Discovery and Development of Cyclic Peptide Inhibitors of CIB1.

Calcium and integrin binding protein 1 (CIB1) is a small, intracellular protein recently implicated in survival and proliferation of triple-negative breast cancer (TNBC). Considering its interactions with PAK1 and downstream signaling, CIB1 has been suggested as a potential therapeutic target in TNBC. As such, CIB1 has been the focus of inhibitor discovery efforts. To overcome issues of potency and stability in previously reported CIB1 inhibitors, we deploy mRNA display to discover new cyclic peptide inhibitors with improved biophysical properties and cellular activity. We advance UNC10245131, a cyclic peptide with low nanomolar affinity and good selectivity for CIB1 over other EF-hand domain proteins and improved permeability and stability over previously identified linear peptide inhibitor UNC10245092. Unlike UNC10245092, UNC10245131 lacks cytotoxicity and does not affect downstream signaling. Despite this, UNC10245131 is a potent ligand that could aid in clarifying roles of CIB1 in TNBC survival and proliferation and other CIB1-associated biological phenotypes.

Discovery and Characterization of Peptide Inhibitors for Calcium and Integrin Binding Protein 1.

Calcium and integrin binding protein 1 (CIB1) is an EF-hand-containing, small intracellular protein that has recently been implicated in cancer cell survival and proliferation. In particular, CIB1 depletion significantly impairs tumor growth in triple-negative breast cancer (TNBC). Thus, CIB1 is a potentially attractive target for cancer chemotherapy that has yet to be validated by a chemical probe. To produce a probe molecule to the CIB1 helix 10 (H10) pocket and demonstrate that it is a viable target for molecular intervention, we employed random peptide phage display to screen and select CIB1-binding peptides. The top peptide sequence selected, UNC10245092, was produced synthetically, and binding to CIB1 was confirmed by isothermal titration calorimetry (ITC) and a time-resolved fluorescence resonance energy transfer (TR-FRET) assay. Both assays showed that the peptide bound to CIB1 with low nanomolar affinity. CIB1 was cocrystallized with UNC10245092, and the 2.1 Å resolution structure revealed that the peptide binds as an α-helix in the H10 pocket, displacing the CIB1 C-terminal H10 helix and causing conformational changes in H7 and H8. UNC10245092 was further derivatized with a C-terminal Tat-derived cell penetrating peptide (CPP) to demonstrate its effects on TNBC cells in culture, which are consistent with results of CIB1 depletion. These studies provide a first-in-class chemical tool for CIB1 inhibition in cell culture and validate the CIB1 H10 pocket for future probe and drug discovery efforts.

Ether lipid metabolism by AADACL1 regulates platelet function and thrombosis.

We previously reported the discovery of a novel lipid deacetylase in platelets, arylacetamide deacetylase-like 1 (AADACL1/NCEH1), and that its inhibition impairs agonist-induced platelet aggregation, Rap1 GTP loading, protein kinase C (PKC) activation, and ex vivo thrombus growth. However, precise mechanisms by which AADACL1 impacts platelet signaling and function in vivo are currently unknown. Here, we demonstrate that AADACL1 regulates the accumulation of ether lipids that impact PKC signaling networks crucial for platelet activation in vitro and in vivo. Human platelets treated with the AADACL1 inhibitor JW480 or the AADACL1 substrate 1-O-hexadecyl-2-acetyl-sn-glycerol (HAG) exhibited decreased platelet aggregation, granule secretion, Ca2+ flux, and PKC phosphorylation. Decreased aggregation and secretion were rescued by exogenous adenosine 5'-diphosphate, indicating that AADACL1 likely functions to induce dense granule secretion. Experiments with P2Y12-/- and CalDAG GEFI-/- mice revealed that the P2Y12 pathway is the predominate target of HAG-mediated inhibition of platelet aggregation. HAG itself displayed weak agonist properties and likely mediates its inhibitory effects via conversion to a phosphorylated metabolite, HAGP, which directly interacted with the C1a domains of 2 distinct PKC isoforms and blocked PKC kinase activity in vitro. Finally, AADACL1 inhibition in rats reduced platelet aggregation, protected against FeCl3-induced arterial thrombosis, and delayed tail bleeding time. In summary, our data support a model whereby AADACL1 inhibition shifts the platelet ether lipidome to an inhibitory axis of HAGP accumulation that impairs PKC activation, granule secretion, and recruitment of platelets to sites of vascular damage.

Targeted Delivery of siRNA Lipoplexes to Cancer Cells Using Macrophage Transient Horizontal Gene Transfer.

Delivery of nucleic acids into solid tumor environments remains a pressing challenge. This study examines the ability of macrophages to horizontally transfer small interfering RNA (siRNA) lipoplexes to cancer cells. Macrophages are a natural candidate for a drug carrier because of their ability to accumulate at high densities into many cancer types, including, breast, prostate, brain, and colon cancer. Here, it is demonstrated that macrophages can horizontally transfer siRNA to cancer cells during in vitro coculture. The amount of transfer can be dosed depending on the amount of siRNA loaded and total number of macrophages delivered. Macrophages loaded with calcium integrin binding protein-1 (CIB1)-siRNA result in decreased tumorsphere growth and decreased mRNA expression of CIB1 and KI67 in MDA-MB-468 human breast cancer cells. Adoptive transfer of macrophages transfected with CIB1-siRNA localizes to the orthotopic MDA-MB-468 tumor. Furthermore, it is reported that macrophage activation can modulate this transfer process as well as intracellular trafficking protein Rab27a. As macrophages are heavily involved in tumor progression, understanding how to use macrophages for drug delivery can substantially benefit the treatment of tumors.

Machine Learning to Quantitate Neutrophil NETosis.

We introduce machine learning (ML) to perform classification and quantitation of images of nuclei from human blood neutrophils. Here we assessed the use of convolutional neural networks (CNNs) using free, open source software to accurately quantitate neutrophil NETosis, a recently discovered process involved in multiple human diseases. CNNs achieved >94% in performance accuracy in differentiating NETotic from non-NETotic cells and vastly facilitated dose-response analysis and screening of the NETotic response in neutrophils from patients. Using only features learned from nuclear morphology, CNNs can distinguish between NETosis and necrosis and between distinct NETosis signaling pathways, making them a precise tool for NETosis detection. Furthermore, by using CNNs and tools to determine object dispersion, we uncovered differences in NETotic nuclei clustering between major NETosis pathways that is useful in understanding NETosis signaling events. Our study also shows that neutrophils from patients with sickle cell disease were unresponsive to one of two major NETosis pathways. Thus, we demonstrate the design, performance, and implementation of ML tools for rapid quantitative and qualitative cell analysis in basic science.

CIB1 depletion with docetaxel or TRAIL enhances triple-negative breast cancer cell death.

BACKGROUND: Patients diagnosed with triple negative breast cancer (TNBC) have limited treatment options and often suffer from resistance and toxicity due to chemotherapy. We previously found that depleting calcium and integrin-binding protein 1 (CIB1) induces cell death selectively in TNBC cells, while sparing normal cells. Therefore, we asked whether CIB1 depletion further enhances tumor-specific killing when combined with either the commonly used chemotherapeutic, docetaxel, or the cell death-inducing ligand, TRAIL. METHODS: We targeted CIB1 by RNA interference in MDA-MB-436, MDA-MB-231, MDA-MB-468, docetaxel-resistant MDA-MB-436 TNBC cells and ME16C normal breast epithelial cells alone or combination with docetaxel or TRAIL. Cell death was quantified via trypan blue exclusion using flow cytometry and cell death mechanisms were analyzed by Western blotting. Cell surface levels of TRAIL receptors were measured by flow cytometry analysis. RESULTS: CIB1 depletion combined with docetaxel significantly enhanced tumor-specific cell death relative to each treatment alone. The enhanced cell death strongly correlated with caspase-8 activation, a hallmark of death receptor-mediated apoptosis. The death receptor TRAIL-R2 was upregulated in response to CIB1 depletion, which sensitized TNBC cells to the ligand TRAIL, resulting in a synergistic increase in cell death. In addition to death receptor-mediated apoptosis, both combination treatments activated a non-apoptotic mechanism, called paraptosis. Interestingly, these combination treatments also induced nearly complete death of docetaxel-resistant MDA-MB-436 cells, again via apoptosis and paraptosis. In contrast, neither combination treatment induced cell death in normal ME16C cells. CONCLUSION: Novel combinations of CIB1 depletion with docetaxel or TRAIL selectively enhance naive and docetaxel-resistant TNBC cell death while sparing normal cell. Therefore, combination therapies that target CIB1 could prove to be a safe and durable strategy for treatment of TNBC and potentially other cancers.

The human CIB1-EVER1-EVER2 complex governs keratinocyte-intrinsic immunity to ß-papillomaviruses.

Patients with epidermodysplasia verruciformis (EV) and biallelic null mutations of TMC6 (encoding EVER1) or TMC8 (EVER2) are selectively prone to disseminated skin lesions due to keratinocyte-tropic human ß-papillomaviruses (ß-HPVs), which lack E5 and E8. We describe EV patients homozygous for null mutations of the CIB1 gene encoding calcium- and integrin-binding protein-1 (CIB1). CIB1 is strongly expressed in the skin and cultured keratinocytes of controls but not in those of patients. CIB1 forms a complex with EVER1 and EVER2, and CIB1 proteins are not expressed in EVER1- or EVER2-deficient cells. The known functions of EVER1 and EVER2 in human keratinocytes are not dependent on CIB1, and CIB1 deficiency does not impair keratinocyte adhesion or migration. In keratinocytes, the CIB1 protein interacts with the HPV E5 and E8 proteins encoded by α-HPV16 and γ-HPV4, respectively, suggesting that this protein acts as a restriction factor against HPVs. Collectively, these findings suggest that the disruption of CIB1-EVER1-EVER2-dependent keratinocyte-intrinsic immunity underlies the selective susceptibility to ß-HPVs of EV patients.

CIB1 protects against MPTP-induced neurotoxicity through inhibiting ASK1.

Calcium and integrin binding protein 1 (CIB1) is a calcium-binding protein that was initially identified as a binding partner of platelet integrin αIIb. Although CIB1 has been shown to interact with multiple proteins, its biological function in the brain remains unclear. Here, we show that CIB1 negatively regulates degeneration of dopaminergic neurons in a mouse model of Parkinson's disease using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Genetic deficiency of the CIB1 gene enhances MPTP-induced neurotoxicity in dopaminergic neurons in CIB1-/- mice. Furthermore, RNAi-mediated depletion of CIB1 in primary dopaminergic neurons potentiated 1-methyl-4-phenyl pyrinidium (MPP+)-induced neuronal death. CIB1 physically associated with apoptosis signal-regulating kinase 1 (ASK1) and thereby inhibited the MPP+-induced stimulation of the ASK1-mediated signaling cascade. These findings suggest that CIB1 plays a protective role in MPTP/MPP+-induced neurotoxicity by blocking ASK1-mediated signaling.

CLIC1 null mice demonstrate a role for CLIC1 in macrophage superoxide production and tissue injury.

We generated and studied CLIC1 null (C1KO) mice to investigate the physiological role of this protein. C1KO and matched wild-type (WT) mice were studied in two models of acute toxic tissue injury. CLIC1 expression is upregulated following acute injury of WT kidney and pancreas and is absent in C1KOs. Acute tissue injury is attenuated in the C1KOs and this correlates with an absence of the rise in tissue reactive oxygen species (ROS) that is seen in WT mice. Infiltration of injured tissue by inflammatory cells was comparable between WT and C1KOs. Absence of CLIC1 increased PMA-induced superoxide production by isolated peritoneal neutrophils but dramatically decreased PMA-induced superoxide production by peritoneal macrophages. CLIC1 is expressed in both neutrophils and macrophages in a peripheral pattern consistent with either plasma membrane or the cortical cytoskeleton in resting cells and redistributes away from the periphery following PMA stimulation in both cell types. Absence of CLIC1 had no effect on redistribution or dephosphorylation of Ezrin/ERM cytoskeleton in macrophages. Plasma membrane chloride conductance is altered in the absence of CLIC1, but not in a way that would be expected to block superoxide production. NADPH oxidase redistributes from an intracellular compartment to the plasma membrane when WT macrophages are stimulated to produce superoxide and this redistribution fails to occur in C1KO macrophages. We conclude that the role of CLIC1 in macrophage superoxide production is to support redistribution of NADPH oxidase to the plasma membrane, and not through major effects on ERM cytoskeleton or by acting as a plasma membrane chloride channel.

Mice Expressing Low Levels of CalDAG-GEFI Exhibit Markedly Impaired Platelet Activation With Minor Impact on Hemostasis.

OBJECTIVE: The tight regulation of platelet adhesiveness, mediated by the αIIbß3 integrin, is critical for hemostasis and prevention of thrombosis. We recently demonstrated that integrin affinity in platelets is controlled by the guanine nucleotide exchange factor, CalDAG-GEFI (CD-GEFI), and its target, RAP1. In this study, we investigated whether low-level expression of CD-GEFI leads to protection from thrombosis without pathological bleeding in mice. APPROACH AND RESULTS: Cdg1(low) mice were generated by knockin of human CD-GEFI cDNA into the mouse Cdg1 locus. CD-GEFI expression in platelets from Cdg1(low) mice was reduced by ≈90% when compared with controls. Activation of RAP1 and αIIbß3 was abolished at low agonist concentrations and partially inhibited at high agonist concentrations in Cdg1(low) platelets. Consistently, the aggregation response of Cdg1(low) platelets was weaker than that of wild-type platelets, but more efficient than that observed in Cdg1(-/-) platelets. Importantly, Cdg1(low) mice were strongly protected from arterial and immune complex-mediated thrombosis, with only minimal impact on primary hemostasis. CONCLUSIONS: Together, our studies suggest the partial inhibition of CD-GEFI function as a powerful new approach to safely prevent thrombotic complications.

CIB1: a small protein with big ambitions.

Calcium- and integrin-binding protein 1 (CIB1) is a small, ubiquitously expressed protein that was first identified as an intracellular binding partner of a platelet-specific α-integrin cytoplasmic tail. Although early studies revealed a role for CIB1 in regulating platelet integrin activity, recent studies have indicated a more diverse role for CIB1 in many different cell types and processes, including calcium signaling, migration, adhesion, proliferation, and survival. Increasing evidence also points to a novel role for CIB1 in cancer and cardiovascular disease. In addition, an array of CIB1 binding partners has been identified that provide important insight into how CIB1 may regulate these processes. Some of these binding partners include the serine/threonine kinases, p21-activated kinase 1 (PAK1), apoptosis signal-regulating kinase 1 (ASK1), and polo-like kinase 3 (PLK3). Structural and mutational studies indicate that CIB1 binds most or all of its partners via a well-defined hydrophobic cleft. Although CIB1 itself lacks known enzymatic activity, it supports the PI3K/AKT and MEK/ERK oncogenic signaling pathways, in part, by directly modulating enzymes in these pathways. In this review, we discuss our current understanding of CIB1 and key questions regarding structure and function and how this seemingly diminutive protein impacts important signaling pathways and cellular processes in human health and disease.-Leisner, T. M., Freeman, T. C., Black, J. L., Parise, L. V. CIB1: a small protein with big ambitions.

The Plasma Membrane as a Reservoir, Protective Shield, and Light-Triggered Launch Pad for Peptide Therapeutics.

Although peptide-based therapeutics are finding increasing application in the clinic, extensive structural modification is typically required to prevent their rapid degradation by proteases in the blood. We have evaluated the ability of erythrocytes to serve as reservoirs, protective shields (against proteases), and light-triggered launch pads for peptides. We designed lipidated peptides that are anchored to the surface of red blood cells, which furnishes a protease-resistant environment. A photocleavable moiety is inserted between the lipid anchor and the peptide backbone, thereby enabling light-triggered peptide release from erythrocytes. We have shown that a cell-permeable peptide, a hormone (melanocyte stimulating hormone), and a blood-clotting agent can be anchored to erythrocytes, protected from proteases, and photolytically released to create the desired biological effect.

CIB1 depletion impairs cell survival and tumor growth in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) is an aggressive breast cancer subtype with generally poor prognosis and no available targeted therapies, highlighting a critical unmet need to identify and characterize novel therapeutic targets. We previously demonstrated that CIB1 is necessary for cancer cell survival and proliferation via regulation of two oncogenic signaling pathways, RAF-MEK-ERK and PI3K-AKT. Because these pathways are often upregulated in TNBC, we hypothesized that CIB1 may play a broader role in TNBC cell survival and tumor growth. Methods utilized include inducible RNAi depletion of CIB1 in vitro and in vivo, immunoblotting, clonogenic assay, flow cytometry, RNA-sequencing, bioinformatics analysis, and Kaplan-Meier survival analysis. CIB1 depletion resulted in significant cell death in 8 of 11 TNBC cell lines tested. Analysis of components related to PI3K-AKT and RAF-MEK-ERK signaling revealed that elevated AKT activation status and low PTEN expression were key predictors of sensitivity to CIB1 depletion. Furthermore, CIB1 knockdown caused dramatic shrinkage of MDA-MB-468 xenograft tumors in vivo. RNA sequence analysis also showed that CIB1 depletion in TNBC cells activates gene programs associated with decreased proliferation and increased cell death. CIB1 expression levels per se did not predict TNBC susceptibility to CIB1 depletion, and CIB1 mRNA expression levels did not associate with TNBC patient survival. Our data are consistent with the emerging concept of non-oncogene addiction, where a large subset of TNBCs depend on CIB1 for cell survival and tumor growth, independent of CIB1 expression levels. Our data establish CIB1 as a novel therapeutic target for TNBC.

Potential compensation among group I PAK members in hindlimb ischemia and wound healing.

PAKs are serine/threonine kinases that regulate cytoskeletal dynamics and cell migration. PAK1 is activated by binding to the small EF hand protein, CIB1, or to the Rho GTPases Rac1 or Cdc42. The role of PAK1 in angiogenesis was established based only on in vitro studies and its role in angiogenesis in vivo has never been examined. Here we tested the hypothesis that PAK1 is an essential regulator of ischemic neovascularization (arteriogenesis and angiogenesis) and wound healing using a global PAK1 knockout mouse. Neovascularization was assessed using unilateral hindlimb ischemia. We found that plantar perfusion, limb use and appearance were not significantly different between 6-8 week old PAK1-/- and PAK1+/+ mice throughout the 21-day period following hindlimb ischemia; however a slightly delayed healing was observed in 16 week old PAK1-/- mice. In addition, the wound healing rate, as assessed with an ear punch assay, was unchanged in PAK1-/- mice. Surprisingly, however, we observed a notable increase in PAK2 expression and phosphorylation in ischemic gastrocnemius tissue from PAK1-/- but not PAK1+/+ mice. Furthermore, we observed higher levels of activated ERK2, but not AKT, in ischemic and non-ischemic muscle of PAK1-/- mice upon hindlimb ischemic injury. A group I PAK inhibitor, IPA3, significantly inhibited endothelial cell sprouting from aortic rings in both PAK1-/- and PAK1+/+ mice, implying that PAK2 is a potential contributor to this process. Taken together, our data indicate that while PAK1 has the potential to contribute to neovascularization and wound healing, PAK2 may functionally compensate when PAK1 is deficient.

Identification of novel integrin binding partners for calcium and integrin binding protein 1 (CIB1): structural and thermodynamic basis of CIB1 promiscuity.

The short cytoplasmic tails of the α- and ß-chains of integrin adhesion receptors regulate integrin activation and cell signaling. Significantly less is known about proteins that bind to α-integrin cytoplasmic tails (CTs) as opposed to ß-CTs to regulate integrins. Calcium and integrin binding protein 1 (CIB1) was previously identified as an αIIb binding partner that inhibits agonist-induced activation of the platelet-specific integrin, αIIbß3. A sequence alignment of all α-integrin CTs revealed that key residues in the CIB1 binding site of αIIb are well-conserved, and was used to delineate a consensus binding site (I/L-x-x-x-L/M-W/Y-K-x-G-F-F). Because the CIB1 binding site of αIIb is conserved in all α-integrins and CIB1 expression is ubiquitous, we asked if CIB1 could interact with other α-integrin CTs. We predicted that multiple α-integrin CTs were capable of binding to the same hydrophobic binding pocket on CIB1 with docking models generated by all-atom replica exchange discrete molecular dynamics. After demonstrating novel in vivo interactions between CIB1 and other whole integrin complexes with co-immunoprecipitations, we validated the modeled predictions with solid-phase competitive binding assays, which showed that other α-integrin CTs compete with the αIIb CT for binding to CIB1 in vitro. Isothermal titration calorimetry measurements indicated that this binding is driven by hydrophobic interactions and depends on residues in the CIB1 consensus binding site. These new mechanistic details of CIB1-integrin binding imply that CIB1 could bind to all integrin complexes and act as a broad regulator of integrin function.

Chemoproteomic discovery of AADACL1 as a regulator of human platelet activation.

A comprehensive knowledge of the platelet proteome is necessary for understanding thrombosis and for envisioning antiplatelet therapies. To discover other biochemical pathways in human platelets, we screened platelets with a carbamate library designed to interrogate the serine hydrolase subproteome and used competitive activity-based protein profiling to map the targets of active carbamates. We identified an inhibitor that targets arylacetamide deacetylase-like 1 (AADACL1), a lipid deacetylase originally identified in invasive cancers. Using this compound, along with highly selective second-generation inhibitors of AADACL1, metabolomics, and RNA interference, we show that AADACL1 regulates platelet aggregation, thrombus growth, RAP1 and PKC activation, lipid metabolism, and fibrinogen binding to platelets and megakaryocytes. These data provide evidence that AADACL1 regulates platelet and megakaryocyte activation and highlight the value of this chemoproteomic strategy for target discovery in platelets.

A pilot study of eptifibatide for treatment of acute pain episodes in sickle cell disease.

INTRODUCTION: The contribution of platelet activation to the pathogenesis of sickle cell disease (SCD) remains uncertain. We evaluated the safety and efficacy of eptifibatide, a synthetic peptide inhibitor of the αIIbß3 receptor, in SCD patients during acute painful episodes. MATERIALS AND METHODS: In this single site, double-blind, placebo-controlled trial, eligible patients with SCD admitted for acute painful episodes were randomized to receive eptifibatide or placebo at a ratio of 2:1. RESULTS: Thirteen patients (SS - 10, Sß(0) - 2, SC - 1) were randomized to receive either eptifibatide (N=9; 6 females; median age - 25years) or placebo (N=4; 3 females; median age - 31years). In the intent-to-treat analysis, there were no major bleeding episodes in either the eptifibatide or placebo arms (point estimate of difference: 0.00, 95% CI; -0.604, 0.372). There was one minor bleeding episode in the eptifibatide arm (point estimate of difference for any bleeding: 0.11, 95% CI: -0.502, 0.494). There was no significant difference in the proportion of patients with thrombocytopenia between the treatment groups (point estimate of difference: 0.11, 95% CI: -0.587, 0.495). There were no differences in the median times to discharge, median times to crisis resolution or the median total opioid use. CONCLUSIONS: In this small study, eptifibatide appeared to be safe, but did not improve the times to crisis resolution or hospital discharge. Adequately powered studies are required to evaluate the safety and efficacy of eptifibatide in SCD. Clinicaltrials.gov Identifier: NCT00834899.