A stimulatory thyrotropin receptor antibody enhances hyaluronic Acid synthesis in graves' orbital fibroblasts: inhibition by an igf-I receptor blocking antibody.
Kumar S, Iyer S, Bauer H, Coenen M, Bahn RS. JCEM 2012 May;97(5):1681-
Graves' ophthalmopathy (GO) is characterized by expanded volume of the orbital fat and extraocular muscle tissues and elevated levels of TSH receptor autoantibodies (TRAb). The expansion of orbital tissues involves accumulation of hyaluronic acid (HA) within the orbit.: The objective of the study was to determine whether a monoclonal stimulatory TRAb (M22) impacts HA synthesis in GO orbital cells and, if so, whether this might be blocked by an IGF-I receptor (IGF-IR)-blocking antibody (1H7) or inhibitors of various downstream signaling cascades. GO orbital fibroblast cultures (n = 6) were treated with M22, bovine TSH (bTSH), or IGF-I in serum-free medium. Some cultures also received 1H7, LY294002, rapamycin, or protein kinase A inhibitor: M22 or bTSH stimulated HA synthesis (2.1-fold with 100 ng/ml M22 and 1.9-fold with 10 U/liter bTSH; P < 0.05 each). M22-induced HA synthesis was inhibited by LY294002 or rapamycin but not by protein kinase inhibitor. HA synthesis stimulated by M22 or IGF-I was inhibited by 1H7 (mean 36.6 ± 5.6% and mean 45.8 ± 7.6%, respectively; P < 0.05 each). Similarly, M22- or IGF-I-stimulated Akt phosphorylation was inhibited by 1H7 (mean 54 ± 9.6 and 36.1 ±8.8%, respectively; P = 0.01 each).
The stimulatory TRAb M22 increases HA production in undifferentiated GO orbital fibroblasts via phosphoinositide 3-kinase/phosphorylated AKT/mammalian target of rapamycin activation. Blockade of IGF-IR inhibits both HA synthesis and Akt phosphorylation induced by M22 or IGF-I in these cells, suggesting that TSH receptor and IGF-IR signaling may be closely linked in the GO orbit.
COMMENT- TSH and Graves’ IgG stimulate differentiation of orbital fibroblasts into adipocytes and increases TSHR expression.. It has long been known that in GO there is excess production of hyaluronic acid in the orbit. Recent studies by Smith and colleagues have suggested that a second antibody in Graves’ Ig, directed to the IGF-1-R, plays an important role in GO, separately from anti-TSH-R antibodies. This study documents that a powerful human monoclonal anti-TSHR antibody, and bTSH, can induce HA synthesis in un-differentiated orbital firboblasts primarily through the PI3K/pAkt/mTOR pathway rather than via cAMP. This action involves the TSHR, and in an as-yet unidentified manner, downstream signaling from the IGF1-R. The study appears to offer a unitary explanation for the relation of anti-TSH-R antibodies to the features of GO. L De Groot, MD
Complete Inhibition of rhTSH-, Graves' Disease IgG-, and M22-Induced cAMP Production in Differentiated Orbital Fibroblasts by a Low-Molecular-Weight TSHR Antagonist.
van Zeijl CJ, van Koppen CJ, Surovtseva OV, de Gooyer ME, Plate R, Conti P, Karstens WJ, Timmers M, Saeed P, Wiersinga WM, Miltenburg AM, Fliers E, Boelen A. JCEM 2012 May;97(5):E781-5. Epub 2012 Mar 14.
The TSH receptor (TSHR) on orbital fibroblasts (OF) is a proposed target of the autoimmune attack in Graves' ophthalmopathy. In the present study, we tested whether the novel low-molecular-weight (LMW) TSHR antagonist Org-274179-0 inhibits cAMP production induced by rhTSH, Graves' disease IgG (GD-IgG), or M22 (a potent human monoclonal TSHR stimulating antibody) in cultured and differentiated OF from Graves' ophthalmopathy patients. cAMP production significantly increased after incubation either with 10 mU/ml rhTSH (3-fold; P ≤ 0.05), 1 mg/ml GD-IgG (2-fold; P ≤ 0.05), or 500 ng/ml M22 (5-fold; P ≤ 0.05). Incubation with the LMW TSHR antagonist dose dependently inhibited rhTSH, GD-IgG as well as the M22-induced cAMP production at nanomolar concentrations; complete blockade was affected at 10(-6) m. Our results suggest that GD-IgG- and M22-induced cAMP production in differentiated OF is exclusively mediated via the TSHR because it can be completely blocked by the LMW TSHR antagonist.
COMMENT-That cAMP production caused by Graves’ IgG can be completely blocked in a TSH receptor antagonist, fits with a single-function antibody in the IgG, but does not rule out other possible antibodies, or other stimulatory pathways. However this study raises hope for cliically useful low molecular weight TSHR antagonists that could be safe and effective in GD and GO.