TSH stimulates the proliferation of vascular smooth muscle cells.

Subclinical hypothyroidism (SCH) was reported to be associated with atherosclerosis (AS) in recent studies. Thyroid hormone levels are normal in patients with SCH, but the levels of thyroid-stimulating hormone (TSH) are increased. Thyroid-stimulating hormone receptor (TSHR) in extra-thyroidal tissues plays a pathophysiological role in these conditions. Our previous results demonstrated that TSHR was functional in hepatocytes and revealed elevated total cholesterol levels in the serum, which were an independent risk factor for AS. TSHR is expressed in vascular smooth muscle cells (VSMCs), and VSMC proliferation plays an important role in the development of AS. Cell proliferation was measured by the MTT assay. Intracellular cyclic AMP (cAMP) was measured using a cAMP ELISA kit. Cells were analyzed using a flow cytometer to determine the cell-cycle phase of each cell. For the purpose of detecting cyclin A and cyclin D, immunohistochemical staining and western blots were performed. Real-time PCR was used to assess the VSMC phenotypes. TSH increased cell progression into the G2/M phases and induced VSMC proliferation; thus, functional TSHR was present on VSMCs. Furthermore, the expression of cyclin D1 and cyclin A was increased. In addition, the results indicated that VSMCs undergo a phenotypic transformation from a contractile state to a synthetic state after treatment with different concentrations of TSH. Elevated TSH can promote VSMC proliferation through the cAMP-dependent pathway.