Defective processing of the insulin receptor in an endoprotease-deficient Chinese hamster cell strain is corrected by expression of mouse furin.

Characterization of an endoprotease-deficient mutant Chinese hamster ovary (CHO) cell, designated RPE.40, revealed that it bound less than 10% as much insulin as did its parent, CHO-K1. We examined processing of the endogenous insulin receptor in CHO-K1 and RPE.40 cells, and processing of the human insulin receptor expressed in these cells. RPE.40 cells did not process the endogenous insulin proreceptor to its subunit forms, and processed the human insulin proreceptor inefficiently. Accumulation of the proreceptor form of the insulin receptor was seen in both cases. Furin is a mammalian endoprotease that cleaves proproteins at a consensus sequence of basic amino acids found in the insulin proreceptor. Expression of mouse furin in RPE.40 cells restored normal processing of the endogenous and the human insulin receptor in these cells. In addition, expression of mouse furin corrected the reduced binding of insulin in RPE.40 cells, indicating that receptor function as well as processing was restored.