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ABSTRACT: The mitochondrial uncoupling protein-2 (UCP2) can uncouple phosphorylation to subserve several functions. It has been reported
that the insulin sensitizers, thiazolidinediones (TZDs), increase UCP2 mRNA levels and, more recently, that TZDs stimulate
UCP2 reporter genes but that the sequences involved do not bind peroxisome proliferator-activated receptor γ (PPARγ). We report
here that TZDs stimulated UCP2 gene (ucp2) transcription in L6 myotubules involving an indirect mechanism. L6 cells contained comparatively small amounts of PPARγ
mRNA but clearly detectable amounts of PPARγ2 protein. UCP2 mRNA levels were increased in a time- and concentration-dependent
manner by TZDs. UCP2 mRNA had slow turnover (t
1/2 ≈ 38 h), and this was not affected by TZDs. Bisphenol A diglycidyl ether, a PPARγ antagonist, concentration dependently inhibited
the TZD-induced increase in UCP2 mRNA. Blockade of protein synthesis with cycloheximide as well as abrogation of mitogen-activated
protein kinase (MAPK) activity with PD98059 or U0126 also prevented the TZD-induced increase in UCP2 mRNA. As with autologous
UCP2 mRNA, TZDs stimulated reporter gene expression directed by ucp2 sequences in transiently transfected L6 cells. The effect was enhanced by cotransfection of PPARγ + retinoid X receptor γ
and prevented by MEK blockade. TZDs, however, did not increase the activation of MAPK, nor did its activation by other means
(change of medium, insulin-like growth factor-1, insulin) increase UCP2 mRNA, indicating that phosphorylation is not limiting.
These results suggest that TZDs indirectly stimulate ucp2 transcription by inducing—via PPARγ—limiting amounts of a protein, which must be phosphorylated by MAPK to stimulate the
gene.
Endocrine 04/2012; 19(2):197-208. · 1.42 Impact Factor
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ABSTRACT: The stimulation of the uncoupling protein-2 gene (ucp2) by thyroid hormone (triiodothyronine [T3]) in vivo is variable, suggesting complex interactions and even the possibility of indirect effects. We investigated the effect of T3 on ucp2 expression in L6 myotubules. Alone, T3 did not significantly stimulate ucp2 expression in L6 cells, but it amplified the stimulation by thiazolidinediones (TZDs). L6 cells expressed both alpha1 and beta1 thyroid hormone receptors and the data were consistent with the effect being mediated by these receptors. T3 also enhanced the stimulation of ucp2 by the nonselective peroxisome proliferator-activated receptor (PPAR) ligands bezafibrate and carbacyclin, but not that by oleic acid or norepinephrine. L6 cells expressed PPARbeta and PPARgamma, but not PPARalpha. As short as a 1-h preexposure of L6 cells to T3 was sufficient to amplify the effect of PPAR ligands. Neither transcription nor translation was needed for this effect of T3. T3 did not affect the t1/2 of UCP2 mRNA. The histone deacetylases inhibitor trichostatin A (TSA) stimulated the expression of ucp2 but did not add to the effect of T3 nor did this hormone enhance the effect of TSA. These results suggest that T3 selectively enhances the transcriptional stimulation of ucp2 by TZDs and nonselective PPAR ligands by priming the gene to a transactivating signal(s) generated by such ligands.
Endocrine 12/2002; 19(2):209-17. · 1.42 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: The mitochondrial uncoupling protein-2 (UCP2) can uncouple phosphorylation to subserve several functions. It has been reported that the insulin sensitizers, thiazolidinediones (TZDs), increase UCP2 mRNA levels and, more recently, that TZDs stimulate UCP2 reporter genes but that the sequences involved do not bind peroxisome proliferator-activated receptor gamma (PPARgamma). We report here that TZDs stimulated UCP2 gene (ucp2) transcription in L6 myotubules involving an indirect mechanism. L6 cells contained comparatively small amounts of PPARgamma mRNA but clearly detectable amounts of PPARgamma2 protein. UCP2 mRNA levels were increased in a time- and concentration-dependent manner by TZDs. UCP2 mRNA had slow turnover (t 1/2 approximately 38 h), and this was not affected by TZDs. Bisphenol A diglycidyl ether, a PPARy antagonist, concentration dependently inhibited the TZD-induced increase in UCP2 mRNA. Blockade of protein synthesis with cycloheximide as well as abrogation of mitogen-activated protein kinase (MAPK) activity with PD98059 or U0126 also prevented the TZD-induced increase in UCP2 mRNA. As with autologous UCP2 mRNA, TZDs stimulated reporter gene expression directed by ucp2 sequences in transiently transfected L6 cells. The effect was enhanced by cotransfection of PPARgamma + retinoid X receptor gamma and prevented by MEK blockade. TZDs, however, did not increase the activation of MAPK, nor did its activation by other means (change of medium, insulin-like growth factor-1, insulin) increase UCP2 mRNA, indicating that phosphorylation is not limiting. These results suggest that TZDs indirectly stimulate ucp2 transcription by inducing-via PPARgamma-limiting amounts of a protein, which must be phosphorylated by MAPK to stimulate the gene.
Endocrine 12/2002; 19(2):197-208. · 1.42 Impact Factor
