PGC-1α decreases XBP1s protein levels and suppresses XBP1s activity. (A) XBP1s, PGC-1α, and tubulin protein levels in MEFs infected with equal doses of Ad-PGC-1α plus increasing doses of Ad-XBP1s. The same dose of Ad-XBP1s was used in lanes 5 and 6, except that Ad-LacZ was used in lane 6 instead of Ad-PGC-1α for co-expression. (B, C) MEFs were infected with the same amount of Ad-XBP1s, plus increasing amount of Ad-PGC-1α. (B) XBP1s, PGC-1α and actin protein levels in MEFs. (C) XBP1s mRNA levels measured by real time PCR (qPCR) (n = 5 for each group). (D) Levels of unspliced and spliced XBP1 mRNAs analyzed by reverse transcription PCR. MEFs were infected with increasing doses of Ad-PGC-1α, and treated with tunicamycin (Tm; 3 μg/ml, 90 min). (E) Levels of endogenous Xbp1s mRNA and protein in MEFs that were infected overnight with Ad-LacZ or Ad-PGC-1α then treated with increasing doses (0, 0.125, 0.25, 0.5, 1, 2 μM) of tunicamycin for 5 h. (F) ERSE-luciferase assay in MEFs infected with a fixed amount of Ad-XBP1s, plus increasing doses of Ad-PGC-1α, as in B and C. Renilla luciferase activity was used for normalization (n = 3 for each group). (G) Levels of endogenous FoxO1 protein in Fao cells infected with increasing doses of Ad-PGC-1α. (H) Levels of endogenous FoxO1 protein in Fao cells infected with equal amounts of Ad-XBP1s and Ad-LacZ or Ad-PGC-1α. We infected cells with the equal amount of virus by co-infecting with Ad-LacZ, and each experiment was repeated at least two times. Values are means ± s.e.m. Significance was determined by two-way analysis of variance (ANOVA), with Bonferroni multiple-comparison analysis (E) or Student's t test (C,F). AU, arbitrary units. **P < 0.01, ***P < 0.001.