Signaling in human osteoblasts by extracellular nucleotides - Their weak induction of the c-fos proto-oncogene via Ca2+ mobilization is strongly potentiated by a parathyroid hormone/cAMP-dependent protein kinase pathway independently of mitogen-activated protein kinase

Extracellular nucleotides acting through specific P2 receptors activate int racellular signaling cascades. Consistent with the expression of G protein- coupled P2Y receptors in skeletal tissue, the human osteosarcoma cell line SaOS-2 and primary osteoblasts express P2Y(1) and P2Y(2) receptors, respect ively. Their activation by nucleotide agonists (ADP and ATP for P2Y(1); ATP and UTP for P2Y(2)) elevates [Ca2+](i) and moderately induces expression o f the c-fos proto-oncogene. A synergistic effect on c-fos induction is obse rved by combining ATP and parathyroid hormone, a key bone cell regulator. P arathyroid hormone elevates intracellular cAMP levels and correspondingly a ctivates a stably integrated reporter gene driven by the Ca2+/cAMP-responsi ve element of the human c-fos promoter. Nucleotides have little effect on e ither cAMP levels or this reporter, instead activating luciferase controlle d by the full c-fos promoter. This induction is reproduced by a stably inte grated serum response element reporter independently of mitogen-activated p rotein kinase activation and ternary complex factor phosphorylation. This n ovel example of synergy between the cAMP-dependent protein kinase/CaCRE sig naling module and a non-mitogen-activated protein kinase/ternary complex fa ctor pathway that targets the serum response element shows that extracellul ar ATP, via P2Y receptors, can potentiate strong responses to ubiquitous gr owth and differentiative factors.