PAPP-A as a Potential Target in Thyroid Eye Disease

Context: Proptosis in thyroid eye disease (TED) can result in facial disfigurement and visual dysfunction. Treatment with insulin-like growth factor I receptor (IGF-IR) inhibitors has been shown to be effective in reducing proptosis but with side effects. Objective: To test the hypothesis that inhibition of IGF-IR indirectly and more selectively with PAPP-A inhibitors attenuates IGF-IR signaling in TED. Informed consent was obtained from patients with TED undergoing surgery, and retro-orbital tissue was collected for fibroblast isolation and culture. Operations were performed in Mayo Clinic operating suites. Cell culture was performed in a sterile tissue culture facility. Retro-orbital tissue was collected from 19 patients with TED. Methods: Treatment of TED fibroblasts with proinflammatory cytokines. Flow separation of CD34⁻ and CD34⁺ orbital fibroblasts, the latter representing infiltrating fibrocytes into the orbit in TED. PAPP-A expression and proteolytic activity, IGF-I stimulation of phosphatidylinositol 3 kinase/Akt pathway, and inhibition by immuno-neutralizing antibodies against PAPP-A, CD34⁺ status, and associated PAPP-A and IGF-IR expression were measured. Results: Proinflammatory cytokines markedly increased PAPP-A expression in TED fibroblasts. IGF-IR expression was not affected by cytokine treatment. Inhibition of PAPP-A’s proteolytic activity suppressed IGF-IR activation in orbital fibroblasts from patients with TED. TED fibroblasts that were CD34⁺ represented ∼80% of the cells in culture and accounted for ∼70% of PAPP-A and IGF-IR–expressing cells. Conclusion: These results support a role for PAPP-A in TED pathogenesis and indicate the potential for novel therapeutic targeting of the IGF axis.