Activated nongenomically by L-thyroxine (T4), mitogen-activated protein kinase (MAPK) complexed in 19-20 min with endogenous nuclear thyroid hormone receptor (TRβ1 or TR) in nuclear fractions of 293T cells, resulting in serine phosphorylation of TR. Treatment of cells with the MAPK kinase inhibitor, PD 98059, prevented both T4-induced nuclear MAPK-TR co-immuno-precipitation and serine phosphorylation of TR. T4 treatment caused dissociation of TR and SMRT (silencing mediator of retinoid and thyroid hormone receptor), an effect also inhibited by PD 98059 and presumptively a result of association of nuclear MAPK with TR. Transfection into CV-1 cells of TR gene constructs in which one or both zinc fingers in the TR DNA-binding domain were replaced with those from the glucocorticoid receptor localized the site of TR phosphorylation by T4-activated MAPK to a serine in the second zinc finger of the TR DNA-binding domain. In an in vitro cell- and hormone-free system, purified activated MAPK phosphorylated recombinant human TRβ1 (102-461). Thus, T4 activates MAPK and causes MAPK-mediated serine phosphorylation of TRβ1 and dissociation of TR and the co-repressor SMRT.
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