Antagonistic effect of cyclin-dependent kinases and a calcium-dependent phosphatase on polyglutamine-expanded androgen receptor toxic gain of function

Spinal and bulbar muscular atrophy is caused by polyglutamine (polyQ) expansions in androgen receptor (AR), generating gain-of-function toxicity that may involve phosphorylation. Using cellular and animal models, we investigated what kinases and phosphatases target polyQ-expanded AR, whether polyQ expansions modify AR phosphorylation, and how this contributes to neurodegeneration. Mass spectrometry showed that polyQ expansions preserve native phosphorylation and increase phosphorylation at conserved sites controlling AR stability and transactivation. In small-molecule screening, we identified that CDC25/CDK2 signaling could enhance AR phosphorylation, and the calcium-sensitive phosphatase calcineurin had opposite effects. Pharmacologic and genetic manipulation of these kinases and phosphatases modified polyQ-expanded AR function and toxicity in cells, flies, and mice. Ablation of CDK2 reduced AR phosphorylation in the brainstem and restored expression of Myc and other genes involved in DNA damage, senescence, and apoptosis, indicating that the cell cycle-regulated kinase plays more than a bystander role in SBMA-vulnerable postmitotic cells.

Novel interactions of the von Hippel-Lindau (pVHL) tumor suppressor with the CDKN1 family of cell cycle inhibitors

Germline inactivation of the von Hippel-Lindau (VHL) tumor suppressor predisposes patients to develop different highly vascularized cancers. pVHL targets the hypoxia-inducible transcription factor (HIF-1α) for degradation, modulating the activation of various genes involved in hypoxia response. Hypoxia plays a relevant role in regulating cell cycle progression, inducing growth arrest in cells exposed to prolonged oxygen deprivation. However, the exact molecular details driving this transition are far from understood. Here, we present novel interactions between pVHL and the cyclin-dependent kinase inhibitor family CDKN1 (p21, p27 and p57). Bioinformatics analysis, yeast two-hybrid screening and co-immunoprecipitation assays were used to predict, dissect and validate the interactions. We found that the CDKN1 proteins share a conserved region mimicking the HIF-1α motif responsible for pVHL binding. Intriguingly, a p27 site-specific mutation associated to cancer is shown to modulate this novel interaction. Our findings suggest a new connection between the pathways regulating hypoxia and cell cycle progression.