Overcoming chemotherapy drug resistance by targeting inhibitors of apoptosis proteins (IAPs).

Inhibitors of apoptosis (IAPs) are a family of proteins that play a significant role in the control of programmed cell death (PCD). PCD is essential to maintain healthy cell turnover within tissue but also to fight disease or infection. Uninhibited, IAPs can suppress apoptosis and promote cell cycle progression. Therefore, it is unsurprising that cancer cells demonstrate significantly elevated expression levels of IAPs, resulting in improved cell survival, enhanced tumor growth and subsequent metastasis. Therapies to target IAPs in cancer has garnered substantial scientific interest and as resistance to anti-cancer agents becomes more prevalent, targeting IAPs has become an increasingly attractive strategy to re-sensitize cancer cells to chemotherapies, antibody based-therapies and TRAIL therapy. Antagonism strategies to modulate the actions of XIAP, cIAP1/2 and survivin are the central focus of current research and this review highlights advances within this field with particular emphasis upon the development and specificity of second mitochondria-derived activator of caspase (SMAC) mimetics (synthetic analogs of endogenously expressed inhibitors of IAPs SMAC/DIABLO). While we highlight the potential of SMAC mimetics as effective single agent or combinatory therapies to treat cancer we also discuss the likely clinical implications of resistance to SMAC mimetic therapy, occasionally observed in cancer cell lines.

Calcium-regulatory proteins as modulators of chemotherapy in human neuroblastoma.

Neuroblastoma (NB) is a pediatric cancer treated with poly-chemotherapy including platinum complexes (e.g. cisplatin (CDDP), carboplatin), DNA alkylating agents, and topoisomerase I inhibitors (e.g. topotecan (TOPO)). Despite aggressive treatment, NB may become resistant to chemotherapy. We investigated whether CDDP and TOPO treatment of NB cells interacts with the expression and function of proteins involved in regulating calcium signaling. Human neuroblastoma cell lines SH-SY5Y, IMR-32 and NLF were used to investigate the effects of CDDP and TOPO on cell viability, apoptosis, calcium homeostasis, and expression of selected proteins regulating intracellular calcium concentration ([Ca2+]i). In addition, the impact of pharmacological inhibition of [Ca2+]i-regulating proteins on neuroblastoma cell survival was studied. Treatment of neuroblastoma cells with increasing concentrations of CDDP (0.1-10 µM) or TOPO (0.1 nM-1 µM) induced cytotoxicity and increased apoptosis in a concentration- and time-dependent manner. Both drugs increased [Ca2+]i over time. Treatment with CDDP or TOPO also modified mRNA expression of selected genes encoding [Ca2+]i-regulating proteins. Differentially regulated genes included S100A6, ITPR1, ITPR3, RYR1 and RYR3. With FACS and confocal laser scanning microscopy experiments we validated their differential expression at the protein level. Importantly, treatment of neuroblastoma cells with pharmacological modulators of [Ca2+]i-regulating proteins in combination with CDDP or TOPO increased cytotoxicity. Thus, our results confirm an important role of calcium signaling in the response of neuroblastoma cells to chemotherapy and suggest [Ca2+]i modulation as a promising strategy for adjunctive treatment.

G-Protein-Coupled Receptor 35 Mediates Human Saphenous Vein Vascular Smooth Muscle Cell Migration and Endothelial Cell Proliferation.

Vascular smooth muscle cell (VSMC) migration and proliferation is central to neointima formation in vein graft failure following coronary artery bypass. However, there are currently no pharmacological interventions that prevent vein graft failure through intimal occlusion. It is hence a therapeutic target. Here, we investigated the contribution of GPR35 to human VSMC and endothelial cell (EC) migration, using a scratch-wound assay, and also the contribution to proliferation, using MTS and BrdU assays, in in vitro models using recently characterized human GPR35 ortholog-selective small-molecule agonists and antagonists. Real-time PCR studies showed GPR35 to be robustly expressed in human VSMCs and ECs. Stimulation of GPR35, with either the human-selective agonist pamoic acid or the reference agonist zaprinast, promoted VSMC migration in the scratch-wound assay. These effects were blocked by coincubation with either of the human GPR35-specific antagonists, CID-2745687 or ML-145. These GPR35-mediated effects were produced by inducing alterations in the actin cytoskeleton via the Rho A/Rho kinase signaling axis. Additionally, the agonist ligands stimulated a proliferative response in ECs. These studies highlight the potential that small molecules that stimulate or block GPR35 activity can modulate vascular proliferation and migration. These data propose GPR35 as a translational therapeutic target in vascular remodeling.

The antiallergic mast cell stabilizers lodoxamide and bufrolin as the first high and equipotent agonists of human and rat GPR35.

Lack of high potency agonists has restricted analysis of the G protein-coupled receptor GPR35. Moreover, marked variation in potency and/or affinity of current ligands between human and rodent orthologs of GPR35 has limited their productive use in rodent models of physiology. Based on the reported modest potency of the antiasthma and antiallergic ligands cromolyn disodium and nedocromil sodium, we identified the related compounds lodoxamide and bufrolin as high potency agonists of human GPR35. Unlike previously identified high potency agonists that are highly selective for human GPR35, both lodoxamide and bufrolin displayed equivalent potency at rat GPR35. Further synthetic antiallergic ligands, either sharing features of the standard surrogate agonist zaprinast, or with lodoxamide and bufrolin, were also shown to display agonism at either human or rat GPR35. Because both lodoxamide and bufrolin are symmetric di-acids, their potential mode of binding was explored via mutagenesis based on swapping between the rat and human ortholog nonconserved arginine residues within proximity of a key conserved arginine at position 3.36. Computational modeling and ligand docking predicted the contributions of different arginine residues, other than at 3.36, in human GPR35 for these two ligands and were consistent with selective loss of potency of either bufrolin or lodoxamide at distinct arginine mutants. The computational models also suggested that bufrolin and lodoxamide would display reduced potency at a low-frequency human GPR35 single nucleotide polymorphism. This prediction was confirmed experimentally.