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Short-term hypergravity does not affect protein-ubiquitination and proliferation in rat L6 myoblastic cells.     K. Hirasaka1, Y. Onishi1, H. Furouchi1, J. Goto1, R. Nakao1, T. Shimazu2, K. Kishi1 and T. Nikawa1 

1Dept of Nutrition, Univ of Tokushima School of Medicine, Tokushima, JAPAN;
2Japan Space Fourum, Tokyo, JAPAN.
   We previously reported that spaceflight (STS-90) and tail-suspension stimulated muscle protein ubiquitination and accumulated the degradation
fragments.  However, in space experiments the side-effects of hypergravity on samples are inevitable during the launch of a space shuttle into space or  the reentry.  To examine whether hypergravity also caused  protein-ubiquitination in skeletal muscle cells, we exposed rat myoblastic
L6 cells to various hypergravity conditions.  Immunoblot analysis showed  that the centrifugation at 2, 3, 30 or 100G for 10 min did not increased the  amount of ubiquitinated proteins in L6 cells, whereas the centrifugation at  100G for 1 or 2 hrs significantly induced the protein-ubiquitination.  In contrast, heat shock protein 70 (HSP70), another stress-responsive protein, in L6 cells was accumulated only by centrifugation at 100G for more than 10 min.  short-term (10 min) hypergravity including 3 or 100G did not affect the proliferation and morphological changes in L6 cells.  Our present results suggest that the ubiquitination of muscle proteins in less sensitive to hypergravity than the induction of HSP70, and that the effect of hypergravity on protein-ubiquitination and proliferation of skeletal muscle cells may be negligible, as far as its duration in short-term.