Study On The Regulation Mechanisms And Biological Function Of KLF4

Kruppel-like factor4(KLF4) is a zinc-finger protein of the Kriippel-like factor (KLF) family, which is involved in the regulation of cell proliferation and differentiation. KLF4is of particular interest since it shows dramatically different functions depending on cell context.In the first part, we demonstrate that the level of KLF4expression is inversely correlated with the extent of DNA damage. KLF4is activated by p53following cytostatic, mild DNA damage, whereas it is strongly repressed via enhanced turnover of mRNA upon severe DNA damage that irreversibly drives cells to apoptosis. To further determine the cis-acting factors on3’UTR of KLF4mRNA that confer the enhanced turnover of KLF4mRNA upon apoptotic DNA damage, we searched for putative RNA regulatory elements in the human KLF43’UTR. Remarkably, we identified five putative HuR binding sites, suggesting that KLF4mRNA might be a direct target of HuR. Our study shows that HuR bound to the KLF4mRNA and affected its stability. Notably, apoptotic doses of ADR treatment triggers the rapid dissociation of KLF4mRNA from HuR, in turn destabilizing the KLF4mRNA.The fact that KLF4protein level is inversely correlated with the extent of genotoxic stress indicates that KLF4may play a role in controlling the switch in p53response. In concert with this notion, blocking the repression of KLF4upon apoptotic DNA damage suppresses p53-mediated apoptosis, while ablation the KLF4induction upon cytostatic doses of ADR shifts the p53response from cell repair to cell death. We demonstrate that blocking the repression of KLF4upon apoptotic DNA damage resulted in significantly reduced Bax induction while the induction of p21was enhanced in response to apoptotic DNA damage, suggesting that repression of KLF4upon apoptotic DNA damage may contribute to ability of p53to preferentially transactivate proapoptotic gene Bax over proarrest gene p21. In summary, the finding presented here demonstrates that KLF4expression is tight regulated depending on the extent of DNA damage, which may be an important mechanism that dictates the life and death decisions of p53.This may have important implications in chemo-resistance of tumor retaining wild-type p53and for the development of overcoming strategies, as most chemotherapeutic strategies are aimed at triggering the apoptosis of tumor cells.In the second part, we demonstrate that KLF4is strongly induced by PPARy ligands, and this increase is PPARy dependent. Furthermore, the induction of KLF4by PPARy ligands does not require de novo protein synthesis, and could not be explained by changes in KLF4mRNA stability, suggesting that KLF4may be a direct target gene of PPARy and may be involved in the PPARy signal pathway.To determine if KLF4was a direct transcriptional target of PPARy, we searched the KLF4gene for consensus PPAR response element (PPRE), and a potential binding site was identified in promoter region of KLF4. Further analysis by ChIP, EMS A and luciferase assay revealed that PPARy can specifically bind to the PPAR response element in promoter region of KLF4, which was required for PPARy to transactive KLF4. Finally, we further studied the role of KLF4on the PPARy signal pathway. We found that knowdown of KLF4obviously suppressed the Gl/S arrest induced by PPARy ligands, suggesting that KLF4is important for the PPARy mediated G1/S arrest. In addition, block K.LF4increased the chemosensitivity of the HCT116cells with PPARy ligands. Taken together, the results indicated that KLF4, as a new PPARy target gene, is necessary for the PPARy mediated G1/S arrest and block KLF4can obviously increase the chemosensitivity of HCT116cells with PPARy ligands.