INVOLVEMENT OF HEAT-SHOCK-PROTEIN-90 IN THE DEGRADATION OF MUTANT INSULIN-RECEPTORS BY THE PROTEASOME

We previously reported three families with type A insulin-resistant sy ndrome who had mutations, either Asp(1179) or Leu(1193), in the kinase domain of the insulin receptor. The extreme insulin resistance of the se patients was found to be caused by the decreased number of insulin receptors on the cell surface, due to the intracellular rapid degradat ion (Imamura, T., Takata, Y., Sasaoka, T., Takada, Y., Morioka, H., Ha ruta, T., Sawa, T., Iwanishi, M., Yang, G. H., Suzuki, Y., Hamada, J., and Kobayashi, M. (1994) J. Biol. Chem. 269, 31019-31027). In the pre sent study, we first examined whether these mutations caused rapid deg radation of unprocessed proreceptors, using the exon 13 deleted mutant insulin receptors (Delta Ex13-IR), which were accumulated in the endo plasmic reticulum as unprocessed proreceptors. The addition of Asp(117 9) or Leu(1193) mutation to Delta Ex13-IR caused accelerated degradati on of the unprocessed Delta Ex13-IR in the transfected COS-7 cells. Ne xt, we tested whether these mutant receptors were degraded by the prot easome. Treatment with proteasome inhibitors Z-Leu-Leu-Nva-H (MG-115) or Z-Leu-Leu-Leu-H (MG-132) prevented the accelerated degradation of t hese mutant receptors, resulting in increased amounts of the mutant re ceptors in the COS-7 cells. Essentially the same results were obtained in the patient's transformed lymphocytes. Finally, we found that thes e mutant receptors bound to heat shock protein 90 (Hsp90). To determin e whether Hsp90 played an important role in the accelerated receptor d egradation, we examined the effect of anti-Hsp90 antibody on the mutan t receptor degradation. The microinjection of anti-Hsp90 antibody into cells prevented the accelerated degradation of both Asp(1179) and Leu (1193) mutant insulin receptors. Taken together, these results suggest that Hsp90 is involved in dislocation of the mutant insulin receptors out of the endoplasmic reticulum into the cytosol, where the mutant r eceptors are degraded by the proteasome.