Insulin-ricin B hybrid molecules: receptor binding and biological activity in a minimal-deviation hepatoma cell line

Hybrid molecules were produced by covalently coupling the hormone insulin to the binding chain B of the plant toxin ricin. Binding of the insulin-ricin B hybrid to minimal-deviation hepatoma cells occurred primarily through ricin-specified cell-surface carbohydrates (galactose, N-acetylgalactosamine) since 125I-insulin-ricin B binding to cells could be 90% displaced by 50 mM lactose. [14C]Glucose incorporation into glycogen was maximally stimulated approximately 80% by insulin, whereas maximum stimulation by insulin-ricin B hybrid was greater than 100%. Ricin B chain alone was non-stimulating at concentrations tested (10(-9)-10(-7) M). Furthermore, the stimulation of [14C]glycogen labeling mediated by the hybrid was markedly inhibited by 1 mM lactose, while this sugar had no effect on the stimulation mediated by native insulin. Additionally, a preparation of ricin B shown to actively displace up to 80% of the binding of 125I-hybrid to cells also inhibited hybrid-mediated [14C]glycogen production. These results indicate that insulin-ricin B hybrid molecules possess toxin-specified binding abilities while evoking the insulin-associated cellular response of stimulated incorporation of [14C]glucose into glycogen. Such results thus suggest the possibility that alternate cell-surface receptors may play a role in conveying insulin's intracellular metabolic-control signals.