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CLINOSTAT ROTATION CULTURE MODULATES GENE EXPRESSION OF OSTEOCLASTOGENESIS-REGULATING FACTORS VIA A CYCLIC-AMP DEPENDENT MECHANISM.    M. Kanematsu¹, H.Takai², M.Takaoki¹ and A.Sato¹.  ¹National Space Development Agency of Japan, Ibaraki, Japan.  ²National Institute for Longevity Science, Aichi, Japan.

  Bone loss observed in astronauts may be induced by an acceleration of osteoclastic bone resorption as well as a decline of osteoblastic bone formation.  Recently, an osteoclastogenesis inhibitory factor (OPG) and an osteoclast differentiation factor (RANKL) were cloned and identified as the products for osteoblasts/bone marrow stromal cells.  In the last year, we reported that gene expression level of RANKL was increased while that of OPG was decreased revealed by northern analysis in mouse bone marrow-derived stromal cell line (ST2) cultured on a single axis clinostat.  The modulation was not due to alteration in mRNA stability.  The present study was undertaken to clarify the mechanism how the modulation of RANKL and OPG gene expressions observed in clinostat culture was induced.

  Osteotropic factors such as PGE₂ and PTH has been shown to up-regulate RANKL gene expression, and the effect was suggested to be mediated by cAMP.  The clinostat culture caused an increase in the intracellular cAMP level in ST2 cells.  Both of forskolin, an intracellular cAMP elevating agent, and db-cAMP mimicked the modulation of RANKL and OPG gene expression in clinostat culture.  The modulation of these gene expressions in clinostat culture was blocked by a PKA inhibitor (H89).  The modulation was not blocked by a cyclooxygenase inhibitor. 

  Our results showed that clinostat culture increase RANKL while decrease OPG gene expression via a cAMP dependent mechanism.  This modulation was not mediated by an autocrine loop for PGE₂.