Subcellular Localization Dynamics of Thyroid Hormone Receptor and Mediator Complex Subunit 1

Intracellular trafficking of transcription factors is an essential cellular function that has implications in regulating gene expression. For thyroid hormone receptor (TR), nuclear localization is fundamental to its function of mediating gene expression in response to thyroid hormone (T3). Yet, we’ve previously shown that TR contains both nuclear localization signals and nuclear export signals, and shuttles rapidly between the nucleus and cytosol. Mislocalization of TR, and loss of transcriptional control, may lead to negative consequences for growth, development, and metabolism. Here, we explore factors that enhance nuclear retention of TR. Emerging studies suggest that Mediator complex subunit 1 (MED1 or TRAP220), a TR-interacting protein, may modulate nuclear retention of TR. to investigate this possibility, nucleocytoplasmic distribution and mobility of mCherry-tagged TR subtypes, TRα1, TRβ1, and the oncoprotein v-ErbA, were assessed in response to MED1 overexpression in HeLa cells. TRα1, which has a predominantly nuclear distribution at steady state showed no change in distribution pattern or intranuclear mobility when co-transfected with MED1. In contrast, overexpression of MED1 caused increased nuclear localization of TRβ1 and v-ErbA, subtypes with cytosolic populations at steady state, as well as a decrease in intranuclear mobility of TRβ1. Interestingly, in the presence of T3, which is known to induce phosphorylation of MED1, there was a decrease in both nuclear mobility and nuclear retention of TR subtypes. Using TRAP220-/- and TRAP220+/+ mouse embryonic fibroblasts (MEF), TR localization in the absence of MED1 was subsequently analyzed. Compared to TRAP220+/+ cells, TRα1 and TRβ1 showed an increase in cytosolic localization when expressed in TRAP220-/- MEFs. Taken together, our data provide evidence for MED1 promoting the nuclear retention of TRα1, TRβ1, and v-ErbA. Whether increased nuclear retention correlates with increased TR gene transactivation requires further analysis. Collectively, our findings implicate MED1 as a potential target in the pathogenesis of diseases that are linked to TR mislocalization and dysfunction