The thyroid hormone-αvβ3 integrin axis in ovarian cancer: regulation of gene transcription and MAPK-dependent proliferation

Ovarian carcinoma is the fifth common cause of cancer death in women, despite advanced therapeutic approaches. alpha v beta 3 integrin, a plasma membrane receptor, binds thyroid hormones (L-thyroxine, T4; 3,5,3'-triiodo-L-thyronine, T3) and is overexpressed in ovarian cancer. We have demonstrated selective binding of fluorescently labeled hormones to alpha v beta 3-positive ovarian cancer cells but not to integrin-negative cells. Physiologically relevant T3 (1 nM) and T4 (100 nM) concentrations in OVCAR-3 (high alpha v beta 3) and A2780 (low alpha v beta 3) cells promoted alpha v and beta 3 transcription in association with basal integrin levels. This transcription was effectively blocked by RGD (Arg-Gly-Asp) peptide and neutralizing alpha v beta 3 antibodies, excluding T3-induced beta 3 messenger RNA, suggesting subspecialization of T3 and T4 binding to the integrin receptor pocket. We have provided support for extracellular regulated kinase (ERK)-mediated transcriptional regulation of the alpha v monomer by T3 and of beta 3 monomer by both hormones and documented a rapid (30-120 min) and dose-dependent (0.1-1000 nM) ERK activation. OVCAR-3 cells and alpha v beta 3-deficient HEK293 cells treated with alpha v beta 3 blockers confirmed the requirement for an intact thyroid hormone-integrin interaction in ERK activation. In addition, novel data indicated that T4, but not T3, controls integrin's outside-in signaling by phosphorylating tyrosine 759 in the beta 3 subunit. Both hormones induced cell proliferation (cell counts), survival (Annexin-PI), viability (WST-1) and significantly reduced the expression of genes that inhibit cell cycle (p21, p16), promote mitochondrial apoptosis (Nix, PUMA) and tumor suppression (GDF-15, IGFBP-6), particularly in cells with high integrin expression. At last, we have confirmed that hypothyroid environment attenuated ovarian cancer growth using a novel experimental platform that exploited paired euthyroid and severe hypothyroid serum samples from human subjects. To conclude, our data define a critical role for thyroid hormones as potent alpha v beta 3-ligands, driving ovarian cancer cell proliferation and suggest that disruption of this axis may present a novel treatment strategy in this aggressive disease.