Membrane anchored polyQ-expanded androgen receptors: a role for S-palmitoylation in spinal and bulbar muscular atrophy (SBMA)?

Androgen receptor (AR) is a cytoplasmic transcription factor whose function is regulated by testosterone/dihydrotestosterone (T/DHT). PolyQ expansion in the AR gene causes SBMA, a neuromuscular disease triggered by hormone binding to AR. S-palmitoylation is a post-translational modification referring to the covalent attachment of palmitate moieties to cysteine residues. Regulated cycles of palmitoylation and depalmitoylation lead to reversible membrane association. This study aims to explore the regulatory mechanism of AR palmitoylation and its potential implications in SBMA. By cDNA transfection, membrane fractionation and confocal microscope analysis performed in HEK 293T cells and motor neuron derived MN1cells, we present evidence that both wild-type AR24Q and mutant AR65Q are in part localized to intracellular membranes and that membrane binding occurs uniquely through palmitoylation. In silico analysis revealed several predicted palmitoylation sites on AR, in addition to the previously described cysteine 807. We performed site-directed mutagenesis to abolish putative AR palmitoylation sites, changing cysteine 577, 580 and 615 into serines and are now evaluating which mutants lose membrane-binding and 3H-palmitate incorporation abilities. AR phosphorylation at serines 215 and 792 by Akt has been shown to reduce polyQ-AR toxicity by impairing AR nuclear translocation, due to acquired DHT treatment resistance. Interestingly, we show that AR molecules bearing Ser215Asp and Ser792Asp mutations, where aspartate mimics constitutive AR phosphorylation, are entirely recovered in membrane fractions, suggesting a correlation between phosphorylation by Akt and AR palmitoylation. We propose reversible palmitoylation as a way to control polyQ-expanded AR function by down-modulating its nuclear translocation and therefore reducing gene transactivation.