No abstract

Thyroid hormone (triiodothyronine [T3]) regulates gene expression by binding to high-affinity nuclear receptors. Thyroid hormone receptors (TRs) recognize specific response element sequences in the promoters of T3-target genes and activate or repress transcription in response to hormone. In this paper, we review the TR proteins and thyroid hormone response elements (TREs) to which they bind, the mechanisms of action of TRs bound to the TRE in basal and liganded conformations, and the interacting proteins implicated in these complexes. We then briefly consider the cross-talk with other signaling pathways and introduce the idea that T3 may also act rapidly via nongenomic actions located on membranes. We discuss patterns of gene expression and specific actions of the various TR isoforms and consider the novel TR isoform specific ligands.

Thyroid hormone receptors are encoded by the TR␣ (NR1A1) and TR␤ (NR1A2) loci. These genes are transcribed into multiple variants whose functions are unclear. Analysis by gene inactivation in mice has provided new insights into the functional complexity of these products. Different strategies designed to modify the TR␣ locus have led to strikingly different phenotypes. In order to analyze the molecular basis for these alterations, we generated mice devoid of all known isoforms produced from the TR␣ locus (TR␣ 0/0 ). These mice are viable and exhibit reduced linear growth, bone maturation delay, moderate hypothermia, and reduced thickness of the intestinal mucosa. Compounding TR␣ 0 and TR␤ ؊ mutations produces viable TR␣ 0/0 ␤ ؊/؊ mice, which display a more severe linear growth reduction and a more profound hypothermia as well as impaired hearing. A striking phenotypic difference is observed between TR␣ 0/0 and the previously described TR␣ ؊/؊ mice, which retain truncated TR⌬␣ isoforms arising from a newly described promoter in intron 7. The lethality and severe impairment of the intestinal maturation in TR␣ ؊/؊ mice are rescued in TR␣ 0/0 animals. We demonstrate that the TR⌬␣ protein isoforms, which are natural products of the TR␣ locus, are the key determinants of these phenotypical differences. These data reveal the functional importance of the non-T3-binding variants encoded by the TR␣ locus in vertebrate postnatal development and homeostasis.

By means of differential RNA display, we have isolated a cDNA corresponding to transcripts that are down-regulated upon differentiation of the goblet cell-like HT-29-M6 human colon carcinoma cell line. These transcripts encode proteins originally identified as CROC-1 on the basis of their capacity to activate transcription of c-fos. We show that these proteins are similar in sequence, and in predicted secondary and tertiary structure, to the ubiquitin-conjugating enzymes, also known as E2. Despite the similarities, these proteins lack a critical cysteine residue essential for the catalytic activity of E2 enzymes and, in vitro, they do not conjugate or transfer ubiquitin to protein substrates. These proteins constitute a distinct subfamily within the E2 protein family and are highly conserved in phylogeny from yeasts to mammals. Therefore, we have designated them UEV (ubiquitin-conjugating E2 enzyme variant) proteins, defined as proteins similar in sequence and structure to the E2 ubiquitin-conjugating enzymes but lacking their enzymatic activity (HW/GDB-approved gene symbol, UBE2V). At least two human genes code for UEV proteins, and one of them, located on chromosome 20q13.2, is expressed as at least four isoforms, generated by alternative splicing. All human cell types analyzed expressed at least one of these isoforms. Constitutive expression of exogenous human UEV in HT-29-M6 cells inhibited their capacity to differentiate upon confluence and caused both the entry of a larger proportion of cells in the division cycle and an accumulation in G 2 -M. This was accompanied with a profound inhibition of the mitotic kinase, cdk1. These results suggest that UEV proteins are involved in the control of differentiation and could exert their effects by altering cell cycle distribution.The intestinal epithelium is comprised of cells with different mature phenotypes that are believed to derive from common precursor cells resident in special anatomic compartments, called the crypts of Lieberkühn. Through asymmetric divisions and migration along the crypt, such precursor cells undergo phenotypic conversion into mucosecretory, absorptive, enteroendocrine or Paneth cells (19), with each expressing a distinct set of molecules characteristic of their specialized mature functions. The life cycle of mature cells terminates by apoptosis, followed by shedding from the tip of the villus to the intestinal lumen (19).A number of cellular models, such as the human colorectal cancer-derived cell lines HT-29-M6, HS174T, and Caco-2, have allowed the analysis of the molecular mechanisms that control the differentiated phenotypes of human intestinal epithelial cells (24,36,44,47,73). Several of these models appear to recapitulate some of the differentiation processes that accompany developmentally regulated events, such as the establishment of cephalocaudal and crypt-villus axes (59, 62). Using either in vitro or in vivo models, systematic approaches have led to the identification of differentially expressed genes and proteins involved in th...