Conditioned medium differentially regulates matrix protein gene expression in cells of the intervertebral disc

Study Design. Matrix protein gene expression was determined for cells of the anulus fibrosus (AF) and nucleus pulposus (NP) regions of the intervertebral disc when cultured in AF or NP cell-conditioned medium. Objectives. To investigate changes in mRNA levels for type I collagen, type II collagen and aggrecan in cells of the AF and NP in response to cell-conditioned medium. Summary of Background Data. Cells of the intervertebral disc have been shown to respond to exogenous soluble mediators such as the growth factors TGF-beta and IGF-1. Little is known of their biologic response to endogenous factors that may be secreted locally or by cells of neighboring regions. Methods. Porcine cells were cultured for 48 hours in alginate gel in the presence or absence of conditioned medium. Gene expression for aggrecan and collagens was quantified using real-time reverse transcriptase polymerase chain reaction. Results. AF cell gene expression was generally stimulated by the conditioned medium of either AF or NP cells. In contrast, the notochordal cell-containing NP cells showed little change in gene expression with either source of conditioned medium. Conclusions. Cells of the NP and AF secrete soluble factors in culture at similarly effective doses to stimulate matrix protein gene expression in AF cells of the intervertebral disc. Unlike AF cells, however, NP cell gene expression was not stimulated by any conditioned medium, suggesting that differences exist in the responsiveness of cells of notochordal (NP) and fibrocartilaginous (AF) phenotypes. Understanding these differences between cells of the intervertebral disc may reveal unique stimulatory factors important to repair and regeneration of the degenerated intervertebral disc.