Sobolewski 2013

Histone Deacetylase inhibitors activate Tristetraprolin expression through induction of Early growth response protein 1 in colorectal cancer cells

Cyril Sobolewski and Dan. A Dixon Kansas University Medical Center.

The University of Kansas Cancer Center Department of Cancer Biology, 3020 Wahl Hall East, Kansas City, KS 66160

Messenger RNA turnover is a tightly regulated mechanism central to regulating gene expression. In the normal intestinal epithelium, cell growth and proliferation is controlled by rapid decay of growth-related mRNAs bearing 3’UTR AU-rich elements (ARE). The transacting mRNA-binding protein tristetraprolin (TTP) mediates expression of ARE-containing mRNAs by binding to AREs and promoting rapid mRNA decay. Recently, it has been shown that TTP expression is lost in many human malignancies, thus compromising ARE-mediated mRNA decay. The physiological consequence of this results in a pathologic overexpression of these genes, which favor tumors growth. In the present study, we sought to determine the underlying mechanisms behind TTP loss in colorectal cancer. Many tumor suppressor genes are silenced at the epigenetic level via DNA methylation on CpG islands or by chromatin modeling (e.g., histone deacetylation). In our study, we investigated the effect of the histone deacetylase (HDAC) inhibitors trichostatin A (TSA), SAHA, and sodium butyrate on TTP expression in a panel of colorectal cancer cell lines (HCA-7, HCT-116, Moser, SW480 and HT-29). All the HDAC inhibitors were able to rescue TTP expression and promote increased TTP mRNA and protein expression. As a result of this, rapid degradation of the TTP target mRNA COX-2 was observed, showing that TTP induction is functional. In addition, using siRNA against TTP, we found that TTP contributes to the growth inhibitory effect of HDAC inhibitors. Chromatin immunoprecipitation (ChIP) assays demonstrate that TSA treatment promotes phosphorylated RNA polymerase II recruitment to the proximal region of the TTP promoter, along with histone H3 enrichment, suggesting a potential mechanism involving chromatin remodeling. Furthermore, we demonstrate that siRNA-mediated knockdown of Early Growth Response protein 1 (EGR-1), prevented TSA-mediated induction of TTP expression. These results indicate that HDAC inhibitors activate TTP expression in a EGR-1- dependent manner. Our findings show for the first time that HDAC inhibitors can activate the expression of TTP in human colon cancer cells, suggesting that loss of TTP in colon cancer occurs through epigenetic silencing. Moreover our data underline a therapeutic strategy for TTP reactivation in cancer cells by using HDAC inhibitors.