Gas6/Axl is the sensor of arginine-auxotrophic response in targeted chemotherapy with arginine-depleting agents

Reactive Oxygen Species Mechanisms that Regulate Protein-Protein Interactions in Cancer

Reactive oxygen species (ROS) are produced during cellular metabolism and in response to environmental stress. While low levels of ROS play essential physiological roles, excess ROS can damage cellular components, leading to cell death or transformation. ROS can also regulate protein interactions in cancer cells, thereby affecting processes such as cell growth, migration, and angiogenesis. Dysregulated interactions occur via various mechanisms, including amino acid modifications, conformational changes, and alterations in complex stability. Understanding ROS-mediated changes in protein interactions is crucial for targeted cancer therapies. In this review, we examine the role that ROS mechanisms in regulating pathways through protein-protein interactions.

The addition of arginine deiminase potentiates Mithramycin A-induced cell death in patient-derived glioblastoma cells via ATF4 and cytochrome C

BACKGROUND

Arginine auxotrophy constitutes a shortcoming for ~ 30% of glioblastoma multiforme (GBM). Indeed, arginine-depleting therapy using arginine deiminase from Streptococcus pyogenes (SpyADI) has proven activity against GBM in preclinical studies. The good safety profile of SpyADI renders this agent an ideal combination partner for cytostatic therapy.

METHODS

In this study, we combined the antineoplastic antibiotic Mithramycin A (MitA) with SpyADI to boost single-agent activity and analyzed underlying response mechanisms in-depth.

RESULTS

MitA monotherapy induced a time- and dose-dependent cytotoxicity in eight patient-derived GBM cell lines and had a radiosensitizing effect in all but one cell line. Combination treatment boosted the effects of the monotherapy in 2D- and 3D models. The simultaneous approach was superior to the sequential application and significantly impaired colony formation after repetitive treatment. MitA monotherapy significantly inhibited GBM invasiveness. However, this effect was not enhanced in the combination. Functional analysis identified SpyADI-triggered senescence induction accompanied by increased mitochondrial membrane polarization upon mono- and combination therapy. In HROG63, induction of lysosomes was seen after both monotherapies, indicative of autophagy. These cells seemed swollen and had a more pronounced cortically formed cytoskeleton. Also, cytochrome C and endoplasmatic reticulum-stress-associated proteins ATF4 and Calnexin were enhanced in the combination, contributing to apoptosis. Notably, no significant increases in glioma-stemness marker were seen.

CONCLUSIONS

Therapeutic utilization of a metabolic defect in GBM along with cytostatic therapy provides a novel combination approach. Whether this SpyADI/MitA regimen will provide a safe alternative to combat GBM, will have to be addressed in subsequent (pre-)clinical trials.

Enhancing the Effect of Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand Signaling and Arginine Deprivation in Melanoma

Melanoma as a very aggressive type of cancer is still in urgent need of improved treatment. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and arginine deiminase (ADI-PEG20) are two of many suggested drugs for treating melanoma. Both have shown anti-tumor activities without harming normal cells. However, resistance to both drugs has also been noted. Studies on the mechanism of action of and resistance to these drugs provide multiple targets that can be utilized to increase the efficacy and overcome the resistance. As a result, combination strategies have been proposed for these drug candidates with various other agents, and achieved enhanced or synergistic anti-tumor effect. The combination of TRAIL and ADI-PEG20 as one example can greatly enhance the cytotoxicity to melanoma cells including those resistant to the single component of this combination. It is found that combination treatment generally can alter the expression of the components of cell signaling in melanoma cells to favor cell death. In this paper, the signaling of TRAIL and ADI-PEG20-induced arginine deprivation including the main mechanism of resistance to these drugs and exemplary combination strategies is discussed. Finally, factors hampering the clinical application of both drugs, current and future development to overcome these hurdles are briefly discussed.

Arginine Signaling and Cancer Metabolism

Arginine is an amino acid critically involved in multiple cellular processes including the syntheses of nitric oxide and polyamines, and is a direct activator of mTOR, a nutrient-sensing kinase strongly implicated in carcinogenesis. Yet, it is also considered as a non- or semi-essential amino acid, due to normal cells' intrinsic ability to synthesize arginine from citrulline and aspartate via ASS1 (argininosuccinate synthase 1) and ASL (argininosuccinate lyase). As such, arginine can be used as a dietary supplement and its depletion as a therapeutic strategy. Strikingly, in over 70% of tumors, ASS1 transcription is suppressed, rendering the cells addicted to external arginine, forming the basis of arginine-deprivation therapy. In this review, we will discuss arginine as a signaling metabolite, arginine's role in cancer metabolism, arginine as an epigenetic regulator, arginine as an immunomodulator, and arginine as a therapeutic target. We will also provide a comprehensive summary of ADI (arginine deiminase)-based arginine-deprivation preclinical studies and an update of clinical trials for ADI and arginase. The different cell killing mechanisms associated with various cancer types will also be described.

Genome-wide CRISPR/Cas9 knockout screening uncovers a novel inflammatory pathway critical for resistance to arginine-deprivation therapy

Arginine synthesis deficiency due to the suppressed expression of ASS1 (argininosuccinate synthetase 1) represents one of the most frequently occurring metabolic defects of tumor cells. Arginine-deprivation therapy has gained increasing attention in recent years. One challenge of ADI-PEG20 (pegylated ADI) therapy is the development of drug resistance caused by restoration of ASS1 expression and other factors. The goal of this work is to identify novel factors conferring therapy resistance.

Methods: Multiple, independently derived ADI-resistant clones including derivatives of breast (MDA-MB-231 and BT-549) and prostate (PC3, CWR22Rv1, and DU145) cancer cells were developed. RNA-seq and RT-PCR were used to identify genes upregulated in the resistant clones. Unbiased genome-wide CRISPR/Cas9 knockout screening was used to identify genes whose absence confers sensitivity to these cells. shRNA and CRISPR/Cas9 knockout as well as overexpression approaches were used to validate the functions of the resistant genes both in vitro and in xenograft models. The signal pathways were verified by western blotting and cytokine release.

Results: Based on unbiased CRISPR/Cas9 knockout screening and RNA-seq analyses of independently derived ADI-resistant (ADIR) clones, aberrant activation of the TREM1/CCL2 axis in addition to ASS1 expression was consistently identified as the resistant factors. Unlike ADIR, MDA-MB-231 overexpressing ASS1 cells achieved only moderate ADI resistance both in vitro and in vivo, and overexpression of ASS1 alone does not activate the TREM1/CCL2 axis. These data suggested that upregulation of TREM1 is an independent factor in the development of strong resistance, which is accompanied by activation of the AKT/mTOR/STAT3/CCL2 pathway and contributes to cell survival and overcoming the tumor suppressive effects of ASS1 overexpression. Importantly, knockdown of TREM1 or CCL2 significantly sensitized ADIR toward ADI. Similar results were obtained in BT-549 breast cancer cell line as well as castration-resistant prostate cancer cells. The present study sheds light on the detailed mechanisms of resistance to arginine-deprivation therapy and uncovers novel targets to overcome resistance.

Conclusion: We uncovered TREM1/CCL2 activation, in addition to restored ASS1 expression, as a key pathway involved in full ADI-resistance in breast and prostate cancer models.

Targeting the Proline-Glutamine-Asparagine-Arginine Metabolic Axis in Amino Acid Starvation Cancer Therapy

Proline, glutamine, asparagine, and arginine are conditionally non-essential amino acids that can be produced in our body. However, they are essential for the growth of highly proliferative cells such as cancers. Many cancers express reduced levels of these amino acids and thus require import from the environment. Meanwhile, the biosynthesis of these amino acids is inter-connected but can be intervened individually through the inhibition of key enzymes of the biosynthesis of these amino acids, resulting in amino acid starvation and cell death. Amino acid starvation strategies have been in various stages of clinical applications. Targeting asparagine using asparaginase has been approved for treating acute lymphoblastic leukemia. Targeting glutamine and arginine starvations are in various stages of clinical trials, and targeting proline starvation is in preclinical development. The most important obstacle of these therapies is drug resistance, which is mostly due to reactivation of the key enzymes involved in biosynthesis of the targeted amino acids and reprogramming of compensatory survival pathways via transcriptional, epigenetic, and post-translational mechanisms. Here, we review the interactive regulatory mechanisms that control cellular levels of these amino acids for amino acid starvation therapy and how drug resistance is evolved underlying treatment failure.

Targeting TAM to Tame Pancreatic Cancer

Pancreatic cancer is expected to become the second leading cause of cancer-related death within the next few years. Current therapeutic strategies have limited effectiveness and therefore there is an urgency to develop novel effective therapies. The receptor tyrosine kinase subfamily TAM (Tyro3, Axl, MerTK) is directly implicated in the pathogenesis of the metastatic, chemoresistant, and immunosuppressive phenotype in pancreatic cancer. TAM inhibitors are promising investigational therapies for pancreatic cancer due to their potential to target multiple aspects of pancreatic cancer biology. Specifically, recent mechanistic investigations and therapeutic combinations in the preclinical setting suggest that TAM inhibition with chemotherapy, targeted therapy, and immunotherapy should be evaluated clinically.

Anti-inflammation and anti-apoptosis effects of growth arrest-specific protein 6 in acute liver injury induced by LPS/D-GalN in mice

Purpose

To investigate the effect of growth arrest-specific protein 6 (Gas6) on acute liver injury in mice and related mechanisms.

Methods

Thirty C57BL/6 (6-8 weeks old) mice were randomly divided into control, LPS/D-GalN, and LPS/D-GalN+Gas6 groups (10 mice in each group). The LPS/D-GalN group was intraperitoneally administered with LPS (0.25 mg/Kg) and D-GalN (400 mg/Kg) for 5h. The LPS/D-GalN+Gas6 group was intraperitoneally administered with rmGas6 one hour before intraperitoneal application of LPS/D-GalN. All subjects were sacrificed at 5 h for blood and tissue analysis. The expression of protein and mRNA was assessed by western blotting and RT-PCR, respectively.

Results

Compared with the control group, AST, ALT, IL-1β, TNF-α, IL-6 IL-10, MPO activity were increased in the LPS/D-GalN group. However, they were significantly inhibited by Gas6. Gas6 markedly suppressed the expression of apoptosis-related protein induced by LPS/D-GalN. Moreover, Gas6 attenuated the activation of the NF-κB signaling pathway in acute liver injury induced by LPS/D-GalN.

Conclusions

Gas6 alleviates acute liver injury in mice through regulating NF-κB signaling pathways. Gas6 can be a potential therapeutic agent in treating LPS/D-GalN-induced acute liver injury in the future.

Collaboration Between RSK-EphA2 and Gas6-Axl RTK Signaling in Arginine Starvation Response That Confers Resistance to EGFR Inhibitors

Many human malignancies require extracellular arginine (Arg) for survival because the key enzyme for de novo Arg biosynthesis, argininosuccinate synthetase 1 (ASS1), is silenced. Recombinant arginine deiminase (ADI-PEG20), which digests extracellular Arg, has been in clinical trials for treating ASS1-negative tumors. Reactivation of ASS1 is responsible for the treatment failure. We previously demonstrated that ASS1 reactivation is transcriptionally regulated by c-Myc via the upstream Gas6-Axl tyrosine kinase (RTK) signal. Here, we report that another RTK EphA2 is coactivated via PI3K-ERK/RSK1 pathway in a ligand-independent mechanism. EphA2 is also regulated by c-Myc. Moreover, we found that knockdown Axl upregulates EphA2 expression, demonstrating cross-talk between these RTKs. ADI^R cell lines exhibits enhanced sensitivities to nutrient deprivation such as charcoal-stripped FBS and multiple RTK inhibitor foretinib but resistance to EGFR inhibitors. Knockdown EphA2, and to lesser extent, Axl, overcomes EGFRi resistance. c-Myc inhibitor JQ1 can also sensitize ADI^R cells to ADI-PEG20. This study elucidates molecular interactions of multiple RTKs in Arg-stress response and offers approaches for developing strategies of overcoming ADI-PEG20 resistance.

Cancer Stem Cells: Emergent Nature of Tumor Emergency

A functional analysis of 167 genes overexpressed in Krebs-2 tumor initiating cells was performed. In the first part of the study, the genes were analyzed for their belonging to one or more of the three groups, which represent the three major phenotypic manifestation of malignancy of cancer cells, namely (1) proliferative self-sufficiency, (2) invasive growth and metastasis, and (3) multiple drug resistance. 96 genes out of 167 were identified as possible contributors to at least one of these fundamental properties. It was also found that substantial part of these genes are also known as genes responsible for formation and/or maintenance of the stemness of normal pluri-/multipotent stem cells. These results suggest that the malignancy is simply the ability to maintain the stem cell specific genes expression profile, and, as a consequence, the stemness itself regardless of the controlling effect of stem niches. In the second part of the study, three stress factors combined into the single concept of "generalized cellular stress," which are assumed to activate the expression of these genes, were defined. In addition, possible mechanisms for such activation were identified. The data obtained suggest the existence of a mechanism for the de novo formation of a pluripotent/stem phenotype in the subpopulation of "committed" tumor cells.

Cisplatin-induced synthetic lethality to arginine-starvation therapy by transcriptional suppression of ASS1 is regulated by DEC1, HIF-1α, and c-Myc transcription network and is independent of ASS1 promoter DNA methylation

Many human tumors require extracellular arginine (Arg) for growth because the key enzyme for de novo biosynthesis of Arg, argininosuccinate synthetase 1 (ASS1), is silenced. These tumors are sensitive to Arg-starvation therapy using pegylated arginine deiminase (ADI-PEG20) which digests extracellular Arg. Many previous studies reported that ASS1 silencing is due to epigenetic inactivation of ASS1 expression by DNA methylation, and that the demethylation agent 5-aza-deoxycytidine (Aza-dC) can induce ASS1 expression. Moreover, it was reported that cisplatin suppresses ASS1 expression through ASS1 promoter methylation, leading to synthetic lethality to ADI-PEG20 treatment. We report here that cisplatin supppresses ASS1 expression is due to upregulation of HIF-1α and downregulation of c-Myc, which function as negative and positive regulators of ASS1 expression, respectively, by reciprocal bindings to the ASS1 promoter. In contrast, we found that Aza-dC induces ASS1 expression by downregulation of HIF-1α but upregulation of c-Myc. We further demonstrated that the clock protein DEC1 is the master regulator of HIF-1α and c-Myc that regulate ASS1. cDDP upregulates DEC1, whereas Aza-dC suppresses its expression. Using two proteasomal inhibitors bortezomib and carfilzomib which induce HIF-1α accumulation, we further demonstrated that HIF-1α is involved in ASS1 silencing for the maintenance of Arg auxotrophy for targeted Arg-starvation therapy.

Chromatin remodeling system p300-HDAC2-Sin3A is involved in Arginine Starvation-Induced HIF-1α Degradation at the ASS1 promoter for ASS1 Derepression

Argininosuccinate synthetase 1 (ASS1) is the key enzyme that controls biosynthesis of arginine (Arg). ASS1 is silenced in many human malignancies therefore, these tumors require extracellular Arg for growth. The Arg-degrading recombinant protein, pegylated arginine deiminase (ADI-PEG20), has been in clinical trials for targeting Arg auxotrophic tumors by Arg starvation therapy. Resistance to Arg starvation is often developed through reactivation of ASS1 expression. We previously demonstrated that ASS1 silencing is controlled by HIF-1α and Arg starvation-reactivated ASS1 is associated with HIF-1α downregulation. However, mechanisms underlying ASS1 repression and HIF-1α turnover are not known. Here, we demonstrate that interplay of p300-HDAC2-Sin3A in the chromatin remodeling system is involved in HIF-1α degradation at the ASS1 promoter. The histone acetyltransferase p300 is normally associated with the ASS1 promoter to maintain acetylated H3K14ac and H3K27ac for ASS1 silencing. Arg starvation induces p300 dissociation, allowing histone HDAC2 and cofactor Sin3A to deacetylate these histones at the ASS1 promoter, thereby facilitating HIF-1α-proteasomal complex, driven by PHD2, to degrade HIF-1α in situ. Arg starvation induces PHD2 and HDAC2 interaction which is sensitive to antioxidants. This is the first report describing epigenetic regulation of chromosomal HIF-1α turnover in gene activation that bears important implication in cancer therapy.

Argininosuccinate synthetase 1 (ASS1) is a common metabolic marker of chemosensitivity for targeted arginine- and glutamine-starvation therapy

Argininosuccinate synthetase 1 (ASS1) is the rate-limiting enzyme that catalyzes the biosynthesis of arginine (Arg). Many malignant human tumors are auxotrophic for Arg because ASS1 is silenced. ASS1 has been established as a sensor of Arg auxotrophic response and a chemosensitivity marker for Arg starvation therapy. Here, we report that ASS1 is also a sensor for glutamine (Gln)-deprivation response, and that upregulation of ASS1 expression is associated with resistance to Gln-starvation treatments. Knockdown of ASS1 expression resulted in increased sensitivity to both Arg- and Gln-starvation, whereas increased ASS1 expression by ectopic transfection is associated with resistance to both Arg- and Gln-starvation. The addition of permeable fumarate, a metabolite that bridges the tricarboxylic acid and urea cycles, resulted in downregulation of ASS1 expression and increased sensitivity to both Arg- and Gln-deprivation treatments. Mechanistically, the Gln-deprivation response, like the arginine-auxotrophic response, downregulates HIF-1α resulting in de-silencing of ASS1. Our results demonstrate that ASS1 is a common biosensor for Arg and Gln deprivation response and a shared target for Arg- and Gln-starvation therapies which have been in several current clinical trials.