Thyroxine promotes association of mitogen-activated protein kinase and nuclear thyroid hormone receptor (TR) and causes serine phosphorylation of TR

Activated nongenomically by L-thyroxine (T-4), mitogen-activated protein ki nase (MAPK) complexed in 10-20 min with endogenous nuclear thyroid hormone receptor (TR beta1 or TR) in nuclear fractions of 293T cells, resulting in serine phosphorylation of TR, Treatment of cells with the MAPK kinase inhib itor, PD 98059, prevented both T-4-induced nuclear MAPK-TR co-immunoprecipi tation and serine phosphorylation of TR. T-4 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 assoc iation of nuclear MAPK with TR, Transfection into CV-1 cells of TR gene con structs 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 T-4-activated MAPK to a serine in the second zinc fi nger of the TR DNA-binding domain. In an in vitro cell- and hormone-free sy stem, purified activated MAPK phosphorylated recombinant human TR beta1 (10 2-461). Thus, T-4 activates MAPK and causes MAPK-mediated serine phosphoryl ation of TR beta1 and dissociation of TR and the co-repressor SMRT.