Role of Growth Factors in Periodontal Wound Healing and Regeneration

INTRODUCTION

Periodontal disease activity results in destruction of periodontal tissues (periodontal ligament [PDL], alveolar bone and cementum) eventually leading to tooth loss if untreated. Periodontal therapy can be undertaken by nonsurgical and by surgical methods. Periodontal therapeutic procedures are directed to arrest disease progression and achieve regeneration of lost tissues. In the recent years increased efforts have been focused on understanding the mechanism of and factors required for restoring periodontal tissues, in order to increase the predictability of regenerative therapy. The basic cellular mechanisms of periodontal regeneration are proliferation and migration of periodontal ligament cells, differentiation of cementoblasts and synthesis of extracellular matrix. These events are controlled by biological mediators like growth factors, Bone morphogenic proteins (BMPs), extra cellular matrix proteins which are produced by monocytes, platelets and resident tissue cells like PDL cells, osteoblasts, cementoblasts and endothelial cells. Furthermore the process of periodontal regeneration also involves inflammatory aspects. Several studies have shown that periodontal regeneration may be enhanced by therapeutic application of specific growth factors .Growth factors are a group of polypeptides that are natural to every cell. They are involved in a variety of cell to cell and cell to matrix interactions during development of particular organ, in normal health state and in wound healing.¹ These growth factors mediate cellular processes like DNA synthesis, mitosis, chemotaxis and also conversion of the cells in resting phase of cell cycle into replicating cells. Various growth factors have been isolated and studied such as Platelet derived growth factor; Fibroblast derived growth factor, Transforming growth factor, Bone morphogenic proteins. Growth factors are used alone or in combination to achieve regeneration. The combination being the preferred one as it is thought that one growth factor stimulates the formation of mineralised tissues whereas the other one non mineralised tissue. Studies have shown bright future for the use of these in field of periodontal regeneration.

RATIONALE FOR THE USE OF GROWTH FACTORS IN DENTISTRY

A key factor for enhancing the predictability of regenerative therapies is an understanding of cellular and molecular events required to regenerate periodontal tissues. It is now recognised that an important link, although not exact, to understand the requirements for regeneration of tissues is to acquire knowledge as to mechanisms involved in development of tissues. In wound healing and regeneration the early events include, recruitment of marrow cells and clot formation, followed by migration of inflammatory cells and release of cell cytokines and growth factors at the healing site. During development it is recognised that specific growth factors and morphogens trigger differentiation of epithelial and mesenchymal derived cells during tooth formation. Another event considered critical for both development and regeneration of periodontal tissues is attracting appropriate cells to the site of repair and development. Once at the site, the cells attach to the extracellular matrix of local environment and become biologically active to differentiate into osteoblasts, cementoblasts, or PDL cells and lay down appropriate matrix required for the formation of hard and soft connective tissues.² To synthesize sufficient matrix, the appropriate cells must be stimulated to proliferate at the local site.

BASIS OF ACTION OF BIOLOGIC MODIFIERS

Mode of action

Once growth factor is synthesized by cell, it reaches its target receptor or binding protein, on interaction with these receptors causes activation of secondary messengers or terminal effectors. Growth factors are mainly associated with local modes of action. Various modes being – Paracrine, Autocrine, Juxtacrine and Intracrine.²

Paracrine – Biological modifiers are produced by one cell in soluble manner and interact with the target receptors present in another cell in local micro environment to evoke its effect. E.g. – Platelet derived growth factor (PDGF).

Autocrine - Factors are synthesized by one cell and secreted in soluble form outside the cell and then bind to surface receptors present on the same cell to evoke its effect. e.g. - BMPs (produced by and acts on osteoblastic cells).

Juxtacrine –Factor which is produced by one cell requires cell to cell contact with the target cell to evoke its response. e.g. - stem cell factor.

Intracrine –factor is produced by one cell and it acts intracellularly to facilitate its effect e.g. - parathyroid hormone related protein.²

Receptors

Receptors are the components of the cell membrane, which are modified to act in a particular manner. To exert its effect, a growth factor has to be received by the receptors on a cell membrane. They should be present in sufficient quantity, orientation and functional activity to transmit appropriate stimuli.² Growth factor receptors are divided into two main categories; Cell surface receptors and. Intracellular receptors. Cell surface receptors are further divided into three main categories, G-protein linked, Receptor tyrosine kinases, Serine threonine kinases Cell surface receptors commonly bind peptide factors that are soluble in water but not easily transported across lipophilic cell membrane. Intracellular receptors are commonly described for steroids such as vitamin D3, oestrogens and gluco corticoids. Steroid receptors have been described in both the cytoplasm and nucleus of target cells. Additionally, intra cellular receptors or binding proteins for factors that act in an Intracrine manner are located within the nucleus. Once a cell surface receptor has been bound and activated a series of second messengers are responsible for taking the next step in evoking biologic activity

Four main second messengers: Adenyl cyclase-Enzyme activated by G-proteins in the cell membrane in response to activation of G-protein linked receptors such as parathyroid hormone. Adenyl cyclase catalyses the reaction from adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP) which activates protein kinase A to cause protein phosphorylation. G-protein linked receptors Couple to membrane bound phospholipase C with activation of protein kinase C to evoke protein phosphorylation. Receptor tyrosine kinases and serine threonine kinase are also responsible for phosphorylating their target proteins. Protein phosphorylation is a key factor in growth factor activity and is responsible for mediating changes in cell proliferation and differentiation which are considered to be the hall mark of growth factor activity .³

CELL PROLIFERATION

The basic part for tissue growth is cell proliferation and it is a prerequisite for repair and regeneration. Duplication of the cells occurs in a manner in which daughter cell receives identical copy of genetic material. Cells from different tissues grow and divide at different rates. There are mainly four phases of cell cycle, S-phase –DNA synthesis takes place, Mphase –mitotic phase in which cell is actually dividing .The two G phases represent gap phases G1-(first gap), G2-(second gap) between the S and M phase, Go - If a cell is not active in the cell cycle (i.e. it is terminally differentiated or at rest). For a cell to re-enter the cell cycle a stimulus designated as competence factor is required. After the cell has been rendered competent to undergo cell division it requires a progression factor (Insulin Growth Factor – I). Once the cell has progressed to synthesis phase it is committed to undergo cell division, there are growth factors which may act at later stage to delay or block cells in the G2 phase. Biologic modifiers are key regulators of this process of cell proliferation via their action at different stages of cell cycle.

CELL DIFFERENTIATION

It is a process by which undifferentiated cells undergo transformation into specific type of cells. Differentiation is a critical component of regeneration. It brings about the structural and functional changes in the cells which are necessary for formation of a type of tissue for e.g. Fibroblasts for periodontal ligament.⁴

PLATELET DERIVED GROWTH FACTOR

This was originally isolated from alpha granules of platelets and was seen to have mitogenic activity on smooth muscle. Platelet derived growth factor consists of two Di-sulphide bonded polypeptide chains and is encoded by two different genes. PDGF–A and PDGF–B. PDGF is highly mitogenic and chemotactic for periodontal ligament cells. Regulates osteoblastic transformation of cells, Stimulates mitogenic activity and chemotaxis.^5,6,7

TRANSFORMING GROWTH FACTOR (TGF)

TGFs – α and β, were discovered in 1978 as substances secreted into serum-free media by cultured 3T3 cells transformed by a murine sarcoma virus. Transforming growth factor –beta –selectively stimulates the synthesis of connective tissue matrix components such as collagen, fibronectin, proteoglycan and glycosaminoglycan both in vivo and in vitro.⁷

INSULIN LIKE GROWTH FACTOR (IGF)

The term insulin like growth factor is used to describe growth promoting activities that share structural homology with pro insulin. There are 2 different types of IGFs – I and II. It acts synergistically with platelet derived growth factor and both are chemotactic for periodontal ligament cells.⁷

EPIDERMAL GROWTH FACTORS (EGF)

EGF is a 6045 Dalton polypeptide consists of epidermal growth factor, TGF-α, amphi regulin and heparin binding epidermal growth factor. All these bind to the same receptor and have similar biologic effects. Their effects on periodontal ligament include mitogenic to fibroblasts, increased keratinocyte proliferation, and inhibition of collagen synthesis.⁸

VASCULAR-ENDOTHELIAL GROWTH FACTOR (VEGF)

VEGF has been assayed from the gingival crevicular fluid of a cross-section of periodontal patients and healthy control subjects. Higher levels of VEGF were found in diseased sites than in healthy sites.³ In addition to its role in the regulation of angiogenesis, it increases micro vascular permeability. VEGF has also been recently suggested to play an important role in the regulation of bone remodeling by attracting endothelial cells and osteoclasts and by stimulating osteoblast differentiation.⁹

CEMENTUM-DERIVED GROWTH FACTOR (CGF)

CGF is mitogenic to gingival, periodontal ligament and skin fibroblasts and to bovine and human aortic smooth muscle cells. The presence of CGF and other growth factors in cementum indicates that these substances influence the cells present in cementum and or adjacent periodontal ligament, gingiva and dentin.⁹

EFFECT OF BMPS ON PERIODONTAL LIGAMENT CELLS

BMPs stimulate matrix synthesis, cementoblast proliferation and production. They also regulate proliferation and mitogenesis of the cells of osteoblastic lineage, stimulate maturation of osteoblastic cells, and stimulate alkaline phosphatase activity thus in turn stimulating increased bone formation. They induce osteoblastic transformation of stromal cells. Along with basic fibroblast growth factor, it stimulates angiogenesis.^{10,11,12,13,14}

CONCLUSION

The role of growth factors in periodontal wound healing and regeneration has been studied by various investigators. It is known that growth factors are effective at various concentrations. The effective concentration of growth factors has to be studied which results in optimum regeneration .Growth factors act over an area on application and for these delivery systems have to be developed, so that there are equal and sustained delivery of growth factors which results in regeneration.