Insulin-degrading enzyme (IDE) functions in the catabolism of bioactive peptides. Established roles include degrading insulin and the amyloid beta peptide (Aβ), linking it to diabetes and Alzheimer’s disease. IDE is primarily located in the cytosol, and a longstanding question is how it gains access to its peptide substrates. Reports suggest that IDE secreted by an unconventional pathway participates in extracellular hydrolysis of insulin and Aβ. We find that IDE release from cultured HEK-293 or BV-2 cells represents only ~1% of total cellular IDE, far less than has been reported previously. Importantly, lactate dehydrogenase (LDH) and other cytosolic enzymes are released at the same relative level, indicating that extracellular IDE results from a loss of cell integrity, not secretion. Lovastatin increases IDE release from BV-2 cells as reported, but this release is mirrored by LDH release. Cell viability assays indicate lovastatin causes a loss of cell integrity, explaining its effect on IDE release. IDE is present in an exosome-enriched fraction from BV-2 cell conditioned media, however it represents only ~0.01% of the total cellular enzyme and is unlikely to be a significant source of IDE. These results call into question the secretion of IDE and its importance in extracellular peptide degradation.
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This work was supported by National Institutes of Health grants GM 11787 (to LBH) and NS38041 (to DWR) and National Science Foundation grant IIA-1355438 (to DWR). We acknowledge use of facilities in the Kentucky Center for Structural Biology as well as support from and use of core facilities belonging to the Center for Molecular Medicine (National Institutes of Health grant P20 GM103486).
Supplementary information accompanies this paper at: https://doi.org/10.1038/s41598-018-20597-6.
Song, Eun Suk; Rodgers, David W.; and Hersh, Louis, "Insulin-Degrading Enzyme Is Not Secreted From Cultured Cells" (2018). Molecular and Cellular Biochemistry Faculty Publications. 140.