By Dean A. Haycock
Special to BioWorld Today
It is a common miracle that probably gets more attention than other common miracles. You cut yourself or lose a chunk of skin completing a simple home repair. You monitor the wound in your spare moments. In a few days, you notice the speed with which it is healing, the completeness with which the flesh is replaced.
The process you have witnessed is actually three processes. One involves the attraction of inflammatory cells to the injured site. These fight bacteria and prevent infection. Another lays down collagen that provides a foundation for the growth of replacement cells. The third regulates the proliferation of skin cells that fill in the wound.
For simple injuries in healthy people, this works well. But there are instances in which the wounded could benefit from a little help. Anyone with severe injuries, or patients whose healing ability is slowed by poor nutrition, are two examples.
A report in the September issue of Nature Cell Biology suggests an approach that eventually could lead to therapies to speed wound healing. The paper, titled "Mice lacking Smad3 show accelerated wound healing and an impaired local inflammatory response," shows that blocking the activity of the Smad3 gene results in significantly faster healing in mice.
The Smad3 protein mediates the function of transforming growth factor-beta (TGF-beta). Applying TGF-beta to wounds improves healing. This is likely due to TGF-beta's ability to increase the deposition of collagen at wound sites and to attract inflammatory cells. The factor does not, however, stimulate the growth of skin cells. In fact, it seems to inhibit their proliferation. Without Smad3, TGF-beta can't do its job. Therefore, Smad3 is closely linked to the attraction of inflammatory cells and to the inhibition of skin cell proliferation seen when TGF-beta is applied to wounds. Based on this limited information, one might predict that blocking the action of Smad3 would not speed wound healing.
Not The Expected Outcome In Wounded Mice
Anita Roberts, chief of the Laboratory of Cell Regulation and Carcinogenesis at the National Cancer Institute in Bethesda, Md., and her colleagues from the National Institutes of Health noticed something different when they studied wound healing in mice that lacked the Smad3 gene (Smad3-null mice). These genetically engineered, knockout mice healed faster than controls that had functioning Smad3 genes. Their skin cells grew at a faster rate while immune cells, monocytes, infiltrated the wound more slowly. Overall, these mice healed about two and a half times quicker than wild type mice.
"We are probably being too simplistic to say that this is completely orthogonal to our expectations," Roberts told BioWorld Today. "I would say that we get less granulation tissue [vascular connective tissue that forms granular projections on wound surfaces] in these wounds. We get less matrix protein deposition. Certainly those are two of the things that the addition of TGF-beta had always produced. The surprising outcome was just the narrowness of these wounds and the rapidity which with they restore their epithelial covering. It is not always so simple to say that it is better. We have not tested the tensile strength of these wounds either."
Nevertheless, Roberts and her co-authors wrote in their paper that their data "indicate that disruption of the Smad3 pathway in vivo, coupled with exogenous TGF signaling through intact alternate pathways, may be of therapeutic benefit in accelerating all aspects of impaired wound healing."
Of the multiple pathways downstream of the TGF-beta superfamily of receptors, Smad is certainly a prominent one, according to Roberts. "We have ablated a particular Smad," she explained, "TGF-beta signals are known to be mediated minimally by Smad2 and Smad3 and in certain cases even Smad1. I think with this selective elimination of one pathway, we have shown that certain of the responses to TGF-beta, but not others, are modulated.
Small Molecules Eyed For Suppression
"So I think that from this we have learned that we can possibly selectively modulate the response of different cells in a particular cellular response pattern," she continued. "And it may be, for example, in trying to cover a large surface area that has no more epidermal covering, we could use Smad3 null keratinocytes [skin cells that produce the protein keratin and eventually make up the outer surface of the skin]. Or more likely, and especially in terms of the biotech companies, the direction this could likely move toward is the identification of small-molecule inhibitors that could possibly, even transiently, be able to suppress Smad3."
There are three laboratories that have knocked out the Smad3 gene, according to Roberts. One, at the University of Texas Southwestern Medical Center in Dallas, published a paper in the journal Cell reporting 100 percent penetrance of colon cancer in the Smad3 knockout mice. Another was developed by Chuxia Deng at the NIH and used in the experiments described in September's Nature Cell Biology. The third was described in Molecular Cellular Biology. The latter two knockouts have the same phenotype.
"It is principally an immune cell phenotype and we do not see any colon cancers," Roberts said. "So this really is still a problem. It has to be investigated. I would guess that there are collaborating factors but it has not been worked out yet."
Roberts said, "In experiments we have done that aren't published in this paper we show that in the keratinocytes and the macrophages, the exposure to TGF-beta significantly downregulates the expression of Smad3. It looks like the natural wound healing actually involves suppression of this Smad, perhaps in a transient way. What we've done is to completely eliminate it and show that that increases the response even more."
The NCI and NIH researchers now are interested in investigating the response of Smad3-null mice to agents that induce fibrosis, the formation of fibrous tissue during repair or in reaction to a disease or injury. "Our prediction would be that these mice would be much more resistant to pulmonary fibrosis or cirrhosis or radiation-induced fibrosis," she said. "We feel that one of the primary factors in the disease process that perpetuates the response is the continued production of TGF-beta."