Silencing of Wnt Signaling and Activation of Multiple Metabolic Pathways in Response to Thyroid Hormone-Stimulated Cell Proliferation

Abstract

To investigate the transcriptional program underlying thyroid hormone (T3)-induced cell proliferation, cDNA microarrays were used to survey the temporal expression profiles of 4,400 genes. Of 358 responsive genes identified, 88% had not previously been reported to be transcriptionally or functionally modulated by T3. Partitioning the genes into functional classes revealed the activation of multiple pathways, including glucose metabolism, biosynthesis, transcriptional regulation, protein degradation, and detoxification in T3-induced cell proliferation. Clustering the genes by temporal expression patterns provided further insight into the dynamics of T3 response pathways. Of particular significance was the finding that T3 rapidly repressed the expression of key regulators of the Wnt signaling pathway and suppressed the transcriptional downstream elements of the ␤-catenin-T-cell factor complex. This was confirmed biochemically, as ␤-catenin protein levels also decreased, leading to a decrease in the transcriptional activity of a ␤-catenin-responsive promoter. These results indicate that T3-induced cell proliferation is accompanied by a complex coordinated transcriptional reprogramming of many genes in different pathways and that early silencing of the Wnt pathway may be critical to this event.Thyroid hormone (3,3Ј,5-triiodo-L-thyronine [T3]) receptors (TRs) are ligand-dependent transcription factors which are members of the steroid hormone/retinoic acid (RA) receptor superfamily. Two TR genes, TR␣ and TR␤, located on chromosomes 17 and 3, respectively, give rise, by alternative splicing, to three T3-binding TR isoforms, ␣1, ␤1, and ␤2 (reviewed by Cheng [12]). TRs mediate the biological activities of T3 by binding to specific DNA sequences known as the T3 response elements present in the promoter regions of T3 target genes (reviewed by Cheng [12]). The transcriptional regulatory activity of TRs depends not only on T3 and the types of T3 response elements but also on a host of coregulatory proteins, including corepressors, coactivators, and the tumor suppressor p53 (3, 38).The growth-stimulatory effect of T3 has long been recognized. In humans, lack of T3 during development leads to growth retardation and cretinism. Growth retardation is also evident in patients with resistance to T3, a genetic disease due to mutations in the TR␤ gene (54). TR␤1 mutants act in a dominant negative fashion to cause growth retardation and delayed bone maturation (54). Moreover, mutant mice harboring a potent dominant negative mutant TR␤1 also exhibit a similar phenotype (25,60).GC is a rat pituitary cell line that expresses functional TRs and has long been used as a model cell line to understand the mechanisms of T3 action. Previously we have shown that GC cells are induced to proliferate by T3 in cultured medium containing 10% T3-depleted (Td) serum (2, 27). This induction is T3 specific, because the inactive T3 analogs, L-thyronine and reverse T3, failed to stimulate proliferation of GC cells in the same culture medium (27). Recently...

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