Regulation of transcription factors by nuclear degradation

固醇调节元件结合蛋白(SREBPs)被合成为膜结合的前体，并被加工以产生转录活性形式。活性的SREBPs移位到细胞核，诱导应答基因的表达，并很快被降解。蛋白酶体抑制剂的治疗提高了HeLa细胞中内源性核SREBPs的数量，但不是前体。在稳定的中国仓鼠卵巢细胞系(CHO-487和-481)中瞬时表达的人SREBP-1a(氨基酸1-487)和SREBP-2(氨基酸1-481)的核形式也被蛋白酶体抑制剂稳定，这表明核SREBPs可能是蛋白酶体依赖的蛋白分解的底物。稳定的核SREBPs能主动诱导应答基因的表达，包括HMG辅酶A合成酶、脂肪酸合成酶和低密度脂蛋白受体。核内SREBP-1a的快速周转不受细胞内固醇水平的影响，半衰期约为3h。核内SREBP-1a与多泛素链结合。当这种连接被泛素化缺陷的突变泛素的过表达抑制时，核SREBPs部分稳定并诱导反应基因的表达，这表明泛素连接的SREBPs是蛋白酶体的底物。综上所述，这些结果表明，泛素-蛋白酶体系统降解SREBPs，该系统控制SREBP反应基因的表达。

Sterol regulatory element-binding proteins (SREBPs) are synthesized as membrane-bound precursors and processed to generate transcriptionally active forms. The active SREBPs translocate to the nucleus, induce the expression of responsive genes and are degraded very rapidly. Treatment with proteasome inhibitors elevates the amount of the endogenous nuclear SREBPs, but not the precursors, in HeLa cells. Nuclear forms of human SREBP-la (amino acids 1-487) and SREBP-2 (amino acids 1-481), which are transiently expressed in stable Chinese hamster ovary cell lines (CHO-487 and -481), are also stabilized by proteasome inhibitors, suggesting that the nuclear SREBPs are likely to be substrates for the proteasome-dependent proteolysis. The stabilized nuclear SREBPs actively induce the expression of responsive genes including HMG CoA synthase, fatty acid synthase and the low density lipoprotein receptor. The rapid turnover of nuclear SREBP-la is not affected by the intracellular sterol levels and the half-life is estimated to be approximately 3 h. The nuclear SREBPs are found conjugated with a polyubiquitin chain. When this conjugation is inhibited by overexpression of mutant ubiquitin, defective in polyubiquitination, the nuclear SREBPs are partly stabilized and induce the expression of the responsive gene, suggesting that the ubiquitin conjugated-SREBPs are substrates for the proteasome. Taken together, these results demonstrate that the ubiquitin-proteasome system degrades SREBPs and that this system controls the expression of SREBP-responsive genes.