PBK/TOPK promotes tumour cell proliferation through p38 MAPK activity and regulation of the DNA damage response

Abstract

The contribution of the insulin-like growth-factor-I receptor (IGF-IR) to tumour progression is well documented. To identify new mediators of IGF-IR function in cancer, we recently isolated genes differentially expressed in cells overexpressing the IGF-IR. Among these was the serine/threonine kinase PBK/TOPK (PDZ-binding kinase/T-LAK cell-originated protein kinase), previously associated with highly proliferative cells and tissues. Here, we show that PBK is expressed at high levels in tumour cell lines compared with non-transformed cells. IGF-I could induce PBK expression only in transformed cells, whereas epidermal growth factor could induce PBK in non-transformed MCF-10A breast epithelial cells. Suppression of PBK expression using small interfering RNA did not prevent progression through the cell cycle, but caused decreased proliferation over time in culture, and reduced clonogenic growth in soft agarose. PBK knockdown impaired p38 activation after long-term stimulation with different growth factors and reduced DU145 cells motility. Suppressed PBK expression also resulted in an impaired response to DNA damage that was evident by the decreased generation of c-H2AX, increased DNA damage and decreased cell survival. Taken together, the data indicate that PBK is necessary for appropriate activation and function of the p38 pathway by growth factors. Thus, enhanced expression of PBK may facilitate tumour growth by mediating p38 activation and by helping cells to overcome DNA damage.

PBK/TOPK interacts with the DBD domain of tumor suppressor p53 and modulates expression of transcriptional targets including p21

“…Our findings also agree with previous studies showing that reduced PBK expression in MCF-7 breast cancer cells led to slower growth and reduced clonogenicity in soft agar assays (Ayllo´n and O'Connor, 2007). These investigators also showed that MCF-7 tumor cells with knocked down PBK expression exhibited decreased survival following DNA damage from UV or chemotherapeutic agents as well as impaired generation of phosphorylated H2AX (g-H2AX), an early response to double-strand DNA breaks (Ayllo´n and O'Connor, 2007). Other investigators showed that knocked down expression of PBK in HCT116 colorectal carcinoma cells decreased tumorigenicity .…”

“…Cellcycle-specific regulation of PBK/TOPK is mediated partly by transcription factors E2F and CREB/ATF (Nandi and Rapoport, 2006). Previous studies also suggest a role for PBK/TOPK in cytokinesis and DNA damage sensing and repair through phosphorylation of histone H2AX (Matsumoto et al, 2004;Zykova et al, 2006;Ayllo´n and O'Connor, 2007).…”

“…PBK/TOPK functions partly as an MAP kinase kinase by phosphorylation of p38 mitogen-activated protein kinase (MAPK) (Abe et al, 2000;Nandi et al, 2004;Ayllo´n and O'Connor, 2007) and is active during mitosis through phosphorylation of residue Thr9 by cyclin B1/Cdk1 (Abe et al, 2000;Gaudet et al, 2000). During mitosis, PBK/TOPK forms a complex with cyclinB1/Cdk1 on microtubules that promotes cytokinesis through phosphorylation of PRC1 .…”

“…Indeed PBK/TOPK is overexpressed in breast cancer, and siRNA-mediated reduction of PBK/ TOPK expression suppresses breast cancer cell growth and clonogenicity (Park et al, 2006;Ayllo´n and O'Connor, 2007). In addition to its role in the MAPK oncogenic signaling pathway, PBK/TOPK has recently been found to be a downstream target of the EWS-FLI1 chimeric fusion protein that is present in 90% of cases of Ewing sarcoma (Herrero-Martin et al, 2009).…”

Mitochondrial pyrimidine nucleotide carrier (PNC1) regulates mitochondrial biogenesis and the invasive phenotype of cancer cells

“…Adhesion, migration, soft agar and proliferation assays These assays were conducted as previously described (Ayllon and O'Connor, 2007) with the following modifications. For adhesion assays, 1 Â 10 4 cells per well were added to previously coated wells (with fibronectin) in triplicate and allowed to attach for 30 min.…”