Effects of intermittent parathyroid hormone treatment on osteoprogenitor cells in postmenopausal women

Abstract

Intermittent parathyroid hormone (PTH) 1-34 treatment stimulates bone formation, but the molecular mechanisms mediating this effect have not been previously studied in humans. Thus, we used magnetic activated cell sorting to isolate hematopoietic lineage negative (lin-)/alkaline phosphatase positive (AP+) osteoprogenitor cells from bone marrow of 20 postmenopausal women treated with PTH (1-34) for 14 days and 19 control subjects. Serum PINP and CTX increased in PTH-treated subjects (by 97% and 30%, respectively, P<0.001). Bone marrow lin-/AP+ cells from PTH-treated subjects showed an increase in the RANKL/OPG mRNA ratio (by 7.5-fold, P=0.011) and in the mRNAs for c-fos (a known PTH-responsive gene, by 42%, P=0.035) and VEGF-C (by 57%, P=0.046). Gene Set Enrichment Analysis (GSEA, testing for changes in pre-specified pathways) demonstrated that PTH had no effect on osteoblast proliferation, apoptosis, or differentiation markers. However, PTH treatment resulted in a significant decrease (GSEA P-value, 0.005) in a panel of BMP target genes in the lin-/AP+ cells. Our findings thus identify several future directions for studying mechanisms of PTH action in humans. First, given the increasing evidence that PTH induces angiogenesis, the role of increased VEGF-C production by bone marrow osteoprogenitor cells in mediating this effect and the anabolic response to PTH warrants further study. Second, while the observed inhibition of BMP target gene expression by PTH is not consistent with the anabolic effects of PTH on bone and requires further validation, these data do generate the hypothesis that an inhibition of BMP signaling by PTH may, over time, limit the availability of mature osteoblasts on bone surfaces and thereby contribute to the observed waning of the anabolic response to PTH.

Figure 1

Percent change from baseline in (A) bone formation markers (PINP, osteocalcin) and (B) bone resorption markers (CTX, TRAP5b) in the two groups of study subjects. Data are means and SEMs. ^*P < 0.05; ^***P < 0.001 versus baseline. P-values for comparison of changes in each marker between groups are as indicated.

Figure 2

Differences in BMP target genes in lin−/AP+ cells from control versus PTH treated subjects. Data are median and interquartile (25^th–75^th percentile) ranges. The numerical P-value is by the GSEA analysis, which provides a more robust means to analyze gene expression data and increases the power of detecting changes in genes occurring in pre-specified clusters, rather than in isolation (see Methods). ^**P < 0.01; ^*P < 0.05; ^†P = 0.07 versus control.

Figure 3

Expression of c-fos, IFNγ, IL-18, and latent TGFβ binding protein 1 in lin−/AP+ cells from control versus PTH treated subjects. Data are median and interquartile (25^th–75^th percentile) ranges. ^*P < 0.05; ^aP = 0.07; ^bP = 0.08 versus control.