Download Polyubiquitination in diseases: implications in skeletal muscle

Polyubiquitination in diseases: implications in skeletal muscle atrophy and
vascular calcification
Hyun Kook
Department of Pharmacology, Medical Research Center for Gene Regulation, and National Research
Laboratory for Epigenetic Regulation of Heart and Muscle Diseases, Chonnam National University
Medical School, Gwangju 501-746.Chonnam National University Medical School
Polyubiquitination of lysine initiates proteasome-dependent degradation of target proteins, a process
involved in both the pathogenic mechanisms of various diseases and in normal biological function.
Recently we investigated polyubiquitination involves skeletal muscle atrophy by degradation of
MyoD and vascular calcification by decreasing histone deacetylase 1 (HDAC1). Skeletal muscle
atrophy results from the net loss of muscular proteins and organelles and is caused by pathologic
conditions such as nerve injury, immobilization, cancer, and other metabolic diseases. Ret finger
protein (RFP), also known as TRIM27, works as an E3 ligase in Pax7-induced degradation of MyoD.
Diverse muscle injury states, including sciatic nerve transection, cardiotoxin treatment, and fasting,
up-regulated RFP. RFP induced Pax7 expression. RFP also physically interacted with both Pax7 and
MyoD. RFP and Pax7 synergistically reduced the protein amounts of MyoD but not the mRNA. RFPinduced reduction of MyoD protein was also blocked by MG132, a proteasome inhibitor. The Pax7induced reduction MyoD was attenuated by RFP siRNA and by MG132. RFPR, an RFP construct
that lacks the RING domain, failed to reduce MyoD amounts. RFP ubiquitinated MyoD, but RFP R
failed to do so. Forced expression of RFP, but not RFPR, enhanced Pax7-induced polyubiquitination
of MyoD, whereas RFP siRNA blocked the polyubiquitination. Sciatic nerve injury-induced muscle
atrophy as well the reduction in MyoD was attenuated in RFP knockout mice. Taken together, our
results show that RFP works as a novel E3 ligase in the Pax7-mediated degradation of MyoD in
response to skeletal muscle atrophy. We also demonstrated that MDM2-induced polyubiquitination of
HDAC1 mediates vascular calcification (VC). Loss of HDAC1 activity enhanced VC in vivo and in
vitro. HDAC1 protein was reduced in cell and animal calcification models and in human calcified
coronary artery. Calcification stresses induced MDM2 E3 ligase, which resulted in HDAC1 K74
polyubiquitination. Forced expression of MDM2 enhanced VC, whereas loss of MDM2 by either
siRNA or genetic disruption blunted it. Small peptide spanning HDAC1 K74 worked as decoys to
prevent VC in vitro and in vivo. These results demonstrate a previously unknown polyubiquitination
pathway as well as the involvement of HDAC1 in VC and its signal cascades. Our results suggest
MDM2-mediated HDAC1 polyubiquitination as a new therapeutic target in vascular calcification.