its effects are mediated principally through triiodothyronine ( t3 ) , which acts as a ligand for the th receptors ( trs ) 1 , 2 and 1 [ harvey and williams , 2002 ; yen , 2001 ] . in the classical model of genes positively regulated by th ,
the tr first binds as a heterodimer or homodimer on th response elements ( tre ) located in the promoter regions of target genes , where it interacts with corepressors . upon ligand
binding , the tr homodimers are dissociated in favor of heterodimer formation with the retinoid - x receptor ( rxr ) , resulting in release of the corepressors and recruitment of coactivators .  
this new complex attracts a large number of proteins which engage the rna polymerase ii in the transcription of the targeted gene ( figure 1 , part 1 ) .
this classical mechanism can also lead to increased expression of genes devoid of tres , if they are target genes for transcription factors that are induced by this mechanism .
in addition to the classical ,  nuclear  mode of th action , a number of rapid effects taking place in the cytosol and at the plasma membrane have been subsequently identified .
th can control ca entry , intracellular protein trafficking and regulation of protein kinase c [ davis and davis , 2002 ; davis et al .
the mapk pathway can be activated by th binding to the integrin v3 , located in the cell membrane , without entering the cell .
this mechanism leads to phosphorylation of nuclear receptors and can induce angiogenesis and promote cell growth [ bergh et al . , 2005 ; tang et al . ,
a derivative of th , 3-iodothyronamine ( t1am ) , can induce bradycardia and hypothermia within minutes through a mechanism that remains unknown [ scanlan et al .
these nongenomic actions of th are mostly extranuclear , appear to be independent of trs and have rapid effects on proteins rather than modulate gene expression .
as all proteins , trs are synthesized in the cytoplasm from where they are translocated into the nucleus to exert their genomic effect summarized above . a dynamic nucleo - cytoplasmic shuttling has been described [ baumann et al . , 2001 ] .
we recently identified a new mechanism of th action in which the liganded tr interacts with the regulatory subunit of pi3k ( p85 ) in the cytosol [ cao et al . ,
this leads to activation of pi3k ( figure 2 ) and its downstream signaling cascade ( figure 1 part 2 ) , sequential phosphorylation and activation of the serine / threonine kinase akt , mammalian target of rapamycin ( mtor ) and its substrate p70 .
mtor activation is rapid , with detectable phosphorylation as early as 10 minutes after t3 treatment , and not sensitive to cycloheximide ( chx ) treatment , indicating that this effect of th uses preexisting proteins .
th acts through the tr , because in human fibroblasts that express the wt tr , introduction of a dominant negative mutant tr abrogated the effect of th .
furthermore , a direct interaction between tr and pi3k could be demonstrated by coimmunoprecipitation of tr1 with the p85 subunit of pi3k .
the interaction between tr and pi3k most likely takes place in the cytosol . within minutes after activation by t3 , phosphorylated akt , as part of the pi3k pathway , is translocated from the cytosol into the nucleus ( figure 1 part 2 ) .
this th action is very rapid and independent of protein synthesis , which is typical of nongenomic action . yet , two aspects distinguish this mechanism of th action from most other nongenomic effects of the hormone : 1 ) it requires tr binding and 2 ) its ultimate effect is 
for example , the calcineurin inhibitor zaki-4 had been previously described as a th responsive gene [ cao et al . , 2002 ; cao and seo , 2003 ; miyazaki et al . , 1996 ] .
this effect was blocked by inhibitors of pi3k and by a dominant negative p85 subunit of pi3k [ cao et al . ,
rapamycin , an inhibitor of mtor also abrogated the th - dependent induction of zaki-4 , suggesting the requirement of sequential activation of kinases initiated by the activation of pi3k .
zaki-4 expression is also chx sensitive , indicating that protein synthesis is necessary for the induction of zaki-4. these results show that zaki-4 represents an example of an indirectly induced gene downstream of nongenomic action of th .
the transcription factor linking the pi3k pathway to zaki-4 expression has not yet been identified .
a similar activation of pi3k has been observed for other steroid hormone nuclear receptors [ simoncini et al . , 2000 ]
to determine the broad scale effect of th on gene expression in normal human cells , we measured the expression of more than 15,000 genes in fibroblasts of normal individuals by quantitative fluorescent cdna microarray [ moeller et al .
fibroblasts from two subjects with resistance to thyroid hormone ( rth ) due to mutations in the tr gene were used to confirm the specificity of the hormonal effect by the ability to discriminate between normal cells and cells with a defect in th action .
microarray analysis identified 91 up - regulated and 5 down - regulated genes and confirmation by real - time pcr was obtained in 8 of 10 induced and 2 of 3 repressed genes that were tested .
several new th responsive genes were identified , both positively ( akr1c1 - 3 , pfkp , rab3b , hif-1 , colvia3 ) and negatively regulated ( fgf7 , adh1b ) .
genes that had been found to respond to th in other species were also found in these human cells ( bteb1 , glut1 , mct4 ) .
further evidence for t3-specific induction was provided by a graded dose response for induced genes that was absent in fibroblasts from the patients with rth .
of special interest among the upregulated genes were the transcription factor subunit hypoxia - inducible factor ( hif)-1 , its target genes , glucose transporter ( glut)1 and platelet - type phosphofructokinase ( pfkp ) , and the monocarboxylate transporter ( mct)4 .
these genes are functionally related as they have important roles in cellular glucose metabolism , from glucose uptake ( glut1 ) to glycolysis ( pfkp ) and lactate export ( mct4 ) .
the response of these genes to th was reduced or absent in the th resistant fibroblasts , which confirms the role of the tr.
the transcription factor hif-1 consists of two subunits ,  and . while the  subunit is constitutively expressed , the  subunit is tightly regulated .
cellular hypoxia leads to increase of hif-1 protein levels by decreased ubiquitination [ semenza , 2001 ] and in normoxia , growth factors and hormones ( e.g. egf , igf-1 , insulin , androgens ) lead to increased hif-1 by posttranscriptional mechanisms [ bardos and ashcroft , 2004 ; kasuno et al . ,
the latter is mediated by cellular signal transduction pathways , especially the pi3k - akt / pkb - mtor pathway and the mapk pathways [ fukuda et al . , 2002 ; kasuno et al . ,
we examined the involvement of the pi3k pathway by treating human skin fibroblasts with t3 .
mrna of hif-1 was increased after 3 hours and the effect lasted for the 24 hours of the experiments .
mrna induction led to significant increase in the protein , demonstrated by western blotting for hif-1 as well as pfkp .
addition of the pi3k inhibitor ly294002 abrogated th - dependent induction of mrnas for hif-1 as well as glut1 , pfkp and mct4 [ moeller et al .
hif-1 induction was direct and translation - independent , as chx pretreatment did not inhibit hif-1 mrna increase after t3 treatment .
as expected , expression of hif-1 s target genes , glut1 and pfkp , was chx - sensitive .
the mapk pathway was examined by pretreatment with pd98059 , but inhibition of t3 effect was not observed .
the mapk pathway seems not to be involved expression of these genes in human fibroblasts .
these results further demonstrate that activation of the pi3k signaling pathway by th and tr in the cytosol can lead to direct ( hif-1 ) and indirect ( glut1 , pfkp , mct4 ) gene expression .
until recently , th - mediated changes in gene expression were thought to be primarily , if not solely , initiated by direct nuclear tr binding to a tre in the promoter of a target gene .
it is capable of activating cellular signaling pathways in the cytosol , as demonstrated for the pi3k pathway , which leads to induction of zaki4 , hif-1 and its target genes .
this cytosolic mechanism of th - mediated tr-dependent action contributes to the overall effect of th on gene expression .
surprisingly , some of the most highly induced genes in our experiments were later found to be downstream of the pi3k pathway ( e.g. zaki-4 , pfkp ) .
a possible explanation is that kinase cascades have the potential to amplify the signal from kinase to kinase , whereas on a tre amplification steps are limited .
the definition of the trs therefore has broadened beyond being merely ligand - dependent nuclear transcription factors .
we conclude that th responsive genes can be directly induced by nuclear ( e.g. bteb1 ) and cytosolic action of th ( e.g. hif-1 ) without requirement of prior protein synthesis .
indirectly induced genes , following expression of a transcription factor , exist for both modes of th action .
nongenomically induced and that more signaling pathways will be found to be activated by the trs .
genomic ( 1 ) and non - genomic ( 2 ) actions of th are illustrated .
genomic action requires thyroid hormone responsive elements ( tres ) for the recognition of genes for direct transcriptional regulation .
non - genomic action is initiated by the th - dependent activation of pi3k as illustrated in figure 2 .
activation of pi3k leads to sequential activation of akt / pkb - mtor - p70 .
although not well defined , this cascade leads to transcriptional upregulation of some genes such as zaki-4 and hif-1. gtf : general transcription factors . for details
tr present in the cytosol forms a complex with p85 subunit ( regulatory subunit of pi3k ) in a ligand independent manner .
 thyroid hormone ( th ) action is mediated principally through binding of the hormone ligand , 3,3,5-triiodothyronine ( t3 ) , to th receptors ( trs ) . 
 this hormone - receptor interaction recruits other proteins to form complexes that regulate gene expression by binding to dna sequences in the promoter of target genes . 
 we recently described an extranuclear mechanism of th action that consists of the association of th - liganded tr with p85 [ regulatory subunit of phosphatidylinositol 3-kinase ( pi3k ) ] in the cytosol and subsequent activation of the pi3k , generating phosphatidylinositol 3,4,5-triphosphate [ ptdins(3,4,5)p3 ] . 
 this initiates the activation of a signaling cascade by phosphorylation of akt , mammalian target of rapamycin ( mtor ) and its substrate p70s6k , leading to the stimulation of zaki-4 synthesis , a calcineurin inhibitor . 
 furthermore , we found that this same mechanism leads to induction of the transcription factor hypoxia - inducible factor ( hif-1 ) , and its target genes , glucose transporter ( glut)1 , platelet - type phosphofructokinase ( pfkp ) , and monocarboxylate transporter ( mct ) 4 . 
 these genes are of special interest , because their products have important roles in cellular glucose metabolism , from glucose uptake ( glut1 ) to glycolysis ( pfkp ) and lactate export ( mct4 ) . 
 these results demonstrate that the th - tr complex can exert a non - genomic action in the cytosol leading to changes in gene expression by direct ( hif-1 ) and indirect ( zaki-4 , glut1 , pfkp ) means . 
Classical, genomic, thyroid hormone action
Nongenomic action of thyroid hormone
Cytosolic activation of the PI3K pathway by TR
TH target genes
The cytosolic action of TH induces HIF-1 and glycolytic genes
Conclusion and perspective
Figures and Tables