June 27, 2013
An estimated 14,380 Americans this year will get the news. They have head and neck cancer. Most will receive carefully calibrated doses of radiation therapy each day over several weeks to kill cancer cells and shrink the tumor. At some point, more than 90 percent will notice an unpleasant parched sensation in their mouths. It is a side effect of the radiation treatment called xerostomia, or dry mouth. For most, the persistent dryness will become a permanent part of their lives, causing difficulty with eating and speaking, oral infections, dental caries, tissue inflammation, and decreased quality of life.
This common scenario has led scientists to delve deeper into the biological processes that underlie radiation-induced xerostomia. They have determined that when exposed to radiation, cells in the salivary gland tend to activate an age-old suicide response called apoptosis. In Greek, the word literally means, “as petals falling from a rose.” The response involves ramping up the production of a number of proteins to induce death. As more cells die and are not replaced over the course of the radiotherapy, the ability of the salivary gland to make saliva permanently diminishes. These basic findings have raised a major clinical research question: If the process of apoptosis is blocked in the salivary glands during radiation treatment, can xerostomia be prevented?
In the June issue of the journal Molecular Therapy, a team of NIDCR supported scientists report initial success in reducing apoptosis in the submandibular salivary gland in mice. They do so using short interfering RNAs (siRNAs), which are small RNA molecules that can be introduced into a cell to block temporarily the expression of a protein.
For those who study siRNAs, two broad and interrelated issues loom large: efficient delivery of the molecules to a target organ and hitting their target to produce a desired outcome. First, the delivery issue. The scientists successfully combined two important techniques. One, they complexed the siRNAs with nanoparticles (50-60 nanometers in length, or about the size of common virus) assembled from a specially designed, pH-responsive polymer material. The nanoparticle protects the siRNAs from degradation and helps them enter the cell. Two, the scientists administered the siRNA-nanoparticles with a direct, non-invasive approach called retroductal injection. In this case, the siRNA-nanoparticle complexes were injected into the long, narrow ducts of the salivary gland. Once the nanoparticles pass through the duct and into the gland, they are actively imported into the cells like Trojan horses, via a common cellular process called endocytosis. The altered pH causes the nanoparticles to collapse, releasing the customized siRNA to block production of its pre-selected target protein. The target was, in this case, a pro-apoptotic enzyme called protein kinase C (Pkcδ).
In the mouse study, the scientists introduced siRNA-nanoparticles into the salivary glands prior to treatment with a single moderate dose of radiation (10.0 Gy). They soon determined the siRNAs hit their mark, greatly reducing the amount of Pkcδ protein after radiation treatment. As further confirmation, the researchers found that the pre-treated mice showed less apoptosis immediately after treatment compared to those that did not receive siRNA. But, most notably, those that were pre-treated with siRNA-nanoparticles showed a less severe drop in saliva secretion. After three months, they had over 50 percent more salivary flow compared to untreated mice following irradiation.
The authors concluded “Our results suggest that optimization of in vivo siRNA-mediated silencing for clinical application could be an effective means of protecting salivary glands in the radiation treatment of head and neck cancer.” They also pointed out that the approach has significant advantages over alternative methods, as it is limited to the salivary glands, does not involve viruses, and the block in Pkcδ protein expression is only temporary.
The article is titled, “Nanoparticle-mediated gene silencing confers radioprotection to salivary glands in vivo” and is published in the June issue of the journal Molecular Therapy. The authors are: Szilvia Arany, Danielle SW Benoit, Stephen Dewhurst, and Catherine E. Ovitt.