Exogenous insulin-like growth factor-I stimulates an autoinductive IGF-I autocrine/paracrine response in chondrocytes

The modulation of insulin-like growth factor-I (IGF-I) gene expression by chondrocytes following exogenous IGF-I supplementation of culture was assessed to examine the hypothesis that constitutive IGF-I mRNA activity is suppressed by exogenous administration of IGF-I to cartilage in situ. Chondrocytes in monolayer culture were treated with 0, 10 or 100 ng/ml IGF-I for 48 h and resultant IGF-I and matrix gene expression patterns were assessed by quantitative polymerase chain reaction (qPCR) and northern blotting, respectively. Effective translation of proteoglycan (PG) as a response to IGF-I was determined by dimethylmethylene blue (DMMB) dye-binding assay. To determine the temporal nature of the IGF-I autocrine/paracrine response to exogenous IGF-I, chondrocyte cultures were treated with 100 ng/ml IGF-I and the IGF-I mRNA response was assessed at 0, 4, 12, 24, 48 and 72 h. Significant increases in chondrocyte density and in PG synthesis occurred during treatment of chondrocyte cultures with 10 and 100 ng/ml IGF-I. Persistent exposure of chondrocytes to 100 ng/ml IGF-I resulted in maximal IGF-I mRNA response at 24 h, with declining message accumulation at 48 and 72 h. These data suggest that IGF-I induces an autoinductive IGF-I autocrine/paracrine transcriptional response. The clinical ramifications of these findings include support for the use of exogenous IGF-I for cartilage repair where it could conceivably amplify and extend the effect of exogenous IGF-I beyond the transitory persistence of supplemental IGF-I ligand in repair constructs.