The relationship between mucosal cyclooxygenase-2 (COX-2) expression and experimental radiation-induced mucositis

doi:10.1016/S1368-8375(03)00148-9

Oral Oncology

Volume 40, Issue 2, February 2004, Pages 170-176

https://doi.org/10.1016/S1368-8375(03)00148-9 Get rights and content

Abstract

Although cycloooxygenase-2 (COX-2) is upregulated by factors associated with oral mucositis, its role in the pathogenesis of mucositis has not been studied. We investigated the kinetics of mucosal COX-2 expression following radiation exposure, and assessed its relationship to the development of oral mucositis in an established animal model using immunohistochemical endpoints. While little or no COX-2 expression was observed in unirradiated mucosa or in tissue taken 2 days after radiation, COX-2 expression was dramatic on days 10 and 16, especially in submucosal fibroblasts and endothelium. The kinetics of COX-2 expression paralleled mucositis severity. A burst of angiogenic activity was seen on day 21 following peak COX-2 expression. The kinetics of COX-2 expression relative to mucositis progression suggests that COX-2 is not a primary driver of radiation injury, but instead plays an amplifying role.

Introduction

Oral mucositis is a common acute toxicity elicited by head and neck radiation and many drugs used to treat cancer. Clinically, mucositis is characterized by erythema, ulceration and pseudomembrane formation. Ulceration results in severe pain that often necessitates changes in diet and the need for parenteral narcotics for palliation. In some cases, mucositis is so symptomatic that it leads to breaks in therapy, which adversely effect tumor response. In myeloablated patients, ulcerative mucositis serves as a conduit for the systemic invasion of bacteria or bacterial cell wall products.¹

The biological events underlying the pathogenesis of mucositis are still being defined. However, there is increasing evidence to support a role for pro-inflammatory cytokines in either primary or secondary roles in the development of mucositis. A relationship between peripheral blood levels of tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) and chemotherapy-induced nonhematologic toxicities has been recognized.² Furthermore, tissue levels of TNF-α and interleukin-1beta (IL-1β) track closely with radiation-induced mucosal injury³ as does pro-inflammatory cytokine gene expression.4 Experimental and clinical evidence suggests that attenuation of these cytokines is associated with a reduction in mucositis.

It seems likely that the nuclear transcription factor, NF-κB, may be important in the development of oral mucositis as it acts as a “gatekeeper” for various pathways.⁵ Both ionizing radiation and chemotherapeutic drugs are effective activators of NF-κB.⁶ Once activated, NF-κB may effectively upregulate over 200 genes, among them those controlling TNF-α and IL-1β .7, 8 In addition, NF-κB is also activated by these cytokines,9, 10, 11 a mechanism that permits tissue amplification of injurious elements.

Among the genes that have been shown to be upregulated by NF-κB is cyclooxygenase II (COX-2).12, 13 COX-2 is an inducible enzyme involved in inflammation that has a key role in the production of prostaglandins from arachadonic acid.14, 15, 16 COX-2 is expressed upon stimulation in vascular endothelial cells, inflammatory cells, cancerous epithelium, and fibroblasts. Unlike COX-1, COX-2 is not expressed in healthy tissue.17, 18 COX-2 expression is induced by various cytokines, growth factors, as well as by LPS,19, 20, 21, 22, 23 and radiation.²⁴ NF-κB is a key component of the pathway by which these stimuli upregulate COX-2.

Previous studies on COX-2 have focused on the enzyme's role in inflammation, as well as its role in tumor development and radiosensitivity. Not only has it been found to be involved in inflammation, but it has been shown to increase tumor angiogenesis through the activation of VEGF (vascular endothelial growth factor).¹⁸ Radiation has also been shown to stimulate angiogenesis via the nitric oxide pathway.²⁵ In addition, COX-2 has been shown to decrease the rate of apoptosis of tumor cells and increase tumor radiosensitivity.²⁶ As a result, COX-2 inhibitors have become a popular target as potential antineoplastic interventions.

The relationship between COX-2 expression and normal tissue toxicity following antineoplastic therapy has not been studied. Given the response of NF-κB to radiation and chemotherapy, it seems likely that COX-2 expression would take place and could have implications in the pathogenesis of mucositis. The objectives of this investigation were to determine if changes in COX-2 expression were present in mucosa following radiation, and to determine if there was a relationship between COX-2 expression and the course of oral mucositis.

Section snippets

Animals

Twenty Golden Syrian Hamsters (Charles River Laboratories; Wilmington, MA), aged 6–8 weeks and weighing approximately 90 g, were caged in small groups (with wood chip bedding), and given water and food ad libitum. A 12 h light/dark cycle was set. Animals were acclimatized for a minimum of 48 h prior to radiation to minimize the effects of stress due to shipping. Animals were weighed prior to radiation and at the time of sacrifice. All studies were approved by the Standing Committee of Animal

Clinical course of mucositis

The clinical course of mucositis in this animal model is well established. The development of ulcerative mucositis in this study was consistent with earlier findings. Mucosal erythema was noted within a week of radiation. Ulcerative mucositis developed shortly thereafter, with peak mucosal breakdown occurring between days 14 and 18. Subsequently, spontaneous resolution is typically complete by day 35. In this study, the last day of evaluation was day 22.

Immunohistochemical analysis (Figs. 1 and 2)

There was no evidence of measurable COX-2

Discussion

Our results demonstrate that COX-2 expression in submucosal tissues increased in response to targeted radiation, and that the extent of COX-2 expression paralleled the development of ulcerative mucositis. Specifically, we noted that COX-2 expression, localized to the vascular endothelium and fibroblasts, increased from day 2 to day 16, when peak intensity was seen. The tissue distribution of COX-2 that we observed is similar to what has been reported in the neovasculature of malignant tumors.18

Acknowledgements

This study was supported in part by Pharmacia Inc.

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^☆: Preliminary reports of some of the elements of this study were presented at the 32nd Annual Meeting of the American Association for Dental Research/27th Annual Meeting of the Canadian Association for Dental Research, San Antonio, Texas, USA, March 2003.

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The relationship between mucosal cyclooxygenase-2 (COX-2) expression and experimental radiation-induced mucositis☆

Abstract

Introduction

Section snippets

Animals

Clinical course of mucositis

Immunohistochemical analysis (Figs. 1 and 2)

Discussion

Acknowledgements

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The influence of serum tumor necrosis factor-α and interleukin-6 concentrations on non-hematologic toxicity and hematologic recovery in patients with acute myelogenous leukaemia

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