DNA-dependent protein kinase catalytic subunit modulates the stability of c-Myc oncoprotein.
BACKGROUND
C-Myc is a short-lived oncoprotein that is destroyed by ubiquitin-mediated proteolysis. Dysregulated accumulation of c-Myc commonly occurs in human cancers. Some of those cases with the dysregulated c-Myc protein accumulation are attributed to gene amplification or increased mRNA expression. However, the abnormal accumulation of c-Myc protein is also a common finding in human cancers with normal copy number and transcription level of c-Myc gene. It seems that the mechanistic dysregulation in the control of c-Myc protein stabilization is another important hallmark associated with c-Myc accumulation in cancer cells. Here we report a novel mechanistic pathway through which DNA-dependent protein kinase catalytic subunit (DNA-PKcs) modulates the stability of c-Myc protein.


RESULTS
Firstly, siRNA-mediated silencing of DNA-PKcs strikingly downregulated c-Myc protein levels in HeLa and HepG2 cells, and simultaneously decreased cell proliferation. The c-Myc protein level in DNA-PKcs deficient human glioma M059J cells was also found much lower than that in DNA-PKcs efficient M059K cells. ATM deficiency does not affect c-Myc expression level. Silencing of DNA-PKcs in HeLa cells resulted in a decreased stability of c-Myc protein, which was associated the increasing of c-Myc phosphorylation on Thr58/Ser62 and ubiquitination level. Phosphorylation of Akt on Ser473, a substrate of DNA-PKcs was found decreased in DNA-PKcs deficient cells. As the consequence, the phosphorylation of GSK3 beta on Ser9, a negatively regulated target of Akt, was also decreased, and which led to activation of GSK 3beta and in turn phosphorylation of c-Myc on Thr58. Moreover, inhibition of GSK3 activity by LiCl or specific siRNA molecules rescued the downregulation of c-Myc mediated by silencing DNA-PKcs. Consistent with this depressed DNA-PKcs cell model, overexpressing DNA-PKcs in normal human liver L02 cells, by sub-chronically exposing to very low dose of carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), increased c-Myc protein level, the phosphorylation of Akt and GSK3 beta, as well as cell proliferation. siRNA-mediated silencing of DNA-PKcs in this cell model reversed above alterations to the original levels of L02 cells.


CONCLUSIONS
A suitable DNA-PKcs level in cells is necessary for maintaining genomic stability, while abnormal overexpression of DNA-PKcs may contribute to cell proliferation and even oncogenic transformation by stabilizing the c-Myc oncoprotein via at least the Akt/GSK3 pathway. Our results suggest DNA-PKcs a novel biological role beyond its DNA repair function.
An
Jing
J
Department of Radiation Toxicology and Oncology, Beijing Institute of Radiation Medicine, Beijing, 100850, PR China. peace74839@shu.edu.cn
Yang
Dong-Yan
DY
Xu
Qin-Zhi
QZ
Zhang
Shi-Meng
SM
Huo
Yan-Ying
YY
Shang
Zeng-Fu
ZF
Wang
Yu
Y
Wu
De-Chang
DC
Zhou
Ping-Kun
PK
D049109:Cell Proliferation; D051747:DNA-Activated Protein Kinase / Q000378:metabolism* / Q000502:physiology; D015536:Down-Regulation; D042822:Genomic Instability; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D009687:Nuclear Proteins / Q000502:physiology*; D051058:Oncogene Protein v-akt / Q000378:metabolism; D010766:Phosphorylation; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics* / Q000378:metabolism; D034741:RNA, Small Interfering; D015398:Signal Transduction
C489427:MYC protein, human; D009687:Nuclear Proteins; D016271:Proto-Oncogene Proteins c-myc; D034741:RNA, Small Interfering; D051747:DNA-Activated Protein Kinase; D051058:Oncogene Protein v-akt; C497263:PRKDC protein, human; D038362:Glycogen Synthase Kinase 3