FAK, SMAD and MAPK Pathways Diverge During Osteogenic and Chondrogenic Differentiation of Adipose-Derived Mesenchymal Cells

Adipose-derived mesenchymal cells (AMCs) are a promising cell source for orthopaedic tissue engineering applications due to their accessibility and multi-lineage potential [1]. However, future use in bone and cartilage regeneration requires a comprehensive understanding of the pathways driving AMCs to osteogenic and chondrogenic lineages. We have previously demonstrated the dual function of a single medium containing bone morphogenetic protein-6 (BMP-6) on differentiation of AMCs; in the presence of BMP-6, monolayer culture induces osteogenic differentiation while pellet culture stimulates chondrogenesis [2]. Additionally, BMP-6 has been demonstrated to be both osteogenic and chondrogenic on marrow-derived stem cells [3,4], but the mechanisms driving the effect of BMP-6 in these conditions remains poorly understood. Recent studies have implicated focal adhesion kinase (FAK) and cell/matrix attachment in directing osteogenesis, while an absence of these signals support chondrogenesis [5,6]. We hypothesized that the focal adhesions present in monolayer prompt different pathway activation than in pellet culture, resulting in either osteogenic or chondrogenic differentiation of AMCs in response to BMP-6. Our goal was to elucidate the cellular mechanisms employed by BMP-6 during differentiation. Therefore, we examined the activation of the FAK, MAP kinase (MAPK) pathways, and the canonical BMP-6 pathway via SMAD signaling in both monolayer and pellet culture.