p27(Kip1) signaling: Transcriptional and post-translational regulation.

p27(Kip1) is an inhibitor of a broad spectrum of cyclin-dependent kinases (CDKs), and the loss of a single p27(Kip1) allele is thereby sufficient to increase tumor incidence via CDK-mediated cell cycle entry. As such, down-regulation of p27(Kip1) protein levels, in particular nuclear expressed p27(Kip1), is implicated in both disease progression and poor prognosis in a variety of cancers. p27(Kip1) expression is positively regulated by the transcription factor MENIN, and inhibited by oncogenic transcription factors MYC and PIM. However, regulation of p27(Kip1) protein expression and function is predominantly through post-translational modifications that alter both the cellular localization and the extent of E3 ubiquitin ligase-mediated degradation. Phosphorylation of p27(Kip1) at Thr(187) and Ser(10) is a prerequisite for its degradation via the E3 ubiquitin ligases SKP2 (nuclear) and KPC (cytoplasmic), respectively. Additionally, Ser(10) phosphorylated p27(Kip1) is predominantly localized in the cytoplasm due to the nuclear export protein CRM1. Another E3 ubiquitin ligase, PIRH2, degrades p27(Kip1) in both the cytoplasm and nucleus independent of phosphorylation state. As such, inhibition of cell cycle entry and progression in a variety of cancers may be achieved with therapies designed to correct p27(Kip1) localization and/or block its degradation.