Diverse effects of cyclic AMP variants on osteogenic and adipogenic differentiation of human mesenchymal stromal cells

Osteogenic differentiation of human mesenchymal stromal cells (hMSCs) may potentially be used in cell based bone tissue engineering applications to enhance the bone forming potential of these cells. Osteogenic and adipogenic differentiation are thought to be mutually exclusive and, although several signaling pathways and cues that induce osteogenic or adipogenic differentiation respectively have been identified, there is no general consensus on how to optimally differentiate hMSCs into the osteogenic lineage. Some pathways have also been reported to be involved in both adipogenic and osteogenic differentiation, as for example the protein kinase A (PKA) pathway, and the aim of this study was to investigate the role of cAMP / PKA signaling in differentiation of hMSCs in more detail. We show that activation of this pathway with dibutyryl-cAMP results in enhanced alkaline phosphatase expression whereas another cAMP analogue induces adipogenesis in long-term mineralization cultures. Adipogenic differentiation, induced by 8-bromo-cAMP, was accompanied by stronger PKA activity and higher expression of cAMP-responsive genes, suggesting that stronger activation correlates with adipogenic differentiation. In addition, whole genome expression analysis showed an increase in expression of adipogenic genes in 8-br-cAMP-treated cells. Furthermore, by means of qPCR, we show differences in peroxisome proliferator activated receptor-γ (PPARγ) activation, either alone, or in combination with dexamethasone, thus demonstrating differential effects of the PKA pathway, most likely depending on its mode of activation.