STIMULATION OF EXTRAOCULAR-MUSCLE FIBROBLASTS BY CYTOKINES AND HYPOXIA - POSSIBLE ROLE IN THYROID-ASSOCIATED OPHTHALMOPATHY

OBJECTIVE Smoking is a risk factor for the development of thyroid-associated ophthalmopathy, an inflammatory process primarily affecting the fibroblasts in extraocular muscles. We wished to determine whether the extraocular muscle fibroblasts are more sensitive than dermal fibroblasts to T-cell derived cytokines, as a reason for this anatomical localization, and whether hypoxia alters fibroblast function, as one explanation for the susceptibility conferred by smoking. DESIGN Fibroblasts derived from the skin or extraocular muscles of healthy subjects were cultured with cytokines under normal (5% CO2:95% air) and hypoxic (5% CO2:95% N-2) conditions. MEASUREMENTS Glycosaminoglycan, protein and DNA synthesis were measured by assessing incorporation of D-6-H-3-glucosamine, H-3-amino acids, and H-3-thymidine respectively. RESULTS alpha-lnterferon and interleukin-6 had no effect on fibroblasts. gamma-lnterferon, tumour necrosis factor and interleukin-1 stimulated glycosaminoglycan synthesis; this effect was greater in orbital than in dermal fibroblasts with gamma-interferon and interleukin-1 (P<0.05). The same cytokines stimulated total protein with a greater response in orbital fibroblasts with gamma-interferon. Interleukin-1 inhibited DNA synthesis in orbital fibroblasts but stimulated DNA synthesis in dermal fibroblasts (P<0.01); tumour necrosis factor also displayed a differential effect (P<0.01). Hypoxia caused a significant increase in glycosaminoglycan, protein and DNA synthesis in both types of fibroblasts, under both basal and cytokine-treated conditions (P<0.05). CONCLUSIONS Extraocular muscle fibroblasts respond differently from dermal fibroblasts following cytokine stimulation, which may explain in part the anatomical localization of ophthalmopathy. Hypoxia stimulates fibroblasts and this could contribute, as an enhancing factor, to the adverse effects of smoking on thyroid eye disease.