Enzymology. Proteases: to each his own

Regulation of basic fibroblast growth factor (bFGF) gene and protein expression following its release from sublethally injured endothelial cells

Basic fibroblast growth factor (bFGF; FGF-2) lacks a signal sequence and thus is not secreted by classical pathways. It has been speculated that one mode of bFGF release may be injury, either sublethal or lethal; and, transient disruption of the plasma membrane has been shown to release bFGF [Muthukrihnan et al. (1991): J Cell Physiol 148:1-16]. This observation has led to the concept of bFGF as a "wound hormone," involved in tissue integrity and repair. Findings of elevated bFGF following injury in vivo support this concept. Using an in vitro model, we have examined the regulation of bFGF gene expression following its release by sublethal injury. Analysis of bFGF protein by ELISA revealed that scraping subconfluent bovine aortic EC (BAE) released up to 80% of their bFGF. Following scraping, there was a 4- to 10-fold increase in the steady state level of bFGF mRNA, which reached a maximum at 2-3 h. There was a parallel increase in protein so that by 6 h after the scrape-induced release, bFGF levels were restored to those measured prior to scraping. Since bFGF has been reported to induce its own expression, we hypothesized that the released bFGF might be responsible for the increase in bFGF mRNA. However, inclusion of neutralizing antibodies against bFGF had a negligible effect on the scrape-induced increase in bFGF mRNA levels. Because of the important role of transforming growth factor type-beta 1 (TGF-beta 1), the plasminogen/plasminogen activator system, and thrombin in wound healing, we investigated their potential contributions to the increase in bFGF expression. Addition of anti-TGF-beta 1 antibodies, plasminogen activator inhibitor-1 (PAI-1), or the thrombin inhibitory combination of heparin and anti-thrombin III (AT III) to the cells at the time of scraping blocked about 50% of the increase in bFGF mRNA; the effects of these agents were not additive. The suppression of bFGF mRNA was associated with a proportional reduction in bFGF protein. Inclusion of the antagonists for 2 h at the time of scraping led to reduced cell proliferation, suggesting that cell-associated bFGF may be required for recovery and growth. Finally, studies to characterize the molecular mechanisms underlying the increased bFGF mRNA following sublethal injury revealed an increase in the transcriptional activation of bFGF gene. These results indicate that in spite of the fact that bFGF is not a secreted protein, levels of bFGF in the cell are tightly regulated. Furthermore, these findings suggest a role for bFGF in recovery from cell injury.

NMR conformational study of a model tetradecapeptide mimicking the RXVRG consensus cleavage site of a Xenopus laevis skin endoprotease

A model tetradecapeptide, used for the purification of the RXVRG-endoprotease from Xenopus laevis skin exudate, has been studied by two-dimensional NMR, correlation (COSY) and NOE (NOESY) spectroscopy. This peptide has the 5-9 consensus sequence (RXVRG), along with an acidic moiety (1-4) and a hydrophobic domain (10-14). Variations with temperature of NH chemical shifts in a dimethyl sulfoxide solution (low thermal coefficients at residues 6, 7 and 8) and quantified NOE values from four spectra at different mixing times clearly showed a structural organization in the consensus domain with psi-angles around [-40, -10 degrees] for residues 7 and 8, and two NOE correlations of alpha HiNHi + 2 type (5-7 and 6-8). Moreover, a privileged rotamer in the side chain is established for three residues (Val2, Asp3 and Val7) and limited possibilities are discussed for seven others. Most of the folding trends were not observed in the [Ser7] derivative, underlying the relationship between the conformations and a full consensus sequence. In the model tetradecapeptide an equilibrium between two beta-turns of type I, fragments 4-7 and 5-8, seems the most probable. Comparison between this tetradecapeptide and its 4-14 fragment, also a substrate for RXVRG-endoprotease, shows that the 1-3 moiety (DVD) influences the consensus domain structure(s) and clearly stabilizes the folded one(s). Finally, two analytical methods are developed in order to determine: (1) the trifluoroacetic acid content of the peptide samples, on the basis of 19F NMR spectroscopy; (2) the mean phi- and psi-angles of each residue, from the whole set of NH/alpha H coupling constants (3JN alpha) and NOE data at a local level.

Two-dimensional 1H NMR study of a tetradecapeptide with the consensus sequence Arg5-Asp-Val-Arg-Gly9: structural effects of the outside substitution Ser12 by Ala12

Conformation of a tetradecapeptide with a RXVRG consensus sequence, Arg5-Asp-Val-Arg-Gly9, found in several precursors of antibacterian peptides, was investigated in dimethylsulfoxide solution by proton NMR spectroscopy. Complete resonance assignments and conformational parameters were obtained through correlated (COSY) and nuclear Overhauser (NOESY) techniques. The 3J(alpha H, beta H) coupling constants and the intramolecular NOE, NH...beta H, were used to analyse the conformers around the C alpha-C beta bond and, in four cases, to obtain stereospecific assignments. Use of restraints derived from NOE connectivities and 3J(NH, alpha H) coupling constants allows the determination of a range of phi and psi dihedral angles for all the residues in the sequence. The present NMR results provide favourable evidence for the formation of two bends in the consensus sequence of the tetradecapeptide. The first one has most of the features of a Glu4-Val7 beta-turn (low temperature coefficient of the Val7NH chemical shift, Arg5 alpha H...Val7NH and Asp6NH...Val7NH NOE correlations). The second one exhibits only the Asp6 alpha H...Arg7NH and Val7NH...Arg8NH NOE interactions. These consensus sequence organizations proposed were confirmed by molecular modeling based on low potential energy structure on the [4-9] fragment with high agreement of NOE data. Overall, the substitution of Ser12 by Ala12 shifts the conformation of the hydrophobic moiety [10-14] towards a quite random coil structure in this fragment and strongly destabilizes the folded structures of the consensus domain where only one NH (Val7) is solvent-shielded opposed to three (Asp6 to Arg8) in the [Ser12] tetradecapeptide. These conformational changes could be related to the processing enzyme activities on these model oligopeptides.

Conformational studies of an undecapeptide reproducing the consensus sequence around the cleavage site of the RXVRG endoprotease from Xenopus laevis skin

Two synthetic fragments, corresponding to the 4-9 and 4-14 sequences of a tetradecapeptide used as a model to test the RXVRG-endoprotease activity from Xenopus laevis skin, have been studied by two-dimensional nmr spectroscopies, correlated spectroscopy, and nuclear Overhauser effect (NOE) spectroscopy. Both peptides wore the 5-9 consensus sequence found in several hormonal precursors. The nmr data for the 4-9 hexapeptide did not indicate any particular organization, either in water or in dimethylsulfoxide (DMSO), whereas, the 4-14 undecapeptide, a substrate for the RXVRG endoprotease, showed, in DMSO solution, significant trends of structural organization involving the amino acids pertaining to the consensus domain. From variations of integrated NOE peaks with temperature, the apparent interproton correlation times tau c were estimated and the maxima observed with Val7, the central residue in the consensus sequence. A defined tertiary structure in that domain was also supported by medium- and long-range NOEs between Asp6 and Arg8, Glu4 and Gly9, and by the likely involvement of Arg8 and Gly9 NHs in intramolecular hydrogen bonds. Most of these observations could be rationalized by an equilibrium between a 5-8 beta-turn and a 9 > 4 H-bonded loop. The predominance of one rotamer for the C alpha-C beta bond was established in four residues. Finally, the average phi and psi angles were derived from two models taking, or not, into account variations in the correlation times along the sequence. This allowed us to discuss the artefacts generated by using an average correlation time through the whole molecule.