The role of Rac1-modulated gene transcription in tumorigenesis

Abstract

Gene expression regulation is a dynamic and multi-step process, in which transcription plays a major role. Transcription initiation depends on binding of transcription regulators to DNA elements located in promoter or enhancer regions, a process often controlled by signalling pathways. One such pathway is regulated by Rac1, a member of the Rho family of small GTPases involved in cell proliferation, adhesion and migration. In this work, novel links between Rac1 signalling and transcriptional regulation in colorectal tumour cells are described. First, it is shown that Rac1 activation leads to PAK1-mediated phosphorylation of the transcriptional repressor BCL-6 in colorectal cancer cells, inactivating its repressor function. In the presence of active Rac1, BCL-6 redistribution within the nucleus, a reduction in its affinity to chromatin and increased expression of the endogenous target genes NFKB1 and CD44, and of a BCL-6-controlled luciferase reporter construct were observed. Next, it was found that Rac1 signalling promotes gene transcription by inducing a transcriptional switch from the repressor BCL-6 to the activator STAT5A at the promoter of certain target genes. Using chromatin immunoprecipitation, it is demonstrated in different colorectal cell lines that active Rac1 promotes release of BCL-6 with concomitant nuclear translocation and binding of STAT5A at the same promoter site. Three endogenous cell-cycle-related genes (CCND2, CDKN2B, SUMO1) were identified to be inversely regulated by BCL-6 and STAT5A and shown to respond to Rac1 signalling with promoter occupancy switches that correlate directly with changes in their expression levels.(...)