Mechanical stimulation by intermittent hydrostatic compression promotes bone-specific gene expression in vitro

Mechanical stimulation of osteopontin mRNA expression and synthesis in bone cell cultures

We have shown earlier that mechanical stimulation by intermittent hydrostatic compression (IHC) promotes alkaline phosphatase and procollagen type I gene expression in calvarial bone cells. The bone matrix glycoprotein osteopontin (OPN) is considered to be important in bone matrix metabolism and cell-matrix interactions, but its role is unknown. Here we examined the effects of IHC (13 kPa) on OPN mRNA expression and synthesis in primary calvarial cell cultures and the osteoblast-like cell line MC3T3-E1. OPN mRNA expression declined during control culture of primary calvarial cells, but not MC3T3-E1 cells. IHC upregulated OPN mRNA expression in late released osteoblastic cell cultures, but not in early released osteoprogenitor-like cells. Also, in both proliferating and differentiating MC3T3-E1 cells, OPN mRNA expression and synthesis were enhanced by IHC, differentiating cells being more responsive than proliferating cells. These results suggest a role for OPN in the reaction of bone cells to mechanical stimuli. The severe loss of OPN expression in primary bone cells cultured without mechanical stimulation suggests that disuse conditions down-regulate the differentiated osteoblastic phenotype.

Demonstration of feasibility of automated osteoblastic line culture in space flight

There is a large body of evidence that microgravity- or immobilization-induced bone loss is mainly related to osteoblastic cell impairment. Osteoblasts are sensitive to increased mechanical stress and could therefore be responsible for unloading-induced bone changes. However, the nature of osteoblast involvement remains unclear. The effects of the space environment on cells have been studied extensively, but little information about anchorage-dependent cell cultures of the 25 different cell types flown in space has been published. We studied the effects of long-term weightlessness on the cell shape of cultured osteoblasts during the Russian Bion 10 space-flight. This experiment required the development of special automatic culture devices (the plunger-box culture system) finalized with the constructors. Multiple feasibility experiments were performed to allow osteoblast culture for 6 days in microgravity. The study revealed plunger-box biocompatibility; optimization of ROS 17/2.8 (mammalian adherent cells) culture under closed conditions (without gas exchange); and transport of viable cells for 5 days. During the 6 days of microgravity, the growth curves of ground controls and cells in space were roughly similar. Alkaline phosphatase activity was enhanced twofold in microgravity. ROS 17/2.8 cell morphology began to change significantly after 4 days of microgravity; they became rounder and covered with microvilli. At the end of the flight, the cells exhibited mixed morphological types, piling cells, stellar shape, and spread out cells, resembling ground controls or 1g flight controls (centrifuge). We demonstrated that ROS 17/2.8 cells were viable during a 6 day automatic culture in space and were sensitive to space related conditions. They adapted their structure and function to this environment, characterized by loss of mechanical stimuli.

Pulsating fluid flow stimulates prostaglandin release and inducible prostaglandin G/H synthase mRNA expression in primary mouse bone cells

Bone tissue responds to mechanical stress with adaptive changes in mass and structure. Mechanical stress produces flow of fluid in the osteocyte lacunar-canalicular network, which is likely the physiological signal for bone cell adaptive responses. We examined the effects of 1 h pulsating fluid flow (PFF; 0.7 +/- 0.02 Pa, 5 Hz) on prostaglandin (PG) E2, PGI2, and PGF2alpha production and on the expression of the constitutive and inducible prostaglandin G/H synthases, PGHS-1, and PGHS-2, the major enzymes in the conversion of arachidonic acid to prostaglandins, using mouse calvarial bone cell cultures. PFF treatment stimulated the release of all three prostaglandins under 2% serum conditions, but with a different time course and to a different extent. PGF2alpha was rapidly increased 5-10 minutes after the onset of PFF. PGE2 release increased somewhat more slowly (significant after 10 minutes), but continued throughout 60 minutes of treatment. The response of PGI2 was the slowest, and only significant after 30 and 60 minutes of treatment. In addition, PFF induced the expression of PGHS-2 but not PGHS-1. One hour of PFF treatment increased PGHS-2 mRNA expression about 2-fold relative to the induction by 2% fresh serum given at the start of PFF. When the addition of fresh serum was reduced to 0.1%, the induction of PGHS-2 was 8- to 9-fold in PFF-treated cells relative to controls. This up-regulation continued for at least 1 h after PFF removal. PFF also markedly increased PGHS activity, measured as the conversion of arachidonic acid into PGE2. One hour after PFF removal, the production of all three prostaglandins was still enhanced. These results suggest that prostaglandins are important early mediators of the response of bone cells to mechanical stress. Prostaglandin up-regulation is associated with an induction of PGHS-2 enzyme mRNA, which may subsequently provide a means for amplifying the cellular response to mechanical stress.