Singh Anurag, Karnoub Antoine E, Palmby Todd R, Lengyel Ernst, Sondek John, Der Channing J
Department of Pharmacology, Lineberger Comprehensive Cancer Center, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
Oncogene. 2004 Dec 16;23(58):9369-80. doi: 10.1038/sj.onc.1208182.
A novel splice variant of Rac1, designated Rac1b, is expressed in human breast and colon carcinoma cells. Rac1b contains an additional 19 amino-acid insert immediately behind the switch II domain, a region important for Rac1 interaction with regulators and effectors. Recent studies showed that Rac1b exhibited the biochemical properties of a constitutively activated GTPase, yet it showed impaired interaction with downstream effectors, suggesting that Rac1b may be defective in biological activity. Whether Rac1b is a biologically active protein was not addressed. Therefore, we evaluated the biochemical, signaling and growth-promoting properties of authentic Rac1b. Similar to previous observations, we found that Rac1b showed enhanced intrinsic guanine nucleotide exchange activity, impaired intrinsic GTPase activity, and failed to interact with RhoGDI. Surprisingly, we found that Rac1b, like the constitutively-activated and transforming Rac1(Q61L) mutant, promoted growth transformation of NIH3T3 cells. Rac1b-expressing cells also showed a loss of density-dependent and anchorage-dependent growth. Surprisingly, unlike activated Rac1(61L), Rac1b did not show enhanced activation of the nuclear factor kappaB (NF-kappaB) transcription factor or stimulate cyclin D1 expression, the signaling activities that best correlate with Rac1 transforming activity. However, Rac1b did promote activation of the AKT serine/threonine kinase. Therefore, we suggest that Rac1b selectively activates a subset of Rac1 downstream signaling pathways to facilitate cellular transformation.
一种名为Rac1b的Rac1新型剪接变体在人乳腺癌和结肠癌细胞中表达。Rac1b在开关II结构域（对Rac1与调节因子和效应器相互作用很重要的区域）之后紧接着含有一个额外的19个氨基酸的插入序列。最近的研究表明，Rac1b表现出组成型激活的GTP酶的生化特性，但它与下游效应器的相互作用受损，这表明Rac1b在生物活性方面可能存在缺陷。Rac1b是否是一种具有生物活性的蛋白质尚未得到解决。因此，我们评估了天然Rac1b的生化、信号传导和促进生长的特性。与之前的观察结果相似，我们发现Rac1b表现出增强的内在鸟嘌呤核苷酸交换活性、受损的内在GTP酶活性，并且无法与RhoGDI相互作用。令人惊讶的是，我们发现Rac1b与组成型激活并具有转化作用的Rac1（Q61L）突变体一样，能促进NIH3T3细胞的生长转化。表达Rac1b的细胞还表现出密度依赖性和锚定依赖性生长的丧失。令人惊讶的是，与激活的Rac1（61L）不同，Rac1b没有表现出核因子κB（NF-κB）转录因子的激活增强，也没有刺激细胞周期蛋白D1的表达，而这些信号传导活性与Rac1的转化活性最相关。然而，Rac1b确实促进了AKT丝氨酸/苏氨酸激酶的激活。因此，我们认为Rac1b选择性地激活Rac1下游信号通路的一个子集以促进细胞转化。
