Bladder Cancer Diagnosis, Treatment, and Prevention May Be Aided by Tumor Suppressor Gene

¹Continued development of urine-based biomarker diagnostic tests may help replace the costly and more invasive techniques of imaging, cytology, and cystoscopy for cancer detection. Genome-wide expression and sequencing studies have identified genes and pathways considered key drivers of bladder cancer.²Unfortunately, none of these markers have proven to be sufficiently useful for clinical application. An improved understanding of the complex picture is therefore still required along with novel markers for more efficient diagnosis and treatment.

ANature Scientific Reportsstudy, details how METTL13 is down-regulated in bladder carcinoma and suppresses cell proliferation migration and invasion. Zhang et al have identified METTL13 as a tumor suppressor gene that has the potential to be developed as a marker for bladder cancer detection, treatment, and prevention.³The METTTL13 protein has aberrant expression in variety of human cancers and is involved in oncogenic processes. Its role in bladder cancer was unexplored till now.

The authors report lower METTL13 expression in bladder cancer tissue samples and cancer cell lines compared with normal tissue and cell lines. Additionally, they observed lower METTL13 expression with disease progression. Low levels were maintained through tumor progression based on tumor node metastasis (TNM) staging.

The study conducted between 2011 and 2014 in the Department of Urology of the First Affiliated Hospital of China Medical University in China, collected clinical samples by partial and radical cystectomies from 83 patients with bladder urothelial cell carcinoma. Additionally, normal urothelial specimens (83) were also collected more than 3 cm away from the bladder cancer tissues, at the same time, to use as normal controls. The study included 61 men and 22 women with 49 low-grade and 34 high-grade papillary urothelial carcinomas.

To study the mechanisms underlying METTL13 mode of action, the team transfected urinary bladder and uroepithelial cell lines with wild-type METTL13 (for over-expression) and METTL13 siRNA (for knocking down expression) expression vectors. Over-expression significantly inhibited cell proliferation while down-regulation of METTL13 enhanced cell proliferation, indicating that METTL13 inhibits cell proliferation.

Flow cytometric analysis of METTL13-transfected cells revealed significantly increased number of cells in the G0 through G1 phases of the cell cycle and decreased S-phase population, suggesting that the overexpression of METTL13 induces G1/S arrest. The mRNA levels of cyclin-dependent kinase 4 (CDK4), cyclin-dependent kinase 6 (CDK6), and cyclin-D1 (CCND1), all proteins involved in the G1/S checkpoint, decreased following an increase in METTL13. The level of phospho-Rb were also decreased in cells that overexpressed METTL13, affecting the final step in the G1 to S transition checkpoint.

Based on these observations, the study team hypothesized and tested if treatment of cancer cells with METTL13 can reinstate the G1/S checkpoint via the coordinated down-regulation of CDK6, CDK4, and CCND1, decreased Rb phosphorylation, and delayed cell-cycle progression.

They found that METTL13 overexpression, related to reduced cell proliferation, could not be strengthened or weakened by palbociclib, a CDK4/6 inhibitor.

The group also investigated the effect of METTL13 expression on bladder cancer cell growth in vivo,

Xenograft tumors were induced in nude mice using bladder cancer cell line alone or cell line transfected with METTL13 expression vector. Tumors derived from cells transfected with WT-METTL13 grew much slower than those derived from nontransfected cells, showing that overexpression of METTL13 significantly suppressed tumor growth relative to the growth of mock cells and vector control cells.

The research study suggests that METTL13 negatively regulates cell proliferation and reinstates G1/S checkpoint via the coordinated down-regulation of CDK6, CDK4, and CCND1, decreased phosphorylation of Rb, and subsequent delayed cell cycle progression. Moreover, METTL13-dependent inhibition of bladder cancer cell migration and invasion is mediated by down-regulation of FAK (Focal adhesion kinase) phosphorylation, AKT (v-akt murine thymoma viral oncogene) phosphorylation, β-catenin expression, and MMP-9 expression.

Collectively, these findings can be highly relevant in the clinical management and therapy of human bladder cancers. Down-regulated METTL13 expression levels have the potential to be used for bladder cancer diagnosis. Since the cell proliferation, migration, and invasion of bladder cancer is dependent on METTL13 inhibition, increasing METTL13 expression level might have a therapeutic effect on bladder cancer progression. The possible therapeutic role of METTL13 in the management of bladder cancer is therefore warranted.

References

1. James AC, Gore JL. The costs of non-muscle invasive bladder cancer.Urol Clin North Am. 2013;40(2):261-9.
2. U.S. Department of Health & Human Services. Emerging approaches to diagnosis and treatment of non-muscle-invasive bladder cancer.Research protocol. http://effectivehealthcare.ahrq.gov/index.cfm/search-for-guides-reviews-and-reports/?pageaction=displayproduct&productid=1941. Accessed January 22, 2016
3. Zhang Z, Zhang G, Kong C et al. METTL13 is downregulated in bladder carcinoma and suppresses cell proliferation, migration and invasion.Nature Scientific Reports. 2016; 19261.