Epac1 Inhibits Orbital Fibroblast Activation to Ameliorate Thyroid-Associated Orbitopathy-Like Features Through the JAK/STAT Signaling Pathway

Purpose: Orbital fibroblast (OF) activation plays an important role in thyroid-associated orbitopathy (TAO) development. Epac1, as a cyclic adenosine monophosphate (cAMP) effector, has been recognized as a pivotal mediator of the anti-fibrosis properties of cAMP. This study investigated the role and mechanisms of Epac1 in OF activation.

Methods: Clinical orbital samples were collected from patients with TAO and normal healthy volunteers, and a TAO mouse model was established using adenovirus expressing the human TSHR A subunit (Ad-TSHR), which was evaluated for histopathological features and detected for Epac1 and vimentin expression levels. Clinical TAO samples- (TAO OFs) and healthy people-derived OFs (normal OFs) were isolated and identified using immunofluorescent (IF) staining. Healthy and TAO OFs were transfected with Epac1-overexpressing plasmid (Epac1) for Epac1 overexpression or sh-Epac1 plasmid for Epac1 knockdown, treated with TGFβ1, and then examined for cell phenotypes and the expression level of Epac1 and fibrosis-related markers (α-SMA, fibronectin, and collagens). Furthermore, adeno-associated virus (AAV) overexpressing Epac1 (AAV-Epac1) was injected into TAO mouse orbital tissues to investigate the anti-fibrotic effects of Epac1 on TGFβ1-stimulated OFs in vivo. Moreover, the phosphorylation of STAT3 in the TGFβ1-stimulated TAO OFs and TAO mice model was investigated. The JAK/STAT signaling inhibitor Stattic was used to explore the involvement of the JAK/STAT signaling in the functions of Epac1.

Results: Both clinical TAO samples and the TAO mouse model demonstrated significantly changed histopathology, reduced Epac1 expression, and increased vimentin level. TGFβ1 stimulation significantly facilitated cell viability and migration of normal OFs and TAO OFs, and elevated α-SMA, fibronectin, vimentin, Collagen I, and Collagen III expression levels. However, Epac1 overexpression in OFs could notably attenuate these pro-fibrosis effects of TGFβ1 on both normal and TAO OFs, shown as inhibiting cell viability and migration and decreasing expression of fibrotic markers. Whereas, Epac1 knockdown aggravated TGFβ1-induced fibrosis. In vivo, Epac1 overexpression also improved TAO-like symptoms in TAO model mice. Mechanically, Epac1 overexpression inhibited STAT3 phosphorylation in vitro and in vivo. Stattic effectively attenuated the effects of Epac1 knockdown on TGFβ1-treated TAO OFs.

Conclusions: Epac1 was a critical regulator of fibrotic and inflammatory processes in TAO through mediating the JAK/STAT signaling pathway.