A quantitative study of the in vitro binding of the C-terminal domain of p21 to PCNA: affinity, stoichiometry, and thermodynamics.

Proliferating cell nuclear antigen (PCNA) plays an essential role in DNA replication, repair, and control of cell proliferation, and its activity can be modulated by interaction with p21(Waf1/Cip1) [Cox, L. S., (1997) Trends Cell Biol. 7, 493-497]. This protein-protein interaction provides a particularly good model target for designing therapeutic agents to treat proliferative disorders such as cancer. In this study, the formation of complexes between PCNA and peptides derived from the C-terminus of p21 has been investigated at the molecular level and quantified using a competitive PCNA binding assay and isothermal titration calorimetry (ITC). The affinity constant for the interaction between p21 (141-160) peptide and PCNA has been determined to be 1.14 x 10(7) M(-)(1), corresponding to a K(d) of 87.7 nM. Measurement of the interaction of truncation and substitution analogues based on the p21 (141-160) sequence with PCNA revealed that the N-terminal part (residues 141-152) of the above peptide is the minimum recognition motif, required for PCNA binding. Truncation of the C-terminal region p21 (153-160), though, inhibited significantly the ability of the peptides to compete with the full-length p21 (141-160) for binding to PCNA. Alanine mutation of Met 147 or Asp 149 completely abolished or significantly decreased, respectively, the level of the PCNA binding and the inhibition of SV40 DNA replication. Comparison of the data obtained by the competitive PCNA binding assay and the ITC measurements demonstrated the usefulness of this assay for screening for compounds that could modulate the PCNA-p21 interaction. Using this assay, we have screened rationally designed peptides for binding to PCNA and interruption of the PCNA-p21 (141-160) complex. As a result of this screening, we have identified a 16-residue peptide (consensus motif 1 peptide) with the following sequence: SAVLQKKITDYFHPKK. Consensus motif 1 peptide and p21 (141-160) have similar affinities for binding PCNA and abilities to inhibit in vitro replication of DNA originated from SV40. Such peptides could prove useful in assessing p21-mimetic strategies for cancer treatment.

Regulation of gamma-glutamylcysteine synthetase regulatory subunit (GLCLR) gene expression: identification of the major transcriptional start site in HT29 cells.

gamma-Glutamylcysteine synthetase (GCS) is of major importance in glutathione homeostasis. The GCS heterodimer is composed of catalytic (heavy subunit, GCSh) and regulatory (light subunit, GCSl) subunits. Regulation of the human GCSl subunit gene (GLCLR) expression was studied as GCSl has a critical role in glutathione synthesis. The minimal basal expression of GLCLR was found to be mediated by a region between nt -205 and -318. The major transcriptional start site in HT29 cells was located within this region at nt -283. A region between nt -411 and -447 was identified as having a potential involvement in the negative regulation of GLCLR expression. In order to study the transcriptional regulation of GCSl by oxidant stress, HepG2 cells were treated with sodium nitroprusside (SNP). SNP (1.5 mM) was found to increase glutathione levels by 2-fold, as well as GCS activity by 6-fold. This is accompanied by a co-ordinate increase in the levels of the both the GCSl and GCSh subunits, each by approximately 2-fold. The transcriptional activity of the GLCLR gene was increased by approximately 2.5-fold in SNP-treated cells.

Mutants of Tn3 resolvase which do not require accessory binding sites for recombination activity.

Tn3 resolvase promotes site-specific recombination between two res sites, each of which has three resolvase dimer-binding sites. Catalysis of DNA-strand cleavage and rejoining occurs at binding site I, but binding sites II and III are required for recombination. We used an in vivo screen to detect resolvase mutants that were active on res sites with binding sites II and III deleted (that is, only site I remaining). Mutations of amino acids Asp102 (D102) or Met103 (M103) were sufficient to permit catalysis of recombination between site I and a full res, but not between two copies of site I. A double mutant resolvase, with a D102Y mutation and an additional activating mutation at Glu124 (E124Q), recombined substrates containing only two copies of site I, in vivo and in vitro. In these novel site Ixsite I reactions, product topology is no longer restricted to the normal simple catenane, indicating synapsis by random collision. Furthermore, the mutants have lost the normal specificity for directly repeated sites and supercoiled substrates; that is, they promote recombination between pairs of res sites in linear molecules, or in inverted repeat in a supercoiled molecule, or in separate molecules.

Overexpression of the regulatory subunit of gamma-glutamylcysteine synthetase in HeLa cells increases gamma-glutamylcysteine synthetase activity and confers drug resistance.

gamma-Glutamylcysteine synthetase (GCS) is reported to catalyse the rate-limiting step in glutathione biosynthesis, and is a heterodimer composed of a catalytic subunit [heavy subunit (GCSh) of Mr 73000] and a regulatory subunit [light subunit (GCSl) of Mr 31000]. In the present study, we have demonstrated for the first time a potential role for GCSl in resistance towards doxorubicin and cadmium chloride. Addition of recombinant GCSl to HeLa cell extracts in vitro was found to result in an increase in GCS activity of between 2- and 3-fold. Transient transfections of COS-1 cells with the GCSl cDNA cause an increase in GCS activity of approx. 2-fold, and a small but significant (P=0.008) increase in glutathione levels from 126.9+/-4. 2 nmol/mg protein to 178.8+/-19.1 nmol/mg protein. We proceeded to make a HeLa cell line (LN73), which stably overexpresses GCSl. These cells overexpress GCSl approx. 20-fold above basal levels. LN73 was found to have a 2-fold increase in GCS activity (437.3+/-85.2 pmol/min per mg) relative to the control cell line, HL9 (213.4+/-71. 8 pmol/min per mg). In contrast with the transient transfections in COS-1 cells, stable overexpression of GCSl was found not to be associated with an increase in glutathione content. However, when the LN73 and HL9 cells were treated with the glutathione-depleting agent, diethylmaleate, the LN73 cells were found to have an enhanced ability to regenerate glutathione, compared with HL9 cells. The cell lines were treated with various anti-cancer drugs, and their cytotoxicity was examined. No obvious differences in toxicity were observed between the different cell lines following treatment with cisplatin and melphalan. The redox-cycling agent doxorubicin, however, was found to be more toxic (approx. 2-fold) to the HL9 cells than the LN73 cells. When the cells were treated with the carcinogenic transition-metal compound, cadmium chloride, LN73 cells were found to be approx. 3-fold more resistant than HL9 cells.

Regulation of human gamma-glutamylcysteine synthetase: co-ordinate induction of the catalytic and regulatory subunits in HepG2 cells.

We have shown that in HepG2 cells treatment with 75 microM t-butylhydroquinone (tBHQ) results in a 2.5-fold increase in glutathione concentration, as part of an adaptive response to chemical stress. In these cells the elevation in intracellular glutathione level was found to be accompanied by an increase of between 2-fold and 3-fold in the level of the 73 kDa catalytic subunit of gamma-glutamylcysteine synthetase (heavy subunit, GCSh) and the 31 kDa regulatory subunit (light subunit, GCSl). Levels of GCSh and GCSl mRNA were increased by up to 5-fold in HepG2 cells in response to tBHQ. To study the transcriptional regulation of GCSl, we subcloned 6.7 kb of the upstream region of the human GCSl gene (GLCLR) from a genomic clone isolated from a P1 lymphoblastoid cell line genomic library. HepG2 cells were transfected with GLCLR promoter reporter constructs and treated with tBHQ. This resulted in an induction of between 1.5-fold and 3.5-fold in reporter activity, indicating that transcriptional regulation of GLCLR is likely to contribute to the induction of GCSl by tBHQ in HepG2 cells. Sequence analysis of the promoter region demonstrated the presence of putative enhancer elements including AP-1 sites and an antioxidant-responsive element, which might be involved in the observed induction of the GLCLR promoter.

Cooperative binding of Tn3 resolvase monomers to a functionally asymmetric binding site.

BACKGROUND: The inverted repeat is a common feature of protein-binding sites in DNA. The two-fold symmetry of the inverted repeat corresponds to the two-fold symmetry of the protein that binds to it. In most natural inverted-repeat binding sites, however, the DNA sequence does not have perfect two-fold symmetry. Our study of how a site-specific recombinase recognizes an inverted-repeat binding site indicates that such sequence asymmetry can be functionally important. RESULTS: Tn3 resolvase forms two complexes with the 34 base-pair binding site II of its recombination region, res. A resolvase monomer first binds at the left end of the site; a second monomer then binds cooperatively at the right end. In both complexes, the DNA is bent by resolvase. In contrast, the closely related gamma delta resolvase binds to site II mainly as a dimer. Insertion of 5 or 10 base pairs at the centre of the site does not prevent cooperative binding of two Tn3 resolvase subunits. The fully occupied site II has a very asymmetric structure. Reversal of the orientation of site II in res blocks recombination; thus, its asymmetric properties are functionally important. We propose a structure for the two-subunit complex formed with site II, based on our results and by analogy with the co-crystal structure of gamma delta resolvase bound to res site I. CONCLUSIONS: Deviations from perfect inverted-repeat symmetry in a resolvase-binding site lead to ordered binding of subunits, structural asymmetry of resolvase-DNA complexes, and asymmetric function.

Fc gamma bearing T cells in non-Hodgkin lymphoma.

T-cell subpopulations have been implicated in the regulation of normal human B-cell reactivity. As the non-Hodgkin lymphomas (NHL) represent predominantly clonal B-cell malignancies, we examined the relationship of total T-cell [Sheep Red Blood Cell (SRBC) binding] and Fc gamma bearing T-cell populations in these disorders. Peripheral blood from seven low-grade (Rai stage 0), six high-grade (Rai stage 3 or 4) CLL patients, lymph node specimens from five patients with WDLL, seven patients with PDLL-D, three patients with MC-D, and eight patients with DHL were studied. All values were compared to normal controls. The percentage of total T cells in each disease category was decreased compared to controls. In addition, there was a reproducible correlation between the percentage of Fc gamma bearing T cells and the histopathologic diagnosis. The percentage of Fc gamma bearing T cells was highest in low-grade CLL and decreased incremently from high-grade CLL and WDLL, to MC-D, and PDLL-D. In DHL, we found no Fc gamma bearing T cells. Finally, the percentage of Fc gamma bearing T cells in each disease category was decreased compared to controls. These findings suggest a correlation between Fc gamma bearing T cells and the clinical aggressiveness of disease in NHL. In addition, they may raise important questions about therapy. Finally, they may offer a useful clinical test as an adjunct to histopathology although this will need to be confirmed in larger series.

Immunoregulatory abnormalities in chronic lymphocytic leukemia. I. Correlations of immunoregulatory subpopulations with stage of disease.

Previous studies have demonstrated multiple immunologic abnormalities in Chronic Lymphocytic Leukemia (CLL). We, therefore, investigated the relationship of immunoregulatory mononuclear cell subpopulations and disease activity in inactive and active CLL. The absolute number of T cells is increased in both groups compared to controls and a significant increase (p less than 0.001) in the number of monocytes was found in active patients. When the number of Fc gamma bearing T cells was compared to the number of B cells, active patients revealed a 993% decrease and inactive patients a 93% decrease compared to controls. Inactive patients also revealed increased proliferation when stimulated (PHA) after 48 hours in media alone compared to fresh cells. The active group revealed no increase in preincubated (PHA) stimulated cultures. As this suggested the possibility of immunoregulatory differences, suppressor cells were studied. Con A induced suppressor cells decreased proliferation of PHA stimulated cultures in inactive and control groups but had no effect in active patients. Isolated fresh autochthonous T cells (1:1) decreased PHA-induced proliferation 86% in the inactive group, 50% in the control group but had no effect in active patients. Pre-incubation (48 hr) followed by T cell separation revealed decreased Fc gamma T cells and abrogation of this suppressive effect in both inactive and control groups. Finally, isolated adherent cells decreased PHA stimulation by 86% in active patients but had insignificant effects on preincubated PHA stimulated cultures in the other groups. These data suggest that inactive CLL is characterized by a population of T suppressor cells that are more active than similar numbers of this population in control cultures. This population is short-lived and correlated with the Fc gamma bearing T cell population. This population appears inactive or non-functional in active CLL where adherent suppressor cells are increased.