Insulin‐like growth factor‐I is expressed by avian flexor tendon cells

Abstract

Cells in normal tendon are in a resting G0 state, performing maintenance functions. However, traumatic injury introduces growth factors such as platelet-derived growth factor and insulin-like growth factor from blood as well as activates endogenous growth factors. These factors stimulate migration and proliferation of tendon cells at the wound area. Tendon cells require growth-promoting factors to transit the cell cycle. To evaluate the contribution of endogenous growth factors in tendon, extracts of the epitenon and internal compartment of avian flexor tendon as well as medium of cultured cells from the epitenon (tendon surface cells) and internal tendon (tendon internal fibroblasts) were collected to assess their ability to stimulate DNA synthesis. Acid-ethanol extracts of tissues and medium were chromatographed on a P-30 molecular sieve column and assayed for mitogenic activity by quantitating [3H]thymidine incorporation into tendon cell DNA. The extract from the internal tendon compartment was more stimulatory for DNA synthesis than that from the epitenon, particularly when tested on tendon internal fibroblasts. However, conditioned medium fractions from surface epitenon cells stimulated DNA synthesis to a high degree on both tendon surface cells and tendon internal fibroblasts. Conditioned medium from tendon internal fibroblasts was also stimulatory. An anti-insulin-like growth factor-I antibody ablated most of the mitogenic activity present in both tissues and conditioned medium. The levels of acid-extractable insulin-like growth factor-I in tendon were determined by competitive radioimmunoassay as 1.48+/-0.05 ng/g tissue for the epitenon and 3.83+/-0.03 ng/g tissue for the internal compartment. Results of Western immunoblots of conditioned medium revealed insulin-like growth factor-I at the 7.5 kDa position. Cultured tendon surface cells and tendon internal fibroblasts as well as cells in intact flexor tendon expressed insulin-like growth factor-I mRNA detected by reverse transcriptase-polymerase chain reaction. In situ hybridization histochemistry positively identified insulin-like growth factor-I mRNA in tendons from 52-day-old chickens. Platelet-derived growth factor was not detected at the protein or message levels. Furthermore, tendon surface cells and tendon internal fibroblasts both expressed receptors for insulin-like growth factor-I detected by flow cytometry. These data suggest that tendon cells express insulin-like growth factor-I mRNA and synthesize insulin-like growth factor-I in both the epitenon and the internal compartment of tendon, which is present in an inactive form, most likely bound to insulin-like growth factor-binding proteins.

Effect of estrogen on tendon collagen synthesis, tendon structural characteristics, and biomechanical properties in postmenopausal women

“…Although speculative, such an explanation could be supported by the fact that IGF-I was reduced in ERT users, whereas IGFBPs were unchanged, which indicates a lower IGF-I bioavailability, as observed by others (6,8,27). It is well documented, at least in young animals, that IGF-I exerts anabolic effects on tendon fibroblasts by increasing collagen synthesis in a dose-dependent manner (2,43,45,56). In support of this, a lower tendon and muscle collagen FSR, and at the same time a markedly lower bioavailability of IGF-I within these tissues, have been observed in young oral contraceptive users compared with control (21).…”

“…These findings suggest a differential effect of estrogen in bone and tendons and estrogen in combination with mechanical loading. The effect may either be direct or an indirect effect of estrogen on other growth factors and cytokines known to influence collagen turnover, such as insulin-like growth factor-I (IGF-I) (2,6,8,43,56).…”

Expression of insulin‐like growth factor binding proteins in healing tendon lesions

“…11,12 The IGF system plays an important role in tendon healing. Previous studies have shown that IGF-I is synthesized locally in the tendon, 13,14 and that its expression in the tendon increases in response to vibration trauma, mechanical load, and tissue injury. 1,15,16 The patterns of expression of the IGFBPs in the healing tendon are currently unknown; however, given their vital regulatory and independent functions, it would seem important to better understand the role of this component of the IGF system in tendon healing.…”

Histological and molecular analysis of the biceps tendon long head post‐tenotomy

“…6,17,18 MMP-3, a broad spectrum proteolyte that cleaves collagen and proteoglycan (PG) is often found to be down-regulated in tendinopathy, a potential explanation for the increased proteoglycan content observed in tendinopathic samples. 6,18 In addition to matrix degrading enzymes, several growth factors such as TGF-b, 19 IL-6, 20 and insulin-like growth factor-I (IGF-I) 21 are produced in tendon and are responsible for induction of ECM production. While the specific products of growth-factor-mediated ECM deposition are unknown, IGF-I is thought to function in the modulation of collagen synthesis in response to mechanical loading.…”