COX Inhibitors Downregulate PDE4D Expression in a Clinical Model of Inflammatory Pain

Genome-wide association study on chronic postsurgical pain after abdominal surgeries in the UK Biobank

INTRODUCTION

Chronic pain is one of the most common and severe complications after surgery, affecting quality of life and overall wellbeing of patients. Several risk factors have been identified but the mechanisms of chronic postsurgical pain development remain unclear. This study aimed to identify single-nucleotide polymorphisms associated with developing chronic postsurgical pain after abdominal surgery, one of the most common types of surgery.

METHODS

A genome-wide association study was performed on 27,603 patients from the UK Biobank who underwent abdominal surgery. The robustness of identified loci was validated by split-half validation analysis. Functionally related top loci were selected for expression validation in clinical samples of adhesions from patients with and without pain.

RESULTS

One locus (rs185545327) reached genome-wide significance for association with chronic postsurgical pain development, and 10 loci surpassed the suggestively significant threshold (p < 1 × 10-6). In the robustness analysis, eight loci had at least nominal significance. The loci passing the suggestively significant threshold were mapped to 15 genes, of which two loci contained pain-related genes (SRPK2, PDE4D). Although marginally approaching statistical significance in the expression validation of clinical samples, the detection rate and expression level of PDE4D were modestly higher in patients with pain compared with those in the control group.

DISCUSSION

This study provides preliminary evidence for genetic risk factors implicated in chronic postsurgical pain following abdominal surgery, particularly the PDE4D gene, which has been associated with pain in previous studies. The findings add to evidence suggesting potential for the future development of a clinically applicable tool for personalised risk prediction, aiding clinicians in stratifying patients and enhancing clinical decision-making through individualised risk assessments.

Genomic methods for clinical and translational pain research

Pain is a complex sensory experience for which the molecular mechanisms are yet to be fully elucidated. Individual differences in pain sensitivity are mediated by a complex network of multiple gene polymorphisms, physiological and psychological processes, and environmental factors. Here, we present the methods for applying unbiased molecular-genetic approaches, genome-wide association study (GWAS), and global gene expression analysis, to help better understand the molecular basis of pain sensitivity in humans and variable responses to analgesic drugs.

Advancements in pain research

After the publication of the First Edition of this book in the series of Methods in Molecular Medicine (volume 99 in the series) in 2004, pain research continues its rapid acceleration until 2009, during which it experienced a plateau of growth that likely resulted from the economic downturn started in 2008 (Fig. 1.1). This rapid growth in pain research could be the driving force for an impressive 66% increase in new randomized, double-blind, placebo-control trials for neuropathic pain medications in the past 5 years compared with the last four decades. Unfortunately, little improvement in pain medications has been obtained [1] due to primarily our limited understanding of mechanisms mediating different pain states, especially that for chronic pain. It is highly possible that the growth in pain research will continue for decades to come due to three main reasons. First, there is an urgent need for more efficacious and safer pain medications that are necessary for better and individualized pain management. The increase in life expectancy of the general population and patients due to improvements in quality of health care and medicine is likely to increase the demand for better pain medications for improving the quality of daily life of those living with pain. It is estimated that the continuous increase in percentage of patients suffering from chronic pain (pain conditions lasting more than 6 months) arranges from 11 to 47% between 40 and 75 years of age [2], which will inevitably and continually increase the demand for better pain medications. Second, the cost of pain conditions to our society is high, estimated $55 billion per year in loss of productivity from full-time workers alone [3], so better pain management can significantly help economic growth and stability. Third, the swift advancement in technologies and our better understanding of sensory circuitries and pain pathways serves as a driving force for timely drug discovery research and development at an unprecedented pace to meet the demand for better pain medications.

Nitric oxide is negatively correlated to pain during acute inflammation

BACKGROUND

The role that nitric oxide (NO) plays in modulating pain in the periphery is unclear. We show here, the results of two independent clinical studies (microdialysis and gene expression studies) and a pilot dose finding study (glyceryl trinitrate study), to study the role of NO in the early phase of acute inflammatory pain following oral surgery. The effect of ketorolac on NO production and nitric oxide synthase (NOS) gene expression was also studied.

RESULTS

Microdialysis samples showed significantly higher levels of NO at the first 100 min compared to the last 80 minutes in the placebo treated group. In the ketorolac group, on the other hand, NO levels gradually decreased over the first 60 min but were similar to placebo over the later 100-180 min, with no significant change in NO level over time. The levels of NO were negatively correlated to pain intensity scores. Local infusion of the NO donor glyceryl trinitrate at the site of surgery, showed a small analgesic effect that did not reach statistical significance in the sample size used. While the gene expression of iNOS and eNOS were not up-regulated, 3 hours after surgery, nNOS was downregulated in both treatment groups and eNOS gene expression was significantly lower in the ketorolac group compared to the placebo group. Further, there was a positive correlation between the change in gene expression of nNOS and eNOS in the placebo group but not in the ketorolac group.

CONCLUSION

We suggest that at this early stage of inflammatory pain in man, NO is analgesic in the periphery. Further, ketorolac down-regulates eNOS gene expression.

Mechanisms of non-opioid analgesics beyond cyclooxygenase enzyme inhibition

Non-opioid analgesics including both selective and non-selective cyclooxygenase (COX) inhibitors and acetaminophen are the most widely used treatments for pain. Inhibition of COX is thought to be largely responsible for both the therapeutic and adverse effects of this class of drugs. Accumulating evidence over the past two decades has demonstrated effects of non-opioids beyond the inhibition of COX and prostaglandin synthesis that might also explain their therapeutic and adverse effects. These include their interaction with endocannabinoids, nitric oxide, monoaminergic, and cholinergic systems. Moreover, the recent development of microarray technology that allows the study of human gene expression suggests multiple pathways that may be related to the analgesic and anti-inflammatory effects of non-opioids. The present review will discuss the multiple actions of non-opioids and their interactions with these systems during inflammation and pain, suggesting that COX inhibition is an incomplete explanation for the actions of non-opioids and proposes the involvement of multiple selective targets for their analgesic, as well as, their adverse effects.

Kinin B1 receptors contributes to acute pain following minor surgery in humans

Background

Kinins play an important role in regulation of pain and hyperalgesia after tissue injury and inflammation by activating two types of G-protein-coupled receptors, the kinin B₁ and B₂ receptors. It is generally accepted that the B₂ receptor is constitutively expressed, whereas the B₁ receptor is induced in response to inflammation. However, little is known about the regulatory effects of kinin receptors on the onset of acute inflammation and inflammatory pain in humans. The present study investigated the changes in gene expression of kinin receptors and the levels of their endogenous ligands at an early time point following tissue injury and their relation to clinical pain, as well as the effect of COX-inhibition on their expression levels.

Results

Tissue injury resulted in a significant up-regulation in the gene expression of B₁ and B₂ receptors at 3 hours post-surgery, the onset of acute inflammatory pain. Interestingly, the up-regulation in the gene expression of B₁ and B₂ receptors was positively correlated to pain intensity only after ketorolac treatment, signifying an interaction between prostaglandins and kinins in the inflammatory pain process. Further, the gene expression of both B₁ and B₂ receptors were correlated. Following tissue injury, B₁ ligands des-Arg⁹-BK and des-Arg¹⁰-KD were significantly lower at the third hour compared to the first 2 hours in both the placebo and the ketorolac treatment groups but did not differ significantly between groups. Tissue injury also resulted in the down-regulation of TRPV1 gene expression at 3 hours post-surgery with no significant effect by ketorolac treatment. Interestingly, the change in gene expression of TRPV1 was correlated to the change in gene expression of B₁ receptor but not B₂ receptor.

Conclusions

These results provide evidence at the transcriptional level in a clinical model of tissue injury that up-regulation of kinin receptors are involved in the development of the early phase of inflammation and inflammatory pain. The up-regulation of B₁ receptors may contribute to acute inflammatory pain through TRPV1 activation.

Upregulation of IL-6, IL-8 and CCL2 gene expression after acute inflammation: Correlation to clinical pain

Tissue injury initiates a cascade of inflammatory mediators and hyperalgesic substances including prostaglandins, cytokines and chemokines. Using microarray and qRT-PCR gene expression analyses, the present study evaluated changes in gene expression of a cascade of cytokines following acute inflammation and the correlation between the changes in the gene expression level and pain intensity in the oral surgery model of tissue injury and acute pain. Tissue injury resulted in a significant upregulation in the gene expression of interleukin-6 (IL-6; 63.3-fold), IL-8 (8.1-fold), chemokine (C-C motif) ligand 2 (CCL2; 8.9-fold), chemokine (C-X-C motif) ligand 1 (CXCL1; 30.5-fold), chemokine (C-X-C motif) ligand 2 (CXCL2; 26-fold) and annexin A1 (ANXA1; 12-fold). The upregulation of IL-6 gene expression was significantly correlated to the upregulation of IL-8, CCL2, CXCL1 and CXCL2 gene expression. Interestingly, the tissue injury-induced upregulation of IL-6, IL-8 and CCL2 gene expression, was positively correlated to pain intensity at 3h post-surgery, the onset of acute inflammatory pain. However, ketorolac treatment did not have a significant effect on the gene expression of IL-6, IL-8, CCL2, CXCL2 and ANXA1 at the same time point of acute inflammation. These results demonstrate that the upregulation of IL-6, IL-8 and CCL2 gene expression contributes to the development of acute inflammation and inflammatory pain. The lack of effect of ketorolac on the expression of these gene products may be related to the ceiling analgesic effects of non-steroidal anti-inflammatory drugs.