Itch/AIP4 Associates with and Promotes p63 Protein Degradation

Abstract

P63, a protein related to the tumor suppressor p53, is a transcription factor that plays an important role in epidermal differentiation and limb development. The gene has two distinct promoters that allow the formation of proteins that either contain (TA) or lack (ΔN) a transactivation domain. In addition, alternative splicing at the 3' end generates proteins with different C-termini, denoted α, β and γ for a total of six isoforms. ΔNp63α isoform is the main isoform expressed at all stages of development, however the relative contribution of individual p63 isoform during ectodermal differentiation and organogenesis is still far from understood. Overexpression of ΔNp63 led to increased growth of transformed cells in vitro and in vivo while treatment of keratinocytes with ultraviolet irradiation causes down-regulation of ΔNp63 proteins and their corresponding mRNA. The p63 gene locus is often amplified in squamous cell carcinomas while alterations in the relative levels of TA and ΔNp63 correlate with prognosis in several human cancers suggesting that fine regulation of p63 intracellular levels must be of pivotal importance in controlling cell proliferation, death and differentiation. Despite its relevance little is known on the mechanisms controlling p63 protein levels. Here we show that Itch/AIP4, a HECT E3-ubiquitin ligase, promotes p63 degradation. Using a set of p63 deletion mutants, we have identified a region and two critical lysine residues of p63, associated to human Split-Hand and Foot Malformation-4 (SHFM-4) syndrome, which are involved in the mechanism of Itch-mediated p63 degradation.