Growth Factors in Wound Healing

If the wound healing process could be likened to the performance of a symphony with macrophages, fibroblasts, endothelial cells, and platelets, the major players, growth factors, provide the means of communication. They are important for the completion of all three phases of the wound healing process: inflammatory, proliferative, and matrix remodeling. Without the timely release of growth factors, chemotaxis, proliferation of cellular structures, and extracellular matrix production would not occur. For example, the right players need to enter the wound at the proper time so that the coordination of blood vessel formation and the deposition of fibronectin scaffolding and collagen matrix can form granulation tissue. Similar coordinated efforts are required for epithelialization and, later, scar formation.

Growth factors not only have to be produced in the correct amounts, they must be released at the right times. Thus, platelet-derived growth factor (PDGF), released relatively early, is an important factor in the formation of granulation tissue. Transforming growth factor ß (TGFß), released later, is considered to play a significant role in the formation of scar. Prolonged or abnormal release of this growth factor might be the cause of excessive scarring. Basic fibroblast growth factor (bFGF) is known to be an important stimulator of angiogenesis.

Platelet-Derived Growth Factor (PDGF)

PDGF is a 30,000 to 32,000 molecular weight glycoprotein consisting of two subunits linked by disulfide bonds. This results in a family of two homodimers and a heterodimer: AA, BB, and AB.¹ The BB homodimer has been the subject of recent clinical studies.

Many cells in addition to platelets are capable of releasing PDGF. These include fibroblasts, endothelial cells, macrophages, and even keratinocytes.² Similarly, there are several cells, such as macrophages,³ polymorphonuclear cells, and fibroblasts,⁴ that are capable of binding this protein. Once binding occurs, intracellular chemical changes follow that eventually affect nuclear and cytoplasmic processes.

PDGF has wide-ranging effects on the cells capable of binding it. It has been found to be chemotactic for macrophages and polymorphonuclear cells.² This would seem to make sense, as PDGF is a growth factor that is released relatively early in the acute wound healing process. Of course, these early inflammatory cells function to rid the wound of foreign debris and dying cells. As the inflammatory phase continues, the platelet plug diminishes, and the macrophage becomes a more important source. PDGF also supports the migration and proliferation of fibroblasts that enter the wound by crawling along the fibronectin scaffolding. This process requires the binding of cell-surface proteins, ß1 integrins, with receptors on fibronectin. PDGF also acts as a mitogen for fibroblasts; however, it has recently been found that the proliferative response depends on the type of wound. Fibroblasts from older, nonhealing venous leg ulcers grow poorly in tissue culture compared to those derived from healing ulcers. In addition, the poor mitogenic response to applied PDGF-BB correlated with increased ulcer age.⁴ Histologically, the fibroblasts from these poorly healing wounds resembled in-vitro aged cells. The results of this very interesting study suggest that one possible reason for nonhealing of some chronic wounds is that the fibroblasts are senescent and unable to respond to growth factors such as PDGF. This growth factor is also able to stimulate the formation of collagen⁵ and ground substance.⁶

From a theoretical point of view, it would seem PDGF is able to support the initial inflammatory stages of healing and, later, plays an important role in the formation of granulation tissue.

In summary, PDGF has been found to be efficacious in the management of diabetic ulcers. It has been released by the regulatory authorities in the USA and Canada for the treatment of diabetic neurotropic ulcers. Its use in pressure ulcers is undergoing further study.

Transforming Growth Factor Beta (TGFß)

Like PDGF, TGFß is a glycoprotein family of at least three isoforms: TGFß1, ß2, and ß3. Initially, there were thought to be no significant differences in activity among the isoforms,⁷ although more recent work has suggested there may be a slightly different role for the ß3 subtype.⁸

Cells capable of releasing TGFß include platelets and macrophages. As with PDGF, TGFß is released early in the acute wound healing process as platelets degranulate. Also similar to PDGF, TGFß is chemotactic for neutrophils and macrophages and, thus, participates in the inflammatory phase of healing. It also is chemotactic for fibroblasts.⁹ Almost all cells possess receptors for TGFß and, theoretically, can respond.¹⁰ The most important cell in this regard is the fibroblast. TGFß stimulates both fibroblast chemotaxis and proliferation.¹¹ It is a very potent stimulator of the synthesis of collagen and fibronectin¹² and stimulates the expression of what are known as "integrins" on the fibroblast.¹³ These integrins are cell-surface proteins that bind extracellular matrix such as fibronectin and collagen. This binding allows motile cells to "crawl" through the wound along the fibronectin scaffolding. TGFß has also been found to be able to induce the expression of these integrins on the surface of keratinocytes.¹⁴ This enables keratinocytes to migrate over granulation tissue in a similar fashion.

Because of the prominent effects TGFß has on extracellular matrix formation, most research has centered on its role in scarring. One recent study¹⁵ suggests that TGFß may play a role in the formation of proliferative scars. TGFß-1 and TGFß-2 have been found to be elevated in tissue culture fibroblasts derived from keloids compared to those derived from normal dermis.⁸

Fibroblast Growth Factor

The fibroblast growth factor family consists of over nine isoforms. The best-characterized member is basic FGF (b-FGF). There is also an acidic FGF that is 50-percent homologous with b-FGF.

FGF is released by macrophages and fibroblasts. Several cells are capable of responding to this growth factor. Basic FGF is chemotactic for fibroblasts and especially endothelial cells making it a potent angiogenic factor.¹⁶ It stimulates proliferation of fibroblasts and keratinocytes.¹⁷ Basic FGF, like TGFß, also stimulates fibroblast production of extracellular matrix, namely collagen, fibronectin, and proteoglycan.¹⁸ Overall, FGF is an important growth factor for angiogenesis and the formation of granulation tissue.

Despite encouraging results in animal studies, there are few good human studies involving the FGF family.

Epidermal Growth Factor

Epidermal growth factor (EGF) is a 53-amino-acid protein that is identical to urogastrone. Of interest is that EGF has been isolated from the salivary glands of animals, which may explain why cats lick their wounds. It is released by platelets, along with TGFß, as they degranulate. It is also derived from macrophages. Many cells possess receptors for EGF, although the significance is unknown. As the name suggests, EGF is both chemotactic and mitogenic for keratinocytes,¹⁹ fibroblasts,²⁰ and endothelial cells.²¹ EGF, despite its name, also stimulates the production and release of matrix proteins, such as fibronectin and collagen. Thus, in addition to its effects on epithelialization, EGF also plays a role in the formation of granulation tissue. It has also been found to stimulate collagenase activity.²²

The results of human studies involving EGF in chronic wounds have been disappointing.

Other Growth Factors

Other growth factors have been studied. Clinical trials on granulocyte-macrophage colony stimulating factor (GM-CSF) have been promising. As its name suggests, GM-CSF can stimulate leukocyte and macrophage activity. In wounds, it can stimulate migration and proliferation of endothelial cells.²³ Clinical studies have shown encouraging results.

Interleukin 1 was evaluated in the management of pressure ulcers.²⁴ It was thought to be potentially beneficial, since its main activity is to stimulate macrophages, which, when activated, secrete many growth factors.

Summary

The availability of recombinant DNA technology has revolutionized the study of growth factors that can now be mass produced. This has made it relatively easy to perform animal and human clinical studies. It can be said that with respect to growth factors in chronic wounds, the great expectations borne of theory have not been met by great results in practice. This should not be reason to despair. Current research is in its infancy. More needs to be known about how these growth factors fit in the puzzle that is the chronic wound. In the future, combinations of growth factors will likely be more successful. Similarly, the status of the wound will dictate which growth factor should be added and when. All of this will require a quantum leap in our knowledge of the wound healing process. The future should be quite interesting.

Excerpted and adapted with permission from HMP Communications. Kunimoto BT. Growth factors in wound healing. In: Krasner DL, Rodeheaver GT, Sibbald RG (eds). Chronic Wound Care: A Clinical Source Book for Healthcare Professionals, Third Edition. Wayne, PA: HMP Communications, 2001:391–7. Copyright © 2001 HMP Communications.

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