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Parathyroid hormone inhibits mitogen-activated protein kinase activation in osteosarcoma cells via a protein kinase
1Hubrecht Laboratory, Netherlands Institute for Developmental Biology, Vtrecht.
Abstract:
Osteoblast-like cells, such as UMR 106 osteosarcoma cells, are known to be growth stimulated by growth factors such as EGF. In contrast, factors such as PTH and prostaglandin E2 inhibit their growth. The exact signal transduction mechanisms by which these latter factors act remain to be elucidated. Here we show that simultaneous treatment of UMR 106 cells with EGF and PTH-(1-34) resulted in a level of DNA synthesis intermediate between the levels of treatment with epidermal growth factor (EGF) and PTH alone. This correlated with the interference of PTH-(1-34) early in an EGF receptor-linked signal transduction pathway, i.e. the EGF-induced activation of p42 mitogen-activated protein (MAP) kinase. This effect was also found for prostaglandin E2, and could be potentiated by the phosphodiesterase inhibitor isobutyl-methylxanthine and mimicked by forskolin and 8-bromo-cAMP. There was a strict correlation between the lowest concentration of PTH-(1-34) required to enhance protein kinase A (PKA) activity and that required to inhibit MAP kinase activation, whereas saturating amounts of PTH-(3-34), a PTH analog unable to elevate PKA activity, had no effect. Lysophosphatidic acid- and 12-O-tetracanoylphorbol-13-acetate-induced MAP kinase activation were also inhibited by PTH-(1-34) and forskolin in these cells. Similar effects were seen on basic fibroblast growth factor-mediated MAP kinase activation in ROS 17/2.8 cells, indicating that this mechanism is a general feature of PTH in osteosarcoma cells. The inhibition of this mitogenic pathway through activation of PKA might play an important role in PTH-induced changes in proliferation and differentiation of osteoblasts.
Insights
Parathyroid hormone (PTH) and prostaglandin E2 inhibit osteosarcoma cell growth by interfering with epidermal growth factor (EGF)-induced signaling pathways, specifically blocking mitogen-activated protein kinase (MAP) activation through protein kinase A (PKA).
Area of Science:
- Cell Biology
- Molecular Signaling
- Endocrinology
Background:
- Osteoblast-like cells, including UMR 106 osteosarcoma cells, exhibit growth responses to various factors.
- Epidermal growth factor (EGF) stimulates cell growth, while parathyroid hormone (PTH) and prostaglandin E2 (PGE2) inhibit it.
- The precise signal transduction mechanisms underlying PTH and PGE2-mediated growth inhibition are not fully understood.
Purpose of the Study:
- To investigate the signal transduction pathways involved in PTH and PGE2 inhibition of osteosarcoma cell growth.
- To elucidate how PTH-(1-34) interferes with EGF-induced signaling.
- To determine the role of protein kinase A (PKA) in mediating these inhibitory effects.
Main Methods:
- Treatment of UMR 106 cells with EGF, PTH-(1-34), PGE2, forskolin, and various analogs.
- Measurement of DNA synthesis levels.
- Assessment of mitogen-activated protein (MAP) kinase activation.
- Evaluation of protein kinase A (PKA) activity.
Main Results:
- Simultaneous treatment with EGF and PTH-(1-34) resulted in intermediate DNA synthesis levels.
- PTH-(1-34) inhibited EGF-induced p42 MAP kinase activation, an effect mimicked by PGE2.
- Inhibition of MAP kinase activation correlated strictly with enhanced PKA activity, and PTH analogs lacking PKA-stimulating capacity did not inhibit MAP kinase.
- PTH-(1-34) and forskolin also inhibited lysophosphatidic acid-, 12-O-tetracanoylphorbol-13-acetate-, and basic fibroblast growth factor-mediated MAP kinase activation in osteosarcoma cells.
Conclusions:
- Parathyroid hormone (PTH) and prostaglandin E2 inhibit EGF receptor-linked signaling pathways in osteosarcoma cells.
- The inhibition of MAP kinase activation by PTH is mediated through the activation of protein kinase A (PKA).
- PKA-mediated inhibition of this mitogenic pathway likely plays a significant role in PTH-induced alterations in osteoblast proliferation and differentiation.