Growth factors are substances capable of initiating the proliferation of cells that are in a quiescent state (Hefti 1993) by stimulating DMA synthesis and progression of the cell cycle (O'Neal at al. 1994). They have primarily a paracrine or autocrine action and exert their effect via binding to specific transmembrane receptors on target cells (Alexander and Damoulis 1994) which generate a cascade of intracellular molecular signals (Sporn and Roberts 1991). These polypeptide growth factors thus regulate a number of processes in vitro in addition to activation and proliferation, including cell migration and the synthesis of ECM proteins, all of which are essential events in wound healing (Deuel at al. 1991; Kiritsy at al. 1993). Moreover, Terranova and Wikesjo (1987) suggested that growth factors may also have the potential to biochemically mediate periodontal wound healing and regeneration when applied topically. The nature and role of some of these factors [e.g. PDGF, TGF, fibroblast growth factor (FGF), epidermal growth factor (EGF), IGF and bone morphogenetic proteins (BMPs)] in wound healing and their effect on periodontal regeneration are discussed below.
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1.3.6.1 Platelet-derived growth factor
PDGF plays an important role not only in wound healing but also in embryogenesis, neoplasia and fibrotic responses associated with inflammatory disease (Sporn and Roberts 1991; Hu at at. 1995; Horner et a i 1996; Ataliotis and Mercola 1997). It is released mainly from platelets and is also synthesised by macrophages, fibroblasts, endothelial cells, skeletal myoblasts and kidney epithelial cells (Sporn and Roberts 1991; Plemons et a i 1996). PDGF consists of a dimer of two glycoprotein subunits, A and B (Sporn and Roberts 1991 ). There are therefore three combinations of the two chains, PDGF-AA, PDGF-BB and PDGF-AB. PDGF acts by binding to two distinct cell surface receptors, termed PDGFR-a and PDGFR-p, on target cells.
A number of in vitro studies have demonstrated that PDGF can stimulate proliferation (Piche and Graves 1989; Boyan et a i 1994; Dennison
et a i 1994; Anderson et a i 1998), DMA synthesis (Matsuda et a i 1992;
Gates et a i 1993; Blom et a i 1994) and collagen production by PL cells (Matsuda et a i 1992). It is also chemotactic for PL cells (Matsuda et a i 1992; Boyan et a i 1994; Nishimura and Terranova 1996) as well as for osteoblasts (Hughes et a i 1992). PDGF-BB was more effective than the other isoforms in promoting mitogenesis and chemotaxis of PL cells in vitro (Boyan et a i 1994) and also acted synergistically with other growth factors both in vitro and in vivo (Lynch et a i 1987; Rutherford et a i 1992; Matsuda
et a i 1992; Hefti 1993).
Experimental studies in animals examined the efficacy of PDGF- modulated GTR on furcation defects (Cho et a i 1995; Park et a i 1995) and fenestration wounds (Wang et a i 1994b) and concluded that this therapy effectively promoted periodontal regeneration. When PDGF was applied in combination with IGF to periodontal defects in dogs, enhanced formation of new PL, cementum and alveolar bone was observed during the early phases of healing (Lynch et a i 1991). This combination has recently been applied locally to periodontal defects in humans and appears to significantly promote bone regeneration (Howell et a i 1997).
1.3.6.2 Transforming growth factor
The TGFs are a family of structurally and functionally different proteins that have been isolated from normal and neoplastic tissues (Massague, 1987; Barnard at al. 1990; Sporn and Roberts, 1991). The two best characterised are TGF-a, primarily a growth stimulator and TGF-p, primarily a growth inhibitor (Massague 1987; Sporn and Roberts 1991). TGF-a stimulates epithelial and endothelial cells and acts through the receptor of a different growth factor, EGF (Hefti 1993). TGF-p is encoded by 5 different genes yielding the 5 isoforms TGF-pl to TGF-p5 which display different spatial and temporal patterns of expression during healing (Levine at al. 1993; Frank at al. 1996). TGF-p is present in high concentration in platelets (Hefti 1993) and is also produced by both activated macrophages and neutrophils, which are present during the initial phases of wound healing (Igarashi at al. 1993). Three distinct receptors, type I, type II and type III, have been identified on almost all normal cells and most neoplastic cells (Barnard at a i 1990).
The biological effects of TGF-p in vitro are highly diverse. It has been found to be chemotactic for macrophages (Barnard at al. 1990) and gingival and PL cells (Postlethwaite at al. 1987; Nishimura and Terranova 1996), stimulated the proliferation of gingival and PL cells (Postlethwaite at al. 1987; Oates at al. 1993; Anderson at al. 1998), inhibited the growth of epithelial, endothelial and certain mesenchymal cells (Lynch at al. 1989; Barnard at al. 1990; Matsuda at al. 1992; Lu at al. 1997) and selectively stimulated the synthesis of ECM components such as collagen, fibronectin, tenascin and proteoglycans (Irwin at al. 1994b; Matsuda at al. 1992; Lynch at al. 1989; Barnard at al. 1990). In addition, TGF-pi alone and also in combination with PDGF increased the proliferation of PL cells significantly more than gingival cells (Dennison at al. 1994). However, when collagen sponges impregnated with TGF-pi, IGF and FGF were placed into fenestration defects in dogs, this combination was found to have no effect on bone regeneration (Selvig at al. 1994).
1.3.6.3 Fibroblast growth factor
The FGFs are a family of polypeptides which are potent mitogens and chemoattractants for endothelial and mesenchymal cells (Caffesse and Quinones 1993). Two of the most widely studied forms of this family are acidic FGF (aFGF) and basic FGF (bFGF). aFGF stimulates endothelial cell proliferation (Hefti 1993). bFGF is widely distributed and found in nearly all tissues, including brain, retina, kidney, placenta, gingiva, PL and bone (Hefti
1993; Bilezikian eta!. 1996; Gao et al. 1996; Murata et al. 1997).
FGF has been reported to stimulate PL and endothelial cell migration (Terranova et al. 1989b), increase DMA synthesis and enhance proliferation (Blom et al. 1994; Takayama et al. 1997) and inhibit the induction of ALP activity and mineralised nodule formation by PL cells in vitro (Takayama et al. 1997). The in vivo use of bFGF in conjunction with autografts has had a beneficial effect on mandibular bone healing in rabbits (Eppley et al. 1991). However, in combination v\/ith other growth factors it did not show any effect on periodontal healing (Selvig et al. 1994).
1.3.6.4 Epidermal growth factor
EGF is a small polypeptide which stimulates the proliferation of epithelial, endothelial and mesodermal cells (Caffesse and Quinones 1993; Bilezikian et al. 1996). It is present in most human extracellular fluids and secretions including plasma, saliva, milk, amniotic fluid and urine (Sporn and Roberts 1991). EGF is mitogenic for PL cells (Matsuda et al. 1992; Blom et al. 1994) and was found to stimulate the growth of gingival cells in vitro (Irwin
et al. 1994b). It also showed a slightly increased chemotactic effect on PL
cells but suppressed their collagen synthesis (Matsuda et al. 1992). The effects of EGF on periodontal regeneration remain to be investigated.
1.3.6.5 Insulin-like growth factor
The IGFs are a family of single-chain proteins (Sporn and Roberts 1991). IGF-I and IGF-II are anabolic peptides structurally and functionally related to insulin. They are synthesised by liver, smooth muscle and placenta and transported via the plasma (Caffesse and Quinones 1993). They are also present in skeletal tissues, via de novo synthesis by bone cells
and release of the stored peptides from the bone matrix (Bilezikian at al. 1996). Both gingival and PL cells were found to exhibit dose-dependent migratory responses when incubated with IGF-I (Matsuda at al. 1992; Nishimura and Terranova 1996) and IGF-II (Nishimura and Terranova 1996). In addition, IGF-I increased DNA and protein synthesis in PL cells (Blom at al. 1992; Matsuda at al. 1992). IGF-I has been reported to act synergistically with other growth factors to enhance epidermal and connective tissue wound healing (Lynch at al. 1987). Moreover, the short term application of IGF-I in combination with PDGF was found to increase the healing response following periodontal surgery in animals (Lynch at al. 1991; Giannobile at al.
1996) and in humans (Howell at a i 1997).
1.3.6.6 Bone morphogenetic proteins
The BMPs are part of the large TGF-p superfamily (Wozney 1995; Bilezikian at a i 1996). Localisation of members of the BMP family in embryological development of the skeleton has provided strong evidence of an important role in mediating skeletal patterning as well as skeletal cell differentiation (Bilezikian at al. 1996). Furthermore, the BMPs are considered to be responsible for the inductive and regenerative ability of demineralised bone allografts used in periodontal therapy (Urist 1965). In addition, BMP-2 was found to stimulate osteocalcin and ALP expression by cultured PL cells and was chemotactic for connective tissue stem cells in
vitro (Hughes 1995). The BMPs also induced the formation of cartilage and
bone tissues when implanted with a carrier into a soft tissue site (Wozney 1995). Moreover, the topical application of BMPs onto periodontal defects resulted in the establishment of new attachment, regeneration of alveolar bone (Sigurdsson at al. 1995a) and induction of cementum formation in animals (Ripamonti at a i 1996).
1.4 GINGIVAL CREVICULAR FLUID