The putative role of prostaglandin endoperoxide H synthase-2 in the pathogenesis of thyroid-associated orbitopathy

Thyroid-associated orbitopathy (TAO) involves a remodelling of the connective tissue in the orbit, accumulation of the non-sulfated glycosaminoglycan, hyaluronan, and often intense inflammation. Orbital fibroblasts exhibit a remarkable susceptibility to various actions of pro-inflammatory cytokines and these molecular interactions we hypothesize are the basis for the peculiar tissue changes seen in ophthalmopathy, including the accumulation of hyaluronan. We have found that several pro-inflammatory cytokines can dramatically induce prostaglandin endoperoxide H synthase-2 (PGHS-2), the inflammatory cyclooxygenase, and that this induction results in a substantial increase in PGE2 production. The increase in cyclooxygenase expression and PGE2 synthesis can be blocked with glucocorticoids. The magnitude of the up-regulation of the prostanoid biosynthetic machinery in orbital fibroblasts from patients with ophthalmopathy was considerably greater than that found in dermal cultures or in orbital fibroblasts from normal tissue. Orbital fibroblasts, unlike most fibroblasts, express CD40 and when that surface receptor is cross-linked with CD154, its natural ligand, a number of inflammation-related genes are activated. These include IL-1alpha, IL-6, IL-8 and PGHS-2. It would appear that orbital fibroblasts, especially those from patients with ophthalmopathy, exhibit several exaggerated responses to pro-inflammatory signals and that those cellular actions could provide the molecular basis for orbital tissue remodelling.