title: Cucurbitacin I inhibits Rac1 activation in breast cancer cells by a reactive oxygen species-mediated mechanism and independently of Janus tyrosine kinase 2 and P-Rex1.
abstract: The small GTPase Rac1 has been widely implicated in mammary tumorigenesis and metastasis. Previous studies established that stimulation of ErbB receptors in breast cancer cells activates Rac1 and enhances motility via the Rac-guanine nucleotide exchange factor P-Rex1. As the Janus tyrosine kinase 2 (Jak2)/signal transducer and activator of transcription 3 (Stat3) pathway has been shown to be functionally associated with ErbB receptors, we asked if this pathway could mediate P-Rex1/Rac1 activation in response to ErbB ligands. Here we found that the anticancer agent cucurbitacin I, a Jak2 inhibitor, reduced the activation of Rac1 and motility in response to the ErbB3 ligand heregulin in breast cancer cells. However, Rac1 activation was not affected by Jak2 or Stat3 RNA interference, suggesting that the effect of cucurbitacin I occurs through a Jak2-independent mechanism. Cucurbitacin I also failed to affect the activation of P-Rex1 by heregulin. Subsequent analysis revealed that cucurbitacin I strongly activates RhoA and the Rho effector Rho kinase (ROCK) in breast cancer cells and induces the formation of stress fibers. Interestingly, disruption of the RhoA-ROCK pathway prevented the inhibitory effect of cucurbitacin I on Rac1 activation by heregulin. Lastly, we found that RhoA activation by cucurbitacin I is mediated by reactive oxygen species (ROS). The ROS scavenger N-acetyl L-cysteine and the mitochondrial antioxidant Mito-TEMPO rescued the inhibitory effect of cucurbitacin I on Rac1 activation. In conclusion, these results indicate that ErbB-driven Rac1 activation in breast cancer cells proceeds independently of the Jak2 pathway. Moreover, they established that the inhibitory effect of cucurbitacin I on Rac1 activity involves the alteration of the balance between Rho and Rac.
authors.affiliation: Department of Pharmacology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6160, USA.
authors.forename: Cynthia
authors.initials: C
authors.lastname: Lopez-Haber
authors.forename: Marcelo G
authors.initials: MG
authors.lastname: Kazanietz
mesh_terms: D001943:Breast Neoplasms / Q000188:drug therapy / Q000378:metabolism*; D045744:Cell Line, Tumor; D002465:Cell Movement / Q000187:drug effects; D005260:Female; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism*; D006801:Humans; D053614:Janus Kinase 2 / Q000037:antagonists & inhibitors / Q000378:metabolism*; D008024:Ligands; D061986:MCF-7 Cells; D020890:Neuregulin-1 / Q000494:pharmacology; D017382:Reactive Oxygen Species / Q000378:metabolism*; D020893:Receptor, ErbB-3 / Q000378:metabolism; D050796:STAT3 Transcription Factor / Q000378:metabolism; D022502:Stress Fibers / Q000187:drug effects / Q000378:metabolism; D014315:Triterpenes / Q000494:pharmacology*; D020830:rac1 GTP-Binding Protein / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D054460:rho-Associated Kinases / Q000378:metabolism; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D008024:Ligands; D020890:Neuregulin-1; C453280:PREX1 protein, human; C107953:RAC1 protein, human; D017382:Reactive Oxygen Species; D050796:STAT3 Transcription Factor; C494086:STAT3 protein, human; D014315:Triterpenes; C066737:RHOA protein, human; C581292:ERBB3 protein, human; D020893:Receptor, ErbB-3; C507924:JAK2 protein, human; D053614:Janus Kinase 2; D054460:rho-Associated Kinases; D020830:rac1 GTP-Binding Protein; D020742:rhoA GTP-Binding Protein; C038106:cucurbitacin I