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-asso ciated ophthalmopathy, an inflammatory process primarily affecting the fibroblasts in extraocular muscles. We wished to determine whether th e extraocular muscle fibroblasts are more sensitive than dermal fibrob lasts to T-cell derived cytokines, as a reason for this anatomical loc alization, and whether hypoxia alters fibroblast function, as one expl anation for the susceptibility conferred by smoking. DESIGN Fibroblast s derived from the skin or extraocular muscles of healthy subjects wer e 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-glu cosamine, H-3-amino acids, and H-3-thymidine respectively. RESULTS alp ha-lnterferon and interleukin-6 had no effect on fibroblasts. gamma-ln terferon, tumour necrosis factor and interleukin-1 stimulated glycosam inoglycan synthesis; this effect was greater in orbital than in dermal fibroblasts with gamma-interferon and interleukin-1 (P<0.05). The sam e cytokines stimulated total protein with a greater response in orbita l fibroblasts with gamma-interferon. Interleukin-1 inhibited DNA synth esis in orbital fibroblasts but stimulated DNA synthesis in dermal fib roblasts (P<0.01); tumour necrosis factor also displayed a differentia l effect (P<0.01). Hypoxia caused a significant increase in glycosamin oglycan, protein and DNA synthesis in both types of fibroblasts, under both basal and cytokine-treated conditions (P<0.05). CONCLUSIONS Extr aocular muscle fibroblasts respond differently from dermal fibroblasts following cytokine stimulation, which may explain in part the anatomi cal localization of ophthalmopathy. Hypoxia stimulates fibroblasts and this could contribute, as an enhancing factor, to the adverse effects of smoking on thyroid eye disease.