Interaction of basic fibroblast growth factor with bovine growth plate chondrocytes

The integrated processes of hard tissue regeneration with special emphasis on fracture healing

When bone is fractured, a sequence of dynamic events ensue to restore form and therefore function. Many key biologic cell regulators for these events have been identified, expressed through recombinant technology, and their roles posited. Moreover, the availability of recombinantly engineered molecules, such as the bone morphogenetic proteins with their potential to benefit patient care, has ushered in an important era in clinical dentistry that may eliminate either autografting or bank bone allografts. Therefore, in this review article, we have highlighted some of the exciting biologic regulators relevant to bone fracture healing and outlined the dynamic elements in this process.

Basic fibroblast growth factor: an autocrine growth factor for epiphyseal growth plate chondrocytes

Basic fibroblast growth factor (bFGF) is a permissive mitogen for cultured chondrocytes and has been localized in the specific zones of the epiphyseal growth plate. In this study, we demonstrate that bFGF present in cartilage originates from within the cellular constituents of this tissue. Utilizing reverse transcription coupled to the polymerase chain reaction (PCR), bFGF mRNA was found in extracts of cartilage tissue. Immunocytochemical studies revealed that bFGF was present intracellularly in freshly isolated proliferative chondrocytes and in the extracellular matrix (ECM) after 24 h of culture. Western blot analysis of protein extracts from isolated proliferative chondrocytes identified a bFGF immunoreactive species with a molecular weight of approximately 18 kDa. In situ hybridization confirmed the presence of bFGF mRNA in freshly isolated proliferative chondrocytes. The bFGF in the ECM seemed to be sequestered and not available for biological activity, since these cells still required exogenous bFGF for cell proliferation. This sequestered bFGF could be released to stimulate cell proliferation when cultures were treated with plasmin, a proteolytic enzyme. These data support the hypothesis that bFGF is synthesized by chondrocytes and functions as an autocrine/paracrine mitogen via its deposition into the ECM with subsequent release from the ECM of cartilage being a critical step in biological activity. In addition, the study provides further evidence that locally produced bFGF plays an important role in normal growth and development of cartilage tissue.

Differential effects of serum, insulin-like growth factor-I, and fibroblast growth factor-2 on the maintenance of cartilage physical properties during long-term culture

The effects of fetal bovine serum, insulin-like growth factor-I, and fibroblast growth factor-2 on the regulation of the functional physical properties of adult bovine cartilage explants during an incubation period of 18-20 days was determined, and the relationship between the measured functional properties of the cartilage and the tissue composition was assessed. Cartilage disks were tested in the uniaxial radially confined configuration by the application of low amplitude oscillatory displacement and measurement of the resultant load and streaming potential. For the control cartilage terminated just after explant, the modulus was 0.39 +/- 0.28 MPa, the open circuit hydraulic permeability was 2.0 +/- 1.0 x 10(-15) m2/(Pa.sec), and the electrokinetic (streaming potential) coefficient was -2.3 +/- 0.6 mV/MPa. Incubation of cartilage in medium supplemented with serum or insulin-like growth factor-I resulted in maintenance of the modulus and electrokinetic coefficient, whereas incubation in basal medium or medium supplemented with fibroblast growth factor-2 led to a marked decrease from control values in the modulus and the amplitude of the electrokinetic coefficient. All of the culture conditions examined resulted in an increase in permeability that was not statistically significant. The variation in the electromechanical properties of all the cartilage samples tested was related to the density of tissue proteoglycan and collagen (hydroxyproline). The modulus was correlated with both the density of tissue proteoglycan (+0.014 MPa/[mg/ml]) and the density of tissue hydroxyproline (+0.008 MPa/[mg/ml]). The electrokinetic coefficient was also correlated with the density of proteoglycan (-0.080 [mV/MPa]/[mg/ml]) and the density of hydroxyproline (+0.064 [mV/MPa]/[mg/ml]). These data indicate that the regulation of chondrocyte matrix metabolism by growth factors can significantly affect the physical properties and function of cartilage.

Fractionation of growth plate chondrocytes: differential expression of IGF-I and growth hormone and IGF-I receptor mRNA in purified populations

In vitro studies of growth plate cell kinetics have been hindered by the spatial arrangement and heterogeneity of cells within the plate. In this study, we describe a fractionation method that consistently generated five relatively pure populations of growth plate chondrocytes. Each fraction exhibited morphology, proliferative rates, and marker mRNA expression consistent with in vivo positional phenotypes. In characterizing the fractional response, fibroblast growth factor was most effective in stimulating resting cells to proliferate and least effective on cells actively dividing (fraction 3). Insulin-like growth factor-I (IGF-I) was most active on fraction 3 while epidermal growth factor's mitogenic induction was equivalent across all fractions. Growth hormone receptor (GHR) mRNA was most abundant in mature hypertrophic cells and undetectable in resting cells; IGF-I receptor (IGF-IR) mRNA was detectable in resting cells but two-fold higher in the fraction adjacent to cells possessing high GHR mRNA, while proliferating and resting chondrocytes had elevated IGF-I mRNA levels when compared to that for hypertrophic chondrocytes. The growth plate distribution of IGF-IR and GHR mRNA implies distinct roles for circulating IGF-I vs. paracrine produced IGF-I.

Mechanism of longitudinal bone growth and its regulation by growth plate chondrocytes

Growth plate chondrocytes play a pivotal role in promoting longitudinal bone growth. The current review represents a brief survey of the phenomena involved in this process at the cellular level; it delineates the contributions made by various activities during the course of the chondrocyte life cycle, notably proliferation and hypertrophy, and illustrates how the relative contributions may be modulated according to the particular needs of an organism at critical phases of growth. The cellular mechanisms by which a few well characterized growth-promoting substances exert their influences are discussed in the light of recent findings pertaining to epiphyseal plate chondrocytes in vivo.

Regulation of growth-plate chondrocytes by insulin-like growth-factor I and basic fibroblast growth factor

A study was performed in order to investigate the possible functional roles of insulin-like growth-factor I (IGFI) and basic fibroblast growth factor (bFGF) in the regulation of mitotic and metabolic activity of growth-plate chondrocytes. Chondrocytes from the distal radial growth plates of calves and the costal physeal cartilage of rats were exposed to these factors, individually and in combination, in primary monolayer culture, to assess their effects. The data showed that bFGF had both a greater potency and a greater efficacy as a mitogen for bovine growth-plate chondrocytes than did IGF-I. The maximum incorporation of 3H-thymidine by bFGF was 8.3 times that in serum-free (control) cultures; the maximum stimulation of incorporation by IGF-I was 2.5 times that in the control medium. In contrast, IGF-I stimulated a maximum incorporation of 35S-sulfate into glycosaminoglycans that was 2.6 times that in the IGF-I serum-free control cultures, while bFGF had no effect or was mildly inhibitory. When used together, these two factors acted synergistically. Incorporation of 3H-thymidine was more than two times greater than the sum of the effects of the growth factors when used alone and 20.5 times greater than that of the growth factor-free control cultures. Physeal chondrocytes from six-day-old rats were mitotically more responsive to bFGF than to IGF-I, but they were more responsive to IGF-I when they had been derived from twenty-eight-day-old rats. Interaction between bFGF and factors in the serum enhanced the mitotic activity of the rat chondrocytes, but bFGF did not interact with IGF-I under the same experimental conditions. In the presence of bFGF, there was a reduction in the stimulation by IGF-I of incorporation of 35S-sulfate and a decrease in the percentage of chondrocytes containing alkaline phosphatase. These growth factors also influenced cellular morphology in culture. In the presence of IGF-I or serum, the rat chondrocytes manifested the polygonal morphology typical of chondrocytes in culture, while bFGF promoted a more elongated spindle shape. Removal of bFGF and replacement by IGF-I restored the polygonal morphology, indicating that this transition is reversible.