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

DNp63 is a transcription factor that is critical for the development of stratified epithelia and is overexpressed or amplified in 480%of squamous cell carcinomas (SCCs). We identified the RING finger E3 ubiquitin ligase PIR2/Rnf144b as a direct transcriptional targetof DNp63a and showed that its expression parallels that of DNp63a in keratinocytes, SCC cell lines and SCCs. We used primarykeratinocytes as a model system to investigate the function of PIR2/Rnf144b in stratified epithelia. Depletion of PIR2/Rnf144bseverely impaired keratinocyte proliferation and differentiation, associated with accumulation of p21WAF1/CIP1; a known target ofPIR2/Rnf144b. More importantly, we found that PIR2/Rnf144b binds and mediates proteasomal degradation of DNp63a, generatinga hitherto unknown auto-regulatory feedback loop. These findings substantiate PIR2/Rnf144b as a potentially critical componentof epithelial homeostasis, acting downstream of DNp63a to regulate cellular levels of p21WAF1/CIP1 and DNp63a.