Intrathecal administration of proteinase-activated receptor-2 agonists produces hyperalgesia by exciting the cell bodies of primary sensory neurons

Metabolic underpinnings of cancer-related fatigue

Cancer-related fatigue (CRF) is one of the most prevalent and detrimental complications of cancer. Emerging evidence suggests that obesity and insulin resistance are associated with CRF occurrence and severity in cancer patients and survivors. In this narrative review, we analyzed recent studies including both preclinical and clinical research on the relationship between obesity and/or insulin resistance and CRF. We also describe potential mechanisms for these relationships, though with the caveat that because the mechanisms underlying CRF are incompletely understood, the mechanisms mediating the association between obesity/insulin resistance and CRF are similarly incompletely delineated. The data suggest that, in addition to their effects to worsen CRF by directly promoting tumor growth and metastasis, obesity and insulin resistance may also contribute to CRF by inducing chronic inflammation, neuroendocrinological disturbance, and metabolic alterations. Furthermore, studies suggest that patients with obesity and insulin resistance experience more cancer-induced pain and are at more risk of emotional and behavioral disruptions correlated with CRF. However, other studies implied a potentially paradoxical impact of obesity and insulin resistance to reduce CRF symptoms. Despite the need for further investigation utilizing interventions to directly elucidate the mechanisms of cancer-related fatigue, current evidence demonstrates a correlation between obesity and/or insulin resistance and CRF, and suggests potential therapeutics for CRF by targeting obesity and/or obesity-related mediators.

Protease-activated receptors in health and disease

Proteases are signaling molecules that specifically control cellular functions by cleaving protease-activated receptors (PARs). The four known PARs are members of the large family of G protein-coupled receptors. These transmembrane receptors control most physiological and pathological processes and are the target of a large proportion of therapeutic drugs. Signaling proteases include enzymes from the circulation; from immune, inflammatory epithelial, and cancer cells; as well as from commensal and pathogenic bacteria. Advances in our understanding of the structure and function of PARs provide insights into how diverse proteases activate these receptors to regulate physiological and pathological processes in most tissues and organ systems. The realization that proteases and PARs are key mediators of disease, coupled with advances in understanding the atomic level structure of PARs and their mechanisms of signaling in subcellular microdomains, has spurred the development of antagonists, some of which have advanced to the clinic. Herein we review the discovery, structure, and function of this receptor system, highlight the contribution of PARs to homeostatic control, and discuss the potential of PAR antagonists for the treatment of major diseases.

Protease Activated Receptors and Arthritis

The catabolic and destructive activity of serine proteases in arthritic joints is well known; however, these enzymes can also signal pain and inflammation in joints. For example, thrombin, trypsin, tryptase, and neutrophil elastase cleave the extracellular N-terminus of a family of G protein-coupled receptors and the remaining tethered ligand sequence then binds to the same receptor to initiate a series of molecular signalling processes. These protease activated receptors (PARs) pervade multiple tissues and cells throughout joints where they have the potential to regulate joint homeostasis. Overall, joint PARs contribute to pain, inflammation, and structural integrity by altering vascular reactivity, nociceptor sensitivity, and tissue remodelling. This review highlights the therapeutic potential of targeting PARs to alleviate the pain and destructive nature of elevated proteases in various arthritic conditions.

Spinal PAR2 Activation Contributes to Hypersensitivity Induced by Peripheral Inflammation in Rats

The mechanisms of inflammatory pain need to be identified in order to find new superior treatments. Protease-activated receptors 2 (PAR2) and transient receptor potential vanilloid 1 (TRPV1) are highly co-expressed in dorsal root ganglion neurons and implicated in pain development. Here, we examined the role of spinal PAR2 in hyperalgesia and the modulation of synaptic transmission in carrageenan-induced peripheral inflammation, using intrathecal (i.t.) treatment in the behavioral experiments and recordings of spontaneous, miniature and dorsal root stimulation-evoked excitatory postsynaptic currents (sEPSCs, mEPSCs and eEPSCs) in spinal cord slices. Intrathecal PAR2-activating peptide (AP) administration aggravated the carrageenan-induced thermal hyperalgesia, and this was prevented by a TRPV1 antagonist (SB 366791) and staurosporine i.t. pretreatment. Additionally, the frequency of the mEPSC and sEPSC and the amplitude of the eEPSC recorded from the superficial dorsal horn neurons were enhanced after acute PAR2 AP application, while prevented with SB 366791 or staurosporine pretreatment. PAR2 antagonist application reduced the thermal hyperalgesia and decreased the frequency of mEPSC and sEPSC and the amplitude of eEPSC. Our findings highlight the contribution of spinal PAR2 activation to carrageenan-induced hyperalgesia and the importance of dorsal horn PAR2 and TRPV1 receptor interactions in the modulation of nociceptive synaptic transmission.

Ketamine reduces remifentanil-induced postoperative hyperalgesia mediated by CaMKII-NMDAR in the primary somatosensory cerebral cortex region in mice

Remifentanil is commonly used clinically for perioperative pain relief, but it may induce postoperative hyperalgesia. Low doses of ketamine have remained a common choice in clinical practice, but the mechanisms of ketamine have not yet been fully elucidated. In this study, we examined the possible effects of ketamine on calcium/calmodulin-dependent protein kinase II α (CaMKIIα) and N-methyl-d-aspartate receptor (NMDAR) subunit NR2B in a mouse model of remifentanil-induced postoperative hyperalgesia (RIPH) in the primary somatosensory cerebral cortex (SI) region. The paw withdrawal mechanical threshold (PWMT) and paw withdrawal thermal latency (PWTL) were used to assess mechanical allodynia and thermal hyperalgesia, respectively, before and after intraoperative remifentanil administration. Before surgery, mice received intrathecal injections of the following drugs: ketamine, NMDA, BayK8644 (CaMKII activator), and KN93 (CaMKII inhibitor). Immunofluorescence was performed to determine the anatomical location and expression of activated CaMKIIα, phosphorylated CaMKIIα (p-CaMKIIα). Additionally, western blotting was performed to assess p-CaMKIIα and NMDAR expression levels in the SI region. Remifentanil decreased the PWMT and PWTL at 0.5 h, 2 h, and 5 h and increased p-CaMKIIα expression in the SI region. Ketamine increased the PWMT and PWTL and reversed the p-CaMKIIα upregulation. Both BayK8644 and NMDA reversed the effect of ketamine, decreased the PWMT and PWTL, and upregulated p-CaMKIIα expression. In contrast, KN93 enhanced the effect of ketamine by reducing hyperalgesia and downregulating p-CaMKIIα expression. These results suggested that ketamine reversed RIPH by inhibiting the phosphorylation of CaMKIIα and the NMDA receptor in the SI region in mice.

Mechanisms of Probiotic VSL#3 in a Rat Model of Visceral Hypersensitivity Involves the Mast Cell-PAR2-TRPV1 Pathway

BACKGROUND

Mast cells (MCs), PAR2 and TRPV1, play a key role in the regulation of visceral pain. Several studies have found that probiotics regulate visceral sensitivity.

AIMS

The purpose of the current study was to explore the role of MC-PAR2-TRPV1 in VH and the mechanism of VSL#3 in a rat model of VH.

METHODS

A total of 64 rats were randomly divided into eight groups: Control VH, VH + ketotifen, VH + FSLLRY-NH2, VH + SB366791, VH + VSL#3, VH + VSL#3 + capsaicin, and VH + VSL#3 + SLIGRL-NH2. The rat model of VH was induced by acetic acid enema and the partial limb restraint method. VH was assessed by the abdominal withdrawal reflex score. MCs in colonic tissue were detected by the toluidine blue staining assay. The expression of PAR2 and TRPV1 in DRGs (L6-S1) was measured by immunohistochemistry and Western blotting.

RESULTS

The established VH was abolished by treatment with ketotifen, a mast cell stabilizer FSLLRY-NH2, a PAR2 antagonist SB366791 a TRPV1 antagonist, and probiotic VSL#3 in rats. The administration of ketotifen or probiotic VSL#3 caused a decrease in mast cell number in the colon and decreased PAR2 and TRPV1 expression in DRGs. Intrathecal injection of FSLLRY-NH2 or SB366791 caused decreased expression of PAR2 and/or TRPV1 in DRGs in VH rats. SLIGRL-NH2, a PAR2 agonist, and capsaicin, a TRPV1 agonist, blocked the effects of probiotic VSL#3.

CONCLUSIONS

The probiotic VSL#3 decreases VH in rat model of VH. The mechanism may be related with the mast cell-PAR2-TRPV1 signaling pathway.

Cyclic nucleotide signaling in sensory neuron hyperexcitability and chronic pain after nerve injury

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Activation of cAMP-PKA and cGMP-PKG pathways contributes to injury-induced sensory neuron hyperexcitability.

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Activation of cAMP and cGMP contributes to the development of bone cancer pain.

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PAR2 activation mediates injury-induced cAMP-dependent sensory neuron hyperexcitability.

The cyclic nucleotide signaling, including cAMP-PKA and cGMP-PKG pathways, has been well known to play critical roles in regulating cellular growth, metabolism and many other intracellular processes. In recent years, more and more studies have uncovered the roles of cAMP and cGMP in the nervous system. The cAMP and cGMP signaling mediates chronic pain induced by different forms of injury and stress. Here we summarize the roles of cAMP-PKA and cGMP-PKG signaling pathways in the pathogenesis of chronic pain after nerve injury. In addition, acute dissociation and chronic compression of the dorsal root ganglion (DRG) neurons, respectively, leads to neural hyperexcitability possibly through PAR2 activation-dependent activation of cAMP-PKA pathway. Clinically, radiotherapy can effectively alleviate bone cancer pain at least partly through inhibiting the cancer cell-induced activation of cAMP-PKA pathway. Roles of cyclic nucleotide signaling in neuropathic and inflammatory pain are also seen in many other animal models and are involved in many pro-nociceptive mechanisms including the activation of hyperpolarization-activated cyclic nucleotide (HCN)-modulated ion channels and the exchange proteins directly activated by cAMP (EPAC). Further understanding the roles of cAMP and cGMP signaling in the pathogenesis of chronic pain is theoretically significant and clinically valuable for treatment of chronic pain.

Trypsin-protease activated receptor-2 signaling contributes to pancreatic cancer pain

Pain treatment is a critical aspect of pancreatic cancer patient clinical care. This study investigated the role of trypsin-protease activated receptor-2 (PAR-2) in pancreatic cancer pain. Pancreatic tissue samples were collected from pancreatic cancer (n=22) and control patients (n=22). Immunofluorescence analyses confirmed colocalization of PAR-2 and neuronal markers in pancreatic cancer tissues. Trypsin levels and protease activities were higher in pancreatic cancer tissue specimens than in the controls. Supernatants from cultured human pancreatic cancer tissues (PC supernatants) induced substance P and calcitonin gene-related peptide release in dorsal root ganglia (DRG) neurons, and FS-NH₂, a selective PAR-2 antagonist, inhibited this effect. A BALB/c nude mouse orthotopic tumor model was used to confirm the role of PAR-2 signaling in pancreatic cancer visceral pain, and male Sprague-Dawley rats were used to assess ambulatory pain. FS-NH₂ treatment decreased hunch scores, mechanical hyperalgesia, and visceromotor reflex responses in tumor-bearing mice. In rats, subcutaneous injection of PC supernatant induced pain behavior, which was alleviated by treatment with FS-NH₂ or FUT-175, a broad-spectrum serine protease inhibitor. Our findings suggest that trypsin-PAR-2 signaling contributes to pancreatic cancer pain in vivo. Treatment strategies targeting PAR-2 or its downstream signaling molecules might effectively relieve pancreatic cancer pain.

Hypersensitivity Induced by Activation of Spinal Cord PAR2 Receptors Is Partially Mediated by TRPV1 Receptors

Protease-activated receptors 2 (PAR2) and transient receptor potential vanilloid 1 (TRPV1) receptors in the peripheral nerve endings are implicated in the development of increased sensitivity to mechanical and thermal stimuli, especially during inflammatory states. Both PAR2 and TRPV1 receptors are co-expressed in nociceptive dorsal root ganglion (DRG) neurons on their peripheral endings and also on presynaptic endings in the spinal cord dorsal horn. However, the modulation of nociceptive synaptic transmission in the superficial dorsal horn after activation of PAR2 and their functional coupling with TRPV1 is not clear. To investigate the role of spinal PAR2 activation on nociceptive modulation, intrathecal drug application was used in behavioural experiments and patch-clamp recordings of spontaneous, miniature and dorsal root stimulation-evoked excitatory postsynaptic currents (sEPSCs, mEPSCs, eEPSCs) were performed on superficial dorsal horn neurons in acute rat spinal cord slices. Intrathecal application of PAR2 activating peptide SLIGKV-NH₂ induced thermal hyperalgesia, which was prevented by pretreatment with TRPV1 antagonist SB 366791 and was reduced by protein kinases inhibitor staurosporine. Patch-clamp experiments revealed robust decrease of mEPSC frequency (62.8 ± 4.9%), increase of sEPSC frequency (127.0 ± 5.9%) and eEPSC amplitude (126.9 ± 12.0%) in dorsal horn neurons after acute SLIGKV-NH₂ application. All these EPSC changes, induced by PAR2 activation, were prevented by SB 366791 and staurosporine pretreatment. Our results demonstrate an important role of spinal PAR2 receptors in modulation of nociceptive transmission in the spinal cord dorsal horn at least partially mediated by activation of presynaptic TRPV1 receptors. The functional coupling between the PAR2 and TRPV1 receptors on the central branches of DRG neurons may be important especially during different pathological states when it may enhance pain perception.

The role of protease-activated receptor type 2 in nociceptive signaling and pain

Protease-activated receptors (PARs) belong to the G-protein-coupled receptor family, that are expressed in many body tissues especially in different epithelial cells, mast cells and also in neurons and astrocytes. PARs play different physiological roles according to the location of their expression. Increased evidence supports the importance of PARs activation during nociceptive signaling and in the development of chronic pain states. This short review focuses on the role of PAR2 receptors in nociceptive transmission with the emphasis on the modulation at the spinal cord level. PAR2 are cleaved and subsequently activated by endogenous proteases such as tryptase and trypsin. In vivo, peripheral and intrathecal administration of PAR2 agonists induces thermal and mechanical hypersensitivity that is thought to be mediated by PAR2-induced release of pronociceptive neuropeptides and modulation of different receptors. PAR2 activation leads also to sensitization of transient receptor potential channels (TRP) that are crucial for nociceptive signaling and modulation. PAR2 receptors may play an important modulatory role in the development and maintenance of different pathological pain states and could represent a potential target for new analgesic treatments.

Role of PAR2 in regulating oxaliplatin-induced neuropathic pain via TRPA1

Oxaliplatin (OXL) is a third-generation chemotherapeutic agent commonly used to treat metastatic digestive tumors; however, one of the main limiting complications of OXL is neuropathic pain. In this study, the underlying mechanisms responsible for OXL evoked-neuropathic pain were examined. Using a rat model, the results demonstrated that intraperitoneal (i.p.) injection of OXL significantly increased mechanical pain and cold sensitivity as compared with control animals (P < 0.05 vs. control rats). Blocking proteinase-activated receptor 2 (PAR2) significantly attenuated mechanical pain and cold sensitivity observed in control rats and OXL rats (P < 0.05 vs. vehicle control). The attenuating effect of PAR2 on mechanical pain and cold sensitivity were significantly smaller in OXL-rats than in control rats. The role played by PAR2 downstream signaling pathways [namely, transient receptor potential ankyrin 1 (TRPA1)] in regulating OXL evoked-neuropathic pain was also examined. The data shows that TRPA1 expression was upregulated in the lumbar dorsal root ganglion (DRG) of OXL rats and blocking TRPA1 inhibited mechanical pain and heightened cold sensitivity (P < 0.05 vs. control rats). Blocking PAR2 also significantly decreased TRPA1 expression in the DRG. Findings in this study show that OXL intervention amplifies mechanical hyperalgesia and cold hypersensitivity and PAR2 plays an important role in regulating OXL-induced neuropathic pain via TRPA1 pathways.

Engagement of signaling pathways of protease-activated receptor 2 and μ-opioid receptor in bone cancer pain and morphine tolerance

Pain is one of the most common and distressing symptoms suffered by patients with progression of cancer. Using a rat model of bone cancer, recent findings suggest that proteinase-activated receptor 2 (PAR2) signaling pathways contribute to neuropathic pain and blocking PAR2 amplifies antinociceptive effects of systemic morphine. The purpose of our study was to examine the underlying mechanisms responsible for the role of PAR2 in regulating bone cancer-evoked pain and the tolerance of systemic morphine. Breast sarcocarcinoma Walker 256 cells were implanted into the tibia bone cavity of rats and this evoked significant mechanical and thermal hyperalgesia. Our results showed that the protein expression of PAR2 and its downstream pathways (protein kinases namely, PKCε and PKA) and transient receptor potential vanilloid 1 (TRPV1) were amplified in the dorsal horn of the spinal cord of bone cancer rats compared to control rats. Blocking spinal PAR2 by using FSLLRY-NH2 significantly attenuated the activities of PKCε/PKA signaling pathways and TRPV1 expression as well as mechanical and thermal hyperalgesia. Also, inhibition of PKCε/PKA and TRPV1 significantly diminished the hyperalgesia observed in bone cancer rats. Additionally, blocking PAR2 enhanced the attenuations of PKCε/PKA and cyclic adenosine monophosphate induced by morphine and further extended analgesia of morphine via μ-opioid receptor (MOR). Our data revealed specific signaling pathways, leading to bone cancer pain, including the activation of PAR2, downstream PKCε/PKA, TRPV1 and resultant sensitization of MOR. Targeting one or more of these signaling molecules may present new opportunities for treatment and management of bone cancer pain often observed in clinics.

New insights of nociceptor sensitization in bone cancer pain

INTRODUCTION

Numerous studies have shown that an intact CNS is required for the conscious perception of cancer-induced bone pain (CIBP) and that changes in the CNS are clearly evident. Accordingly, the blockage of nociceptive stimulus into the CNS can effectively relieve or markedly attenuate CIBP, revealing the clinical implication of the blockage of ongoing peripheral inputs for the control of CIBP.

AREAS COVERED

In this review, the heterogeneity and excitability of nociceptors in bone are covered. Furthermore, their role in initiating and maintaining CIBP is also described.

EXPERT OPINION

Developing mechanistic therapies to treat CIBP is a challenge, but they have the potential to fundamentally change our ability to effectively block/relieve CIBP and increase the functional status and quality of life of patients with bone metastasis. Further studies are desperately needed at both the preclinical and clinical levels to determine whether the targets as mentioned in this review are viable and feasible for patient populations.

Proteinase-activated receptors 1 and 2 and the regulation of porcine coronary artery contractility: a role for distinct tyrosine kinase pathways

BACKGROUND AND PURPOSE

Because angiotensin-II-mediated porcine coronary artery (PCA) vasoconstriction is blocked by protein tyrosine kinase (PYK) inhibitors, we hypothesized that proteinase-activated receptors (PARs), known to regulate vascular tension, like angiotensin-II, would also cause PCA contractions via PYK-dependent signalling pathways.

EXPERIMENTAL APPROACH

Contractions of intact and endothelium-free isolated PCA rings, stimulated by PAR1 /PAR2 -activating peptides, angiotensin-II, PGF2α , EGF, PDGF and KCl, were monitored with/without multiple signalling pathway inhibitors, including AG-tyrphostins AG18 (non-specific PYKs), AG1478 (EGF-receptor kinase), AG1296 (PDGF receptor kinase), PP1 (Src kinase), U0126 and PD98059 (MEK/MAPKinase kinase), indomethacin/SC-560/NS-398 (COX-1/2) and L-NAME (NOS).

KEY RESULTS

AG18 inhibited the contractions induced by all the agonists except KCl, whereas U0126 attenuated contractions induced by PAR1 /PAR2 agonists, EGF and angiotensin-II, but not by PGF2α , the COX-produced metabolites of arachidonate and KCl. PP1 only affected the responses to PAR1 /PAR2 -activating peptides and angiotensin-II. The EGF-kinase inhibitor, AG1478, attenuated contractions initiated by the PARs (PAR2 >> PAR1 ) and EGF itself, but not by angiotensin-II, PGF2α or KCl. COX-1/2 inhibitors blocked the contractions induced by all the agonists, except KCl and PGF2α .

CONCLUSION AND IMPLICATIONS

PAR1/2 -mediated contractions of the PCA are dependent on Src and MAPKinase and, in part, involve EGF-receptor-kinase transactivation and the generation of a COX-derived contractile agonist. However, the PYK signalling pathways used by PARs are distinct from each other and from those triggered by angiotensin-II and EGF. These signalling pathways may be therapeutic targets for managing coagulation-proteinase-induced coronary vasospasm.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

Increased expression of protease-activated receptor 2 and 4 within dorsal root ganglia in a rat model of bone cancer pain

In an effort to understand the underlying mechanisms of cancer-induced bone pain, we investigated the presence of two protease-activated receptors, protease-activated receptor 2 (PAR2), and protease-activated receptor 4 (PAR4), in dorsal root ganglia (DRGs) neurons in an animal model of bone cancer pain. Female Wistar rats were randomized into three groups: tumor-bearing animals killed after 14 days (D14) and tumor-bearing animals killed after 21 days (D21) group and a sham operation group. After establishment of the Walker 256 carcinoma bone cancer pain model, behavioral tests were carried out to determine both the spontaneous nocifensive behavior and the paw withdrawal threshold (PWT) of mechanical and thermal hyperalgesia in these rats. Subsequently, real-time RT-PCR, Western bolt, and immunofluorescence were used to determine the messenger RNA (mRNA) and protein expression of PAR2 and PAR4 in the ipsilateral lumbar 4-5 DRG neurons. Rats in the D21 treatment group displayed a significant increase in spontaneous nocifensive behavior scores compared with the sham group as well as a considerably decreased withdrawal threshold in mechanical allodynia and thermal stimulation. Compared to sham group, the relative mRNA and protein expression of PAR2 and PAR4 was significantly upregulated in the D14 group and D21 groups, concurrent with tumor growth and proliferation. In addition, we identified the co-expression of PAR2 and PAR4 in the DRG neurons. The upregulation of mRNA and protein levels as well as the co-localization of PAR2 and PAR4 in DRG neurons suggests their novel involvement in the development and maintenance of bone cancer pain.

Suppression of network activity in dorsal horn by gabapentin permeation of TRPV1 channels: implications for drug access to cytoplasmic targets

The effectiveness of gabapentin (GBP) in the treatment of neuropathic pain depends on access to the α2δ-1 accessory subunit of voltage-gated Ca(2+) channels. Access may be limited by its rate of entry via the neuronal system L-neutral amino acid transporter. The open pore of capsaicin-activated TRPV1 channel admits organic molecules such as local anesthetics and we calculated that GBP entry via this route would be 500× more rapid than via the transporter. Capsaicin should therefore increase GBP effectiveness. We used a quaternary GBP derivative (Q-GBP) as sole charge carrier in whole-cell recording experiments on rat dorsal root ganglion (DRG) neurons. Under these conditions, capsaicin produced a capsazepine-sensitive inward current thereby confirming Q-GBP permeation of TRPV1 channels. We have previously established that 5-6 days exposure to 100 μM GBP decreases excitability of dorsal horn neurons whereas 10 μM is ineffective. Excitability was monitored using confocal Ca(2+) imaging of rat spinal cord slices in organotypic culture. GBP effectiveness was augmented by transient exposures of cultures to capsaicin and robust suppression of excitability was seen with 10 μM GBP. Experiments with an inhibitor of the neutral amino acid transporter, 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid (BCH 300 μM), showed the actions of GBP seen in the presence of capsaicin were independent of its entry by this route. Capsaicin potentiation of GBP depression of dorsal horn activity may therefore reflect drug permeation of TRPV1 channels. Agonist activation of TRP channels may provide a means for improving drug access to cytoplasmic targets in selective neuronal populations defined on the basis of type of TRP channel expressed.

Plasma cytokine profiles in HIV-1 infected patients developing neuropathic symptoms shortly after commencing antiretroviral therapy: a case-control study

BACKGROUND

In patients infected with human immunodeficiency virus 1 (HIV-1) neuropathic symptoms may develop within weeks of starting combination antiretroviral therapy (cART). This timing coincides with the occurrence of immune reconstitution inflammatory syndrome. Our objective was to investigate the longitudinal association of plasma cytokine and soluble receptor concentrations with incident neuropathic symptoms within 12 weeks of starting programme-based cART in a nested case-control study.

METHODS

One hundred and twenty adults without neuropathic symptoms and about to initiate cART were followed longitudinally for 24 weeks after cART initiation. Subjects were examined for peripheral neuropathy at baseline (pre-cART) and 2-, 4-, 12- and 24 weeks thereafter. Individuals developing neuropathic symptoms within 12 weeks of starting cART were matched in a nested case-control design with those remaining symptom-free for at least 24 weeks. Plasma was collected at each visit. Cytokines and soluble receptors were quantified using multiplex immunometric assays.

RESULTS

Incident neuropathic symptoms occurred in 32 (27%) individuals within 12 weeks of starting cART for the first time. Cytokine concentrations increased at 2 weeks, irrespective of symptom-status, returning to baseline concentrations at 12 weeks. Compared to the control group, the symptomatic group had higher baseline levels of interleukin-1 receptor (IL-1R)-antagonist. The symptomatic group also showed greater increases in soluble interleukin-2 receptor-alpha and tumour necrosis factor (TNF) receptor-II levels at week 2 and soluble interleukin-6 receptor levels at week 12. Ratios of pro-inflammatory- vs anti-inflammatory cytokines were higher for TNF-alpha/IL-4 (p = 0.022) and interferon-gamma/IL-10 (p = 0.044) in those developing symptoms. After 24 weeks of cART, the symptomatic group showed higher CD4+ counts (p = 0.002).

CONCLUSIONS

The initiation of cART in previously treatment naïve individuals was associated with a cytokine 'burst' between 2- and 4 weeks compared with pre-cART levels. Individuals developing neuropathic symptoms within 12 weeks of starting cART showed evidence of altered cytokine concentrations even prior to initiating cART, most notably higher circulating IL-1R-antagonist levels, and altered ratios of "pain-associated" cytokine and soluble receptors shortly after cART initiation.

Involvement of protease-activated receptor 2 in nociceptive behavior in a rat model of bone cancer

Treatment for bone cancer pain remains a clinical challenge due to a poor understanding of the underlying mechanisms. Protease-activated receptor 2 (PAR2), a receptor for inflammatory proteases, has been implicated in nociceptive signaling under both normal and pathologic pain states. However, little is known of the role of PAR2 in cancer-induced bone pain. Here we investigated the potential role of PAR2 in a rat model of bone cancer pain. The model of bone cancer pain was induced by inoculating Walker 256 into the tibia bone cavity of rats and verified by X-ray imaging, pathology, and behavior assessments. The rats with bone cancer exhibited marked mechanical allodynia, thermal hyperalgesia, and signs of spontaneous nocifensive behavior. Subcutaneous administration of the PAR2 antagonist FSLLRY-NH2 almost completely abolished mechanical allodynia and thermal hyperalgesia but had no effects on spontaneous pain behavior in the rats with bone cancer. Immunohistochemical study revealed that the expression of PAR2 was significantly increased in large- and medium-sized dorsal root ganglia (DRG) neurons but not in small-sized neurons after Walker 256 inoculation. These results suggest that the increased expression of PAR2 in the DRG may contribute to the development of mechanical allodynia and thermal hyperalgesia associated with bone cancer rats. PAR2 might become a novel target for the treatment of pain in patients with bone cancer.

Toward drugs for protease-activated receptor 2 (PAR2)

PAR2 has a distinctive functional phenotype among an unusual group of GPCRs called protease activated receptors, which self-activate after cleavage of their N-termini by mainly serine proteases. PAR2 is the most highly expressed PAR on certain immune cells, and it is activated by multiple proteases (but not thrombin) in inflammation. PAR2 is expressed on many types of primary human cells and cancer cells. PAR2 knockout mice and PAR2 agonists and antagonists have implicated PAR2 as a promising target in inflammatory conditions; respiratory, gastrointestinal, metabolic, cardiovascular, and neurological dysfunction; and cancers. This article summarizes salient features of PAR2 structure, activation, and function; opportunities for disease intervention via PAR2; pharmacological properties of published or patented PAR2 modulators (small molecule agonists and antagonists, pepducins, antibodies); and some personal perspectives on limitations of assessing their properties and on promising new directions for PAR2 modulation.

Protease-activated receptor 2 in dorsal root ganglion contributes to peripheral sensitization of bone cancer pain

BACKGROUND

Treating bone cancer pain continues to be a major clinical challenge, and the underlying mechanisms of bone cancer pain remain elusive. Protease-activated receptor 2 (PAR2) has been reported to be involved in neurogenic inflammation, nociceptive pain and hyperalgesia. Here, we investigated the role of PAR2 in bone cancer pain development.

METHORDS

Expression of PAR2, mechanical allodynia, thermal hyperalgesia and neurochemical alterations induced by bone cancer pain were analysed in male, adult C3H/HeJ mice with tumour cell implantation (TCI). To investigate the contribution of PAR2 to bone cancer pain, PAR2 antagonist peptide and PAR2 knockout mice were used.

RESULTS

TCI produced bone cancer-related pain behaviours. Production and persistence of these pain behaviours were well correlated with TCI-induced up-regulation of PAR2 in sciatic nerve and dorsal root ganglia (DRG). PAR2 knockout and spinal administration of PAR2 antagonist peptide prevented and/or reversed bone cancer-related pain behaviours and associated neurochemical changes in DRG and dorsal horn (DH). TCI also induced proteases release in tumour-bearing tibia, sciatic nerve and DRG. Plantar injection of supernatant from sarcoma cells induced PAR2 up-regulation and intracellular calcium [Ca(2+) ]i increase in DRG, and calcitonin gene-related peptide accumulation in DH, as well as significant thermal and mechanical hyperalgesia, which were all in PAR2-dependent manners.

CONCLUSION

These findings suggest that PAR2 may be a key mediator for peripheral sensitization of bone cancer pain. Inhibiting PAR2 activation, especially during the early phase, may be a new therapy for preventing/suppressing development of bone cancer pain.

Activation of PAR(2) receptors sensitizes primary afferents and causes leukocyte rolling and adherence in the rat knee joint

BACKGROUND AND PURPOSE

The PAR(2) receptors are involved in chronic arthritis by mechanisms that are as yet unclear. Here, we examined PAR(2) activation in the rat knee joint.

EXPERIMENTAL APPROACH

PAR(2) in rat knee joint dorsal root ganglia (DRG) cells at L3-L5, retrogradely labelled with Fluoro-gold (FG) were demonstrated immunohistochemically. Electrophysiological recordings from knee joint nerve fibres in urethane anaesthetized Wistar rats assessed the effects of stimulating joint PAR(2) with its activating peptide, 2-furoyl-LIGRLO-NH(2) (1-100 nmol·100 μL(-1) , via close intra-arterial injection). Fibre firing rate was recorded during joint rotations before and 15 min after administration of PAR(2) activating peptide or control peptide. Leukocyte kinetics in the synovial vasculature upon PAR(2) activation were followed by intravital microscopy for 60 min after perfusion of 2-furoyl-LIGRLO-NH(2) or control peptide. Roles for transient receptor potential vanilloid-1 (TRPV1) or neurokinin-1 (NK(1) ) receptors in the PAR(2) responses were assessed using the selective antagonists, SB366791 and RP67580 respectively.

KEY RESULTS

PAR(2) were expressed in 59 ± 5% of FG-positive DRG cells; 100 nmol 2-furoyl-LIGRLO-NH(2) increased joint fibre firing rate during normal and noxious rotation, maximal at 3 min (normal; 110 ± 43%, noxious; 90 ± 31%). 2-Furoyl-LIGRLO-NH(2) also significantly increased leukocyte rolling and adhesion over 60 min. All these effects were blocked by pre-treatment with SB366791 and RP67580 (P < 0.05 compared with 2-furoyl-LIGRLO-NH(2) alone).

CONCLUSIONS AND IMPLICATIONS

PAR(2) receptors play an acute inflammatory role in the knee joint via TRPV1- and NK(1) -dependent mechanisms involving both PAR(2) -mediated neuronal sensitization and leukocyte trafficking.

Development of highly potent protease-activated receptor 2 agonists via synthetic lipid tethering

Protease-activated receptor-2 (PAR₂) is a G-protein coupled receptor (GPCR) associated with a variety of pathologies. However, the therapeutic potential of PAR₂ is limited by a lack of potent and specific ligands. Following proteolytic cleavage, PAR₂ is activated through a tethered ligand. Hence, we reasoned that lipidation of peptidomimetic ligands could promote membrane targeting and thus significantly improve potency and constructed a series of synthetic tethered ligands (STLs). STLs contained a peptidomimetic PAR₂ agonist (2-aminothiazol-4-yl-LIGRL-NH₂) bound to a palmitoyl group (Pam) via polyethylene glycol (PEG) linkers. In a high-throughput physiological assay, these STL agonists displayed EC₅₀ values as low as 1.47 nM, representing a ∼200 fold improvement over the untethered parent ligand. Similarly, these STL agonists were potent activators of signaling pathways associated with PAR₂: EC₅₀ for Ca(2+) response as low as 3.95 nM; EC₅₀ for MAPK response as low as 9.49 nM. Moreover, STLs demonstrated significant improvement in potency in vivo, evoking mechanical allodynia with an EC₅₀ of 14.4 pmol. STLs failed to elicit responses in PAR2(-/-) cells at agonist concentrations of >300-fold their EC₅₀ values. Our results demonstrate that the STL approach is a powerful tool for increasing ligand potency at PAR₂ and represent opportunities for drug development at other protease activated receptors and across GPCRs.

Chronic compression or acute dissociation of dorsal root ganglion induces cAMP-dependent neuronal hyperexcitability through activation of PAR2

Chronic compression (CCD) or dissociation of dorsal root ganglion (DRG) can induce cyclic adenosine monophosphate (cAMP)-dependent DRG neuronal hyperexcitability and behaviorally expressed hyperalgesia. Here, we report that protease-activated receptor 2 (PAR2) activation after CCD or dissociation mediates the increase of cAMP activity and protein kinase A (PKA) and cAMP-dependent hyperexcitability and hyperalgesia in rats. CCD and dissociation, as well as trypsin (a PAR2 activator) treatment, increased level of cAMP concentration, mRNA, and protein expression for PKA subunits PKA-RII and PKA-c and protein expression of PAR2, in addition to producing neuronal hyperexcitability and, in CCD rats, thermal hyperalgesia. The increased expression of PAR2 was colocalized with PKA-c subunit. A PAR2 antagonistic peptide applied before and/or during the treatment, prevented or largely diminished the increased activity of cAMP and PKA, neuronal hyperexcitability, and thermal hyperalgesia. However, posttreatment with the PAR2 antagonistic peptide failed to alter either hyperexcitability or hyperalgesia. In contrast, an adenylyl cyclase inhibitor, SQ22536, administrated after dissociation or CCD, successfully suppressed hyperexcitability and hyperalgesia, in vitro and/or in vivo. Trypsin-induced increase of the intracellular calcium [Ca(2+)](i) was prevented in CCD or dissociation DRG neurons. These alterations were further confirmed by knockdown of PAR2 with siRNA. In addition, trypsin and PAR2 agonistic peptide-induced increase of cAMP was prevented by inhibition of PKC, but not Gαs. These findings suggest that PAR2 activation is critical to induction of nerve injury-induced neuronal hyperexcitability and cAMP-PKA activation. Inhibiting PAR2 activation may be a potential target for preventing/suppressing development of neuropathic pain.

Proteinase-activated receptor-1 mediates dorsal root ganglion neuronal degeneration in HIV/AIDS

Distal sensory polyneuropathy is a frequent complication of lentivirus infections of the peripheral nervous system including both human immunodeficiency virus and feline immunodeficiency virus. Proteinase-activated receptors are G protein-coupled receptors implicated in the pathogenesis of neuroinflammation and neurodegeneration. Proteinase-activated receptor-1 is expressed on different cell types within the nervous system including neurons and glia, but little is known about its role in the pathogenesis of inflammatory peripheral nerve diseases, particularly lentivirus-related distal sensory polyneuropathy. Herein, the expression and functions of proteinase-activated receptor-1 in the peripheral nervous system during human immunodeficiency virus and feline immunodeficiency virus infections were investigated. Proteinase-activated receptor-1 expression was most evident in autopsied dorsal root ganglion neurons from subjects infected with human immunodeficiency virus, compared with the dorsal root ganglia of uninfected subjects. Human immunodeficiency virus or feline immunodeficiency virus infection of cultured human or feline dorsal root ganglia caused upregulation of interleukin-1β and proteinase-activated receptor-1 expression. In the human immunodeficiency virus- or feline immunodeficiency virus-infected dorsal root ganglia, interleukin-1β activation was principally detected in macrophages, while neurons showed induction of proteinase-activated receptor-1. Binding of proteinase-activated receptor-1 by the selective proteinase-activated receptor-1-activating peptide resulted in neurite retraction and soma atrophy in conjunction with cytosolic calcium activation in human dorsal root ganglion neurons. Interleukin-1β exposure to feline or human dorsal root ganglia caused upregulation of proteinase-activated receptor-1 in neurons. Exposure of feline immunodeficiency virus-infected dorsal root ganglia to the interleukin-1 receptor antagonist prevented proteinase-activated receptor-1 induction and neurite retraction. In vivo feline immunodeficiency virus infection was associated with increased proteinase-activated receptor-1 expression on neurons and interleukin-1β induction in macrophages. Moreover, feline immunodeficiency virus infection caused hyposensitivity to mechanical stimulation. These data indicated that activation and upregulation of proteinase-activated receptor-1 by interleukin-1β contributed to dorsal root ganglion neuronal damage during lentivirus infections leading to the development of distal sensory polyneuropathy and might also provide new targets for future therapeutic interventions.

Acute PAR2 activation reduces GABAergic inhibition in the spinal dorsal horn

We investigated the mechanism underlying inhibition of spinal dorsal horn GABAergic neurotransmission to elucidate the role of protease-activated receptor-2 (PAR2). Initially, we confirmed that PAR2 agonist SL-NH(2) applied intrathecally produced mechanical hyperalgesia. Then we performed patch-clamp experiments in substantia gelatinosa neurons of spinal cord slice, and found that spontaneous inhibitory post-synaptic currents (sIPSCs) were significantly decreased in both frequency and amplitude when neurons were incubated with PAR2 agonist SL-NH(2) for a brief time period (2 min). The GABA-mediated currents were significantly reduced, and there was no impact on glycine-mediated currents during this SL-NH(2) treatment. These results suggest that PAR2 activation enhanced the pain response, potentially via inhibition of dorsal horn GABAergic neurotransmission.

Proteinase-activated receptor 2 sensitizes transient receptor potential vanilloid 1, transient receptor potential vanilloid 4, and transient receptor potential ankyrin 1 in paclitaxel-induced neuropathic pain

Paclitaxel chemotherapy is limited by a long-lasting painful neuropathy that lacks an effective therapy. In this study, we tested the hypothesis that paclitaxel may release mast cell tryptase, which activates protease-activated receptor 2 (PAR2) and, subsequently, protein kinases A and C, resulting in mechanical and thermal (both heat and cold) hypersensitivity. Correlating with the development of neuropathy after repeated administration of paclitaxel, mast cell tryptase activity was found to be increased in the spinal cord, dorsal root ganglia, and peripheral tissues in mice. FSLLRY-amide, a selective PAR2 antagonist, blocked paclitaxel-induced neuropathic pain behaviors in a dose- and time-dependent manner. In addition, blocking downstream signaling pathways of PAR2, including phospholipase C (PLC), protein kinase A (PKA), and protein kinase Cε (PKC), effectively attenuated paclitaxel-induced mechanical, heat, or cold hypersensitivity. Furthermore, sensitized pain response was selectively inhibited by antagonists of transient receptor potential (TRP) V1, TRPV4, or TRPA1. These results revealed specific cellular signaling pathways leading to paclitaxel-induced neuropathy, including the activation of PAR2 and downstream enzymes PLC, PKCε, and PKA and resultant sensitization of TRPV1, TRPV4, and TRPA1. Targeting one or more of these signaling molecules may present new opportunities for the treatment of paclitaxel-induced neuropathy.

Structure, function and pathophysiology of protease activated receptors

Discovered in the 1990s, protease activated receptors(1) (PARs) are membrane-spanning cell surface proteins that belong to the G protein coupled receptor (GPCR) family. A defining feature of these receptors is their irreversible activation by proteases; mainly serine. Proteolytic agonists remove the PAR extracellular amino terminal pro-domain to expose a new amino terminus, or tethered ligand, that binds intramolecularly to induce intracellular signal transduction via a number of molecular pathways that regulate a variety of cellular responses. By these mechanisms PARs function as cell surface sensors of extracellular and cell surface associated proteases, contributing extensively to regulation of homeostasis, as well as to dysfunctional responses required for progression of a number of diseases. This review examines common and distinguishing structural features of PARs, mechanisms of receptor activation, trafficking and signal termination, and discusses the physiological and pathological roles of these receptors and emerging approaches for modulating PAR-mediated signaling in disease.

Mechanism of cancer pain

Ongoing and breakthrough pain is a primary concern for the cancer patient. Although the etiology of cancer pain remains unclear, animal models of cancer pain have allowed investigators to unravel some of the cancer-induced neuropathologic processes that occur in the region of tumor growth and in the dorsal horn of the spinal cord. Within the cancer microenvironment, cancer and immune cells produce and secrete mediators that activate and sensitize primary afferent nociceptors. Pursuant to these peripheral changes, nociceptive secondary neurons in spinal cord exhibit increased spontaneous activity and enhanced responsiveness to three modes of noxious stimulation: heat, cold, and mechanical stimuli. As our understanding of the peripheral and central mechanisms that underlie cancer pain improves, targeted analgesics for the cancer patient will likely follow.

HIV-1 viral protein R causes peripheral nervous system injury associated with in vivo neuropathic pain

Painful peripheral neuropathy has become the principal neurological disorder in HIV/AIDS patients. Herein, we investigated the effects of a cytotoxic HIV-1 accessory protein, viral protein R (Vpr), on the peripheral nervous system (PNS). Host and viral gene expression was investigated in peripheral nerves from HIV-infected individuals and in HIV-infected human dorsal root ganglion (DRG) cultures by RT-PCR and immunocytochemistry. Cytosolic calcium ([Ca(2+)]) fluxes and neuronal membrane responses were analyzed in cultured DRGs. Neurobehavioral responses and cytokine levels were assessed in a transgenic mouse model in which the vpr transgene was expressed in an immunodeficient background (vpr/RAG1(-/-)). Vpr transcripts and proteins were detected in peripheral nerves and DRGs from HIV-infected patients. Exposure of rat or human cultured DRG neurons to Vpr rapidly increased [Ca(2+)] and action potential frequency while increasing input resistance. HIV infection of human DRG cultures caused neurite retraction (P<0.05), accompanied by induction of interferon-α (IFN-α) transcripts (P<0.05). vpr/RAG1(-/-) mice expressed Vpr together with increased IFN-α (P<0.05) in the PNS and also exhibited mechanical allodynia, unlike their vpr/RAG1(-/-) littermates (P<0.05). Herein, Vpr caused DRG neuronal damage, likely through cytosolic calcium activation and cytokine perturbation, highlighting Vpr's contribution to HIV-associated peripheral neuropathy and ensuing neuropathic pain.

Proteinase-activated receptor-1 activation presynaptically enhances spontaneous glutamatergic excitatory transmission in adult rat substantia gelatinosa neurons

Proteinase-activated receptors (PARs) have a unique activation mechanism in that a proteolytically exposed N-terminal region acts as a tethered ligand. A potential impact of PAR on sensory processing has not been fully examined yet. Here we report that synthetic peptides with sequences corresponding to PAR ligands enhance glutamatergic excitatory transmission in substantia gelatinosa (SG) neurons of adult rat spinal cord slices by using the whole cell patch-clamp technique. The frequency of spontaneous excitatory postsynaptic current (EPSC) was increased by PAR-1 agonist SFLLRN-NH2 (by 47% at 1 microM) with small increases by PAR-2 and -4 agonists (SLIGKV-NH2 and GYPGQV-OH, respectively; at >3 microM); there was no change in its amplitude or in holding current at -70 mV. The PAR-1 peptide action was inhibited by PAR-1 antagonist YFLLRNP-OH. TFLLR-NH2, an agonist which is more selective to PAR-1 than SFLLRN-NH2, dose-dependently increased spontaneous EPSC frequency (EC50=0.32 microM). A similar presynaptic effect was produced by PAR-1 activating proteinase thrombin in a manner sensitive to YFLLRNP-OH. The PAR-1 peptide action was resistant to tetrodotoxin and inhibited in Ca2+-free solution. Primary-afferent monosynaptically evoked EPSC amplitudes were unaffected by PAR-1 agonist. These results indicate that PAR-1 activation increases the spontaneous release of L-glutamate onto SG neurons from nerve terminals in a manner dependent on extracellular Ca2+. Considering that sensory processing within the SG plays a pivotal role in regulating nociceptive transmission to the spinal dorsal horn, the PAR-1-mediated glutamatergic transmission enhancement could be involved in a positive modulation of nociceptive transmission.

Inhibition of M current in sensory neurons by exogenous proteases: a signaling pathway mediating inflammatory nociception

Inflammatory pain is thought to be mediated in part through the action of inflammatory mediators on membrane receptors of peripheral nerve terminals, however, the downstream signaling events which lead to pain are poorly understood. In this study we investigated the nociceptive pathways induced by activation of protease-activated receptor 2 (PAR-2) in damage-sensing (nociceptive) neurons from rat dorsal root ganglion (DRG). We found that activation of PAR-2 in these cells strongly inhibited M-type potassium currents (conducted by Kv7 potassium channels). Such inhibition caused depolarization of the neuronal resting membrane potential leading, ultimately, to nociception. Consistent with this mechanism, injection of the specific M channel blocker XE991 into rat paw induced nociception in a concentration-dependent manner. Injection of a PAR-2 agonist peptide also induced nociception but coinjection of XE991 and the PAR-2 agonist did not result in summation of nociception, suggesting that the action of both agents may share a similar mechanism. We also studied the signaling pathway of M current inhibition by PAR-2 using patch-clamp and fluorescence imaging of DRG neurons. These experiments revealed that the PAR-2 effect was mediated by phospholipase C (PLC). Further experiments demonstrated that M current inhibition required concurrent rises in cytosolic Ca(2+) concentration and depletion of membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)). We propose that PLC- and Ca(2+)/PIP(2)-mediated inhibition of M current in sensory neurons may represent one of the general mechanisms underlying pain produced by inflammatory mediators, and may therefore open up a new therapeutic window for treatment of this major clinical problem.