Improved radiotracing of oxytocin receptor-expressing tumours using the new [111In]-DOTA-Lys8-deamino-vasotocin analogue.

Oxytocin receptors (OTR) have been described in a number of tumours of different origin, and represent a new target for specific radiolabelled oxytocin (OT) analogues in cancer diagnosis and therapy. By linking the DOTA chelating agent to position 8 of the deamino derivative of Lys(8)-vasotocin (dLVT), we obtained a new compound (DOTA-dLVT) with the following characteristics: (1) it forms a monomeric and stable compound that binds to OTR with an affinity comparable to that of the endogenous OT ligand; (2) it is characterised by a very good selectivity profile for the human OTR, with a low affinity binding to the closely related V1a, V1b and V2 vasopressin receptor subtypes; (3) it induces rapid and persistent receptor internalisation and (4) when radiolabelled, [(111)In]-DOTA-dLVT is efficiently and selectively taken up by OTR-positive tumours grown in mice. These features makes radiolabelled DOTA-dLVT a very good candidate for the radiotargeting of OTR-expressing tumours.

Structure and regulation of the CDK5-p25(nck5a) complex.

CDK5 plays an indispensable role in the central nervous system, and its deregulation is involved in neurodegeneration. We report the crystal structure of a complex between CDK5 and p25, a fragment of the p35 activator. Despite its partial structural similarity with the cyclins, p25 displays an unprecedented mechanism for the regulation of a cyclin-dependent kinase. p25 tethers the unphosphorylated T loop of CDK5 in the active conformation. Residue Ser159, equivalent to Thr160 on CDK2, contributes to the specificity of the CDK5-p35 interaction. Its substitution with threonine prevents p35 binding, while the presence of alanine affects neither binding nor kinase activity. Finally, we provide evidence that the CDK5-p25 complex employs a distinct mechanism from the phospho-CDK2-cyclin A complex to establish substrate specificity.

An effector region in Eps8 is responsible for the activation of the Rac-specific GEF activity of Sos-1 and for the proper localization of the Rac-based actin-polymerizing machine.

Genetic and biochemical evidence demonstrated that Eps8 is involved in the routing of signals from Ras to Rac. This is achieved through the formation of a tricomplex consisting of Eps8-E3b1-Sos-1, which is endowed with Rac guanine nucleotide exchange activity. The catalytic subunit of this complex is represented by Sos-1, a bifunctional molecule capable of catalyzing guanine nucleotide exchange on Ras and Rac. The mechanism by which Sos-1 activity is specifically directed toward Rac remains to be established. Here, by performing a structure-function analysis we show that the Eps8 output function resides in an effector region located within its COOH terminus. This effector region, when separated from the holoprotein, activates Rac and acts as a potent inducer of actin polymerization. In addition, it binds to Sos-1 and is able to induce Rac-specific, Sos-1-dependent guanine nucleotide exchange activity. Finally, the Eps8 effector region mediates a direct interaction of Eps8 with F-actin, dictating Eps8 cellular localization. We propose a model whereby the engagement of Eps8 in a tricomplex with E3b1 and Sos-1 facilitates the interaction of Eps8 with Sos-1 and the consequent activation of an Sos-1 Rac-specific catalytic ability. In this complex, determinants of Eps8 are responsible for the proper localization of the Rac-activating machine to sites of actin remodeling.

Protein kinase C in cell signaling: strategies for the development of selective inhibitors.

Protein kinase C plays a central role in the cellular signaling pathway for the lipophilic second messenger sn-1,2-diacylglycerol, which is involved in many biological responses, including tumor promotion and inflammation. A major effort has been directed at understanding diversity within this system in order to develop strategies for selective inhibition. Two classes of ligands for the regulatory domain of protein kinase C have been identified which, although they function in vitro as activators of the enzyme, paradoxically behave in vivo as partial antagonists. Identification of targets for the phorbol esters distinct from protein kinase C argues that antagonists acting on the regulatory and catalytic domains of protein kinase C will have different spectra of action.

Close similarity of baculovirus-expressed n-chimaerin and protein kinase C alpha as phorbol ester receptors.

n-Chimaerin is a recently described phorbol ester receptor that shares homology in its N-terminal region with the cysteine-rich zinc finger domain of protein kinase C. We have expressed n-chimaerin in insect cells using the baculovirus system and have used the isolated, recombinant n-chimaerin to characterize phorbol ester binding and structure-activity relations, lipid requirements, and inhibitor sensitivity. We find that n-chimaerin expressed in the baculovirus system bound [3H]phorbol 12,13-dibutyrate with high affinity (0.17 +/- 0.01 nM). Although having only a single cysteine-rich zinc finger region compared to two for protein kinase C, n-chimaerin thus closely resembled protein kinase C alpha. n-Chimaerin was likewise virtually indistinguishable from protein kinase C alpha in phorbol ester structure-activity relations, in phospholipid requirements, and in inhibition of binding by sphingosine and calphostin C, protein kinase C inhibitors acting on the regulatory domain. We conclude that a number of typical approaches used to implicate protein kinase C in biological function in cells do not discriminate between the n-chimaerin and protein kinase C classes of phorbol ester receptors.

Functional specificity of cytoplasmic and transmembrane tyrosine kinases: identification of 130- and 75-kilodalton substrates of c-fps/fes tyrosine kinase in macrophages.

c-fps/fes encodes a 92-kDa protein-tyrosine kinase (NCP92) that is expressed at the highest levels in macrophages. To determine if c-fps/fes can mediate the action of the colony-stimulating factor 1 (CSF-1) receptor (CSF-1R) and to identify potential targets of c-fps/fes in macrophages, we have overexpressed c-fps/fes in a CSF-1-dependent macrophage cell line. A 30- to 50-fold overexpression of c-fps/fes partially released these cells from their factor dependence by a nonautocrine mechanism, and this correlated with the tyrosine phosphorylation of two proteins of 130 and 75 kDa (P130 and P75). c-fps/fes did not cause tyrosine phosphorylation or activation of CSF-1 dependent targets, including CSF-1R, Shc, and phosphatidylinositol 3-kinase, and conversely, CSF-1 did not induce tyrosine phosphorylation of P130 and P75. P75 appears to be a novel phosphotyrosyl protein, whereas P130 cross-reacts with a known substrate of v-src. P130 and P75 may be direct substrates of c-fps/fes: P130 was tightly associated with NCP92, and the src homology 2 domain of NCP92 specifically bound phosphorylated P130 and P75 but not the CSF-1-induced phosphotyrosyl proteins, consistent with the possibility that P130 and P75 are physiological targets of c-fps/fes. We conclude that although c-fps/fes can functionally substitute for CSF-1R to a certain extent, these tyrosine kinases act largely independently of each other and that P130 and P75 are novel targets whose mechanisms of action may be unrelated to the signalling pathways utilized by receptor tyrosine kinases.

Characterization of ligand and substrate specificity for the calcium-dependent and calcium-independent protein kinase C isozymes.

Analysis of [3H]phorbol-12,13-dibutyrate (PDBu) binding was performed with protein kinase C (PKC)-alpha, -beta 1, -gamma, -delta, -epsilon, -eta, and -zeta produced in Sf9 insect cells using the baculovirus expression system. With the exception of PKC-zeta, all of the PKC isozymes bound [3H]PDBu with high affinity (Kd < 1 nM), either in the presence or in the absence of calcium. Scatchard analysis using 100% phosphatidylserine vesicles revealed slightly lower affinity for the calcium-independent isozymes (PKC-delta, -epsilon, and -eta) than for the calcium-dependent isozymes (PKC-alpha, -beta, and -gamma). Competition for [3H]PDBu binding by different classes of PKC activators showed that 12-deoxyphorbol esters, mezerein, and octahydromezerein likewise possessed lower affinity for the calcium-independent isozymes. The mezerein analog thymeleatoxin was the most marked example, being almost 20-fold less potent for binding to PKC-epsilon and -eta than to PKC-beta 1. In contrast, the indole alkaloids (-)-indolactam V and (-)-octylindolactam V and the postulated endogenous activator 1,2-diacylglycerol bound with similar affinities to all of the PKC isoforms, suggesting that different residues/configurations in the binding sites of the different PKC isozymes might be involved in interaction with the pharmacophore of the activators. The seven PKC isozymes also showed clearly different substrate specificities with exogenous peptide and protein substrates. The heterogeneous behavior of the different members of the PKC family with ligands and substrates may contribute to the heterogeneity of PKC-mediated pathways at the cellular level.

Ligand-dependent transformation by the receptor for human granulocyte/macrophage colony-stimulating factor and tyrosine phosphorylation of the receptor beta subunit.

The receptor for human granulocyte/macrophage colony-stimulating factor (hGMR) is composed of two subunits, alpha and beta, which are both required for high-affinity binding of the ligand. To examine the transforming potential of hGMR, we have transfected cDNAs encoding the receptor alpha and beta subunits into NIH 3T3 cells, which normally do not express GMRs. Introduction of the receptor subunits into these cells resulted in focal transformation, which was dependent on the presence of human granulocyte/macrophage colony-stimulating factor (hGM-CSF) in the culture medium. No transformation was observed when hGM-CSF was replaced with other growth factors such as human epidermal growth factor or human interleukin 3 or when cells were transfected with the alpha or beta subunit alone. Individual conditional transformants isolated after transfection expressed functional hGMRs, were susceptible to transformation by picomolar levels of the ligand, and were capable of anchorage-independent growth in soft agar in the presence but not in the absence of hGM-CSF. Biochemical analysis showed that treatment of these cells with hGM-CSF caused a rapid phosphorylation of the beta subunit and other cellular proteins on tyrosine residues, recapitulating some of the events that take place during GM-CSF signaling in myeloid cells. We conclude that coexpression of the alpha and beta subunits of hGMR in established murine fibroblasts is sufficient to reconstitute a functional receptor, which is capable of causing ligand-dependent transformation. The oncogenic potential of hGMR lends support to the hypothesis that its deregulated or abnormal expression may play a role in leukemogenesis.

Purification and characterization of a milk clotting protease from Mucor bacilliformis.

An acid protease having milk clotting activity has been isolated from Mucor bacilliformis cultures. The enzyme was basically purified by ionic exchange chromatography. An average yield of 29 mg purified product was obtained from 100 mL crude extract. As purity criteria, SDS-PAGE, reverse-phase HPLC, and N-terminal analysis were performed. The protease is a protein composed of a single polypeptide chain with glycine at the N-terminus. The mol wt is approx 32,000, and its amino acid composition is very similar to those of other fungal proteases. As expected, its clotting activity was drastically inhibited by pepstatin A action. On the other hand, its instability against heat treatment and its clotting/proteolytic activity ratio indicate that it may be considered as a potential substitute for bovine chymosin.

Antigenicity of basement membrane fractions obtained from rat seminiferous tubules.

A noncollagenous fraction of basement membrane (D-STBM) obtained from rat testes was submitted to sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis (SDS-PAGE), and eight well-defined bands were detected. A cross-reaction with an antiserum against laminin was revealed by immunoblotting in five bands, with molecular weights ranging from 54 to 64 kDa. No further resolution of these components could be obtained by size exclusion and ionic exchange chromatography. Fifty-two percent of the rats immunized with D-STBM and adjuvants developed a mild multifocal damage of the testis. The lesions were characterized by foci of seminiferous tubules with different degrees of sloughing and/or atrophy of the germinal epithelium. Giant multinucleated cells were frequently seen, and mild interstitial mononuclear cell infiltrates were also detected. By immunofluorescence, deposits of rat IgG with a faint discontinuous linear pattern were observed along the walls of the seminiferous tubules. Circulating antibodies to D-STBM were detected by ELISA in 100% of the rats, whereas in a cross-reaction with laminin antibodies were detected in only 63%. All rats studied revealed a positive delayed type of hypersensitivity (DTH) response to D-STBM. None of the control rats injected with saline and adjuvants presented circulating antibodies to D-STBM or laminin or a positive DTH reaction to D-STBM. Some control group rats (10%) revealed few isolated seminiferous tubules with some degree of sloughing of the germinal epithelium.