Intestinal glycoproteins

Holmgren J, Fredman P, Lindblad M et al. Rabbit intestinal glycoprotein receptor for Escherichia coli heat-labile enterotoxin lacking affinity for cholera toxin. Infect Immun 1982 38 2) 424-433. 

Alkaline borohydride degradation of rat intestinal glycoprotein releases... 

P-glycoprotein (P-gp) works as a transporter at the intestinal mucosa pumping drugs out into the lumen. Absorption of P-gp substrates, such as digoxin, cyclosporine, etc., can be increased by inhibitors of P-gp and reduced by inducers. 

Vitamin B12 is special in as far as its absorption depends on the availability of several secretory proteins, the most important being the so-called intrinsic factor (IF). IF is produced by the parietal cells of the fundic mucosa in man and is secreted simultaneously with HC1. In the small intestine, vitamin B12 (extrinsic factor) binds to the alkali-stable gastric glycoprotein IF. The molecules form a complex that resists intestinal proteolysis. In the ileum, the IF-vitamin B 12-complex attaches to specific mucosal receptors of the microvilli as soon as the chymus reaches a neutral pH. Then either cobalamin alone or the complex as a whole enters the mucosal cell. 

Both influx and efflux transporters are located in intestinal epithelial cells and can either increase or decrease oral absorption. Influx transporters such as human peptide transporter 1 (hPEPTl), apical sodium bile acid transporter (ASBT), and nucleoside transporters actively transport drugs that mimic their native substrates across the epithelial cell, whereas efflux transporters such as P-glycoprotein (P-gp), multidrug resistance-associated protein (MRP), and breast cancer resistance protein (BCRP) actively pump absorbed drugs back into the intestinal lumen. 

Sparreboom A, van Asperen J, Mayer U, Schinkel AH, Smit JW, Meijer DK, et al. Limited oral bioavailability and active epithelial excretion of paclitaxel (Taxol) caused by P-glycoprotein in the intestine. Proc Natl Acad Sci USA 1997 94 2031-5. 

FIG. 2 Mechanisms of drug transfer in the cellular layers that line different compartments in the body. These mechanisms regulate drug absorption, distribution, and elimination. The figure illustrates these mechanisms in the intestinal wall. (1) Passive transcellular diffusion across the lipid bilayers, (2) paracellular passive diffusion, (3) efflux by P-glycoprotein, (4) metabolism during drug absorption, (5) active transport, and (6) transcytosis [251]. 

JV Asperen, OV Tellingen, JH Beijnen. The pharmacological role of / -glycoprotein in the intestinal epithelium. Pharmacol Res 37 429—435, 1998. 

A Sparreboom, J van Asperen, U Mayer, AH Schinkel, JW Smit, DKF Meijer, P Borst, WJ Nooijen, JH Beijnen, O van Tillingen. Limited oral bioavailability of taxol and active epithelial secretion of pa-clitaxel (Taxol) caused by -glycoprotein in the intestine. Proc Nat Acad Sci USA 94 2031-2035, 1997. 

K Westphal, A Weinbrenner, T Giessmann, M Stuhr, G Franke, M Zschiesche, R Oertel, B Terhaag, HK Kroemer, W Siegmund. Oral bioavailability of digoxin is enhanced by talinolol evidence for involvement of intestinal p-glycoprotein. Clin Pharmacol Ther 68 6-12, 2000. 

Current understanding of parasitism by T. spiralis is compartmentalized, and so glycoprotein function has been considered in the context of one compartment or another. However, LI larvae of T. spiralis have evolved under selective pressure to parasitize both intestine and muscle. Biological economy may require a duality of function in larval glycoproteins such that they are able to perform distinct roles in each of the two niches. Dualism is common in proteins, and elucidation of such properties in parasitic nematode products would provide unique insights into the basis of host adaptation. 

Attempts to study the entry of ES products into cells using markers of fluid phase endocytosis yielded unexpected results. When larvae browse resistant IEC-6 cells in the presence of extracellular fluorescent dextran, dextran enters the cytoplasm of a significant proportion of the cells in the mono-layer (Butcher et al., 2000). The parameters of dextran entry are most compatible with the conclusion that larvae wound the plasma membranes of IEC-6 cells that is, they create transient breaches in the membrane that allow impermeant markers to enter the cell (McNeil and Ito, 1989). Wounding is considered to be a common occurrence in intestinal epithelia (McNeil and Ito, 1989). Injured cells are able to heal their wounds by recruiting vesicles to seal the breach (Steinhardt et al., 1994). In an experimental system, healing allows the injured cell to retain cytoplasmic dextran. In epithelial cell cultures inoculated with T. spiralis larvae, the relationship between glycoprotein delivery and injury of plasma membranes is not clear, i.e. dextran-laden cells do not always stain with Tyv-specific antibodies and... 

This fraction was termed Haemonchus galactose-containing glycoprotein complex (H-gal-GP). The microvillar surface of the intestinal cells of worms retrieved from vaccinated lambs was coated with sheep immunoglobulin and protection observed, over a series of trials, was correlated with systemic antibody titre (Smith et al., 1999). 

Kindon, H., Pothoulakis, C., Thim, L., Lynch-Devaney, K. and Podolsky, D.K. (1995) Trefoil peptide protection of intestinal epithelial barrier function cooperative interaction with mucin glycoprotein. Gastroenterobgy 109, 516-523. 

In conclusion, there are several drawbacks to the use of Caco-2 cells in studies of active drug transport. Despite these drawbacks, we note that a recent comprehensive study comparing various P-glycoprotein drug efflux assays in drug discovery came to the conclusion that the Caco-2 transport assay is the method of choice, since it displays a biased responsiveness towards compounds with low or moderate permeability - in other words, towards compounds whose intestinal permeability is most likely to be significantly affected by drug efflux mechanisms [101]. 

N. L. Simmons, and B. H. Hirst. Functional expression of P-glycoprotein in apical membranes of human intestinal Caco-2 cells. Kinetics of vinblastine secretion and interaction with modulators, J. Biol. Chem. 1993, 268, 14991-14997... 

L. Z. Benet. Unmasking the dynamic interplay between intestinal P-glycoprotein and CYP3A4, /. 

N. L., Drugs absorption limited by P-glycoprotein-mediated secretory drug transport in human intestinal epithelial Caco-2 cell layers, Pharm. Res. 1993, 10, 743-749. 

Hunter, J., Hirst, B. H., Intestinal secretion of drugs the role of P-glycoprotein and related drug efflux systems in limiting oral drug absorption, A dr. Drug Del. Rev. 1997, 25, 129-157. 

Stephens, R. H., O Neill, C. A., Warhurst, A., Carlson, G. L., Rowiand, M., Warhurst, G., Kinetic profiling of P-glycoprotein-mediated drug efflux in rat and human intestinal epithelia, J. Pharmacol. Exp. Ther. 2001, 296, 584-591. 

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