Analysis of signaling pathways used by parathyroid hormone to activate the c-fos gene in human SaOS2 osteoblast-like cells

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Abstract
We have evaluated the signaling pathways activated by parathyroid hormone (PTH) in SaOS2 human osteoblast-like cells correlating with induction of the c-fos proto-oncogene. Human PTH (1–34) (hPTH[1–34]) and hPTH(1–34) Nle8,18 Tyr34 induced the expression of c-fos mRNA in quiescent SaOS2 cells in a concentration-dependent manner. N-terminal truncations of hPTH(1–34) that fail to activate protein kinase A (PKA) also abolished c-fos mRNA induction. In gel retardation assays hPTH(1–34) led to a change in the mobility of specific, cyclic adenosine monophosphate (cAMP) response element binding protein (CREB)-containing protein-DNA complexes identical to that caused by other activators of PKA. The appearance of this altered mobility complex correlated temporally with the induction of c-fos mRNA. Using a c-fos serum response element probe, a slowed protein DNA complex appeared upon serum, epidermal growth factor, and basic fibroblast growth factor treatment. This slowed complex reflects phosphorylation of the transcription factor ternary complex factor (TCF) mediated via activation of the mitogen-activated protein (MAP) kinase pathway. The MAP kinase cascade is also activated by protein kinase C (PKC), and treatment with phorbol ester led to the induced TCF shift. In contrast, PTH did not produce this shift, ruling out PTH activation of c-fos via PKC and the MAP kinase signaling cascade. Further evidence for this was the lack of effect of the highly selective PKC inhibitor CGP 41251 on c-fos induction by hPTH(1–34). The janus kinase (JAK)/signal transducers and activators of transcription (STAT) signaling cascade targets the v-sis-inducible element in the c-fos promoter via the induced binding of STATs. Interferon γ rapidly induced STAT binding in SaOS2 cells, unlike PTH. Thus, PTH induction of c-fos transcription appears to occur principally through activation of PKA that then targets CREB and the c-fos calcium/cAMP response element.