Insulin inhibition of the proteasome is dependent on degradation of insulin by insulin-degrading enzyme

A consequence of insulin-dependent diabetes mellitus is the loss of lean muscle mass as a result of accelerated proteolysis by the proteasome. Insulin inhibition of proteasomal activity requires interaction with insulin-degrading enzyme (IDE), but it is unclear if proteasome inhibition is dependent merely on insulin-IDE binding or if degradation of insulin by IDE is required. To test the hypothesis that degradation by IDE is required for proteasome inhibition, a panel of insulin analogues with variable susceptibility to degradation by IDE binding was used to assess effects on the proteasome. The analogues used were [Lys^B28, Pro^B29]-insulin (lispro), [Asp^B10]-insulin (Asp^B10) and [Glu^B4, Gln^B16, Phe^B17]-insulin (EQF). Lispro was as effective as insulin at inhibition of degradation of iodine-125 (¹²⁵I)-labeled insulin, but Asp^B10 and EQF were somewhat more effective. All agents inhibited cross-linking of ¹²⁵I-insulin to IDE, suggesting that all were capable of IDE binding. In contrast, although insulin and lispro were readily degraded by IDE, Asp^B10 was degraded more slowly, and EQF degradation was undetectable. Both insulin and lispro inhibited the proteasome, but Asp^B10 was less effective, and EQF had little effect. In summary, despite effective IDE binding, EQF was poorly degraded by IDE, and was ineffective at proteasome inhibition. These data suggest that insulin inhibition of proteasome activity is dependent on degradation by IDE. The mechanism of proteasome inhibition may be the generation of inhibitory fragments of insulin, or by displacement of IDE from the proteasome.