells and fibers.
The main component of the matrix is proteoglycan (also called protein polysaccharide), a polymer formed by glycosaminoglycans (about 80%-90%) covalently bound to proteins. Glycosaminoglycans, also called amino polysaccharides or mucopolysaccharides, are mainly divided into sulfated and non-sulfated types. The former includes chondroitin sulfate, keratan sulfate, dermatan sulfate, and heparan sulfate, which are relatively small molecules; the latter is mainly hyaluronic acid, a long, coiled, winding macromolecule up to 2.5 micrometers long that forms the backbone of the proteoglycan.
The small glycosaminoglycan molecules are like the bristles on a test-tube brush; they bind covalently to the core protein and radiate outward around it, forming a proteoglycan subunit. These subunits then bind through link proteins to the hyaluronic acid backbone, forming a proteoglycan aggregate.
Large numbers of proteoglycans aggregate to form a molecular sieve with many micropores—a sieve-like structure that can filter molecules, hence the name molecular sieve. Its function is like a semipermeable membrane with selective permeability: it allows water, nutrients, metabolites, hormones, and gas molecules to pass, while blocking larger macromolecules and bacteria beyond the pores, so that the matrix becomes a defensive barrier limiting the spread of harmful substances such as bacteria.
Hemolytic streptococci and cancer cells can produce hyaluronidase, which destroys the matrix's molecular-sieve structure and thereby enables spread or metastasis. Theoretically, if we could control the hyaluronidase produced by cancer cells, we might hope to slow cancer progression. But research in this area is still scarce, and no drug or therapy that effectively exploits this molecular-sieve mechanism to suppress tumors has yet appeared.