A Study On The Roles Of AURKA In Tumorigenesis

AURKA encodes an evolutionally conserved serine/threonine kinase that belongs to the Aurora kinase family. During mitosis, AURKA is involved in centrosome maturation and separation, as well as bipolar spindle formation, ensuring that the chromosomes are properly segregated and cytokinesis smoothly executed. Therefore AURKA plays a vital role in cell cycle. The aberrant amplification and overexpression of AURKA is commonly seen in various types of cancers. Recent studies unravel roles for AURKA in multiple critical cancer-associated signaling pathways. As a kinase. AURKA functions to activate a number of oncogenic proteins as well as to inactivate tumor suppressor proteins, thereby promoting the pathogenesis of cancer.To further elucidate the mechanism by which AURKA promotes tumorigenesis, we set out to investigate how the transcription factors that play a central role in controlling cellular gene expression respond to AURKA overexpression. We took advantage of a transcription factor activity array, and within an AURKA-inducible system, we identified E2F1 as a novel transcription factor that showed increased transcriptional activity upon AURKA induction. We studied the relationship between AURKA and E2F1, and discovered that AURKA expression upregulated E2FI on the protein level, and importantly, AURKA expression positively correlated with E2F1 level in cancer specimens. Subsequent degradation analysis revealed that AURKA alleviated the proteosome-dependent degradation of E2F1, thus inducing E2F1 protein accumulation. Furthermore, we discovered that AURKA induced the upregulation of microRNA-17-92 cluster, and we demonstrated that this stimulation was at least in part mediated by E2F1 transcription factor.To better understand how AURKA contributes to chemoresistance of cancer cells, we generated a stable cell line in ESCC cells and found that AURKA induced anti-apoptotic effects in ESCC cells. With an apoptotic PCR array, we were able to identify PAK7 as a downstream effector that mediated the anti-apoptotic effects of AURKA. We found that AURKA elevated PAK7 expression on both mRNA and protein level, and this elevation was mediated by the enhanced occupation of E2F1 transcription factor to the promoter of PAK7. Knockdown of PAK7 at least partly overcame the anti-apoptotic activity of AURKA. Further investigation showed that PAK7 was overexpressed in both ESCC cell lines and cancer samples, and clinical significance analysis revealed that PAK7 overexpression was correlated positively with the lymph node metastasis and TNM stage in ESCC. Functional analysis showed that PAK7 overexpression increased the migration of ESCC cells, and was able to transform NIH3T3 cells, leading to tumor formation in nude mice, indicating that PAK7 is a novel potent candidate oncogene.To fully understand the transcriptional regulation of E2F1, we performed ChlP-chip assays to detect the genome-wide localization of E2F1. With two separate analytical tools, we identified pathway dynamics regulated by E2F1:in the first 24 hours, E2F1-regulated genes tended to be enriched in the EGF signaling pathway, while in the next 24 hours E2FI downstream genes mainly were enriched in Wnt/β-catenin signaling and VEGF signaling pathways. When the changes within the 48 hours were taken in all. the E2F1 target genes were mostly enriched in pathways of tumorigenesis, and implied the possible cell cycle progression prompted by AURKA overexpression via E2F1.In summary, AURKA is an important oncogene with multiple roles in tumorigenesis. We identified E2F1 and its two target genes miR-17-92 and PAK7, as well as some pathways, as crucial players acting downstream of AURKA, providing new insights into the role of AURKA in cancer development. These downstream effectors may also serve as therapeutic targets for cancer patients with AURKA abnormalities.