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Department of Medicine, McGill University Health Center, Montreal, Quebec H3A 1A1, Canada
Parathyroid hormone
(PTH)-related peptide (PTHrP) can modulate the proliferation and
differentiation of a number of cell types including osteoblasts. PTHrP
can activate a G protein-coupled PTH/PTHrP receptor, which can
interface with several second-messenger systems. In the current study,
we have examined the signaling pathways involved in stimulated type I
collagen and alkaline phosphatase expression in the human
osteoblast-derived osteosarcoma cells, MG-63. By use of Northern
blotting and histochemical analysis, maximum induction of these two
markers of osteoblast differentiation occurred after 8 h of
treatment with 100 nM PTHrP-(1-34). Chemical inhibitors of adenylate cyclase (H-89) or of protein kinase C (chelerythrine chloride) each diminished PTHrP-mediated type I collagen
and alkaline phosphatase stimulation in a dose-dependent manner. These
effects of PTHrP could also be blocked by inhibiting the
Ras-mitogen-activated protein kinase (MAPK) pathway with a Ras
farnesylation inhibitor, B1086, or with a MAPK inhibitor, PD-98059.
Transient transfection of MG-63 cells with a mutant form of G
, which
can sequester 
-subunits, showed significant downregulation of
PTHrP-stimulated type I collagen expression, as did inhibition of
phosphatidylinositol 3-kinase (PI 3-kinase) by wortmannin.
Consequently, the 
-PI 3-kinase pathway may be involved in PTHrP
stimulation of Ras. Collectively, these results demonstrate that,
acting via its G protein-coupled receptor, PTHrP can induce indexes of
osteoblast differentiation by utilizing multiple, perhaps parallel,
signaling pathways.
parathyroid hormone-related peptide; osteoblast; cell differentiation; hypercalcemia
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