Thyroid Hormone Is a MAPK-Dependent Growth Factor for Human Myeloma Cells Acting via avb3 Integrin

Experimental and clinical observations suggest that thyroid hormone [L-thyroxine (T4) and 3,5,30-triiodo-Lthyronine (T3)] can support cancer cell proliferation. T3 and T4 promote both tumor cell division and angiogenesis by activating mitogen-activated protein kinase (MAPK) via binding to a hormone receptor on the avb3 integrin, overexpressed on many cancer cells. We have studied the responsiveness of several MM cell lines to T3 and T4 and characterized hormonal effects on cell survival, proliferation, and MAPK activation. Overnight T3 (1–100 nmol/L) and T4 (100 nmol/L) incubation enhanced, up to 50% (P < 0.002), MM cell viability (WST-1 assay) and increased cell proliferation by 30% to 60% (P < 0.01). Short exposure (10 minutes) to T3 and T4 increased MAPK activity by 2.5to 3.5-fold (P < 0.03). Pharmacologic MAPK inhibition blocked the proliferative action of T3 and T4. Antibodies to the integrin avb3 dimer and av and b3 monomers (but not b1) inhibited MAPK activation and subsequent cell proliferation in response to thyroid hormone, indicating dependence upon this integrin. Moreover, tetraiodothyroacetic acid (tetrac), a non-agonist T4 analogue previously shown to selectively block T3/T4 binding to avb3 receptor site, blocked induction of MAPK by the hormones in a dose-dependent manner. This demonstration of the role of thyroid hormones as growth factors for MM cells may offer novel therapeutic approaches. Mol Cancer Res; 2011 AACR.