title: RAC1 GTPase promotes the survival of breast cancer cells in response to hyper-fractionated radiation treatment.
abstract: Radiation therapy is a staple approach for cancer treatment, whereas radioresistance of cancer cells remains a substantial clinical problem. In response to ionizing radiation (IR) induced DNA damage, cancer cells can sustain/activate pro-survival signaling pathways, leading to apoptotic resistance and induction of cell cycle checkpoint/DNA repair. Previous studies show that Rac1 GTPase is overexpressed/hyperactivated in breast cancer cells and is associated with poor prognosis. Studies from our laboratory reveal that Rac1 activity is necessary for G2/M checkpoint activation and cell survival in response to IR exposure of breast and pancreatic cancer cells. In this study, we investigated the effect of Rac1 on the survival of breast cancer cells treated with hyper-fractionated radiation (HFR), which is used clinically for cancer treatment. Results in this report indicate that Rac1 protein expression is increased in the breast cancer cells that survived HFR compared with parental cells. Furthermore, this increase of Rac1 is associated with enhanced activities of extracellular signal-regulated kinases 1 and 2 (ERK1/2) and nuclear factor-κB (NF-κB) signaling pathways and increased levels of anti-apoptotic protein Bcl-xL and Mcl-1, which are downstream targets of ERK1/2 and NF-κB signaling pathways. Using Rac1-specific inhibitor and dominant-negative mutant N17Rac1, here we demonstrate that Rac1 inhibition decreases the phosphorylation of ERK1/2 and inhibitory κBα (IκBα), as well as the levels of Bcl-xL and Mcl-1 protein in the HFR-selected breast cancer cells. Moreover, inhibition of Rac1 using either small molecule inhibitor or dominant-negative N17Rac1 abrogates clonogenic survival of HFR-selected breast cancer cells and decreases the level of intact poly(ADP-ribose) polymerase, which is indicative of apoptosis induction. Collectively, results in this report suggest that Rac1 signaling is essential for the survival of breast cancer cells subjected to HFR and implicate Rac1 in radioresistance of breast cancer cells. These studies also provide the basis to explore Rac1 as a therapeutic target for radioresistant breast cancer cells.
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: A L
authors.initials: AL
authors.lastname: Hein
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: C M
authors.initials: CM
authors.lastname: Post
authors.affiliation: Department of Pathology and Microbiology, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: Y M
authors.initials: YM
authors.lastname: Sheinin
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: I
authors.initials: I
authors.lastname: Lakshmanan
authors.affiliation: Eppley Institute for Research in Cancer and Allied Diseases, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: A
authors.initials: A
authors.lastname: Natarajan
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: C A
authors.initials: CA
authors.lastname: Enke
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: S K
authors.initials: SK
authors.lastname: Batra
authors.affiliation: Department of Internal Medicine, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: M M
authors.initials: MM
authors.lastname: Ouellette
authors.affiliation: Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE, USA.|Department of Biochemistry and Molecular Biology, University of Nebraska Medical Center, Omaha, NE, USA.
authors.forename: Y
authors.initials: Y
authors.lastname: Yan
mesh_terms: D001943:Breast Neoplasms / Q000201:enzymology* / Q000235:genetics / Q000473:pathology / Q000532:radiotherapy*; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000502:physiology; D005260:Female; D006801:Humans; D011836:Radiation Tolerance; D015398:Signal Transduction; D020830:rac1 GTP-Binding Protein / Q000235:genetics / Q000378:metabolism*
chemical_list: C107953:RAC1 protein, human; D020830:rac1 GTP-Binding Protein