Soccio Raymond E, Adams Rachel M, Maxwell Kara N, Breslow Jan L
Laboratory of Biochemical Genetics and Metabolism, Rockefeller University, New York, New York 10021, USA.
J Biol Chem. 2005 May 13;280(19):19410-8. doi: 10.1074/jbc.M501778200. Epub 2005 Mar 10.
The StarD4 and StarD5 proteins share approximately 30% identity, and each is a steroidogenic acute regulatory protein (StAR)-related lipid transfer (START) domain. We previously showed StarD4 expression is sterol-repressed, consistent with regulation by sterol regulatory element-binding proteins (SREBPs), whereas StarD5 is not sterol-regulated. Here we further address the regulation and function of StarD4 and StarD5. Unlike StAR, the START family prototype, StarD4 and StarD5 were not induced by steroidogenic stimuli in Leydig cells. However, StarD4 and StarD5 showed StAR-like activity in a cell culture steroidogenesis assay, indicating cholesterol transfer. In transgenic mice expressing active SREBPs, StarD4 was predominantly activated by SREBP-2 rather than SREBP-1a. The mouse and human StarD4 proximal promoters share approximately 70% identity, including several potential sterol regulatory elements (SREs). Reporters driven by the StarD4 promoter from either species were transfected into NIH-3T3 cells, and reporter activity was highly repressed by sterols. Site-directed mutagenesis of potential SREs identified a conserved functional SRE in the mouse (TCGGTCCAT) and human (TCATTCCAT) promoters. StarD5 was not sterol-repressed via SREBPs nor was it sterol-activated via liver X receptors (LXRs). Even though StarD4 and StarD5 were not LXR targets, their overexpression stimulated LXR reporter activity, suggesting roles in cholesterol metabolism. StarD5 expression increased 3-fold in free cholesterol-loaded macrophages, which activate the endoplasmic reticulum (ER) stress response. When NIH-3T3 cells were treated with agents to induce ER stress, StarD5 expression increased 6-8-fold. Because StarD4 is regulated by sterols via SREBP-2, whereas StarD5 is activated by ER stress, they likely serve distinct functions in cholesterol metabolism.
StarD4和StarD5蛋白的同源性约为30%，且二者均含有一个与类固醇生成急性调节蛋白（StAR）相关的脂质转运（START）结构域。我们之前的研究表明，StarD4的表达受固醇抑制，这与固醇调节元件结合蛋白（SREBP）的调控一致，而StarD5不受固醇调节。在此，我们进一步探讨StarD4和StarD5的调控及功能。与START家族原型StAR不同，StarD4和StarD5在睾丸间质细胞中不会被类固醇生成刺激所诱导。然而，在细胞培养类固醇生成试验中，StarD4和StarD5表现出类似StAR的活性，表明存在胆固醇转运。在表达活性SREBP的转基因小鼠中，StarD4主要由SREBP-2而非SREBP-1a激活。小鼠和人类StarD4近端启动子的同源性约为70%，包括几个潜在的固醇调节元件（SRE）。将来自任一物种的由StarD4启动子驱动的报告基因转染到NIH-3T3细胞中，报告基因活性受到固醇的高度抑制。对潜在SRE进行定点诱变，在小鼠（TCGGTCCAT）和人类（TCATTCCAT）启动子中鉴定出一个保守的功能性SRE。StarD5既不受SREBP的固醇抑制，也不受肝脏X受体（LXR）的固醇激活。尽管StarD4和StarD5不是LXR的靶标，但它们的过表达会刺激LXR报告基因活性，提示其在胆固醇代谢中发挥作用。在负载游离胆固醇的巨噬细胞中，StarD5的表达增加了3倍，这些巨噬细胞会激活内质网（ER）应激反应。当用诱导ER应激的药物处理NIH-3T3细胞时，StarD5的表达增加了6至8倍。由于StarD4受SREBP-2的固醇调节，而StarD5受ER应激激活，它们在胆固醇代谢中可能发挥不同的功能。
