Insulin-mimicking effects on adipocytes and other cells

The finding that lectins, such as concanavalin A, wheatgerm agglutinin, PHA and others, can mimic the effects of insulin on adipocytes is one of the most dramatic examples of interactions between lectins and cell membranes. Thus, these lectins stimulate the synthesis of triglycerides and the transport and oxidation of glucose in fat cells in vitro (Cuatrecasas & Tell, 1973; Pusztai & Watt, 1974; Livingstone & Purvis, 1980). Similar effects have also been observed in vivo (Katzen et al., 1981).

It is clear that, depending on their carbohydrate specificities, various lectins display significant differences in their insulin-mimicking activities. These differences become clearer if the insulin effect is not tested by the commonly used glucose oxidation assay, but rather by assessing their effects on lipogenesis or from their inhibition of lipolytic effects of hormones, such as epinephrine or other lipolytic hormones, on rat or hamster adipocytes (Ng, Li & Yeung, 1989). Rat cells lack functional αadrenergic receptors, while hamster adipocytes possess both α- and βdrenergic receptors (Carpene, Berlan & Lafontan, 1983) and these variations help to differentiate between the insulin effects of the different lectins. Thus, the D-mannose and N-acetyl-D-glucosamine-specific lectins generally have both potent antilipolytic and lipogenic activities while the fucose-binding lectins possess antilipolytic activity only. With some exceptions, most of the D-galactose-specific lectins show slight insulinlike effects. Furthermore, the N-acetyl-D-galactosamine-specific Dolichos biflorus and Maclura pomifera lectins exert antilipolytic activity in hamster but not in rat adipocytes.