Cdc7 is an evolutionarily conserved kinase that regulates S phase by promoting replication origin activation. Down-regulation of Cdc7 by small interfering RNA in a variety of tumor cell lines causes an abortive S phase, leading to cell death by either p53-independent apoptosis or aberrant mitosis. Unlike replication fork blockade, Cdc7-depleted tumor cells do not elicit a robust checkpoint response; thus, inhibitory signals preventing additional cell cycle progression are not generated. In normal fibroblasts, however, a p53-dependent pathway actively prevents progression through a lethal S phase in the absence of sufficient Cdc7 kinase. We show that in this experimental system, p53 is required for the lasting maintenance of this checkpoint and for cell viability. With this work we reveal and begin to characterize a novel mechanism that regulates DNA synthesis in human cells, and we suggest that inhibition of Cdc7 kinase represents a promising approach for the development of a new generation of anticancer agents.

Human bronchial epithelial cells were treated in vitro with interferon-gamma or tumor necrosis factor-alpha to assess their effect on transepithelial ion transport. Short-circuit current measurements revealed that Na(+) absorption was markedly inhibited by interferon-gamma (10-1,000 U/ml). The cystic fibrosis transmembrane conductance regulator was also downregulated by interferon-gamma as evident at the protein level and by the decrease in the cAMP-dependent current. On the other hand, interferon-gamma caused an increase of the current elicited by apical UTP application, which is due to the activity of Ca(2+)-dependent Cl(-) channels. Tumor necrosis factor-alpha caused few changes in ion transport. Transepithelial fluid transport was measured in normal and cystic fibrosis cells. At rest, both types of cells showed an amiloride-sensitive fluid absorption that was inhibited by interferon-gamma but not by tumor necrosis factor-alpha. Our results show that interferon-gamma alters the transepithelial ion transport of cultured bronchial cells. This effect may change the ion composition and/or volume of periciliary fluid.

In contrast to two-chain urokinase (uPA), a chemical conjugate between uPA and native saporin (a cytotoxic plant seed ribosome-inactivating protein) did not require plasminogen activator inhibitors to be internalized. To dissect this pathway, we constructed a chimera consisting of the amino-terminal fragment (ATF) of human urokinase fused to a saporin isoform (SAP-3). The chimeric ATF-SAP toxin was expressed in Escherichia coli, purified, and characterized for its ribosome-inactivating activity. Besides being a potent inhibitor of protein synthesis in cell-free assays, ATF-SAP was specifically cytotoxic toward cells expressing human uPAR. Competition experiments indicated that both the human uPAR and the LDL-related receptor protein are involved in mediating the cell killing ability of ATF-SAP. We conclude that neither plasminogen activator inhibitors nor the catalytic moiety of urokinase are necessary to initiate these internalization pathways. Thus, saporin may play a role similar to plasminogen activator inhibitors in its ability to trigger internalization of uPAR-bound ligands through endocytic receptors.

We have expressed in Escherichia coli five isoforms of saporin, a single-chain ribosome-inactivating protein (RIP). Translation inhibition activities of the purified recombinant polypeptides in itro were compared with those of recombinant dianthin 30, a less potent and closely related RIP, and of ricin A chain. Dianthin 30, and a saporin isoform encoded by a cDNA from leaf tissue (SAP-C), both had about one order of magnitude lower activity in translation inhibition assays than all other isoforms of saporin tested. We recently demonstrated that saporin extracted from seeds of Saponaria officinalis binds to α2-macroglobulin receptor (α2MR ; also termed low density lipoprotein-receptor-related-protein), indicating a general mechanism of interaction of plant RIPs with the α2MR system [Cavallaro, Nykjaer, Nielsen and Soria (1995) Eur. J. Biochem.

CF respiratory epithelial cells exhibit a basal dysregulated production of IL-8 that partially correlates to enhanced NF-kappaB activity. Our data corroborate the hypothesis of a basal exaggerated inflammatory response in the CF respiratory epithelium.

In this work, we have devised an intracellular immunization strategy for the expression in high amounts of ATF-saporin, a targeted chimeric toxin constituted by the ATF receptor binding domain of human urokinase and the plant ribosome-inactivating protein saporin, which has been shown to be highly cytotoxic to target cells. This strategy may allow the production of highly toxic secretory proteins in eukaryotic cells, avoiding cell suicide caused by autointoxication. The procedure consists of equipping host cells with cytosolic neutralizing antibodies directed toward the toxic domain of the heterologous polypeptide. We show that this intracellular immunization is essential for the synthesis of correctly folded, biologically active ATF-SAP in the high amounts needed to investigate its in vivo anti-metastatic potential. Such a strategy should be generally useful for the production of toxic molecules of therapeutic value whose folding and maturation require transit through the eukaryotic secretory pathway. Fabbrini, M. S., Carpani, D., Soria, M. R., Ceriotti, A. Cytosolic immunization allows the expression of preATF-saporin chimeric toxin in eukaryotic cells.

Artificial chromosomes have been claimed to be the ideal vector for gene therapy, but their use has been hampered by an inability to produce stable and well designed molecules. We have used a structurally defined minichromosome to clone the human cystic fybrosis transmembrane conductance regulator (CFTR) locus. To guarantee the presence of the proper regulatory elements, we used the 320 kb yeast artificial chromosome (YAC) 37AB12 with the intact CFTR gene and upstream sequences. The resulting minichromosome was analyzed for the presence of the entire CFTR gene and for its functional activity by molecular and functional methods. We have identified clones showing the presence of both the transcript and the CFTR protein. Moreover, in the same clones, a chloride secretory response to cAMP was detected. Mitotic and molecular stability after prolonged growth without selection demonstrated that the constructs were stable. This is the first example of a structurally known minichromosome made to contain an active therapeutic gene.

A fluorescent derivative of a chimeric toxin between human pro-urokinase and the plant ribosome-inactivating protein saporin (p-uPA-Sap(TRITC)), has been prepared in order to study the endocytosis of this potentially antimetastatic conjugate in the murine model cell line LB6 clone19 (Cl19) transfected with the human urokinase receptor gene. The physiological internalization of urokinase-inhibitor complexes is triggered by the interaction of plasminogen inhibitors (PAIs) with receptors belonging to the low density lipoprotein-related receptor protein (LRP) family, and involves a macro-quaternary structure including uPAR, LRP, and PAIs. However, in contrast to this mechanism, we observed a two-step process: first, the urokinase receptor (uPAR) acts as the anchoring factor on the plasma membrane; subsequently, LRP acts as the endocytic trigger. Once the chimera is bound to the plasma membrane by interaction with uPAR, we suggest that a possible exchange may occur to transfer the toxin to LRP via the saporin moiety and begin the internalization. So an unusual endocytic process is described, where the toxin enters the cell via a receptor different from that used to bind the plasma membrane.