Different signaling and functionality of Rac1 and Rac1b in the progression of lung adenocarcinoma

Rac1 is a ubiquitously expressed Rho GTPase and an important regulator of the actin cytoskeleton. Its splice variant Rac1b exhibits a 19-amino acid (aa) in-frame insertion and is predominantly active. Both proteins were described in tumorigenesis or metastasis. We investigated the contribution of Rac1and Rac1b to tumor progression of human non-small-cell lung adenocarcinoma (NSCLA). Rac1 protein was present in 8/8 NSCLA cell lines analyzed, whereas Rac1b was expressed in only 6/8. In wound-healing assays, enhanced green fluorescence protein (EGFP)-Rac1 slightly decreased cell migration, whereas proliferation was increased in both, Rac1- and Rac1b-expressing cells. In the in vivo chorioallantoic invasion model, EGFP-Rac1-expressing cells formed more invasive tumors compared to EGFP-Rac1b. This increased invasiveness correlated with enhanced phosphorylation of p38 alpha, AKT and glycogen synthase kinase 3 beta (GSK3 beta), and activation of serum response- and Smad-dependent gene promoters by Raci. In contrast, Racib solely activated the mitogen-activated protein kinase (MAPK) JNK2, together with TCF/LEF1- and nuclear factor kappa B (NF kappa B)-responsive gene reporters. Rac1b, as Rac1, phosphorylated p38 alpha, AKT and GSIC3 beta. Knockdown of the splicing factor epithelial splicing regulatory protein 1 (ESRP1), which mediates out-splicing of exon 3b from Racl pre-messenger RNA, resulted in increased Rac1b messenger RNA (mRNA) and suppression of the epithelial-mesenchymal transition (EMT)-associated transcription factor ZEB1. Our data demonstrate different signaling and functional activities of Rac1 and Rac1b and an important role for Rac1 in lung cancer metastasis.