Parathyroid hormone induces differentiation of mesenchymal stromal/stem cells by enhancing bone morphogenetic protein signaling

Abstract

Parathyroid hormone (PTH) stimulates bone remodeling and induces differentiation of bone marrow mesenchymal stromal/stem cells (MSCs) by orchestrating activities of local factors such as bone morphogenetic proteins (BMPs). The activity and specificity of different BMP ligands are controlled by various extracellular antagonists that prevent binding of BMPs to their receptors. Low-density lipoprotein receptor-related protein 6 (LRP6) has been shown to interact with both the PTH and BMP extracellular signaling pathways by forming a complex with PTH1R and sharing common antagonists with BMPs. We hypothesized that PTH-enhanced differentiation of MSCs into the osteoblast lineage through enhancement of BMP signaling occurs by modifying the extracellular antagonist network via LRP6. In vitro studies using multiple cell lines, including Sca-1+CD45−CD11b− MSCs showed that a single injection of PTH enhanced phosphorylation of Smad1 and could also antagonize the inhibitory effect of noggin. PTH treatment induced endocytosis of a PTH1R/LRP6 complex and resulted in enhancement of phosphorylation of Smad1 that was abrogated by deletion of PTH1R, β-arrestin, or chlorpromazine. Deletion of LRP6 alone lead to enhancement of pSmad1 levels that could not be further increased with PTH treatment. Finally, knockdown of LRP6 increased the exposure of endogenous cell-surface BMPRII significantly in C2C12 cells and PTH treatment significantly enhanced cell surface binding of 125I-BMP2 in a dose- and time-dependent manner, implying that LRP6 organizes an extracellular network of BMP antagonists that prevent access of BMPs to BMP receptors. In vivo studies in C57BL/6J mice and of transplanted GFP-labeled Sca-1+CD45−CD11b− MSCs into bone marrow cavity of Rag2−/− immunodeficient mice showed PTH-enhanced phosphorylation of Smad1 and increased commitment of MSCs to osteoblast lineage, respectively. These data demonstrate that PTH-enhancement of MSCs differentiation to the osteoblast lineage occurs through a PTH and LRP6 dependent pathway by endocytosis of LRP6/PTH1R complex, allowing enhancement of BMP signaling.

Transient Increased Calcium and Calcitriol Requirements After Discontinuation of Human Synthetic Parathyroid Hormone 1-34 (hPTH 1-34) Replacement Therapy in Hypoparathyroidism

“…(14) When PTH is used intermittently as a treatment for osteoporosis, it similarly stimulates bone turnover increasing the amount of younger, less mineralized bone. (15) In contrast to hyperparathyroidism, however, this increased turnover seen with intermittent administration appears to favor anabolism, rather than catabolism, stimulating osteoblast progenitors, (16,17) inhibiting osteoblast apoptosis (18) and increasing bone density at the lumbar spine and hip while decreasing BMD at the distal radius. (19,20) When PTH therapy for osteoporosis is discontinued, spinal and hip gains in BMD are gradually lost and radial BMD increases.…”

Bone marrow mesenchymal stem cells and TGF-β signaling in bone remodeling

“…Thus, the fate of MSCs and other cells in the microenvironment are indirectly regulated by PTH to integrate systemic control of bone remodeling (Figure 1 and refs. [7][8][9][10][11][12][13][14][15].…”

“…Endocytosis of growth factors and GPCRs is known to integrate different signaling pathways (49). PTH binding to PTH1R can induce endocytosis of a PTH1R-LRP6 complex, resulting in enhancement of downstream BMP-phospho-SMAD1 signaling, which ultimately enhances the number of MSCs differentiating into the osteoblast lineage (15). Specifically deleting Lrp6 in mature murine osteoblasts results in a loss of anabolic response to PTH (50).…”

“…Improving the osteogenic microenvironment may help restore coupling by enhancing the osteogenic potential of MSCs during the reversal phase of bone remodeling, and therefore may be another potential therapeutic target. PTH modifies the bone marrow microenvironment by orchestrating the signaling of local factors for bone remodeling, reducing ROS, and stimulating Wnt signaling in the bones of old mice (7)(8)(9)(10)(11)(12)(13)(14)(15)(16). PTH is an FDA-approved anabolic therapy for osteoporosis, and daily injection of PTH increases bone formation with normal microarchitecture by coupling bone remodeling (7)(8)(9)(10)(11)(12)(13)(14)(15).…”

“…During osteoclastmediated bone resorption, multiple factors are released from the bone matrix and/or secreted locally, creating an osteogenic microenvironment that promotes MSC recruitment and osteoblast differentiation for new bone formation (3)(4)(5)(6). Parathyroid hormone (PTH), the systemic hormone that regulates calcium homeostasis, plays a major role in orchestrating bone remodeling by modulating the bone marrow microenvironment and regulating osteogenic signaling pathways (7)(8)(9)(10)(11)(12)(13)(14)(15)(16). Latent TGF-β1, which is abundant in the bone matrix, is released and activated by osteoclasts to specifically induce migration of MSCs to bone-resorptive sites (17)(18)(19).…”