The cdk5 Kinase Regulates the STAT3 Transcription Factor to Prevent DNA Damage upon Topoisomerase I Inhibition

Abstract

The STAT3 transcription factors are cytoplasmic proteins that induce gene activation in response to growth factor stimulation. Following tyrosine phosphorylation, STAT3 proteins dimerize, translocate to the nucleus, and activate specific target genes involved in cell-cycle progression. Despite its importance in cancer cells, the molecular mechanisms by which this protein is regulated in response to DNA damage remain to be characterized. In this study, we show that STAT3 is activated in response to topoisomerase I inhibition. Following treatment, STAT3 is phosphorylated on its C-terminal serine 727 residue but not on its tyrosine 705 site. We also show that topoisomerase I inhibition induced the up-regulation of the cdk5 kinase, a protein initially described in neuronal stress responses. In co-immunoprecipitations, cdk5 was found to associate with STAT3, and pulldown experiments indicated that it associates with the C-terminal activation domain of STAT3 upon DNA damage. Importantly, the cdk5-STAT3 pathway reduced DNA damage in response to topoisomerase I inhibition through the up-regulation of Eme1, an endonuclease involved in DNA repair. ChIP experiments indicated that STAT3 can be found associated with the Eme1 promoter when phosphorylated only on its serine 727 residue and not on tyrosine 705. We therefore propose that the cdk5-STAT3 oncogenic pathway plays an important role in the expression of DNA repair genes and that these proteins could be used as predictive markers of tumors that will fail to respond to chemotherapy.Signal transducer and activator of transcription 3 (STAT3) 3 proteins are cytoplasmic transcription factors that translocate into the nucleus following growth factor stimulation. In contrast to normal cells where its phosphorylation is only transient, constitutive activation of STAT3 has been reported in several primary cancers and tumor cell lines (1-3). This abnormal activation is due to oncogenic kinases such as epidermal growth factor receptor, Her2/Neu, src, or bcr-abl, which induce STAT3 activation through phosphorylation of its tyrosine 705 residue (4). This phosphorylation allows the nuclear translocation and DNA binding of the STAT3 dimer and the up-regulation of several genes involved in cell-cycle and cell survival such as cyclin D1, Myc, or bclxl. The up-regulation of these cancer genes mediates the oncogenic activity of STAT3 and its ability to transform cells (5).

Cyclin-dependent kinase 5 modulates STAT3 and androgen receptor activation through phosphorylation of Ser727 on STAT3 in prostate cancer cells

“…Several lines of evidence have revealed that STAT3 can be modulated by Cdk5-dependent phosphorylation at the Ser 727 site in mouse neurons (12), muscle cells (12), and liver cancer (34). A recent study indicates that Cdk5 prevents DNA damage through Ser 727 phosphorylation of STAT3 (9). Our previous results also show that Cdk5 modulates STAT3 activation and cell proliferation of thyroid cancer (26).…”

Escape from p21-mediated Oncogene-induced Senescence Leads to Cell Dedifferentiation and Dependence on Anti-apoptotic Bcl-xL and MCL1 Proteins

“…Chromatin Immunoprecipitation Assays-ChIP experiments were performed as described previously (21)(22)(23). Briefly, HT29 cells were fixed with 1% formaldehyde.…”

Phosphoregulation of the RNA-binding Protein Hu Antigen R (HuR) by Cdk5 Affects Centrosome Function

“…Cdk5 is required for synapses formation and is important in local protein synthesis and synaptic transmission (16,17). In tumor cells, Cdk5 overexpression or activation by p35 and growth factors promotes an aggressive metastatic behavior, resistance to the chemotherapeutic drugs, and, in some cases, alterations of cell cycle progression (7,8,18,19). The mechanisms implicated in Cdk5 function include phosphorylation and activation of transcriptional factors, ion channels, cytoskeleton proteins, and kinases (19 -21).…”