Growth Factors in Space (MyoLab) Cbl-b-Mediated Protein Ubiquitination Downregulates the Response of Skeletal Muscle Cells to
MUSCLE CELLS TO GROWTH FACTORS IN SPACE (MYOLAB)
Research Area: Cellular Biology
Expedition(s): 23 and 24
Principle Investigator(s): ● Takeshi Nikawa, MD, PhD, The University of Tokushima,
Tokushima, Japan
RESEARCH OBJECTIVES
Cbl-b-Mediated Protein Ubiquitination Downregulates the Response of Skeletal Muscle Cells to
Growth Factors in Space (MyoLab) studies a rat muscle gene modified cell line to determine the
effects of microgravity.
EARTH BENEFITS
The number of bedridden elderly people in Japan is remarkably increasing, which can be
considered a serious social problem. However, there is no effective countermeasure for muscle
atrophy (decrease in muscle mass), which is a main cause for bedridden conditions. The few
countermeasures for unloading mediated muscle atrophy include: rehabilitation, diet, and
drugs. The MyoLab investigation focuses on the inhibition of Cbl-b-mediated ubiquitination
(enzyme found in humans) to improve IGF-1 (insulin-like growth hormone) resistance of
skeletal muscle cells. Ubiquitin ligase Cbl-b is inhibited by focusing on the competitively
inhibitory function of oligopeptides (molecules containing a small number of peptides).
SPACE BENEFITS
The data collected during this investigation may lead to the discovery of the underlying
mechanism behind muscle atrophy caused by microgravity. Novel countermeasures (ubiquitin
ligase inhibitors and nutrients) against muscle atrophy are being examined in order to allow
crew members to stay in space and perform various projects without muscle atrophy.
RESULTS
Researchers investigated the transcription factors that regulate Cbl-b expression using rat L6
myoblasts and differentiated myotubes. The biological relevance of Cbl-b expression as a sensor
of unloading is strengthened by the findings that both oxidative stress and 3-D-clinorotation
induced Cbl-b expression in L6 myoblasts and myotubes. These findings suggest that increased
levels of ROS link mechanical stress to downstream signaling pathways. In the present study,
we observed that H 2 O 2 treatment promoted the binding of Egr to the 5'-franking region of Cbl-b
gene. Moreover, 3-D-clinorotation and H 2 O 2 each induced the expression of Cbl-b in a manner
accompanied by the early expression of Egrs 1-3. This is consistent with the findings of another
laboratory using Egr-2 or Egr-3 knockout mice. The results obtained in Egr knockdown studies
(siRNA) confirm that Egr transcription factors play a major role in 3-D-clinorotation-mediated
Cbl-b induction. Together, these data uncover the molecular mechanism through which
mechanical unloading is transduced into biochemical signaling in skeletal muscle.