Inositol phosphates and phosphoinositides activate insulin-degrading enzyme, while phosphoinositides also mediate binding to endosomes

Abstract

Insulin-degrading enzyme (IDE) hydrolyzes bioactive peptides, including insulin, amylin, and the amyloid β peptides. Polyanions activate IDE toward some substrates, yet an endogenous polyanion activator has not yet been identified. Here we report that inositol phosphates (InsPs) and phosphatdidylinositol phosphates (PtdInsPs) serve as activators of IDE. InsPs and PtdInsPs interact with the polyanion-binding site located on an inner chamber wall of the enzyme. InsPs activate IDE by up to ∼95-fold, affecting primarily V max . The extent of activation and binding affinity correlate with the number of phosphate groups on the inositol ring, with phosphate positional effects observed. IDE binds PtdInsPs from solution, immobilized on membranes, or presented in liposomes. Interaction with PtdInsPs, likely PtdIns(3)P, plays a role in localizing IDE to endosomes, where the enzyme reportedly encounters physiological substrates. Thus, InsPs and PtdInsPs can serve as endogenous modulators of IDE activity, as well as regulators of its intracellular spatial distribution.insulin-degrading enzyme | inositols | phosphatidylinositols | activation | subcellular localization I nsulin-degrading enzyme (IDE), also known as insulysin (EC 3.4.24.56), hydrolyzes a broad range of bioactive peptides in vitro, including angiotensin, glucagon, β-endorphin, amylin, and amyloid β peptides (Aβ), with the two most established in vivo substrates being insulin and Aβ. Evidence supporting a role for IDE in degrading insulin and Aβ in vivo includes associations of IDE gene variants with type 2 diabetes (1, 2) and Alzheimer's disease (3, 4), decreased clearance of the two peptides in IDE-deficient mice (5, 6), and antidiabetic activity produced by IDE inhibitors (7, 8), although some reports have questioned its role in insulin degradation (9, 10). In addition, IDE has been reported to have noncatalytic functions, such as acting as a receptor for varicella-zoster virus (11) and serving as a heat shock protein in stressed cells (12). IDE also modulates the activity of the proteasome (13), reportedly in conjunction with the retinoblastoma tumor-suppressor protein (14).We previously established that polyanions, such as free ATP and triphosphate, increase IDE activity by up to 100-fold toward a synthetic peptide substrate (15) and that ATP binds to a strongly electropositive inner surface of IDE that forms one-half of the substrate-binding chamber (16,17). Significantly, mutations in IDE that reduce its activation by polyanions also decrease its ability to rescue production of a mature yeast mating factor, indicating that activation by polyanions plays an important physiological role in cells (16). Physiological polyanionic activators of IDE have not yet been identified, however.Inositol phosphates (InsPs) are important intracellular second messengers generated by activation of many cell surface receptors (18,19). Phosphatidylinositol phosphates (PtdInsPs; phosphoinositides) participate in signaling and help define the identity of subcellular compartment...

Insulin-degrading enzyme is not secreted from cultured cells

“…It is possible that nearly all degradation of Aβ and insulin by IDE occurs in one or more intracellular compartments, most likely the endosomal system 4 , 34 – 36 . We recently showed that phosphoinositols can serve as intracellular signals for IDE localization to endosomes 7 adding support to the hypothesis that the endosomal pool of IDE might represent the major site of Aβ and insulin catabolism.…”

“…With some of its smaller peptide substrates and with amyloid β-peptide, IDE exhibits allosteric kinetic behavior 1 . IDE is mostly localized to the cytosol of the cell, but has also been observed in mitochondria 2 , peroxisomes 3 and endosomes 4 – 7 . There have been a number of studies reporting the secretion of IDE from cells 6 , 8 – 13 and this secreted form of IDE has been suggested to play an important role in degrading insulin and Aβ.…”

“…We recently uncovered a mechanism for the localization of IDE to endosomes via interaction with phosphoinositides 7 . During the course of these studies we asked whether the phosphatidylinositol dependent trafficking of IDE to endosomes or other intracellular compartments could play a role in IDE secretion.…”

The cell biology of systemic insulin function

“…The degradation process begins on the membrane immediately after insulin binding, where some insulin is reported to be partially degraded by extracellular IDE before internalization (Yokono et al, 1982). After internalization, additional IDE is thought to be targeted to endosomal membranes through its interaction with phosphatidylinositol phosphates (Song et al, 2017), where it begins to degrade receptor-bound insulin in endosomes (Yonezawa et al, 1988) before acidification occurs (Hamel et al, 1991). As endosomes acidify, any remaining insulin or partially degraded insulin that escaped complete degradation by IDE dissociates from the IR (Murphy et al, 1984; Fig.…”

Modulation of Insulin Sensitivity by Insulin-Degrading Enzyme

“…The subcellular localization of IDE is mainly cytosolic [ 37 , 156 , 157 , 158 ], but it has been reported to be present in several other subcellular compartments, including endosomes [ 58 , 159 , 160 ], peroxisomes [ 43 ], mitochondria [ 107 ], plasma membrane [ 161 , 162 , 163 , 164 , 165 ], endoplasmic reticulum [ 166 ], exosomes [ 167 ], the extracellular space [ 166 ] and even in human cerebrospinal fluid [ 168 ]. If IDE is primarily cytosolic, its role as an insulin protease seems to be called into question.…”