HERBIMYCIN-A INDUCES THE 20S PROTEASOME-DEPENDENT AND UBIQUITIN-DEPENDENT DEGRADATION OF RECEPTOR TYROSINE KINASES

Herbimycin A is an ansamycin antibiotic isolated as an agent that reverses morphological transformation induced by v-src. Although herbimycin A is widely used as a tool for inhibiting multiple tyrosine protein kinases and tyrosine kinase-activated signal transduction, its mechanism of action is not well defined and includes a decrease in both tyrosine kinase protein levels and activity (Uehara, Y., Murakami, Y., Sugimoto, Y., and Mizuno, S, (1989) Cancer Res. 49, 780-785). We now show that herbimycin A induces a profound decrease in the total cellular activity of transmembrane tyrosine kinase receptors, such as insulin-like growth factor, insulin, and epidermal growth factor receptors. A substantial proportion of the in vivo inhibition could be explained by an increase in the rate of degradation, The enhanced degradation of insulin-like growth factor-insulin receptor was prevented by inhibitors of the 20S proteasome, whereas neither lysosomotropic agents nor general serine- and cysteine-protease inhibitors were active in preventing receptor degradation induced by herbimycin A. Moreover, in a temperature-sensitive mutant cell line defective in the E1-catalyzed activation of ubiquitin, herbimycin A treatment at the restrictive temperature did not result in the degradation of insulin receptor, These results suggest that herbimycin A represents a novel class of drug that targets the degradation of tyrosine kinases by the 20S proteasome. The ubiquitin dependence of this process indicates that this degradation of tyrosine kinases might involve the 20S proteasome as the proteolytic core of the ubiquitin-dependent 26S protease.