The relative effectiveness of 6-thioguanine and 8-azaguanine in selecting resistant mutants from two V79 Chinese hamster cells in vitro

The frequency of surviving colonies in two V79 cell lines exposed to either 6-thioguanine or 8-azaguanine was dependent on initial plating density. Different degrees of metabolic-co-operation were found to occur in the two cell lines and the loss of both spontaneous and added mutants occurred at a lower cell density when 6TG was used for selection than when 8 AZ was used in both cell lines. Both analogues were degraded on incubation in medium plus serum in the absence of added cells. Variation in serum batch had little effect on the rate of degradation or on the frequency of colonies recovered after treatment of V79 cell lines with 8AZ. The reasons for preferring 8AZ to 6TG as a selective agent are discussed.     

Mutational analysis of the antizyme-binding element reveals critical residues for the function of ornithine decarboxylase

Background

Ornithine decarboxylase (ODC), the key enzyme in the polyamine biosynthetic pathway, is highly regulated by antizymes (AZs), small proteins that bind and inhibit ODC and increase its proteasomal degradation. Early studies delimited the putative AZ-binding element (AZBE) to the region 117-140 of ODC. The aim of the present work was to study the importance of certain residues of the region 110-142 that includes the AZBE region for the interaction between ODC and AZ1 and the ODC functionality.

Methods

Computational analysis of the protein sequences of the extended AZBE site of ODC and ODC paralogues from different eukaryotes was used to search for conserved residues. The influence of these residues on ODC functionality was studied by site directed mutagenesis, followed by different biochemical techniques.

Results

The results revealed that: a) there are five conserved residues in ODC and its paralogues: K115, A123, E138, L139 and K141; b) among these, L139 is the most critical one for the interaction with AZs, since its substitution decreases the affinity of the mutant protein towards AZs; c) all these conserved residues, with the exception of A123, are critical for ODC activity; d) substitutions of K115, E138 or L139 diminish the formation of ODC homodimers.

Conclusions

These results reveal that four of the invariant residues of the AZBE region are strongly related to ODC functionality.

General significance

This work helps to understand the interaction between ODC and AZ1, and describes various new residues involved in ODC activity, a key enzyme for cell growth and proliferation.     

Discovery and antiparasitic activity of AZ960 as a Trypanosoma brucei ERK8 inhibitor

Human African trypanosomiasis (HAT) is a lethal, vector-borne disease caused by the parasite Trypanosoma brucei. Therapeutic strategies for this neglected tropical disease suffer from disadvantages such as toxicity, high cost, and emerging resistance. Therefore, new drugs with novel modes of action are needed. We screened cultured T. brucei against a focused kinase inhibitor library to identify promising bioactive compounds. Among the ten hits identified from the phenotypic screen, AZ960 emerged as the most promising compound with potent antiparasitic activity (IC₅₀ = 120 nM) and was shown to be a selective inhibitor of an essential gene product, T. brucei extracellular signal-regulated kinase 8 (TbERK8). We report that AZ960 has a K_i of 1.25 μM for TbERK8 and demonstrate its utility in establishing TbERK8 as a potentially druggable target in T. brucei.     

Effects of somatic mutations on CDR loop flexibility during affinity maturation

Prior studies suggest that antibody affinity maturation is achieved, in part, via prearranging the CDRs for binding. The implication is that the entropy cost of binding is reduced and that this rigidification occurs as a consequence of somatic mutations during maturation. However, how these mutations modulate CDR flexibility is unclear. Here, molecular dynamics simulations captured CDR flexibility differences between four mature antibodies (7G12, AZ28, 28B4, and 48G7) and their germline predecessors. Analysis of their trajectories: (1) rationalized how mutations during affinity maturation restrict CDR motility, (2) captured the equilibrium between bound and unbound conformations for the H3 loop of unliganded 7G12, and (3) predicted a set of new mutations that, according to our simulations, should diminish binding by increasing flexibility.     

Dynamically engaged smiling predicts cooperation above and beyond average smiling levels

Smiling has been conceptualized as a signal of cooperative intent, yet smiles are easy to fake. We suggest that contextually appropriate, dynamically engaged smiling imposes an attentional cost, thereby making engaged smiling a plausible “honest signal” of cooperative intent. To test this hypothesis, we analyzed data from 123 pairs of same-sex strangers having “getting-to-know-you” conversations who subsequently played a one-shot prisoner's dilemma together. We calculated the strength of engagement in smiling using a cross-lagged auto-regressive model for dyadic data. We found that when an individual's partner (the signaler) tended to smile in a more responsive way, that individual (the receiver) was more likely to cooperate. Conversely, when a signaler tended to smile in a less responsive way, the receiver was less likely to cooperate. These effects were present over-and-above the effects of average levels of smiling and self-reported liking, which also predicted likelihood of cooperation. However, dynamically engaged smiling did not predict cooperation on the part of the signaler, suggesting that receivers weight the importance of engagement more highly than they should, or even that engaged smiling might be a manipulative display. These results illustrate how conversational dynamics can influence evolutionary signaling.     

Effects of metabolic inhibitors on cell lethality and mutation induction in Chinese hamster cells. II. The effect of posttreatment with non-toxic concentrations of thymidine

The ability of posttreatment exposure to non-toxic concentrations of thymidine (TdR) to enhance the lethal effects of a number of alkylating agents, X-rays and UV and the lethal and mutagenic effects of N′-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU) has been examined in V79 cell lines. TdR posttreatment enhanced the cytotoxic effects of ethyl methanesulphonate (EMS), MNU and ENU but not of UV or X-rays and increased both the spontaneous and MNU- and ENU-induced frequencies of azaguanine resistant (AZ^R) mutants. No significant effect of TdR on the spontaneous frequency of thioguanine resistant (TG^R) mutants was demonstrated but the frequency of MNU-induced mutants to TG^R was enhanced. The effects on expression of both potentially lethal and premutagenic damage were reversed by addition of an equimolar concentration of deoxycytidine (dCdR). The enhancement in spontaneous and induced mutant frequency (IMF) at the HGPRT locus appears to be due to an alteration in the selective efficiency of purine analogous due to alteration in growth kinetics of cells exposed to TdR or treated with alkylated agents or posttreated with thymidine after alkylation damage and not to an alteration in the miscoding potential of alkylated bases.     

Mechanism of cytotoxic action of azaguanine and thioguanine in wild-type V79 cell lines and their relative efficiency in selection of structural gene mutants

The cytotoxic effects of azaguanine and thioguanine have been compared in two wild-type V79 cells. To achieve equitoxic effects in both cell lines a 10–20-fold higher concentration of azaguanine than thioguanine was required. Affinity of HGPRT for azaguanine was 10-fold lower than for hypoxanthine in both cell lines and was similar to that for thioguanine in V79S cells. Affinity for thioguanine differed by a factor of 3 in the two cell lines. The rate of cell kill by azaguanine was markedly slower than by thioguanine in both cell lines. Reduction of whole cell uptake of [¹⁴C]hypoxanthine incorporation by unlabelled azaguanine was only demonstrable after prolonged incubation periods as was incorporation of [¹⁴C]azaguanine into acid-insoluble material. Experiments with cell-free extracts indicated that hypoxanthine acts as a non-competitive inhibitor of the enzyme. The slow rate of dissociation of the HGPRT—azaguanine complex is reflected in the slow rate of killing of wild-type cells. Clones resistant to the cytotoxic effects of these analogues have been selected from both cell lines and have been shown to possess HGPRT with altered kinetic properties. Our data suggest that azaguanine and thioguanine may select for mutations at different sites on the HGPRT molecule in V79 cells and provide possible explanations for the differences in effectiveness of these two agents reported in other cell lines.