Proceedings of the National Academy of Sciences

Farrell, F. T.; Jones, L. E.

doi: N/Apmid: 16594041

Let M be a complete (connected) Riemannian manifold having finite volume and whose sectional curvatures lie in the interval c 1, c 2 with -∞ < c 1unkc 2 < 0. Then any proper homotopy equivalence h:N → M from a topological manifold N is properly homotopic to a homeomorphism, provided the dimension of M is >5. In particular, if M and N are both compact (connected) negatively curved Riemannian manifolds with isomorphic fundamental groups, then M and N are homeomorphic provided dim M unk 3 and 4. {If both are locally symmetric, this is a consequence of Mostow's rigidity theorem Mostow, G. D. (1967) Publ. Inst. Haut. Etud. Sci. 34, 53-104.} When M has infinite volume we can still calculate the surgery L-groups of π1 M, even when dim M = 3, 4, or 5, provided M is locally symmetric. An identification of the weak homotopy type of the homeomorphism group of (finite volume) M is also made through a stable range. We have previously announced these results for the special case that c 1 = c 2 = -1.

Harpel, P C; Gordon, B R; Parker, T S

doi: N/Apmid: 2524834

Lipoprotein (a) Lp(a) is a plasma component whose concentration is related to the development of atherosclerosis, although the underlying mechanisms are not known. Lp(a) contains a unique structure, apolipoprotein (a), that shares partial homology with plasminogen. We now report that plasmin catalyzes the binding of Lp(a) to both immobilized fibrinogen and fibrin in a manner analogous to our previously reported studies with plasminogen. Plasmin treatment of immobilized fibrinogen induces a 3.7-fold increase in Lp(a) binding. Low density lipoprotein, molecules similar to Lp(a) but lacking apolipoprotein (a), bind poorly to immobilized fibrinogen and binding is not increased by plasmin. Trypsin but not neutrophil elastase also increases the binding of Lp(a) to fibrinogen. Lp(a) also complexes to plasmin-fibrinogen digests, and binding increases in proportion to the time of plasmin-induced fibrinogen degradation. Lp(a) binding is lysine-binding site dependent as it is inhibited by epsilon-aminocaproic acid. Lp(a) inhibits the binding of plasminogen to plasmin-modified immobilized fibrinogen, indicating that both molecules compete for similar lysine-binding sites. These findings demonstrate an affinity between Lp(a) and protease-modified fibrinogen or fibrin and thereby provide a potential mechanism to explain the association between thrombosis, coronary atherosclerosis, and increased blood concentrations of Lp(a).

Robbins, A H; Stout, C D

doi: N/Apmid: 2726740

The structure of activated pig heart aconitase citrate(isocitrate) hydro-lyase, EC 4.2.1.3 containing a 4Fe-4S cluster has been refined at 2.5-A resolution to a crystallographic residual of 18.2%. Comparison of this structure to the recently determined 2.1-A resolution structure of the inactive enzyme containing a 3Fe-4S cluster, by difference Fourier analysis, shows that upon activation iron is inserted into the structure isomorphously. The common atoms of the 3Fe-4S and 4Fe-4S cores agree within 0.1 A; the three common cysteinyl S gamma ligand atoms agree within 0.25 A. The fourth ligand of the Fe inserted into the 3Fe-4S cluster is a water or hydroxyl from solvent, consistent with the absence of a free cysteine ligand in the enzyme active site cleft and the isomorphism of the two structures. A water molecule occupies a similar site in the crystal structure of the inactive enzyme.

Fefferman, C. L.; Seco, L. A.

doi: N/Apmid: 16594042

Let E(Z, N) be the ground-state energy of N quantized electrons and a single nucleus of charge Z. For fixed Z, E(Z, N) is independent of N for N ≥ N critical(Z). Physically, this means that at most N critical electrons can bind to the nucleus. We prove that N critical ≤ Z + CZa with a = 0.84.

Aboud-Pirak, E; Hurwitz, E; Bellot, F; Schlessinger, J; Sela, M

doi: N/Apmid: 2786202

Monoclonal antibodies that recognize the extracellular domain of the epidermal growth factor receptor (mAb108) were conjugated with doxorubicin through a dextran bridge. Several antibody-drug conjugates, containing different amounts of doxorubicin, retained binding capacity to human epidermoid carcinoma (KB) cells overexpressing epidermal growth factor receptors. Slight decrease in drug cytotoxicity was seen in in vitro tests, as determined either by inhibition of thymidine incorporation into cells or by reduction in number and size of KB-cell colonies. Yet, when tested in vivo against KB tumor xenografted into nude mice, the anti-epidermal growth factor-receptor drug conjugates with high drug-substitution levels were significantly more effective than free doxorubicin, antibody alone, mixture of dextran-doxorubicin and antibody, or drug conjugated with irrelevant antibody. When the labile covalent bonds linking antibody to dextran bridge were stabilized by reduction, the therapeutic efficacy of the conjugate was markedly decreased. These results show that antibodies against the extracellular domain of the epidermal growth factor can deliver doxorubicin specifically and efficiently to tumor sites that express high receptor levels exerting a specific antitumor effect.

Pierce, G B; Lewellyn, A L; Parchment, R E

doi: N/Apmid: 2726743

The malignant growth potential of embryonal carcinoma cells may be controlled by environmental factors. For example, embryonal carcinoma cells placed into normal blastocysts may not exhibit the continued growth expected of malignant cells but rather may lose all aspects of the malignant phenotype and become apparently normal embryonic cells. Loss of the malignant phenotype of embryonal carcinoma cells occurs early in these injected blastocysts and has been used as the basis of assays to study the mechanisms of regulation of embryonal carcinoma by the blastocyst. In this regard, P19, an embryonal carcinoma that makes midgestation chimeras, was regulated by blastocele fluid plus contact with trophectoderm but not by blastocele fluid plus contact with inner cell mass (ICM). In contrast, ECa 247, which makes trophectoderm, was regulated by exposure to blastocele fluid plus contact with trophectoderm or ICM. During the course of these experiments, dead embryonal carcinoma and ICM cells were observed, and blastocele fluid was then shown to kill ECa 247 and normal ICM cells of early blastocysts with trophectodermal potential. P19 cells and ICM cells with potential to make the embryo were not killed by blastocele fluid. Programmed cell death occurs in the ICM of the blastocyst during the transition from early (when ICM has the potential to make trophectoderm) to late (when the ICM lacks the potential to make trophectoderm). It is postulated that this programmed cell death is designed to eliminate redundant ICM cells with trophectodermal potential, and its mechanism of action is mediated by epigenetic factors in blastocele fluid.

Terwilliger, E F; Godin, B; Sodroski, J G; Haseltine, W A

doi: N/Apmid: 2726755

The construction and properties of an infectious human immunodeficiency virus (HIV) that expresses the bacterial gene chloramphenicol acetyltransferase are described. This virus can be used in vitro to screen for drugs that inhibit HIV infection. The marked virus may also be used to trace the routes of infection from the site of inoculation in animal experiments.

Shuman, S

doi: N/Apmid: 2542933

Vaccinia virus encapsidates a Mr 32,000 type IDNA topoisomerase. Although the vaccinia gene encoding the topoisomerase is essential for virus growth, the role of the enzyme in vivo remains unclear. In the present study, the physiologic consequences of vaccinia topoisomerase action have been examined in a heterologous system, Escherichia coli. The vaccinia topoisomerase gene was inducibly expressed in an int-lambda lysogen BL21(DE3) using a T7 RNA polymerase-based transcription system. Expression of active topoisomerase in this context resulted in recA-dependent lysogenic induction as well as cell lysis. Surprisingly, topoisomerase expression also effected a 200-fold increase in the titer of infectious lambda phage, apparently by promoting int-independent prophage excision. This effect was not observed during lysogenic induction with nalidixic acid. Restriction analysis of genomic DNA from plaque-purified excisants revealed (in 10 of 10 cases) gross alterations of the DNA structure around the att site relative to the structure of the parental phage DE3. It is construed therefore that vaccinia DNA topoisomerase I acts to promote illegitimate recombination in E. coli.

Thompson, C C; Evans, R M

doi: N/Apmid: 2726731

The effects of thyroid hormones are mediated through nuclear receptor proteins that modulate the transcription of specific genes in target cells. We previously isolated cDNAs encoding two different mammalian thyroid hormone receptors, one from human placenta (hTR beta) and the other from rat brain (rTR alpha), and showed that their in vitro translation products bind thyroid hormones with the characteritistic affinities of the native thyroid hormone receptor. We now demonstrate that both of the cloned receptors activate transcription from a thyroid hormone-responsive promoter in a hormone-dependent manner, with rTR alpha eliciting a greater response than hTR beta. The putative functional domains of the thyroid hormone receptors were examined by creating chimeric thyroid hormone/glucocorticoid receptors, producing receptors with hybrid functional properties. These experiments support the proposal that the thyroid hormone receptors are composed of interchangeable functional domains, and indicate that the mechanism of hormone-inducible gene regulation has been conserved in steroid and thyroid hormone receptors.

Lew, A M; Langford, C J; Anders, R F; Kemp, D J; Saul, A; Fardoulys, C; Geysen, M; Sheppard, M

doi: N/Apmid: 2471191

The monoclonal antibody 5C10/66 was shown to afford strong protection in mice against fulminating Plasmodium chabaudi adami infection. This was remarkable, as immunity to this organism is regarded to be mainly T-cell mediated. This antibody identified a 250-kDa molecule in schizonts and an 83-kDa fragment in merozoites. A cDNA clone selected by 5C10/66 was the homologue of the Plasmodium falciparum precursor to the major merozoite surface antigen (PMMSA). Comparison with the P. falciparum sequence showed that the P. chabaudi adami clone encoded the middle portion of the gene and that it can also be divided into variable and conserved blocks. Screening of a set of all possible octamer peptides predicted by the cDNA clone revealed that the core epitope of 5C10/66 was Glu-Thr-Thr-Glu-Thr. This region resides in a variable block of PMMSA.