A novel interaction of PAK4 with PPARγ to regulate Nox1 and radiation-induced epithelial-to-mesenchymal transition in glioma.

Tumor recurrence in glioblastoma (GBM) is, in part, attributed to increased epithelial-to-mesenchymal transition (EMT) and enhanced tumor cell dissemination in adjacent brain parenchyma after ionizing radiation (IR). EMT is associated with aggressive behavior, increased stem-like characteristics and treatment resistance in malignancies; however, the underlying signaling mechanisms that regulate EMT are poorly understood. We identified grade-dependent p21-activated kinases 4 (PAK4) upregulation in gliomas and further determined its role in mesenchymal transition and radioresistance. IR treatment significantly elevated expression and nuclear localization of PAK4 in correlation with induction of reactive oxygen species (ROS) and mesenchymal transition in GBM cells. Stable PAK4 overexpression promoted mesenchymal transition by elevating EMT marker expression in these cells. Of note, transcription factor-DNA-binding arrays and chromatin immunoprecipitation experiments identified the formation of a novel nuclear PAK4/PPARγ complex which was recruited to the promoter of Nox1, a peroxisome proliferator-activated receptor gamma (PPARγ) target gene. In addition, IR further elevated PAK4/PPARγ complex co-recruitment to Nox1 promoter, and increased Nox1 expression and ROS levels associated with mesenchymal transition in these cells. Conversely, specific PAK4 downregulation decreased PPARγ-mediated Nox1 expression and suppressed EMT in IR-treated cells. In vivo orthotopic tumor experiments showed inhibition of growth and suppression of IR-induced PPARγ and Nox1 expression by PAK4 downregulation in tumors. Our results provide the first evidence of a novel role for PAK4 in IR-induced EMT and suggest potential therapeutic efficacy of targeting PAK4 to overcome radioresistance in gliomas.

G-overhang dynamics at Tetrahymena telomeres.

To learn more about the structure of the DNA terminus at Tetrahymena thermophila telomeres, we have devised a ligation-mediated primer extension protocol to accurately measure the length of the G-strand overhang. We show that overhang length and the identity of the 3'-terminal nucleotide are tightly regulated. The majority of overhangs terminate in the sequence 5'-TTGGGGT and >80% are either 14-15 or 20-21 nucleotides in length. No significant changes in overhang length were detected as cells traversed the cell cycle. However, changes in length distribution were observed when cells exited the cell cycle, indicating an altered balance between DNA synthesis and degradation or end protection. We also provide evidence that rDNA molecules have overhangs on both telomeres. Full-length rDNA could be cloned by a strategy that depends on overhangs being present at both ends. Moreover, analysis of leading strand telomeres revealed that a significant fraction have overhangs > or =5 nucleotides. Our results indicate that generation of the terminal telomeric DNA structure is highly regulated and requires several distinct DNA-processing events.