Patched-1 pro-apoptotic activity is downregulated by modification of K1413 by the E3 ubiquitin-protein ligase Itchy homolog

ABSTRACT

The Hedgehog (Hh) receptor Patched1 (PTCH1) opposes the activation of Gli transcription factors and induces cell death through a Gli-independent pathway. Here, we report that the C-terminal domain (CTD) of PTCH1 interacts with and is ubiquitylated on K1413 by E3 ubiquitin-protein ligase Itchy homolog (Itch), a Nedd4 family member. Itch induces ubiquitylation of K1413, reduction of PTCH1 at the plasma membrane and degradation, activating Gli transcriptional activity in the absence of Hh ligands. Silencing of Itch stabilizes PTCH1 and increases its plasma membrane retention. Itch is the preferential PTCH1 E3 ligase in the absence of Hh ligands, since of the other seven Nedd4 family members only WW domain-containing protein 2 (WWP2) showed a minor redundant role. Like Itch depletion, mutation of the ubiquitylation site (K1314R) resulted in accumulation of PTCH1 at the plasma membrane, prolongation of its half-life, and increase cell death by hyperactivation of caspase-9. Remarkably, Itch is the main determinant of PTCH1 stability under resting conditions but not in response in the presence of Sonic Hedgehog. In conclusion, our findings reveal that Itch is a key regulator of ligand-independent Gli activation and noncanonical Hh signaling by the governance of basal PTCH1 internalization and degradation.