P2X Receptors: Potential Therapeutic Targets for Symptoms Associated With Lung Cancer - A Mini Review.

Lung cancer is one of the most common types of cancer, annually causing about 1.8 million deaths worldwide ( 1 ). The early detection of lung cancer and improvements in treatment have increased the survival of patients ( 2 ). The number of lung cancer survivors is anticipated to increase to 673,000 by 2026 ( 3 ). Cancer survivors experience a complex profile of diverse symptomatology, adversely affecting their quality of life. Consequently, patients complain of a wide variety of symptoms that are mainly categorized as physical and psychological symptoms. Dodd et al. defined the cancer-related symptoms as “symptom clusters” ( 4 ). Specifically, in lung cancer patients, respiratory clusters are identified as pain, cough, fatigue, and dyspnea ( 5 , 6 ). However, cancer patients present multiple symptoms, and lung cancer studies illustrate the complex correlations between different clusters of symptoms. Clinically, opiate-derivative treatment is the mainstay of the drug management of cancer-related symptoms, such as pain, fatigue, cough, and dyspnea ( 7 – 9 ), but benefits relatively few cancer patients. Currently, physicians prescribe opioid therapy without high-quality evidence, which might heighten the potential risk of opioid-induced side effects in patients with cancer, such as sedation, respiratory depression, tolerance development, and gastrointestinal dysmotility ( 10 , 11 ). Therefore, an in-depth longitudinal exploration of lung cancer related-symptoms is essential to developing a targeted intervention to improve the quality of life of cancer survivors. Accumulated adenosine triphosphatez (ATP) and other extracellular nucleotides shape tumor microenvironment (TME), significantly influencing cancer proliferation, progression, tumor/immune-cell cross-talk, and related symptoms ( 12 , 13 ). Accordingly, receptors for extracellular ATP (G-coupled P2Y and P2X ion channels) are involved in driving several functions during tumor initiation and development. To note, the ATP-gated P2X purine family consists of seven subtypes (P2X1–7 receptors) ( 14 ), with some being demonstrated to directly or indirectly regulate tumor proliferation, angiogenesis, and dissemination. For example, P2X4, P2X5, and P2X7 receptors exist on the membrane of multiple tumor cells, such as non-small cell lung cancer, colorectal cancer, bladder cancer, renal cancer, as well as haematological malignancies, to promote the proliferation and metastatic potential of the tumor ( 15 – 20 ). In particular, the P2X7 receptor is the subtype associated with cancer proliferation ( 21 ), and its activation promotes VEGF-dependent angiogenesis and extracellular matrix degradation via protease releasing and cytoskeletal remodeling, playing a prometastatic role in cancer ( 22 – 25 ). A study analyzing P2X7 mRNA expressions in patients with non-small cell lung cancer (NSCLC) revealed an upregulated P2X7 expression in bronchoalveolar lavage fluid of tumor with distant metastases ( 20 ). To understand its proliferative and prometastatic roles in tumor, the potential interaction of P2X7R splice variants and cancer cell determination should be discussed. Evidence which appeared on non-pore functional P2X7R (nfP2X7) and P2X7B isoforms in a wide range of tumors suggested that lacking the pore-forming cytotoxic activity enables them to retain a distinct pro-survival trophic property and promote oncologic progression ( 26 , 27 ). Collectively, the purinergic/adenosinergic system regulates the growth, metastasis, and invasion of cancer, thus rendering P2X purine receptors as potential targets for tumor therapy ( 13 ). More data demonstrated that ATP, its hydrolyzation products, ectonucleotidases, (degrading enzymes, like CD39), and purinergic receptors play a significant role in the modulation of the TME immune component. Extracellular nucleotides and P2 purinergic signaling drive the recruitment of inflammatory cells (such as macrophages, neutrophils, DCs, and microglia) and adjust immunomodulation on tumor sites ( 12 ). The purinergic/adenosinergic system modulates cytokine gene expression within the nervous and immune systems and also regulates the secretion of pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α ( 28 – 30 ). With the collaboration between anti-CD39 and P2X7 activation in the TME, immune cells can bring an antitumor response by P2X7-mediated NLRP3 inflammasome activation and IL-18 release from myeloid cells ( 31 , 32 ). The relationship of receptor polymorphism and inflammatory responses (including NLRP3 inflammasome activation and IL-1β and IL-8 release) was reported by Hu and colleagues ( 33 ). Besides purinergic receptors’ involvement, sensory nerves are also found to be involved in the stimulation of cancer progression, indicating the existence of tumor-nerve interactions. Reportedly, the denervation of vagus nerves and ablation of sensory neurons inhibit tumor initiation and progression in mouse models with cancer ( 34 , 35 ). Herein, we hypothesize that ATP acts as a pivotal transmitter to convey sensory stimuli from peripheral nerves to the CNS, to activate P2X purine receptors (P2X2, P2X3, P2X4, and P2X7 receptors) expressed on sensory nerve fibers and microglia, to enhance peripheral neural information transmission, as well as to sensitize the CNS ( 36 ). A study reinforced this hypothesis that ATP is transported into secretory vesicles in primary afferents and spinal cord by vesicular nucleotide transporter (VNUT) to stimulate related purinergic receptors (i.e. P2X4R), which has been proved in genetic knockout or VNUT inhibitors to relieve neuropathic and inflammatory pain sensation ( 37 ). Marked upregulation of P2X4 receptors was detected in C6 glioma tissue; these receptors also activate microglia in the central nervous system (CNS) and tumor-associated macrophages in the peripheral system to mediate inflammatory reactions ( 38 ). Taken together, those evidence highlighted the crosstalk between nervous and immune systems via P2X pathways. Thus, dissecting the neuro-immune pathways via P2X receptors may provide new therapeutic strategies in cancer treatment. Intriguingly, a high concentration of extracellular ATP in the tumor milieu is able to regulate cancer cell death by exploiting ATP-dependent cytotoxicity ( 39 ). Purinergic receptors’ cytotoxic functions are shown under the condition of persistent over-stimulation of high levels of ATP. Prolonged stimulation of P2X7 receptor via high dosage ATP leads to the opening of a larger conductance membrane pore, which in turn induces tumor cell death and inhibits tumor growth ( 15 ). Sustained opening of the P2X7R macropore stimulated by high extracellular ATP concentration in the TME triggers caspase-3 cleavage and then leads to membrane progression and ultimate cell death via different pathways ( 15 , 40 ). Therefore, it makes sense that the application of P2X receptor agonists, such as ATP and P2X receptor activators, restrains tumor growth. Preclinical cancer models revealed the efficacy of administration of ATP at a high dosage to suppress tumor growth ( 41 , 42 ). Clinical trials in treating advanced NSCLC patients with intravenous infusion of ATP showed a significant improvement in quality of life and cachexia effects ( 43 ). A recent study demonstrated the pro-apoptotic mechanism of P2X7R stimulation, showing that combined with an αPD-1 immune checkpoint inhibitor, HEI3090, as a P2X7R agonist, activates tumor regression in 80% of Lewis lung carcinoma tumor-bearing mice. During tumor inhibition, P2X7R’s cytotoxic activities were described in immune cells, especially dendritic cells, to release pro-inflammatory cytokines (IL-1β and IL-18) via an NLRP3-dependent pathway ( 44 ). Despite the promising efficacy of P2XR agonist therapy being displayed, side effects should be noticed. In intravenous ATP infusions, dyspnea emerged as the most common side effects, followed by chest tightness, urge to take a deep breath, and cardiac disorders ( 45 ). We speculated that ATP agonists activate various P2X receptors, especially P2X3R, P2X4R, and P2X7R, in other tissues or organs to induce a series of symptoms. Their predominant roles are also reported in mediating immune responses and the nervous system (see below). Based on the aforementioned side effects, it is tempting to hypothesize that compared with P2X antagonists/inhibitors, therapies based on P2X agonists would get patients more exposed to more risks of discomforts (i.e. pain sensations, dyspnea, immune system disorders). Whether P2X agonists or antagonists should be adopted depends on multiple factors, including genetic differences ( 46 , 47 ), different cancer types and onset sites, P2X receptor tumor expression ( 48 ), and cancer-associated symptoms (pain, dyspnea, fatigue, etc.). More studies are required to elucidate further use of P2X receptors-targeted therapies in cancer patients in the future. A large number of studies have highlighted the role of ATP and P2X purine receptors on the development and progression of cancer. Thus, it could be speculated that a putative P2X-dependent mechanism affects cancer-related symptoms, including pain, fatigue, cough, and dyspnea. Herein, we summarize the related findings on the potential correlation between different types of cancer symptom clusters and P2X purine signaling, providing an outline for the potential treatment of distressing cancer-related symptoms using P2X receptor antagonists ( Figure 1 ). The anatomical and molecular mediators of lung cancer symptom cluster. Ca 2+ , calcium; ATP, adenosine triphosphate; BDNF, brain-derived neurotrophic factor.

YM, ZG, and WY drafted the manuscript. NZ, PD, and RC designed and revised the manuscript. All authors contributed to the article and approved the submitted version.

Cancer-related symptoms may significantly and adversely affect patients with lung cancer, causing a marked diminution in the quality of life. Hence, the potential therapeutic roles of P2X receptors in cancer as well as cancer-related symptoms need to be further elucidated. Intratumoral ATP, ATP-P2X receptors, and the vagus nerve are involved in the neuro-immune interactions in cancer. Increased extracellular ATP and its associated compounds activate P2X receptors expressed on bronchopulmonary nerves to evoke unpleasant sensations (such as the urge to cough, dyspnea, chest tightness, fatigue, and pain) and stimulate/modulate reflexes (such as cough, bronchoconstriction, respiratory rate, and inspiratory drive). Various cancer-related symptoms may share similar pathogenic mechanisms. To date, a significant clinical investigation has established a role for P2X receptor antagonists in the treatment of chronic cough and cancer-related pain. Thus, a basis has been formed for the further evaluation of potential therapeutic roles for P2X antagonists in the treatment of other cancer-induced symptoms.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Moderate or severe dyspnea is reported in 20–80% of cancer patients. Dyspnea is a common symptom in patients with advanced cancer, accompanied by cough due to primary lung cancer in the airway or metastasis of other cancers to the bronchus. Especially, intraluminal tumor in the trachea or a mainstem bronchus activates cough receptors and mechanically obstructs airflow to elicit the sensation of dyspnea ( 117 ). Dyspnea is a multifactorial sensation associated with central, peripheral, and chemoreceptor modulation ( 118 ). Notably, vagal nerves innervating both the airways and lungs and transmitting to the sensorimotor cortex are capable of inducing dyspnea ( 119 ). Several studies have indicated that vagal C-fibers are of dyspneic origin and can be triggered by ATP and adenosine. Furthermore, some Aδ stretch fibers can be directly stimulated by ATP similar to C-fibers ( 53 , 120 , 121 ). In clinical trials, dyspnea could be evoked by the direct C-fiber activators ATP and adenosine during bronchial challenge testing ( 122 ). Inhaling ATP and adenosine induced dyspnea in subjects with asthma ( 123 ) and COPD ( 124 ). Additionally, intravenous injection of adenosine induced distressing symptoms such as dyspnea, hyperventilation, urge for deep breathing, and chest tightness in healthy volunteers ( 125 ), and pre-terminal cancer patients ( 45 , 126 ). Taken together, extracellular ATP is likely to give rise to dyspnea, and thus may be a potential target for the treatment of dyspnea. Hypoxemia may be one of the factors stimulating ventilation and producing dyspnea in lung cancer patients ( 127 ). Furthermore, central cardiovascular and respiratory neuronal networks are intertwined during and post-hypoxia ( 128 ). In knockout P2X2 mice, the activation of the chemoreceptor of the carotid sinus nerve transmits information about arterial pO 2 to the respiratory centers of the brain to mediate ventilatory responses to hypoxia. The P2X2 receptor antagonist PPADS was shown to block neural discharge evoked by hypoxia ( 129 ). Furthermore, Zhang et al. reported that suramin, a P2X receptor antagonist, combined with a nicotinic Ach receptor antagonist, blocked the hypoxia-induced increase in chemoreceptor afferent neuronal discharge ( 130 ). Collectively, these studies suggest that the inhibition of specific ATP signal transduction pathways, such as with P2X3 receptor antagonists in the lungs, constitutes an attractive target for the development of new therapies to ameliorate dyspnea. However, additional randomized, double-blinded, placebo-controlled clinical studies are required to confirm whether P2X antagonists are able to eliminate or improve cancer-induced dyspnea.

This study was supported by grants from the National Natural Science Foundation of China (81870079) and National Outstanding Youth Incubation Program of Guangzhou Medical University(2020-207).