Fibroblast Growth Factor-2 Maintains the Differentiation Potential of Nucleus Pulposus Cells In Vitro: Implications for Cell-Based Transplantation Therapy

Study Design. To investigate effects of FGF-2 on nucleus pulposus cell growth and differentiation. Objectives. To elucidate the phenotypic changes that occur during expansion of nucleus pulposus cells in monolayer culture, and to investigate the effects of fibroblast growth factor (FGF)-2 on cell growth and differentiation. Summary of Background Data. Nucleus pulposus cells would have a limited application for autologous cell transplantation if phenotypic dedifferentiation takes place during culture expansion. FGF-2 has been shown to retain the differentiation potential of monolayer expanded chondrocytic cells. However, its effect on nucleus pulposus cells is not known. Methods. Bovine nucleus pulposus cells were serially passaged in the presence or absence of FGF-2 (1 and 10 ng/mL). After passage numbers 1 and 7, cells were immobilized in alginate beads and treated with transforming growth factor (TGF)-&bgr;1 for 1 week to assess their differentiation. Results. During culture expansion in monolayer, nucleus pulposus cells maintained the expression of aggrecan messenger ribonucleic acid (mRNA). However, mRNA levels of collagen type I, collagen type II, Sox-9, and versican decreased with increasing passage number for both control (untreated) cells and FGF-2 treated cells. When grown in alginate with TFG-&bgr;1, passage 7 cells that received FGF-2 during culture expansion restored the mRNA expression of type II collagen, Sox-9, COMP, chondroadherin, and fibromodulin. Moreover, FGF-2 treatment resulted in increased sulfated proteoglycan synthesis and lower aggrecan turnover compared to untreated controls under identical culture conditions. FGF-2 treated cells continued to express HIF-1&agr; protein till passage 7, while MMP-2 expression was evident in cells treated with TGF-&bgr;1. In addition, cells pretreated with FGF-2 showedhigher induction of phospho ERK1/2 after treatment withTGF-&bgr;1. Also, FGF-2 maintained smad 2/smad 3 mediatedsignaling in cells after TGF-&bgr; treatment. FGF-2 action resulted in reduced actin stress fiber formation and migratory cell morphology, with no effect on cell proliferation. Conclusions. The presence of FGF-2 during culture expansion of nucleus pulposus cells in monolayer can sustain a differentiated cell phenotype by maintaining responsiveness to TGF-&bgr;1. Our results suggest that FGF-2 should be tested for its ability to maintain the reactivity of the nucleus pulposus cells to other morphogenic factors that may be used for cell-based transplantation therapy.

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