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 Thr¹⁸⁷ and Ser¹⁰ is a prerequisite for its degradation via the E3 ubiquitin ligases SKP2 (nuclear) and KPC (cytoplasmic), respectively. Additionally, Ser10 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.