Thrombosis, or blood clot formation, and its sequelae remain a leading cause of morbidity and mortality, and recurrent thrombosis is common despite current optimal therapy. Protein disulfide isomerase (PDI) is an oxidoreductase that has recently been shown to participate in thrombus formation. While currently available antithrombotic agents inhibit either platelet aggregation or fibrin generation, inhibition of secreted PDI blocks the earliest stages of thrombus formation, suppressing both pathways. Here, we explored extracellular PDI as an alternative target of antithrombotic therapy. A high-throughput screen identified quercetin-3-rutinoside as an inhibitor of PDI reductase activity in vitro. Inhibition of PDI was selective, as quercetin-3-rutinoside failed to inhibit the reductase activity of several other thiol isomerases found in the vasculature. Cellular assays showed that quercetin-3-rutinoside inhibited aggregation of human and mouse platelets and endothelial cell-mediated fibrin generation in human endothelial cells. Using intravital microscopy in mice, we demonstrated that quercetin-3-rutinoside blocks thrombus formation in vivo by inhibiting PDI. Infusion of recombinant PDI reversed the antithrombotic effect of quercetin-3-rutinoside. Thus, PDI is a viable target for small molecule inhibition of thrombus formation, and its inhibition may prove to be a useful adjunct in refractory thrombotic diseases that are not controlled with conventional antithrombotic agents.

Protein disulfide isomerase (PDI) is an oxidoreductase essential for folding proteins in the endoplasmic reticulum. The domain structure of PDI is a–b–b′–x–a′, wherein the thioredoxin-like a and a′ domains mediate disulfide bond shuffling and b and b′ domains are substrate binding. The b′ and a′ domains are connected via the x-linker, a 19-amino-acid flexible peptide. Here we identify a class of compounds, termed bepristats, that target the substrate-binding pocket of b′. Bepristats reversibly block substrate binding and inhibit platelet aggregation and thrombus formation in vivo. Ligation of the substrate-binding pocket by bepristats paradoxically enhances catalytic activity of a and a′ by displacing the x-linker, which acts as an allosteric switch to augment reductase activity in the catalytic domains. This substrate-driven allosteric switch is also activated by peptides and proteins and is present in other thiol isomerases. Our results demonstrate a mechanism whereby binding of a substrate to thiol isomerases enhances catalytic activity of remote domains.

Extracellular protein disulfide isomerase (PDI) is required for platelet thrombus formation and fibrin generation after arteriolar wall injury in live mice. PDI is secreted from platelets and endothelial cells on cellular activation, but the mechanism of capture of secreted PDI within the injured vasculature is unknown. We establish that, like the endothelial β3 integrin α(V)β(3), the platelet integrin α(IIb)β(3) binds PDI. PDI also binds to recombinant β3. Using intravital microscopy, we demonstrate that PDI accumulation at the site of laser-induced arteriolar wall injury is markedly reduced in β3-null (β3(-/-)) mice, and neither a platelet thrombus nor fibrin is generated at the vessel injury site. The absence of fibrin after vascular injury in β3(-/-) mice is because of the absence of extracellular PDI. To evaluate the relative importance of endothelial α(V)β(3) versus platelet α(IIb)β(3) or α(V)β(3), we performed reciprocal bone marrow transplants on wild-type and β3(-/-) mice. PDI accumulation and platelet thrombus formation were markedly decreased after vessel injury in wild-type mice transplanted with β3(-/-) bone marrow or in β3(-/-) mice transplanted with wild-type bone marrow. These results indicate that both endothelial and platelet β3 integrins contribute to extracellular PDI binding at the vascular injury site.

The radiomimetic enediyne C-1027 induces almost exclusively DNA double-strand breaks (DSB) and is extremely cytotoxic. Unique among radiomimetics, ataxia-telangiectasia mutated (ATM) is dispensable for cellular responses to C-1027-induced DNA damage. This study explores the biological activity of three recently bioengineered C-1027 analogues: 7 00

The ability of the radiomimetic anticancer enediyne C-1027 to induce ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR)-independent damage responses was discovered to reside in its unique ability to concurrently generate robust amounts of double-strand breaks (DSBs) and interstrand cross-links (ICLs) in cellular DNA. Furthermore, a single substitution to the chromophore's benzoxazolinate moiety shifted DNA damage to primarily ICLs and an ATR-but not ATM-dependent damage response. In contrast, single substitutions of the chromophore's ␤-amino acid component shifted DNA damage to primarily DSBs, consistent with its induction of conventional ATM-dependent damage responses of the type generated by ionizing radiation and other radiomimetics. Thus, phosphatidylinositol 3-kinase-like protein kinase regulation of DNA damage responses is dictated by the relative proportions of DSBs and ICLs.DNA double-strand break ͉ C-1027 ͉ radioimetic ͉ ATM ͉ ATR C ells use several members of phosphatidylinositol 3-kinaselike protein kinases (PIKKs), including ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR), to initiate cell cycle checkpoint responses to DNA damage (1). The induction of double-strand breaks (DSBs) into cellular DNA rapidly triggers ATM activation, which promotes a kinase cascade involving phosphorylation of the signal transducers Chk1 and Chk2 and downstream targets such as p53, all of which contribute to cell cycle arrest (1, 2). Similar to ATM, ATR activates Chk1, p53, and, to a lesser degree, Chk2 to initiate cell cycle checkpoints in response to lesions such as interstrand cross-links (ICLs), which stall DNA replication (2).Although ATM's role in responding to DSBs was primarily based on ionizing radiation (IR) studies, radiomimetic enediynes such as neocarzinostatin, C-1027, and calicheamicin also have provided insight (3). Enediynes are a structurally diverse group of compounds whose chromophores bind to the DNA minor groove and subsequently undergo Bergman cycloaromatization, generating two free radicals (4, 5). These radicals can induce DSBs when hydrogen atoms in close proximity, but on opposite DNA strands (6), are abstracted from deoxyribose.Cellular responses to radiomimetic treatment are similar to IR, which include ATM-dependent activation of p53-Ser-15 and Chk2-Thr-68 (7, 8) and enhanced cell death in cells lacking ATM (5). That responses to DSBs are ATM-dependent regardless of the damaging agent has led to the general conclusion that cells require ATM to activate DNA damage responses to DSBs (5, 6). The sole exception is C-1027, where cells deficient in either ATM or ATR phosphorylate p53-Ser-15 and Chk2-Thr-68 as readily as wild type and are diminished only when both PIKKs are absent (9, 10). Furthermore, C-1027 is similarly toxic to wild-type, ATM-, or ATR-deficient cells, because cell death hypersensitivity occurs only in the absence of both (10).Remarkably, minor modifications of the C-1027 chromophore can alter the PIKK dependence of the DNA damage response (11). ...