Ligand Independent and Subtype-Selective Actions of Thyroid Hormone Receptors in Human Adipose Derived Stem Cells

Thyroid hormone (TH) receptors (TR alpha a and beta) are homologous ligand-dependent transcription factors (TFs). While the TRs display distinct actions in development, metabolic regulation and other processes, comparisons of TR alpha and TR beta dependent gene regulation mostly reveal similar mechanisms of action and few TR subtype specific genes. Here, we show that TRa predominates in multipotent human adipose derived stem cells (hADSC) whereas TR beta is expressed at lower levels and is upregulated during hADSC differentiation. The TRs display several unusual properties in parental hADSC. First, TRs display predominantly cytoplasmic intracellular distribution and major TRa variants TR alpha 1 and TR alpha 2 colocalize with mitochondria. Second, knockdown experiments reveal that endogenous TRs influence hADSC cell morphology and expression of hundreds of genes in the absence of hormone, but do not respond to exogenous TH. Third, TR alpha and TR beta affect hADSC in completely distinct ways; TR alpha regulates cell cycle associated processes while TR beta may repress aspects of differentiation. TR alpha splice variant specific knockdown reveals that TR alpha 1 and TR alpha 2 both contribute to TR alpha-dependent gene expression in a gene specific manner. We propose that TRs work in a non-canonical and hormone independent manner in hADSC and that prominent subtype-specific activities emerge in the context of these unusual actions.