Human Molecular Genetics, 2003, Vol. 12, No. 13 1497-1506
DOI: 10.1093/hmg/ddg161
© 2003 Oxford University Press
Transglutaminase potentiates ligand-dependent proteasome dysfunction induced by polyglutamine-expanded androgen receptor
1Lady Davis Institute for Medical Research, Sir Mortimer B. Davis-Jewish General Hospital, Montreal, Quebec, H3T 1E2, Canada, 2Department of Human Genetics and 3Department of Medicine, McGill University, Montreal, Quebec, H3A 1B1, Canada, 4Montreal Children's Hospital Research Institute, Montreal, Quebec, H3Z 2Z3, Canada and 5Department of Biochemistry and Pharmacology, Thomas Jefferson University, Philadelphia, PA 19107, USA
Received February 13, 2003; Revised April 14, 2003; Accepted April 28, 2003
Expansion of the CAG trinucleotide repeat encoding glutamine in the androgen receptor gene leads to spinobulbar muscular atrophy (SBMA), a neurodegenerative disorder in a family of polyglutamine diseases with enigmatic pathogenic mechanisms. One established property of glutamine residues is their ability to act as an amine accepter in a transglutaminase-catalyzed reaction, resulting in a proteolytically resistant glutamyl-lysine cross-link. To examine underlying disease mechanisms we investigated the relationship between polyglutamine-expanded androgen receptor and transglutaminase. We found androgen receptor N-terminal fragments are a substrate for transglutaminase. Western blots of the proteins following incubation with transglutaminase show that several different epitopes of the AR appear to be lost. We propose that this is due to the transglutaminase cross-linking of the AR, which interferes with antibody recognition. Furthermore, HEK GFPu-1 cells expressing polyglutamine-expanded androgen receptor and transglutaminase exhibit ligand-dependent proteasome dysfunction; this effect was not observed in the presence of cystamine, a transglutaminase inhibitor. In addition, transglutaminase-mediated isopeptide bonds were detected in brains of SBMA transgenic mice, but not in controls, suggesting involvement of transglutaminase-catalyzed reactions in polyglutamine disease pathogenesis. Our hypothesis is that cross-linked AR cannot to be degraded by the proteasome and obstructs the proteasome pore, preventing normal function. Because of the central role the ubiquitin–proteasome degradation system plays in fundamental cellular processes, any alteration in its function could cause cell death, ultimately contributing to SBMA pathogenesis.
* To whom correspondence should be addressed. Email: lenore.beitel{at}mcgill.ca