Regulation of Insulin Secretion by SIRT4, a Mitochondrial ADP-ribosyltransferase

Abstract

Sirtuins are homologues of the yeast transcriptional repressor Sir2p and are conserved from bacteria to humans. We report that human SIRT4 is localized to the mitochondria. SIRT4 is a matrix protein and becomes cleaved at amino acid 28 after import into mitochondria. Mass spectrometry analysis of proteins that coimmunoprecipitate with SIRT4 identified insulindegrading enzyme and the ADP/ATP carrier proteins, ANT2 and ANT3. SIRT4 exhibits no histone deacetylase activity but functions as an efficient ADP-ribosyltransferase on histones and bovine serum albumin. SIRT4 is expressed in islets of Langerhans and colocalizes with insulin-expressing ␤ cells. Depletion of SIRT4 from insulin-producing INS-1E cells results in increased insulin secretion in response to glucose. These observations define a new role for mitochondrial SIRT4 in the regulation of insulin secretion.Histone deacetylases are enzymes that catalyze the removal of acetyl groups from the ⑀-amino group of lysine residues and are separated into three classes. Sirtuins, the class III histone deacetylases, are homologous to the yeast transcriptional repressor, Sir2p, and are NAD ϩ -dependent enzymes (1-3). Seven sirtuins have been identified in the human genome (4, 5). They share a conserved Sir2 catalytic core domain and exhibit variable amino-and carboxyl-terminal extensions that contribute to their unique subcellular localization and may also regulate their catalytic activity.The subcellular distribution, substrate specificity, and cellular functions of sirtuins are quite diverse (reviewed in Refs. 1-3). SIRT1 is found in the nucleus, where it functions as a transcriptional repressor via histone deacetylation. SIRT1 can also regulate transcription by modifying the acetylation levels of transcription factors, such as MyoD, FOXO, p53, and NF-B (6 -12). The SIRT2 protein is found in the cytoplasm, where it associates with microtubules and deacetylates lysine 40 of ␣-tubulin (13). The SIRT3 protein is localized in the mitochondrial matrix (14, 15), where it is proteolytically processed at its NH 2 terminus, yielding a mature protein that has protein deacetylase activity (14). These observations indicate that the targets of sirtuins are not restricted to histone proteins but extend to acetylated proteins in other subcellular compartments.Sirtuins also differ in their substrate specificities. For instance, SIRT1, -2, and -3 have robust activity on chemically acetylated histone H4 peptides, whereas SIRT5 has weak but detectable activity, and SIRT4, -6, and -7 have no detectable activity on the same substrate (13). Interestingly, a sirtuin from Archaeoglobus fulgidus, Sir2-Af1, which has close homology with SIRT5, also has weak activity on a histone peptide but significantly stronger activity on an acetylated bovine serum albumin substrate (16,17). Similarly, both SIRT1 and SIRT2 can deacetylate p53; however, only SIRT2 deacetylates lysine 40 of ␣-tubulin (13, 17).Recently, SIRT6 was demonstrated to be a nuclear ADP-ribosyltransferase (18), whereas a T. brucei SI...

NAD + and SIRT3 control microtubule dynamics and reduce susceptibility to antimicrotubule agents

“…Despite the effectiveness of SIRT3 in this overexpression assay, the other mitochondrial sirtuins, SIRT4 and SIRT5, were much less effective. Recent studies suggest that these sirtuins remove acyl modifications in proteins or deacetylate only a small subset of proteins in mitochondria (46)(47)(48). This finding contrasts with SIRT3, which is the major protein deacetylase in mitochondria (49), affecting the acetylation status of ∼100 proteins in mitochondria (40).…”

Glucose inhibits the insulin-induced activation of the insulin-degrading enzyme in HepG2 cells

“…Moreover, IDE-interacting proteins might play a role in the disturbance of regulation of IDE activity in high glucose conditions. The recently described IDE interaction with the mitochondrial protein SIRT4 can alter IDE protease activity by ADP-ribosylation [38]. IDE participates in a functional interaction with the 26 S proteasome, which is regarded as the principal site of ubiquitin-dependent intracellular protein degradation [39].…”

Mitochondrial Sirtuins and Their Relationships with Metabolic Disease and Cancer

“…Although SIRT4 possesses a conserved sirtuin deacetylase domain (42), initial reports did not identify any deacetylase activity of this sirtuin (2,52,107). Recently, however, SIRT4 has been reported to possess specific deacetylase activity toward at least one particular substrate (see subsequent section on SIRT4 and fatty acid metabolism) (84).…”

“…In recent years, protein posttranslational modifications (PTMs) such as ADP-ribosylation and lysine acetylation, succinylation, malonylation, and glutarylation on diverse mitochondrial proteins have emerged as a novel mechanism of mitochondrial regulation. These modifications are directly regulated by members of the sirtuin enzyme family (2,34,91,121,156).…”

“…However, certain sirtuins are now known to possess alternative catalytic functions-for example, deacylase, decrotonylase, desuccinylase, demalonylase, deglutarylase, and ADP-ribosyltransferase activities (Fig. 1)-that play crucial roles in the regulation of diverse cellular processes (2,10,32,34,36,48,65,72,121,156).…”