The human asparaginase enzyme (ASPG) inhibits growth in leukemic cells

<div><p>The human protein ASPG is an enzyme with a putative antitumor activity. We generated in bacteria and then purified a recombinant GST-ASPG protein that we used to characterize the biochemical and cytotoxic properties of the human ASPG. We demonstrated that ASPG possesses asparaginase and PAF acetylhydrolase activities that depend on a critical threonine residue at position 19. Consistently, ASPG but not its T19A mutant showed cytotoxic activity in K562, NALM-6 and MOLT-4 leukemic cell lines but not in normal cells. Regarding the mechanism of action of ASPG, it was able to induce a significant apoptotic death in K562 cells. Taken together our data suggest that ASPG, combining different enzymatic activities, should be considered a promising anti-cancer agent for inhibiting the growth of leukemia cells.</p></div

PAF-AH activity of GST-ASPG.

<p>(A) The PAF-AH activity of recombinant GST-ASPG (●) and its point mutant GST-ASPG (T19A) (▲) was measured as function of enzyme concentration using Abcam's PAF acetylhydrolase Assay Kit as reported in Materials and Methods. Data points are represented as means ± SD of triplicate sample measurements and were fitted with a second order polynomial equation in GraphPad Prism software. (B) The residual PAF-acetylhydrolase activity of GST-ASPG was measured after pre-incubation for 10 min of 1.5 μg of GST-ASPG with 40 mM and 200 mM of D-asparagine using Abcam's PAF-AH assay. Data are shown as means ± SD of triplicate measurements. ** (<i>p</i> < 0.01); *** (<i>p</i> < 0.001).</p

Caspase assay on K562.

<p>(A) Representative Guava caspase assay graphs of K562 cells lines treated with 0.7 μg of GST, GST-ASPG or GST-ASPG (T19A) for 6, 12 and 24h. (B). Bar graphs representing the percentage of caspase and/or 7AAD positive cells after treatment with 0.7 μg of GST, GST-ASPG or GST-ASPG (T19A) for 6, 12 and 24h. (C) Table representing the statistical significances between groups. *(p < 0.05); **(<i>p</i>< 0.01); ***(<i>p</i>< 0.001).</p

SDS Page of purified GST-fusion proteins.

<p>Line 1: marker, line 2: GST, line 3: GST-ASPG, line 4: GST-ASPG (T19A), line 5–7: 50, 100 and 250 ng of bovine serum albumin respectively.</p

Schematic representation of ASPG action on the growth and survival of leukemia cells.

<p>ASPG with its L-asparaginase activity could deprive leukemia cells of L-asparagine, which is an essential metabolite for its malignant growth, inducing apoptosis and blocking cell proliferation. ASPG could also inducing the arrest of cell proliferation converting the active PAF in the inactive form Lyso-PAF and down-regulating the expression of the Epithelial Sodium Channel [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0178174#pone.0178174.ref003" target="_blank">3</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0178174#pone.0178174.ref026" target="_blank">26</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0178174#pone.0178174.ref027" target="_blank">27</a>].</p

Cytotoxicity of GST-ASPG on human leukemia cell lines and normal cells.

<p>The percentage of cell survival was evaluated by CCK8 assay in K562 (A), NALM-6 (B) and MOLT-4 (C) cell lines after 24h of treatment with GST-ASPG (●) and its inactive catalytically mutant GST-ASPG (T19A) (▲). HDFA (□) and PBMCs (◊) were used to analyze the cytotoxicity of GST-ASPG in normal cells (D). The results were fitted using GraphPad Prism software and represent the average and the standard deviation of three independent experiments. * (<i>p</i> < 0.05); *** (<i>p</i> < 0.001).</p

Cell growth inhibition by GST-ASPG.

<p>(A) K562 cells after 24h of incubation with 1000 ng (115 nM) of GST-ASPG and its inactive mutant GST-ASPG (T19A). (B) Time course of K562 cells treatment with 115 nM of GST-ASPG (T19A) (▲), 115 nM of GST ASPG (●) and after further addition of 115 nM of GST-ASPG at 12h (ο).Viability of K562 (C) and NALM-6 (D) cells after 24h of treatment with increasing concentrations of GST-ASPG (●) and its inactive catalytically mutant GST-ASPG (T19A) (▲). The results (the average and the standard deviation of three independent experiments) were evaluated by Trypan Blue exclusion assay and fitted using GraphPad Prism software. * (<i>p</i> < 0.05); ** (<i>p</i> < 0.01); *** (<i>p</i> < 0.001).</p

Characterization of L-asparaginase activity of GST-ASPG.

<p>L-Asparaginase activity of GST-ASPG (●) and its catalytically inactive mutant GST-ASPG (T19A) (▲) was evaluated by Nessler’s method in a time dependent manner (A), as a function of enzyme concentration (B) and substrate concentration (C). A hill slope of 6.9 and an S_0.5 value of ≈13 mM were estimated. Ammonia release of GST-ASPG was measured using 15 mM of L-asparagine alone or in presence of 15 mM of D-asparagine; No detectable enzymatic activity was found using 15 mM of D-asparagine or 15 mM of L-glutamine as substrates (D). Steady state kinetic of recombinant GST-ASPG was calculated in presence of 0 mM (●), 10 mM (■) and 30 mM (▲) of D-asparagine and a Ki value of 71 mM was estimated (E). All the experiments were performed at 37°C as reported in Materials and Methods using 1.5 μg of each protein; data points are represented as means ± SD of triplicate sample measurements. *** (<i>p</i> < 0.001).</p

In Silico Identification and Biological Evaluation of Novel Selective Serum/Glucocorticoid-Inducible Kinase 1 Inhibitors Based on the Pyrazolo-Pyrimidine Scaffold

The serum/glucocorticoid-inducible kinase 1 (Sgk1) has demonstrated antiapoptotic function and the capability to regulate cell survival, proliferation, and differentiation. A pivotal role of Sgk1 in carcinogenesis and in resistance to anticancer therapy has been suggested. With the aim of identifying new Sgk1 modulators, 322 pyrazolo-pyrimidine derivatives have been virtually screened with respect to a crystallographic model of Sgk1. The top five ranked compounds have been evaluated demonstrating Sgk1 inhibition in vitro and selectivity compared to RAC-alpha serine/threonine-protein kinase (Akt1)