Short-term hypergravity does not affect protein-ubiquitination and proliferation in rat L6 myoblastic cells

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 100 G for 10 min did not increase the amount of ubiquitinated proteins in L6 cells, whereas the centrifugation at 100 G 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 100 G for more than 10 min. Short-term (10 min) hypergravity including 3 or 100 G did not affect the proliferation and morphological changes in L6 cells. Our present results suggest that the ubiquitination of muscle proteins is 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 is short-term.