High-resolution crystal structure of apolipoprotein(a) kringle IV type 7: insights into ligand binding

Apolipoprotein(a) [apo(a)] consists of a series of tandemly repeated modules known as kringles that are commonly found in many proteins involved in the fibrinolytic and coagulation cascades, such as plasminogen and thrombin, respectively. Specifically, apo(a) contains multiple tandem repeats of domains similar to plasminogen kringle IV (designated as KIV(1) to KIV(10)) followed by sequences similar to the kringle V and protease domains of plasminogen. The KIV domains of apo(a) differ with respect to their ability to bind lysine or lysine analogs. KIV(10) represents the high-affinity lysine-binding site (LBS) of apo(a); a weak LBS is predicted in each of KIV(5)-KIV(8) and has been directly demonstrated in KIV(7). The present study describes the first crystal structure of apo(a) KIV(7), refined to a resolution of 1.45 A, representing the highest resolution for a kringle structure determined to date. A critical substitution of Tyr-62 in KIV(7) for the corresponding Phe-62 residue in KIV(10), in conjunction with the presence of Arg-35 in KIV(7), results in the formation of a unique network of hydrogen bonds and electrostatic interactions between key LBS residues (Arg-35, Tyr-62, Asp-54) and a peripheral tyrosine residue (Tyr-40). These interactions restrain the flexibility of key LBS residues (Arg-35, Asp-54) and, in turn, reduce their adaptability in accommodating lysine and its analogs. Steric hindrance involving Tyr-62, as well as the elimination of critical ligand-stabilizing interactions within the LBS are also consequences of this interaction network. Thus, these subtle yet critical structural features are responsible for the weak lysine-binding affinity exhibited by KIV(7) relative to that of KIV(10).     

Solution structure of human apolipoprotein(a) kringle IV type 6.

The structure of apo(a) KIVT6 was investigated by two- and three-dimensional homo- and heteronuclear NMR spectroscopy. The solution structure of apo(a) KIVT6 contains only a small amount of regular secondary structure elements, comprising a short piece of antiparallel beta-sheet formed by residues Trp62-Tyr64 and Trp72-Tyr74, a short piece of parallel beta-sheet formed by the residues Cys1-Tyr2 and Thr78-Gln79, and a small 3(10)-helix within residues Thr38-Tyr40. The backbone as well as the side chains are arranged in a way similar to those of apo(a) KIVT7, apo(a) KIVT10, and plasminogen K4. We determined additionally the K(d) value of 0.31 +/- 0.04 mM for the binding of epsilon-aminocaproic acid (EACA) to apo(a) KIVT6 and mapped the binding region on apo(a) KIVT6 by means of chemical shift perturbation. This lysine binding activity, which was reported to occur within apo(a) KIVT5-8, is functionally different from the lysine binding activity found for apo(a) KIVT10.     

Nuclear magnetic resonance (NMR) solution structure, dynamics, and binding properties of the kringle IV type 8 module of apolipoprotein(a)

The plasma lipoprotein lipoprotein(a) [Lp(a)] comprises a low-density lipoprotein (LDL)-like particle covalently attached to the glycoprotein apolipoprotein(a) [apo(a)]. Apo(a) consists of multiple tandem repeating kringle modules, similar to plasminogen kringle IV (designated KIV1-KIV10), followed by modules homologous to the kringle V module and protease domain of plasminogen. The apo(a) KIV modules have been classified on the basis of their binding affinity for lysine and lysine analogues. The strong lysine-binding apo(a) KIV10 module mediates lysine-dependent interactions with fibrin and cell-surface receptors. Weak lysine-binding apo(a) KIV7 and KIV8 modules display a 2-3-fold difference in lysine affinity and play a direct role in the noncovalent step in Lp(a) assembly through binding to unique lysine-containing sequences in apolipoproteinB-100 (apoB-100). The present study describes the nuclear magnetic resonance solution structure of apo(a) KIV8 and its solution dynamics properties, the first for an apo(a) kringle module, and compares the effects of epsilon-aminocaproic acid (epsilon-ACA) binding on the backbone and side-chain conformation of KIV7 and KIV8 on a per residue basis. Apo(a) KIV8 adopts a well-ordered structure that shares the general tri-loop kringle topology with apo(a) KIV6, KIV7, and KIV10. Mapping of epsilon-ACA-induced chemical-shift changes on KIV7 and KIV8 indicate that the same residues are affected, despite a 2-3-fold difference in epsilon-ACA affinity. A unique loop conformation within KIV8, involving hydrophobic interactions with Tyr40, affects the positioning of Arg35 relative to the lysine-binding site (LBS). A difference in the orientation of the aromatic side chains comprising the hydrophobic center of the LBS in KIV8 decreases the size of the hydrophobic cleft compared to other apo(a) KIV modules. An exposed hydrophobic patch contiguous with the LBS in KIV8 and not conserved in other weak lysine-binding apo(a) kringle modules may modulate specificity for regions within apoB-100. An additional ligand recognition site comprises a structured arginine-glycine-aspartate motif at the N terminus of the KIV8 module, which may mediate Lp(a)/apo(a)-integrin interactions.     

Baboon lipoprotein(a) binds very weakly to lysine-agarose and fibrin despite the presence of a strong lysine-binding site in apolipoprotein(a) kringle IV type 10

Human apolipoprotein(a) kringle IV type 10 [apo(a) KIV(10)] contains a strong lysine-binding site (LBS) that mediates the interaction of Lp(a) with biological substrates such as fibrin. Mutations in the KIV(10) LBS have been reported in both the rhesus monkey and chimpanzee, and have been proposed to explain the lack of ability of the corresponding Lp(a) species to bind to lysine and fibrin. To further the comparative analyses with other primate species, we sequenced a segment of baboon liver apo(a) cDNA spanning KIV(9) through the protease domain. Like rhesus monkey apo(a), baboon apo(a) lacks a kringle V (KV)-like domain. Interestingly, we found that the baboon apo(a) KIV(10) sequence contains all of the canonical LBS residues. We sequenced the apo(a) KIV(10) sequence from an additional 10 unrelated baboons; 17 of 20 alleles encoded Trp at position 70, whereas only two alleles encoded Arg at this position and thus a defective LBS. Despite the apparent presence of a functional KIV(10) LBS in most of the baboons, none of the Lp(a) in the plasma of the corresponding baboons bound specifically to lysine-Sepharose (agarose) even upon partial purification. Moreover, baboon Lp(a) bound very poorly to plasmin-modified fibrinogen. Expression of baboon and human KIV(10) in bacteria allowed us to verify that these domains bind comparably to lysine and lysine analogues. We conclude that presentation of KIV(10) in the context of apo(a) lacking KV may interfere with the ability of KIV(10) to bind to substrates such as fibrin; this paradigm may also be present in other non-human primates.     

Secreted human apolipoprotein(a) kringle IV-10 and kringle V inhibit angiogenesis and xenografted tumor growth

Abstract Angiogenesis plays an important role in normal physiology of blood vessel growth, but can contribute to the pathogenesis of diseases, such as cancer. A new anti-angiogenic recombinant kringle protein, composed of the fused domains of human apolipoprotein(a) carboxyl-terminal kringle IV-10 and kringle V, was expressed in Pichia pastoris and human colorectal carcinoma (HCT 116) cells to investigate its influence on angiogenesis and tumor growth. The mature recombinant protein exhibited the characteristic features of kringle-containing proteins (glycosylation and disulfide bond formation) and, when added to cultures of human umbilical vein endothelial cell, resulted in a 31% decrease in proliferation relative to untreated controls (p<0.05). The neo-angiogenesis was diminished by 63% in chick embryos treated with 10 mug recombinant protein compared with 7% for phosphate buffer solution-treated embryos (p<0.01). Transfection of a kringle IV-10-kringle V fusion protein construct into HCT 116 cells decreased tumorigenesis and inhibited tumor growth in vivo without affecting tumor cell proliferation. HCT 116 cells that expressed recombinant protein displayed a much lower relative growth ratio of 8% (p<0.01) against the control tumor cells. From these results, we conclude that human apolipoprotein(a) carboxyl-terminal kringle IV-10-kringle V fusion protein is an effective inhibitor of angiogenesis and angiogenesis-dependent tumor growth.     

The lysine binding sites of human plasminogen. Evidence for a critical tryptophan in the binding site of kringle 4

Chemical modification of human degraded form of plasminogen with NH2-terminal lysine (Lys-plasminogen) and the elastase fragments kringle 1 + 2 + 3 and kringle 4 with the tryptophan reagent [14C]dimethyl(2-hydroxy-5-nitrobenzyl)sulfonium bromide results in the incorporation of label and the parallel loss of lysine binding ability. In the case of kringle 4, only one-half of the lysine binding sites could be inactivated, but the modified and unmodified forms could be separated by affinity chromatography. The modified form contained 1 mol of 2-hydroxy-5-nitrobenzyl groups/mol of kringle 4 and did not bind to lysine-Sepharose. Lysine analogs such as 6-aminohexanoic acid protected kringle 4 against modification. Peptide-mapping studies on this form showed that essentially all of the label was in two chymotryptic peptides containing a tryptophan corresponding to Trp426 in the plasminogen sequence. Competition experiments with anti-kringle 4 antibodies having an affinity for the lysine binding site showed that the binding of 2-hydroxy-5-nitrobenzyl-kringle 4 to antibodies was about 10 times weaker than for unmodified kringle 4. These results indicate that the integrity of specific tryptophan residue is critical to the binding of lysine and related amino acids to kringle 4of human plasminogen.     

Purification and characterization of a recombinant anti-angiogenic kringle fragment expressed in Escherichia coli: Purification and characterization of a tri-kringle fragment from human apolipoprotein (a) (kringle IV (9)-kringle IV (10)-kringle V)

A kringle fragment (type IV (9)-IV (10)-V) from human apolipoprotein (a) (called LK68) was expressed in an inclusion body in Escherichia coli. The LK68 in this inclusion body was rendered soluble with urea, and efficiently refolded via oxidation in the presence of re-dox couple. The refolded LK68 was then purified via two steps of ion exchange chromatography, concentrated via preparative reversed-phase chromatography, and freeze-dried, at a final yield of approximately 30%. The purified LK68 exhibited profound affinity for lysine and fibrinogen, which suggests the proper folding of the kringle fragment, and also indicates that the native characteristics of apolipoprotein (a) were preserved. The purified LK68 was determined to be highly homogeneous upon reversed-phase HPLC analysis and size-exclusion HPLC analysis, in the presence of 20% (v/v) acetonitrile. However, on size-exclusion HPLC analysis without acetonitrile, it was determined to be somewhat heterogeneous, and this was corroborated by native analyses, including native PAGE and IEF.     

Oxidation of apolipoprotein(a) inhibits kringle-associated lysine binding: the loss of intrinsic protein fluorescence suggests a role for tryptophan residues in the lysine binding site.

Lipoprotein(a) [Lp(a)] is a low-density lipoprotein complex consisting of apolipoprotein(a) [apo(a)] disulfide-linked to apolipoprotein B-100. Lp(a) has been implicated in atherogenesis and thrombosis through the lysine binding site (LBS) affinity of its kringle domains. We have examined the oxidative effect of 2,2'-azobis-(amidinopropane) HCl (AAPH), a mild hydrophilic free radical initiator, upon the ability of Lp(a) and recombinant apo(a), r-apo(a), to bind through their LBS domains. AAPH treatment caused a time-dependent decrease in the number of functional Lp(a) or r-apo(a) molecules capable of binding to fibrin or lysine-Sepharose and in the intrinsic protein fluorescence of both Lp(a) and r-apo(a). The presence of a lysine analogue during the reaction prevented the loss of lysine binding and provided a partial protection from the loss of tryptophan fluorescence. The partial protection of fluorescence by lysine analogues was observed in other kringle-containing proteins, but not in proteins lacking kringles. No significant aggregation, fragmentation, or change in conformation of Lp(a) or r-apo(a) was observed as assessed by native or SDS-PAGE, light scattering, retention of antigenicity, and protein fluorescence emission spectra. Our results suggest that AAPH destroys amino acids in the kringles of apo(a) that are essential for lysine binding, including one or more tryptophan residues. The present study, therefore, raises the possibility that the biological roles of Lp(a) may be mediated by its state of oxidation, especially in light of our previous study showing that the reductive properties of sulfhydryl-containing compounds increase the LBS affinity of Lp(a) for fibrin.     

Quantitative evaluation of the contribution of weak lysine-binding sites present within apolipoprotein(a) kringle IV types 6-8 to lipoprotein(a) assembly

During lipoprotein(a) (Lp(a)) assembly, non-covalent interactions between apolipoprotein(a) (apo(a)) and low density lipoprotein precede specific disulfide bond formation. Studies have shown that the non-covalent step involves an interaction between the weak lysine-binding sites (WLBS) present within each of apo(a) kringle IV types 6, 7, and 8 (KIV(6-8)), and two lysine residues (Lys(680) and Lys(690)) within the NH(2) terminus of the apolipoprotein B-100 (apoB) component of low density lipoprotein. In the present study, we introduced single point mutations (E56G) into each of the WLBS present in apo(a) KIV(6-8) and expressed these mutations in the context of a 17-kringle (17K) recombinant apo(a) variant. Single mutations that disrupt the WLBS in KIV(6), KIV(7), and KIV(8), as well as mutants that disrupt the WLBS in both KIV(6) and KIV(7), or both KIV(7) and KIV(8), were assessed for their ability to form non-covalent and covalent Lp(a) complexes. Our results demonstrate that both apo(a) KIV(7) and KIV(8), but not KIV(6), are required for maximally efficient non-covalent and covalent Lp(a) assembly. Single mutations in the WLBS of KIV(7) or KIV(8) resulted in a 3-fold decrease in the affinity of 17K recombinant apo(a) for apoB, and a 20% reduction in the rate of covalent Lp(a) formation. Tandem mutations in the WLBS in both KIV(7) and KIV(8) resulted in a 13-fold reduction in the binding affinity between apo(a) and apoB, and a 75% reduction in the rate of the covalent step of Lp(a) formation. We also showed that KIV(7) and KIV(8) specifically bind with high affinity to apoB-derived peptides containing Lys(690) or Lys(680), respectively. Taken together, our data demonstrate that specific interactions between apo(a) KIV(7) and KIV(8) and Lys(680) and Lys(690) in apoB mediate a high affinity non-covalent interaction between apo(a) and low density lipoprotein, which dictates the efficiency of covalent Lp(a) formation.     

Recombinant kringle IV-10 modules of human apolipoprotein(a): structure, ligand binding modes, and biological relevance

The kringle modules of apolipoprotein(a) [apo(a)] of lipoprotein(a) [Lp(a)] are highly homologous with kringle 4 of plasminogen (75-94%) and like the latter are autonomous structural and functional units. Apo(a) contains 14-37 kringle 4 (KIV) repeats distributed into 10 classes (1-10). Lp(a) binds lysine-Sepharose via a lysine binding site (LBS) located in KIV-10 (88% homology with plasminogen K4). However, the W72R substitution that occurs in rhesus monkeys and occasionally in humans leads to impaired lysine binding capacity of KIV-10 and Lp(a). The foregoing has been investigated by determining the structures of KIV-10/M66 (M66 variant) in its unliganded and ligand [epsilon-aminocaproic acid (EACA)] bound modes and the structure of recombinant KIV-10/M66R72 (the W72R mutant). In addition, the EACA liganded structure of a sequence polymorph (M66T in about 42-50% of the human population) was reexamined (KIV-10/T66/EACA). The KIV-10/M66, KIV-10/M66/EACA, and KIV-10/T66/EACA molecular structures are highly isostructural, indicating that the LBS of the kringles is preformed anticipating ligand binding. A displacement of three water molecules from the EACA binding groove and a movement of R35 bringing the guanidinium group close to the carboxylate of EACA to assist R71 in stabilizing the anionic group of the ligand are the only changes accompanying ligand binding. Both EACA structures were in the embedded binding mode utilizing all three binding centers (anionic, hydrophobic, cationic) like plasminogen kringles 1 and 4. The KIV-10/T66/EACA structure determined in this work differs from one previously reported [Mikol, V., Lo Grasso, P. V. and, Boettcher, B. R. (1996) J. Mol. Biol. 256, 751-761], which crystallized in a different crystal system and displayed an unbound binding mode, where only the amino group of EACA interacted with the anionic center of the LBS. The remainder of the ligand extended into solvent perpendicular to the kringle surface, leaving the hydrophobic pocket and the cationic center of the LBS unoccupied. The structure of recombinant KIV-10/M66R72 shows that R72 extends along the ligand binding groove parallel to the expected position of EACA toward the anionic center (D55/D57) and makes a salt bridge with D57. Thus, the R72 side chain mimics ligand binding, and loss of binding ability is the result of steric blockage of the LBS by R72 physically occupying part of the site. The rhesus monkey lysine binding impairment is compared with that of chimpanzee where KIV-10 has been shown to have a D57N mutation instead.     

Vanadyl(IV) conalbumin. I. Metal binding site configurations

Electron paramagnetic resonance (epr) and ultraviolet difference spectroscopy of vanadyl conalbumin indicate a binding capacity of two vanadyl ions, VO²⁺, per protein molecule in the pH 8–11 range; the binding capacity drops in the pH 6–8 range with an apparent pK_a′ = 6.6. Iron-saturated conalbumin does not bind vanadyl ions, which suggests common binding sites for iron and vanadium. Ultraviolet difference spectroscopy indicates 2–3 tyrosines are involved in the binding of each metal ion; pH titrations show that three protons are released per vanadyl ion bound by conalbumin. Room and liquid nitrogen temperature X-band (ca. 9.2–9.5 gHz) epr spectra show that the vanadyl ion binds in three magnetically distinct environments (A, B, and C) that arise from interconvertible metal site configurations. These configurations are probably examples of conformational substrates of the protein. Q-band (ca 34 gHz) epr spectra resolve the spectral features more clearly and show that two configurations (A and B) have axially symmetric epr parameters but angles of noncoincidence of 12° and 8°, respectively, between the z components of the g and nuclear hyperfine tensors. The third (C) configuration has rhombic magnetic symmetry and a 6° angle of noncoincidence. These observations demonstrate that the metal sites are of low symmetry and are flexible in their geometry about the metal.The isotropic g and nuclear hyperfine tensor values and the line widths used in computer-simulated epr spectra are consistent with four oxygen or three oxygen and one nitrogen donor atoms binding equatorially to the VO²⁺ group. The apparent stability constant indicates that vanadyl ion binds to conalbumin approximately twelve orders of magnitude more weakly than iron to human serotransferrin but still sufficiently strongly to overcome hydrolysis.     

Lysine-phosphatidylcholine adducts in kringle V impart unique immunological and potential pro-inflammatory properties to human apolipoprotein(a)

Lipoprotein(a), Lp(a), an athero-thrombotic risk factor, reacts with EO6, a natural monoclonal autoantibody that recognizes the phophorylcholine (PC) group of oxidized phosphatidylcholine (oxPtdPC) either as a lipid or linked by a Schiff base to lysine residues of peptides/proteins. Here we show that EO6 reacts with free apolipoprotein(a) apo(a), its C-terminal domain, F2 (but not the N-terminal F1), kringle V-containing fragments obtained by the enzymatic digestion of apo(a) and also kringle V-containing apo(a) recombinants. The evidence that kringle V is critical for EO6 reactivity is supported by the finding that apo(a) of rhesus monkeys lacking kringle V did not react with EO6. Based on the previously established EO6 specificity requirements, we hypothesized that all or some of the six lysines in human kringle V are involved in Schiff base linkage with oxPtdPC. To test this hypothesis, we made use of a recombinant lysine-containing apo(a) fragment, rIII, containing kringle V but not the protease domain. EO6 reacted with rIII before and after reduction to stabilize the Schiff base and also after extensive ethanol/ether extraction that yielded no lipids. On the other hand, delipidation of the saponified product yielded an average of two mol of phospholipids/mol of protein consistent with direct analysis of inorganic phosphorous on the non-saponified rIII. Moreover, only two of the six theoretical free lysine amino groups per mol of rIII were unavailable to chemical modification by 2,4,6-trinitrobenzene sulfonic acid. Finally, rIII, like human apo(a), stimulated the production of interleukin 8 in THP-1 macrophages in culture. Together, our studies provide evidence that in human apo(a), kringle V is the site that reacts with EO6 via lysine-oxPtdPC adducts that may also be involved in the previously reported pro-inflammatory effect of apo(a) in cultured human macrophages.     

Expression and purification of kringle 4-type 2 of human apolipoprotein (a) in Escherichia coli.

The most frequently occurring kringle 4 domain of human apolipoprotein (a), Kringle 4-subtype 2 (K4(2)), was expressed as a fusion protein with the maltose binding protein in Escherichia coli using the "tac" promoter. Although the fusion protein was expressed without a signal sequence, 25% was secreted into the periplasmic space; the remainder was found associated with the soluble cytosolic fraction. The fusion protein was readily isolated from whole cell lysate by amylose agarose affinity chromatography. Although a factor Xa cleavage site was engineered into the fusion protein, it was found that release of the K4(2) protein was most conveniently achieved by proteolysis with subtilisin A. The cleavage product produced in this way was shown to be intact K4(2) with only the first three amino acid residues of the leading flanking peptide missing, as judged by N-terminal sequence analysis. K4(2) was isolated from the hydrolysate by FPLC on a Mono-Q column with a yield of 170 +/- 30 micrograms/g wet cells. The resulting protein was monomeric in phosphate-buffered saline as judged by size-exclusion chromatography and appeared to be folded as shown by spectroscopic and immunological assays. The recombinant K4(2) did not bind to either lysine- or proline-Sepharose, suggesting that the ligand binding activities of lipoprotein (a) may reside in the other kringle domains of apolipoprotein (a).     

Production of the kringle fragments of human apolipoprotein(a) by continuous lactose induction strategy

A novel lactose induction strategy for the production of rhLK68, the kringle fragments of human apolipoprotein(a) (apo(a)) as a novel anti-angiogenic protein, was investigated. A scale-up of the production was accompanied by a decrease in expression level, and severe aggregation occurred during the solubilization of rhLK68 from the inclusion body during a conventional single introduction of lactose. To overcome this problem, a continuous induction strategy was applied where lactose was mixed with glycerol and fed continuously in a dissolved oxygen (DO)-stat manner. With the sub-optimal feed medium consisted of 1:50 of lactose/glycerol (w/w), the expression level reached 16% of the total cellular protein, which was 1.6-fold higher than that obtained from the conventional lactose induction. Moreover, the solubilization yield of rhLK68 from the inclusion body increased from 30 +/- 5 to 85 +/- 3% compared to the conventional single introduction of lactose. This result suggests that the continuous lactose induction strategy beneficially influenced the expression level of rhLK68 and the quality of its inclusion body.     

Comparative binding properties of metallobleomycins with DNA 10-mers.

Properties of the interaction of bleomycin (Blm) and metallobleomycins [M = Zn, Cu(II), Fe(III), and HO(2)-Co(III)] with site-specific and nonspecific DNA oligomers, d(GGAAGCTTCC)(2) (I) and d(GGAAATTTCC)(2) (II), respectively, were investigated. With both 10-mers association constants increased in the series Blm A(2), ZnBlm A(2), Cu(II)Blm A(2), Fe(III)Blm A(2), and HO(2)-Co(III)Blm A(2). Generally, the metallobleomycins were bound with a modestly higher affinity to I. One-dimensional (1)H NMR spectra of the imino proton region of I in the presence of this series of compounds revealed that Blm and Zn- and CuBlm bind in fast exchange on the NMR time scale, while the Fe and Co complexes bind in slow exchange. Blm, ZnBlm, and Cu(II)Blm caused little perturbation of the UV circular dichroism spectrum of I or II. In contrast, Fe(III)Blm and HO(2)-Co(III)Blm induced hypochromic effects in the CD spectrum of I and altered the spectrum of II to a smaller extent. On the basis of these results, the DNA binding structures and properties of Blm A(2), ZnBlm A(2), and CuBlm A(2) differ substantially from those of Fe(III)Blm A(2) and HO(2)-Co(III)Blm A(2).     

Inhibition of angiogenesis and angiogenesis-dependent tumor growth by the cryptic kringle fragments of human apolipoprotein(a)

Apolipoprotein(a) (apo(a)) contains tandemly repeated kringle domains that are closely related to plasminogen kringle 4, followed by a single kringle 5-like domain and an inactive protease-like domain. Recently, the anti-angiogenic activities of apo(a) have been demonstrated both in vitro and in vivo. However, its effects on tumor angiogenesis and the underlying mechanisms involved have not been fully elucidated. To evaluate the anti-angiogenic and anti-tumor activities of the apo(a) kringle domains and to elucidate their mechanism of action, we expressed the last three kringle domains of apo(a), KIV-9, KIV-10, and KV, in Escherichia coli. The resultant recombinant protein, termed rhLK68, exhibited a dose-dependent inhibition of basic fibroblast growth factor-stimulated human umbilical vein endothelial cell proliferation and migration in vitro and inhibited the neovascularization in chick chorioallantoic membranes in vivo. The ability of rhLK68 to abrogate the activation of extracellular signal-regulated kinases appears to be responsible for rhLK68-mediated anti-angiogenesis. Furthermore, systemic administration of rhLK68 suppressed human lung (A549) and colon (HCT-15) tumor growth in nude mice. Immunohistochemical examination and in situ hybridization analysis of the tumors showed a significant decrease in the number of blood vessels and the reduced expression of vascular endothelial growth factor, basic fibroblast growth factor, and angiogenin, indicating that suppression of angiogenesis may have played a significant role in the inhibition of tumor growth. Collectively, these results suggest that a truncated apo(a), rhLK68, is a potent anti-angiogenic and anti-tumor molecule.     

Localization of a lysine residue near the site of initiating substrate binding of T7 bacteriophage RNA polymerase

A highly selective affinity label was introduced into the T7 phage RNA polymerase by means of GMP ortho-formylphenyl ester and [alpha-32P]UTP nearby the enzyme's active site, which was located using limited cleavage technique. Hydroxylamine, bromine, N-chlorosuccinimide, and cyanogen bromide were employed as the reagents. Analysis of gel-electrophoretic patterns of the cleavage products led to a conclusion that Lys631 is the target of labelling. The region nearby this residue has a high degree of sequence homology with regions of RNA polymerases from T3 and SP6 phages and yeast mitochondria.