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

Cells in normal tendon are in a resting Go state, performing maintenance fu nctions. However, traumatic injury introduces growth factors such as platel et-derived growth factor and insulin-like growth factor from blood as well as activates endogenous growth factors. These factors stimulate migration a nd proliferation of tendon cells at the wound area. Tendon cells require gr owth-promoting factors to transit the cell cycle. To evaluate the contribut ion of endogenous growth factors in tendon, extracts of the epitenon and in ternal compartment of avian flexor tendon as well as medium of cultured cel ls from the epitenon (tendon surface cells) and internal tendon (tendon int ernal 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 qua ntitating [H-3]thymidine incorporation into tendon cell DNA. The extract fr om the internal tendon compartment was more stimulatory for DNA synthesis t han that from the epitenon, particularly when tested on tendon internal fib roblasts. 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 fibro blasts was also stimulatory. An anti-insulin-like growth factor-I antibody ablated most of the mitogenic activity present in both tissues and conditio ned 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 co mpartment. Results of Western immunoblots of conditioned medium revealed in sulin-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 te ndon expressed insulin-like growth factor-I mRNA detected by reverse transc riptase-polymerase chain reaction. In situ hybridization histochemistry pos itively identified insulin-like growth factor-I mRNA in tendons from 52-day -old chickens. Platelet-derived growth factor was not detected at the prote in or message levels. Furthermore, tendon surface cells and tendon internal fibroblasts both expressed receptors for insulin-like growth factor-I dete cted by flow cytometry. These data suggest that tendon cells express insuli n-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-bindin g proteins.