[Effect of dexamethasone on degradation of cultured myotube protein and its mechanism].
OBJECTIVE
To study the effect of dexamethasone on degradation of long-lived protein in myotubes and to elucidate its possible mechanism.


METHODS
After isolating skeletal muscles of rat's hind limb under sterile condition, the myoblasts were proliferated in tissue-block culture. After being passaged and purified, the myoblasts fused into myotubes. The protein in myotubes was radiolabelled with L-[3,5-(3)H]-tyrosine, and then the myotubes were divided into two groups. In group A, the myotubes were cultured in Dubbecco's modified Eagle's medium (DMEM) containing 10% fetal bovine serum (FBS) and 2 mmol/L tyrosine without dexamethasone (control group), with dexamethasone 10 nmol/L (group A1), dexamethasone 100 nmol/L (group A2), and dexamethasone 1,000 nmol/L (group A3), respectively. The amounts of L-[3,5-(3)H]-tyrosine in culture medium and cells were determined and the degradation rates of long-lived protein were calculated at 12, 24, 36, 48 hours of culture. In group B, the myotubes were cultured in DMEM containing 10% FBS and 2 mmol/L tyrosine with 50 micromol/L proteasome inhibitor MG132 (group B1), or 50 micromol/L MG132 and 100 nmol/L dexamethasone (group B2). Proteolytic rates of protein were calculated with the same way as group A at 24 hours.


RESULTS
The proteolytic rate of long-lived protein in group A1, group A2 and group A3 was all increased significantly compared with that in control group (all P<0.01), and that in group A2 and A3 rose markedly compared with that in group A1 (both P<0.01), respectively. There was no statistically significant difference between group A2 and group A3 (P>0.05). Proteolytic rate of long-lived protein in group B1 was significantly lower than that in control group (P<0.01). The degradation rate of long-lived protein in group B2 was markedly lower than that in group A2 (P<0.01).


CONCLUSIONS
The results suggest that the ubiquitin-proteasome system is one of the important pathways of degradation of long-lived protein in cultured myotubes, and dexamethasone could enhance the degradation of long-lived protein in cultured myotubes by activating ubiquitin proteasome pathway in dose-dependent manner.
Shen
Chuan-an
CA
Burns Institute, First Affiliated Hospital of General Hospital of PLA, Beijing 100037, China.
Chai
Jia-ke
JK
Yao
Yong-ming
YM
Sheng
Zhi-yong
ZY
D000818:Animals; D002478:Cells, Cultured; D003907:Dexamethasone / Q000494:pharmacology*; D018485:Muscle Fibers, Skeletal / Q000378:metabolism*; D009124:Muscle Proteins / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D051381:Rats; D025801:Ubiquitin / Q000378:metabolism
D009124:Muscle Proteins; D025801:Ubiquitin; D003907:Dexamethasone; D046988:Proteasome Endopeptidase Complex