Ariyoshi N, Kim Y C, Artemenko I, Bhattacharyya K K, Jefcoate C R
Department of Pharmacology, University of Wisconsin Medical School, Madison, Wisconsin 53706, USA.
J Biol Chem. 1998 Mar 27;273(13):7610-9. doi: 10.1074/jbc.273.13.7610.
The steroidogenic acute regulatory protein (STAR) participates in steroidogenesis through the mitochondrial transfer of cholesterol to cytochrome P450scc. The rat adrenal Star gene is transcribed as a 3. 5-kilobase pair (kb) and 1.6-kb mRNA with the larger mRNA predominating ( approximately 85% of total) in vivo. Hypophysectomy (HPX) produced a 3-5-fold decrease in Star mRNA along with a loss of adrenal steroids, whereas P450scc mRNA decreased by less than 2-fold. Adrenocorticotropic hormone (ACTH) treatment of HPX rats maximally stimulated steroidogenesis rates within 5 min with over 10-fold elevation of steady state blood levels occurring within 10 min. For intact rats there was a 5-10-fold larger increase, paralleling previously observed elevations of cholesterol-cytochrome P450scc association and metabolism in subsequently isolated adrenal mitochondria. ACTH did not increase either total STAR protein or a group of modified forms until at least 30 min after completion of acute stimulation, indicating that elevated translation of STAR protein cannot alone mediate this acute stimulation. Parallel slow changes in STAR protein and corticosterone formation after ACTH treatment are consistent with participation of STAR forms as co-regulators of these hormonal responses. ACTH stimulation of HPX rats increased Star mRNA by 2.5-fold within 20 min and by 4.5-fold after 1 h, thus preceding the rise in the STAR protein. A 3.5-kb Star cDNA clone isolated from a rat adrenal cDNA library exhibited a 0.9-kb open reading frame and a 2.5-kb 3'-untranslated region (3'-UTR). The open reading frame sequence differed at only 12 amino acids from that of the mouse Star. The rat Star gene seven exons with exon 7 encoding the entire 2.5 kb of 3'-UTR of the 3.5-kb mRNA. The 3'-UTR sequence suggests that 1.6- and 3.5-kb mRNA are formed by an alternative usage of different polyadenylation signals. Multiple UUAUUUA(U/A)(U/A) motifs also suggest additional regulation through this extended 3'-UTR. Although elevation of STAR protein by ACTH does not cause the acute increase in adrenal cholesterol metabolism, changes in the turnover or distribution of an active STAR subfraction cannot be excluded.
类固醇生成急性调节蛋白（STAR）通过将胆固醇线粒体转运至细胞色素P450scc参与类固醇生成。大鼠肾上腺Star基因转录产生3.5千碱基对（kb）和1.6 kb的mRNA，其中较大的mRNA在体内占主导（约占总量的85%）。垂体切除术（HPX）使Star mRNA减少3 - 5倍，同时肾上腺类固醇减少，而P450scc mRNA减少不到2倍。对HPX大鼠进行促肾上腺皮质激素（ACTH）治疗，在5分钟内最大程度地刺激了类固醇生成率，10分钟内稳态血药浓度升高超过10倍。对于完整大鼠，增加幅度大5 - 10倍，这与先前观察到的随后分离的肾上腺线粒体中胆固醇 - 细胞色素P450scc结合和代谢的升高情况相似。直到急性刺激完成后至少30分钟，ACTH才增加总STAR蛋白或一组修饰形式，这表明STAR蛋白翻译的升高不能单独介导这种急性刺激。ACTH治疗后STAR蛋白和皮质酮形成的平行缓慢变化与STAR形式作为这些激素反应的共调节因子的参与情况一致。ACTH刺激HPX大鼠在20分钟内使Star mRNA增加2.5倍，1小时后增加4.5倍，因此先于STAR蛋白的升高。从大鼠肾上腺cDNA文库中分离出的一个3.5 kb的Star cDNA克隆显示有一个0.9 kb的开放阅读框和一个2.5 kb的3'非翻译区（3'-UTR）。该开放阅读框序列与小鼠Star的序列仅在12个氨基酸上不同。大鼠Star基因有七个外显子，其中外显子7编码3.5 kb mRNA的整个2.5 kb的3'-UTR。3'-UTR序列表明，1.6 kb和3.5 kb的mRNA是通过不同聚腺苷酸化信号的交替使用形成的。多个UUAUUUA(U/A)(U/A)基序也表明通过这个延长的3'-UTR存在额外的调控。虽然ACTH使STAR蛋白升高不会导致肾上腺胆固醇代谢的急性增加，但不能排除活性STAR亚组分的周转或分布发生变化。
