Determination of Dual Effects of Parathyroid Hormone on Skeletal Gene Expression in Vivo by Microarray and Network Analysis

Abstract

Parathyroid hormone (PTH) stimulates bone formation when injected daily but causes severe bone loss with continuous infusion. The mechanism of its paradoxical effects is still elusive. In this study, we compared changes in the gene expression profile in bone induced by intermittent or continuous treatment with three different PTH peptides, PTH-(1-34), -(1-31), and -(3-34), in Sprague-Dawley female rats. PTH-(1-34) regulated numerous genes (ϳ1,000), but differentially, in both regimes. PTH-(1-31) regulated a similar number of genes in the intermittent regimen but fewer in the continuous regimen, consistent with its less potent catabolic effect. PTH-(3-34) regulated very few genes in both regimes, which suggests the protein kinase C pathway plays a limited role in mediating the dual effects of PTH, whereas the cAMPdependent protein kinase A pathway appears to predominate. In the intermittent treatment, many genes encoding signaling mediators, transcription factors, cytokines, and proteases/ protease inhibitors are regulated rapidly and cyclically with each PTH injection; genes associated with skeletal development show a slowly accruing pattern of expression. With continuous treatment, some genes are regulated from 6 h, and the mRNA levels are sustained with a longer infusion, whereas others show a kinetic decrease and then increase later. Significant up-regulation of genes stimulating osteoclastogenesis in the anabolic regime suggests a provocative and paradoxical theme for the anabolic effect of PTH that a full anabolic response requires a transient up-regulation of genes classically associated with a resorptive response. Ingenuity pathway analysis was performed on the microarray data. A novel signaling network was established that is differentially regulated in the two PTH treatment regimes. Key regulators are suggested to be AREG, CCL2, WNT4, and cAMP-responsive element modulator. PTH,2 administered intermittently, stimulates bone formation in contrast to antiresorptive agents, which reduce bone resorption. Concerns with the limited improvement of antiresorptive treatments in severe osteoporosis led to a search for alternative anabolic agents (1). Teraparatide, recombinant human parathyroid hormone consisting of the first 34 of the 84 amino acids of human parathyroid hormone, has been shown to reduce significantly the risk of both vertebral and nonvertebral fractures in postmenopausal women and to significantly increase bone mineral density (2). It has become the only anabolic agent currently approved for the treatment of osteoporosis in the United States.Bone is a highly specialized form of connective tissue and dynamic organ in all higher vertebrates; it undergoes continuous regeneration. Bone remodeling occurs at discrete sites within the skeleton and proceeds in an orderly fashion, with coupling of osteoclastic resorption and osteoblastic formation (3). This physiological process is coordinated and tightly regulated by local and endocrine factors to ensure that there is a balance between bone formation and ...

Parathyroid hormone regulates the distribution and osteoclastogenic potential of hematopoietic progenitors in the bone marrow

“…It also may inhibit apoptosis in mature osteoblasts, which might prolong the anabolic effects of PTH. (12)(13)(14)(15)(16)(17)(18)(19)30) However, to fully appreciate the responses of bone to PTH, it is also necessary to understand how PTH administration affects other type of cells in the bone microenvironment that are implicated in bone turnover. Osteoclasts are another potential target of PTH action because the physiologic activities of osteoblasts and osteoclasts are tightly coupled.…”

“…Continuous administration of PTH induces catabolic effects on bone, whereas intermittent PTH administration exerts anabolic effects. (12)(13)(14)(15)(16)(17)(18)(19) Currently, once-daily injection of human PTH is approved by the Food and Drug Administration as treatment for osteoporosis in the United States. In addition, it could have potential for the treatment of other conditions that require new bone formation.…”

Proteoglycan 4, a Novel Immunomodulatory Factor, Regulates Parathyroid Hormone Actions on Hematopoietic Cells

“…With the exception of specific PPRexpressing T-cell subpopulations, 6,7 PTH actions on hematopoiesis are thought to be mediated via paracrine or juxtacrine signaling from PPR-activated stromal osteoblastic cells. 1,2,8 PPR signaling in stromal osteoblastic cells alters the expression of osteoclastic cytokines, most notably receptor activator of nuclear factor-B ligand (RANKL) and osteoprotegerin (OPG), which regulate the differentiation of hematopoietic cells to mature bone resorbing osteoclastic cells. 8 -12 More recent studies have focused on the role of PTH actions on HPCs.…”

Building bone with a SOST-PTH partnership

“…A number of microarray studies of PTH-treated whole bone or osteoblasts have identified thousands of genes regulated by PTH. (21)(22)(23) Many of these genes have been identified to play a role in PTH anabolic action in studies of knockout mice, including transcription factors ATF4 (18) and CREM (24) ; signaling molecules such as b-arrestin (25) ; cytokines such as fibroblast growth factor 2 (FGF2), (16) interleukin 18 (IL-18), (15) and insulin-like growth factor 1 (IGF-1) (26) ; matrix proteins including osteonectin (27) ; and other factors that regulate Wnt pathway signaling including secreted Frizzled-related protein 1 (sFRP-1). (28,29) Surprisingly, given the results in SOST transgenic mice, anabolic treatment with PTH was just as effective in the absence of low density lipoprotein receptorrelated protein 5 (LRP5) as in wild-type mice.…”