Effects of lectins on CCK-8-stimulated enzyme secretion and differentiation of the rat pancreatic cell line AR42J

Toxic proteins in plants

Plants have evolved to synthesize a variety of noxious compounds to cope with unfavorable circumstances, among which a large group of toxic proteins that play a critical role in plant defense against predators and microbes. Up to now, a wide range of harmful proteins have been discovered in different plants, including lectins, ribosome-inactivating proteins, protease inhibitors, ureases, arcelins, antimicrobial peptides and pore-forming toxins. To fulfill their role in plant defense, these proteins exhibit various degrees of toxicity towards animals, insects, bacteria or fungi. Numerous studies have been carried out to investigate the toxic effects and mode of action of these plant proteins in order to explore their possible applications. Indeed, because of their biological activities, toxic plant proteins are also considered as potentially useful tools in crop protection and in biomedical applications, such as cancer treatment. Genes encoding toxic plant proteins have been introduced into crop genomes using genetic engineering technology in order to increase the plant's resistance against pathogens and diseases. Despite the availability of ample information on toxic plant proteins, very few publications have attempted to summarize the research progress made during the last decades. This review focuses on the diversity of toxic plant proteins in view of their toxicity as well as their mode of action. Furthermore, an outlook towards the biological role(s) of these proteins and their potential applications is discussed.

Plant lectins: potential antineoplastic drugs from bench to clinic

Plant lectins, carbohydrate-binding proteins distributed widely in a variety of plant species, have drawn a rising attention for cancer biologists due to their remarkable anti-tumour properties. In this review, we present a brief outline of the up-to-date advances of plant lectins in elucidating their complex anti-cancer mechanisms implicated in apoptosis and autophagy. In addition, we further discuss the pre-clinical and clinical studies of plant lectins for their potential therapeutic applications. In conclusion, these inspiring findings would open a new perspective for plant lectins as potential antineoplastic drugs from bench to clinic.

Inhibitory effect of the lectin wheat germ agglutinin (WGA) on the proliferation of AR42J cells

The rat pancreatic acinar tumour cell line AR42J is a widely used model to study the secretion, proliferation and differentiation of cells under the influence of hormones. These so-called amphicrine cells synthesize and secrete digestive enzymes as well as neuroendocrine peptides. They possess both subtypes of the highly glycosylated cholecystokinin (CCK) receptor which are important for the regulation of secretion and for cell growth. AR42J cells extrude CCK and gastrin-like hormone peptides and have the ability of an autostimulation (autocrine loop). The lectins wheat germ agglutinin (WGA) and Ulex europaeus agglutinin (UEA-I) bind to the glycosylated sites of these CCK receptors with the effect inhibiting CCK binding and thus inhibiting the CCK-induced Ca2+ release and alpha-amylase secretion. The so-called trophic hormones CCK and gastrin stimulate the secretion and proliferation of AR42J cells within the autocrine loop via autostimulation of their CCK receptors. In preceding papers, we described the inhibitory effect of WGA on the binding of 125I-CCK-8s to the CCK-A and -B receptors and the subsequent enzyme secretion of AR42J cells. In the present work, we studied the influence of the lectins WGA, UEA-I and galectin-1, as well as of the lectin-like enzyme alpha-amylase, on the proliferation of AR42J cells and prevention of autostimulation. The proliferation inhibition of the growth fraction was measured by estimation of the S-phase fraction by DNA flow cytometry. Whereas WGA inhibited the growth fraction significantly, UEA-I, human galectin-1 and human alpha-amylase had no significant effect. In transmission electron microscopy, we observed the accumulation of typical zymogen granules under the effect of WGA and a better differentiation of cells.

Lectins as Bioactive Plant Proteins: A Potential in Cancer Treatment

Plant lectins, a unique group of proteins and glycoproteins with potent biological activity, occur in foods like wheat, corn, tomato, peanut, kidney bean, banana, pea, lentil, soybean, mushroom, rice, and potato. Thus, dietary intakes by humans can be significant. Many lectins resist digestion, survive gut passage, and bind to gastrointestinal cells and/or enter the circulation intact, maintaining full biological activity. Several lectins have been found to possess anticancer properties in vitro, in vivo, and in human case studies; they are used as therapeutic agents, preferentially binding to cancer cell membranes or their receptors, causing cytotoxicity, apoptosis, and inhibition of tumor growth. These compounds can become internalized into cells, causing cancer cell agglutination and/or aggregation. Ingestion of lectins also sequesters the available body pool of polyamines, thereby thwarting cancer cell growth. They also affect the immune system by altering the production of various interleukins, or by activating certain protein kinases. Lectins can bind to ribosomes and inhibit protein synthesis. They also modify the cell cycle by inducing non-apoptotic G1-phase accumulation mechanisms, G2/M phase cell cycle arrest and apoptosis, and can activate the caspase cascade. Lectins can also downregulate telomerase activity and inhibit angiogenesis. Although lectins seem to have great potential as anticancer agents, further research is still needed and should include a genomic and proteomic approach.

Inhibitory effects of human and porcine alpha-amylase on CCK-8-stimulated lipase secretion of isolated rat pancreatic acini

Previously we have demonstrated inhibitory effects of the plant lectin wheat germ agglutinin (WGA) on (125)I-CCK-8 binding to pancreatic AR42J cells as well as on CCK-8-stimulated Ca(2+) release and alpha-amylase secretion of rat pancreatic acini or acinar cells. Therefore, it is entirely conceivable that alpha-amylase having several lectin-like carbohydrate recognition domains can modulate the CCK-8 stimulated lipase secretion. Human alpha-amylase, purified from pancreatic juice by affinity chromatography to homogeneity, and commercial porcine pancreatic alpha-amylase inhibit CCK-8-stimulated lipase secretion of rat pancreatic acini in a concentration-dependent manner. Acarbose, a specific inhibitor of alpha-amylase, was without effect on CCK-8-induced cellular lipase secretion. The data presented here provide evidence for a regulatory function of alpha-amylase in CCK-8-stimulated pancreatic secretion.

Inhibitory effect of the lectin wheat germ agglutinin on the binding of 125I-CCK-8s to the CCK-A and -B receptors of AR42J cells

INTRODUCTION

Cholecystokinin (CCK) is a peptide hormone and plays a major role both in the regulation of pancreatic enzyme secretion and growth of the gastrointestinal tract. The pancreatic CCK receptors are highly glycosylated membrane proteins that are able to bind plant lectins such as wheat germ agglutinin (WGA) and Ulex europaeus agglutinin (UEA-I).

AIM AND METHODOLOGY

In preceding papers, we demonstrated an inhibition of CCK-8s induced Ca2+ signaling and secretion of rat pancreatic acini and AR42J cells by the lectins WGA and UEA-I (Pancreas 2001;23:368-374). Here we studied the influence of WGA, UEA-I, and 22 other lectins on 125I-CCK-8s binding on AR42J cells. A binding assay was used with 125I-CCK-8s and dexamethasone-stimulated AR42J cells, bearing CCK-A as well as CCK-B receptors.

RESULTS

WGA inhibits 125I-CCK-8s binding in a dose-dependent manner. The binding is affected at concentrations of WGA >1 microg/mL. The EC50 for inhibition is 8 microg/mL. At a concentration of 25 microg/mL, WGA inhibits the hormone binding 70%. This inhibition can be abolished by the specific sugars for WGA N,N',N"-triacetylchitotriose and N-acetylglucosamine, but not by N-acetylneuraminic acid. UEA-I diminished hormone binding but without significance, although UEA-I binds to the fucose residues of receptor glycosylations. All other 22 lectins tested here were ineffective.

CONCLUSION

The blockage of CCK receptors by WGA explains the inhibition of CCK-8s induced Ca2+ signaling and the secretion of pancreatic acinar cells and AR42J cells. Although the inhibitory effect of WGA is in agreement with the findings of Santer et al, the results with UEA-I are in contrast to those of Santer et al (1990), who described a strong increase in 125I-CCK-8s binding to isolated crude rat pancreatic cell membranes in the presence of UEA-I.