Insulin-like growth factor-I maintains the phenotype of rat endplate chondrocytes in vitro via distinct signaling pathways

The goal of this study was to investigate whether the degeneration of rat endplate chondrocytes during in vitro monolayer culture is able to imitate that in vivo and whether insulin-like growth factor-I (IGF-I) is able to slow down this process. Rats were divided into three groups according to their age (young, adult, and older rats). From passage-2 (P2), the chondrocytes of young rats were passaged every third day, finally reaching P6, irrespective of whether IGF-I was used. Three cell staining methods were performed to examine the cell morphology and extracellular matrix. Chondrocyte protein and gene expression was examined by Western blot and immunofluorescence, and reverse-transcription quantitative PCR, respectively. The results indicated that cartilaginous end plates underwent age-associated degenerative changes. 40 ng/ml IGF-I was found to be the optimal concentration for enhancing the expression of COL-2A in endplate chondrocytes. With increasing passage, chondrocytes in monolayer cultures exhibited significant phenotypic and morphological changes. However, IGF-I maintained the chondrocyte phenotype and activated ERK and AKT signaling pathways during in vitro monolayer culture. Of note, the increased quantity of phosphorylated ERK and AKT was mainly located in the nucleus. Therefore, the degeneration of chondrocytes cultured in vitro, to a certain extent, may serve as a model of that caused in chondrocytes by age in vivo, and IGF-I delays the aging process, probably mediated by the ERK and AKT signaling pathways.