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PROTEIN AND VESICLE TRAFFICKING, CYTOSKELETON

Influence of increased mechanical loading by hypergravity on the microtubule cytoskeleton and prostaglandin E₂ release in primary osteoblasts

¹Life Sciences Division, National Aeronautics and Space Administration Ames Research Center, Moffett Field; ²Department of Mechanical Engineering, Stanford University, Stanford; and ³Department of Stomatology, University of California, San Francisco, San Francisco, California

Submitted 22 November 2003 ; accepted in final form 16 February 2005

Cells respond to a wide range of mechanical stimuli such as fluid shear and strain, although the contribution of gravity to cell structure and function is not understood. We hypothesized that bone-forming osteoblasts are sensitive to increased mechanical loading by hypergravity. A centrifuge suitable for cell culture was developed and validated, and then primary cultures of fetal rat calvarial osteoblasts at various stages of differentiation were mechanically loaded using hypergravity. We measured microtubule network morphology as well as release of the paracrine factor prostaglandin E₂ (PGE₂). In immature osteoblasts, a stimulus of 10x gravity (10 g) for 3 h increased PGE₂ 2.5-fold and decreased microtubule network height 1.12-fold without affecting cell viability. Hypergravity (3 h) caused dose-dependent (5–50 g) increases in PGE₂ (5.3-fold at 50 g) and decreases (1.26-fold at 50 g) in microtubule network height. PGE₂ release depended on duration but not orientation of the hypergravity load. As osteoblasts differentiated, sensitivity to hypergravity declined. We conclude that primary osteoblasts demonstrate dose- and duration-dependent sensitivity to gravitational loading, which appears to be blunted in mature osteoblasts.

mechanotransduction; differentiation; bone