Selvaraj Ahalya, Prywes Ron
Department of Biological Sciences, Columbia University, New York, New York, USA.
BMC Mol Biol. 2004 Aug 25;5:13. doi: 10.1186/1471-2199-5-13.
Serum Response Factor (SRF) is a transcription factor that is required for the expression of many genes including immediate early genes, cytoskeletal genes, and muscle-specific genes. SRF is activated in response to extra-cellular signals by its association with a diverse set of co-activators in different cell types. In the case of the ubiquitously expressed immediate early genes, the two sets of SRF binding proteins that regulate its activity are the TCF family of proteins that include Elk1, SAP1 and SAP2 and the myocardin-related MKL family of proteins that include MKL1 and MKL2 (also known as MAL, MRTF-A and -B and BSAC). In response to serum or growth factors these two classes of co-activators are activated by different upstream signal transduction pathways. However, it is not clear how they differentially activate SRF target genes.
In order to identify the serum-inducible SRF target genes that are specifically dependent on the MKL pathway, we have performed microarray experiments using a cell line that expresses dominant negative MKL1. This approach was used to identify SRF target genes whose activation is MKL-dependent. Twenty-eight of 150 serum-inducible genes were found to be MKL-dependent. The promoters of the serum-inducible genes were analyzed for SRF binding sites and other common regulatory elements. Putative SRF binding sites were found at a higher rate than in a mouse promoter database but were only identified in 12% of the serum-inducible promoters analyzed. Additional partial matches to the consensus SRF binding site were found at a higher than expected rate in the MKL-dependent gene promoters. The analysis for other common regulatory elements is discussed.
These results suggest that a subset of immediate early and SRF target genes are activated by the Rho-MKL pathway. MKL may also contribute to the induction of other SRF target genes however its role is not essential, possibly due to other activation mechanisms such as MAPK phosphorylation of TCFs.
血清反应因子（SRF）是一种转录因子，许多基因（包括即早基因、细胞骨架基因和肌肉特异性基因）的表达都需要它。在不同细胞类型中，SRF通过与多种共激活因子结合来响应细胞外信号从而被激活。对于普遍表达的即早基因，调控其活性的两组SRF结合蛋白分别是TCF蛋白家族（包括Elk-1、SAP-1和SAP-2）和与心肌素相关的MKL蛋白家族（包括MKL-1和MKL-2，也称为MAL、MRTF-A和-B以及BSAC）。响应血清或生长因子时，这两类共激活因子由不同的上游信号转导通路激活。然而，尚不清楚它们如何差异激活SRF靶基因。
为了鉴定特别依赖MKL通路的血清诱导型SRF靶基因，我们使用表达显性负性MKL-1的细胞系进行了微阵列实验。该方法用于鉴定其激活依赖MKL的SRF靶基因。150个血清诱导基因中有28个被发现是MKL依赖的。分析了血清诱导基因的启动子中的SRF结合位点和其他常见调控元件。在血清诱导基因启动子中发现推定的SRF结合位点的比例高于小鼠启动子数据库，但在所分析的血清诱导启动子中仅12%被鉴定出。在MKL依赖的基因启动子中发现与SRF结合位点共有序列的其他部分匹配的比例高于预期。讨论了对其他常见调控元件的分析。
这些结果表明，一部分即早基因和SRF靶基因由Rho-MKL通路激活。MKL也可能有助于诱导其他SRF靶基因，但其作用并非必不可少，这可能是由于其他激活机制，如TCF的MAPK磷酸化。
