Role of angiocrine signals in bone development, homeostasis and disease

Skeletal vasculature plays a central role in the maintenance of microenvironments for osteogenesis and haematopoiesis. In addition to supplying oxygen and nutrients, vasculature provides a number of inductive factors termed as angiocrine signals. Blood vessels drive recruitment of osteoblast precursors and bone formation during development. Angiogenesis is indispensable for bone repair and regeneration. Dysregulation of the angiocrine crosstalk is a hallmark of ageing and pathobiological conditions in the skeletal system. The skeletal vascular bed is complex, heterogeneous and characterized by distinct capillary subtypes (type H and type L), which exhibit differential expression of angiocrine factors. Furthermore, distinct blood vessel subtypes with differential angiocrine profiles differentially regulate osteogenesis and haematopoiesis, and drive disease states in the skeletal system. This review provides an overview of the role of angiocrine signals in bone during homeostasis and disease.

Extrahepatic 25-Hydroxylation of Vitamin D3 in an Engineered Osteoblast Precursor Cell Line Exploring the Influence on Cellular Proliferation and Matrix Maturation during Bone Development

Osteoblastic precursors experience distinct stages during differentiation and bone development, which include proliferation, extracellular matrix (ECM) maturation, and ECM mineralization. It is well known that vitamin D plays a large role in the regulation of bone mineralization and homeostasis via the endocrine system. The activation of vitamin D requires two sequential hydroxylation steps, first in the kidney and then in the liver, in order to carry out its role in calcium homeostasis. Recent research has demonstrated that human-derived mesenchymal stem cells (MSCs) and osteoblasts can metabolize the immediate vitamin D precursor 25-dihydroxyvitamin D3 (25OHD3) to the active steroid 1α,25-dihydroxyvitamin D3 (1,25OH2D3) and elicit an osteogenic response. However, reports of extrahepatic metabolism of vitamin D3, the parental vitamin D precursor, have been limited. In this study, we investigated whether osteoblast precursors have the capacity to convert vitamin D3 to 1,25OH2D3 and examined the potential of vitamin D3 to induce 1,25OH2D3 associated biological activities in osteoblast precursors. It was demonstrated that the engineered osteoblast precursor derived from human marrow (OPC1) is capable of metabolizing vitamin D3 to 1,25OH2D3 in a dose-dependent manner. It was also demonstrated that administration of vitamin D3 leads to the increase in alkaline phosphatase (ALP) activity associated with osteoblast ECM maturation and calcium deposits and a decrease in cellular proliferation in both osteoblast precursor cell lines OPC1 and MC3T3-E1. These findings provide a two-dimensional culture foundation for future three-dimensional engineered tissue studies using the OPC1 cell line.

Leukemogenic Transformation of Hematopoietic Cells by Constitutive Activation of Canonical Wnt Signaling in Osteoblast Precursors

Abstract 509 Osteoblasts, the bone forming cells, are implicated in the fate of healthy and malignant stem cells. They affect self-renewal and expansion of hematopoietic stem cells (HSCs) and homing of tumor cells into the bone marrow. Here we show that constitutive activation of canonical Wnt signaling in osteoblast precursors disrupts hematopoiesis in mice by shifting the differentiation potential of HSC progenitors to the myeloid lineage which results in accumulation of granulocyte/monocyte progenitors and concomitant development of acute myeloid leukemia (AML). B-lymphopoiesis is also decreased. The AML phenotype is associated with clonal evolution at the cytogenetic level since clonal abnormalities could be detected in leukemic blasts from mice with constitutive activation of the canonical Wnt target β-catenin in osteoblast precursors (βcateninosb mice). Bone marrow transplantation experiments from βcateninosb mice to wild type lethally irradiated mice resulted in development of AML within 8 weeks following transplantation, demonstrating progression towards AML. At the molecular level, cell-specific gene inactivation mouse models demonstrate that β-catenin interacts with FoxO1 in osteoblasts to induce development of AML. Downstream signaling events that confer osteoblast signaling to normal HSCs and lead to their leukemogenic transformation will be presented. Importantly, malignancy-inducing osteoblasts, detected by nuclear accumulation of β-catenin in bone marrow biopsies, were identified in > 25% of patients with myelodysplasia (MDS), acute myeloid leukemia (AML) or AML arising from a prior MDS. Specifically, 15 out of 53 patients with MDS (n=17 patients), AML (n=20 patients), or MDS that had transformed to AML (n=16) chosen at random showed nuclear localization of β-catenin in osteoblasts. Of note, 12 of the 15 (80%) patients with nuclear localization of β-catenin in osteoblasts had abnormalities of chromosome 5 and/or 7, very common cytogenetic abnormalities in patients with MDS and AML. The same signaling pathways mediating AML development in βcateninosb mice were also found to be activated in osteoblasts and hematopoietic cells from the patients with nuclear accumulation of β-catenin in osteoblasts. These findings demonstrate that genetic alterations in osteoblast precursors (1) can induce AML in mice and (2) are associated with AML development in humans. They also provide a molecular basis for the leukemogenic transformation. Disclosures: No relevant conflicts of interest to declare.