Cytosolic action of thyroid hormone leads to induction of hypoxia-inducible factor-1alpha and glycolytic genes.

Thyroid hormone (TH) effects are mediated through T(3), which regulates gene expression by binding to the nuclear TH receptors, TRalpha and TRbeta. Using microarrays and real-time PCR we found mRNAs of the following genes increased in response to T(3) in a TRbeta-specific manner: the transcription factor hypoxia-inducible factor (HIF)-1alpha, its target genes glucose transporter (GLUT)1 and platelet-type phosphofructokinase (PFKP), and the monocarboxylate transporter (MCT)4. The products of these genes have important roles in cellular glucose metabolism. HIF-1alpha expression and activity can be regulated through phosphatidylinositol-OH-3-kinase (PI3K) and MAPK signaling; thus the possibility of alternative, nonnuclear pathways of TH action was raised. We examined the involvement of these pathways in mediating TH effects by treating human skin fibroblasts with 2 nm T(3) in the absence or presence of either the PI3K inhibitor LY294002 or the MAPK inhibitor PD98059. T(3) induced HIF-1alpha mRNA by 2.7-fold (+/-0.4; P < 0.013). This increase was completely abrogated by LY294002 (1.1 +/- 0.1; nonsignificant = 0.57), but preserved in the presence of PD98059 (2.2 +/- 0.2; P < 0.009). Western blotting confirmed these results at the protein level, indicating dependency on the PI3K pathway. The same pattern of response was observed for GLUT1, PFKP, and MCT4 expression. To examine whether HIF-1alpha is directly induced, we used the translation inhibitor cycloheximide (CHX). T(3) induction of HIF-1alpha mRNA was not affected by CHX, whereas T(3) effect on GLUT1, PFKP, and MCT4 mRNA was completely abrogated by CHX. These results demonstrate that cytosolic activation of the PI3K signaling pathway has a role in TH-mediated direct (HIF-1alpha) and indirect (GLUT1, PFKP, MCT4) gene expression, and possibly provides a link between TH and cellular glucose metabolism in human fibroblasts.