Kırık Onarımının Büyüme Faktörü Düzenlenmesi

Özet (abstract)

Fracture repair can be considered as a biologically optimal process resulting in the restoration of injured skeletal tissue to a state of normal structure and function. Although the process leads to healing in the vast majority of cases, a small but significant proportion of fractures result in delayed union or persistent nonunion. Surgical interventions have been directed toward enhancing the fracture repair process, normalizing the rate of healing, and decreasing the likelihood of nonunion. During fracture repair, a number of growth factors, cytokines, and their cognate receptors are present at elevated levels in and around the fracture site. Many of these proteins are normally expressed in skeletal tissue, and others are released from associated inflammatory cells at the site of injury. The induction of these proteins is regulated both spatially and temporally, suggesting that they play an active role in promoting fracture repair. The following review will summarize the current literature on the roles of the major cytokines and growth factors involved in fracture repair. In addition, the signaling cascades induced by these molecules will be discussed. While many cytokine and growth factor signaling events have not been specifically examined in the context of fracture repair, a large body of literature on signal transduction has emanated from studies on these molecules in embryonic bone development. Given the conserved nature of these molecules and their signaling cascades from Drosophila to humans, and the similarities between the fracture repair process and embryonic bone development, it seems highly probable that these downstream signaling events are conserved in fracture repair. Fracture repair can be envisioned as involving five distinguishable processes, including the immediate response to injury, intramembranous bone formation, chondrogenesis, endochondral bone formation leading to the reestablishment of load bearing function, and bone remodeling. While these processes may be discussed individually, it should be recognized that the first four occur simultaneously during fracture repair and are likely to influence one another. Extensive remodeling of the newly formed bone follows these four concurrent processes and facilitates the reestablishment of the full biomechanical integrity of the bone. A number of investigators have commented on the similarities between the repair process and embryonic bone formation and indeed a number of specific events characteristic of embryonic bone formation are reiterated in fracture repair. This is especially evident as one begins to examine the specifics of local factors regulating fracture repair, specifically those involved in the events of endochondral bone formation. The immediate response to injury from the fracture trauma is the initiating event of the fracture repair process. Fracture trauma involves not only an interruption of skeletal integrity but also a disruption of the normal vascular structures and nutrient flow at the fracture site. This leads to reduce oxygen tension, disruption of the marrow architecture, and results in the infiltration of inflammatory cells, macrophages, and degranulating platelets during formation of a hematoma.1, 2 While it is likely that the mechanical stresses, changes in oxygen tension, and loss of vascular nutrients at the fracture site may signal some aspects of the healing process, the dominant initiators of fracture repair are most likely the numerous cytokines and growth factors released into the fracture site.3 To date the majority of research on fracture repair has focused on the actions of a relatively limited number of cytokines including interleukin-1 (IL-1), interleukin-6 (IL-6), tumor necrosis factor alpha, and macrophage colony-stimulating factor as well as the local growth factors fibroblast growth factor (FGF), platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-β), and the bone morphogenetic proteins (BMPs)-2, -3, -4, and -7. It is interesting to note that the majority of the factors mentioned have well described roles in the formation and patterning of the embryonic skeleton or in the homeostatic maintenance of the mature skeleton. During the immediate injury response, the first detectable factors include PDGF and TGF-β released by the degranulating platelets.3 This is closely followed by infiltrating macrophages and other inflammatory cells that secrete FGF and additional PDGF and TGF-β. Presumably, these inflammatory cells also release a variety of cytokines including IL-1 and IL-6; however, this has only been demonstrated indirectly and no comprehensive data are available on the spatiotemporal profile of the proinflammatory and immune cytokine expression during the fracture repair process.4 Thus within the first hours post fracture, a complex milieu of factors involved in bone repair can be detected in the fracture site. Some or all of these factors are presumably the initiators of the active repair process acting on the cells of the bone marrow, periosteum, and external soft tissues adjacent to the fracture site. One of the first tissues to visibly respond to the milieu of factors released by the immediate response to injury is the bone marrow, and in fact preliminary evidence suggests the bone marrow may be a major contributor of some of these factors.2 It has been demonstrated that within hours following the fracture event, the bone marrow undergoes a loss of normal architecture, disappearance of the blood vessels adjacent to the fracture callus clot, and a reorganization of the cellular complement into regions of high and low cellular density (Fig. 1A). Within 24 h post fracture, cells in the high cellular density regions undergo differentiation and take on an osteoblastic phenotype.2 In addition to changes in the bone marrow, osteoblasts lining the cortical bone surface are activated, and adjacent periosteal cambium derived preosteoblasts divide and begin differentiating.1 These resident and differentiating osteoblasts lay down new bone via an intramembranous pathway, thus forming the woven bone or so-called “hard” callus adjacent to the fracture site. These events can be detected as early as 3 days post fracture and continue with proliferation peaking between 7–10 days and disappearing in this region by 14 days post fracture (Figs. 1B, and 2A, 2B).5 Osteocalcin expression in this region is maximal around day 15, indicating continued osteoblastic activity post cessation of proliferation. Under specific conditions in which the fracture is rigidly fixed by a surgical plate and screws, this newly formed bone can bridge the fracture gap and produce fracture repair without the production of a cartilaginous callus.6 This, however, is a relatively rare situation. In the majority of fractures, healing involves an additional chemotactic response by periosteally derived undifferentiated mesenchymal cells and fibroblasts from the external soft tissues that infiltrate and divide replacing the hematoma. Photomicrographs of early fracture callus formation in the mouse. (A) Fracture site 24 h after fracture showing no detectable fracture callus. (B) Fracture site at day 7 post fracture showing early callus formation and organization. Intramembranous bone formation is detectable under the periosteum (open arrow) and the early chondroid callus representing the initial stages of the endochondral repair process is visible adjacent to the fracture site (closed arrow) (hematoxylin and eosin, ×40). Low power photomicrographs of fracture callus in the rat. (A) Fracture callus at day 9 post fracture showing increased intramembranous bone formation under the periosteum and increased chondroid tissue overlying the fracture site. (B) Fracture callus at 14 days post fracture showing the continued intramembranous bone formation under the periosteum referred to as the hard callus and increased volume of chondroid tissue referred to as the soft callus. (C) Fracture callus 21 days after fracture showing newly formed woven bone adjacent to the fracture site formed as a result of endochondral bone formation (closed arrow) and the residual chondroid tissue still to be replaced with new bone. (hematoxylin and eosin, ×4). Mesenchymal cell proliferation can be detected as early as day 3 post fracture and remains high for several days.5 The subsequent chondrogenesis by these mesenchymal cells and proliferation of these new chondrocytes continues from day 7 to day 21, leading to the formation of a cartilaginous callus that bridges the fracture site. This cartilaginous callus is referred to as the “soft” callus and provides the initial stabilization of the fracture site (Figs. 1B and 2A–2C). This cascade of events is the initiation of an endochondral bone formation process and is remarkably reminiscent of the proliferative zone of the growth plate. By 9 days post fracture, the chondrocytes of the soft callus adjacent to the woven bone of the hard callus begin to elongate, and the and expression characteristic of These chondrocytes undergo and the characteristic expression of of chondrocytes of the growth In addition, these cells produce and release that begin down the cartilaginous including in for 9 expression by day 14 and is followed days by a in indicating an in The of the soft callus in a spatially with of chondrocytes and at the between the and newly formed woven bone (Figs. and closely after of the is and the formation of new vascular structures which infiltrate with The resident chondrocytes begin to undergo cell or and the is replaced by woven bone down by the osteoblasts (Fig. the fracture site is with new bone which will be by bone which is from While the process by which fracture repair is well relatively is the of the events leading to repair. is the process can leading to of delayed and repair. It is that fracture repair involves the of cellular and differentiation and that these events are likely to be regulated signaling via growth factors The major growth factors as present in the fracture site are and and and FGF and Given the well described roles of these factors in embryonic bone and in on bone cells, these molecules are likely to be of fracture repair. It is interesting to note that the signaling by these molecules into those signaling and and those receptors and The is signal transduction a in which activity is induced by by The of a is the that this facilitates between signaling and the to the signal cascade the cell after the initial (Fig. the between the signaling of the major growth factors and cytokines involved in fracture repair. involved in and are not The are a of by a high to The FGF expressed in The most in normal tissues are FGF and also and and are expressed in a variety of tissues and are in the bone presumably to their for The FGF of molecules to the via a of receptors with have been five FGF receptors to The FGF receptors for of the of the FGF and are expressed in a variety of tissues including no have the FGF profile during the fracture repair process. with many of the of the activity in response to additional may be to the the of FGF by or in These molecules may by the to the thus the complex and the downstream The major downstream signaling cascades include the associated and (Fig. and can be detected in the early stages of fracture repair in the tissue at the fracture and other inflammatory cells and this is the likely of FGF in the are expressed by mesenchymal cells, chondrocytes and osteoblasts and have been demonstrated to TGF-β expression in osteoblastic are also expressed by The as on a variety of mesenchymal cells including chondrocytes and osteoblasts In the of the of this to be on This is by the expression profile of that during In addition, in a fracture of into fracture during the first 9 days post fracture associated with an increased callus of which to increased proliferation of biomechanical of fractures but preliminary data a of fracture with This result the that fracture callus is a of fracture repair and the major role of in proliferation. is and has been in fracture repair is expressed by osteoblasts and has also been detected in the of the growth plate during development, an that suggests a role in and endochondral bone formation In addition to and also bone These that has the to influence many of fracture repair, from early events to remodeling of the callus. This is by the in In a a of associated with an early in callus This to the of periosteal cell proliferation and associated with increased mechanical at post also associated with a of the soft that at post fracture no in mechanical between fractures with or with In a fracture at the of fracture to the fracture site of normal and via In these increased the volume and the of the callus and the mechanical of the healing results have been by in a formation of an and as a to in the of most other growth factors, to be a between fracture callus and mechanical It is to that the of fibroblast and proliferation by result in within the callus and mechanical This studies have that fracture repair in A of into days after fracture no on callus or of bone and on day not in this and it remains that the of not optimal for It is also of that a with days after fracture in callus formation, in studies are within 24 h after fracture, an in callus is This suggests that the for with FGF may be limited to the early post fracture of with this in normal a of the proliferation or of mesenchymal cells around within 3 This response associated with a significant and in bone but only after post In this and in may be a induction of mesenchymal cells, which is followed by a significant of cell increased bone formation or repair are PDGF is a of and PDGF can as a or a to the levels of a of in cells in which both are The PDGF their on cells by with to as the and The PDGF receptors are by and subsequent of the PDGF a one molecules The receptors also in that the will an or but the only the or and all leads to which the signal transduction molecules with PDGF their 2 demonstrated as to with PDGF and include the of and signal and of of the (Fig. PDGF is released by degranulating platelets in the fracture and may be in promoting In this early is and most cells to be the PDGF is also expressed by macrophages that into the fracture site in response to the fracture trauma and initial release of PDGF by in fracture repair, PDGF is detectable in both early and mature this is only suggesting the is the in these PDGF in include a in PDGF or in a and into PDGF associated with a in callus days post as as still These PDGF also associated with an to normal This a small number of and these results The by which PDGF influence fracture repair have to be One demonstrated that PDGF the bone induced by in a The for this to a significant of soft tissue repair with which may have the of on These studies on the for PDGF to influence bone formation and on in the full cascade of events for fracture repair. The TGF-β molecules are of a large of factors referred to as the TGF-β This not only the TGF-β but also the the growth and differentiation factors or and several other proteins are in all the and the of the TGF-β are as large which are to mature These conserved molecules play roles in cell differentiation and proliferation during and to play a variety of roles in the of TGF-β have been to and have the most fracture repair, and for may be referred to as TGF-β. TGF-β signaling involves TGF-β and cells within the fracture site as well as in the body TGF-β receptors on their The of downstream is to which of the receptors is associated with the TGF-β an of followed by the of a into the also in an resulting in a complex associated with the The has a active which the in the thus the The is for the downstream of the signal the signal of the downstream of molecules is characteristic of the TGF-β receptors (Fig. Given the of the signaling and the for between of the TGF-β of several of which will be discussed in the context of the fracture repair process, a on signaling molecules is TGF-β is a growth factor released by the degranulating platelets in the and by the bone at the fracture In the initial stages of fracture repair, TGF-β can be to the region of the hard callus it the region of periosteal proliferation and intramembranous bone suggests that TGF-β is likely to be involved in the of proliferation by the preosteoblasts in this In addition, the expression of TGF-β is elevated during chondrogenesis and bone formation with an initial in levels detected around day post fracture followed by a at day TGF-β expression by day 14 and remains elevated The of TGF-β expression with the in and the subsequent with TGF-β is to be a of undifferentiated mesenchymal cell and proliferation and production during chondrogenesis and endochondral bone formation TGF-β may also be involved in the normal of bone formation with The role of or of growth in normal fracture repair is to the of TGF-β to this process is by the of TGF-β to fracture repair in several evidence for the of TGF-β from studies on repair in the A of TGF-β induced of that with While healing in this via intramembranous bone formation, this the first of a growth factor bone repair in the of bone morphogenetic The of TGF-β to bone healing first demonstrated in in with a of the site with high of TGF-β for in a in callus volume and increased mechanical with In a fracture TGF-β around the fracture site 2 days during a healing TGF-β increased the of the and mechanical demonstrated a load in fractures with the high of TGF-β. The results of these studies be to the or into the fracture site have limited and studies have the healing of fractures after a of into the callus days after by or and healing 14 days post This cartilaginous callus to but is not to cortical not in this of associated with significant around the site. In fractures, the high of an in callus volume but not the bone of the callus. In fractures, not callus volume or bone but the of the callus. The result is with callus with and the for surgical of fractures growth factors are to be to studies involving a at the of have also to significant bone healing in with a of the of bone and in with not in without In a or in a of and and to a callus to be and to in with this callus response not associated with union or of the which at not also demonstrated that not those The results of these studies that the of TGF-β to fracture repair may persistent or high The are a of the TGF-β of are from other of the by in conserved in the mature The play roles in regulating cell and in a variety of cells during development, including osteoblasts and with have on bone and also have results in of fracture The of by the by that bone the to bone formation in an are of this that can be into several on signal transduction by a to the other of the TGF-β (Fig. can with several including and and as well as the receptors and with the to the and this leads to of the complex with an forming an active This can be by the of and which can and activity by This of and has been specifically demonstrated in osteoblastic The expression of the receptors and is increased in cells of the periosteum the of the fracture in the early post fracture and levels are also elevated in and mature chondrocytes in day 7 but are not highly expressed in is with and but at The is also expressed in and mature A is only expressed around the signaling involves a complex in addition to the of expressed during fracture repair. as with the is the of signal for a high of between by of the TGF-β of (Fig. During fracture repair, the to be expressed include and and are closely to the Drosophila and signal a have demonstrated that are expressed in the early stages of fracture repair it is likely that small are released from the of the bone During intramembranous bone formation, cells in the cambium of the periosteum may respond to this initial low of release from the and begin levels in cells in this and an in detectable the fracture in the cambium region of the In this it that are the cells to their differentiation into mature for and is around the but is in mesenchymal cells as they infiltrate into the fracture site and By days post fracture, the expression of is maximal in chondroid chondrocytes and osteoblasts only levels of expression not detectable in day 14 The current of the role of in fracture repair is that these molecules are of differentiation in and mesenchymal cells to osteoblasts and This by specifically is by the molecules and which have been demonstrated to with these cells expression is expression in chondrocytes and osteoblasts during their of formation, and to low levels during callus It is interesting to note that mature osteoblasts and chondrocytes not significant levels of in normal they both have increased expression in fracture repair. The studies on fracture repair of from bone These and include bone without bone is an in promoting the union of large in the have also been in with bone to repair large for involved to union in with In one union in all with a of and of to these to respond to an of surgical at In a into in with and union in 24 of While these early data are the of a the of studies with have significant evidence that are of the union of large that to in the of In the addition of to a significant in new bone formation within not In this without or with significant bone formation in the indicating for at in the of also as in an with a to union or evidence of new bone formation, the addition of to the associated with of the by of the that the of these to that of the results have been with a with a or with no no significant bone formation or during an induced within The mechanical of the with to that of has to be in a number of The first demonstrated that new bone formation in the of The of associated with union in of with a to demonstrated that the of with to that of the has also been demonstrated to union in In one a in the of that with an bone or with in nonunion. with the bone all union within 3 bone that not include also This result that the levels of in bone are to bone formation and that union bone or mechanical that to the also demonstrated to bone formation in a In this union in all with a high of a low in union in the without to result in union in all that those that union after and that at as as has been in a a with demonstrated a induction of bone both within the and within the biomechanical not of on of mechanical has also been to the healing of in the of studies be with biomechanical as one to the of will have to that with or are a of proteins that as signaling for the TGF-β of signal their as a of the of the and Drosophila the first of this conserved of have a structure that a conserved and by a conserved are of the into including the and and the and TGF-β signaling the and are and The the by the thus initiating the of the with the This complex is to the it by with other factors and (Fig. Given with of the and is for signaling it is likely that this an role in the of simultaneously by of the TGF-β While the for signal transduction growth factors and their associated receptors are well still no of these factors expressed in a context can a complex process as fracture repair. is it these growth factors with the signaling by cytokines and their associated is a that the initiation of fracture repair may by the cytokines expressed as of the initial trauma response and that and are expressed early in the fracture repair process but no comprehensive has been of the spatiotemporal profile of the cytokines expressed in the fracture site during the repair cytokine receptors is a of research that has a large body of of the nature of these signaling for the most not in their and these molecules not under one of the These receptors molecules that with the of the receptors for signal In the of these include the of These molecules are indirectly by and signal by the downstream These molecules to the and in this may be also involved in downstream by the PDGF it has been demonstrated that can be a between and activity suggesting between by the and those from TGF-β (Fig. of between signaling involved in fracture repair is the FGF and PDGF in signal this cascade via of the of the TGF-β associated has been to be by TGF-β and and is evidence that of the the growth factor or growth factor in the region to the These data an active of between the and the the signaling cascades (Fig. In addition, is evidence that IL-1 and signaling also the signaling an signaling with similarities to as well as (Fig. While the is only one of several signaling these this of in the signaling from growth factor and cytokine receptors is an site for from the cell surface (Fig. the volume of data on the of fracture repair and the molecules involved in a number of complex still to be not a of which of the molecules are for fracture repair and which are is a of cells are these factors and which the receptors and signal transduction to respond to their is on which factors may the of the discussed to One in which this is evident is in signaling and by and molecules that or growth factor and cytokine are likely to the fracture repair process, and the of these molecules is as as the growth factors this review has focused on growth factors and cytokines and their signaling associated however, one also that for many of these molecules are and of the process of fracture repair. remains to be a of fracture repair can be The for the photomicrographs and in this

Yazarların özeti; kaynağından alınmıştır. Journal of Bone and Mineral Research, 1999 · DOI ↗