Novel role: EZH2 boosts creation of ovarian cancer blood vessels

A protein associated with cancer progression when abundant inside of tumours also unexpectedly regulates the creation of new blood vessels that feed the tumour outside, a research team led by scientists at The University of Texas MD Anderson Cancer Centre reports in the August edition of Cancer Cell.

By using a nanoparticle-based gene-silencing system to block production of the protein, the researchers inhibited formation of new blood vessels (angiogenesis) to the tumour and caused a steep reduction in tumour burden in a mouse model of ovarian cancer.

'We've discovered that EZH2 promotes tumour growth by shutting down genes that block formation of new blood vessels,' said study senior author Anil Sood, M.D., professor in UT MD Anderson's departments of Gynaecologic Oncology and Cancer Biology. 'Tumours treated with current anti-angiogenesis drugs eventually progress. This study presents a new mechanism for angiogenesis that opens the door for development of new treatment approaches.'

EZH2 is a member of a group of proteins known to repress gene expression. It has been associated with the progression and spread of bladder, breast, prostate and gastric cancers and one type of cancer of the pharynx.

An examination of 180 ovarian cancer tumours found that the protein was overexpressed in the tumour in 66 percent of cases and in the endothelial cells of 67 percent of samples.

Endothelial cells line the inside of blood vessels and are crucial to angiogenesis.

Increased expression of the protein in either tumour or endothelial cells was associated with late-stage and high-grade disease and decreased median survival. Patients with increased EZH2 levels in their tumours had a median survival of 2.5 years compared to 7.33 years for those without. For overexpression in the endothelial cells, the difference was 2.33 years versus 8.33 years for those with normal levels.

In a series of lab experiments, the team found that vascular endothelial growth factor (VEGF), a known stimulator of angiogenesis, boosts the level of EZH2 in endothelial cells. EZH2 then silences the vasohibin1 (VASH1) gene, which normally inhibits blood-vessel-formation. Silencing the EZH2 gene in the tumour's endothelial cells reactivates VASH1, reducing angiogenesis and ovarian cancer growth in mice.

The ezh2 gene was targeted separately in tumour cells and in endothelial cells by delivery of small interfering RNA (siRNA) - short snippets of RNA that block gene expression - to mice with one of two strains of ovarian cancer.

- Treating mice with siRNA that silenced ezh2 in the tumour-associated endothelial cells reduced average tumour weight by 62 percent and 40 percent in the two strains of cancer compared with control mice.

- Hitting the gene only in the tumour had little significant effect on tumour burden.

- Silencing in both tumour and endothelial cells reduced average tumour weight by 83 percent and 65 percent in the two cancer strains.

Additional tests showed that silencing ezh2 reduced both he number of blood vessels serving the tumours and ovarian cancer cell proliferation while increasing programmed death of tumour cells.

Sood and co-author Gabriel Lopez-Berestein, M.D., professor in UT MD Anderson's Department of Experimental Therapeutics, have developed delivery systems that package siRNA with a fatty ball called a liposome to silence specific genes in cancer cells.

'Those systems are quite effective for delivery to tumours and tumour cells but not as effective for delivery to tumour vasculature,' Sood said. They jointly developed a new delivery system that packages siRNA into chitosan nanoparticles. Chitosan is derived from a chitin, a structural component in the shells of crustaceans.

Chitosan nanoparticles carry a slight positive electrical charge, making them attractive to the mostly negatively charged endothelial cells. The nanoparticles penetrate the tumour by way of its vasculature, so the new system hits both targets.

The nanoparticles accumulate in the cancer cell and vasculature passively as they circulate in the blood stream. Chitosan nanoparticles are so small that they can flow through tiny holes in the tumour vasculature. They also accumulate in other organs, so the researchers are working to add a targeting molecule that will limit nanoparticle uptake to tumours and their vasculature.