Trichostatin A Modulates Thiazolidinedione-Mediated Suppression of Tumor Necrosis Factor α-Induced Lipolysis in 3T3-L1 Adipocytes

In obesity, high levels of tumor necrosis factor α (TNFα) stimulate lipolysis in adipocytes, leading to hyperlipidemia and insulin resistance. Thiazolidinediones (TZDs), the insulin-sensitizing drugs, antagonize TNFα-induced lipolysis in adipocytes, thereby increasing insulin sensitivity in diabetes patients. The cellular target of TZDs is peroxisome proliferator-activated receptor γ (PPARγ), a nuclear receptor that controls many adipocyte functions. As a transcription factor, PPARγ is closely modulated by coregulators, which include coactivators and corepressors. Previous studies have revealed that in macrophages, the insulin-sensitizing effect of PPARγ may involve suppression of proinflammatory gene expression by recruiting the corepressor complex that contains corepressors and histone deacetylases (HDACs). Therefore, we investigated whether the corepressor complex is involved in TZD-mediated suppression of TNFα-induced lipolysis in 3T3-L1 adipocytes. Trichostatin A (TSA), a pan HDAC inhibitor (HDACI) that inhibits class I and II HDACs, was used to examine the involvement of HDACs in the actions of TZDs. TSA alone increased basal lipolysis and attenuated TZD-mediated suppression of TNFα-induced lipolysis. Increased basal lipolysis may in part result from class I HDAC inhibition because selective class I HDACI treatment had similar results. However, attenuation of TZD-mediated TNFα antagonism may be specific to TSA and related hydroxamate-based HDACI rather than to HDAC inhibition. Consistently, corepressor depletion did not affect TZD-mediated suppression. Interestingly, TSA treatment greatly reduced PPARγ levels in differentiated adipocytes. Finally, extracellular signal-related kinase 1/2 (ERK1/2) mediated TNFα-induced lipolysis, and TZDs suppressed TNFα-induced ERK phosphorylation. We determined that TSA increased basal ERK phosphorylation, and attenuated TZD-mediated suppression of TNFα-induced ERK phosphorylation, consistent with TSA’s effects on lipolysis. These studies suggest that TSA, through down-regulating PPARγ, attenuates TZD-mediated suppression of TNFα-induced ERK phosphorylation and lipolysis in adipocytes.在肥胖状态下，高水平的肿瘤坏死因子α（tumor necrosis factor α, TNFα）可刺激脂肪细胞（adipocytes）发生脂解作用，进而引发高脂血症与胰岛素抵抗。噻唑烷二酮类（Thiazolidinediones, TZDs）作为胰岛素增敏药物，能够拮抗TNFα诱导的脂肪细胞脂解，从而提升糖尿病患者的胰岛素敏感性。TZDs的细胞靶点为过氧化物酶体增殖物激活受体γ（peroxisome proliferator-activated receptor γ, PPARγ），这是一类调控诸多脂肪细胞功能的核受体。作为转录因子，PPARγ受到辅调节因子（coregulators）的紧密调控，这类辅调节因子包括辅激活因子（coactivators）与辅阻遏因子（corepressors）。既往研究表明，在巨噬细胞（macrophages）中，PPARγ的胰岛素增敏效应可能通过招募包含辅阻遏因子与组蛋白去乙酰化酶（histone deacetylases, HDACs）的辅阻遏复合物，抑制促炎基因的表达。因此，本研究旨在探究辅阻遏复合物是否参与了TZDs介导的、在3T3-L1脂肪细胞中对TNFα诱导脂解的抑制作用。曲古抑菌素A（Trichostatin A, TSA）作为一种可抑制I类与II类HDAC的广谱HDAC抑制剂（pan HDAC inhibitor, HDACI），被用于探究HDAC是否参与TZDs的作用过程。单独使用TSA可提升基础脂解水平，并削弱TZDs介导的对TNFα诱导脂解的抑制作用。基础脂解水平的提升可能部分源于I类HDAC的抑制，因为选择性I类HDACI处理也得到了相似的实验结果。然而，TZDs介导的TNFα拮抗作用被削弱这一效应，可能仅特异性地出现于TSA及相关的羟肟酸类HDAC抑制剂，而非广谱HDAC抑制所导致。与之相符的是，辅阻遏因子的敲低并未影响TZDs介导的抑制作用。值得注意的是，TSA处理可显著降低分化成熟脂肪细胞中的PPARγ水平。最后，细胞外信号调节激酶1/2（extracellular signal-related kinase 1/2, ERK1/2）介导了TNFα诱导的脂解作用，而TZDs可抑制TNFα诱导的ERK磷酸化。本研究证实，TSA可提升基础ERK磷酸化水平，并削弱TZDs介导的对TNFα诱导ERK磷酸化的抑制作用，这与TSA对脂解的影响相一致。综上，本研究表明TSA通过下调PPARγ的表达，削弱了TZDs在脂肪细胞中对TNFα诱导的ERK磷酸化与脂解的抑制作用。