Thyroid hormone induces artery smooth muscle cell proliferation: discovery of a new TRalpha1-Nox1 pathway

Thyroid hormone (T3) can stimulate protein synthesis and cell growth. NOX1 is a mitogenic oxidase. The aim of this study was to test a novel hypothesis that T3 induces artery smooth muscle cell proliferation by up-regulating NOX1. Immunofluoresence confocal microscopy was used to visualize the sub-cellular localization of NOX1 and TRalpha1 in rat aorta smooth muscle (RASM) cells. Optical sectioning showed that TRalpha1 and NOX1 co-localized around the nucleus. T3 promoted RASM cell proliferation as determined by the fact that T3 significantly increased the number of cytokinesis cells, proliferating cellular nuclear antigen (PCNA) and smooth muscle alpha-actin (SM alpha-actin). T3 increased NOX1 expression at both the transcription (mRNA) and translation (protein) levels as evaluated by RT-PCR and Western blot, respectively. T3 also significantly increased the intracellular ROS production based on the oxidation of 2',7'-dichlorodihydrofluoresein (H2DCF) to a fluorescent 2',7'-dichlorofluoresein (DCF). RNAi silence of TRalpha1 or NOX1 abolished T3-induced intracellular ROS generation and PCNA and SM alpha-actin expression, indicating that TRalpha1 and NOX1 mediated T3-induced RASM cell proliferation. Notably, RNAi silence of TRalpha1 blocked the T3-induced increase in NOX1 expression, whereas silence of NOX1 did not affect TRalpha1 expression, disclosing a new pathway, i.e. T3-TRalpha1-NOX1-cell proliferation. TRalpha1 and NOX1 co-localized around the nucleus. T3 induced RASM cell proliferation by up-regulating NOX1 in a TRalpha1-dependent manner.