Hepatic lectins. Concept, biochemistry, and role

Identification of receptors in the liver that mediate endocytosis of circulating tissue kallikreins

The liver plays an important role in the clearance, by receptor-mediated endocytosis, of circulating glycoproteins. It has been demonstrated that tissue kallikreins, which are acid glycoproteins, circulate in plasma, where they are poorly inhibited by plasma proteins. We have shown that the liver is the main organ that clears tissue kallikreins from the circulation. We now report the identification of receptors involved in this clearance. Using a perfused rat-liver system, and as models, pig pancreatic (PPK) and horse urinary (HoUK) kallikreins, we have found that: (a) the binding of PPK to the perfused liver was inhibited by 50 mM methyl alpha-D-mannoside and 20 microM mannan, was partially inhibited by 50 mM mannose and was unaffected by 1.5 microM asialofetuin; (b) binding of HoUK to the perfused liver was inhibited by 1.5 microM asialofetuin, 50 mM galactose and 50 mM lactose and was unaffected by 50 mM mannose; (c) the clearance rate of both kallikreins followed the equation y = a.xb; (d) their binding was Ca2+-dependent and their clearance was inhibited by 3 mM chloroquine and 10 mM methylamine. Using isolated liver cells and tritiated HoUK, we calculated that 500,000 receptors/cell were present and the Scatchard plot showed that there were two apparent affinity constants: 0.24.10(9) l/M) (high-affinity) and 0.3.10(8) l/M (low-affinity). These results show that PPK is recognized by a liver mannose receptor and HoUK by the galactose receptor. The liver uptake of native and circulating tissue kallikreins thus emerges as a mechanism by which their levels in plasma are regulated.

Analysis of differentiation and transformation of cells by lectins

During differentiation cells are known to change their biological behavior according to their genotype. This is thought to be accompanied by a modulation of cell surface determinants expressed on the outer cell membrane. Vice versa, cell surface molecules are suggested to mediate extracellular signals to the genome. Most of these molecules integrated in the cell membrane have been proven to be glycoconjugates. The carbohydrate moieties of these molecules can be detected by means of lectins that are characterized by their ability to react specifically with distinct terminal sugar sequences. Thus, lectins have been used as appropriate tools for studying the modulation of functionally important membrane-associated molecules during the differentiation of cells, in particular of B- and T-lymphocytes. Moreover, lectins have been proven to distinguish between differentiated cells and malignant cell clones, according to the hypothesis that transformed cells possess a glycoconjugate profile that corresponds to the stage of differentiation at which they are arrested. Since lectins, like monoclonal antibodies, make it possible to study functionally important molecules that are associated with differentiation and malignancy, they might be of value for diagnostic purposes and, moreover, for analyzing malignant transformation.