Shedding light on wound healing

Hank Hogan, hank.hogan@photonics.com

A little light may go a long way toward healing a wound, investigators reported at the recent annual meeting of the American Association for Dental Research. They found that a single small dose of blue-violet light caused certain cells to produce minute amounts of reactive oxygen species, which contributed to increased activity and growth.

Their findings suggest that blue light could be useful in enhancing cell growth in a wound. However, lead researcher Capt. Claudia P. Millan, a third-year periodontal resident in the US Army, noted that not all cell types respond favorably to such light and that too much reactive oxygen can cause damage.

The right light helps some cells heal but not others. Human gingival fibroblasts (HGF, left) don’t close a wound (black circle in the middle) when exposed to blue light. The red is DRAQ5 dye that attaches to replicating DNA in dividing cells. On the other hand, normal human epidermal keratinocytes (right) respond to the same light with increased growth factor, imaged here with fluorescent antibodies. Courtesy of Claudia P. Millan, US Army.

“This truly is a balance between the good and the bad of the reactive oxidative species that are produced in response to the light,” said Millan, who was part of the team that did the research. Others involved were from the Medical College of Georgia in Augusta.

The investigators worked with two types of cells, normal human epidermal keratinocytes and human gingival fibro-blasts. The former are normally exposed to the environment, whereas the latter are typically found below the surface. Another key difference between the two is the speed with which they grow and wounds heal. The epithelium, the outermost layer of cells, is more active and moves to close an opening at a rate of 0.5 mm per day. On the other hand, connective tissue, such as gingival fibroblasts, can take weeks to mature and thereby heal a wound.

For this investigation, the researchers first plated both types of cells around cloning cylinders. They let them grow until there was no more room left to do so. They then removed the cylinders, creating a wound.

In previous studies, Millan exposed cells to light for various lengths of time and at different intensities. Thus, she had an idea of the optimal exposure settings. For this work, she treated the cells with a 20-J/cm² dose from a quartz-tungsten-halogen light source, filtering its output so that only light from 400 to 500 nm reached the cells.

The researchers evaluated the effect of the light on the tissue in a number of ways. One was by looking at mitochondrial activity, which they measured by using a standard assay to gauge production of succinate dehydrogenase. This enzyme complex plays a role in mitochondrial energy generation. They also assessed cell proliferation using the DRAQ5 DNA dye.

They found that normal human epidermal keratinocytes, the first cell type, responded to the blue light exposure by increasing the production and secretion of several growth factors. The list included insulinlike growth factor binding protein-1, amphiregulin, epidermal growth factor and fibroblast growth factor. They also found enhanced mitochondrial dehydrogenase activity and cell proliferation.

In contrast, human gingival fibroblasts, the second cell type, did nothing. Millan noted two possible reasons for this lack of response. One is that such cells are not normally exposed to light and so might have no reaction mechanisms. Another is that these cells are not normally as active metabolically as the first type and so might lack the mitochondria needed to respond to the light.

She noted that filtered quartz-tungsten-halogen light can do more than speed wound healing. For example, squamous carcinoma cells, a type of skin cancer, die upon exposure, which could be a beneficial outcome. Exposure also may kill bacteria, which likewise could be a good thing.

This variability in response means that more studies must be done before light can be safely used to speed wound healing, Millan said. “It would be great to have a human trial, but I think we still have to test it on other cell-culture types.”