Tang E D, Nuñez G, Barr F G, Guan K L
Department of Biological Chemistry, University of Michigan Medical School Ann Arbor, Michigan 48109, USA.
J Biol Chem. 1999 Jun 11;274(24):16741-6. doi: 10.1074/jbc.274.24.16741.
The FKHR gene was first identified from its disruption by the t(2;13) chromosomal translocation seen in the pediatric tumor alveolar rhabdomyosarcoma. It encodes for a member of the forkhead family of transcription factors. Recently, a homolog of FKHR in the nematode Caenorhabditis elegans was identified called DAF-16, which is a downstream target of two Akt homologs in an insulin-related signaling pathway. We have examined the possible role of Akt in the regulation of FKHR. We find that FKHR can bind in vitro to the insulin-responsive sequence (IRS) in the insulin-like growth factor-binding protein 1 promoter and can activate transcription from a reporter plasmid containing multiple copies of the IRS. Expression of active but not inactive Akt can suppress FKHR-mediated transcriptional activation. Akt can phosphorylate FKHR in vitro on three phosphoacceptor sites, at least a subset of which can also be phosphorylated by Akt in vivo. Importantly, mutation of these three sites to alanine residues enhances the transcriptional activity of FKHR and renders it resistant to inhibition by Akt. Expression of an Akt-resistant mutant of FKHR causes apoptosis in 293T cells in a manner dependent on DNA binding. These results suggest that FKHR may be a direct nuclear regulatory target for Akt in both metabolic and cell survival pathways.
FKHR基因最初是在小儿肿瘤肺泡横纹肌肉瘤中发现的t(2;13)染色体易位导致其被破坏时被鉴定出来的。它编码一种叉头转录因子家族的成员。最近，在秀丽隐杆线虫中鉴定出了FKHR的一个同源物，称为DAF-16，它是胰岛素相关信号通路中两个Akt同源物的下游靶点。我们研究了Akt在FKHR调控中的可能作用。我们发现FKHR在体外可与胰岛素样生长因子结合蛋白1启动子中的胰岛素反应序列（IRS）结合，并能激活含有多个IRS拷贝的报告质粒的转录。活性而非无活性的Akt的表达可抑制FKHR介导的转录激活。Akt可在体外使FKHR的三个磷酸化位点磷酸化，其中至少一部分位点在体内也可被Akt磷酸化。重要的是，将这三个位点突变为丙氨酸残基可增强FKHR的转录活性，并使其对Akt的抑制产生抗性。FKHR的Akt抗性突变体的表达以依赖于DNA结合的方式在293T细胞中诱导凋亡。这些结果表明，FKHR可能是Akt在代谢和细胞存活途径中的直接核调节靶点。
