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Peroxisome proliferator-activated receptor gamma (PPARγ) is a nuclear receptor whose activation is dependent on a ligand. PPARγ activation by exogenous ligands, such as thiazolidinediones (TZDs), is a strategy in the treatment of type 2 diabetes mellitus for the improvement of insulin sensitivity. In addition to a ligand, PPARγ function is also regulated by posttranslational modifications, such as phosphorylation, sumoylation, and ubiquitination. Herein, we report that the PPARγ protein is modified by acetylation, which induces the PPARγ function in the absence of an external ligand. We observed that histone deacetylase 3 (HDAC3) interacted with PPARγ to deacetylate the protein. In immunoprecipitation assays, the HDAC3 protein was associated with the PPARγ protein. Inhibition of HDAC3 using RNAi-mediated knockdown or HDAC3 inhibitor increased acetylation of the PPARγ protein. Furthermore, inhibition of HDAC3 enhanced the expression of PPARγ target genes such as adiponectin and aP2. The expression was associated with an increase in glucose uptake and insulin signaling in adipocytes. HDAC3 inhibition enhanced lipid accumulation during differentiation of adipocytes. PPARγ acetylation was also induced by pioglitazone and acetylation was required for PPARγ activation. In the absence of TZDs, the acetylation from HDAC3 inhibition was sufficient to induce the transcriptional activity of PPARγ. Treating diet-induced obesity mice with HDAC3 inhibitor or pioglitazone for 2 weeks significantly improved high-fat-diet-induced insulin resistance. Our results indicate that acetylation of PPARγ is a ligand-independent mechanism of PPARγ activation. HDAC3 inhibitor is a potential PPARγ activator for the improvement of insulin sensitivity.

Placental-derived corticotropin-releasing hormone (CRH) seems to play a major role in the mechanisms controlling human pregnancy and parturition. It has been suggested that CRH directly modulates the endocrine function of placental trophoblasts, including the production of estrogen, ACTH, and prostaglandin. In this study, we sought to investigate the effect of CRH, locally produced by placenta, on progesterone production. Percoll-purified placental trophoblasts were obtained from uncomplicated term pregnancies and cultured for 72 h. Progesterone concentration in culture media was measured by RIA. The mRNA transcripts encoding CYP11A1 and HSD3B1, the enzymes for progesterone synthesis, were determined by quantitative real-time reverse transcription (RT)-PCR. Results showed that CRH (10⁻⁸–10⁻⁶ mol/l) caused a significant decrease in progesterone levels in a dose-dependent manner. The CRH antagonist, α-helical CRH 9-41, at 10⁻⁷–10⁻⁵ mol/l stimulated progesterone secretion. Consistent with this thesis, CRH decreased, whereas α-helical CRH increased, the mRNA levels of CYP11A1 and HSD3B1. Since CRH has been shown to activate the phospholipase C–protein kinase C (PKC) signal pathway in placenta, we examined whether the effect of CRH on progesterone synthesis was dependent on PKC signal pathway. Treatment of cells with PKC inhibitor, Gö6976, resulted in a significant increase in progesterone production, and exogenous CRH restored progesterone production. In conclusion, placental CRH exhibits a tonic inhibitory effect on progesterone production in a PKC-dependent fashion.

It is known that decreased apoptosis of thyrocytes may be involved in the formation of goiters in patients with Graves’ disease, and growth factors are involved in regulating the size of the thyroid gland. The purpose of our study was to investigate mRNA and protein levels of an antiapoptotic protein, namely, Fas-associated death domain-like interleukin-1-converting enzyme (FLICE)-inhibitory protein (FLIP). The results showed that in FRTL thyroid cells, treatment with IGF-I upregulated mRNA and protein levels of FLIP in a dose-dependant manner. While a specific nuclear factor-κB (NF-κB) inhibitor, BAY11-7082, blocked this effect. Further study demonstrated that IGF-I induced the DNA-binding activity of NF-κB in association with decreased expression of the NF-κB inhibitory protein IκBα . These findings implied that IGF-I increased FLIP expression by enhancing the activation of NF-κB in FRTL thyroid cells. Using a specific phosphatidylinositol 3-kinase (PI3K) inhibitor, LY294002, we also found that PI3K was involved in the pathway by which IGF-I activated NF-κB and increased FLIP expression. When treated with IGF-I and LY294002, decreased NF-κB DNA binding activity and increased expression of IκBα protein were detected in cultured thyroid cells, which further confirmed that NF-κB was under the control of the PI3K pathway. Taken together, our results suggest that IGF-I regulates the expression of FLIP in FRTL cells by activating the PI3K/NF-κB cascade.