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See article on page 709
Lectins are proteins or glycoproteins of non-immune origin that bind specifically to carbohydrates. They usually, and arguably by definition, have at least two binding sites per molecule and tend to agglutinate cells to which they bind. They are ubiquitous in living matter, whether of plant or animal origin.1 Animal lectins include the selectins which are responsible for leucocyte–endothelial interactions, the hepatocyte galactose binding lectin responsible for removing aging, desialylated, glycoproteins from circulation (the asialoglycoprotein receptor), the circulating mannose binding lectin which functions as a complement protein, and the intracellular galectins (galactose binding lectins) whose natural functions have yet to be determined. Microbial lectins include the adhesins that are essential for the pathogenicity of many enteric organisms. Plant lectins are particularly plentiful in seeds and nuts. They are typically globular proteins which are highly resistant to digestion by mammalian enzymes and survive passage though the digestive tract. Their functions in the plant are unclear but probably include growth promoting and antifungal effects. Lectins usually have an effect on the cells to which they bind. Mitogenic functions have long been recognised—for example, for concanavalin A and phytohaemagglutinin. Although the effects of toxic lectins, such as phytohaemagglutinin (red kidney bean lectin) in undercooked chilli con carne, on the gut are well recognised, the interaction between non-toxic dietary lectins and the intestine has been relatively little studied until recently.
In this issue (see page 709), Jordinson and colleagues report that the broad bean lectin inhibits proliferation without apparent cytotoxicity and stimulates differentiation and protein synthesis. This is an unusual and intriguing combination of effects. As is currently the case for most of the known lectin effects, the mechanism is unclear but evidence is presented that the effect on differentiation is related to the adhesion molecule ep-CAM. The lectin is similar in some respects to the non-toxic antiproliferative lectin in the common edible mushroom (Agaricus bisporus).2 We have recently found that this lectin becomes internalised and selectively blocks nuclear-localising-sequence-dependent nuclear protein import.3 It differs from the broad bean lectin however in that it inhibits rather than stimulates protein synthesis. Care needs to be taken however not to extrapolate too far from results in one cell line to another and particularly when extrapolating from a malignant cell line to a whole animal. It is notable that the stimulation of differentiation is only seen in LS174T and not in HT29 or SW1222 cells. LS174T, unlike HT29, tend to form well differentiated goblet cells in confluent culture.4 Further studies are needed to determine which cell surface glycoproteins bind the lectin, remembering that quite different glycoproteins may express the same carbohydrate structure and it may be just one of these glycoprotein–lectin interactions that is responsible for initiating the differentiation effect. It will then be important to determine whether this broad bean lectin binding glycoprotein is present in the normal or diseased human intestine.
Jordinson and colleagues point out that lectins are plentiful in fruit and vegetables yet intake of these foods is protective against colon cancer, implying that this makes unlikely any connection between pro-proliferative plant lectins and colon cancer. However, this incompletely represents our lectin–galactose hypothesis for diet and colon cancer.5 The evidence that peanut ingestion stimulates rectal mucosal proliferation in individuals who express galactose on their mucosal glycoproteins6 we take as proof of concept of the principle that important functional interactions are likely to occur between intraluminal lectins and the increased galactose expressed by mucosal glycoproteins in colon cancer and premalignant disease,7 rather than proof that dietary galactose binding lectins will prove to be a major cause of colon cancer. We have pointed out that many of the intraluminal lectins will be of microbial origin and that the role of dietary galactose, which will competitively bind and therefore inhibit many of these lectins, may be a more important mechanism to explain the protective effect of fruit and vegetable fibre against colon cancer. A recent case control study of diet and colon cancer in Liverpool supports the protective effect of dietary galactose.8
It is not possible to predict what effects the broad bean lectin will have on the intact human intestine and Jordinsonet al’s conclusion that the broad bean lectin taken in the diet “may slow progression of colon cancer” is interesting but highly speculative. There are many unpredictable factors which include possible interactions between the lectin and dietary carbohydrates, interaction between the lectin and intestinal bacteria, the ability of the lectin to resist heat and digestion, and the possibility that the lectin might have effects on cells other than colon epithelial cells. Pusztai and colleagues have shown that many of the toxic effects of some lectins are dependent on their interaction with the intestinal flora9 and we have shown that dietary lectins may become internalised and circulate intact in the peripheral blood.10
The whole field of epithelial glycobiology and its implications for interaction between the mucosa and intraluminal lectins of dietary or microbial origin is fascinating and ripe for further study. Many of the glycosylation abnormalities found in colon cancer have been shown to correlate with invasive potential and ultimate prognosis. Some of the glycosylation changes are likely to be under genetic control—that is, as mucosally expressed blood group carbohydrate antigens. Much remains to be discovered about the nature of lectin–epithelial cell interactions and their implications for the functional importance of the regulation of glycosylation on cell surface and intracellular epithelial glycoproteins. Some of the plant lectins, such as the broad bean and mushroom lectins, may prove very useful tools in helping to identify key cellular glycoproteins involved in the regulation of proliferation and differentiation and its alteration in malignant disease.