Oberkofler Hannes, Schraml Elisabeth, Krempler Franz, Patsch Wolfgang
Department of Laboratory Medicine, Landeskliniken Salzburg, Muellner Hauptstrasse 48, A-5020, Salzburg, Austria.
Biochem J. 2004 Jul 15;381(Pt 2):357-63. doi: 10.1042/BJ20040173.
The expression of SREBP-1 (sterol-regulatory-element-binding protein-1) isoforms differs between tissues and cultured cell lines in that SREBP-1a is the major isoform in established cell lines, whereas SREBP-1c predominates in liver and most other human tissues. SREBP-1c is transcriptionally less active than SREBP-1a, but is a main mediator of hepatic insulin action and is selectively up-regulated by LXR (liver X receptor) agonists. LXR-mediated transactivation is co-activated by PGC-1alpha (peroxisome-proliferator-activated receptor-gamma co-activator-1alpha), which displays deficient expression in skeletal-muscle-derived cell lines. In the present paper, we show that PGC-1alpha expression is also deficient in HepG2 cells and in a human brown adipocyte cell line (PAZ6). In transient transfection studies, PGC-1alpha selectively amplified the LXR-mediated transcription from the human SREBP-1c promoter in HepG2 and PAZ6 cells via two LXR-response elements with extensive similarity to the respective murine sequence. Mutational analysis showed that the human LXR-response element-1 (hLXRE-1) was essential for co-activation of LXR-mediated SREBP-1c gene transcription by PGC-1alpha. Ectopic overexpression of PGC-1alpha in HepG2 cells enhanced basal SREBP-1c and, to a lesser extent, -1a mRNA expression, but only SREBP-1c expression was augmented further in an LXR/RXR (retinoic X receptor)-dependent fashion, thereby inducing mRNA abundance levels of SREBP-1c target genes, fatty acid synthase and acetyl-CoA carboxylase. These results indicate that PGC-1alpha contributes to the regulation of SREBP-1 gene expression, and can restore the SREBP-1 isoform expression pattern of HepG2 cells to that of human liver.
固醇调节元件结合蛋白-1（SREBP-1）亚型的表达在不同组织和培养细胞系中存在差异，即SREBP-1a是已建立细胞系中的主要亚型，而SREBP-1c在肝脏和大多数其他人体组织中占主导。SREBP-1c的转录活性低于SREBP-1a，但它是肝脏胰岛素作用的主要介导因子，并被肝脏X受体（LXR）激动剂选择性上调。LXR介导的反式激活由过氧化物酶体增殖物激活受体γ共激活因子-1α（PGC-1α）共同激活，而PGC-1α在骨骼肌来源的细胞系中表达不足。在本文中，我们发现PGC-1α在HepG2细胞和人棕色脂肪细胞系（PAZ6）中表达也不足。在瞬时转染研究中，PGC-1α通过两个与相应小鼠序列具有广泛相似性的LXR反应元件，选择性地增强了HepG2和PAZ6细胞中人类SREBP-1c启动子的LXR介导的转录。突变分析表明，人类LXR反应元件-1（hLXRE-1）对于PGC-1α共同激活LXR介导的SREBP-1c基因转录至关重要。PGC-1α在HepG2细胞中的异位过表达增强了基础SREBP-1c以及程度较轻的-1a mRNA表达，但只有SREBP-1c的表达以LXR/视黄酸X受体（RXR）依赖的方式进一步增加，从而诱导SREBP-1c靶基因脂肪酸合酶和乙酰辅酶A羧化酶的mRNA丰度水平。这些结果表明，PGC-1α有助于SREBP-1基因表达的调控，并能将HepG2细胞的SREBP-1亚型表达模式恢复为人类肝脏的模式。
