IGF axis expression and activity in differentiating dental pulp cells

The insulin-like growth factor (IGF) axis comprises two growth factors (IGF-I and IGF-II), two cell surface receptors (IGF1R and IGF2R), six high affinity soluble IGF binding proteins (IGFBP1-6) and various families of IGFBP proteases. The IGF axis acts co-ordinately to control several cellular processes including mitogenesis, apoptosis, cell migration and differentiation. In this latter respect the IGFs are the most abundant growth factors present in bone matrix and the IGF axis is believed to play an important role in the differentiation of osteoblast precursors and also to control bone accretion and resorption which occurs throughout adult life. There is much interest in the use of mesenchymal stem cells (MSCs) as a resource for tissue engineering approaches in the restoration of bone and other hard tissue lesions. As such a thorough knowledge of the effects of the IGF axis on osteogenic differentiation is essential to the success of such tissue engineering approaches. In our laboratories we use dental pulp/stromal cells (DPCs) as a source of precursor cells which can be differentiated to an osteogenic phenotype by culture under appropriate conditions. In order to address some of the issues raised above we have examined the expression and activity of the IGF axis in DPCs which have been induced to differentiate to an osteogenic/matrix mineralising phenotype in vitro. We found that DPCs express all components of the IGF axis (except IGF-I and IGFBP-1) under both basal and osteogenic conditions. With respect to IGFBPs we report that IGFBP protein concentrations in DPC conditioned medium closely follow mRNA levels prepared from DPCs. IGFBP-4 is the most abundant IGFBP in both basal and osteogenic DPCs and levels are not altered between basal and osteogenic cultures. IGFBP-4 and -5 inhibit IG-I and IGF-II stimulated osteogenic differentiation in DPCs. Of some interest although IGFBP-4 expression is not altered by differentiation of DPCs IGFBP-4 proteolysis is enhanced under osteogenic conditions. Subsequently we found that pregnancy associated plasma protein-A (PAPP-A) is the sole IGFBP-4 protease in DPC conditioned medium and its concentration is increased under osteogenic conditions. Further IGF-II (an activator of PAPP-A) and stanniocalcin-2 (STC2- an inhibitor of PAPP-A) concentrations are respectively increased and decreased during osteogenic differentiation of DPCs. We suggest that our data may represent a series of coordinated changes in IGF axis expression and activity which represent a novel osteogenic “signature” of differentiating DPCs.