Nicotinamide is a specific inhibitor of dark-operative protochlorophyllide oxidoreductase, a nitrogenase-like enzyme, from Rhodobacter capsulatus.

Dark-operative protochlorophyllide oxidoreductase (DPOR) is a nitrogenase-like enzyme consisting of two components, L-protein as a reductase component and NB-protein as a catalytic component. Elucidation of the crystal structures of NB-protein (Muraki et al., Nature 2010, 465: 110-114) has enabled us to study its reaction mechanism in combination with biochemical analysis. Here we demonstrate that nicotinamide (NA) inhibits DPOR activity by blocking the electron transfer from L-protein to NB-protein. A reaction scheme of DPOR, in which the binding of protochlorophyllide (Pchlide) to the NB-protein precedes the electron transfer from the L-protein, is proposed based on the NA effects.

Differential regulation of CYP3A4 promoter activity by a new class of natural product derivatives binding to pregnane X receptor.

The pregnane X receptor (PXR) regulates drug metabolism by regulating the expression of drug-metabolizing enzymes such as cytochrome P450 3A4 (CYP3A4), which is involved in the metabolism of >50% of clinically prescribed drugs. The activity of PXR can be controlled by the binding of small molecule agonists or antagonists. Because of its unique ligand binding pocket, PXR binds promiscuously to structurally diverse chemicals. To study the structure-activity relationship, novel modulators for PXR are needed. Here we report the virtual screening of ∼25,000 natural product derivatives from the ZINC database using the Molecular Operating Environment docking software tool against the PXR-rifampicin complex X-ray crystal structure. Our screening resulted in identification of compounds based on the lowest S score, which measures Gibbs free energy. Interestingly, we found that the compounds that bind directly to PXR, as revealed in an intrinsic tryptophan fluorescence assay, modulate CYP3A4 promoter activity differentially in HepG2 cells. Mutational analysis and docking studies showed that these compounds bind broadly in the ligand binding pocket but interact with different amino acid residues. We further investigated the mechanism of binding by analyzing the functional groups that are important for distinguishing agonists from antagonists. The approach we used to identify novel modulators that bind to PXR can be useful for finding novel modulators of PXR.

Allosteric inhibitor specificity of Thermotoga maritima 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase.

3-Deoxy-D-arabino-heptulosonate 7-phosphate synthase (DAH7PS) catalyses the first step of the shikimate pathway for the biosynthesis of aromatic amino acids. Allosteric regulation of Thermotoga maritima DAH7PS is mediated by L-Tyr binding to a discrete ACT regulatory domain appended to a core catalytic (ß/α)8 barrel. Variants of T. maritima DAH7PS (TmaDAH7PS) were created to probe the role of key residues in inhibitor selection. Substitution Ser31Gly severely reduced inhibition by L-Tyr. In contrast both L-Tyr and L-Phe inhibited the TmaHis29Ala variant, while the variant where Ser31 and His29 were interchanged (His29Ser/Ser31His), was inhibited to a greater extent by L-Phe than L-Tyr. These studies highlight the role and importance of His29 and Ser31 for determining both inhibitory ligand selectivity and the potency of allosteric response by TmaDAH7PS.

Latent myostatin has significant activity and this activity is controlled more efficiently by WFIKKN1 than by WFIKKN2.

Myostatin, a negative regulator of skeletal muscle growth, is produced from myostatin precursor by multiple steps of proteolytic processing. After cleavage by a furin-type protease, the propeptide and growth factor domains remain associated, forming a noncovalent complex, the latent myostatin complex. Mature myostatin is liberated from latent myostatin by bone morphogenetic protein 1/tolloid proteases. Here, we show that, in reporter assays, latent myostatin preparations have significant myostatin activity, as the noncovalent complex dissociates at an appreciable rate, and both mature and semilatent myostatin (a complex in which the dimeric growth factor domain interacts with only one molecule of myostatin propeptide) bind to myostatin receptor. The interaction of myostatin receptor with semilatent myostatin is efficiently blocked by WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing protein 1 or growth and differentiation factor-associated serum protein 2 (WFIKKN1), a large extracellular multidomain protein that binds both mature myostatin and myostatin propeptide [Kondás et al. (2008) J Biol Chem 283, 23677-23684]. Interestingly, the paralogous protein WAP, Kazal, immunoglobulin, Kunitz and NTR domain-containing protein 2 or growth and differentiation factor-associated serum protein 1 (WFIKKN2) was less efficient than WFIKKN1 as an antagonist of the interactions of myostatin receptor with semilatent myostatin. Our studies have shown that this difference is attributable to the fact that only WFIKKN1 has affinity for the propeptide domain, and this interaction increases its potency in suppressing the receptor-binding activity of semilatent myostatin. As the interaction of WFIKKN1 with various forms of myostatin permits tighter control of myostatin activity until myostatin is liberated from latent myostatin by bone morphogenetic protein 1/tolloid proteases, WFIKKN1 may have greater potential as an antimyostatic agent than WFIKKN2.

Development and characterization of a non-cell-based assay to assess the presence of neutralizing antibodies to interferon-beta in clinical samples.

Assessment of immunogenicity is an integral part of product development and involves evaluation of binding and neutralizing antibodies. The use of cell-based assays for detection of neutralizing antibodies (NAbs) is usually a regulatory expectation. Different cell-based assay formats are available for detection of anti-interferon-beta (IFN-ß) NAbs but all present technical difficulties and limitations. In this paper, a non-cell-based NAb assay which overcomes the limitations of cell-based assays is described. This NAb assay utilizes an electrochemiluminescence detection platform and is based on the first step involved in all IFN-ß-induced biological activities, namely the binding of IFN-ß to its receptor, which is inhibited when NAbs are present. Using this approach, NAb titers in clinical samples from multiple sclerosis patients treated with IFN-ß were determined and compared with those obtained using existing cell-based NAb assays. The sensitivity of the assays was not comparable, the cell-based approach having superior sensitivity. However a good correlation between the two approaches was observed. This study illustrates the practicality and feasibility of non-cell-based neutralization assays in the context of immunogenicity, however the utility of this approach would need to be assessed on a case-by-case basis for each therapeutic.

The carbonylation and covalent dimerization of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity is inhibited by the radical scavenger tempol.

Tempol (4-hydroxy-2,2,6,6-tetramethyl piperidine-1-oxyl) reduces tissue injury in animal models of various diseases via mechanisms that are not completely understood. Recently, we reported that high doses of tempol moderately increased survival in a rat model of ALS (amyotrophic lateral sclerosis) while decreasing the levels of oxidized hSOD1 (human Cu,Zn-superoxide dismutase) in spinal cord tissues. To better understand such a protective effect in vivo, we studied the effects of tempol on hSOD1 oxidation in vitro. The chosen oxidizing system was the bicarbonate-dependent peroxidase activity of hSOD1 that consumes H2O2 to produce carbonate radical, which oxidizes the enzyme. Most of the experiments were performed with 30 µM hSOD1, 25 mM bicarbonate, 1 mM H2O2, 0.1 mM DTPA (diethylenetriaminepenta-acetic acid) and 50 mM phosphate buffer at a final pH of 7.4. The results showed that tempol (5-75 µM) does not inhibit hSOD1 turnover, but decreases its resulting oxidation to carbonylated and covalently dimerized forms. Tempol acted by scavenging the carbonate radical produced and by recombining with hSOD1-derived radicals. As a result, tempol was consumed nearly stoichiometrically with hSOD1 monomers. MS analyses of turned-over hSOD1 and of a related peptide oxidized by the carbonate radical indicated the formation of a relatively unstable adduct between tempol and hSOD1-Trp32•. Tempol consumption by the bicarbonate-dependent peroxidase activity of hSOD1 may be one of the reasons why high doses of tempol were required to afford protection in an ALS rat model. Overall, the results of the present study confirm that tempol can protect against protein oxidation and the ensuing consequences.

Biological and structural evaluation of 10R- and 10S-methylthio-DDACTHF reveals a new role for sulfur in inhibition of glycinamide ribonucleotide transformylase.

Glycinamide ribonucleotide transformylase (GAR Tfase) is a folate-dependent enzyme in the de novo purine biosynthesis pathway, which has long been considered a potential target for development of anti-neoplastic therapeutics. Here we report the biological and X-ray crystallographic evaluations of both independent C10 diastereomers, 10S- and 10R-methylthio-DDACTHF, bound to human GAR Tfase, including the highest-resolution apo GAR Tfase structure to date (1.52 Å). Both diastereomers are potent inhibitors (Ki = 210 nM for 10R, and Ki = 180 nM for 10S) of GAR Tfase and exhibit effective inhibition of human leukemia cell growth (IC50 = 80 and 50 nM, respectively). Their inhibitory activity was surprisingly high, and these lipophilic C10-substituted analogues show distinct advantages over their hydrophilic counterparts, most strikingly in retaining potency in mutant human leukemia cell lines that lack reduced folate carrier protein activity (IC50 = 70 and 60 nM, respectively). Structural characterization reveals a new binding mode for these diastereoisomers, in which the lipophilic thiomethyl groups penetrate deeper into a hydrophobic pocket within the folate-binding site. In silico docking simulations of three other sulfur-containing folate analogues also indicate that this hydrophobic cleft represents a favorable region for binding lipophilic substituents. Overall, these results suggest sulfur and its substitutions play an important role in not only the binding of anti-folates to GAR Tfase but also the selectivity and cellular activity (growth inhibition), thereby presenting new possibilities for the future design of potent and selective anti-folate drugs that target GAR Tfase.

Plumbagin inhibits cytokinesis in Bacillus subtilis by inhibiting FtsZ assembly--a mechanistic study of its antibacterial activity.

The assembly of FtsZ plays a central role in construction of the cytokinetic Z-ring that orchestrates bacterial cell division. A naturally occurring naphthoquinone, plumbagin, is known to exhibit antibacterial properties against several types of bacteria. In this study, plumbagin was found to perturb formation of the Z-ring in Bacillus subtilis 168 cells and to cause elongation of these cells without an apparent effect on nucleoid segregation, indicating that it may inhibit FtsZ assembly. Furthermore, it bound to purified B. subtilis FtsZ (BsFtsZ) with a dissociation constant of 20.7 ± 5.6 µM, and inhibited the assembly and GTPase activity of BsFtsZ in vitro. Interestingly, plumbagin did not inhibit either the assembly or GTPase activity of Escherichia coli FtsZ (EcFtsZ) in vitro. Using docking analysis, a putative plumbagin-binding site on BsFtsZ was identified, and the analysis indicated that hydrophobic interactions and hydrogen bonds predominate. Based on the in silico analysis, two variants of BsFtsZ, namely D199A and V307R, were constructed to explore the binding interaction of plumbagin and BsFtsZ. The effects of plumbagin on the assembly and GTPase activity of the variant BsFtsZ proteins in vitro indicated that the residues D199 and V307 may be involved in the binding of plumbagin to BsFtsZ. The results suggest that plumbagin inhibits bacterial proliferation by inhibiting the assembly of FtsZ, and provide insight into the binding site of plumbagin on BsFtsZ, which may help in the design of potent FtsZ-targeted antibacterial agents.

A general anaesthetic propofol inhibits aquaporin-4 in the presence of Zn²âº.

AQP4 (aquaporin-4), a water channel protein that is predominantly expressed in astrocyte end-feet, plays an important role in the brain oedema formation, and is thereby considered to be a potential therapeutic target. Using a stopped-flow analysis, we showed that propofol (2,6-diisopropylphenol), a general anaesthetic drug, profoundly inhibited the osmotic water permeability of AQP4 proteoliposomes in the presence of Zn²âº. This propofol inhibition was not observed in AQP1, suggesting the specificity for AQP4. In addition, the inhibitory effects of propofol could be reversed by the removal of Zn²âº. Other lipid membrane fluidizers also similarly inhibited AQP4, suggesting that the modulation of protein-lipid interactions plays an essential role in the propofol-induced inhibition of AQP4. Accordingly, we used Blue native PAGE and showed that the profound inhibition caused by propofol in the presence of Zn²âº is coupled with the reversible clustering of AQP4 tetramers. Site-directed mutagenesis identified that Cys²5³, located at the membrane interface connecting to the C-terminal tail, is responsible for Zn²âº-mediated propofol inhibition. Overall, we discovered that propofol specifically and reversibly inhibits AQP4 through the interaction between Zn²âº and Cys²5³. The findings provide new insight into the functional regulation of AQP4 and may facilitate the identification of novel AQP4-specific inhibitors.

FDA-approved drugs and other compounds tested as inhibitors of human glutathione transferase P1-1.

OBJECTIVE: Glutathione transferase P1-1 (GST P1-1) is often overexpressed in tumor cells and is regarded as a contributor to their drug resistance. Inhibitors of GST P1-1 are expected to counteract drug resistance and may therefore serve as adjuvants in the chemotherapy of cancer by increasing the efficacy of cytostatic drugs. Finding useful inhibitors among compounds used for other indications would be a shortcut to clinical applications and a search for GST P1-1 inhibitors among approved drugs and other compounds was therefore conducted. METHODS: We tested 1040 FDA-approved compounds as inhibitors of the catalytic activity of purified human GST P1-1 in vitro. RESULTS: We identified chlorophyllide, merbromine, hexachlorophene, and ethacrynic acid as the most effective GST P1-1 inhibitors with IC50 values in the low micromolar range. For comparison, these compounds were even more potent in the inhibition of human GST A3-3, an enzyme implicated in steroid hormone biosynthesis. In distinction from the other inhibitors, which showed conventional inhibition patterns, the competitive inhibitor ethacrynic acid elicited strong kinetic cooperativity in the glutathione saturation of GST P1-1. Apparently, ethacrynic acid serves as an allosteric inhibitor of the enzyme. CONCLUSION AND PRACTICAL IMPLICATIONS: In their own right, the compounds investigated are less potent than desired for adjuvants in cancer chemotherapy, but the structures of the most potent inhibitors could serve as leads for the synthesis of more efficient adjuvants.

microRNA-133a regulates the cell cycle and proliferation of breast cancer cells by targeting epidermal growth factor receptor through the EGFR/Akt signaling pathway.

microRNAs are small, highly conserved, non-coding RNAs that regulate gene expression of target mRNAs through cleavage or translational inhibition, and are widely involved in carcinogenesis and cancer development. In this study, the expression profile of microRNA-133a (miR-133a) was examined in breast cancer cells and breast cancer tissues. The results showed that expression of miR-133a in both breast cancer cells and breast cancer tissues was significantly down-regulated. Over-expression of miR-133a in tumor cells arrested the cell cycle by drastically decreasing the G2 /S phase and retarded the newly synthesized DNA, suggesting a regulatory role for miR-133a in proliferation of breast cancer cells. Bioinformatics prediction showed that epidermal growth factor receptor (EGFR) is a potential target for miR-133a. A dual luciferase reporter gene assay showed that miR-133a bound to the 3' UTR of EGFR but not a mutated 3' UTR, thereby down-regulating the protein expression level. Accordingly, we found that expression of EGFR protein decreased with increased expression of miR-133a in MCF-7 and MDA-MB-231 cells. Over-expression of miR-133a in breast cancer cells resulted in suppression of the level of phosphorylated Akt protein (p-Akt) and inhibition of p-Akt nuclear translocation. These results demonstrate that miR-133a, which may act as a tumor suppressor in breast cancer, regulates the cell cycle and proliferation in tumorigenesis by targeting EGFR through the downstream signal molecule Akt. Overall, these results show that miR-133a may be used as biomarker and/or therapeutic target for diagnosis and therapy of breast cancer.

MiR-133a modulates osteogenic differentiation of vascular smooth muscle cells.

Arterial calcification is a key pathologic component of vascular diseases such as atherosclerosis, coronary artery disease, and peripheral vascular disease. A hallmark of this pathological process is the phenotypic transition of vascular smooth muscle cells (VSMCs) to osteoblast-like cells. Several studies have demonstrated that microRNAs (miRNAs) regulate osteoblast differentiation, but it is unclear whether miRNAs also regulate VSMC-mediated arterial calcification. In the present study, we sought to characterize the role of miR-133a in regulating VSMC-mediated arterial calcification. Northern blotting analysis of VSMCs treated with ß-glycerophosphate demonstrated that miR-133a was significantly decreased during osteogenic differentiation. Overexpression of miR-133a inhibited VSMC transdifferentiation into osteoblast-like cells as evidenced by a decrease in alkaline phosphatase activity, osteocalcin secretion, Runx2 expression, and mineralized nodule formation. Conversely, the knockdown of miR-133a using an miR-133a inhibitor promoted osteogenic differentiation of VSMCs by increasing alkaline phosphatase activity, osteocalcin secretion, and Runx2 expression. Runx2 was identified as a direct target of miR-133a by a cotransfection experiment in VSMCs with luciferase reporter plasmids containing wild-type or mutant 3'-untranslated region sequences of Runx2. Furthermore, the pro-osteogenic effects of miR-133a inhibitor were abrogated in Runx2-knockdown cells, and the inhibition of osteogenic differentiation by pre-miR-133a was reversed by overexpression of Runx2, providing functional evidence that the effects of miR-133a in osteogenic differentiation were mediated by targeting Runx2. These results demonstrate that miR-133a is a key negative regulator of the osteogenic differentiation of VSMCs.

MicroRNA-503 suppresses proliferation and cell-cycle progression of endometrioid endometrial cancer by negatively regulating cyclin D1.

MicroRNAs (miRNAs) are post-transcriptional inhibitor regulators of gene expression that act by directly binding complementary mRNA and are key determinants of cancer initiation and progression. In this study, we revealed a role for the tumor-suppressor miRNA miR-503 in endometrioid endometrial cancer (EEC) cells. The miR-503 expression level gradually decreases across normal endometrial tissues, endometrial tissues with complex atypical hyperplasia, and EEC tissues. A relatively high level of miR-503 in EEC tissues indicates a longer survival time in EEC patients. The expression of a cell cycle-associated oncogene encoding cyclin D1 (CCND1) was inversely correlated with miR-503 expression in EEC tissues and cell lines. CCND1 has a binding sequence of miR-503 within its 3' untranslated region, and was confirmed to be a direct target of miR-503 by the fluorescent reporter assays. Increasing the miR-503 level in EEC cells suppressed cell viability, colon formation activity and cell-cycle progression, and the inhibited oncogenic phenotypes induced by miR-503 were alleviated by ectopic expression of CCND1 without the untranslated region sequence. Furthermore, in vivo studies also suggested a suppressive effect of miR-503 on EEC cell-derived xenografts. miR-503 increased in cell cycle-arrested EEC cells, and was restored to a normal level in EEC cells after cell cycle re-entry, while CCND1 displayed the opposite expression pattern. Collectively, this study suggested that miR-503 plays a tumor-suppressor role by targeting CCND1. Abnormal suppression of miR-503 leads to an increase in the CCND1 level, which may promote carcinogenesis and progression of EEC.

Structure-guided optimization of protein kinase inhibitors reverses aminoglycoside antibiotic resistance.

Activity of the aminoglycoside phosphotransferase APH(3')-Ia leads to resistance to aminoglycoside antibiotics in pathogenic Gram-negative bacteria, and contributes to the clinical obsolescence of this class of antibiotics. One strategy to rescue compromised antibiotics such as aminoglycosides is targeting the enzymes that confer resistance with small molecules. We demonstrated previously that ePK (eukaryotic protein kinase) inhibitors could inhibit APH enzymes, owing to the structural similarity between these two enzyme families. However, limited structural information of enzyme-inhibitor complexes hindered interpretation of the results. In addition, cross-reactivity of compounds between APHs and ePKs represents an obstacle to their use as aminoglycoside adjuvants to rescue aminoglycoside antibiotic activity. In the present study, we structurally and functionally characterize inhibition of APH(3')-Ia by three diverse chemical scaffolds, anthrapyrazolone, 4-anilinoquinazoline and PP (pyrazolopyrimidine), and reveal distinctions in the binding mode of anthrapyrazolone and PP compounds to APH(3')-Ia compared with ePKs. Using this observation, we identify PP derivatives that select against ePKs, attenuate APH(3')-Ia activity and rescue aminoglycoside antibiotic activity against a resistant Escherichia coli strain. The structures described in the present paper and the inhibition studies provide an important opportunity for structure-based design of compounds to target aminoglycoside phosphotransferases for inhibition, potentially overcoming this form of antibiotic resistance.

Diacylglycerol kinase θ couples farnesoid X receptor-dependent bile acid signalling to Akt activation and glucose homoeostasis in hepatocytes.

DGKs (diacylglycerol kinases) catalyse the conversion of diacylglycerol into PA (phosphatidic acid), a positive modulator of mTOR (mammalian target of rapamycin). We have found that chenodeoxycholic acid and the synthetic FXR (farnesoid X receptor) ligand GW4064 induce the mRNA and protein expression of DGKθ in the HepG2 cell line and in primary human hepatocytes. Reporter gene studies using 1.5 kB of the DGKθ promoter fused to the luciferase gene revealed that bile acids increase DGKθ transcriptional activity. Mutation of putative FXR-binding sites attenuated the ability of GW4046 to increase DGKθ luciferase activity. Consistent with this finding, ChIP (chromatin immunoprecipitation) assays demonstrated that bile acid signalling increased the recruitment of FXR to the DGKθ promoter. Furthermore, GW4064 evoked a time-dependent increase in the cellular concentration of PA. We also found that GW4064 and PA promote the phosphorylation of mTOR, Akt and FoxO1 (forkhead box O1), and that silencing DGKθ expression significantly abrogated the ability of GW4046 to promote the phosphorylation of these PA-regulated targets. DGKθ was also required for bile-acid-dependent decreased glucose production. Taken together, our results establish DGKθ as a key mediator of bile-acid-stimulated modulation of mTORC2 (mTOR complex 2), the Akt pathway and glucose homoeostasis.

ROS inhibitor N-acetyl-L-cysteine antagonizes the activity of proteasome inhibitors.

NAC (N-acetyl-L-cysteine) is commonly used to identify and test ROS (reactive oxygen species) inducers, and to inhibit ROS. In the present study, we identified inhibition of proteasome inhibitors as a novel activity of NAC. Both NAC and catalase, another known scavenger of ROS, similarly inhibited ROS levels and apoptosis associated with H2O2. However, only NAC, and not catalase or another ROS scavenger Trolox, was able to prevent effects linked to proteasome inhibition, such as protein stabilization, apoptosis and accumulation of ubiquitin conjugates. These observations suggest that NAC has a dual activity as an inhibitor of ROS and proteasome inhibitors. Recently, NAC was used as a ROS inhibitor to functionally characterize a novel anticancer compound, piperlongumine, leading to its description as a ROS inducer. In contrast, our own experiments showed that this compound depicts features of proteasome inhibitors including suppression of FOXM1 (Forkhead box protein M1), stabilization of cellular proteins, induction of ROS-independent apoptosis and enhanced accumulation of ubiquitin conjugates. In addition, NAC, but not catalase or Trolox, interfered with the activity of piperlongumine, further supporting that piperlongumine is a proteasome inhibitor. Most importantly, we showed that NAC, but not other ROS scavengers, directly binds to proteasome inhibitors. To our knowledge, NAC is the first known compound that directly interacts with and antagonizes the activity of proteasome inhibitors. Taken together, the findings of the present study suggest that, as a result of the dual nature of NAC, data interpretation might not be straightforward when NAC is utilized as an antioxidant to demonstrate ROS involvement in drug-induced apoptosis.

S100A11 is involved in the regulation of the stability of cell cycle regulator p21(CIP1/WAF1) in human keratinocyte HaCaT cells.

The cyclin-dependent kinase inhibitor p21(CIP1/WAF1) is a regulatory factor of the cell cycle. Its transcriptional activation and protein stability are tightly controlled by several distinct mechanisms. S100A11 is a member of the S100 family of Ca²âº-binding proteins involved in several biological processes, including cell cycle progression and signal transduction. In the present study, we show that down-regulation of S100A11 results in the reduction of p21 protein in human HaCaT keratinocytes. It appears that a ubiquitin-independent proteasomal degradation process is involved in p21 degradation in S100A11 down-regulated cells. The application of a proteasome inhibitor stabilized p21 protein in these cells. Analysis of distinct signal transduction pathways revealed a disturbed phosphatidylinositol-3-kinase/Akt pathway after S100A11 knockdown. We determined that the glycogen synthase kinase-3, which is negatively regulated by phosphatidylinositol 3-kinase/Akt, was activated in cells possessing knocked-down S100A11 and appears to be involved in p21 protein destabilization. The application of a specific inhibitor of glycogen synthase kinase 3 resulted in an increase of the p21 protein level in S100A11 down-regulated HaCaT cells. Glycogen synthase kinase 3 is able to phosphorylate p21 at T57, which induces p21 proteasomal turnover. Mutation of the glycogen synthase kinase 3 site threonine 57 into alanine (T57A) stabilizes p21 in HaCaT cells lacking S100A11. Beside decreased p21 protein, down-regulation of S100A11 triggered the induction of apoptosis in HaCaT cells. These observations suggest that S100A11 is involved in the maintenance of p21 protein stability and appears to function as an inhibitor of apoptosis in human HaCaT keratinocyte cells. Thus, the data shed light on a novel pathway regulating p21 protein stability.

(-)-Epigallocatechin-3-gallate, a potential inhibitor to human dicarbonyl/L-xylulose reductase.

Dicarbonyl/l-xylulose reductase (DCXR), mainly catalysing the reduction of α-dicarbonyl compounds and l-xylulose, belongs to the short-chain dehydrogenase/reductase superfamily. Its enzyme activity can be inhibited by short-chain fatty acids. In this study, a novel DCXR inhibitor named (-)-epigallocatechin-3-gallate (EGCG) was reported. First, we overexpressed recombinant human DCXR in Escherichia coli, purified the enzyme by affinity chromatography and measured its activity. The inhibition effects of EGCG and its analogues on DCXR were determined subsequently, and EGCG showed the strongest inhibition with 50% inhibition concentration value of 78.8 µM. The surface plasmon resonance analysis also indicated that the equilibrium dissociation constant (KD) reached to 7.11 × 10(-8) M, which implied a high affinity between EGCG and DCXR. From enzyme kinetic analysis, EGCG acted as a mixed inhibitor against its forward and reverse substrates and the coenzyme, reduced nicotinamide adenine dinucleotide phosphate (NADPH). However, the inhibition is pH dependent. The molecular docking finally showed that EGCG formed several hydrogen bonds with the Thr190 residue of DCXR, and the model was further verified by site-directed mutagenesis. Therefore, EGCG is a potential inhibitor to human DCXR.

PD173074, a selective FGFR inhibitor, reverses ABCB1-mediated drug resistance in cancer cells.

PURPOSE: Specific tyrosine kinase inhibitors were recently reported to modulate the activity of ABC transporters, leading to an increase in the intracellular concentration of their substrate drugs. In this study, we determine whether PD173074, a specific fibroblast growth factor receptor (FGFR) inhibitor, could reverse ABC transporter-mediated multidrug resistance. METHODS: 3-(4,5-Dimethylthiazol-yl)-2,5-diphenyllapatinibrazolium bromide assay was used to determine the effect of PD173074 on reversal of ABC transporter-mediated multidrug resistance (MDR). In addition, [³H]-paclitaxel accumulation/efflux assay, western blotting analysis, ATPase, and photoaffinity labeling assays were done to study the interaction of PD173074 on ABC transporters. RESULTS: PD173074 significantly sensitized both ABCB1-transfected and drug-selected cell lines overexpressing this transporter to substrate anticancer drugs colchicine, paclitaxel, and vincristine. This effect of PD173074 is specific to ABCB1, as no significant interaction was detected with other ABC transporters such as ABCC1 and ABCG2. The observed reversal effect seems to be primarily due to the decreased active efflux of [³H]-paclitaxel in ABCB1 overexpressing cells observed in efflux assay. In addition, no significant change in the ABCB1 expression was observed when ABCB1 overexpressing cells were exposed to 5 µM PD173074 for up to 3 days, thereby further suggesting its role in modulating the function of the transporter. In addition, PD173074 stimulated the ATPase activity of ABCB1 in a concentration-dependent manner, indicating a direct interaction with the transporter. Interestingly, PD173074 did not inhibit photolabeling of ABCB1 with [¹²5I]-iodoarylazidoprazosin (IAAP), showing that it binds at a site different from that of IAAP in the drug-binding pocket. CONCLUSIONS: Here, we report for the first time, PD173074, an inhibitor of the FGFR, to selectively reverse ABCB1 transporter-mediated MDR by directly blocking the efflux function of the transporter.

Manipulation of PBF/PTTG1IP phosphorylation status; a potential new therapeutic strategy for improving radioiodine uptake in thyroid and other tumors.

CONTEXT: The clinical effectiveness of ablative radioiodine treatment of thyroid tumors is limited by the availability of the sodium iodide symporter (NIS) at the plasma membrane (PM) for uptake of ¹³¹I. A significant proportion of well-differentiated thyroid tumors are unable to concentrate sufficient radioiodine for effective therapy, and in other tumor models such as breast tumors, where radioiodine uptake would be an attractive therapeutic option, uptake is insufficient. OBJECTIVE: Pituitary tumor-transforming gene-binding factor (PBF; PTTG1IP) is overexpressed in multiple cancers and significantly decreases NIS expression at the PM. The goal of this study was to identify a method by which PBF repression of NIS may be overcome in human tumors. RESULTS: Here, we identify PBF as a tyrosine phosphoprotein that specifically binds the proto-oncogene tyrosine protein kinase Src in mass spectrometry, glutathione S-transferase pulldown and coimmunoprecipitation assays. Src induction leads to phosphorylation at PBF residue Y174. Abrogation of this residue results in PM retention and a markedly reduced ability to bind NIS. The Src inhibitor PP1 inhibits PBF phosphorylation in multiple cell lines in vitro, including human primary thyroid cells. Of direct clinical importance to the treatment of thyroid cancer, PP1 stimulates iodide uptake by transfected NIS in TPC1 thyroid carcinoma cells and entirely overcomes PBF repression of iodide uptake in human primary thyroid cells. CONCLUSIONS: We propose that targeting PBF phosphorylation at residue Y174 via tyrosine kinase inhibitors may be a novel therapeutic strategy to enhance the efficacy of ablative radioiodine treatment in thyroid and other endocrine and endocrine-related tumors.