Aurora-A overexpression associates with Ha-ras codon-12 mutation and blackfoot disease endemic area in bladder cancer

Summary

Our data revealed that 59.4% of the bladder cancer specimens showed Aurora-A overexpression, of which 31.8% also had Ha-ras codon-12 mutation; 45.5% were from blackfoot-disease endemic areas in which arsenic exposure is a major environment factor associated with various cancer formation. We further demonstrated that arsenic treatment of the immortalized bladder cell line, E7, increased Aurora-A expression. All together, co-existence of Aurora-A overexpression and Ha-ras mutation suggests a possible additively effect on the tumorigenesis of bladder cancer. In addition, Aurora-A overexpression and up-regulated by arsenic exposure opens a new direction for exploring the occurrence of bladder cancer occurrence in Taiwan.

Introduction

Overexpression of Aurora-A has been detected in several human cancer cell lines and cancers [1], [2], [3]. In proliferation cells, expression levels of Aurora-A mRNA and protein are low at the G1 and S phases and peak at the G2 phase of the cell cycle. The expression levels fall during mitotic exit and into the G1 phase of the next cell cycle [4]. The translated Aurora-A peptide consists of 403 amino acids and has a molecular weight of 46 kDa [1]. Ectopic expression of Aurora-A in mouse NIH3T3 cells and Rat1 fibroblasts causes centrosome amplification and transformation [5], [6], suggesting its crucial role in the development of human cancers.

Ras proteins are important for controlling the activities of cell survival, transformation, and apoptosis through several signaling pathways [7], [8], [9]. Multiple effectors have been found downstream of Ras, including Raf, PI3K, RalGDS, RIN1, MEKK, GAP, NF1, and AF6 [10]. The ras-gene encoded proteins become constitutively active because of the point mutations in their coding sequences, especially at amino acid codons 12, 13, and 61 [11]. Overexpression of Ha-ras^V12 oncogene not only transforms NIH3T3 cells but also sensitizes them to various stresses, such as serum deprivation, and lovastatin, tumor necrosis factor-α and 5-FU treatments [12], [13], [14], [15]. In addition, urothelial expression of activated Ha-ras in transgenic mice induced urothelial hyperplasia and superficial papillary bladder cancer [16]. About 30% of human tumors, including bladder cancers, contain mutated Ras oncogene [11]. Some of these cancers also showed overexpression of Aurora-A. Whether there is any relation between the Aurora-A overexpression and Ras gene mutation remains unclear.

The blackfoot-disease (BFD) endemic area is defined in the Southwestern coast of Taiwan [17], [18], [19]. High levels of arsenic and fluorescent substances are detected in the artesian well water in this area [20]. The association between the effects of chronic arsenic exposure in drinking water and the occurrence of a variety of disorders includes higher rate of skin, bladder, renal, and lung cancers in BFD endemic area have been reported [21], [22], [23], [24], [25], [26]. In addition, epidemiological studies have correlated the arsenic exposure and the incidence of bladder cancer [25], [26], which was confirmed by reduced bladder cancer mortality after installation of the tap-water supply system in these arsenic endemic regions [27]. Elevated expression of Aurora-A has been strongly associated with the parameters of clinical aggressiveness, including high histological grade, invasion, increasing rate of metastasis, and decreasing overall survival of patients with bladder cancers [28], but the factors that induce the overexpression of Aurora-A are still unclear.

In the present study, we identified the association between Aurora-A overexpression with Ha-ras codon-12 mutation as well as the association between Aurora-A environmental and environmental factor(s) in bladder cancers. A total of 37 bladder cancer patients were conducted to measure the expression levels of Aurora-A mRNA using quantitative real time PCR, and to detect the Ha-ras codon-12 mutations by single nucleotide polymorphism analysis. The assessment of the environmental factors was obtained from the epidemiological studies [17], [18], [19]. The association between Aurora-A mRNA overexpression level and Ha-ras codon-12 mutation as well as environmental factor (BFD endemic area) were evaluated.