Oncogenesis driven by the Ras/Raf pathway requires the SUMO E2 ligase Ubc9

The small GTPase KRAS is frequently mutated in human cancer and currently there are no targeted therapies for KRAS mutant tumors. Here, we show that the small ubiquitin-like modifier (SUMO) pathway is required for KRAS-driven transformation. RNAi depletion of the SUMO E2 ligase Ubc9 suppresses 3D growth of KRAS mutant colorectal cancer cells in vitro and attenuates tumor growth in vivo. In KRAS mutant cells, a subset of proteins exhibit elevated levels of SUMOylation. Among these proteins, KAP1, CHD1, and EIF3L collectively support anchorage-independent growth, and the SUMOylation of KAP1 is necessary for its activity in this context. Thus, the SUMO pathway critically contributes to the transformed phenotype of KRAS mutant cells and Ubc9 presents a potential target for the treatment of KRAS mutant colorectal cancer.The Ras family of small GTPases are signal transduction molecules downstream of growth factor receptors. Ras activates a number of downstream effector pathways to regulate cell proliferation, survival and motility, these effectors include the MAP kinase (MAPK) pathway, the PI3-kinase (PI3K) pathway, the small GTPases RalA, RalB, and Rho, and phospholipase-Cε (1). Activating mutations in Ras are frequently found in human malignancies, with mutations in the KRAS gene being particularly prevalent. KRAS mutations occur in ∼60% of pancreatic ductal carcinomas, 26% of lung adenocarcinomas, and 45% of colorectal carcinomas, as well as a significant fraction of ovarian, endometrial, and biliary track cancers (2, 3). A salient hallmark of the Ras oncogene is its ability to transform cells to enable anchorage-independent 3D colony growth in vitro and tumor growth in vivo. Consequently, Ras mutant cancer cells often exhibit oncogene addiction to Ras such that extinction of the Ras oncogene leads to either a reversion of the transformed phenotype or loss of viability (4, 5). Therapeutically, the Ras oncoprotein has proven pharmacologically intractable thus far: intensive drug screening efforts have not yielded high-affinity, selective Ras inhibitors. Farnesyltransferase inhibitors that aimed to block Ras membrane localization are ineffective against KRAS because of its alternative geranylgeranylation. Inhibitors targeting Ras effector kinases, including MEK, PI3K, and Akt, are currently undergoing clinical evaluations, but they have yet to demonstrate clear clinical benefits (6). Thus, KRAS mutant tumors represent a class of “recalcitrant cancer” with urgent, unmet therapeutic needs.To gain new insight into the genetic dependencies of Ras mutant cancers and discover new therapeutic targets, we and others have previously carried out genome-wide synthetic lethal screens in KRAS mutant and WT cells to identify genes whose depletion leads to greater toxicity in KRAS mutant cells. In our screen we found a wide array of genes, many of which are involved in cellular stress response, that are required to maintain the viability of KRAS mutant cells (7). We proposed the concept of “nononcogene addiction” to explain the heightened dependency of cancer cells on stress response and other indirect cellular pathways for survival, and we suggested that this form of addiction could be exploited for therapeutic gain (8).In our primary screen we identified the small ubiquitin-like modifier (SUMO) E2 ligase Ubc9 (encoded by the UBE2I gene) and the E1 ligase subunit SAE1 as candidate KRAS synthetic lethal partners. Similar to the ubiquitin pathway, the SUMO pathway modulates the function and stability of cellular proteins through the reversible conjugation of SUMO on their lysine residues, often in a “ΨKxE” motif (9). In human, the SUMO pathway consists of three SUMO proteins (SUMO1, SUMO2, and SUMO3), a single heterodimeric E1 ligase SAE1/UBA2, a single E2 ligase Ubc9, and several E3 ligases. SUMO proteins are conjugated onto target proteins either directly by Ubc9 or through a family of E3s, and removed by the sentrin/SUMO-specific protease (SENP) family of SUMO peptidases. SUMOylation occurs in a highly dynamic manner in the cell and substrate proteins can be modified with either mono- or poly-SUMOylation. The SUMO pathway plays a critical role in cellular stress response, such as DNA damage, genomic stability, and heat shock (10–12), and it has also been recently implicated in prostate and breast cancer (13–16). However, the role of this pathway in KRAS mutant cancers is not clear.In this study we provide evidence for the requirement for the SUMO pathway in the transformation growth of KRAS mutant colorectal cancer (CRC) cells. We found that these cells are highly dependent on Ubc9 for their clonogenic growth under both anchorage-dependent (AD) and anchorage-independent (AI) conditions. Quantitative proteomics analysis revealed that the SUMOylation patterns of a subset of cellular proteins are altered by the KRAS oncogene, and these SUMO target proteins functionally support the 3D growth of KRAS mutant cells. Our findings thus provide evidence that the SUMO pathway is critical for the transformation growth of KRAS mutant cancer cells, and suggests Ubc9 could be a potential drug target.     

Nedd4-2-dependent ubiquitylation and regulation of the cardiac potassium channel hERG1

The voltage-gated cardiac potassium channel hERG1 (human ether-à-gogo-related gene 1) plays a key role in the repolarization phase of the cardiac action potential (AP). Mutations in its gene, KCNH2, can lead to defects in the biosynthesis and maturation of the channel, resulting in congenital long QT syndrome (LQTS). To identify the molecular mechanisms regulating the density of hERG1 channels at the plasma membrane, we investigated channel ubiquitylation by ubiquitin ligase Nedd4-2, a post-translational regulatory mechanism previously linked to other ion channels. We found that whole-cell hERG1 currents recorded in HEK293 cells were decreased upon neural precursor cell expressed developmentally down-regulated 4-2 (Nedd4-2) co-expression. The amount of hERG1 channels in total HEK293 lysates and at the cell surface, as assessed by Western blot and biotinylation assays, respectively, were concomitantly decreased. Nedd4-2 and hERG1 interact via a PY motif located in the C-terminus of hERG1. Finally, we determined that Nedd4-2 mediates ubiquitylation of hERG1 and that deletion of this motif affects Nedd4-2-dependent regulation. These results suggest that ubiquitylation of the hERG1 protein by Nedd4-2, and its subsequent down-regulation, could represent an important mechanism for modulation of the duration of the human cardiac action potential.     

Locations of the beta1 transmembrane helices in the BK potassium channel

BK channels are composed of α-subunits, which form a voltage- and Ca²⁺-gated potassium channel, and of modulatory β-subunits. The β1-subunit is expressed in smooth muscle, where it renders the BK channel sensitive to [Ca²⁺]_i in a voltage range near the smooth-muscle resting potential and slows activation and deactivation. BK channel acts thereby as a damped feedback regulator of voltage-dependent Ca²⁺ channels and of smooth muscle tone. We explored the contacts between α and β1 by determining the extent of endogenous disulfide bond formation between cysteines substituted just extracellular to the two β1 transmembrane (TM) helices, TM1 and TM2, and to the seven α TM helices, consisting of S1–S6, conserved in all voltage-dependent potassium channels, and the unique S0 helix, which we previously concluded was partly surrounded by S1–S4. We now find that the extracellular ends of β1 TM2 and α S0 are in contact and that β1 TM1 is close to both S1 and S2. The extracellular ends of TM1 and TM2 are not close to S3–S6. In almost all cases, cross-linking of TM2 to S0 or of TM1 to S1 or S2 shifted the conductance–voltage curves toward more positive potentials, slowed activation, and speeded deactivation, and in general favored the closed state. TM1 and TM2 are in position to contribute, in concert with the extracellular loop and the intracellular N- and C-terminal tails of β1, to the modulation of BK channel function.     

Location of KCNE1 relative to KCNQ1 in the IKS potassium channel by disulfide cross-linking of substituted cysteines

The cardiac-delayed rectifier K⁺ current (I_KS) is carried by a complex of KCNQ1 (Q1) subunits, containing the voltage-sensor domains and the pore, and auxiliary KCNE1 (E1) subunits, required for the characteristic I_KS voltage dependence and kinetics. To locate the transmembrane helix of E1 (E1-TM) relative to the Q1 TM helices (S1–S6), we mutated, one at a time, the first four residues flanking the extracellular ends of S1–S6 and E1-TM to Cys, coexpressed all combinations of Q1 and E1 Cys-substituted mutants in CHO cells, and determined the extents of spontaneous disulfide-bond formation. Cys-flanking E1-TM readily formed disulfides with Cys-flanking S1 and S6, much less so with the S3-S4 linker, and not at all with S2 or S5. These results imply that the extracellular flank of the E1-TM is located between S1 and S6 on different subunits of Q1. The salient functional effects of selected cross-links were as follows. A disulfide from E1 K41C to S1 I145C strongly slowed deactivation, and one from E1 L42C to S6 V324C eliminated deactivation. Given that E1-TM is between S1 and S6 and that K41C and L42C are likely to point approximately oppositely, these two cross-links are likely to favor similar axial rotations of E1-TM. In the opposite orientation, a disulfide from E1 K41C to S6 V324C slightly slowed activation, and one from E1 L42C to S1 I145C slightly speeded deactivation. Thus, the first E1 orientation strongly favors the open state, while the approximately opposite orientation favors the closed state.     

Gating of the MlotiK1 potassium channel involves large rearrangements of the cyclic nucleotide-binding domains

Cyclic nucleotide-regulated ion channels are present in bacteria, plants, vertebrates, and humans. In higher organisms, they are closely involved in signaling networks of vision and olfaction. Binding of cAMP or cGMP favors the activation of these ion channels. Despite a wealth of structural and studies, there is a lack of structural data describing the gating process in a full-length cyclic nucleotide-regulated channel. We used high-resolution atomic force microscopy (AFM) to directly observe the conformational change of the membrane embedded bacterial cyclic nucleotide-regulated channel MlotiK1. In the nucleotide-bound conformation, the cytoplasmic cyclic nucleotide-binding (CNB) domains of MlotiK1 are disposed in a fourfold symmetric arrangement forming a pore-like vestibule. Upon nucleotide-unbinding, the four CNB domains undergo a large rearrangement, stand up by ～1.7 nm, and adopt a structurally variable grouped conformation that closes the cytoplasmic vestibule. This fully reversible conformational change provides insight into how CNB domains rearrange when regulating the potassium channel.     

肌肉萎缩蛋白Fbox-1基因沉默对糖皮质激素作用下肌管萎缩的影响

目的:研究糖皮质激素地塞米松(dexamethasone,DEX)对体外培养肌管的形态、蛋白质合成/分解代谢及肌肉萎缩蛋白Fbox-1(Atrogin-1)的表达,Atrogin-1基因沉默是否可减轻肌管萎缩.方法:体外培养小鼠肌纤维细胞株C2C12细胞并分化为肌管后,用DEX处理48h,同位素3H-酪氨酸掺入法检测蛋白合成,3H-酪氨酸释放率检测蛋白分解代谢;荧光显微镜观察肌管形态并拍照;Northern blot检测Atrogin-1 mRNA水平;应用小分子干扰RNA片段(siRNA)技术使Atrogin-1基因沉默后,观察DEX作用下肌管形态的改变.结果:DEX 5μmol/L作用后,肌管酪氨酸(tyrosine)掺入率下降17.4%,同时tyrosine释放率升高24.7%,使肌管形态变细、萎缩;剂量依赖性地升高Atroign-1蛋白表达水平,使Atroign-1 mRNA表达升高3倍;Atrogin-1基因沉默可改善DEX引起的肌管萎缩. 结论:DEX促进肌管蛋白质分解代谢,并抑制蛋白合成代谢,导致肌肉萎缩;Atrogin-1基因沉默可改善DEX引起的肌肉萎缩,Atrogin-1基因可能是逆转肌肉消耗性营养不良的有效靶点.     

A single weekly dose of imatinib is sufficient to induce and maintain remission of chronic eosinophilic leukaemia in FIP1L1-PDGFRA-expressing patients

Hypereosinophilic syndrome (HES) is defined as chronic, unexplained hypereosinophilia with organ involvement. A subset of HES patients presents an interstitial deletion in chromosome 4q12, which leads to the expression of an imatinib-responsive fusion gene, FIP1L1-PDGFRA. These patients are diagnosed as chronic eosinophilic leukaemia (CEL). We treated seven CEL and HES patients, six of which expressed FIP1L1-PDGFRA , with imatinib using initial daily doses ranging from 100 to 400 mg. In a remission maintenance phase, the patients were treated with imatinib once weekly. All imatinib-treated patients achieved a complete haematological remission (CHR), and five of the six patients with FIP1L1-PDGFRA expression exhibited molecular remission. The decreased imatinib doses were as follows: 200 mg/week in three patients, 100 mg/week in two patients and 100 mg/d in the remaining two patients. For remission maintenance, imatinib doses were set at 100 mg/week in five patients and 200 mg/week in two patients. At a median follow-up of 30 months all patients remained in CHR and FIP1L1-PDGFRA expression was undetectable in five of the six FIP1L1-PDGFRA -expressing patients. These data suggest that a single weekly dose of imatinib is sufficient to maintain remission in FIP1L1-PDGFRA - positive CEL patients.     

Intracellular ATP binding is required to activate the slowly activating K+ channel IKs

Gating of ion channels by ligands is fundamental to cellular function, and ATP serves as both an energy source and a signaling molecule that modulates ion channel and transporter functions. The slowly activating K⁺ channel I_Ks in cardiac myocytes is formed by KCNQ1 and KCNE1 subunits that conduct K⁺ to repolarize the action potential. Here we show that intracellular ATP activates heterologously coexpressed KCNQ1 and KCNE1 as well as I_Ks in cardiac myocytes by directly binding to the C terminus of KCNQ1 to allow the pore to open. The channel is most sensitive to ATP near its physiological concentration, and lowering ATP concentration in cardiac myocytes results in I_Ks reduction and action potential prolongation. Multiple mutations that suppress I_Ks by decreasing the ATP sensitivity of the channel are associated with the long QT (interval between the Q and T waves in electrocardiogram) syndrome that predisposes afflicted individuals to cardiac arrhythmia and sudden death. A cluster of basic and aromatic residues that may form a unique ATP binding site are identified; ATP activation of the wild-type channel and the effects of the mutations on ATP sensitivity are consistent with an allosteric mechanism. These results demonstrate the activation of an ion channel by intracellular ATP binding, and ATP-dependent gating allows I_Ks to couple myocyte energy state to its electrophysiology in physiologic and pathologic conditions.Significant energy is required to sustain both the electrical and contractile events that accompany each heart beat, suggesting that the level of ATP is one key to normal cardiac functions. Not surprisingly, a reduction in ATP concentration ([ATP]) plays a key role in the pathogenesis and progression of ischemic heart diseases, including heart failure. Intracellular ATP is important not only in providing energy (1) and as a substrate for protein kinases (2), but also as signaling molecules to bind and modulate proteins. Only a handful of results have shown that intracellular ATP serves as a signal for membrane channels and transporters (3). The best-studied example is the K_ATP channel (4, 5). This channel is inhibited in physiologic conditions by [ATP]s of 5–10 mM (6), but when the ATP levels drop to submillimolar concentrations, as in cardiac ischemia, the K_ATP channels open, shortening the action potential duration and providing metabolic protection against the insult of ischemia (7). However, at normal physiologic conditions, whether and how ATP serves as a signal connecting the energetic state of the cell to membrane excitability is still unknown.The slowly activating K⁺ channel I_Ks plays an important role in controlling the action potential duration (APD) in cardiac myocytes; it opens in response to depolarization to conduct potassium ions out of the cell, which contributes to repolarization of the membrane, terminating the cardiac action potential and thereby the myocyte contraction. The I_Ks channel consists of pore-forming KCNQ1 subunits and the single-transmembrane auxiliary subunits KCNE1 (8, 9). Loss-of-function mutations in either KCNQ1 or KCNE1 lead to prolongation of ventricular action potentials and long QT syndrome (LQTS) that manifests as QT (interval between the Q and T waves in electrocardiogram) prolongation in the electrocardiogram. LQTS predisposes patients to cardiac arrhythmias that lead to syncope and sudden death (10). Previous studies show that ATP is required for activation of I_Ks channels (11), but the molecular mechanism and the physiologic function of this ATP modulation is not known. It was the purpose of this study to investigate the mechanism by which ATP regulated I_Ks. Our results show that ATP directly binds to the KCNQ1 protein to regulate channel function at concentrations ([ATP]) close to the physiologic intracellular [ATP] in cardiac myocytes. Our studies also reveal a unique ATP binding site and mechanism for modulation of ion channel function. Further, we found that several LQT-associated mutations alter I_Ks function by reducing ATP sensitivity. These results demonstrate that ATP regulation is vital for I_Ks channel function; a disruption of these regulations predisposes to life-threatening cardiac arrhythmias.     

ADP ribose is an endogenous ligand for the purinergic P2Y1 receptor

The mechanism by which extracellular ADP ribose (ADPr) increases intracellular free Ca(2+) concentration ([Ca(2+)](i)) remains unknown. We measured [Ca(2+)](i) changes in fura-2 loaded rat insulinoma INS-1E cells, and in primary β-cells from rat and human. A phosphonate analogue of ADPr (PADPr) and 8-Bromo-ADPr (8Br-ADPr) were synthesized. ADPr increased [Ca(2+)](i) in the form of a peak followed by a plateau dependent on extracellular Ca(2+). NAD(+), cADPr, PADPr, 8Br-ADPr or breakdown products of ADPr did not increase [Ca(2+)](i). The ADPr-induced [Ca(2+)](i) increase was not affected by inhibitors of TRPM2, but was abolished by thapsigargin and inhibited when phospholipase C and IP(3) receptors were inhibited. MRS 2179 and MRS 2279, specific inhibitors of the purinergic receptor P2Y1, completely blocked the ADPr-induced [Ca(2+)](i) increase. ADPr increased [Ca(2+)](i) in transfected human astrocytoma cells (1321N1) that express human P2Y1 receptors, but not in untransfected astrocytoma cells. We conclude that ADPr is a specific agonist of P2Y1 receptors.     

Uridine adenosine tetraphosphate is a novel neurogenic P2Y1 receptor activator in the gut

Enteric purinergic motor neurotransmission, acting through P2Y1 receptors (P2Y1R), mediates inhibitory neural control of the intestines. Recent studies have shown that NAD⁺ and ADP ribose better meet criteria for enteric inhibitory neurotransmitters in colon than ATP or ADP. Here we report that human and murine colon muscles also release uridine adenosine tetraphosphate (Up4A) spontaneously and upon stimulation of enteric neurons. Release of Up4A was reduced by tetrodotoxin, suggesting that at least a portion of Up4A is of neural origin. Up4A caused relaxation (human and murine colons) and hyperpolarization (murine colon) that was blocked by the P2Y1R antagonist, MRS 2500, and by apamin, an inhibitor of Ca²⁺-activated small-conductance K⁺ (SK) channels. Up4A responses were greatly reduced or absent in colons of P2ry1^−/− mice. Up4A induced P2Y1R–SK-channel–mediated hyperpolarization in isolated PDGFRα⁺ cells, which are postjunctional targets for purinergic neurotransmission. Up4A caused MRS 2500-sensitive Ca²⁺ transients in human 1321N1 astrocytoma cells expressing human P2Y1R. Up4A was more potent than ATP, ADP, NAD⁺, or ADP ribose in colonic muscles. In murine distal colon Up4A elicited transient P2Y1R-mediated relaxation followed by a suramin-sensitive contraction. HPLC analysis of Up4A degradation suggests that exogenous Up4A first forms UMP and ATP in the human colon and UDP and ADP in the murine colon. Adenosine then is generated by extracellular catabolism of ATP and ADP. However, the relaxation and hyperpolarization responses to Up4A are not mediated by its metabolites. This study shows that Up4A is a potent native agonist for P2Y1R and SK-channel activation in human and mouse colon.Uridine adenosine tetraphosphate (Up4A) is, to the authors’ knowledge, the first dinucleotide isolated from living organisms that contains both purine and pyrimidine moieties. Up4A is a recently-identified, nonpeptide, endothelium-derived vasoconstrictor (1, 2). Up4A is likely associated with blood pressure regulation, because its levels in plasma are elevated in hypertensive subjects (3) and it causes vasoconstriction in deoxycorticosterone acetate-salt hypertensive rats (4) and type 2 diabetic rats (5). Up4A also contracts rat and human airways (6) and rat gastric smooth muscles (7). Pharmacological studies suggest that Up4A causes vasoconstriction via activation of P2X1, P2Y2, and P2Y4 receptors (1) and endothelium-dependent vasodilatation via activation of P2Y1 and P2Y2 receptors (8). In porcine coronary artery Up4A causes vasodilatation via adenosine (P1) receptors (9). Furthermore, Up4A causes vascular smooth muscle cell proliferation and migration (10), stimulates monocyte and lymphocyte oxidative burst activities (11), is a potent proinflammatory agent in the vascular wall (12), and may contribute to the proinflammatory status in patients with chronic kidney disease (11). Plasma of healthy human subjects contains ∼50 nmol/L Up4A, which is sufficient to elicit vascular effects (1). The role of Up4A in the gastrointestinal (GI) tract is unknown.Enteric neural regulation of GI motility includes motor neurotransmission mediated by inhibitory neurons releasing purines that act via P2Y1 receptors (P2Y1Rs) (13–17) and apamin-sensitive small-conductance Ca²⁺-activated K⁺ (SK) channels (13, 14, 18, 19). ATP (20), NAD⁺ (14, 15, 21), and adenosine 5′-diphosphate ribose (ADPR) (17) activate P2Y1R and SK channels and might be inhibitory neurotransmitters in the colon (22). Because Up4A appears to stimulate P2Y1Rs in endothelium, and P2Y1Rs are important for purinergic signaling in the colon, we investigated whether Up4A is released in colonic muscle, whether Up4A affects membrane potentials and contractions of colonic muscles, whether Up4A is an agonist for P2Y1R, whether cells expressing PDGF receptor α (PDGFRα) are targets of Up4A, and how Up4A is metabolized in colons of humans and mice.     

Operation of the voltage sensor of a human voltage- and Ca2+-activated K+ channel

Voltage sensor domains (VSDs) are structurally and functionally conserved protein modules that consist of four transmembrane segments (S1–S4) and confer voltage sensitivity to many ion channels. Depolarization is sensed by VSD-charged residues residing in the membrane field, inducing VSD activation that facilitates channel gating. S4 is typically thought to be the principal functional component of the VSD because it carries, in most channels, a large portion of the VSD gating charge. The VSDs of large-conductance, voltage- and Ca²⁺-activated K⁺ channels are peculiar in that more gating charge is carried by transmembrane segments other than S4. Considering its “decentralized” distribution of voltage-sensing residues, we probed the BK_Ca VSD for evidence of cooperativity between charge-carrying segments S2 and S4. We achieved this by optically tracking their activation by using voltage clamp fluorometry, in channels with intact voltage sensors and charge-neutralized mutants. The results from these experiments indicate that S2 and S4 possess distinct voltage dependence, but functionally interact, such that the effective valence of one segment is affected by charge neutralization in the other. Statistical-mechanical modeling of the experimental findings using allosteric interactions demonstrates two mechanisms (mechanical coupling and dynamic focusing of the membrane electric field) that are compatible with the observed cross-segment effects of charge neutralization.     

The P2Y1 receptor, necessary but not sufficient to support full ADP-induced platelet aggregation, is not the target of the drug clopidogrel

Recently we showed that the P2Y₁ receptor coupled to calcium mobilization is necessary to initiate ADP-induced human platelet aggregation. Since the thienopyridine compound clopidogrel specifically inhibits ADP-induced platelet aggregation, it was of interest to determine whether the P2Y₁ receptor was the target of this drug. Therefore we studied the effects of clopidogrel and of the two specific P2Y₁ antagonists A2P5P and A3P5P on ADP-induced platelet events in rats. Although clopidogrel treatment (50 mg/kg) greatly reduced platelet aggregation in response to ADP as compared to untreated platelets, some residual aggregation was still detectable. In contrast, A2P5P and A3P5P totally abolished ADP-induced shape change and aggregation in platelets from both control and clopidogrel-treated rats. A2P5P and A3P5P (100 μM ) totally inhibited the [Ca²⁺]_i rise induced by ADP (0.1 μM ) in control and clopidogrel-treated platelets, whereas clopidogrel treatment had no effect. Conversely, the inhibition of adenylyl cyclase induced by ADP (5 μM ) was completely blocked by clopidogrel but not modified by A2P5P or A3P5P (100 μM ). A3P5P (1 m M ) reduced the number of [³³P]2MeSADP binding sites on control rat platelets from 907 ± 50 to 611 ± 25 per platelet. After clopidogrel treatment, binding of [³³P]2MeSADP decreased to 505 ± 68 sites per platelet and further decreased to 55 ± 12 sites in the presence of A3P5P (1 m M ). In summary, these results demonstrate that the platelet P2Y₁ receptor responsible for the initiation of aggregation in response to ADP is not the target of clopidogrel. Platelets may express another, as yet unidentified, P2Y receptor, specifically coupled to the inhibition of adenylyl cyclase and necessary to induce full platelet aggregation, which could be the target of this drug.     

C1q binding to surface-bound IgG is stabilized by C1r2s2 proteases

Complement is an important effector mechanism for antibody-mediated clearance of infections and tumor cells. Upon binding to target cells, the antibody’s constant (Fc) domain recruits complement component C1 to initiate a proteolytic cascade that generates lytic pores and stimulates phagocytosis. The C1 complex (C1qr₂s₂) consists of the large recognition protein C1q and a heterotetramer of proteases C1r and C1s (C1r₂s₂). While interactions between C1 and IgG-Fc are believed to be mediated by the globular heads of C1q, we here find that C1r₂s₂ proteases affect the capacity of C1q to form an avid complex with surface-bound IgG molecules (on various 2,4-dinitrophenol [DNP]-coated surfaces and pathogenic Staphylococcus aureus). The extent to which C1r₂s₂ contributes to C1q–IgG stability strongly differs between human IgG subclasses. Using antibody engineering of monoclonal IgG, we reveal that hexamer-enhancing mutations improve C1q–IgG stability, both in the absence and presence of C1r₂s₂. In addition, hexamer-enhanced IgGs targeting S. aureus mediate improved complement-dependent phagocytosis by human neutrophils. Altogether, these molecular insights into complement binding to surface-bound IgGs could be important for optimal design of antibody therapies.

Antibodies are important mediators of the human complement response, which offers critical protection against microbial infections and damaged host cells (1). In order to initiate a complement response, an antibody molecule first needs to bind antigens on the target cell via its antigen-binding (Fab) domains (2–5). Subsequently, the antibody’s constant (Fc) domain recruits the first complement protein complex, C1, to the cell surface (SI Appendix, Fig. S1A). The large C1 complex (also denoted as C1qr₂s₂, 766 kDa) consists of the recognition protein C1q (410 kDa) and a heterotetramer of serine proteases C1r and C1s (denoted C1r₂s₂, 356 kDa) (SI Appendix, Fig. S1B). While C1q is responsible for antibody recognition, its attached proteases C1r₂s₂ induce the activation of downstream enzymatic complexes (i.e., C3 convertases [C4b2b (6)]) that catalyze the covalent deposition of C3-derived molecules (e.g., C3b and its degradation product iC3b) onto the target cell surface (SI Appendix, Fig. S1A) (7, 8). C3b opsonizes the target cell surface and can induce formation of lytic pores (membrane attack complex [MAC]) in the target cell membrane (9–11). In contrast to human cells and gram-negative bacteria, gram-positive bacteria are not susceptible to the MAC due to their thick cell wall (12). On these bacteria, efficient decoration with C3b and iC3b is essential for triggering effective phagocytic uptake of target cells via complement receptors (CR) expressed on phagocytes of which the integrin CR3 (also denoted CD11b/CD18) is considered most important (13, 14).In recent years, our insights into IgG-dependent complement activation have increased significantly. A combination of structural, biophysical, and functional studies revealed that surface-bound IgG molecules (after Fab-mediated antigen binding) require organization into higher-ordered structures, namely hexamers, to induce complement activation most effectively (15–19). Hexamerized IgGs are being held together by noncovalent Fc–Fc interactions and form an optimal platform for C1q docking (SI Appendix, Fig. S1A). C1q has a “bunch of tulips–” like structure, consisting of six collagen arms that each end in a globular (gC1q) domain (SI Appendix, Fig. S1B) that binds the Fc region of an IgG. As the affinity of C1q for a single IgG is very weak (20, 21), avidity achieved through simultaneous binding of its globular domains to six oligomerized IgG molecules is paramount for a strong response (15, 17–19). Furthermore, it was found that IgG hexamerization could be manipulated by specific point mutations in the Fc–Fc contact region that enhance such oligomerization (15, 18, 22). While these hexamer-enhancing mutations in IgG potentiate the efficacy of MAC-dependent cytotoxicity on tumor cells and gram-negative bacteria (15, 23), their effect on complement-dependent phagocytosis is not known.Because complement is an important effector mechanism to kill bacteria and tumor cells, development of complement-enhancing antibodies represents an attractive strategy for immune therapies (1, 24). Immunotherapy based on human monoclonal antibodies is not yet available for bacterial infections (25–28). Such developments are mainly hampered by the fact that little is known about the basic mechanisms of complement activation on bacterial cells. For instance, we do not understand why certain antibodies induce complement activation on bacteria and others do not. In this study, we set out to investigate how antibacterial IgGs induce an effective complement response. By surprise, we noticed that C1q–IgG stability differs between human IgG subclasses. More detailed molecular investigations revealed that C1r₂s₂ proteases are important for generating stable C1q–IgG complexes on various target surfaces. Furthermore, we demonstrate that C1q–IgG stability is influenced by antibody oligomerization. These molecular insights into C1q binding to surface-bound IgGs may pave the way for optimal design of antibody therapies.     

浙江汉族人群散发性结直肠腺癌易感性与谷胱甘肽S-转移酶P1基因型相关性

[目的]谷胱甘肽S转移酶(GST)是参与人体内多种致癌物代谢的Ⅱ相代谢酶,本研究旨在探讨GSTP1基因型是否与浙江汉族人群散发性结直肠腺癌(SCRAC)遗传易感性相关.[方法]收集168例SCRAC患者(SCRAC组)和204名健康体检者(健康对照组),采用聚合酶链反应-限制性片段长度多态性(PCR-RFLP)检测GSTP1基因型频率在2组之间的差异.[结果]SCRAC组中,GSTP1突变等位基因及纯合突变基因型频率均显著高于健康对照组(43.75％∶35.78％,P＜0.05;29.76％∶ 17.16％,P＜0.01).根据临床病理特征对SCRAC患者进行分层分析,DukesC期和低分化腺癌患者中,纯合突变基因型频率均显著高于Dukes A、B期(P＜0.01)和高、中分化腺癌(P＜0.01).[结论]GSTP1基因突变与SCRAC相关,纯合突变基因型携带者能增加SCRAC的易感性.     

构建高效抑制细胞色素P450 2E1基因转录位点的siRNA表达载体

目的 寻找高效抑制细胞色素P450 3E1(CYP2E1)基因转录的位点,构建该位点的siRNA表达载体.方法 用PCR的扩增物作为表达框架,直接转染E47细胞,产生siRNA抑制CYP 2E1基因转录,筛选高效抑制CYP 2E1基因表达的位点,并构建BS/U6/CYP2E1的表达质粒,转染E47细胞,观察该质粒在E47细胞中发生RNAi效果.结果 4个备选位点的抑制效率最高,用该位点构建BS/U6/CY2E1的表达质粒可有效地、特异地抑制CYP2E1基因的表达.结论 用PCR产物作为表达框架,在体内产生双链的siRNA作为筛选高效的RNA干扰位点的方法,既快速又简便.用该位点的构建BS/U6/CYP2E1的表达质粒可用于CYP2E1基因的功能研究、酒精性脂肪肝病和非酒精性脂肪性肝病的发病机制及肿瘤发生的研究等领域中.     

谷胱甘肽-S-转移酶M1和T1空白基因型与肝癌遗传易感性的研究

目的谷胱甘肽－Ｓ－转移酶Ｍ１和Ｔ１（ＧＳＴＭ１和ＧＳＴＴ１）空白基因型与肝癌遗传易感性的关系。方法应用多重ＰＣＲ技术检测６３例肝癌患者和８８例健康对照的ＧＳＴＭ１和ＧＳＴＴ１空白基因型。结果病例组ＧＳＴＭ１空白基因型的频率为５７．１％,对照组则为４２．０％,二者差异无显著性（χ２＝３．３５,Ｐ＝０．０６７）,处于临界水平。ＯＲ值为１．８４（９５％ＣＩ＝０．９１～３．７３）。病例组ＧＳＴＴ１非空白基因型的频率为８７．３％,对照组则为６２．５％,二者差异有非常显著性（χ２＝１１．４２,Ｐ＝０．０００７２７４）,ＯＲ值为４．１３（９５％ＣＩ＝１．６４～１０．７０）。叉生分析表明,ＧＳＴＴ１非空白基因型与肝癌的关联大于ＧＳＴＭ１空白基因型,两因素在肝癌发生中存在协同作用。结论具有ＧＳＴＭ１空白基因型和ＧＳＴＴ１非空白基因型的个体,患肝癌的危险性增加。     

Cytochrome P450 (CYP) 1A2, sulfotransferase (SULT) 1A1, and N-acetyltransferase (NAT) 2 polymorphisms and susceptibility to urothelial cancer

Purpose Arylamines are suspected to be the primary causative agent of urothelial cancer in tobacco smoke. In the human liver, arylamines are N-hydroxylated by a cytochrome P450 (CYP)1A2-catalyzed reaction, which produces a substrate for O-esterification that can be catalyzed by N-acetyltransferases (NAT) or sulfotransferases (SULT). Recently, several polymorphisms of CYP1A2, SULT1A1, and NAT2 that affect their activities have been reported.Methods In this study, 306 Japanese patients with urothelial transitional cell carcinoma and 306 healthy controls were compared for frequencies of CYP1A2, SULT1A1, and NAT2 genotypes.Results The frequencies of NAT2 intermediate or slow acetylator genotype were significantly higher in the urothelial cancer patients than in the healthy control subjects [odds ratio (OR)=1.49, 95% confidence interval (95% CI) 1.06–2.09, OR=3.23, 95% CI 1.72–6.08, respectively]. Stratifying by amount of smoking, among subjects who consumed >33.5 pack-years and carried the SULT1A1 *1/*1 or NAT2 slow acetylator genotype, the OR was 1.73 (95% CI 1.01–2.97) whereas it was 7.31 (95% CI 1.90–28.05) in non-smokers who carried the homozygous wild genotype, respectively. The relationships between CYP1A2, SULT1A1, and NAT2 polymorphisms and clinical findings including tumor differentiation, stage, and recurrence rate were analyzed. Only associations between NAT2 genotype and pathological findings were admitted, and the higher OR of NAT2 intermediate and slow acetylator genotype was more likely to present to a low-grade tumor (G1) among heavy-smokers.Conclusions Our results suggest that SULT1A1 *1/*1 and NAT2 slow acetylator genotypes might modulate the effect of carcinogenic arylamines contained in tobacco smoke, and that the modulation of NAT2 intermediate and slow acetylator genotype has a tendency to present a higher risk for highly differentiated tumors among heavy-smokers.