The transcription factor NF-kappaB is activated by the degradation of its inhibitor IkappaBalpha, resulting in its nuclear translocation. However, the mechanism by which nuclear NF-kappaB is subsequently regulated is not clear. Here we demonstrate that NF-kappaB function is regulated by Pin1-mediated prolyl isomerization and ubiquitin-mediated proteolysis of its p65/RelA subunit. Upon cytokine treatment, Pin1 binds to the pThr254-Pro motif in p65 and inhibits p65 binding to IkappaBalpha, resulting in increased nuclear accumulation and protein stability of p65 and enhanced NF-kappaB activity. Significantly, Pin1-deficient mice and cells are refractory to NF-kappaB activation by cytokine signals. Moreover, the stability of p65 is controlled by ubiquitin-mediated proteolysis, facilitated by a cytokine signal inhibitor, SOCS-1, acting as a ubiquitin ligase. These findings uncover two important mechanisms of regulating NF-kappaB signaling and offer new insight into the pathogenesis and treatment of some human diseases such as cancers.

The prolyl isomerase Pin1 is a conserved enzyme that is intimately involved in diverse biological processes and pathological conditions such as cancer and Alzheimer's disease. By catalysing cis-trans interconversion of certain motifs containing phosphorylated serine or threonine residues followed by a proline residue (pSer/Thr-Pro), Pin1 can have profound effects on phosphorylation signalling. The structural and functional differences that result from cis-trans isomerization of specific pSer/Thr-Pro motifs probably underlie most, if not all, Pin1-dependent actions. Phosphorylation-dependent prolyl isomerization by Pin1 remains a unique mode for the modulation of signal transduction. Here, we provide an overview of the plethora of regulatory events that involve this unique enzyme, with a particular focus on oncogenic signalling and neurodegeneration.

SUMMARY Pin1 is a phospho-specific prolyl isomerase that regulates numerous key signaling molecules and whose deregulation contributes to disease notably cancer. However, since prolyl isomerases are often believed to be constitutively active, little is known whether and how Pin1 catalytic activity is regulated. Here we identify death associated protein kinase 1 (DAPK1), a known tumor suppressor, as a kinase responsible for phosphorylation of Pin1 on Ser71 in the catalytic active site. Such phosphorylation fully inactivates Pin1 catalytic activity and inhibits its nuclear location. Moreover, DAPK1 inhibits the ability of Pin1 to induce centrosome amplification and cell transformation. Finally, Pin1 pSer71 levels are positively correlated with DAPK1 levels and negatively with centrosome amplification in human breast cancer. Thus, phosphorylation of Pin1 Ser71 by DAPK1 inhibits its catalytic activity and cellular function, providing strong evidence for an essential role of the Pin1 enzymatic activity for its cellular function.