Insulin Internalization and Intracellular Protein Degradation: A Quantitative Correlation

Summary

We have recently demonstrated that internalization of Insulin is essential for insulin's action upon intracellular proteolysis (Draznin and Trowbridge 1982). In this study we have investigated the quantitative relationship between the rate of insulin internalization and its ability to inhibit intracellular proteolysis. We have used the acidification technique to separate surface bound ¹²⁵I-insulin (sur) from internalized ligand (In). The In/Sur ratio plotted as a function of time permits the calculation of the rate of insulin internalization (K_e) (Draznin, Trowbridge and Ferguson 1984).

Insulin in a dose dependent manner increased the rate of C¹⁴-glucose incorporation into glycogen and inhibited the rate of degradation of intracellular proteins prelabelled in vivo with C¹⁴-valine. When insulin internalization was blocked by phenylarsine oxide (10^-5 M), the amount of surface bound ligand and its effect on glucose incorporation into glycogen were unaffected whereas insulin's effect on intracellular proteolysis was markedly diminished. There was a direct and significant correlation between K_e and insulin induced inhibition of intracellular proteolysis (r= .72, P < .05). The correlation between the amount of internalized insulin and intracellular proteolysis was also significant (r = .84, P < .01).

Key-Words

Insulin - Internalization - Action - Protein Degradation