Activity and expression of ecto-5'-nucleotidase/CD73 are increased by thyroid hormones in vascular smooth muscle cells

Extracellular nucleotides ATP, ADP, AMP and adenosine are well known signaling molecules of the cardiovascular system that are involved in several physiological processes: cell proliferation, platelet aggregation, inflammatory processes and vascular tonus. The levels of these molecules are controlled by ecto-NTPDases and ecto-5'-nucleotidase/CD73 (ecto-5'-NT/CD73) actions, which are responsible for the complete ATP degradation to adenosine. The thyroid hormones, thyroxine (T-4) and triiodothyronine (T-3), play important roles in the vascular system promoting vasodilatation. Here we investigated the influence of thyroid hormones on the enzyme cascade that catalyzes the interconversion of purine nucleotides in vascular smooth muscle cells (VSMC). Exposure of VSMCs to 50nM T-3 or T-4 did not change ATP and ADP hydrolysis significantly. However, the same treatment caused an increase of 75% in AMP hydrolysis, which was time-dependent but dose-independent. Moreover, T-3 treatment significantly increased ecto-5'-NT/CD73 mRNA expression, which suggests a genomic effect of this hormone upon ecto-5'-NT/CD73. In addition to the importance of the ecto-5'-NT in cell proliferation and differentiation, its overexpression could result in higher extracellular levels of adenosine, an important local vasodilatator molecule.