A molecule that treats multiple age-related diseases would have a major impact on global health and economics. The SIRT1 deacetylase has drawn attention in this regard as a target for drug design. Yet controversy exists around the mechanism of sirtuin-activating compounds (STACs). We found that specific hydrophobic motifs found in SIRT1 substrates such as PGC-1α and FOXO3a facilitate SIRT1 activation by STACs. A single amino acid in SIRT1, Glu230, located in a structured N-terminal domain, was critical for activation by all previously reported STAC scaffolds and a new class of chemically distinct activators. In primary cells reconstituted with activation-defective SIRT1, the metabolic effects of STACs were blocked. Thus, SIRT1 can be directly activated through an allosteric mechanism common to chemically diverse STACs.

In human airways, epithelial cells lining the lumen and intraluminal cells (e.g., polymorphonuclear cells) participate in the innate immune response. These cells secrete or express on their surfaces arginine-specific ADP ribosyltransferases. Defensins, antimicrobial proteins secreted by immune cells, are arginine-rich, leading us to hypothesize that ADP ribosylation could modify their biological activities. We found that an arginine-specific ADP ribosyltransferase-1 present on airway epithelial cells modifies Arg-14 of ␣ defensin-1. ADP-ribosylated defensin-1 had decreased antimicrobial and cytotoxic activities but still stimulated T cell chemotaxis and IL-8 release from A549 cells. Further, ADP-ribosylated defensin-1 inhibited cytotoxic and antimicrobial activities of unmodified defensin-1. We identified ADP-ribosylated defensin-1 in bronchoalveolar lavage fluid from smokers but not from nonsmokers, confirming its existence in vivo. Thus, airway mono-ADP-ribosyltransferases could have an important regulatory role in the innate immune response through modification of ␣ defensin-1 and perhaps other basic molecules, with alteration of their biological properties. In the human airway, epithelial cells, in concert with intraluminal cells (e.g., macrophages), contribute to the innate immune response. Activation of airway epithelial cells releases proinflammatory mediators and chemotaxins, leading to the recruitment of inflammatory cells (e.g., neutrophils) (1, 2). Activated neutrophils, in turn, release azurophilic granules that contain human neutrophil peptides (HNP), small cationic peptides ranging in size from 29-33 aa (3). HNPs are characterized by a high arginine content and contain three pairs of disulfidelinked cysteines (4). They have a broad spectrum of antimicrobial and cytotoxic activities and play an important role in the innate immune response against both Gram-positive and -negative bacteria, fungi, and certain enveloped viruses (5-8). High HNP levels have been found in airway secretions of patients with inflammatory lung diseases (9-11).A family of eukaryotic and prokaryotic mono-ADP ribosyltransferases (ARTs) catalyze the transfer of ADP ribose from NAD ϩ to arginine residues in proteins (12). Among the monoARTs are bacterial products (e.g., cholera toxin), which are responsible for the clinical syndrome in disease (13). A family of mammalian ARTs that are secreted or localized on the cell surface through glycosylphosphatidylinositol anchors (14-19) are expressed preferentially on epithelial and inflammatory cells such as lymphocytes and neutrophils (17,20). Substrates of the five known mammalian ARTs (ART-1-5) include proteins that are involved in critical cellular events (e.g., lymphocyte activation, neutrophil chemotaxis) (21-23). Two of these transferases, ART-1 and -5, specifically modify arginine residues in proteins. The finding of ARTs on airway epithelial and inflammatory cells prompted us to evaluate potential substrates for ADP ribosylation in the airway (24). Because of the high arginine c...

Epithelial cells lining human airways and cells recruited to airways participate in the innate immune response in part by releasing human neutrophil peptides (HNP). Arginine-specific ADP-ribosyltransferases (ART) on the surface of these cells can catalyze the transfer of ADP-ribose from NAD to proteins. We reported that ART1, a mammalian ADP-ribosyltransferase, present in epithelial cells lining the human airway, modified HNP-1, altering its function. ADP-ribosylated HNP-1 was identified in bronchoalveolar lavage fluid (BALF) from patients with asthma, idiopathic pulmonary fibrosis, or a history of smoking (and having two common polymorphic forms of ART1 that differ in activity), but not in normal volunteers or patients with lymphangioleiomyomatosis. Modified HNP-1 was not found in the sputum of cystic fibrosis patients or in leukocyte granules of normal volunteers. The finding of ADP-ribosyl-HNP-1 in BALF but not in leukocyte granules suggests that the modification occurred in the airway. Most of the HNP-1 in the BALF from individuals with a history of smoking was, in fact, monoor di-ADP-ribosylated. ART1 synthesized in Escherichia coli, glycosylphosphatidylinositol-anchored ART1 released with phosphatidylinositol-specific phospholipase C from transfected NMU cells, or ART1 expressed endogenously on C2C12 myotubes modified arginine 14 on HNP-1 with a secondary site on arginine 24. ADPribosylation of HNP-1 by ART1 was substantially greater than that by ART3, ART4, ART5, Pseudomonas aeruginosa exoenzyme S, or cholera toxin A subunit. Mouse ART2, which is an NAD:arginine ADP-ribosyltransferase, was able to modify HNP-1, but to a lesser extent than ART1. Although HNP-1 was not modified to a significant degree by ART5, it inhibited ART5 as well as ART1 activities. Human ␤-defensin-1 (HBD1) was a poor transferase substrate. Reduction of the cysteine-rich defensins enhanced their ability to serve as ADP-ribose acceptors. We conclude that ADP-ribosylation of HNP-1 appears to be primarily an activity of ART1 and occurs in inflammatory conditions and disease.