PIR2/Rnf144B regulates epithelial homeostasis by mediating degradation of p21WAF1 and p63

Delta Np63 is a transcription factor that is critical for the development of stratified epithelia and is overexpressed or amplified in >80% of squamous cell carcinomas (SCCs). We identified the RING finger E3 ubiquitin ligase PIR2/Rnf144b as a direct transcriptional target of Delta Np63 alpha and showed that its expression parallels that of Delta Np63 alpha in keratinocytes, SCC cell lines and SCCs. We used primary keratinocytes as a model system to investigate the function of PIR2/Rnf144b in stratified epithelia. Depletion of PIR2/Rnf144b severely impaired keratinocyte proliferation and differentiation, associated with accumulation of p21(WAF1/CIP1); a known target of PIR2/Rnf144b. More importantly, we found that PIR2/Rnf144b binds and mediates proteasomal degradation of Delta Np63 alpha, generating a hitherto unknown auto-regulatory feedback loop. These findings substantiate PIR2/Rnf144b as a potentially critical component of epithelial homeostasis, acting downstream of Delta Np63 alpha to regulate cellular levels of p21(WAF1/CIP1) and Delta Np63 alpha.