Thyroid hormones are important for physiology and homeostasis. In addition to nuclear thyroid hormone receptors, the plasma membrane protein integrin αvβ3 has been recognized as a receptor for both thyroxine (T4) and triiodothyronine (T3). Here, we studied whether thyroid hormone promotes growth of murine lung cancer via αvβ3 in vivo. Murine Lewis lung carcinoma cells (3LL), stably transfected with luciferase, were injected into mouse lungs. Tumor growth in untreated mice was compared to hypothyroid mice and hypothyroid mice treated with T3 or T4 with or without the αvβ3 inhibitor 3,5,3',5'-tetraiodothyroacetic acid (Tetrac). Tumor progression was determined by serial in vivo imaging of bioluminescence emitted from the tumor. Tumor weight was recorded at the end of the experiment. Neoangiogenesis was determined by immunohistochemistry for CD31. Tumor growth was reduced in hypothyroidism and increased by T4 treatment. Strikingly, only T4 but not T3 treatment promoted tumor growth. This T4 effect was abrogated by the αvβ3 inhibitor Tetrac. Tumor weight and neoangiogenesis were also significantly increased only in T4-treated mice. The T4 effect on tumor weight and neoangiogenesis was abolished by Tetrac. In vitro, T4 did not stimulate 3LL cell proliferation or signaling pathway activation. We conclude that T4 promotes lung cancer growth in this orthotopic mouse model. The tumor-promoting effect is mediated via the plasma membrane integrin αvβ3 and increased neoangiogenesis rather than direct stimulation of 3LL cells. These data suggest that such effects of levothyroxine may need to be considered in cancer patients on T4 substitution.
Keywords: Lewis lung carcinoma; Tetrac; integrin αvβ3; thyroid hormone; thyroxine; tumor growth.