Oxygen tension is an important mediator of the transformation of osteoblasts to osteocytes

Abstract

Osteocytes are derived from osteoblasts, but reside in the mineralized bone matrix under hypoxic conditions. Osteocyte-like cells show higher expression of ORP150, which is induced by hypoxia, than osteoblast-like cells. Accordingly, we hypothesized that the oxygen tension may regulate the transformation of osteoblasts to osteocytes. MC3T3-E1 cells and calvariae from 4-day-old mice were cultured under normoxic (20% O(2)) or hypoxic (5% O(2)) conditions. To investigate osteoblastic differentiation and tranformation to osteocytes, alizarin red staining was done and the expression of various factors was assessed. Hypoxic culture promoted the increased synthesis of mineralized matrix by MC3T3-E1 cells. Alkaline phosphatase activity was initially increased during hypoxic culture, but decreased during osteogenesis. Osteocalcin production was also increased by hypoxic culture, but decreased after mineralization. Furthermore, expression of Dmp1, Mepe, Fgf23, and Cx43, which are osteocyte-specific or osteocyte-predominant proteins, by MC3T3-E1 cells was greater under hypoxic than under normoxic conditions. In mouse calvarial cultures, the number of cells in the bone matrix and cells expressing Dmp1 and Mepe were increased by hypoxia. In MC3T3-E1 cell cultures, ORP150 expression was only detected in the mineralized nodules under normoxic conditions, while its expression was diffuse under hypoxic conditions, suggesting that the nodules were hypoxic zones even in normoxic cultures. These findings suggest that a low oxygen tension promotes osteoblastic differentiation and subsequent transformation to osteocytes.

Hypoxia mediates osteocyte ORP150 expression and cell death in vitro

“…The ORP150 protein levels were increased in MLO-Y4 osteocytes compared with MC3T3-E1 pre-osteoblasts grown under normoxic conditions. These observations are in agreement with previous studies demonstrating that low oxygen tension serves an important role in the differentiation of osteoblasts into osteocytes (45,56). Guo et al (30) demonstrated that a greater amount of hypoxia-associated proteins in osteocytes (such as ORP150), compared with in osteoblasts.…”

“…A concentration of approximately 5% O 2 is more likely to be physiological for osteocytes than a concentration of 20%O 2 , as osteocytes are embedded deep inside the mineralized bone matrix and their nutrient availability is greatly dependent on diffusion (43)(44)(45). The actual concentration of oxygen inside of the osteocyte lacunae during skeletal unloading remains unknown.…”

Osteogenic differences in cultured rat periosteal cells under hypoxic and normal conditions

“…Generally, it is known that the cell proliferation ratio changes according to the type of hypoxic condition that the cells are placed in (25)(26)(27)(28)(29)(30). Also, the hypoxic condition affects the cell kinetics even when the same cells are used in the condition.…”

223Ra and other bone-targeting radiopharmaceuticals—the translation of radiation biology into clinical practice

“…19 This may be particularly true in bone metastases since, physiologically, bone is a relatively hypoxic tissue. 20 In clonogenic experiments, this effect can be expressed as the OER, that is, the ratio of dose required to kill a given fraction of cells in hypoxic vs oxic conditions. Various investigators have demonstrated that OER decreases with increasing LET, with Barensden 9 finding that OER reached 1 with LET of 165 keV mm 21 .…”