Effects of adenosine metabolism in astrocytes on central nervous system oxygen toxicity

Hyperbaric oxygen attenuates chronic postsurgical pain by regulating the CD73/adenosine/A1R axis of the spinal cord in rats

Chronic postsurgical pain (CPSP) affects postoperative rehabilitation and quality of life in patients, but its mechanisms are still poorly understood. Hyperbaric oxygen (HBO) attenuates neuropathic pain in animal and human studies, but its efficacy for CPSP treatment and its underlying mechanism have not been elucidated. This study aimed to investigate the analgesic effect of HBO in a CPSP rat model and the role of spinal cord adenosine circulation in HBO-induced analgesia. A skin/muscle incision and retraction (SMIR) rat model was used to mimic CPSP, and HBO treatment (2.5 ATA, 60 min) was administered once daily for five consecutive days beginning three days after surgery. The role of spinal cord adenosine circulation in HBO-induced analgesia was investigated using APCP (a CD73 inhibitor), DPCPX (an A1R antagonist) or an intrathecal injection of adenosine. The mechanical paw withdrawal threshold (PWT) was determined at different time points before and after surgery. The spinal cord adenosine and ATP contents were analyzed using high-performance liquid chromatography (HPLC), and the spinal cord expression of A1R, CD73, and ADK was examined by Western blotting and immunofluorescence staining. The results showed that the mechanical PWT of the ipsilateral hind paw and the adenosine content decreased, and the spinal cord expression of A1R, CD73, and ADK and ATP content increased within 14 days after surgery. HBO treatment alleviated mechanical allodynia, reduced ATP content, and increased adenosine content by activating CD73 but downregulated the spinal cord expression of A1R, CD73, and ADK. Intrathecal adenosine alleviated mechanical allodynia after SMIR and downregulated the spinal cord expression of A1R and CD73, and intrathecal APCP or DPCPX attenuated the analgesic effect of HBO treatment on SMIR-induced CPSP. PERSPECTIVE: Spinal cord adenosine is involved in the occurrence and development of CPSP, and HBO treatment alleviates CPSP by regulating adenosine production/metabolism in the spinal cord. Thus, HBO may be employed for the treatment of CPSP with favorable efficacy.

Adenosine Kinase on Deoxyribonucleic Acid Methylation: Adenosine Receptor-Independent Pathway in Cancer Therapy

Methylation is an important mechanism contributing to cancer pathology. Methylation of tumor suppressor genes and oncogenes has been closely associated with tumor occurrence and development. New insights regarding the potential role of the adenosine receptor-independent pathway in the epigenetic modulation of DNA methylation offer the possibility of new interventional strategies for cancer therapy. Targeting DNA methylation of cancer-related genes is a promising therapeutic strategy; drugs like 5-Aza-2′-deoxycytidine (5-AZA-CdR, decitabine) effectively reverse DNA methylation and cancer cell growth. However, current anti-methylation (or methylation modifiers) are associated with severe side effects; thus, there is an urgent need for safer and more specific inhibitors of DNA methylation (or DNA methylation modifiers). The adenosine signaling pathway is reported to be involved in cancer pathology and participates in the development of tumors by altering DNA methylation. Most recently, an adenosine metabolic clearance enzyme, adenosine kinase (ADK), has been shown to influence methylation on tumor suppressor genes and tumor development and progression. This review article focuses on recent updates on ADK and its two isoforms, and its actions in adenosine receptor-independent pathways, including methylation modification and epigenetic changes in cancer pathology.

Hypoxia-alleviated sonodynamic therapy based on a hybrid protein oxygen carrier to enhance tumor inhibition

Sonodynamic therapy (SDT) is a highly attractive therapy due to its advantages of being non-invasive and having good penetration depth, but tumor hypoxia extremely restricts its therapeutic effect. Here, a novel oxygen-enhanced hybrid protein nanosonosensitizer system (MnPcS@HPO) is designed using human serum albumin (HSA) and hemoglobin (Hb) through disulfide reconfiguration, followed by encapsulating Mn-phthalocyanine (MnPcS), aiming to develop O₂ self-supplementing nanoparticles (NPs) for enhanced SDT. Benefitting from the O₂-carrying ability of Hb and the tumor-targeting property of HSA, the MnPcS@HPO NPs are able to target tumor sites and alleviate hypoxia. Meanwhile, as a sonosensitizer, MnPcS is excited under US irradiation and activates dioxygen to generate abundant singlet oxygen (¹O₂), resulting in oxidative damage of tumor cells. Guided by photoacoustic and magnetic resonance dual-modal imaging, the MnPcS@HPO NPs alleviate tumor hypoxia and achieve good SDT efficiency for suppressing tumor growth. This work presents a novel insight into enhanced SDT antitumor activity through natural protein-mediated tumor microenvironment improvement.

Exogenous Adenosine Antagonizes Excitatory Amino Acid Toxicity in Primary Astrocytes

Excitatory toxicity is still a hot topic in the study of ischemic stroke, and related research has focused mainly on neurons. Adenosine is an important neuromodulator that is known as a "biosignature" in the central nervous system (CNS). The protective effect of exogenous adenosine on neurons has been confirmed, but its mechanism remains elusive. In this study, astrocytes were pretreated with adenosine, and the effects of an A2a receptor (A2aR) inhibitor (SCH58261) and A2b receptor (A2bR) inhibitor (PSB1115) on excitatory glutamate were investigated. An oxygen glucose deprivation/reoxygenation (OGD/R) and glutamate model was generated in vitro. Post-model assessment included expression levels of glutamate transporters (glt-1), gap junction protein (Cx43) and glutamate receptor (AMPAR), Na+-K+-ATPase activity, and diffusion distance of dyes. Glutamate and glutamine contents were determined at different time points. The results showed that (1) adenosine could improve the function of Na+-K+-ATPase, upregulate the expression of glt-1, and enhance the synthesis of glutamine in astrocytes. This effect was associated with A2aR activation but not with A2bR activation. (2) Adenosine could inhibit the expression of gap junction protein (Cx43) and reduce glutamate diffusion. Inhibition of A2aR attenuated adenosine inhibition of gap junction intercellular communication (GJIC) in the OGD/R model, while it enhanced adenosine inhibition of GJIC in the glutamate model, depending on the glutamate concentration. (3) Adenosine could cause AMPAR gradually entered the nucleus from the cytoplasm, thereby reducing the expression of AMPAR on the cell membrane. Taken together, the results indicate that adenosine plays a role of anti-excitatory toxicity effect in protection against neuronal death and the functional recovery of ischemic stroke mainly by targeting astrocytes, which are closely related to A2aR. The present study provided a scientific basis for adenosine prevention and ischemic stroke treatment, thereby providing a new approach for alleviating ischemic stroke.

Selective adenosine A2A receptor inhibitor SCH58261 reduces oligodendrocyte loss upon brain injury in young rats

Cellular elements of maturing brain are vulnerable to insults, which lead to neurodevelopmental defects. There are no established treatments at present. Here we examined the efficacy of selective adenosine A_2A receptor inhibitor SCH58261 to combat brain injury, particularly oligodendrocyte (OL) lineage cells, in young rats. Wistar rats (n = 24, 6.5 days old) were randomly divided into equal groups of four. The sham (SHAM) group received no treatment, the vehicle (VEHICLE) group received 0.1% dimethylsufoxide, the injury (INJ) group was exposed to oxygen-glucose deprivation insult, and the injury+SCH58261 (INJ+SCH58261) group was exposed to the insult and received 1 μM SCH58261. Immunocytochemical experiments revealed that there was a significant reduction in the populations of mature OL (MBP⁺ OLs) and immature OL precursors (NG2⁺ OPCs) in the INJ group compared to SHAM group. Furthermore, there was also a significant increase in the percent of apoptotic MBP⁺ OL and NG2⁺ OPC populations as evidenced by TUNEL assay. In addition, there was a significant reduction in the proliferation rate among NG2⁺ OPCs, which was confirmed by BrdU immunostaining. On the other hand, treatment with SCH58261 significantly enhanced survival, evidenced by the reduction in apoptotic indices for both cell types, and it is preserved the NG2⁺ OPC proliferation. Activation of adenosine A_2A receptors may contribute to OL lineage cell loss in association with decreased mitotic behavior of OPCs in neonatal brains upon injury. Future investigations assessing ability of SCH58261 to regenerate myelin will provide insights into its wider clinical relevance.

An Ethacrynic Acid-Brominated BODIPY Photosensitizer (EA-BPS) Construct Enhances the Lethality of Reactive Oxygen Species in Hypoxic Tumor-Targeted Photodynamic Therapy

Despite being a clinically approved intervention for cancer, photodynamic therapy (PDT) still suffers from limitations. Prime among these is a therapeutic response that is mostly oxygen dependent. This limits the utility of PDT in treating hypoxic tumors since lower levels of cytotoxic reactive oxygen species (ROS) are generated in regions of low oxygen tension. Glutathione-pi (GST-pi) is a key enzyme that militates against ROS-mediated apoptosis. We report herein a new construct, EA-BPS, that contains both a brominated BODIPY photosensitizer (BPS) and an ethacrynic acid (EA) GST-pi inhibitor. Photoirradiation of EA-BPS induces a synergistic antitumor effect that results from the combination of ROS production and GST-pi inhibition. Relative to BPS alone, an enhanced cell-killing effect is seen under hypoxic conditions both in vitro and in vivo. We conclude that by making better use of the available oxygen in tumor environments, improved therapeutic PDT outcomes should be achievable even under hypoxic conditions.

In Situ Photocatalysis of TiO-Porphyrin-Encapsulated Nanosystem for Highly Efficient Oxidative Damage against Hypoxic Tumors

Reactive oxygen species (ROS)-mediated cell apoptosis has been a significant strategy for tumor oxidative damage, while tumor hypoxia is a major bottleneck for efficiency. Here, a novel TiO-porphyrin nanosystem (FA-TiOPs) is designed by encapsulating TiO-porphyrin (TiOP) in folate-liposome. The nanosysytem can photocatalyze H₂O and tumor-overexpressed H₂O_2, in situ generating sufficient ROS. TiOP can photosplit water to produce ·OH radical, H₂O₂, and O₂. Generated O₂ not only conquers the hypoxia of tumor environment but also can be further excited by TiOP to ¹O₂ for killing tumor cells. Density functional theory calculations indicate that high energy in excited state (S₁) of TiOP and narrow gap energy between S₁ and the triplet excited state (T_n) might contribute to the efficient photocatalytic action. Moreover, the generated and overexpressed H₂O₂ in tumors can also be photocatalyzed to generate ¹O₂ especially in acid condition, helpful to specific anticancer effect while harmless to normal tissues. This research might pave a new way to bypass the hypoxia-triggered problem for cancer therapy.

Audit of practice in Australasian hyperbaric units on the incidence of central nervous system oxygen toxicity

INTRODUCTION

Central nervous system oxygen toxicity (CNS-OT) is an uncommon complication of hyperbaric oxygen treatment (HBOT). Different facilities have developed local protocols in an attempt to reduce the risk of CNS-OT. This audit was performed to elucidate which protocols might be of benefit in mitigating CNS-OT and to open discussion on adopting a common protocol for Treatment Table 14 (TT14) to enable future multicentre clinical trials.

METHODS

Audit of CNS-OT events between units using different compression profiles for TT14, performed at 243 kPa with variable durations of oxygen breathing and 'air breaks', to assess whether there is a statistical diference between protocols. Data were collected retrospectively from public and private hyperbaric facilities in Australia and New Zealand between 01 January 2010 and 31 December 2014.

RESULTS

Eight of 15 units approached participated. During the five-year period 5,193 patients received 96,670 treatments. There were a total of 38 seizures in 33 patients when all treatment pressures were examined. In the group of patients treated at 243 kPa there were a total of 26 seizures in 23 patients. The incidence of seizure per treatment was 0.024% (2.4 per 10,000 treatments) at 243 kPa and the risk per patient was 0.45% (4.5 in 1,000 patients). There were no statistically significant differences between the incidences of CNS-OT using different TT14 protocols in this analysis.

CONCLUSION

HBOT is safe and CNS-OT is uncommon. The risk of CNS-OT per patient at 243 kPa was 1 in 222 (0.45%; range 0-1%) and the overall risk irrespective of treatment table was 0.6% (range 0.31-1.8%). These figures are higher than previously reported as they represent individual patient risk as opposed to risk per treatment. The wide disparity of facility protocols for a 243 kPa table without discernible influence on the incidence of CNS-OT rates should facilitate a national approach to consensus.

Effect of Interaction between Adenosine and Nitric Oxide on Central Nervous System Oxygen Toxicity

The metabolism of adenosine (ADO) and nitric oxide (NO) in brain tissues is closely associated with the change of oxygen content. They have contrary effects in the onset of hyperbaric oxygen (HBO)-induced central nervous system oxygen toxicity (CNS OT): ADO can suppress the onset, while NO promotes it. We adopted the ADO-augmenting measure and NO-inhibiting measure in this study and found the combined use had a far superior preventive and therapeutic effect in protecting against CNS OT compared with the use of either measure alone. So we hypothesized that there is an interaction between ADO and NO which has an important impact on the onset of CNS OT. On this basis, we administered ADO-augmenting or ADO-inhibiting drugs to rats. After exposure to HBO, the onset of CNS OT was evaluated, followed by the measurement of NO content in brain tissues. In another experiment, rats were administered NO-augmenting or NO-inhibiting drugs. After exposure to HBO, the onset of CNS OT was evaluated, followed by measurement of the activities of ADO metabolism-related enzymes in brain tissues. The results showed that, following ADO augmentation, the content of NO and its metabolite was significantly reduced, and the onset of CNS OT significantly improved. After ADO inhibition, just the opposite was observed. NO promotion resulted in a decrease in the activity of ADO-producing enzyme, an increase in the activity of ADO-decomposing enzyme, and an aggravation in CNS OT. The above results were all reversed after an inhibition in NO content. Studies have shown that exposure to HBO has a significant impact on the content of ADO and NO in brain tissues as well as their biological effects, and ADO and NO might have an intense interaction, which might generate an important effect on the onset of CNS OT. The prophylaxis and treatment effects of CNS OT can be greatly enhanced by augmenting ADO and inhibiting NO.

A Multifunctional Biomimetic Nanoplatform for Relieving Hypoxia to Enhance Chemotherapy and Inhibit the PD-1/PD-L1 Axis

Hypoxia is reported to participate in tumor progression, promote drug resistance, and immune escape within tumor microenvironment, and thus impair therapeutic effects including the chemotherapy and advanced immunotherapy. Here, a multifunctional biomimetic core-shell nanoplatform is reported for improving synergetic chemotherapy and immunotherapy. Based on the properties including good biodegradability and functionalities, the pH-sensitive zeolitic imidazolate framework 8 embedded with catalase and doxorubicin constructs the core and serves as an oxygen generator and drug reservoir. Murine melanoma cell membrane coating on the core provides tumor targeting ability and elicits an immune response due to abundance of antigens. It is demonstrated that this biomimetic core-shell nanoplatform with oxygen generation can be partial to accumulate in tumor and downregulate the expression of hypoxia-inducible factor 1α, which can further enhance the therapeutic effects of chemotherapy and reduce the expression of programmed death ligand 1 (PD-L1). Combined with immune checkpoints blockade therapy by programmed death 1 (PD-1) antibody, the dual inhibition of the PD-1/PD-L1 axis elicits significant immune response and presents a robust effect in lengthening tumor recurrent time and inhibiting tumor metastasis. Consequently, the multifunctional nanoplatform provides a potential strategy of synergetic chemotherapy and immunotherapy.

Oxygenating the way for enhanced chemophototherapy

Hypoxia is behind tumor resistance in both chemotherapy and photodynamic therapy. This editorial highlights a study by Cai et al. [12] that a hemoglobin and human serum albumin hybrid protein nanoparticle can simultaneously deliver O₂, chemotherapeutics, and photosensitizers to tumors for enhanced chemophototherapy.

Tumor-targeted hybrid protein oxygen carrier to simultaneously enhance hypoxia-dampened chemotherapy and photodynamic therapy at a single dose

Hypoxia is a characteristic feature of solid tumors and an important causation of resistance to chemotherapy and photodynamic therapy (PDT). It is challenging to develop efficient functional nanomaterials for tumor oxygenation and therapeutic applications.

Methods: Through disulfide reconfiguration to hybridize hemoglobin and albumin, tumor-targeted hybrid protein oxygen carriers (HPOCs) were fabricated, serving as nanomedicines for precise tumor oxygenation and simultaneous enhancement of hypoxia-dampened chemotherapy and photodynamic therapy. Based on encapsulation of doxorubicin (DOX) and chlorin e6 (Ce6) into HPOCs to form ODC-HPOCs, the mechanism and therapeutic efficacy of oxygen-enhanced chemo-PDT was investigated in vitro and in vivo.

Results: The precise oxygen preservation and release of the HPOC guaranteed sufficient tumor oxygenation, which is able to break hypoxia-induced chemoresistance by downregulating the expressions of hypoxia-inducible factor-1α (HIF-1α), multidrug resistance 1 (MDR1) and P-glycoprotein (P-gp), resulting in minimized cellular efflux of chemodrug. Moreover, the oxygen supply is fully exploited for upgrading the generation of reactive oxygen species (ROS) during the photodynamic process. As a result, only a single-dose treatment of the HPOCs-based chemo-PDT exhibited superior tumor suppression. The combination therapy was guided by in vivo fluorescence/photoacoustic imaging with nanoparticle tracking and oxygen monitoring.

Conclusion: This well-defined HPOC as a versatile nanosystem is expected to pave a new way for breaking multiple hypoxia-induced therapeutic resistances to achieve highly effective treatment of solid tumors.