The macrophage scavenger receptor CD36 plays an important role in the uptake of oxidized forms of low density lipoprotein (LDL) and contributes to lesion development in murine models of atherosclerosis. However, the structural basis of CD36 lipoprotein ligand recognition is unknown. We now identify a novel class of oxidized phospholipids that serve as high affinity ligands for CD36 and mediate recognition of oxidized forms of LDL by CD36 on macrophages. Small unilamellar vesicles of homogeneous phosphatidylcholine (PC) molecular species were oxidized by the myeloperoxidase (MPO)-H 2 O 2 -NO 2 ؊ system, and products were separated by sequential LC/ESI/MS/MS. In parallel, fractions were tested for their ability to bind to CD36. Four major structurally related phospholipids with CD36 binding activity were identified from oxidized 1-palmitoyl-2-arachidonyl-PC, and four corresponding structural analogs with CD36 binding activity were identified from oxidized 1-palmitoyl-2-linoleoyl-PC. Each was then synthetically prepared, its structure confirmed by multinuclear NMR and high resolution mass spectrometry, and shown to possess identical CD36 binding activity and LC/ESI/MS/MS characteristics in both native and derivatized forms. Based upon the structures of the active compounds identified, and structure-function studies with a variety of synthetic analogs, we conclude that the structural characteristics required for high affinity binding of oxidized PC species to CD36 are a phospholipid with an sn-2 acyl group that incorporates a terminal ␥-hydroxy(or oxo)-␣,␤-unsaturated carbonyl (oxPC CD36 ). LC/ESI/MS/MS studies demonstrate that oxPC CD36 are formed during LDL oxidation by multiple distinct pathways. Formation of this novel class of oxidized PC species contributes to CD36-mediated recognition of LDL oxidized by MPO and other biologically relevant mechanisms. The present results offer structural insights into the molecular patterns recognized by the scavenger receptor CD36 and provide a platform for the development of potential therapeutic inhibitory agents.CD36 is a heavily glycosylated, single chain, integral plasma membrane protein that belongs to an evolutionarily conserved family of proteins that serve as scavenger and lipid receptors (1, 2). It is expressed on the surface of adipocytes, microvascular endothelial cells, macrophages, platelets, and specialized epithelial cells (1, 2). CD36 functions in vivo in scavenger recognition of oxidized lipoproteins and senescent or apoptotic cells, fatty acid transport, cell-matrix interactions, and antiangiogenic actions (3-5). Its deficiency in humans has been correlated with alterations in myocardial fatty acid uptake, hypertrophic cardiac myopathy, and insulin resistance (6 -8). Recent studies (3, 4, 9 -11) have focused attention on CD36 as a participant in the atherosclerotic process because of its ability to recognize oxidized forms of LDL (oxLDL).1 CD36 mediates lipid accumulation and macrophage foam cell formation in vitro and in vivo (3,12,13). It is heavily expre...

A series of 6-substituted classical pyrrolo [2,3-d]pyrimidine antifolates with a 3-to 6-carbon bridge between the heterocycle and the benzoyl-L-glutamate (compounds 2, 3, 4 and 5, respectively) was synthesized starting from methyl 4-formylbenzoate and a Wittig reaction with the appropriate triphenylphosphonium bromide, followed by reduction and conversion to the α-bromomethylketones. Cyclocondensation of 2,4-diamino-4-oxopyrimidine with the α-bromoketones, coupling with diethyl-L-glutamate and saponification afforded 2-5. Compounds 2-5 had negligible substrate activity for RFC but showed variably potent (nanomolar) and selective inhibitory activities toward Chinese hamster ovary cells that expressed FRα or FRβ, and toward FRα-expressing KB and IGROV1 human tumor cells. Inhibition of KB cell colony formation was also observed. Glycinamide ribonucleotide formyl transferase (GARFTase) was identified as the primary intracellular target of the pyrrolo [2,3-d]pyrimidines. The combined properties of selective FR targeting, lack of RFC transport, and GARFTase inhibition resulting in potent antitumor activity are unprecedented and warrant development of these analogs as antitumor agents.

The macrophage scavenger receptor CD36 plays an important role in binding and uptake of oxidized forms of low-density lipoprotein (LDL), foam cell formation, and lesion development during atherosclerosis. The structural basis of CD36-lipoprotein ligand recognition is an area of intense interest. In a companion article we reported the characterization of a structurally conserved family of oxidized choline glycerophospholipids (ox-PC CD36 ) that serve as novel high affinity ligands for cells stably transfected with CD36, mediating recognition of multiple oxidized forms of LDL (Podrez, E. A., Poliakov, E., Shen, Z., Zhang, R., Deng, Y., Sun, M., Finton, P., Shan, L., Gugiu, B., Fox, P. L., Hoff, H. F., Salomon, R. G., and Hazen, S. L. (July 8, 2002) J. Biol. Chem. 277, 10.1074/ jbc.M203318200). Here we use macrophages from wildtype and CD36 null mice to demonstrate that CD36 is the major receptor on macrophages mediating recognition of oxPC CD36 species when presented (؉/؊ plasma) in pure form, within PC bilayers in small unilamellar vesicles, and within liposomes generated from lipid extracts of native LDL. We also show that oxPC CD36 promote CD36-dependent recognition when present at only a few molecules per particle, resulting in macrophage binding, uptake, metabolism, cholesterol accumulation, and foam cell formation. Finally, using high performance liquid chromatography with on-line electrospray ionization tandem mass spectrometry (LC/ESI/MS/MS), we demonstrate that oxPC CD36 are generated in vivo and are enriched in atherosclerotic lesions. Collectively, our data suggest that formation of this novel family of oxidized phospholipids participates in CD36-mediated recognition of oxidized lipoproteins and foam cell formation in vivo.CD36 is a multifunctional cellular receptor with broad ligand specificity (1, 2). It is expressed in a number of cells including microvascular endothelial cells, platelets, adipocytes, striated muscles, macrophages, and some vascular smooth muscle cells (1, 2). CD36 regulates cellular adhesion and angiogenesis, serving as a receptor for thrombospondin; it also serves as a scavenger receptor in macrophages, mediating uptake of apoptotic cells and modified lipoproteins, and participates in carbohydrate and lipid metabolism, modulating insulin resistance and long chain fatty acid transport (3-8). CD36 has recently been implicated in a variety of pathologic conditions, including atherosclerosis, diabetes, and cardiomyopathy. Perhaps the most compelling data on the role of CD36 in atherosclerosis are from studies of CD36-deficient mice, which show a 70 -80% reduction in aortic lesion size (9). In vitro experiments demonstrate that macrophages from CD36-deficient mice take up different forms of oxidized LDL 1 poorly and are resistant to foam cell formation, providing a mechanism for the atheroprotection observed in CD36 null mice (9, 10). Given the potential clinical significance of this receptor, it is important to know the nature of the ligand(s) in oxidized LDL (oxLDL) that are recognize...

This review attempts to provide a comprehensive overview of the biology of the physiologically and pharmacologically important transport system termed the "reduced folate carrier" (RFC). The ubiquitously expressed RFC has unequivocally established itself as the major transport system in mammalian cells and tissues for a group of compounds including folate cofactors and classical antifolate therapeutics. Loss of RFC expression or function may have potentially profound pathophysiologic consequences including cancer. For chemotherapeutic antifolates used for cancer such as methotrexate or pemetrexed, synthesis of mutant RFCs or loss of RFC transcripts and proteins results in antifolate resistance due to incomplete inhibition of cellular enzyme targets and insufficient substrate for polyglutamate synthesis. Since RFC was first cloned in 1994, tremendous advances have been made in understanding the complex transcriptional and posttranscriptional regulation of RFC, in identifying structurally and functionally important domains and amino acids in the RFC molecule as a prelude to establishing the mechanism of transport, and in characterizing the molecular defects in RFC associated with loss of transport in antifolate resistant cell line models. Many of the insights gained from laboratory models of RFC portend opportunities for modulating carrier expression in drug resistant tumors, and for designing a new generation of agents with improved transport by RFC or substantially enhanced transport by other folate transporters over RFC. Many of the advances in the basic biology of RFC in cell line models are now being directly applied to human cancers in the clinical setting, most notably pediatric acute lymphoblastic leukemia and osteogenic sarcoma.

A series of seven 2-amino-4-oxo-6-substituted thieno [2,3-d]pyrimidines, with bridge length variations (from 2-8 carbon atoms) were synthesized as selective folate receptor (FR) α and β substrates and as antitumor agents. The syntheses were accomplished from appropriate allylalcohols and 4-iodobenzoate to afford the aldehydes which were converted to the appropriate 2-amino-4-carbethoxy-5-substituted thiophenes 23-29. Cyclization with chlorformamidine afforded the thieno [2,3-d]pyrimidines 30-36 which were hydrolyzed and coupled with diethyl-L-glutamate, followed by saponification to give the target compounds 2-8. Compounds 3-6 were potent growth inhibitors (IC 50 4.7 to 334 nM) of human tumor cells (KB and IGROV1) that express FRs. In addition, compounds 3-6 inhibited the growth of Chinese hamster ovary (CHO) cells that expressed FRs but not the reduced folate carrier (RFC) or proton-coupled folate transporter (PCFT). However, the compounds were inactive toward CHO cells that lacked FRs but contained either the RFC or PCFT. By nucleoside and 5-amino-4-imidazole carboxamide (AICA) protection studies, along with in vitro and in situ enzyme activity assays, the mechanism of antitumor activity was identified as the dual inhibition of glycinamide ribonucleotide formyltransferase and, likely, AICA ribonucleotide formyltransferase. The dual inhibitory activity of the active thieno [2,3-d]pyrimidine antifolates and the FR specificity represent unique mechanistic features for these compounds distinct from all other known antifolates. The potent inhibitory effects of compounds 3-6 toward cells expressing FRs but not PCFT provide direct evidence that cellular uptake of this series of compounds by FRs does not depend on the presence of PCFT and argues that direct coupling between these transporters is not obligatory.

Abstract2-Amino-4-oxo-6-substituted pyrrolo [2,3-d]pyrimidines with a thienoyl side chain and 4-6 carbon bridge lengths (compounds 1-3) were synthesized as substrates for folate receptors (FRs) and the proton-coupled folate transporter (PCFT). Conversion of acetylene carboxylic acids to α-bromomethylketones and condensation with 2,4-diamino-6-hydroxypyrimidine afforded the 6-substituted pyrrolo [2,3-d]pyrimidines. Sonogashira coupling with (S)-2-[(5-bromo-thiophene-2-carbonyl)-amino]-pentanedioic acid diethyl ester, followed by hydrogenation and saponification, afforded 1-3. Compounds 1 and 2 potently inhibited KB and IGROV1 human tumor cells that express FRα, reduced folate carrier (RFC), and PCFT. The analogs were selective for FR-and PCFT over RFC. Glycinamide ribonucleotide formyltransferase was the principal cellular target. In SCID mice with KB tumors, 1 was highly active against both early (3.5 log kill, 1/5 cures) and advanced (3.7 log kill, 4/5 complete remissions) stage tumors. Our results demonstrate potent in vitro and in vivo antitumor activity for 1 due to selective transport by FRs and PCFT over RFC.

The acquisition of apoptosis resistance is a fundamental event in cancer development. Among the mechanisms used by cancer cells to evade apoptosis is the dysregulation of inhibitor of apoptosis (IAP) proteins. The activity of the IAPs is regulated by endogenous IAP antagonists such as SMAC (also termed DIABLO). Antagonism of IAP proteins by SMAC occurs via binding of the N-terminal tetrapeptide (AVPI) of SMAC to selected BIR domains of the IAPs. Small molecule compounds that mimic the AVPI motif of SMAC have been designed to overcome IAP-mediated apoptosis resistance of cancer cells. Here, we report the preclinical characterization of birinapant (TL32711), a bivalent SMAC-mimetic compound currently in clinical trials for the treatment of cancer. Birinapant bound to the BIR3 domains of cIAP1, cIAP2, XIAP, and the BIR domain of ML-IAP in vitro and induced the autoubiquitylation and proteasomal degradation of cIAP1 and cIAP2 in intact cells, which resulted in formation of a RIPK1:caspase-8 complex, caspase-8 activation, and induction of tumor cell death. Birinapant preferentially targeted the TRAF2-associated cIAP1 and cIAP2 with subsequent inhibition of TNF-induced NF-kB activation. The activity of a variety of chemotherapeutic cancer drugs was potentiated by birinapant both in a TNF-dependent or TNF-independent manner. Tumor growth in multiple primary patient-derived xenotransplant models was inhibited by birinapant at well-tolerated doses. These results support the therapeutic combination of birinapant with multiple chemotherapies, in particular, those therapies that can induce TNF secretion.

Birinapant (1) is a second-generation bivalent antagonist of IAP proteins that is currently undergoing clinical development for the treatment of cancer. Using a range of assays that evaluated cIAP1 stability and oligomeric state, we demonstrated that 1 stabilized the cIAP1-BUCR (BIR3-UBA-CARD-RING) dimer and promoted autoubiquitylation of cIAP1 in vitro. Smac-mimetic 1-induced loss of cIAPs correlated with inhibition of TNF-mediated NF-κB activation, caspase activation, and tumor cell killing. Many first-generation Smac-mimetics such as compound A (2) were poorly tolerated. Notably, animals that lack functional cIAP1, cIAP2, and XIAP are not viable, and 2 mimicked features of triple IAP knockout cells in vitro. The improved tolerability of 1 was associated with (i) decreased potency against cIAP2 and affinity for XIAP BIR3 and (ii) decreased ability to inhibit XIAP-dependent signaling pathways. The P2' position of 1 was critical to this differential activity, and this improved tolerability has allowed 1 to proceed into clinical studies.