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N. Guo, S. E. Feldon, P. J. Sime, R. P. Phipps; Peroxisome Proliferator-Activated Receptor- is a Negative Regulator of Tissue Transglutaminase in Human Orbital Fibroblasts. Invest. Ophthalmol. Vis. Sci. 2010;51(13):3928.
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© ARVO (1962-2015); The Authors (2016-present)
Thyroid Eye Disease (TED) is an autoimmune condition in which intense inflammation leads to orbital tissue remodeling, including the accumulation of extracellular matrix (ECM) and fat. Tissue transglutaminase (TG2) cross-links a wide variety of ECM proteins, producing a protease-resistant matrix, and is reported to be expressed at sites of inflammation. Peroxisome proliferator-activated receptor-γ (PPARγ) is a transcription factor that regulates adipocyte differentiation, inflammation and fibrosis. We hypothesize that PPARγ is a negative regulator of TG2 in human orbital fibroblasts and thus regulates the matrix in TED.
Primary orbital fibroblasts were isolated from TED patients undergoing orbital decompression surgery. The cells were grown in RPMI media and infected with wild-type (WT), dominant-negative (DN) PPARγ1 or control lentivirus. The mRNA level of TG2 was detected by real-time RT-PCR. TG2 protein was detected by immunostaining, western blot, and flow cytometry. The differentiation of orbital fibroblasts to adipocytes was detected using an adipo-red assay or oil-red O staining. Fatty-acid-binding protein4 (FABP4) was detected by western blot.
Over-expression of WT-PPARγ led to accumulation of orbital fat and expression of FABP4 in orbital fibroblasts. Cells infected with the DN-PPARγ virus did not differentiate into adipocytes after treatment with PPARγ ligands and failed to express FABP4. Over-expression of WT-PPARγ completely suppressed TG2 expression while the DN-PPARγ virus increased TG2 mRNA and protein 5-fold compared to control. TG2 is a TGF-β activating enzyme and treatment with TGF-β markedly increased TG2 mRNA and protein levels in DN-PPARγ and control virus infected cells. Over-expression of WT-PPARγ prevented TGFβ-induced TG2 upregulation. Furthermore, the TGF-β induced TG2 mRNA expression in DN-PPARγ virus infected cells was 12-fold more than that in control virus infected cells.
Our findings reveal that PPARγ dampens TG2 expression at baseline and in response to TGFβ treatment, likely through transcriptional regulation of the TG2 gene. This work implicates PPARγ as an important regulator of fibroblast differentiation, matrix stabilization in TED and possibly in other diseases.
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