Functional regulation of thyroid hormone receptor variant TR alpha 2 by phosphorylation

Tissue-specific stabilization of the thyroid hormone beta1 nuclear receptor by phosphorylation

The present study evaluated the expression and regulation of endogenous thyroid hormone receptors (TRs) in cultured cells. In COS-1 cells, the endogenous TR, subtype beta1 (TRbeta1), but not subtype beta2 or alpha1, was induced to express by okadaic acid (OA) in a concentration-dependent manner. The induced TRbeta1 had immunoreactivity and partial V8 proteolytic maps similar to those of the transfected and in vitro translated human TRbeta1 (h-TRbeta1). The OA-induced expression of endogenous TRbeta1 was, however, not observed in a variety of other cultured cell lines tested, indicating that the induction was cell type-dependent. TRbeta1 induced by OA was a multisite phosphorylated protein, in which serine and threonine in a ratio of 10:1 were phosphorylated. The induced TRbeta1 was functional as it could mediate the thyroid hormone-dependent transcriptional activity via several thyroid hormone response elements. The induction of endogenous TRbeta1 expression by OA was not accompanied by an increase in mRNA levels but was the result of an increase in the stability of the TRbeta1 protein. This is the first report to indicate that one of the mechanisms by which the TR isoforms are differentially expressed is via the tissue-specific stabilization of the TR isoform proteins. Furthermore, this selective stability of TRbeta1 could be conferred by phosphorylation.

DNA-independent and DNA-dependent mechanisms regulate the differential heterodimerization of the isoforms of the thyroid hormone receptor with retinoid X receptor

Thyroid hormone receptors (TRs) require heterodimerization with retinoid X receptor (RXR) for maximum DNA binding affinity. Interaction with RXR occurs via two dimerization interfaces, one in the DNA-binding domain and one in the C-terminal "ninth heptad" of the receptors. We studied the relative importance of these two dimerization domains in naturally occurring C-terminal TR variants. TRalpha1 has a conserved ninth heptad and formed stable heterodimers with RXR in solution. TRalpha1.RXR heterodimers bound similarly to direct repeat 4 (DR4) sites with different 5'-flanking and spacer sequences. In contrast, TRalpha2, which contains a highly divergent ninth heptad, did not interact with RXR in solution and bound as a heterodimer with RXR only to specific DR4 sequences in which the downstream half-site was the preferred octameric binding site of TR (TNAGGTCA). Although the ninth heptad of TRalpha2 was insufficient for interaction with RXR off DNA, this region was required for DNA-dependent heterodimerization with RXR. TRalpha3, another naturally occurring TRalpha isoform whose ninth heptad differs from those of both TRalpha1 and TRalpha2, displayed intermediate behavior in heterodimerization with RXR. Thus, in the absence of a strong ninth heptad interaction an octameric downstream half-site allosterically promotes RXR heterodimerization with TRalpha2. Differential dependence upon DNA-binding for heterodimerization with RXR may influence transcriptional regulation by TRalpha isoforms.

Thyroid hormone receptor variant alpha2. Role of the ninth heptad in dna binding, heterodimerization with retinoid X receptors, and dominant negative activity

Thyroid hormone receptors bind DNA with highest affinity as heterodimers with retinoid X receptors, and such heterodimers generally are thought to be the biological mediators of thyroid hormone action. An alternative splice product of the thyroid hormone receptor alpha gene, thyroid hormone receptor variant alpha2, does not bind thyroid hormone and functions as a weak dominant negative inhibitor of thyroid hormone action. Thyroid hormone receptor variant alpha2 is missing one-half of the ninth heptad, a region of the bona fide receptor thought to be important for heterodimerization with retinoid X receptors. The role of the ninth heptad in heterodimerization has been evaluated further. Thyroid hormone receptor variant alpha2-retinoid X receptor heterodimers form on a subset of direct repeat response elements but not on palindromic or inverted palindromic elements. Restoration of the missing ninth heptad sequence is critical for restoring heterodimerization on the palindromic DNA, but either the ninth heptad amino acids or a stretch of alanines is equally able to restore heterodimerization on the inverted palindrome. Thus, the role of the ninth heptad in heterodimerization differs on direct repeat, palindromic, and inverted palindromic response elements, suggesting that the protein-protein interactions differ on each of these elements. The dominant negative activity of thyroid hormone receptor variant alpha2 requires DNA binding, but the relatively weak nature of the dominant negative activity is only partially explained by the weak DNA binding.

Steroid hormone receptors and their regulation by phosphorylation

The steroid/thyroid hormone receptor superfamily of ligand-activated transcription factors encompasses not only the receptors for steroids, thyroid hormone, retinoids and vitamin D, but also a large number of proteins whose functions and/or ligands are unknown and which are thus termed orphan receptors. Recent studies have highlighted the importance of phosphorylation in receptor function. Although most of the phosphorylation sites are serine and threonine residues, a few of the family members are also phosphorylated on tyrosine. Those steroid receptor family members that are bound to heat-shock proteins in the absence of ligand typically are basally phosphorylated and exhibit increases in phosphorylation upon ligand binding. Most of these sites contain Ser-Pro motifs, and there is evidence that cyclin-dependent kinases and MAP kinases (mitogen-activated protein kinases) phosphorylate subsets of these sites. In contrast, phosphorylation sites identified thus far in members of the family that bind to DNA in the absence of hormone typically do not contain Ser-Pro motifs and are frequently casein kinase II or protein kinase A sites. Phosphorylation has been implicated in DNA binding, transcriptional activation and stability of the receptors. The finding that some of the steroid receptor family members can be activated in the absence of ligand by growth factors or neurotransmitters that modulate kinase and/or phosphatase pathways underscores the role of phosphorylation in receptor function. Hence this family of transcription factors integrates signals from ligands as well as from signal transduction pathways, resulting in alterations in mRNA and protein expression that are unique to the complex signals received.

Phosphorylation of tyrosine 537 on the human estrogen receptor is required for binding to an estrogen response element

We report here that the phosphorylation of tyrosine 537 on the human estrogen receptor (hER) controls the receptor's dimerization and DNA binding ability. The DNA-binding form of both the hER from human MCF-7 mammary carcinoma cells and the hER overexpressed in Sf9 insect cells was isolated using estrogen response element (ERE) affinity chromatography. Western blot analyses demonstrated that the DNA-binding form of the hER from MCF-7 or Sf9 cells was (i) phosphorylated at tyrosine 537, (ii) localized in the nucleus of estradiol-treated MCF-7 cells with an apparent molecular mass of 67 kDa, and (iii) hyperphosphorylated at serine residue(s). The non-DNA-binding form of the hER was (i) devoid of phosphorylation at tyrosine 537, (ii) cytosolic with an apparent molecular mass of 66 kDa, and (iii) hypophosphorylated at serine residue(s). The dephosphorylation of the purified hER at phosphotyrosine 537 with a tyrosine phosphatase eliminated binding to an ERE in a gel mobility shift assay. The binding of the tyrosine-dephosphorylated hER to an ERE was restored by the rephosphorylation of tyrosine 537 with Src family tyrosine kinases, p60c-src or p56lck. Mutation of tyrosine 537 to phenylalanine confirmed that the phosphorylation of tyrosine 537 is necessary for the hER to bind an ERE. An anti-hER antibody restored the binding of the tyrosine-dephosphorylated hER to an ERE, indicating that the bivalent anti-hER antibody brought together the two inactive hER monomers. A far-Western blot confirmed that phosphotyrosine 537 is required for hER homodimerization. These experiments establish that dimerization of the hER and DNA binding are regulated by phosphorylation at tyrosine 537. This is the first demonstration of the regulation of dimerization of a steroid hormone receptor by phosphorylation. These results are significant since p60c-src is overexpressed in estrogen-dependent breast cancers and may act to enhance the activity of the hER.