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Impact of the mitogen-activated protein kinase pathway on parathyroid hormone-related protein actions in osteoblasts

Chen Chen1, Amy J Koh, Nabanita S Datta

  • 1Department of Periodontics Prevention Geriatrics, University of Michigan, Ann Arbor, Michigan 48109, USA.

The Journal of Biological Chemistry
|May 7, 2004
PubMed

Insights

Parathyroid hormone-related protein (PTHrP) signaling in osteoblasts involves the MAPK pathway, which selectively regulates gene expression and cell differentiation. Inhibition of MAPK impacts PTHrP

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Endocrinology

Background:

  • Parathyroid hormone-related protein (PTHrP) is crucial for osteoblastic cell proliferation and differentiation, primarily through the parathyroid hormone receptor (PTH-1R).
  • While the cAMP-dependent protein kinase A pathway mediates many PTHrP effects, the involvement of the MAPK pathway is increasingly recognized.

Purpose of the Study:

  • To investigate the specific role of the MAPK pathway in mediating the effects of PTHrP on osteoblastic cells (MC3T3-E1 subclone 4).
  • To elucidate the molecular mechanisms by which PTHrP, via MAPK signaling, influences gene expression related to osteoblast differentiation.

Main Methods:

  • Treatment of MC3T3-E1 subclone 4 cells with PTHrP and the MAPK inhibitor U0126.
  • Analysis of gene expression for osteocalcin, bone sialoprotein, PTH-1R, c-fos, fra-2, and interleukin-6 (IL-6) using quantitative PCR.
  • Luciferase promoter assays to assess the transcriptional regulation of IL-6, involving activator protein-1 (AP-1) sites.
  • Investigation of signaling pathways including cAMP/Epac/Rap1/MAPK and CREB phosphorylation.

Main Results:

  • PTHrP and MAPK inhibition both down-regulated osteocalcin and bone sialoprotein gene expression in differentiated MC4 cells.
  • PTHrP-mediated regulation of PTH-1R and c-fos mRNA was MAPK-independent, while fra-2 and IL-6 mRNA regulation was MAPK-dependent.
  • IL-6 regulation involved both cAMP-dependent protein kinase A and MAPK pathways, requiring an AP-1 site for transcription.
  • PTHrP exhibited bidirectional effects on ERK phosphorylation: increasing it in undifferentiated cells and decreasing it in differentiated cells.

Conclusions:

  • The MAPK pathway plays a selective but significant role in PTHrP actions on osteoblasts.
  • MAPK inactivation differentially regulates PTHrP-stimulated AP-1 members, inhibits PTHrP-stimulated IL-6, and synergizes with PTHrP to down-regulate osteoblastic differentiation markers.
  • These findings highlight the complex interplay between PTHrP, MAPK, and cAMP signaling in osteoblast biology.

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