The cellular abundance of topoisomerase II (TOP2A) critically maintains DNA topology

The cellular abundance of topoisomerase II (TOP2A) critically maintains DNA topology after replication and determines the efficacy of TOP2 inhibitors in chemotherapy. include those that drive cell cycle progression (e.g., cyclins) and those required for the cellular response to the different metabolic requirements of each cell cycle phase. 918505-61-0 Among the latter group is usually topoisomerase II (TOP2A), an enzyme that helps to maintain proper DNA topology by introducing double-strand breaks to relieve the tension created by processes like DNA replication (12, 38). Expression of TOP2A peaks during G2 and mitosis, unlike expression of the related protein TOP2B, whose abundance is constant throughout the cell division cycle (19, 39). This pattern of expression supports a role for TOP2A in relaxing the positive supercoiling that develops as the replication fork advances during the S phase and in mitotic events, such as chromosome decatenation, and kinetochore and centromere function (28, 31, 33). TOP2A is also important in chemotherapy; a growing body of literature indicates that the effectiveness 918505-61-0 of several anticancer drugs depends on TOP2A levels (29). Since transcription by RNA polymerase II is usually repressed during mitosis (30), posttranscriptional processes are particularly important for controlling protein abundance in mitotic cells. The expression of TOP2A peaks in mitotic cells (19, 39); thus, the underlying mechanisms regulating TOP2A expression are crucial. In mammalian cells, TOP2A function has been linked to its posttranslational modification (sumoylation, phosphorylation) MEKK and its conversation with other proteins (reviewed in reference 28). However, the transcriptional and posttranscriptional mechanisms that control TOP2A expression are virtually unknown. The posttranscriptional gene regulation (e.g., changes in mRNA splicing, 918505-61-0 transport, storage, stability, and translation) is typically controlled by the conversation of mRNA, in competition with binding of miR-548c-3p to the mRNA, whose conversation with mRNA led to its recruitment to processing bodies (PBs), cytoplasmic foci specialized in mRNA decay and translational repression. The antagonistic influence of HuR and miR-548c-3p upon TOP2A expression selectively affected the extent of DNA damage after treatment with TOP2A inhibitors. Our results underscore the usefulness of chemotherapeutic strategies that include modulating TOP2A translation. MATERIALS AND METHODS Cell culture, treatment, and transfection. HeLa cells were cultured in Dulbecco’s altered essential medium (DMEM; Invitrogen) supplemented with 10% fetal bovine serum (FBS) and antibiotics. Lipofectamine-2000 (Invitrogen) was used to transfect cells with small RNAs and plasmids. Small RNAs used (at 100 nM) to silence HuR were AATCTTAAGTTTCGTAAGTTA (HuR U1) and TTCCTTTAAGATATATATTAA (HuR U2), the control small interfering RNA (Ctrl siRNA) was AATTCTCCGAACGTGTCACGT (Qiagen), and the TOP2A siRNA was from Santa Cruz Biotech. Plasmid DNAs were transfected at 50 ng/ml [pEGFP, pEGFP-TOP2A(3), pEGFP-TOP2A(3mut), pEGFP-TOP2A(3)HuR] or at 1 to 2 2 g/ml [pFlag, pHuR-Flag, pMS2, pMS2-TOP2A(3), pMS2-YFP]. Treatment with nocodazole (100 ng/ml) lasted 16 h. Double thymidine block and flow cytometry were performed as described previously (21). 3-untranslated region (3UTR) reporter constructs were made by inserting the 3UTR into pEGFP-C1 or pMS2. I. E. Gallouzi kindly provided pHuR-Flag; pMS2 and pMS2-YFP plasmids were described previously (25). Microscopy. Fluorescence microscopy was performed as described previously (25). Briefly, cells were fixed with 2% formaldehyde, permeabilized with 0.2% Triton X-100, and blocked with 5% bovine serum albumin (BSA). After incubation with a primary antibody recognizing DCP1a (Abcam), an Alexa 568-conjugated secondary antibody (Invitrogen) was used to detect primary antibody-antigen complexes (red). Yellow fluorescent protein (YFP) fluorescence was green. Images were acquired using an Axio Observer microscope (Zeiss) with AxioVision 4.7 Zeiss image processing software or with LSM 510 Meta (Zeiss). Confocal microscopy images were acquired with mRNA, TGCACCACCAACTGCTTAGC and GGCATGGACTGTGGTCATGAG to detect (glyceraldehyde-3-phosphate dehydrogenase) mRNA, and TGACCGCAGAGTCTTTTCCCT and TGGGTTGGTCATGCTCACTA to detect (enhanced GFP).